《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_Microstructure and properties of monazite（LaPO4）coated saphikon ?ber/alumina matrix composites.pdf_大学文库

E驅≈3S ournal of the European Ceramic Society 20(2000)551-559 Microstructure and properties of monazite (LaPO4)coated saphikon fiber / alumina matrix composites KK. Chawlaa, *H. Liu.J. Janczak-Rusch. s. sambasivand a Department of Materials and Mechancial Engineering, University of Alabama at Birmingham, 254 BEC, 1530 Third Ave. South, Birmingham, AL35294. Sumitomo, Sitix, Albuquerque, NM 87131, USA FEMPA Thun, Feuerwerkstrasses 39, CH-3602 Diibendorf. Switzerland BIRL, Northwestern University, Evanston, IL 60201, US.A Accepted 10 August 1999 Abstract The objective of this research was to engineer a weak interfacial bond in single crystal a-alumina( Saphikon) fiber/polycrystalline alumina matrix composites by incorporating a monazite (lanthanum phosphate, LaPO4) coating onto Saphikon fibers via soh-gel dip process. Uniaxial hot pressing was used to densify LaPOa-coated AL,O3 fiber in an AlO3 matrix composites. Characterization of the composites was done by optical microscopy, SEM(scanning electron microscopy), EDS(energy dispersive spectrometer). dentation tests, three-point bend and fiber pushout tests. The results showed that the Saphikon fiber/monazite interface was weaker than the polycrystalline alumina/monazite interface Crack deflection, interfacial debonding and fiber pullout occurred at this interface. This was attributed to the fact that the Saphikon fiber /monazite interface was smoother than the monazite/poly- crystalline alumina matrix interface. Monazite coating obtained by sol-gel dip coating method withstood high fabrication tem- peratures(1400C)and was conducive to the toughness properties of the composites. 2000 Elsevier Science Ltd. All rights Keywords: Al,O, fibers; AL2O3 matrix; Composites: Interfaces: LaPOa 1. Introduction between fiber and matrix plays a crucial role in determining the strength and toughness of the composite. 3 For Ceramic matrix composites(CMCs)consisting of instance, in a composite consisting of Al2O3 fiber in a nonoxide fiber/nonoxide matrix, nonoxide fiber/oxide SiO2-based matrix, a strong interfacial bond(chemical matrix or oxide fiber/nonoxide matrix are susceptible to bond) causes the failure mode to be similar to that of oxidation in oxidizing environments at high tempera- monolithic ceramics(brittle). 4 In some simple eutectic tures, causing loss of strength and rapid decrease in type oxide systems, such as Al,O -SnO: and Al2O3- toughness. The degradation of properties of CMCs at ZrO2, no chemical reactions would be expected and elevated temperatures may be due to oxidation of the these composites are relatively stable, moreover, a weak fiber, matrix, and/ or interface, thermal expansion- interface can change the failure mode from brittle to induced residual stresses, and matrix microcracking. 2 non-brittle. 4 In Al2O3/ Al2O3 system, very stron g lonIc Thus, for high temperature applications, in air, an and or covalent bonding leads to low toughness. It thus oxide/oxide composite system would be desirable becomes necessary to apply an interface engineering because of its inherent stability at high temperatures and in approach to increase the toughness by adding a suitable oxidizing atmospheres. In all composites, the interface interphase material between the fiber and matrix. 5 Morgan and Marshall6-s investigated a number of interphase materials including simple metal oxides and Corresponding author. Tel :+1-205-934-8450; fax:+1-205-934. mixed oxides for oxide/oxide composite systems.More significantly, for Al2O3/Al2O3 composites, they found E-mail address; kchawla(@uab.edu(KK. Chawla). that lanthanum phosphate, LaPO4(monazite)was a 0955-2219/00/S. see front matter C 2000 Elsevier Science Ltd. All rights reserved PII:S0955-2219(99)00253-8

Microstructure and properties of monazite (LaPO4) coated saphikon ®ber/alumina matrix composites K.K. Chawlaa,*, H. Liub, J. Janczak-Ruschc , S. Sambasivand a Department of Materials and Mechancial Engineering, University of Alabama at Birmingham, 254 BEC, 1530 Third Ave. South, Birmingham, AL 35294- 4461, USA bSumitomo, Sitix, Albuquerque, NM 87131, USA c EMPA Thun, Feuerwerkstrasses 39, CH-3602 DuÈbendorf, Switzerland dBIRL, Northwestern University, Evanston, IL 60201, USA Accepted 10 August 1999 Abstract The objective of this research was to engineer a weak interfacial bond in single crystal a-alumina (Saphikon) ®ber/polycrystalline alumina matrix composites by incorporating a monazite (lanthanum phosphate, LaPO4) coating onto Saphikon ®bers via sol±gel dip process. Uniaxial hot pressing was used to densify LaPO4-coated Al2O3 ®ber in an Al2O3 matrix composites. Characterization of the composites was done by optical microscopy, SEM (scanning electron microscopy), EDS (energy dispersive spectrometer), indentation tests, three-point bend and ®ber pushout tests. The results showed that the Saphikon ®ber/monazite interface was weaker than the polycrystalline alumina/monazite interface. Crack de¯ection, interfacial debonding and ®ber pullout occurred at this interface. This was attributed to the fact that the Saphikon ®ber/monazite interface was smoother than the monazite/polycrystalline alumina matrix interface. Monazite coating obtained by sol±gel dip coating method withstood high fabrication temperatures (1400C) and was conducive to the toughness properties of the composites. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Al2O3 ®bers; Al2O3 matrix; Composites; Interfaces; LaPO4 1. Introduction Ceramic matrix composites (CMCs) consisting of nonoxide ®ber/nonoxide matrix, nonoxide ®ber/oxide matrix or oxide ®ber/nonoxide matrix are susceptible to oxidation in oxidizing environments at high temperatures, causing loss of strength and rapid decrease in toughness.1 The degradation of properties of CMCs at elevated temperatures may be due to oxidation of the ®ber, matrix, and/or interface, thermal expansioninduced residual stresses, and matrix microcracking.2 Thus, for high temperature applications, in air, an oxide/oxide composite system would be desirable because of its inherent stability at high temperatures and in oxidizing atmospheres. In all composites, the interface between ®ber and matrix plays a crucial role in determining the strength and toughness of the composite.3 For instance, in a composite consisting of Al2O3 ®ber in a SiO2-based matrix, a strong interfacial bond (chemical bond) causes the failure mode to be similar to that of monolithic ceramics (brittle).4 In some simple eutectic type oxide systems, such as Al2O3±SnO2 and Al2O3± ZrO2, no chemical reactions would be expected and these composites are relatively stable, moreover, a weak interface can change the failure mode from brittle to non-brittle.4 In Al2O3/Al2O3 system, very strong ionic and/or covalent bonding leads to low toughness. It thus becomes necessary to apply an interface engineering approach to increase the toughness by adding a suitable interphase material between the ®ber and matrix.5 Morgan and Marshall6±8 investigated a number of interphase materials including simple metal oxides and mixed oxides for oxide/oxide composite systems. More signi®cantly, for Al2O3/Al2O3 composites, they found that lanthanum phosphate, LaPO4 (monazite) was a 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(99)00253-8 Journal of the European Ceramic Society 20 (2000) 551±559 * Corresponding author. Tel.: +1-205-934-8450; fax: +1-205-934- 8485 E-mail address; kchawla@uab.edu (K.K. Chawla)

KK Chawla et al. / Journal of the European Ceramic Society 20(2000)551-559 suitable and effective oxide interphase material, which easily washed out. For example, methyl cellulose coat exhibited high stability at high temperature in both ing is usually used on Saphikon fibers. This coating is a reducing and oxidizing environments and good chemical food grade coating that can be removed with cold water compatibility with alumina. They observed that the and agitation. Frequently, sinusoidal asperities have monazite/alumina fiber (Saphikon) interface had a low been observed on Saphikon fiber surface, which ca enough fracture resistance to satisfy the condition for affect both debonding and sliding abilities of this fiber interfacial debonding, when a crack grew from monazite within the matrix. In the present work, single crystal to alumina. The monazite/alumina interface was weak alumina fiber with c-orientation, i.e. with the basal enough to prevent crack growth by interfacial debond plane perpendicular to the fiber axis, was used ing and crack deflection. They obtained monazite coat- ng by manually dipping Saphikon fibers in a monazite 2. 2. Monazite precursor sol slurry, which was made by precipitation from aqueous solution with potassium phosphate. After hot pressing Monazite sol was synthesized by using alcohol-based the Saphikon/monazite/polycrystalline alumina system solutions of La(as lanthanum nitrate)and P(as phos at 1400 and 1600oC, minor liquid phase rich in potas- phorus pentoxide) in appropriate proportions. Fibers sium was found, which was thought to cause creep at were passed through the sol and heated to 600%CCon high temperatures. Kuo et al. also used slurry dipping version to monazite occurred around 275C, and heat method to coat oxide fibers with monazite and studied ing to 600C eliminated organic impurities. After dip he effect of coating thickness on the interfacial shear coating and heat treatment, the coated fibers were stress during fiber pushout embedded in alumina powder, put into a graphite die, In this work, we used a sol-gel technique to apply the and hot pressed at 1400 C for I h. The initial heating monazite coatings on Saphikon fibers Sol-gel technique rate was 900oC per h. When the temperature reached is advantageous o because it is generally simple, it can 1400 C, a pressure of 30 MPa was applied for I h. The provide better reproducibility of coating thickness and system was then allowed to cool to room temperature control of coating composition, and temperature of sol- gel processing is relatively low. A low processing tem- 2.3. Microstructural characterization perature will not only reduce the cost of fabrication, but also reduce the extent of coating interaction with fibers The desized Saphikon fiber surfaces were first exam- and minimize potential coating degradation during ined using optical microscopy and SEM. After hot processing. So far, some promising coating materials pressing, the five- and 10-dip LaPOa-coated Saphikon have been deposited uniformly on monofilament fibers fiber/alumina matrix composites were sectioned and and multifilament tows by sol-gel processing. ,12 In the polished to allow the observation of microstructure of present work, a sol-gel dip coating method was used to the composites. In addition, the fracture behavior, such coat Saphikon monofilament with monazite precursor. as interfacial debonding, crack deflection and fiber The objective of this research was to use a sol-gel dip pullout, was observed under SEM. Compositional ana- coating process to incorporate the LaPO4 coating on lysis of the monazite coating was carried out by X-ray Saphikon fibers and thus obtain a weak interfacial bond diffraction and energy dispersive spectroscopy(EDs) in Saphikon(single crystal a-alumina) fiber/alumina matrix composite. 2.4. Mechanical characterization The ability of the monazite/alumina interfaces to 2. Materials and experimental procedure exhibit interfacial debonding and crack deflection was investigated by using an indentation technique. Inden- from a vickers hardness indentor with 9 8N (30 S), 49N(15 s), or 98N(15 s) load were made in the Alpha-alumina (a-AlO3)is a thermodynamically matrix near the matrix/monazite interface, and in the stable phase of alumina Single crystal a-alumina fibers fiber near the fiber /monazite interface. The indentations (trade name"Saphikon")are produced by an edge- were oriented so that cracks from the indentation would defined film-fed jowth(EFG)technique. 13. 4 The shape intersect the monazite/alumina interfaces. A three-point of the crystal is defined by the external shape of the die. bend test was performed to measure bend strength and This technique permits the growth of a crystal from a test fiber pullout ability. Fiber pushout tests were also molten film between the growing crystal and the die As performed to measure the debonding and frictional soon as the fiber comes out of the crucible, a"size"is shear stresses at the monazite/alumina interfaces usually applied for ease of handling during manu The fiber pushout tests were performed in an in-situ facturing without damaging the surface. The size is SEM-pushout apparatus(Touchstone Ltd, WV). The generally a water-based emulsion coating, which can be specimens were cut and ground to a thickness between

suitable and eective oxide interphase material, which exhibited high stability at high temperature in both reducing and oxidizing environments and good chemical compatibility with alumina. They observed that the monazite/alumina ®ber (Saphikon) interface had a low enough fracture resistance to satisfy the condition for interfacial debonding, when a crack grew from monazite to alumina. The monazite/alumina interface was weak enough to prevent crack growth by interfacial debonding and crack de¯ection. They obtained monazite coating by manually dipping Saphikon ®bers in a monazite slurry, which was made by precipitation from aqueous solution with potassium phosphate. After hot pressing the Saphikon/monazite/polycrystalline alumina system at 1400 and 1600C, minor liquid phase rich in potassium was found,7 which was thought to cause creep at high temperatures. Kuo et al.9 also used slurry dipping method to coat oxide ®bers with monazite and studied the eect of coating thickness on the interfacial shear stress during ®ber pushout. In this work, we used a sol±gel technique to apply the monazite coatings on Saphikon ®bers. Sol±gel technique is advantageous10 because it is generally simple, it can provide better reproducibility of coating thickness and control of coating composition, and temperature of sol± gel processing is relatively low. A low processing temperature will not only reduce the cost of fabrication, but also reduce the extent of coating interaction with ®bers and minimize potential coating degradation during processing. So far, some promising coating materials have been deposited uniformly on mono®lament ®bers and multi®lament tows by sol±gel processing.11,12 In the present work, a sol±gel dip coating method was used to coat Saphikon mono®lament with monazite precursor. The objective of this research was to use a sol±gel dip coating process to incorporate the LaPO4 coating on Saphikon ®bers and thus obtain a weak interfacial bond in Saphikon (single crystal a-alumina) ®ber/alumina matrix composite. 2. Materials and experimental procedure 2.1. Saphikon ®ber Alpha-alumina (a-Al2O3) is a thermodynamically stable phase of alumina. Single crystal a-alumina ®bers (trade name ``Saphikon'') are produced by an edgede®ned ®lm-fed jowth (EFG) technique.13,14 The shape of the crystal is de®ned by the external shape of the die. This technique permits the growth of a crystal from a molten ®lm between the growing crystal and the die. As soon as the ®ber comes out of the crucible, a ``size'' is usually applied for ease of handling during manufacturing without damaging the surface. The size is generally a water-based emulsion coating, which can be easily washed out. For example, methyl cellulose coating is usually used on Saphikon ®bers. This coating is a food grade coating that can be removed with cold water and agitation. Frequently, sinusoidal asperities have been observed on Saphikon ®ber surface, which can aect both debonding and sliding abilities of this ®ber within the matrix.15 In the present work, single crystal alumina ®ber with c-orientation, i.e. with the basal plane perpendicular to the ®ber axis, was used. 2.2. Monazite precursor sol Monazite sol was synthesized by using alcohol-based solutions of La (as lanthanum nitrate) and P (as phosphorus pentoxide) in appropriate proportions. Fibers were passed through the sol and heated to 600C. Conversion to monazite occurred around 275C, and heating to 600C eliminated organic impurities. After dip coating and heat treatment, the coated ®bers were embedded in alumina powder, put into a graphite die, and hot pressed at 1400C for 1 h. The initial heating rate was 900C per h. When the temperature reached 1400C, a pressure of 30 MPa was applied for 1 h. The system was then allowed to cool to room temperature. 2.3. Microstructural characterization The desized Saphikon ®ber surfaces were ®rst examined using optical microscopy and SEM. After hot pressing, the ®ve- and 10-dip LaPO4-coated Saphikon ®ber/alumina matrix composites were sectioned and polished to allow the observation of microstructure of the composites. In addition, the fracture behavior, such as interfacial debonding, crack de¯ection and ®ber pullout, was observed under SEM. Compositional analysis of the monazite coating was carried out by X-ray diraction and energy dispersive spectroscopy (EDS). 2.4. Mechanical characterization The ability of the monazite/alumina interfaces to exhibit interfacial debonding and crack de¯ection was investigated by using an indentation technique. Indentations from a Vickers hardness indentor with 9.8 N (30 s), 49 N (15 s), or 98 N (15 s) load were made in the matrix near the matrix/monazite interface, and in the ®ber near the ®ber/monazite interface. The indentations were oriented so that cracks from the indentation would intersect the monazite/alumina interfaces. A three-point bend test was performed to measure bend strength and test ®ber pullout ability. Fiber pushout tests were also performed to measure the debonding and frictional shear stresses at the monazite/alumina interfaces. The ®ber pushout tests were performed in an in-situ SEM-pushout apparatus (Touchstone Ltd., WV). The specimens were cut and ground to a thickness between 552 K.K. Chawla et al. / Journal of the European Ceramic Society 20 (2000) 551±559

K K. Chawla et al. Journal of the European Ceramic Society 20(2000)551-559 125 and 150 um and a finish of 0. 1 um, perpendicular to the debonding observed in 10-dip coated composite. Fig the fiber orientation. A cylindrical indentor with a 114.3 3 shows that cracks passed through the alumina matrix um diameter was used to apply a load at a constant monazite interface and debonded the monazite/Saphi displacement rate of 0.3125 um/s through the debond- kon fiber interface. Comparatively, monazite/Saphikon ing and sliding stages fiber interface was weaker than the monazite/alumina matrix interface. Fig 4 shows two indentations made by 98N load in the matrix on the two sides of a five-dir 3. Results and discussion coated fiber. One can see smooth and continuous inter. facial debonding In Fig. 4(b)and(c), high magnification Fig. I shows the different parameters that should be secondary electron images show clearly the crack in alu- considered in the precursor design of a coating. In the mina penetrating the monazite interphase and debond- present case, the ethanol-based solution was clear and ing at the monazite/Saphikon fiber interface. The bright relatively stable. It had a low viscosity and gave a high area in the backscattered electron image [ Fig 4(c)) is the ield of monazite(160 g/). The monazite coating formed monazite, which separated from the Saphikon fiber below 600oC. It showed good wetting and film forming Clearly, interfacial debonding occurred at the Saphikon characteristics. Fig. 2. shows the X-ray u: ndicating the polycrystalline alumina/monazite interface. This is con- diffraction pat fiber/monazite interface, which was less rough than formation of stoichiometric lanthanum phosphate. sistent with the Morgan and Marshall analysis of inter- Indentation cracks, produced in the matrix with 98n facial debonding in this system. 7 For the 10-dip coate force, showed interfacial debonding. However, in the fiber composite, when two indentations with 98 N force case of five-dip coating, the interfacial debonding was were put in the matrix on the two sides of the fiber,as more clear and larger areas were debonded compared to seen in Fig. 5, interfacial debonding was also observed However, the crack surface was rough. It was not possi ble to determine which interface, if any, debonded It is Low Temperature High Yield SEM cursor Wetting Saphikon Solution Efficient Drying Characteristics Fig l. Various parameters to be considered in the coating precursor Saphikon Alumina Monazite interface with cracks produced by a 98N indentation in the matrix: (a) low magnification SEM image and(b) high magnification SEM image (98 N load, 15 s). Note that cracks passed through the alumina matrix/ Fig. 2. X-ray diffraction pattern showing the formation of stoichio- monazite interface and debonded the monazite/Saphikon fiber inter- metric LaPO4 face

125 and 150 mm and a ®nish of 0.1 mm, perpendicular to the ®ber orientation. A cylindrical indentor with a 114.3 mm diameter was used to apply a load at a constant displacement rate of 0.3125 mm/s through the debonding and sliding stages. 3. Results and discussion Fig. 1 shows the dierent parameters that should be considered in the precursor design of a coating. In the present case, the ethanol-based solution was clear and relatively stable. It had a low viscosity and gave a high yield of monazite (160 g/l). The monazite coating formed below 600C. It showed good wetting and ®lm forming characteristics. Fig. 2. shows the X-ray diraction pattern of the coating obtained from the sol indicating the formation of stoichiometric lanthanum phosphate. Indentation cracks, produced in the matrix with 98 N force, showed interfacial debonding. However, in the case of ®ve-dip coating, the interfacial debonding was more clear and larger areas were debonded compared to the debonding observed in 10-dip coated composite. Fig. 3 shows that cracks passed through the alumina matrix/ monazite interface and debonded the monazite/Saphikon ®ber interface. Comparatively, monazite/Saphikon ®ber interface was weaker than the monazite/alumina matrix interface. Fig. 4. shows two indentations made by 98 N load in the matrix on the two sides of a ®ve-dip coated ®ber. One can see smooth and continuous interfacial debonding. In Fig. 4(b) and (c), high magni®cation secondary electron images show clearly the crack in alumina penetrating the monazite interphase and debonding at the monazite/Saphikon ®ber interface. The bright area in the backscattered electron image [Fig. 4(c)] is the monazite, which separated from the Saphikon ®ber. Clearly, interfacial debonding occurred at the Saphikon ®ber/monazite interface, which was less rough than polycrystalline alumina/monazite interface. This is consistent with the Morgan and Marshall analysis of interfacial debonding in this system.7 For the 10-dip coated ®ber composite, when two indentations with 98 N force were put in the matrix on the two sides of the ®ber, as seen in Fig. 5, interfacial debonding was also observed. However, the crack surface was rough. It was not possible to determine which interface, if any, debonded. It is Fig 1. Various parameters to be considered in the coating precursor design. Fig. 2. X-ray diraction pattern showing the formation of stoichiometric LaPO4. Fig. 3. Interfacial debonding at the (10-dip) monazite/Saphikon ®ber interface with cracks produced by a 98 N indentation in the matrix: (a) low magni®cation SEM image and (b) high magni®cation SEM image (98 N load, 15 s). Note that cracks passed through the alumina matrix/ monazite interface and debonded the monazite/Saphikon ®ber interface. K.K. Chawla et al. / Journal of the European Ceramic Society 20 (2000) 551±559 553

K K. Chawla et al. Journal of the European Ceramic Society 20(2000)551-559 50 um Monazites Saphikon 10 um 10μm Fig 4. Interface ng at the(five-dip) monazite/fiber interface with cracks produced by two 98N(15 s) indentations in the matrix:(a) low magnification SEl, and (c) backscattered electron image of the same area shown in(b). The bright phase surrounding the Saphikon fi Note the interfacial debonding occurred along the Saphikon fiber/ monazite interface, but not along the polycrystalline just as likely that the interphase cracked, rather than ated clamping pressures, the polycrystalline alumina, separation occurring at one the two interfaces. This monazite interface was much rougher than the single could be caused by the microcracking of the coating. crystal Saphikon fiber/monazite interface When a vickers indentor with a 49n indentation was Fiber pushout tests showed that both five- and 10-dip used to produce cracks in the fiber, it resulted in crack coated fibers slid smoothly from the alumina matrix and arrest, crack deflection, and interfacial debonding in five shout load did not lead to matrix cracking. The and 10-dip coated composites. Again, debonding was shear stress-displacement curve and SEM images for a observed at the smooth Saphikon fiber/monazite inter- five-dip coated fiber composite with 122 um fiber diam face but not at the rough polycrystalline alumina / mon- eter and 134 um specimen thickness are shown in Fig. 6 azite interface. This is contrary to the analysis of In Fig. 6(b), the debonded monazite coating is stuck to interfacial debonding due to Morgan and Marshall, the matrix, i.e. the debonding occurred mostly along the According to Morgan and Marshall, only when a crack Saphikon fiber/monazite interface. Fig. 7 shows the grew from monazite to polycrystalline alumina was shear stress-displacement curve and SEM images for a interfacial debonding expected and observed, not vice five-dip coated fiber composite with 163 um fiber diam versa. However, the present work showed that when eter and 145 um specimen thickness. A sinusoidal var cracks approached any one of the two interfaces, poly- iation in the shear stress-displacement curve can be crystalline alumina monazite or single crystal(Saphi- seen. This was probably caused by sinusoidal asperities kon)alumina/monazite interfaces, interfacial debonding existing on the fiber surface. occurred at the smooth single crystal alumina/monazite A comparison of pushout shear stress/displacement interface rather than at the rough interface between curves of five- and 10-dip coated composites showed polycrystalline alumina/monazite. It would appear that that the five-dip coated composites debonded at higher in our case, the interfacial roughness played a very shear stress values than the 10-dip coated composites, important role in interfacial debonding. In spite of a and also the frictional shear stress in these composites certain surface roughness on the Saphikon fiber, which was higher. The reasons are as follows. Debonding was grown into the fiber during manufacture and gener- usually involves a Mode II (shear) fracture phenomenon

just as likely that the interphase cracked, rather than separation occurring at one the two interfaces. This could be caused by the microcracking of the coating. When a Vickers indentor with a 49 N indentation was used to produce cracks in the ®ber, it resulted in crack arrest, crack de¯ection, and interfacial debonding in ®ve and 10-dip coated composites. Again, debonding was observed at the smooth Saphikon ®ber/monazite interface but not at the rough polycrystalline alumina/monazite interface. This is contrary to the analysis of interfacial debonding due to Morgan and Marshall,7 According to Morgan and Marshall,7 only when a crack grew from monazite to polycrystalline alumina was interfacial debonding expected and observed, not vice versa. However, the present work showed that when cracks approached any one of the two interfaces, polycrystalline alumina/monazite or single crystal (Saphikon) alumina/monazite interfaces, interfacial debonding occurred at the smooth single crystal alumina/monazite interface rather than at the rough interface between polycrystalline alumina/monazite. It would appear that in our case, the interfacial roughness played a very important role in interfacial debonding. In spite of a certain surface roughness on the Saphikon ®ber, which was grown into the ®ber during manufacture and generated clamping pressures, the polycrystalline alumina/ monazite interface was much rougher than the single crystal Saphikon ®ber/monazite interface. Fiber pushout tests showed that both ®ve- and 10-dip coated ®bers slid smoothly from the alumina matrix and the pushout load did not lead to matrix cracking. The shear stress±displacement curve and SEM images for a ®ve-dip coated ®ber composite with 122 mm ®ber diameter and 134 mm specimen thickness are shown in Fig. 6. In Fig. 6(b), the debonded monazite coating is stuck to the matrix, i.e. the debonding occurred mostly along the Saphikon ®ber/monazite interface. Fig. 7 shows the shear stress±displacement curve and SEM images for a ®ve-dip coated ®ber composite with 163 mm ®ber diameter and 145 mm specimen thickness. A sinusoidal variation in the shear stress±displacement curve can be seen. This was probably caused by sinusoidal asperities existing on the ®ber surface. A comparison of pushout shear stress/displacement curves of ®ve- and 10-dip coated composites showed that the ®ve-dip coated composites debonded at higher shear stress values than the 10-dip coated composites, and also the frictional shear stress in these composites was higher. The reasons are as follows. Debonding usually involves a Mode II (shear) fracture phenomenon. Fig. 4. Interfacial debonding at the (®ve-dip) monazite/®ber interface with cracks produced by two 98 N (15 s) indentations in the matrix: (a) low magni®cation SEI, (b) high magni®cation SEI, and (c) backscattered electron image of the same area shown in (b). The bright phase surrounding the Saphikon ®ber is monazite. Note the interfacial debonding occurred along the Saphikon ®ber/monazite interface, but not along the polycrystalline alumina/monazite interface. 554 K.K. Chawla et al. / Journal of the European Ceramic Society 20 (2000) 551±559

K K. Chawla et al. Journal of the European Ceramic Society 20(2000)551-559 Alumina Matrix (a) 25μm SEM SEI Fiber Coating Matrix Fig. 5. Interfacial debonding at the(10-dip) monazite/fiber interfac th cracks produced by two 98N(1 s)indentations in the matrix (a) low magnification SEM image and (b) high magnification SEM 50m image. Note the roughness of the debonded fracture surface. In brittle systems, Mode II fracture typically occurs by Fig. 6. Fiber pushout test r Saphikon/five-dip LaPO4/Al2O3 er and 134 um specimen thickness e coalescence of microcracks in a layer In the case of (a)SEM image showing tl ed fiber is pushed out of the alu- a thick coating, the 10 dip coating in the present case, mina matrix,(b)SEM image showing the debonded monazite coating this layer coincides with the coating itself such that is stuck to the matrix debonding involves a diffuse zone of microcrack damage. In other cases, the layer is very thin and the matic of the interfacial debonding models for thin and thick interphase is shown in Fig 8. The 10-dip coating may have some defects in it, making it easier to form microcracks in the coating itself and debonding may occur by the coalescence of microcracks within the monazite coating. The coefficient of friction in the frac- Fig. 7 D4A1 0, cor stress-displacement curve for a Saphikon/ five-dip tured monazite is likely to be higher than that of mon- LaPo4 mposite with 163 um fiber diameter and 145 um spe- azite/Saphikon interface with a thinner coating(five clmen Note a sinusoidal variation in the curve which corre. sponds to the asperities on the as received fiber surface. dip), where initiation of debonding occurs by a single crack along the Saphikon fiber/monazite interface when the matrix failed. The monolithic alumina failed The stress-displacement curves in three-point bend catastrophically; its bend strength was very low (140 tests at room temperature of five-dip fiber coated com- MPa). Comparatively, the work of fracture of monazite posites and monolithic alumina are shown in Fig 9. The coated Saphikon fiber/alumina matrix composites is composite failed in a non-brittle manner. The load much higher than that of monolithic alumina. Note that increased until a stress of about 180 MPa. where the he fiber volume fraction in the composite was very fibers started to debond and pullout from the matrix. small, about 0.01. The fracture surfaces of five-dip spe- After debonding and pullout, the matrix still could cimens fiber pullout was observed and the average transfer some load. Finally, the ultimate stress (230 length of pullout fiber was about 130 um. In most cases, MPa)was reached. There, the stress suddenly decreased the monazite coating was largely peeled off the fiber

In brittle systems, Mode II fracture typically occurs by the coalescence of microcracks in a layer. In the case of a thick coating, the 10 dip coating in the present case, this layer coincides with the coating itself such that debonding involves a diuse zone of microcrack damage. In other cases, the layer is very thin and the debond has the appearance of a single crack. A schematic of the interfacial debonding models for thin and thick interphase is shown in Fig. 8. The 10-dip coating may have some defects in it, making it easier to form microcracks in the coating itself and debonding may occur by the coalescence of microcracks within the monazite coating. The coecient of friction in the fractured monazite is likely to be higher than that of monazite/Saphikon interface with a thinner coating (®vedip), where initiation of debonding occurs by a single crack along the Saphikon ®ber/monazite interface. The stress±displacement curves in three-point bend tests at room temperature of ®ve-dip ®ber coated composites and monolithic alumina are shown in Fig. 9. The composite failed in a non-brittle manner. The load increased until a stress of about 180 MPa, where the ®bers started to debond and pullout from the matrix. After debonding and pullout, the matrix still could transfer some load. Finally, the ultimate stress (230 MPa) was reached. There, the stress suddenly decreased when the matrix failed. The monolithic alumina failed catastrophically; its bend strength was very low (140 MPa). Comparatively, the work of fracture of monazite coated Saphikon ®ber/alumina matrix composites is much higher than that of monolithic alumina. Note that the ®ber volume fraction in the composite was very small, about 0.01. The fracture surfaces of ®ve-dip specimens ®ber pullout was observed and the average length of pullout ®ber was about 130 mm. In most cases, the monazite coating was largely peeled o the ®ber. Fig. 7. Shear stress±displacement curve for a Saphikon/®ve-dip LaPO4/Al2O3 composite with 163 mm ®ber diameter and 145 mm specimen thickness. Note a sinusoidal variation in the curve which corresponds to the asperities on the as-received ®ber surface. Fig. 6. Fiber pushout test results on a Saphikon/®ve-dip LaPO4/Al2O3 composite with 122 mm ®ber diameter and 134 mm specimen thickness: (a) SEM image showing the debonded ®ber is pushed out of the alumina matrix, (b) SEM image showing the debonded monazite coating is stuck to the matrix. Fig. 5. Interfacial debonding at the (10-dip) monazite/®ber interface with cracks produced by two 98 N (1 5 s) indentations in the matrix; (a) low magni®cation SEM image and (b) high magni®cation SEM image. Note the roughness of the debonded fracture surface. K.K. Chawla et al. / Journal of the European Ceramic Society 20 (2000) 551±559 555

K.K. Chawla et al. Journal of the European Ceramic Society 20(2000)551-559 strain A/r is about 0.00011. Thermal strain AoAT in 4. Conclusions Saphikon/monazite interface is equal to 0.0008 if AT=1000.C. Comparatively, the thermal strain is From the results and discussions presented in this about 8 times greater than the roughness-induced strain. present work, we can make the following conclusions Thus, the effect of interfacial roughness of monazite/ Saphikon fiber would not be significant. Specifically 1. The incorporation of monazite interphase coating when compared with that of monazite/polycrystalline by sol-gel dip coating method is an effective way alumina, the roughness-induced radial clamping strain of creating weak interfacial bonds between mon- at the monazite/Saphikon interface is very small azide an umina 2. Two-phase layered liquid dipping method for 3.2. Thermal stress analysis coating monofilament Saphikon fibers with mon- azite sol was effective We used a three-layer model to calculate the thermal 3. The presence of monazite as an interphase was stresses for the five- and 10-dip coated composites, uccessful in providing weak interface to both sin- respectively. A 0.01 fiber volume fraction we used a gle-crystal alumina and polycrystalline alumina. 1000C temperature change and 0.5 and 1 um coating 4. The roughness-induced clamping was much thickness for five- and 10-dip, respectively. The distribu greater at the rough monazite/polycrystalline alu tion of thermal stresses in Saphikon fiber/Lapo/alu mina interface as compared with the smooth single mina, matrix composite for two coating thicknesses is crystal alumina/monazite interface. Therefore, the Figs. 10 and 11. the difference between the Saphikon fiber/monazite interface was relatively two figures is insignificant. One can see that all the stress weak and interfacial debonding and fiber pullout components are constant in the central fiber. The o, is were easier to initiate at this interface than at the constant in the coating as well as the matrix. The e is polycrystalline alumina/monazite interface. It did discontinuous at the interface the o maintains continuity not matter in which phase the crack originated at the interface, and goes to zero at the free surface 5. A sinusoidal variation in the five-dip shear stress- This analysis shows radial gripping increased the displacement curve due to the sinusoidal asperities strength of the fiber/monazite and monazite/matrix on fiber surface was observed interfaces. However, the magnitude of the thermal 6. The energy expended in interfacial debonding radial stress is low and gripping due to thermal stress is ading to fiber pullout caused an increase not significant. The total radial stress is the sum of the toughness or work of fracture of such composite hermal stress and the stress due to the interfacial materials roughness. The roughness-induced radial stress is much 7. Thermal stress analysis showed radial gripping, greater at the monazite/polycrystalline alumina inter- which increased the strength of the interfaces face than at the single-crystal alumina/monazite inter However, the magnitude was not significant face, although they are both mechanically bonded. This because of the small thickness of coating probably is the reason why debonding occurred more often at the fiber/monazite interface instead of the monazite/matrix interface References The present work has demonstrated the effective interface engineering approach by sol-gel dip coating L. Prewo, K. M. and Batt, J.A., The oxidative stability of carbon method. There are, however, some unanswered ques fiber reinforced glass-matrix composites. J. Mater. Sci., 1988, 23 tions. a detailed study on the coating thickness effects 523-527 on the debonding behavior is necessary. More uniform 2. Chawla, K. K, Ceramic Matrix Composite Champion and Hall, coating thickness should be obtained. Measurements of interface shear stress as a function of thickness should 3. Chawla, K.K., Composite Materials, 2nd ed. Springer Verlag, New York. 1998 be done in a systematic manner. We also recognize that 4. Chawla. K.K. Ferber. M. K. Xu. Z.R. and venkatesh. R. very low fiber volume fraction was used in present work Interface engineering in alumina/glass composites. Materials Sci- (0.001)due to the high cost of the single-crystal alumina ence and Engineering. 1993.A 162. 35-44. fiber and small amount of monazite precursor sol. We 5. Chawla, K. K. Schneider, H. Schmuicker. M. and Xu. Z.R. in suggest increasing fiber volume fraction and decreasing Processing and Design Issues in High Temperature Materials, TMS, Warrendale, PA, 1997, pp 235-245 fiber diameter to create statistically better measurements 6. Morgan, P. E. D and Marshall, D. B, Functional interfaces for One practical way is to use NextelTM 610 oxide alumina oxide/oxide composites. Mater. Sci. Eng, 1993, A162, 15-25. fibers instead, which are less expensive. Also, because of 7. Morgan, P. E D. and Marshall, D. B. Ceramic composites of the ambiguous estimates of interfacial energy for LaPO4 monazite and alumina. J. m Ceram. Soc.. 1995 78. 1553-1563. 8. Morgan, P.E. D, Marshall, D. B. and Housley, R. M, High 12O3 interface and for LaPO4/ single crystal AlO3, future temperature stability of monazite-alumina composites. Mater work to measure these values would be useful Sci.Eng.1995,A195,215-22

strain A/r is about 0.00011. Thermal strain aT in Saphikon/monazite interface is equal to 0.0008 if T=1000C. Comparatively, the thermal strain is about 8 times greater than the roughness-induced strain. Thus, the eect of interfacial roughness of monazite/ Saphikon ®ber would not be signi®cant. Speci®cally, when compared with that of monazite/polycrystalline alumina, the roughness-induced radial clamping strain at the monazite/Saphikon interface is very small. 3.2. Thermal stress analysis We used a three-layer model2 to calculate the thermal stresses for the ®ve- and 10-dip coated composites, respectively. A 0.01 ®ber volume fraction we used a 1000C temperature change and 0.5 and 1 mm coating thickness for ®ve- and 10-dip, respectively. The distribution of thermal stresses in Saphikon ®ber/LaPO4/alumina, matrix composite for two coating thicknesses is shown in Figs. 10 and 11. The dierence between the two ®gures is insigni®cant. One can see that all the stress components are constant in the central ®ber. The z is constant in the coating as well as the matrix. The y is discontinuous at the interface. The r maintains continuity at the interface, and goes to zero at the free surface. This analysis shows radial gripping increased the strength of the ®ber/monazite and monazite/matrix interfaces. However, the magnitude of the thermal radial stress is low and gripping due to thermal stress is not signi®cant. The total radial stress is the sum of the thermal stress and the stress due to the interfacial roughness.2 The roughness-induced radial stress is much greater at the monazite/polycrystalline alumina interface than at the single-crystal alumina/monazite interface, although they are both mechanically bonded. This probably is the reason why debonding occurred more often at the ®ber/monazite interface instead of the monazite/matrix interface. The present work has demonstrated the eective interface engineering approach by sol±gel dip coating method. There are, however, some unanswered questions. A detailed study on the coating thickness eects on the debonding behavior is necessary. More uniform coating thickness should be obtained. Measurements of interface shear stress as a function of thickness should be done in a systematic manner. We also recognize that very low ®ber volume fraction was used in present work (0.001) due to the high cost of the single-crystal alumina ®ber and small amount of monazite precursor sol. We suggest increasing ®ber volume fraction and decreasing ®ber diameter to create statistically better measurements. One practical way is to use NextelTM 610 oxide alumina ®bers instead, which are less expensive. Also, because of the ambiguous estimates of interfacial energy for LaPO4/ Al2O3 interface and for LaPO4/single crystal Al2O3, future work to measure these values would be useful. 4. Conclusions From the results and discussions presented in this present work, we can make the following conclusions: 1. The incorporation of monazite interphase coating by sol±gel dip coating method is an eective way of creating weak interfacial bonds between monazite and alumina. 2. Two-phase layered liquid dipping method for coating mono®lament Saphikon ®bers with monazite sol was eective. 3. The presence of monazite as an interphase was successful in providing weak interface to both single-crystal alumina and polycrystalline alumina. 4. The roughness-induced clamping was much greater at the rough monazite/polyerystalline alumina interface as compared with the smooth single crystal alumina/monazite interface. Therefore, the Saphikon ®ber/monazite interface was relatively weak and interfacial debonding and ®ber pullout were easier to initiate at this interface than at the polycrystalline alumina/monazite interface. It did not matter in which phase the crack originated. 5. A sinusoidal variation in the ®ve-dip shear stress± displacement curve due to the sinusoidal asperities on ®ber surface was observed. 6. The energy expended in interfacial debonding leading to ®ber pullout caused an increase in toughness or work of fracture of such composite materials. 7. Thermal stress analysis showed radial gripping, which increased the strength of the interfaces. However, the magnitude was not signi®cant because of the small thickness of coating. References 1. Prewo, K. M. and Batt, J. A., The oxidative stability of carbon ®ber reinforced glass-matrix composites. J. Mater. Sci., 1988, 23, 523±527. 2. Chawla, K. K., Ceramic Matrix Composite. Champion and Hall, London, 1993. 3. Chawla, K. K., Composite Materials, 2nd ed. Springer Verlag, New York, 1998. 4. Chawla, K. K., Ferber, M. K., Xu, Z. R. and Venkatesh, R., Interface engineering in alumina/glass composites. Materials Science and Engineering, 1993, A 162, 35±44. 5. Chawla, K. K., Schneider, H., SchmuÈcker, M. and Xu, Z. R. in Processing and Design Issues in High Temperature Materials, TMS, Warrendale, PA, 1997, pp. 235±245. 6. Morgan, P. E. D. and Marshall, D. B., Functional interfaces for oxide/oxide composites. Mater. Sci. Eng., 1993, A162, 15±25. 7. Morgan, P. E. D. and Marshall, D. B., Ceramic composites of monazite and alumina. J. Am. Ceram. Soc., 1995, 78, 1553±1563. 8. Morgan, P. E. D., Marshall, D. B. and Housley, R. M., Hightemperature stability of monazite±alumina composites. Mater. Sci. Eng., 1995, A195, 215±222. 558 K.K. Chawla et al. / Journal of the European Ceramic Society 20 (2000) 551±559

《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Microstructure and properties of monazite（LaPO4）coated saphikon ?ber/alumina matrix composites

《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_Microstructure and properties of monazite（LaPO4）coated saphikon ?ber/alumina matrix composites