# Grain Orientation and Hardness in the Graded Interlayer of Plasma Sprayed W on CuCrZr
## 3 Results and Discussion
The interlayer consists of Cu and W splats with irregular shapes (Figure 3) and, as shown for residual stresses in a previous paper [19], the properties of W and Cu vary along the interlayer. The results presented in this article evidence how the hardness of the two phases changes with the distance from the substrate. In fusion reactors, during service, W armors are irradiated by energetic neutrons, leading to the formation and accumulation of lattice defects and to transmutation with products such as Re, Os, Ta, H, and He [26]. Moreover, W is also exposed to direct bombardment by H and He from plasma [27]. An extensive description of the physical phenomena occurring in W during H and He interactions is given in [28]; here, it is noteworthy that at high temperatures, H and He atoms have high mobility for interstitial diffusion towards defects, resulting in increasing hardness, which is to the detriment of  
Figure 10: Hardness, \(H\), trends of Cu and W across the interlayer. ductivity. This is a serious concern for the long-term structural integrity of armors; thus, the presence of a hardness gradient in the interlayer before irradiation must be well understood. Therefore, to find the causes of the specific trends displayed in Figure 10, the grain size, dislocation density and texture of both phases were considered the main factors that play a role in the plastic deformation of pure metals. The results of the microstructural examinations clearly indicate that variations in grain size or dislocation density could not have been responsible for the hardness trend of W: the average thickness \(d_{T}\) and width \(d_{W}\) of W lamella-shaped grains is substantially the same in the whole interlayer (Figure 4), while the dislocation density slightly decreases, moving towards the coating. Analogous considerations can be made for Cu. Thus, our attention was focused on the texture of both phases, which shows significant changes depending on the specific position in the interlayer. The characteristics of the deposited metal are critically affected by the interaction with the substrate; however, the specific mechanisms of nucleation and growth depend on the technology used for depositing the metal and process parameters (for an example, see [29; 30; 31]). The topic is too complex to be explored exhaustively owing to the large variety of deposition techniques and the many parameters involved in each of them. Recently, [PERSON] et al. [31] described the growth of yttria-stabilized zirconia (YSZ) coatings prepared by means of plasma spray-physical vapor deposition (PS-PVD) as a three-stage process, as follows: (i) the formation of many randomly orientated grains of small size, (ii) the development of these small grains into elongated grains without preferred crystallographic orientation, and (iii) the reduction of the number of columnar grains and the coating growth along preferred orientations. The study clearly highlights the complexity of phenomena occurring in PS deposition; however, the model cannot be directly adopted to explain the textures observed in the experiments reported in this article because a large part of powder feedstock in PS-PVD is evaporated whereas in PS the metal is deposited in the liquid state (droplets of Cu or W). As a consequence, textures originated from preferential grain growth in PS-PVD, and from repeated nucleation and growth in PS. As shown by Figure 7a, the substrate of CuCrZr alloy exhibits a strong \(<\)110\(>\) texture with a secondary \(<\)100\(>\) component. Thus, in the first deposited layer (90% Cu and 10% W), the droplets of liquid Cu impact directly on the substrate and in large part solidify with the same orientation of the CuCrZr grains. This phenomenon also occurs in the successive layers up to a distance of \(\sim\)450 \(\upmu\)m from the substrate; it then progressively weakens as the amount of W in the interlayer increases and completely disappears near the W coating, where Cu grains have a nearly random orientation.