Formation of Grain Boundaries in Liquid-Phase-Sintered WC–Co Alloys

The origin of WC/WC grain boundaries in liquid-phase-sintered WC–Co alloys has been investigated in a WC–Mo₂C–Co model system using coarse WC polygrain powder. The evolution of grain shape during liquid phase sintering was able to be identified by observing a growth layer that contained Mo. During liquid phase sintering, most of the grain boundaries in the powder were penetrated by a Co liquid but some of them were not. Electron backscattered diffraction analysis confirmed that some boundaries in the powder, in particular, Σ2 twist boundaries and Σ97 special boundaries, remained intact during liquid phase sintering. These experimental results confirm that the grain boundaries of WC grains in liquid-phase-sintered WC–Co alloys originated from those present in the starting powder.     

Processing of ZrC–Mo Cermets for High Temperature Applications, Part II: Pressureless Sintering and Mechanical Properties

Pressureless sintering of ZrC–Mo cermets was investigated in a He/H₂ atmosphere and under vacuum. A large density increase was observed for specimens with >20 vol% Mo after heating at 2150°C for 60 min in a He/H₂ atmosphere. The increase in density was attributed to the formation of Mo₂C during heating and its subsequent eutectic reaction with Mo, which produced rounded ZrC grains in a Mo–Mo₂C matrix. Sintering in vacuum did not produce the same increase in density, due to the lack of Mo₂C formation and subsequent lack of liquid formation, which resulted in a microstructure with irregular ZrC grains with isolated areas of Mo. Mechanical properties testing showed a decrease in Young's modulus with increasing Mo content that was consistent with the models presented. Flexure strength of ZrC–Mo cermets sintered in He/H₂ atmosphere materials increased with increasing Mo content from 320 MPa at 20 vol% Mo to 410 MPa at 40 vol% Mo. Strength was predicted by adapting theories developed previously for WC–Co cermets.     

Corrosion of Ceramics in Aqueous Hydrofluoric Acid

A variety of commercially available ceramic-based oxides, carbides, nitrides, and borides were evaluated for chemical attack in an azeotropic aqueous hydrofluoric acid (HF) test protocol at 90°C. Weight change measurements and microstructure analysis showed that HF corrosion in polycrystalline ceramics generally occurred at grain boundaries by the dissolution of grain boundary phases although the bulk single crystal may inherently resist attack. Virtually all commercially prepared polycrystalline oxide ceramics (i.e., Al₂O₃, TiO₂, ZrO₂) and nonoxide ceramics (i.e., Si₃N₄, AlN, BN) were extensively corroded while polycrystalline pure carbides (i.e., SiC, TiC, B₄C, WC) resisted corrosion. Equilibrium thermodynamic calculations show that these materials are soluble in HF; however, the kinetics of dissolution are slow enough in some cases to permit useful engineering lifetimes.     

Constitution of Ternary Ta-Mo-C Alloys

The ternary system Ta-Mo-C was investigated by X-ray, melting point, DTA, and metallographic methods on chemically and thermally characterized specimens; a phase diagram for temperatures from 1500°C through the melting range was established. At high temperatures, the cubic monocarbides, TaC and αMOc_1-x, as well as the hexagonal subcarbides, Ta₂C and Mo₂C₃ form continuous series of solid solutions. Below 1960°C, the homogeneity range of the β phase is temperature-dependent. At 2230°C, the ternary subcarbide solid solution disproportionates into metal and monocarbide alloys. The sub-lattice order-disorder transformation temperatures in both subcarbides are lowered by the mutual exchange. The ternary range of the η-MoC_1-x, phase is very restricted, and the metastable ζ-Ta-C phase is stabilized in the ternary. Seven isothermal reactions occur in the system in the range from melting to 1500°C; two further isotherms between 1400° and 1500°C are indicated to result from the order-disorder transition in Mo₂C. Using existing literature data for the thermodynamic properties of the binary carbides, the phase relations in the Ta-Mo-C system were thermodynamically analyzed. The calculated equilibria are in good agreement with experimental observations.     

Synthesis of nonstoichiometric high entropy Nb-added carbides

A novel class of nonstoichiometric high-entropy carbide (HEC_x) materials, namely, Nb/TiC/TaC, Nb/TiC/TaC/VC and Nb/TiC/TaC/VC/WC, were produced by mechanically milled and spark plasma sintering (SPS) from Nb and carbides. XRD, SEM-EDS and S/TEM-EDS were used to characterize the phase constitution, microstructure and compositional distribution of samples, respectively. HEC_x exhibits a single-phase rock-salt crystal structure with a relatively uniform elemental distribution. Among the three different HEC_x materials, Nb/TiC/TaC/VC/WC with an average grain size of 2.15 μm sintered at 1600 °C shows an enhanced fracture toughness of 5.1 ± 0.1 MPa m^1/2 compared with transition metal carbides. The mechanically mixed and low sintering temperature lead to the formation of finer grains. The higher fracture toughness can be attributed to atomic relaxation resulting from carbon vacancies and solid solution strengthening.     

Hot Pressing of Tantalum Carbide With and Without Sintering Additives

Densification of tantalum carbide (TaC) was studied by hot pressing at temperatures ranging from 1900° to 2400°C with and without sintering additives. Without sintering additives, the relative density increased from 75% at 1900°C to 96% at 2400°C. A microstructural examination showed no observable grain growth up to 2300°C. Densification was enhanced with carbon (C) and/or B₄C additions. TaC with a 0.78 wt% C addition achieved a relative density of 97% at 2300°C. Additions of 0.36 wt% B₄C or 0.43 wt% B₄C and 0.13 wt% C increased the relative density to 98% at 2200°C, accompanied by rapid grain growth at 2100°C and higher temperatures.     

Processing of ZrC–Mo Cermets for High-Temperature Applications, Part I: Chemical Interactions in the ZrC–Mo System

The chemical compatibility of ZrC and Mo was investigated in carburizing and carbon-free environments at temperatures from 1700° to 2200°C. Heating in the carburizing atmosphere resulted in the complete reaction of Mo with C, while the carbon-free atmosphere resulted in retained metallic phase with a maximum of 13.8 mol% Mo₂C formed. The presence of Mo₂C was not detected at 2100°C in the carbon-free atmosphere, confirming the existing phase equilibria in the Zr–Mo–C system. Heat treatments in the carbon-free atmosphere also showed liquid formation at 2200°C, as evident from microstructure analysis. Liquid formation was consistent with the interaction between Mo and Mo₂C. The liquid was found to comprise at least 7 vol% of the total component, based on a phase diagram for the Mo–C system. The formation of a liquid should allow for the processing of ZrC–Mo cermets by liquid-phase pressureless sintering.     

Solid-State Diffusion Bonding of Carbon–Carbon Composites with Borides and Carbides

Solid-state diffusion bonding of carbon–carbon (C─C) composites by using boride and carbide interlayers has been investigated. The interlayer materials used in this study were single-phase borides (TiB₂ or ZrB₂), eutectic mixtures of borides and carbides (ZrB₂+ ZrC or TiB₂+ B₄C), and mixtures of TiB₂+ SiC + B₄C produced in situ by chemical reactions between B₄C, Ti, and Si or between TiC, Si, and B. The double-notch shear strengths of the joints produced by solid-state reaction sintering of B₄C + Ti + Si interlayers were much higher than those of joints produced with other interlayers. The maximum strength was achieved for C─C specimens bonded at 2000°C with a 2:1:1 mole ratio of Ti, Si, and B₄C powders. The reaction products identified in the interlayers, after joining, were TiB₂, SiC, and TiC. The joint shear strength increased with the test temperature, from 8.99 MPa at room temperature to an average value of 14.51 MPa at 2000°C.     

Reaction Processing of Ultra-High Temperature W/Ta2C-Based Cermets

A reactive processing method was used to produce dense (>99.5% relative density) W/Ta₂C/Ta₂WO₈ cermets from Ta₂O₅, WC, TaC, and phenolic resin. The powder compacts were reacted under vacuum at 1450°C and then densified at 1850°C. The microstructure of the cermets was examined using scanning electron microscopy and found to contain fine grains on the order of 1–3 μm in diameter. X-ray diffraction, transmission electron microscopy, and parallel energy electron loss spectroscopy were used to identify the composition and structure of the phases in the composites. The composites were found to contain approximately 47.5 wt% W, 47.3 wt% Ta₂C, and 5.2 wt% Ta₂WO₈. Crack-free and dense W/Ta₂C-based cermets were prepared by an in situ reactive sintering process without the application of pressure during densification.     

Molybdenum Oxycarbide and Tungsten Oxycarbide Coatings on Silicon Carbide Fibers

The strength and toughness of fibrous composites depend on the interface properties which control the bonding between the fibers and matrices. One method of controlling the interface involves coating the fiber with an appropriate material. In a previous study, it was found that there is a definite advantage in using low coating temperatures to prevent fibers from degrading. We therefore were interested in a report that Mo₂C could be deposited from Mo(CO)₆ at temperatures as low as 300° to 475°C. Our studies indicated that the material was not Mo₂C, but an oxycarbide, which, with an analogous tungsten oxycarbide coating, was applied to SiC yarns. Both oxycarbides could be converted to the metals by heat-treating in N₂.     

In Situ TiC/TiB2 Particulate Locally Reinforced Steel Matrix Composites Fabricated Via the SHS Reaction of Ni–Ti–B4C System

The composites synthesized with three kinds of B₄C particles mainly consist of TiC, TiB₂, and the alloy austenite containing Ni element. Ceramic particulate sizes in the composites synthesized with ∼3.5 and ∼45 μm B₄C particles are larger than that synthesized with ∼140 μm B₄C particle. No pores are found between the reinforcing region and matrix in the composites synthesized with ∼3.5 and ∼45 μm B₄C particles, while some large pores exist in the composites synthesized with ∼140 μm B₄C particle. With the decrease of B₄C particle size, the pores in the composites become fewer and the hardness and wear resistance of the composites increase.     

Tribological Behavior of Mo5Si3-Particle-Reinforced Silicon Nitride Composites

The tribological behavior of Mo₅Si₃-particle-reinforced silicon nitride (Si₃N₄) composites was investigated by pin-on-plate wear testing under dry conditions. The friction coefficient of the Mo₅Si₃–Si₃N₄ composites and Si₃N₄ essentially decreased slowly with the sliding distance, but showed sudden increase for several times during the wear testing. The average friction coefficient of the Si₃N₄ decreased with the incorporation of submicrometer-sized Mo₅Si₃ particles and also as the content of Mo₅Si₃ particles increased. When the Mo₅Si₃–Si₃N₄ composites were oxidized at 700°C in air, solid-lubricant MoO₃ particles were generated on the surface layer. Oxidized Mo₅Si₃–Si₃N₄ composites showed self-lubricating behavior, and the average friction coefficient and wear rate of the oxidized 2.8 wt% Mo₅Si₃–Si₃N₄ composite were 0.43 and 0.72 × 10⁻⁵ mm³ (N·m)⁻¹, respectively. Both values were ∼30% lower than those for the Si₃N₄ tested in an identical manner.     

Development of WC–ZrO2 Nanocomposites by Spark Plasma Sintering

In the present work, we report the processing of ultrahard tungsten carbide (WC) nanocomposites with 6 wt% zirconia additions. The densification is conducted by the spark plasma sintering (SPS) technique in a vacuum. Fully dense materials are obtained after SPS at 1300°C for 5 min. The sinterability and mechanical properties of the WC–6 wt% ZrO₂ materials are compared with the conventional WC–6 wt% Co materials. Because of the high heating rate, lower sintering temperature, and short holding time involved in SPS, extremely fine zirconia particles (∼100 nm) and submicrometer WC grains are retained in the WC–ZrO₂ nanostructured composites. Independent of the processing route (SPS or pressureless sintering in a vacuum), superior hardness (21–24 GPa) is obtained with the newly developed WC–ZrO₂ materials compared with that of the WC–Co materials (15–17 GPa). This extremely high hardness of the novel WC–ZrO₂ composites is expected to lead to significantly higher abrasive-wear resistance.     

Tool life and wear of WC–TiC–Co ultrafine cemented carbide during dry cutting of AISI H13 steel

WC–5TiC–10Co ultrafine cemented carbides were prepared and used for the cutting tool for AISI H13 hardened steel. The effect of cutting parameters on the tool life and tool wear mechanism was investigated, and conventional cemented carbide with the same composition and medium grain size were prepared for comparison. The results showed that WC–5TiC–10Co ultrafine cemented carbides possess higher hardness and transverse rupture strength, and showed better cutting performance than conventional insert with the same cutting condition. Tool life was analyzed by an extended Taylor's tool life equation, indicating that cutting speed played a profound effect on the tool life and wear behavior of both cutting inserts. SEM and EDS analysis revealed that there were major adhesive wear and minor abrasive wear on the rake of WC–5TiC–10Co ultrafine inserts, and increase of cutting speed resulted in a transition from abrasion predominant wear mechanism to adhesive wear on the flank face. As for the conventional inserts, there were combination of more serious abrasive and adhesive wear on the rake and flank. The favorable cutting performance of ultrafine WC–5TiC–10Co inserts was attributed to the higher hardness and less thermal softening during machining.     

Synthesis, Microstructural Characterization, and Mechanical Property Evaluation of Vacuum Plasma Sprayed Tantalum Carbide

Tantalum carbide (TaC) is an ultra-high-temperature ceramic for potential applications as protective coating, furnace components, propulsion liners for space shuttles and aircrafts, etc. Microstructural and mechanical behavior of vacuum plasma-sprayed (VPS) TaC has been investigated in the present study. Apart from major TaC phase, microstructural definitions elucidated Ta₂C, non-stoichiometric TaC _x phases (0.83≤ x ≤0.94), partial grain formation, polygonization of grains, and inhomogeneous C/Ta ratios in the sprayed structure. Near-isotropy in the fracture–toughness ratio ( K _axial/ K _trans=1.01) is attributed to compact coating, fine-closed porosity, and distribution of non-stoichiometric phases.     

Microstructure of Ti(CN)–WC–NbC–Ni Cermets

An investigation of the microstructural evolution and dissolution phenomena in a Ti(C_0.7N_0.3)– x WC– y NbC–20Ni system is reported. In Ti(C_0.7N_0.3)– y NbC–20Ni systems, a phase separation occurs between the Ti(CN) core and the (Ti,Nb)(CN) rim phases when the system contains >15 wt% NbC. This phase separation results from the increased misfit between the cores and the solid-solution rim phases with the addition of NbC. Based on data obtained from a previous study and compositional analyses of the rim structure of the Ti(C_0.7N_0.3)– y NbC–20Ni system, the average dissolution rates of WC and NbC appear to be approximately the same with respect to that of Ti(CN), under given sintering conditions (1510°C for 1 h). In addition, compositional changes in the rim structure of the Ti(C_0.7N_0.3)– x WC– y NbC–20Ni system are compared with those for a Ti(C_0.7N_0.3)– x WC–20Ni system to explain the effect of NbC on WC dissolution in the Ti(C_0.7N_0.3)–WC–NbC–Ni system. The presence of NbC in the Ti(C_0.7N_0.3) –x WC–20Ni system is found to suppress the dissolution of WC.     

Wear Mechanisms of TiB2 and TiB2–TiSi2 at Fretting Contacts with Steel and WC–6 wt% Co

Unlubricated fretting wear tests on TiB₂ and TiB₂–5 wt% TiSi₂ ceramics against two different mating materials (bearing grade steel and WC–6 wt% Co balls) were performed with a view to understand the counterbody-dependent difference in friction and wear properties. The fretting experiments were conducted systematically by varying load (2–10 N) at an oscillating frequency of 4 Hz and 100 μm linear stroke, for a duration of 100,000 cycles. Adhesion, abrasion, and three-body wear have been observed as mechanisms of material damage for both the TiB₂/steel and TiB₂/WC–Co tribosystems. The third body is predominantly characterized as tribochemical layer for TiB₂/steel and loose wear debris particles for TiB₂/WC–Co tribocouple. An explanation on differences in tribological properties has been provided in reference to the counterbody material as well as microstructure and mechanical properties of flat materials.     

Synthesis-Fabrication of an Al2O3-Tungsten Carbide Composite

A composite of 70 vol% Al₂O₃ and 30 vol% tungsten carbide was formed by hot-pressing. Simultaneously carbon reacts with an intimate mixture of WO₃ and Al₂O₃ to form a dense body. The composite approached theoretical density; a 1- to -10-μm carbide phase was uniformly dispersed in a 2-μm Al₂O₃ matrix. Maximum density and fine-grained microstructure were obtained when pressure was applied during heating from 1200° to 1600°C and temperature and pressure were then maintained for 20 min. At an initial ratio of 2.8 and 2.9 mol C/mol WO₃, the tungsten appeared as free W, WC, and W₂C. For C/WO₃=3.0 to 3.6, mixtures of W₂C and WC were present, whereas for C/WO₃>3.6, free C appeared with WC. The effects of the hot-pressing parameters are discussed.     

High-Temperature Cermets: I, Compatibility

The stability of refractory oxides (Y₂O₃,-stabilized HfO₂ and ZrO₂, Tho₂, CeO₂), carbides (HfC, NbC, TaC, and ZrC), borides (NbB₂ and TaB₂), and HfN was determined in combination with the Groups VIA and VIIA refractory metals and combinations thereof. Thermodynamic calculations were made to predict stability up to 2500°K between the ceramic oxides and carbides in contact with Mo, Re, and W. Reaction studies were conducted between the ceramics and the Groups VIA and VIIA refractory metals in vacuum and in helium to 2750°C. The Mo-40 wt% Re, Re, and W were stable in contact with the carbides, nitrides, and oxides to 2450°C with two exceptions. These occurred when CeO₂ reacted with W at 1700°C and Mo-40 wt% Re reacted with WC. The Mo-40 wt% Re and Re also reacted with the NbB₂ and TaB_a above 2200°C to form very hard single-phase compounds.     

Elevated-Temperature Toughness and Hardness of a Hot-Pressed Al2O3—WC—Co Composite

The fracture toughness and hardness of an Al₂O₃80WC10Co composite were investigated in air at elevated temperatures. The primary phases in the composite were WC, α-Al₂O₃, and Co₃W₃C, but small amounts of Co and C (graphite) appeared at elevated temperatures, related to decomposition of the Co₃W₃C phase. The fracture toughness of the composite was constant with increasing temperature up to 330°C and then increased in the range 400° to 550°C. A transition of brittle to ductile behavior occurred at about 700°C. The enhancement of fracture toughness at elevated temperature is attributed to the decomposition of Co₃W₃C to Co and C, and enhanced crack deflection and bridging. Decreases in hardness with increasing temperature are attributed to the softening of WC matrix and decomposition of Co₃W₃C.