Compound Layer Formation during Plasma Nitriding and Plasma Nitrocarburising

Abstract

The models described in the literature for compound layer formation during plasma nitriding and nitrocarburising are discussed with reference to the mechanisms and process steps involved (activation, implantation, sputtering, adsorption, etc.), which generally run in parallel to one another. Experimental results are presented for pure iron, plain carbon and low alloy steels, and ledeburitic cast iron. The flexibility of plasma processing allows the structure of the compound layer to be varied within broad limits, including complete suppression if the application demands. Pure γ’ phase layers with low porosity can be produced by plasma nitriding even on high carbon materials, owing to decarburisation of the layer. It is possible to obtain satisfactory compound layers in atmospheres containing oxygen, providing sufficient amounts of hydrogen are also present. During nitrocarburising, the addition of even small amounts of carbon to the treatment gas promotes formation of 8 rather than γ'; at contents > 3 at.-%C in the atmosphere absorption of nitrogen is completely suppressed, resulting in pure Fe3C layers being formed.