The selection and tribological performance of coated/non-coated materials for high temperature tribological systems for turbomachinery : innovation report

At the hot end of the turbocharger, there are components that undergo surface-related degradation mechanisms, which include the ones that arise when the components move against each other. Not only do these degradation mechanisms affect the long-term performance of the component, they could eventually lead to catastrophic failure of the entire turbocharger. In addition, future regulations dictate higher efficiency engine systems to meet emission restrictions. These require the turbocharger to run at higher temperatures and loads, leading to reduced service life of current components if new materials are not found. As conventional materials, like stainless steels, do not tend to provide adequate friction and/or wear behaviour and cobalt-based superalloys being expensive, alternative solutions need to be explored. As the dominant degradation processes are surface-related, treating or coating the material’s surface to obtain the properties required for the application offers an alternative. As coating solutions will open a wide range of materials to investigate, a time efficient process needs to be implemented by industry in order to reduce the time in obtaining an appropriate solution. From an industrial perspective, the solution must not only fit with the required engineering requirements, it must also meet business and purchasing requirements. The present study was conducted as a research and development project with Cummins Turbo Technologies (CTT) to implement new solutions for variable geometry (VG) turbochargers that will operate in increasingly aggressive conditions. A description of how sets of selected materials behaved at three critical temperatures for turbocharger operation is discussed in terms of microstructural, mechanical and chemical changes in order to understand their behaviour in the relevant environments. The fundamental degradation aspects of a range of coatings wear are compared to stainless steels and Co-superalloys, offering new scientific knowledge in high temperature dry wear of engineering alloys and coatings. In order to understand the performance of the materials performace in the aggressive environments experienced a series of environmental tests were conducted and characterized before and after testing. A room temperature corrosion and high temperature oxidation test were done to obtain information about the material behaviour when the turbocharger is not or is in operation, respectively. A tribology test at different temperratures within the temperature range of operation of a VG mechanism were done in order to obtain information regarding their performance while in contact. A thermal shock test was done in order to understand whether or not the coating system is able to withstand high temperature changes. Finally, an adhesion test was done in order to understand how to coating system will behave under overstressed conditions. From Cummins’s perspective, this knowledge was enhanced by the development of comprehensive selection methodology, which will be discussed and applied throughout this thesis. The methodology includes not only engineering requirements but also accounts for business requirements. This provides an innovative aspect to the research work that has been implemented by Cummi