Atomic layer deposition of ZnO/Al2O3/ZrO2 nanolaminates for improved thermal and wear resistance in carbon-carbon composites

Abstract

Carbon-carbon composites (CCCs) are well recognized structural materials which are generally known for their high strength and thermal conductivity, low density, and open cell structure. They have attracted great interest for aerospace applications, such as bushings used in jet engines, and potentially in future hypersonic aircraft wing structures, which function in an oxidizing environment. The enhancement in thermal (oxidation) and wear resistance of CCCs is desirable since carbon-based materials start to oxidize around 400 °C and uncoated bushings are susceptible to wear. To this end, thermodynamically stable protective oxide nanolaminates (ZnO/Al2O3/ZrO2) have been deposited by atomic layer deposition (ALD) to infiltrate porous CCCs and graphite foams in order to improve their thermal stability and sliding wear resistance. It was determined with x-ray tomography imaging and energy dispersive x-ray spectroscopy mapping that ALD ZnO/Al2O3/ZrO2 nanolaminates and baseline ZrO2 coatings exhibited pore-filling down to ∼55 μm and 1.5 mm in the porous CCCs and graphite foam, respectively. Furthermore, x-ray diffraction and high resolution transmission electron microscopy determined that the {0002}-textured nanocrystalline ZnO (wurtzite), amorphous Al2O3, and {101}-textured tetragonal ZrO2 nanolaminates exhibited excellent coating conformality and uniformity inside the pores. A significant reduction in the sliding wear factor (2.3 × 10−5 to 4.8 × 10−6 mm3/Nm) and friction coefficient (0.22 to 0.15) was observed with the ALD ZnO/Al2O3/ZrO2 nanolaminate in comparison to uncoated CCCs. The mechanism for improved tribological properties was attributed to intrafilm shear (slip) of partial dislocations along the ZnO {0002} basal stacking faults by a dislocation glide process.