Property enhancement of diffusion borided layers by laser treatment

Abstract

Tribological properties such as abrasion and erosion resistance of boride coatings on steels have been proven to be outstanding when compared with conventional hardening techniques, e.g., carburizing and nitriding. Pack boriding is a relatively simple and economical technique among all the methods available for boriding. A single phase Fe₂B layer is desirable in the boride coating for its superior performance and this can be achieved in several ways. However, the boride coating is often associated with inherent porosity, especially near the surface and this is undesirable from the standpoint of structural integrity of the coating particularly for applications involving highly loaded parts. Laser-based postprocessing is an effective technique for eliminating these undesirable microstructural features without deteriorating the mechanical properties of the coating. In the present investigation, a low activity boronizing mixture was used for pack boriding of a medium carbon steel to obtain a single phase Fe₂B layer. The coating was laser treated with a pulsed Nd-yttrium-aluminum-garnet laser at different laser powers to control the extent of melting of the boride layer. Characterization of both as-borided and laser treated coatings was carried out to examine changes in surface roughness, microstructural features, and microhardness profile and phases as a result of laser treatment. Testing of the coating under erosive and abrasive wear conditions was also carried out to ascertain the influence of laser treatment on tribological performance of the coating. It was found that, under controlled processing parameters, it is possible to improve the durability of coating properties through laser treatment.