Strength of Porcelain and Zirconia Submitted to Different Cooling Methods

Objectives: To investigate the three-point flexural strength of monolayer and bilayer porcelain/zirconia (Y-TZP) bars submitted to three cooling methods.

Methods: Forty-five specimens were made for each following design groups: (PM)monolithic specimens of veneer porcelain Vita VM9 (Vita, Germany); (ZM)monolithic specimens of zirconia ZiHP (ProtMat, Brazil); (PB)bilayer specimens zirconia/porcelain with porcelain in lower surface; (ZB)bilayer specimens porcelain/zirconia with zirconia in lower surface. Each group was cooled by three different methods after porcelain sintering: slow – specimens were cooled inside the turned off furnace; normal – specimens were removed from the furnace and cooled in ambient air at room temperature; and fast – specimens were removed at about 900^oC from the furnace and cooled by compressed air for 10sec. Specimens were polished and tested in water at 37^oC (n=15) in a universal test machine. Maximum load at fracture was recorded and equations for simple and composite structures were used to calculate the flexural strength. Results were analyzed by one-way ANOVA (p<0.05) and Tukey test separately for each design.

Results: Data are presented in Table 1.

Table 1. Mean of flexural strength, standard deviation, coefficient of variance, significance level and Tukey test.

GROUPS	COOLING	AVERAGE	STDEV	COVAR	p	TUKEY
PM	SLOW	74.4	9.0	12.1%	0.582	ns
	NORMAL	75.4	7.7	10.2%		ns
	FAST	77.9	11.1	14.2%		ns
ZM	SLOW	835.5	129.9	15.5%	0.541	ns
	NORMAL	873.0	140.1	16.1%		ns
	FAST	817.1	95.2	11.6%		ns
PB	SLOW	49.3	10.1	20.5%	0.002	B
	NORMAL	56.1	11.0	19.6%		AB
	FAST	67.5	16.7	24.8%		A
ZB	SLOW	572.5	136.5	23.8%	0.066	ns
	NORMAL	590.6	117.4	19.9%		ns
	FAST	691.3	177.8	25.7%		ns

Conclusions: Different cooling methods affected only the flexural strength of bilayer specimens with porcelain in lower surface. The complex residual thermal stresses generated in bilayer specimens might be associated to thermal tempering stress, which probably did not occur on slow cooling method.