Dissertations / Theses on the topic 'Carbides'

Phase formation and transformation in mechanically alloyed iron-rich Sm-Fe-C is the principal subject of this thesis. Ternary Sm-Fe-C is a complicated system. The strategy was therefore to start with a binary system. A series of mechanically alloyed R$ sb2$Fe$ sb{17}$ powders were investigated for a better understanding of both the Sm-Fe alloy system in general and the Sm$ sb2$Fe$ sb{17}$ compound in particular. The objective was to learn (1) what is the steady-state in the mechanically alloyed R$ sb2$Fe$ sb{17},$ and (2) how the 2-17 structure is formed from the mechanically alloyed precursors. Phase formation and transformation in the mechanically alloyed Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ with various carbon contents was then studied. The objective in this case was to learn (1) how the 2-17 structure with interstitial carbon is formed, (2) what is the maximum C content in the 2-17 structure, the critical content $x sb{c},$ and (3) what phase(s) is (are) formed with $x>x sb{c}.$
Phase transformation from Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ to Sm$ sb2$Fe$ sb{14}$C was the second subject for study. As required by this study, the grain refinement process was investigated first. The objective was to prepare the nanocrystalline Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ with various grain sizes. Emphasis was on the ball milling of Sm$ sb2$Fe$ sb{17}$/graphite mixture in the hope of forming a nano-scale mixing of Sm$ sb2$Fe$ sb{17}$ and graphite by ball milling. Solid-solid reaction between the Sm$ sb2$Fe$ sb{17}$ and graphite leading to the formation of nanocrystalline Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ was then studied. The phase transformation from Sm$ sb2$Fe$ sb{17}$ was carried out with nanocrystalline Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ samples. Samples prepared by other methods were also studied. The objective was to learn (1) what the transformation product is and (2) what the kinetics of the phase transformation and its grain size dependence are. (Abstract shortened by UMI.)

Oxidation of uranium carbide (UC) was investigated because it is used as a preliminary treatment prior to storage and permanent disposal of carbide nuclear fuels. Working with UC present some challenges, mainly related to its radioactivity and pyrophoricity, therefore initial studies were conducted on zirconium carbide (ZrC) which is isostructural and exhibits similar chemistry to UC. High temperature environmental scanning electron microscopy (HT-ESEM) was used to examine in situ the oxidation of both ZrC and UC. Oxide products were subsequently analysed using macro to nano characterisation techniques, such as focused ion beam secondary ions mass spectroscopy (FIB-SIMS) and high resolution transmission electron microscopy (HRTEM). Oxidation of ZrC was studied from 1073 to 1373 K in air and at 1073 K in a 200 Pa oxygen atmosphere. UC oxidation was studied from 723 to 1173 K at different oxygen atmospheres (2–100 Pa) and from 873 to 1173 K in air. A key result was the improved understanding of the role of cracking in the oxidation mechanism of both carbides. Cyclic cracking parallel to the carbide/oxide interface and crack propagation at corners was found to be responsible for the Maltese cross shape of the oxide in ZrC. The oxidation mechanism of ZrC was governed by oxygen diffusion through a layer of constant thickness formed by the cyclic debonding of the interface after the oxide layer reached approximately 20 μm at 1073 K. The interface was an approximately 2 μm thick intermediate layer comprising zirconia nanocrystals (≤5 nm) in an amorphous carbon matrix. Crack length stabilisation was characteristic of UC oxidation to UO2+x while an exponential increase of crack length triggered an explosive transformation producing U3O8 in samples oxidised from 723 to 848 K in 2–100 Pa O2 atmosphere. The explosive transformation was caused by UC self-ignition which proceeded as a self-propagating high-temperature synthesis (SHS) reaction through the previously fragmented sample. UC oxidised in air from 873 K to 1173 K showed that better oxide conversion can be achieved at lower temperatures, 873 K, as oxide sintering at higher temperatures, 1173 K, limited further oxidation only on cracked surfaces. Oxide cracking was ascribed to the stresses generated from the volumetric transformation from the carbide to the oxide.

This project treats the W-C-Co-Cr quaternary system at low carbon contents. The main goals with this project were to find a four phase equilibrium between WC, Co-binder phase, η-phase (M6C) and a fourth unknown phase, to discover which the fourth phase is and to establish whether or not WC dissolves any chromium. In order to answer these questions a number of alloys were prepared, the compositions were chosen using thermodynamic calculations and observations from previous alloys. The samples were prepared using a powder metallurgical route and they were sintered at 1410oC. The samples were prepared for analysis via grinding and polishing and then analyzed using LOM, SEM, EDS, WDS and XRD. The results showed that no four phase equilibrium could be found in the analyzed spectra and therefore neither could the fourth phase. However other interesting phenomena were found. The η-phase showed a much higher solubility of chromium than previously thought. According to the current Sandvik-Seco database the η-phase should dissolve a maximum of 0.21 wt% and the maximum value detected using WDS was 8.42 wt%. The solubility of chromium in WC was found to be approximately 5 wt%. The most important conclusion that could be drawn from this project was that the Sandvik-Seco database for the W-C-Co-Cr system still needs to be improved, especially concerning the solubility of chromium in the η-phase.

This thesis concerns thermodynamic modeling of carbides in multicomponent systems. Focus has been made on systems interesting for cemented carbide production but the results are also useful for many other application were the material consist of different carbides, for example tool steels/high speed steels. The Co-W-C system forms the basis in cemented carbide production. An accurate thermodynamic description of this system is therefore crucial for extrapolation into higher order systems. New experimental results on the liquid+fcc+graphite+WC and liquid+fcc+WC+M6C equilibrium temperatures, that has recently been published, shows that these equilibrium temperatures are higher than the values used in the available assessment of Co-W-C. Since an accurate description of these equilibrium temperatures are very important for production of cemented carbides and when extrapolating into higher order systems a reassessment of the Co-W-C system is presented. Cr is sometimes deliberately added to lower the melting point, reduce grain growth and/or increase corrosion resistance in the production of cemented carbides. When adding chromium there is a risk of forming an unwanted M7C3 carbide. It is therefore of great interest to know the stability of this carbide. New experimental results on the maximum solubility of Co in the M7C3 is presented as well as a new thermodynamic description of the Co-Cr-C system which accurately describes the solubility of Co in the M7C3 carbide in the temperature range 1373- 1723 K. The assessment of a system, and the determination of Gibbs energy functions, is straightforward when reliable and consistent thermochemical and phase equilibrium information is available. However, reliable experimental information is often lacking or does not give a unique set of model parameters, and therefore different strategies to estimate information have been developed. In the present work the excess energies for A1-xBxC mixed carbides (where A and B are metals) have been calculated using ab-initio calculations, for 14 systems. A thorough comparison has been made with experimentally assessed excess energies. The comparison shows that ab-initio calculations can be used to predict the sign, magnitude and symmetry of the excess energy for A1-xBxC mixed carbides. The calculated excess energies have also successfully been used to describe several AC-BC systems where the experimental information does not give a unique determination of the excess energy in traditional CALPHAD modeling. Experimental work has also been done on the C-Co-Ti-V-W-Zr system in order to determine the extension of the miscibility gaps in TiC-ZrC and VC-ZrC into the (TiC or VC)-ZrC-WC system. Thermodynamic calculations were used to design samples that will form a miscibility gap in equilibrium with liquid, WC and graphite. Samples were produced from powder and sintered for 1 week in controlled atmosphere at 1300, 1410 and 1500 °C. From the microstructure it could be concluded that the samples form a miscibility gap in equilibrium with liquid, WC and graphite at all temperatures. The composition of the MCx carbides was measured using an analytic SEM. The new experimental information was used to assess the thermodynamic description for the TiC-ZrC system.

The purpose of this work was to model carbide precipitation in steels of a quaternary system which includes two substitutional elements. The work focuses on secondary hardening steels which are used for high-strength components, where hydrogen embrittlement is one of the major factors responsible for failure. It is believed that carbide particles can act as hydrogen trapping sites, thus reducing the risk of embrittlement. The thesis begins with a review of the physical metallurgy of secondary hardening steels and the phenomena of hydrogen embrittlement and hydrogen trapping. The basic theory for the precipitation processes, including the nucleation and diffusion-controlled growth of particles, is reviewed in Chapter 2. Significant progress has recently been made in modelling the overall kinetics of transformations which occur simultaneously. The new theory, also reviewed, adopts classical nucleation and diffusion-controlled growth concepts and takes into account the capillarity effect. In the present work, a modified model has been developed for the precipitation of needle-shaped carbides, and a new model for plate-shaped carbides. The models are then verified experimentally, using five steels designed specifically for this purpose. Using the chemical compositions of the steels and thermodynamic data, the carbide precipitation, dissolution and coarsening kinetics at 600°C were estimated. It is found that reasonable agreement can be obtained between experiment and theory for ternary steels, when multicomponent diffusion and capillarity effects are taken into account. This applies to both needle and plate-shaped particles. The same approach was then used successfully for quaternary steels. For the specific steels studied, M2C- and M4C3-type carbides are expected to be hydrogen trapping sites which improve the hydrogen embrittlement properties. Experimental results on the hydrogen trapping capacity of the steels confirm this expectation and the relationships between the hydrogen trapping capacity and the features of M4C3 carbide particles are discussed. Finally, conclusions are drawn and suggestions are made for future work.

WC cemented carbides are composites consisting of WC and a binder phase. WC/Co is widely used as cutting tools due to its excellent combination of hardness and toughness. This thesis work was performed at the R&D department of Sandvik Coromant and aimed to find the alternative binder phase to substitute cobalt. Several compositions of Fe-Ni and Fe-Ni-Co binder have been investigated in this study. The WC/Co reference samples were also prepared. The initial compositions were decided by the CALPHAD method. The samples were then produced by the means of powder metallurgy. The producing conditions, especially the sintering conditions, were manipulated to achieve full dense and uniform samples. The samples were analyzed by XRD, LOM, SEM, and EDS. Mechanical properties test has also been performed.The results showed that adjustment on carbon content is necessary to attain desirable structure. Increasing Fe content in the binder tends to make the materials harder. For Fe-Ni and Fe-Ni-Co, the martensitic transformation is essential to the mechanical performance. The induced “transformation toughening” in 72Fe28Ni and 82Fe18Ni binders significantly promoted the toughness. Furthermore, the grain growth inhibition by Fe was confirmed. The relations between sintering temperature, grain size and mechanical properties have been discussed. Compared with the WC/Co references, several compositions showed close and even superior mechanical performance which might provide solutions for the future alternative binder phase.

In view of the extensive use of Cr as a grain growth inhibitor in WC-Co cemented carbides this thesis comprises a combined experimental and ab initio study of a number of critical issues pertaining to phase equilibria of the subsystems to the W-Co-Cr-C system. To be able to predict which of the stable Cr-carbides forms above the solubility limits of Cr and C in the fcc phase the respective solubility of Co in Cr23C6, Cr7C3 and Cr3C2 are investigated experimentally. Furthermore, the site occupancies of Co in Cr7C3 are investigated by neutron diffraction as a first step towards a more realistic model for this phase. The energetics of the ternary intermetallic R-phase and the unstable Co3C2 end-member compound are investigated by density functional theory. For Co3C2, the quasi-harmonic vibrational Gibbs energy is also calculated. By subsequent CALPHAD assessment an improved thermodynamic description is developed and its agreement with experiment is investigated. The resulting thermodynamic description allows for improved accuracy in predicting the formation of carbide phases as a function of C content and Co/Cr ratio as well as liquidus and solidus temperatures. Remaining issues may be attributed to the thermodynamic description of the W-Cr-C system and the stability of the Cr-based carbides in the Cr-C system. In the case of the Cr-based carbides, severe experimental scatter prevents an accurate determination of the stability of either of them. As a first attempt to resolve the situation, a state of the art ab initio approach is applied to calculate the finite temperature thermodynamic properties of Cr3C2, benchmarked with reported heat capacity and relative thermal expansion measurements.

QC 20151029

The aim of the present work is to study the formation of the cubic carbide phase (gamma phase) free gradient zone and the gamma cone structure at the edges of gradient sintered cemented carbides. Four types of cemented carbides; WCTi(C,N)-Co, WC-Ti(C,N)-Ni, WC-Ti(C,N)-Fe, WC-(Ti,Ta,NB)(C,N)-Co were gradient sintered and the thicknesses of the gradients were measured. Formation of the gradients is simulated and the simulations results are compared with experimental data. For all of the one-dimensional simulations, the DICTRA [1] software is used. The two-dimensional simulations are carried out by using a new simulation tool which is called “YAPFI”. The YAPFI software is a tool for simulation of diffffusion in multiphase systems along one, two, or three spatial coordinates. Various numerical parameters have been studied by running less computationally demanding one-dimensional simulations. The optimized parameters are used to setup the two-dimensional simulations. Two different kinetic databases were used in the simulations. The effect of different so-called labyrinth factors were studied systematically. The simulation results are in close agreement with the experimental observations, although some anomalies are present in the results. Results of the two-dimensional simulations show the formation of the gamma cone at the edges of the insert.

Alloy B is relatively new precipitation hardening superalloy. It´s applications are in the hot sections of the aero engines, rocket nozzles, gas turbines and in the chemical and petro-leum applications. The alloy is characterized by keeping high strength at elevated tempera-tures and high creep resistance. It´s excellent mechanical properties and corrosion resis-tance are due to the balanced amount of the coherent γ' matrix, combined with other alloy-ing elements and carbides. There are three types of carbides which can be found in nickel-based superalloys: MC, M 23C6 and M6C. Primary MC carbides act as source of carbon for the secondary carbides, which precipitate at the grain boundaries. They can have strengthening effect by hindering the movement of dislocations. In this work both simulation and experimental analysis are conducted in order to investi-gate the behaviour of the secondary carbides. JMatPro simulation is used to predict the behaviour of the material. Heat treatments are conducted at soak temperatures ranging from 920 °C to 1130 °C, with steps of 30 °C, and dwell times of 0.5, 1, 2 and 24 hours. Experimental methods included analysis at LOM, SEM, EDS, manual point counting and hardness tests. Main results show chromium rich M 23C6 carbides are stable at lower temperature compared to molybdenum rich M6C. Both appear as fine and discrete particles at the grain boundaries at 1070 °C. This morphology is believed to be beneficial for the mechanical properties of the alloy. The volume fraction varies between 0.6 and 1.3%. Hardness values are relevant in the range of 920-1010 °C. Above this range there is sudden drop of the hardness

A tutorial review on cellular as well as nanoporous carbides covering their structure, synthesis and potential applications. Especially new carbide materials with a hierarchical pore structure are in focus. As a central theme silicon carbide based materials are picked out, but also titanium, tungsten and boron carbides, as well as carbide-derived carbons, are part of this review
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Steel turning inserts cemented carbides have a binder phase consisting of cobalt (Co). However, in recent years a study from the United States National Toxicity Program (NTP) found that cobalt powder is carcinogenic upon inhalation. The European Union's REACH have therefore also classified cobalt powder as carcinogenic upon inhalation. The worldwide search to find a replacement has therefore lately intensified. It is important that the alternative binder phase has no negative effects on the properties of the insert. In this thesis the adhesion between a multilayer ceramic chemical vapor deposition (CVD) coating and a cemented carbide with the alternative binder phases consisting of iron (Fe), nickel (Ni) and cobalt (Co) has been studied. First of all, the fracture surfaces showed that the CVD coating was able to grow on all cemented carbides, regardless of which binder phase. To evaluate the adhesion, scratch tests were performed on all samples. The results from the scratch tests were not as expected. No chipping of the coating down to the cemented carbide occurred on any of the samples and the samples with the hardest cemented carbide did not get the highest critical load, which it should according to the literature if all other parameters were the same. Instead the sample with the binder phase consisting of 73 wt% iron and 27 wt% nickel had the highest critical load. This is thought to be due to that during the scratch test the binder phase in this cemented carbide would most likely transform into deformation martensite.

The development of novel efficient catalytic materials to improve the efficiency of industrial processes has been the driving force for many academic and industrial studies. The general approach adopted to enhance the activity of a given catalytic formulation is usually based on enhancing the structural and structural properties (e.g. crystal size and surface area) by adopting new synthesis methods, by supporting the active phase or by modifying the reactivity of the parent materials by adding dopants. However, in a less studied approach, it has been shown that the presence of interstitial species such as carbon or nitrogen can modify the electronic structure of parent metals apparently conferring, in the case of systems such as molybdenum carbide, properties akin to precious metals. This approach allows not just improvement of the catalytic activity in an incremental manner but also the design entirely new catalytic formulations. In this context, the effect of the interstitial elements carbon and nitrogen upon the activity of a range binary and ternary molybdenum based materials for ammonia synthesis and methane cracking has been investigated within this thesis. The performance of Co3Mo3N, Co3Mo3C, and Co6Mo6C for ammonia synthesis has been compared. Depending on the chemical composition, significant difference in catalytic activity was apparent. In contrast to Co3Mo3N, which is active at 400 °C, Co3Mo3C was found to be only active at a reaction temperature of 500 °C. Furthermore, in-situ NPD revealed that the catalytic activity of ternary cobalt molybdenum systems is associated with the presence of N in the 16c Wyckoff crystallographic site. Co6Mo6C was found to be inactive under the conditions tested. The same comparison between the chemical composition and the catalytic activity has been made in the context of methane cracking. Although all the prepared materials (i.e. Co3Mo3N, Co6Mo6N, Co3Mo3C, and Co6Mo6C) displayed catalytic activity, differences as a function of chemical composition were observed. Among the evaluated catalysts, the Co6Mo6N sample showed the highest activity. However, in-situ and post-reaction analysis revealed a significant phase transformation during reaction which explains the differences in terms of catalytic reactivity. In summary, this thesis details a comparison between the catalytic performance of a range of binary and ternary molybdenum based materials presenting different chemical compositions. Particular attention has been directed towards the role of, and the interconversion between, lattice C and N species with the intention of elucidating their influence upon catalytic behaviour.

The usage of cobalt (Co) as binder phase material in cemented carbides has been questioned becauseof the potential health hazards associated with cobalt particle inhalation. Cobalt is used because ofits excellent adhesive and wetting properties, combined with adequate mechanical properties. Thepurpose of this work is to investigate the mechanical properties of Fe-Ni bulk alloys and WC-Cocemented carbides using Integrated Computational Materials Engineering (ICME) methods com-bined with FEM data. The report investigates the mechanical properties of several bulk alloys inthe Fe-Ni system as a function of void size and fraction. FEM indentation and FEM fracture datais interpolated and used to model the hardnessHand fracture toughnessKIc. A precipitationhardening model based on the Ashby-Orowan’s equation is implemented to predict the e↵ect on theyield strength from precipitated particles. A model for solid solution hardening is also implemented.Existing models are used to simulate the properties of WC-Co cemented carbides together with thesolid solution hardening model. Results show that the simulated properties of the Fe-Ni bulk alloysare comparable to those of cobalt. However, the results could not be confirmed due to a lack ofexperimental data. The properties of WC-Co cemented carbides are in reasonable agreement withexisting experimental data, with an average deviation of the hardness by 11.5% and of the fracturetoughness by 24.8%. The conclusions are that experimental data for di↵erent Fe-Ni bulk alloys isneeded to verify the presented models and that it is possible to accurately model the properties ofcemented carbides.
Anv¨andandet av kobolt (Co) som bindefas-material i h°ardmetall har blivit ifr°agasatt som en f¨oljdav av de potentiella h¨alsoriskerna associerade med inhalering av koboltpartiklar. Kobolt anv¨ands p°agrund av dess utm¨arkta vidh¨aftande och v¨atande egenskaper, kombinerat med tillr¨ackliga mekaniskaegenskaper. Syftet med detta arbete ¨ar att unders¨oka de mekaniska egenskaperna hos Fe-Ni bulklegeringarochWC-Co h°ardmetall genom att anv¨anda Integrated Computational Materials Engineering(ICME) metoder kombinerat med FEM-data. Rapporten unders¨oker de mekaniska egenskapernahos flera bulklegeringar i Fe-Ni systemet. FEM-indentering och FEM-fraktur data interpoleras ochanv¨ands f¨or att modellera h°ardheten H och brottsegheten KIc. En modell f¨or utskiljningsh¨ardningbaserad p°a Ashby-Orowans ekvation implementeras f¨or att f¨oruts¨aga e↵ekten p°a brottgr¨ansen av utskiljdapartiklar. ¨Aven en modell f¨or l¨osningsh¨ardning implementeras. Existerande modeller anv¨andsf¨or att simulera egenskaperna hos WC-Co h°ardmetall tillsammans med modellen f¨or l¨osningsh¨ardning.Resultaten visar att de simulerade egenskaperna hos Fe-Ni bulklegeringar ¨ar j¨amf¨orbara medde f¨or kobolt. Dock kan de inte bekr¨aftas p°a grund av avsaknad av experimentell data. Egenskapernahos WC-Co h°ardmetall st¨ammer rimligt ¨overens med existerande experimentell data, meden genomsnittlig avvikelse av h°ardheten med 11.5% och av brottsegheten med 24.8%. Slutsatserna¨ar att det beh¨ovs experimentell data f¨or Fe-Ni bulklegeringar f¨or att kunna verifiera modellernasnoggrannhet och att det ¨ar m¨ojligt att f¨oruts¨aga egenskaperna hos h°ardmetall.

In this Master Thesis work, the effect of pressure on sintering of cemented carbides is investigated. Special focus hasbeen given to the residual porosity after sintering. It is well known that sintering shrinkage depends on binder phasecontent, grain size, temperature and pressure. Thus 4 different cemented carbides grades were selected. The gradeswere pressed into standard products and TRS (Tensile Rupture Strength) rods with two different shrinkage factors.These were then sintered at different pressures and temperatures. Thereafter the impact of pressure on propertiessuch as Density, Coercivity, Porosity and TRS were analyzed. The observations were further supported with Weibullprobability analysis and fracture surface analysis in SEM.It has been shown that there exists a distinct threshold pressure at which significant reduction in porosity occurs forgiven compacted densities. For 3 out of 4 cemented carbide grades it has been observed that P2 sintering pressure issufficient to meet desired product characteristics. The fourth grade required a minimum sintering pressure of P4 toachieve desired quality criteria. Moreover it has been concluded that an increased sintering pressure increasesmaterial strength, as compared to vacuum sintering, thereby reducing the amount of early failures caused by poresduring TRS test.                                    Keywords: Cemented Carbides, Pressurized Sintering, Tungsten carbide, Porosity, Vacuum sintering, TensileRupture Strength, Weibull analysis, Fracture surface analysis.

A tutorial review on cellular as well as nanoporous carbides covering their structure, synthesis and potential applications. Especially new carbide materials with a hierarchical pore structure are in focus. As a central theme silicon carbide based materials are picked out, but also titanium, tungsten and boron carbides, as well as carbide-derived carbons, are part of this review.
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Ternary transition metal carbides with inherently nanolaminated crystal structure are a class of materials with typically higher damage tolerance, better machinability and lower brittleness compared to the binary counterparts, yet retaining their satisfactory electrical and thermal conductivity. Their interesting properties can be related to the laminated structure. Though studies of their properties based on calculations and bulk materials have suggested potential thin film applications, such as high temperature hard coatings and electrical contacts, a relatively small number of these phases have been synthesized as thin films.  Investigation of thin film deposition of these inherently nanolaminated materials further the understanding of their phase formation and crystal growth. Motivated by predicted superconductivity and thermoelectric properties of molybdenum carbides and related layered molybdenum compounds, nanolaminated materials in the Mo-Ga-C ternary system were studied. Apart from the previously reported Mo2GaC, a new layered carbide, Mo2Ga2C, was synthesized in both thin film and bulk form with a postulated crystal structure related to Mo2GaC. The proposed structure was further validated by first principles calculations, showing higher stability compared to other crystal structure as well as other competing phases. The calculated lattice parameters were consistent with values from Rietveld analysis of X-ray and neutron diffraction patterns. In addition, both scanning transmission electron microscopy and X-ray photoelectron spectroscopy showed experimental evidence of the close structural-chemical relation between Mo2Ga2C and Mo2GaC. Driven by a need of high temperature protective coatings in nuclear applications, Zr-based nanolaminated carbides have become more attractive. In this work, another nanolaminated carbide, Zr2Al3C4, was synthesized in thin film form by pulsed cathodic arc deposition. Formation of the Zr2Al3C4 phase and its competing phases was studied with X-ray diffraction of thin films deposited with varying incoming flux compositions, temperatures and substrate materials. On 4H-SiC(001) substrates, highly phase-pure epitaxial Zr2Al3C4 films were formed, whereas depositions on Al2O3(001) substrates resulted in competing phases. A growth behavior similar to that of nanolaminated Mn+1AXn phases (M is a group 3-7 transition metal; A is commonly a group 13-14 element; X is C or N; n = 1 - 3) was observed, despite the structuraland chemical differences between Zr2Al3C4 and MAX phases.

The series name Linköping Studies in Science and Technology Licentiate Thesis is incorrect. Correct series name is Linköping Studies in Science and Technology. Thesis.

Metal dusting corrosion has been known for more than a 100 years as an industrial problem. As a result of extensive research over the last five decades several mechanisms have been evolved involving ferritic materials. However, a complete understanding is yet lacking. One of the most referred models, developed by Hochman-Grabke, suggests that formation of metastable cementite and its subsequent decomposition is the central aspect of the process. To verify this hypothesis, an Fe-Si model was designed based on silicon's ability to retard cementite formation. However, this strategy was unsuccessful because silicon oxidized and amount of silicon remaining after silica formation was not sufficient to suppress cementite. On the other hand, germanium does not form a stable oxide in the conditions employed. A alloying with germanium did prevent Fe3C formation, but not dusting, which resulted from an alternative mechanism. Dusted particles were confirmed to be cementite for pure iron specimens (where cementite scale formed) and ferrite for alloys that did not form cementite. These observations are inconsistent with the prior model. In addition, the general features of metal dusting corrosion have been characterized. Kinetics of coking and metal wastage for ferritic materials (Fe, Fe-Si, Fe-Ge and Fe-Ge-Ni) were found to be linear in nature, though respective rates may vary due to the differences in alloy catalytic activity and reaction morphologies. The carbon diffusion coefficient in cementite was evaluated from Fe3C scaling rates. Crystallographic orientations of different forms of cementite were established. Internal cementite precipitates in pure iron accounted for by a very high degree of supersaturation with respect to carbon, indicating a non-equilibrium situation. Coking and dusting rates were found to be strongly correlated and their gas composition dependence indicate the contribution of the Boudouard reaction. Reactions with fixed carbon activity gases demonstrated that kinetics rather than thermodynamics control the reaction rates. However, at a particular temperature, these rates increase with carbon activity. Activation energies for coking and dusting are equal for a given alloy, meaning that the same process controls them. For Fe-lOGe alloy, in the early stages of reaction, grains with near (001) surfaces were more susceptible to graphitization than grains having near (110) surfaces, but the underlying cause has not been revealed.

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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Hardmetal industry is continuously seeking for high-performance products at reduced costs. In addition, it is strongly struggled by the high and volatile prices of raw materials. At this juncture, producers and end-users are deeply concerned in increasing the performance and enhancing service-life and reliability of engineering products, and replacing current constituents by alternative and less critical materials. Premature and unexpected fracture, together with wear, is the main damage phenomenon limiting the life in most cemented carbide applications. In the vast majority of cases such ruptures stem from the combination of high monotonic and cyclic stresses, together with different damage-related features associated with harsh service conditions, such as corrosion, and thermal shock. Therefore, relevant consideration of fracture toughness and fatigue resistance is required if reliability and lifetime of hardmetals applications is to be increased. Following the above ideas, the purpose of this thesis is to improve the performance and increase the reliability of cemented carbides in rupture-limited applications on the basis of enhanced damage tolerance and reduced fatigue sensitivity through an optimal microstructural design. Within this framework, this investigation is composed of three main subjects covering different aspects related to the performance of hardmetals under service-like conditions. The first two sections are devoted to conduct a comprehensive study on the influence of the microstructure on fracture and fatigue behaviour of hardmetals. The aim of the third section is to evaluate microstructural effects on the tolerance of cemented carbides to service-like damage, induced either by localised corrosion or thermal shock. Main contribution to toughness in cemented carbides derives from plastic stretching of crack-bridging ductile enclaves at the crack wake, referred to as the multiligament zone. Hence, the development of a multiligament zone implies the existence of a rising crack growth resistance (R-curve) behaviour in cemented carbides. This effectiveness of this toughening mechanism is intimately related to the microstructural characteristics. Within this context, the first section of this thesis is dedicated to carry out a detailed investigation of fracture mechanics and mechanisms in cemented carbides, and to propose a relation to capture microstructural effects on the R-curve characteristics of these materials. Strength reduction of hardmetals under the application of cyclic stresses is related to the inhibition of the crack-tip bridging mechanism. For WC¿Co cemented carbides, the degradation of bridging ligaments is mainly associated with an accumulation of the fcc to hcp phase transformation. However, this mechanism does not apply for Ni binders; therefore, it remains unclear if effective fatigue susceptibility of Co-base hardmetals is comparable to that of cemented carbides consisting of alternative binders. Moreover, hardmetals exhibit crack-deflection as an additional toughening mechanism, but contrary to the case of crack-bridging, it is immune to fatigue loads. The effective action of this toughening mechanism is speculated to increase with rising carbide mean grain size. Hence, the second part of this thesis is devoted to study and understand the fatigue sensitivity of cemented carbides consisting of binders with deformation mechanisms beyond phase transformation as well as medium/coarse microstructures. Finally, the third section of this thesis consists of a systematic study on the influence of the microstructure on damage-related features induced by either thermal shock or corrosion, in order to set out guidelines for optimal microstructural design. In doing so, the structural integrity of damaged cemented carbides is assessed on the basis of residual strength, and microstructural effects on damage tolerance are captured by means of considering induced damage level as a critical parameter.
Por un lado, la industria del metal duro está sumergida en una búsqueda constante de materiales de altas prestaciones a un coste reducido. Por el otro lado, las materias primas tienen precios altos y volátiles, que comprometen la estabilidad del mercado. En esta coyuntura, los productores y los usuarios finales están muy interesados, tanto en aumentar el rendimiento, incrementar la vida útil y mejorar la fiabilidad de estos productos, como en su sustitución por materiales alternativos y considerados menos críticos. En este contexto, el desgaste y la ruptura prematura son los dos principales mecanismos que limitan la vida útil de las aplicaciones de metal duro. En la gran mayoría de los casos las rupturas prematuras derivan de la combinación de altas tensiones, tanto monótonas como cíclicas, con el daño inducido durante la vida en servicio, como la corrosión, y el choque térmico. Por lo tanto, con el fin de aumentar fiabilidad en estas aplicaciones, es necesario entender los mecanismos de daño y fallo en estos materiales. Así, el propósito de esta tesis es mejorar el rendimiento y aumentar la fiabilidad de los carburos cementados a partir del desarrollo de materiales con una mayor tolerancia al daño y una menor sensibilidad a fatiga, a través de un óptimo diseño microestructural. La presente investigación se compone de tres partes que abarcan diferentes aspectos relacionados con el desempeño de los metales duros en condiciones de servicio. Las dos primeras secciones están dedicadas a realizar un estudio general sobre la influencia de la microestructura en el comportamiento a fractura y fatiga del metal duro. El objetivo de la tercera sección es evaluar los efectos microestructurales en la tolerancia al daño de los carburos cementados, ya sea inducido por corrosión o por choque térmico. El principal mecanismo de tenacidad en los carburos cementados reside en el estiramiento plástico de ligamentos metálicos de puenteo que se forman detrás de la punta de la grieta, llamada la zona de multiligamentos. El desarrollo del mecanismo de puenteo implica un incremento de la resistencia a fractura a medida que aumenta la longitud de la grieta. Este mecanismo es conocido como curva-R y su eficacia está íntimamente relacionada con las características microestructurales del material. Así, la primera parte de esta tesis doctoral está dedicada a llevar a cabo una investigación detallada de los mecanismos de fractura en los carburos cementados, y a proponer una relación que permita captar los efectos microestructurales en las características de curva-R de estos materiales. Por otro lado, la segunda parte de la tesis está dedicada a estudiar la influencia de la microestructura, incluyendo tanto el tamaño de grano de la fase carburo como el contenido de la fase ligante y su naturaleza química, en la sensibilidad a fatiga de los carburos cementados. Así, se ha prestado una atención particular en estudiar el comportamiento a fatiga de los carburos cementados con base níquel y en su comparación con los de base cobalto. Por otro lado, también se ha estudiado la influencia del tamaño de grano en la deflexión de grieta como un mecanismo adicional de aumento de tenacidad, inmune a las solicitaciones cíclicas. Por último, la tercera sección de esta tesis consiste en un estudio sistemático de la influencia de la microestructura de los carburos cementados en su tolerancia al daño, inducido tanto por corrosión como por choque térmico, con el fin de establecer las directrices para un diseño microestructural óptimo. De este modo, la integridad estructural de carburos cementados se evalúa sobre la base de su resistencia residual a flexión después de la inducción de daño

Cemented carbides are composite materials widely used in different industry fields within applications involving wear, due to their outstanding wear resistance. The most commonly used are WC-Co grades, for Co wettability with the carbide and adhesion characteristics. Emergence of new applications, the existence of advanced characterization techniques, economic and environmental aspects, among others, encourages the development of a new cemented carbides generation containing other binding phases as Ni and Fe or alloys of them. Furthermore, Co powder has been classified as very toxic for the human health and the combination carbide-cobalt hardmetals dust has shown to be even more toxic than both pure cobalt and tungsten. The success of substitution of the main constituents of cemented carbides, have been commonly measured in terms of their final mechanical properties at macroscale such as hardness, toughness and transverse rupture strength; and structural integrity under service-like conditions, such as corrosion resistance, thermal shock and fatigue resistance. In this sense, general framework of microstructural effects – carbide mean grain size, volume fraction and chemical nature of constitutive phases - on the mechanical response of cemented carbides is well established at the macroscale. However, assessment of the individual role of the binder and carbide phases at local scale i.e. microscale, is yet to be studied in depth. Within micromechanical testing, special attention has being paid to the micropillar compression approach because its advantages: the stress-state is nominally uniaxial, allowing a straight conversion of the measured load-displacement data into flow curves; sample preparation by means of Focused Ion Beam (FIB) milling is a relatively easy machining route; it involves the use of a conventional nanoindenter with a flat-end tip; and, it can be performed ex-situ or in-situ by using Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM) techniques. However, attention have to be paid to sample sizes since it has been well established that intrinsic properties of crystalline materials such as yield stress and strength, can be greatly influenced by extrinsic factors such as volume. For instance, results have evidenced an inverse relation between hardness and the indentation depth at the micro- and nanometric length scales. Regarding cemented carbides, recent studies showed that changes in volume fraction of binder and carbides in samples can lead to wide scatter in results of Young’s modulus measured at the microscale. Following the above ideas, in this PhD thesis uniaxial compression of micropillars and nanoindentation have been selected to evaluate the role of binder and carbides regarding their chemical nature and microstructural dimensions, i.e. carbide mean grain size and binder mean free path, in the mechanical properties and response of cemented carbides at local scales. This thesis is presented by a compendium of scientific publications in which several specific objectives are studied individually. In the first and second publications the sample size and the volume fraction of constitutive phases within the micropillar are studied respectively. Results allowed to overcome the size effect issue – usually found when testing in the micro or nanometer regime – by selecting an appropriate sample size, to accomplish reliability on the mechanical properties evaluated at local length scales. Third and fourth publications are devoted to investigating the mechanical properties of cemented carbides with partial or total substitution of WC or Co as main constitutive phases based on their intrinsic mechanical properties and behavior. Outcomes evidence that small scale testing of complex composite materials such as cemented carbides by means of uniaxial compression of micropillars and nanoindentation, allows to evaluate the role of each constitutive phase on their mechanical behavior.
Los carburos cementados son materiales compuestos ampliamente utilizados en diferentes campos de la industria dentro de aplicaciones que implican desgaste, debido a su excelente resistencia al mismo. Los más utilizados son los grados WC-Co, debido a la buena mojabilidad del Co con el carburo. La aparición de nuevas aplicaciones, la existencia de técnicas avanzadas de caracterización, y aspectos económicos y ambientales, fomenta el desarrollo de una nueva generación de carburos cementados que contiene otras fases ligantes como Ni y Fe o sus aleaciones. Además, el polvo de Co ha sido clasificado como muy tóxico para la salud humana y la combinación de polvo de metal duro de carburo y cobalto ha demostrado ser aún más tóxico que el cobalto y el tungsteno puros. El éxito en la sustitución del Co y WC en carburos cementados es medido comúnmente en términos de sus propiedades mecánicas finales a escala macro, como dureza, tenacidad y resistencia; y de su integridad estructural en condiciones de servicio, como resistencia a la corrosión, choque térmico y resistencia a fatiga. En este sentido, los efectos microestructurales (tamaño medio de WC, fracción de volumen y naturaleza química de las fases constitutivas) sobre la respuesta mecánica de estos materiales están bien establecidos a macroescala. Sin embargo, el papel individual cada fase a escala local, es decir, microescala, aún no se ha estudiado en profundidad. Dentro de los ensayos micromecánicos, se ha prestado especial atención a la compresión de micropilares debido a sus ventajas: estado de tensión nominalmente uniaxial, permitiendo la conversión directa de los datos medidos de desplazamiento y carga en curvas de flujo; la preparación de la muestra mediante fresado con haz de iones focalizados (FIB) es una ruta de mecanizado relativamente fácil; implica el uso de un nanoindentador convencional con punta plana; y, puede realizarse ex situ o in situ utilizando técnicas de microscopía electrónica de barrido (SEM) o microscopía electrónica de transmisión (TEM). Sin embargo, se debe prestar atención a los tamaños de muestra, ya que las propiedades intrínsecas de los materiales cristalinos, como el límite elástico y la resistencia, pueden verse muy influidas por factores extrínsecos como el volumen. Por ejemplo, resultados han evidenciado una relación inversa entre la dureza y la profundidad de indentación en las escalas de longitud micro y nanométrica. Con respecto a los carburos cementados, estudios recientes mostraron que cambios en la fracción de volume de ligante y carburos conducen a una amplia dispersión en los resultados del módulo de Young medido a microescala. Siguiendo las ideas anteriores, en esta tesis doctoral se ha seleccionado la compresión uniaxial de micropilares y nanoindentación para evaluar el papel del ligante y los carburos con respecto a su naturaleza química y dimensiones microestructurales, es decir, el tamaño medio del grano de carburo y el camino libre medio del ligante, en las propiedades y respuesta mecánica de carburos cementados a escalas locales. Esta tesis es presentada por un compendio de publicaciones científicas en los que varios objetivos específicos se estudian individualmente. En la primera y segunda publicación se estudia el efecto del diámetro del micropilar y la fracción volumétrica de las fases constitutivas dentro del mismo para superar el problema del efecto del tamaño de la muestra, seleccionando un tamaño apropiado para lograr confiabilidad en las propiedades mecánicas evaluadas localmente. Las publicaciones tercera y cuarta se dedican a investigar las propiedades mecánicas de los carburos cementados con sustitución parcial o total de WC o Co, en función del comportamiento mecánico intrínseco de las fases constitutivas. Los resultados demuestran que las pruebas a pequeña escala de materiales compuestos complejos – como los carburos cementados – mediante compresión uniaxial de micropilares y nanoindentación, permiten evaluar el papel de cada fase constitutiva en su respuesta y propiedades mecánicas. Al hacerlo, se debe seleccionar un tamaño de muestra apropiado para obtener resultados confiables del comportamiento general del material.
Els carburs cimentats – també coneguts com a metalls durs – són materials compostos àmpliament usats a diversos camps industrials en aplicacions que comporten desgast, com en eines de tall, mecanitzat o trepat, a causa de la seva excepcional resistència al mateix. Els carburs cimentats més comunament usats són graus de WC-Co, per les característiques d’humectabilitat de cobalt (Co) amb el carbur de tungstè (WC) i la seva adhesió. L’aparició de noves aplicacions, l’existència de tècniques de caracterització avançades, aspectes econòmics i ambientals, entre d’altres, fomenta a el desenvolupament d’una nova generació de carburs cimentats que continguin altres fases d’unió com níquel (Ni) i ferro (Fe) o els seus aliatges. A més, la pols de Co ha estat classificada com a molt tòxica per a la salut humana i la combinació de pols de metall dur carbur-cobalt ha demostrat ser encara més tòxica que el Co o el W purs. L’èxit de la substitució dels constituents principals dels carburs cimentats es mesura habitualment en termes de propietats mecàniques finals, com la duresa, la tenacitat de fractura Palmqvist i la resistència a fractura transversal (TRS) a escala macroscòpica; i en termes d’integritat estructural en condicions similars a servei, com ara la resistència a corrosió, resistència a xocs tèrmics i fatiga, etc. En aquest sentit, el marc general dels efectes de les característiques microestructurals – mida mitjana dels carburs i fracció de volum i naturalesa química de les fases constitutives – en la resposta mecànica dels carburs cimentats està ben establerta en l’escala macroscòpica. No obstant això, encara cal estudiar en profunditat el paper individual de la fase lligant i dels carburs en l’escala local, és a dir, a l’escala micromètrica. Pel que fa als assajos micromecànics, s’ha prestat especial atenció a la compressió de micropilars gràcies als seus avantatges: l’estat de tensions és nominalment uniaxial, permetent una conversió directa de les mesures càrrega-desplaçament a corbes de flux; la preparació de mostres mitjançant un microscopi de feix de ions (FIB) és una tècnica de mecanitzat relativament senzilla; implica l’ús d’un nanoindentador convencional amb una punta plana; i es pot realitzar ex-situ o in-situ mitjançant un microscopi electrònic de rastreig (SEM) o de transmissió (TEM). Tot i això, cal parar atenció a les dimensions de les mostres, ja que està ben establert que les propietats intrínseques dels materials cristal·lins, com ara la tensió i la resistència, poden estar molt influïdes per factors extrínsecs com ara el volum. Per exemple, els resultats han evidenciat una relació inversa entre la duresa i la profunditat d’indentació a les escales micro- i nanomètriques. Respecte als carburs cimentats, estudis recents han demostrat que canvis en la fracció volumètrica de lligant i carburs comporta una àmplia dispersió en els resultats de mòdul de Young mesurat a la microescala. Seguint aquestes idees, en aquesta tesi doctoral s’ha seleccionat la compressió uniaxial de micropilars i nanoindentació per avaluar el paper del lligant i els carburs respecte la seva naturalesa química i dimensions microestructurals, és a dir, grandària mitjana del carbur i camí lliure mig del lligant, en les propietats mecàniques dels carburs cimentats i la seva resposta mecànica a escales locals. Aquesta tesi es presenta com a compendi de publicacions científiques en les quals s’estudien objectius específics individualment. La primera publicació té com a objectiu avaluar l’efecte del diàmetre del micropilar en la resposta micromecànica del WC-Co. A la segona publicació, s’investiguen l’efecte de la mitja mitjana del gra de WC i la fracció de volum de les fases de carbur i lligant. Els resultats han permès superar el problema de l’efecte de mida – habitual quan s’assaja a escales micro- i nanomètrica – seleccionant una mida de mostra adequada per tal d’aconseguir propietats mecàniques fiables avaluades a escales locals. La tercera i quarta publicacions estan dedicades a investigar les propietats mecàniques dels carburs cimentats amb substitució parcial o total de WC o Co com a fase constitutiva principal. En aquest sentit, en la tercera publicació s’usa la tècnica de nanoindentació per avaluar la duresa intrínseca de les fases constitutives i la tensió de flux del lligant constret en un carbur cimentat WC-(W,Ti,Ta,Nb)C-Co. Finalment, en el quart treball s’han estudiat tres materials, un amb Co i dos amb substitució parcial o total de Co com a lligant, respectivament, per tal d’investigar la influència de la naturalesa química del lligant en la resposta mecànica global dels carburs cimentats, segons fenòmens de deformació plàstica i mecanismes de fallada induïts per compressió uniaxial de micropilars. Els resultats derivats de la investigació realitzada durant aquesta tesi doctoral demostren que els assajos a escala petita de materials compostos complexos com ara els carburs cementats mitjançant compressió uniaxial de micropilars i tècniques de nanoindentació permeten avaluar el rol de cada fase constitutiva en les propietats i resposta mecàniques. Per fer-ho, cal seleccionar una mida de mostra adequada per tal d’obtenir resultats fiables del comportament global del material.

For sustainable development, it is highly important to limit the loss of energy and materials in machines used for transportation, manufacturing, and other purposes. Large improvements can be achieved by reducing friction and wear in machine elements, for example by the application of coatings. This work is focused on triboactive coatings, for which the outermost layer changes in tribological contacts to form so-called tribofilms. The coatings are deposited by magnetron sputtering (a physical vapor deposition method) and thoroughly chemically and structurally characterized, often theoretically modelled, and tribologically evaluated, to study the connection between the composition, structure and tribological performance of the coatings. Tungsten disulfide, WS2, is a layered material with the possibility of ultra-low friction. This work presents a number of nanocomposite or amorphous coatings based on WS2, which combine the low friction with improved mechanical properties. Addition of N can give amorphous coatings consisting of a network of W, S and N with N2 molecules in nanometer-sized pockets, or lead to the formation of a metastable cubic tungsten nitride. Co-deposition with C can also give amorphous coatings, or nanocomposites with WSx grains in an amorphous C-based matrix. Further increase in coating hardness is achieved by adding both C and Ti, forming titanium carbide. All the WS2-based materials can provide very low friction (down to µ<0.02) by the formation of WS2 tribofilms, but the performance is dependent on the atmosphere as O2 and H2O can be detrimental to the tribofilm functionality. Another possibility is to form low-friction tribofilms by tribochemical reactions between the two surfaces in contact. Addition of S to TiC/a-C nanocomposite coatings leads to the formation of a metastable S-doped carbide phase, TiCxSy, from which S can be released. This enables the formation of low-friction WS2 tribofilms when a Ti-C-S coating is run against a W counter-surface. Reduced friction, at a moderate level, also occurs for steel counter-surfaces, likely due to formation of beneficial iron sulfide tribofilms. The studied coatings, whether based on WS2 or TiC, are thus triboactive, with the ability to form low-friction tribofilms in a sliding contact.

The formation and oxidation of transition metal nitrides and carbides is reviewed and the crystal structures and types of bonding are discussed. Types of nitrides and carbides are categorized in terms of physical and chemical properties and type of bonding. The principles of sintering are summarised. Tlie theory and applications of thermal analytical techniques are reviewed. Surface area determination and estimation of average crystallite size by the BET method utilizing the adsorption of nitrogen gas at -196 X are explained along with the application of x-ray dififractometry ajid scanning electron microscopy to work in this area. In this present research selected transition metal nitrides and carbides have been oxidised in air and carbon dioxide. Activation energies have been determined for these reactions from isothennal oxidations utilizing the Arrhenius equation and from oxidations at different heating rates utilizing the Kissinger equation. Kinetic schemes for the isothermal oxidations have been proposed based on two models of the reactions, that is half order kinetics in which the rate of reaction is determined by the diffijsion of oxidising gas througli the oxide product and two-thirds order kinetics in which the reaction takes place at the surface of a spherical particle of diminishing size as the reaction pioceeds. Surface area measurements and electron microscopy have been utilized to study the ability of the product formed during the oxidations to sinter. X-ray diffractometry has been used to identify the crystal phases present in the initial nitride or carbide and in the oxidation products. The activation energies of the carbides were found to be lower than that found for the respective nitrides. At low temperatures the carbides oxidised more extensively than the respective nitrides, but at high temperatures the situation was reversed. This is explained in terms of the difference in the preexponential term of the Arrhenius equation. Tlie kinetics were found to be dependent on whether the oxide produced was structurally compatible with the remaining reactant and whether the oxide produced was able to sinter at the temperature used in the experiment.

The magnetic properties, coercivity, Hc, and weight-specific magnetic saturation, CoM, are two important quality characteristics in cemented carbides and ceramic metals, cermets. These properties give information about grain size and binder phase content, and are influenced by the different stages in the sintering process. This master thesis aim to investigate how the magnetic properties in cemented carbides are influenced by the top temperature during sintering and how the sintering processes used for cermets can be optimized in order to gain better magnetic properties in the final products. During the first part of the project, the temperature range investigated was 1380°C–1520°C. The results indicate that Hc in cemented carbides has a strong temperature dependence, where increasing top temperature results in lower Hc. In order to have approval limits for the furnace control pieces that follow the process directives, the limits used today need to become narrower. Furthermore, the results show that CoM also has a temperature dependence, although not as strongly as Hc. During the second part of the project, already existing data of the magnetic properties in four different cermet grades were evaluated. The results indicate that the two sintering processes used in the DDK furnace are generating too high results in Hc and CoM. Optimization tests were conducted and changes implemented in order to gain better results, where the DJ1430 process now has an increased time during the solid state sintering and the DF1480 process now has an increased time during the liquid phase sintering.

Magnetic measurements are useful tools for quality control of cemented carbides. Previous work at Sandvik Mining and Rock Technology has shown that the coercivity increases with increased cooling rate during sintering for a specific grade.  This study aims to investigate why the coercivity changes with the cooling rate and if this is true for other cemented carbide grades as well. Three different cemented carbide grades were sintered with different cooling rates and evaluated with coercivity, Cobalt-magnetic saturation and hardness measurements, and with microscopy and Electron Backscatter Diffraction analysis. It was found that the coercivity increased with increasing cooling rates for the previously studied grade, but not for the two other grades. It was expected that the increased coercivity would indicate a decrease in WC grain size, but the results showed that the WC grain size of the fastest and slowest cooling rate were the same. However, a change in size of the Co areas between the WC grains was found. The fast cooled sample showed smaller Co areas than the slow cooled sample. These Co/WC grain boundaries increase the coercivity. An increased fraction of hcp-Co/fcc-Co was also found for the fast cooled material which also increases the coercivity. No relationship between the hardness and the coercivity or the cooling rate was found.   The contiguity for the different grades was also calculated.  No significant difference in contiguity between the different cooling rates of each material was found but the contiguity values between the different materials differed.  This is probably mainly dependent on the different binder contents of the materials.
Magnetiska egenskaper är en viktig del av kvalitetskontrollen av hårdmetaller.  Tidigare forskning hos Sandvik Mining and Rock Technology har visat att koerciviteten hos en hårdmetallsort ökar med ökad kylhastighet vid sintring.  Målet med den här studien är att undersöka varför koerciviteten ökar med kylhastigheten och om detta även gäller andra hårdmetallsorter.  Tre olika hårdmetallsorter sintrades med olika kylhastigheter undersöktes med mätningar av koercivitet, magnetisk mättnat i koboltfasen och hårdhet,  samt med mikroskopi och Electron Backscatter Diffraction analys. Det  visade  sig  att  koerciviteten  ökade  med  ökad  kylhastighet  för  hårdmetallsorten  som  studerats tidigare, men inte för de andra två sorterna.  En ökad koercivitet förväntas visa på en minskad WC-kornstorlek, men resultaten visar att det långsamt kylda och snabbkylda provet hade samma WC-kornstorlek.  Däremot upptäcktes en ändring av storleken på Co-områdena mellan WC-kornen.  Det snabbkylda provet visade en mindre storlek av Co-områdena jämfört med det långsamt kylda provet. Fler Co/WC-korngränser i det snabbkylda provet leder till en ökad koercivitet.  Fraktionen av hcp-Co/fcc-Co-korngränser ökade också för det snabbkylda provet vilket också ökar koerciviteten. Inget samband mellan hårdheten och koerciviteten eller kylhastigheten upptäcktes.  Beräkning av kontiguiteten för de olika sorterna genomfördes också. Ingen signifikant skillnad i koercivitet mellan de olika kylhastigheterna för the olika sorterna hittades,  men kontiguiteten mellan de olika sorterna varierade. Det beror mest troligt på att dom olika sorterna har olika mängd matrismaterial.

This thesis describes the rapid microwave synthesis and subsequent structural characterisation of the group 13 carbides; specifically aluminium carbide, Al4C3 and boron carbide, typically B4C or B13C2. Due to practical considerations, which will be described in Chapters Three and Four, syntheses were conducted using elemental precursors. Using a multi-mode microwave cavity (MMC) with an operating frequency of 2.45 GHz it was necessary to make use of a sealed and inert environment for the synthesis of oxide-free products. This was rationalised by the high stability of the oxides of aluminium (for example, Al2O3 ΔH= -1675.69 kJ mol-1, Al4C3 ΔH= -206.9 kJ mol-1). For boron carbide, this observation was explained by the tendency for the oxides of boron to volatilise (for example as B2O3). The use of sealed, evacuated tubes facilitated the synthesis of high-purity carbide products in 30 minute timescales. This represents the first report of successful Al4C3 synthesis in a microwave cavity. In addition, the synthesis of boron carbide was achieved in air in just 90 seconds using a 2.45 GHz single-mode microwave cavity (SMC) for the first time. Following synthesis, products were characterised by powder X-ray diffraction (PXD), powder neutron diffraction (PND), Raman spectroscopy and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX). PXD was used for phase identification and preliminary structure refinement. Raman spectroscopy offered supporting information to PXD to confirm phase identity. SEM-EDX offered an insight into product morphology, for example, as a function of time, microwave power and cavity type, along with quantitative purity information. There was no PXD or EDX evidence for oxygen contamination across carbide samples. PND was used to probe defect structure and structural stability at elevated temperature. Microwave synthesised Al4C3 was structurally stable up to 1000 °C and boron carbide up to 400 °C (the maximum temperatures of the respective experiments). Aluminium carbide is reported elsewhere to interact with water. Some initial experiments surrounding the nature of this interaction have been conducted. The preliminary results have indicated potential for intercalation of water into the aluminium carbide structure. Obtaining a definitive structural model for boron carbide is complicated by the difficulty in distinguishing between boron and carbon experimentally. Boron and carbon are neighbours in the periodic table and near-isoelectronic. This complicates characterising the boron-carbon system by PXD, since PXD is mediated by electrons. There are also challenges posed by the system when using PND. The 10B isotope has an extremely high absorption cross section for neutrons; 3835.0(9) barn for 2200 m s-1 neutrons. In addition, the bound coherent neutron scattering length of 11B is very similar to that of carbon; C: 6.6460 fm and 11B: 6.65 fm. In this thesis, natural boron (abundance typically ~80% 11B and 20% 10B) was used to prepare boron carbide for PND experiments. By taking advantage of the contrast in scattering lengths of carbon and 10B, while obtaining high quality data by the presence of the non-absorbing isotope 11B, it was possible to derive a structural model for microwave-synthesised boron carbide. It is expected that the synthetic methods developed in this thesis can be applied to more complex carbide systems and beyond. A particularly encouraging result from this work is the feasibility of synthesising high-purity, crystalline boron carbide in 90 second timescales in open air. Such a synthesis would be compatible with an SMC based continuous flow process which may offer a step reduction in energy usage compared to conventional batch processes.

The microstructure and physical properties of hot pressed (Hf, Ti) C have been investigated with the aim of producing a cutting tool material with similar hardness to that of WC-Co and TiC-based cermets. Sintered samples were hot pressed from HfC0.7 and TiC0.9 powders using powder metallurgical techniques and the processing cycle was optimized for this system. Ni was used as a binder in selected samples and C black was added to compensate for sub-stoichiometry and to aid in the reduction of oxides formed during milling. Microstructural analyses were performed by scanning and transmission electron microscopy (SEM and TEM) and the composition was determined from X-ray diffraction (XRD) and energy dispersive X-ray spectrometry (EDS). The physical properties measured are density and Vickers hardness, and the indentation fracture toughness was determined using the Shetty formula. The fundamental interactions between HfC, TiC and Ni during hot pressing were investigated and the results obtained used to explain the microstructure that develops in samples made from powder mixtures. The interactions studied are the inter-diffusion of HfC and TiC through the solid state, and the dissolution and re-precipitation rate of the carbides in a liquid Ni binder. EDS analysis revealed that the rate at which Ti diffuses into HfC is higher than the rate at which Hf diffuses into TiC. Upper limits to the diffusion coefficients for these processes are determined and show that solid solution carbides will form from HfC + TiC powder mixtures at 2000 ºC in 1 hour if the average powder particle size is less than 5 μm. The diffusion rates decrease with a decrease in hot pressing temperature but mass transport between the phases can be enhanced by addition of a metallic binder. TEM and EDS analysis shows that Ni wets TiC more efficiently than HfC and that the solubility of TiC in Ni is also higher than that of HfC. The grain size of the carbide phases increases with an increase in the rate at which they dissolve into and re-precipitate from the liquid binder. The crystal structure of the binder phase depends on the concentration of Ti and Hf that remain in the binder after cooling and the carbide phase in which the binder is embedded. Analysis of TEM electron diffraction patterns show that the binder phase consists of cubic solid solutions as well as intermetallic and cubic phases in which atomic ordering is observed.

This thesis contains the results of a study focused on cubic carbide and carbonitride grain growth retardation in hardmetals. Large additions of VC, or VC and TiC, or (W,V)C were made to the WC-Co hardmetal, which was then sintered in vacuum or nitrogen. The effectiveness of Ti as the grain growth inhibitor, and the influence of nitrogen sintering on grain coarsening were investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffractrometry (XRD) and atom probe field ion microscope (APFIM). Analysis of vacuum-sintered WC-VC-Co revealed that the cubic carbide grains have a core-rim structure. Ti was found to be a core-rim inhibitor in vacuum-sintered WCVC- TiC-Co. The average cubic carbide grain size distribution for the vacuumsintered materials was narrowed in the Ti-containing hardmetal. The nitrogen-sintered WC-VC-TiC-Co consisted of two phases of cubic carbonitride, one with high Ti-content and the other with high (W,V)-content. The surface region of the nitrogen-sintered WC-VC-TiC-Co was covered with a cubic carbonitride phase. Similar phenomena was observed in nitrogen-sintered WC-VC-Co where two types of carbonitrides were found, those with high V-content and the others with low Vcontent. A gradient zone, consisting of fine WC grains in a Co-rich binder and free from cubic carbonitride grains, was created in the surface region. The nitrogensintered materials consisted of a narrow grain size distribution. The use of (W,V)C as a starting powder affected the mechanical properties of the material with the WC-(W,V)C-Co material being the hardest of those produced in this study. Addition of (W,V)C powder to WC-Co was shown to be the most effective way to limit the cubic carbide grain size during sintering and produce a hard material. The cubic carbide grain size in the material produced this way was the smallest of all studied.

Thesis (Ph. D.)--West Virginia University, 2006.
Title from document title page. Document formatted into pages; contains x, 174 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 155-166).

Thesis (M.S.)--University of Delaware, 2007.
Principal faculty advisors: Jingguang G. Chen, Dept. of Chemical Engineering; and Thomas P. Beebe, Jr., Dept. of Chemistry & Biochemistry. Includes bibliographical references.

In today’s inserts used for metal cutting the binder phase consists of cobalt (Co).However, EU’s REACH programme and the U.S’s National Toxicity Programme(NTP) classified Co as toxic/carcinogenic. Therefore, there is a strong need toinvestigate alternative binder phases. This thesis covers sintering and characterisationof cemented carbide with a binder phase consisting of nickel (Ni) and iron (Fe) withthe composition of 85Ni:15Fe. The aim was to study the gradient formation of turninginsert and find sintering processes to achieve a gradient structure with the targetedthickness of 26 microns. Simulations in ThermoCalc provided a suitable composition and a starting point forsintering parameters. The influences of sintering process parameters, such as holdingtime, temperature and counter pressure on the formation of the gradient zone wereinvestigated. Hot isostatic pressing (HIP) sintering was done in order to study thegradient formation as well as to reduce the porosity when needed. Sintered insertswere analysed by light optical microscopy. It was found that there are at least three possible ways to control the formation ofthe gradient: sintering in vacuum with a holding time of 20 min at 1450°C, sintering at1450°C with a counter pressure of 5 mbar nitrogen, and sintering with a counterpressure of 11.5 mbar followed by a double sinter-hip with 55 bar argon atmosphere.However, only the last process fulfilled the microstructure criteria in terms ofporosity and binder phase distribution. It is clear that the formation of gradient zonesin 85Ni:15Fe can be predicted, however calculations and simulations need to beoptimized in order to get more accurate results.

This thesis aims to describe the ultra-rapid synthesis of a number of important transition metal carbides as well as investigating their reaction mechanisms. 4 binary systems are discussed; Nb-C, Mo-C, Ta-C and W-C, and work carried out on the ternary system, Nb-Ta-C, is also evaluated. Carbide production was investigated from both the oxide and elemental precursors. Ultra-rapid synthesis has been achieved through the development of a reproducible experimental technique and the investigation into a plethora of reaction variables as well as microwave applicators and powers. This resulted in, specifically within the single mode cavity, the completion of the majority of reactions within 20 s. Further development was then built upon the direct relationship observed between phase fraction results (obtained from Powder X-ray Diffraction (PXD) data), in-situ temperature and ex-situ dielectric property measurements; allowing reaction profiles of the various carbides to be mapped, as well as a crucial understanding of the effects of microwave energy on materials at various temperatures. Powder Neutron Diffraction (PND) was also used to evaluate product purity and the C occupancy of the final products, revealing non-stoichiometry which relates directly to the Tc onset observed for the superconducting transition metal carbides. This, in turn, allowed the trends observed for the ternary carbides to be explained, a linear trend does not exist between Tc and C occupancy. In an effort to develop on the understanding of solid state microwave heating, in-situ reaction monitoring techniques were investigated. Through the use of thermal imaging and high speed photography, the W-C system was observed during the crucial initial stages of the reaction process. The information obtained both corroborated previously collected data and allowed a possible reaction mechanism to be alluded to. The observation of localised heating, prior to the beginning of carbide formation, suggests possible high temperatures far exceeding those observed by optical pyrometry. This could well explain the rapid reaction times as well as suggest an interaction mechanism between carbon, an efficient microwave absorber, and tungsten, a low dielectric loss metal.

Two classes of materials are investigated using ab intio methods based on density functional theory. The structural properties, electronic structure and thermodynamic properties of binary and ternary transition metal carbides are discussed in details. In addition, two actinide compounds will be presented. A new actinide monoxide, ThO, is predicted to be stable under pressure, and the weakly correlated UN is investigated as regards to its magnetic properties and electronic structure. The atomic and electronic structures of various types of single defects in TiC such as vacancies, interstitial defects, and antisite defects are investigated systematically. Both the C-poor and C-rich off-stoichiometric Ti1-cCc composition (0.49≤c≤0.51) have been studied. For the electronic structure, the difference of density of states (dDOS) is introduced to characterize the changes produced by the defects. Concerning the atomic structures, both interstitial defects and antisites are shown to induce the formation of C dumbbells or Ti dumbbells. To date, the Ti self-diffusion mechanism in TiC has not been fully understood, and particularly the Ti diffusion is much less studied in comparison with the C diffusion. Therefore, the self-diffusion of Ti in sub-stoichiometric TiC is studied, and the formation energies, migration barriers for Ti interstitials, dumbbells and dumbbell-vacancy clusters are reported. Some of the calculated activation energies are close to the experimental values, and the migration of Ti dumbbell terminated by C vacancies gives the lowest activation energy, which is in good agreement with the experimental data. These studies must be continued to obtain a full description (including phonon contributions, prefactors, etc.) of all the feasible diffusion mechanisms in TiC. The focus is then shifted from the light transition metal carbides to the heavy transition metal carbides. Various structures of Ru2C under ambient conditions are explored by using an unbiased swarm structure searching algorithm. The structures with R3m (one formula unit) and R-3m symmetry (two formula units) have been found to be lower in energy than the P-3m1 structure, and also to be dynamically stable at ambient conditions. The R-3m structure is characterized by emergence of the Ru-Ru metallic bonding, which has a crucial role in diminishing the hardness of this material. The study of correlation and relativistic effects in Ta2AlC is also presented. We have shown that going from a scalar relativity to a fully relativistic description does not have a significant effect on the computed electronic and mechanical properties of Ta2AlC. In addition, the calculations show that the structural and mechanical properties of Ta2AlC are strongly dependent on other details of theoretical treatment, such as the value of the Hubbard U parameter. The comparison between our results and experimental data point to that Ta2AlC is a weakly correlated system, which originates from that the 5d band is relatively wide in comparison with that of the 3d band. The existence of a rock salt Thorium monoxide (ThO) under high pressure is theoretically predicted. A chemical reaction between Th and ThO2 can produce a novel compound thorium monoxide under sufficient external pressure. To determine the pressure range where this reaction can be observed, we have identified two extreme boundaries by means of different theoretical approaches. The first one is given by a fully relativity DFT code in local density approximation (LDA). The second one is given by a scalar relativistic DFT code in generalized gradient approximation (GGA). It is found that ThO is energetically favored between 14 and 26 GPa. The f orbitals are filled at the expense of s and d electrons states of Th metal, under the action of pressure. The d-p hybridization leads to the stability of metallic ThO. Dynamical stability is also investigated by computing the phonon dispersions for the considered structures at high pressure. The electronic structure and magnetic properties of a promising nuclear fuel material, uranium mononitride (UN), are studied by means of density functional theory (DFT) and several extensions, such as dynamical mean-field theory (DMFT), disordered local moment (DLM) approach, and the GW method. The role of the relativistic corrections is analyzed for different levels of approximation. The importance of correlation effects is assessed through a detailed comparison between calculated electronic structure and measured photoemission spectrum, which helps to clarify the dual itinerant/localized nature of the 5f states of U in UN. Important effects are also observed for the 2p states of nitrogen, which are positioned at much lower energies that are difficult to be well treated in the conventional electronic structure calculations.

QC 20141219

The atomic-scale dynamical processes at play during film growth cannot be resolved by even the most advanced experimental methods. As such, computational methods, and chiefly classical molecular dynamics, are the only available research tools to study these processes. The investigation of key dynamical processes during thin film growth yields a deeper understanding of the film growth evolution, ultimately allowing for the optimization of experimental parameters and tailoring of film properties. This thesis details the study of fundamental surface dynamics processes, and the role played by primary diffusing species, during TiN film growth, here employed as a model system for transition metal nitrides in general. It is found that Ti adatoms and TiN2 admolecules are the fastest diffusing species, and the species which most rapidly descend from islands onto the growing film. Thus, they are the main contributors and players in driving the layer-by-layer growth mode. TiN3 admolecules, in contrast, are essentially stationary and thereby promote multilayer growth. Large-scale growth simulations reveal that tailoring the incident N/Ti ratio and N kinetic energy significantly affects the growth mode and film microstructure. The mechanical properties of ternary transition metal nitride and carbide alloys, investigated using density functional theory, are also discussed herein, in comparison to recent experimental results. By optimizing the valence electron concentration in these compounds, the occupation of shear-compliant d‑t2g electronic states can be maximized. The investigation of M1M2N alloys, where M1 = Ti or V and M2 = W or Mo, with different structures demonstrates that this optimization leads to enhanced ductility, and thereby toughness, in transition metal nitride alloys regardless of the degree of ordering on the metal sublattice. Estimations based on the calculation of the mechanical properties of the corresponding M1M2C transition metal carbide alloys indicate that these materials remain brittle. However, charge density analysis and calculations of stress/strain curves reveal features commonly associated with ductile materials.

La demande urgente d'innovations dans le domaine du stockage de l'énergie est liée au développement récent de la production d'énergie renouvelable ainsi qu'à la diversification des produits électroniques portables qui consomment de plus en plus d'énergie. Il existe plusieurs technologies pour le stockage et la conversion électrochimique de l'énergie, les plus notables étant les batteries aux ions lithium, les piles à combustible et les supercondensateurs. Ces systèmes sont utilisés de façon complémentaire des uns aux autres dans des applications différentes. Par exemple, les batteries sont plus facilement transportables que les piles à combustible et ont de bonne densité d'énergie alors que les supercondensateurs ont des densités de puissance plus élevés et une meilleure durée de vie. L'objectif principal de ces travaux est d'étudier les performances électrochimiques d'une nouvelle famille de matériaux bidimensionnel appelée MXène, en vue de proposer de nouvelles solutions pour le stockage de l'énergie. Pour y arriver, plusieurs directions ont été explorées. Dans un premier temps, la thèse se concentre sur les supercondensateurs dans des électrolytes aqueux et aux effets des groupes de surface. La seconde partie se concentre sur les systèmes de batterie et de capacités à ions sodium. Une cellule complète comportant une anode en carbone et une cathode de MXène a été développées. La dernière partie de la thèse présente l'étude des MXènes pour les supercondensateur en milieu organique. Une attention particulière est apportée à l'étude du mécanisme d'intercalation des ions entre les feuillets de MXène. Différentes techniques de caractérisations ont été utilisées, en particulier la voltampérométrie cyclique, le cyclage galvanostatique, la spectroscopie d'impédance, la microscopie électronique et la diffraction des rayons X
An increase in energy and power densities is needed to match the growing energy storage demands linked with the development of renewable energy production and portable electronics. Several energy storage technologies exist including lithium ion batteries, sodium ion batteries, fuel cells and electrochemical capacitors. These systems are complementary to each other. For example, electrochemical capacitors (ECs) can deliver high power densities whereas batteries are used for high energy densities applications. The first objective of this work is to investigate the electrochemical performances of a new family of 2-D material called MXene and propose new solutions to tackle the energy storage concern. To achieve this goal, several directions have been explored. The first part of the research focuses on MXene behavior as electrode material for electrochemical capacitors in aqueous electrolytes. The next part starts with sodium-ion batteries, and a new hybrid system of sodium ion capacitor is proposed. The last part is the study of MXene electrodes for supercapacitors is organic electrolytes. The energy storage mechanisms are thoroughly investigated. Different characterization techniques were used in this work, such as cyclic voltammetry, galvanostatic charge-discharge, electrochemical impedance spectroscopy, scanning electron microscopy and X-ray diffraction

The development of alternative binder systems for tungsten carbide (WC) based cemented carbides has again become of relevance due to possible changes in EU regulations regarding the use of Cobalt (Co). A framework for the ICME (Integrated Computational Materials Engineering) based Materials Design is presented to accelerate the development of alternative binder systems. Part one of this work deals with the design of the cemented carbide composite hardness. It has been shown that the intrinsic binder hardness is comparable to a bulk metal alloy and that based on the binder solubilities a solid solution strengthening model developed in this work can be employed. Using a method presented in this work the non-equilibrium, frozen-in binder solubilities can be obtained. Both the design of the binder phase and composite hardness is presented based on a general Materials Design approach. Part two deals with a multiscale approach to model the surface gradient formation. The experimentally missing data on liquid binder diffusion has been calculated using AIMD (Ab initio Molecular Dynamics). The diffusion through the liquid cemented carbide binder has to be reduced to an effective diffusion value due to the solid carbides acting as obstacles that increase the diffusion path. The geometrical reduction of the diffusion has been investigated experimentally using the SIMS (secondary ion mass spectroscopy) technique in WC-Nickel-58Nickel diffusion couples. The geometrical contribution of the so-called labyrinth factor has been proven by the combination of the experiments and in conjunction with DICTRA simulations using the precise liquid AIMD diffusivities. Unfortunately, despite the improved kinetic database and the geometrical diffusion reduction, the surface gradient formation cannot be explained satisfactory in complex cemented carbide grades. Additional, but so far unidentified, contributions have to be considered to predict the surface gradient thickness.

QC 20170919

Current estimations show hydrogen production to be in excess of 55 million tonnes annually. With consumption growing globally by six percent annually, new feedstocks such as ammonia need to be explored to meet demand. Ammonia has many attractive characteristics as a source for hydrogen, such as its product stream when decomposed producing no carbon oxides or sulfurs. Research currently is limited on the decomposition of ammonia to produce hydrogen supported by transitional metal catalysts. This lack of research has suppressed the development of this technology. This thesis project is intended to develop on the original research conducted at the King Abdulaziz University where the development of high surface area molybdenum nitride was researched as a catalyst for the ammonia decomposition reaction to produce hydrogen. The work completed in this thesis aims to capitalise on the findings in the original experiment by accurately determining the mechanisms and kinetics associated with the production of hydrogen. The modelling was separated into three distinct components which investigated the ammonia decomposition on a cobalt-molybdenum catalyst. The first component of the model was focused on optimising the surface which the mechanisms of the decomposition reaction would be situated on. The optimisation calculations found the (111) index to be the most stable of all the surfaces modelled. The data collected displayed a directly proportional relationship between the increasing number of atoms in the structure and its stability. The second component of the project modelled the ammonia decomposition mechanisms on the surface of the optimum (111) indexed surface. Multiple iterations were tested for different locations of the reacting molecule above the surface of the catalyst. The results from the optimised models found the desorption mechanisms were slightly more stable than the adsorption component of the decomposition reaction. The concluding component of the model investigated the transient states associated with the ammonia decomposition reaction. Six transient states where identified, however only five could be modelled as a result of an atomic imbalance between the adsorption and desorption phases. Multiple transient states were calculated, with the rate limiting step determined to be the final desorption of the hydrogen molecule from the catalyst surface. From the transient state models it was determined that the Co3Mo3N catalyst was kinetically unsuitable for the ammonia decomposition reaction.

Aluminium is one of the metals playing a prominent role in automobile industry after cast iron. Because of its light weight property and good mechanical properties. When aluminium reinforced with silicon carbide showing good tribological properties and improved strength. Aluminium silicon carbide needs some good wear and frictional properties to use it as break disc. Aluminium reinforced with 15% and 20% silicon carbide and casted in two different ways, liquid casting and stir casting. Four different composites are compared in this paper. Hardness test was carried out on the samples. Increase in the Vickers hardness with increase in silicon carbide reinforcement for both the castings is observed. Rockwell C hardness is showing decreasing trend with increase in SiC reinforcement. The scratch resistance of the surface under micro level was analysed with the help of nano scratch test. The SiC particles in the aluminium matrix are resisting the indenter from deep deformation of the surface. Frictional forces are dropped whenever the indenter met the SiC particles. In other cases, SiC particles are deforming the aluminium matrix in the form of broken particles. The plastic deformation of aluminium is observed, and material is piled up on sideways of groove at high load.Sliding wear behaviour of the composites are investigated by means of reciprocating pin on plate wear rig. The test was carried out at load of 20N for five different sliding duration. Aluminium with 20% silicon carbide of liquid casting is used as a base metal. The worn-out surface of the samples is analysed in SEM. The metallography of the worn-out samples is showing some deep grooves and abrasion of the material. Wear debris from both the surfaces are forming into a cluster of layers. These layers are protecting the surface from wear in some areas were observed. Composition of tribo layer formed during the test was investigated with the help of EDS analysis. The tribo layer are rich in aluminium and silicon elements because both the samples are made of aluminium silicon carbide.

Cemented carbides are composite materials formed by high amount of WC bonded by a soft phase, usually Co. They are used in many applications, such as drawing dies, cutting tools and hot rolls due to theirs remarkable properties of high hardness and wear resistance. Mechanical properties are strongly related to microstructure, namely the binder amount and the carbide grain size. Increasing the binder content and the carbide grain size, the hardness decreases ad the fracture toughness increases. In this PhD, the correlations between the mechanical properties of WC-Co and the microstructural characteristics, in parts taken from industrial production, were defined. After that, the influence of the residual microporosity on the mechanical properties was evaluated. Considering the production process, another important modification of the final microstructure of WC-Co occurs due to the liquid cobalt migration phenomenon. Based on this, also the liquid cobalt migration that occurs during sintering was investigated. At the end of the thesis, since a few data are available in literature, Thermal Fatigue and oxidation damage in WC-Co were studied. The main results of this PhD thesis show that the hardness and fracture toughness of WC-Co are defined by the mean binder free path and not by the contiguity since the high standard deviations, the microstructural fineness and also the high carbide grain size scatter. Differently, in case of mechanical strength, also the residual microporosity that depends on the dewaxing stage must be defined. Furthermore, the dewaxing stage acts on the liquid cobalt migration that affects the surface properties and also the final microstructure of the WC-Co part in industrial production. At the end, considering the damages that occur during high temperature applications, the TF and oxidation resistance of WC-Co results affected by the Co content: high cobalt content leads to a better condition of TF damage and s higher oxidation resistance.

Cemented carbides are composite materials formed by high amount of WC bonded by a soft phase, usually Co. They are used in many applications, such as drawing dies, cutting tools and hot rolls due to theirs remarkable properties of high hardness and wear resistance. Mechanical properties are strongly related to microstructure, namely the binder amount and the carbide grain size. Increasing the binder content and the carbide grain size, the hardness decreases ad the fracture toughness increases. In this PhD, the correlations between the mechanical properties of WC-Co and the microstructural characteristics, in parts taken from industrial production, were defined. After that, the influence of the residual microporosity on the mechanical properties was evaluated. Considering the production process, another important modification of the final microstructure of WC-Co occurs due to the liquid cobalt migration phenomenon. Based on this, also the liquid cobalt migration that occurs during sintering was investigated. At the end of the thesis, since a few data are available in literature, Thermal Fatigue and oxidation damage in WC-Co were studied. The main results of this PhD thesis show that the hardness and fracture toughness of WC-Co are defined by the mean binder free path and not by the contiguity since the high standard deviations, the microstructural fineness and also the high carbide grain size scatter. Differently, in case of mechanical strength, also the residual microporosity that depends on the dewaxing stage must be defined. Furthermore, the dewaxing stage acts on the liquid cobalt migration that affects the surface properties and also the final microstructure of the WC-Co part in industrial production. At the end, considering the damages that occur during high temperature applications, the TF and oxidation resistance of WC-Co results affected by the Co content: high cobalt content leads to a better condition of TF damage and s higher oxidation resistance.

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

In this work the chromium solubility in MC, and M in Cr3C2 and Cr7C3 carbides in the Ti-Cr-C and Ta-Cr-C system have been examined experimentally. Special attention is given to the cubic MC phase due to its frequent use in industrial cemented carbides. A sample series was made where half of the samples were arc-melted and all samples were heat-treated at different temperatures. By arc-melting some of the samples it was possible to compare the arc-melted and non arc-melted samples to confirm equilibrium. Three phases were expected in each sample. The microstructure was examined by LOM and SEM. The phases were identified by XRD and the amount of Cr in each phase was measured by WDS in FEG-SEM or by microprobe analysis. A higher temperature for the heat-treatment allows more Cr to dissolve in the cubic carbide. Arc-melted samples allow more Cr to dissolve than the same system which has not been arc-melted. The Cr solubility in the cubic carbide in non arc-melted samples at 1400 degree Celcius is 8,1±0,4 at% in (Ti, Cr)C and 7,6±0,3 at% in (Ta, Cr)C. According to the samples the phase diagrams based on thermodynamic calculations are different to experimental data. Therefore, more experimental data should be made to update existing ternary diagrams.