Processing, Structure, and Properties of Nanostructured Multifunctional Tribological Coatings

Abstract

Nanostructured, nanocomposite binary (TiC-a:C), ternary (Cr-Al-N), quaternary (Ti-B-C-N) and quinternary (Ti-Si-B-C-N) multicomponent films have been deposited using unbalanced magnetron sputtering (UBMS) and closed field unbalanced magnetron sputtering (CFUBMS) from both elemental and composite targets. Approaches to control the film chemistry, volume fraction and size of the multicomponent species, and pulsed ion energy (ion flux) bombardment to tailor the structure and properties of the films for specific tribological applications, e.g., low friction coefficient and low wear rate, are emphasized. The synthesized films are characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), nanoindentation, and microtribometry. The relationships between processing parameters (pulsed ion energy and ion flux), thin film microstructure, mechanical and tribological properties are being investigated in terms of the nanocrystalline-nanocrystalline and nanocrystalline-amorphous composite thin film systems that are generated. In the Ti-Si-B-C-N films, nanocomposites of solid solutions, e.g., nanosized (Ti,C,N)B2 and Ti(C,N) crystallites are embedded in an amorphous TiSi2 and SiC matrix including some carbon, SiB4, BN, CN(x), TiO2 and B2O3 components. The Ti-Si-B-C-N coating with up to 150 W Si target power exhibited a hardness of about 35 GPa, a high H/E ratio of 0.095, and a low wear rate of from approximately 3 to approximately 10 x 10(-6) mm3/(Nm). In another aspect, using increased ion energy and ion flux, which are generated by pulsing the power of the target(s) in a closed field arrangement, to provide improved ion bombardment on tailoring the structure and properties of TiC-a:C and Cr-Al-N coatings are demonstrated. It was found that highly energetic species (up to hundreds eV) were found in the plasma by pulsing the power of the target(s) during magnetron sputtering. Applying higher pulse frequency and longer reverse time (lower duty cycle) will result in higher ion energy and ion flux in the plasma, which can be utilized to improve the film structure and properties. For example, optimum properties of the TiC-a:C coating were a hardness of 35 to 40 GPa and a COF of 0.2 to 0.22 for moderate maximum ion energies of 70 to 100 eV, and a super high hardness of 41 GPa and low wear rate of 3.41 x 10(-6) mm3N(-1) m(-1) was obtained for Cr-Al-N coatings deposited with a maximum ion energy of 122 eV. These conditions can be achieved by adjusting the pulsing parameters and target voltages. However, the pulsed ion energy together with the applied substrate bias are need to be carefully controlled in order to avoid excessive ion bombardment (e.g., the maximum ion energy is larger than 180 eV in the current study), which will responsible for an increase in point and line defects, and high residual stress in the crystalline structure.