Part A: applied scienc and manufacturing ELSEVIER Composites: Part A 30(1999)483-488 Ceramic composites for thermal protection systems J B. Davis, D B. Marshall, K.S. Oka",R. M. Housley, P.E. D Morgan Abstract d coating systems based on monazite, a weak interphase for oxide composites, are being in as a means to increase the service temperatures of thermal protection blankets for re-entry space craft. Preliminary evaluations, including chemical compatibility tensile strengths of coated, heat-treated fibers and fabrics, and durability in a modulated wind tunnel facility have been conducted. c 1999 Elsevier Science Ltd. All rights reserved Keywords: Thermal protection systems; Coatings; Oxide composites 1. Introduction alumino-silicate fiber (Nextel 440), while the coating (matrix) is a tw Thermal protection blankets, consisting of refractory and Al2O,. Recent studies have shown that LaPO4 forms fiber batting sandwiched between two sheets of woven cera- weak interfaces in oxide composites and is compatible at mic fabric, are of interest as a lower cost alternative to rigid high temperatures(> 1400C)with several oxides that have tiles for protection of re-entry vehicles. These blankets potential as high temperature reinforcements(e.g. Al2O3, require a coating on the outer woven sheet that infiltrates ZrO2, mullite and YAG)[2-4] and stiffens the fabric to provide an aerodynamic surface The coating must act as a high temperature starch, without causing embrittlement of the fabric. Since the coated fabric 2 Experimental procedure layer is essentially a thin ceramic matrix composite(the infiltrated coating being the matrix) the requirements for 2/Coating development blanket durability are the same as those for damage toler ance in structural CMCs a weak bond is needed between the To achieve optimum properties, LaPOa-based coatings matrix and the fibers to prevent embrittlement must be formed with a 1: I ratio of La: P. The presence of Blankets consisting of silica fiber fabrics and insula excess La or P causes detrimental reactions. In this study, are used to protect the upper surface of the Space Shuttle the monazite was formed from an aqueous precursor solu- Orbiter. These blankets which are coated with a silica-based tion of La ions and P-containing complexes. The precursor coating commonly referred to as C-9 are rated for multiple was used both to coat fabrics directly and to produce mona use at service temperatures to 650C [1]. At higher temp tures, the silica-based coatings become strongly bonded to coatings. The La P ratio in the precursor was adjusted using the fibers, embrittling the outer fabric and limiting their an iterative method. A sample of the precursor was used to useful life. Development of more refractory blanket materi form a powder by calcining at low temperatures in a closed als and compatible coatings, with temperature capabilities crucible; the powder was mixed with small sapphire crystals in the range 1000-1100C. would allow use of blankets on and cold pressed into a pellet; the pellet was packed in additional surfaces of the Orbiter, as well as on new spa additional loose monazite buffer powder in a closed crucible vehicles and fired to 1200-1400C, the fired pellet was then broken This paper presents a preliminary assessment of a new apart and the sapphire crystals which were exposed on the fabric/coating system suitable for use in this temperature fracture surface were examined by scanning electron micro- range. The fabric consists of a commercially available scopy and energy-dispersive X-ray spectroscopy to identify any reaction products. When the solutions were rich in La either LaAlO3 or LaAl1O1s phases formed on the sapphire orresponding author depending on the firing temperature: when the solutions (/99/.see front matter e 1999 Elsevier Science Ltd. All rights reserved 59-835X(98)00138-9Ceramic composites for thermal protection systems J.B. Davisa , D.B. Marshalla , K.S. Okab,*, R.M. Housley, P.E.D. Morgan a Rockwell Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360, USA b The Boeing Company, Boeing Defense and Space Group, 12214 Lakewood Boulevard, Downey, CA 90242, USA Abstract Advanced coating systems based on monazite, a weak interphase for oxide composites, are being investigated as a means to increase the service temperatures of thermal protection blankets for re-entry space craft. Preliminary evaluations, including chemical compatibility, tensile strengths of coated, heat-treated fibers and fabrics, and durability in a modulated wind tunnel facility have been conducted. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Thermal protection systems; Coatings; Oxide composites 1. Introduction Thermal protection blankets, consisting of refractory fiber batting sandwiched between two sheets of woven cera￾mic fabric, are of interest as a lower cost alternative to rigid tiles for protection of re-entry vehicles. These blankets require a coating on the outer woven sheet that infiltrates and stiffens the fabric to provide an aerodynamic surface. The coating must act as a high temperature starch, without causing embrittlement of the fabric. Since the coated fabric layer is essentially a thin ceramic matrix composite (the infiltrated coating being the matrix) the requirements for blanket durability are the same as those for damage toler￾ance in structural CMCs: a weak bond is needed between the matrix and the fibers to prevent embrittlement. Blankets consisting of silica fiber fabrics and insulation are used to protect the upper surface of the Space Shuttle Orbiter. These blankets which are coated with a silica-based coating commonly referred to as C-9 are rated for multiple use at service temperatures to 6508C [1]. At higher tempera￾tures, the silica-based coatings become strongly bonded to the fibers, embrittling the outer fabric and limiting their useful life. Development of more refractory blanket materi￾als and compatible coatings, with temperature capabilities in the range 1000–11008C, would allow use of blankets on additional surfaces of the Orbiter, as well as on new space vehicles. This paper presents a preliminary assessment of a new fabric/coating system suitable for use in this temperature range. The fabric consists of a commercially available alumino-silicate fiber (Nextel 440), while the coating (matrix) is a two-phase mixture of LaPO4 (La-monazite) and Al2O3. Recent studies have shown that LaPO4 forms weak interfaces in oxide composites and is compatible at high temperatures ( . 14008C) with several oxides that have potential as high temperature reinforcements (e.g. Al2O3, ZrO2, mullite and YAG) [2–4]. 2. Experimental procedures 2.1. Coating development To achieve optimum properties, LaPO4-based coatings must be formed with a 1:1 ratio of La:P. The presence of excess La or P causes detrimental reactions. In this study, the monazite was formed from an aqueous precursor solu￾tion of La ions and P-containing complexes. The precursor was used both to coat fabrics directly and to produce mona￾zite powders which were dispersed in water to form slurry coatings. The La:P ratio in the precursor was adjusted using an iterative method. A sample of the precursor was used to form a powder by calcining at low temperatures in a closed crucible; the powder was mixed with small sapphire crystals and cold pressed into a pellet; the pellet was packed in additional loose monazite buffer powder in a closed crucible and fired to 1200–14008C; the fired pellet was then broken apart and the sapphire crystals which were exposed on the fracture surface were examined by scanning electron micro￾scopy and energy-dispersive X-ray spectroscopy to identify any reaction products. When the solutions were rich in La, either LaAlO3 or LaAl11O18 phases formed on the sapphire, depending on the firing temperature: when the solutions Composites: Part A 30 (1999) 483–488 1359-835X/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S1359-835X(98)00138-9 * Corresponding author