MIATERIALS SENE S ENGEERING ELSEVIER Materials Science and Engineering A342(2003)23-27 Celsian formation in fiber-reinforced barium aluminosilicate glass-ceramic matrix composites Narottam p. bansal s National Aeronautics and Space Administration, Glenn Research Center, Cleveland, OH 44135-3191, USA Received 20 November 2001: received in revised form 28 March 2002 Hot pressing of barium aluminosilicate(BAS) glass or its composites reinforced with large diameter Textron chemical vapor deposited(CVD)silicon carbide SCS-6 monofilaments or small diameter multifilament Nicalon or Hi-Nicalon fibers resulted in the crystallization of both hexacelsian and monoclinic celsian phases. Effects of additions of monoclinic celsian seeds and strontium aluminosilicate(SAS)glass on crystal phase formation during hot pressing has been investigated. On doping BAs with 5 wt% monoclinic celsian seeds or 10 wt %SAS, only the celsian phase was formed in hot pressed monolithic specimens. However, in fiber reinforced composites hot pressed under similar conditions, a small concentration of hexacelsian was still present as hexacelsian nucleates preferentially on surfaces and the presence of fibers provides a large surface area. When the additive concentration was increased to 10 wt celsian seeds or 20 wt SAs, celsian was the only phase detected from X-ray diffraction, with complete elimination of hexacelsian, in the hot pressed composites reinforced with large or small diameter SiC fibers. C 2002 Elsevier Science B.v. All rights reserved Keywords: Celsian; Hexacelsian; Composite; X-ray diffraction 1. Introduction applications as high temperature structural materials in hot sections of gas turbine engines On heat treatment. both hexacelsian and monoclinic [1-8] are being investigated as matrix materials for celsian phases crystallize in Bas glass [9, 10]. Hexacel fiber-reinforced composites for high temperature struc- sian is the high temperature polymorph and is thermo- tural applications. Maximum use temperatures of dynamically stable above 1590C, but once formed, matrices in composite form are compared [1, 2] in can metastably exist in the entire temperature range 1. Barium aluminosilicate glass-ceramic having mono- down to room temperature(Fig. 1). The kinetics of clinic celsian, BaAl2Si2Os, as the crystalline phase transformation of hexacelsian into monoclinic celsian is efractory with a projected very sluggish [ll]. Hexacelsian she nows a large thermal maximum use temperature of 1590 C and a melting expansion coefficient of c80x 10 perC and point of 1760C. Celsian is oxidation resistant, has undergoes a rapid, reversible structural transformation good thermal shock resistance because of its low thermal nto the orthorhombic fc expansion coefficient of~23×10-°per°C, and does by a large volume change of x 3%. In the presence of 1590C. Therefore, at NASA Glenn, BaO. A1, O3. result in microcracking of the BAS matrix. Hence, it is 2SIO2(BAS) glass-ceramic is being investigated [3-8 quite obvious that if BAS is to be successfully used as as a matrix material for fiber reinforced composites for matrix material for fiber reinforced composites, the processing conditions need to be designed to completely eliminate the undesirable hexacelsian phase. The objectives of the present study were to optimize *Tel.+1-216-433-3855;fax:+1-216-433-5544 the concentrations of two dopants for celsian formation E-mail address: narottam p bansal@grc. nasa. gov(N P. Bansal) and to develop hot pressing conditions which would 0921-5093/02/.see front matter C 2002 Elsevier Science B.v. All rights reserved. PI:s0921-5093(02)00313
Celsian formation in fiber-reinforced barium aluminosilicate glass/ceramic matrix composites Narottam P. Bansal * National Aeronautics and Space Administration, Glenn Research Center, Cleveland, OH 44135-3191, USA Received 20 November 2001; received in revised form 28 March 2002 Abstract Hot pressing of barium aluminosilcate (BAS) glass or its composites reinforced with large diameter Textron chemical vapor deposited (CVD) silicon carbide SCS-6 monofilaments or small diameter multifilament Nicalon or Hi-Nicalon fibers resulted in the crystallization of both hexacelsian and monoclinic celsian phases. Effects of additions of monoclinic celsian seeds and strontium aluminosilicate (SAS) glass on crystal phase formation during hot pressing has been investigated. On doping BAS with 5 wt.% monoclinic celsian seeds or 10 wt.% SAS, only the celsian phase was formed in hot pressed monolithic specimens. However, in fiberreinforced composites hot pressed under similar conditions, a small concentration of hexacelsian was still present as hexacelsian nucleates preferentially on surfaces and the presence of fibers provides a large surface area. When the additive concentration was increased to 10 wt.% celsian seeds or 20 wt.% SAS, celsian was the only phase detected from X-ray diffraction, with complete elimination of hexacelsian, in the hot pressed composites reinforced with large or small diameter SiC fibers. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Celsian; Hexacelsian; Composite; X-ray diffraction 1. Introduction Glasses and glass/ceramics of various compositions [1/8] are being investigated as matrix materials for fiber-reinforced composites for high temperature structural applications. Maximum use temperatures of these matrices in composite form are compared [1,2] in Table 1. Barium aluminosilicate glass/ceramic having monoclinic celsian, BaAl2Si2O8, as the crystalline phase appears to be the most refractory with a projected maximum use temperature of /1590 8C and a melting point of 1760 8C. Celsian is oxidation resistant, has good thermal shock resistance because of its low thermal expansion coefficient of /2.3/106 per 8C, and does not undergo any phase transformation up to / 1590 8C. Therefore, at NASA Glenn, BaO / Al2O3 / 2SiO2 (BAS) glass/ceramic is being investigated [3/8] as a matrix material for fiber reinforced composites for applications as high temperature structural materials in hot sections of gas turbine engines. On heat treatment, both hexacelsian and monoclinic celsian phases crystallize in BAS glass [9,10]. Hexacelsian is the high temperature polymorph and is thermodynamically stable above 1590 8C, but once formed, can metastably exist in the entire temperature range down to room temperature (Fig. 1). The kinetics of transformation of hexacelsian into monoclinic celsian is very sluggish [11]. Hexacelsian shows a large thermal expansion coefficient of /8.0/106 per 8C and undergoes a rapid, reversible structural transformation into the orthorhombic form at /300 8C, accompanied by a large volume change of /3%. In the presence of hexacelsian, thermal cycling of the composite would result in microcracking of the BAS matrix. Hence, it is quite obvious that if BAS is to be successfully used as a matrix material for fiber reinforced composites, the processing conditions need to be designed to completely eliminate the undesirable hexacelsian phase. The objectives of the present study were to optimize the concentrations of two dopants for celsian formation and to develop hot pressing conditions which would * Tel.: /1-216-433-3855; fax: /1-216-433-5544 E-mail address: narottam.p.bansal@grc.nasa.gov (N.P. Bansal). Materials Science and Engineering A342 (2003) 23/27 www.elsevier.com/locate/msea 0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 3 1 3 - 1
N P. Bansal Materials Science and engineer A342(2003)23 Table i Glass and glass-ceramic matrices of interest for fiber-reinforced composites [1, 2 Matrix type Major constituents Minor constitu- Major crystalline Maximum use temperature (C)in composite ents phases Glasses 7740 Borosilicate B2O3, Sio Na o, Al2O3 1723 Aluminosili- Al2O3, MgO, Cao, Sio 700 7930 High silica SiO, B2O3 LASI Li2O, Al2O3, MgO, Sio Zno, ZrO2, Bao B-Spodumene 1000 Zno, ZrO2, Bao R-Spodumene l100 BroS LAS-IIl LizO, Al,O3, ZrO, B-Spodumene 1200 MAS Mgo, AlO3. SiO2 Bao Cordierite 1200 BMAS Bao, Mgo, Al2O3, SiO2 Barium osumilite 1250 Ternary mullite BaO, Al2O3, Sic Mullite ~1500 Celsian 1600 7740 1723 and 7930 are Corning Glass Works designations melt between water cooled metallic rollers, Glass flakes Celsian were attrition milled to a powder having an average particle size of 2.5+0.05 um as described elsewhere [9, 10]. Monoclinic celsian seeds were synthesized by heat 0.8F M.P. of hexacelsian=1760"C treatment of Bas glass powder at 1250C for 3 days followed by grinding. This process of heat treatment and Hexacelsian grinding was repeated until X-ray diffraction g0.5 showed the presence of only the monoclinic phase and complete absence of hexacelsian. Ty 0.3 this was achieved after three heat treatment cycles. The material was then ground and passed through a 325 mesh screen. The -325 mesh(<44 um)powder was d with the bas gla Fig. 1. Linear thermal expansion of hexacelsian and monoclinic powder celsian bas Continuous CVD SiC(SCS-6) monofilaments from Textron Specialty Materials having a protective double lead to the formation of only the monoclinic celsian coating of carbon-rich silicon carbide on the outer phase, with complete elimination of hexacelsian, in small surface and an overall diameter of -142 um were diameter Nicalon or Hi-Nicalon or large diameter used as the reinforcement. Hi-Nicalon or ceramic grade Textron CVD SiC SCS-6 fiber reinforced BAS compo- Nicalon multifilament tow(500 filaments/yarn) fibers sites. Fine monoclinic celsian(BaAl2Si2Os)seeds as well from Nippon Carbon Co (Japan)with an average fiber as SrO. Al2O3 2Sio2(SAS)glass were the two addi tives investigated diameter of 14 um were also employed Nicalon fibers were coated with a surface protective dual coating of 0. 4 um thick boron nitride over-coated with a 0.2 um hick layer of silicon carbide using the CVd technique. 2. Experimental methods Hi-Nicalon fibers were used without any surface pro- tective coating, but only after the polyvinyl alcohol Glasses of Bao. Al,O3 2SiO2(BAS) and SrO (PVA)sizing had been removed by pyrolysis AlO3 2SiO2(SAS)compositions were melted at Monolithic glass-ceramic samples were prepared by 2000C in a continuous electric melter with Mo packing the matrix powder into a graphite die followed electrodes using laboratory grade BaCO,, SrCO y hot pressing. Unidirectional SCS-6 fiber-reinforced Al2O3, and SiOz. Clear, colorless, and homogeneous composites were prepared by alternately stacking matrix glass flakes were produced by rapid quenching of the tapes and fiber mats and hot pressing as described
lead to the formation of only the monoclinic celsian phase, with complete elimination of hexacelsian, in small diameter Nicalon or Hi-Nicalon or large diameter Textron CVD SiC SCS-6 fiber reinforced BAS composites. Fine monoclinic celsian (BaAl2Si2O8) seeds as well as SrO / Al2O3 / 2SiO2 (SAS) glass were the two additives investigated. 2. Experimental methods Glasses of BaO / Al2O3 / 2SiO2 (BAS) and SrO / Al2O3 / 2SiO2 (SAS) compositions were melted at / 2000 8C in a continuous electric melter with Mo electrodes using laboratory grade BaCO3, SrCO3, Al2O3, and SiO2. Clear, colorless, and homogeneous glass flakes were produced by rapid quenching of the melt between water cooled metallic rollers. Glass flakes were attrition milled to a powder having an average particle size of /2.59/0.05 mm as described elsewhere [9,10]. Monoclinic celsian seeds were synthesized by heat treatment of BAS glass powder at 1250 8C for 3 days followed by grinding. This process of heat treatment and grinding was repeated until X-ray diffraction analysis showed the presence of only the monoclinic celsian phase and complete absence of hexacelsian. Typically, this was achieved after three heat treatment cycles. The material was then ground and passed through a 325 mesh screen. The /325 mesh (B/44 mm) powder was used as seeds during processing. Either of the two dopants, 5/10 wt.% of celsian seeds or 10/20 wt.% of SAS glass powder, were mixed with the BAS glass powder. Continuous CVD SiC (SCS-6) monofilaments from Textron Specialty Materials having a protective double coating of carbon-rich silicon carbide on the outer surface and an overall diameter of /142 mm were used as the reinforcement. Hi-Nicalon or ceramic grade Nicalon multifilament tow (500 filaments/yarn) fibers from Nippon Carbon Co. (Japan) with an average fiber diameter of /14 mm were also employed. Nicalon fibers were coated with a surface protective dual coating of / 0.4 mm thick boron nitride over-coated with a /0.2 mm thick layer of silicon carbide using the CVD technique. Hi-Nicalon fibers were used without any surface protective coating, but only after the polyvinyl alcohol (PVA) sizing had been removed by pyrolysis. Monolithic glass/ceramic samples were prepared by packing the matrix powder into a graphite die followed by hot pressing. Unidirectional SCS-6 fiber-reinforced composites were prepared by alternately stacking matrix tapes and fiber mats and hot pressing as described Table 1 Glass and glass/ceramic matrices of interest for fiber-reinforced composites [1,2] Matrix type Major constituents Minor constituents Major crystalline phases Maximum use temperature (8C) in composite form Glasses a 7740 Borosilicate B2O3, SiO2 Na2O, Al2O3 / 600 1723 Aluminosilicate Al2O3, MgO, CaO, SiO2 B2O3, BaO / 700 7930 High silica SiO2 B2O3 / 1150 Glass /ceramics LAS-I Li2O, Al2O3, MgO, SiO2, ZnO, ZrO2, BaO b-Spodumene 1000 LAS-II Li2O, Al2O3, MgO, SiO2, Nb2O5 ZnO, ZrO2, BaO b-Spodumene 1100 LAS-III Li2O, Al2O3, MgO, SiO2, Nb2O5 ZrO2 b-Spodumene 1200 MAS MgO, Al2O3, SiO2 BaO Cordierite 1200 BMAS BaO, MgO, Al2O3, SiO2 / Barium osumilite 1250 Ternary mullite BaO, Al2O3, SiO2 / Mullite 1500 Celsian BaO, Al2O3, SiO2 / Celsian 1600 a 7740, 1723 and 7930 are Corning Glass Works designations. Fig. 1. Linear thermal expansion of hexacelsian and monoclinic celsian BAS. 24 N.P. Bansal / Materials Science and Engineering A342 (2003) 23 /27
N P. Bansal Materials Science and Engineering 4342(2003)23-2 earlier [3-6]. Nicalon or Hi-Nicalon fiber-reinforced composites were fabricated [7, 8] by drawing the fiber c-ces sian 5 wt celsian seeds slurry composed of glass powder and organic additives(binder, surfactant, de- flocculant), winding on a rotating drum, cutting the prepreg tape, and laying up in the 0/90 orientation. The organics were burned out in air prior to loading the prepreg into a graphite die for consolidation by hot pressing. Crystalline phases present were identified from X-ray diffraction patterns recorded at room temperature using a step scan procedure(0.03 per 20 step, count time 0.5 s)on a Philips ADP-3600 automated powder diffractometer equipped with a crystal monochromator and employing copper Ko radiation 3. Results and discussion Crystalline phases present in Bas glass, in the absence and presence of 5 wt. of celsian seeds, hot pressed at various temperatures for 2 h under 24 MPa 20. deg (3.5 ksi) pressure are shown in Table 2. Hexacelsian, Fig. 2. Effect of monoclinic celsian seeds on powder X-ray diffraction along with monoclinic celsian, is present in all the patterns of BAS glass hot pressed at 1400C for 2 h followed by unseeded samples. However, only the monoclinic celsian 1200"C for 2 h phase is detected in seeded monoliths hot pressed at or higher than 1300 C. X-ray diffraction patterns of treatments of the SAs glass [12]. Also, SAS forms solid seeded and unseeded BAS hot pressed at 1400C for solutions in the entire composition range with BAs. It 2 h followed by 2 h at 1200C are compared in Fig. 2. was, therefore, justified to use SAS as an additive to The CVD SiC (SCS-6) fiber reinforced BAs glass BAS for accelerating the rate of celsian formation. A ceramic composite containing 5 wt. of celsian seeds monolithic specimen containing 90 wt. BAS and 10 still showed the presence of hexacelsian in the hot wt. SAs glasses, when hot pressed at 1000C for 10 pressed material. However, when the amount of celsian min under 13.8 MPa(2 ksi)pressure resulted in the seeds was raised to 10 wt % the hexacelsian was formation of both hexacelsian and celsian as shown in completely absent from the composite as shown in the the powder X-ray diffraction pattern in Fig 4. However, X-ray diffraction pattern in Fig. 3 when this material was annealed at 1200 C for 2 h in The kinetics of hexacelsian to celsian transformation air, the hexacelsian completely transformed into celsian in pure SAS is quite fast [12]. In fact, it is difficult to Another monolithic specimen of the same composition synthesize SAs as hexacelsian by the usual approach of hot pressed at 1300C for 2 h under 13.8 MPa(2 ksi) solid state reaction between the constituent oxides. It pressure showed the formation of only celsian, with can, however, be crystallized from controlled heat complete absence of hexacelsian, in the powder X-ray Table 2 Effect of celsian seed additions on properties of BAs glass hot-pressed at various temperatures at 24 MPa(3.5 ksi) for 2 h in vacuum Hot pressing temperature ( C) Celsian seeds(wt Density (g cm-5)" Crystalline phases(XRD) 3.32+0.005 C(M), HC(m) 3.34+0.004 CM), HC(m) 1400,1200 1400,1300 000005555 3.39+0.003 CM), HC(m) 3.36+0.002 CM), HC(m) 3.36+0.003 C(M), HC(trace) 3.38+0.003 C(M), HC(m 3.38+0.00 3.41+0004 1400,1200 3.38+0.003 ccc Theoretical density of BaAl2Si2Os; celsian, 3. 390 g cm; hexacelsian, 3.296 g cm hree measurements per sar b C. celsian: HC, hexacelsian; M, major: m, minor
earlier [3/6]. Nicalon or Hi-Nicalon fiber-reinforced composites were fabricated [7,8] by drawing the fiber tows through the matrix slurry composed of glass powder and organic additives (binder, surfactant, deflocculant), winding on a rotating drum, cutting the prepreg tape, and laying up in the 0/908 orientation. The organics were burned out in air prior to loading the prepreg into a graphite die for consolidation by hot pressing. Crystalline phases present were identified from X-ray diffraction patterns recorded at room temperature using a step scan procedure (0.038 per 2u step, count time 0.5 s) on a Philips ADP-3600 automated powder diffractometer equipped with a crystal monochromator and employing copper Ka radiation. 3. Results and discussion Crystalline phases present in BAS glass, in the absence and presence of 5 wt.% of celsian seeds, hot pressed at various temperatures for 2 h under 24 MPa (3.5 ksi) pressure are shown in Table 2. Hexacelsian, along with monoclinic celsian, is present in all the unseeded samples. However, only the monoclinic celsian phase is detected in seeded monoliths hot pressed at or higher than 1300 8C. X-ray diffraction patterns of seeded and unseeded BAS hot pressed at 1400 8C for 2 h followed by 2 h at 1200 8C are compared in Fig. 2. The CVD SiC (SCS-6) fiber reinforced BAS glass ceramic composite containing 5 wt.% of celsian seeds still showed the presence of hexacelsian in the hot pressed material. However, when the amount of celsian seeds was raised to 10 wt.%, the hexacelsian was completely absent from the composite as shown in the X-ray diffraction pattern in Fig. 3. The kinetics of hexacelsian to celsian transformation in pure SAS is quite fast [12]. In fact, it is difficult to synthesize SAS as hexacelsian by the usual approach of solid state reaction between the constituent oxides. It can, however, be crystallized from controlled heat treatments of the SAS glass [12]. Also, SAS forms solid solutions in the entire composition range with BAS. It was, therefore, justified to use SAS as an additive to BAS for accelerating the rate of celsian formation. A monolithic specimen containing 90 wt.% BAS and 10 wt.% SAS glasses, when hot pressed at 1000 8C for 10 min under 13.8 MPa (2 ksi) pressure resulted in the formation of both hexacelsian and celsian as shown in the powder X-ray diffraction pattern in Fig. 4. However, when this material was annealed at 1200 8C for 2 h in air, the hexacelsian completely transformed into celsian. Another monolithic specimen of the same composition hot pressed at 1300 8C for 2 h under 13.8 MPa (2 ksi) pressure showed the formation of only celsian, with complete absence of hexacelsian, in the powder X-ray Table 2 Effect of celsian seed additions on properties of BAS glass hot-pressed at various temperatures at 24 MPa (3.5 ksi) for 2 h in vacuum Hot pressing temperature (8C) Celsian seeds (wt.%) Density (g cm3 ) a Crystalline phases (XRD)b 1200 0 3.3290.005 C(M), HC(m) 1300 0 3.3490.004 C(M), HC(m) 1400 0 3.3990.003 C(M), HC(m) 1400, 1200 0 3.3690.002 C(M), HC(m) 1400, 1300 0 3.3690.003 C(M), HC(trace) 1200 5 3.3890.003 C(M), HC(m) 1300 5 3.3890.002 C 1400 5 3.4190.004 C 1400, 1200 5 3.3890.003 C Theoretical density of BaAl2Si2O8; celsian, 3.390 g cm3 ; hexacelsian, 3.296 g cm3 . a Three measurements per sample. b C, celsian; HC, hexacelsian; M, major; m, minor. Fig. 2. Effect of monoclinic celsian seeds on powder X-ray diffraction patterns of BAS glass hot pressed at 1400 8C for 2 h followed by 1200 8C for 2 h. N.P. Bansal / Materials Science and Engineering A342 (2003) 23 /27 25
N P. Bansal Materials Science and Engineering 4342(2003)23-2 5x102 Celsian only Celsian +siC phases 607080 M Fig. 3. X-ray diffraction spectra of CVD SiC (SCS-6) fiber-reinforced 101520 BAs glass-ceramic matrix composite containing 10 wt. monoclinic celsian seeds, hot pressed at 1350 C for 2 h: Vr= 19%. Fig. 5. Powder X-ray diffraction pattern of 90BAS-1OSAS(wt % glass ceramic hot pressed at 1300 C for 2 h under 13.8 MPa(2 ksi) As hot pressed Ledley 1200C, 2 hr, air Fig. 6. Surface X-ray diffraction of CVD SiC(SCS-6)fiber reinforced 90BAS-1OSAS (wt %) glass-ceramic matrix composite hot pressed at 1400 C for 2 h under 27.6 MPa(4 ksi): Vr=23%. (Fig. 7). Hexacelsian phase is known to nucleate preferentially on the surface [9, 10]. The large surface area provided by the presence of reinforcing fibers in the composite causes the formation of hexacelsian and 20, deg resists its transformation to monoclinic celsian. how Fig. 4. Effect of annealing on powder X-ray diffractograms of 90 ever, when the amount of sas additive in the matrix was raised to 20 wt % hexacelsian was completely pressed, (b)hot pressed and annealed at 200 .C for 2 h in air; peaks eliminated from the Nicalon reinforced composite arked with asterisk correspond to hexacelsian fabricated under same processing conditions as above, as seen from the X-ray diffraction patterns presented in diffraction pattern(Fig. 5). X-ray diffraction(Fig. 6)of Fig. 7. The X-ray diffraction spectrum recorded from 90 wt% BAS and 10 wt sas glass ceramic matrix the surface of a 0/90 cross-ply Hi-Nicalon fiber reinforced with 23 vol. of CVD SiC(SCS-6)mono- reinforced 80 wt. BAS-20 wt %SAS glass-ceramic filaments hot pressed at 1400C for 2 h under 27.6 matrix composite hot pressed at 1300C for 2 h under MPa (4 ksi) pressure showed the presence of both 27.6 MPa (4 ksi) pressure containing 37 vol. of hexacelsian and celsian. Another 0/90 cross-ply com- fibers is shown in Fig 8. This shows the presence of only posite of similar matrix composition, but reinforced the monoclinic celsian phase in the matrix with 40 vol. of small diameter Nicalon fibers having The above results clearly indicate that hexacelsian the dual surface protective coating of BN/SiC was hot formation can be avoided in continuous SiC fiber pressed at 1300C for 2 h followed by ceraming at reinforced composites by doping the bas glass matrix 1200C for 24 h in argon. This also resulted in the with either 10 wt %o of celsian seeds or with 20 wt. of formation of both the hexacelsian and celsian phases SAs. Melting a glass of 0.8 Bao-0 2Sro-Al2O3-2Sio
diffraction pattern (Fig. 5). X-ray diffraction (Fig. 6) of 90 wt.% BAS and 10 wt.% SAS glass ceramic matrix reinforced with 23 vol.% of CVD SiC (SCS-6) monofilaments hot pressed at 1400 8C for 2 h under 27.6 MPa (4 ksi) pressure showed the presence of both hexacelsian and celsian. Another 0/908 cross-ply composite of similar matrix composition, but reinforced with 40 vol.% of small diameter Nicalon fibers having the dual surface protective coating of BN/SiC was hot pressed at 1300 8C for 2 h followed by ceraming at 1200 8C for 24 h in argon. This also resulted in the formation of both the hexacelsian and celsian phases (Fig. 7). Hexacelsian phase is known to nucleate preferentially on the surface [9,10]. The large surface area provided by the presence of reinforcing fibers in the composite causes the formation of hexacelsian and resists its transformation to monoclinic celsian. However, when the amount of SAS additive in the matrix was raised to 20 wt.%, hexacelsian was completely eliminated from the Nicalon reinforced composite fabricated under same processing conditions as above, as seen from the X-ray diffraction patterns presented in Fig. 7. The X-ray diffraction spectrum recorded from the surface of a 0/908 cross-ply Hi-Nicalon fiberreinforced 80 wt.% BAS/20 wt.% SAS glass/ceramic matrix composite hot pressed at 1300 8C for 2 h under 27.6 MPa (4 ksi) pressure containing /37 vol.% of fibers is shown in Fig. 8. This shows the presence of only the monoclinic celsian phase in the matrix. The above results clearly indicate that hexacelsian formation can be avoided in continuous SiC fiberreinforced composites by doping the BAS glass matrix with either 10 wt.% of celsian seeds or with 20 wt.% of SAS. Melting a glass of 0.8BaO/0.2SrO/Al2O3 /2SiO2 Fig. 3. X-ray diffraction spectra of CVD SiC (SCS-6) fiber-reinforced BAS glass/ceramic matrix composite containing 10 wt.% monoclinic celsian seeds, hot pressed at 1350 8C for 2 h; Vf/19%. Fig. 4. Effect of annealing on powder X-ray diffractograms of 90 BAS-10 SAS (wt.%) glass hot pressed at 1000 8C for 10 min; (a) as hot pressed, (b) hot pressed and annealed at 200 8C for 2 h in air; peaks marked with asterisk correspond to hexacelsian. Fig. 5. Powder X-ray diffraction pattern of 90BAS-10SAS (wt.%) glass ceramic hot pressed at 1300 8C for 2 h under 13.8 MPa (2 ksi). Fig. 6. Surface X-ray diffraction of CVD SiC (SCS-6) fiber reinforced 90BAS-10SAS (wt.%) glass/ceramic matrix composite hot pressed at 1400 8C for 2 h under 27.6 MPa (4 ksi); Vf/23%. 26 N.P. Bansal / Materials Science and Engineering A342 (2003) 23 /27
N P. Bansal Materials Science and Engineering 4342(2003)23-2 and hexacelsian. Doping with 5 wt. celsian seeds or 10 wt. sro is sufficient to form X monoclinic celsian in monolithic BAS glass-ceramics However. in the diameter Nicalon or Hi-Nicalon fibers or large diameter Textron cvd SiC SCS-6 monofilaments as reinforce- ments, the associated large increase in available surface area for hexacelsian nucleation makes the above levels of dopant insufficient. With 10 wt. celsian seeds or 20 wt% SrO, hexacelsian formation in hot pressed fiber- reinforced composites was avoided. The results of this study would be useful in complete elimination of the undesired hexacelsian phase from large or small dia- meter fiber-reinforced Bas glass-ceramic matrix com- posites 清 5. Conclusior 7. Powder X-ray diffraction patterns of 0/90 Nicalon/BN/SIC/ S-SAS ceramic composites of different matrix It may be concluded the addition of 10% hot pressed at 1300 oC for 2 h and ceramed at 1200 C for 24 h in monoclinic celsian seeds or 20 wt SAS results in the argon; Vr=40%. The peaks marked with asterisks are for hexacelsian formation of phase pure monoclinic celsian in SiC fiber- and the remaining peaks correspond to monoclinic celsian. reinforced BAs glass-ceramic matrix composites hot pressed at 1300C for 2 h. This would avoid the detrimental volume changes associated with hexacelsian and also circumvent microcracking of the matrix during thermal cycling of the composites References [K.M. Prewo, Fiber reinforced M.H. Lewis(Ed ) Glasses and Glass-ceramics, Chapman Hall, New York, NY, 1989, pp. Glass and glass-ceramic matrix composites, in: KS 101520 33540 2]JJ. Mazdiyasni (Ed. ) Fiber Reinforced Ceramic Composites Mate. als. P Fig. 8. Surface X-ray diffraction of 0/90 Hi-Nicalon/80BAS-20SAS Ridge,NJ,190.pp.222-259 3 N.P. Bansal, Method of Producing a Ceramic Fiber-Reinforced Glass-ceramic Matrix Composite, US Patent 5, 281, 559, January composition, rather than separately melting and mixing 4 N.P. Bansal, Ceramic Fiber Reinforced Glass-Ceramic Matrix of BAS and SAs glasses, should yield the same results Composite, US Patent 5, 214, 004, May 25(1993) and also would be more convenient and economical [N.P. Bansal, Mater Sci Eng. A220(1996)129 [6 N.P. Bansal, Mater Sci Eng. A231(1997)117. [7 N.P. Bansal, J. Am. Ceram Soc. 80(1997)2407 [8 N.P. Bansal, J I. Eldridge, J. Mater Res. 13(1998)1530 4. Summary of results 9 N.P. Bansal, M.J. Hyatt, J. Mater. Res. 4(1989)1257 [10 M.J. Hyatt, N P Bansal, J. Mater. Sci. 31(1996)172. [11 D. Bahat, J Mater. Sci. 5(1970)805 Hot pressing of BAS glass of feldspar composition (12) N.P. Bansal, C H Drummond, Ill, J. Am. Ceram Soc. 76(1993) results in the crystallization of both monoclinic celsian
composition, rather than separately melting and mixing of BAS and SAS glasses, should yield the same results and also would be more convenient and economical. 4. Summary of results Hot pressing of BAS glass of feldspar composition results in the crystallization of both monoclinic celsian and hexacelsian. Doping with 5 wt.% celsian seeds or 10 wt.% SrO is sufficient to form X-ray phase pure monoclinic celsian in monolithic BAS glass/ceramics after hot pressing. However, in the presence of small diameter Nicalon or Hi-Nicalon fibers or large diameter Textron CVD SiC SCS-6 monofilaments as reinforcements, the associated large increase in available surface area for hexacelsian nucleation makes the above levels of dopant insufficient. With 10 wt.% celsian seeds or 20 wt.% SrO, hexacelsian formation in hot pressed fiberreinforced composites was avoided. The results of this study would be useful in complete elimination of the undesired hexacelsian phase from large or small diameter fiber-reinforced BAS glass/ceramic matrix composites. 5. Conclusions It may be concluded that the addition of 10% monoclinic celsian seeds or 20 wt.% SAS results in the formation of phase pure monoclinic celsian in SiC fiberreinforced BAS glass/ceramic matrix composites hot pressed at /1300 8C for 2 h. This would avoid the detrimental volume changes associated with hexacelsian and also circumvent microcracking of the matrix during thermal cycling of the composites. References [1] K.M. Prewo, Fiber reinforced glasses and glass/ceramics, in: M.H. Lewis (Ed.), Glasses and Glass/ceramics, Chapman & Hall, New York, NY, 1989, pp. 336/368. [2] J.J. Brennan, Glass and glass/ceramic matrix composites, in: K.S. Mazdiyasni (Ed.), Fiber Reinforced Ceramic Composites. Materials, Processing and Technology, Noyes Publications, Park Ridge, NJ, 1990, pp. 222/259. [3] N.P. Bansal, Method of Producing a Ceramic Fiber-Reinforced Glass/ceramic Matrix Composite, US Patent 5,281,559, January 25 (1994). [4] N.P. Bansal, Ceramic Fiber Reinforced Glass/Ceramic Matrix Composite, US Patent 5,214,004, May 25 (1993). [5] N.P. Bansal, Mater. Sci. Eng. A220 (1996) 129. [6] N.P. Bansal, Mater. Sci. Eng. A231 (1997) 117. [7] N.P. Bansal, J. Am. Ceram. Soc. 80 (1997) 2407. [8] N.P. Bansal, J.I. Eldridge, J. Mater. Res. 13 (1998) 1530. [9] N.P. Bansal, M.J. Hyatt, J. Mater. Res. 4 (1989) 1257. [10] M.J. Hyatt, N.P. Bansal, J. Mater. Sci. 31 (1996) 172. [11] D. Bahat, J. Mater. Sci. 5 (1970) 805. [12] N.P. Bansal, C.H. Drummond, III, J. Am. Ceram. Soc. 76 (1993) 1321. Fig. 7. Powder X-ray diffraction patterns of 0/908 Nicalon/BN/SiC/ BAS-SAS glass ceramic composites of different matrix compositions hot pressed at 1300 8C for 2 h and ceramed at 1200 8C for 24 h in argon; Vf/40%. The peaks marked with asterisks are for hexacelsian and the remaining peaks correspond to monoclinic celsian. Fig. 8. Surface X-ray diffraction of 0/908 Hi-Nicalon/80BAS-20SAS (wt.%) glass/ceramic matrix composite hot pressed at 1300 8C for 2 h under 27.6 MPa (4 ksi); Vf/37%. N.P. Bansal / Materials Science and Engineering A342 (2003) 23 /27 27