Surface Modification Of Titanium Alloy Micro-dynamic Fatigue Behavior Research

Poor tribological property and high susceptivity to fretting fatigue (FF) influence wide application of titanium alloy in aeronautical industry. It is important to improve FF resistance of titanium alloy by surface treatment techniques for developing advanced aero-engine. The behavior and mechanisms of FF of the titanium alloy at elevated temperature were investigated. Ion Beam Enhanced Deposition (IBED), unbalance magnetron sputtering (UBMS) and thermal spraying techniques were solely employed or combined with shot peening (SP) to treat the titanium alloy surface in order to improve FF resistance of the alloy at high temperature. The main contributions are as follows:1. Delimitation is the character of FF damage of Ti811 titanium alloy at elevated temperature. The life of FF is changed non-monotonically with increasing slip amplitude or contact pressure. Because slip amplitude and contact pressure affect the action of fatigue and wear in FF process, result in the change of crack initiation probability and crack propagating rate. Creep is the important factor that influenced fretting fatigue failure process of Ti811 alloy at elevated temperature as creep deformation is easy and the plain fatigue property becomes poor under the high temperature conditions.2. IBED coating has high density, fine grain, low porosity and good bonding strength. The FF life of the IBED OCrl 8Ni9 coated specimens increases 66% of that of the base titanium alloy at 350"C, because the coating with higher hardness and high ductility improves the wear-resistant and plain fatigue resistance of the titanium alloy. However, IBED CuNi coating has not the good effect due to its low ductility and low adhesion strength to base materials.3. An increase in FF life of 78% was observed for specimens coated with UBMS MoS2-Ti at 350"C. This is due to the fact that the UBMS MoS2-Ti with good bonding strength lubricates Ti alloy surface and reduces surface stress at contact area and inhibits FF crack initiation and propagation.4. Co-WC coatings prepared by HVOF and plasma spraying techniques have high hardness and good wear-resistant. But those coatings can't improve FF resistance of the titanium alloy at elevated temperature because of their low ductility.5. It relies on the properties of the surface coating whether SP and IBED coating can synergistically improve FF resistance of titanium alloy. IBED 0Cr18Ni9 has good bonding strength and ductility, therefore the coating can bear the shock of SP andsynergistic effect of improving FF resistance was realized by SP post treating IBED 0Crl8Ni9 coating. IBED CuNi coating combined SP hasn't the synergistic effect as the coating is brittle. Because the process of UBMS MoS2-Ti coating reduces the residual compressive stress of the shot peened surface, the FF life of the duplex treated specimen by UBMS MoS₂-Ti and shot peening is shorter than that of shot peened one.