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 betweensuitable and e€ective 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 debond￾ing and crack de¯ection. They obtained monazite coat￾ing 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 potas￾sium 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 e€ect 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 tem￾perature 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 edge￾de®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 manu￾facturing without damaging the surface. The size is generally a water-based emulsion coating, which can be easily washed out. For example, methyl cellulose coat￾ing 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 a€ect 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 phos￾phorus pentoxide) in appropriate proportions. Fibers were passed through the sol and heated to 600C. Con￾version to monazite occurred around 275C, and heat￾ing 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 exam￾ined 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 ana￾lysis of the monazite coating was carried out by X-ray di€raction 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. Inden￾tations 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