Self-passivating materials have an innate ability to spontaneously form a tightly adhering oxide layer when exposed to an oxygen-containing environment. This behavior is characteristic of metals with high affinity to oxygen, but also of alloys that contain a sufficient content of (strong) oxide forming elements. This “feature” is exploited in many technical alloy systems – perhaps best known is stainless steel, where the presence of more than 10 wt% Cr provides the self-passivating ability and thus results in enhanced corrosion resistance. The same mechanism is also responsible for the excellent corrosion resistance and biocompatibility of titanium and its alloys. However, the special “nature” of self-passivating materials/alloys requires special processes when it comes to thermochemical surface engineering. The present contribution provides an overview of the present status and future trends of thermochemical surface engineering of self-passivating materials, with special emphasis on stainless steels and titanium alloys. For stainless steels there are essentially two options for thermochemical surface engineering without impairing the corrosion performance: high temperature solution nitriding (HTSN) and low temperature surface hardening (LTSH). The concepts of these processes will be presented and the scientific and technological background will illustrated. LTSH comprises nitriding, nitrocarburizing and carburizing and can be performed with or without a preceding HTSN treatment. For titanium alloys thermochemical surface engineering is presently more niche, but is expected to gain momentum in the future. Here there are many different options in terms of the “hardening” species (nitrogen, carbon, oxygen and boron) and in terms of surface hardening temperatures and different process variants. The different strategies for thermochemical surface engineering of titanium alloys will be show-cased by assorted examples of patent pending processes recently developed in the authors’ laboratories.
Thomas L. Christiansen is an Associate Professor at the Department of Mechanical Engineering at the Technical University of Denmark, Section for Materials and Surface Engineering. He has been working with gaseous thermochemical surface treatment of metallic materials for more than 20 years. His main focus of academic attention has been surface hardening and heat treatment of self-passivating alloys such as titanium and stainless steels. Besides his academic career, he is the co-founder of two companies working with surface hardening of metallic materials.