Investigation of nitrogen mass transfer within an industrial plasma nitriding system I: The role of surface deposits

It has been proposed that in plasma nitriding, sputtering of material from biased components within the chamber assists in the nitrogen mass transfer process. Here, we investigate the effects of this sputter deposition process on the nitriding response of biased and unbiased AISI P20 steel samples mounted in a large-scale plasma nitriding system operated at 520 °C. Films with nanostructured morphologies resulting from Volmer–Weber film growth were observed on Si substrates placed adjacent to AISI P20 steel substrates after nitriding experiments. Auger depth profiling revealed that the films on the Si substrates had a stoichiometric ratio of 4:1 Fe:N. This suggested that the particles consisted largely of Fe4N and it was concluded that they were formed from atoms and small clusters sputtered from biased components in the chamber. Despite the deposition of these films, no significant improvement in surface hardness was observed in the steel samples unless bias was applied to them. Furthermore, the maximum hardness achieved in biased P20 samples after the nitriding process occurred in the samples positioned adjacent to the Si samples supporting the thinnest deposited films. These findings do not support the proposition that in plasma nitriding, nitrogen mass transfer occurs predominantly by sputter deposition of iron nitride.