Date of Degree
MS (Master of Science)
Isabelle L. Denry
Purpose: the purpose of this study was to evaluate the effect of annealing heat treatment on biaxial flexural strength and reliability of 3Y-TZP, sintered at various temperatures.
Materials & methods: 3Y-TZP blanks were pre-sintered at 850°C for 2 hours and sliced into discs (20x1.2mm). Specimens were randomly assigned to 5 groups and subsequently sintered at various temperatures ranging from 1350°C to 1650°C for 2 hours. For each sintering temperature, specimens were divided into three treatment groups. One group (n=20) was left as-sintered as control. One group was air-abraded with 50 micron aluminum oxide powder (n=20). The last group was air-abraded and heat-treated at 1250°C for 20 minutes (n=20). In addition, polished specimens (n=5 per sintering temperature) were prepared to study microstructure, grain size and indentation crack patterns. The mean density was measured by helium pycnometry. The percent porosity was calculated from measured and theoretical density. The mean grain size was determined by the linear intercept method on atomic force micrographs. Crystalline phases were analyzed by x-ray diffraction (XRD). Biaxial flexural strength (BFS) was tested according to ISO standard 6872 using a Universal Testing Machine. Polished specimens were thermally etched, gold coated and Vickers indentations were produced under a 98N load. Indentation crack patterns were analyzed by Scanning Electron Microscopy (SEM) on digital images. The length ratio of trans-granular to inter-granular fracture was determined. Results were analyzed by Kruskal-Wallis test and Turkey's adjustment for multiple comparisons. A 0.05 level of segnificance was used. Reliability was evaluated by weibull analysis.
Results: There was an inverse relationship between density and sintering temperature Spearman rank correlation r = -0.982, p<0.0001). Statistically significant differences were found between all the groups (p<0.0001, exact Kruskal-Wallis test). There was strong evidence of an increase in the percentage of porosity with increasing sintering temperature Spearman rank correlation r=1.00, exact p = 0.017). The mean real grain size increased with sintering temperature. Analysis of XRD data showed that the monoclinic phase as well as ferro-elastic domain switching were present for all air-abraded groups. A small amount of monoclinic phase was also present in groups sintered at 1600°C and 1650°C. In The mean BFS was higher for all air-abraded groups compared to as-sintered or air abraded and heat-treated groups. Air-abraded groups sintered at 1350, 1450, 1550, and 1600°C showed the highest mean BFS (1552.97±200.85, 1502.29±102.36, 1391.4±108.3, 1258.5±114.8 MPa), respectively. The highest Weibull moduli (reliability) were obtained with the heat-treated group sintered at 1550°C (19.8), air-abraded group sintered at 1450°C (17.6) and heat-treated group sintered at 1350°C (15.4). The group sintered at 1650°C was the least reliable, independently of treatment state. Based on the data analysis of both the biaxial flexural strength and Weibull modulus, it was found that, the optimal treatment combination was obtained for the air-abraded group sintered at 1450°C, followed by the air-abraded group sintered at 1550°C. Crack patterns analyses showed that the proportion of trans-granular fracture increased with sintering temperature.
Conclusions: Annealing heat treatment is not recommended after any adjustments, as it does not improve the reliability of the material. Crack patterns and flexural strength are strongly influenced by the crystalline phase composition of the material. Sintering at 1600°C and 1650°C is not recommended due to the corresponding decrease in mechanical properties independently of treatment. Air-abrasion of zirconia sintered at 1450°C or 1550°C led to the best combination of high strength and reliability.
Zirconia is one of the most commonly used dental ceramics that has gained a lot of popularity in the last few years, due to its outstanding mechanical properties that permit its use as framework material in the fabrication of crowns, bridges, or as dental abutments and dental implants.
However, one key factor influencing the clinical performance of dental zirconia is the presence of flaws or microcracks created either at the fabrication stage or during chair side adjustments in the dental office. The presence of these flaws and microcracks is likely to be detrimental to the long-term performance of zirconia dental restorations.
Heat treatment after surface adjustments of zirconia is recommended by some manufacturers to restore the material to its initial stress-free state. However, it is also likely to decrease the strength of dental zirconia. Meanwhile, this annealing heat treatment may also increase the reliability of the material and therefore would be beneficial to the clinical performance.
The purpose of this study was to evaluate the effect of heat treatment on strength properties and reliability of zirconia.
It was found that heat treatment relieved the stresses that were induced by air abrasion, however it decreased both the strength and the reliability of the material. Therefore, we concluded that annealing heat treatment is not recommended after air abrasion. It was also found that firing the material at temperatures above 1550°C compromised its mechanical properties. However, air abrasion alone was found to improve the strength properties of zirconia ceramics fired at either 1450 or 1550°C.
Copyright 2016 Maged Abdelaal