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
D.-H. Kuo, W.M. Kriven/ Materials Science and Engineering A210(1996)123-134 layers are about 25, 30 and 25 um for LP/A, LP/YAG and LP/LA1 laminates respectively, with 70 um thick Apoa ness of Al2O3, 38 um of YAG and 35 um of LAul.The volume fractions of LP in LP/A, LP/YAG and LP/ LAn laminates are about 26%, 44% and 42% respec tively. Although the three LP-containing laminates were fabricated successfully, the pure LP material was LaPO4 CAlso 1 mmw Y3Al6012 LaPo LaAl11018 50 Fig. 9. Optical micrographs of three as-fabricated LaAl11o18 LaPO4/Al O3 hot pressed at 1300C for 3 h:(b)LaP hot pressed at 1600C for 3 h;(c)LaPO4/ LaAl, O1s he Lipoate++ l600°for3h. 3. 2. Microstructure of laminated composite Three multilayered composites were fabricated by hot pressing at 1300C and 1600c giving uniform layer thicknesses. The laminates are shown in the optI- nates with indentation cracks:(a)LaPo/Al20, hot pressed at 1300 ree as-fabricated la cal micrographs of F and the scanning electron C for 3 h;(b)LaPO/Y3, O,2 hot pressed at 1600C for 3 h;(c) micrographs of Fig. 10. The thicknesses of the LP LaPO./ LaAl,O hot pressed at 1600%C for 3 h
128 D.-H. Kuo, W.M. Kriven / Materials Science and Engineering A210 (1996) 123-134 layers are about 25, 30 and 25/xm for LP/A, LP/YAG and LP/LAll laminates respectively, with 70/tm thickness of A1203, 38/tm of YAG and 35/zm of LA~1. The volume fractions of LP in LP/A, LP/YAG and LP/ LAI1 laminates are about 26%, 44% and 42% respectively. Although the three LP-containing laminates were fabricated successfully, the pure LP material was Fig. 9. Optical micrographs of three as-fabricated laminates: (a) LaPO4/AI203 hot pressed at 1300 °C for 3 h; (b) LaPO4/Y3AIsOI2 hot pressed at 1600 °C for 3 h; (c) LaPO4/LaA12~O18 hot pressed at 1600 °C for 3 h. 3.2. Microstructure of laminated composites Three multilayered composites were fabricated by hot pressing at 1300 °C and 1600 °C giving uniform layer thicknesses. The laminates are shown in the optical micrographs of Fig. 9 and the scanning electron micrographs of Fig. 10. The thicknesses of the LP Fig. 10. Scanning electron micrographs of three as-fabricated laminates with indentation cracks: (a) LaPO4/AI203 hot pressed at 1300 °C for 3 h; (b) LaPO4/Y3AIsOt2 hot pressed at 1600 °C for 3 h; (c) LapO4/LaAlliOls hot pressed at 1600 °C for 3 h
D-H. Kuo, W.M. Kricen/ Materials Science and Engineering A210 (1996)123-1.34 after five cycles. In this way, interfacial delamination between LP and Lau was shown by sem at the notch tip(Fig. 12) To examine the interactions between the cracks and the microstructure, indentation cracks were introduced (Fig. 10). Radial cracks easily propagate across LP/A Fig 10(a))and LP/YAG( Fig. 10(b))interfaces. On the other hand, indentation cracks in the LP/LAI(Fig 10(c))system display a preferred LPE along the oundaries between Lp and LA. In 10(c), the broken line indicates the supposed direction of the 00400600800100012001400 Vickers radial crack, whereas the crack is actually Temperature(°C) deflected along the LP/LAn interface. An enlarged micrograph of the interfacial delamination in the in- ig. 11, Thermal expansion of LaAl, Os as measured by dilatome- dented LP/ LAu laminate, which in Fig. 10(c), is shown in Fig. 13. This is consistent with difficult to make. Densification by either hot pressing or the sem observation in Fig. 12 sintering at 1600C produced cracked specimens The morphological stability of the interface in fiber 3. 4. Pushout tests of fiber model systems reinforced composites is of concern in considering long term high temperature applications. Morphologic instability has been found between B-alumina-related 3.4.I. Fiber model systems materials and single-crystal Al2O3 fibers [11], which can The interfacial behavior of the Al,O, fiber/ LaPO4 fiber sliding to be difficult. Al2O3 matrix and YAG fiber/ LaPO. In the LP/A, LP/YAG and LP/LA, laminates, mor Al,O3 matrix model systems was studied by fiber phological instability is not evident. All the interfaces around the fiber coating for the two model systems are between the components of the laminates are stable and flat(Fig. 10) shown in Fig. 14. After sintering at 1550 some elongated magnetoplumbite B-alumina(LAu) crystals The CTE of LA, was not known before this study. are formed between the coating and the matrix. The After fabrication of single-phase LA, by hot pressing formation of LAn during sintering at 1550C may be at 1600C for 3 h, a density of 3.0 g cm-3was measured. The la, bar tested for cte had dimen- facilitated by the use of the high surface area Al2o powders for the matrix. This reaction is also observed sions of 17. 1 mm x 2. I mm x 1.7 mm. The thermal between the Al-O, fiber and the LP interphase, giving a expansion data for hot pressed LA u are shown in Fig 11. The ctes for la are8.8×10-6Cat200°C thin LAu layer. 10.0×106°C-lat600°andl0.8×10-6°-at 1000C, with an average value over this temperature 3. 4.2 Pushout test Examples of the pushout curves(load vs, crosshead displacement) for Al, O3 and YAG fiber model systems are shown in Fig. 15. On reaching the peak load (Pp), 3.3. Mechanical responses of laminated composites there is a subsequent load drop, which indicates that the bottom surface of the test fiber protrudes out of the Table 2 summarizes the four-point flexural strengths thin slice [22]. In other systems, after the fiber slide of single-phase and laminated materials, accompanied with its bottom surface protruded, the sliding resistance by the microstructural data. No available test data for decreases with increasing fiber displacement [23.How- the flexural strength of LP were obtained due to the ever, in the present case, both fiber systems in Fig. 15 difficulty in preparing specimens without cracking. The ide at about constant force, after passing the peak low strength of the LP/A laminate may be attributed to load the lower densification temperature of 1300C. Most The peak load(P), the easiest parameter to measure, with their tensile surfaces ground, indented or was used as the debonding load(Pa) in Eq. (1). Fig. 16 notched show brittle fracture under flexural tests. One gives the peak load(P)as a function of the embedded exception is the annealed and notched LP/LAu lami- fiber length(L). The experimental data plotted are the nate. The three notched LP /LA, bars show non- average peak loads, and the error bars represent one catastrophic fracture to some degree. One of the bars, standard deviation. By using an iterative regressive after the first load drop under a test, was subjected to curve fitting procedure, the interfacial shear strength td multiple unloading and re-loading. This bend bar broke was obtained. td values calculated by the linear and
D.-H. Kuo, W.M. Kriven Materials Science and Engineering A210 (1996) 123-134 129 0 ;,< 1,4 ' '' I' ' ' I ' '' I ' ' ' I , ' ' I ' ' ' I 1.2 1 0.8 0.6 0.4 0.2 0 I 200 400 600 800 1000 1200 1400 Temperature (°C) Fig. 11. Thermal expansion of LaAl~Ot8 as measured by dilatometry. difficult to make. Densification by either hot pressing or sintering at 1600 °C produced cracked specimens. The morphological stability of the interface in fiberreinforced composites is of concern in considering longterm high temperature applications. Morphological instability has been found between fl-alumina-related materials and single-crystal A1203 fibers [11], which can degrade the fiber and cause fiber sliding to be difficult. In the LP/A, LP/YAG and LP/LA~ laminates, morphological instability is not evident. All the interfaces between the components of the laminates are stable and flat (Fig. 10). The CTE of LA~ was not known before this study. After fabrication of single-phase LA~ by hot pressing at 1600 °C for 3 h, a density of 3.0 g cm -3 was measured. The LAt~ bar tested for CTE had dimensions of 17.1 mm x 2.1 mm × 1.7 mm. The thermal expansion data for hot pressed LAt~ are shown in Fig. 11. The CTEs for LA~ are 8.8 x 10 -6 °C I at 200 °C, 10.0 x 10 6 o C ~ at 600 °C and 10.8 x 10 6 °C ~ at 1000 °C, with an average value over this temperature range ofl0.0x 10 6oc 1. 3.3. Mechanical responses of laminated composites Table 2 summarizes the four-point flexural strengths of single-phase and laminated materials, accompanied by the microstructural data. No available test data for the flexural strength of LP were obtained due to the difficulty in preparing specimens without cracking. The low strength of the LP/A laminate may be attributed to the lower densification temperature of 1300 °C. Most laminates with their tensile surfaces ground, indented or notched show brittle fracture under flexural tests. One exception is the annealed and notched LP/LA1~ laminate. The three notched LP/LA11 bars show noncatastrophic fracture to some degree. One of the bars, after the first load drop under a test, was subjected to multiple unloading and re-loading. This bend bar broke after five cycles. In this way, interfacial delamination between LP and LA~ was shown by SEM at the notch tip (Fig. 12). To examine the interactions between the cracks and the microstructure, indentation cracks were introduced (Fig. 10). Radial cracks easily propagate across LP/A (Fig. 10(a)) and LP/YAG (Fig. 10(b)) interfaces. On the other hand, indentation cracks in the LP/LA~ (Fig. 10(c)) system display a preferred path along the boundaries between LP and LA~. In Fig. 10(c), the broken line indicates the supposed direction of the Vickers radial crack, whereas the crack is actually deflected along the LP/LA~ interface. An enlarged micrograph of the interfacial delamination in the indented LP/LA~ laminate, which is marked by arrows in Fig. 10(c), is shown in Fig. 13. This is consistent with the SEM observation in Fig. 12. 3.4. Pushout tests of fiber model systems 3.4. I. Fiber model systems The interfacial behavior of the A1203 fiber/LaPO4 coating/Al203 matrix and YAG fiber/LaPO4 coating/ A1203 matrix model systems was studied by fiber pushout testing. Scanning electron micrographs taken around the fiber coating for the two model systems are shown in Fig. 14. After sintering at 1550 °C, some elongated magnetoplumbite/fl-alumina (LA~) crystals are formed between the coating and the matrix. The formation of LAI~ during sintering at 1550 °C may be facilitated by the use of the high surface area A1203 powders for the matrix, This reaction is also observed between the A1203 fiber and the LP interphase, giving a thin LA~ layer. 3.4.2. Pushout test Examples of the pushout curves (load vs. crosshead displacement) for A1203 and YAG fiber model systems are shown in Fig. 15. On reaching the peak load (Pp), there is a subsequent load drop, which indicates that the bottom surface of the test fiber protrudes out of the thin slice [22]. In other systems, after the fiber slides with its bottom surface protruded, the sliding resistance decreases with increasing fiber displacement [23]. However, in the present case, both fiber systems in Fig. 15 slide at about constant force, after passing the peak load. The peak load (Pp), the easiest parameter to measure, was used as the debonding load (Pd) in Eq. (1). Fig. 16 gives the peak load (Pp) as a function of the embedded fiber length (L). The experimental data plotted are the average peak loads, and the error bars represent one standard deviation. By using an iterative regressive curve fitting procedure, the interfacial shear strength ra was obtained, rd values calculated by the linear and
D.-H. Kuo, W M, Kriven/ Materials Science and Engineering A210(1996)123-134 mammary of four point fexural strengths of single phase and laminates Y3AlsO12 LaAlO,s LaPO4/Al,O, LaPO/YAl O LaPO4/laA hickness (u allot shear-lag approaches are very similar for the YaG fiber reaction is the P+ ions remaining in the La+-deficient system(100 MPa) and Al2O3 fiber system(128 MPa). LP coating after the La+ has diffused into the Al,O3 Both systems have similar test sizes and coating thick phase, formed vaporizable P2Os and created porous nesses.The average LP coating thickness is approxi- voids. Vaporization of phosphorus in the form of P2O. mately 6.5 um for the Al O3 fiber system and results in no detectable P in zones ()and (ii).The approximately 9 um for the YAG fiber system. The results in Fig. 7 also suggest that La+ ions diffuse latter has a lower interfacial shear strength. Table 3 more slowly in single-crystal Al,O, than in polycrys- gives a summary of the experimental pushout results. talline Al,O3. This is confirmed by the relative widths of Scanning electron micrographs of the pushed-out fibers the LAu and LA layers at the fiber/coating(approxi- are shown in Fig 17. Because of the formation of LAu mately 2 um)and coating/matrix(approximately 7 um) between the Al,O3 fibers and the Lp coating, deter- interfaces respectively mined from the chemical compatibility study, EDS The reaction between Al,O and LP was also noted analysis was used to examine the wall of the pushed-out by Morgan et al. [12]. After prolonged firing at 1600C fibers. The result confirms that interfacial debonding for 24 h, small amounts of B-alumina/ magnetoplumbite occurs between the thin LAu layer on the Al,, fiber platelets were observed in localized regions of the alu surface and the lp coating. mina layers in multilayered composites. They pointed out that the reaction was confined to the near-surface egion and was very sensitive to impurities which could 4. Discussion Phenate from raw materials or from the furnace atmo- orig 4.1. Chemical compatibility 4.2. Microstructure of laminated composites The observation of compound formation in zones of finite thickness in Fig. 7 indicates that interdiffusion The laminated composites show flat, stable inter- occurs between A12O3 and LP. a possible source of faces, and no residual stress-induced cracking after 题1243a,Qg四 Fig. 12. Scanning electron micrograph of interfacial delamination in Fig. 13. Enlarged micrograph of interfacial delamination in the a notched LaPO/LaAl, O1s laminate after a flexural test. indented laminate shown in Fig. 10(c)(marked by arrows)
130 D.-H. Kuo, W.M. Kriven / Materials Science and Engineering A210 (1996) 123-134 Table 2 Summary of four point flexural strengths of single phase and laminates YsA150~2 LaAll i O t 8 LaPO4/AI2Os LaPO4/YsAI5012 LaPO4/LaAll IO1 s LaPO4 N/A (vol.%) LaPO4 N/A thickness (#m) Strength 180 (MPa) One standard 14 deviation (MPa) N/A 26 44 42 N/A 25 30 25 217 79 107 122 3 16 13 20 shear-lag approaches are very similar for the YAG fiber system (100 MPa) and A1203 fiber system (128 MPa). Both systems have similar test sizes and coating thicknesses. The average LP coating thickness is approximately 6.5 /tm for the A1203 fiber system and approximately 9 /zm for the YAG fiber system. The latter has a lower interfacial shear strength. Table 3 gives a summary of the experimental pushout results. Scanning electron micrographs of the pushed-out fibers are shown in Fig. 17. Because of the formation of LA11 between the A1203 fibers and the LP coating, determined from the chemical compatibility study, EDS analysis was used to examine the wall of the pushed-out fibers. The result confirms that interfacial debonding occurs between the thin LA~ layer on the A1203 fiber surface and the LP coating. 4. Discussion 4.1. Chemical compatibility The observation of compound formation in zones of finite thickness in Fig. 7 indicates that interdiffusion occurs between A1203 and LP. A possible source of reaction is the p5 + ions remaining in the La 3 ÷ -deficient LP coating after the La 3 ÷ has diffused into the A12Oa phase, formed vaporizable P205 and created porous voids. Vaporization of phosphorus in the form of P205 results in no detectable P in zones (i) and (ii). The results in Fig. 7 also suggest that La 3+ ions diffuse more slowly in single-crystal AI203 than in polycrystalline AI203. This is confirmed by the relative widths of the LAI~ and LA layers at the fiber/coating (approximately 2/~m) and coating/matrix (approximately 7/tm) interfaces respectively. The reaction between A1203 and LP was also noted by Morgan et al. [12]. After prolonged firing at 1600 °C for 24 h, small amounts of fl-alumina/magnetoplumbite platelets were observed in localized regions of the alumina layers in multilayered composites. They pointed out that the reaction was confined to the near-surface region and was very sensitive to impurities which could originate from raw materials or from the furnace atmosphere. 4.2. Microstructure of laminated composites The laminated composites show flat, stable interfaces, and no residual stress-induced cracking after Fig. 12. Scanning electron micrograph of interfacial delamination in Fig. 13. Enlarged micrograph of interfacial delamination in the a notched LaPOa/LaAI~tO~s laminate after a flexural test. indented laminate shown in Fig. 10(c) (marked by arrows)
D.-H. Kuo, W.M. Kriven/ Materials Science and Engineering A210(1996)123-134 z30 Alo, Fiber/Lapo, Coating/Alo, Matr 10 0 L,,,L,A,,L,,,,L, Crosshead Displacement(um Coating/A°3 合m出Hh=m Crosshead Displacement( um) Fig. 15. Load vs. crosshead displacement curves of fiber pushout tests for Al2O, fiber/ LaPO4 coating/Al,O, matrix(a)and Y,Al O12 fiber/ LaPOa coating/ Al,, matrix(b)model systems between the components is not strong. In this study, the LP/LA, laminate undergoes inte Fig. 14. Scanning electron micrographs of the fiber/coating interface facial delamination and the interfacial bonding between region for Al2O3 fiber/ LaPO. coating/Al2O3 matrix(a)and Y3AlsO, iber/LaPO, coating/Al,O3 matrix(b) model systems firing and annealing. These observations result from the “128g chemical compatibility and small mi smatch in the Ctes between the components of the laminates. The CTE of 2 monazite-type LP is9.6×10-6℃C-1(20-1000133 Linear FEng which is close to 8.8 x 10-6 oC-I of A12O3[25]and 8.0 x 10-6C- of YAG [26]. In comparison, the CTE 1o, ber/LAPo, coating/Al, matrix Coating Thickness: 6 um of Lau was measured in this study to be 100x 10-6 °C-1(200-1000°C Embedded Fiber Length, L(mm) 4.3. Mechanical evaluation of laminated composites 60 Model Fitting Whether or not delamination occurs in laminates τ,100M luring testing is decided by complex factors, such as the chemical bonding, thermal and mechanical proper- ties of the laminae, the testing methods used and the relative thicknesses of the laminae. Although laminates YAG,/LPO, /Alo can be used for interfacial study, a laminated composite with a weak interface may not show delamination easily by indentation and flexural tests. This complexity also Embedded Fiber Length, L occurred in the work of LP/A laminates [13], where Fig. 16. Variation of the peak load (Po) as a function of the delamination occurred within thicker layers, but not within thinner layers. However, if a laminate shows embedded fiber length(L)in fiber pushout tests of Al2O, fiber/ LaPO coating/Al2O3 matrix (a) and Y3Als O,, fiber/ LaPO4 coating/AlO delamination, it suggests that the interfacial bonding matrix(b)model systems
D.-H. Kuo, W.M. Kriven / Materials Science and Engineering A210 (1996) 123-134 131 5O 4O A z 3o 10 0 50 ' ' ' ' I ' ' ' ' I " ' ' ' I .... I ' ' ' ' (a) P = 36.5 N P F.mbedded Hber ~ O~ mm 10 20 30 40 Crosshead Displacement lima) 50 ' ' ° ' I " ' ' ' ' ' ' ' I .... I ' ' " ' (b) P :' 4o.5 N 40 P 4 Coating / ~d20 ~ Matrix 10 / A~ C~I~ Thicka~!~: - 9 itm ~b~ld~ l~,un~t Length: 0.8 =w~ 0 10 20 30 40 50 Crosshead Displacement (~xm) Fig. 15. Load vs. crosshead displacement curves of fiber pushout tests for AI203 fiber/LaPO4 coating/Al203 matrix (a) and Y3A15OI2 fiber/ LaPO4 coating/Al203 matrix (b) model systems. Fig. 14. Scanning electron micrographs of the fiber/coating interface region for AI203 fiber/LaPO4 coating/A1203 matrix (a) and Y3AIsOt2 fiber/LaPO4 coating/A1203 matrix (b) model systems. firing and annealing. These observations result from the chemical compatibility and small mismatch in the CTEs between the components of the laminates. The CTE of monazite-type LP is 9.6 x 10- 6 °C-1 (20-1000 °C) [13] which is close to 8.8 x l0 -6 °C-1 of A1203 [25] and 8.0 × 10 -6 °C- 1 of YAG [26]. In comparison, the CTE of LA~I was measured in this study to be 10.0 × 10 6 o C 1 (200-1000 °C). 4.3. Mechanical evaluation of laminated composites Whether or not delamination occurs in laminates during testing is decided by complex factors, such as the chemical bonding, thermal and mechanical properties of the laminae, the testing methods used and the relative thicknesses of the laminae. Although laminates can be used for interfacial study, a laminated composite with a weak interface may not show delamination easily by indentation and flexural tests. This complexity also occurred in the work of LP/A laminates [13], where delamination occurred within thicker layers, but not within thinner layers. However, if a laminate shows delamination, it suggests that the interfacial bonding between the components is not strong. In this study, the LP/LAll laminate undergoes interfacial delamination and the interfacial bonding between 70 (a) 6O Sheaur-l~ Model Pitting i 50 ~d "~n~ ~~ g ¢~" 30 ~ ~d = 128 l~gpa 2O o .... ," • o~. 0.4 0.6 o~ ~ a.~ F.~bedded ~ ~ L (-,~) 6O ' " " I " ' ' I = ' " lH.i~a. ~ ' I " " ' I " " " 0 o o.2 0.4 0.6 o.s a ~.2 F.adx.dded ~ ~ L (ram) Fig. 16. Variation of the peak load (Pp) as a function of the embedded fiber length (L) in fiber pushout tests of AI203 fiber/LaPO 4 coating/Al203 matrix (a) and Y3AIsOl~ fiber/LaPO4 coating/Al203 matrix (b) model systems
D.H. Kuo, W.M. Kriven/ Materials Science and Engineering A210(1996)123-134 Table 3 occur. A similar behavior to that of the LP/A laminate Summary of intertacial shear strengths for two fiber model systems takes place in the LPYAg combination No interfacial Interfacial shear strength delamination occurs(Fig. 10(b)), but fiber debonding thickness (um)(MP and sliding(Fig. 17(b))are observed. The difficult achieving the higher shear stress under four-point bend Shear lag Linear ing tests for the three laminates causes the laminated 1,Oa fiber, composites to fracture in brittle behavior. On the other LaPOa coating hand, the notched LP/LA, beam(Fig. 12)with the stronger LAu second component (Table 2)has a stress Y3AlsO12 fibe concentration on the notch tip, which raises the stres LaPO coating. and causes delamination Al,O Two delamination mechanisms have been reported in laminate does not show interfacial delamination by sigmo/ tes [13]. Delamination has been observed LP/A laminat LP and LAu is not strong. In contrast, our LP/A along the LI JA interface, but nation Is icrocracks within the LP layer. As shown in dentation(Fig. 10(a)). As mentioned by Morgan and Marshall [13 this may be attributed to the insufficient Figs. 12 and 13, our observations of delamination in the LP/Lau system correspond to the former mecha strength of Al O3(compare the flexure test data listed nism, i.e. interfacial delamination in Table 2). As strong Al,O3 fibers with LP coatings were incorporated in the Al2O, matrix, interfacial de- 4.4. Pushout tests of fiber model systems lamination by indentation tests(Fig. 6)and fiber debonding and sliding by pushout tests(Fig. 17(a)) Examples of pushout curves for Al,O3 fiber and YAG fiber model systems are shown in Figs. 15(a)and 15(b) respectively. After reaching the peak load (PD) fibers of both systems slide at constant, rather than ith increasing fiber displac has been shown that fibers which have as-manufactured surface when used in composites. These rough inter faces slide over one another via a mechanism of asper ity lock and friction [27. Observations of the surface morphologies of the pushed-out fibers(Figs. 17(a)and 17(b)suggest that the mechanism of asperity lock and formed. The debris forms when the shear stress is higher than the strength of LP. The surface asperity and debris formation can contribute to the constant sliding loads after passing the peak load. Some unusual pushout curves with increasing sliding resistance during fiber displacement have also been mentioned in SiC/ Si3N4 composites [28]and in fatigued Ti-15-3/SiC mate- als [29] An average shear stress along the interface nset of sliding for the Al,O, fiber /LP coating/AlO matrix system has been estimated to be 85 MPa by the linear approach [13]. In this study, the interfacial shear strength calculated using the shear-lag model or the linear approach for the same system is 128 MPa. The interfacial shear strength of the YAG fiber system has also been obtained (100 MPa). Several factors can have an influence on the pushout results. As expressed by Liang and Hutchinson [22], the residual stress, debond Fig. 17. Scanning electron micrographs of the pushed-out fibers of ng toughness and friction are the factors contributing AL,o, fiber/ LaPO, coating/AL,O, matrix (a)and Y, A1,O, 2 fiber/ to the pushout stress at breakthrough. The thin LAll LaPO. coating/Al2O3 matrix(b) model systems layer between the Al2O, fiber and coating may also
132 D.-H. Kuo, W.M. Kriven / Materials Science and Engineering ,4210 (1996) 123-134 Table 3 Summary of interfacial shear strengths for two fiber model systems System Coating Interfacial shear strength thickness (/~m) (MPa) Shear lag Linear A1203 fiber/ ~ 6.5 LaPO 4 coating/ AI203 matrix Y3A15OI2 fiber/ ~ 9 LaPO4 coating/ A1203 matrix 128 128 100 99 LP and LA~ is not strong. In contrast, our LP/A laminate does not show interfacial delamination by indentation (Fig. 10(a)). As mentioned by Morgan and Marshall [13], this may be attributed to the insufficient strength of AI203 (compare the flexure test data listed in Table 2). As strong A1203 fibers with LP coatings were incorporated in the A1203 matrix, interfacial delamination by indentation tests (Fig. 6) and fiber debonding and sliding by pushout tests (Fig. 17(a)) Fig. 17. Scanning electron micrographs of the pushed-out fibers of AI203 fiber/LaPO4 coating/Al203 matrix (a) and Y3A15012 fiber/ LaPO 4 coating/A1203 matrix (b) model systems. occur. A similar behavior to that of the LP/A laminate takes place in the LP/YAG combination. No interracial delamination occurs (Fig. 10(b)), but fiber debonding and sliding (Fig. 17(b)) are observed. The difficulty in achieving the higher shear stress under four-point bending tests for the three laminates causes the laminated composites to fracture in brittle behavior. On the other hand, the notched LP/LA11 beam (Fig. 12) with the stronger LAI1 second component (Table 2) has a stress concentration on the notch tip, which raises the stress and causes delamination. Two delamination mechanisms have been reported in LP/A laminates [13]. Delamination has been observed along the LP/A interface, but most commonly delamination is associated with the initiation of an array of sigmoidal microcracks within the LP layer. As shown in Figs. 12 and 13, our observations of delamination in the LP/LAI~ system correspond to the former mechanism, i.e. interfacial delamination. 4.4. Pushout tests of fiber model systems Examples of pushout curves for A120 3 fiber and YAG fiber model systems are shown in Figs. 15(a) and 15(b) respectively. After reaching the peak load (Pp), fibers of both systems slide at constant, rather than decreasing, forces with increasing fiber displacement. It has been shown that fibers which have as-manufactured smooth surfaces form a much more inhomogeneous surface when used in composites. These rough interfaces slide over one another via a mechanism of asperity lock and friction [27]. Observations of the surface morphologies of the pushed-out fibers (Figs. 17(a) and 17(b)) suggest that the mechanism of asperity lock and friction during fiber sliding can also cause debris to be formed. The debris forms when the shear stress is higher than the strength of LP. The surface asperity and debris formation can contribute to the constant sliding loads after passing the peak load. Some unusual pushout curves with increasing sliding resistance during fiber displacement have also been mentioned in SiC/ SiaN 4 composites [28] and in fatigued Ti-15-3/SiC materials [29]. An average shear stress along the interface at the onset of sliding for the A1203 fiber/LP coating/A1203 matrix system has been estimated to be 85 MPa by the linear approach [13]. In this study, the interfacial shear strength calculated using the shear-lag model or the linear approach for the same system is 128 MPa. The interfacial shear strength of the YAG fiber system has also been obtained (100 MPa). Several factors can have an influence on the pushout results. As expressed by Liang and Hutchinson [22], the residual stress, debonding toughness and friction are the factors contributing to the pushout stress at breakthrough. The thin LA11 layer between the Al203 fiber and coating may also