Cavitation erosion characteristics and mechanisms of hydraulic turbine substrates and their coatings

This study systematically investigated the cavitation erosion resistance and failure mechanisms of 06Cr13Ni5Mo(S135) stainless steel and four coatings (HVOF-WC10Co4Cr, HVOF-Cr₃C₂37WC18, HVAF-WC10Co4Cr, Laser-Clad(LC)) using ultrasonic cavitation testing. The cavitation weight losses of the HVOF-WC10Co4Cr, HVAF-WC10Co4Cr, HVOF-Cr₃C₂37WC18, and LC coatings were 1.58, 0.88, 0.91, and 0.34 times that of the S135 stainless-steel substrate, respectively. Their surface roughness values were 5.43, 3.48, 2.30, and 0.56 times that of the substrate. Among the four coatings, the LC coating showed the lowest weight loss and the smallest roughness increase. The HVOF-WC10Co4Cr coating exhibited the poorest cavitation erosion resistance despite having the highest microhardness, which indicated that high hardness alone did not ensure superior performance. For the thermal spray coatings, pre-existing pores served as the primary initiation sites for damage. Fatigue cracks nucleated at these pores and propagated along inter-splat boundaries, leading to spallation. The HVAF-WC10Co4Cr coating, with 29% lower porosity than its HVOF counterpart, demonstrated significantly enhanced resistance. The LC coating had a unique lamellar stacking structure that effectively confined damage to the surface. However, the through-thickness cracks caused by process defects accelerated local delamination and resulted in funnel-shaped pits. These findings provide critical insights into the design and selection of cavitation-resistant coatings for hydraulic turbines.