Effect of borotitanizing on microstructure and wear behavior of Inconel 625

Inconel 625, a nickel-based superalloy, is used in a wide range of applications including the marine and petroleum industries under where it is subjected to harsh conditions such as high temperatures and highly corrosive environments. However, its wear resistance is limited and can be often considered unsatisfactory in some applications. If this alloy were to be used under abrasive wear conditions, its surface would have to be protected by a wear resistant coating. In this study, a two-step thermo-chemical borotitanizing treatment (including an initial boriding step followed by titanium diffusion) is proposed. Microstructural characterization (optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction) and mechanical properties (including micro-hardness and micro-abrasion wear) of the coated samples were conducted. Microstructural studies revealed a compact, homogenous, silicide-free coating, consisting of four distinct regions: a TiB2 layer, a multi-phase boride layer, a diffusion zone and the substrate. Hardness values were significantly higher than those obtained by standard boriding treatments. Due to the nano-sized boriding agents used, the coatings formed on the surface were thicker than coatings obtained by methods such as nitriding, paste boriding and pack-boriding, and comparable to that of laser boriding. The wear resistance was improved by up to ten times in comparison with untreated Inconel 625. Grooving was the effective wear mechanism in untreated Inconel 625. However the increase in surface hardness achieved by the borotitanizing treatment changed the wear mechanism in the coated samples from grooving to rolling.