Replacement of Hard Cr on AISI 316L Steel in Sliding Contacts for the Pharmaceutical Industry: Failure Analysis and Dry Sliding Tests

Austenitic stainless steels (SS) such as AISI 316L are widely used in the pharmaceutical industry in components such as pumps and distributors. However, the poor tribological behaviour of SS 316L requires the use of coatings or surface hardening techniques. Among overlay coating, also electroplated hard chromium coatings have been applied for improving the tribological behaviour of SS 316L. However, hard chromium plating is today recognized as a major source of environmental pollution and different solution for its replacement are available. In the present work, several alternatives are compared, concerning a SS 301 vs. SS 316L contact in packaging components for solid dose processing, where the surface modifications are applied to the SS316L rotating component, dry sliding against a stationary SS 301 part. In particular, the tribological performance of thermal spray coatings (Air Plasma Sprayed (APS) Al2O3-13TiO2 and High Velocity Oxy-Fuel (HVOF) WC-17Co) deposited onto SS 316L is compared with that of the same steel, thermochemically treated by Low Temperature Carburised (LTC). The main advantage of LTC is the ability to increase the hardness of the austenitic steel, owing to the formation of carbon-supersaturated austenite, without adversely affecting corrosion resistance. In the first step of the work, failure analysis techniques, such as 3-D digital microscopy and SEM/EDS, were used for identifying the main wear mechanisms in components from the SS301/Cr-coated SS 316L contact of the packaging device. Subsequently, laboratory dry sliding tests by a flat-on-cylinder tribometer were carried out, so as to investigate and compare the tribological behaviour of alternative solutions for the replacement of hard Cr. Both Hard Cr and thermal sprayed coatings or LTC treated SS 316L were used as the rotating cylinder, whereas the stationary slider consisted of SS 301. Tests were carried out at room temperature, with applied loads ranging from 5 to 15 N, sliding speed of 0.3 m s-1 and a sliding distance of 1000 m. Among the alternatives for the replacement of hard Cr, LTC treated SS 316 L showed the better tribological behavior in terms of wear (even slightly lower than hard Cr), also leading to lower wear rates in the SS 301 counterpart.