M B. Ruggles-Wrenn et al /Composites Science and Technology 66(2006 )2089-2099 not substantially improve the high temperature oxidation objective of this study was to investigate the effectiveness resistance[12]. The need for environmentally stable com- of using monazite coatings in a crossply NextelM610/alu posites led to the development of CMCs based on oxide mina composite to improve the creep resistance of the constituents [13-21 CMC. Keller et al. [39] investigated the effectiveness of It is widely accepted that in order to avoid brittle frac- monazite coatings in Nextel 610/alumina composites after ture behavior in CMCs and improve the damage tolerance, long-term exposure at 1100 and 1200C Coated fiber sam- a weak fiber/matrix interface is needed, which serves to ples exhibited better tensile strength retention after 1000 h deflect matrix cracks and to allow subsequent fiber pull- at 1200C when compared to the control(uncoated fiber out [22-25]. A matrix consisting of finely distributed poros- material. This effort investigates the creep-rupture behavior o y can provide a similar crack-deflecting behavior instead of the Nextel 610/Monazite/Alumina(N610/M/A)com- of a separate interface between matrix and fibers [26]. In posite developed at the Air Force Research Laboratory this case, there is strong bonding between the fiber and(AFRL/MLLN), Materials and Manufacturing Director matrix; consequently, there is a minimum matrix porosity ate. Creep-rupture tests were conducted at temperatures needed for this concept to work [27]. For a dense (90%) in the 900-1 100C range for creep stresses ranging from matrix composite, however, an interfacial coating is needed 40 to 150 MPa Resulting creep performance imposes lim oxide coatings for oxide and non-oxide composites has applications. The composite microstructure, as wlle for crack deflection. An extensive review of the research on itations on the use of this material in high-temperature been given by Kerans et al. [28]. damage and failure mechanisms are discussed The development of oxide-oxide composites that rely on a weak fiber/matrix interface for crack deflection prompte 2. Material and experimental arrangements research into oxidation resistant fiber coatings that are hemically stable with the composite constituents. Various Nextel 610 fiber tows were desized in air at 1100C and oxidation-resistant coating materials have been investi- then coated with a monazite precursor solution at 1100oC gated, including monazite(LaPO4) and scheelite Of these, As described elsewhere [35] this monazite sol was washed a significant amount of work has been completed on com- multiple times to remove any residual ions, which have been posites containing monazite coatings. The initial works of associated with fiber strength loss. After coating, the fibers Morgan and Marshall [20, 29, 30] showed the potential of were filament wound using an alumina slurry consisting of a monazite as a weak interface material, particularly for alu- polymeric alumina sol and alumina powder (AKP-53 mina-based composites, due to the chemical compatibility Sumitomo Corp. ) In addition, uncoated fibers were wound of monazite with alumina at high temperature. Since then, for control samples. As described by Keller et al. [39], dur- numerous studies have examined the production of mona- ing the winding process, the number of wheel revolutions zite coatings and its use with different fiber/matrix combi- was recorded to determine the number of fiber tows within nations [31-35]. Fiber strength degradation caused by the a given tape width. This value was then used to calculate the coating and long-term, high-temperature heat treatments volume percentage of fibers in the composite. The resultant was identified as the key problem with monazite coatings unidirectional tapes were cut from the spool, stacked into a Boakye et al. [35] investigated the effects of different metal mold in an 8-layer symmetric cross-ply orientation of liquid precursors on coating characteristics and tensile [(0%/90%)]s, and warm vacuum-molded to form green cross- strength of coated fibers, and developed monazite coating ply composites. After drying, the billets were sintered at that did not cause significant loss of fiber strength 1200C for 5 h in air. During the heating cycle, a one hour Porous-matrix oxide/oxide CMCs exhibit several behav- hold at low temperature was performed to remove any or trends that are distinctly different from those exhibited residual organics in the matrix. Control billets, N610/Alu by traditional CMCs with a fiber-matrix interface. Most mina(N610/A), containing uncoated fibers were produced Sic-fiber-containing CMCs exhibit longer life under static with the same procedure. Billet properties, namely fiber loading and shorter life under cyclic loading [36]. For these volume fraction and density, are summarized in Table I materials, fatigue is significantly more damaging than Micrographs of the as-processed material shown in Fig. I creep. Zawada et al. [37] examined the high-temperature reveal shrinkage and sintering cracks that occurred during mechanical behavior of a porous matrix Nextel 610/alumi- the cooling stage of the composite processing. Extensive nosilicate composite Results revealed excellent fatigue per- surface microcracking is seen in Fig. 1(a), while Fig. I(b) formance at 1000C, the material exhibited high fatigue shows interlaminar matrix cracks. limit, long fatigue life and near 100% strength retention The N610/M/A specimens were cut from seven different Conversely, creep lives were short, indicating low creep billets, and N610/A specimens from three different billets. resistance and limiting the use of that CMC to tempera- Specimen numbers contain reference to the billet number. tures below 1000 For example, number Bl-I refers to the specimen I from Because creep is shown to be considerably more damag- billet 1. Billets were cut into flat rectangular coupons, which ing to porous-matrix oxide/oxide CMCs [37,38) creep test- were machined into dog bone-shaped specimens shown in ing is well suited for assessing the long term durability an nd Fig. 2. Diamond-grit grinding was used for billets Bl-B8 gh-temperature erformance of these materials. The and the abrasive water-jet machine, for billets B9-Bllnot substantially improve the high temperature oxidation resistance [12]. The need for environmentally stable com￾posites led to the development of CMCs based on oxide constituents [13–21]. It is widely accepted that in order to avoid brittle frac￾ture behavior in CMCs and improve the damage tolerance, a weak fiber/matrix interface is needed, which serves to deflect matrix cracks and to allow subsequent fiber pull￾out [22–25]. A matrix consisting of finely distributed poros￾ity can provide a similar crack-deflecting behavior instead of a separate interface between matrix and fibers [26]. In this case, there is strong bonding between the fiber and matrix; consequently, there is a minimum matrix porosity needed for this concept to work [27]. For a dense (>90%) matrix composite, however, an interfacial coating is needed for crack deflection. An extensive review of the research on oxide coatings for oxide and non-oxide composites has been given by Kerans et al. [28]. The development of oxide–oxide composites that rely on a weak fiber/matrix interface for crack deflection prompted research into oxidation resistant fiber coatings that are chemically stable with the composite constituents. Various oxidation-resistant coating materials have been investi￾gated, including monazite (LaPO4) and scheelite. Of these, a significant amount of work has been completed on com￾posites containing monazite coatings. The initial works of Morgan and Marshall [20,29,30] showed the potential of monazite as a weak interface material, particularly for alu￾mina-based composites, due to the chemical compatibility of monazite with alumina at high temperature. Since then, numerous studies have examined the production of mona￾zite coatings and its use with different fiber/matrix combi￾nations [31–35]. Fiber strength degradation caused by the coating and long-term, high-temperature heat treatments was identified as the key problem with monazite coatings [34]. Boakye et al. [35] investigated the effects of different liquid precursors on coating characteristics and tensile strength of coated fibers, and developed monazite coating that did not cause significant loss of fiber strength. Porous-matrix oxide/oxide CMCs exhibit several behav￾ior trends that are distinctly different from those exhibited by traditional CMCs with a fiber–matrix interface. Most SiC-fiber-containing CMCs exhibit longer life under static loading and shorter life under cyclic loading [36]. For these materials, fatigue is significantly more damaging than creep. Zawada et al. [37] examined the high-temperature mechanical behavior of a porous matrix Nextel 610/alumi￾nosilicate composite. Results revealed excellent fatigue per￾formance at 1000 C, the material exhibited high fatigue limit, long fatigue life and near 100% strength retention. Conversely, creep lives were short, indicating low creep resistance and limiting the use of that CMC to tempera￾tures below 1000 C. Because creep is shown to be considerably more damag￾ing to porous-matrix oxide/oxide CMCs [37,38], creep test￾ing is well suited for assessing the long term durability and high-temperature performance of these materials. The objective of this study was to investigate the effectiveness of using monazite coatings in a crossply NextelTM610/alu￾mina composite to improve the creep resistance of the CMC. Keller et al. [39] investigated the effectiveness of monazite coatings in Nextel 610/alumina composites after long-term exposure at 1100 and 1200 C. Coated fiber sam￾ples exhibited better tensile strength retention after 1000 h at 1200 C when compared to the control (uncoated fiber) material. This effort investigates the creep-rupture behavior of the Nextel 610/Monazite/Alumina (N610/M/A) com￾posite developed at the Air Force Research Laboratory (AFRL/MLLN), Materials and Manufacturing Director￾ate. Creep-rupture tests were conducted at temperatures in the 900–1100 C range for creep stresses ranging from 40 to 150 MPa. Resulting creep performance imposes lim￾itations on the use of this material in high-temperature applications. The composite microstructure, as well as damage and failure mechanisms are discussed. 2. Material and experimental arrangements Nextel 610 fiber tows were desized in air at 1100 C and then coated with a monazite precursor solution at 1100 C. As described elsewhere [35], this monazite sol was washed multiple times to remove any residual ions, which have been associated with fiber strength loss. After coating, the fibers were filament wound using an alumina slurry consisting of a polymeric alumina sol and alumina powder (AKP-53, Sumitomo Corp.). In addition, uncoated fibers were wound for control samples. As described by Keller et al. [39], dur￾ing the winding process, the number of wheel revolutions was recorded to determine the number of fiber tows within a given tape width. This value was then used to calculate the volume percentage of fibers in the composite. The resultant unidirectional tapes were cut from the spool, stacked into a metal mold in an 8-layer symmetric cross-ply orientation of [(0/90)]2S, and warm vacuum-molded to form green cross￾ply composites. After drying, the billets were sintered at 1200 C for 5 h in air. During the heating cycle, a one hour hold at low temperature was performed to remove any residual organics in the matrix. Control billets, N610/Alu￾mina (N610/A), containing uncoated fibers were produced with the same procedure. Billet properties, namely fiber volume fraction and density, are summarized in Table 1. Micrographs of the as-processed material shown in Fig. 1 reveal shrinkage and sintering cracks that occurred during the cooling stage of the composite processing. Extensive surface microcracking is seen in Fig. 1(a), while Fig. 1(b) shows interlaminar matrix cracks. The N610/M/A specimens were cut from seven different billets, and N610/A specimens from three different billets. Specimen numbers contain reference to the billet number. For example, number B1-1 refers to the specimen 1 from billet 1. Billets were cut into flat rectangular coupons, which were machined into dog bone-shaped specimens shown in Fig. 2. Diamond-grit grinding was used for billets B1–B8, and the abrasive water-jet machine, for billets B9–B11. 2090 M.B. Ruggles-Wrenn et al. / Composites Science and Technology 66 (2006) 2089–2099