Ion treatment by low pressure arc plasma immersion surface engineering processes

Abstract

The gaseous plasma of low pressure arc discharges has been used extensively for various surface treatment applications including heat treatment, ion nitriding, ion implantation, PECVD and duplex processes. Highly ionized low pressure arc plasmas with electron density up to ~1018m−3 can be generated by a shielded vacuum arc cathode, a hollow cathode or by a thermionic cathode. In this paper, plasma properties are characterized by electrostatic probes and optical emission spectroscopy. A range of different species can be produced in low pressure arc plasma immersion processes via decomposition of precursor molecules by electron collisions. Surface treatment of different steels and metal alloys in such a dense plasma environment can substantially affect the surface profile. The ion nitriding of different types of steel in a low pressure arc plasma environment is investigated. The rate of ion nitriding as a function of plasma parameters, such as ion current density, pressure and gas composition is established for several types of steel and ranging from 0.1 to 1μm/min. Ionitrided layers can be produced in arc plasma immersion processes at substrate biases as low as −30V and substrate temperatures as low as 200°C, depending on the type of steel. Alternatively, low energy ion implantation of nitrogen can be produced at bias voltages exceeding −500V and substrate temperatures less than 100°C. The distribution of plasma density and the uniformity of ion nitriding layers in industrial scale vacuum processing chambers are investigated. Duplex coatings were also produced by ion nitriding in an arc plasma immersion environment followed by deposition of TiN coatings. The ion nitriding and duplex coating layers are characterized by structure, thickness, microhardness depth profile, surface roughness and coating adhesion. Surface treatment in conventional glow discharge compared to low pressure arc plasma immersion processes is presented. The results of processing complex shape components are discussed.