Slurry erosion is a degrading phenomenon usually observed in machineries dealing with particle-laden fluid such as hydro power plants, ship propellers, pump impellers, valves, and connecting pipes. The low erosion resistance of commonly employed structural materials prompts the use of different surface modification techniques. Among several types of surface modification techniques, thermal spraying has achieved a significant recognition worldwide due to its versatile nature. In this chapter, slurry erosion behavior of thermal sprayed coatings has been discussed with special emphasis on the contribution of different coating related parameter. It has been observed that microstructure play an important role in determining the slurry erosion performance of thermal spray coatings. Different microstructural features such as splat boundaries, pores, un-melted particles, and cracks are detrimental for the thermal spray coatings exposed to erosive environment. A parameter useful for identification of primary erosion mechanism for thermal sprayed coatings is also discussed.
TopIntroduction
Slurry erosion is a type of wear phenomenon in which the degradation of the target surface takes place due to impact of solid particles entrained in liquid medium. During impact, significant amount of complex stresses accompanied by high strain rates originate in the target surface (Hutchings & Winter, 1974). This result in significant deformation of the target surface eventually leading to loss of material. The problem caused by slurry erosion is highly severe and this results in the loss of performance of many fluid machineries and components such as pumps, hydroturbines, propeller, control values and connecting pipelines to name a few (Mann & Arya, 2002). The direct and indirect economic costs involved in repair and maintenance of the slurry affected components could be enormous (Grewal, Bhandari, & Singh, 2012; Mann, 2000; Mann & Arya, 2001).
Various factors that affect the slurry erosion performance of a target material can be categorized into following three categories(Finnie, 1995)
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Factors dependent upon the operating parameters such as impact velocity, impingement angle, concentration of erodent particles, viscosity, density and temperature of the carrier fluid.
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Factors dependent upon the erodent particles such as size, shape, composition, hardness.
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Factors dependent upon the target material such as yield and ultimate strength, ductility, hardness, fracture toughness, microstructure and composition.
Other than these factors, it has been observed that restitution coefficient, which is a complex function of different mechanical properties of target and erodent materials and the operating parameters such as impact velocity and angle (Hussein & Tabakoff, 1974) has been observed to play an important role (Grewal, Agrawal, & Singh, 2013a; Papini, 2003).
It is understood that with an increase in velocity of the erodent particles, the slurry erosion rate also increases (Kleis & Kulu, 2008). It has been observed that slurry erosion rate exhibits a power-law relation with velocity as shown in Equation (1)
E = KVn(1)Here ‘E’ represents the slurry erosion rate, ‘K’ is the proportionality constant, ‘V’ is the erodent velocity and ‘n’ the velocity exponent. It has been normally observed that the value of ‘n’ varies in between 2 to 5 (Kleis & Kulu, 2008). However, value below two has also been reported (Grewal, Agrawal, & Singh, 2013a; Shivamurthy, Kamaraj, Nagarajan, Shariff, & Padmanabham, 2009).
Impingement angle of the erodent particles has also been observed to significantly influence the slurry erosion performance of the materials. It has been generally observed that materials exhibit maximum slurry erosion either at the normal impingement angle or at bleak angles typically between 20° to 30°. Materials showing maximum erosion rate at bleak angles are said to exhibit ductile mode of erosion (Finnie, 1995; A.V. Levy, 1995). However, materials for which maximum erosion takes place at 90° are termed to exhibit brittle mode of erosion. This type of subtle categorization based upon the dependence on impingement angle can be related to the erosion mechanism (Levy, 1995; Stachowiak & Batchelor, 2005).