The purpose of this study was to evaluate the subcritical crack growth parameters (SCG) through constant stress-rate testing and the critical time to transformation (CTT) for stress-induced toughening of a Y-TZP ceramic.
Pre-sintered ceramic plates were cut from yttrium-stabilized zirconium oxide ceramic blocks. One surface was finished to create a controlled surface flaw population. The samples were then sintered according to the manufacturer’s recommendation. The width and thickness were measured and recorded. Experimental groups consisted of five constant stress-rate groups for SCG determination and six critical time to transformation groups. Each group contained approximately 10-15 samples. For constant stress-rate testing, biaxial flexural strength was tested in air and oil using a ball on ring-of-balls fixture on a Universal Instron Testing Machine with crosshead speeds of 1.0, 0.1, 0.01 and 0.001 mm/min. The biaxial flexural strength data was analyzed using Weibull statistics. The stress rate for each specimen was determined using linear regression analysis. SCG parameters n and D were calculated and compared. For critical time to transformation testing, control non-stress groups were stored and pre-stress groups were loaded 80% to maximum/failure in water. A ball on a ring-of-balls fixture on a static load apparatus was used. Time periods of two-weeks, two-days and two-seconds were evaluated. All specimens were removed and loaded to failure in water on a Universal Instron testing machine with a crosshead speed of 0.1mm/min. Fractured samples were examined utilizing SEM. Additional samples were stressed on the static load apparatus for periods of 1 hour, 2 hours, 2 days, 4 days and 2 weeks and were analyzed by X-ray diffraction (XRD). The mass fraction of monoclinic phase was determined. Statistical significance was determined using ANOVA, Tukey’s test and the JMP statistical analysis package.
Results showed no significant difference between SCG groups, suggesting that this zirconia (Y-TZP) ceramic material is not susceptible to subcritical crack growth. The Weibull fatigue constants (m and σ0) and SCG parameters (n and D) agree with other studies. Results for CTT testing showed a difference in mean biaxial flexural strength of approximately 10% for the pre-stress and non-stress two-day and two-week groups. Using these testing parameters, it was concluded that the critical time to transformation for stress-induced toughening is greater than two seconds and less than two days. XRD results showed significant amounts of tetragonal to monoclinic phase transition after only two hours of stress application. Indicating that the window of critical time to stress-induce transformation occurs in as little as one to two hours. The most important factor tested affecting the CTT is the experimental condition of pre-stress.