Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Journal of the European Ceramic Society 29(2009)363-367 www.elsevier.comlocate/jeurceramsoc Unidirectional all-oxide mini-composites with crack-deflecting ndPOa interface E. G. Butler Yildiz Technical University, Faculry of Chemistry and Metallurgy. Department of Metallurgical and Materials Engineering, Davutpasa Campus, Esenler, Istanbul, Turkey b The University of Birmingham, IRC in Materials Processing and School of Metallurgy and Materials, Edgbaston, Birmingham B152TT,UK Received 6 February 2008: received in revised form 20 May 2008: accepted 6 June 2008 Available online 21 July 2008 Unidirectional Nextel 720M fibers were coated with crack-deflecting NdPOa interface using dip coating and int manufactur th silica-free alumina matrix using electrophoretic deposition followed by drying and pressureless sir at1200° In an attempt Ire oxide-based model 'mini-composites' which are less complex to produce in a short time and which therefore allow rapid results for cal and microstructural characterisation. This material is targeted for use at 1200C in an oxidising atmosphere and has shown an excellent tensile strength value of 1.2 GI and flexural strengths of 894 MPa at room temperature and 761 MPa at 1300 C in unidirectional form. o 2008 Elsevier Ltd. all rights reserved. Keywords: Composites: AlO3; Interface; Mini-composites: NdPO4 Introduction anisms that all contribute to increasing toughness. -79-15, 2To obtain ideal damage-tolerant behaviour, two concepts have been High strength continuous fiber-reinforced ceramic compos- developed so far+, 12, 13, 22; the first one is to create a relatively ites(CMCs) have emerged as leading candidates in gas-turbine weak bonding between fibers and the matrix by coating the fibers applications and power generation systems where future require- with suitable crack-deflecting interface materials, such as ZrO ments for increased operating temperature, reduction in weight or NdPO4 that do not react with the ceramic matrices/fibers or and in exhaust emissions are becoming difficult to meet using fugitive carbon that creates a gap at the fiber/matrix interface; the conventional metallic alloys. The physical and mechanical second one is to use highly porous matrices to isolate fibers from properties of new generation oxide/oxide CMcs enable innova- matrix cracks so that quasi-ductile deformation can be obtained tive solutions for problems with materials in thermal protection as the cracks do not have a continuous front since the matrix is systems and liners in gas-turbine engines, rocket engine, hot held together by grain pairs but the overall strength is quite low in gas filter technologies, fire prevention, catalytic converters, soot this case due to the presence of high porosity content up to 50% filters and medical applications. 7-9As a consequence, many In the present work, alumina ceramic matrix is reinforced gas-turbine manufactures are now placing greater emphasis with NdPO4-coated unidirectional ceramic fibers in an attempt on the evaluation of oxide/oxide components with enhanced to manufacture small damage-tolerant model composite sam- high temperature stability and long service life in oxidising ples called 'mini-composites'suitable for mechanical testin environments.-20 Although the reinforcement fibers and matri- and microstructural observations in a short processing time ces are brittle, the obtained composites display quasi-ductile deformation behaviour due to operation of crack deflection, 2. Experimental work fiber-matrix debonding, crack bridging and fiber pull-out mech- 2.. Materials Corresponding author. Tel. +90 2123834713: fax: +90 2123834665. Sinter-active high purity alumina powders (Tai-Micron E-mail address: cngzky(@ yahoo. co uk(C. Kaya) Japan)with an average particle size of 160 nm and surface area 0955-2219/S-see front matter o 2008 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2008.06.009
Available online at www.sciencedirect.com Journal of the European Ceramic Society 29 (2009) 363–367 Unidirectional all-oxide mini-composites with crack-deflecting NdPO4 interface C. Kaya a,∗, E.G. Butler b a Yildiz Technical University, Faculty of Chemistry and Metallurgy, Department of Metallurgical and Materials Engineering, Davutpasa Campus, Esenler, Istanbul, Turkey b The University of Birmingham, IRC in Materials Processing and School of Metallurgy and Materials, Edgbaston, Birmingham B15 2TT, UK Received 6 February 2008; received in revised form 20 May 2008; accepted 6 June 2008 Available online 21 July 2008 Abstract Unidirectional Nextel 720TM fibers were coated with crack-deflecting NdPO4 interface using dip coating and infiltrated with silica-free alumina matrix using electrophoretic deposition followed by drying and pressureless sintering at 1200 ◦C in an attempt to manufacture oxide-based model ‘mini-composites’ which are less complex to produce in a short time and which therefore allow rapid results for mechanical and microstructural characterisation. This material is targeted for use at 1200 ◦C in an oxidising atmosphere and has shown an excellent tensile strength value of 1.2 GPa and flexural strengths of 894 MPa at room temperature and 761 MPa at 1300 ◦C in unidirectional form. © 2008 Elsevier Ltd. All rights reserved. Keywords: Composites; Al2O3; Interface; Mini-composites; NdPO4 1. Introduction High strength continuous fiber-reinforced ceramic composites (CMCs) have emerged as leading candidates in gas-turbine applications and power generation systems where future requirements for increased operating temperature, reduction in weight and in exhaust emissions are becoming difficult to meet using conventional metallic alloys.1–6 The physical and mechanical properties of new generation oxide/oxide CMCs enable innovative solutions for problems with materials in thermal protection systems and liners in gas-turbine engines, rocket engine, hot gas filter technologies, fire prevention, catalytic converters, soot filters and medical applications.7–9 As a consequence, many gas-turbine manufactures are now placing greater emphasis on the evaluation of oxide/oxide components with enhanced high temperature stability and long service life in oxidising environments.10–20 Although the reinforcement fibers and matrices are brittle, the obtained composites display quasi-ductile deformation behaviour due to operation of crack deflection, fiber–matrix debonding, crack bridging and fiber pull-out mech- ∗ Corresponding author. Tel.: +90 2123834713; fax: +90 2123834665. E-mail address: cngzky@yahoo.co.uk (C. Kaya). anisms that all contribute to increasing toughness.1–7,9–15,21 To obtain ideal damage-tolerant behaviour, two concepts have been developed so far4,12,13,22; the first one is to create a relatively weak bonding between fibers and the matrix by coating the fibers with suitable crack-deflecting interface materials, such as ZrO2 or NdPO4 that do not react with the ceramic matrices/fibers or fugitive carbon that creates a gap at the fiber/matrix interface; the second one is to use highly porous matrices to isolate fibers from matrix cracks so that quasi-ductile deformation can be obtained as the cracks do not have a continuous front since the matrix is held together by grain pairs but the overall strength is quite low in this case due to the presence of high porosity content up to 50%. In the present work, alumina ceramic matrix is reinforced with NdPO4-coated unidirectional ceramic fibers in an attempt to manufacture small damage-tolerant model composite samples called ‘mini-composites’ suitable for mechanical testing and microstructural observations in a short processing time. 2. Experimental work 2.1. Materials Sinter-active high purity alumina powders (Tai-Micron, Japan) with an average particle size of 160 nm and surface area 0955-2219/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2008.06.009
C. Kaya, E.G. Butler / Journal of the European Ceramic Sociery 29(2009)363-367 of 14.3 m-/g were used as the matrix materials. Alumina pow ders were first dispersed in distilled water with the additions of dispersant, liquid binder which is a water-based acrylic polymer uramax B1014 Chesham Chemicals Ltd. UK)and a mixture of boehmite(y-AlOOH) and colloidal Y,O3 as sintering aid with average particle sizes of 20 and 10 nm, respectively and magnetically stirred for 2 h followed by mechanical ball-milling Coated and with alumina balls for h and finally ultrasonication for 0.5 h. impregnated The amount of sintering aids was 2 wt %o of the total powder and Uni-directional the solid-loading of the final suspension was 85 wt %o and the oH was adjusted to be around 4(the weight ratio of boehmite to fiber bundle Y2O3 was 1: 1) Unidirectional fiber bundles extracted from eight-harness satin woven mullite fiber mats(Nextel M 720, 1500-denier yarn 3M, USA)were used as reinforcement materials. Each fiber bundle contains approximately 1500 filaments with an average diameter of 12 um. Before the extraction process, the woven fiber mats were pre-treated by desizing at 500C for I h to remove the organic protection layer from the fiber surface and then the bundles were extracted from the desized mats NdPO4 interface material was prepared by the neutral reaction of neodymium nitrate with ammonium di-hydrogen phosphate(ADPH) at room temperature. Equimolar amounts of Cylindirical Nd(NO3 )3 and AdPh were dissolved into water to make 0.25 M hollow plastic solutions. Mixing of the two solutions by vigorous stirring and tube heating was followed by filtration. The resultant gel filtrate was then dried and calcined at 1000C for 3 h to yield stoichiomet- ric NdPO4 monazite powder with an average particle size of 60nm. A 15-wt %o aqueous-based suspension was prepared by ball milling for 4 h with the pH value adjusted to be 3 2.2. Fiber coating he required number of bundles was extracted from the mat and then immersed in an ammonia-based solution consisting of an ammonium salt of polymethacrylic acid (Versical KA21, pH Allied colloids, UK)in orderto create a strong negative surface charge on the fiber surface. Surface-modified fiber bundle was Fig. 1. Schematic representation of the model mini-composites produced en dipped in NdPO4 suspension for 1 min to allow NdPO4 particles to fully cover the fibers. Coated fiber bundles were then with a diameter of 2 mm and 40 vol% fiber loading were then sintered at 600 C for O5 h to increase the adhesion between the cut for mechanical testing and microstructural observat fiber and the coating layer 2.4. Mechanical tests 2.3. Processing of mini-composite Tubular tensile and flexural test specimens with a length of Dip-coated fiber bundle is impregnated with nano-size alu- 10 cm were cut from the sintered composites. Tensile ends of mina ceramic particles by electrophoretic deposition(EPD) the specimen were fixed in tubular aluminium tabs using a poly Ising a deposition voltage of 10V for 3 min. The details of meric resin which was cured at 280C for I h in order to provide the tech nique can be found elsewhere. 7-19,23-27 Coated and a strong adhesion between the sample and aluminium tabs so electrophoretically deposited fiber bundles were then put in a that sliding of the sample from grips was prevented during ten- polymer-based tube with an inner diameter of 4 mm as shown in sile tests. Room and high temperature four-point bend tests were Fig. 1. Then a hot gun was used to heat the surface of the plastic performed on an Instron Testing machine fitted with a furnace tube up to 150C so that it shrinks and squeezes the bundles which has tungsten mesh elements enabling tests to be carried homogeneously. The specimens compacted within the plastic out at temperatures up to 1500C Specimens to be tested at tube were then removed by cutting the plastic tube and pressure- 1300C, were held at the test temperature for at least I h prior less sintered at 1200C for 2h. The sintered mini-composites the testing to allow the system to equilibrate and to ensure that
364 C. Kaya, E.G. Butler / Journal of the European Ceramic Society 29 (2009) 363–367 of 14.3 m2/g were used as the matrix materials. Alumina powders were first dispersed in distilled water with the additions of dispersant, liquid binder which is a water-based acrylic polymer (Duramax B1014, Chesham Chemicals Ltd., UK) and a mixture of boehmite (-AlOOH) and colloidal Y2O3 as sintering aids with average particle sizes of 20 and 10 nm, respectively and magnetically stirred for 2 h followed by mechanical ball-milling with alumina balls for 8 h and finally ultrasonication for 0.5 h. The amount of sintering aids was 2 wt.% of the total powder and the solid-loading of the final suspension was 85 wt.% and the pH was adjusted to be around 4 (the weight ratio of boehmite to Y2O3 was 1:1). Unidirectional fiber bundles extracted from eight-harness satin woven mullite fiber mats (NextelTM 720, 1500-denier yarn, 3M, USA) were used as reinforcement materials. Each fiber bundle contains approximately 1500 filaments with an average diameter of 12m. Before the extraction process, the woven fiber mats were pre-treated by desizing at 500 ◦C for 1 h to remove the organic protection layer from the fiber surface and then the bundles were extracted from the desized mats. NdPO4 interface material was prepared by the neutral reaction of neodymium nitrate with ammonium di-hydrogen phosphate (ADPH) at room temperature. Equimolar amounts of Nd(NO3)3 and ADPH were dissolved into water to make 0.25 M solutions. Mixing of the two solutions by vigorous stirring and heating was followed by filtration. The resultant gel filtrate was then dried and calcined at 1000 ◦C for 3 h to yield stoichiometric NdPO4 monazite powder with an average particle size of 60 nm. A 15-wt.% aqueous-based suspension was prepared by ball milling for 4 h with the pH value adjusted to be 3. 2.2. Fiber coating The required number of bundles was extracted from the mat and then immersed in an ammonia-based solution, consisting of an ammonium salt of polymethacrylic acid (Versical KA21, pH: 9, Allied colloids, UK) in order to create a strong negative surface charge on the fiber surface. Surface-modified fiber bundle was then dipped in NdPO4 suspension for 1 min to allow NdPO4 particles to fully cover the fibers. Coated fiber bundles were then sintered at 600 ◦C for 0.5 h to increase the adhesion between the fiber and the coating layer. 2.3. Processing of mini-composites Dip-coated fiber bundle is impregnated with nano-size alumina ceramic particles by electrophoretic deposition (EPD) using a deposition voltage of 10 V for 3 min. The details of the technique can be found elsewhere.17–19,23–27 Coated and electrophoretically deposited fiber bundles were then put in a polymer-based tube with an inner diameter of 4 mm as shown in Fig. 1. Then a hot gun was used to heat the surface of the plastic tube up to 150 ◦C so that it shrinks and squeezes the bundles homogeneously. The specimens compacted within the plastic tube were then removed by cutting the plastic tube and pressureless sintered at 1200 ◦C for 2 h. The sintered mini-composites Fig. 1. Schematic representation of the model mini-composites produced. with a diameter of 2 mm and 40 vol.% fiber loading were then cut for mechanical testing and microstructural observations. 2.4. Mechanical tests Tubular tensile and flexural test specimens with a length of 10 cm were cut from the sintered composites. Tensile ends of the specimen were fixed in tubular aluminium tabs using a polymeric resin which was cured at 280 ◦C for 1 h in order to provide a strong adhesion between the sample and aluminium tabs so that sliding of the sample from grips was prevented during tensile tests. Room and high temperature four-point bend 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. Specimens to be tested at 1300 ◦C, were held at the test temperature for at least 1 h prior the testing to allow the system to equilibrate and to ensure that
C. Kaya, E.G. Butler /Joumal of the European Ceramic Society 29(2009)363-367 the specimen was at this temperature. Tensile tests were per- formed at room temperature. For flexural and tensile tests, constant cross-head speed of 0.5 mm/min was used For all the mechanical results reported, seven samples were used for each value by eliminating the highest and the lowest values and taking the average value of the remaining five readings 2.5. Microstructural characterisation Microstructural examinations were carried out on sintered Fiber and fractured composite samples using a high-resolution field emission gun(FEG)SEM(FX-4000, Jeol Ltd, Japan). Further detail observations were carried out using transmission electron microscopy (TEM, Jeol Ltd, Japan, 4000 FX TEM) operat- ng at 400 kV, and equipped with an energy dispersive X-Ray Fiber/matrix analysis(EDX) unit. Porosity(%)and pore size were mea- interface sured using a mercury porosimeter(Hg Por, Micromechanics Instrument Corp, USA)using a penetrometer weight of 62.79 g, NdPO head pressure of 4. 45 psia and penetration volume of 6. 188 mL Coating Archimedes technique was also used for density measurements 3. Results and discussion Fan wme 00 nm Fig. 2 shows the SEM microstructure of dip-coated unidirec tional Nextel 720 fibers with NdPO4 interface material after Fig 3. Transmission electron microscopy (TEM)image of the composite sample sintering at 600C for 0.5 h indicating that the coating layer containing NdPO4 interface indicating that there is no reaction between the is very homogeneous, dense and its thickness is less than 2 um. interface and the fiber When oxide/oxide composites are designed for high temperature pplications, it is fundamental that a compatible weak interface actually caused by the fiber damage during processing and there- between the reinforcement fiber and ceramic matrix should be fore the dense coating structure shown in Fig. 2 helps to protect provided in order to obtain a damage-tolerant behaviour due to the fiber's surface from faws during manufacturing steps, esp mechanisms of crack deflection, debonding and fiber pull-out cially during impregnation and consolidation within the hollow that all contribute to increasing the toughness. Dense interfaces are also desirable plastic tube subjected to heat. Fig. 2 also shows that the NdPO4 as the protect the fibers at high tempera- coating layer is non-porous and quite dense and there is no vis- ture against heat and oxygen diffusion which may cause grain ible evidence of reaction taking place between the coating layer growth and loss in mechanical properties. Furthermore, it is well and the fiber. Further examinations were also conducted using established that significant reduction in mechanical properties is transmission electron microscopy to confirm that there was no reaction zone at the interfaces between NdPOa and fiber as in the TEM micrograph shown in Fig 3. As shown in Fig 3, the inter facial zone between the fiber and the NdPO4 interface is very clear and there is no strong g at this point that proves the NdPO4 absence of any chemical reaction between the interface mate- coating rial and the fibers. TEM EDX analysis on the interfacial zone laver also indicated that there was no reaction product at that region that explained the absence of any undesirable reactions(see also Fig 5b showing the clear surface of the pulled out fibers that also proves the absence of any strong bonding between the fibers and the interface materials). This is very important to improve the flaw tolerance and work of fracture of th to obtain non-catastrophic mode of failure due to presence of an interface which is weak enough to deflect propagating cracks which will lead to substantial energy dissipation and improve ed unidirectionalNextelzz0M ber with fracture toughness. Fig. 3 also shows that the grain size of a crack-deflecting NdPO4 interface after sintering at 600C for 0.5 h indicating he homogeneous and dense structure of the coating layer with a thickness of coating microstructure(see Fig. 2)resulted from sinter-active starting particles of NdPO4
C. Kaya, E.G. Butler / Journal of the European Ceramic Society 29 (2009) 363–367 365 the specimen was at this temperature. Tensile tests were performed at room temperature. For flexural and tensile tests, a constant cross-head speed of 0.5 mm/min was used. For all the mechanical results reported, seven samples were used for each value by eliminating the highest and the lowest values and taking the average value of the remaining five readings. 2.5. Microstructural characterisation Microstructural examinations were carried out on sintered and fractured composite samples using a high-resolution field emission gun (FEG) SEM (FX-4000, Jeol Ltd., Japan). Further detail observations were carried out using transmission electron microscopy (TEM, Jeol Ltd., Japan, 4000 FX TEM) operating at 400 kV, and equipped with an energy dispersive X-Ray analysis (EDX) unit. Porosity (%) and pore size were measured using a mercury porosimeter (Hg Por, Micromechanics Instrument Corp., USA) using a penetrometer weight of 62.79 g, head pressure of 4.45 psia and penetration volume of 6.188 mL. Archimedes technique was also used for density measurements. 3. Results and discussion Fig. 2 shows the SEM microstructure of dip-coated unidirectional Nextel 720TM fibers with NdPO4 interface material after sintering at 600 ◦C for 0.5 h indicating that the coating layer is very homogeneous, dense and its thickness is less than 2 m. When oxide/oxide composites are designed for high temperature applications, it is fundamental that a compatible weak interface between the reinforcement fiber and ceramic matrix should be provided in order to obtain a damage-tolerant behaviour due to mechanisms of crack deflection, debonding and fiber pull-out that all contribute to increasing the toughness. Dense interfaces are also desirable as they protect the fibers at high temperature against heat and oxygen diffusion which may cause grain growth and loss in mechanical properties. Furthermore, it is well established that significant reduction in mechanical properties is Fig. 2. SEM microstructure of dip-coated unidirectional Nextel 720TM fiber with a crack-deflecting NdPO4 interface after sintering at 600 ◦C for 0.5 h indicating the homogeneous and dense structure of the coating layer with a thickness of less than 2m. Fig. 3. Transmission electron microscopy (TEM) image of the composite sample containing NdPO4 interface indicating that there is no reaction between the interface and the fiber. actually caused by the fiber damage during processing and therefore the dense coating structure shown in Fig. 2 helps to protect the fiber’s surface from flaws during manufacturing steps, especially during impregnation and consolidation within the hollow plastic tube subjected to heat. Fig. 2 also shows that the NdPO4 coating layer is non-porous and quite dense and there is no visible evidence of reaction taking place between the coating layer and the fiber. Further examinations were also conducted using transmission electron microscopy to confirm that there was no reaction zone at the interfaces between NdPO4 and fiber as in the TEM micrograph shown in Fig. 3. As shown in Fig. 3, the interfacial zone between the fiber and the NdPO4 interface is very clear and there is no strong bonding at this point that proves the absence of any chemical reaction between the interface material and the fibers. TEM EDX analysis on the interfacial zone also indicated that there was no reaction product at that region that explained the absence of any undesirable reactions (see also Fig. 5b showing the clear surface of the pulled out fibers that also proves the absence of any strong bonding between the fibers and the interface materials). This is very important to improve the flaw tolerance and work of fracture of the composite and also to obtain non-catastrophic mode of failure due to presence of an interface which is weak enough to deflect propagating cracks which will lead to substantial energy dissipation and improve fracture toughness. Fig. 3 also shows that the grain size of the NdPO4 interface is about 250 nm which explains the dense coating microstructure (see Fig. 2) resulted from sinter-active starting particles of NdPO4.
366 C. Kaya, E.G. Butler / Journal of the European Ceramic Sociery 29(2009)363-367 0.015 of matrix Bundle failure Fig. 4. An optical microscopy photo of the tensile test specimens of mini- Displacement mm omposite containing unidirectional Nextel 720 M fiber reinforced alumina ceramic composite with NdPOa interface An optical microscopy image of the unidirectional fiber reinforced mini-composites with NdPO4 interface in tubular ape before and after tensile tests is shown in Fig 4 indicat ing the size of the test sample(about 2 mm in diameter) and the arrangement of the tensile specimen using aluminium tabs. The final mini-composites tested contain a fiber volume fraction of 0.4, 16% porosity with an average pore size of 75 nm. The tensile strength of the mini-composite was determined to be 1203 MPa (1. 2 GPa) with the presence of damage-tolerant behaviour. The tensile strength of the fiber bundle itself is 800 MPa 100m In order to analyse the fracture behaviour in deta load-displacement curves of the mini-composites subjected to Fig.5.(a)Load-displacement curves of four-point bend tested mini-composites flexural tests at room temperature and 1300C are shown in (at room temperature and 1300C)with NdPO4 interfaces and (b)fracture Fig 5a that indicates the behaviour of a typical fiber-reinforced surface of tensile-tested mini-composite showing the presence of long fiber composite behaviour with the absence of catastrophic failure. pull-out. Flexural strengths of the mini-composites were determined to be 894 MPa at room temperature and 761 MPa at 1300C which subjected to tensile test at room temperature is shown in Fig 5b indicated that only 15% reduction in flexural strength was seen indicating the presence of long fiber pull-out and some degree of at 1300C but no distinct change of damage-tolerant behaviour bundle fracture due to mechanisms, such as fiber/matrix debond- was seen at both test temperatures(see Fig. 5a). As shown ing and crack deflection both initiated by the weak nature of in Fig 5a, mini-composites display quasi-ductile deformation the NdPO4 interface. Fig. 5b also proves that crack-deflecting behaviour due to operation of crack deflection, fiber-matrix interface plays an important role in obtaining long fiber pull debonding, crack bridging and fiber pull-out mechanisms that out(pulled-out fiber lengths were observed to be longer than are all operational at room temperature and 1300C. Up to 300 um under scanning electron microscope)which eventually the maximum load, the mini-composite sample showed linear- leads to a pseudoplastic damage-tolerant behaviour. Combining stic deformation behaviour without any obvious damage until the results presented in Figs. 2-5, it can be concluded that the the first matrix cracks occurred (between stress levels of 40 and mini-composites showed a very good room and high temperature 100 MPa as expected), indicated by a step-wise decrease of load flexural strength value with non-catastrophic failure behaviour (as circled in Fig 5a)at both temperatures. But at this load, suf- due to possible two main reasons: (i) the presence of dense but ficient load transfer from the matrix to the fibers was achieved crack-deflecting NdPO4 coating layer that protects the fiber to be nd as a result the composite was still able to carry loads without damaged during processing and against heat and (ii) low poros catastrophic failure by the operation of fiber/matrix debond- ity content of 16% and small pore size. Therefore, the matrix ng and pull-out mechanisms. The weak nature of the NdPO4 properties(porosity level and pore size) and interface structure interface also allowed crack deflection to take place at the inter- play a role and determine the overall damage-tolerant behaviour. face region that consumed significant part of the energy Due to Overall, it is presented in the present work that repro high strength and stiffness of the reinforcement fibers they are ducible model mini-CMCs are manufactured in a very short expected to fracture at very high level of stresses. As shown in processing time using relatively simple and rapid processing Fig 5a, there were sharp step-wise decreases in loads for the techniques, namely dip coating and electrophoretic deposition mini-composite samples subjected to flexural tests at both room to obtain quick mechanical and microstructural results that are temperature and 1300C that indicated the bundle type of fail- both necessary for design and modelling purposes. Therefore ure. This type of fracture generally leads to a considerable fiber time consuming several processing steps used for bulk compos- pull-out. This case was well proven by the SEM microstructure ite processing(for example woven fiber-reinforced 2-D CMCs) shown in Fig 5b. Fracture surface of the mini-composite sample to obtain mechanical test results can be eliminated. These
366 C. Kaya, E.G. Butler / Journal of the European Ceramic Society 29 (2009) 363–367 Fig. 4. An optical microscopy photo of the tensile test specimens of minicomposite containing unidirectional Nextel 720TM fiber reinforced alumina ceramic composite with NdPO4 interface. An optical microscopy image of the unidirectional fiberreinforced mini-composites with NdPO4 interface in tubular shape before and after tensile tests is shown in Fig. 4 indicating the size of the test sample (about 2 mm in diameter) and the arrangement of the tensile specimen using aluminium tabs. The final mini-composites tested contain a fiber volume fraction of 0.4, 16% porosity with an average pore size of 75 nm. The tensile strength of the mini-composite was determined to be 1203 MPa (1.2 GPa) with the presence of damage-tolerant behaviour. The tensile strength of the fiber bundle itself is 800 MPa.28 In order to analyse the fracture behaviour in detail, load–displacement curves of the mini-composites subjected to flexural tests at room temperature and 1300 ◦C are shown in Fig. 5a that indicates the behaviour of a typical fiber-reinforced composite behaviour with the absence of catastrophic failure. Flexural strengths of the mini-composites were determined to be 894 MPa at room temperature and 761 MPa at 1300 ◦C which indicated that only 15% reduction in flexural strength was seen at 1300 ◦C but no distinct change of damage-tolerant behaviour was seen at both test temperatures (see Fig. 5a). As shown in Fig. 5a, mini-composites display quasi-ductile deformation behaviour due to operation of crack deflection, fiber–matrix debonding, crack bridging and fiber pull-out mechanisms that are all operational at room temperature and 1300 ◦C. Up to the maximum load, the mini-composite sample showed linearelastic deformation behaviour without any obvious damage until the first matrix cracks occurred (between stress levels of 40 and 100 MPa as expected), indicated by a step-wise decrease of load (as circled in Fig. 5a) at both temperatures. But at this load, suf- ficient load transfer from the matrix to the fibers was achieved and as a result the composite was still able to carry loads without catastrophic failure by the operation of fiber/matrix debonding and pull-out mechanisms. The weak nature of the NdPO4 interface also allowed crack deflection to take place at the interface region that consumed significant part of the energy. Due to high strength and stiffness of the reinforcement fibers they are expected to fracture at very high level of stresses. As shown in Fig. 5a, there were sharp step-wise decreases in loads for the mini-composite samples subjected to flexural tests at both room temperature and 1300 ◦C that indicated the bundle type of failure. This type of fracture generally leads to a considerable fiber pull-out. This case was well proven by the SEM microstructure shown in Fig. 5b. Fracture surface of the mini-composite sample Fig. 5. (a) Load–displacement curves of four-point bend tested mini-composites (at room temperature and 1300 ◦C) with NdPO4 interfaces and (b) fracture surface of tensile-tested mini-composite showing the presence of long fiber pull-out. subjected to tensile test at room temperature is shown in Fig. 5b indicating the presence of long fiber pull-out and some degree of bundle fracture due to mechanisms, such as fiber/matrix debonding and crack deflection both initiated by the weak nature of the NdPO4 interface. Fig. 5b also proves that crack-deflecting interface plays an important role in obtaining long fiber pullout (pulled-out fiber lengths were observed to be longer than 300m under scanning electron microscope) which eventually leads to a pseudoplastic damage-tolerant behaviour. Combining the results presented in Figs. 2–5, it can be concluded that the mini-composites showed a very good room and high temperature flexural strength value with non-catastrophic failure behaviour due to possible two main reasons: (i) the presence of dense but crack-deflecting NdPO4 coating layer that protects the fiber to be damaged during processing and against heat and (ii) low porosity content of 16% and small pore size. Therefore, the matrix properties (porosity level and pore size) and interface structure play a role and determine the overall damage-tolerant behaviour. Overall, it is presented in the present work that reproducible model mini-CMCs are manufactured in a very short processing time using relatively simple and rapid processing techniques, namely dip coating and electrophoretic deposition to obtain quick mechanical and microstructural results that are both necessary for design and modelling purposes. Therefore time consuming several processing steps used for bulk composite processing (for example woven fiber-reinforced 2-D CMCs) to obtain mechanical test results can be eliminated. These
C. Kaya, E.G. Butler /Journal of the European Ceramic Sociery 29(2009)363-367 model mini-composites can be used to derive useful composite 10. Lewis, M. H, Tye, A, Butler, E. G and Al-Dawery, L,Development of parameters including load transfer and fiber/matrix interfacial interfaces in oxide matrix composites. Key. Eng. Mater, 1999, 164-165, parameters that can all be used to design more complex com- 351-356. 1. Zawada. L P, Longitudinal and transthickness tensile behaviour of several oxide/oxide composites Ceram. Eng. Sci. Proc. 1998, 18(3), 327-339. 12. Tu, w. C, Lange, F. F. and Evans, A. G, Concept for damage-tolerant 4. Conclusions ceramic composites with interfaces. J. Am. Ceram. Soc., 1996, 7902),417-424 Unidirectional Nextel 720M fibers-reinforced alumin 13. Faber, K. T, Ceramic composite interfaces: Properties and design. Annu. ceramic matrix mini-composites are fabricated using rapid and Rev. Mater Sci.,1997,27,499-524. 14. Kanka, B and Schneider, H, Aluminosilicate fiber/mullite matrix compos- simple processing techniques of dip coating and electrophoretic ites with favorable high-temperature properties. J. Eur. Ceram. Soc., 2000, deposition in order to obtain quick results from mechanical 20.619-623. tests and microstructural observations Pressureless sintered 15. Kaya, C, Gu, X, Al-Dawery, I and Butler, E. G, Microstructural develop- (1200oC, 2 h) model mini-composites with an overall poros- nt of woven mullite fibre-reinforced mullite ceramic matrix composites by infiltration processing. Sci. Technol. Adv. Mater., 2002, 3(1), 35-44. ty content of 16%, fiber volume fraction of 0. 4 and average 16. Kaya, C, He, j.Y. Gu, X and Butler, E.G, Nanostructured ceramic pow pore size of less than 100 nm provide excellent room tempera- ders by hydrothermal synthesis and their applications. Micropor. Mesopor ture tensile strength value of 1.2 GPa and flexural strengths of Mater,2002,54(1-2),37-49. 894 MPa at room temperature and 761 MPa at 1300C Quasi- 17. Stoll, E, Mahr, P, Kruger, H G, Kern, H, Thomas, B.J.C. and Boccaccini, ductile deformation( damage-tolerant) behaviour is achieved by A.R., Fabrication technologies for oxide-oxide ceramic matrix compos- dense but weak NdPO4 interface that provides crack deflec es based on electrophoretic deposition. J. Eur. Cerm. Soc., 2006, 26(9). 1567-1576 tion, fiber-matrix debonding and fiber pull-out mechanisms( that 18. Stoll, E, Mahr, P. Kruger, H. G, Kern, H, Dlouhy and Boccaccini all contribute to increasing toughness) to take place. All-oxide A.R. Progress in the characterisation of structural oxide/oxide ceramic model mini-composites can be used to obtain quick mechanical composites fabricated by electrophoretic deposition(EPD).Ad Eng and microstructural results that are both necessary for design Mater,2006,8(4),282-285 and modelling purposes 19. Boccaccini, A.R., Kaya, C and Kruger, H -G, Application of the elec- trophoretic deposition technique in the fabrication of fibre reinforced eramic and glass matrix composites. Che. Ing. -Tech, 2001, 73(5 Acknowledgements 443-452. 20. Bansall, P N. Handbook of Ceramic Composites. Kluwer Academic Pub- TUBITAK (Turkish Science and Technological Research fishers. Boston. USA. 2005 Counsel) and YildIz Technical University of Istanbul are 21. Naslain, N, Lamon, J, Pailler,R, Bourrat, X. Guette, A and Langlais, E, acknowledged for financial support Micro/minicomposites: a useful approach to the design and development of on-oxide CMCs Composite Part A, 1999, 30, 537-547. 22. Lange, F. F, Tu, W. and Evans, A. G, Processing of damage-tolerant, References oxidation-resistant ceramic matrix composites by a precursor infiltration I pyrolysis method. Mater. Sci. Eng. A, 1995, 195, 145-1 2. Kaya, C, Butler, E. G, Selcuk nL. Mate: Rew, 2000, 45, 16- in oxide 23. Kaya, C, Kaya, E and Boccaccini, A.R,Electrophoretic deposition infil- 1. Chawla, K. K. Coffin, C. and Xu, Z.R. Interface fibre/oxide matrix o Boccaccini.AR. and Lewis M.H. Mul- shape.J. Mater sci,2002,37(19),4145-4153. lite(NextelTM 720)fibre-reinforced mullite matrix composites exhibiting 24. Kaya, C- Kaya, E. and Boccaccini, A.R Fabrication of stainless. favourable thermomechanical properties. J. Eur Ceram. Soc., 2002, 22(13), eel-fiber-reinforced cordierite- matrix composites of tubular shape using 2333-2342. ectrophoretic deposition J. Am. Ceram. Soc., 2002, 85(10), 2575-2577 3. Chawla, KK Ceramic Matrix Composites. Chapman& Hall, London, UK, Boccaccini, A.R., Kaya, C. and Chawla, K.K., Use of electrophoretic 1998 deposition in the processing of fibre reinforced ceramic and glass matrix 4. Lewis, M. H, Tye, E, Butler, E. G. and Doleman, P. A, Oxide CMCs: omposites: a review. Composites Part A: Appl. Sci. Manuf, 2001, 32(8). interphase synthesis and novel fibre development. J. Eur. Ceram. Soc., 2000, 997-1006. 20.639644 25. Kaya, C, Kaya, F, Boccaccini, A. R. and Chawla, K. K, Fabrication 5. Warren, R and Deng, S, Continuous fibre reinforced ceramic composites and characterisation of Ni-coated carbon fibre- reinforced alumina ceramic for very high temperatures. Silic. Ind, 1996. 5(6), 96-10 matrix composites using electrophoretic deposition. Acta Mater, 200 6. Peters, P W. M, Daniels, B, Clemens, F and Vogel, W.D. Mechanical char- 4907),1189-1197 at test temperatures 26. Kaya, C, Boccaccini, A. R and Chawla, KK. Electrophoretic deposition up to 1200.C J. Eur Ceram. Soc., 2000, 20, 531-535 forming of nickel-coated-carbon-fiber- reinforced borosilicate-glass--matrix 7. Holmquist, M. G, Radsick, T. C, Sudre. O. H and Lange, FF. Fabrication composites.J. A. Ceram Soc., 2000. 83(8), 1885-1888. and testing of all-oxide CFCC tubes. Composite Part A, 2003, 34. 163-170 27. Kaya, C, Boccaccini, A.R. and Trusty, P. A Processing and characteris- 8. Razzel, A G, Holquist, M, Molliex, L and Sudre, O, Oxide/oxide cerar tion of 2-D woven metal fibre-reinforced multilayer silica matrix composites matrix composites in gas turbine combuster. ASME Paper 98-GT-30 using electrophoretic deposition and pressure filtration. J. Eur. Ceram. So ASME. New York, 1998 1999.19(16,28592866. 9. Beesley, C. P, The applications of CMCs in high integrity gas turb 28. Bansal, P N, Handbook of Ceramic Composites. Kluwer Academic Pub- nes. Key Eng. Mater,1997,127-131,165-174 sher, Boston, 2005, PP 3-33
C. Kaya, E.G. Butler / Journal of the European Ceramic Society 29 (2009) 363–367 367 model mini-composites can be used to derive useful composite parameters including load transfer and fiber/matrix interfacial parameters that can all be used to design more complex composite systems. 4. Conclusions Unidirectional Nextel 720TM fibers-reinforced alumina ceramic matrix mini-composites are fabricated using rapid and simple processing techniques of dip coating and electrophoretic deposition in order to obtain quick results from mechanical tests and microstructural observations. Pressureless sintered (1200 ◦C, 2 h) model mini-composites with an overall porosity content of 16%, fiber volume fraction of 0.4 and average pore size of less than 100 nm provide excellent room temperature tensile strength value of 1.2 GPa and flexural strengths of 894 MPa at room temperature and 761 MPa at 1300 ◦C. Quasiductile deformation (damage-tolerant) behaviour is achieved by dense but weak NdPO4 interface that provides crack deflection, fiber–matrix debonding and fiber pull-out mechanisms (that all contribute to increasing toughness) to take place. All-oxide model mini-composites can be used to obtain quick mechanical and microstructural results that are both necessary for design and modelling purposes. Acknowledgements TUBITAK (Turkish Science and Technological Research Counsel) and Yıldız Technical University of Istanbul are acknowledged for financial support. References 1. Chawla, K. K., Coffin, C. and Xu, Z. R., Interface engineering in oxide fibre/oxide matrix composites. Int. Mater. Rev., 2000, 45, 165–189. 2. Kaya, C., Butler, E. G., Selcuk, A., Boccaccini, A. R. and Lewis, M. H., Mullite (NextelTM 720) fibre-reinforced mullite matrix composites exhibiting favourable thermomechanical properties. J. Eur. Ceram. Soc., 2002, 22(13), 2333–2342. 3. Chawla, K. K., Ceramic Matrix Composites. Chapman & Hall, London, UK, 1998. 4. Lewis, M. H., Tye, E., Butler, E. G. and Doleman, P. A., Oxide CMCs: interphase synthesis and novel fibre development. J. Eur. Ceram. Soc., 2000, 20, 639644. 5. Warren, R. and Deng, S., Continuous fibre reinforced ceramic composites for very high temperatures. Silic. Ind., 1996, 5(6), 96–107. 6. Peters, P. W. M., Daniels, B., Clemens, F. and Vogel, W. D., Mechanical characterisation of mullite-based ceramic matrix composites at test temperatures up to 1200 ◦C. J. Eur. Ceram. Soc., 2000, 20, 531–535. 7. Holmquist, M. G., Radsick, T. C., Sudre, O. H. and Lange, F. F., Fabrication and testing of all-oxide CFCC tubes. Composite Part A, 2003, 34, 163–170. 8. Razzel, A. G., Holquist, M., Molliex, L. and Sudre, O., Oxide/oxide ceramic matrix composites in gas turbine combusters. ASME Paper 98-GT-30. ASME, New York, 1998. 9. Beesley, C. P., The applications of CMCs in high integrity gas turbine engines. Key Eng. Mater., 1997, 127–131, 165–174. 10. Lewis, M. H., Tye, A., Butler, E. G. and Al-Dawery, I., Development of interfaces in oxide matrix composites. Key. Eng. Mater., 1999, 164–165, 351–356. 11. Zawada, L. P., Longitudinal and transthickness tensile behaviour of several oxide/oxide composites. Ceram. Eng. Sci. Proc., 1998, 18(3), 327–339. 12. Tu, W. C., Lange, F. F. and Evans, A. G., Concept for damage-tolerant ceramic composites with ‘strong’ interfaces. J. Am. Ceram. Soc., 1996, 79(2), 417–424. 13. Faber, K. T., Ceramic composite interfaces: Properties and design. Annu. Rev. Mater. Sci., 1997, 27, 499–524. 14. Kanka, B. and Schneider, H., Aluminosilicate fiber/mullite matrix composites with favorable high-temperature properties. J. Eur. Ceram. Soc., 2000, 20, 619–623. 15. Kaya, C., Gu, X., Al-Dawery, I. and Butler, E. G., Microstructural development of woven mullite fibre-reinforced mullite ceramic matrix composites by infiltration processing. Sci. Technol. Adv. Mater., 2002, 3(1), 35–44. 16. Kaya, C., He, J. Y., Gu, X. and Butler, E. G., Nanostructured ceramic powders by hydrothermal synthesis and their applications. Micropor. Mesopor. Mater., 2002, 54(1–2), 37–49. 17. Stoll, E., Mahr, P., Kruger, H. G., Kern, H., Thomas, B. J. C. and Boccaccini, A. R., Fabrication technologies for oxide-oxide ceramic matrix composites based on electrophoretic deposition. J. Eur. Cerm. Soc., 2006, 26(9), 1567–1576. 18. Stoll, E., Mahr, P., Kruger, H. G., Kern, H., Dlouhy and Boccaccini, A. R., Progress in the characterisation of structural oxide/oxide ceramic matrix composites fabricated by electrophoretic deposition (EPD).Adv. Eng. Mater., 2006, 8(4), 282–285. 19. Boccaccini, A. R., Kaya, C. and Krüger, H.-G., Application of the electrophoretic deposition technique in the fabrication of fibre reinforced ceramic and glass matrix composites. Chem.-Ing.-Tech., 2001, 73(5), 443–452. 20. Bansall, P. N., Handbook of Ceramic Composites. Kluwer Academic Publishers, Boston, USA, 2005. 21. Naslain, N., Lamon, J., Pailler, R., Bourrat, X., Guette, A. and Langlais, F., Micro/minicomposites: a useful approach to the design and development of non-oxide CMCs. Composite Part A, 1999, 30, 537–547. 22. Lange, F. F., Tu, W. and Cevans, A. G., Processing of damage-tolerant, oxidation-resistant ceramic matrix composites by a precursor infiltration and pyrolysis method. Mater. Sci. Eng. A, 1995, 195, 145–150. 23. Kaya, C., Kaya, F. and Boccaccini, A. R., Electrophoretic deposition infiltration of 2-D metal fibre-reinforced cordierite matrix composites of tubular shape. J. Mater. Sci., 2002, 37(19), 4145–4153. 24. Kaya, C., Kaya, F. and Boccaccini, A. R., Fabrication of stainlesssteel-fiber-reinforced cordierite-matrix composites of tubular shape using electrophoretic deposition. J. Am. Ceram. Soc., 2002, 85(10), 2575–2577; Boccaccini, A. R., Kaya, C. and Chawla, K. K., Use of electrophoretic deposition in the processing of fibre reinforced ceramic and glass matrix composites: a review. Composites Part A: Appl. Sci. Manuf., 2001, 32(8), 997–1006. 25. Kaya, C., Kaya, F., Boccaccini, A. R. and Chawla, K. K., Fabrication and characterisation of Ni-coated carbon fibre-reinforced alumina ceramic matrix composites using electrophoretic deposition. Acta Mater., 2001, 49(7), 1189–1197. 26. Kaya, C., Boccaccini, A. R. and Chawla, K. K., Electrophoretic deposition forming of nickel-coated-carbon-fiber-reinforced borosilicate-glass–matrix composites. J. Am. Ceram. Soc., 2000, 83(8), 1885–1888. 27. Kaya, C., Boccaccini, A. R. and Trusty, P. A., Processing and characterisation of 2-D woven metal fibre-reinforced multilayer silica matrix composites using electrophoretic deposition and pressure filtration. J. Eur. Ceram. Soc., 1999, 19(16), 2859–2866. 28. Bansal, P. N., Handbook of Ceramic Composites. Kluwer Academic Publisher, Boston, 2005, pp. 3–33