An Investigation of the Mechanical Properties of Different Types of Zirconia

Objectives: The mechanical properties of any zirconia restoration have an important impact on its serviceability and its lifespan. This study aimed to investigate the biaxial flexural strength and hardness of three different types of zirconia processed by two methods, simulating the production of blanks by either (i) uniaxial pressing alone or (ii) uniaxial in combination with cold isostatic pressing.
Methods: Three types of zirconia powders were used in this study; a full contour zirconia, Zpex, and two different grades of conventional zirconia, TZ-3Y-E and TZ-3YS-E (TOSOH, Japan). The three materials were processed by two methods; uniaxially pressing (UP) or uniaxially + cold isostatically pressing (UPCIP). All the groups were then sintered according to the manufacturers’ instructions. The biaxial flexural strength of the six groups (n=20 discs, 1,2±0,2*12±4mm) corresponding to each combination was measured according to ISO standard 6872:2008. For the Vickers microhardness test, six groups of 5 discs each were subjected to a load of 3N for 5Sec and the VHN measured. Following normality testing, statistical analysis was carried out using one-way ANOVA, followed by Tukey's HSD test (p≤0.05) and Independent sample T test.
Results: There was a statistically significant increase in mean strength (p≤0.05) for Zpex and TZ-3YE-following cold isostatic pressing, but not for TZ-3YS-E. The hardness results showed an increase in mean hardness (p≤0.05) for Zpex and TZ-3YS-E but not for TZ-3YE.
Conclusions: The ‘green body’ processing method had an important impact on the mechanical properties of the sintered zirconia. UPCIP could enhance mechanical properties; however this was not true for all starting powders. Care must be given when selecting zirconia powders with an emphasis placed on the processability of the green body during uniaxial pressing.