The role of energetic neutrals in reactive magnetron sputtering of nitrogen-doped austenitic stainless steel coatings

A series of nitrogen-doped austenitic stainless steel coatings was prepared by magnetron sputtering with a range of argon/nitrogen gas mixtures. The nitrogen content of the coatings was found to be proportional to the nitrogen flow rate. The coatings consisted of an fee phase, which had a strong [100] fibre texture. The strain-free lattice parameters, measured with the crystallite group method, confirmed that this phase was simply an austenitic structure that had been uniformly expanded by the dissolution of nitrogen. The residual macrostress of the coating was related to the amount of nitrogen in the sputter gas. Furthermore, in coatings deposited with high nitrogen flow rates, the fibre axis was tilted away from the surface normal by up to several degrees. Simulations with TRIM revealed that a significant proportion of high-energy neutrals are backscattered from a stainless steel target when sputtered with nitrogen ions. In mixtures of Ar and N-2, it follows that as the N-2 content of the sputter gas is increased, the subsequent proportion of energetic nitrogen neutrals also increases. It is argued that energetic neutral nitrogen bombardment is the principal mechanism for burial of nitrogen in the coatings, and modification of the microstructure and residual stress. (C) 1997 Elsevier Science S.A.