Interfacial fracture behavior of tungsten wire/tungsten matrix composites with copper-coated interfaces
Introduction
Tungsten is attracting increasing interest in specific high temperature applications, especially as a plasma-facing armor material for fusion reactors, due to its refractory nature, minimal sputtering yield, large heat removal capability and thermal shock resistance. On the other hand, the potential as a structural material is still strongly restricted by its inherent brittleness which prevails below the ductile-to-brittle transition temperature (DBTT). There have been intensive efforts to improve poor tungsten toughness by means of conventional metallurgical methods including alloying with rhenium, severe plastic deformation to form a nano-scale microstructure, mechanical alloying followed by HIP also to form a nano-microstructure, and increase of the degree of deformation (e.g. foils or wires) [1], [2], [3]. Oxide particles dispersion could increase the creep strength but reduced the tensile elongation [4]. But it seems very difficult to achieve substantial enhancement in near future despite of the decades of research. Hence, in order to accelerate the progress, one needs to devise a completely novel toughening technique which is not affected by the ductile-to-brittle transition mechanism.
Since mid-1980s, ceramic matrix composites (CMC) reinforced with continuous strong fibers have been developed. In the meantime they were approved as a structural material equipped with extensive global toughness [5], [6]. Numerous experimental studies demonstrated that the long fiber reinforcement could endow the brittle ceramic matrix with significant toughness by interfacial energy dissipation. The effective energy absorption is caused by the controlled crack deflection (debonding) and frictional sliding at the fiber/matrix interfaces [6]. Hence, the fracture energy of the interface plays a key role in this process.
Recently, the authors have been exploring the applicability of this CMC toughening concept for tungsten. To this end, single fiber specimens of tungsten wire–reinforced tungsten matrix composites (Ww/W) were fabricated and investigated by means of fiber push-out test. Their study was focused on the interface engineering using submicron-thick ceramic coatings. The chemical composition of the composites was only minimally changed by the thin coatings. The extensive push-out tests revealed considerable amount of energy dissipation produced by the brittle cracking and frictional sliding of the coated interfaces. In addition, the strength and toughness of the tungsten composite can be further enhanced by the toughness of the tungsten wire itself since the commercially available tungsten wires are normally very strong (2.5–3 GPa) and ductile.
Another possibility to realize the interfacial energy dissipation would be to use a soft metallic coating to exploit substantial plastic deformation. In this work, we discuss the feasibility of metallic coatings for the interface of the Ww/W composite. First results obtained from the push-out experiments are presented. Two kinds of copper coatings were investigated, namely, copper single-layer and copper/tungsten multi-layers. Interfacial material parameters were identified. The microscopic fracture feature of the debonded interfaces was examined.
Section snippets
Specimen preparation
Commercial tungsten wire with diameter of 150 μm was used as reinforcement. The interface coatings were deposited by magnetron sputtering (MS). The single-layer copper coating was 420 nm think, whereas the multi-layer coating consisted of four 55 nm thick copper films and five 110 nm thick tungsten films in an alternating way. The coatings were further coated with a thin tungsten film to protect the surfaces. The coated fibers were then further coated with tungsten using chemical vapor deposition
Push-out curve
The measured push-out load vs. fiber-end displacement curves are plotted in Fig. 5. The circle markers indicate the interface with the copper single-layer coating whereas the triangles the W/Cu multi-layer coating, respectively. In the initial loading phase the both interfaces showed a nearly linear response until it reached the maximum load Pd (point A). The slopes of the two curves were almost identical. It should be noted that the total displacements include the bending deflection of the
Conclusion
In this work the properties of copper-coated fiber/matrix interfaces for a Ww/W composite were successfully investigated using fiber push-out test. Cu single-layer and Cu/W multi-layer coatings were considered for interface engineering. Until the global debonding event at the maximum load, both coatings exhibited nearly identical load-displacement responses. In the frictional sliding stage, however, the two coatings revealed an essentially different energy dissipation behavior from each other.
Acknowledgements
The authors are grateful to the colleagues of IPP Garching, Dr. A. Brendel, Mr. F. Koch and G. Matern for their support for magnetron sputtering and metallographical preparation. The author Juan Du is also grateful to Chinese Scholarship Council (CSC) for the grant.
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