Yoshitaka Kubota

Thermal shock behavior of Y2O3-partially stabilized zirconia

The thermal shock behavior of 3mol% Y2O3-partially stabilized zirconia with average grain size of 0.4μm (Z3Y-I) and 1.0μm (Z3Y-II) has been studied by the water quench method. Twenty three specimens of rectangular cross-section bar (3 by 3 by 45mm) were quenched at 300°, 350° and 400deg;C temperature differentials (ΔT) and their retained strength (σf) were plotted using the Weibull distribution function and doubly exponential distribution function. The retained strength distribution at ΔT=350°C are similar to the original strength distribution. On the other hand, the distribution at ΔT=350° and 400°C showed a shoulder at F=0.458 and 0.875, respectively, They are the initiation points of thermal shock damage, Assumming the surface heat transfer coefficient (h) of 0.19 and 0.4cal/cm2·°C·s, calculated stress intensity factors KIs for crack initiation by water quench were 73-75% and 91-92% of the critical stress intensity factor KIC for Z3 Y-I, and 69-70% and 86-88% for Z3 Y-II, respectively. A σf-δT curve for a certain failure probability level F showed instantaneous decline at a critical temperature differential δTc. High failure probability level F resulted in a high critical temperature diffrential δTc. Estimated δTcs are 310°, 360° and 400°C for F=0.2, 0.5 and 0.9, respectively.

Fatigue behavior of 2 and 4 mol% Y2O3-containing tetragonal zirconia polycrystals (Y-TZP)

The fatigue behavior of 2 and 4mol% Y2O3-containing tetragonal zirconia polycrystals (Y-TZP) was studied by measuring their fracture strength σf as a function of stressing rate σ (dynamic fatigue technique). The average grain size of 2mol% Y2O3-containing tetragonal zirconia polycrystals (Z2Y) was 1.0-1.5μm, while Z4Y was composed of 2.0-2.5μm grains in the most part and above 3μm grains in the other part. Stress-deflection relationship of Z2Y at 20°C and Z4Y at 20° and 250°C showed elastic behavior prior to failure. Stress-deflection relationship of Z2Y at 250°C showed elastic behavior at the stressing rate over 3.02MPa/s, too. On the other hand, stress-deflection relationship of 0.301MPa/s (crosshead speed 0.005mm/min) showed inelastic behavior. The logσf-logσ relationships of Z2Y at 20° and 250°C were linear. Their crack growth parameter Ns were 67.3 at 20° and 8.7 at 250°C. The logσf-logσ relationship of Z4Y at 20°C was linear, too. The N was 45.3. On the other hand, the fracture strength of Z4Y at 250°C increased below the stressing rate of 3.02MPa/s and kept a constant value over 3.02MPa/s, if stressing rate increased. The N value obtained from the linear relationship below 3.02MPa/s was 4.7. The fracture surface of Z2Y and Z4Y at 250°C tested at lower stressing rates showed fatigue fracture surface in the vicinity of the tensile surface side. The area of the fatigue fracture surface in Z2Y at stressing rate of 3.01MPa/s and in Z4Y at 0.302MPa/s occupied about one-fourth and one-half the fracture surface, respectively. Many cracks were observed on the tensile surface of Z2Y which showed inelastic behavior. The fracture surface of those specimens showed larger fatigue fracture surface than the specimens which showed the elastic behavior.

Thernal fatigue behavior of Y2O3-containing tetragonal zirconia polycrystals (Y-TZP)

The theremal fatigue behavior of 3mol% Y2O3-containing tetragonal zirconia polycrystals (Y-TZP) with average grain sizes of 0.4μm (Z3Y-I) and 1.0μm (Z3Y-II) was studied by quenching into water bath at 20°C. The retained strength distribution of Z3Y-I subjected to multiple cycle quench at ΔT=350° and 300°C showed a shoulder at a certain F level, which corresponds to the initiation point of thermal shock damage. The F level increased with increasing thermal shock cycle number. For Z3Y-I at ΔT=350°, the proportion of damaged specimens was half for 1 cycle quenching and most for 10 cycles. At ΔT=300°C, the proportion of damaged specimens was none for 1 cycle, a few for 8 cycles and half for 60 cycles, but at 250°C, no crack was generated at 90 thermal shock cycles. On the other hdnd, the strength distribution of Z3Y-II subjected to multiple cycle quench at 350°C was similar to that of Z3Y-I at ΔT=350°C, but those of Z3Y-II at ΔT=300° and 250° Cwere followed by decreasing standard deviation, and the difference between maximum and minimum strengths was within about 3.1% for the thermal shock cycles above fifteen. The average strength of Z3Y-II subjected to 75 thermal shock cycles at ΔT=300°Cand 90 cycles at ΔT=250°C were about 60 and 76% of the original strength before water quench, respectively. The thickness of zone transformed from tetragonal to monoclinic phase by chemical reaction with hot water was 103μm for 30 thermal shock cycles, 155μm for 50 cycles and 356μm for 75 cycles at ΔT=300°C, and 36μm for 60 cycles and 211μm for 90 cycles at ΔT=250°C.

Crack configuration beneath vickers indentation and fracture toughness evaluation in Y2O3-partially stabilized zirconia

Crack configurations beneath Vickers indentation in 3mol% Y2O3-partially stabilized zirconia ceramics with average grain sizes of 0.4μm (Z3Y-I) and 1.0μm (Z3Y-II) were examined and their fracture toughness was measured. Crack configurations were assessed by the dye penetrant technique and observations of the flaw distribution on the fracture surfaces by using scanning electron microscopy. The crack configurations of partially stabilized zirconia were found to be of Palmqvist type even at a high indentation load (50kg). However, the Palmqvist type cracks were well-developed in the direction of indentation depth and appeared to be different in shape from the typical Palmqvist type cracks like those observed in WC-Co. Fracture toughnesses KIC measured by the controlled surface flaw technique and the chevron notch technique were 7.13-7.26MPa·m1/2. KIC values were also measured by the indentation fracture technique using Niihara et al. equation, Lankford equation and Evans equation based on polynominal curve fitting. KIC values obtained by using Niihara et al. equation were the lowest and those obtained by Lankford equation were the largest in three equations. The KICs obtained by using Evans equation were intermediate between them, and 6.58MPa·m1/2 for Z3Y-I and 7.25MPa·m1/2 for Z3Y-II.