TUNGSTEN AND TUNGSTEN-CARBON PVD MULTILAYERED STRUCTURES AS EROSION-RESISTANT COATINGS

The aerospace industry is faced with a major erosive wear problem of engine components operating in dust environments. Various physical vapor deposition (PVD) or chemical vapor deposition (CVD) protective coatings have been used previously with limited success. The present paper deals with the elaboration and characterization of W/W-C multilayer coatings produced by magnetron sputtering from a W target in Ar and in an Ar+methane mixture respectively. This low temperature PVD process, compared with CVD processes, enables the deposition of protective coatings on Ti6Al4V substrates at temperatures below 400-degrees-C. The microhardness of W-C films was found to be strongly dependent on the carbon concentration; two maximum hardness values of 26000 MPa were obtained with W-C layers containing either 14-15 at.% or 40-45 at.% C. This change in microhardness with the carbon content was correlated with the evolution of the crystallographic structure. Multilayer coatings of total thickness 60 mum composed of various W/W-C stacking arrangements have been elaborated with hard W-C films containing 14-15 at.% or 40-45 at.% C. These samples were subjected to erosive wear tests using a sand blast unit. As a comparison, Mo/Mo-C and Cr/Cr-C structures containing more than 10 at.% C were also tested. W/W-C coatings with an appropriate stacking arrangement and a C concentration of 14-15 at.% in W-C layers exhibited an erosion resistance improved by more than two orders of magnitude compared with that of uncoated Ti6Al4V substrates. Complementary tests of the fatigue performance of Ti6Al4V specimens coated with these very promising erosion-resistant coatings were also performed.