ELSEVIER Materials Science and Engineering A210 (1996)123-134 Chemical stability, microstructure and mechanical behavior of LaPOa-containing ceramics Dong-Hau Kuo, Waltraud M. Kriven Received 3 May 1995: in revised form 27 October 1995 The use of LaPO4 as a weak interface in composites for high temperature applications was investigated using tape-cast aminates and fiber model systems. Three laminates were fabricated with LaPO4 as one component and Al,O, YAlsO,2or LaAlo,s as the other. The chemical compatibility between the different components of the laminates, as well as the mechanical responses to flexural deformation and the propagation of indentation cracks, were examined. Two fiber model systems(Al,O fiber/ LaPO4 coating/Al,O, matrix and Y3,o,? fiber/ LaPO4 coating/AL,O3 matrix)were studied by fiber pushout tests to measure the interfacial shear strengths. The interfacial shear strengths were calculated by the linear and shear-lag approaches for different embedded fiber lengths. The results suggest that Y3AlsO12 fiber-reinforced composites with LaPO4 coatings have potential as high Keywords: Lanthanum phosphate: Alumina: Aluminates: Laminates: Fibers; Pushout test 1. Introduction eutectic filaments [9 have shown good mechanical high For most non-oxide ceramics, high temperature oxi- (3AL,O3,' 2SiO2) fibers have also been considered fo dation which can degrade the performance of materials applications above 1370C [10]. Of these, single-crystal is a main concern [1-7]. Therefore oxide/oxide(fiber/ cubic-YAG fibers have shown the required creep resis- matrix) continuous fiber-reinforced ceramic composites tance above 1600C [7, 8]. The next challenge is to find with weak interfaces are preferred for high temperature a weak interface or interlayer for an oxide oxide sys- applications in air [6,7]. The weak interface allows debonding, fiber sliding and load transfer to occur, LapO4, a monazite structure, has recently been intro- thereby improving the toughness at room temperature, duced as a possible functional interface for oxide/oxide while the strong oxide fibers supply the required composites by Morgan and Marshall [11-13]. The in- strength and creep resistance at high temperatures formation presented is encouraging because it enables air. Although oxide ceramics are stable in oxidizing fiber-reinforced oxide/oxide composites to withstand environments, they often suffer mechanical degradation high temperatures in oxidizing environments, while at high temperature due to strong bonding between maintaining high strength from the strong fibers as well dissimilar oxides as high toughness from fiber debonding and sliding The use of ceramic materials at high temperatures in mechanisms. Thus LaPO4 is a candidate for preventing air faces many challenges. To overcome room tempera- strong bonding between an oxide fiber and oxide ma ture brittleness and the degradation of the mechanical trix properties at high temperature, new materials need to The tape casting technique has been used in ceramic be introduced. With regard to fiber materials, some processing [14-16] to fabricate laminated composites single-crystal alumina (AL,O3) and yttrium aluminate by stacking tapes of different compositions, as well as (Y,AlSOn2 or"YAG") fibers [7, 8] and Al,O3/YAG to incorporate fibers and whiskers into the laminates 0921-509396S15000 1996- Elsevier Science S.A. All rights res SSD09215093(95)100849
A ELSEVIER Materials Science and Engineering A210 (1996) 123 134 Chemical stability, microstructure and mechanical behavior of LaPO4-containing ceramics Dong-Hau Kuo, Waltraud M. Kriven Department o1' Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 3 May 1995; in revised form 27 October 1995 Abstract The use of LaPO4 as a weak interface in composites for high temperature applications was investigated using tape-cast laminates and fiber model systems. Three laminates were fabricated with LaPO4 as one component and AI203, Y3AIsO12 or LaAI~IO~8 as the other. The chemical compatibility between the different components of the laminates, as well as the mechanical responses to flexural deformation and the propagation of indentation cracks, were examined. Two fiber model systems (A1203 fiber/LaPO4 coating/A1203 matrix and Y3A~O12 fiber/LaPO4 coating/A1203 matrix) were studied by fiber pushout tests to measure the interfacial shear strengths. The interfacial shear strengths were calculated by the linear and shear-lag approaches for different embedded fiber lengths. The results suggest that Y3AlsOl2 fiber-reinforced composites with LaPO 4 coatings have potential as high temperature materials in oxidizing environments. Keywords: Lanthanum phosphate; Alumina; Aluminates; Laminates; Fibers; Pushout test I. Introduction For most non-oxide ceramics, high temperature oxidation which can degrade the performance of materials is a main concern [1-7]. Therefore oxide/oxide (fiber/ matrix) continuous fiber-reinforced ceramic composites with weak interfaces are preferred for high temperature applications in air [6,7]. The weak interface allows debonding, fiber sliding and load transfer to occur, thereby improving the toughness at room temperature, while the strong oxide fibers supply the required strength and creep resistance at high temperatures in air. Although oxide ceramics are stable in oxidizing environments, they often suffer mechanical degradation at high temperature due to strong bonding between dissimilar oxides. The use of ceramic materials at high temperatures in air faces many challenges. To overcome room temperature brittleness and the degradation of the mechanical properties at high temperature, new materials need to be introduced. With regard to fiber materials, some single-crystal alumina (A1203) and yttrium aluminate (Y~AlsO12 or "YAG") fibers [7,8] and A1203/YAG 0921-5093/96/$15.00 © 1996 - Elsevier Science S.A. All rights reserved SSDI 0921-5093(95)10084-9 eutectic filaments [9] have shown good mechanical properties at high temperatures. Mullite (3A1203'2SIO2) fibers have also been considered for applications above 1370 °C [10]. Of these, single-crystal cubic-YAG fibers have shown the required creep resistance above 1600 °C [7,8]. The next challenge is to find a weak interface or interlayer for an oxide/oxide system. LaPO 4, a monazite structure, has recently been introduced as a possible functional interface for oxide/oxide composites by Morgan and Marshall [11--13]. The information presented is encouraging because it enables fiber-reinforced oxide/oxide composites to withstand high temperatures in oxidizing environments, while maintaining high strength from the strong fibers as well as high toughness from fiber debonding and sliding mechanisms. Thus LaPO4 is a candidate for preventing strong bonding between an oxide fiber and oxide matrix. The tape casting technique has been used in ceramic processing [14-16] to fabricate laminated composites by stacking tapes of different compositions, as well as to incorporate fibers and whiskers into the laminates
D.-H. Kuo, W.M. Kriten/ Materials Science and Engineering A210(1996)123-134 Material properties can be controlled by adjusting the After drying at 200-300C on a hot plate, the powders tape compositions, reinforcement orientation and stack- were calcined at 950C (LP)and 1200C(LA, and ing sequence. Tough laminated composites can be ob- YAG). The calcined powders were ball milled for 3 tained by introducing ductile interlayers, e.g. metallic days, dried and passed through a number 100 sieve layers [17, 18] or carbon fiber/ epoxy prepregs [19], or by inserting a weak interlayer, e. g. carbon [20], in between 2. 2. Chemical compatibility and microstructural ceramic substrates. Nevertheless . these laminates have characterization problems in high temperature oxidizing applications The fiber pushout test has been widely used to char Studies of chemical compatibility were carried out on acterize the nature of interfaces in fiber-reinforced ce- pressed pellets composed of Lp powder as one compo- ramic composites. This test can be a cost-saving nent and Al,O3, YAG or LAu powder as the other screening test on model systems when used to evaluate Studies were also carried out on an LP-coated Al2O3 the mechanical responses(i.e debonding and sliding)of fiber (Saphikon, Inc, Milford, NH)/ AL2O3 matrix fibers in a matrix. Theoretical models [21, 22] and a nodel system. These materials were fired at 1550C shear-lag approach [23] can then be applied to calculate and 1600C for 3-6 h and the phases were identified the interfacial properties, although the models need to using X-ray diffractometry (XRD, model D-Max, be modified to allow for the effect of the coating on the Rigaku/USA, Inc., Danvers, MA)and scanning elec- interface behavior during the pushout experiment tron microscopy (SEM, model DS-130, International In this paper, LaPO4 was investigated as a weak Scientific Instruments, Santa Clara, CA)equipped with interlayer in three laminates and two fiber model sys- energy dispersive spectroscopy (EDS). Microstructural tems. The laminates were fabricated by a tape casting characterization was performed by optical microscopy (doctor blade) process. Al,O3(A),Y3AlSO12(YAG) and SEM, using as-fabricated specimens for better con and LaAlyOIs(LAu) were combined with LaPO4(LP) trast. The coefficient of thermal expansion(CTE)for to make the following laminates: LP/A, LP/YAG and LAu was measured on a NETZSCH dilatometer P/LAu. The combinations of LP/A and LP/ YAG are (model 402 ES, Selb, Germany)for temperatures up to related to the developments of Al, O3 fiber- and YAG 1200C fiber-reinforced ceramic composites, LAu is a member magnetoplumbite/B-alumina group 23 Laminate fa which contains weak basal planes [11]. Thus it was hoped that the LP/LA, combination would also have a The procedure for making laminated composites by weak interface. Flexural testing and the indentation tape casting is summarized in Fig. I. The formulation method were used to measure the flexural strengths and followed the technique of Plucknett et al. [ 14-16. The to examine the interfacial bonding slurry formulation contained approximately 20 vol. The interfacial shear strengths were measured by oxide powders, approximately 60 vol. solvent(con fiber pushout tests on the systems Al2O3 fiber/LP/ AL2O3 sisting of a mixture of trichloroethylene and ethanol) matrix and YAG fiber/LP/Al, O3 matrix, and compared and a dispersant, binder and plasticizers. The slurry with calculations made by the shear-lag and linear formulation for tape casting of the different materials is given in Table 1. a slight change in the amount of solvent was made for the slurry viscosity when needed. Slurries were tape cast to 2. Experimental procedures yield laminae 100-200 um thick with a doctor blade opening of 250-350 um. Eighty-layer laminated com- sites were fabricated by alternatively stacking two 2.1. Powder preparation kinds of oxide laminae having dimensions of 25 mm x 51 mm. Thermocompression was performed by Powdered 99.8% Al6-SG(Alcoa Aluminum Co., holding for I h at 50-80C under a pressure of 10 Pittsburgh, PA)Al,O3 was used. The LP, LAu and MPa. The organic additives were removed by heating YAG powders were prepared by dissolving 99.9% to 500C at a rate of 3C h, followed by a 3 h La, O3 or Y,O, powders(Molycorp, Inc, White Plains, holding time. Subsequently, the bulk materials were NY in nitric acid. Ammonium phosphate, dibasic isostatically cold pressed at approximately 170 MPa for Fisher Scientific, Pittsburgh, PA)or aluminum nitrate 10 min, and then loaded into a graphite die with Al,O nonahydrate (J.T. Baker Chemical Co., Phillipsburg, YAG and LAu powders surrounding the pressed LP/ NJ)was then added to the solution. An organic resin A, LP/YAG and LP/LAu specimens respectively. Con- ormed by mixing ethylene glycol(Fisher Scientific)and solidation was performed by hot pressing, under an for citric acid monohydrate(EM Science, Gibbstown, NJ) argon atmosphere at 28 MPa, at temperatures of 1600 was added to control drying and to form fine powders C for 3 h in the case of LP/YAG and LP/LAul
124 D.-H. Kuo, W.M. Kriven / Materials Science and Engineering A210 (1996) 123-134 Material properties can be controlled by adjusting the tape compositions, reinforcement orientation and stacking sequence. Tough laminated composites can be obtained by introducing ductile interlayers, e.g. metallic layers [17,18] or carbon fiber/epoxy prepregs [19], or by inserting a weak interlayer, e.g. carbon [20], in between ceramic substrates. Nevertheless, these laminates have problems in high temperature oxidizing applications. The fiber pushout test has been widely used to characterize the nature of interfaces in fiber-reinforced ceramic composites. This test can be a cost-saving screening test on model systems when used to evaluate the mechanical responses (i.e. debonding and sliding) of fibers in a matrix. Theoretical models [21,22] and a shear-lag approach [23] can then be applied to calculate the interfacial properties, although the models need to be modified to allow for the effect of the coating on the interface behavior during the pushout experiments. In this paper, LaPO4 was investigated as a weak interlayer in three laminates and two fiber model systems. The laminates were fabricated by a tape casting (doctor blade) process. AI203 (A), Y3A15012 (YAG) and LaAll ~O18 (LAI~) were combined with LaPO 4 (LP) to make the following laminates: LP/A, LP/YAG and LP/LA~. The combinations of LP/A and LP/YAG are related to the developments of A1203 fiber- and YAG fiber-reinforced ceramic composites. LAl~ is a member of the refractory magnetoplumbite/fl-alumina group which contains weak basal planes [11]. Thus it was hoped that the LP/LA~ combination would also have a weak interface. Flexural testing and the indentation method were used to measure the flexural strengths and to examine the interfacial bonding. The interfacial shear strengths were measured by fiber pushout tests on the systems A1203 fiber/LP/A1203 matrix and YAG fiber/LP/A1203 matrix, and compared with calculations made by the shear-lag and linear approaches. 2. Experimental procedures 2.1. Powder preparation Powdered 99.8% A16-SG (Alcoa Aluminum Co., Pittsburgh, PA) A1203 was used. The LP, LAll and YAG powders were prepared by dissolving 99.9% La203 or Y203 powders (Molycorp, Inc., White Plains, NY) in nitric acid. Ammonium phosphate, dibasic (Fisher Scientific, Pittsburgh, PA) or aluminum nitrate nonahydrate (J.T. Baker Chemical Co., Phillipsburg, N J) was then added to the solution. An organic resin formed by mixing ethylene glycol (Fisher Scientific) and citric acid monohydrate (EM Science, Gibbstown, N J) was added to control drying and to form fine powders. After drying at 200-300 °C on a hot plate, the powders were calcined at 950 °C (LP) and 1200 °C (LA~l and YAG). The calcined powders were ball milled for 3 days, dried and passed through a number 100 sieve. 2.2. Chemical compatibility and microstructural characterization Studies of chemical compatibility were carried out on pressed pellets composed of LP powder as one component and A1203, YAG or LAI~ powder as the other. Studies were also carried out on an LP-coated A1203 fiber (Saphikon, Inc., Milford, NH)/A1203 matrix model system. These materials were fired at 1550 °C and 1600 °C for 3-6 h and the phases were identified using X-ray diffractometry (XRD, model D-Max, Rigaku/USA, Inc., Danvers, MA) and scanning electron microscopy (SEM, model DS-130, International Scientific Instruments, Santa Clara, CA) equipped with energy dispersive spectroscopy (EDS). Microstructural characterization was performed by optical microscopy and SEM, using as-fabricated specimens for better contrast. The coefficient of thermal expansion (CTE) for LAtl was measured on a NETZSCH dilatometer (model 402 ES, Selb, Germany) for temperatures up to 1200 °C. 2.3. Laminate fabrication The procedure for making laminated composites by tape casting is summarized in Fig. I. The formulation followed the technique of Plucknett et al. [14-16]. The slurry formulation contained approximately 20 vol.% oxide powders, approximately 60 vol.% solvent (consisting of a mixture of trichloroethylene and ethanol) and a dispersant, binder and plasticizers. The slurry formulation for tape casting of the different materials is given in Table 1. A slight change in the amount of solvent was made for the purpose of adjusting the slurry viscosity when needed. Slurries were tape cast to yield laminae 100-200 /~m thick with a doctor blade opening of 250-350 pro. Eighty-layer laminated composites were fabricated by alternatively stacking two kinds of oxide laminae having dimensions of 25 mm x 51 mm. Thermocompression was performed by holding for 1 h at 50-80 °C under a pressure of 10 MPa. The organic additives were removed by heating to 500 °C at a rate of 3 °C h-l, followed by a 3 h holding time. Subsequently, the bulk materials were isostatically cold pressed at approximately 170 MPa for 10 min, and then loaded into a graphite die with A1203, YAG and LAll powders surrounding the pressed LP/ A, LP/YAG and LP/LA~ specimens respectively. Consolidation was performed by hot pressing, under an argon atmosphere at 28 MPa, at temperatures of 1600 °C for 3 h in the case of LP/YAG and LP/LA~1
D.-H. Kuo, W.M. Kriven/ Materials Science and Engineering A210(1996)123-1.34 Powder+dispersant annealed at 1500C for 6 h and on LP/A laminates (ball mill for 48 hrs) Shurry compositions: annealed at 1250C for 6 h Lower annealing tempera- tures than used for hot pressing were employed to compensate for the oxygen deficiency found in oxide ene gycol/docta phthalate ceramics after hot pressing. Four-point flexural tests (ball mil for 24 hrs) were performed with the tensile surface parallel to the laminate, at room temperature, using a screw-driven machine (model 4502, Instron Corp, Canton, MA) pe casting rate: 1 cm/sec ctor blade opening: 25 with a crosshead speed of 0.05 mm min Three bend bars with ground surfaces were tested for flexural strength. Two or three bend bars with indented surfaces and two or three bars with notches were also tested in essive lamination: flexure. Five indents parallel to the width direction and laminator der an uniaxial compression the center of the tensile surface were produced under a 3 kg indentation load. Radial cracks were generated under a 5 kg indentation load in order to study crack propagation profiles and interaction with the mi- Binder removal crostructure. The notched specimens were cut with a 160 um thick diamond-edged blade CIP condition 2.5. Pushout tests of fiber model systems Cold isostatic pressing 170 MPa for 10 min 2.5. 1. Sample prepard Since the available amount of single-crystal YAG fibers was limited, fiber model systems wer Hot pressing to obtain the interfacial shear strengths. To the densification problem without using hot or hot isostatic pressing techniques, a high surface area Fig. 1. Tape casting procedures for making laminated composites. (75-90 m2g ')Al2 O, powder(Praxair Surface Tech nologies, Inc, Indianapolis, IN) was mixed with A16- laminates, and at 1300C for 3 h in the case of LP a SG Al,O3 powder (60 vol %) to form the powder laminates. Single phase LP, YAG and Lau were also for the matrix. The mixture of powders was intended hot pressed at 1600C for comparison. The holding to lower the sintering temperature and to control time at this temperature was 3 h shrinkage a slurry was prepared by ball milling a mixture 2. 4. Mechanical evaluation of laminated composites of lp powder(approximately 70 wt. ) ethanol(ap proximately 27 wt %)and polyvinyl butyral (approxi The hot pressed slabs were cut into bars with dimen mately 3 wt %) AL,O fibers (diameter, 140 um) sions of 25 mm x 2 mm x 2 mm. Mechanical testing and YAG fibers(diameter, 160 um) were subsequently was conducted on LP/YAG and LP/LA, laminates dip coated with the LP slurry. Next, the dip-coated fibers and a marker (SCS-8 SiC fiber, Textron Specialty Materials, Lowell, MA) were embedded in the matrix The marker fiber Slurry formulation for tape casting of different materials dded to facilitate the fiber alignment and positioning before sintering, as well Constituent Amount (g) Function as the cutting of thin slices for pushout testing. After embedding, the fiber-containing pellets were dry pressed Ceramic powder at approximately 1 MPa, isostatically cold pressed at about 70 MPa and sintered in air at 1550oC for 3 h YAlsO, rather than at 1600oC for 3 h as when Al6-SG Al, O, Dispersant powders were used alone. The 50C difference in sintering temperature had a large effect on the Al,O3 Solvent fibers. Damage was observed on the Al2O3 fibers after Polyvinyl butyral Plasticizer sintering at 1600C, but they stayed intact at 1550oC Dioctyl phthalate Plasticizer This damage, which was observable by SEM, was also reported for Al,O, fibers under high temperature load Emphos PS-21A (Witco Chemicals. Houston. TX) ng[24]
D.-H. Kuo, W.M. Kriven /Materials Science and Engineering A210 (1996) 123-134 125 Powder + dispersant +solvent (ball mill for 48 hrs) Slurry compositions: Ceramic powders ~r Dispersant: phosphate ester Slovents: trichioroathylene / ethanol I Binder: polyvinyl butyral Add: ptasticizers Plasticizers: polyethylene glycol / dioctyt phthalate + binder (ball mill for 24 hrs) Tape casting rate: 1 cm / sec Tape casting Doctor blade opening: 250 - 350 p,m and drying Drying under solvent - saturated atmosphere I I Condition of thermocompressive lamination: Cutting, stacking 50 - 80°C for 1 hr and lamination under an uniaxial compression , of t0 MPa l Binder removal cycles: Binder removal R.T. - 150°C at 60"C / hr 150 - 500°C at 3°C / hr I 1 CIP condition: Cold isostaticpressing -170 MPa for 10 min (CIP) I HP conditions Hot pressing 1300"C &1600"C for 3 hr (HP) at a pressure of 28 MPa Fig. 1. Tape casting procedures for making laminated composites. laminates, and at 1300 °C for 3 h in the case of LP/A laminates. Single phase LP, YAG and LA~ were also hot pressed at 1600 °C for comparison. The holding time at this temperature was 3 h. 2.4. Mechanical evaluation of laminated composites The hot pressed slabs were cut into bars with dimensions of 25 mm × 2 mm × 2 mm. Mechanical testing was conducted on LP/YAG and LP/LA~I laminates Table 1 Slurry formulation for tape casting of different materials Constituent Amount (g) Function A1203 100 Ceramic powder LaPO 4 128 Y3AIsOI2 105 or LaAlllOi8 100 Phosphate ester" 1.8 Dispersant Trichlorethylene 62 --75 Solvent Ethanol 24 35 Solvent Polyvinyl butyral 8.4 Binder Polyethylene glycol 5.9 Plasticizer Dioctyl phthalate 5.9 Plasticizer "Emphos PS-21A (Witco Chemicals, Houston, TX). annealed at 1500 °C for 6 h and on LP/A laminates annealed at 1250 °C for 6 h. Lower annealing temperatures than used for hot pressing were employed to compensate for the oxygen deficiency found in oxide ceramics after hot pressing. Four-point flexural tests were performed with the tensile surface parallel to the laminate, at room temperature, using a screw-driven machine (model 4502, Instron Corp., Canton, MA) with a crosshead speed of 0.05 mm min ~. Three bend bars with ground surfaces were tested for flexural strength. Two or three bend bars with indented surfaces and two or three bars with notches were also tested in flexure. Five indents parallel to the width direction in the center of the tensile surface were produced under a 3 kg indentation load. Radial cracks were generated under a 5 kg indentation load in order to study crack propagation profiles and interaction with the microstructure. The notched specimens were cut with a 160/lm thick diamond-edged blade. 2.5. Pushout tests of fiber model systems 2.5.1. Sample preparation Since the available amount of single-crystal YAG fibers was limited, fiber model systems were studied to obtain the interfacial shear strengths. To overcome the densification problem without using hot pressing or hot isostatic pressing techniques, a high surface area (75 90 m 2 g 1) A12Os powder (Praxair Surface Technologies, Inc., Indianapolis, IN) was mixed with A16- SG AI203 powder (60 vol.%) to form the powder for the matrix. The mixture of powders was intended to lower the sintering temperature and to control shrinkage. A slurry was prepared by ball milling a mixture of LP powder (approximately 70 wt.%), ethanol (approximately 27 wt.%) and polyvinyl butyral (approximately 3 wtY,,). A1203 fibers (diameter, 140 /zm) and YAG fibers (diameter, 160/~m) were subsequently dip coated with the LP slurry. Next, the dip-coated fibers and a marker (SCS-8 SiC fiber, Textron Specialty Materials, Lowell, MA) were embedded in the matrix. The marker fiber was added to facilitate the fiber alignment and positioning before sintering, as well as the cutting of thin slices for pushout testing. After embedding, the fiber-containing pellets were dry pressed at approximately 1 MPa, isostatically cold pressed at about 70 MPa and sintered in air at 1550 °C for 3 h rather than at 1600 °C for 3 h as when A16-SG AI20~ powders were used alone. The 50 °C difference in sintering temperature had a large effect on the A1203 fibers. Damage was observed on the A1203 fibers after sintering at 1600 °C, but they stayed intact at 1550 °C. This damage, which was observable by SEM, was also reported for A1203 fibers under high temperature loading [24]
D-H, Kuo, W M. Riven/ Materials Science and Engineering A210(1996)123-134 load crosshead ￥3A13 probe substrate A1:8 LLLLLLLLLLLLLLLA ig. 3. XRD of an LaPO4/Y3AlsO12 pellet fired at 1600C for 6 h 2.5.2. Pushout test procedure calculation of the interfacial frictional strength by the The sintered slabs were sliced to produce samples hear-lag model involved an unknown parameter, as an with different thicknesses. The surface of these samples interfacial coating existed between the fiber and the was then polished. Pushout tests were conducted matrix. For this reason the interfacial frictional screw-driven machine with a l kg load cell. A diamond strengths were not calculated probe with a 95 um diameter flat tip was fixed into a cylinder, which was threaded to the load cell. a 3. Results crosshead displacement rate of 60 um min was used The specimen mounted on a slotted alumina substrate 3. 1. Chemical compatibility and stability for testing was positioned under a stereomicroscope ith an X-y micropositioning stage. The experimental Studies of the chemical compatibility can guide the set-up for the fiber pushout tests is represented sche- choice of suitable materials for use at high tempera matically in Fig. 2. Four or five pushout results on each tures. The XRD results are shown in Fig. 3 for a A1,O, fiber and YAG fiber system were obtained for mixture of LP and YAG(50: 50 vol %)and in Fig each slice thickness for a mixture of LP and LAu(50: 50 vol %)after firing Although small amounts 2.5.3. Method of analysis cubic form are detected. these results indicate that no The shear-lag approach [23] was used to calculate the chemical reactions occur between LP and YAG and interfacial shear strength. In the shear-lag model, th between LP and LAu. The JCPDS files 9-310 and 33-40 debonding load (Pa) can be related to the interfacial yielded data to identify YAG in the tetragonal and shear strength(ta) by cubic forms. The XRD results in Fig. 5 indicate that chemical compatibility exists between LP and Al2O, at Pa= aorta (1) 1600"C/h where L is the embedded fiber length(slice thickness),r is the fiber radius and a is a shear-lag parameter which n the Youngs modulus, Poisson ratio, co efficient of thermal expansion and geometric configura tion of the components. Since Eq (1)cannot be educed to a linear form, the conventional linear regres- sion procedure cannot be used to s the optimum values of the parameters. An iterative regressive curve fitting procedure was thus applied to obtain ta by fitting the above equation to the experimental data(Pa vs. L) [23]. As the embedded length(L)in Eq (1)approaches zero, the shear-lag approach reduces to a linear equa tion: Pa=2xrLta. This linear form was also applied to calculate ta in this study. On the other hand, the Fig 4 XRD of an LaPO /Laal Og pellet fired at 1600C for h
126 D.-H. Kuo, W.M. Kriven / Materials Science and Engineering A210 (1996) 123-134 crosshead load eell "~'--" ~ diamond ~r probe ~ Stereom icroseope specimen ~ [~ alumina~ ~-rfiberL~ ~[ su bstrate [--==i~ [ \ "~r-'~ x ~=L//////l 'r .... i Y stagl I/r/"/~//A ~ I r] Fig. 2. Schematic representation of the experimental set-up used in the pushout tests. 2.5.2. Pushout test procedure The sintered slabs were sliced to produce samples with different thicknesses. The surface of these samples was then polished. Pushout tests were conducted in a screw-driven machine with a 1 kg load cell. A diamond probe with a 95 #m diameter fiat tip was fixed into a cylinder, which was threaded to the load cell. A crosshead displacement rate of 60 #m min- 1 was used. The specimen mounted on a slotted alumina substrate for testing was positioned under a stereomicroscope with an X-Y micropositioning stage. The experimental set-up for the fiber pushout tests is represented schematically in Fig. 2. Four or five pushout results on each A1203 fiber and YAG fiber system were obtained for each slice thickness. 2.5.3. Method of analysis The shear-lag approach [23] was used to calculate the interfacial shear strength. In the shear-lag model, the debonding load (Pd) can be related to the interfacial shear strength (%) by Pd = 2rcrzd tanh~L (1) where L is the embedded fiber length (slice thickness), r is the fiber radius and 7 is a shear-lag parameter which depends on the Young's modulus, Poisson ratio, coefficient of thermal expansion • and geometric configuration of the components. Since Eq. (1) cannot be reduced to a linear form, the conventional linear regression procedure cannot be used to assess the optimum values of the parameters. An iterative regressive curve fitting procedure was thus applied to obtain rd by fitting the above equation to the experimental data (Pd vs. L) [23]. As the embedded length (L) in Eq. (1) approaches zero, the shear-lag approach reduces to a linear equation: Pd = 2nrLrd. This linear form was also applied to calculate r d in this study. On the other hand, the 1600"C/6 hrs 0 ,O • LaPO 4 [] YsA15012 O [] [] O [] [] [] • • [] • 15 20 25 30 35 40 45 50 55 60 20 Fig. 3. XRD of an LaPO4/Y3AIsOI2 pellet fired at 1600 °C for 6 h. calculation of the interfacial frictional strength by the shear-lag model involved an unknown parameter, as an interfacial coating existed between the fiber and the matrix. For this reason, the interfacial frictional strengths were not calculated. 3. Results 3. I. Chemical compatibility and stability Studies of the chemical compatibility can guide the choice of suitable materials for use at high temperatures. The XRD results are shown in Fig. 3 for a mixture of LP and YAG (50:50 vol.%) and in Fig. for a mixture of LP and LA11 (50:50 vol.%) after firing at 1600 °C. Although small amounts of YAG in cubic form are detected, these results indicate that no chemical reactions occur between LP and YAG and between LP and LA11. The JCPDS files 9-310 and 33-40 yielded data to identify YAG in the tetragonal and cubic forms. The XRD results in Fig. 5 indicate that chemical compatibility exists between LP and AI203 at 1600"C/3 hrs • LaPO t a LaAliiOis A • ° ° A° • ,, ,,i,, I,i .... i .... i .... la, i,i .... ! .... i i i i i I 15 20 25 30 35 40 45 50 55 60 20 Fig. 4. XRD of an LaPO4/LaA1HOts pellet fired at 1600 °C for 3 h
D.H. Kuo, w M. Kriven/Materials Science and Engineering A210 (1996)123-134 1550C/5hrs+1600C/6hrs 米 1559C/6h Fig. 7. Scan ctron micrograph of a single-crystal AlO, fiber D of an Al,O/LaPO4 pellet fired at 1550 C for 6 h AdAl,O, ) system having LaPO4(Lp)as an interlayer (bottom) and subsequently fired at 1600C for 6 h(top) and subsequently fired at 1600C for 6 h 1550C. However, chemical reaction occurs after sub- dispersive spectra with those of synthesized pure sequent firing at 1600 C, as shown by the peaks phases. The widths of the LAl and La zones on the corresponding to laAm O18 outer surface of the fiber are approximately 2 um, while In the A12O3 fiber/Al,O3 matrix system with LP as an between the coating and matrix the width is approxi- interlayer, no chemical reaction takes place between mately 7 um. Zone (iii) is an La-rich porous region AL,O, fibers and LP after firing at 1550C in air for 6 with small amounts of Al and P. From the known h. Indentations placed near the LP interlayers in the coating thickness, the porous regions in Fig. 7 can be 1550C composite cause indentation cracks to be located at the positions of the original LP/matrix and deflected by fibers or the LP monazite to chip. Fig. 6 LP/fiber interfaces. Zone(iv)is LP On the other hand, shows such a chipped region around a fiber. It can be the different diffusion configurations in the Al,O3/LP seen that the chipping fracture does not damage the (50: 50 vol %)pellet, after firing at 1600C for 6 h fiber, but is deflected by interfacial debonding. Addi ause a third phase, LAu, to be formed (Fig. 5). Due to tional firing at 1600C for 6 h was performed on the these chemical reactions, the LP/A laminates in this specimen in Fig. 6. Reaction layers and void regions are study were hot pressed only at 1300( formed at the fiber/coating and coating/matrix inter After annealing, XRD and EDS of bulk LP, and faces(Fig. 7). The corresponding energy dispersive EDS of the laminates, indicated that LP was stable, and spectra taken from the zones marked (i)to(iv)in Fig no decomposition or reduction reactions occurred dur 7 are shown in Fig. 8. The compounds formed are ing consolidation by hot pressing identified as LAu in zone(i) and LaAlO,(La)in zone (ii). This is confirmed by comparison of the energy ENERGY (Kev) ENERGY(KeV ENERGY (Kev) Fig. 6. Scanning electron micrograph of a chipped region in single-crystal Al2O, fiber(Ad)/AlO, matrix(A)system having LaPO4 Fig. 8. Energy dispersive spectra taken from the reaction zones LP)as an interlayer, sintered at 1550C for indicated as(i)to(iv) in Fig. 7 between the Al,O, matrix and LaPo interlay
D.-H. Kuo, W.M. Kriven /Materials Science and Engineering A210 (1996) 123-134 127 1550"C/6 h.rs + 1600"C/6 hrs A 1550"C/6 hrs 20 25 o A1~ 03 • LaPO 4 A LaAIL1018 A A • • 0 A •A & 0 0 A • A 0 •• • 30 35 40 45 50 20 Fig. 5. XRD of an A1203/LaPO4 pellet fired at 1550 °C for 6 h (bottom) and subsequently fired at 1600 °C for 6 h (top). 1550 °C. However, chemical reaction occurs after subsequent firing at 1600 °C, as shown by the peaks corresponding to LaAll]O~8. In the A1203 fiber/A1203 matrix system with LP as an interlayer, no chemical reaction takes place between AI20 3 fibers and LP after firing at 1550 °C in air for 6 h. Indentations placed near the LP interlayers in the 1550 °C composite cause indentation cracks to be deflected by fibers or the LP monazite to chip. Fig. 6 shows such a chipped region around a fiber. It can be seen that the chipping fracture does not damage the fiber, but is deflected by interfacial debonding. Additional firing at 1600 °C for 6 h was performed on the specimen in Fig. 6. Reaction layers and void regions are formed at the fiber/coating and coating/matrix interfaces (Fig. 7). The corresponding energy dispersive spectra taken from the zones marked (i) to (iv) in Fig. 7 are shown in Fig. 8. The compounds formed are identified as LA11 in zone (i) and LaAIO3 (LA) in zone (ii). This is confirmed by comparison of the energy Fig. 7. Scanning electron micrograph of a single-crystal A1203 fiber (Af)/AI203 matrix (A) system having LaPO4 (LP) as an interlayer, sintered at 1550 °C and subsequently fired at 1600 °C for 6 h. dispersive spectra with those of synthesized pure phases. The widths of the LAtl and LA zones on the outer surface of the fiber are approximately 2/lm, while between the coating and matrix the width is approximately 7 /~m. Zone (iii) is an La-rich porous region with small amounts of A1 and P. From the known coating thickness, the porous regions in Fig. 7 can be located at the positions of the original LP/matrix and LP/fiber interfaces. Zone (iv) is LP. On the other hand, the different diffusion configurations in the AI203/LP (50:50 vol.%) pellet, after firing at 1600 °C for 6 h, cause a third phase, LA11, to be formed (Fig. 5). Due to these chemical reactions, the LP/A laminates in this study were hot pressed only at 1300 °C. After annealing, XRD and EDS of bulk LP, and EDS of the laminates, indicated that LP was stable, and no decomposition or reduction reactions occurred during consolidation by hot pressing. Z [- z AI , , i , ITI iJ~l i i" , i i i i i i i 2 4 6 8 10 ENERGY (KeV) La (ii) 2 4 6 8 10 ENERGY (KeV) .... Fig. 6. Scanning electron micrograph of a chipped region in a single-crystal A1203 fiber (Af)/AI203 matrix (A) system having LaPO4 (LP) as an interlayer, sintered at 1550 °C for 6 h. La (iii) ,i ,i ,i ,i 2 4 6 8 10 ENERGY (KeV) ~ (iv) ENERGY (KeV) Fig. 8. Energy dispersive spectra taken from the reaction zones indicated as (i) to (iv) in Fig. 7 between the A1203 matrix and LaPO4 interlayer