Journal of Dental Research, Vol 70, 1009-1013, Copyright © 1991 by International & American Associations for Dental Research Online Journals
Effect of thermal tempering on strength and crack propagation behavior of feldspathic porcelains
K. J. Anusavice and B. Hojjatie
Department of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville 32610-0446.
The objective of this study was to test the hypothesis that tempering
stress can retard the growth of surface cracks in layered porcelain discs
with variable levels of contraction mismatch. Porcelain discs, 16 mm in
diameter and 2 mm thick, were prepared with a 0.5-mm-thick layer of opaque
porcelain (O) and a 1.5-mm-thick layer of body porcelain (B). The materials
were selected to produce contraction coefficient differences, alpha O-alpha
B, of +3.2, +0.7, -0.9, and -1.5 ppm/degrees C. Body porcelain discs with a
thickness of 2 mm were used as the thermally compatible control specimens
(delta alpha = 0). The discs were fired to the maturing temperature of body
porcelain (982 degrees C) and were then subjected to three cooling
procedures: slow cooling (SC) in a furnace, fast cooling (FC) in air, and
tempering (T) by blasting the surface of the body porcelain with compressed
and dried air for 90 s. The dimensions of cracks induced by a Vickers
microhardness indenter under a load of 4.9 N were measured at baseline and
six months after indentation at 80 points along diametral lines within the
surface of body porcelain. In addition, biaxial flexure tests were
performed to determine the influence of mismatch and tempering on flexure
strength. The results of ANOVA indicate that crack dimensions were
influenced significantly by the interaction of cooling rate and contraction
mismatch (p less than 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)
