Nanocomposite coatings formed by cosputtering metals with solid lubricants like MoS₂ have been extensively studied. Recently, it was discovered that increasing the metal content above 50 at% gave coatings that could operate effectively at contact stresses far below that previously used (i.e., below 1 MPa).¹ The ability to use these films at lower contact stresses opens up new application areas, including electrical contacts. Previous studies of these coatings have been extended to include atomic force microscopy (AFM), X-ray diffraction, and electrical resistance measurements. The MoS₂ nanophase was shown to be amorphous in the as-grown films. However, contact AFM analysis showed that a thin crystalline MoS₂ layer forms at the surface during sliding, in agreement with macroscale tribological testing and surface analysis via Auger Electron Spectroscopy. Based on the widely different electrical resistivities of the Au and MoS₂ phases, contact resistance measurements were used to elucidate the nanostructure of the films. Analysis of the contact resistance variations with Au:MoS₂ ratio indicates that the Au particles are separated from each other by MoS₂ within the films. Overall, the results have better defined the compositions useful for electrical contact applications, and also increased our scientific understanding of sputter-deposited solid lubricant films by showing clear correlations between their nanostructure, tribological performance, and electrical properties.

¹ J. R. Lince, "Tribology of Co-Sputtered Nanocomposite MoS₂ Solid Lubricant Films over a Wide Contact Stress Range," Tribol. Lett., 17(3) (2004) 419-428.

Recently the oxide materials have become more interesting for tribological applications because of their oxidation stability and low tribo-oxidation sensitivity. Tungsten oxide coatings are considered as promising candidates for tribological application at elevated temperature due to their ability to form oxygen deficient Magnéli phases leading to significant decrease of friction coefficient. To verify these theoretical assumptions, tungsten oxide coatings with different stoichiometry prepared by d.c. reactive magnetron sputtering were tested in situ at high temperatures.

W-O coatings were deposited on the steel substrates. The evolution of the structure with the temperature was studied in situ and after annealing by X-ray diffraction (XRD). The mechanical properties were investigated by depth sensing indentation before and after annealing of the coatings. Morphology of coating surface, ball scars, wear tracks and wear debris were examined by scanning electron microscopy (SEM); the chemical composition was obtained by electron probe microanalysis (EPMA). Wear testing was done using a high temperature tribometer (pin-on-disc); the maximum testing temperature was 800°C. Evaluation of friction coefficient was measured at different temperatures and the wear rates of coatings and balls were determined. Three different materials were used as counter-parts: 100Cr6 bearing steel, Si₃N₄ and Al₂O₃ balls.

The XRD results showed a strong dependence of the structure on the temperature and the environment. In the air atmosphere, the oxidation of W and β-W₃O revealed at 600 and 800°C, respectively; only an increase in the crystallinity was observed in case of annealing of WO₃. The friction coefficient was higher than expected, particularly at room temperature. The dominant wear mechanisms were described using analysis of the wear tracks and the shape and distribution of the wear debris particles.

The deposition of ternary hard coatings within the system Ti-N-B by thermal CVD is well-established in cutting industry. Nevertheless, only little work has been published and a comprehensive study on the influence of deposition parameters on structure and tribological properties is still missing.

In this work Ti-N-B coatings were deposited on cemented carbide between 850 and 1000°C by thermal CVD using BCl₃, TiCl₄, Ar, N₂, and H₂ gas mixtures at atmospheric pressure with varying BCl₃ partial pressures. The boron content in the coatings increased with increasing BCl₃ partial pressure and decreasing deposition temperature. Contents up to 35 at.% B were measured by glow discharge optical emission spectrometry (GDOES). The microstructure was investigated by scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and transmission electron microscopy (TEM). Hardness values between 20 GPa for TiN up to 45 GPa for the highest boron contents were measured by micro-indentation tests. The tribological properties were investigated by dry sliding experiments against alumina balls between 25 and 600°C at ambient air using a high temperature tribometer. The wear tracks after ball-on-disc tests were investigated using an optical profiler, SEM, Raman spectroscopy as well as infrared (IR) spectroscopy. At room temperature, the friction coefficient increases from 0.75 for pure TiN to about 1.1 for boron contents of 4 at.%, while low friction values down to about 0.3 were found for the highest boron contents. The friction coefficient increases from 0.6 to 1.0 at 500°C and from about 0.5 to 0.9 at 600°C with increasing B content. At room temperature, the lowest wear rates were found for the highest boron contents. The opposite trend was observed at 500 and 600°C, where the presence of cracks and the consequent oxidation throughout the coating favors the wear at higher temperatures.