High Temperature Comparative Tribological Study of CoCrTaAlY Coatings Reinforced with Different Percentages of Alumina

In several aerospace applications the contact surfaces undergo a repeated cyclical rubbing, known as fretting, and then they are coated with low wear rate material. Moreover the contact surfaces rub at a high temperature. In recent years materials engineers have been working to improve performance of coatings, in order to withstand such high demands for wear resistance, by developing new materials and new coating methods. It is then of great interest to investigate the tribological behavior of new contact materials in the real operating condition and the request for fretting wear experimental test is rising quickly. This work is focused on coatings applied on the Z-notches of turbine shrouded blades in order to minimize the damage due to fretting wear. The more severe operational environment conditions lead to evaluate the behavior of coatings at a high temperature. CoCrTaAlY alloys coatings reinforced with ceramics and applied with thermal spray process are widely applied to Z-notches due to their wear, oxidation and thermal shock resistance at high temperature. The tribological behavior of coatings is affected by several parameters such as i) the contact conditions – normal load, relative speed, surface morphology - ii) the environmental conditions - temperature, gases, lubrication - and iii) the contact materials, in term of mechanical and chemical properties. Because of the complexity of the tribological contact mechanisms, the contact parameters, namely coefficient of friction and contact stiffness, or the wear rate cannot be accurately predicted based solely on theoretical analyses. An experimental campaign was carried out to predict and evaluate the wear behavior of a CoCrTaAlY coating, when the ceramic component (i.e. alumina) increased from 10% wt to 30% wt.