Role of atomic layer deposited solid lubricants in the sliding wear reduction of carbon–carbon composites at room and higher temperatures

Abstract

High frequency reciprocating sliding wear behavior and mechanisms were investigated at 25, 150 and 400°C for solid lubricant oxide coated carbon–carbon composites (CCC). Atomic layer deposition (ALD) was employed to infiltrate a trilayer nanolaminate coating of ZnO/Al2O3/ZrO2 into the porous CCC. The ALD coatings exhibited excellent conformality and pore-filling down to ~55μm into the CCC, which cannot be achieved by other coating techniques. Compared to uncoated CCC, ALD infiltration resulted in a 64% improvement in the wear rate to 1.5×10−6mm3/N·m at 25°C, while a 31% improvement was determined during 150°C sliding. Conversely, higher temperature sliding at 400°C resulted in increasing wear rates by an order of magnitude due to combined oxidation of the steel counterface and exposed CCC. Cross-sectional focused ion beam-transmission electron microscopy (FIB-TEM) studies of the worn surfaces were performed to elucidate the solid lubrication mechanisms. It was determined that a highly deformed composite surface tribolayer of nanolaminate coating and amorphous C was responsible for improved sliding wear compared to the uncoated CCC that just exhibited an amorphous C tribolayer. Inside the deformed composite tribolayer, a high density of near surface ZnO (0002) basal plane sliding/shear-induced stacking faults was observed in comparison to unworn ALD nanolaminates. Activating subsurface basal stacking faults in ALD ZnO promotes intrafilm shear/slip and hence improves wear resistance. The slip of partial dislocations results from a dislocation glide process along the ZnO (0002) basal planes. Thus, ALD ZnO/Al2O3/ZrO2 nanolaminate coatings are potential candidates for providing wear reduction from low to intermediate temperatures in moving mechanical assembles, such as CCC bushings that experience fretting-type wear.