Pseudoplastic deformation pits on polished ceramics due to cavitation erosion

First Results of Cavitation Erosion Behavior of Plasma Nitrided Niobium: Surface Modification

“…But, in the specialized literature, cavitation erosion behavior of plasma nitrided niobium has not been studied. Despite that significant research works have been done on the cavitation behavior of pure metals and metallic alloys, a much lesser degree of published works has studied the behavior of engineering ceramics [6][7][8][9][10][11][12][13] . Plasma nitriding technique was used here in order to obtain niobium nitride (ceramic-like material) surfaces formed in pure niobium substrates.…”

“…The presence of microcracks at the pits boundaries would result in a coalescence mechanism or pits bridging formation, leading more pits to be successively removed as the surface exposition to cavitation erosion goes on 10 . Finally, possible deformation and cracking mechanisms in the pit formation are presented in a study of cavitation exposure carried out in silicon nitride and zirconia 8 . In general terms, according to the authors 8 , it has been established that pseudoplastic deformation pits are formed when tensile stress induced by the impact of the collapsing bubble, leading the surface to be elastically deformed, is high enough to create cracks in intergranular phase next to the surface.…”

“…Finally, possible deformation and cracking mechanisms in the pit formation are presented in a study of cavitation exposure carried out in silicon nitride and zirconia 8 . In general terms, according to the authors 8 , it has been established that pseudoplastic deformation pits are formed when tensile stress induced by the impact of the collapsing bubble, leading the surface to be elastically deformed, is high enough to create cracks in intergranular phase next to the surface. These cracks release the slip systems in the crystals, making possible the occurrence of a small plastic deformation, as a result of the water pressure action, within the remaining grains after the impact 8 .…”

“…It was also observed by studying different ceramic materials that the possibility of exploring compressive stresses in surface layers would be an interesting prospect to improve the incubation period 6 . As presented in 8 , silicon nitride and zirconia has been reported as the technical ceramics with the best cavitation erosion resistance in the literature [6][7][8] , comprising the main subject for studies aiming to improve the performance of bearings, injectors or valves submitted to cavitation problem in real and different operating conditions, such as those verified in rocket engines 8,10,11 . Karunamurthy et al have also shown that cracking in cavitation eroded silicon nitride is preferentially intergranular, leading erosion pits to be produced.…”

“…In general terms, according to the authors 8 , it has been established that pseudoplastic deformation pits are formed when tensile stress induced by the impact of the collapsing bubble, leading the surface to be elastically deformed, is high enough to create cracks in intergranular phase next to the surface. These cracks release the slip systems in the crystals, making possible the occurrence of a small plastic deformation, as a result of the water pressure action, within the remaining grains after the impact 8 . Local plastic deformation tends to occur at the same time as the surface is still highly elastically deformed, being that the plastically deformed surface fits the new geometry of the elastically deformed bulk material 8 .…”