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Alpha-Si3N4 powders were explosively shock loaded at levels of 260 and 570 kbar pressure. Specimens exhibited densification without additives into the 93 to 98% dense range as a result of shock-induced consolidation. X-ray line broadening investigations indicated that significant residual, internal strain levels of the order of 0.4% were developed in the densified material. Hardness and indentation fracture toughness values for shock-densified regions were nearly equivalent to those for ultra-high-pressure hot-pressed Si3N4 without densification aids.

Reproducible controlled surface flaws were introduced in reaction-bonded Si3N4 by Knoop microhardness indentation. Fracture was initiated at a single suitably oriented flaw on the tensile surface of a four-point bend specimen, and the stress required to propagate the controlled flaw catastrophically was used to calculate the critical stress intensity factor KIC from standard fracture-mechanics formulae for semi-elliptical surface flaws in bending. KIC was unchanged up to 1200° C, but values above this temperature together with those at room temperature after vacuum annealing were reduced approximately 25%. This reduction is thought to be due to reaction with oxygen concentrations present in the vacuum “atmosphere” rather than to a change in material properties. Although flaw “healing” was observed for annealing in air, it was accompanied by a decrease in fracture stress. It is suggested that this is due to volume mismatch between Si3N4 and the oxidation product, cristobalite, which introduces tensile stresses in the matrix material.

The R-curve behaviour of a B4C-Al cermet was studied experimentally using the indentation/strength technique. Microstructural observations clearly indicate that the dominant mechanism contributing towards increasing toughness with crack length is due to bridging of ductile Al phases. It is inferred that ductile toughening, along with toughening due to residual stresses generated upon cooling of the cermet, contribute to this toughening behaviour. Close agreement between experimental results and a simple model is also established.

A 30 vol % partially stabilized ZrO2 particle-MoSi2 matrix composite was synthesized by hot pressing to 96% theoretical density at 1700°C. No chemical reactions between the PSZ and MoSi2 were observed after hot pressing, indicating thermodynamic stability of these species. The composite formed an adherent and coherent glassy-appearing oxidation layer after oxidation at 1500°C. The room temperature indentation fracture toughness of the composite was 2.5 times that of pure MoSi2. These results demonstrate the feasibility of PSZ particle-MoSi2 matrix composites.

Carbon as a second-phase particulate addition to a MoSi2 matrix has been investigated for possible improved mechanical property effects. MoSi2 with 0, 1, 2 and 4 wt% C was fabricated using hot pressing and containerless hot isostatic pressing at temperatures of 1400, 1600 and 1800°C. These processing conditions showed the formation of an in-situ SiC phase and the ternary Nowotny phase C<1Mo<5Si3, which affected the microstructure and fracture behaviour of the materials. Microindentation techniques were used to evaluate the hardness and fracture toughness of the MoSi2 and C-MoSi2 materials. Increasing the processing temperature increased the hardness due to in-situ SiC formation as a result of carbon reaction with silica, although only modest improvements in fracture toughness were observed.