Investigation of residual stress effects in an alloy reinforced ceramic/metal composite

This study aims at investigating the thermal expansion behavior and internal residual strains in metal reinforced ceramic matrix composites (CMCs). A variety of Al₂O₃/A356 CMCs composites with an interpenetrating network structure and varying metal content, ranging from 10 to 40 vol.%, were produced using the pressure infiltration technique of Squeeze casting. Values of coefficients of thermal expansion (CTEs) were found to vary significantly with temperature, indicating an influence of the flow characteristics of the metal. Comparisons are made with well known methods for predicting CTEs values of metal/ceramic composites. The overall strain was found to increase with temperature and scaled proportionally with the metal content of the composite. Comparisons were also made with non-infiltrated porous ceramic preforms and a pure metallic sample. The uniform heating and cooling curves for the composite samples were found to exhibit hysterisis. Residual stress analysis and failure simulation were performed based on thermomechanics and the finite element method (FEM). This analysis is often utilized for the analysis of stress distribution or deformation of a structure. High angle X-ray and CTEs mismatch equation analysis were utilized to analyze the residual stresses at the ceramic/metal interface of the Al₂O₃/A356 composites. The relationship of residual stresses and the contact area of the ceramic/metal interface are also discussed.     

Improving Erosion Resistance of Plasma-Sprayed Ceramic Coatings by Elevating the Deposition Temperature Based on the Critical Bonding Temperature

Interlamellar bonding within plasma-sprayed coatings is one of the most important factors dominating the properties and performance of coatings. The interface bonding between lamellae significantly influences the erosion behavior of plasma-sprayed ceramic coatings. In this study, TiO₂ and Al₂O₃ coatings with different microstructures were deposited at different deposition temperatures based on the critical bonding temperature concept. The erosion behavior of ceramic coatings was investigated. It was revealed that the coatings prepared at room temperature exhibit a typical lamellar structure with numerous unbonded interfaces, whereas the coatings deposited at the temperature above the critical bonding temperature present a dense structure with well-bonded interfaces. The erosion rate decreases sharply with the improvement of interlamellar bonding when the deposition temperature increases to the critical bonding temperature. In addition, the erosion mechanisms of ceramic coatings were examined. The unbonded interfaces in the conventional coatings act as pre-cracks accelerating the erosion of coatings. Thus, controlling interlamellar bonding formation based on the critical bonding temperature is an effective approach to improve the erosion resistance of plasma-sprayed ceramic coatings.     

VC and Cr3C2 doped WCoB-TiC ceramic composites prepared by hot-pressing

The grain growth inhibitors (GGIs) VC and Cr₃C₂ doped WCoB-TiC ceramic composites were fabricated by hot-pressing. The microstructure, hardness, transverse rupture strength (TRS), fracture toughness (K_IC) and wear-resistance of WCoB-TiC ceramic composites were investigated. The results reveal that the grains can obviously become refined and the densification temperature of WCoB-TiC ceramic composites will be increased due to the VC and Cr₃C₂. The typical microstructure of WCoB-TiC ceramic composites mainly consist of bright W₂CoB₂ grains, gray TiC particles, dark TiB₂ and pores. WCoB-TiC ceramic composites doped with 0.3 wt% VC and 0.3 wt% Cr₃C₂ hot-pressing at 1420 °C show the optimum mechanical properties (hardness, TRS and K_IC are 92.6 HRA, 1976 MPa and 14.8 MPa m^1/2, respectively) and the best dry sliding wear-resistance.     

Thermal ablation resistance of melt-infiltrated titanium diboride-(copper, nickel) composites

TiB₂/(Cu, Ni) melt-infiltrated composites with high ceramic volume fraction were prepared via powder pre-sintering and melt-infiltration processing in vacuum. Infiltrated TiB₂/(Cu, Ni) composite shows enhanced damage tolerance and good mechanical properties, i.e., fracture toughness up to 12.5 MPa m^0.5 and bending strength up to 844.7 MPa. The thermal shock and plasma ablation behavior of TiB₂/(Cu, Ni) composites were conducted by plasma arc heater. The initial microstructure, interfacial bonding and ablated morphology were investigated. The experimental results show that the ablation mechanism of TiB₂/(Cu, Ni) composites are mainly attributed to the transpiration cooling of metallic phase to bring away inner heat at elevated temperature. The microstructure analysis and properties measurement before and after ablation test reveals a clue of redistribution of infiltrated (Cu, Ni) so as to deduce and explain the functioning of transpiration cooling at ultra-high temperature.     

Formation of high-density TiN/Ti<Subscript>5</Subscript>Si<Subscript>3</Subscript> ceramic composites using preceramic polymer

The formation, microstructure and properties of high-density TiN/Ti₅Si₃ ceramic composites created by the pyrolysis of preceramic polymer with filler were investigated. Methylpolysiloxane was mixed with TiH₂ as filler and ceramic composites prepared by pyrolysis at 1200°C to 1600°C under N₂, Ar and vacuum were studied. When a specimen with 70 vol.% TiH₂ was pyrolyzed up to 1600°C in a vacuum after a preheat treatment at 850°C in a N₂ atmosphere and subsequently heat-treated at 1600°C for 1 h under Ar at a pressure of 2 MPa, a ceramic composite with full density was obtained. The microstructure of the ceramic composite was composed of TiN and Ti₅Si₃ phases. Under specific pyrolysis conditions, a ceramic composite with a density of 99.2 TD%, a Vickers hardness of 18 GPa, a fracture toughness of 3.5 MPam^1/2, a flexural strength of 270 MPa and a electrical conductivity of 6200 ohm⁻¹·cm⁻¹ was obtained.     

Interconnected metal-ceramic composites by chemical means

Using a sol-gel technique, Ni/α-Al₂O₃ composites with ceramic content ranging from 33–95 vol.% have been synthesized. The process involves dispersing the metal and ceramic precursors in solution followed by calcination in a reducing environment. Fully dense composites are obtained by hot pressing at 1,400°C in an oxygen partial pressure of 10^-12 atm. For the composites containing 20 vol.% metal or higher, the metallic phase was been found to be completely continuous. All of the composites exhibited enhanced fracture toughness behavior relative to the pure ceramic.     

Dielectric and microwave properties of ZrO2 doped CaO-SiO2-B2O3 ceramic matrix composites

The behavior of dielectric and microwave properties against sintering temperature has been carried out on CaO-SiO₂-B₂O₃ ceramic matrix composites with ZrO₂ addition. The results indicated that ZrO₂ addition was advantageous to improve the dielectric and microwave properties. X-ray diffraction (XRD) patterns show that the major crystalline β-CaSiO₃ and a little SiO₂ phase existed at the temperature ranging from 950 °C to 1050 °C. At 0.5 wt% ZrO₂, CaO-SiO₂-B₂O₃ ceramic matrix composites sintered at 1000 °C possess good dielectric properties: ?_r = 5.85, tan δ = 1.59 × 10⁻⁴ (1 MHz) and excellent microwave properties: ?_r = 5.52, Q · f = 28,487 GHz (11.11 GHz). The permittivity of Zr-doped CaO-SiO₂-B₂O₃ ceramic matrix composites exhibited very little temperature dependence, which was less than ±2% over the temperature range of −50 to 150 °C. Moreover, the ZrO₂-doped CaO-SiO₂-B₂O₃ ceramic matrix composites have low permittivity below 5.5 over a wide frequency range from 20 Hz to 1 MHz.     

Microstructure and Cavitation Erosion Properties of Ceramic Coatings Fabricated on Ti-6Al-4V Alloy by Pack Carburizing

In this study, Ti-6Al-4V alloy was processed by pack carburizing to improve the cavitation erosion behavior. X-ray diffraction and scanning-electron microscopy (SEM) analysis showed that a uniform and crack-free ceramic coating formed on the surface of the treated samples. The coating layer comprised primary TiC and less oxide. Cavitation erosion experiment results indicated that the treated samples have the factor of 3.44 to 6.68 increase in cavitation erosion resistance (R _e) as compared with the as-received sample. The ceramic coatings with high hardness and good metallurgical bonding were responsible for the enhanced cavitation erosion properties. When the coatings were treated at condition of high temperature and/or long time, the R _e was enervated due to the thin oxide film formed at the outermost surface. Cavitation erosion mechanism for the coatings was characterized as brittle mode by SEM observation of the worn surfaces.     

C/Al2O3复合材料的固体粒子冲蚀行为与磨擦磨损性能研究

以溶胶浸渍热处理技术路线制备的碳纤维布叠层缝合预制件增强Al₂O₃(C/Al₂O₃)复合材料为对象,以刚玉粉为介质,研究了复合材料的固体粒子冲蚀行为,按照GB5763-2008规定的条件研究了复合材料的磨擦磨损性能。室温下,复合材料冲蚀率随着冲击角度与送粉量的增大而增加;温度升高,由于机械冲击和热冲击的双重作用,冲蚀率显著变大。在GB5763-2008规定的条件下,C/Al₂O₃复合材料具有稳定的摩擦系数和很低的磨损率。结合微观形貌分析,探讨了复合材料的冲蚀与磨损机理。得益于连续碳纤维的补强增韧作用,即使基体致密度低于单体Al2O3陶瓷,C/Al₂O₃复合材料在冲蚀和磨损时不会发生脆性断裂,使用安全性优于单体Al₂O₃陶瓷。     

Plasma Sprayed Coatings of High-Purity Ceramics for Semiconductor and Flat-Panel-Display Production Equipment

High-purity oxide ceramic powders of alumina (Al₂O₃) and yttria (Y₂O₃) have been developed to apply to semiconductor and flat-panel-display (FPD) production equipment. The ceramic coatings on the inside chamber wall of the equipment are required to have high erosion resistance against CFx plasma in dry etching process for microfabrications of the devices. It is found that the yttria coating formed from agglomerated-and-sintered powder consisting of large primary particles has smoother eroded surface with high erosion resistance. Considering the particle deposition on the devices, this coating will be effective in decreasing generation of large-sized particles, which easily deposit on the devices. Electric insulating properties of the coatings are also investigated to apply to electrostatic chuck. Electric breakdown voltage of yttria coatings is comparable to that of alumina coatings. Smaller powder is effective for improving the electric properties, and the influence of coating purity is lower than that of the powder size.     

Characterization of hot pressed SiC whisker reinforced TiB2 based composites

TiB₂–SiC ceramic composites, with different contents of SiC whiskers (SiC_w), as a ceramic sinter-additive, were prepared by the hot pressing process at 1850 °C for 2 h under a pressure of 20 MPa. For comparison, a monolithic TiB₂ ceramic was also fabricated under the identical temperature, pressure, atmosphere, and holding time by the hot pressing process. The effects of fabrication process and SiC whiskers on microstructural features, phase evolution and mechanical properties were investigated. Hardness measurements revealed an initial increase in hardness for TiB₂–SiC compared to TiB₂. Also the improvement of the fracture toughness was attributed to the toughening and strengthening effects of SiC whiskers such as crack deflection. The results showed that promoted densification of TiB₂–SiC ceramic composites is due to addition of SiC whiskers and reduction of oxide impurities by reacting with SiC whiskers and removing them from the surface layer of TiB₂ particles. The reaction between TiB₂ particles and SiC whiskers led to in-situ formation of TiC phase in the matrix as well. In general, it is concluded that the sinterability of TiB₂-based composites was remarkably improved by introducing SiC whiskers compared to the single phase TiB₂ ceramic.     

铜基自润滑复合材料的电弧烧蚀性能研究

研究了铜-30%(体积分数,下同)石墨、铜-30%二硫化钨和铜-30%二硫化钼3种铜基自润滑复合材料的抗电弧烧蚀性能。结果表明:石墨熔点较高,在电弧放电瞬间主要以氧化的形式损耗,而二硫化钨和二硫化钼则会在电弧放电造成的高温下发生熔化甚至与铜基体发生化学反应,所以铜-30%石墨复合材料的抗电弧烧蚀性能要优于铜-30%二硫化钨和铜-30%二硫化钼复合材料。铜基自润滑复合材料的电弧烧损机制主要有材料的氧化、熔化飞溅、内部化学反应以及疲劳脱落。     

High temperature erosion of Cr3C2-NiCr thermal spray coatings — The role of phase microstructure

Cr₃C₂-NiCr thermal spray coatings are extensively applied to mitigate erosion at temperatures above 450-550 °C. The aim of this work was to extend the current comparison based knowledge towards a mechanistic interpretation of the high temperature erosion of Cr₃C₂ based thermal spray coatings. Coatings that span the range of industrial quality were assessed. They were eroded under high temperature (700 °C and 800 °C), aggressive (impact velocity 225-235 m/s) conditions designed to simulate the high velocity erodent impacts within a turbine environment. The influence on the erosion response of high temperature induced changes in the coating microstructure and composition with extended in-service exposure was assessed by heat treating selected samples to generate a steady state microstructure prior to testing. In spite of the marked variation in coating microstructure the erosion rates were comparable across the range of coatings tested. The significance of this conclusion is discussed in terms of the erosion mechanism.     

Fabrication,microstructures and High-Strain-Rate properties of TiC-reinforced titanium matrix composites

TiC ceramic particulate-reinforced titanium matrix composites were fabricated and the resultant densification, microstructure, and static and dynamic mechanical properties were studied. Comparing Ti with TiH₂ powders as host materials for TiC ceramic reinforcement by pressureless vacuum sintering, TiH₂-started composites showed better sinterability and resistance to both elastic and plastic deformation than Ti-started ones. When TiH₂ and TiH₂-45 vol.% TiC samples were hot pressed, TiH₂ matrices transformed to alpha prime Ti and alpha Ti phase, respectively. It is interpreted that the diffusion of an alpha stabilizer carbon from TiC into the matrix is one of the plausible reasons for such a microstructural difference. The 0.2% offset yield strengths of the hot pressed TiH₂ and TiH₂-45 vol.% TiC samples were 1008 and 1446 MPa, respectively, in a static compressive mode (strain rate of 1×10⁻³/s). Dynamic compressive strengths of the samples were 1600 and 2060 MPa, respectively, at a strain rate of 4×10³/s.     

Synthesis and consolidation of TiN/TiB2 ceramic composites via reactive spark plasma sintering

The simultaneous synthesis and densification of TiN/TiB₂ ceramic composites via reactive spark plasma sintering (RSPS) was investigated. Different component ratios (TiH₂/BN (TiN, B)) and heating rates (112.5-300 °C/min) were used to initiate the chemical reaction for TiN/TiB₂ synthesis. The omit RSPS process was revealed to have three stages, which are described separately. The relationships between the RSPS conditions, the microstructure and the properties of sintered ceramic composites were established. A Vickers hardness of 16-25 GPa and a fracture toughness of 4-6.5 MPa m^1/2 were measured for various compositions. Sintered ceramic composites containing 36 wt% TiB₂ with the highest relative density of 97.4 ± 0.4% and an average grain size of 150-550 nm have been obtained.     

Preparation of Al2O3–ZrO2–SiO2 ceramic composites by high-gravity combustion synthesis

By a furnace-free technique of high-gravity combustion synthesis, Al₂O₃–ZrO₂–SiO₂ ceramic composites were prepared via melt solidification instead of conventional powder sintering. The solidification kinetics and microstructure evolution of the ceramic composites in high-gravity combustion synthesis were discussed. The phase assemblage of the ceramic composites depended on the chemical composition, where both (Al₂O₃ + ZrO₂) and (mullite + ZrO₂) composites were obtained. The ceramic composites consisted of ultrafine eutectics and sometimes also large primary crystals. In the (mullite + ZrO₂) composites, two different morphologies and orientations were observed for the primary mullite crystals, and the volume fraction of mullite increased with increasing SiO₂ content. The ceramic composites exhibited a hardness of 11.2–14.8 GPa, depending on the chemical composition and phase assemblage.     

Using coated ceramic particles to increase wear resistance in high-speed steels

The use of chemical-vapor-deposition (CVD)-coated ceramic particle reinforcements in metal-matrix composites allows the control of reactivity at the particle/matrix interface. Wear-resistant, high-speed, steel-based composites containing uncoatedAl₂O₃ uncoated TiC, and CVD-coated A1₂O₃ were liquid-phase sintered and characterized using pin-on-disk wear testing. TiC or TiN CVD coating of Al₂O₃ resulted in a porosity decrease at the particle/matrix interface in addition to better ceramic/metal cohesion due to improved wettability. Lower wear rates were obtained with the composites containing TiC-or TiN-coated Al₂O₃.     

Joining of ZrB2 Ceramics to Ti6Al4V by Ni-Based Interlayers

Pure ZrB₂ and ZrB₂-SiC composites were joined to Ti6Al4V at 1100 °C using B-Ni50 (at.%) as a filler alloy. The brazing medium and the processing parameters were chosen on the basis of specific wetting tests which showed the good adhesion properties of the B-Ni alloy with both the ceramic and the Ti alloy; interfacial reactions were foreseen and interpreted by phase diagram analysis. A multilayer metal-ceramic interfacial structure was observed in the joints and a key role was played by Ti coming from Ti6Al4V: it worked as the active element enhancing the adhesion of the liquid to the ceramic and segregated at the interface forming TiB. A satisfactory mechanical performance was obtained for ZrB₂-SiC/Ti6Al4V joints, which exhibited a room temperature shear strength of 74 MPa.     

Thermomechanical properties of TiC particle-reinforced tungsten composites for high temperature applications

Tungsten and tungsten alloys are widely used in high temperature environments where arc ablation or mechanical deformation and damage are the main sources of materials failure. For high temperature critical applications in thermomechanical environments, however, the low strength limits the use of tungsten and tungsten alloys. Hence, new tungsten based materials with good high temperature thermomechanical properties need to be developed in order to extend the use of tungsten. TiC particle-reinforced tungsten based composites (TiC_p/W) were fabricated by hot pressing at 2000 °C, 20 MPa in a vacuum of 1.3×10⁻³ Pa. The composites were examined with respect to their thermophysical and mechanical properties at room temperature and at elevated temperature. Vickers hardness and elastic modulus increased with increasing TiC content from 0 to 40 vol.%. The highest flexural strength, 843 MPa, and the highest toughness, 10.1 MPa m^1/2, of the composites at room temperature were all obtained when 20 vol.% TiC particle were added. As the test temperature rose, the flexural strength of the TiC_p/W composites firstly increased and then decreased, except in the monolithic tungsten. The highest strength of 1155 MPa was measured at 1000 °C in the composite containing 30 vol.% TiC particles. The strengthening effect of TiC particles on the tungsten matrix is more significant at high temperatures. With the addition of TiC particles, the thermal conduction of tungsten composites was drastically decreased from 153 W m⁻¹ K⁻¹ for monolithic W to 27.9 W m⁻¹ K⁻¹ for 40 vol.% TiC_p/W composites, and the thermal expansion was also increased. The new composites are successfully used to make high temperature grips and moulds.     

Oxidation of niobium particles embedded in a sintered ceramic matrix

The toughness of ceramic materials can be improved through the inclusion of metallic particles in the ceramic matrix. The plastic deformation of the particles limits the rupture of the ceramic matrix. Al₂O₃‐Nb composites show the ability of niobium to form a strong bond with Al₂O₃, but the poor resistance to oxidation of niobium hinders the use of niobium for high temperature applications. The study of the oxidation mechanism was done using thermogravimetric analysis under an oxygen atmosphere in order to obtain the oxidation reaction rate constants of the Al₂O₃‐Nb composite. At the beginning the oxidation kinetics are linear, probably due to a great availability of oxygen. Later on, the oxidation mechanism showed to be parabolic, indicating that the process depends on the oxygen diffusion through the ceramic matrix.