Mechanical properties of cold-sprayed and thermally sprayed copper coatings

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Abstract

The present investigation compares the mechanical properties of cold-sprayed and thermally sprayed copper coatings. The mechanical properties of the Cu-coatings are determined by in plane tensile test using micro-flat tensile specimen technique. A deeper view into the type of obtained defects, their stability and their influence on coating performance, is supplied by subsequent failure analyses and the comparison to annealed copper coatings. The results demonstrate that cold-sprayed coatings, processed with helium as propellant gas, show similar performance as highly deformed bulk copper sheets and respective changes in properties after annealing. In the as-sprayed condition, cold-sprayed coatings processed with nitrogen and thermally sprayed coatings show rather brittle behavior. Whereas subsequent annealing can improve the properties of the cold-sprayed coating, processed with nitrogen, such heat treatments have only minor influence on the tensile properties of thermally sprayed copper coatings. The investigation of failure modes for the as-sprayed states and after different heat treatments provided further information concerning particle–particle bonding and the effect of oxides on mechanical properties.

Introduction

In thermal spraying, major developments within the last three decades aimed to operate spray systems at lower process temperatures and respectively increased gas and particle velocities [1]. These attempts to shift the balance between thermal and kinetic energies towards the latter were driven by the goal to reduce disadvantageous effects on coating properties like oxidation, phase transformations or crack formation due to stresses introduced during rapid solidification of the spray material on the substrate. In the comparison of currently available spray techniques, cold spraying (CS) just represents a range at the extreme end with process temperatures far below the melting point of respective spray materials and with very high gas and particle velocities [2]. These process characteristics make cold spraying particularly suitable for oxidation-sensitive materials [3], [4].

The adhesion of particles in cold-sprayed coatings is solely the result of the high-energy impact of solid particles. As in explosive cladding or explosive powder compaction, bonding can be attributed to the degree of deformation and the associated temperature rise at particle–particle and particle–substrate interfaces [5]. The localized rise in plastic flow by shear instabilities is a necessary requirement for successful bonding. Criteria for bonding are only met if the particles exceed a certain critical impact velocity, which is specific to the coating and the substrate material. For copper powder with a particle sizes distribution from 5 to 22 μm, the critical velocity was determined to range from 530 to 560 m/s [5], [6]. The high gas and particle velocities in cold spraying are obtained by the expansion and acceleration of a heated and highly pressurized gas during the flow through a DeLaval-type nozzle. The principle of the technical set-up for cold spraying is described elsewhere [3], [4].

For the current study, copper was chosen as spray material with respect to the high deformability, the available variety of powder feedstock with different oxygen contents, a high amount of reference data and interests concerning applications [7]. To cover a wide range between thermal and kinetic particle impact energy in thermal spraying, arc spraying (AS) and high velocity oxy-fuel flame (HVOF) spraying were chosen as processes, serving as reference [7]. As further reference, respective analyses were also performed for cold rolled, high purity copper (OFHC). A number of defects introduced by thermal or cold spraying, can heal out during annealing. To study their nature and respective consequences on coating properties, subsequent heat treatments were applied to the various copper coatings. Recent investigations already demonstrated that high velocity impacts as obtained in HVOF or cold spraying result in a type of persistent defect in the form of dislocation loops, which enhance hardness and correlate well to the recombination of non-equilibrium vacancies and interstitials observed after heavy ion implantation [8]. The study of coating properties after annealing is also of interest with respect to applications. The first industrial use of cold spraying involves soldering of the coated parts and thus a heat treatment after coating deposition [9].

Recent investigations on the comparison of thermally sprayed and cold-sprayed copper coatings supplied information concerning microstructural details, deposition efficiencies, and properties like bond strength, hardness and electrical conductivity [7]. The present study aims to supply a more detailed view on intrinsic tensile properties of the copper coatings and thus on particle–particle bonding attained in cold spraying in comparison to thermal spraying in as-sprayed and heat-treated states.

Mechanical properties of thermal spray coatings are often investigated by bend tests [10], [11], [12] with specimens containing coating and substrate materials together. Such analyses need careful consideration to distinguish influences by the substrate material, the adhesive strength of the coating–substrate interface and the internal stress in the coating. The real performance of the coatings with respect to mechanical properties can only be investigated by direct measurements of the stress–stain curves of the samples, prepared from the coating itself. This can depend on the thickness of the coating layer. For the thin layer of coating, a micro-sized specimen extraction and respectively testing is essential.

For the plasma- and HVOF-sprayed copper, tensile tests in combination of the results obtained by X-ray diffraction have already supplied valuable information on coating performance [13]. Recently, in a similar combination of methods, mechanical properties of cold-sprayed copper coatings, processed with helium, were also reported [14]. In that work, as well as in studies of cold-sprayed aluminum coatings or investigating titanium alloy coatings, comparatively thick coatings were prepared to comply with the sample geometries of EN or ASTM requirements [15], [16]. However, building up such thick layers leads to heating of the coatings already during the spray process.

Thus, the present study aims to minimize such thermal effects during the coating process by limiting coating thickness to 5 mm. For determination of the full stress–strain curves of the narrow welds (e.g. laser beam) or interface regions of bi-material systems, the micro-flat tensile testing technique was developed by Kocak et al. [17]. The present investigation has used a similar approach for the evaluation of the mechanical properties of the thin copper coatings by extracting multiple thin specimens to determine the properties of the coating in detail.

After tensile testing, metallographic methods are applied to investigate fractured surfaces to improve the understanding of the bonding processes and intrinsic properties of the coatings. The obtained failure morphologies provided valuable information on the effect of defects, present in the different types of coatings, on respective strengths. As reference, the results are compared to those obtained for similarly deformed and heat-treated bulk copper samples.

Section snippets

Cold spraying, thermal spraying and annealing

Cold spray and thermal spray experiments were performed under previously optimized standard parameter settings [7], whereas substrate material, form and dimensions, as well as coating thickness were chosen with regard to the further investigations. The whole set of parameters used in cold spraying (CS), high velocity oxy-fuel spraying (HVOF) and arc spraying (AS) is summarized in Table 1. Samples obtained from the various spray methods were subsequently annealed in vacuum for 1 h at

Applicability of micro-flat tensile tests to determine mechanical properties of pure copper

The results presented in Fig. 2 and Table 2 demonstrate that miniaturized test samples show slightly higher yield strengths for the highly deformed and the annealed state and very similar ultimate strengths for both conditions. In that comparison, the miniaturized test samples also show a higher elongation to failure than the samples tested according to EN 10002-1. Those differences might be attributed to a slightly more non-uniform deformation of the small geometries, but within the given

Coating performance

Fig. 10 summarizes the tensile strengths (Table 2, Table 3) for the different coating and annealing conditions. The decrease of ultimate strength with increasing annealing temperature of CS copper coatings, sprayed with helium as process gas is similar to that of bulk material cold rolled to 70% in thickness reduction (%). The slight differences can be attributed to the higher degree of work hardening, attained by cold spraying with helium. Taking hardness as a measure for strain hardening by

Summary and conclusions

In the present study, mechanical properties and micro-mechanisms of failure modes of cold-sprayed and thermally sprayed copper coatings are investigated for as-sprayed states and different annealing conditions. The results demonstrate that cold-sprayed coatings, which are processed with helium, show a similar performance as highly deformed bulk material. Also after subsequent annealing, strength and elongation to failure develop in a similar manner as for cold rolled sheets. In the as-sprayed

Acknowledgements

Significant parts of the research were supported by the Deutsche Forschungsgemeinschaft (DFG), grant number KR 808 1103/3-3, and the Arbeitsgemeinschaft industrieller Forschungsvereinigungen (AiF) grant number 12.671 N, which are greatly acknowledged.

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