Repeated loading can start a fatigue mechanism in the material, leading to the nucleation of a small crack, followed by crack growth, and ultimately complete failure. Cyclic loading, which is below the yield stress (in the elastic region) initiates very local plastic deformation as a result of repeated dislocation slipping and initiation of non-reversable changes in the microstructure. Fatigue failure requires first the initiation of a crack, usually on the surface, thus it is dependent on the surface quality. After initiation, the crack growth rate is dependent on the conditions at the crack tip and is controlled by the load amplitude, frequency, and the material’s ability to resist crack growth. As the crack initiation and growth rate are dependent on the stress field in the crack tip, it can be influenced by residual stresses. A compressive stress state on the surface is one of the clear benefits observed to improve the fatigue resistance of the components [111]. Several surface treatment processes that produce compressive residual stress, such as shot peening and laser shot peening, have been utilized to improve the fatigue resistance of shafts, gear
wheels, and other components. It is also known that many surface thermo-chemical treatments such as nitriding and carburizing increase the fatigue life of steel due to the compressive residual stresses they produce [112].
Studies on the effect of applied thermally sprayed coatings on the fatigue life of components show that the coating may have either a positive or negative influence.
Several studies show that various HVOF coatings decrease the fatigue life of the component [3,62,113–115]. Only a few studies show that a positive influence on the fatigue life can be achieved by thermal sprayed coatings with compressive residual stresses [73,116], or by increases in the total stiffness of the component thanks to the application of the coating, thus increasing the fatigue life [117]. In the research of Vackel and Sampath [73], shown in Fig. 12, it was shown that a HVOF-sprayed WC-CoCr coating (S + DJB) with high compressive stresses increased the fatigue performance compared to steel (S), while a HVOF-sprayed WC-CoCr coating (S + DJA) with tensile residual stresses decreased it.
Figure 12. Fatigue life of WC-CoCr coatings sprayed by HVOF compared to steel. Coating can either improve the fatigue performance if the coating is in compression (S+DJB) or worsen the fatigue performance if the coating is in tension ((S+DJA), compared to steel (S). [73]
It is agreed that the fatigue failures of solid materials usually start on the surface of a fatigue specimen or at the locations where the highest tensile stress concentrations have been generated [118–120]. The mechanism of fatigue crack initiation and growth in coated structures have been investigated in relatively few studies but various factors have been shown to influence fatigue life. In the case of coatings,
under the coating. In several publications, cracks have shown to be initiated directly from the surface of the substrate, most typically from irregularities or grit blasting residues at the coating-substrate interface [115,121–123]. García et al. pointed out that surface roughening produces irregularities for crack initiation, which decreases the fatigue life; on the other hand, the grit blasting process produces compressive stress in the surface, which may increase the fatigue life [116]. However, in some situations, cracks may initiate in the coating. The coating structure, which is full of voids, increases the number of potential locations for crack initiation. Several authors have shown that cracks can initiate in the coating at the locations of weak lamella bonding or pores in the coating [116,117,124]. Zhu et al. also showed crack initiation on the coating surface [125]. Thermally sprayed coatings are relatively brittle and their load carrying capacity is limited. Therefore, perhaps the most important aspect is how the coating influences the crack initiation of the component if the crack has initiated from the coating. Research on the fatigue behavior of bi-layered structures shows that, once initiated, crack propagation is associated with the direction in which the crack approaches the interface. Moreover, it is influenced by the plastic properties of the coating and substrate. If the crack approaches from the less brittle material toward the brittle material, the crack continues to advance through the interface. If the crack approaches the interface from the plastically weak material to the material which can plastically deform, the surface layer can behave as a crack arrestor [126]. Moreover, coating adhesion plays an important role on crack growth in the vicinity of the interface. It has been shown that if the adhesion of the coating is low, the crack can start to advance along the interface and thus delamination of the coating may occur [124]. [3,54,62,73,113,116,117,123,127–129]
5 MATERIALS AND METHODS
There are several parameters in high kinetic thermal spraying that affect the deposit formation and hence the coating properties. The differences in parameters are the greatest between the coating processes such as first generation HVOF, high pressure HP-HVOF, and HVAF. In addition, each coating device may have different hardware setups, which significantly affect the coatings. Finally, the processing parameters are important controllable factors for optimizing the coating microstructure and properties. The main controllable outputs in the high kinetic process are particle temperature, velocity, and melting state. By utilizing in-situ monitoring of the thermal spray process, direct measurements from the spray process can be collected. In this way the relationship between the processing, structure, properties, and performance of the coatings can be determined. By using the on-line particle sensor in flight conditions, T and v, can be monitored, and by using the in-situ curvature monitoring technique direct measurements of the particle impact, quenching, and thermal history can be recorded. Finally, curvature monitoring and the data that it generates allow the determination of the residual stress distribution in the coating by using analytic models, for example.