Fracture of oxynitride glasses and SiC particulate composites

The fracture behaviour of YMgSiAlON oxynitride glasses and SiC/oxynitride glass composites was assessed by means of flexural strength measurements and a fractographic approach. The glasses were either produced by pouring of a glass melt (`bulk' glass) or by sintering of a glass powder. The composites were produced by sintering of a mixture of glass and SiC powders. 3-point bending test bars were prepared with different polished surfaces. Some samples were submitted to a flame polishing heat treatment. The strength of the glasses increased with the improvement of the surface finish. The flame polishing treatment led to an increase of the strength of the bulk glass but no further improvement was observed for the sintered glass. For the composites, fracture originated from the particles and their strength only changed with the finish when the defects created by machining were bigger than the defects introduced by the particles. The consequence is that composites are less sensitive to machining flaws, but, when the finish of the surface was good, the strength of the matrix was only increased for small particle sizes. The fracture toughness of the different materials was evaluated using fractographic analysis. For the glasses, the values were similar to values obtained previously by other techniques. For the composites, the values were similar to those obtained for the matrix but significantly different to what has been measured previously using indentation or chevron notched techniques.