Copper helps to accelerate heat transfer during the braking process, allowing the brake materials to produce a stable coefficient of friction (COF), which in turn reduces wear loss and braking noise. However, its properties are also quite harmful to aquatic organisms. Finding a suitable replacement that fits all functions of copper for brake materials is not an easy feat. In this paper, six different carbonaceous components (coke, carbon black, carbon fiber, artificial graphite, natural graphite and expanded graphite) were substituted for copper in non-asbestos organic (NAO) friction materials. The hardness, thermal conductivity and tribological behaviors of these copper-free NAO friction materials were examined. Experimental results indicate that carbonaceous components improve lubrication and assist the friction composites with generating friction layers on the worn surface. Specimens containing coke, carbon black or carbon fiber exhibit broken friction layers, whereas specimens containing artificial graphite, natural graphite or expanded graphite exhibit quite adherent and smooth friction layers. Among all the copper-free carbon containing specimens, the specimen containing expanded graphite appears to be the best choice. It has the highest thermal conductivity, a relatively low wear loss and a relatively high and stable COF.

The effects of annealing temperature on the structural, surface morphological and nanomechanical properties of Cu-doped (Cu-10 at %) NiO thin films grown on glass substrates by radio-frequency magnetron sputtering are investigated in this study. The X-ray diffraction (XRD) results indicated that the as-deposited Cu-doped NiO (CNO) thin films predominantly consisted of highly defective (200)-oriented grains, as revealed by the broadened diffraction peaks. Progressively increasing the annealing temperature from 300 to 500 °C appeared to drive the films into a more equiaxed polycrystalline structure with enhanced film crystallinity, as manifested by the increased intensities and narrower peak widths of (111), (200) and even (220) diffraction peaks. The changes in the film microstructure appeared to result in significant effects on the surface energy, in particular the wettability of the films as revealed by the X-ray photoelectron spectroscopy and the contact angle of the water droplets on the film surface. The nanoindentation tests further revealed that both the hardness and Young’s modulus of the CNO thin films increased with the annealing temperature, suggesting that the strain state and/or grain boundaries may have played a prominent role in determining the film’s nanomechanical characterizations.