Improving the Fracture Resistance of Dental Ceramic B. DUNN, M. N. LEVY, and M. H. REISBICK

School of Dentistry, University of California, Los Angeles, California 90007 and The University of Texas Health Science Center at San Antonio, San Antonio, Texas 78284, USA A time-temperature sequence for optimally strengthening dental ceramic by ion exchange was investigated. Stress relaxation occurs at higher temperatures and/or longer times. Improved strengths are obtained in a reasonable period of time that could be used for clinical

application. J Dent Res 56(10) :1209-1213 October 1977.

The fracture of glass and ceramic materials may often be traced to the propagation of surface flaws through the bulk material. The presence of these defects is inevitable since they are caused by ordinary handling and accentuated by slight chemical attack from moisture. Tensile rather than compressive forces are generally responsible for crack extension and ultimate failure. A practical method of increasing the tensile strength of ceramics is to create a compressive skin on the surface of the ceramic; the applied tensile forces must then overcome the Received for publication July 9, 1976. Accepted for publication December 17, 1976. Read before the 54th Annual Session of the American Association for Dental Research, Miami Beach, Florida, 1976.

compression layer before they become instrumental in causing failure. The compressive layer must extend beyond the flaw tip in order to be eflective. Ion exchange is a low temperature method of obtaining surface compression.' The ion exchange process is a direct consequence of the ionic nature of glass and the accompanying rules of electrical neutrality. In Figure 1, the solid glass is immersed in a molten salt bath at a temperature below the glass transition temperature (approximately 500 C for dental porcelain). In this temperature range, the glass is still rigid. However, the temperature is sufficient for relatively rapid ionic motion to occur, although only alkali ions are mobile enough to migrate the appreciable distances necessary. The molten salt is suitably selected so that small cations in the solid will diffuse into the bath while the larger ions in the melt replace the smaller ones. In the case of dental porcelain, Na+ ions are replaced by K+ ions from the KNO3 bath. Since K+ occupies a much larger volume than Na+ (by more than 25%) the remaining silicate network is forced together and compression is produced. As long as the treatment is subAfter

Ion Exchange

Molten Salt

FIG 1. Ion exchange of K+ for Na+ in porcelain. Ceramic

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j Dent Res October 1977

-Furnace Furnace Insulation-

KNO3 Bath

FIG 2.-Apparatus for ion exchange treatment.

-Sample holder

stantially below the glass transition temperature the viscosity of the glass is so great that the stresses introduced by the exchange of ions cannot be relieved during the time period of exchange. However, this compression may be attenuated if the sample is treated at elevated temperatures for prolonged periods of time.2 The ion exchange process has been successfully utilized in commercial glass and ceramic industries to produce high strength materials. Compressive layers of at least 50 microns are necessary for practical applications. The treatment produces a significant increase in mechanical strength but does not affect the optical properties of the solid. Therefore, the ion exchange treatment appears attractive for dental porcelain. The objective of our work was to investigate a variety of time-temperature parameters of ion exchange strengthening that

might be applied to dental porcelain. Previous results reported by Southan3 indicated that strengthening could be achieved, although only one treatment was reported in that paper. Thus in our work, dental porcelain was subjected to treatments of various temperatures and times in an attempt to fully characterize the process so that we might obtain good strengthening in a

reasonable period of time.

Materials and Methods A weighed quantity of ceramic powder* was charged into a split steel mold and compacted at a pressure of approximately 3,000 psi. This method helped to standardize compaction, a most important factor in sample preparation. * Ceramco Body Porcelain #62, Ceramco Inc., New York, NY. * Instron Corp., Canton, Ma.

Specimens were then taken from the mold and dried thoroughly. The sample dimensions were such that during vacuum firing full density was reached by following the manufacturer's recommendations for porcelain restorations. Upon removal from the furnace the samples were cooled in ambient. They were then annealed at 500 C and slowly cooled to room temperature in order to relieve any residual stresses. The dimensions of the finished samples were 0.1 in. X 0.1 in. X 1 in. long. The ion exchange treatment was carried out in the apparatus shown in Figure 2. The porcelain bars were placed in the sample holder and lowered into the molten KNO3 bath. The furnace temperature was kept constant by means of a controller while a thermocouple reported the temperature of the bath. The treatment temperature ranged from 375 C to 475 C with times of '2 hour to 9 hours. After ion exchange, the samples were abraded in a ball mill using 50-mesh sand for 15 minutes. Surface abrasion is a standard procedure used in determining the mechanical properties of all ceramics.4 It serves to normalize the porcelain surface by introducing a large number of surface flaws to each sample. The treatment was also intended to approximate the type of abrasion which may be created during normal service. The compressive skin must extend beyond the deepest flaw, or it will not be effective in strengthening.5 The modulus of rupture was measured by using a four-point bend test at an Instron* crosshead speed of 0.02 inches/minute. The span to diameter ratio was nearly 8: 1. The number of specimens tested at each condition was between 5 and 10.

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FRACTURE RESISTANCE OF DENTAL CERAMIC

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FIG 3. - Strengthening of dental porcelain by ion exchange at 375 C and 400 C.

Untreated

( Hours)1

Results The results in Figure 3 indicate that appreciable strengthening may be achieved by the ion exchange treatment.4 In most ion exchange work, strength is generally plotted as a function of time to the Y2 power because it has been shown that alkali ion exchange is diffusion controlled reaction.5 The linearity at 375 C is indicative of this behavior with the diffusion of K+ producing a compressive stress at the surface. At higher temperatures, diffusion is more rapid and greater strengths are achieved in shorter periods of time. However, this high rate of increase is not sustained. After sufficient time at 400 C, stress release will occur. Thus the

linearity of the a versus tl½2 curve is altered and the slope decreases. Stress relaxation occurs within the structure and acts as a competing mechanism to the compressive stress produced by the K+ for Na+ exchange. At higher temperatures (Fig 4) relaxation occurs at shorter times (9 hours) and the measured modulus of rupture exhibits a decrease. Although the diffusion of K+ at this temperature is faster, the lower viscosity of the material enables it to undergo slight rearrangements over a period of time thereby accommodating the larger ion better and reducing the degree of compression. At short exchange times, very little relaxation occurs and a high rate of

FIG 4. Ion exchange of dental porcelain at 450 C. Excessive treatment time reduces the strengthening effect.

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j Dent Res October 1977

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Modulus of Rupture (psi

El-One Hour El-our Hours

15,000 -

I

10,000

5000

Standard Deviation

FIG 5. -Composite curve for ion exchange treatments at various temperatures and times of 1 and 4 hours.

r

Untreated

375 C

400 c

450 C

strength increase is observed due to the larger diffusion coefficient of K+ at this temperature. The effect of ion exchange temperature is shown in Figure 5. A significant decrease in strength is observed at 475 C. This temperature is close to the glass transition temperature and therefore considerable stress relaxation occurs at the treatment times indicated. The optimum temperature is 400 C for the two treatment times plotted. The value at 4 hours is comparable to that previously reported.3 Discussion From Figure 5 it is evident that average strength values which are nearly twice that of the untreated samples are observed after only one hour of treatment. Futhermore, these values are those of abraded strengths. If a strengthening process is to be of practical importance the resulting strength must be retained during the service of the article. The abraded strength values more nearly reflect the strength of that article during later use when surface flaws are certain to occur.4 Thus the ion exchange treatment is an effective means of strengthening dental porcelain. In addition, the treatment produces no changes in the optical properties of the ceramic. The strength of a ceramic subjected to ion exchange is dependent upon both the degree of compression produced at the surface as well as the depth. In these experiments, the compressive skin was deep enough to withstand the abrasion procedure practiced. Otherwise, strengthening would not have been observed. The degree of compression may be expressed by the relation. 6 UC IE- OR [l]

475;C

where ac is the net compressive stress, CIE is the stress resulting from the ion exchange process and ap is the stress attenuated from relaxation. As a result a time at each temperature of treatment yields a maximum strength. This behavior was shown at 450 C, but requires longer treatment times in order to be observed at 400 C and 375 C. The position of the maximum shifts toward shorter times as the temperature increases because stress relaxation occurs sooner. The maximum strength value should also be lower at the higher temperatures;5 however, this feature was not clearly shown in the present work. Another variable liable to have a substantial effect on the strengthening produced by ion exchange is the composition of the porcelain. This influence has been investigated with glasses and the effect is considerable. Conventional soda-lime silicate glasses do not lend themselves to practical ion exchange treatments6 whereas alkali-aluminosilicate compositions are extremely conductive to strengthening.5 Similar effects would be expected with dental porcelains; that is, ion exchange behavior should be sensitive to composition.

Summary and Conclusions Dental porcelain can be strengthened by an ion exchange treatment using a KNO3 bath. Average abraded strengths greater than two times that of untreated samples have been obtained. The treatment has no influence on optical properties and appears to be an effective means of increasing the strength of dental porcelain. The measured strength is dependent upon the compressive stress produced by the ion ex-

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FRACTURE RESISTANCE OF DENTAL CERAMIC

change process. There are primarily two contributions. The compressive stress generated by cation exchange is a diffusion controlled mechanism which predominates at low treatment temperatures and/or short times. Stress relaxation occurs at high temperatures and/or long times and reduces the degree of compression. This behavior allows good strengths to be achieved in reasonable periods of time (e.g. 440 C for 1 hour). Application of this process would be useful clinically for strengthening porcelain jacket crowns; these restorations are often chosen over ceramic-metal crowns because of their better optical properties and necessary tooth conservation during preparation. Their prime disadvantage, that of poorer strength, may be overcome by ion exchange treatment. The reported results were obtained using one type of dental porcelain. It is expected that ion exchange strengthening of dental porcelain will be sensitive to composition. We are grateful for the partial support of this work by the Academic Senate of UCLA. The authors also

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appreciate the helpful advice of their colleague, A. A.

Caputo.

References 1. KISTLER, S.S.: Stresses in Glass Produced by Non-Uniform Exchange of Monovalent Ions, J Am Ceram Soc 45:59-68, 1962. 2. GARFINKEL, H.M.: The Thermal Fatigue of Glasses and Glass-Ceramics Strengthened by Ion Exchange, Symp sur la Surface du Verre, Union Sci. Continentale du Verre: 165-180, 1967. 3. SOUTHAN, D.E.: Strengthening Modern Dental Porcelain by Ion Exchange, Austr Dent J 15:507-510, 1970. 4. SHAND, E.B.: Glass Engineering Handbook, McGraw-Hill, New York, p. 134, 1958. 5. NORDBERG, M.E.; MOCHEL, E.L.; GARFINKEL, N.M.; and OLCOTT. J.S.: Strengthening by Ion Exchange, J Am Ceram Soc 47: 215-219, 1964. 6. WARD, J.B.; SURGARMAN, B.; and SYMMERS, C.: Studies on the Chemical Strengthening of Soda-Lime-Silica Glass, Glass Tech 6:9097, 1965.

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Improving the fracture resistance of dental ceramic.

Improving the Fracture Resistance of Dental Ceramic B. DUNN, M. N. LEVY, and M. H. REISBICK School of Dentistry, University of California, Los Angele...
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