Fabrication and characterization of vacuum plasma sprayed W/Cu-composites for extreme thermal conditions
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- Pintsuk, G., Smid, I., Döring, J. et al. J Mater Sci (2007) 42: 30. doi:10.1007/s10853-006-1039-y
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The joining of tungsten to copper and the ongoing search for commercially viable production techniques is one of the challenging issues in the field of composite materials. The reason why this material combination is of essential importance is its ability to withstand erosion and high temperatures on the tungsten side and to remove big quantities of heat on the copper side. Due to the mismatch of thermal expansion and Young’s moduli, the direct joining of these two materials results in high residual and thermal stresses at the interface, ultimately reducing component lifetime. One potential answer to this problem is functionally graded structures of W and Cu, which smoothen the transition of material properties. The present study focuses on vacuum plasma spraying (120 mbar, Ar) of W/Cu-gradients and composites with defined mixing ratios. The influence of the fabrication process and the W:Cu ratio on the microstructure has been investigated and results from thermo-mechanical and thermo-physical results analyses are presented. Finite element modeling has been used to demonstrate the positive effect of gradients on the elastic and elastic–plastic response within two different model-geometries. Partial gradients, ranging from pure tungsten to 75 vol.% tungsten, exhibit the best results and improve the expected life-time performance significantly by reducing the stresses at both interfaces, W/FGM and FGM/Cu, compared to a reference interface between W and Cu.
The field of possible applications of W/Cu-composites varies over a wide range, which includes components in military, space and nuclear fusion technologies as well as parts in electronic devices (heat sinks) and energy systems (e.g., contact material in power switches) [1, 5]. In every case it is used to deal with thermal and often extreme thermal conditions.
The choice of joining techniques for tungsten and copper is strongly connected to big differences in melting temperatures and densities. Various methods of forming combinations of these materials either with fixed compositions or as graded structures have been investigated [6–19]. The motivation to determine new ways of designing gradients is in their ability to reduce thermal stresses [20–23]. At the W/Cu-interface, huge differences in Young’s moduli (ΔE ≈ 300 GPa) and thermal expansion coefficients (Δα ≈ 12 × 10−6 K−1) occur, which is critical for lifetime. By smoothing the transition of properties at the interface of W/Cu-laminates via a functionally graded material (FGM), an increase in the lifetime of thermally loaded structural parts can be obtained.
This paper focuses on vacuum plasma spraying (VPS) and the evaluation of microstructural, mechanical, and thermo-physical properties of W/Cu-composites with a defined mixing ratio. By inserting this data into elastic and elasto-plastic finite element analyses, further optimizations of the graded structure can be achieved.
Vacuum plasma spraying—experimental
The W/Cu-coatings were deposited by VPS in a plasma-spray facility manufactured by Sulzer-Metco, Wohlen, Switzerland. In the VPS process the powder particles of the material to be deposited on a given substrate are blown into a plasma plume produced by a certain gas mixture in an ambient Ar-atmosphere of 120 mbar. Therein they are melted, accelerated towards the substrate and finally each particle is deposited on the substrate in a splat like structure using mechanical bonding. The current was 740 A, which results in 50 kW power output, the plasma gas flow was 40/10 SLPM (standard liters per minute) Ar/H2 and the feeding gas flow amounted 1 SLPM argon gas in the case of copper and 0.7 SLPM in the case of tungsten .
Additionally, transferred arc-cleaning (TAC) has been used during all experiments for in situ purification of the layer surface by the removal of loose particles [12, 24]. TAC is used for the removal of loose material, which leads to a densification of the material and an improved connectivity of the single phases.
Density before and after heat treatment (1,050 °C, 1 h, hydrogen atmosphere) of plasma-sprayed layers
Difference, Δρ (g/cm3)
Before heat treatment
After heat treatment
Material testing and characterization
Mechanical testing: static
Yield strength Rp,0.2 from RT to 600 °C for different material compositions
The Young’s modulus, a measure of the stiffness, remains constant up to 300 °C, observed for all compositions with 43 vol.% Cu or higher. A consequence of this is the existence of a strengthening mechanism, with a dependence on the Cu-content, the thermo-mechanical behavior of Cu and the porosity within the composite. Filling of gaps, which have been formed during cool-down and have not become evident in microstructural analyses due to smearing of copper, and in relation to this the formation of compressive stresses within the material are assumed to be the main reasons. A limitation to this strengthening effect is set by the softening of Cu and the resulting significant increase in ductile deformation with increasing temperature, caused by a temperature dependent reduction in yield strength. Therefore, with increasing temperature and Cu-content, a reduction in stiffness is observed (Fig. 8).
At a temperature of 600 °C, another effect becomes apparent comparing samples (b) and (c) (see Fig. 3). Within this range a loss of more than 30% of stiffness (and strength, too) is measured, reflecting the transition in microstructure from a stabilizing W-skeleton to a Cu-skeleton.
Mechanical testing: dynamic
Finite element modeling
Composition (vol.% W)
Extrapolated temperature dependent mechanical properties of plasma sprayed W/Cu-composites with selected compositions as input data for elastic–plastic analyses
Dealing with the ability of FGMs to reduce thermal stresses (point 2) it becomes evident that for both reference models without FGM high compressive stresses in W and high tensile stresses in Cu arise at the W/OFHC-Cu interface (Figs. 15 and 16), apparently the most crucial part in the structure. Further, the reference stresses in W are considered as representative for the overall model behavior. By introducing an FGM between W and OFHC-Cu these stresses are, now at the W/FGM-interface and irrespective of the component size, reduced by a continuous gradient by 55% (∅ = 5 mm) and 36% (∅ = 20 mm) and by a partial gradient by 87% (∅ = 5 mm), and 53% (∅ = 20 mm). This shows that a reduction depends on the composition of the graded interface and is significantly enhanced by the use of a partial gradient with a high W-content.
The stress evolution within the gradients is strongly influenced by the compositional differences. For the partial gradient, the stresses within the FGM and at the FGM/OFHC-Cu interface continue to be compressive and are increased to σFGM/Cu = −527 MPa (∅ = 5 mm, Fig. 15) and σFGM/Cu = −627 MPa (∅ = 20 mm, Fig. 16), but are still below the maximum values of the reference model without FGM. In contrast, with increasing Cu content in the gradient layer a transition from compressive to tensile stresses occurs leading to tensile stresses at the FGM–OFHC–Cu joint of σFGM/Cu = 113 MPa (∅ = 5 mm, Fig. 15) and σFGM/Cu = 269 MPa (∅ = 20 mm, Fig. 16). Additionally, the stresses in the OFHC-Cu interlayer are, especially for the smaller diameter (Fig. 15), strongly reduced by the continuous gradient.
Vacuum plasma spraying was found to be a very good method to produce graded W/Cu-composites which exhibit homogeneous distributions of W and Cu in the composite, no oxidation, sufficiently high density and good thermal conductivity, especially at higher temperatures. Variations within the whole composition range are possible and, due to a low deposition rate during plasma spraying of 6–10 μm per cycle, every desired gradient function and continuous gradients are achieved.
From elastic and elastic-plastic FEM-analyses, partial gradients from 75 vol.% to pure W and small component geometries (∅ ≤ 5 mm) are able to minimize the thermally induced stresses in a W/Cu-component. For a given cylindrical geometry with a diameter of 5 mm, the life limiting stresses at the interface are reduced by ∼35% which will extend the expected life-time performance significantly.
The supply of W-powders by the Plansee AG and funding by the Austrian “Friedrich Schiedelstiftung für Energietechnik”, which made this project possible, are gratefully acknowledged.