In the search of new wear resistant coatings for applications such as cutting tools and turbine refurbishing, a wide range of coatings, belonging to emerging classes of materials, namely, nanocrystalline materials and nanolayered composites, have been produced and tested. These coatings were produced using an rf magnetron sputtering system and include monolithic nanocrystalline metals (Al,Ti,Cu), nanolaminated composites composed of alternating layers of metal/ceramic (Al/Al₂O₃, Ti/TiN) and metal/metal (Ti/Cu). The metal layer thickness in the as-sputtered films of Al/Al₂O₃ ranged from 70 to 500 nm, and 150 to 450 nm in Ti/TiN. The nonmetals (Al/Al₂O₃, TiN) layer thicknesses ranged from 10 to 40 nm and total film thicknesses of 10-15 μm. As-sputtered nanocrystalline aluminum films with an average grain size of 16.4 nm were isothermally annealed at 573 K to increase the grain size up to 98.0. nm.

All materials were characterized and tested for their tribological properties. Friction and wear tests were performed under unlubricated sliding conditions using pin-on-disc type tribometer which was designed and constructed for measuring wear rates and coefficients of friction of thin films in air and in vacuum. The coefficient of friction of the materials tested against the stainless steel pin varied with the sliding distance. At the early stages of sliding the coefficient of friction rose to a peak, followed by a decrease to a steady-state value. The transition to the steady-state in the friction curve corresponded to a transition from severe wear to mild wear. These were discussed in terms of work hardening, texture evolution and roughness of worn surface in monolithic materials.

In aluminum the value of the peak coefficient of friction decreased from μ^p= 1.4 for a coarse grain size of 10⁶ nm to μ^p=0.6 for a grain size of 16.4 nm when tested under ambient conditions. The coefficient of friction of nanocrystalline aluminum showed a 30% increase when tested in vacuum (10^-6 ton). Within the grain size range of 15-100 nm, the wear rates were found to be linearly dependent on the square root of the grain size (W_s = 8.5 × 10^-4 + (2.44 × 10^-4) . D^1/2 for severe wear and W_m = -1.9 × 10^-4 + (5.1 × 10^-6) . D^1/2 for mild wear). The peak value of the coefficient of friction decreased about 70% in Al/Al₂O₃ (with 200 nm Al layer thickness) while a 60% improvement in the steady-state coefficient of friction was measured in Ti/TiN (with 150 nm Ti layer thickness) in comparison to the as-sputtered monolithic aluminum and titanium films, respectively. An increase in wear resistance with decreasing layer thickness was also observed (for example, W_s = 7.0 × 10^-5 + (2.9 × 10^-7) . λ^-0.5_Ti).

Mechanical properties (hardness and elastic moduli) of the films were measured using an ultra-microindentation system. Hardness measurement of nanocrystalline aluminum revealed that within the grain size range 15-100 nm the hardness-grain size data obeys a Hall-Petch type relationship (i.e., H = 34 [MPa] + 0.21 [MPa.m^0.5] D^-0.5 [m^-0.5]). The hardness of Al/Al₂O₃ and Ti/TiN could also be described in the formalism of the Hall-Petch type indicating that ceramic layers inhibit slip transfer across metallic layers.

Recommendations are made, based on a proposed modified Archrad's law which directly relates wear rate to structure refinement, on future directions for producing improved wear resistant coatings.