Journal of Dental Research, Vol 70, 131-136, Copyright © 1991 by International & American Associations for Dental Research Online Journals
Influence of initial flaw size on crack growth in air-tempered porcelain
K. J. Anusavice, A. Gray and C. Shen
Department of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville 32610-0446.
Tempering of dental porcelain by forced convective cooling in air has been
shown previously to inhibit the sizes of cracks induced in porcelain
surfaces. However, the depth of compressive stress that is produced by
tempering of feldspathic porcelain has not been determined. It can be
assumed that the potential of tempered specimens to inhibit crack formation
is reduced as the flaw size exceeds some critical value, since the
compressive stress decreases with distance from the surface. The objective
of this study was to test the hypothesis that the effectiveness of
tempering stress in porcelain decreases with the increasing size of initial
surface flaws. Body porcelain discs, 16 mm in diameter and 2 mm in
thickness, were tempered (T) by forced convective cooling in air from an
initial temperature of 982 degrees C at a pressure of 0.34 MPa for 90 s.
Nontempered control discs (NT) were slowly cooled by termination of power
to the furnace. The half crack-lengths (c) of surface cracks induced by a
Vickers microhardness indenter at loads ranging from 2.0 to 39.2 N were
determined initially and over a period of 24 h. The initial half
crack-lengths, co, for the tempered specimens were significantly smaller (p
less than 0.05) than the corresponding values for the slow-cooled
specimens. The mean value of the stress corrosion susceptibility
coefficient (n), which is inversely related to the slope of log crack size
vs. log time, was only slightly smaller for the NT specimen groups (15.3),
compared with the mean value for the T groups (17.1).(ABSTRACT TRUNCATED AT
250 WORDS)
