Galvanic deposition of Co–Mo layers and their influence on tensile strength of carbon fibers

Abstract

Aluminum and copper are of interest for application in the electronic industry due to high thermal and electric conductivity, but their coefficient of thermal expansion is too high for some applications. The use of carbon fibers with these metals as a composite material can decrease the thermal expansion and the density of the material [1–7]. The problem with using copper–carbon fibers (Cu/CF) composite is the low adhesion between copper and carbon fibers. Due to poor wettability and low adhesion between the composite components the transfer of load from matrix to fibers is poor and a very low value of shear strength in unidirectional composite is obtained. To increase the adhesion between the composite components, either carbide forming elements (for example Cr, W, Mo, Ti, etc.) are added to copper matrix, or carbon fibers are coated with layers containing these elements [4–6, 8]. The large interface layer formed by these reactions reduces carbon fibers properties. Higher content of these elements in copper considerably decreases some properties of the composite, mainly the thermal conductivity. The carbon fibers are also used as reinforcement in aluminum matrix composites [1–3]. To prevent the formation of aluminum carbide the carbon fibers are usually coated by a thin carbide or oxide protective layer. Galvanic metal coating is a widely used method in which different thicknesses can be obtained, from about several tenths of micrometer up to micrometers, which should be sufficient for the matrix to produce satisfaction composites. In our experiments we choose molybdenum as the carbide forming element. Unfortunately, it is not possible to deposit this metal alone either from aqueous or organic solutions. Therefore, in this letter we present a method of deposition of Co-Mo layers onto carbon fiber tow from an alkaline Co-Mo plating bath in which the metals were presented as a complex carbonate [9]. High strength carbon fibers (Torayca T300) with 3000 monofilaments in tow (diameter of a filament is about 7 μm) were used for the present study. All galvanic layers were deposited on carbon fibers in the continuous coating apparatus schematically described in Fig. 1. The sizing from the original C-fibers was removed in the furnace (2) at a temperature about 550 ◦C. To obtain active surface nuclei the commercial Sn (3) and Pd (4) baths were used (the same as at the chemical copper plating [10]). The individual fibers (original and heat treated at 700 ◦C for 30 s in argon atmosphere) were glued onto a paper frame with gauge length 20 mm and inserted into a tensile testing machine with a load cell of maximum force 0.5N. To explain some facts we used the Weibull statistical method [11]. The fibers were galvanically coated in aqueous electrolyte (5) with Na2MoO4 and CoCl2 at 90 ◦C. The deposition conditions which assured the forming of a continuous layer around each monofilament with the highest amount of Mo were optimized. The thickness of coating layer on carbon fibers was from 0.1 to 0.2 μm. On some carbon fibers tows with CoMo layer, in an other galvanic bath (6), copper was deposited with thickness approximately five times higher than the first layer. Because it is difficult to determine Mo content in such a thin layer directly onto the fiber, we deposited (at similar conditions as the fiber coating) the Co-Mo layer on a carbon plate with thickness of several micrometers. Energy dispersive X-rays analysis measurements showed that the alloy consisted of ∼40 wt.% of molybdenum and ∼60 wt.% of cobalt. From scanning electron microscopy observation one can see that all fibers are uniformly coated. The structure of Co-Mo layer on carbon fibers is fine-grained (Fig. 2), whereas the surface of the Cu layer (Fig. 3) is coarse. The tensile strength of the coated carbon fibers is practically the same as that of the original T300 fibers— neither galvanic deposition conditions nor thin deposited layers influence the surface of the fibers. Heat treatment of all types of coated fibers at 700 ◦C has different effect on their strength (Fig. 4). Whereas the strength of the original fibers and that, of the one with