Effect of the Diffusion Annealing Process on the Sliding Wear Resistance of Cobalt Boride Layer

Abstract

New results about the sliding wear resistance under dry and wet conditions of cobalt boride layer exposed to a diffusion annealing process (DAP) were estimated in this study. Firstly, the cobalt boride layer (CoB-Co2B) 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. Then, the borided ASTM F1537 alloy was exposed to the DAP at 1273 K with 2 h of exposure in an argon atmosphere. The cobalt boride layers, obtained by the PPBP and PPBP + DAP, were characterized by different physicochemical techniques and depth-sensing Vickers microindentation to determine the nature of phases formed on the surface, the chemical composition along the depth of the cobalt boride layers, and the indentation properties, respectively. The wear sliding tests on the PPBP, PPBP + DAP, and the reference material (ASTM F1537 alloy) were performed using a ball-on-flat configuration comprised with an WC–Co ball as a counterpart using a constant normal force; particularly, for the wet (lubricated) sliding tests, the materials were immersed in a Hanks’ balanced salt solution. The failure mechanisms on the worn tracks were analyzed and correlated with the behavior of the friction coefficient (CoF) and wear resistance of the tested materials. Finally, and according to the entire set of experimental conditions, the results showed that after DAP, the CoB layer was dissolved completely on the surface of the borided ASTM F1537 alloy, with a change of the magnitude of residual stresses (compressive) on the cobalt boride layer. In addition, under dry and wet conditions, the presence of a stiff and hard (CoB) layer formed by the PPBP on the surface of the ASTM F1537 alloy increased the wear resistance compared with the values estimated on the PPBP + DAP and reference material.