Understanding and Mitigating Water Droplet Impingement Erosion

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Water droplet impingement erosion (WDIE) is caused by high speed collision between water droplets and solid surfaces. This phenomenon is encountered in gas and steam turbines, helicopters, and jet engines. Although large efforts have been performed to address this problem, the WDIE process is still not well understood because these efforts have been mostly directed towards studying the water droplets or the solid material separately. However, in order to understand this phenomenon, the water droplets and the solid surface should be considered at the same time. Therefore, this will be attempted in this project through combining experimental studies with numerical modeling. To date, it is not easy to accurately relate the WDIE lab test results to the in-service blade life. What makes things more complicated is the difficulty in comparing WDIE results obtained by different groups using different rigs. Direct comparison is very difficult because of different design and testing conditions. This makes the large data produced by different labs not useful for the industry. For example, in the literature some data is reported in terms of time of exposure without accurate account of the actual amount of droplets impacting the solid surface. Other papers report material loss versus number of rotations of the solid sample which again does not take into account the actual number of droplets and their size distribution. In this work we will develop a more representative method that will enable comparing results obtained using different rigs. This will also help the industry in relating the lab test results to the actual blades life. Large number of researchers tried to explain WDIE as a fatigue process. Nevertheless, three processes that are known to improve fatigue performance have been studied by the applicant's group. These are laser shock peening (LSP), low plasticity burnishing (LPB) and ultrasonic nanocrystalline surface modification (UNSM). The choice of these processes was because each one modifies the surface through different attributes that result in improving the fatigue life. While all the three result in deep and high compressive residual stresses, LPB causes surface hardening but without any noticeable microstructural changes unlike the UNSM process. Among these processes, only UNSM causes noticeable WDIE improvement in Ti64 alloy. This means that fatigue mechanism is not the only dominating damage mechanism in WDIE. This also suggests that for improving WDIE resistance, a synergy between compressive residual stress, microstructural changes and surface hardening is required. We would like to further understand the underlying damage mechanisms at different WDIE stages. For instance, to understand the combined effect of residual stresses and microstructural changes, WDIE performance of LPB-treated 17-4PH stainless steel will be studied because it does not show significant strain hardening.