Tribological Performance Analysis of Coated Hydro Turbine Blade Materials

Abstract

Hydro turbines are one of the most vital and essential components for the production of hydroelectric power. Since the hydro turbine is always operating under the influence of water and the silt transported by the water from the hilly region, it poses a lot of threat and damage to the turbine blades working constantly under the action of silt laden water. Erosion wear is the most common type of damage, and drastically affects the life of turbine runners and blades. The introduction of thermal spray coatings, namely high-velocity oxy-fuel (HVOF) technique has resulted in a major improvement in stainless steel erosion resistance. On stainless steel substrates of grade 304, ceramic coating powders primarily Al2O3, Cr2O3, and a composite mixture of composition 55% Cr2O3+45% Al2O3 were deposited. The thorough experiments utilizing silt slurry are conducted at different particle size of silt in the range 212-250 µm, 150-212 µm, and 53-106 µm. The silt concentration has been varied in the range 12-42% by weight and the substrate rotated at rotational speed in the range 600-1500 rev/min for the test time of 4 hours. SEM images were used to assess the surface morphology of the required testing samples. The erosion observance and the material removal mechanism were investigated and discussed on the basis of microstructure studies. Under all test conditions, Cr2O3 coating is found to be more resistant to erosion than other coatings. Also, the composite coatings of composition 65% Cr2O3+35% Al2O3 has been modified by the addition of CeO2 to the percentage of 0.5%,1% and 1.5%, a rare earth element for analysing the silt erosion behaviour at particle size 212-250µm, 150-212µm, 53-106µm and the concentration of the silt range from 10-40% by weight. The rotational speed of the pot tester has been varied between 500-1500 revolutions per minute and the test duration has been kept to 4 hours. It has been found that the erosion wear resistance of the uncoated SS-304 has been greatly enhanced by the application of HVOF sprayed Al2O3-Cr2O3 composite coatings. Also, the addition of CeO2 has a significant impact in reducing the erosive wear caused by silt slurry. The composite coating powder composition of 65%Cr2O3+34.5%Al2O3+ 0.5%CeO2 has shown the highest erosion resistance. The developed coatings have the potential to be used for hydro turbines as subjected to silt slurry conditions. Also, the Taguchi approach has been applied for tribo-erosion wear of Al2O3-Cr2O3 composite coatings. It has been analysed that of all the control variables such as particle size and silt concentration, rotational speed is the most dominant factor among all.