The boriding process in CoCrMo alloy: Fracture toughness in cobalt boride coatings

New fracture toughness (K_C) data for cobalt boride (CoB and Co₂B) coatings were obtained using the Vickers depth-sensing microindentation technique. The coatings were developed on the surface of a CoCrMo alloy using the powder-pack boriding process at temperatures between 1223 and 1273K using various exposure times for each temperature. The mechanical characterization of the boride coatings was divided into two procedures: first, Vickers indentations were conducted at constant distances from the surface using loads ranging from 15 to 450mN. For the entire set of experimental conditions, the behavior of the indentation load as a function of the diagonal length (d_m) was examined on the CoB and Co₂B coatings. Second, the crack lengths emanated on the corners of the indentations marks (with applied loads above 250mN) were measured on both coatings using a scanning electron microscope (SEM).Based on the indentation results, the K_C values of the cobalt boride coatings were estimated using the universal crack equation, which is applicable independent of the cracking mode. The results indicated that the CoB and Co₂B coatings exhibited two types of cracking modes (intermediate and radial-median, respectively), and that the fracture toughness of the Co₂B coating was ten-fold greater than that of the CoB coating.