《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_electrophoretic deposition-8

Availableonlineatwww.sciencedirect.com SCIENCE E噩≈S Journal of the European Ceramic Society 23(2003)1655-1660 www.elsevier.com/locate/jeurceramsoc AlO3-Y-TZP/Al2O3 functionally graded composites of tubular shape from nano-sols using double-step electrophoretic deposition Z K Interdisciplinary Research Centre (IRC) in Materials Processing and School of Metallurgy and materials The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 12 March 2002: accepted 5 October 2002 Al2Ox-Y-TZP/Al2O3 functionally graded composites of tubular shape incorporating a very tough central layer with graded composition(Al2O3-Y-TZP)and a hard outer surface layer of pure alumina were produced from sols using electro- phoretic deposition(EPD)in an attempt to generate a continuously inhomogeneous property variation across the final component d to control the microstructure at a nanometer scale. It is shown that hardness, fracture toughness and alumina grain size within the graded layer are controlled by the volume fraction of Tzp grains and the highest volume fraction(71%)of Tzp phase provides a fracture toughness value of 7. 1 MPa m/ and Vicker's hardness of 10.4 GPa whilst the lowest volume fraction(13%)results in obtaining a fracture toughness value of 3. 8 MPa m/ and hardness value of 15.7 GPa. The pure alumina surface layer(100 um in thickness) with a high hardness value of 19.4 GPa is considered to be beneficial for tribological applications where high wear C 2003 Elsevier Science Ltd. All rights reserved Keywords: Al2O3-ZrO2; Electrophoretic deposition; Functionally graded materials; Hardness: Toughness 1. Introduction thermal barriers, and materials for energy conversion. A variety of processing techniques including powder proces- The need for new materials with tailored properties sing, electrochemical processing and filtration, field acti- and different functions at different surfaces has led to vated synthesis, thermal or plasma spraying, microwave exploration of the phase gradation that has been long processing and chemical vapour deposition have been established in nature. These graded structures, seen in proposed to manufacture such graded components. -I1 such examples of culms of bamboo and barley, on bone However, these methods are generally complicated, time and seashell, have been the main driving force for exten- consuming and also expensive for large scale production sive work to produce similar microstructures from metal/ in industry; therefore electrophoretic deposition(EPD) ceramic, metal/metal or ceramic/ceramic compositions. -b has been introduced as an alternative to produce gra The concept behind this significant research effort is dient materials. 12. 3 EPD is accepted as a novel, rela- that the chemical composition and or microstructure tively simple, cost-effective and high forming-rate across the component can be controlled in order to technique for producing monolithic/multilayer ceramics control the property gradation. The resultant compo- and fiber-reinforced composites. 4-20 This process relies sites are referred to as functionally graded materials on the presence of charged particles in liquid suspen FGMs)and they possess unique properties for a variety sion, i.e. a sol, which, on the application of an electric of applications such as fuel cells, biomaterial implants, field, will move and be deposited on an oppositely charged electrode. The rate of deposition is high and can be controlled by controlling the applied potential *Tel:+44-121-414-3537;fax:+44-121-414-344l In the present work, tubular Al2O3-Y-TZP/AlO E-mail address: c kaya(@ bham ac uk(C. Kaya) functionally graded composites incorporating a tough 0955-2219/03/S. see front matter C 2003 Elsevier Science Ltd. All rights reserved. PII:S0955-2219(02)00370-9

Al2O3–Y-TZP/Al2O3 functionally graded composites of tubular shape from nano-sols using double-step electrophoretic deposition CengizKaya* Interdisciplinary Research Centre (IRC) in Materials Processing and School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 12 March 2002; accepted 5 October 2002 Abstract Al2O3–Y-TZP/Al2O3 functionally graded composites of tubular shape incorporating a very tough central layer with graded composition (Al2O3–Y-TZP) and a hard outer surface layer of pure alumina were produced from nano-size sols using electro￾phoretic deposition (EPD) in an attempt to generate a continuously inhomogeneous property variation across the final component and to control the microstructure at a nanometer scale. It is shown that hardness, fracture toughness and alumina grain size within the graded layer are controlled by the volume fraction of TZP grains and the highest volume fraction (71%) of TZP phase provides a fracture toughness value of 7.1 MPa m1/2 and Vicker’s hardness of 10.4 GPa whilst the lowest volume fraction (13%) results in obtaining a fracture toughness value of 3.8 MPa m1/2 and hardness value of 15.7 GPa. The pure alumina surface layer (100 mm in thickness) with a high hardness value of 19.4 GPa is considered to be beneficial for tribological applications where high wear resistance is required. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Al2O3–ZrO2; Electrophoretic deposition; Functionally graded materials; Hardness; Toughness 1. Introduction The need for new materials with tailored properties and different functions at different surfaces has led to exploration of the phase gradation that has been long established in nature. These graded structures, seen in such examples of culms of bamboo and barley, on bone and seashell, have been the main driving force for exten￾sive work to produce similar microstructures from metal/ ceramic, metal/metal or ceramic/ceramic compositions.1
6 The concept behind this significant research effort is that the chemical composition and/or microstructure across the component can be controlled in order to control the property gradation. The resultant compo￾sites are referred to as functionally graded materials (FGMs) and they possess unique properties for a variety of applications such as fuel cells, biomaterial implants, thermal barriers, and materials for energy conversion. A variety of processing techniques including powder proces￾sing, electrochemical processing and filtration, field acti￾vated synthesis, thermal or plasma spraying, microwave processing and chemical vapour deposition have been proposed to manufacture such graded components.7
11 However, these methods are generally complicated, time consuming and also expensive for large scale production in industry; therefore electrophoretic deposition (EPD) has been introduced as an alternative to produce gra￾dient materials.12,13 EPD is accepted as a novel, rela￾tively simple, cost-effective and high forming-rate technique for producing monolithic/multilayer ceramics and fiber-reinforced composites.14
20 This process relies on the presence of charged particles in liquid suspen￾sion, i.e. a sol, which, on the application of an electric field, will move and be deposited on an oppositely charged electrode. The rate of deposition is high and can be controlled by controlling the applied potential. In the present work, tubular Al2O3–Y-TZP/Al2O3 functionally graded composites incorporating a tough 0955-2219/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(02)00370-9 Journal of the European Ceramic Society 23 (2003) 1655–1660 www.elsevier.com/locate/jeurceramsoc * Tel.: +44-121-414-3537; fax: +44-121-414-3441. E-mail address: c.kaya@bham.ac.uk (C. Kaya)

C. Kaya/Journal of the European Ceramic Society 23(2003)1655-1660 central layer with graded composition (Al2O3-Y-TZP) EPD was performed with the Al_ Y-TZP sol under a and a hard outer surface layer of pure alumina were constant voltage of 10 V using a deposition time of 2.5 produced from nano-size sols using EPD in an attempt min and with magnetic stirring during the EPD. Under to generate a continuous property variation across thethe applied electric field, the boehmite and zirconia final component and to improve the microstructural particles(possessing a net negative surface charge at pH features in terms of grain size, hardness and fracture 9.2) migrated towards the positive electrode, i.e., the toughnes deposition electrode (see Fig. 1). The particles were deposited until a matrix thickness of 1.4 mm was achieved. The central deposition electrode was con 2. Experimental work nected to a balance linked to a computer. The apparatus Two different colloidal sols, i. e. pure alumina and alumina plus Y-TZP, to be used in the EPd experi- Electrode什+) ments were prepared. In order to obtain the Al2O3-Y Electrode(-) TZP structure after sintering, equal amounts of very fine (the average particle size is 30 nm, VP zirconia, Degussa td, Germany), medium size(150 nm, BDH Chemicals, UK) and coarse(400 nm, Dai-chi Ltd, Japan) zirconia powders were first dispersed in distilled water with the addition of 3 mol% Y2O3. To prepare a kinetically First step of stable zirconia sol. the distilled water was stirred vigo AlOOH-Y-TZP sol ously while the nano-size zirconia powders were added at a rate of 0.5 g min=. The low rate of addition pre- rented the formation of large heteroflocculated clusters The resultant sol and 0.5 wt. binder(Celacol, in order to increase the green strength) were then added to a boehmite sol (r-AlOOH) containing spherical particles with an average particle size of 100 nm(Alcan Chemi- cals, UK)and then ball-mixed all together for I day(the EPD formed boehmite sol was seeded with 2 wt. T1O2 particles in order to lower the a-alumina transformation tempera- ture). The solids-loading of the final sol was adjusted to be 20 wt. of the dispersion liquid with simultaneous ultrasonic agitation to enhance powder dispersion Homogeneous, well-dispersed and agglomerate-free stable suspensions were obtained at a pH value of 9.2. A commercial a-Al2O3 powder (Tai-micron, Japan) (+) was used as the pure alumina source. As received alu mina powders contain spherical particles with an aver age particle size of 150 nm. These powders were dispersed in distilled water at a pH value of 4 with the addition of 0.5 wt. binder(Celacol)and the solids- ding of the prepared suspension was adjusted to be Second step of 20 wt. of the dispersion medium. The final sol was Pure AlO3 sol ball-mixed for I day An in-situ electrophoretic deposition cell was used in order to manufacture functionally graded compo- nents. 7 A stainless steel rod 0.3 mm in diameter was used as deposition electrode (+) whilst a tubular stain Pure AhO3 surface less steel electrode 40 mm in diameter was used as the negative electrode (cathode). The central electrode was EPD formed coated with a very thin C layer in order to allow easy FGM of removel of the EPD formed green deposit before sin- tubula Graded Al,O3-Y-TZP tering. The distance between the two electrodes was hape chosen to be 17 mm The epd cell electrodes were con Fig. I. Processing of Al2Or-Y-TZP/Al2O3 FGM of tubular shape nected to a 0-60 v d.c. power supply. The first stage of using double-step EPD

central layer with graded composition (Al2O3–Y-TZP) and a hard outer surface layer of pure alumina were produced from nano-size sols using EPD in an attempt to generate a continuous property variation across the final component and to improve the microstructural features in terms of grain size, hardness and fracture toughness. 2. Experimental work Two different colloidal sols, i.e. pure alumina and alumina plus Y–TZP, to be used in the EPD experi￾ments were prepared. In order to obtain the Al2O3–Y￾TZP structure after sintering, equal amounts of very fine (the average particle size is 30 nm, VP zirconia, Degussa Ltd, Germany), medium size (150 nm, BDH Chemicals, UK) and coarse (400 nm, Dai-chi Ltd, Japan) zirconia powders were first dispersed in distilled water with the addition of 3 mol% Y2O3. To prepare a kinetically stable zirconia sol, the distilled water was stirred vigor￾ously while the nano-size zirconia powders were added at a rate of 0.5 g min
1 . The low rate of addition pre￾vented the formation of large heteroflocculated clusters. The resultant sol and 0.5 wt.% binder (Celacol, in order to increase the green strength) were then added to a boehmite sol (g-AlOOH) containing spherical particles with an average particle size of 100 nm (Alcan Chemi￾cals, UK) and then ball-mixed all together for 1 day (the boehmite sol was seeded with 2 wt.% TiO2 particles in order to lower the a-alumina transformation tempera￾ture). The solids-loading of the final sol was adjusted to be 20 wt.% of the dispersion liquid with simultaneous ultrasonic agitation to enhance powder dispersion. Homogeneous, well-dispersed and agglomerate-free stable suspensions were obtained at a pH value of 9.2. A commercial a-Al2O3 powder (Tai-micron, Japan) was used as the pure alumina source. As received alu￾mina powders contain spherical particles with an aver￾age particle size of 150 nm. These powders were dispersed in distilled water at a pH value of 4 with the addition of 0.5 wt.% binder (Celacol) and the solids￾loading of the prepared suspension was adjusted to be 20 wt.% of the dispersion medium. The final sol was ball-mixed for 1 day. An in-situ electrophoretic deposition cell was used in order to manufacture functionally graded compo￾nents.17 A stainless steel rod 0.3 mm in diameter was used as deposition electrode (+) whilst a tubular stain￾less steel electrode 40 mm in diameter was used as the negative electrode (cathode). The central electrode was coated with a very thin C layer in order to allow easy removel of the EPD formed green deposit before sin￾tering. The distance between the two electrodes was chosen to be 17 mm. The EPD cell electrodes were con￾nected to a 0–60 V d.c. power supply. The first stage of EPD was performed with the Al2O3–Y-TZP sol under a constant voltage of 10 V using a deposition time of 2.5 min and with magnetic stirring during the EPD. Under the applied electric field, the boehmite and zirconia particles (possessing a net negative surface charge at pH 9.2) migrated towards the positive electrode, i.e., the deposition electrode (see Fig. 1). The particles were deposited until a matrix thickness of 1.4 mm was achieved. The central deposition electrode was con￾nected to a balance linked to a computer. The apparatus Fig. 1. Processing of Al2O3–Y-TZP/Al2O3 FGM of tubular shape using double-step EPD. 1656 C. Kaya / Journal of the European Ceramic Society 23 (2003) 1655–1660

C. Kaya/Journal of the European Ceramic Society 23(2003)1655-1660 is able to record the weight gain per millisecond during the deposition process, i.e., in real time. After the first AlO, Surface stage EPD, the deposition electrode surrounded with he boehmite plus zirconia deposit layer in green state was put in the pure alumina suspension and the central deposition electrode was connected to the negative terminal of the power supply as alumina particles have positive surface charge at the working pH value of 4 The second stage EPD was performed using the same Graded AL Oj-Y-T7P voltage of 10 V d. c. for a deposition time of 1.5 min in order to obtain a thin alumina layer around the first formed thicker boehmite plus zirconia layer. The final green body specimens in tubular shape containing an inner layer of boehmite plus zirconia surrounded by a thin layer of pure alumina were dried under humidity Fig. 2. SEM micrograph of the EPD-formed FGM of tubular shape controlled atmosphere for I day and left in ambient air showing the XRD and SEM analysed points within the graded layer for another day before being pressureless sintered at and the thickness of the pure alumina surface layer to be 100 um 1400°Cfor2h. The surface properties of the colloidal sol particles in terms of their electrophoretic mobility and net surface using X-ray diffraction and also an image analyser for harge were determined using a surface charge analyser comparison (DELSA 440 surface charge analyser). Microstructural The graded internal layer of Al2O3-Y-TZP is formed observations were carried out by using a field emission by the controlled and engineered EPD using nano sol un scanning electron microscope(FEG SEM, Hitachi particles having different particle sizes. The electro- FX-4000, Japan) on sintered, fractured and thermally phoretic mobility of each particle within the mixed sol etched(1300C for 20 min) surfaces. Fracture tough- will be different as this depands on the particle sizes, and ness and hardness were determined using the Vickers hence the masses. 14, 17 Particles with smaller diameter indentation technique. 2I Grain size measurements were will have higher mobility than those of larger size.The conducted using the linear intercept technique. 2 and SEM pictures shown in Fig 3 indicate the difference in the interfacial behaviour of the composites between the composition from the centre to the alumina layer prov graded layer and the pure alumina surface layer was ing the validity of the 'particle mobility-particle size characterised using the crack path propagation tech- relationships concept. The SEM micrograph shown in Fig. 3a represents the microstructure at the point A (dark phase is alumina and the light phase is TZP). The volume fraction of the TZP phase was found to be 71% 3. Results and discussion at this point (r=150 um). The composition of the gra- ded layer shows a gradual change in composition as The sintered (1400oC for 2 h) microstructure of the shown in Fig. 3b and c. The volume fraction of TZP tubular Al2O3-Y-TZP/Al2O3 functionally graded com- phase decreases from 35%(point B, r=350 um) to 13% posite incorporating a tough central layer with graded (point C, r=550 um). sEM pictures taken along the composition(AlOxY-TZP)and a hard outer surface radius of the sintered FGM show an increase in alumina layer of pure alumina is shown in Fig. 2. From the SEM volume fraction from the centre to the pure alumina picture it is clear that the thickness of the outer alumina layer and they also show a significant change in micro layer is about 100 um indicating the ability of electro- structure as shown in Fig. 3. Table I shows the rela- phoretic deposition to produce a homogeneous and tionships between the composition of the graded layer and controlled surface layer. It is also seen from Fig. 2 that the grain size of the alumina and tzp grains. The finest the pure alumina surface layer is continuous and alumina grain size(0.65 um)was found at the point A homogeneous in thickness and also that there is no where the volume fraction of TZP is 71% whilst this value crack formation within the graded layer or alumina increases to 1.85 um at the point C when the TZP volume surface layer. The FGM produced has a central hole fraction decreases to 13%. However, the TzP grain size 0.25 mm in diameter, a layer of Al2OxY-TZP 1 4 mm along the radius of the FGM seems to be independent of in diameter and a surface alumina layer 100 um in he composition. The SEM microstructure of the pure thickness. In order to analyse the composition within alumina surface layer is shown in Fig. 3d indicating the the graded section of the composite, detail SEM pictures presence of a dense microstructure with an average were taken from the points shown in Fig. 2 and the alumina grain size of 2.6 um. It can be concluded from amount of alumina and Y-TZP phases were calculated the results presented in Fig. 3 that the Tzp grains

is able to record the weight gain per millisecond during the deposition process, i.e., in real time. After the first stage EPD, the deposition electrode surrounded with the boehmite plus zirconia deposit layer in green state was put in the pure alumina suspension and the central deposition electrode was connected to the negative terminal of the power supply as alumina particles have positive surface charge at the working pH value of 4. The second stage EPD was performed using the same voltage of 10 V d.c. for a deposition time of 1.5 min in order to obtain a thin alumina layer around the first formed thicker boehmite plus zirconia layer. The final green body specimens in tubular shape containing an inner layer of boehmite plus zirconia surrounded by a thin layer of pure alumina were dried under humidity controlled atmosphere for 1 day and left in ambient air for another day before being pressureless sintered at 1400 C for 2 h. The surface properties of the colloidal sol particles in terms of their electrophoretic mobility and net surface charge were determined using a surface charge analyser (DELSA 440 surface charge analyser). Microstructural observations were carried out by using a field emission gun scanning electron microscope (FEG SEM, Hitachi FX-4000, Japan) on sintered, fractured and thermally etched (1300 C for 20 min) surfaces. Fracture tough￾ness and hardness were determined using the Vickers indentation technique.21 Grain size measurements were conducted using the linear intercept technique.22 and the interfacial behaviour of the composites between the graded layer and the pure alumina surface layer was characterised using the crack path propagation tech￾nique.23 3. Results and discussion The sintered (1400 C for 2 h) microstructure of the tubular Al2O3–Y-TZP/Al2O3 functionally graded com￾posite incorporating a tough central layer with graded composition (Al2O3–Y-TZP) and a hard outer surface layer of pure alumina is shown in Fig. 2. From the SEM picture it is clear that the thickness of the outer alumina layer is about 100 mm indicating the ability of electro￾phoretic deposition to produce a homogeneous and controlled surface layer. It is also seen from Fig. 2 that the pure alumina surface layer is continuous and homogeneous in thickness and also that there is no crack formation within the graded layer or alumina surface layer. The FGM produced has a central hole 0.25 mm in diameter, a layer of Al2O3–Y-TZP 1.4 mm in diameter and a surface alumina layer 100 mm in thickness. In order to analyse the composition within the graded section of the composite, detail SEM pictures were taken from the points shown in Fig. 2 and the amount of alumina and Y–TZP phases were calculated using X-ray diffraction and also an image analyser for comparison. The graded internal layer of Al2O3–Y-TZP is formed by the controlled and engineered EPD using nano sol particles having different particle sizes. The electro￾phoretic mobility of each particle within the mixed sol will be different as this depands on the particle sizes, and hence the masses.14,17 Particles with smaller diameter will have higher mobility than those of larger size. The SEM pictures shown in Fig. 3 indicate the difference in composition from the centre to the alumina layer prov￾ing the validity of the ‘particle mobility-particle size relationships’ concept. The SEM micrograph shown in Fig. 3a represents the microstructure at the point A (dark phase is alumina and the light phase is TZP). The volume fraction of the TZP phase was found to be 71% at this point (r=150 mm). The composition of the gra￾ded layer shows a gradual change in composition as shown in Fig. 3b and c. The volume fraction of TZP phase decreases from 35% (point B, r=350 mm) to 13% (point C, r=550 mm). SEM pictures taken along the radius of the sintered FGM show an increase in alumina volume fraction from the centre to the pure alumina layer and they also show a significant change in micro￾structure as shown in Fig. 3. Table 1 shows the rela￾tionships between the composition of the graded layer and the grain size of the alumina and TZP grains. The finest alumina grain size (0.65 mm) was found at the point A where the volume fraction of TZP is 71% whilst this value increases to 1.85 mm at the point C when the TZP volume fraction decreases to 13%. However, the TZP grain size along the radius of the FGM seems to be independent of the composition. The SEM microstructure of the pure alumina surface layer is shown in Fig. 3d indicating the presence of a dense microstructure with an average alumina grain size of 2.6 mm. It can be concluded from the results presented in Fig. 3 that the TZP grains Fig. 2. SEM micrograph of the EPD-formed FGM of tubular shape showing the XRD and SEM analysed points within the graded layer and the thickness of the pure alumina surface layer to be 100 mm. C. Kaya / Journal of the European Ceramic Society 23 (2003) 1655–1660 1657

C. Kaya/Journal of the European Ceramic Society 23(2003)1655-1660 A- 7I voL% TZP B 35 voL% TZP (o) C- 13 voL, %TZI Pure AL, O, Layer Fig 3. SEM micrographs taken from the points within the Al2OrY-T'ZP graded layer shown in Fig. 2: (a)71 vol. TZP(point A), (b)35 vol% TZP (point B),(c)13 vol. TZP (point C)and (d) the microstructure of pure alumina surface layer(dark phase represents alumina and the light phase represents TzP grains Table I Propertices of electrophoretically formed Al,0-Y-TZPJALO, FGM of tubular shape rea in graded layer Grain size(um) Volume fraction of Hardness Fracture toughness (see Fig. 2) TZP (%) (GPa) MPa m2) TZP 000 3.8 Pure Al2O3 layer 3.1 within the graded layer play an important role in sup The SEM micrographs of fracture surfaces of the pressing alumina grain growth, the highest volume graded layer and pure alumina surface layer are shown fraction providing sub-micron alumina grains within in Fig. 4a and b, respectively. Inter-granular fracture the graded layer is the dominant failure mode for both layers, however, The relationships between the composition of the a few alumina grains within the pure alumina surface FGM and hardness/fracture toughness are also given in layer fail trans-granularly, as shown in Fig. 4b. The Table 1. As the volume fraction of TzP phase within the presence of fine, medium size and also coarser TZP functionally graded Al2O3-Y-TZP layer decreases, the grains within the graded layer is visible from the hardness value increases whilst the fracture toughness image shown in Fig. 4a. The nature of the interfacial decreases. The highest toughness value of 7. 1 MPa m/2 region between the graded Al,O3-Y-TZP and the pure was determined at the point A and the highest hardness Al2O3 surface layer in terms of the crack deflection value of 15.7 GPa at the point C. It is also seen from the and propagation behaviour was characterised by using Table I that the pure alumina surface layer has a hard- a crack path propagation test, as shown in Fig. 4c ness value of 19.4 GPa and a moderate fracture tough- An indenter-induced crack propagates intergranularly ness value of 3. 1 MPa m, resulting from the dense and from the pure alumina surface layer and when it fine grained alumina microstructure. interacts with the graded layer, it propagates along the

within the graded layer play an important role in sup￾pressing alumina grain growth, the highest volume fraction providing sub-micron alumina grains within the graded layer. The relationships between the composition of the FGM and hardness/fracture toughness are also given in Table 1. As the volume fraction of TZP phase within the functionally graded Al2O3–Y-TZP layer decreases, the hardness value increases whilst the fracture toughness decreases. The highest toughness value of 7.1 MPa m1/2 was determined at the point A and the highest hardness value of 15.7 GPa at the point C. It is also seen from the Table 1 that the pure alumina surface layer has a hard￾ness value of 19.4 GPa and a moderate fracture tough￾ness value of 3.1 MPa m1/2, resulting from the dense and fine grained alumina microstructure. The SEM micrographs of fracture surfaces of the graded layer and pure alumina surface layer are shown in Fig. 4a and b, respectively. Inter-granular fracture is the dominant failure mode for both layers, however, a few alumina grains within the pure alumina surface layer fail trans-granularly, as shown in Fig. 4b. The presence of fine, medium size and also coarser TZP grains within the graded layer is visible from the image shown in Fig. 4a. The nature of the interfacial region between the graded Al2O3–Y-TZP and the pure Al2O3 surface layer in terms of the crack deflection and propagation behaviour was characterised by using a crack path propagation test, as shown in Fig. 4c. An indenter-induced crack propagates intergranularly from the pure alumina surface layer and when it interacts with the graded layer, it propagates along the Fig. 3. SEM micrographs taken from the points within the Al2O3–Y-TZP graded layer shown in Fig. 2: (a) 71 vol.% TZP (point A), (b) 35 vol.% TZP (point B), (c) 13 vol.% TZP (point C) and (d) the microstructure of pure alumina surface layer (dark phase represents alumina and the light phase represents TZP grains. Table 1 Properties of electrophoretically formed Al2O3–Y-TZP/Al2O3 FGM of tubular shape Area in graded layer (see Fig. 2) Grain size (mm) Volume fraction of TZP (%) Hardness (GPa) Fracture toughness (MPa m1/2) Al2O3 TZP A 0.65 0.70 71 10.4 7.1 B 1.35 0.75 35 12.9 6.3 C 1.85 0.85 13 15.7 3.8 Pure Al2O3 layer 2.6 – 0 19.4 3.1 1658 C. Kaya / Journal of the European Ceramic Society 23 (2003) 1655–1660

C. Kaya/Journal of the European Ceramic Society 23(2003)1655-1660 :始 Fig. 4. SEM micrographs of the fracture surfaces of (a) Al2O]Y-TZP graded layer, (b) pure alumina surface layer, both showing dominantly inter- granular fracture behaviour and (c) intergranular propagation of an indenter-induced crack along the graded and surface alumina grain boundary graded layer and pure alumina boundary. Fig. 4c of 7. 1 MPa m/2 and Vicker's hardness of 10.4 GPa hows that the bonding between these two phases is whilst the lowest volume fraction(13%)results in a weak enough to deflect a crack which should con- fracture toughness of 3. 8 MPa m/2 and hardness of 15.7 tribute to the damage-tolerant behaviour of the overall GPa. Increase in volume fraction of Tzp phase within FGM composites. the graded layer decreases hardness and increases the fracture toughness and also controls the alumina grain growth during sintering. Pure alumina surface layer 4. Conclusions (100 um in thickness) with a hardness value of 19.4 GPa is introduced for tribological applications. The FGM Electrophoretic deposition(EPD) was used to pro- composite shows mainly intergranular fracture beh luce tubular Al2Ox-Y-TZP/AlO3 functionally graded viour and the interfacial region between the graded composites incorporating a tough central layer with layer and the alumina surface layer is weak as an graded composition(Al2O3-Y-TZP)and a hard outer indenter-induced crack propagates intergranularly surface layer of pure alumina from nano-size sols in an along the boundary between these two phases. The attempt to generate a continuous property variation results presented in this work suggest that the overall across the final component and to control the micro- FGM produced could be an ideal micro-component for structure at a nanometer scale. Hardness, fracture room and high temperature applications where toughness and alumina grain size within the graded"damage-tolerantbehaviour and high wear resistance layer were found to be controlled by the volume frac- are required. Experimental work is being conducted at tion of Tzp grains and the highest volume fraction present in order to determine the thermomechanical (71%)of Tzp phase provides a fracture toughness value properties of the Fgm

graded layer and pure alumina boundary. Fig. 4c shows that the bonding between these two phases is weak enough to deflect a crack which should con￾tribute to the damage-tolerant behaviour of the overall FGM composites. 4. Conclusions Electrophoretic deposition (EPD) was used to pro￾duce tubular Al2O3–Y-TZP/Al2O3 functionally graded composites incorporating a tough central layer with graded composition (Al2O3–Y-TZP) and a hard outer surface layer of pure alumina from nano-size sols in an attempt to generate a continuous property variation across the final component and to control the micro￾structure at a nanometer scale. Hardness, fracture toughness and alumina grain size within the graded layer were found to be controlled by the volume frac￾tion of TZP grains and the highest volume fraction (71%) of TZP phase provides a fracture toughness value of 7.1 MPa m1/2 and Vicker’s hardness of 10.4 GPa whilst the lowest volume fraction (13%) results in a fracture toughness of 3.8 MPa m1/2 and hardness of 15.7 GPa. Increase in volume fraction of TZP phase within the graded layer decreases hardness and increases the fracture toughness and also controls the alumina grain growth during sintering. Pure alumina surface layer (100 mm in thickness) with a hardness value of 19.4 GPa is introduced for tribological applications. The FGM composite shows mainly intergranular fracture beha￾viour and the interfacial region between the graded layer and the alumina surface layer is weak as an indenter-induced crack propagates intergranularly along the boundary between these two phases. The results presented in this work suggest that the overall FGM produced could be an ideal micro-component for room and high temperature applications where ‘‘damage-tolerant’’ behaviour and high wear resistance are required. Experimental work is being conducted at present in order to determine the thermomechanical properties of the FGM. Fig. 4. SEM micrographs of the fracture surfaces of (a) Al2O3–Y-TZP graded layer, (b) pure alumina surface layer, both showing dominantly inter￾granular fracture behaviour and (c) intergranular propagation of an indenter-induced crack along the graded and surface alumina grain boundary. C. Kaya / Journal of the European Ceramic Society 23 (2003) 1655–1660 1659

C. Kaya/Journal of the European Ceramic Society 23(2003)1655-1660 Acknowledgements 95.40.239-265 Profs. P. Bowen and s. blackburn are acknowledged 12. Kirihara, S, Tomota, Y. and Tsijimoto, T, Application of an for the provision of laboratory facilities in the School of termetallic compound Ti5si3 to functionally graded materials. Sci.Eng.A,1997,240,600-604 Metallurgy and Materials and the IrC in materials pro- 13. Sarkar, P, Datta, S and Nicholson, P.S., Fuctionally graded cessing, respectively. Partial financial support by the Eur- ceramic/ceramic and metal/ceramic opean Commission under the contract number brite deposition. Comp. Part B--Eng, 1997, 28, 49- EURAM. BRPR-CT97-0609 is also acknowledged 14. Moritz, K. Thauer, R. and Muller, E, Electrophoretic deposi- tion of nano-scaled zirconia powders prepared by laser evapora- References Mullite Ceramic Matrix Composites. PhD Thesis, The Uni of Birmingham, UK. June 1999 1. Lin. C. Y. McShane. H. B. and Rawlings, R. D. Extrusion 16. Harbach F and Nienburg. H. Homogeneous functional ceramic rocess for manufacture of bulk functionally graded materials. mposnents through electrophoretic deposition from stable col- Powder Metall. 1996. 39. 219-222 lodal suspensions-lI Beta-alumina and concepts for industrial 2. Rawlings, R. D, Tailoring properties-functionally graded roduction. J. Eur. Ceram. Soc.. 1998. 18. 685-692. naterials. Mater. World. 1995.3. 474-475 17. Kaya, C, Boccaccini, A. R and Chawla, K. K, Electrophoretic 3. Zhang W.F Xi.N.S.. Tao C.H. Han.J C and Du s. Y deposition forming of nickel-coated-carbon-fibre-reinforced bor isting thermal shock behaviours silicate-glass-matrix composites J. A. Ceram. Soc., 2000, 83, Al2O3/Fe functionally graded materials prepared by SHS/PHIP. 1885-1888 J. Mater. Sci. Techno. 2001. 17. 65-66. 18. Kaya, C, Kaya, F, Boccaccini, A. R and Chawla, K. 4. Zhao, C, Vandeperre, L, Basu, B. and Van Der Biest, O. rication and characterisation of Ni-coated carbon fibre nationally graded materials by El umina ceramic matrix composites using electrophoreti Brit Ceram. Trans. 2000.99 284-287 tion. Acta Mater. 2001. 49. 1189-1197 5. Oike, S. and Watanabe, Y, Development of in-situ AF-Al2Cu 19. Negishi, H, Sakai, N, Yamaji, K, Horita, T. and Yoke functionally graded materials by a centrufugal method. Int.J. H, Application of electrophoretic deposition technique Mater. Prod. Tec. 2001. 16. 40-49 oxide fuel cells. J. Electrochemical soc. 2000. 147. 1682-1687 6. Ishibashi, H, Tobimatsu, H, Matsumoto, T, Hayashi, K, Tom 20. Kaya, C, Butler, E. G, Boccaccini, A. R. and Lewis, M. H sia, A. P and Saiz. E, Characterization of Mo-Sio2 functionally graded materials. Metall. Mater. Trans. 2000. 31A. 299-306. reinforced mullite matrix composites from hydrothermally pro- 7. Daniel, M. P, Levin, L. and Frage, N, Graded ceramic pre. cessed mullite precursors In High Temperature Ceramic Matri orms: various processing approaches. Materials Chem Composites (HT-CMC 4), ed. w. KrenkeL, R. Naslain and Physics,2001,67,192-198. H Schneider. WILEY-VCH, Weinheim, Germany, 2001, pp 639- 8. Miyamoto, Y, Kaysser, w.A.Rabin, B. H, Kawasa 21. Anstis. G. R. Chantikul. P. Lawn. B. R. and Marshall. D d Applications, 1999 critical evaluation of indentation technique for measuring materials by electrochemical processing and infiltration: application Soc.1982,64,53-538 to tungsten/copper composites. J. Mater. Sci., 2000, 35, 477-486 Mendelson, I M., Average grain size in polycrystalline ceramics. 10. Koizumi, M, FGM activities in Japan. Compos. Part B. Eng J.Am. Ceran.Soc.1969,52,443446 23. Morgan, P. E. D. and Marshall, D. B, Ceramic I1. Mortensen, A. and Suresh, S, Functionally graded metals and monazite and alumina. J. Am. Ceram. Soc. 1995. 78. 1553-

Acknowledgements Profs. P. Bowen and S. Blackburn are acknowledged for the provision of laboratory facilities in the School of Metallurgy and Materials and the IRC in materials pro￾cessing, respectively. Partial financial support by the Eur￾opean Commission under the contract number BRITE￾EURAM, BRPR-CT 97-0609 is also acknowledged. References 1. Lin, C. Y., McShane, H. B. and Rawlings, R. D., Extrusion process for manufacture of bulk functionally graded materials. Powder Metall., 1996, 39, 219–222. 2. Rawlings, R. D., Tailoring properties—functionally graded materials. Mater. World, 1995, 3, 474–475. 3. Zhang, W. F., Xi, N. S., Tao, C. H., Han, J. C. and Du, S. Y., Microstructure and resisting thermal shock behaviours of TiC– Al2O3/Fe functionally graded materials prepared by SHS/PHIP. J. Mater. Sci. Technol., 2001, 17, 65–66. 4. Zhao, C., Vandeperre, L., Basu, B. and Van Der Biest, O., Cylindirical Al2O3/TZP functionally graded materials by EPD. Brit. Ceram. Trans., 2000, 99, 284–287. 5. Oike, S. and Watanabe, Y., Development of in-situ Al–Al2Cu functionally graded materials by a centrufugal method. Int. J. Mater. Prod. Tec., 2001, 16, 40–49. 6. Ishibashi, H., Tobimatsu, H., Matsumoto, T., Hayashi, K., Tom￾sia, A. P. and Saiz, E., Characterization of Mo–SiO2 functionally graded materials. Metall. Mater. Trans., 2000, 31A, 299–306. 7. Daniel, M. P., Levin, L. and Frage, N., Graded ceramic pre￾forms: various processing approaches. Materials Chemistry and Physics, 2001, 67, 192–198. 8. Miyamoto, Y., Kaysser, W. A., Rabin, B. H., Kawasaki, A. and Ford, R. G., Functionally Graded Materials: Design, Processing and Applications. , 1999. 9. Jedamzik, R., Neubrand, A. and Ro¨del, J., Functionally graded materials by electrochemical processing and infiltration: application to tungsten/copper composites. J. Mater. Sci., 2000, 35, 477–486. 10. Koizumi, M., FGM activities in Japan. Compos. Part B. Eng., 1997, 28, 1–4. 11. Mortensen, A. and Suresh, S., Functionally graded metals and metal-ceramic composites: part 1, processing. Int. Mater. Rew., 1995, 40, 239–265. 12. Kirihara, S., Tomota, Y. and Tsijimoto, T., Application of an intermetallic compound Ti5Si3 to functionally graded materials. Mat. Sci. Eng. A, 1997, 240, 600–604. 13. Sarkar, P., Datta, S. and Nicholson, P. S., Fuctionally graded ceramic/ceramic and metal/ceramic composites by electrophoretic deposition. Comp. Part B—Eng., 1997, 28, 49–56. 14. Moritz, K., Thauer, R. and Muller, E., Electrophoretic deposi￾tion of nano-scaled zirconia powders prepared by laser evapora￾tion. CFI, 2000, 77, E8. 15. Kaya, C. Processing and Properties of Alumina Fibre-reinforced Mullite Ceramic Matrix Composites. PhD Thesis, The University of Birmingham, UK, June 1999. 16. Harbach, F. and Nienburg, H., Homogeneous functional ceramic composnents through electrophoretic deposition from stable col￾loidal suspensions—II. Beta-alumina and concepts for industrial production. J. Eur. Ceram. Soc., 1998, 18, 685–692. 17. Kaya, C., Boccaccini, A. R. and Chawla, K. K., Electrophoretic deposition forming of nickel-coated-carbon-fibre-reinforced bor￾osilicate-glass-matrix composites. J. Am. Ceram. Soc., 2000, 83, 1885–1888. 18. Kaya, C., Kaya, F., Boccaccini, A. R. and Chawla, K. K., Fab￾rication and characterisation of Ni-coated carbon fibre-reinforced alumina ceramic matrix composites using electrophoretic deposi￾tion. Acta Mater., 2001, 49, 1189–1197. 19. Negishi, H., Sakai, N., Yamaji, K., Horita, T. and Yokokawa, H., Application of electrophoretic deposition technique to solid oxide fuel cells. J. Electrochemical Soc., 2000, 147, 1682–1687. 20. Kaya, C., Butler, E. G., Boccaccini, A. R. and Lewis, M. H., Processing and characterisation of mullite (NextelTM 720) fibre￾reinforced mullite matrix composites from hydrothermally pro￾cessed mullite precursors. In High Temperature Ceramic Matrix Composites (HT-CMC 4), ed. W. Krenkel, R. Naslain and H. Schneider. WILEY-VCH, Weinheim, Germany, 2001, pp. 639– 644. 21. Anstis, G. R., Chantikul, P., Lawn, B. R. and Marshall, D. B., A critical evaluation of indentation technique for measuring frac￾ture toughness. I—direct crack measurement. J. Am. Ceram. Soc., 1982, 64, 533–538. 22. Mendelson, I. M., Average grain size in polycrystalline ceramics. J. Am. Ceram. Soc., 1969, 52, 443–446. 23. Morgan, P. E. D. and Marshall, D. B., Ceramic composites of monozite and alumina. J. Am. Ceram. Soc., 1995, 78, 1553–1563. 1660 C. Kaya / Journal of the European Ceramic Society 23 (2003) 1655–1660