Toughness evaluation of HVOF WC–Co coatings using non-linear regression analysis

The models for predicting indentation toughness of materials are usually functions of both indenter and induced crack (i.e. Palmqvist or Half-Penny) geometries. Palmqvist cracks have already been identified in sintered WC–Co cermets. In the present work, the indentation toughness of high velocity oxy-fuel thermally sprayed WC–12%Co coatings has been evaluated using a standard metallographic procedure. Two crack regimes were found to occur in such coatings, depending on the indentation load: at low indentation loads, Palmqvist cracks appeared whilst Half-Penny cracks were detected at high indentation loads. A statistical methodology was developed in order to determine the indentation toughness of WC–12%Co coatings, which were heat-treated at 1173 and 1463 K, from induced crack sizes measured at different loads. The non-linear regression analysis statistically confirmed the existence of two regimes for crack propagation and two distinct toughness values were recorded for the same material under different load conditions. The present results indicate that adequate post-heat treatments can lead to an increase in coating toughness.