# Grain Orientation and Hardness in the Graded Interlayer of Plasma Sprayed W on CuCrZr
## 1 Introduction  
The selection of plasma-facing materials (PFMs) to protect structural components, of divertor and cooling systems, is of fundamental importance for realizing future nuclear fusion reactors. In the International Thermonuclear Experimental Reactor (ITER), PFMs withstand thermal loads ranging from 5 to 20 MW/m\({}^{2}\)[1,2] and high particle fluxes. Therefore, they must have a high melting point, excellent thermal conductivity, low activation under neutron irradiation, and very low physical sputtering and tritium retention [3]. Among all the plasma-facing materials (PFMs) considered for building armors, W is the most promising [4, 5, 6, 7, 8, 9], even though some of its problems are not yet completely solved (e.g., see the review of [PERSON] et al. [10]).  
The main drawback of bulk W is its bad machinability, which makes it difficult for it to be used to manufacture complex components, such as the armors protecting the pipes and fittings of cooling systems. To overcome this problem, a possible approach is based on the deposition of W coatings on the cooling system components made of a CuCrZr alloy [11]; however the coefficients of thermal expansion (CTE) of W and Cu alloys are quite different and high residual stresses may arise during cooling to ambient temperature, with consequent crack formation and coating detachment.  
Plasma spray (PS) in vacuum or a controlled atmosphere is a highly attractive coating technology owing to the possibility it offers of covering large surfaces, as well as itsflexibility and low costs. By using PS in a controlled atmosphere, 5 mm-thick W coatings on CuCrZr substrates were prepared [12; 13]; they were able to withstand heat fluxes up to 5 MW/m\({}^{2}\) without significant deterioration of integrity or mechanical properties [14]. An interlayer (\(\approx\)800 \(\upmu\)m thick) made of about 20 layers with different compositions was deposited between W and CuCrZr to provide suitable thermo-mechanical compatibility. The interlayer was built by first depositing a layer of pure Ni and then a stratification of layers consisting of graded mixtures of Al-12% Si and Ni-20% Al. The thermo-mechanical performance of the system's substrate interlayer coating was excellent [15; 16]; however, the presence of Ni represents a serious drawback for applications in nuclear fusion reactors owing to its high activation under neutron irradiation. Therefore, a new type of interlayer was investigated without other elements, except for those forming the coating and substrate. This consisted of Cu + W layers with relative amounts of the two metals, which gradually changed from substrate to pure W coating. The same approach was also used by other investigators to deposit W on martensitic steels for structural applications in fusion reactors [17; 18].  
This article reports some of the results of an extended study carried out through X-ray diffraction (XRD), light microscopy (LM), scanning electron microscopy (SEM), and nano-indentation to characterize the properties of each phase (W and Cu) along the interlayer from substrate to coating. In particular, XRD provides information about the microstructural features averaged over hundreds of W and Cu splats while nano-indentation was performed on each splat. In a previous paper [19], residual stresses were examined, and it was demonstrated that they had different signs in Cu (tensile) and in W (compressive); their maximum value reached in the inner part of the interlayer, where the relative amounts of the two deposited metals were similar. Here, our attention is focused on the trend of hardness in both metals along the interlayer.