The tribological performance of coated and non-coated materials in high temperature environments

Variable geometry (VG) and wastegate mechanisms are commonplace within modern turbochargers and contain critical components without which the correct and efficient operation of a turbomachine would be affected. Such mechanisms rely upon components repeatedly and accurately moving against one another, even when subjected to extreme conditions such as high temperatures, a range of exhaust gas chemistries and in dry conditions. Therefore, the materials used within typical VG and wastegate mechanisms need to possess corrosion and oxidation resistance as well as the required mechanical properties at high temperatures. Conventional materials with such characteristics, however, do not necessarily possess adequate friction and wear behaviour. Hence, there is a desire to investigate the benefit of using alternative materials, such as nickel and cobalt based superalloys, but their use in these applications is limited by their cost. A coated base alloy provides a wider pool of possible solutions and the ability to achieve a more cost-effective solution. By taking into consideration the environments to which VG and wastegate components are subjected, a time efficient selection process was developed that takes engineering and purchasing requirements into consideration. The material and coating samples were subjected to different conditions which are typically experienced during turbocharger operation in order to understand their corrosion and oxidation behaviour, with the worst performing concepts removed from the testing matrix. Subsequently, tribology testing was conducted using the remaining concepts, at temperatures typical of turbocharger operation, and the friction and wear results were compared. All concepts were characterised by using confocal microscopy, high resolution microscopy, chemical analysis and nanoindentation in order to understand their performance. The results were compiled in a matrix in order to determine, from an engineering and purchasing perspective, which concepts were most suitable for application within high temperature, turbocharger-based, tribological environments.