W.Z. Zhu bainite transformation, which is quite different where Im(111)and Im(1l1)refer to the relative peak from results of their previous studies. Heuer et strengths of the (111)and(111)planes of the al. pointed out that the t-m transformation in monoclinic phase, respectively, and Ic(lll)repre Y-TZP ceramics displays both athermal and iso- sents the relative peak strength of the (111) plane thermal kinetics; they are both stress-assisted mar- of the tetragonal or cubic pha ple fo tensitic transformations. They viewed the athermal TEM inspection was prepared by the conventional character usually found in a cooling transforma- procedure of grinding, dimpling and ion-thinning tion as due to rapid isothermal transformation of to perforation, followed by coating with a thin film the most easily nucleated regions at a succession of of amorphous carbon to avoid charging and Temperatures. In other words, a range of activation JEM100 type microscopy was operated with energies exists for such martensitic transforma- 120 v as the accelerating voltage I data has been obtained to confirm this deduction. The intent of the present paper is to interpret the character of the 3 RESULTS AND DISCUSSION kinetics of the t-m transformation in ZrO2 (3 mol%Y2O3) ceramics, which consist of cubic, tet ragonal and monoclinic phases, and to propose an 3. 1 Thermal expansion curves of ZrO2 (2 mo/%y203 appropriate mechanism and ZrO2 3 mo/%Y2O3)specimens Figure I shows the thermal expansion curves of 2 EXPERIMENTAL PROCEDURES ZrO2(2 mol%Y2O3) and ZrO2(3 mol%Y2O3) ceramics obtained at a heating and cooling rate of Powders of zirconia containing 2 mol% and 10 C/min. Specimen size initially increases with 3 mol% yttria in solid solution, respectively, with increase in temperature, then drastically decreases purity higher than 99% and average diameter of as the temperature is increased above the a, point about 0-1 um, were cold-pressed into pellets of (starting temperature of the mt transformation), 24 mmx6mmx6mm in size under a pressure of finally it increases again as the temperature is fur- 300 MPa, followed by pressureless sintering at ther increased above the Af point (ending tem 1 600C with 5 h holding time. a relative density of perature of the m+t transformation). It is more than 98% could be obtained. Thermal apparent that at temperatures above the Ar point expansion analysis experiments were performed on only t-phase exists in the specimen. Similarly, the a Perkin-Elmer 7 Series Thermal Analysis System specimen size initially decreases linearly with with polished specimens, machined to be decrease in temperature until the Ms point(starting 4 mmx4mmx3 mm in size. The point at which a temperature of the t-m transformation), below tangential line deviates from the thermal expansion which it increases significantly due to the dilata curve was defined as the transformation tempera- tional nature of the t-m transformation. When ture. The relative amount of monclinic phase was the temperature is further decreased below the m calculated using the formula proposed by Garvie, point(ending temperature of the t-m transfor according to the X-ray diffraction results mation), thermal expansion curves reverse to the linear shrinkage section until ambient temperature (l11+lm(1l It should be noted that, in the present study, Im(111)+Im(111)+lct(111) ZrO2 (2 mol%Y2O3) ceramics were sintered the single t-phase region according to the phas .Oh 00500600700 00500600700 Fig. 1. Thermal expansion curves of ZrO2(Y203)ceramics obtained at a heating and cooling rate of 10C/min (a)ZrO2(2 mol%Y2O3);(b)Zro2(3 mol% Y 2O3)36 bainite transformation, which is quite different from results of their previous studies.9 Heuer et aLlo pointed out that the t-m transformation in 3Y-TZP ceramics displays both athermal and iso￾thermal kinetics; they are both stress-assisted mar￾tensitic transformations. They viewed the athermal character usually found in a cooling transforma￾tion as due to rapid isothermal transformation of the most easily nucleated regions at a succession of temperatures. In other words, a range of activation energies exists for such martensitic transforma￾tions. Up to now, no experimental data has been obtained to confirm this deduction. The intent of the present paper is to interpret the character of the kinetics of the t-m transformation in ZrOz(3 mol% Y203) ceramics, which consist of cubic, tet￾ragonal and monoclinic phases, and to propose an appropriate mechanism, 2 EXPERIMENTAL PROCEDURES Powders of zirconia containing 2mol% and 3mol% yttria in solid solution, respectively, with purity higher than 99% and average diameter of about O-1 grn, were cold-pressed into pellets of 24 mmx6mm x6 mm in size under a pressure of 300MPa, followed by pressureless sintering at 1600°C with 5 h holding time. A relative density of more than 98% couid be obtained. Thermal expansion analysis experiments were performed on a Perkin-Elmer 7 Series Thermal Analysis System with polished specimens, machined to be 4 mm x4 mmx 3 mm in size. The point at which a tangential line deviates from the thermal expansion curve was defined as the transformation tempera￾ture. The relative amount of monclinic phase was calculated using the formula proposed by Garvie,’ ’ according to the X-ray diffraction results: M% = I,(lii) +~,(lil) I,(lli) +1nl(lll) +Lt(lll) -0.6 - I I I I IT I 0 100 200 300 400 500 600 700 Temperature (“C) W. Z. Zhu where I,( 11 i) and Im( 111) refer to the relative peak strengths of the (11 i) and (111) planes of the monoclinic phase, respectively, and I& 111) repre￾sents the relative peak strength of the (111) plane of the tetragonal or cubic phase. A sample for TEM inspection was prepared by the conventional procedure of grinding, dimpling and ion-thinning to perforation, followed by coating with a thin film of amorphous carbon to avoid charging and JEMlOO type microscopy was operated with 120 kV as the accelerating voltage. 3 RESULTS AND DISCUSSION 3. I Thermalexpansion curves of ZrQ(2 moi% Y&) and ZrUz(3 mol% Y203) specimens Figure 1 shows the thermal expansion curves of ZrOz(2 mol% Y203) and ZrOz(3 mol% Y203) ceramics obtained at a heating and cooling rate of lO”C/min. Specimen size initially increases with increase in temperature, then drastically decreases as the temperature is increased above the A, point (starting temperature of the m-+t transformation), finally it increases again as the temperature is fur￾ther increased above the Af point (ending tem￾perature of the m-t transformation). It is apparent that at temperatures above the Ar point, only t-phase exists in the specimen. Similarly, the specimen size initially decreases linearly with decrease in temperature until the M, point (starting temperature of the t+m transformation), below which it increases significantly due to the dilata￾tional nature of the t-+m transformation. When the temperature is further decreased below the Mf point (ending temperature of the t-+m transfor￾mation), thermal expansion curves reverse to the linear shrinkage section until ambient temperature. It should be noted that, in the present study, Zr02(2 mol% Y203) ceramics were sintered in the single t-phase region according to the phase g 0.2 g ‘;: 0 3 a -0.2 - 5 6h -0.4- .s z z -0.6 - p! I I MS I I I _ 0 100 200 300 400 500 600 700 Temperature (“Cl Fig. 1. Thermal expansion curves of Zr02(Y203) ceramics obtained at a heating and cooling rate of lO”C/min: (a) Zr02(2mol% Y,O,); (b) Zr02(3 mol% Y20,)