oJ opaque porcelain-body porcelain discs, with contraction coefficient differentces .... greater rate relative to the cooler, rigid, external region, while compressive ...
Journal of Dental Research http://jdr.sagepub.com/
Influence of Tempering and Contraction Mismatch on Crack Development in Ceramic Surfaces K.J. Anusavice, P.H. Dehoff, B. Hojjatie and A. Gray J DENT RES 1989 68: 1182 DOI: 10.1177/00220345890680070801 The online version of this article can be found at: http://jdr.sagepub.com/content/68/7/1182
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Influence of Tempering and Contraction Mismatch on Crack Development in Ceramic Surfaces K.J. ANUSAVICE, P.H. DEHOFF', B. HOJJATIE, and A. GRAY Department of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville, Florida 32610-0446; and 'Department of Mechanical Engineering and Engineering Science, University of North Carolina at Charlotte, Charlotte, North Carolina 28223
Tempering of glass produces a state of compressive stress in surface regions which can enhance the resistance to crack initiation and growth. The objective of this study was to determine the influence of tempering on the sizes of surface cracks induced within the tempered surfaces oJ opaque porcelain-body porcelain discs, with contraction coefficient different ces (ao- ca) of + 3. 2, + 0. 7, 0.0, 0. 9, and - 1. 5 ppm/'C. We fired the discs to the maturing temperature (982°C) of body porcelain and then subjected them to three cooling procedures: slow cooling in a furnace (SC), fast cooling in air (EC), and tempering (T) by blasting the body porcelain surface with compressed air for 90 s. We used body porcelain discs as the thermally compatible (AAa = 0) control specimens. We measured tile diameters of cracks induced by a microhardness indenter at an applied load of 4.9 N at 80 points along diametral lines within the surface of body porcelain. The mean values of the crack diameters varied from 75.9 pm (Aa - 1. 5 ppm/'C) to 103.3 pm (,Aa = + 3.2 ppm/'C). The results of ANOVA indicate that significant differences in crack dimensions were controlled by cooling rate, contraction mismatch, and their combined effect (p < 0. 0001). Multiple contrast analysis (Tukey 's HSD Test) revealed significantly lower (p
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1
Materials and methods. Feldspathic porcelain formulations with a typical range of thermal contraction coefficients were prepared by a dental ceramics manufacturer. Listed in Table 1 are the opaque and body porcelains used in this study, and the relative contraction differences for the five states of thermal mismatch. We prepared the porcelain specimens as circular discs 16 mm in diameter and 2 mm thick. These discs consisted of a 0.5-mm-thick layer of opaque porcelain and a 1.5-mm-thick layer of body porcelain. The body porcelain control discs were 2 mm thick. We placed the porcelain inside a cylindrical brass mold (approximately 3.5 mm in depth) and condensed it by means of the Vibra II handpiece (J.F. Jelenko and Co., Armonk, NY). Excess moisture was blotted dry. The body porcelain was trimmed flush with the top surface of the mold, and the opaque porcelain was applied manually. The specimens were dried in front of the open door of a Ney Mark IV Digital furnace (J.M. Ney Company, Bloomfield, CT) for ten min, and were then placed inside the furnace at an initial temperature of 6480C. The specimens were subjected to two firing cycles, and in each cycle the furnace temperature was raised to a maximum temperature of 9820C at a heating rate of 550C/ min. The specimens were kept at this temperature for 15 s. In
DISTANCE Fig. 1-Transient stress profiles for a glass plate subjected to a tempering treatment [based on study by Gardon (1980)].
the first firing cycle, the specimens were fired in a vacuum; the second firing cycle was carried out in air. Following the initial firing cycle, the discs were ground through 0.05-pm alumina abrasive to a thickness of 2 mm by means of a Buehler polishing wheel (Buehler Ltd., Lake Bluff, IL). After the second firing cycle, the discs were subjected to one of the three cooling procedures: slow cooling (SC), fast cooling (FC), and tempering (T). We tempered the discs by blasting compressed air directly on them as they were removed from the furnace. A nozzle with a 4-mm diameter (Fig. 2) was placed 20 mm above the disc surface, and compressed air was blasted on each disc at a pressure of 0.34 MPa (50 psi) for 90 s. We prepared three discs of each opaque-body porcelain combination, and of body porcelain only, one for each of the three cooling rates. To minimize the introduction of dust particles
TABLE 1 THERMAL CONTRACTION CHARACTERISTICS OF PORCELAIN SYSTEMS USED FOR EXPERIMENTAL DISCS Body Porcelain Opaque Porcelain Mismatch Type o - 0tB (ppm/0C) OtB (ppm/0C) %O (ppm/0C) (°( - %B) +3.2 11.0 14.2 High Positive +0.7 13.5 14.2 Low Positive 0.0* 13.5 13.5* Zero -0.9 11.0 12.6 Low Negative -1.5 13.5 12.0 Medium Negative for substituted been has opaque porcelain. *Represents the all-body discs wherein body porcelain Downloaded from jdr.sagepub.com by guest on July 10, 2011 For personal use only. No other uses without permission.
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J Dent Res
ANUSA VICE et al.
July
1989
MULTIPLE ORIFICE NOZZLE (4 mm) 0.34 MPa (50 psi)
SUPPORT BASE
INSULATING BLANKET
SAGGERTRAY
20 mm
-
DISC
Fig. 2-Design of tempering apparatus.
At--
20 INDENTATIONS / LINE
16 mm DIAMETER (NOT DRAWN TO SCALE)
[ I
BODY nrtmEE
I
Al .....Zl
1.5 mnI __
, lz z zz Fig. 3-Arrangement of indentations on experimental discs. .
.rtD
I2mm
_
in the surface during the air-blasting procedure, we supported the specimens on a layer of refractory cloth (Thermoz, American Dental Supply, Easton, PA). To determine the temperature vs. time profiles during each cooling technique, we installed a chromel-alumel thermocouple (k type) at the center of a typical porcelain disc, such that the distance between the center of the thermocouple tip and the disc surface was 0.5 mm. The thermocouple was interfaced to the RS2-32 serial port of an IBM PC-XT (IBM Corporation, Boca Raton, FL) microcomputer through an analog-to-digital converter (OMEGA Engineering, Inc., Stamford, CT). A computer program in BASIC language was developed to record the temperature values as a function of time. The time and temperature data were stored on the microcomputer hard disc, and the thermal history for each cooling technique was plotted. For the discs that were slow-cooled (SC) from an initial temperature of 9820C, the ambient temperature of 30TC was reached after approximately 4 h. For the fast-cooled (FC) specimens, ambient temperature was reached after 7.5 min. Forced-air convective cooling of the tempered (T) specimens caused the disc temperature to drop to 30'C after only 45 s. A Vickers indenter was applied to the body porcelain surfaces at a load of 4.9 N. Each disc was indented along four diametral lines (Fig. 3). Each line contained 20 indentations,
Fig. 4-Schematic illustration of microhardness indentation and radial crack dimensions.
1000
900
-I -
800 -
700-
Slow Cooling
600 -a
500 400 -
Fast Cooling
| \.s
300
-
200 -
I
Tempered
100-
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.
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60
120
_
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.
180
240
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_
-_
l
300
-_
_
l
360
_
-_
_
-_ 420
TIME (sec)
Fig. 5-Profiles of surface temperature of porcelain discs the three cooling conditions.
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vs.
time, for
Vol. 68 No. 7
CRACK DEVELOPMENT IN CERAMIC SURFACES
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TABLE 2 RESULTS OF TUKEY'S HSD ANALYSIS BASED ON MEAN CRACK DIAMETER* AS A FUNCTION OF COOLING RATE FOR EACH CONTRACTION DIFFERENCE Mean Crack Diameter (pm) Slow-cooled Fast-cooled Tempered a, a3(ppm/0C) + 3.2 103.7 - 2.6 89.4 - 2.5 103.3 + 2.9 + 0.7
102.1 + 3.0
101.6
0.0 0.9
115.0 + 3.5 89.1 + 2.0
101.9 + 2.9 87.2 + 2.7
75.1 + 1.6 70.3 + 1.6
1.5
91.9 + 2.3
91.7 + 3.0
75.9 ± 1.8
-
3.4
81.2
-
2.2
*The 95% confidence interval is represented by a + value after each mean crack diameter. The mean crack diameters of groups that are joined by a horizontal line are not significantly different from one another. cance to determine the specific conditions effect on the crack diameter (2c).
that had a significant
The fracture toughness values for the slowly cooled (SC) specimens were determined from the following relation, with use of values of load (P), crack size (c), and hardness (H) derived from microhardness indentation data:
FC
110T
K,
w
=
1.6
x
10 -8 (P/C3/2)
(E/H)1/2
(1)
90
where KC P
5~~~~~~ U 70
c
=
E H
= = =
a 50
+3.2
+0.7
0.0
-0.9
-1.5
CONTRACTION DIFFERENCE (ppm/JC) Fig. 6-Mean crack diameters as a function of cooling condition and contraction difference.
which were spaced 300 A.rm apart, and each disc thus received 80 indentations. We made the measurements within 30-45 s after indentation, to minimize errors in crack diameter as a result of continuing crack propagation in the presence of residual indentation stress and environmental moisture. The 2c-dimension (diameter) of the radial half-penny cracks (Fig. 4), which were measured for each of the five states of contraction mismatch and cooling technique, were analyzed by a two-factor analysis of variance (ANOVA). Tukey's (HSD) multiple contrast analysis was used at a 5% level of signifi-
= =
fracture toughness in MPa-m"2 indentation load = 4.9 N crack size (m) elastic modulus = 73.4 GPa hardness (GPa) = 0.47 P/a2 projected length of the indenter half-diagonal dimension (m)
The surface stress values in MPa for the tempered (T) specimens were calculated from the following relation, which was developed by Marshall and Lawn (1978a): Kc ( -M a= 2 CT
1/2
I
(CSCjCT)3/2
(2)
where C, and Csc are crack sizes (Fig. 4) for tempered and slowly cooled specimens, respectively.
Results. The temperature vs. time profiles of porcelain discs during cooling (from an initial temperature of 9820C to 30'C) are shown in Fig. 5 for the three cooling conditions. For slow
TABLE 3
RESULTS OF STATISTICAL ANALYSIS BASED ON MEAN CRACK DIAMETER* AS A FUNCTION OF CONTRACTION DIFFERENCE FOR EACH COOLING RATE Mean Crack Diameter (prm) O- (Xc (ppm/0C) 0.0 + 0.7 - 0.9 - 1.5 + 3.2 Cooling Mode 91.9 89.9 115.0 102.1 103.3 Slow-cooled 87.2 91.7 101.9 101.7 103.7 Fast-cooled 75.8 70.3 75.2 81.2 89.4 Tempered sig. diff. (p5O.O5) *Except for the mean crack diameters of 81.2 aLm and 70.3 pum, which arc significantly different, the horizontal line arc not significantly different.
mean
crack diameters of groups joined by
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a
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ANUSA VICE et al.
TABLE 4 COMPUTED SURFACE STRESSES* IN BODY PORCELAIN AS A RESULT OF SLOW COOLING (SC), FAST COOLING (FC), AND TEMPERING (T) Cxn - tX,3 (ppm/lC) ar (MPa) USc (MPa) arC (MPa) 34.5 + 47.5 - 7.7 + 3.2 64.1 - 17.0 + 10.4 + 0.7 86.7 - 16.5 0.0 0.0 - 0.9 - 46.3 -109.5 - 13.4 - 1.5 - 83.5 - 35.5 - 22.3 element from finite values were determined analyses. *(Ts, ,-c and err were determined from Eq. 2. Negative values of stress represent a state of compression. Positive values of stress represent a state of tension.
cooling (SC) and fast cooling (FC), only small segments of the curves are shown because of space limitations. The surface temperature of the tempered discs reached ambient temperature shortly after the air-blasting procedure was initiated. However, because the interior was still at a higher temperature, the airblasting process was continued for approximately 90 s, until the entire specimen reached ambient temperature. Summarized in Table 2 are the mean crack diameters (2c) and the 95% confidence intervals for the five mismatch cases. Shown in Fig. 6 is a comparison of the mean crack diameters for the three cooling procedures. The vertical bars represent the mean values, plus and minus the 95% confidence intervals. Based on a two-factor analysis of variance, the influence of the cooling technique, thermal contraction mismatch, and their interaction effect on the crack size were highly significant (p
