ICMCTF2011 Session E3-1: Tribology of Nanostructured and Amorphous Films

Nanoparticles addition in DLC films can improve tribomechanical properties and produce adaptive coatings that change their tribological behaviour according environmental atmosphere [1]. Usual experimental approach to study material behaviour to vary one parameter at time while keeping the others constant, thus measuring influence of each variable. This approach requires more experimentation, ignores the interactions between the parameters and can lead to wrong conclusions. Two level factorial design (2-LFD) was used to describe overall effect when you vary all parameters at the same time allows representing the parameters influence in a simple way[2]. This paper presents the use of 2-LFD to study the effect of diamond nanoparticles addition and test conditions (environment, sliding speed and applied load) on tribological behaviour of DLC thin films. Tribological tests were performed in high vacuum and in environment air on rotational mode and in a wide range of load and sliding speed. Scratching tests was also used to study the effect of diamond nanoparticles addition on DLC films. Raman spectroscopy and images of worn surfaces was also used to characterize the films and wear debris.

Diamond-like Carbon (DLC) coatings show excellent properties such as high hardness, low friction and low wear, which expand the wide range of their applications. However, there are some disadvantages. Especially, tribological behavior of DLC coatings is highly sensitive to environment and strongly influenced by contact pressure. These materials also exhibit very poor electrical conductivity. These drawbacks can sometimes cause problems for industrial applications. To address such issues, we have focused our attention on nanocomposite DLC coatings, in order to modify surface properties and provide new functionalities. For instance, electrical conductivity can be drastically improved by inclusion of metallic elements/clusters into the DLC coating. We have developed a combined vacuum process composed of plasma enhanced chemical vapor deposition (PECVD) and DC magnetron sputtering devices. Main parameters for the deposition are: gas mixture ratio of the hydrocarbon precursor and argon (controlling the DLC phase growth); and DC sputtering power (controlling additional phase growth). Thanks to the independent and adjustable parameters of CVD and sputtering, various types of nanocomposite coatings with different micro/nano-structures can be fabricated. Based on electron micrographs, structural changes of the coatings are presented depending on the gas mixture ratio of hydrocarbon precursor and argon. Tribological behavior of such coatings is of high interest, since it appears to be quite insensitive to environment (ambient air or ultra-high vacuum) or to normal load. Such results can be discussed in a viewpoint of shearing force of the material transferred from the coating to the counterface. Finally, we discuss “how to design nanocomposite coatings” as a strategic concept to achieve a desired performance.

Amorphous hydrogenated silicon-based multilayer coatings were deposited on 301 stainless steel (SS301) and Ti-6Al-4V alloy substrates using plasma enhanced chemical vapor deposition (PECVD), in order to integrate the advantages of the respective layers. Corrosion and tribo-corrosion behavior of the complete coating/substrate system on different substrates was investigated. The SiN/SiC double layer coating substantially improved the corrosion resistance of the metals: For SS301, the corrosion current, i_corr, was reduced by more than three orders of magnitude, and the breakdown voltage was increased from 0.34 to 1.37 V. For Ti-6Al-4V, the i_corr was decreased by a factor of ~50. Particularly, the Ti-6Al-4V/SiN/SiC multilayer system exhibited excellent anti-corrosion properties according to potentiodynamic polarization measurements, due to the superior corrosion resistance of both the Ti-6Al-4V substrate and the silicon-based coatings. Further enhancement of the tribo-corrosion resistance has been achieved by applying an a-C coating as a top layer in the three-layer system. In the tribo-corrosion test in 1 wt.% NaCl solution, the SiN/SiC/a-C coating reduced the wear rate and the friction coefficient by a factor of ~175 and ~4, respectively, compared with the bare Ti-6Al-4V. The Ti-6Al-4V/SiN/SiC/a-C multilayer system integrates in synergy the advantages of the respective layers, and its versatility makes it a particularly attractive candidate for applications in different harsh working environments.