Surface morphology and compound layer pores of plasma nitrocarburized low carbon steel
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INTRODUCTION
NITROCARBUR/ZING is a thermochemical heat-treatment process which involves the diffusion of the alloying elements of nitrogen and carbon into steel at elevated temperatures. The main objective of this process is to upgrade the surface properties of low alloyed and plain carbon steel through the formation of an antiwear and anticorrosion compound layer. Components such as gears, shafts, bearings, etc. usually need to be nitrocarburized to improve the surface mechanical properties. Nitrocarburizing can be achieved through different media: solid, liquid, gas, or glow discharge (plasma). Plasma nitrocarburizing has recently gained more acceptance in industry in view of the fact that it is an environmentally harmless process. Plasma nitrocarburizing is a low pressure glow discharge surface treatment which employs a generator to establish an electropotential between the anode and the cathode. The gas mixture used in the process contains carbon, nitrogen, and hydrogen in varying amounts. The plasma generated by the electric field results in unidirectional movement of the nitrogen and carbon containing species toward the treated workpiece which is usually used as cathode in the electric field. The direct contact of the nitrogen and carbon containing species (charged particles or neutrals) with the workpiece surface leads to further dissociation of these species releasing C and N atoms which then diffuse into the sample surface at the treatment temperature.t1,2.3] The parameters commonly known to affect the nitrocarburizing results are gas mixture, pressure, flow rate, current density, treatment temperature, and treatment time. The gas mixture in particular has been found to have a significant
SHILI, Postgraduate Student, Metallurgical Engineering, and RAFAEL R. MANORY, Senior Lecturer, are with the Department of Chemical and Metallurgical Engineering, Royal Melbourne Institute of Technology, Melbourne, Victoria 3001, Australia. Manuscript submitted October 3, 1994. METALLURGICALAND MATERIALS TRANSACTIONS A
influence on the final microstructure of the components, whereas current density has been identified as the main factor influencing the thickness of the compound layer54J Although plasma nitriding and nitrocarburizing have been studied quite extensively, some basic questions conceming this treatment are yet to be solved. For example, so far, no consensus has been reached about a proper model of the process: several plasma nitriding models have been proposed,~5 sj but at this stage, it seems that these models contradict each other. The K61belt51 and Hudist6]models in particular are very well known, but the mechanism of nitriding described in each of them is totally different. In Krlbel's model, atomic nitrogen is released as a result of the higher nitrides dissociating into lower nitrides, i.e., FeN Fe2N --~ Fe3N ~ FeaN. The initial higher nitride such as FeN is formed as the result of sputtered Fe atoms combining with the incoming N atoms. Because FeN is not a thermally stable compound, it then furthe
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