Influence of annealing temperature on the strength of dental glass ceramic materials.
OBJECTIVE
Dental glass ceramic components are always loaded by residual stresses because of the manufacturing process. For a reliable mechanical characterization, residual stress-free specimens are needed. Residual stresses can be removed by an annealing process. The hypothesis of this work was, that the annealing temperature must be individually chosen to reliably remove possible residual stresses without changing the characteristics of the glass ceramic material.


METHODS
Knoop-indented specimens made of the glass ceramic materials Duceram Opaker (Degudent, Hanau, Germany, formerly Ducera, Rosbach, FRG), Empress 1 (Ivoclar Vivadent, Schaan, Liechtenstein), In-Ceram Alumina (Vita Zahnfabrik, Bad Säckingen, Germany), Optec OPC (Jeneric Pentron, Wellingford, USA), and Vita Omega Opaker (Vita Zahnfabrik, Bad Säckingen, Germany) were annealed at various temperatures below and above the respective glass transition temperature. The Weibull strength parameters and the lengths of the indentations before and after annealing were determined.


RESULTS
A strength increase caused by relaxation of residual stresses was found after annealing at elevated temperatures. A local maximum of increased strength was determined after annealing at a temperature of 100K below the glass transition temperature. After annealing at temperatures above the glass transition temperature, microplasticity was observed.


CONCLUSIONS
It is recommended to anneal glass ceramic components that are intended to be mechanically characterized at a temperature 100K below the respective glass transition temperature before testing to determine 'true' mechanical values that are not influenced by possible residual stresses.
Department of Ceramics and Refractory Materials, Institute of Mineral Engineering, RWTH Aachen University, Mauerstrasse 5, D-52064 Aachen, Germany. h.fischer@rwth-aachen.de
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D000538:Aluminum Silicates; D002516:Ceramics; D003080:Cold Temperature; D003776:Dental Porcelain; D003799:Dental Stress Analysis; D006244:Hardness; D006358:Hot Temperature; D008422:Materials Testing; D018583:Pliability; D013314:Stress, Mechanical; D016019:Survival Analysis; D044366:Transition Temperature
D000538:Aluminum Silicates; C095718:Duceram; C075815:IPS-Empress ceramic; C483026:In-Ceram Alumina; C110711:vita omega; D003776:Dental Porcelain; C073101:Optec; C023506:Glass ceramics