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

Abstract

Abstract New fracture toughness ( K C ) data for cobalt boride (CoB and Co 2 B) 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 1273 K 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 450 mN. 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 Co 2 B coatings. Second, the crack lengths emanated on the corners of the indentations marks (with applied loads above 250 mN) 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 Co 2 B coatings exhibited two types of cracking modes (intermediate and radial-median, respectively), and that the fracture toughness of the Co 2 B coating was ten-fold greater than that of the CoB coating.