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Top1. Introduction
The semi-empirical nature of crack growth relationships implies that, for every new material, the fatigue crack growth behavior has to be determined experimentally. Additionally, description of fatigue crack growth rates using the linear elastic fracture mechanics methodology has been shown to be invalid for flaws that are small in size. Small cracks can grow considerably faster than long cracks at equivalent values of the stress intensity factor range and this phenomenon was first identified by Pearson (1975) for an aluminum alloy. Suresh and Ritchie (1984) suggested the following broad classification of small cracks: (i) Micro-structurally small cracks, where crack size is comparable to the scale of some characteristic micro-structural dimension such as grain size; (ii) Mechanically small cracks, where the zone of permanent deformation around a crack may be comparable to its size; (iii) Physically small cracks, which do not lie in any of the above two categories but are merely smaller than 1–2 mm; and (iv) Chemically small cracks, where environmental stress corrosion mechanisms become more active below a certain crack size.
The 7075 series aluminum alloys are used for plate, extrusions, hand and die forgings in structural aircraft parts (Handbook, 1992; Starke & Staley, 1996). However, aluminum alloys have poor corrosion resistance and tribological properties at room and high temperatures (Picas et al., 2005; Wenzelburger, 2004). Therefore, for many applications like the aeronautic (Handbook, 1992; Starke & Staley, 1996), automobile (Gérard, 2006; Kim et al., 2005) and textile industries (Ravi et al., 2007) the improvement of the surface properties of aluminum alloys is necessary. The surface modification of mechanical structures and machinery improves resistance against severe environments (Ray, 1997). Surface hardening by special heat treatment or surface coating is one of the most popular methods for obtaining surface-enhanced properties. However, coating methods provide more variety in surface properties based on selecting the proper coating material according to the required surface characteristic.
Ceramic coatings are often used to provide mechanical, thermal and corrosion protection to industrial equipment components which, for reasons of cost, size or complex shape, would be impractical to fabricate from bulk advanced ceramics. Plasma sprayed ceramic coatings are widely used but, because of their complex anisotropic, lamellar structure, their characteristics are quite different from those of bulk advanced and they are still not well- understood(McPherson, 1997). WC-Co is one of the most useful cermets thermal spraying materials. But for the traditional thermal spraying technology, it's difficult to gain the high quality cermets coatings for high porosity and poor bonding of the coatings (Jin et al., 2007). The combination of alumina ceramics (Al2O3) with metals is widely used in aircraft, automotive industry and microelectronics as an electro-insulating and substrate material (Tummala & Rymaszewski, 1989). Few researchers have worked on the oxidation behavior of INAL Al2O3 composite coatings with varied Al2O3 (Vasudev et al., 2020), have shown that the coating with 30% of Al2O3 will have maximum oxidation resistance. Much amount of work has been documented on the wear behavior of the Ni, Al2O3, and WC coated materials (Prashar et al., 2020; Vasudev, 2020; Vasudev et al., 2018).