Thermal spray is a generic term used to define a group of coating processes used to apply both metallic and non-metallic coatings. These coatings are usually defined by their hardness, strength, porosity, roughness, and wear resistance. In this chapter, the authors turn their attention to the mechanical and tribological properties of thermal spray coatings. The individual phase plays a very important role in determining the performance of the coating. However, evaluating the mechanical and tribological properties at a nano-level requires new test methods and their validation. In this chapter, elaborate discussion of some techniques to evaluate and analyze the mechanical and tribological properties of different thermal spray coatings is done. This chapter is intended to help the reader to firstly understand the basic principle and methods of characterization of thermal spray coatings using instrumented nanoindentation, nanoscratch, abrasive wear testing techniques, and secondly to get an idea of the recent techniques and review the research and development in the same field.
TopIntroduction
The most important advantage of thermal spray process, compared to the other coating processes, is that it has a great range of coating materials, coating thickness and possible coating characteristics. To predict their behavior, lifetime and application area, it is necessary to completely understand the relationships between technology, process parameters, microstructure and properties of the coatings. In the characterization of thermally sprayed coatings it is necessary to take into consideration their unique lamellar microstructure as shown in Figure 1.
Figure 1.
Diagrammatic representation of the cross-sectional view of thermal spray coatings
The purpose of this chapter in tribology is understandably the minimization and elimination of losses resulting from friction and wear at all levels of technology where the rubbing of surfaces is involved. Now a days, cermets (i.e ceramics and metals) are used for coating purposes. The hardness of ceramics and the toughness of metals when combined gives unconventional strength and wear resistance to these thermally sprayed coatings. In order to achieve the best performance of a thermal spray coating we need to know the relationships between the mechanisms of coating formation and its mechanical properties. While the mechanism of formation of coating has rather been well discussed and understood, the detailed information on its mechanical properties still remains a difficult job. The primary reason is the heterogeneity of the coating as shown in Figure 2.
Figure 2.
Typical microstructure of a wc-17co HVOF-sprayed coating showing WC grains and binding metal matrix
Now a days, analytical and numerical modeling of thermal spray coatings is possible using various software. These models are capable of demonstrating the complexity of gas-particle interactions like distribution of non-molten particles in a spray. Modeling and software simulation is also done to validate new spray techniques. Due to splattering up of molten coating particles at supersonic speed, pores are generated in thermally sprayed coatings. As a result of these pores, residual stresses are developed. Various research works are aimed at determining the mechanical and tribological properties of these coatings in order to develop coatings with superior performance.
In view of the previous discussion the objective of this chapter is to help the reader to firstly understand the basic principles and methods of characterization of thermal spray coatings using instrumented nanoindentation, nanoscratch techniques and abrasion wear testing and secondly to get an idea of the recent techniques, related sources of error and review the research and development in the same field. This chapter is not intended to describe the tribological properties and aspects of thermal spray coatings, rather it elaborately demonstrates the working principle of some of the most important characterization techniques to evaluate the mechanical and tribological properties.