Low-Temperature Plasma Nitriding of Martensitic and Austenitic Steels to Increase Tribocorrosion Resistance

Tribocorrosion is the simultaneous occurrence of wear and corrosion in a tribosystem and their interaction. In many applications, such as media-lubricated rolling bearings and (cutting-)tools in the food industry or medicine, tribocorrosion occurs and leads to a high material loss and damage to materials. The tribocorrosion resistance of mechanically and chemically stressed steel surfaces can be significantly increased by low-temperature plasma nitriding at T < 400 degrees C. In this process, nitrogen is forcibly dissolved in the surface area (up to approx. 20 mu m) in high contents of 15 wt.-% without precipitation. This results in an extreme expansion and distortion of the metal lattice ("expanded martensite", "expanded austenite"), which leads to an increase in hardness of up to 1000 HV with the same or even increased pitting corrosion resistance. Due to the formation of expanded martensite/austenite, the tribocorrosion resistance of the martensitic steels X40Cr14 and X54CrMnN13-2 and that of an austenitic CrMn steel can be significantly improved compared to the initial state, which is expressed in a 40-70 % lower material loss under tribocorrosive attack. It was found that the tribocorrosion resistance depends on the process parameters of the surface treatment and on the chemical composition of the steels and their crystal lattice.