Zirconia and organotitanate film formation on graphite fiber reinforcement for metal matrix composites
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The formation and characterization of zirconia coatings on graphite/carbon filaments were investigated. The objective was to eliminate or minimize degradative chemical reactions and improve bonding at the metal/carbon-fiber interface when the coated fiber is used as reinforcement in metal matrix composites. Thin, homogeneous films of zirconium oxide, ZrO 2 , less than 1 nva thick, were formed on carbon monofilaments (35 /xm diameter) from a zirconium oxychloride solution in water (less than 1.0 wt. % ZrO2) by dip coating and heating. Chemical changes during thermal decomposition and polycondensation were examined by FTIR spectroscopy. Through dynamic x-ray diffraction tests, the zirconia coating was found to transform first to a metastable tetragonal phase on heating to 330 °C, and then upon cooling, to a stable monoclinic structure. Organotitanate coatings were formed by electrodeposition of the ionizable organometallic complex, titanium di(dioctylpyrophosphate) oxyacetate (TDPA). Single fiber tests revealed a slight reduction in the strength of fibers with thicker coatings, probably due to crack initiation by brittle fracture of the ZrO 2 coating. Thin coatings applied from 0.25% ZrOCl2 did not cause such strength reduction. Effective bonding of ZrO 2 coatings to the filaments was revealed in single filament composite tests which showed the interfacial shear strength (IFSS) of the coated filaments in an epoxy matrix to be higher than that of the uncoated filaments. The IFSS of monofilaments electrocoated by the titanate was also higher. Examination of the fracture surfaces showed fiber pull-out associated with poor bonding in the case of specimens prepared from uncoated monofilaments. The coated monofilaments showed no fiber pull-out, suggesting that maximum fiber strength was achieved and transferred through the interface to the matrix. Finally, the chemical and thermal stability of the interfacial region of coated and uncoated graphite rods embedded in an aluminum matrix were evaluated. The uncoated rod showed little or no interfacial bonding to the metal matrix, suggesting poor wetting, while the dip-coated and electrocoated rods showed good wetting and compatibility with the metal matrix.
I. INTRODUCTION The fiber-matrix interface plays a critical role in controlling the performance and properties of fiber reinforced composites. A limiting factor in the production and use of graphite fiber reinforced metal matrix composites has been the deleterious reaction that occurs at the fiber-matrix interface to form weak or brittle interfacial regions.1 Many studies have been concerned with the interfacial region degrading and leading to premature failure of the composite.2'3 Research programs in our laboratory have focused on interphase modification in polymer composites by electropolymerization and electrodeposition on graphite fibers4 and in basalt fiber reinforced cement composites.5 a)
Present address: Ris0 National Laboratory, DK4000 Roskilde, Denmark. J. Mater. Res., Vol. 7, No. 3, Mar 1992 http://journals.cam
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