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.     

Electrochemical Behavior of TiCN–Ni-Based Cermets

The electrochemical behavior of TiCN–20 wt% Ni cermets containing different secondary carbides (10 wt% WC, NbC, TaC, and HfC) was investigated in freely aerated 0.2 mol/L sulfuric acid. A comparison has also been made with polarization behavior of pure Ni. All the materials (except HfC-containing cermet) exhibited active–passive polarization behavior, characterized by two passive regions. The first passive region, obtained on polarizing past the zero current potential, was attributed to passive film formation due to TiCN, while the second passive film is presumably due to the presence of a Ni binder phase. The passivation behavior in the case of cermets containing various secondary carbides (WC, TaC, and NbC) was similar to baseline TiCN–20Ni cermets, while poor passivation behavior was observed in the case of TiCN–20Ni–10HfC. Efforts have been made to correlate the passivation behavior with the microstructural characteristics of sintered cermets.     

Erosion Wear Behavior of TiCN–Ni Cermets Containing Secondary Carbides (WC/NbC/TaC)

The present communication reports the results of a first set of erosion wear experiments, conducted on TiCN–20 wt% Ni cermets containing different secondary carbides (WC, NbC, and TaC). The cermets are eroded by SiC particles (66 μm size) with a constant mass flow rate (2.33 g/s) at different angles of impingement (30°, 60°, 90°) on an in-house fabricated erosion wear tester. An important experimental observation is that the investigated cermets exhibit behavior similar to ceramics, as evident from the systematic increase in erosion wear rate with impact angles. Among all investigated cermets, WC-added TiCN–20 wt% Ni cermets exhibit superior erosion wear resistance. SEM investigations of eroded surfaces (normal incidence) reveal grain pullout with intergranular fracture as the dominant material removal process.     

High-Temperature Cermets: 11, Wetting and Fabrication

Wetting tests were performed with refractory metal-ceramic combinations selected on the basis of previous compatibility studies. Sessile drop contact angles between liquid Cr, V, Pt, Rh, Mo, and Mo-Re alloy and solid ThO₂, HfO₂, ZrO₂, HfC, TaC, and ZrC were measured at temperatures up to 2800° C in helium. Little wetting occurred between these metals and the refractory oxides, but Mo and the Mo-Re alloy did wet the refractory carbides. Microscopic examination of the metal-ceramic interfaces showed that HfC-Mo-40 wt% Re and ZrC-Mo-40 wt% Re had highly desirable wetting and solubility characteristics for making cermets. Specimens were fabricated from these two systems with 25 and 50 vol% metal by cold-pressing and sintering and by hot-pressing. The microstructures of these cermets consisted of small, uniformly dispersed carbide particles in Mo-Re matrixes. The hardnesses and transverse rupture strengths of these cermets indicate that they have considerable promise for high-temperature applications.     

Microstructural Evolution during the Sintering of TiC–Mo–Ni Cermets

TiC-Ni-Mo cermet specimens were prepared by using a mixture of fine (1.5 μm) and coarse (30 μm) TiC powders. When the fraction of fine TiC particles was 80%, a (Ti,Mo,Ni)C complex carbide phase was observed deposited on the coarse TiC particles and resulted in a typical cored structure. As the fraction of fine TiC particles decreased, the coarse TiC particles exhibited a unique microstructural evolution with the development of a concave interface. This microstructural change of the coarse TiC grains can be explained in terms of the coherency strain energy.     

Sliding Wear Behavior of TaC‐Containing Ti(CN)‐WC‐Ni/Co Cermets

Four compositions of TiCN‐WC‐Ni/Co cermets with or without TaC were prepared by pressureless sintering of respective powders, and unlubricated sliding wear behavior against bearing grade steel ball was studied at 5, 10, or 20 N load. Maximum hardness and fracture toughness were obtained for Ti(CN)‐5WC‐10Ni‐10Co‐5TaC (in wt%) cermet. With change in the cermet composition and sliding load, coefficient of friction varied from 0.3 to 1.0 and wear rate varied from 3 × 10^?7 to 7 × 10^?7 mm³/Nm. The increased material transfer and formation of iron oxide‐rich tribochemical layer were responsible for the reduction in friction and wear for Ti(CN)‐5WC‐10Ni‐10Co‐5TaC cermet.     

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.     

YSZ-Induced Crystallographic Reorientation of Ni Particles in Ni–YSZ Cermets

Metal–ceramic interfaces in Ni–YSZ (YSZ, yttria-stabilized zirconia)-textured porous cermets prepared by reduction of NiO–YSZ directionally solidified eutectics have been studied by transmission electron microscopy and X-ray pole figure experiments. Before reduction of NiO, the interfacial plane is but after reduction, the Ni phase does not maintain the same crystallographic orientation as the NiO parent phase. Ni undergoes an interface-induced crystallographic reorientation to form the lower energy (002)_Ni∥(002)_YSZ interface. This process has been studied as a function of the reduction temperature, and it seems to be more effective at ∼800°C. Metal–ceramic low-energy interfaces prevent Ni particle coarsening and impart long-term stability to the cermet.     

Load-Dependent Transition in Sliding Wear Properties of TiCN–WC–Ni Cermets

The present investigation aims at evaluating the sliding wear behavior of TiCN–20 wt%Ni cermets, containing varying amounts of WC (5–25 wt%) against 100Cr6 steel at different loads (5, 20, and 50 N). The dominant wear mechanisms were established by using surface topography analysis (Stylus profilometry), scanning electron microscopy, and X-ray diffraction. The steady-state coefficient of friction varies over a wide range: 0.73–0.27 at different loads for the investigated cermets. The volumetric wear loss of the cermets in general increases with load, while the wear rate varies in the order of 10⁻⁷ mm³·(N·m)⁻¹. The worn surface characterization reveals that a transition in wear mechanism occurs from abrasion and mild tribo-oxidation at low loads to the formation of a dense tribo-oxide layer, containing oxides of Fe and Ti (essentially Fe₉TiO₁₅) at high loads. Efforts have been made to discuss the possible reaction pathways to explain the formation of the tribochemical layer. The addition of WC to TiCN–Ni-based cermets results in increased abrasion at low loads and severe fracture of the tribo-oxide layer at high loads.     

Determination of Residual Stresses in Titanium Carbide-Base Cermets by High-Temperature X-Ray Diffraction

The crystal structure and thermal expansion of titanium carbide, nickel, and two titanium carbide-base cermets were determined between room temperature and 1100°C. (2012°F.). No structural changes were observed. An abnormal rate of expansion was observed for pure nickel near the Curie temperature, 353°C. (665°F.), and for the nickel and carbide phases in the cermets at about 816°C. (1500°F.). The expansion coefficient of pure nickel and the nickel phase in the cermets was found to be approximately twice that of pure titanium carbide and the carbide phase in the cermets. The brittleness and poor impact strength of the cermets was attributed to the large residual stresses present in these materials as a result of this difference in thermal expansion. The stress-strain relations were interpreted on the basis of a mechanical interaction between the phases in the cermets. The carbide phase was found to be essentially under triaxial compression and the nickel phase under a triaxial tension of 158,000 lb. per sq. in. At elevated temperatures, increased solid solubility of carbide in the nickel phase and plastic deformation of this phase was believed to influence the stress-strain relations and the thermal-expansion behavior of the phases. It was concluded that replacement of the nickel phase in the cermets with a metal or alloy, such as a nickel-chromium-molybdenum alloy, which has a coefficient of thermal expansion similar to the carbide phase in the cermets, would improve the impact strength of these bodies. Equations were developed for the thermal expansion of titanium carbide and nickel. Values of the expansion coefficient were computed for each of the materials by differentiation of these equations.     

High-Temperature Plastic Deformation of Polycrystalline Beryllium Oxide

High-density specimens were plastically deformed under four-point transverse bending. Tests were conducted in vacuum in the region 1400° to 1700°C under stresses of 1000 to 4500 psi. The activation energy for creep was 99.0 kcal/mole. Creep rate was directly proportional to the applied stress and inversely proportional to the square of the grain diameter. The deformation behavior is ascribed to a Nabarro-Herring type mechanism. Results show that creep was the same in tension and compression.     

TiC基金属陶瓷的高温氧化行为

研究了以Fe-Cr-Si或Ni-Fe-Cr-Si为粘结相的新型TiC基金属陶瓷在850℃空气中的氧化行为,测定并计算了氧化增重与时间的函数关系及氧化速率.结果表明,这类陶瓷材料的氧化为"钝化型氧化",显示出较好的抗氧化性和力学性能.通过X射线衍射(XRD)、扫描电镜(SEM)等手段,分析了氧化膜的组成与结构,探讨了TiC基金属陶瓷高温下的耐氧化机理.     

High-Temperature Strength Behavior of Synroc-C

Comparative measurements have been made of the high-temperature flexure strength characteristics of synroc-C in air and argon environments. The stress–strain curves for synroc show a large deviation from linearity with increasing temperature in both environments, indicating a brittle–ductile transition. Strength is relatively constant at ≤800°C, followed by a discernible increase, with a peak at ∼920°C in air and 940°C in argon, and then a dramatic drop-off. The strengthening response is explored with reference to microstructural changes, in particular oxidation effects, and the implications of the observations are discussed.     

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.     

Resistance of Titanium Diboride to High-Temperature Plastic Yielding

Polycrystalline TiB₂ specimens of 3 μm average grain size but free of lamellar precipitates were compression-tested to 1900°C and 500 MPa. No plastic yielding was detected. Under such conditions, most other structural ceramics do exhibit yield behavior. The results suggest the existence of a high Peierls stress, presumably related to Ti—B bonding. This finding contrasts with a report on ZrB₂ in which resistance to yielding was attributed to lamellar precipitates.     

High-Temperature Fatigue Behavior of Polycrystalline Alumina

The strength and fatigue behavior of a 99.5% polycrystalline alumina were measured as a function of temperature. Both the strength and fatigue behavior remained essentially constant up to 500°C; from 800° to 1100°C the strength and fatigue resistance decreased markedly and at >1100°C macroscopic creep was observed. It is believed that the decrease in strength and fatigue resistance is caused by a grain-boundary glassy phase enhancing subcritical crack growth. Proof-testing at room temperature was effective in improving the strength distributions at both room temperature and 1000°C; however, at 1000°C it was not effective, due to crack growth during the proof test. The good agreement between proof-test results and fracture-mechanics theory indicates that the same flaws control the strength at room temperature and at high temperatures.     

Microstructure and High-Temperature Corrosion Behavior of a Cr–Al–C Composite

A Cr–Al–C composite was successfully synthesized by a hot-pressing method using Cr, Al, and graphite as starting materials. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses revealed that the composite contained Cr₂AlC, AlCr₂, Al₈Cr₅, and Cr₇C₃. The orientation relationships and atomic-scale interfacial microstructures among Cr₂AlC, AlCr₂, and Al₈Cr₅ are presented. This composite displays both excellent high-temperature oxidation resistance in air and hot-corrosion resistance against molten Na₂SO₄ salt. The parabolic rate constants for the oxidation in air at 1000°, 1100°, and 1200°C are 3.0 × 10⁻¹², 6.2 × 10⁻¹¹, and 6.2 × 10⁻¹⁰ kg² (m⁴·s)⁻¹, respectively, while the linear weight gain rates for the hot corrosion of Na₂SO₄-coated samples at 900° and 1000°C are, respectively, 1.2 × 10⁻³ and 4.4 × 10⁻³ mg (cm²·h)⁻¹. The mechanism of the excellent high-temperature corrosion resistance can be attributed to the formation of a protectively alumina-rich scale.     

High-Temperature Mechanical Behavior of Stoichiometric Magnesium Spinel

The elastic and mechanical behavior, from room temperature up to 1300^deg;C, of Stoichiometric polycrystalline magnesium aluminum spinel is studied. Elastic modulus, fracture toughness, and modulus of rupture measurements and observations of polished and fracture surfaces have been performed. Two well-differentiated regions of fracture behavior as a function of temperature have been found. In the low-temperature region, this material behaves elastically, whereas in the high-temperature (>800^deg;C) region, plastic phenomena take place.     

High-Temperature Mechanical Behavior of Fiber-Reinforced Glass-Ceramic-Matrix Composites

Lithium aluminosilicate glass-ceramic-matrix composites reinforced with Sic fibers were tested at 900° and 1000°C in flexural and tensile configurations. The composites showed severe thermomechanical degradation when tested in oxidizing atmospheres. This degradation was found to depend on oxygen partial pressure. For P _{o 2}≳10³ Pa the composites failed in a brittle fashion; a single crack initiated and propagated through the matrix and fibers. At lower P_o2, multiple matrix cracking was observed, and the fibers bridged the matrix cracks.