Pressureless sintering of AlN-SiC composites

SiC-AlN composites have been successfully pressureless sintered by using commercial SiC and AlN powders with the optimum amount of sintering aid. The important parameters during pressureless sintering, including the amount and type of sintering aids, sintering temperature, sintering period and packing powder have been studied. Yttria was found to be a better sintering aid than alumina or calcia. The yttria sintering aid reacts with AlN and SiC powders and forms a Y-Al-Si-O-N grain-boundary phase to assist densification during pressureless sintering. With 2 wt% yttria, SiC-AlN composites can be pressureless sintered to high density at 2050–2100 °C for 2 h under the firing conditions where alpha-pp packing powder is used during firing. The microstructure and phases of the composites were characterized by using scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectrometry and X-ray diffractometry.     

SiC-TiC and SiC-TiB2 composites densified by liquid-phase sintering

Particle-reinforced SiC composites with the addition of TiC or TiB₂ were fabricated at 1850 °C by hot-pressing. Densification was accomplished by utilizing a liquid phase formed with added Al₂O₃, Y₂O₃, and surface SiO₂ on SiC. Their mechanical and electrical properties were measured as a function of TiC or TiB₂ content. Adding TiC or TiB₂ to the SiC matrix increased the toughness, and decreased the strength and electrical resistivity. The fracture toughnesses of SiC-50 wt% TiC and SiC-50 wt% TiB₂ composites were approximately 60% and 50%, respectively, higher than that of monolithic SiC ceramics. Microstructural analysis showed that the toughening was due to crack deflection, with some possible contribution from microcracking in the vicinity of TiC or TiB₂ particles.     

Pressureless sintering and mechanical properties of alumina-sialon composites

Compositions of Al₂O₃, Si₃N₄ and AlN were sintered to produce six different alumina-sialon composites. The composites reached the highest density at 1700 °C in a nitrogen atmosphere. Sialon acted as a binder, promoting the densification and suppressing the grain growth of alumina. The composites had a bending strength of 450 MPa at room temperature and maintained high strength over 300 MPa at 1400 °C. The composite with 30 wt % sialon had the maximum fracture toughness of 4.3 MPa·m^1/2 and exhibited good oxidation resistance even at 1400 °C. Mullite was formed in the oxidation layer and little degradation in strength was observed after oxidation.     

Pressureless sintering of Al2O3-SiC whisker composites

High-density compacts, up to 88% theoretical density, of Al₂O₃-SiC whiskers were prepared by a pressure casting and impregnation technique. Starting with these green bodies, composites of Al₂O₃–20 vol% SiC whiskers were pressureless sintered to higher than 95% theoretical density. They were further densified by hot isostatic pressing up to 99% theoretical density, resulting in a rupture strength of 680 MPa and a fracture toughness of 4.70 Mpa m^1/2.     

Pressureless sintering and characterization of cordierite/ZrO2 composites

Cordierite/ZrO₂ composites with 5 to 25 wt% ZrO₂ were fabricated by conventional powder mixing and pressureless sintering method. Their densification behavior, microstructure, mechanical and thermal properties were studied. By dispersing 25 wt% (9.57 vol%) ZrO₂, densified cordierite/ZrO₂ composite with a relative density of 98.5% was obtained at an optimum sintering condition of 1440 °C and 2 h. ZrO₂ particles were homogenously dispersed within matrix grains and at the grain boundaries. The intragranular particles were finer than 100 nm and the intergranular particles were coarser. Both fracture strength and toughness could be enhanced more than two times higher, compare to those of monolithic cordierite, by dispersing 25 wt% ZrO₂ into the cordierite matrix. The toughening mechanism in the present composites was mainly attributed to martensitic transformation due to ZrO₂ dispersion. Electronic Publication     

Hot pressing of SiC-TiC composites

Sintering behaviour during hot pressing of SiC-TiC composite ceramics has been investigated with special emphasis on the effect of various processing parameters on the density and mechanical properties of the sintered body. At hot pressing temperatures greater than 2000° C, significant densification occurred in SiC-50 wt%TiC (–0.5 wt%B-1 wt%C) composites. The room temperature flexural strength of the sintered body increased with the hot pressing temperature up to 2000° C and reached a highest value of 710 MPa in accordance to the variation of density with temperature. In sintering of composites without additives, densification was enhanced with the addition of up to 25 wt%TiC, with relative densities higher than 98% observed when hot pressing at 2150° C for 2h.     

The fracture toughness of composites reinforced with weakened fibres

Fibre fractures which occur near, but not at, the plane of matrix failure in a composite, lead to fibre pull-out during fracture. Energy absorbed in this process contributes directly to the work of fracture and hence to the toughness of the composite.Factors which determine the mean length of fibre pulled out during fracture are discussed for the case of composites reinforced with continuous fibres having variously spaced points of weakness. The presence of such weak points also affects the strength of the composite, but not all composites of the same strength have the same toughness. The greatest toughness for a given strength is always found in composites reinforced with discontinuous fibres.     

Fracture toughness and fracture of WC-Co composites

Critical stress intensity factor, and related parameters have been measured in three-point bending for 18 different combinations of different volume fractions of cobalt (5 to 37%) and grain size of tungsten carbide (0.7, 1.1 and 2.2 m). In particular, a study was made of the correlations between the strength and mechanical and microstructural parameters, such as ¯L _Co,C _WC, ¯L _Co/¯D _WC, ¯L _Co ² /¯D _WC,H _V and wear resistance. A hypothesis for the mechanism of fracture has been proposed following an analysis of these results and a study of the mode of fracture.     

Mixed mode fracture toughness of GFRP composites

Glass fibre reinforced polymer (GFRP) composites are used in a wide range of applications as a structural material. They have high specific mechanical properties but are prone to delamination as a result of manufacturing defects and impact/shock loading. The ability of the structure to continue to carry load after damage and the subsequent propensity of the damage to propagate are important considerations for the design of damage tolerant composite structures. In order to accurately predict the stability of damage under load, relevant mechanical properties of the material must be accurately determined. In particular, mixed mode fracture toughness data is required in order to study the damage criticality in such structures. This paper describes an experimental study to determine Mixed Mode fracture toughness for thick glass/vinylester specimens. The test methodology used for the experiments and its difficulties will be discussed. Mixed mode fracture toughness results are presented, as are Mode I and Mode II fracture toughness results obtained via Double Cantilever Beam (DCB) and End Notch Flexure (ENF) tests, respectively.     

Prediction of the fracture toughness of fibrous composites

A statistical/micromechanical model is developed for the prediction of the fracture toughness of fibrous composites. The fracture resistance of the material is assumed to be related to the statistical distribution of the fiber pull-out length. The distribution of the fiber pull-out length is derived from the fiber strength distribution. The R-curve behavior of the fibrous composite is predicted and interpreted based on the present model. The limiting fracture toughness is predicted to be proportional to the square root of the ineffective length, or proportional to the square root of the fiber length if the fiber length is less than the ineffective length.     

Interfacial fracture energy and the toughness of composites

The premises upon which prevailing composite toughness theories are based are discussed in the light of observed strength variations in boron-epoxy composites with differing shear strengths of the interfacial bond. None of the extant toughness theories (pull-out, debonding, stress redistribution) successfully predicts the work of fracture of the boronepoxy system. However, incorporation of the work to create new surfaces into the total toughness analysis gives better agreement with experiment, and work of fracture predictions for other sytems, such as carbon-polyester, can also be modified. The approach is more generalized than the Outwater/Murphy debonding explanation for toughness, which in the way usually presented only applies when the filament fracture strain is greater than the matrix fracture strain. The present analysis suggests how to tailor the interfacial shear strength in order to obtain a reasonable toughness yet still maintain strengths of the order of the rule of mixtures.