**1.2 Erosion wear**

Erosive wear of the solid bodies is caused by the action of sliding or impact of solids, liquids, gases or a combination of these [1]. Erosion can be divided into three basic types: Solid particle erosion, liquid impact erosion and cavitations erosion. Cavitation erosion is the loss of material due to the repeated formation and collapse of bubbles in a liquid. Liquid impact erosion is the damage caused by water droplets. Solid particle erosion is a wear process where they strike against surfaces and promote material loss. It is also caused by the impact of hard particles carried by a fluid stream onto a material surface.

Solid particle erosion is an important material degradation mechanism encountered in a number of engineering systems such as thermal power plants, aircraft gas turbine engines, IC engines, pneumatic bulk transport systems, coal liquefaction/gasification plants and ore or coal slurry pipe lines. At the same time, the erosion process has been used to advantage in number of situations like sand blasting of castings, shot peening of rotating components, cutting of hard and brittle materials by abrasive jets and rock drilling [2, 3].

Manifestations of solid particle erosion in service usually include thinning of components, a macroscopic scooping appearance following the gas/particle flow field, surface roughening and lack of the directional grooving characteristic of abrasion and in some the formation of ripple patterns on metals. Solid particle erosion can occur in a gaseous or liquid medium containing solid particles. In both the cases, particles can be accelerated or decelerated and their directions of motion can be changed by the fluid [4].

Power station boiler-walls and other utility parts of coal-fired plants are subjected to frequent degradation by erosion–corrosion problems relevant to the reliability and economics of these installations. The environment inside the furnaces is characterized by high-temperature conditions together with aggressive atmospheres, leading to corrosive deposits adhering into the walls and to erosion processes caused by the ash particles [5].

In erosion, several forces of different origins may act on a particle in contact with a solid surface. Neighboring particles may exert contact forces and a flowing fluid, if present, will cause the drag. On some situations, gravity may also be important. However, the dominant force on an erosive particle, which is mainly responsible for decelerating it from its initial impact velocity, is usually the contact force exerted by the surface. Erosion of metals usually involves plastic flow, whereas more brittle materials may wear predominantly either by flow or by fracture depending on the impact conditions [6].

Solid particle erosion behavior of most of the materials can be categorized as being either brittle or ductile in nature [7]. The major differentiating characteristic of the two types of mechanism is the dependence of erosion rate on impact angle i.e. the angle between the moving erodent particle and the material surface [8]. There is general agreement that maximum erosion occurs at a low angle (about 300) for ductile material and at 900 for brittle

Fine microstructures with equiaxed grains and without any type of columnar defects

Erosive wear of the solid bodies is caused by the action of sliding or impact of solids, liquids, gases or a combination of these [1]. Erosion can be divided into three basic types: Solid particle erosion, liquid impact erosion and cavitations erosion. Cavitation erosion is the loss of material due to the repeated formation and collapse of bubbles in a liquid. Liquid impact erosion is the damage caused by water droplets. Solid particle erosion is a wear process where they strike against surfaces and promote material loss. It is also caused by the

Solid particle erosion is an important material degradation mechanism encountered in a number of engineering systems such as thermal power plants, aircraft gas turbine engines, IC engines, pneumatic bulk transport systems, coal liquefaction/gasification plants and ore or coal slurry pipe lines. At the same time, the erosion process has been used to advantage in number of situations like sand blasting of castings, shot peening of rotating components,

Manifestations of solid particle erosion in service usually include thinning of components, a macroscopic scooping appearance following the gas/particle flow field, surface roughening and lack of the directional grooving characteristic of abrasion and in some the formation of ripple patterns on metals. Solid particle erosion can occur in a gaseous or liquid medium containing solid particles. In both the cases, particles can be accelerated or decelerated and

Power station boiler-walls and other utility parts of coal-fired plants are subjected to frequent degradation by erosion–corrosion problems relevant to the reliability and economics of these installations. The environment inside the furnaces is characterized by high-temperature conditions together with aggressive atmospheres, leading to corrosive deposits adhering into the walls and to erosion processes caused by the ash particles [5].

In erosion, several forces of different origins may act on a particle in contact with a solid surface. Neighboring particles may exert contact forces and a flowing fluid, if present, will cause the drag. On some situations, gravity may also be important. However, the dominant force on an erosive particle, which is mainly responsible for decelerating it from its initial impact velocity, is usually the contact force exerted by the surface. Erosion of metals usually involves plastic flow, whereas more brittle materials may wear predominantly either by

Solid particle erosion behavior of most of the materials can be categorized as being either brittle or ductile in nature [7]. The major differentiating characteristic of the two types of mechanism is the dependence of erosion rate on impact angle i.e. the angle between the moving erodent particle and the material surface [8]. There is general agreement that maximum erosion occurs at a low angle (about 300) for ductile material and at 900 for brittle

 High deposition rates are possible without huge investments on capital equipment. The process can be carried out virtually in any environment such as air, encoded inert

are the characteristics of this process.

**1.2 Erosion wear** 

low and high-pressure environments, or underwater.

impact of hard particles carried by a fluid stream onto a material surface.

cutting of hard and brittle materials by abrasive jets and rock drilling [2, 3].

their directions of motion can be changed by the fluid [4].

flow or by fracture depending on the impact conditions [6].

material. Figure shows the schematic of the expected variation in erosion behavior with impact angles

Figure Expected Variation of Erosion Rate with Particle Impact Angle (Ref. 8)

Most applications involve low impact angles at which erosion resistance of ceramics happens to be significant. It should also be noted, that the microstructure of plasma sprayed coatings often differs significantly from that of corresponding bulk material. The structure of plasma sprayed coatings consists of many overlapped lenticular splats which conform more or less either to the morphology of the underlying substrate or to that of previous splats. Although plasma sprayed coatings are anisotropic, their erosion rates tend to exhibit the same dependence on impact angle similar to that of the bulk material of ceramics [9]. On the other both Kingswell [10] and Zhang [11] have noted that the erosion mechanism in plasma sprayed alumina coating is different from those in bulk sintered alumina. Erosion of bulk ceramics generally occurs by a number of fracture mechanisms [12, 13]. During particle impact upon a ceramic surface, median and radial cracks develop at the impact site [14]. Upon rebounding of the particles i.e. unloading of the impact site, lateral cracks develop parallel to the surface and finally follows a curved path before propagating towards the surface, leading to chipping and loss of material. Erosion in plasma sprayed ceramics has been attributed to the failure of the individual splat boundaries.
