Liquid Precursor Infiltration Processing of Powder Compacts: II, Fracture Toughness and Strength

Si 3 N 4 powder compacts were infiltrated with liquid precursors which produce either Zr(Y)O 2 (3 mol% Y 2 O 3 ) solid solution or amorphous Si 3 N 4 after pyrolysis at relative low temperatures and without shrinkage. Results show that cracks which occur within a thin, surface layer of the precursor during pyrolysis can extend into the powder compact. As suggested by theory, this cracking phenomenon could be avoided either by making the powder compact stronger before infiltration or by removing the thin precursor layer before pyrolysis. The mechanical properties of these materials were studied as a function of residual porosity. It was observed that crack extension occurred within the second phase produced by infiltration and pyrolysis. The second phase appeared to govern the critical stress intensity factor ( K c ) of the material. K c was found to be a linear function of the change in residual, relative porosity divided by the initial, relative porosity in the powder compact. Reasonable flexural strengths (∼300 MPa) could be achieved despite considerable residual porosity.