Investigating fundamental processes in erosive wear of titanium alloys by water droplets

Water droplet erosion (WDE) occurs when water droplets impact a surface at high speed, causing erosive wear, in areas such as aerospace engine fan blades, steam turbine blades and wind turbine blades. WDE resistant materials have previously been identified and used to prolong component life, such as cobalt overlays on steam turbine blades, but these can fail dramatically. A fundamental understanding of the wear mechanism is required to effectively design a surface which can resist WDE for longer.

This project sought to better understand the fundamental mechanisms of water droplet erosion through the use of a literature review and experiments.

Two titanium alloys were used in this investigation; Ti-15Mo and Ti-6Al-4V. Ti-15Mo was developed for biomedical applications such as dental implants, while Ti-6Al-4V is the most commonly used titanium alloy, appearing in areas such as aero-engine fan blades and offshore components. Since the Ti-15Mo had a grain size around twenty times larger than the Ti-6Al-4V, some Ti-6Al-4V was annealed to a larger grain size, to provide a means of comparison between the two differing alloys.

Several models are suggested in the literature to describe the physical phenomena of WDE, such as direct deformation, stress wave propagation, hydraulic penetration, and lateral outflow jetting. WDE is likely produced as a synergy of these effects, however the relative importance of each is still poorly understood. Direct deformation and stress wave propagation cause initial roughening and (near) surface cracking. Hydraulic penetration and lateral outflow jetting are secondary in widening cracks or interacting with uneven surface material. Ultimately, large spalls of material are removed due to cyclic fatigue and cumulative damage caused by repeated droplet impact.

In this work, WDE experiments conducted on a whirling arm-type test machine showed good agreement with the literature. Advanced erosion was characterised by deep craters and quantified by mass loss-time plots. Initial erosion showed characteristic features similar to those already noted by other authors in the published literature; micropits, slip bands, depressions. EBSD was used to aid damage tracking; it’s use for the evaluation of WDE damage has previously not been reported in the literature.

Several characteristic WDE material deformation phenomena noted by other researchers are fatigue related, including slip bands, striations, and twinning. Despite noting such features, few researchers conduct comparative studies between WDE and fatigue. Low cycle fatigue tests were planned, and tensile tests were carried out to help plan the fatigue tests. Some material mechanical properties - such as resilience - have been reported in the literature to correlate to improved WDE performance (again, few have researched this area), therefore tensile testing was also useful in recording other characteristic material properties.

WDE tends to occur at high strain rate, though there are limited WDE studies which seek to also investigate the materials in dedicated high strain rate tests. Split Hopkinson pressure bar (SHPB) testing was conducted to obtain material properties at high strain rates. The results showed the Ti-alloys could withstand much higher stresses than under conventional quasi-static loading. Deformation occurred by a combination of slip and twinning, as characterised by SEM and EBSD analysis.

It is likely that cavitation within an impinging droplet contributes to the erosion of surfaces. Some authors in the literature have found that the initial stages of cavitation erosion and the early damage seen in WDE are similar. Cavitation erosion tests were therefore also performed in this work, producing grain tilting and micropitting features similar to those seen in the early stages of WDE by other authors.

By considering how the WDE test materials behaved in other experiments with known conditions, notably high strain rate tests, certain deformations could be compared. The findings of this thesis will be of use for researchers and manufacturers when considering materials to withstand erosion.

Recommendations arising from the work performed in this thesis are to investigate the early stages of erosion with further analysis techniques, including low voltage SEM, FIB sectioning, XRD, TEM, and XCT, and to fully investigate low cycle fatigue. This would give further insights into how materials behave in the early stages of WDE. Cavitation erosion could be used as a simple test for ranking candidate substrate and/or coating materials, before committing to full-scale WDE tests.