Improvement of surface properties of AISI 316L stainless steel coatings with electrochemical boronizing process

Austenitic stainless-steel coatings applied using the high-velocity oxy-fuel (HVOF) technique are extensively utilized as a means of safeguarding ordinary steel from corrosion in various settings, including chemical plants, mechanical machinery, and equipment components. The hardness of AISI 316L austenitic stainless-steel coatings is slightly greater than that of AISI 316L steel plates, as indicated by the Vickers hardness values. The HVOF 316L coating exhibits a hardness of approximately 289±36 HV0.1, while the steel plates demonstrate a hardness of approximately 200 HV. Nevertheless, it should be noted that 316L coatings do not possess the same level of hardness as sprayed ceramic or cermet coatings, as the latter exhibit hardness values over 600 HV. Under severe friction conditions, HVOF 316L coatings are ineffective as a wear-resistant coating. As an alternative to chromium plating and ceramic coating, as repair coatings for machine parts and mold tools, etc., the development of more durable sprayed 316L coatings would significantly expand their range of applications. The purpose of this study is to improve the surface properties of HVOF 316L coatings by applying thermal diffusion-based boriding by forming a thick single-phase Fe2B layer and dual-phase boride structures composed of Fe2B (inner) and FeB (outer) on HVOF 316L coatings, and then to examine the dry sliding wear and tribocorrosion behaviors of samples. Structural characterization of borided and unborided HVOF 316L coatings was conducted using an optical microscope and surface hardness measurements. Wear tracks of samples subjected to dry sliding wear and tribocorrosion tests were examined under an optical microscope in order to determine their wear mechanism. A 2-D profilometer was also used to measure the wear loss. On the basis of the microstructure and surface hardness analysis results, it has been determined that a single-layered (Fe2B) borided structure with a thickness of approximately 14 μm and a surface hardness of 1419±378 HV0.1 is formed on the surface of HVOF 316L coating during 15 min of electrochemical boriding (EB) at 950 °C with 45 min of soaking time. At EB in 30 min and 60 min at 950 °C with 45 min of soaking time, however, a two-layered (FeB+Fe2B) borided structure with approximate thicknesses of 23 μm and 30 μm and surface hardness of 1839±436 HV0.1 and 2085±356 HV0.1 is formed on the surface of the HVOF 316L coating, respectively. Under dry wear conditions, the structure of the HVOF 316L stainless steel coating consists of stacked molten particles and hard oxide layers along with porosity, leading to a higher wear rate (96.12×10-5 mm3/Nm) and higher friction coefficient values (~1.0-1.5). As a result of the electrochemical boriding process applied to the HVOF 316L coating, dry wear rates decrease between 75 and 200 times, depending on the EB duration. In tribocorrosion experiments conducted under open-circuit conditions (OCP), the friction coefficient and wear rate of electrochemically borided coatings increase as the surface contact angle rises.