Pre- and post-surface treatments using electron beam technology for load-related application of thermochemical and PVD hard coatings on soft substrate materials

Abstract

With their specific layer features and properties, surface treatments such as thermochemical treatment (nitriding, boriding) and hard coating cover a broad field of application in the wear and corrosion protection of steels. Limitations exist, however, when applying these surface treatments to softer materials, such as cast irons and aluminum alloys in terms of both their treatability and load-bearing capacity.This contribution deals with investigations into duplex surface treatments, where a pre- and post-electron beam (EB) surface treatment (e.g., hardening, remelting, alloying etc.) was combined with one of the above-mentioned treatments. Among other characteristics, the thermal EB surface treatments were characterized by high heating and cooling rates that facilitated the generation of a variety of non-equilibrium microstructures, which exhibited increased hardness and had minimal thermal effects on the surrounding base material. Furthermore, the layer thicknesses were one or two orders of magnitude higher than those generated by thermochemical treatment or hard coating.Based on the comprehensive results and using cast irons and Al alloys as examples, the study demonstrates the extent to which duplex treatments can overcome the aforementioned limitations, and how the tribological and/or corrosive load behavior is affected. The property profiles achieved after duplex surface treatment were strongly dependent on the inherent microstructural and chemical processes. These complex processes were influenced by a range of factors, such as the thermal stability of the EB surface layer generated in the first process step, the respective temperature and time period of the secondary process, etc.Hardness measurements, scratch tests, unlubricated pin-on-disc wear tests using different normal loads and potentiodynamic corrosion tests were realized to facilitate characterization of the different load behaviors of the single- and duplex-treated layers.