Residual stresses in tungsten deposited by plasma spraying: effect of substrate and processing methods

Owing to its high melting point, favourable thermo-mechanical properties and good thermal conductivity tungsten is a candidate material for the divertor armour of the international thermonuclear experimental reactor (ITER) [1-6]. Wthin coatings on CFC and graphite substrates have been already used in JET [7] and ASDEX-U [8] tokamaks. However, its high elastic modulus, high brittleness and the high thermal expansion mismatch (e.g. aCu ≈ 4αw and αFe-α ≈3αw) make the joining of W with other metals really challenging. This paper reports results of an experimental campaign carried out for investigating W thick coatings on different metal substrates. For realizing the joints, plasma spray (PS) technique has been used for its simplicity, possibility to cover complex extended surfaces and relatively low cost. Particular attention was paid to study the effect of different substrates and a suitable interlayer on residual stresses in W. Residual stresses were measured by X-ray diffraction at room temperature and high temperature. Deposition of W coatings has been performed on 3 different substrates: CuCrZr alloy (with and without interlayer), austenitic steel AISI 316 L, and martensitic steel AISI 420. The CuCrZr alloy has been used in the past as structural material for actively cooled plasma facing components of ; nuclear reactors (Tore Supra and JET) and selected as heat sink material for divertor parts of ITER [5]. It was possible to manufacture mock-ups for ITER by depositing up to 5mm-thick W coatings on tubular substrates of CuCrZr, which iwere able to sustain a remarkable number of thermal fatigue cycles under high heat flux (up to 5 MW/m2) [1-2]. The ;microstructural and mechanical properties of this coating were extensively investigated and the results demonstrated jits good characteristics [9-11]. Similar promising results have been obtained also by Chong et al [12]. For reactors like DEMO, the cooling tubes made of copper based alloys could not be used because of the high neutron fluence, so it is necessary to realize W coatings on tubes made of steel [13-14], In order to determine residual stresses induced by PS process and the effect of increasing temperature, for each sample, XRD peak profiles have been recorded in the 20 angular range 16°- 21° The spacings dM measured from the most intense reflections have been compared to the values dCak calculated taking into account the values reported in < JCPDS-ICDD database and the thermal expansion: (1) a = a0( 1+α-δ7] The strain e was then obtained by the equation: ε = dSper-dCalc/dCalc The tests on the W-CuCrZr system with and without interlayer evidenced the fundamental importance of the interlayer. If there is the interlayer the residual stresses are present only in the interlayer. On the contrary, stresses are present in W and many defects like cracks and pores arise as a consequence of the different thermal expansion of the metals during cooling. The tests on W deposited on steels evidenced that metals forming the interlayer and layering must be chosen with care. In the case of the W-AISI 316 L system the strain of W coating is not negligible reaching a value of ∼ 3 x 103 after heating whereas the strain of Al interlayer is low. In W-AISI 420 system the strain value for W is very low also after heating up to 460 °C. The joining exhibits good adhesion and microstructural characteristics. From XRD tests performed on different W joints it is evident the importance of an appropriate interlayer for a good adhesion between W coatings and substrates. The quality of joints critically depends on the interlayer stratification in order to protect the W coating and concentrate residual stresses in the interlayer.