In this work, the influence of the thermal spraying processes, atmospheric plasma spraying (APS), high velocity oxygen fuel (HVOF), and vacuum plasma spraying (VPS), on the microstructure and properties of copper coatings is discussed. The differences in microstructure, microhardness, and residual stress for each type of coating are shown. The X-ray diffraction (XRD) method is used to evaluate the mechanical anisotropical characteristics of the materials and the residual stress distribution. The particularity of this study is that the thickness of the coating is of the millimetre scale; the massive coating specimens without a substrate were obtained for XRD microelastic modulus measurements and for macrotensile measurement. Microhardness distributions have been obtained in coatings and in substrates that pass through the interface zones. It was observed that dense coatings could be obtained by industrial processes such as APS and HVOF, but they induce oxidation in the copper coating. The copper oxidation is limited in the VPS process. The elastic modulus values measured by the in situ XRD method show that the HVOF coating has more rigidity than the APS coating. HVOF and VPS copper coatings have higher microhardness values than the APS copper coating. The residual stress values are in traction in the APS, VPS, and HVOF coatings: the residual stress level in the HVOF and APS coatings is less important than that in the VPS coating. The microstructure observations show that the VPS copper coating has a recrystallised structure whereas the APS and HVOF copper coatings have a splat type structure. As a consequence, the VPS copper coating has a high Young's modulus, an important mechanical resistance, and a high elongation as compared with the other copper coatings.
Stiffening of bulk niobium SRF cavities is mandatory for reducing the frequency shift induced by Lorentz forces at high accelerating gradients. Experimental and computational data previously reported show that with the actual scheme (i.e. EB welded stiffening rings) the frequency shift of TESLA 9 cells SRF cavities is higher than the cavity bandwidth above Eacc=28 MV/m. We propose a new stiffening method, using a Plasma Sprayed Copper Layer (PSCL) onto bulk niobium cavities. As compared to the actual technique, this method offers several advantages (simplicity, reliability...). The first experimental data obtained with monocell cavities produced by this method demonstrate the efficiency of cavities stiffening with plasma spraying. Thermal and mechanical properties measured on niobium samples with a PSCL are also presented. These data will allow us choose the plasma spraying process suitable for achieving the best cavities performances.
This paper presents a numerical method in order to forecast the thermo-mechanical behavior and the residual stresses in the thermal spray coating process by using high velocity oxygen fuel (HVOF). A set of coupled equations of the heat conduction and stress/strain and solidification based on the metallo-thermo-mechanical theory is introduced into the simulation of thermal spraying. Here, an inelastic constitutive equation with capacity to represent relation of stress/strain during rapid solidification is employed. The numerical modelling based on the finite element method is proposed to solve the heat conduction associated with solidification in the sprayed layer and residual stresses on the interface between multi-layer materials, especially. In this paper, the simulated results of the temperature field, solidified domain and residual stresses in the sprayed layer including interfacial combinations between substrate and spray layer are presented, and the validity of the calculated results is discussed in comparison with the measured results obtained by X-ray diffraction.
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