In this chapter, various layers that are formed during sliding wear of thermal sprayed coatings at elevated temperature are discussed. Glazed layers are formed on the worn surfaces during elevated temperature sliding wear of thermal sprayed coatings. These layers have a characteristic physical appearance, mechanical properties, chemical compositions, and failure mechanisms. Wearing conditions, wearing material, and mating material influence formation and characteristics of glazed layers. Among these parameters, wearing material and mating material are most important. These glazed layers are divided into different types of layers, namely Transfer Layer (TL), Mechanically Mixed Layer (MML), Reaction Layer (RL), and Composite Layer (CL). The recent results on friction of thermal sprayed coatings at elevated temperature are rationalised in the light of different types of glazed layer formation.
TopIntroduction
Advancement of technology has enhanced the demand for tribosystem capable of running under severe conditions such as at higher relative motions, at higher applied loads, at higher temperatures, etc. Sliding wear at elevated temperature is a serious problem in a large number of engineering components in various industrial environment such as power generation, transport, materials processing, high temperature bearing, impeller bearing of slurry pumps operated in waste tank, etc. (Taktak, 2006; Roy et al., 2004; Jiang et al., 2005; Mateos et al., 2001). In order to improve the sliding wear performance of materials at elevated temperature for enhanced elevated temperature performance, sometimes it is imperative to modify the surfaces or use coatings with superior oxidation resistance and mechanical properties without compromising bulk properties. Thermal sprayed coating in this respect is one of the most widely used coating technique which offers several advantages.
Thermal sprayed coatings are used extensively in several engineering components requiring resistance to wear at high temperature, such as fuel rod mandrel, forming tools, gas turbines, etc. (Lai, 1979, Watreme et al., 1996; Li, 1980). Li & Lai (Li & Lai, 1979) noted that at elevated temperature the friction and wear of thermal sprayed Cr3C2-NiCr coating is initially high due to stick-slip phenomenon and as the sliding distance increases these values decrease. Further, friction coefficient increases with decrease of sliding speed. An extensive work on room temperature sliding wear of this coating by Mohanty et al. (Mohanty et al., 1996) indicated decohesion at intersplat controls the wear rate at room temperature. Wear rate is proportional to applied load but friction coefficient decreases with increase of sliding velocity. Mateos et al. (Mateos et al., 2001) reported presence of two distinct zones on the wear versus sliding distance plot. Guilemany et al. (Guilemany et al., 2002) demonstrated an improvement of the sliding wear behaviour of similar coating by heat treatment through formation of Cr3C2 precipitates. However, these studies are confined to the understanding of the parametric effect of wearing condition on the wear rate and carried out primarily at ambient condition. So far, no attempt has been made to compile the state of art on the influence of test temperature on the wear behaviour of thermal sprayed coatings.
The objective of the present work is to give a systematic and brief survey of the current status and future trends of sliding wear of thermal sprayed coatings at elevated temperature. For the purpose of convenience this paper is divided into six sections. After introduction of the elevated-temperature wear in Section 1, background of elevated-temperature wear of metallic materials and thermal sprayed coatings is discussed in Section 2. Formation and mechanisms of formation of wear protective glazed layer is the subject matter in section 3. Classification of various types of glazed layers is made in section 4. The results of recent research are rationalised in section 5. Summary and direction of future research is the subject matter of section 6.