Water droplet erosion (WDE) is a well-known phenomenon. This type of erosion is due to the impingement of water droplets of several hundred microns to a few millimeters size at velocities of hundreds of meters per second on the edges and surfaces of components. The solution to this problem is in high demand especially for the moving blades of gas turbines’ compressors and those operating at the low-pressure (LP) end of steam turbines. Thermal sprayed tungsten carbide based coatings have been the focus of many studies and are industrially accepted for a multitude of wear and erosion resistance applications. The present work studies the microstructural, phase analysis and mechanical properties and their effects on water droplet erosion resistance of such coatings deposited with high velocity oxygen fuel (HVOF) and high velocity air fuel (HVAF) processes. The feed nano-agglomerated tungsten carbide-cobalt powders are in either sintered or non-sintered conditions. The WDE tests were performed using 0.4 mm water droplets at 300 m/s impact velocity. The study shows promising results for this cermet (better than the Ti6Al4V bulk material) as WDE resistant coatings when deposited using HVOF or HVAF processes under optimum conditions.

In this study, suspension plasma spraying is used to deposit pseudo eutectic alumina-yttria stabilized zirconia as a potential thermal barrier coating. Process variables including feed rate, powder size, and plasma gas composition were altered to determine the influence of spray parameters on the formation of phases in the composite coating. The most significant variable was found to be the auxiliary gas. The gas influences the formation of phases primarily through its effect on in-flight particle velocity.

Suspension Plasma Spray (SPS) is a novel process for producing nano-structured coatings with metastable phases using extra small particles as compared to conventional thermal spraying. Suspension spraying involves, atomization, solvent evaporation and melts consolidation, which can cause substantial complexity in the system. Using feedstock mixtures for composite coatings, such as alumina and zirconia, intricacy of the system increases even more. There is consequently a need to better understand the relationship between plasma spray conditions and resulting coating microstructure and defects. In this study, an alumina/ 8 wt% yttria stabilized zirconia was deposited by axial injection SPS process. The effects of principal deposition parameters on the microstructural features are evaluated by using Taguchi design of experiment (DOE). The microstructural features include microcracks, porosities and deposition rate. To better understand the role of the spray parameters, in-flight particle characteristics, i.e. temperature and velocity were also measured. The role of the porosity in this multi-component structure is studied as well. The results indicate that thermal diffusivity of the coatings, an important property for potential thermal barrier applications, is barely affected by the changes in porosity.

Al 2 O 3 -ZrO 2 composite coatings were deposited by suspension thermal spraying of submicron feedstock powders. The suspensions were injected internally into a Mettech Axial III plasma torch and a Sulzer-Metco DJ-2700 HVOF gun. The different spray processes induced a variety of structures ranging from finely segregated ceramic laminates to alloyed amorphous composites. Mechanisms leading to these structures are related to the feedstock size and in-flight particle states. Compositionally segregated crystalline coatings, obtained by plasma spraying, showed the highest hardness of up to 1150 Hv 0.3 , as well as the highest abrasion wear resistance (ASTM G65). The HVOF coating exhibited the highest erosion wear resistance (ASTM G75), which was related to the toughening effect of small dispersed zirconia particles in the alumina-zirconia alloyed matrix. The HVOF microstructures also led to low thermal diffusivity, due to high amorphous phase content and limited particle bonding.