The cavitation erosion result mass loss. Welding is the most common technique used to recover the geometrical profile of the blades. However it is known that tensile residual stress can develop. The search for manufacture process that could reduce or eliminate the residual stress level will contribute for a longer life service. The target in this study to evaluate the potential of ASP thermal spray to recover surfaces. The influence of processing parameters on the cavitation resistance and mechanisms was evaluated for three alloys, AWS309LT1, AWS410NiMo and a Co stainless steel known as Cavitec. Coatings were analyzed by optical and electronic microscopy, microhardness and cavitation tests regarding the effect of air pressure. The results showed that lamellae morphology, oxide volume fraction and cavitation resistance were modified by the ASP parameters. The increase in the pressure modified the oxide fraction from 26 to 37% in AWSI309LT1, 23 to 31% for AWS410NiMo and 16 to 23% for Cavitec. Mass loss varied from 3.5 to 4.8 mg/h for AWSI309LT1, 6.65 to 18.19 mg/h for AWS410NiMo, and 3.4 to 4.0 mg/h for Cavitec; the best performance occurred with Cavitec and was associated with higher pressure of deposition and minor oxide volume fraction.

Many studies have investigated methods to reduce cavitation damage in hydraulic turbines and reduce residual stresses after coating deposition. In this work, a cobalt stainless steel was applied by arc thermal spraying. The influence of air pressure deposition and plasma remelting on the microstructure, oxide volume fraction, porosity, microhardness, and cavitation resistance were studied. Microstructures and properties of the AS895HY cobalt stainless steel coatings were investigated by x-ray diffraction, optical microscopy, scanning electron microscopy, microhardness testing, and ultrasonic cavitation testing (ASTM G32-93). The increase in air pressure, 280 to 550 kPa, modified the oxide fraction from 10.9±1.8% to 24.1±2.8% in the samples. The mass loss results in the cavitation tests were 13.8, 19.2, and 15.0 mg/h for the samples with 280, 410, and 550 kPa, respectively. The remelting of the sprayed coatings eliminated the oxides and porosity. Austenite formation was observed in the two remelted layers with decreases in microhardness; for the first layer, this occurred because of the AWS309L substrate dilution. The PTA remelting reduces the mass loss rate to 0.497 mg/h, with 8.02 hours incubation period. Phase transformations were observed in the remelted coating, but not verified in the arc thermal sprayed coatings.

The deposition of cavitation-resistant materials coatings in turbine blades is an important way to reduce cavitation damage. Fe-Cr-Mn-Si is a cavitation-resistant steel with many deoxidation elements, which can be important for arc thermal spraying materials. The influence of air pressure, arc tension, and chemical composition on the microstructure, area fraction oxide, porosity, microhardness, and cavitation resistance were studied. Microstructures and properties were investigated by XRD, optical and electronic microscopy, microhardness testing, and ultrasonic cavitation testing per ASTM G32-93. Chromium addition promotes an increase in area fraction oxide, and reduces the porosity, changing the microhardness. An increase in air pressure raised the oxide fraction in the SMA_A and 2 alloys. The SMA_A mass loss rates were 31.8, 25.8, and 37.2 mg/h, respectively, for the samples with 280, 410, and 550 kPa of air pressure. For the SMA_3 samples, the increase in the arc voltage reduces the oxide fraction, changing the mass loss rate to 43.8, 32.4, and 29.4 mg/h for 25, 30, and 35 V, respectively. Phase transformations occurred in the arc thermal spray, for all coatings, during cavitation tests. The SEM analysis verified that the mass loss in arc thermal spray coatings occurred because of the oxide fracture and delamination of the splats.