Effect of repeated firing on mechanical properties of different zirconia based ceramic core systems

All ceramic systems have advantages such as esthetics, biocompatibility and colour stability, but their brittleness introduced the usage of zirconia-based ceramics in recent years. The zirconia-based ceramics that demonstrate high mechanical performance and strength increase the reliability of the all ceramic restorations. Indication of all ceramic restoration was restricted with anterior crowns and three unit fixed partial dentures, zirconia core systems made it possible to use all ceramic posterior and long span fixed partial restorations. Unfortunately, current processing technologies cannot make zirconia cores as translucent as natural teeth. To improve the esthetic appearance, the milled cores are veneered with feldspathic ceramic or with ceramics suitable for the zirconia based ceramics. Zirconia cores are very resistant to crack formation. Surface treatments and firing processes performed on the material during the production and veneering process effect the mechanical properties of the zirconia. During the veneering process material is fired usually five times. Firing cycles increase with clinical adaptation and integrity problems. Researches investigated how the repeated firings effect the veneer ceramic but effect of these procedures on core ceramic is still unknown, because of this reason the aim of our study is to investigate the how repeated firing effect the mechanical properties of different zirconia based system frameworks.In our study, 3 different zirconia core systems and 180 zirconia disc samples were used. 60 samples from Zrikonzahn system (Steger, Ahrntal, Italy), 60 samples from Cercon system (DeguDent, Hanau, Germany) and 60 samples from Ceramill system (AmannGirrbach, Koblach, Austria) were prepared. Samples are prepared in disc shape with 15mm radius and 1.4mm thickness. For surface standardization, these zirconia cores were grounded with silicon carbide paper (English abrasives, England) in a sequence of 220, 320, 500, 600, 800 and 1200 grit sizes in grinder-polisher device (Phoenix Beta Grinder/Polisher, Buehler, Germany) under water with 300 rpm/min until discs thickness reduced to 1.2 mm. To mimic the veneering process, the specimens were blasted with 120 µm aluminum oxide particles for 15 seconds with a sandblaster (Microetcher ERC, Danville Eng., CA, USA) at an emission pressure of 0,5 MPa with the nozzle 10 mm from the core surface. Later, their surface roughness were measured (Perthometer M1, Mahr, Germany), according to DIN 4762 standards and obtained standard surfaces. All specimens were ultrasonically cleaned for 3 minutes. Each group was divided into 6 subgroups which contained 10 samples. Subgroups are fired 0, 1, 3, 5, 7 and 9 times respectively according to manufacturer instructions of veneer ceramics which are advised by the manifacturer of zirconia core systems. Ice Keramik firing instruction used for Zirkonzahn, Cercon Ceram Kiss firing instruction used for Cercon and Creation firing instruction used for Ceramill zirconia core system. After the finishing all the preparation of discs of experiment, Universal test machine (Instron, 3345, Instron Corp., Norwood, MA, USA) with a 1 mm/min crosshead speed was used for biaxial flexural strength test. Mean biaxial flexural strength values and standard deviations were calculated. After the biaxial flexural strength test, Vickers microhardness test was performed with ?Vickers? (Buehler, Illınois, ABD) machine applied 9.8 N load for 10 seconds to the each sample for 5 times. Mean biaxial flexural strength values and standard deviations were calculated.Two-way ANOVA was used to appraise whether there was any statistical difference among groups at the 95 % confidence level and series of post hoc tests were used to identify which pairs of groups were different.In the result of our study Zirkonzahn had the highest biaxial flexural strength (1333±114 MPa) and microhardness (1566±159 HVN) values among the all groups. Number of firing did not effect the biaxial flexural strength of Zirkonzahn and Cercon samples significantly. Non-fired group (999±144 MPa) of Ceramill had the highest biaxial flexural strength value and decreased significantly in 3. firing group (659±152 MPa). Biaxial flexural strength values increased respectively during the 5, 7 and 9. firings (respectively; 823±147, 889±148, 889.7±155 MPa). Number of firing did not effect the microhardness of Cercon samples significantly. In Zirkonzahn groups, microhardness values increased significantly in first 5 firings (5.firing; 1720±80 HVN), compared to the non-fired group (1559±103 HVN). Microhardness values decreased again in 7. (1416±78 HVN) and 9. firings (1343±44 HVN). Non-fired group (1503±25 HVN) of Ceramill had the highest microhardness value and decreased significantly in following firing groups.Keywords: Dental ceramic, zirconia, firing, biaxial flexural strength, microhardness.