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

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.