Solid Particle Erosion of Thermal Sprayed Coatings

1 Introduction

Solid particle erosion, more popularly known as erosion or erosive wear is essentially degradation of materials due to impact of moving particles. Erosion is mechanistically different from liquid erosion, cavitation erosion etc. Erosion is a common occurrence and particularly a matter of considerable technical and economical importance. Erosion of choke valves used in oil and gas industry, erosion of steam pipes by entrained iron oxide particles in steam turbines, erosion in fluidized bed combustion systems are some typical examples of erosive wear. Although erosion is generally regarded as a material degradation process, in some cases erosion is a useful phenomenon, as in sand blasting and high-speed abrasive water jet cutting. (Reddy & Sundarrajan,1986; Kosel,1992). The inconsistency in prediction of erosion resistance of engineering materials based on improvement of mechanical properties by various strengthening mechanisms (Roy, Subramaniyan & Sundarrajan, 1992; Roy, Tirupataiah & Sundarrajan, 1993; Roy, Tirupataiah & Sundarrajan, 1995), alteration in microstructure by thermal / mechanical treatment has been the major impetus to the development of erosion resistant surface coatings. Design modification and change of service parameters are often unsuitable solutions to reduce erosion severity. Therefore in many cases especially in industries where service parameters and design are fixed, the most practical route is based on use of alternative material or surface coatings. Since the properties required for wear control are often specific to the surface, use of surface coatings for erosion mitigation had been widely practiced in the past. Several coating techniques ranging from surface hardening, weld hardfacing, chemical vapour deposition, and physical vapour deposition to ion implantation are currently in practice for erosive wear protection. Table 1 gives some of the manufacturing techniques to produce wear resistant surface (Schmidt & Steinhauser, 1996). Weld hardfacing, although considered sometimes as a ‘black art’, offers several economic advantages. The use of hard coatings offers several advantages over bulk material since surface properties can be engineered to be independent of the bulk material to produce a composite system and use of cheaper substrate material can significantly reduce the cost of the product. The selection of a particular coating technique is dictated by several factors such as coating thickness, mechanical properties, morphology and microstructure of the coating as influenced by the manufacturing techniques and its cost, used for coating deposition, coating defects and most importantly the operating conditions such as erosion mode, properties of erodent particles, fluid flow variables such as impact angle and the impact velocity. In this context thermal spray coatings have been widely used for protection of industrial components against solid particle erosion (SPE). Thermal spray coatings offer attractive mechanical properties and are being used for wear, abrasion, erosion and corrosion protection of engineering components in automotive, power plants and mining industries. The erosion resistance of thermal spray coatings relies on use of hard ceramic second phase particles such as carbides and borides in a relatively softer matrix; a binder phase. Figure 1 shows use of different reinforcement phases in thermal spray coatings metals and metalloids. The dispersion of a ceramic phase in a more ductile matrix to improve hardness and toughness has been successfully used for improving and abrasion and erosion resistance of dual phase materials such as tool steels, high chromium cast irons, sintered WC-Co alloys and composite materials. In view of the above, an attempt has been made to examine the performance of solid particle erosion of thermal spray coatings and highlight the development in recent past. Present chapter has been divided under seven sections. After introducing solid particle erosion of thermal sprayed coatings in the first section, evolution of thermal spraying and various testing techniques are discussed in section two and three respectively. Erosion behavior of coatings is elaborated in section four. Applications of thermal sprayed coating for erosion resistant applications are provided in section five. Finally, direction of future research and concluding remark is summarized in section six and seven.