Coating Mechanical and Acoustical Design Considerations for Resistance to Solid and Liquid Particle Impact

The erosion of components subjected to water droplet impact has been documented in various applications e.g. aircraft and wet-steam turbine blades. In wet-steam turbine systems, erosion of the of leading edge turbine blades causes significant loss of efficiency. Despite the efforts that have been put into this field over the past 50 years, no one has solved the problem of water droplet erosion. This may be attributed to the different damage phenomena; extremely high contact pressure; stress wave propagation; jetting and excessive heating etc. that occur in high- speed water droplet erosion. The main purpose of this thesis work was to attempt to link existing (but fragmented) knowledge of different aspects of water droplet erosion and the requirements to construct a protective coating to resist it. Discoveries from critical literature reviews were that the impact energy may be viewed as mechanical dissipation of stresses/strains or acoustic attenuation of stress waves. Therefore, architectural designs of protective WDE coating structures must try to satisfy both of these considerations.
To provide some validation of both the mechanical (stresses/strains) and acoustical (stress waves) considerations, titanium-based monolithic and multilayer PVD coatings will be investigated and characterization techniques (i.e. nano-indentation, x-ray diffractometer (XRD, Optical Microscopy (OM) Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Stylus profilometry) will be performed. Selected (both monolithic and multilayer) coatings will be subjected to both particulate (ball-on-plate) impact testing and water droplet erosion (WDE) testing.
This thesis illustrates the versatility of the triode ion-plating PVD technique and its feasibility to produce thick (Ti, Ti(N) and TiN) monolithic coatings and (TiN/Ti, TiN/Ti(N)) multilayer coatings with reliable controllability in terms of chemical composition and designate
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layer thickness. According to the results of this work, there is a definite distinction between the coating requirements for solid particle impact tests and liquid particle water droplet erosion, due to the differences in the way that the impact energy is delivered (i.e. strain rate, duration of the impact impulse, etc.) However, the results are inconclusive as to whether multilayer or monolithic coatings perform better in water droplet erosion.
Finally, the information gathered experimentally was analyzed (with existing proposed models and theories) and interpreted to propose a coating architecture which will be superior in water droplet erosion performance.