Cavitation erosion of materials

Doctoral Thesis


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University of Cape Town

This investigation was undertaken to find the optimum hydraulic fluids and materials for combating cavitation erosion in hydraulic mining machinery operating,on 5:95 oil-in-water emulsions, Controlled cavitation was generated in the laboratory in a vibratory cavitation apparatus employing the stationery specimen system. The influence of separation distance between the drill tip and specimen and of varying medium temperature on cavitation erosion were also investigated but were of secondary importance to the studies on materials and emulsions, The evaluation of the influence of a number of commercial metal cutting coolants and specific hydraulic emulsions oncavitation erosion,showed that those emulsions with a high mineral oil content suppress erosion the most, This phenomenon is considered to be either the result of a reduction in nucleation of highly damaging surface cavities due to the elimination of surface nuclei by an oil film, or to damping of cavity collapse due to the high dissolved gas content of oil-in-water emulsions, A large number of commercial grade alloys and polymers have been investigated, The alloys ranged from single phase, face centered cubic nickel alloys to complex, multi-phase, cobalt based alloys while the polymeric materials were all linear and crystalline. The modes of erosion and mechanisms of erosion resistance were studied in detail by optical microscopy, scanning and transmission electron microscopy and X-ray diffractometry. The fractographic studies on the alloys revealed, in particular, the role played by second phase particles and other surface features such as protruding grain boundaries and surface steps created by dislocation movement, twinning and phase transformations, Furthermore, X-ray diffractometry coupled with theoretical considerations and the findings of previous investigations indicated the fundamental importance of phase transitions in contributing towards the erosion resistance of alloys, Cobalt based Stellite alloys, nickel based cemented carbides and the intermetallic alloy NiTi have exceptionally high erosion resistance, The Stellites have a high yield stress and also undergo an awe phase transition which results in a high work hardening rate and high strain to fracture, NiTi similarly undergoes a phase transformation which allows absorption of seven per cent reversible strain and also results in a high work hardening rate. The erosion resistance of nickel based tungsten carbides is, in turn, attributable to the very high yield stress of these materials, Ultra high molecular weight polyethylene and nylons have relatively high erosion resistances, Their exceptional performances are due to a high strain to fracture and high yield strength respectively. On the other hand low tensile strength polytetrafluoroethylene has a very poor resistance to erosion. This work has identified various material properties which are required for erosion resistance, These are : (a) a high yield stress, (b) a high work hardening rate and (c) a high strain to fracture.

Bilbiography: p. 141-148.