Powder Synthesis, Microstructure, and Mechanical Properties in CeO2-Y2O3-ZrO2 Ceramic System

• Main Authors: Lin, Jyung-Dong, 林炯棟
• Other Authors: Duh, Jenq-Gong
• Format: Others
• Language: zh-TW
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/82586733758980685457

博士 === 國立清華大學 === 材料科學與工程學研究所 === 85 === Various powder synthesis methods including ammonia coprecipitation and urea hydrolysis, and subsequent crystallization treatment including hydrothermal crystallization (HTX) and calcination are employed to produce the Ce-Y-TZP ceramic powders. Effects of different organic medium washing including alcohol washing and Acetone-Toluene-Acetone (ATA) washing as well as calcination time and temperature on the characteristics of urea-derived Ce-Y-TZP powders are studied in details. It is observed that the employment of various synthesis processes significantly influences the properties of resultant powders, especially, surface area, particle size, crystallite size, microstrain and phase content in powders. These characteristics are explored with the aid of BET, TEM, sedimentation, and X-ray line profile broadening analysis. Three kinds of line profile analysis are used to study the validity of derived data at various conditions. The Warren-Averbach approach gives a reliable and reasonable crystallite size and microstrain. Furthermore, according to the martensitic characteristics and the results of X-ray line profile analysis, the microstructural evolution of tetragonal (t) and monoclinic (m) phases in t->m transformation can be probed. The variations in crystallite size and microstrain reveal the evolution of t->m transformation in zirconia. The presence of m plates causing the formation of partially transformed t crystals reduces the mean size of t crystals. In fact, this t->m transformation could be stress-induced or thermal-induced. In the current study, the former is through ball milling, while the latter includes the as-sintered surface of highly transformable Ce-Y-TZP and thermal ageing of low content ceria- and yttria-doped zirconia. For stress-induced t->m transformation, the ball milling affects the mean sizes of t and m crystals and the microstrain m phase. On the other hand, the preferred t->m transformation and the decrease of t crystallite size is revealed on the as-sintered surface of 5.5 mol% CeO2- 2 mol% YO1.5-ZrO2 as the sintering temperature is higher than 1300℃. It is thought that this t->m transformation occurs at the heating stage of temperature above 1100℃ due to thermal-stress resulted from anisotropic thermal expansion. In the thermal ageing, both ultrafine yttria- and ceria-doped zirconia powder pellets exhibit isothermal t->m transformation after aged at 900℃ for various times. It is argued that a crystallite size effect, rather than the dopant valence, dictates the occurrence of the t->m transformation in ultrafine zirconia powders. The change in crystallite sizes of both t and m phases during ageing depends significantly on the amount of stabilizer, ageing time and mechanism of t->m phase transformation. However, the change of microstrain in both t and m phases is related to the amount of stabilizers present and the matrix constraints. In further considering the monoclinic content in powders, the final phases of zirconia powders depends on the synthesis method employed, washing organic medium, calcination temperature, and the amounts of stabilizer present. The monoclinic content in the powders and the t crystallite size decreases with increasing stabilizer content, while the microstrain of t crystals remains unchanged. The monoclinic content of powders is related to the transformability of t phase with respect to As (the starting temperature of m->t transformation) temperature or tetragonality. It appears that there exists a proportional relation between them. The use of urea hydrolysis followed by ATA washing can produce a finer and more sinterable Ce-Y-TZP powder as compared to that washed by alcohol. Although the raise of calcination temperature (800℃) increase the particle size, it, however, effectively enhances the sintered density of urea-derived Ce-Y-TZP. Finally, the correlation among compositions, grain size, Vickers hardness, fracture toughness, and fracture strength of Ce-Y-TZP are also probed. On the basis of OM observation and the dependence of crack length on indentation load, the crack system of Ce-Y-TZP belongs to Palmqvist crack under loads≦50kg. The hardness and fracture toughness of the as-sintered samples are evaluated by Vickers indentation method. Eleven toughness equations are compared under various loads of 5~50kg. An attempt to understand the variation ofKic value derived from different equations is carried out. The transformability of t phase plays an important role in the toughness of Ce-Y-TZP, which is closely related to Ms temperature and tetragonality. Besides, the toughening mechanisms operated in Ce-Y-TZP is discussed. In fact, the grain size and these mechanical properties are the function of compositions, namely, the contents of ceria and yttria. The hardness of Ce-Y-TZP ceramics increases with increasing mol% Y01.5, but this trend is depressed with the increase of Ce02 content. The regime which lies within compositions of 3.5-5.5 mol% Ce02 and 3.5-5 mol% Y01.5 exhibits the maximum bending strength. Similarly, a maximum toughness regime lies around compositions of 5.5 mol% CeO2 and 2 mol% YO1.5. For thermally ageing of Ce-Y-TZP ceramics, a boundary line to distinguish the stable t phase and unstable t phase is proposed. According to the outcome of the present study, it is demonstrated that the optimum compositions in Ce-Y- TZP ceramic system exist in the composition ranges of 6.5-7 mol% Ce02 and 2~3 mol% Y01.5.