Effects of Nitrogen Concentration on Microstructure of Tungsten Coatings Synthesized by Plasma Sputtering Method

The use of nitrogen seeding to reduce the edge plasma temperature in tokamak raises concern regarding the possible formation of tungsten nitride layers with different properties (melting temperature, hardness, retention, etc.) with respect to the W. Dedicated laboratory experiments are performed to investigate the interaction of W with N plasmas. In this paper, the structure and mechanical performance of WNx coatings with different nitrogen content prepared by rf plasma sputtering have been investigated. The coatings exhibited different phases as a function of the nitrogen content: films with very low N content (<1 atom%) mainly exhibited the W phase, the W and WNx phases were contemporaneously present for N contents in the range 1-5 % and only the WNx phases were observed for nitrogen content higher than 5 %. The mechanical performance of the WNx coatings was strongly influenced by the structure. The coating formed with 5 % of nitrogen showed the highest mechanical performance. The substrate heating during the plasma deposition has also improved the mechanical properties.