Coating carbon fiber to improve the bond to a glass

When carbon fiber is incorporated in a typical borosilicate glass matrix, the bond formed between the carbon fiber and glass is poor, producing composites with measured strengths below that predicted by the rule of mixtures. Taylor believed that the poor bond when using HM fiber originated from the highly nonpolar surface formed by the π-electron cloud of the oriented graphite crystals having no tendency to bond with the oxygen in the silicate matrix. The application of a metal film to the carbon fiber should negate the effect of the non-polar surface and provide a metal cation, which should readily combine with the oxygen in the silicate to form a tight oxide bond. The following metals should be effective—Y, Zr, Nb, Mo, Ag, Cd, Ta, W, Zn, Cu, Co, Fe, Mn, Cr, V, Ti, Sc, Al, Mg and Ni.

Taylor took HMS fiber coated by electroplating with 0.5 um nickel and passed it through a bath containing a slurry of 40% wt% powdered corning 7740 borosilicate glass dispersed in an aqueous or propanol vehicle. The impregnated fiber was wound onto a square drum and when dried, gave approximately 50% glass coating. The coated fiber was formed into disks, which were stacked in a graphite mold, with Mo foil disks placed at the top and bottom to protect the composite from reacting with the graphite mold. The stacked mold was heated to 300C in an inert atmosphere to remove all volatiles and consolidation was achieved in a vacuum hot press, where the pressure was applied with a hydraulic ram via a graphite rod. Heating was undertaken at a rate of 5-30C /min to achieve the maximum rate of outgassing without allowing the vacuum to reduce beyond 1.0*10-1 Nm-2. Finally, a pressure of 10 Mpa was applied and the pack heated to 1095C then holding for 0.5h, to ensure that consolidation was achieved at the center of the composite. The composite was cooled by backfilling with Ar gas to speed the cooling process. The flexural strength of the composite increased from 0.53 Gpa for the uncoated fiber to 0.75 Gpa for the nickel coated fiber.

Other work has been undertaken employing the deposition of Ni on carbon fiber for reinforcing glass matrices.

A novel processing route uses in situ electrophoretic deposition of submicron sized colloidal particles of borosilicate sol onto nickel coated carbon fibers to give a dense fully infiltrated product, which is followed by pressureless sintering. Catastrophic crack growth was prevented by constrained plastic deformation of the interface, fiber debonding and pullout.

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