E噩≈S Journal of the European Ceramic Society 22(2002)2333-2342 www.elsevier.com/locate/jeurceramsoc Mullite(Nextel M 720) fibre-reinforced mullite matrix composites exhibiting favourable thermomechanical properties CKaya,*, E.G. Butler, A Selcuk, A.R. Boccaccinib, M.H. Lewis a Interdisciplinary Research Centre(IRC) in Materials Processing, The University of Birmingham, Edgbas ha, B15 2TT UK dEpartment of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London, SW72BP,UK Department of Physics, Centre for Advanced Materials, University of Warwick, Cor CV4 ZAL. UK Received 25 September 2001; accepted 8 December 200 Abstract A mullite matrix containing homogeneously distributed ultra-fine(70-350 nm) pores was reinforced with NdPOa-coated woven mullite fibre mats(Nextel M 720)leading to damage-tolerant composites with good high temperature(1300C)strength and ther mal cycling resistance. Electrophoretically deposited fibre preforms were placed in a high-load pressure filtration assembly, leading to formation of consolidated compacts with high green densities. After sintering at 1200C for 3 h, the compacts had a density 86. 4% of theoretical density and showed damage-tolerant behaviour up to 1300C, with flexural strength values of 235 MPa and 224 MPa at room temperature and 1300oC, respectively. No significant microstructural damage was detected after thermal cycling the samples between room temperature and 1150 C for up to 300 cycles. The thermomechanical test results combined with detailed electron microscopy observations indicate that the overall composite behaviour in terms of damage-tolerance, thermal capability and thermal cycling resistance is mainly controlled by two microstructural features: (1)the presence of a dense NdPOa interphase but weak bonding with the matrix or fibre and (2) the presence of homogeneously distributed nano pores(<350 nm)within the mullite matrix. C 2002 Elsevier Science Ltd. All rights reserved. Keywords: Composites; Fibres; Mechanical properties; Microstructure-final: Mullite: Mullite fibres; Thermal cycling 1. Introduction temperature applications due to its good thermal shock resistance, low thermal expansion coeficient, high creep Continuous fibre-reinforced ceramic matrix compo- resistance and good chemical and thermal stability. I, I sites have attracted significant scientific and technologi- However, its low fracture toughness(1-3 M Pa m-1/ cal interest for high temperature structural applications and high flaw sensitivity need to be increased for struc due to their damage-tolerant behaviour, decreased flaw tural applications. One way to do this is by incorporating sensitivity and resistance to creep. The development high-strength continuous ceramic fibres into the mullite of oxide fibre-reinforced oxide matrix composites matrix in order to activate debonding, delamination, promising means of achieving lightweight, structural crack deflection, fibre bridging and fibre pull-out materials combining high-temperature strength with mechanisms that will contribute to a non-linear stress- improved fracture toughness, damage tolerance, ther- strain response and achievement of a high fracture mal shock and oxidation resistance. Significant research ffort is being expended in the optimisation of these It has been shown that catastrophic failure of oxide ceramic composites systems, with particular emphasis fibre reinforced oxide matrix composites can be pre being placed on the establishment of reliable and cost- vented using a very porous(up to 50 vol %)ceramic effective fabrication procedures.5-10 Mullite (3Al2O3. matrix with no particularly optimised interface between 2SiO2)is one of the ideal oxide matrix materials for high fibre and matrix. 3 However, most theoretical models and experimental findings suggest that for obtaining Corresponding author. Tel. +44-121-4143537: fax: +44- maximised damage-tolerance and fracture toughness in 4143441. fibre-reinforced brittle-matrix composites, tailored inter- E-mail address: c kaya(@ bham ac uk(C. Kaya) phases between fibre and matrix providing optimised 0955-2219/02/S. see front matter C 2002 Elsevier Science Ltd. All rights reserved. PII:S0955-2219(01)00531-3
Mullite (NextelTM 720) fibre-reinforced mullite matrix composites exhibiting favourable thermomechanical properties C.Kayaa,*, E.G. Butlera , A.Selcukb, A.R. Boccaccinib, M.H. Lewisc a Interdisciplinary Research Centre (IRC) in Materials Processing, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK bDepartment of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London, SW7 2BP, UK c Department of Physics, Centre for Advanced Materials, University of Warwick, Coventry, CV4 7AL, UK Received 25 September 2001; accepted 8 December 2001 Abstract A mullite matrix containing homogeneously distributed ultra-fine (70–350 nm) pores was reinforced with NdPO4-coated woven mullite fibre mats (NextelTM 720) leading to damage-tolerant composites with good high temperature (1300 C) strength and thermal cycling resistance.Electrophoretically deposited fibre preforms were placed in a high-load pressure filtration assembly, leading to formation of consolidated compacts with high green densities.After sintering at 1200 C for 3 h, the compacts had a density of 86.4% of theoretical density and showed damage-tolerant behaviour up to 1300 C, with flexural strength values of 235 MPa and 224 MPa at room temperature and 1300 C, respectively.No significant microstructural damage was detected after thermal cycling the samples between room temperature and 1150 C for up to 300 cycles.The thermomechanical test results combined with detailed electron microscopy observations indicate that the overall composite behaviour in terms of damage-tolerance, thermal capability and thermal cycling resistance is mainly controlled by two microstructural features: (1) the presence of a dense NdPO4 interphase but weak bonding with the matrix or fibre and (2) the presence of homogeneously distributed nano pores (<350 nm) within the mullite matrix. # 2002 Elsevier Science Ltd.All rights reserved. Keywords: Composites; Fibres; Mechanical properties; Microstructure-final; Mullite; Mullite fibres; Thermal cycling 1. Introduction Continuous fibre-reinforced ceramic matrix composites have attracted significant scientific and technological interest for high temperature structural applications due to their damage-tolerant behaviour, decreased flaw sensitivity and resistance to creep.14 The development of oxide fibre-reinforced oxide matrix composites is a promising means of achieving lightweight, structural materials combining high-temperature strength with improved fracture toughness, damage tolerance, thermal shock and oxidation resistance.Significant research effort is being expended in the optimisation of these ceramic composites systems, with particular emphasis being placed on the establishment of reliable and costeffective fabrication procedures.510 Mullite (3Al2O3 . 2SiO2) is one of the ideal oxide matrix materials for high temperature applications due to its good thermal shock resistance, low thermal expansion coefficient, high creep resistance and good chemical and thermal stability.11,12 However, its low fracture toughness (1–3 M Pa m1/2) and high flaw sensitivity need to be increased for structural applications.One way to do this is by incorporating high-strength continuous ceramic fibres into the mullite matrix in order to activate debonding, delamination, crack deflection, fibre bridging and fibre pull-out mechanisms that will contribute to a non-linear stressstrain response and achievement of a high fracture energy.1 It has been shown that catastrophic failure of oxide fibre reinforced oxide matrix composites can be prevented using a very porous (up to 50 vol.%) ceramic matrix with no particularly optimised interface between fibre and matrix.13 However, most theoretical models and experimental findings suggest that for obtaining maximised damage-tolerance and fracture toughness in fibre-reinforced brittle–matrix composites, tailored interphases between fibre and matrix providing optimised 0955-2219/02/$ - see front matter # 2002 Elsevier Science Ltd.All rights reserved. PII: S0955-2219(01)00531-3 Journal of the European Ceramic Society 22 (2002) 2333–2342 www.elsevier.com/locate/jeurceramsoc * Corresponding author.Tel.: +44-121-4143537; fax: +44-121- 4143441. E-mail address: c.kaya@bham.ac.uk (C. Kaya).
C Kaya et al. Journal of the European Ceramic Society 22(2002)2333-2342 fibre/matrix bonding are required. 14-16 The essential 2. Experimental work properties of such an interphase are the ability to debond in the presence of transverse matrix microcracks 2. 1. Matrix material preparation and to transfer applied stress to the high strength /mod ulus fibres I5 Micromechanical parameters such as A sinter-active and stoichiometric mullite (3Al,O3 interfacial shear stress([)and debonding energy(Gi), as 2SiO2) powder containing 5 wt. zirconia was synthe well as long-term thermal stability of the interphase are sised using low-temperature hydrothermal processing, he critical issues that have to be taken into considera- as reported in the literature. The particles tend to be tion when a composite is fabricated. The critical char- highly irregular in morphology with a wide particle size acteristics of the interface can be summarised in terms distribution, as determined by scanning electron micro- of low interfacial fracture energy and moderate inter- scopy(SEM) and particle size analysis, respectively. The facial sliding resistance after debonding. 15, 16 In an ideal as-produced mullite sol was vacuum filtered and the fibre reinforced ceramic matrix composite the interphase obtained powder was dried at 110C for 4 h followed should act as a reaction barrier between fibre and by calcination at 850C for 3 h in order to produce matrix, inhibit diffusion of atmospheric species to the coarser particles and so to reduce the amount of fibres and remain intrinsically stable at high tempera shrinkage that would occur during sintering. The cal tures. Many interphase materials including BN, car- cined powders were ball-milled for 1 day using TZP bon, ZrO2, SnO2, mica, mullite, LaPO4 and porous balls and re-dispersed in water at a pH value of 3. The refractory metals(Pt, Mo, etc. have been introduced solids-loading of the suspension was kept constant at 20 and applied to different matrices. 6, 5-26 In particular, wt %with simultaneous ultrasonic agitation to enhance however, during the last decade oxide based interphase powder dispersion. The electrophoretic mobility and net materials have been the focus point in the oxide-oxide surface charge of the mullite particles were measured as a Bramic composite field due to their long-term high function of dispersion pH using a surface-charge analyser gaseous environments as well as their favourable shear and debonding characteristics and low fracture energies 2. 2. Preparation of interphase material(NdPO4)and also at high temperatures. 6, 24-26 Most studied com- fibre coating pounds have the generic formula MXO4(monazite) where M is a rare earth ion in high oxygen coordination NdPO4 powders suitable for the preparation of a and x is the pentavalent metal ion which is tetra- stable suspension were synthesised by the direct reac hedrally or octahedrally co-ordinated by oxygen. These tion of neodymium oxide and phosphoric acid at room complex oxides have a high probability of low cohesion temperature, as described in the literature: 28 with matrix or fibre due to polarisation of oxygen bonds by high valence cations Phosphates and niobates are Nd2O3+ 2H3 PO4= 2NdPO4+3H20 typical examples having the monazite structure. In the present study, NdPO4 was chosen as a weak interface The main advantage of this direct reaction between material because it is easy to synthesise from liquid Nd,O3 and H PO4 is that there is no by-product other precursors or by hydrolysis of colloidal mixtures of the than water. The resultant gel containing NdPO4 and constituent oxides, allowing for adequate control of the ater was vacuum filtered and washed three times fol- particle characteristics, such as size, shape and purity. lowed by drying at 110C for 3 h. The obtained dried NdPO4 has a high melting point (1975C), is compa- powders were calcined at 1000C for 3 h to yield stoi tible with mullite and less susceptible to B-alumina for- chiometric NdPO4 monazite powders. Both SEM and mation in Al2O3-rich matrices than the La-phosphate transmission electron microsocopy (TEM) were used to In the present work, a mullite matrix containing investigate the NdPO4 particle characteristics. In the nanosized pores was reinforced with NdPOa-coated final stage, NdPO, powders were dispersed in distilled mullite fibres in order to optimise the non-brittle failure water and a 15 wt. stable suspension was prepared of the component by combining both the"porous- after ball-mixing for one day at a ph value of 3. This matrix" and "weak interface" concepts. Thermo- suspension was used to coat the composite reinforcing mechanical tests including high-temperature flexure fibres strength and thermal cycling tests were carried out on the Eight-harness satin woven mullite fibre mats(Nex fabricated composites. The interrelationship between tel M 720, 3M, USA)were used as reinforcement mate interphase structure and damage-tolerant behaviour in rial in the present composites. The fibre mats were pre- terms of fibre debonding, crack deflection and fibre pull- treated by desizing at 500 C for 1 h to remove the out were examined by electron microscopy observations organic protection layer from the fibre surface. The fibre of the interphase/fibre and interphase/matrix interfacial mats were then immersed in an ammonia based solution zones consisting of an ammonium salt of polymethacrylic acid
fibre/matrix bonding are required.1416 The essential properties of such an interphase are the ability to debond in the presence of transverse matrix microcracks and to transfer applied stress to the high strength/modulus fibres.15 Micromechanical parameters such as interfacial shear stress () and debonding energy (Gi), as well as long-term thermal stability of the interphase are the critical issues that have to be taken into consideration when a composite is fabricated.The critical characteristics of the interface can be summarised in terms of low interfacial fracture energy and moderate interfacial sliding resistance after debonding.15,16 In an ideal fibre reinforced ceramic matrix composite the interphase should act as a reaction barrier between fibre and matrix, inhibit diffusion of atmospheric species to the fibres and remain intrinsically stable at high temperatures.16 Many interphase materials including BN, carbon, ZrO2, SnO2, mica, mullite, LaPO4 and porous refractory metals (Pt, Mo, etc.) have been introduced and applied to different matrices.6,1526 In particular, however, during the last decade oxide based interphase materials have been the focus point in the oxide–oxide ceramic composite field due to their long-term high temperature stability with respect to fibre, matrix and gaseous environments as well as their favourable shear and debonding characteristics and low fracture energies also at high temperatures.6,2426 Most studied compounds have the generic formula MXO4 (monazite), where M is a rare earth ion in high oxygen coordination and X is the pentavalent metal ion which is tetrahedrally or octahedrally co-ordinated by oxygen.These complex oxides have a high probability of low cohesion with matrix or fibre due to polarisation of oxygen bonds by high valence cations.Phosphates and niobates are typical examples having the monazite structure.In the present study, NdPO4 was chosen as a weak interface material because it is easy to synthesise from liquid precursors or by hydrolysis of colloidal mixtures of the constituent oxides, allowing for adequate control of the particle characteristics, such as size, shape and purity. NdPO4 has a high melting point (1975 C), is compatible with mullite and less susceptible to b-alumina formation in Al2O3-rich matrices than the La-phosphate. In the present work, a mullite matrix containing nanosized pores was reinforced with NdPO4-coated mullite fibres in order to optimise the non-brittle failure of the component by combining both the ‘‘porousmatrix’’ and ‘‘weak interface’’ concepts.Thermomechanical tests including high-temperature flexure strength and thermal cycling tests were carried out on the fabricated composites.The interrelationship between interphase structure and damage-tolerant behaviour in terms of fibre debonding, crack deflection and fibre pullout were examined by electron microscopy observations of the interphase/fibre and interphase/matrix interfacial zones. 2. Experimental work 2.1. Matrix material preparation A sinter-active and stoichiometric mullite (3Al2O3 . 2SiO2) powder containing 5 wt.% zirconia was synthesised using low-temperature hydrothermal processing, as reported in the literature.27 The particles tend to be highly irregular in morphology with a wide particle size distribution, as determined by scanning electron microscopy (SEM) and particle size analysis, respectively.The as-produced mullite sol was vacuum filtered and the obtained powder was dried at 110 C for 4 h followed by calcination at 850 C for 3 h in order to produce coarser particles and so to reduce the amount of shrinkage that would occur during sintering.The calcined powders were ball-milled for 1 day using TZP balls and re-dispersed in water at a pH value of 3.The solids-loading of the suspension was kept constant at 20 wt.% with simultaneous ultrasonic agitation to enhance powder dispersion.The electrophoretic mobility and net surface charge of the mullite particles were measured as a function of dispersion pH using a surface-charge analyser (Delsa 440). 2.2. Preparation of interphase material (NdPO4) and fibre coating NdPO4 powders suitable for the preparation of a stable suspension were synthesised by the direct reaction of neodymium oxide and phosphoric acid at room temperature, as described in the literature:28 Nd2O3 þ 2H3PO4 ¼ 2NdPO4 þ 3H2O ð1Þ The main advantage of this direct reaction between Nd2O3 and H3PO4 is that there is no by-product other than water.The resultant gel containing NdPO4 and water was vacuum filtered and washed three times followed by drying at 110 C for 3 h.The obtained dried powders were calcined at 1000 C for 3 h to yield stoichiometric NdPO4 monazite powders.Both SEM and transmission electron microsocopy (TEM) were used to investigate the NdPO4 particle characteristics.In the final stage, NdPO4 powders were dispersed in distilled water and a 15 wt.% stable suspension was prepared after ball-mixing for one day at a pH value of 3.This suspension was used to coat the composite reinforcing fibres. Eight-harness satin woven mullite fibre mats (NextelTM 720, 3M, USA) were used as reinforcement material in the present composites.The fibre mats were pretreated by desizing at 500 C for 1 h to remove the organic protection layer from the fibre surface.The fibre mats were then immersed in an ammonia based solution, consisting of an ammonium salt of polymethacrylic acid 2334 C. Kaya et al. / Journal of the European Ceramic Society 22 (2002) 2333–2342
C Kaya et al. Journal of the European Ceramic Society 22(2002)2333-2342 2335 VErsical KA21, pH: 9, Allied Colloids, UK) in order to were bevelled using 6-um diamond paste on a soft cloth create a strong negative surface charge on the fibre sur- in order to minimise possible flaw sources. The tensile face and to improve the wetting behaviour of the fibres. surfaces were then polished to a 0. 25 um finish using This process maximises the mutual electrostatic attrac- diamond paste. Room and high-temperature four-point tion between the fibre surface and the positively charged flexure strength tests were performed on an Instron NdPO4 particles(as determined by electrophoretic mobi- Testing machine fitted with a furnace which has tung- lity measurements)in the coating suspension. During the sten mesh elements enabling tests to be carried out at coating process under vacuum, the fibre mats were temperatures up to 1500C. The pushrods and fixtures immersed in the NdPO4 suspension under sonication are all made from a tungsten-zirconium-moly bdenum causing the positively charged NdPO4 particles to adhere alloy Specimens to be tested at elevated temperatures to the negatively charged fibre surface. The immersed were held at the test temperature for at least an hour fibre mats were removed from the sol and dried in air for prior to testing to allow the system to equilibrate and to one day, and then finally fired at 600C for 0.5 h to ensure that the specimen was at this temperature. Flex- strengthen the coating layer on the fibre surface. ural tests were carried out at a constant displa rate of 0.5 mm/min using a test fixture having outer and 2.3. Composite processing inner spans of 40 and 20 mm, respectively. The span(S) to specimen thickness(h) ratio was chosen to be sufi An in-situ electrophoretic deposition(EPD) cell was cient large(S/h>10) to avoid large shear delamination used to infiltrate the NdPO4-coated mullite fibre mats forces with matrix material. Detailed information about the development and application of this technique can be 2.5. Thermal cycling tests found elsewhere. 7. 29,30 Coated fibre mats were attached to a stainless steel plate acting as deposition electrode, Thermal cycling tests on samples of prismatic shape, which was connected to the negative pole of a power suitable for flexural strength tests, were conducted using supply. Another stainless steel plate served as the counter a computer-controlled rig. A cycling program was electrode. After the fibre preform was placed in the sol, applied to induce heating and cooling between 25 and e system was vacuum degassed to remove any entrap 150oC and an isothermal hold time of 15 min. These ed air. The cell electrodes were connected to a0-60 v tests carried out in laboratory air. The applied d. c power supply. A stable mullite aqueous suspension cycles resulted in a heating/cooling rate of about 900C (pH=3), as described in Section 2.1, was used. EPD was per min at the initial stages, which gradually reduced to performed under constant voltage conditions (10 Vd.c) approximately 500C/min as the temperature approa- using a deposition time of l min An electrode separation ched the limits of the cycle. Temperature was measured distance of 15 mm was used in all experiments and eight by two different thermocouples attached to the samples coated fibre mats were individually deposited. Under After thermal cycling the samples were visually the applied electric field, the very fine mullite particles inspected for the presence of macroscopic surface possessing a net positive surface charge, as determined damage, such as delamination, chipping or fibre protru rom the electrophoretic mobility data, migrated sion. Selected thermal cycled specimens were tested at towards the negative electrode. The particles infiltrated room temperature in flexure strength configuration, as the fibre tows and deposited until a sufficient matrix described above thickness, which enveloped the fibre tows, was achieved Eight electrophoretically deposited fibre mats were then 2.6. Microstructural characterisation placed in a high-load pressure filtration(PF)assembly to form the green body (in the form of flat cylinders of 70 Mullite powder and sintered samples were analysed mm diameter). The PF procedure has been described in using X-ray diffraction(CuK, radiation and nickel filter detail elsewhere. The EPD-infiltrated and pressure fil- to remove the Cu kp peak, Philips X'Pert, Germany) trated green body specimens were subsequently dried operated at 40 keV and 30 mA. The diffractometer under humidity-controlled atmosphere for I day and left scanned from 5 to 80 with a scan step of 0.020 20 and a in normal air for another day before being pressureless count time of 2 s per step. Microstructural examinations sintered at 1200oC for 3 h in air on sintered monolithic mullite and fibre-reinforced composites were also carried out using a high-resolution 2. 4. Flexural tests at room and elevated temperatures Field Emission Gun SEM(FEG SEM FX-4000, Jeol Ltd Japan). The interfacial mi Flexural test bars were cut from the sintered discs NdPO4 interphase and the mullite matrix and between using an Accutom 5 high-speed, precision diamond saw, the NdPO4 interphase and the mullite fibres was exam- and then both surfaces were ground to be parallel using ined using a Jeol 4000 FX TEM operating at 400 keV, a 40-um diamond, resin bonded disc. All sharp edges and equipped with an energy dispersive X-Ray analysis
(Versical KA21, pH: 9, Allied Colloids, UK) in order to create a strong negative surface charge on the fibre surface and to improve the wetting behaviour of the fibres. This process maximises the mutual electrostatic attraction between the fibre surface and the positively charged NdPO4 particles (as determined by electrophoretic mobility measurements) in the coating suspension.During the coating process under vacuum, the fibre mats were immersed in the NdPO4 suspension under sonication, causing the positively charged NdPO4 particles to adhere to the negatively charged fibre surface.The immersed fibre mats were removed from the sol and dried in air for one day, and then finally fired at 600 C for 0.5 h to strengthen the coating layer on the fibre surface. 2.3. Composite processing An in-situ electrophoretic deposition (EPD) cell was used to infiltrate the NdPO4-coated mullite fibre mats with matrix material.Detailed information about the development and application of this technique can be found elsewhere.7,29,30 Coated fibre mats were attached to a stainless steel plate acting as deposition electrode, which was connected to the negative pole of a power supply.Another stainless steel plate served as the counter electrode.After the fibre preform was placed in the sol, the system was vacuum degassed to remove any entrapped air.The cell electrodes were connected to a 0–60 V d.c. power supply. A stable mullite aqueous suspension (pH=3), as described in Section 2.1, was used. EPD was performed under constant voltage conditions (10 V d.c.) using a deposition time of 1 min.An electrode separation distance of 15 mm was used in all experiments and eight coated fibre mats were individually deposited.Under the applied electric field, the very fine mullite particles possessing a net positive surface charge, as determined from the electrophoretic mobility data, migrated towards the negative electrode.The particles infiltrated the fibre tows and deposited until a sufficient matrix thickness, which enveloped the fibre tows, was achieved. Eight electrophoretically deposited fibre mats were then placed in a high-load pressure filtration (PF) assembly to form the green body (in the form of flat cylinders of 70 mm diameter).The PF procedure has been described in detail elsewhere.29 The EPD-infiltrated and pressure filtrated green body specimens were subsequently dried under humidity-controlled atmosphere for 1 day and left in normal air for another day before being pressureless sintered at 1200 C for 3 h in air. 2.4. Flexural tests at room and elevated temperatures Flexural test bars were cut from the sintered discs using an Accutom 5 high-speed, precision diamond saw, and then both surfaces were ground to be parallel using a 40-mm diamond, resin bonded disc.All sharp edges were bevelled using 6-mm diamond paste on a soft cloth in order to minimise possible flaw sources.The tensile surfaces were then polished to a 0.25 mm finish using diamond paste.Room and high-temperature four-point flexure strength tests were performed on an Instron Testing machine fitted with a furnace which has tungsten mesh elements enabling tests to be carried out at temperatures up to 1500 C.The pushrods and fixtures are all made from a tungsten-zirconium-molybdenum alloy.Specimens to be tested at elevated temperatures were held at the test temperature for at least an hour prior to testing to allow the system to equilibrate and to ensure that the specimen was at this temperature.Flexural tests were carried out at a constant displacement rate of 0.5 mm/min using a test fixture having outer and inner spans of 40 and 20 mm, respectively.The span (S) to specimen thickness (h) ratio was chosen to be suffi- cient large (S/h>10) to avoid large shear delamination forces. 2.5. Thermal cycling tests Thermal cycling tests on samples of prismatic shape, suitable for flexural strength tests, were conducted using a computer-controlled rig.A cycling program was applied to induce heating and cooling between 25 and 1150 C and an isothermal hold time of 15 min.These tests were carried out in laboratory air.The applied cycles resulted in a heating/cooling rate of about 900 C per min at the initial stages, which gradually reduced to approximately 500 C/min as the temperature approached the limits of the cycle.Temperature was measured by two different thermocouples attached to the samples. After thermal cycling the samples were visually inspected for the presence of macroscopic surface damage, such as delamination, chipping or fibre protrusion.Selected thermal cycled specimens were tested at room temperature in flexure strength configuration, as described above. 2.6. Microstructural characterisation Mullite powder and sintered samples were analysed using X-ray diffraction (CuKa radiation and nickel filter to remove the Cu Kb peak, Philips X’Pert, Germany), operated at 40 keV and 30 mA.The diffractometer scanned from 5 to 80 with a scan step of 0.02 2y and a count time of 2 s per step.Microstructural examinations on sintered monolithic mullite and fibre-reinforced composites were also carried out using a high-resolution Field Emission Gun SEM (FEG SEM FX-4000, Jeol Ltd.Japan).The interfacial microstructure between the NdPO4 interphase and the mullite matrix and between the NdPO4 interphase and the mullite fibres was examined using a Jeol 4000 FX TEM operating at 400 keV, and equipped with an energy dispersive X-Ray analysis C. Kaya et al. / Journal of the European Ceramic Society 22 (2002) 2333–2342 2335
opean Ceramic Society 22(2002)2333-2342 (EDX) unit. The surfaces of selected thermally cycled of particles analysed, respectively. Particle size distribu samples were also observed by SEM tion is a very critical issue in green body formation using EPD or pressure filtration. A mixture of coarse, medium and very fine particles, where sufficient fine 3. Results and discussion particles are present to fill in the interstices, results in a green body compact of high green density As shown in 3.. Materials characterisation Fig. Ib, there are some very fine particles(as fine as 55 nm)within the synthesised powders as well as very coarse The particle size distribution results and a SEM ones(7. 1 um). However, the mean particle size is in the micrograph of the hydrothermally synthesised mullite range of 2-3 um Fig. Ib also shows the presence of very plus 5 wt zirconia powder used in this work are fine(20-30 nm) zirconia precipitates located within the shown in Fig. 1. The cumulative statistics shown in mullite particles and these very fine particles are expec- Fig. 1(a) for as-produced powder is given as D25, D50, ted to enhance the sintering rates of monophasic mullite D75 and Doo values, which indicate the particle sizes powders via controlling grain growth. Overall, the ver below which are 25, 50, 75 and 90% of the total volume fine particles(<100 nm) are considered to be ideal for D2sD5o D5 D90 Mean Median 6 VolumeSize(pm)0951.7424|7.1|265245 statistics ÷ 00l 0.i Particle Diameter (um) Fig. I(a) Particle size analysis and (b) SEM micrograph of the hydrothermally synthesised mullite plus 5 wt zirconia powder showing the presence of very fine(20-30 nm) zirconia precipitates(as arrowed) within the mullite particles
(EDX) unit.The surfaces of selected thermally cycled samples were also observed by SEM. 3. Results and discussion 3.1. Materials characterisation The particle size distribution results and a SEM micrograph of the hydrothermally synthesised mullite plus 5 wt.% zirconia powder used in this work are shown in Fig.1.The cumulative statistics shown in Fig.1(a) for as-produced powder is given as D25, D50, D75 and D90 values, which indicate the particle sizes below which are 25, 50, 75 and 90% of the total volume of particles analysed, respectively.Particle size distribution is a very critical issue in green body formation using EPD or pressure filtration.A mixture of coarse, medium and very fine particles, where sufficient fine particles are present to fill in the interstices, results in a green body compact of high green density.As shown in Fig.1b, there are some very fine particles (as fine as 55 nm) within the synthesised powders as well as very coarse ones (7.1 mm).However, the mean particle size is in the range of 2–3 mm.Fig.1b also shows the presence of very fine (20–30 nm) zirconia precipitates located within the mullite particles and these very fine particles are expected to enhance the sintering rates of monophasic mullite powders via controlling grain growth.Overall, the very fine particles (<100 nm) are considered to be ideal for Fig.1. (a) Particle size analysis and (b) SEM micrograph of the hydrothermally synthesised mullite plus 5 wt.% zirconia powder showing the presence of very fine (20–30 nm) zirconia precipitates (as arrowed) within the mullite particles. 2336 C. Kaya et al. / Journal of the European Ceramic Society 22 (2002) 2333–2342
C Kaya et al. Journal of the European Ceramic Society 22(2002)2333-2342 2337 deposition behaviour during EPD will be enhanced i,g nd electrophoretic deposition as the particle mobility an weak interfaces), leading to the desired fibre debond- -30 ing, crack deflection and fibre pull-out mechanisms The electrophoretic mobility data in terms of particle Fig 3a shows a SEM microstructure of a fibre, which net surface charge and mean particle size for the mullite as been dip-coated wi ted with NdPO4 without fibre pre- powder and the NdPOa aqueous sol suspensions at a treatment, indicating the poor quality of the coating as constant pH value of 3 are shown in Table 1. From a result of the absence lectrostatic attraction these data, it is evident that both particles are positively- between the fibre and coating particles. However, when charged at a pH value of 3, therefore if any particles are the fibre surface is conditioned with a high pH solution required to be attracted to the target material (for prior to coating in order to create a strong negative example to the electrodes during EPD or to the fibre surface charge, successful results of mullite fibres coated surface during dip coating), the surface charge of such with NdPO4 particles are achievable, as shown in Fig. 3b substrates has to be controlled to be opposite of the after thermal treatment at 600oC for 0.5 h. The image deposited material. The net surface charge around a indicates that the coating is homogeneous over the fibre particle is a very critical parameter in EPD and dip- surface and its thickness is approximately 2.5 um. One coating processes in order to obtain maximum deposi- of the significant disadvantages associated with dip tion and coating efficiency using the"mutual electro- coating is the excess deposition of the coating particles static attraction concept in the inter-fibre regions, generally resulting from the presence of large flocculated chains within the coating 3. 2. Characterisation of the interphase material (NdPO4) sol. As shown in Fig 3c, NdPO4 particles did not fill the regions between individual fibres within the woven mat The key issues in the dip coating process are the structure and during dip coating under the application xtent of wetting of the fibres by the sol and the electro- of vacuum, the capillary pressure difference across the static attraction between the fibre surface and coating particles. 16, 18.31 However, the characteristics of the coating particles dispersed in a suspension in terms of heir shape, size and surface charge have to be taken into consideration as they affect the coating behaviour significantly. The TEM micrograph shown in Fig. 2a indicates that the synthesised NdPO particles are of near spherical shape and the particle size is in the range of 10-70 nm, with a mean particle size of 30 nm(as determined by particle size analysis). In order to deter- mine the microstructure of the interphase after sintering, a sample prepared from the coating suspension contain- ing NdPO4 powders was prepared using simple die press- ing and then sintered at 1200 C for 3 h( the same sintering temperature used for the fin al composite). The SEM micrograph of the sintered microstructure obtained is shown in Fig. 2b. It is seen that the NdPO4 body has a plate-like grain microstructure with the presence of no intra- or inter-granular porosity indicating the dense nature of the material. This microstructure provides when used as an interphase in a composite, a weak bonding with both matrix and fibre (i.e. the required Table Surface charge properties and mean particle size of the hydrothermally nthesised mullite plus 5 wt %o zirconia and NdPOa coating powders dispersed in water at a pH value of 3 Electropho Mean particle mobility (10-mVs-) Mullite+5 wt%0 +1.97 Fig. 2.(a)TEM micrograph of synthesised NdPO4 ZrO2, pH: 3 pherical shape and (b) sEM micre NdPO4, PH: 3 +2.7 30 nm sintered microstructure of NdPO4 material after sintering at for 3 h
electrophoretic deposition as the particle mobility and deposition behaviour during EPD will be enhanced.7,29,30 The electrophoretic mobility data in terms of particle net surface charge and mean particle size for the mullite powder and the NdPO4 aqueous sol suspensions at a constant pH value of 3 are shown in Table 1.From these data, it is evident that both particles are positivelycharged at a pH value of 3, therefore if any particles are required to be attracted to the target material (for example to the electrodes during EPD or to the fibre surface during dip coating), the surface charge of such substrates has to be controlled to be opposite of the deposited material.The net surface charge around a particle is a very critical parameter in EPD and dipcoating processes in order to obtain maximum deposition and coating efficiency using the ‘‘mutual electrostatic attraction’’ concept. 3.2. Characterisation of the interphase material (NdPO4) The key issues in the dip coating process are the extent of wetting of the fibres by the sol and the electrostatic attraction between the fibre surface and coating particles.16,18,31 However, the characteristics of the coating particles dispersed in a suspension in terms of their shape, size and surface charge have to be taken into consideration as they affect the coating behaviour significantly.The TEM micrograph shown in Fig.2a indicates that the synthesised NdPO4 particles are of near spherical shape and the particle size is in the range of 10–70 nm, with a mean particle size of 30 nm (as determined by particle size analysis).In order to determine the microstructure of the interphase after sintering, a sample prepared from the coating suspension containing NdPO4 powders was prepared using simple die pressing and then sintered at 1200 C for 3 h (the same sintering temperature used for the final composite).The SEM micrograph of the sintered microstructure obtained is shown in Fig.2b.It is seen that the NdPO4 body has a plate-like grain microstructure with the presence of no intra- or inter-granular porosity indicating the dense nature of the material.This microstructure provides, when used as an interphase in a composite, a weak bonding with both matrix and fibre (i.e. the required weak interfaces),1 leading to the desired fibre debonding, crack deflection and fibre pull-out mechanisms. Fig.3a shows a SEM microstructure of a fibre, which has been dip-coated with NdPO4 without fibre pretreatment, indicating the poor quality of the coating as a result of the absence of electrostatic attraction between the fibre and coating particles.However, when the fibre surface is conditioned with a high pH solution prior to coating in order to create a strong negative surface charge, successful results of mullite fibres coated with NdPO4 particles are achievable, as shown in Fig.3b after thermal treatment at 600 C for 0.5 h. The image indicates that the coating is homogeneous over the fibre surface and its thickness is approximately 2.5 mm.One of the significant disadvantages associated with dip coating is the excess deposition of the coating particles in the inter-fibre regions, generally resulting from the presence of large flocculated chains within the coating sol.As shown in Fig.3c, NdPO4 particles did not fill the regions between individual fibres within the woven mat structure and during dip coating under the application of vacuum, the capillary pressure difference across the Table 1 Surface charge properties and mean particle size of the hydrothermally synthesised mullite plus 5 wt.% zirconia and NdPO4 coating powders dispersed in water at a pH value of 3 Electrophoretic mobility (108 m2 V.s1 ) Mean particle size Mullite+5 wt.% ZrO2, pH: 3 +1.97 2.65 mm NdPO4, pH: 3 +2.78 30 nm Fig.2. (a) TEM micrograph of synthesised NdPO4 particles indicating their near spherical shape and (b) SEM micrograph showing the dense sintered microstructure of NdPO4 material after sintering at 1200 C for 3 h. C. Kaya et al. / Journal of the European Ceramic Society 22 (2002) 2333–2342 2337