ELSEVIER Materials Science and Engineering A210 (1996)123-134 Chemical stability, microstructure and mechanical behavior of LaPOa-containing ceramics Dong-Hau Kuo, Waltraud M. Kriven Received 3 May 1995: in revised form 27 October 1995 The use of LaPO4 as a weak interface in composites for high temperature applications was investigated using tape-cast aminates and fiber model systems. Three laminates were fabricated with LaPO4 as one component and Al,O, YAlsO,2or LaAlo,s as the other. The chemical compatibility between the different components of the laminates, as well as the mechanical responses to flexural deformation and the propagation of indentation cracks, were examined. Two fiber model systems(Al,O fiber/ LaPO4 coating/Al,O, matrix and Y3,o,? fiber/ LaPO4 coating/AL,O3 matrix)were studied by fiber pushout tests to measure the interfacial shear strengths. The interfacial shear strengths were calculated by the linear and shear-lag approaches for different embedded fiber lengths. The results suggest that Y3AlsO12 fiber-reinforced composites with LaPO4 coatings have potential as high Keywords: Lanthanum phosphate: Alumina: Aluminates: Laminates: Fibers; Pushout test 1. Introduction eutectic filaments [9 have shown good mechanical high For most non-oxide ceramics, high temperature oxi- (3AL,O3,' 2SiO2) fibers have also been considered fo dation which can degrade the performance of materials applications above 1370C [10]. Of these, single-crystal is a main concern [1-7]. Therefore oxide/oxide(fiber/ cubic-YAG fibers have shown the required creep resis- matrix) continuous fiber-reinforced ceramic composites tance above 1600C [7, 8]. The next challenge is to find with weak interfaces are preferred for high temperature a weak interface or interlayer for an oxide oxide sys- applications in air [6,7]. The weak interface allows debonding, fiber sliding and load transfer to occur, LapO4, a monazite structure, has recently been intro- thereby improving the toughness at room temperature, duced as a possible functional interface for oxide/oxide while the strong oxide fibers supply the required composites by Morgan and Marshall [11-13]. The in- strength and creep resistance at high temperatures formation presented is encouraging because it enables air. Although oxide ceramics are stable in oxidizing fiber-reinforced oxide/oxide composites to withstand environments, they often suffer mechanical degradation high temperatures in oxidizing environments, while at high temperature due to strong bonding between maintaining high strength from the strong fibers as well dissimilar oxides as high toughness from fiber debonding and sliding The use of ceramic materials at high temperatures in mechanisms. Thus LaPO4 is a candidate for preventing air faces many challenges. To overcome room tempera- strong bonding between an oxide fiber and oxide ma ture brittleness and the degradation of the mechanical trix properties at high temperature, new materials need to The tape casting technique has been used in ceramic be introduced. With regard to fiber materials, some processing [14-16] to fabricate laminated composites single-crystal alumina (AL,O3) and yttrium aluminate by stacking tapes of different compositions, as well as (Y,AlSOn2 or"YAG") fibers [7, 8] and Al,O3/YAG to incorporate fibers and whiskers into the laminates 0921-509396S15000 1996- Elsevier Science S.A. All rights res SSD09215093(95)100849A ELSEVIER Materials Science and Engineering A210 (1996) 123 134 Chemical stability, microstructure and mechanical behavior of LaPO4-containing ceramics Dong-Hau Kuo, Waltraud M. Kriven Department o1' Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 3 May 1995; in revised form 27 October 1995 Abstract The use of LaPO4 as a weak interface in composites for high temperature applications was investigated using tape-cast laminates and fiber model systems. Three laminates were fabricated with LaPO4 as one component and AI203, Y3AIsO12 or LaAI~IO~8 as the other. The chemical compatibility between the different components of the laminates, as well as the mechanical responses to flexural deformation and the propagation of indentation cracks, were examined. Two fiber model systems (A1203 fiber/LaPO4 coating/A1203 matrix and Y3A~O12 fiber/LaPO4 coating/A1203 matrix) were studied by fiber pushout tests to measure the interfacial shear strengths. The interfacial shear strengths were calculated by the linear and shear-lag approaches for different embedded fiber lengths. The results suggest that Y3AlsOl2 fiber-reinforced composites with LaPO 4 coatings have potential as high temperature materials in oxidizing environments. Keywords: Lanthanum phosphate; Alumina; Aluminates; Laminates; Fibers; Pushout test I. Introduction For most non-oxide ceramics, high temperature oxi￾dation which can degrade the performance of materials is a main concern [1-7]. Therefore oxide/oxide (fiber/ matrix) continuous fiber-reinforced ceramic composites with weak interfaces are preferred for high temperature applications in air [6,7]. The weak interface allows debonding, fiber sliding and load transfer to occur, thereby improving the toughness at room temperature, while the strong oxide fibers supply the required strength and creep resistance at high temperatures in air. Although oxide ceramics are stable in oxidizing environments, they often suffer mechanical degradation at high temperature due to strong bonding between dissimilar oxides. The use of ceramic materials at high temperatures in air faces many challenges. To overcome room tempera￾ture brittleness and the degradation of the mechanical properties at high temperature, new materials need to be introduced. With regard to fiber materials, some single-crystal alumina (A1203) and yttrium aluminate (Y~AlsO12 or "YAG") fibers [7,8] and A1203/YAG 0921-5093/96/$15.00 © 1996 - Elsevier Science S.A. All rights reserved SSDI 0921-5093(95)10084-9 eutectic filaments [9] have shown good mechanical properties at high temperatures. Mullite (3A1203'2SIO2) fibers have also been considered for applications above 1370 °C [10]. Of these, single-crystal cubic-YAG fibers have shown the required creep resis￾tance above 1600 °C [7,8]. The next challenge is to find a weak interface or interlayer for an oxide/oxide sys￾tem. LaPO 4, a monazite structure, has recently been intro￾duced as a possible functional interface for oxide/oxide composites by Morgan and Marshall [11--13]. The in￾formation presented is encouraging because it enables fiber-reinforced oxide/oxide composites to withstand high temperatures in oxidizing environments, while maintaining high strength from the strong fibers as well as high toughness from fiber debonding and sliding mechanisms. Thus LaPO4 is a candidate for preventing strong bonding between an oxide fiber and oxide ma￾trix. The tape casting technique has been used in ceramic processing [14-16] to fabricate laminated composites by stacking tapes of different compositions, as well as to incorporate fibers and whiskers into the laminates