JEC COMPOSITES MAGAZINE - Issue #33 - June 2007 - 68

KNOW-HOW Metal matrix Materials and synthesis g…/… Metal matrix composite for fusion application no interlayer between W fibre and Cu matrix; a thin PVD Cu layer incorporated between W fibre and Cu matrix; and a graded transition between W fibre and Cu matrix. By increasing the concentration gradually from the tungsten reinforcement to the copper matrix, the CTE mismatch between W fibre and Cu matrix can be reduced. The mechanical characterisation of the composite interfaces was performed using fibre push-out and pull-out tests. For the long-fibre reinforcement of the copper matrix, SiC fibres (SCS 6, Specialty Materials) and W fibres (OSRAM) were used (Figure 2). SCS 6 SiC fibres are multi-layered fibres with a diameter of 140 µm consisting of six different structural components: a carbon monofilament, thick fine-grained SiC and coarse-grained SiC-layers, a 0.5 µm thin healing layer consisting of pure amorphous carbon to heal surface defects and thus improve the fibre strength, and two layers of SiC doped carbon (overall thickness of outer layer ≅ 3 µm) to protect the fibre during handling and to improve the wetting properties [11]. An additional titanium layer should improve the fibre/matrix bond strength by reacting with the outer carbon containing layer of the fibre to TiC at temperatures above 350°C [12]. These titanium bond layers with thicknesses of 100 nm were deposited on the fibres by magnetron sputtering. An additional 500nm Cu layer was deposited to protect the Ti against oxidation. The initial W fibres with a diameter of 100 µm were coated by magnetron sputtering with a 500 nm Cu layer or with a graded transition of 500 nm. The copper concentration is thereby gradually increased from the tungsten fibre to the copper matrix. Fig. 2: Synthesis of SiC- and W-fibre reinforced copper matrix composite. 24 hours to obtain an adequate matrix thickness (≅ 1mm) to pull against. After deposition, the fibres were heat-treated at 550°C for 2 hours with a very slow heating rate (0.5 K/min.) to avoid the formation of pores by outgassing of hydrogen and oxygen, which both are normally contained in a galvanic layer. The pores would be a result of a chemical reaction between hydrogen and oxygen to water during fast heating, which would damage the microstructure and decrease the mechanical strength (“pickle brittleness”). In the last step, coated and heat-treated single fibres were packed in a copper capsule (diameter 10 mm, length 45 mm) as dense as possible and subsequently hotisostatically pressed at 650°C with a pressure of 100 MPa for 30 minutes to form the composite material. The reinforced zone had a diameter of 3.5 mm. Fig. 3: Schematic demonstration of a) push-out test and b) pull-out test. Subsequent to the magnetron deposition, both types of fibres were electroplated with a thick copper layer as matrix material in a CuSO4 bath at room temperature. The thickness of the copper layer defines the fibre volume content in the composite: for a fibre volume fraction of 20%, the SiC fibres were coated with an 80 µm thick copper layer within 8 hours. For pull-out measurements, the coated W fibres were electroplated for JEC Composites Magazine / No33 June 2007 For the mechanical characterisation of the fibre/matrix interface, push-out tests on the SiC/Cu composite material and pull-out tests on coated W single fibres were performed (Figure 3). For the push-out test, the composite sample thickness, and thereby the fibre length, varied between 0.4 and 3 mm. Also for the pullout test, the length of the electroplated Cu matrix was varied to obtain embedded fibres with various lengths between 0.5 and 4 mm. A universal test machine was prepared to push or pull single fibres out of the matrix. Both the displacement and the resulting load were acquired continuously. A typical load vs. displacement curve is shown in Figure 4, which results from the push-out as well as from the pull-out tests. The curve shows an elastic increase of the load until the first local maximum indicates the beginning of debonding. This point is called debonding load - Pd. In the further curve progression, the debonding is superposed with friction. At the absolute maximum load - Pmax, the fibre debonds completely. After debonding, the force required to overcome the push/pull-out friction decreases during the final phase of the experiment. For both the push-out and pull-out tests, the Pd values were acquired as a function of embedded length. These values serve as data, which will be fitted with formulas [13] to obtain a characteristic interfacial property, which is the interfacial shear

JEC COMPOSITES MAGAZINE - Issue #33 - June 2007

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