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 acidfibre/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/mod￾ulus 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 considera￾tion when a composite is fabricated.The critical char￾acteristics of the interface can be summarised in terms of low interfacial fracture energy and moderate inter￾facial 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 tempera￾tures.16 Many interphase materials including BN, car￾bon, 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 com￾pounds 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 tetra￾hedrally 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 compa￾tible with mullite and less susceptible to b-alumina for￾mation 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 ‘‘porous￾matrix’’ and ‘‘weak interface’’ concepts.Thermo￾mechanical 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 pull￾out 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 synthe￾sised 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 micro￾scopy (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 cal￾cined 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 reac￾tion 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 fol￾lowed by drying at 110
C for 3 h.The obtained dried powders were calcined at 1000
C for 3 h to yield stoi￾chiometric 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 (Nex￾telTM 720, 3M, USA) were used as reinforcement mate￾rial in the present composites.The fibre mats were pre￾treated 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