Study of shot-peened and PVD coated aerospace-grade titanium alloy

Abstract

Titanium alloys are extensively used in the aerospace industry due to their unique combination of physical and metallurgical properties that enables weight saving in various highly stressed aircraft components. However, the metal is characterised by poor tribological properties that limit its application in sliding components. To solve this problem, engineers resort to surface engineering techniques such as nitriding and physical vapour deposition (PVD) of hard, wear resistant coatings. Nonetheless, improvement in the wear resistance is usually seconded by a drop in the fatigue properties. On the contrary, surface engineering techniques such as shot-peening are designed to induce compressive residual stresses at the surface of the material, which increase the material’s resistance against fatigue. Consequently, the aim of this research was to develop a hybrid treatment consisting of deposition of a tungsten-doped diamond like carbon coating (WC/C) by PVD on shot-peened Ti-6Al-4V surfaces and assess its impact on the fatigue strength. The main factors affecting the fatigue performance were highlighted and characterisation of shot-peened, WC/C coated and hybrid treated surfaces was carried out. Residual stresses, being one of the key properties that dictate the fatigue performance of a material were measured by X-ray diffraction (XRD). Rotating bending fatigue tests were carried out and the fatigue performance was analysed by combination of S-N diagrams and in-depth fractographic analysis. Results show that despite the pronounced surface roughness produced by the shot-peening treatment, the high residual compressive stresses, together with the refined and cold-worked microstructure confirmed by electron backscatter diffraction (EBSD), were capable of improving (3.8% improvement in the fatigue limit as compared to the untreated condition) the fatigue performance of the shot-peened Ti6Al-4V alloy. Conversely, the WC/C coating displayed extensive cracking which propagated prematurely into the substrate and yielded a 4% drop in the fatigue limit. On the other hand, despite the annealing of a fraction of the beneficial compressive residual stresses during the coating deposition treatment, the hybrid treatment yielded a 2.4% improvement in the fatigue limit over the untreated condition. This result coupled with the high coating hardness and adhesion confirm the capability of this hybrid treatment in improving both the tribological and fatigue performance of the Ti6Al-4V alloy.