CrN–Ag nanocomposite coatings: Control of lubricant transport by diffusion barriers

1-μm-thick self-lubricating CrN–Ag composite coatings containing 16at.% Ag were deposited on Si substrates by reactive co-sputtering at Ts=400°C, and were covered with CrN cap layers with a columnar microstructure and a thickness d=0–1000nm. Vacuum annealing at Ta=500 and 600°C for 1h causes Ag transport to the sample surface and the formation of Ag surface grains. Quantitative scanning electron microscopy and energy dispersive spectroscopy analyses show that increasing d from 0 to 10 to 100nm for Ta=500°C leads to a decrease in the areal density of Ag surface grains from 0.86 to 0.45 to 0.04μm−2, while their lateral size remains constant at 360±60nm. However, increasing Ta to 600°C causes a doubling of the Ag grain size, and a 4–30 times larger overall Ag transport. These results are explained by kinetic barriers for Ag diffusion through the porous cap layer with a porosity that decreases with increasing d, resulting in an effective activation barrier for Ag transport that increases from 0.78eV in the absence of a cap layer to 0.89eV for d=10nm and 1.07eV for d=30nm. Auger electron spectroscopy depth profile analyses of annealed layers reveal no detectable Ag within the CrN cap layer and a uniform depletion of the Ag reservoir throughout the composite coating thickness, indicating unhindered Ag transport within the composite. The overall results show that a CrN diffusion barrier cap layer is an effective approach to control Ag lubricant transport to the surface of CrN–Ag composite coatings. ► CrN–Ag composite coatings are capped with CrN diffusion barriers. ► Ag diffuses to the surface during annealing at 500 or 600°C. ► The Ag transport is controlled by the cap thickness d=0–1000nm. ► The activation energy for Ag transport increases with increasing d.