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 di agnostics in the vicinity of the sample, and raises critical questions of how the layers and interfacial microstructure might affect the nea r-surface properties. The composition of the plasma near the surface o f the sample (plasma layer) was examined by optical emission spectrosc opy and mass spectrometry during plasma nitriding and while sputtering the sample after the nitriding process. It was observed that during t he 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 samp le was exposed to argon plasma, Ti, Al and NH were observed. It was fo und that two distinct sublayers, comprising delta-TiN and delta-TiN epsilon-Ti2N phases. were formed with alloying elements in a segregate d zone, followed by a solid solution of nitrogen in titanium. The form ation of the uppermost sublayer (delta-TiN phase), containing H, NH, a nd N, in addition to Ti depleted of Al and V, has a strong effect on t he diffusion of nitrogen into alpha-Ti and on the layer properties. Th is can be enhanced if hydrogen is present in the nitriding atmosphere and is prevented if hydrogen is replaced by argon. Therefore, the nitr ogen content in the layer results in the formation of nitride phases a nd is accompanied by an improvement in mechanical properties.