UC Irvine

With utilization of gas turbine engines in power generation, aerospace and marine propulsion applications, the materials that enable those gas turbine technologies are exposed to a wide range of service temperatures and material exposure environments resulting in application dependent degradation modes. The most severe types of degradation are seen in the hottest section of the turbine with its combined interaction of external contaminants and high temperatures. Although specialized coatings have been developed to try to alleviate the degradation experienced, hot corrosion continues to be a concerning, life-limiting factor, particularly in the case of marine turbines, and it is, therefore, the focus of this study. This work presents the evaluation of new candidate materials for improved marine turbine performance at higher operating temperatures. Three different areas of work are discussed. First, the current methodology for the evaluation of hot corrosion attack in pin-shaped samples, typical of burner rig testing, is presented, and its shortcomings are discussed. A new sample assessment protocol based on image analysis was established and validated. Next, a new nickel-based superalloy and three doped variations, intended to replace current blade and vane substrates, were evaluated under type I hot corrosion conditions in a low-velocity burner rig (LVBR). The tests included both long-term and short-term exposures as well as pre-oxidized and bare materials. Scanning electron microscopy and energy dispersive spectroscopy were used to study the attack mechanisms as a function of doping material and concentration.

It was found that different dopants affected the hot corrosion resistance by promoting the incorporation of certain elements, which changed the types of sulfides and oxides, protective or non-protective, that formed. Silicon was found to be an effective dopant at increasing hot corrosion resistance through two mechanisms: a) by promoting chromia formation and suppressing the activity of titanium, resulting in a more protective oxide able to slow down internal sulfidation, and b) by promoting a different coarsening behavior of the internal sulfides. Co-doping with hafnium and silicon had a synergistic effect where the presence of hafnium enhanced the effects of silicon, and the overall hot corrosion resistance was significantly improved, even though hafnium doping, by itself, had poor performance.

The third area of work is focused on the performance, compatibility, and hot corrosion resistance of substrate-coating material pairs evaluated in a LVBR. The coatings that were evaluated included several commercially available diffusion coatings, and both commercially available and new developmental candidate overlay coatings. In the case of diffusion coatings, it was observed that the formation of topologically closed pack (TCP) phases and elemental segregation along the interdiffusion zone (IDZ) are crucial, limiting factors determining the lifespan of the coating. In the case of overlay coatings, initial observations provided evidence for substrate-dependent performance. However, upon closer inspection, it was revealed that this dependence was a function of differing initial microstructures most likely originating in processing variations. The best performing coatings, evaluated on multiple substrates, were comprised of a modified NiCrAlY and a platinum modified CoCrAlY. As a direct result of this work, new substrate and coating materials with enhanced performance were selected for implementation in the next generation of marine turbine engines, and the testing and sample evaluation framework developed will continue to guide future material selection efforts. The study of commercially processed substrates and coatings led to key findings pointing to the importance of coating microstructure control and prevention of unwanted phase formation and elemental segregation.