22 The most sensitive part of any component from the 23 point of view of fatigue failure is its surface. The surface 24 sensitivity to crack initiation is further enhanced by other 25 forms of surface degradation (caused by loading and/or 26 environmental action). Therefore, the surface properties 27 considerably restrict the usage of classical engineering 28 materials for high-performance applications. Thermally 29 sprayed coatings and other surface modifications are often 30 used to increase component lifetime by preventing surface 31 degradation processes (Ref 1). The thermally sprayed 32 coatings are built of individual splats of different flattening 33 degrees, unmelted powder particles, in-flight resolidified 34 feedstock particles, small ÔdebrisÕ particles resulting from 35 splashing of individual splats at the point of impact, and 36 other features, all of that separated by a porosity network. 37 This complicated nature of thermally sprayed coatings 38 leads to significant differences in mechanical, thermal and 39 fatigue properties compared to bulk material of the same 40 chemical composition (Ref 2-5). Also, the mechanical 41 properties of the substrate material are often significantly 42 changed before and during the deposition process as a 43 result of sample preparation before spraying and/or the 44 thermal and mechanical loading of the substrate during 45 coating deposition.

46 The deposit can influence the fatigue properties of 47 coated part in two principal ways. Hard deposits often 48 show lower than bulk properties due to microcracking of 49 brittle splats. Thus, during fatigue loading the network of 50 small cracks may form or the branching of main fatigue 51 crack may appear in the coating (Ref 6). In this way, the 52 deposit can accommodate the deformation without 53 allowing a major crack to enter the coating/substrate 54 interface. The major fatigue crack then initiates on the 55 substrate surface close to large pores, abrasive particles 56 entrapped by grit-blasting, cracks formed during deposi-57 tion, etc. The crack initiation can be further inhibited by 58 constraining the local plastic deformation by hard deposit 59 as described in (Ref 7). Such mechanism requires dense 60 deposit at least neat the interface and good adherence 61 between the coating and substrate (Ref 7). 62 For softer deposits, near-bulk properties can often be 63 achieved, and considerable residual stresses may develop 64 as a result of particle quenching and deposit cooling. The 65 crack initiation sites can be either in the substrate or in the 66 deposit, depending on the fracture toughness, presence 67 and effectivity of stress concentration centers, and residual 68 stress. Compressive stress can inhibit crack initiation and 69 propagation and prolong fatigue life (Ref 7-10). On the 70 other hand, tensile stress will shorten the fatigue life. The 71 first layer of splats deposited on the substrate makes things 72 more complicated; its residual stress can be different from 73 the rest of the deposit (Ref 11) and may well control the 74 crack initiation in the substrate or act as a crack barrier for 75 cracks propagating from the deposit. Thus, the fatigue 76 resistance of bodies with deposits can be correlated to 77 multiple factors such as residuals stresses at different parts 78 of the deposit, deposit elastic modulus, microstructure, 79 fracture toughness. The properties and characteristics of 80 certain deposits can be found in the literature (Ref 4, 5, 81 12). Some of these deposits were chosen for the present 82 investigation. Alumina, chromia, and olivine sprayed with 83 water-stabilized DC plasma represent ceramics. The alloy 84 Ni-5wt.% Al sprayed by wire arc, DC plasma, and HVOF