urna Am. Ceran.Soc.87151881-887(2004 Effect of Yttrium Aluminum Garnet Additions on Alumina-Fiber Reinforced Porous-Alumina-Matrix Composites Michael K. Cinibulk, *Kristin A Keller, and Tai-Il Mah"+ Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, Ohio 45433-7817 The effects of incorporating yttrium aluminum garnet (YAG) the matrix with the fibers that further degrades fiber strength or degradation of the fiber by volatile byproducts of the matrix fibers were investigated. Composites with various precursors during processing. The presence of silica as a matrix of YAG added to the matrix were prepared to phase accelerates this strength degradation at elevated tempera to 1100-and 1200 C Strengths of YAG-containing composites in atmospheres of high H,o partial pressures. ia-1on temperatures were slightly lower than those of an all-alumina- matrix com- posite after heating for 5 h to 1100 C. However, after heating Nextel 610 fibers were found to retain the strength of the fibers displayed greater strengths and greater strains to failure than the all-alumina composite. At the higher temperature, the 1200 C. 1.I7 However, after heating at 1200C for prolonged presence of YAG is believed to inhibit the densification of the riods(100 h), the porous coating, with initially -20-nm grain matrix, which helps to maintain higher levels of porosity and and pore sizes, densified and the composite strength degraded to weaker interparticle bonding that allows for crack-energy that of a composite without the fiber coating. Subsequently dissipation within the matrix. A reduction in grain growth of initial work with 10 vol% YAG in an alumina matrix showed the fibers by the presence of segregated Y was also observe improved retained strength of both Nextel 610 and 650 fiber which may also contribute to higher fiber strength, thereby reinforced composites compared with similar all-alumina-matrix increasing the retained strengths of the YAG-containing composites after a 1200C heat treatment. The solution-derived composites YAG was originally incorporated during matrix processing with the intent of forming an in situ porous fiber coating in addition to I. Introduction filling particle interstices in the matrix with a fine(20-nm grain and pore size) grained porous YAG phase. However, the formation O Xine composites with poro ts matrices have b en shown to be necessary to provide enhanced strength. Our objective in the distribution of porosity in the matrix is controlled. -Rather than present study was to extend the effort of our initial attempts at relying on a fiber coating to deflect matrix cracks at a weak producing a YAG-alumina-matrix composite by studying the fiber-matrix interface, as in conventional ceramic-matrix compos effects of the Yag volume fraction in the matrix on the micro- ites, porous matrices rely on their inherent weakness to prevent controlled study of the effect of yttrium and YAG on the sintering stress concentrations on the fiber sufficient to fracture it. Matrix and grain growth of alumina at 1200-1300 C to better understand crack energy is dissipated through multiple microfractures in tension and shear in the porous matrix . These materials can their influence on matrices of identical compositions in fiber retain their mechanical properties after long-term exposure to reinforced composites is presented in another paper. 2 1 100C during cyclic tension and fatigue Long-term heat treatments at temperatures above 1 C can n. Experimental Procedure degrade the strength of NextelTM 610 fiber-reinforced porous- alumina-matrix composites. The observed strength degradation is Nextel 610 tows(3M Corp, Minneapolis, MN) were used to more than what would be obtained if the fibers alone were exposed reinforce all composites in this study. Matrices consisted of to a similar thermal treatment. The degree of matrix densifi alumina powder (0. 1-0. 2 um. AKP-53. Sumitomo Chemical Ce cation that occurs in a porous matrix is expected to be limited at Sumitomo, Japan) and/or YAG powder(-I um, Cerelox, Condea emperatures of 1200.C and below and as such would not seem Vista Corp, Tucson. AZ). with a binder of either alumina derived dramatically reduced composite strengths. However, recent work MA)or YAG derived from a polymeric solution. .The matrix has shown that a reduction in matrix porosity, via multiple matrix compositions of each composite, identified as A. YO. 1, AY50 precursor reinfiltrations, of only -17%(29%-24% pore volume YY50. and Y 100. are given in Table L. Composite YO. I was Other reasons forthe th reduction of-64%(220 to 80 MPa). produced using 3000 wt ppm yttria(derived from yttrium nitrate). suIt in with alumina powder to form YAG. This method produced a more omogeneous distribution of a low volume fraction of YA than could be obtained by using the solution-derived YAG. Nextel 610 F. w. Zok--contributing editor tows were filament-wound after passing through a water-based slurry of the appropriate matrix composition. During the filament winding process, the number of tows within a given width was counted to estimate the fiber volume fraction in the final compo pt No. 10272. Received June 9, 2013: approved December 10, 2003 e, which ranged from. 2 to 0.3. After winding, the tape was cut Day m oH es supported by Contract No, F33615-01-C-5214 through and stacked uniaxially in an aluminum mold for warm-press CA under rough vacuum. The green cor e was subsequently d
Journal of the American Ceramic Sociery-Cinibulk et al Vol 87. No 5 Table L. Composition of Composite Matrice Il. Results Composition(vols, solids basis) (1) Processing and Microstructure Alumina YAG Composite powder Binder phase YAG with mean grain sizes of.2 um and uniform porosity distribu- 10 alumina tion, as shown in Figs. I and 2. The alumina-based-matrix Y0.1 3000 ppm yttrium 0. composites contained residual matrix porosity levels of 35-50 nitrate vol%. The microstructure and porosity of the matrix in composite AY50 50 10 alumina Y100 was much coarser than that of the alumina-containing YY50 IO YAG matrices. due to the much larger -I-Hm particle size(Fig. I(c)) Y100 90 IO YAG 000 Incorporation of YAG powder into the alumina matrix was found Assuming complete reaction of 3Y.0,+ 5AL01-2YAl to produce a microstructure of poorly sintered, equiaxed alumina particles containing inclusions of the much larger YAG particles as shown in Figs. I(d)and 2(b)and(e). There was little difference under controlled humidity and then sintered at 1 100 or 1200 for in microstructure when solution-derived YAG or sol-derived either 5 or 100 h in air alumina was used as the binder phase. The purpose of the YAG Scanning and transmission electron microscopies(SEM, Model binder was to form a fine-grained refractory film on all alumina 360FE. Leica. Cambridge, U. K, and TEM. Model CM2OOFEC articles to resist sintering. Figure 3(a) shows the distribution of Philips, Eindhoven, Netherlands, respectively) were used to char- the yag binder in the matrix, where the actual distribution of acterize the microstructures of the composites, Energy-dispersive YAG was discontinuous, with some of the interparticle porosity X-ray spectroscopy(EDS)was used for elemental analysis. TEM filled with YAG. EDS of surfaces of alumina particles and grain specimens were prepared by impregnating the porous samples with boundaries in the matrix detected the presence of Y as either a epoxy and thinning to electron transparency with diamond lapping segregant or as a thin YAG film( Figs. 3(b) and (c), EDS of films, followed by low-angle ion-beam milling, as described in alumina grain boundaries in the fiber near the fiber surface detail elsewhere indicated the presence of trace Y also. in addition to the presence The composites were cut into straight-sided tensile specimens, of trace amounts of Fe and Si(Fig 3(d). The Y is believed to have of -07-cm width and with a 5. 1-cm gauge length, tabbed, and diffused into the fiber from the matrices of all YAG-containing then tested in a universal testing frame. Six specimens were composites during sintering, whereas both Si and Fe were found to obtained for testing from each composite for each heat treatment. be present in the as-received fiber. The microstructure of the Stresses and strains(via digital imaging. VicGauge, Correlated Solutions) were measured to specimen failure. The strengths were the sintering time at 1200'C from 5 to 100 h. ompared by normalizing all composites to a fiber volume fraction Nextel 610 fibers in composites exhibited noticeable grain of 0. 2. assuming the load is carried completely by the fibers. growth after heating at 1200.C for 100 h Grains in the as-received (b) 之 (d)a 10 um Fig. 1. SEM images of polished cross sections of composites(a) A.(b) YO. 1. (c) Y100. and (d) AY50 sintered for 5 h at 1200.C.(a-c)are ondary-electron images, while (d) is a backscattered electron image to highlight YAG in the matrix (bright contrast)
May 2004 Effect of YAG Additions on Porous-Alumina- Matrix composites Fiber (b) Fiber YAG /Fiber 500 nm| AG/S 500nm d Fiber YAG 500nm 2um」 Fig. 2. TEM images of composites (a) YO. I, (b) AY50, (c)YY50, and (d)Y 100 sintered for 5 h at 1200"C. Large matrix grains in(b) and (c)are YAG as noted. Note(a-c) are the same magnification, while(d) is imaged at a lower magnification Nextel 610 fiber average-100 nm in diameter. A comparison of matrix under these conditions. However, after heating at 1200C. tibers in composites A and Y Y50 indicated a larger alumina grain the retained strength is much greater for composites containing size in the all-alumina-matrix composite (Fig. 4) by about a factor YAG than for the all-alumina composite. The highest strength f 2. The extent of fiber grain growth in all composites containing composites have average strengths of over 150 MPa after 5 h of YAG was similar heating, compared with that of A, whose strength is degraded to -50 MPa. After 100 h at 1200C. the Y AG-containing composite (2) Retained Tensile Strength strengths are reduced to-100 MPa, while that of composite A is The average strength of as-received Nextel 610 fiber tows is further reduced to-35 MPa. The strengths obtained with YAGare 1500 MPa, which is reduced to 1250 MPa after heating for 5 h at comparable to those of a similar all-alumina-matrix composite 200 C and is further reduced to 1 100 MPa after heating for 100 h reinforced with Nextel 610 fibers that contain a monazite fiber at 1200 C. 0.l If it is assumed that only fibers are carrying the coating d in the porous matrix composites with a fiber volume fraction Despite the higher strengths of all composites with YAG in of 0.20, a strength of 300 MPa is predicted at full fiber strength the matrix, there does not appear to be a direct correlation However, after heating the fibers alone at 1200 C for 5 h, the between the amount of YAG and the tensile strength. There strength is degraded by-17%, so the strength of the composite does appear to be a correlation between the use of YAG as a would be expected to be -250 MPa if no other fiber degradation binder phase, compared with the use of alumina and strength ccurs due to the presence of a matrix. Similarly, after heating for after heating at 1200C. Figure 6 is a plot of the stress-strain 100 h at 1200oC the strength would be expected to be -220 MPa. behavior of the composites after heating at 1200C for 5 h Figure 5 contains the results of tensile strength tests after Composite A exhibited an average strain to failure of -0.1% heating the composites to 1100 and 1200C. The strengths of All the YAG-containing composites showed greater strains to composites A, AY50, and Y 100 are all very similar after heating failure (0. 2%e-0.3%) than composite A. Figure 7 contain at 1 100C for 5 h, with A having a slightly higher average strength. micrographs of fracture surfaces of composites A and Y Y50 Thus, no apparent benefit is derived by incorporating YAG into the showing very brittle failure of the all-alumina-matrix composite
Journal of the American Ceramic Sociery--Cinibulk et al Vol. 87. No. 5 Grain Y 100m Energy [ke V] (a) Grain Grain Boundary Grain Boundary g Energy [ke V Energy ke V Y. While the signaka trix grain boundary, and (d) Nextel 610 fiber grain boundary near fiber-matrix interface in YO. 1, showing the presence of segregated spectra obtained af e boise ratio is low in the spectra in(d)due to the low amount of Fe, Si, and Y present, these peaks were always discernible in several and a much more fibrous fracture surface for the YAG-alumina Nextel 650 had a tow strength of <200 MPa versus 1 100 MPa for matrix composite after heating for 5 h at 1200C Nextel 610 after heating at 1200 C for 100 h, so a lower strength for the Nextel 650 fiber-reinforced composite would be expecte as well. In the study using Nextel 610 fibers with a porous YAG IV. Discussion fiber coating. it was found that after 100 h at 1200.C, the The results are consistent with previous work that showed that 20-nm-sized pores in the coating had coarsened and then had been climinated when the coating sintered to -90%0 relative YAG-containing alumina-based composites retained greater strengths than an equivalent all-alumina-mat density. This resulted in composites having the same strength shor regardless of whether the YAG fiber coating was present. The In the present study, the strengths were retained even after extended times(100 conclusion was that the dense YAG fiber coating no longer h) at 1200C. Approximately 75% of the strength was retained protected the fiber by deflecting matrix cracks and acted as if no fter the 5-h hold at 1200oC. which is three times greater in ber coating was present, which resulted in decreased strength These observations suggest that the effect of Yag in the matrix in magnitude than that of the control composite A. Two reasons for the strength improvement during longer exposure times compared he present study was to increase the retention of fiber strengt with the earlier studies are the use of Nextel 610 fibers versus the use of an early version of Nextel 650 in one of the prior studies, and the use of a dispersed YAG-matrix phase rather than only a ent version of Nextel 650 had tensile strengths of 1070, 760, and 650 MPa porous YAG fiber coating in another study. The early version of ating at 1200"C for 0. 5, and 100 h, respectively
May 2004 Effect of YAG Additions on Porous-Alumina-Matrix Composites YAA 008 Fiber Matri a AYA Fiber sE Y100 Matrix 100nm Fig. 5. Normalized mean tensile stress for composites heated to (a) 1100 C for 5 h and (b)1200'C for 5 or 100 h. Error bars represent standard Fig. 4. TEM images of Nextel 610 fibers in composites(a) A and(b) YY50 showing the differences in grain size after heating for 100 h at 1200C. Images are at the same magnification in part, for the higher retained strengths. There is much work in the literature on the effect of rare-earth oxide dopants on sintering, grain growth, and creep. 20. 24-3Segregation of yttria and/or the retention of a weak matrix to ensure sequential fracture and lanthanide oxides to alumina grain boundaries generally of particle bonds and a subsequent tortuous crack path through the results in lowered rates of sintering and grain growth at matrix to protect the fiber temperatures of 1350-1650 C,2426,2729, 33 A recent study has Large differences in the initial densities of the processed also found that sintering and grain growth of alumina composites were comparable to differences in final sintered hindered by yttria doping at temperatures as low as 1200C. densities, which made it difficult to measure small changes in For yttria dopant levels of 3000 ppm, densities after sintering at density for comparison. Similarly, an experiment designed to 1200C for 5 and 100 h were 65. 7% and 69.8%, respectively. composite YY50 with the YAG precursor did not result in a direct undoped alumina. Under similar conditions, 50: 50 by volume correlation of density and strength due to the variability of compacts of alumina and YAG powder yielded densities of densities in the initial composites. However, there was a direct%, which can be attributed to a combination of the effects correlation between the increased number of infiltration cycles and of grain-boundary segregation and greater diffusion distances decreased strength. Mattoni et al. showed that matrix porosity imparted by the presence of two mutually insoluble phases could be reduced via multiple infiltration cycles, which resulted in While the magnitude of densification inhibition by the presence decreased composite strengths, where an -17% decrease in of YAG is likely to be less for a matrix in the presence of rigid porosity yielded an-65% reduction in strength. although their inclusi inclusions, as is the case with continuous fiber reinforcements infiltration process resulted in nonuniform infiltrant distribution.A in the present study, local reduction in sintering and grain nore uniform densification would not be expected to reduce growth is expected to retain uniform porosity and aid in the strength as dramatically retention of a weak matrix The effect of YAG and/or Y segregated to alumina A second reason for the increased strength retention of YAG- boundaries in these composites on the stability of the containing matrix composites is the probability of higher fiber microstructure at 1200C is believed to be responsible strengths for these composites compared with the all-alumina