Effect of the diffusion annealing process in the indentation properties of cobalt boride layer

New results about the indentation properties of cobalt boride layer exposed to a diffusion annealing process (DAP) are presented in this study. Initially, the cobalt boride layer (CoB-Co ₂ B) was developed at the surface of the ASTM F1537 alloy using the powder-pack boriding process (PPBP) at 1273 K with 6 h of exposure. The DAP was conducted on the borided ASTM F1537 alloy considering the theoretical values (temperature and exposure time) estimated by a diffusion model to dissolve entirely the CoB layer. After that, the distribution of hardness, Young's modulus, and residual stresses across the depth of cobalt boride layer, obtained by both treatment conditions, were estimated using the depth-sensing Vickers microindentation test at a constant load. In addition, Vickers indentation fracture toughness tests were performed on the “pure zone” of the CoB layer (obtained by PPBP) considering a range of applied loads between 1.2 and 1.8 N, in contrast with the range of applied loads of 3–5 N performed in the cobalt boride layer obtained by the PPBP + DAP. Principally, the effect of the DAP resulted in a change of the microstructure of the cobalt boride layer and the modification of indentation properties; a distribution of compressive residual stresses was estimated along the depth of the cobalt boride layer, which drastically modified the fracture resistance to ~ 2.7 MPa m compared with a lower fracture value of ~ 0.5 MPa m obtained by the cobalt boride layer obtained by PPBP.