Deformation and Fracture of Ceramics and Ceramic Composites
Doctoral thesis, 1993
An understanding of the deformation and fracture behaviour of ceramic materials is of considerable importance in the development of ceramics as reliable structural materials. Of particular interest is how such behaviour is influenced by the microstructure, for example in multiphase materials such as composites. With this in mind a study was made of a variety of ceramics and ceramic composites at temperatures between 20 and 1500 °C.
The fracture toughnesses of alumina and a variety of alumina based composites, containing SiC whiskers and particles of TiC, B4C and Ti(C,N), have been measured. Quantitative characterization of the crack profiles showed that crack deflection does not enhance toughening in these composites and the fracture toughness of the particulate composites was lower than that of alumina. The higher toughness of the SiC whisker composite was shown to be the result of a change in fracture mode from intergranular for unreinforced alumina to transgranular for the whisker composites. A variation in toughness with crack propagation direction was successfully predicted in terms of a crack deflection model in a composite with uniform whisker distribution. The fracture behaviour of the SiC whisker reinforced alumina was studied at temperatures up to 1400 °C in air and up to 1320 °C in vacuum. At temperatures below 1200 °C in air and 1100 °C in vacuum the material fractured in a linear elastic mode. Above these temperatures a rising R-curve and stable crack growth was observed and the fracture toughness rose substantially. The toughness enhancement was shown to be due mainly to such processes as crack deflection, crack branching, microcrack zone formation and crack tip blunting rather than any temperature-related increase in intrinsic fracture resistance of the material. In air, the fracture mechanisms were influenced by the formation of a glass phase.
The fracture toughness of a hot pressed Si3N4 containing 10 vol.% SiC whiskers was studied at temperatures between 20 and 1400oC, and the crack propagation rates were measured for cyclic and static loading at 1300 and 1400 °C. A maximum in fracture toughness observed at 1300 °C is concluded to be due to crack shielding and crack morphology rather than a change in intrinsic toughness. Stable crack growth was shown to occur during the high temperature fracture toughness tests. The present material exhibited crack growth rates, da/dt, that increased with increasing frequency. This is explained by ligaments of the base material bridging the crack.
The tensile creep behaviour of a sintered alumina was studied at temperatures between 1150 and 1350 °C and stresses between 4 and 67 MPa. The stress exponent was 1.3 and the activation energy for creep was 620 kJ/mol. Grain boundary sliding accompanied by diffusion creep (Nabarro-Herring) and cavitation are suggested as the dominating creep mechanisms under the present test conditions. Nucleation, growth and coalescence of cavities is suggested as the failure mechanism at lower loads while microcrack growth determines failure at higher loads.
The knoop indentation hardness in SiC whisker reinforced alumina was shown to fall with increasing temperature up to 1365 °C, and SiC whiskers was shown to harden alumina at temperatures above 600 °C. In pin-on-plate sliding friction in vacuum, the coefficient of friction ( µ) decreases almost continuously with temperature. Stick-slip becomes significant above 900 °C and this is considered to reflect a tendency to increased adhesion. Formation of graphite, combined with very smooth wear track due to extensive plastic deformation of the surface, serves to keep µ low at temperatures above 900 °C. The tribological behaviour of the composite under these conditions was determined more by chemical effects than the micromechanical behaviour.