MECHANISM OF RF PLASMA NITRIDING OF TI-6AL-4V ALLOY

The objective of the current study was the gradual development of the formation of the nitride layer during inductive r.f. plasma nitriding. The study centers on characterization of refined layers and plasma diagnostics in the vicinity of the sample, and raises critical questions of how the layers and interfacial microstructure might affect the near-surface properties. The composition of the plasma near the surface of the sample (plasma layer) was examined by optical emission spectroscopy and mass spectrometry during plasma nitriding and while sputtering the sample after the nitriding process. It was observed that during the nitriding process, the plasma layer contains Ti, NH(n) species, N (or/and N+), H(n) species (or/and H+2). However, when the nitrided sample was exposed to argon plasma, Ti, Al and NH were observed. It was found that two distinct sublayers, comprising delta-TiN and delta-TiN + epsilon-Ti2N phases. were formed with alloying elements in a segregated zone, followed by a solid solution of nitrogen in titanium. The formation of the uppermost sublayer (delta-TiN phase), containing H, NH, and N, in addition to Ti depleted of Al and V, has a strong effect on the diffusion of nitrogen into alpha-Ti and on the layer properties. This can be enhanced if hydrogen is present in the nitriding atmosphere and is prevented if hydrogen is replaced by argon. Therefore, the nitrogen content in the layer results in the formation of nitride phases and is accompanied by an improvement in mechanical properties.