Processing,microstructure and properties of in-situ reinforced SiC matrix composites

SiC matrix composites were fabricated by in-situ formation of transition metal boride and carbide particles from oxide powders by carbothermal reactions. Dense composites with various microstructures were produced by pressureless sintering and additional hot-isostatic pressing. The microstructures and mechanical properties of the composites were dependent upon the pressureless-sintering temperature. The use of submicron-sized TiO₂ lead to fine and equiaxial TiB₂ particulates. The composites exhibited high flexural strengths (>700 MPa). At higher sintering temperatures, the grain growth of SiC swept the boride into clusters with larger sizes and anisotropic shapes, which improved the fracture toughness of the composite at the expense of strength.     

Pressureless-sintered and HIPed SiC-TiB2 composites from SiC-TiO2-B4C-C powder compacts

Dense SiC-TiB₂ composites with prescribed compositions were obtained through pressureless sintering of SiC-TiO₂-B₄C-C powder compacts. During the process, TiO₂, B₄C and C reacted to form TiB₂, followed by the consolidation of SiC matrix with the aid of excess B₄C and C. The effects of the composition of the starting powders on the final density were investigated and the mechanical properties of the composite were evaluated. The sintered body with additional HIPing at 1900 °C exhibited the average four-point flexural strength of more than 700 MPa at both 20 and 1400 °C.     

Fabrication of SiCf/SiC composites by chemical vapor infiltration and vapor silicon infiltration

Three-dimensional (3D) silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites, employing KD-1 SiC fibers (from National University of Defense Technology, China) as reinforcements, were fabricated by a combining chemical vapor infiltration (CVI) and vapor silicon infiltration (VSI) process. The microstructure and properties of the as prepared SiC_f/SiC composites were studied. The results show that the density and open porosity of the as prepared SiC_f/SiC composites are 2.1 g/cm³ and 7.7%, respectively. The SiC fibers are not severely damaged during the VSI process. And the SiC fibers adhere to the matrix with a weak interface, therefore the SiC_f/SiC composites exhibit non-catastrophic failure behavior with the flexural strength of 270 MPa, fracture toughness of 11.4 MPa·m^1/2 and shear strength of 25.7 MPa at ambient conditions. Moreover, the flexural strength decreases sharply at the temperature higher than 1200 °C. In addition, the thermal conductivity is 10.6 W/mk at room temperature.     

Effect of B2O3 on the microstructure and strength of SiC/MoSi2 composite

The effect of boron oxide (B₂O₃) on the microstructure and strength of 20% (by volume) silicon carbide (SiC) reinforced molybdenum disilicide (MoSi₂) composites was examined. Microstructural analysis using optical and electron microscopes was performed. A standard four-point bending test and Weibull statistics were applied to evaluate the flexural strength of the composites. The addition of boron oxide to composite was found to reduce the processing temperature and the grain sizes, result in formation of glassy boundary phase, and to increase the flexural strength of the composite.     

Thermal effects on the mechanical properties of SiC fibre reinforced reaction-bonded silicon nitride matrix composites

The elevated temperature four-point flexural strength and the room-temperature tensile and flexural strength properties after thermal shock were measured for ceramic composites consisting of 30 vol% uniaxially aligned 142 m diameter SiC fibres in a reaction-bonded Si₃N₄ matrix. The elevated temperature strengths were measured after 15 min exposure in air at temperatures upto 1400 ° C. The thermal shock treatment was accomplished by heating the composite in air for 15 min at temperatures up to 1200 ° C and then quenching in water at 25 ° C. The results indicate no significant loss in strength properties either at temperature or after thermal shock when compared with the strength data for composites in the as-fabricated condition.     

SiC matrix composites reinforced with internally synthesized TiB2

A new process of preparing particulate-reinforced ceramic composites by internal synthesis has been developed. SiC powder mixed with TiN and amorphous boron was hot-pressed above 2000° C in an argon atmosphere. The boron molar content in the mixture was designed to be more than twice that of TiN. In the process of hot-pressing, the following reaction took place between 1100 and 1700° C TiN+2B TiB₂+1/2N₂ The synthesis of TiB₂ was followed by the densification of SiC matrix with the aid of the excess boron. The new process provides SiC matrix composites in which fine TiB₂ particulates are dispersed. Compared with hot-pressed monolithic SiC, the composite containing 20 vol % TiB₂ exhibits a 80% increase in fracture toughness and about the same flexural strength of 490 MPa at 20° C in air and 750 MPa at 1400° C in a vacuum.     

Microstructure and thermal shock resistance of Al2O3 fiber/ZrO2 and SiC fiber/ZrO2 composites fabricated by hot pressing

Al₂O₃ chopped fiber/ZrO₂ and SiC continuous fiber/ZrO₂ composites were fabricated by hot pressing at 1550°C and 15 MPa in vacuum. The mechanical properties of thermally shocked composites were measured at room temperature by four-point bending. The addition of Al₂O₃ fibers into ZrO₂ matrix degraded the fracture strength, but improved significantly the thermal shock resistance. In addition, the mechanical properties of SiC fiber/ZrO₂ composites were much lower than those of monolithic ZrO₂ because of the presence of microcracks on the surface. The SiC fiber/ZrO₂ composites showed an excellent thermal shock resistance.     

Oxidation behavior and mechanical properties of C/SiC composites with Si-MoSi2 oxidation protection coating

A new kind of oxidation protection coating of Si-MoSi₂ was developed for three dimensional carbon fiber reinforced silicon carbide composites which could be serviced upto 1550 °C. The overall oxidation behavior could be divided into three stages: (i) 500 °C < T < 800 °C, the oxidation mechanism was considered to be controlled by the chemical reaction between carbon and oxygen; (ii) 800 °C < T < 1100 °C, the oxidation of the composite was controlled by the diffusion of oxygen through the micro-cracks, and; (iii) T > 1100 °C, the oxidation of SiC became significant and was controlled by oxygen diffusion through the SiC layer. Microstructural analysis revealed that the oxidation protection coating had a three-layer structure: the out layer is oxidation layer of silica glass, the media layer is Si + MoSi₂ layer, and the inside layer is SiC layer. The coated C/SiC composites exhibited excellent oxidation resistance and thermal shock resistance. After the composites annealed at 1550 °C for 50 h in air and 1550 °C 100 °C thermal shock for 50 times, the flexural strength was maintained by 85% and 80% respectively. The relationship between oxidation weight change and flexural strength revealed the criteria for protection coating was that the maximum point of oxidation weight gain was the failure starting point for oxidation protection coating.     

The properties of carbon fibre/SiC composites fabricated through impregnation and pyrolysis of polycarbosilane

Unidirectional carbon fibre reinforced SiC composites were prepared from four types of carbon fibres, PAN-based HSCF, pitch-based HMCF, CF50 and CF70, through nine cycles or twelve cycles of impregnation of polycarbosilane and subsequent pyrolysis at 1200°C. The polycarbosilane-derived matrix was found to be -SiC with a crystallite size of 1.95 nm. The mechanical properties of the composites were evaluated by four-point bending tests. The fracture behavior of each composite was investigated based on load-displacement curves and scanning electron microscope (SEM) observation of fracture surfaces of the specimens after tests. It was found that CF50/SiC and CF70/SiC exhibited high strength and non-brittle fracture mode with multiple matrix cracking and extensive fibre pullout, whereas HSCF/SiC and HMCF/SiC exhibited low strength and brittle fracture mode with almost no fibre pullout. The differences in the fracture modes of these carbon fibre/SiC composites were thought to be due to differences in interfacial bonding between carbon fibres and matrix. Values of flexural strengths of CF70/SiC and CF50/SiC were 967 MPa and 624 MPa, respectively, which were approximately 75% and 38% of the predicted values. The relatively lower strength of CF50/SiC, compared with CF70/SiC, was mainly attributed to the shear failure of CF50/SiC during bending tests.     

Effect of C/C preform density on microstructure and mechanical properties of C/C–SiC composites prepared by alloyed reactive melt infiltration

Abstract

Low cost C/C–SiC composites were prepared by alloyed reactive melt infiltration. Effects of the density of C/C preforms on mechanical properties and microstructure of the C/C–SiC composites are reviewed. The results show that with increasing the density of C/C preforms, the flexural strength of the resulting composites increases, while the density of the composites decreases. The flexural strength can reach 341 MPa for the composite produced from the C/C preform of 1·3 g cm^?3. The phases in the composites produced from low density C/C preforms are Si, SiC, ZrSi₂ and carbon, while no Si phase is found in the composites with high density C/C preforms. Furthermore, the mechanism of the microstructure evolution of the C/C–SiC composites is proposed.     

Fracture and flexural characterization of SiCw/SiC composites at room and elevated temperatures

The fracture and flexural behaviour of monolithic SiC and SiC-whisker reinforced SiC composites (SiC_w/SiC) has been investigated at room and elevated temperatures. Flexure and fracture tests were conducted in a four-point beam configuration at 23 °C, 800 °C and 1200 °C to study the effects of whisker reinforcements especially in respect of mechanical and thermal stability at high energy environments. Flexural strengths and fracture toughness data within the test temperature range are presented in graphical as well as in Weibull form, and experimental observations are analysed and discussed. Increase in flexural strength as well as in fracture toughness has been observed with the whisker reinforcement. However, it was found that the trend discontinues after a certain range of temperature. Post-failure analyses have been performed with the scanning electron microscope (SEM). Formation of glass phase has been observed at the whisker/matrix interface and the crack growth was found to be shifting from intergranular to transgranular with the rise in temperature. Effects of whisker reinforcement and the degradation of flexural and fracture properties at elevated temperature are investigated. Ultrasonic velocity measurements have been performed through the thickness of the untested as well as fractured specimen, and the variation in the sonic wave velocity is discussed in this paper.     

Preparation and properties of SiO2 matrix composites doped with AlN particles

SiO₂ matrix composites doped with AlN particles were prepared by hot-pressing process. Mechanical properties of SiO₂ matrix composites can be greatly improved by doping with AlN particles. Flexural strength and fracture toughness of 30 vol%AlN-SiO₂ composite sintered at 1400°C reached 200 MPa and 2.96 MPa·m^1/2. XRD analysis indicated that, up to 1400°C, no chemical reaction occurred between SiO₂ matrix and AlN particles suggesting an excellent chemical compatibility of SiO₂ matrix with AlN particles. The influences of hot-pressing temperature and the content of AlN particles on dielectric properties of SiO₂-AlN composites were studied. The temperature and frequency dependency of dielectric properties of SiO₂-AlN composites were also studied. Residual flexural strength of SiO₂-AlN composites decreased with increasing temperature difference. The critical temperature difference was estimated about 600°C.     

Nicalon-fibre-reinforced silicon-carbide composites via polymer solution infiltration and chemical vapour infiltration

A new, faster process was developed for the fabrication of Nicalon-fibre-reinforced SiC composites by combining polymer solution infiltration (PSI) and chemical vapour infiltration (CVI). The process led to the near-net-shape fabrication of fibre-reinforced ceramic-matrix composites and reduced infiltration time. Typical flexural strength and fracture toughness of these composites were 296 MPa and 10.9 MPa m^1/2 at room temperature (RT) and 252 MPa and 9.6 MPa m^1/2 at 1000 °C, respectively. The composites exhibited load-carrying capability after crack initiation.     

Microstructure and mechanical properties of carbon fiber reinforced multilayered (PyC–SiC)n matrix composites

Carbon fiber reinforced multilayered (PyC–SiC)_n matrix (C/(PyC–SiC)_n) composites were prepared by isothermal chemical vapor infiltration. The phase compositions, microstructures and mechanical properties of the composites were investigated. The results show that the multilayered matrix consists of alternate layers of PyC and β-SiC deposited on carbon fibers. The flexural strength and toughness of C/(PyC–SiC)_n composites with a density of 1.43 g/cm³ are 204.4 MPa and 3028 kJ/m³ respectively, which are 63.4% and 133.3% higher than those of carbon/carbon composites with a density of 1.75 g/cm³. The enhanced mechanical properties of C/(PyC–SiC)_n composites are attributed to the presence of multilayered (PyC–SiC)_n matrix. Cracks deflect and propagate at both fiber/matrix and PyC–SiC interfaces resulting in a step-like fracture mode, which is conducive to fracture energy dissipation. These results demonstrate that the C/(PyC–SiC)_n composite is a promising structural material with low density and high flexural strength and toughness.     

Effect of BN grain size on microstructure and mechanical properties of the ZrB2–SiC–BN composites

ZrB₂–20 vol.%SiC composites containing 10 vol.% h-BN particles (ZSB) with average grain sizes ranging from 1 μm to 10 μm were hot-pressed. The fracture toughness of the ZSB composites was higher than reported results of monolithic ZrB₂ (2.3–3.5 MPa m^1/2) and SiC particle reinforced ZrB₂ composites (4.0–4.5 MPa m^1/2). The improvement in the fracture toughness of the ZSB composites was due to the high aspect ratio of h-BN and weaker interface bonding, which could enhance crack deflection and stress relaxation near the crack-tip. Compared with the flexural strength of the ZrB₂–SiC composites, the reduction in the flexural strength of the ZSB composites was attributed to the weaker interface bonding and the lower relative density. Furthermore, improvement in toughness and the reduction in the strength were valuable to improve the thermal shock resistance of the ZSB composites. The ΔT_c of ZSB5 material is 400 °C which is higher than ZrB₂–20%SiC and ZrB₂–15%SiC–5%AlN.     

ZrB2-ceramic toughened by refractory metal Nb prepared by hot-pressing

ZrB₂–Nb (ZN) composites were prepared through hot-pressing at a temperature of 1800 °C. A contribution of Nb was believed a significant influence on the sinterability, microstructure and mechanical properties of ZN composites. The values of flexural strength of ZN composites rang from 395 to 773 MPa, who are dependent on Nb contents. The highest strength obtained for the ZN composite containing 25 vol.% Nb (773 MPa). A fracture toughness of 7.1 MPa m^1/2 of ZN was revealed, which was much higher than that of monolithic ZrB₂. The improvement in fracture toughness strongly depended on an introduction of Nb–ZrB₂ matrix. Crack deflection and branching were believed to be the toughening mechanism of ZN.     

Pressure pulsed chemical vapour infiltration of SiC to two-dimensional-Tyranno/SiC-C preforms

Preforms of two-dimensional Tyranno fibre (SiC base) of 7×20×1.3 mm³ were chemically vapour infiltrated with SiC at 850–1050 °C from a gas mixture of CH₃SiCl₃ (6%)-H₂ using pressure pulses between below 0.3 kPa and 0.1 MPa. Above 900 °C, films grew on the macrosurface dominantly. At 850 °C, residual porosity decreased to about 10% after 10⁵ pulses, and three point flexural strength reached about 200 MPa. X-ray diffractograms (XRDs) on the surface showed the deposits to be -SiC only.     

Superior high-temperature resistance of aluminium nitride particle-reinforced aluminium compared to silicon carbide or alumina particle-reinforced aluminium

Aluminium-matrix composites containing AlN, SiC or Al₂O₃ particles were fabricated by vacuum infiltration of liquid aluminium into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AlN had similar tensile strengths and higher ductility compared to Al/SiC of similar reinforcement volume fractions at room temperature, but exhibited higher tensile strength arid higher ductility at 300–400 °C and at room temperature after heating at 600 °C for 10–20 days. The ductility of Al/AIN increased with increasing temperature from 22–400 °C, while that of Al/SiC did not change with temperature. At 400 °C, Al/AlN exhibited mainly ductile fracture, whereas Al/SiC exhibited brittle fracture due to particle decohesion. Moreover, Al/AlN exhibited greater resistance to compressive deformation at 525 °C than Al/SiC. The superior high-temperature resistance of Al/AlN is attributed to the lack of a reaction between aluminium and AlN, in contrast to the reaction between aluminium and SiC in Al/SiC. By using Al-20Si-5Mg rather than aluminium as the matrix, the reaction between aluminium and SiC was arrested, resulting in no change in the tensile properties after heating at 500 °C for 20 days. However, the use of Al-20Si-5Mg instead of aluminium as the matrix caused the strength and ductility to decrease by 30% and 70%, respectively, due to the brittleness of Al-20Si-5Mg. Therefore, the use of AIN instead of SiC as the reinforcement is a better way to avoid the filler-matrix reaction. Al/Al₂O₃ had lower room-temperature tensile strength and ductility compared to both Al/AlN and Al/SiC of similar reinforcement volume fractions, both before and after heating at 600 °C for 10–20 days. Al/Al₂O₃ exhibited brittle fracture even at room temperature, due to incomplete infiltration resulting from Al₂O₃ particle clustering.     

Mechanical properties of pulse discharge sintered Cr2AlC at 25-1000 °C

Ternary compound Cr₂AlC was synthesized by a reactive sintering process and its mechanical properties in the 25-1000 °C temperature range were studied by 4-point bending tests. The flexural strength of Cr₂AlC decreases continuously from 555 ± 11 MPa at room temperature down to 100 ± 4 MPa at 1000 °C and this strength decreasing tendency is more obvious as the testing temperature is higher than 900 °C. The ductile-to-brittle transition temperature of Cr₂AlC locates in the range of 800-900 °C. The macro-plastic deformation of Cr₂AlC is mainly attributed to the initiation and propagation of large number of microcracks.     

Fast firing of reaction-bonded aluminium oxide RBAO composites

Al₂O₃/ZrO₂ composites have been prepared by fast firing of oxidized Al/Al₂O₃/ZrO₂ precursors produced by the reaction-bonded aluminium oxide (RBAO) technique. This fabrication route results in high-strength ceramics at relatively low densities. For example, after fast firing for 20 min at 1550 °C, RBAO containing 20 vol% ZrO₂ shows four-point bending strengths of > 600 MPa at a density of 95% theoretical which is comparable to conventionally sintered RBAO.