ournal I Amm Ceram Soc. 81 [10] 2738-40(1998) Platinum as a Weak Interphase for Fiber-Reinforced Oxide-Matrix Composites J. Wendorff. R janssen and N. Claussen Advanced Ceramics Group, Technische Universitat Hamburg-Harburg, D-21071 Hamburg, Germany Alumina/zirconia matrix composites with platinum-coated ready proved effective, and some other investigations show the sapphire fibers were fabricated using the technology of re potential for weak interfaces on oxide ceramic substrates. 9 action bonding of aluminum oxide(RBAO)as the synthesis Some authors have used mixtures of precious metals and a oute. The interfacial behavior of both sintered and an- fugitive phase to form porous coatings during processing, but nealed specimens was investigated by crack path observa- there are still limitations in view of coating homogeneity and ions and fiber pushout tests. The results show that pre degrading mechanical properties of the composites. 10), II The cious metals like platinum or related alloys can be used as compatibility of dense platinum and iridium coatings with alu- igh-temperature interphases for oxide/oxide composites. mina or nonoxide fibers and matrices has already been claimed, Weak interfaces were obtained both between fiber and but the possibility of controlling the interfacial properties with should be suitable to achieve a damage-tolerant behavior of the trin was presnforsed with adhi the a a dense conata mm. coating of the fibers was used to create a weak fiber/matrix L. Introduction interphase. The main emphasis was on investigating the inter- facial behavior of these composites using crack path observa- URE oxide fiber composites can be used at high tions and single fiber pushout tests ven in oxidizing environments even with ctive coatings to prevent degradation. The mage-tolerant fracture behavior of the composi Il. Experimental Procedure material is the control of the fiber/matrix interface. The inter- face usually has to be weak enough to allow fiber/matrix Reaction-bonded aluminum oxide(BAO) was used as ma- debonding and fiber pullout during fracture. For lower trix material, s, I6 The precursor powder was fabricated by at- trition milling of metal ceramic powder mixtures for 7 h in temperature regimes, control of matrix porosity or use of po- acetone. The starting composition was 40 vol% Al(ECKA AS ous oxide coatings is an appropriate tool to adjust the inter- 081, Eckart-Werke GmbH, Furth, FRG), 40 vol% Al203(Cer- facial properties. 2,3 However, at higher temperatures densification of the matrix takes place, leading to strong bond alox MPA 4, RWE-DEA AG, Brunsbuttel, FRG), and 20 vol% ing between fiber and matrix, preventing the required fiber/ partially stabilized ZrO2(TZ-2Y, Tosoh Co., Tokyo, Japan) matrix debonding. Therefore, several attempts have been made After drying and sieving(200 um mesh), the powder was pressed uniaxially together with sapphire fibers followed by to achieve weak interfaces, i.e., by coating the fibers with re- cold isostatic compaction at 300 MPa. The reinforcing fibers fractory metals(W, Mo, Cr), coatings like carbon which form fugitive reaction products, or dense oxides like ZrO2. 4. Lim- with a diameter of =120 um(Saphikon Inc, Milford, NH)we dip coated with a platinum slurry prior to composite fabrica- ited oxidation resistance of metals, poor reproducibility due to tion. First tests were also performed with fibers coated with nonuniform reaction, or severe fiber strength degradation due to reactions between coatings and fibers limits the applicability platinum by PVD. Both coating techniques were applied at of these coating concepts. Recently, improvements have been room temperature. Heat treatment was carried out in a conven- made using phosphate coatings like monazite. Fiber/matrix tional box furnace in air at ambient pressure. A two-step heat debonding and the absence of detrimental reactions between ing cycle was chosen to convert the metallic aluminum into lead to he matrix material at temperatures up to dense composite materials still require pressure-assisted sinter 1 100 and 1550C, respectively. Part of the samples were an- ng procedures like hot pressing, and furthermore, more com- nealed at 1550oC up to 50 h to investigate long-time stability of rehensive studies are necessary to investigate high-temperature havior 6, 7 After sintering and annealing, the samples were cut into slices of 0.4 mm thickness with a high-precision diamond saw 4 Another possibility is the use of precious metals or alloys th high melting temperatures like platinum or iridium. Ap (EXACT GmbH, Norderstedt, FRG). Further polishing was not plications as protective coatings or diffusion barriers have al necessary and therefore avoided in order to reduce damage of the interfaces between fibers coating, and matrix. Previous investigations confirmed that this procedure is suitable to ob- tain damage-free slices of the composite. 7 For initial test of the interfacial behavior. Vickers indentations were used. Crack R.J. Kerans--contributine editor ath propagation at unstable crack growth was investigate with a specially designed cleavage apparatus using notched slices of composites, based on an idea of Morgan and Mar- Manuscript No, 190455. Received December 22, 1997; approved June 30, 1998. shall. Crack propagation during loading was investigated un- der an optical microscope. Furthermore, single fiber pushout tests were performed in Supported by the German Research Foundation(DFG)under Contract No Ja strength from the peak value splacement curves Member, American Ceramic Society The apparatus used is described in elsewhere I8 2738Platinum as a Weak Interphase for Fiber-Reinforced Oxide-Matrix Composites J. Wendorff, R. Janssen,* and N. Claussen* Advanced Ceramics Group, Technische Universität Hamburg-Harburg, D-21071 Hamburg, Germany Alumina/zirconia matrix composites with platinum-coated sapphire fibers were fabricated using the technology of re￾action bonding of aluminum oxide (RBAO) as the synthesis route. The interfacial behavior of both sintered and an￾nealed specimens was investigated by crack path observa￾tions and fiber pushout tests. The results show that pre￾cious metals like platinum or related alloys can be used as high-temperature interphases for oxide/oxide composites. Weak interfaces were obtained both between fiber and coating as well as between matrix and coating, which should be suitable to achieve a damage-tolerant behavior of the composite. I. Introduction PURE oxide fiber composites can be used at high tempera￾tures even in oxidizing environments even without addi￾tional protective coatings to prevent degradation. The key fac￾tor for damage-tolerant fracture behavior of the composite material is the control of the fiber/matrix interface. The inter￾face usually has to be weak enough to allow fiber/matrix debonding and fiber pullout during fracture.1 For lower￾temperature regimes, control of matrix porosity or use of po￾rous oxide coatings is an appropriate tool to adjust the inter￾facial properties.2,3 However, at higher temperatures, densification of the matrix takes place, leading to strong bond￾ing between fiber and matrix, preventing the required fiber/ matrix debonding. Therefore, several attempts have been made to achieve weak interfaces, i.e., by coating the fibers with re￾fractory metals (W, Mo, Cr), coatings like carbon which form fugitive reaction products, or dense oxides like ZrO2. 4,5 Lim￾ited oxidation resistance of metals, poor reproducibility due to nonuniform reaction, or severe fiber strength degradation due to reactions between coatings and fibers limits the applicability of these coating concepts. Recently, improvements have been made using phosphate coatings like monazite. Fiber/matrix debonding and the absence of detrimental reactions between the components lead to encouraging results. However, fully dense composite materials still require pressure-assisted sinter￾ing procedures like hot pressing, and furthermore, more com￾prehensive studies are necessary to investigate high-temperature behavior.6,7 Another possibility is the use of precious metals or alloys with high melting temperatures like platinum or iridium. Ap￾plications as protective coatings or diffusion barriers have al￾ready proved effective, and some other investigations show the potential for weak interfaces on oxide ceramic substrates.8,9 Some authors have used mixtures of precious metals and a fugitive phase to form porous coatings during processing, but there are still limitations in view of coating homogeneity and degrading mechanical properties of the composites.10,11 The compatibility of dense platinum and iridium coatings with alu￾mina or nonoxide fibers and matrices has already been claimed, but the possibility of controlling the interfacial properties with these coatings has not been demonstrated yet.12–14 In the present study, a reaction-bonded alumina/zirconia ma￾trix was reinforced with sapphire fibers. A dense platinum coating of the fibers was used to create a weak fiber/matrix interphase. The main emphasis was on investigating the inter￾facial behavior of these composites using crack path observa￾tions and single fiber pushout tests. II. Experimental Procedure Reaction-bonded aluminum oxide (RBAO) was used as ma￾trix material.15,16 The precursor powder was fabricated by at￾trition milling of metal ceramic powder mixtures for 7 h in acetone. The starting composition was 40 vol% Al (ECKA AS 081, Eckart-Werke GmbH, Fürth, FRG), 40 vol% Al2O3 (Cer￾alox MPA 4, RWE-DEA AG, Brunsbüttel, FRG), and 20 vol% partially stabilized ZrO2 (TZ-2Y, Tosoh Co., Tokyo, Japan). After drying and sieving (200 mm mesh), the powder was pressed uniaxially together with sapphire fibers followed by cold isostatic compaction at 300 MPa. The reinforcing fibers with a diameter of ≈120 mm (Saphikon Inc., Milford, NH) were dip coated with a platinum slurry prior to composite fabrica￾tion. First tests were also performed with fibers coated with platinum by PVD. Both coating techniques were applied at room temperature. Heat treatment was carried out in a conven￾tional box furnace in air at ambient pressure. A two-step heat￾ing cycle was chosen to convert the metallic aluminum into Al2O3 and sinter the matrix material at temperatures up to 1100° and 1550°C, respectively. Part of the samples were an￾nealed at 1550°C up to 50 h to investigate long-time stability of the coatings. After sintering and annealing, the samples were cut into slices of 0.4 mm thickness with a high-precision diamond saw (EXACT GmbH, Norderstedt, FRG). Further polishing was not necessary and therefore avoided in order to reduce damage of the interfaces between fibers, coating, and matrix. Previous investigations confirmed that this procedure is suitable to ob￾tain damage-free slices of the composite.17 For initial test of the interfacial behavior, Vickers indentations were used. Crack path propagation at unstable crack growth was investigated with a specially designed cleavage apparatus using notched slices of composites, based on an idea of Morgan and Mar￾shall.6 Crack propagation during loading was investigated un￾der an optical microscope. Furthermore, single fiber pushout tests were performed in order to determine the interfacial strength from the peak value of the force–displacement curves. The apparatus used is described in detail elsewhere.18 R. J. Kerans—contributing editor Manuscript No. 190455. Received December 22, 1997; approved June 30, 1998. Presented at the 1st Conference on Composites at Lake Louise ’97, Lake Louise, Canada, October 12–17, 1997. Supported by the German Research Foundation (DFG) under Contract No. Ja 655/2.*Member, American Ceramic Society. J. Am. Ceram. Soc., 81 [10] 2738–40 (1998) Journal 2738