Control of lubricant transport by a CrN diffusion barrier layer during high-temperature sliding of a CrN-Ag composite coating

CrN-Ag composite coatings, 2 and 5 pm thick and containing 22 at.% Ag solid lubricant, were grown on Si(001) and 440C stainless steel substrates by reactive co-sputtering at T-s = 500 degrees C, and were covered with 200 nm thick pure CrN diffusion barrier cap layers. Annealing experiments at T-a = 625 degrees C, followed by quantitative scanning electron microscopy, energy dispersive x-ray spectroscopy, and Auger depth profile analyses indicate considerable Ag transport to the top surface for a barrier layer deposited at a substrate floating potential of -30 V, but negligible Ag diffusion when deposited with a substrate bias potential of -150 V. This is attributed to ion-irradiation induced densification which makes the cap layer an effective diffusion barrier. High temperature tribological sliding tests of this coating system against alumina balls at T-t = 550 degrees C indicate an initial friction coefficient mu = 0.43 +/- 0.04 which decreases monotonically to 0.23 +/- 0.03. This is attributed to the development of wear mediated openings in the barrier layer which allow Ag lubricant to diffuse to the sliding top surface. In contrast, pure CrN exhibits a constant mu = 0.41 +/- 0.02 while CrN-Ag composite coatings without cap layer show a low transient mu = 0.16 +/- 0.03, attributed to Ag transport to the surface, that however increases to mu = 0.39 +/- 0.04 after -6000 cycles as the Ag reservoir in the coating is depleted. That is, the dense CrN cap layer reduces the Ag lubricant flow rate and therefore prolongs the time when the coating provides effective lubrication. This results in a cumulative wear rate over 10,000 cycles of 3.1 x 10(-6) mm(3)/Nm, which is 3.3 x lower than without diffusion barrier layer. Published by Elsevier B.V.