Effects of sulfur on the fatigue and fracture resistance of interfaces between γ-Ni(Cr) and α-Al2O3

Dramatic effects of S on the adhesion and fatigue resistance of interfaces between gamma-Ni(Cr) and alpha-Al2O3, have been explicitly demonstrated and quantified. This has been achieved by using two bonding conditions: one involving solid-state diffusion (SSDB) and another through a liquid phase (LPB) that forms at temperatures above a eutectic that releases a S-rich Liquid. Upon SSDB, there is no significant S excess at the interface, whereas LPB forms a thin interphase with a high local concentration of S. The SSDB materials have interfaces with such high toughness (above 300 J m(-2)) and fatigue resistance that mode I cracks divert into the Al2O3, rather than propagate at the interface. Conversely the LPB materials delaminate at the interface, solely as a result of the residual stresses from thermal expansion misfit, with an interface toughness in the range 2 to 7 J m(-2).