A perspective on the optimisation of hard carbon and related coatings for engineering applications

Nanostructured coatings with variable contents of tungsten carbide (WC) and amorphous carbon (a-C) are prepared by controlling the sputtering power ratio using WC and graphite targets. XRD and TEM/ED analysis shows that increasing the C incorporation, the WC nanocrystalline phases evolve from γ-W 2 C to β-WC 1 −  x . Further C enrichment leads to a nanocomposite structure of small WC 1 − x crystals dispersed in a-C matrix. The a-C at.% is estimated by XPS analysis and correlated with the observed tribo-mechanical properties. The hardness and friction properties vary from hard/high friction (36–40 GPa; μ  = 0.6–0.8) to moderate-hard/low friction (16–20 GPa; μ  ∼ 0.2) coatings depending on the film composition. The transition point is found for a-C content of 10 at.%. This correlates with a change from nanocrystalline WC to nanocomposite WC 1 −  x /a-C coatings. The overall study will help to understand the previous literature data and will serve as guide for a tailored synthesis of these WC/a-C nanocomposites.

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Tailored synthesis of nanostructured WC/a-C coatings by dual magnetron sputtering

Deposition and some properties of nanocrystalline WC and nanocomposite WC/a-C:H coatings

Design of hard coating architecture for the optimization of erosion resistance

Deposition and some properties of nanocrystalline, nanocomposite and amorphous carbon-based coatings for tribological applications

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 Ti6A14V substrates at temperatures below 400°C. The microhardness of W-C films was found to be strongly dependent on the carbon concentration; two maximum hardness values of 26 000 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 μm 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 Ti6A14V substrates. Complementary tests of the fatigue performance of Ti6A14V specimens coated with these very promising erosion-resistant coatings were also performed.

M. Brum

Papers

Tungsten and tungsten-carbon PVD multilayered structures as erosion-resistant coatings