Environmental impact on the strength of subcritical cracking AlON material

Subcritical crack growth is often observed in oxide ceramics. This effect is ascribed to the interaction between the strained chemical bonds at the crack tip and water molecules present in the working environment of the material. Owing to this reaction, combined with stresses at the crack tip, slow cracking of the material and relevant strength reduction may occur. In non-oxide ceramics (i.e., without significant oxide sintering additives), no subcritical crack growth was observed. The present work demonstrates slow crack propagation in an aluminum oxynitride (AlON) material. AlON samples were prepared via sintering in nitrogen at 1800 °C. This resulted in dense translucent samples. Constant stress rate tests were carried out in both air and water to assess the influence of water on material strength. The strength results were used to further evaluate the effects of subcritical cracking. A clear decrease in strength was observed at low stress rates. Measurement points below 10 MPa/s were used to determine the lowest stress rate for inert strength. The slope of the fitting line indicated the value of subcritical cracking parameter n.