Effect of multiple firing on the bond strength of selected matched porcelain-fused-to-metal combinations J. G. Stannard, Ph.D., Ed.D.,* L. Marks, D.M.D.,** K. Kanchanatawewat, D.D.S., M.S.** Tufts University, Schoolof Dental Medicine, Boston, Mass.

and

In this study two different opaque porcelain-metal combinations were evaluated for planar shear bond strength. Samples were tested after one, three, five, seven, and nine different firing cycles to evaluate the effect of repeated firing on shear bond strength. For the combination of Will Ceram/Wl and Vita/Olympia materials, no statistical difference was observed either between materials or after repeated firing. For apparently well-matched porcelain-metal combinations, no significant reduction in bond strength occurs during normal firing of the opaque porcelain to themetal.(J PROSTHETDENT~~~O;~~:~~~-~.)

I

n the application of porcelain to metal substrates, there is no universally accepted technique for the firing temperatures of multiple bakes. When analyzing the factors involved in bond strength, consideration should be given to the effects of thermal coefficients of expansion of the various components, rate of heating and cooling, and previous thermal exposure of the porcelain and metal. As the metal-opaque-porcelain combination is baked, each component changes dimension according to its thermal coefficient of expansion (TCE) and degree of bonding to other phases. TCE values should be greater for the metal than for the porcelain.’ Upon cooling, the porcelain ideally would be in compression relative to the metal substrate.2 When that relationship is reversed, high residual stresses can occur during cooling, primarily at the porcelain-metal interface.3B4 The problem of matching porcelain-metal systems can be further compounded as the TCE of porcelain can change with different temperatures and repeated thermocycling.5 Menis et a1.6have shown that the change in TCE with a change in temperature can range from 35% to 136% during the heating cycle and from 14% to 95% during the cooling phase. Ringle et a1.7discovered that the TCE for dental porcelain increased from one to five firings and decreased from five to 10 firings at 75” C and 375” C, respectively. The TCE values at 375” C were higher than at 75” C.7 The increase in TCE for porcelain, with repeated firing, has been partly attributed to the formation of leucite crystals. Smyth et a1.8found that when porcelain body powder was baked three, five, seven and 10 times at 1800° F, leu-

*Assistant Professor,Director of Dental Materials and Restorative Clinical Research, Department of Restorative Dentistry. **Postgraduate student, Department of Graduate and Postgraduate Prosthodontics. 10/l/19471

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cite crystals formed in increasing amounts. Leucite crystals have a TCE of 31 X 10 exp-6/C, which in small amounts increases the TCE of the porcelain to more closely match the TCE of the metal substructure.g This phenomenon may account for the increase in TCE. Repeated firing of porcelain would theoretically lead to a decrease in porcelainmetal compatibility and a subsequent decrease in bond strength. This study determined the effect of repeated firings on the planar shear bond strength of two compatible porcelain-to-metal systems.

MATERIAL

AND

METHODS

Thirty-five samples were prepared for each of the two porcelain-metal combinations to determine planar bond strength. The porcelain-metal combinations selected were: Will-Ceram/Wl (Williams Dental Co., Buffalo, N.Y.), group A, and Vita/Olympia (Jelenko Co., Armonk, N.Y.), opaque, group B. Porcelain cylinders (3.6 mm diameter X 6 mm height) were condensed against cast, sand-blasted (50 cc), degassed metal disks (1.5 mm thick, 13 mm diameter), according to the manufacturer. The opaque porcelain was fused to the metal according to the manufacturer in a calibrated Jelenko Tru-Fire (Jelenko Co.) porcelain furnace, and the samples were embedded in acrylic resin. Sample geometry and the planar shear test configuration are shown in Fig. 1. The surface of the metal was coated with a release agent to prevent adhesion of the metal to the acrylic resin. The number of firing cycles performed on each test combination was varied before embedding and bond strength determination. Firing cycles of one, three, five, seven, and nine times were accomplished for each test combination. The first two firing cycles were at a maximum temperature of 1820’ F for group A and 1850’ F for group B. Additional firing cycles were at a maximum temperature of 1720’ F for group A and 1740’ F for group B. There were seven samples for each different firing cycle. The samples were tested for planar shear bond strength after one, three, five, seven,

627

STANNARD,

-4

i

lb-

MARKS,

AND

KANCHANATAWEWAT

I. Rank order of planar shear bond strength for WillCeram/Wl and Vita/Olympia porcelainfused-to-metal*

15mm

Table

PORCELAIN

Material combination 7

4

Vita/Olympia EMBEDDING

I

I/

\ALLOY

bond

strength

(MN/ms)

(3) (7) (1) (9) (5) 29 + 10 35 f 11 37 & 13 39 f 9 39 + 6 (7)

-i

CUBE

shear

WillCeramiWl 30 + 12 35 + 15 41 +- 17 42 f 14 47 + 23

13mm

3 6mm

Planar

DISC

1. Sample dimensions and test configuration for planar shear bond strength of porcelain cylinder condensed against cast metal disk. Porcelain cylinder portion of sample was embedded in acrylic resin before bond strength determination.

(3)

(5)

(9)

(1)

*Planar bond strength is reported in MN/m2 + SD. In parentheses is number of firing cycles for each sample group. Those groups, which are underlined together, are not statistically different (p = 0.05).

Fig.

and nine firing cycles for a total of 35 tests per combination. An Instron Model 4202 testing instrument (Instron Corp., Canton, Mass.) at a crosshead speed of 0.5 mm/min was used to shear the samples until ultimate failure. An analysis of variance was conducted using an SPSS/ PC computer program (SPSS, Inc., Chicago, Ill.) to determine significant interactive effects between the number of firing cycles and the porcelain-metal combinations. A t-test withp = 0.05 was also performed between all combinations of individual sample groups.

RESULTS Planar shear bond strength of each porcelain-metal combination is listed in Table I. The data are arranged by rank order according to the bond strength. There was no apparent trend between the bond strength and the number of firing cycles. The number of firing cycles and planar shear bond strength, in parentheses (MN/m2 +_SD), for group A was: one (41 + 17), three (30 f 12), five (47 +- 23), seven (35 ? 15), and nine (42 f 14). The number of firing cycles and planar shear bond strength for group B was: one (39 + 6), three (35 + ll), five (37 f 13), seven (29 f lo), and nine (39 + 9). The analysis of variance indicated no significant interaction between the number of firing cycles and the porcelain-metal combination. The t-test comparisons on individual groups produced several groups that were significantly different. This information is summarized in Table I. Those groups that were not statistically different were underlined together. The highest bond strength group for Will-Ceram/Wl materials (firing cycles = five) was not statistically different from the highest bond strength group for Vita/Olympia materials (firing cycles = 1).

DISCUSSION This study compared bond strength of two porcelainalloy combinations after various firing cycles. The condi628

tions of sample geometry and bond strength determination were uniformly controlled and followed accepted test conditions. The selection of porcelain-metal combinations was based upon laboratory and clinical experience as being well-matched, successful materials. Sample failure occurred in all casesat the porcelain-metal interface. In each case, a minute amount of porcelain was also left attached to the metal disk. The planar shear bond strength values obtained from this study agreed with those of Civjan et aLlo for a single firing of different porcelain-metal combinations. The coefficient of variation from both studies is also similar (approximately 30 % to 40 % ). Civjan et al.‘Oalso examined the effect of refiring but showed an increase in mean bond strength after multiple firing of various different alloyporcelain conbinations. The conditions of refiring and statistical significance of the data are unknown. For this study, a Student t-test was performed on the data and no reduction in bond strength for either porcelain-metal combination was observed for increased firing cycles. No significant trends between combinations A and B were observed for these porcelain-metal combinations. Each alloy in this case contained medium to high amounts of noble metal (Wl, 53.5% Pd and Olympia, 51.5% Au + 38.5% Pd). Comparison with the study of Barghi et al.,ll showing differences in bond strength due to alloy composition, was impossible because of differences in sample geometry and test conditions. Barghi et a1.l’ investigated fracture strength of complete crowns. A reduction in bond strength for noble metal alloy, after five to 10 firing cycles, was noted in that study but was not observed in this study. Only opaque-metal combinations were evaluated in this experiment. Thus, any possible stresses from TCE changes at the body porcelain-opaque interface were not present. It has been demonstrated that with repeated firing, the strain in the opaque layer diminishes whereas the strain in the body procelain is elevated under the same conditions.12 This condition, if significant to reduce bond strength, would not be present in this experiment. In this experiment, the geometry of the sample should also be considered. Porcelain-metal disk samples should be less affected by hoop stresses than spherical conformaJUNE

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tion.13 Material thickness affects these results as crack development increases as the porcelain-to-metal ratio increases.13Differences in other material combinations were not observed in this experiment. Sample preparation differences and differences in individual samples also create large coefficient of variation values. Under these test conditions and for well-matched porcelain-to-metal combinations, there was no significant reduction in bond strength despite repeated firing.

SUMMARY

7. Ringle RD, Weber RD, Anusavice KJ, Fairhurst CW. Thermal expansion/contraction behavior of dental porcelain-alloy systems [Abstract]. J Dent Res 1978;57:877. 8. Smyth M, Penugonda B, Sumithra N, Russu F. The effects of multiple firing on the structure of body porcelain [Abstract]. J Dent Res 1986;65:223. 9. Smyth M, Schulman A. The effect of leucite crystals on the physical properties of dental porcelain [Abstract]. J Dent Res 1981;60:383. 10. Civjan S, Huget EF, De Simon LB, Risinger RJ. Determination of apparent bond strength of alloy-porcelain systems [Abstract]. J Dent Res 1974;53:742. 11. Barghi N, McKeehan-Whitmer M, Aranda R. Comparison of fracture strength of porcelain-veneered-to-high noble and base metal alloys. J PROSTHET DENT 1987;57:23-6.

Matched metal-opaque systems were tested for planar shear bond strength after one, three, five, seven, and nine cycles, and there was no statistical reduction in bond strength. This confirmed that repeated firing under controlled conditions did not result in a clinically significant reduction in bond strength for the two apparently wellmatched porcelain-noble metal alloy combinations. We thank the Williams Dental Co., Inc. donation of the alloys used in this study.

and the Jelenko

12. Fairhurst CW, Anusavice KJ, Hashinger DT, Ringle RD, Twiggs SW. Thermal expansion of dental alloys and porcelains. J Biomed Mater Res 1980;14:435-46. 13. Walton TR, O’Brien WJ. Thermal stress failure of porcelain bonded to a palladium-silver alloy. J Dent Res 1985;64:476-80. Reprint

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DR. JAN G. STANNARD SCHOOL OF DENTAL MEDICINE TUFTS UNIVERSITY BOSTON, MA 02111

Contributing

Co. for

authors

D.D.S., Postgraduate student, Department of Graduate and Postgraduate Prosthodontics, School of Dental Medicine, Tufts University, Boston, Mass. D. Lung, D.M.D., Postgraduate student, Department of Graduate and Postgraduate Prosthodontics, School of Dental Medicine, Tufts University, Boston, Mass. F. Varinos, D.M.D., Postgraduate student, Department of Graduate and Postgraduate Prosthodontics, School of Dental Medicine, Tufts University, Boston, Mass. S. Vongkiatkachorn, D.D.S., Instructor, Prosthodontic Department, Chiang-Mai University, Chiang-Mai, Thailand G. Hernandez,

REFERENCES 1. &scone PJ. Effect of thermal properties on porcelain-to-metal compatibility [Abstract]. J Dent Res 1979;58:683. 2. Yamamoto M. Metal ceramics. New Malden, England: Quintessence Publishing Co, 1985:159-61. 3. Dehoff PH, Anusavice KJ. Influence of geometry on incompatibility on metal-ceramic systems [Abstract]. J Dent Res 1986;65:433. 4. Asoaka K. Correlation of thermal transient stress with compatibility data for porcelain/alloy systems [Abstract]. J Dent Res 1986,65:432. 5. Anusavice K. Noble metal alloys for metal ceramic restorations. In: Dental Clinics of North America. Philadelphia: WB Saunders, 1985;29:796. 6. Menis DL, de Rijk WG, Busby RL. Variability of thermal expansion coefficients with temperature for dental porcelains [Abstract]. J Dent Res 1982;61:611.

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Effect of multiple firing on the bond strength of selected matched porcelain-fused-to-metal combinations.

In this study two different opaque porcelain-metal combinations were evaluated for planar shear bond strength. Samples were tested after one, three, f...
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