Residual stress distribution in cermets

Abstract The large difference between the coefficients of thermal expansion of the binder and carbide phases leads to substantial differential thermal residual stresses on colling from the sintering temperature. The stress state is of the grain interaction or microstress type common in composite materials. In this study, neutron diffraction has been used to characterize the degree of strain variance and therefore stress distribution in the carbide phase for two types of cermet. Measurements of diffraction peak breadth in materials with rounded (NbC x Co) and angular ((NbC x TiC)Co) carbide phases have provided evidence of greater variance of residual strain for the angular relative to the rounded carbide. This is attributed to a greater stress distribution due to the angular geometry of the carbide particles. Similar measurements on WC-Co cermets with a low and high binder content indicate that increased strain variance is present in the high relative to the low binder material. This indicates that a greater stress distribution may be present owing to the higher average residual stress level present in the high binder material. The magnitudes of the variance have been estimated for the WC phase and are found to be large, the halfwidth being two to three times the average compressive stress in the carbide. This implies that, assuming a normal (gaussian) stress distribution, in excess of 10% of the WC volume experiences tensile stress.