29 Micromechanics-based Study on Thermo-mechanical Behavior of ZrO 2 /Ti Functionally Graded Materials HIDEAKI TSUKAMOTO * , YOSHIKI KOMIYA, HISASHI SATO and YOSHIMI WATANABE Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi, 466-8555, Japan 1. INTRODUCTION Ti-ZrO 2 composites are to be high-temperature materi- als applied to aerospace and automobile structures. ZrO 2 has superior thermal and mechanical properties, which satis- fes thermal barrier functions. One of superior mechanical properties of ZrO 2 is high fracture toughness due to stress- induced transformation from tetragonal (t-) ZrO 2 to mono- clinic (m-) ZrO 2 under some stress conditions, which leads to enhancement of fracture toughness of other ceramics and ceramic matrix composites [1]. It is also stated that me- chanical properties of ZrO 2 can be enhanced by incorporat- ing Ti. Weber et al. [2] reported that sintered ZrO 2 crucibles containing 15 at% Ti showed superior strength and thermal shock resistance. Arias [3] also found that the thermal shock resistance and strength of ZrO 2 with 15 mol % Ti were in- creased due to the inhabitance of grain growth of ZrO 2 by Ti during sintering processes at 1850°C. In their research, they concluded that these phenomenon were related to dis- solution of oxygen atom into Ti to form titanium oxides without taking into account possible reactions between Ti and ZrO 2 . Functionally graded materials (FGMs) are advanced multi-phase composites that are engineered to have a smooth spatial variation of material constituents. This variation re- sults in an inhomogeneous structure with smoothly varying thermal and mechanical properties [4,5]. In the past decade, ZrO 2 /Ti FGMs have been of high potential as thermal barri- er coating (TBC) structures in aerospace industries. Several studies have been conducted investigating the phase trans- formation of ZrO 2 caused by thermal stresses generated in FGMs [6,7]. Teng et al. [7] showed that only α-Ti, tetragonal (t-) ZrO 2 , and monoclinic (m-) ZrO 2 were found in various Ti-ZrO 2 composites after annealing from 1400 to 1650°C. It was reported that the volume fraction of m-ZrO 2 increased with Ti content in Ti-ZrO 2 composites, while the interfacial stresses, arising from plastic deformation of Ti and thermal expansion mismatch of Ti and ZrO 2 , were driving forces for phase transformation from t-ZrO 2 to m-ZrO 2 . The interfacial reaction between Ti and ZrO 2 in FGMs has been also studied so far [8]. Thermodynamic analysis is essential for chemi- cal design of Ti-ZrO 2 composites [8,9]. It was reported that ABSTRACT The aim of this study is to investigate thermo-mechanical response of ZrO 2 /Ti function- ally graded materials (FGMs) fabricated by spark plasma sintering (SPS) based on a mean-field micromechanics model, which takes account of micro-stress relaxation due to interfacial diffusion between ceramic and metal phases as well as creep of both phases. A resistance to cyclic thermal shock loadings of FGMs with different composi- tional gradation patterns including Ti-rich, linear and ZrO 2 -rich gradation patterns has been investigated. The results demonstrate that Ti-rich FGMs show superior proper- ties among the tested FGM samples. Mean-field micromechanics-based examinations reveal that the range and ratio of thermal stresses in ZrO 2 surface layers in FGMs can affect cyclic thermal shock fracture behaviour but not mean thermal stresses. Creep of ZrO 2 have a large influence on dependence of the range and ratio of thermal stresses in ZrO 2 surface layers on compositional gradation patterns in the FGMs. © 2014 DEStech Publications, Inc. All rights reserved. KEYWORDS mean-field micromechanics functionally graded materials (FGMs) spark plasma sintering (SPS) Ti-ZrO 2 composites *Corresponding author. Email: tsukamoto.hideaki@nitech.ac.jp © 2014 DEStech Publications, Inc. All rights reserved. doi:10.12783/issn.2168-4286/2.1/Tsukamoto