Evaluation of Coatings for Sapphire Fibers in TiAl Using the Fiber Push-Out Test
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EVALUATION OF COATINGS FOR SAPPHIRE FIBERS IN TiAl USING THE FIBER PUSH-OUT TEST T. J. Mackin and J. Yang Materials Department, The University of California at Santa Barbara, Santa Barbara, CA 93106
ABSTRACT
Fiber push-out tests were used to evaluate several double coating systems for sapphire fibers in 'y -TiAl. The double coatings consisted of an inner debond coating, followed by an outer coating of alumina that serves as a diffusion barrier. The inner coatings were of three types: carbon black, colloidal graphite and porous alumina. By comparing estimates of interface toughness computed from the push-out tests, each double coating is found to permit debonding and sliding of the sapphire fibers. Consequently, each of the double coatings would afford improved fracture toughness for sapphire reinforced y -TiAl.
INTRODUCTION
The mechanical properties of fiber-reinforced materials are known to depend upon the mechanical properties of the fiber/matrix interface. 1-3 Optimum longitudinal properties, for both brittle and ductile matrix materials, require that the interface debond and slide with a relatively low shear resistance, T , relative to fiber strength, S. A high composite fracture resistance requires small values of T/S to achieve large fiber pull-out lengths and the consequent frictional dissipation that contributes to the composite's work of rupture. 3 ,4, In many cases, fibers must be 5 coated to permit debonding and sliding. A debonding criterion derived by He and Hutchinson can then be used to screen candidate fiber coatings. This criterion states that the Mode II interfacial toughness must be less than 1/4 of the reinforcement toughness. Subsequent frictional pullout dissipation will depend upon the mechanical properties of the sliding interface. This paper summarizes a comparison of three double coating systems for sapphire fibers in y-TiAl. Sapphire fibers have a sinusoidal surface roughness that exerts an important influence on coating design. 6 Specifically, when the inner coating is chosen to have a thickness less than the peak-to-peak amplitude of the fiber roughness, the debond surfaces would mimic the underlying fiber geometry, leading to explicit roughness effects on T. Conversely, coating thicknesses many times the fiber roughness amplitude would tend to obscure any effects of the underlying fiber geometry.
Mat. Res. Soc. Symp. Proc. Vol. 273. 01992 Materials Research Society
344
MATERIALS
Sapphire fibers [Sapphikon, R=67gtm] were double coated before consolidation into a
y -TiAl matrix. In principle, the inner coating is used to assure debonding between the fiber and matrix during fracture of the composite and is chosen according to the aforementioned debonding criterion. Three different coatings were tested in this regard: carbon black, colloidal graphite, and porous alumina. Since carbon will react with TiAl during consolidation, a second coating must be placed around the inner carbon coating to prevent diffusional interactions. The choice for the outer coating was dense A120 3 . . T
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