Erosion of materials by solid particle impact is a complex wear phenomenon where the material is removed as a result of brittle fracture or plastic deformation [188-190]. Plasma sprayed coatings are used now a days as erosion resistant coatings in a wide variety of applications [191-193, 25]. Extensive research shows that the deposition parameters like energy input to the plasma and the powder properties affect the porosity, splat size, phase composition, coating hardness etc. [194-201]. These in turn, have an influence on the erosion wear resistance of the coatings. Quantitative studies of the combined erosive effect of repeated impacts are very useful in predicting component lifetimes, in comparing the performance of materials and also in understanding the underlying damage mechanisms involved.
Kulu et al. [202-205] have carried out significant research in the field of erosion resistant coatings and have reported that under extreme conditions, solid particle erosion (SPE) is a serious problem for many industrial equipment. Response of a material to SPE is a complex function of the physical properties of the target, the impacting particles and the erosive environment [206]. Many erosion mechanisms have been proposed in the past and have been supported by the experimental data from erosion tests. Various models for the erosion of bulk metals, glass and ceramics have also been proposed [207] usually considering different combinations of micro-cutting, plastic deformation, melting, fatigue and fracture mechanisms [208]. According to Finnie and McFadden [209], there are four principal factors that influence the erosion behaviour of a material: the erodent velocity and size, the impact angle and the properties of the eroded material.
Few reports are available in the existing literature on erosion behaviour of alumina coatings. The resistance to erosion of such coatings depends upon inter- splat cohesion, shape, size and hardness of erodent particles, particle velocity, angle of impact and the presence of cracks and pores [197, 210-213]. The slurry
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and particle erosion response of flame sprayed alumina coatings has also been reported in the literature [70]. It is seen that high particle velocity enhances the erosion rate and it reaches a maximum for an impact angle of 900. The loss of material is by the progressive removal of splats and can be attributed to the presence of defects and pores in the inter-splat regions within the coating. Similar observations have also been reported for the plasma sprayed alumina coatings subjected to an erosive wear caused by the SiO2 particles [214].
Branco et al. [215] examined the room temperature solid particle erosion of zirconia and alumina based ceramic coatings with different levels of porosity and varying microstructure and mechanical properties. The erosion tests were carried out by a stream of alumina particles with an average size of 50 μm at a velocity of 70 m/s, carried by an air jet with impingement angle of 900. The results of this study indicated that there is a strong relationship between the erosion rate and the coating porosity. Similarly, Mishra et al. [216] investigated the erosion characteristics of plasma sprayed alumina-titania coatings deposited on mild steel substrates. This study revealed that premixing of titania in alumina significantly improves the resistance of the coating to solid particle erosion.
Ercenk et al. [217] studied the effects of impingement angle and SiC reinforcement on the erosion wear behaviour of basalt based glass and glass- ceramic coatings. Erosion tests were realized by using corundum media at the different impingement angles and velocities. The test results showed that the addition of SiC in the basalt based coatings resulted in enhancement of erosive wear resistance. Krishnamurthy et al. [218] examined the solid particle erosion behaviour of plasma sprayed alumina and calcia-stabilized zirconia coatings on Al-6061 substrate. Satapathy [88] carried out an extensive research on erosion wear behaviour of plasma sprayed red mud coatings under different test conditions. This study revealed that impact velocity and the impingement angle are the significant factors that influence the erosion rate of the coatings to a great extent.Subsequently, Sahu et al. [219] performed tribo-performance analysis of
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plasma sprayed fly ash-aluminum coatings using experimental design and artificial neural network. Recently, Gupta and Satapathy [220, 221] have reported extensively on solid particle erosion response of plasma sprayed glass microsphere coatings under different operating conditions.
Several erosion models were developed by many researchers to provide a quick answer to design engineers in the absence of a comprehensive practical approach for prediction of erosion response. One of the early prediction correlations is that developed by Finnie [222] expressing the rate of erosion in terms of particle mass and impact velocity. In that correlation, the rate of erosion was proportional to the square of the impact velocity. Nesic [223] found that Finnie’s model over-predicts the erosion rate and presented another formula in terms of a critical velocity rather than the impact velocity. The erosion model suggested by Bitter [5, 224] assumed that the erosion occurred in two main mechanisms; the first was caused by repeated deformation during collisions that eventually results in the breaking loose of a piece of material while the second was caused by the cutting action of the free-moving particles. Glaeser and Dow [225] suggested another two-stage mechanism for explaining different aspects of the erosion process for ductile materials. In the first stage, the particles indent the target surface, causing chips to be removed and some material to be extruded to form vulnerable hillocks around the scar. The second stage was the one in which the particles break up on impact causing fragments to be projected radially to produce a secondary damage. Some other erosion models were also suggested by Laitone [226], Salama and Venkatesh [227], Bourgoyne [228], Chase et al. [229], McLaury [230], Svedeman and Arnold [231] and Jordan [232].
Different models have also been proposed that allow estimation of the stresses that a moving particle will impose on a target during erosion [233]. It has been experimentally observed by many investigators that during the impact, the target can be locally scratched, extruded, melted and/or cracked in different ways [234- 236]. The imposed surface damage will vary with the target material, erodent
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particle, impact angle, erosion time, particle velocity etc. [234, 237]. Over the years, solid particle erosion of metals and coatings has been reviewed time to time by Kosel [238], Engel [239], Preece and Macmillan [240], Hutchings [241], Finnie et al. [242], Ruff and Wiederhom [243], Shewmon and Sundararajan [244], Sundararajan [245], Levy [246] and many others [247-257, 25].
It is well known that resistance of engineering components encountering the attack of erosive environments during operation can be improved by applying hard ceramic coatings on their surfaces. Alonso et al. [258] experimented with the production of plasma sprayed erosion-resistant coatings on carbon-fiber- epoxy composites and studied their erosion behaviour. Tabakoff and Shanov [259] designed a high temperature erosion test facility to obtain erosion data in the range of operating temperatures experienced in compressors and turbines. In addition to the high temperatures, this facility properly simulates all the erosion parameters important from the aerodynamics point of view.