Heat-pressed ionomer glass-ceramics. Part II. Mechanical property evaluation.
OBJECTIVE
A series of ionomer glasses based on the formula: 4.5SiO2-1.5P2O5-(X)Al2O3-4.5CaO-0.5CaF2, were investigated where X was varied from 3.0 to 1.5 in order to develop heat pressable dental ceramics.


METHODS
The glasses were heat-pressed and then subjected to different heat-treatment cycles. The mechanical properties of the glass-ceramics were investigated, specimens were tested for hardness, fracture toughness (indentation method) and flexural strength (biaxial method).


RESULTS
Good mechanical properties were obtained for heat-treatments at lower temperatures (i.e. 1150 degrees C). At intermediate heat-treatment temperatures the glass-ceramics were highly crystalline which did not favor the mechanical properties. There appears to be an inverse relationship between fracture toughness and flexural strength. High fracture toughness values of 2.7 (0.4) MPam0.5 were produced for the X = 2.8 glass heat-treated for 8 h at 1150 degrees C, the flexural strength was lowest for this heat-treatment. High flexural strengths of 194.4 (75.0) MPa were obtained by heat-treating the same glass for 1 h at 1150 degrees C. Increasing the hold time increases crystal size thereby increasing the extent of microcracking in the glass-ceramic thus lowering the flexural strength. Microcracks appear to increase the fracture toughness of the glass-ceramics probably by a crack termination mechanism.


CONCLUSIONS
Good flexural strength and high fracture toughness are attainable in this system, but appear to be mutually exclusive in the materials studied. With further investigation this system could provide clinically useful materials.
Gorman
Catherine M
CM
Department of Restorative Dentistry and Periodontology, School of Dental Science, Trinity College Dublin, Lincoln Place, Dublin 2, Ireland. cgorman@dental.tcd.ie
Hill
Robert G
RG
D000180:Acrylic Resins; D000537:Aluminum Oxide; D000538:Aluminum Silicates; D000704:Analysis of Variance; D001031:Apatites; D017610:Calcium Compounds; D002124:Calcium Fluoride; D002516:Ceramics; D003460:Crystallization; D003776:Dental Porcelain; D003799:Dental Stress Analysis; D004548:Elasticity; D006244:Hardness; D006358:Hot Temperature; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D010087:Oxides; D044367:Phase Transition; D017553:Phosphorus Compounds; D018583:Pliability; D012822:Silicon Dioxide; D013997:Time Factors
D000180:Acrylic Resins; D000538:Aluminum Silicates; D001031:Apatites; D017610:Calcium Compounds; D010087:Oxides; D017553:Phosphorus Compounds; C015897:glass ionomer; D003776:Dental Porcelain; C012500:phosphorus pentoxide; D012822:Silicon Dioxide; C023506:Glass ceramics; C016538:lime; D000537:Aluminum Oxide; D002124:Calcium Fluoride