PR Jackson et al. / Materials Science and Engineering A 454-455(2007)590-601 bstantially improve the high-temperature oxidation resistance Nextel M720/alumina composite and reported excellent fatigue 6. The need for environmentally stable composites moti- resistance and 100% retention of tensile strength at 1200C vated the development of CMCs based on environmentally stable The fatigue limit(based on a run-out condition of 10 cycles) oxide constituents [24-32] was 170 MPa(88% UTS at 1200C). However, the creep pe c More recently it has been demonstrated that similar crack- formance at 1200 C was poor. Creep run-out(defined as 100h filecting behavior can also be achieved by means of a finely at creep stress)was achieved only at stress levels below 50% distributed porosity in the matrix instead of a separate interface UTS between matrix and fibers [33]. This microstructural design phi- Because creep was shown to be considerably more damag losophy implicitly accepts the strong fiber/matrix interface. It ing than cyclic loading to oxide-oxide CMCs with porous matrix builds on the experience with porous interlayers as crack deflec- [50,51], high-temperature creep resistance remains among the tion paths 34,35] and extends the concept to utilize a porous key issues that must be addressed before using these materials matrix as a surrogate. The concept has been successfully demon- in advanced aerospace applications. The objective of this study strated for oxide-oxide composites [24, 28, 32, 36-39]. Resulting is to investigate the effects of monazite fiber coating on com- oxide/oxide Cmcs exhibit damage tolerance combined with pressive creep resistance of NextelM610/alumina(N610/A) inherent oxidation resistance. However, due to the strong bond- composite with a porous matrix. Keller et al. [52] investigated ing between the fiber and matrix, a minimum matrix porosity is the effectiveness of monazite coatings in Nextel M610/alumina needed for this concept to work [40]. An extensive review of the porous matrix composites after long-term exposure at and mechanisms and mechanical properties of porous matrix CMCs 1200.C Coated fiber samples exhibited better tensile strength is given in [41]. retention after 1000 h at 1200C when compared to the control For a dense(>90%)matrix composite, an interfacial coat-(uncoated fiber) material. Ruggles-Wrenn et al. [53]reported ing is needed for crack deflection. An extensive review of the that the use of monazite coating in a NextelM610/alumina research on oxide coatings for oxide and non-oxide compos- porous matrix CMC resulted in improved tensile creep resistance ites has been given by Kerans et al. [42]. The development at 900C. This effort investigates the compressive creep-rupture of oxide-oxide composites that rely on a weak fiber/matrix behavior of the NextelTM610/monazite/alumina(N610/M/A) interface for crack deflection prompted research into oxidation- composite developed at the Air Force Research Labora- resistant fiber coatings that are chemically stable with the tory(AFRL/MLLN), Materials and Manufacturing Directorate omposite constituents. Monazite (Lapo4) and scheelite are Compressive creep-rupture tests were conducted at 900 and among the various oxidation-resistant coating materials that 1100 C for compressive creep stresses ranging from -50 have been investigated. Numerous studies examined composites to -95 MPa. Composite microstructure, damage and failure gan and Marshall [31, 43, mechanisms as well as effects of variation in microstruc Morgan et al. 144] and Chawla et al. [45] showed that due to ture on mechanical response were examined. While differences the chemical compatibility of monazite with alumina at high in processing and consequently in the composite microstruc temperature, monazite was a good candidate for a weak inter- ture did not have a significant effect on tensile respons face material for alumina-based composites. Since then, multiple of the CMC, effects on the compressive properties were investigations into the production of monazite coatings and its dramatic use with different fiber/matrix combinations [46-48] have been carried out. Degradation of fiber strength caused by the coating 2. Material and experimental arrangements nd long-term, high-temperature heat treatments was identified as the key problem with monazite coatings [48]. Boakye et al. The NextelTM610/monazite/alumina and Nextel TM610/ [49]explored the effects of different liquid precursors on coating alumina composites were processed as described elsewhere characteristics and tensile strength of coated fibers, and devel- [ 53]. Both the monazite-containing billets and the control oped monazite coating that did not significantly degrade the fiber (N610/A)billets consisted of eight layers in a symmetric cross- ply orientation of [(0/90))2s. Billet properties, namely fiber Porous matrix oxide/oxide CMCs exhibit several behavior volume fraction (Vr) and density, are summarized in Table 1 trends that are distinctly different from those exhibited by tradi- where properties of N610/M/A and n610/a billets used in tional CMCs with a fiber/matrix interface. For these materials, prior work [53] are also included. Samples produced using fatigue is significantly more damaging than creep. On the other the monazite-coated NextelTM610 fiber exhibited bulk den hand, recent investigations into the high-temperature mechan- sities of 2.48-2.68 g/em,, with 35 vol. composite porosity ical behavior of porous matrix oxide-oxide CMCs [50,51] and 45-50 vol %o matrix porosity (including large microcracks). revealed that creep loading was considerably more damaging Micrographs of the as-processed material shown in Fig. I reveal than fatigue. Zawada et al. [50] have shown that a porous shrinkage and sintering cracks that occurred during the cooling matrix Nextel M610/aluminosilicate composite exhibited high stage of the composite processing. Fig. 1(a)shows extensive sur fatigue limit, long fatigue life and near 100% strength reten- face microcracking, while interlaminar matrix cracks are seen tion at 1000oC. However, creep lives were short, indicating in Fig. 1(b). The N610/M/A specimens were cut from 10 dif- low creep resistance and limiting the use of that CMC to tem- ferent billets, and N610/A specimens from 5 different billets peratures below 1000C. Ruggles-Wrenn et al. [51] examined Specimen numbers contain reference to the billet number. For high-temperature fatigue and creep behaviors of a porous matrix example, number Bl-1 refers to the specimen 1 from billet 1P.R. Jackson et al. / Materials Science and Engineering A 454–455 (2007) 590–601 591 substantially improve the high-temperature oxidation resistance [23]. The need for environmentally stable composites moti￾vated the development of CMCs based on environmentally stable oxide constituents [24–32]. More recently it has been demonstrated that similar crack￾deflecting behavior can also be achieved by means of a finely distributed porosity in the matrix instead of a separate interface between matrix and fibers [33]. This microstructural design phi￾losophy implicitly accepts the strong fiber/matrix interface. It builds on the experience with porous interlayers as crack deflec￾tion paths [34,35] and extends the concept to utilize a porous matrix as a surrogate. The concept has been successfully demon￾strated for oxide–oxide composites[24,28,32,36–39]. Resulting oxide/oxide CMCs exhibit damage tolerance combined with inherent oxidation resistance. However, due to the strong bond￾ing between the fiber and matrix, a minimum matrix porosity is needed for this concept to work [40]. An extensive review of the mechanisms and mechanical properties of porous matrix CMCs is given in [41]. For a dense (>90%) matrix composite, an interfacial coat￾ing is needed for crack deflection. An extensive review of the research on oxide coatings for oxide and non-oxide compos￾ites has been given by Kerans et al. [42]. 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. Monazite (LaPO4) and scheelite are among the various oxidation-resistant coating materials that have been investigated. Numerous studies examined composites containing monazite coatings. Morgan and Marshall [31,43], Morgan et al. [44] and Chawla et al. [45] showed that due to the chemical compatibility of monazite with alumina at high temperature, monazite was a good candidate for a weak inter￾face material for alumina-based composites. Since then, multiple investigations into the production of monazite coatings and its use with different fiber/matrix combinations [46–48] have been carried out. Degradation of fiber strength caused by the coating and long-term, high-temperature heat treatments was identified as the key problem with monazite coatings [48]. Boakye et al. [49] explored the effects of different liquid precursors on coating characteristics and tensile strength of coated fibers, and devel￾oped monazite coating that did not significantly degrade the fiber strength. Porous matrix oxide/oxide CMCs exhibit several behavior trends that are distinctly different from those exhibited by tradi￾tional CMCs with a fiber/matrix interface. For these materials, fatigue is significantly more damaging than creep. On the other hand, recent investigations into the high-temperature mechan￾ical behavior of porous matrix oxide–oxide CMCs [50,51] revealed that creep loading was considerably more damaging than fatigue. Zawada et al. [50] have shown that a porous matrix NextelTM610/aluminosilicate composite exhibited high fatigue limit, long fatigue life and near 100% strength reten￾tion at 1000 ◦C. However, creep lives were short, indicating low creep resistance and limiting the use of that CMC to tem￾peratures below 1000 ◦C. Ruggles-Wrenn et al. [51] examined high-temperature fatigue and creep behaviors of a porous matrix NextelTM720/alumina composite and reported excellent fatigue resistance and 100% retention of tensile strength at 1200 ◦C. The fatigue limit (based on a run-out condition of 105 cycles) was 170 MPa (88% UTS at 1200 ◦C). However, the creep per￾formance at 1200 ◦C was poor. Creep run-out (defined as 100 h at creep stress) was achieved only at stress levels below 50% UTS. Because creep was shown to be considerably more damag￾ing than cyclic loading to oxide–oxide CMCs with porous matrix [50,51], high-temperature creep resistance remains among the key issues that must be addressed before using these materials in advanced aerospace applications. The objective of this study is to investigate the effects of monazite fiber coating on com￾pressive creep resistance of NextelTM610/alumina (N610/A) composite with a porous matrix. Keller et al. [52] investigated the effectiveness of monazite coatings in NextelTM610/alumina porous matrix composites after long-term exposure at 1100 and 1200 ◦C. Coated fiber samples exhibited better tensile strength retention after 1000 h at 1200 ◦C when compared to the control (uncoated fiber) material. Ruggles-Wrenn et al. [53] reported that the use of monazite coating in a NextelTM610/alumina porous matrix CMC resulted in improved tensile creep resistance at 900 ◦C. This effort investigates the compressive creep–rupture behavior of the NextelTM610/monazite/alumina (N610/M/A) composite developed at the Air Force Research Labora￾tory (AFRL/MLLN), Materials and Manufacturing Directorate. Compressive creep–rupture tests were conducted at 900 and 1100 ◦C for compressive creep stresses ranging from −50 to −95 MPa. Composite microstructure, damage and failure mechanisms, as well as effects of variation in microstruc￾ture on mechanical response were examined. While differences in processing and consequently in the composite microstruc￾ture did not have a significant effect on tensile response of the CMC, effects on the compressive properties were dramatic. 2. Material and experimental arrangements The NextelTM610/monazite/alumina and NextelTM610/ alumina composites were processed as described elsewhere [53]. Both the monazite-containing billets and the control (N610/A) billets consisted of eight layers in a symmetric cross￾ply orientation of [(0◦/90◦)]2s. Billet properties, namely fiber volume fraction (Vf) and density, are summarized in Table 1, where properties of N610/M/A and N610/A billets used in prior work [53] are also included. Samples produced using the monazite-coated NextelTM610 fiber exhibited bulk den￾sities of 2.48–2.68 g/cm3, with 35 vol.% composite porosity and 45–50 vol.% matrix porosity (including large microcracks). 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. Fig. 1(a) shows extensive sur￾face microcracking, while interlaminar matrix cracks are seen in Fig. 1(b). The N610/M/A specimens were cut from 10 dif￾ferent billets, and N610/A specimens from 5 different billets. Specimen numbers contain reference to the billet number. For example, number B1-1 refers to the specimen 1 from billet 1