Abstract
Due to the increasing maintenance costs of hydraulic machines related to the damages caused by cavitation erosion and/or erosion of solid particles, as well as in tribological connections, surface protection of these components is very important. Up to now, numerous investigations of resistance of coatings, mainly nitride coatings, such as CrN, TiN, TiCN, (Ti,Cr)N coatings and multilayer TiN/Ti, ZrN/CrN and TN/(Ti,Al)N coatings, produced by physical vapor deposition (PVD) method using different techniques of deposition, such as magnetron sputtering, arc evaporation or ion plating, to cavitation erosion, solid particle erosion and wear have been made. The results of these investigations, degradation processes and main test devices used are presented in this paper. An effect of deposition of mono- and multi-layer PVD coatings on duration of incubation period, cumulative weight loss and erosion rate, as well as on wear rate and coefficient of friction in tribological tests is discussed. It is shown that PVD coating does not always provide extended incubation time and/or improved resistance to mentioned types of damage. The influence of structure, hardness, residence to plastic deformation and stresses in the coatings on erosion and wear resistance is discussed. In the case of cavitation erosion and solid particle erosion, a limit value of the ratio of hardness (H) to Young’s modulus (E) exists at which the best resistance is gained. In the case of tribological tests, the higher the H/E ratio and the lower the coefficient of friction, the lower the wear rate, but there are also many exceptions.
Highlights
Hydraulic machines especially in hydropower plants are exposed to erosion caused by cavitation and/or solid particles that move with the flowing water
As the physical vapor deposition (PVD) method gives a great opportunity to modify the properties of the produced coatings, these coatings are tested as anti-wear and anti-erosion coatings and used in several applications
Increasing the duration of incubation, decreasing the cumulative mass or volume loss, and reducing the erosion rate are indicative of increased anti-erosion properties of PVD coatings
Summary
Hydraulic machines especially in hydropower plants are exposed to erosion caused by cavitation and/or solid particles that move with the flowing water. The improvement of fatigue resistance of the material by deposition of PVD coatings depends on the following factors: coatings properties, substrate properties and test conditions. Because the material property that has largest impact on resistance to cavitation erosion, solid particle erosion and wear is hardness [50,51], hard coatings are tested as anti-erosion and anti-wear coatings [52,53,54,55]. In the case of PVD coatings, the properties affecting their resistance to erosion depend on such factors as the technique and parameters of deposition [40,41,43], the coatings resistance, and presenting common relationships between coating properties and resistance to the mentioned degradation processes
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