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)00370-9Al2O3–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)