Tribological Response of Novel Titanium Boron-Nitride Coatings Against Harsh Thermal Cycles and Radiation Exposure of the Lunar Environment

Ti6Al4V is one of the critical aerospace alloys used in lunar structural components and rovers owing to its excellent mechanical properties including high specific strength. However, its durability deteriorates rapidly when subjected to harsh conditions of the lunar environment, such as extreme thermal cycles, ionizing particle radiation, and abrasive regolith. Atmospheric plasma spray was used to deposit Hexagonal Boron Nitride (hBN) reinforced titanium coatings from cryomilled powders at low and high volume concentrations of hBN to enhance the durability of Ti6Al4V alloy. The coatings exhibited a hardness three times that of conventional Ti6Al4V (335 HV) substrate in the case of low vol.% hBN (900 HV) concentration and 1.5 times in the case of high vol.% hBN coating (516 HV). The coatings were subjected to extreme thermal cycling, solar particle radiation exposure, and thermal-radiation combined environments resembling lunar conditions. Ball-on-disk tribological tests were conducted with JSC-1A lunar regolith simulant to evaluate the wear performance of coatings. Compared with conventional material, 50%, 70%, and 40% reductions in wear volume were observed in the Ti/low vol.% hBN coatings in thermal-only, radiation-only, and combined environments. The effect of thermal and radiation exposure on the microstructure and mechanical properties of the coatings were systematically evaluated.