Protection against Erosive Wear Using Thermal Sprayed Cermet

Abstract

PrefaceThe erosion caused by solid particles under extreme conditions (high hardness and velocity of the erodent, cyclic load, and high temperatures) is problematic for industrial equipments. As function of requests, the tension generated in the particles and/or in the target material is, approximately, one order of magnitude greater than its mechanical strength, and as a result, the material can be damaged.Many industries have invested in technology to minimize damage caused by degradation of the materials under erosive wear. In industrial environment, this type of wear occurs when abrasive particles are mixture or carried through pipelines, fans and cyclones, or, in equipments such as mills and mixers, by the inherent displacement of the load. This type of wear was found in mineral processing and separation industries. At high temperatures, for example, these processes take place in the energy-generation industry (thermal plants), in steel and cement production, paper production, and in the petrochemical industry.The erosion at high temperatures leads to the deterioration of parts or components of machines, turbines, engines, and boilers that operate with particulate flows and, as a consequence, shortens their useful lifespan. In the petrol industry, for example, the erosion in boilers powered by powder fuel contributes to approximately 25% of the failures of these equipments, attributed to erosive wear caused by fly ashes.Regarding erosion at high temperatures, several authors mentioned earlier suggest different regimens that contribute to damage of the material under erosion-oxidation, but there is no accordance among the findings by these authors. It is known that different variables in the process of erosive wear can lead to damage of the material by different mechanisms. However, due to the effect of temperature on the material, an oxide is formed, establishing the wear mechanisms. This mechanism result from erosion of the oxide and/or erosion of the composite (formed by oxide and substrate), making the approach for determining the wear phenomenon more complex.The erosion of cermets (bulk and coating), more specifically at high temperatures, has been the subject of many researchers, although few of these studies are conclusive, and even fewer, agree with each other. In the case of cermet coatings, the complex microstructure, due to the lamellar formation can make even more difficult the understanding of the phenomenon responsible for erosion. Besides, as Stack and Pena (1997) noticed, in their studies of an alloy Ni13%Cr with WC particles at temperatures as high as 650 o C, the change in the mechanical properties of the material due to temperature can contribute to increase in the complexity of the behavior of the wear of this kind of material. VI PrefaceThe use of coatings such as cermets have shown excellent results with respect to the high strength against the erosive wear; however, very little is known about the mechanisms that lead to the degradation of this kind of material, at different work tempera...