Friction and wear of nanocrystalline materials and nanolaminated composites.

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/Al$\sb2$O$\sb3$, Ti/TiN) and metal/metal (Ti/Cu). The metal layer thickness in the as-sputtered films of Al/Al$\sb2$O$\sb3$ ranged from 70 to 500 nm, and 150 to 450 nm in Ti/TiN. The nonmetals (Al$\sb2$O$\sb3$,TiN) layer thicknesses ranged from 10 to 40 nm and total film thicknesses of 10-15 $\mu$m. 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. In aluminum the value of the peak coefficient of friction decreased from $\rm\mu\sb{p} = 1.4$ for a coarse grain size of $10\sp6$nm to $\rm\mu\sb{p} = 0.6$ 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 $(10\sp{-6}$ torr). 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 $\rm(W\sb{s} = 8.5\times 10\sp{-4} + (2.44 \times 10\sp{-4}).\ D\sp{1/2}$ for severe wear and $\rm W\sb{m} = {-}1.9 \times 10\sp{-4} + (5.1 \times 10\sp{-5}).\ D\sp{1/2}$ for mild wear). The peak value of the coefficient of friction decreased about 70% in Al/Al$\sb2$O$\sb3$ (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, $\rm W\sb{s} = 7.0 \times 10\sp{-5} + (2.9 \times 10\sp{-7}).\ \lambda\sp{0.5}\sb{Ti}).$ 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., $\rm H = 34\ \lbrack MPa\rbrack + 0.21 \lbrack MPa\cdot m\sp{0.5}\rbrack\ D\sp{-0.5}\ \lbrack m\sp{-0.5}\rbrack).$ The hardness of Al/Al$\sb2$O$\sb3$ 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. (Abstract shortened by UMI.)Dept. of Mechanical, Automotive, and Materials Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1996 .F37. Source: Dissertation Abstracts International, Volume: 59-08, Section: B, page: 4384. Advisers: D. O. Northwood; A. T. Alpas. Thesis (Ph.D.)--University of Windsor (Canada), 1996