| The focus of this thesis research is to explore a new approach to adaptive solid lubrication using nanoporous hard coatings. To investigate this approach, I deposited prototype coatings for study consisting of a hard chromium nitride (CrN) matrix co-deposited with a lubricious silver (Ag) phase by reactive magnetron co-sputtering. The idea is to exploit the relative immiscibility of the two phases to create nanocomposite structures with intrinsic lubricant transport properties enabled by the presence of the nanopores. Specifically, I develop the scientific understanding of the critical growth parameters that govern nanocomposite structural evolution which in turn control mechanical properties, solid lubricant diffusion, and tribological response.Mechanical properties were analyzed by both micro and nanoindentation measurements for the composites as a function of Ag aggregate morphology. For Ts &le 500°C, hardness as measured by nanoindentation into the surface is relatively uniform giving values of 14.6, 13.6, and 14.3 GPa for Ts = 300, 400, and 500°C respectively. For Ts > 500°C, the cross-sectional microhardness increases with T s from 16.5 to 19.7 to 24.3 GPa for Ts = 500, 600, and 700°C, respectively, which is attributed to a decrease in the effective Ag concentration associated with temperature activated segregation. The average hardness for pure CrN samples is 23.8 and 27.5 GPa as measured by surface nanoindentation and cross-sectional microindentation, respectively.Lubricant transport behavior was characterized by a series of vacuum annealing experiments. Vacuum annealing experiments at Ta = 425, 525, and 625°C show that Ag diffuses to the coating surface to form lubricious surface aggregates and that the rate for Ag lubricant transport increases with increasing DeltaT (Ta - Ts) for Ta > Ts, as determined by quantitative electron microscopy surface analyses. However, the Ag remains in the CrN matrix for Ta < Ts, which is attributed to the Ag aggregate size distribution within the coating which affects the chemical potential.Friction and wear behavior were measured in ambient air at test temperatures, Tt = 20°C to 700°C to study the self-lubricating properties and tribological mechanisms of the coatings as a function of structure and lubricant transport properties. Ball-on-disk tribological tests against 100Cr6 steel at Tt = 20°C indicate that the Ag grains for Ts = 500°C are too small to facilitate an effective lubricious surface layer, resulting in a friction coefficient micro = 0.58 and a composite coating wear rate of 3.8x10-6 mm3/Nm that are nearly identical to those measured for pure CrN with micro = 0.64 and 3.6x10-6 mm3/Nm. The Ts = 600°C coating exhibits a 15% higher Ag concentration on the surface of the wear track than outside the wear track which acts as a lubricious layer that reduces micro to 0.47 and yields a 16x and 2.4x lower wear rate for coating and counterface, respectively. Replacing the steel counterface with an alumina ball results in the lowest micro = 0.50 for Ts = 500°C, attributed to the presence of Ag and the relatively low hardness of 16.5 GPa for this particular coating.High temperature tribological response during ball-on-disk sliding in ambient air against alumina was found to depend strongly on both Ts and the testing temperature Tt = 450, 550, and 650°C. At Tt < Ts, the friction coefficient micro = 0.31--0.34, which is 25--35% below pure CrN with micro = 0.45. This moderate lubrication improvement is attributed to the presence of lubricious Ag within the CrN matrix. In contrast, Tt > Ts results in Ag lubricant transport to the coating surface and the formation of an effective self-lubricating layer, reducing the friction up to 65% to micro = 0.16--0.24. However, raising Tt well above Ts causes relatively rapid lubricant transport to the surface, followed by Ag depletion which results in a short low micro regime followed by a rapid rise in micro and often mechanical collapse of the matrix. The measured wear rate also strongly depends on the Ag solid lubricant transport rate and depletion, which is controlled by T t and Ts. (Abstract shortened by UMI.)... |
