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In this study, CrN and CrAlN coatings were deposited on the surface of aerospace titanium alloys using the arc ion plating technology to investigate the effect of the microstructure of the coatings on the mechanical and erosion resistance properties. The cross-sections of the coatings were observed by Scanning Electron Microscope(SEM), the microstructures were characterized by Transmission Electron Microscope(TEM), and the crystalline structures were tested by X-ray diffraction (XRD); furthermore, the hardness of the coatings was determined by a micro-Vickers hardness tester, the film-base bond strength was measured by a scratch tester, the change in curvature was measured by laser scanning and the residual stress was calculated. The fracture mechanism of the coating was elucidated by simulating the fracture pattern of the cross-section of the erosion crater. The results indicated that the thicknesses of the CrN and CrAlN coatings were almost the same, but the number of large particles on the surface of the CrAlN coating was higher. Both sets of coatings exhibited a nanocrystalline structure with a grain size of around 10 nm. Due to the doping of the Al element, the CrAlN coatings exhibited a hardness of 2846 HV and a higher residual stress of −1.831 GPa. Both the CrN and CrAlN coatings showed lower erosion rates than the titanium substrate at 30° and 90° angles of attack. The results of finite element simulations indicated that the more ductile CrN coating exhibited a better stress absorption capacity, and a typical brittle fracture failure mechanism was observed for both sets of coatings. • CrN and CrAlN coatings with good erosion resistance were obtained. Compared with the titanium alloy substrate, the erosion rate of the coated sample is only 1/8 of the titanium alloy substrate at 30°, and only 1/3 of the titanium alloy at 90°. • The erosion fracture process of the hard coating was explored, in which the cracks were mainly in the direction perpendicular to the film-base interface, and were easily deflected by stress concentration at the microparticles and the interface. • Both CrN and CrAlN coatings are typical brittle fracture failure mechanisms, but the tougher CrN coating has better stress absorption capacity. [ABSTRACT FROM AUTHOR]