C. Verdy

Comparative Study of Mechanical Properties and Residual Stress Distributions of Copper Coatings Obtained by Different Thermal Spray Processes

Abstract 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.

Caractéristiques mécaniques d’assemblages réalisés par projection thermique

Les concepteurs privilegient de plus en plus l’emploi de structures a parois minces, entrainant des pertes consequentes de matiere entre les bruts initiaux et les pieces finies lorsque les methodes classiques d’usinage sont employees. Par ailleurs, certains alliages nouvellement mis au point sont delicats a mettre en forme par les methodes classiques, comme l’alliage a base de nickel NK17CDAT (Astroloy) par exemple. La conjugaison de ces deux aspects constitue le moteur du developpement d’une nouvelle methode de mise en forme utilisant la technique de projection thermique ; cette methode peut etre utilisee pour realiser une forme complete a partir d’un support sacrificiel, ou bien permettre un complement par liaison directe avec une partie plus massive, notamment dans le cas des structures disposant de cavites.

High heat flux thermal cycling of FGM thermal barrier coatings using a HVOF gun

The aim of this work was to determine the behavior of multi-layered and FGM structures under high heat fluxes. A direct simulation by the implicit finite difference method was used to predict the transient temperature distribution in each layer of the multimaterials. Experimentally, high heat fluxes were produced using a HVOF gun operated with a methane-oxygen mixture and internally cooled samples. Multi-layered deposits (Copper alloy / MCrAlY / Partially Stabilized Zirconia) were sprayed using a plasma torch under controlled atmosphere (VPS) onto a copper block containing cooling channels for a circulation of water. Transient and static tests were performed with heat fluxes up to 100 MW/m 2 for durations of a few minutes to several hours. Microstructural characterizations of deposits were performed by optical microscopy after thermal tests in order to determine the erosion resistance of the PSZ coating for both cases. Results show a better erosion resistance of the FGM (MCrAlY / PSZ) thermal barrier coating.