Rain erosion protection for fan blades

Abstract

Rain erosion, or water droplet erosion (WDE), of turbofan blades arises problems in the aeronautics community. The phenomenon changes the profiles on fan blade leading edges, affecting the aerodynamic performance that subsequently leads to a significant efficiency drop for the aircraft engine. A potential risk of blade flutter is also raised consequently. Hence, this research work aims to identify the damage mechanisms induced by water droplet erosion on Ti₆Al₄V turbofan blade in order to ultimately improve the performance with potential coating techniques.

Studies on WDE are restricted to laboratory testing with simplified conditions and samples. Hence, this work will be the first to demonstrate real-life WDE damage on turbofan blades during their ‘complicated’ in-service conditions. Intergranular fracture is observed at the tip of the leading edges, which is induced due to anisotropic behaviour of α grains under high impact pressure and stress waves during early WDE stages. The roughened surface features then interact with lateral outflow jetting, forming cracks. Material removal then takes place in the forms of tunnelling and upheaval of the overlying materials under the effect of hydraulic penetration. Vibratory CE apparatus is employed to replicate the predominant damage induced by WDE on fan blade, so that it can be used as a screening process to predict the performance of the coated systems. A systematic test plan is adopted to cross-check the damage generated by CE with actual damage on the fan blade leading edge, as well as the WDE damage generated on Ti₆Al₄V base material using whirling-arm rig to ensure the relevance of the work to the application. Correlation between the damage mechanisms of CE and WDE are identified as similar damage mechanisms are involved in both processes, especially during the incubation period. Exposed grain boundaries are observed due to the displacement of α grains under dynamic loading induced by CE and WDE during incubation period. The effect of crystal orientation on CE and WDE performance is identified as certain grains show greater resistance better than others.

For the second stage of the work, a series of coatings are proposed and tested under CE to evaluate their potentials for WDE application. As a result, the Hardide CVD-W/WC T1200 coating, Hardide CVD-W/WC LT coatings, HIPIMS deposited CrAlYN/CrN nanoscale multilayer C1 coating and HIPIMS deposited CrAlYN/CrN nanoscale multilayer C2 coating are selected. In-depth understanding on the coating damage mechanisms are established by correlating the coating performance with microstructure, crystallographic texture, interface design, deposition conditions and mechanical properties for the first time. In both Hardide and HIPIMS coating systems, a beneficial crystalline texture is identified that better resists to CE. For Hardide W/WC coating, the damage tends to initiate at the grain boundaries of the exposed surfaces. While in the case of the HIPIMS deposited multilayer coatings, the primary damage mechanism is believed to be the cracks formed at the substrate/interlayer interface. The nanoscale multilayer structure greatly extends the coating lifetime with crack deflection mechanisms at the interfaces between the nanolayers. Therefore, it is found that the coating performance under dynamic compressive loadings cannot be predicted with simple H/E approaches. However, combining the H/E ratios with the factors of microstructure, crystal orientations and nano-layered systems might further facilitate the understanding of coating performances.

Ultimately, WDE testing using the whirling arm rig with 500 µm droplets at 300 m/s impact velocity of the screened coating specimens is attempted to validate their potentials for rain erosion protection of fan blade leading edge. However, the results are considered not representative due to a fault with the test rig.

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Identifiers

ORCID for Robert Wood: orcid.org/0000-0003-0681-9239

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