ournal J An. Ceram Soc, 80[121 2987-96(1997) Control of Interfacial Properties through Fiber Coatings Monazite Coatings in Oxide-Oxide Composites Dong-Hau Kuo and Waltraud M. Kriven Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana. Illinois 61801 Thomas J Mackin Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana. Illinois 61801 Fiber pushout tests were used to quantify the effects of fiber temperature strength and creep resistance, in comparison with oating thickness on the mechanical properties of two other oxide fibers. This fiber is the only oxide fiber with sub- model composite systems: a monazite-coated (LaPO4 stantial creep resistance at temperatures >1600 C 9, 10 To sus- oated) alumina(Al,O3)fiber in an Al,O3 matrix and a tain the concept of an all-oxide system, a weak oxide inter- LaPOr-coated yttrium aluminum garnet(YAG) fiber in an phase is required for strong, tough, and high-temperature AlO3 matrix. Interface properties were quantified using oxidation-resistant fiber composites. A promising, new high he Liang and Hutchinson(LH) pushout model and mecha- temperature fiber-coating material (monazite, LapO4)was re- nistically rationalized by considering the change in residual cently developed by Morgan and coworkers. 1-13 The LaPO hermal stresses with changes in the coating thickness. coating material presents the opportunity of producing a hi Measures of the pure Mode ll interfacial fracture energy, temperature oxide-constituent composite with an inhere he coefficient of friction, and a radial clamping pressure weak interfa re extracted by fitting the lh equations to the experimen Fiber pushout testing has been widely used to quantify in- tal results. Using the approach that has been developed terfacial properties in composites. - Pushout testing affords herein, a methodology is available for measuring the inter- a simple screening test for model composite systems and al- facial properties, predicting the effect of coating thickness, lows the calculation of the key interface properties, which in- and selecting the coating thickness to alter the interfacial clude the following: th e intera ace fracture energy, T the co- properties efficient of sliding friction, u; and the radial clamping pressure at the interface, clamping. Theoretical models8-22 that incor . Introduction porate the elastic properties of the fiber-matrix system have been developed to explain the results of pushout tests and HE fiber/matrix interface is the key to improving the me. quantify the relevant interface properties. Although a modified chanical performance of continuous-fiber-reinforced ce- model is needed for a three-component system(i.e, one whick ramic-matrix composites(CFCCs) I-s A strong interface re- consists of the fiber, the coating, and the matrix),reasonable interfacial debonding and subsequent fiber pullout In genera estimates of the interfacial properties can be made by using the ults in little toughening, whereas a weak interface existing models several fac In this study, fiber pushout tests were used to measure tors, which include the availability of ceramic fibers and the interfacial properties of two model composite systems ( alu- need for thermal and chemical stability among the constituents The choice of constituents is broadened by using a coating stem), and (ii)YAG fiber/LaPO4 coating/Al2 O3 matrix stem that assures chemical stability and, at the same time, AG fiber system). The effect of the LapO4 coating thickness promotes easy debonding. In addition to controlling the inter- face properties, fiber coatings protect fibers from mechanical mens with fiber coatings that varied in thickness from 2 un 24 um. Residual thermal stresses were calculated by the bead- Carbon and boron nitride(bn) are the most commonly used seal solution2, 24 and were used to explain the effect of coating thickness on the interfacial properties. Liang and Hutchin- oxidize in high-temperature environments. Extensive research son's2(LH)model of the fiber pushout test was used to quan- has been undertaken to address these problems. 6-8 Natural tify and rationalize the experimental results an oxide fiber in an oxide matrix circumvents the problem of high-temperature oxidation Single-crystal yttrium aluminum II. Experimental Procedures garnate(YAG, Y3AlsO,2)fibers have shown superior high- (I Sample preparation Model composite systems were fabricated by dip coatin fibers, placing them into a powder compact, and sintering D. K. Shetty--contributing editor them. A LapOa slurry was prepared by ball milling a mixture of LapO, powder(70 wt%), ethanol (27 wt%), and poly(viny butyral)(3 wt%). Continuous single-crystal Al, O,(diameter of Manuscript No. 192227 August 5, 1996, approved April 13, 1997 and YaG (diameter of-160 um) fibers(Saphikon, carch through Dr. A. Milford, NH)were cut, cleaned, and dip coated with the LaPo Pechenik under Grant No slurry. Different coating thicknesses were obtained by repeate letting of the American Ceramic Society, In fiber dipping. To ensure a uniform coating thickness, a quick- drying ethanol-based solution was used. Dip coating was e 2987Control of Interfacial Properties through Fiber Coatings: Monazite Coatings in Oxide–Oxide Composites Dong-Hau Kuo* and Waltraud M. Kriven* Department of Materials Science and Engineering, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801 Thomas J. Mackin* Department of Mechanical and Industrial Engineering, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801 Fiber pushout tests were used to quantify the effects of fiber coating thickness on the mechanical properties of two model composite systems: a monazite-coated (LaPO4- coated) alumina (Al2O3) fiber in an Al2O3 matrix and a LaPO4-coated yttrium aluminum garnet (YAG) fiber in an Al2O3 matrix. Interface properties were quantified using the Liang and Hutchinson (LH) pushout model and mecha￾nistically rationalized by considering the change in residual thermal stresses with changes in the coating thickness. Measures of the pure Mode II interfacial fracture energy, the coefficient of friction, and a radial clamping pressure are extracted by fitting the LH equations to the experimen￾tal results. Using the approach that has been developed herein, a methodology is available for measuring the inter￾facial properties, predicting the effect of coating thickness, and selecting the coating thickness to alter the interfacial properties. I. Introduction THE fiber/matrix interface is the key to improving the me￾chanical performance of continuous-fiber-reinforced ce￾ramic-matrix composites (CFCCs).1–5 A strong interface re￾sults in little toughening, whereas a weak interface promotes interfacial debonding and subsequent fiber pullout. In general, the choice of a fiber–matrix system is limited by several fac￾tors, which include the availability of ceramic fibers and the need for thermal and chemical stability among the constituents. The choice of constituents is broadened by using a coating system that assures chemical stability and, at the same time, promotes easy debonding. In addition to controlling the inter￾face properties, fiber coatings protect fibers from mechanical damage during handling and processing.2 Carbon and boron nitride (BN) are the most commonly used interfacial coatings in CFCCs. However, these coatings readily oxidize in high-temperature environments. Extensive research has been undertaken to address these problems.6–8 Naturally, an oxide fiber in an oxide matrix circumvents the problem of high-temperature oxidation. Single-crystal yttrium aluminum garnate (YAG, Y3Al5O12) fibers have shown superior high￾temperature strength and creep resistance, in comparison with other oxide fibers. This fiber is the only oxide fiber with sub￾stantial creep resistance at temperatures >1600°C.9,10 To sus￾tain the concept of an all-oxide system, a weak oxide inter￾phase is required for strong, tough, and high-temperature oxidation-resistant fiber composites. A promising, new high￾temperature fiber-coating material (monazite, LaPO4) was re￾cently developed by Morgan and coworkers.11–13 The LaPO4 coating material presents the opportunity of producing a high￾temperature oxide-constituent composite with an inherently weak interface. Fiber pushout testing has been widely used to quantify in￾terfacial properties in composites.14–17 Pushout testing affords a simple screening test for model composite systems and al￾lows the calculation of the key interface properties, which in￾clude the following: the interface fracture energy, Gi ; the co￾efficient of sliding friction, m; and the radial clamping pressure at the interface, sclamping. Theoretical models18–22 that incor￾porate the elastic properties of the fiber–matrix system have been developed to explain the results of pushout tests and quantify the relevant interface properties. Although a modified model is needed for a three-component system (i.e., one which consists of the fiber, the coating, and the matrix), reasonable estimates of the interfacial properties can be made by using the existing models. In this study, fiber pushout tests were used to measure the interfacial properties of two model composite systems: (i) alu￾mina (Al2O3) fiber/LaPO4 coating/Al2O3 matrix (Al2O3 fiber system), and (ii) YAG fiber/LaPO4 coating/Al2O3 matrix (YAG fiber system). The effect of the LaPO4 coating thickness on the interfacial properties was evaluated by fabricating speci￾mens with fiber coatings that varied in thickness from 2 mm to 24 mm. Residual thermal stresses were calculated by the bead￾seal solution23,24 and were used to explain the effect of coating thickness on the interfacial properties. Liang and Hutchin￾son’s20 (LH) model of the fiber pushout test was used to quan￾tify and rationalize the experimental results. II. Experimental Procedures (1) Sample Preparation Model composite systems were fabricated by dip coating fibers, placing them into a powder compact, and sintering them. A LaPO4 slurry was prepared by ball milling a mixture of LaPO4 powder (70 wt%), ethanol (27 wt%), and poly(vinyl butyral) (3 wt%). Continuous single-crystal Al2O3 (diameter of ∼140 mm) and YAG (diameter of ∼160 mm) fibers (Saphikon, Milford, NH) were cut, cleaned, and dip coated with the LaPO4 slurry. Different coating thicknesses were obtained by repeated fiber dipping. To ensure a uniform coating thickness, a quick￾drying ethanol-based solution was used. Dip coating was ex￾D. K. Shetty—contributing editor Manuscript No. 192227. Received August 5, 1996; approved April 13, 1997. Supported by the U.S. Air Force Office of Scientific Research through Dr. A. Pechenik under Grant No. AFOSR-F49620-93-1-0027. Presented at the 98th Annual Meeting of the American Ceramic Society, In￾dianapolis, IN, April 14–17, 1996. *Member, American Ceramic Society. J. Am. Ceram. Soc., 80 [12] 2987–96 (1997) Journal 2987