Effect of cooling protocol on mechanical properties and microstructure of dental veneering ceramics

• Published in: Dental materials Vol. 35; no. 10; pp. 1498 - 1505
• Main Authors: Tanaka, Carina B., Ahmad, Nur Hanani Binti, Ellakwa, Ayman, Kruzic, Jamie J.
• Format: Journal Article
• Language: English
• Published: England Elsevier Inc 01.10.2019; Elsevier BV
• Subjects: Aluminum Silicates; Ceramics; Cooling; Cooling effects; Cooling protocol; Crack initiation; Crystal growth; Crystallization; Crystals; Density; Dental Porcelain; Dental restorative materials; Dental Stress Analysis; Dental Veneers; Dentistry; Fracture mechanics; Fracture toughness; Glass; Heat treatments; Materials Testing; Mechanical properties; Microcracks; Microscopy, Electron, Scanning; Microstructure; Microstructure characterization; Modulus of rupture in bending; Porcelain; Potassium aluminum silicates; Scanning electron microscopy; Surface Properties; Temperature; Variance analysis; Veneering; Veneering ceramic; Zirconia; Zirconium dioxide; Mechanical properties; Cooling protocol; Dental restorative materials; Microstructure characterization; Veneering ceramic
• ISSN: 0109-5641; 1879-0097
• DOI: 10.1016/j.dental.2019.07.011

•Mechanical properties of porcelain can be improved with slow cooling protocol.•Strengthening is attributed to enhanced leucite formation during slow cooling.•Property enhancement is partially offset by higher microcrack densities.•Achieving fine grained, well distributed leucite should provide further improvements. Understand how cooling protocols control the microstructure and mechanical properties of veneering porcelains. Two porcelain powders were selected, one used to veneer metallic frameworks (VM13) and one for zirconia frameworks (VM9). After the last firing cycle, the monolithic specimens were subjected to two cooling protocols: slow and fast. Flexural strength (FS) was evaluated by three-point beam bending and fracture toughness (KIC) was evaluated by the single-edge V-notch beam (SEVNB) method. Scanning electron microscopy (SEM) was performed to determine the leucite crystal volume fraction (%), particle size, and matrix microcrack density. The results were compared by analysis of variances (ANOVA) and Tukey’s multiple comparison test. The mechanical properties were significantly (p<0.05) higher for the VM13 porcelain (FS=111.0MPa, KIC=1.01MPa.√m) compared to VM9 (FS=79.6MPa, KIC =0.87MPa.√m) regardless of cooling protocol due to ∼250% higher volume fraction of leucite crystals. The slow cooled VM13 and fast cooled VM9 resulted in the highest and lowest mechanical properties, respectively, while the VM9 slow cooled properties were similar to the VM13 fast cooled. The SEM revealed that the slow cooling significantly increased the volume fraction of leucite crystals by 33–41 %. Across both porcelains, a significant linear correlation between both mechanical properties (strength and toughness) and leucite crystal content was found. Slow cooling was also associated with increased crystal growth resulting in more matrix microcracking. Controlled crystallization using slow cooling can be applied as a means of strengthening dental porcelains. However, the benefits of slow cooling may be partially offset by increasing the microcrack density in the glass matrix. To achieve the maximum benefit of slow cooling, it is recommending to develop heat treatments to produce porcelain with fine-grained and homogenously dispersed leucite crystals to achieve minimal glass matrix microcracking.