Characterization and adhesion strength studies of thick copper coatings deposited on SS316L steel by cold-spraying

Abstract

Thick copper (Cu) claddings/coatings with near bulk Cu properties have been recommended for plasma-facing components inside the Vacuum Vessel (VV) of the Tokamak Reactors for a smooth operation. Work is in progress in the International Thermonuclear Experimental Reactor (ITER) organization for the development of Cu-coatings having desired properties with a partial success. As per ITER recommendation, there is a need to develop such coatings on SS304/SS316 with an acceptable combination of adhesion strength, thermal and electrical conductivity, mechanical and metallurgical properties. Even on fundamental research level, there are contradictions in the literature on adhesion mechanism at the interface of cold-spray coatings. Therefore, the present research work is focused on the development of 2–3 mm-thick Cu-coatings for SS316L steel substrates, for which three coating techniques; laser-cladding, low-pressure cold-spraying (LPCS), and high-pressure cold-spraying (HPCS) have been explored. For in-depth investigation of the developed coatings, various characterization techniques viz. scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), X-ray diffraction (XRD), and mechanical testing including microhardness, nanoindentation, macro-tensile, and adhesion testing were used. Moreover, porosity, density, electrical and thermal conductivity measurements have also been made. Based on the preliminary studies of coatings developed by using the LPCS and laser-cladding techniques, the cold-spraying process was found to be a better option compared to the laser-cladding technique for the intended application. The coatings developed by LPCS were found to be suitable due to their higher conductivities (which is the most significant requirement for the intended application) alongwith lower porosity compared to the coatings developed by using laser-cladding technique. The mechanical properties alongwith adhesion strength were found to be better for the laser-cladding, however with a significant presence of heat-affected zone, which is detrimental for the given application. With regard to strength, the literature indicates that the mechanical properties of the cold-sprayed coatings can be improved by increasing the operational pressure and altering the surface characteristics of the substrate surface. Therefore, it was decided to use HPCS technology to develop 2–3 mm-thick Cu-coatings, which have been investigated in-depth in the current work. In the first approach, the role of substrate surface on the adhesion strength as well as the coating properties has been investigated. In this study, three surface roughness levels (Ra = 5 ± 1.23 μm, Ra = 0.50 ± 0.14 μm, Ra = 0.06 ± 0.01) were used for the substrate surface, which are designated as as-received, semi-polished, and mirror-polished respectively. To understand the bonding mechanism of the coating with the substrates, SEM and EDS analyses were done on interfacial surfaces; exposed after the adhesion testing. Additionally, single-particle deformation behaviour for the developed coatings has been investigated by using SEM/EDS, to elucidate the mechanism behind bonding at particle level. The results of the study validated that the adhesion is governed by both metallurgical bonding, as well as, mechanical interlocking, owing to the jetting formation. The mirror-polished surface resulted in a better adhesion strength compared to the as-received and semi-polished surfaces, which was further validated by ultrasonic testing. This may be attributed to adequate substrate/particle deformation which led to a proper jetting formation. However, it was observed from the single-particle deformation behaviour for the semi-polished surfaces that the bulged-out sharp peaks of the surface create hindrance to the particle deformation and jetting formation, which resulted in poor mechanical interlocking without any metallurgical bonding. Similar hindrance to the deformation was created by peaks and valleys of the as-received substrates, leading to a weak bonding at the interface. Further, the effect of substrate surface roughness on several properties of cold-sprayed Cu-coatings upto a certain depth was investigated. Effect of post-heat-treatment on the properties of the developed coatings was also studied. The coatings developed on the smooth surface were found to have better mechanical and physical properties compared to the coatings developed on the semi-polished and as-received substrate surface; prior as well as post-heat-treatment. In the second approach, interlayers of Cu and nickel (Ni) were introduced by electroplating the steel substrate, in order to improve the adhesion. Subsequently, thick Cu-coatings were deposited on the electroplated steel substrates by HPCS. The effect of these electroplated interlayers on the adhesion mechanism between cold-sprayed copper and steel substrate was studied in detail. Also, the effect of these electroplated interlayers on several properties of cold-spray Cu-coated SS316L steel was investigated. The effect of heat-treatment on the successfully developed coatings was analyzed in this case also. It was observed that the coatings deposited by introducing Ni-electroplating of the steel substrate were found to have better mechanical and physical properties compared to the coatings developed on the Cu-electroplated and as-received substrates; prior as well as post-heat-treatment. Moreover, the Ni-electroplated steel substrate surface resulted in a better adhesion strength compared to the as-received and Cu-electroplated substrate surfaces, which was also validated by ultrasonic testing. This could be attributed to adequate particle/electroplated Ni-layer deformation which led to a proper jetting formation owing to the smooth surface of the electroplated substrate. Moreover, it is mentioned in the literature that Ni has a higher affinity towards metallic bond formation with Cu and steel elements. It was observed from the single-particle deformation behaviour that the particles got penetrated inside the electroplated interlayer and resulted in the jetting formation, hence, mechanical as well as metallurgical bonding may have occurred. However, in the case of Cu-electroplated interlayers, the affinity of Cu to share their electron for making a bond with the steel substrate is low, which may have led to the breakdown of the Cu-electroplated layer during the cold-spray process. The breakdown of electroplated interlayer finally resulted into detachment of the cold-sprayed coating from the steel substrate. Electrical and thermal conductivities have also been evaluated for all the cold-sprayed Cu-coatings. The conductivities were found to be influenced by the material, as well as, the surface asperities of the substrate; attributable to the deformation behaviour of first few cold-sprayed Cu-layers. The deformation of these inner layers also influenced their density and porosity, which in turn influenced the coating properties. Electrical as well as thermal conductivities of the cold-sprayed Cu-coatings deposited on mirror-finished and Ni-electroplated substrates were found to be closest to the actual requirement for the intended application. Moreover, conductivities higher than the required values could be achieved for both these coatings after their heat-treatment, due to the reduced porosity owing to inter-splat diffusion and refined microstructure. Additionally, in-situ micro-tensile testing of the developed high-pressure cold-sprayed coatings was performed to understand their fracture behaviour. The in-situ micro-tensile testing revealed that the failure of the coating took place mainly due to crack initiation from the junctions of the multiple splat boundaries, followed by its growth along the splat boundaries. Moreover, the splat boundaries were found to act as the weaker zones during the testing, which provide path for crack growth leading to the failure. However, crack growth was seen to be arrested after heat-treatment owing to the inter-particle diffusion. Moreover, γ-rays and heavy-nuclei irradiation along with thermal cyclic exposure studies were also performed on the developed coatings to elucidate their actual environment performance. It was observed that the developed high-pressure cold-sprayed Cu-coatings successfully sustained in the radio-active environment without any changes in their microstructures or compositions, which makes them suitable for the given application. Also, the developed coatings could sustain well under thermal cyclic loads, indicating that they have good adherence with the substrate steel. Based upon the overall results of the study, the processing route comprising mirror-finishing of the substrate, prior to the cold-spraying of Cu-particles, followed by heat-treatment of the coated system, has been suggested to develop cold-sprayed Cu-coatings to protect the plasma-facing components of vacuum vessel of Tokamaks from plasma disruptions.