Smart tribological coatings with wear sensing capability

Tribological coatings were developed to allow automatic reporting of remaining wear life while in use. Monitoring of coating health was achieved by embedding sensor layers, known to produce distinctive luminescence spectra when exposed to laser illumination, throughout the thickness of a solid lubricant coating. For the current work, erbium- and samarium-doped yttria stabilized zirconia (YSZ) layers were used as sensor materials. One sensor layer was placed approximately midway through a molybdenum disulfide coating and another was located at the coating/substrate interface. Placement of the luminescent coatings in these positions allowed detection of wear depth and provided a warning of impending coating failure during testing. This smart coating concept is generally applicable to tribological coatings and can easily be implemented to safely increase reliance upon protective materials subject to wear and other damage mechanisms. The soft MoS2 coatings with the imbedded ceramic sensor layers also demonstrated long wear lives (≈200,000 cycles) in humid air compared to monolithic MoS2 coatings (<10,000 cycles) with the same thickness, microstructure, morphology and composition. The mechanism for the observed wear life increase was examined and is discussed together with the general use of embedded wear sensors in smart tribological coatings.