Relative flexural strength of dental restorative ceramics R.R. Seghi T. Daher A. Caputo
UCLA School of Dentistry 10833 Le Conte Ave. Los Angeles, CA 90024 Received August 29, 1989 Accepted March 6, 1990
Dent Mater 6:181-184, July, 1990
Abstract-The relative fiexural strengths of ten brands of dental restorative ceramics were evaluated by a three-point bending test. The materials consisted of three low-fusing and one high-fusing feldspathic porcelains and six reinforced dental restorative ceramics which are currently in clinical use. The reinforced ceramic materials investigated utilized a number of different strengthening processes, including alumina and ceramic fiber reinforcement and controlled crystallization. The results of the investigation indicate that significant differences exist among the measured breaking strengths of the various materials. The processes of controlled crystallization and alumina reinforcement appear to be adequate means of improving the bending strength of restorative dental ceramics.
here is an increased demand for esthetic restorative treatment. Conventional treatment options are no longer sufficient to meet p a t i e n t needs a d e q u a t e l y in all situations. Consequently, both researchers and clinicians have turned their attention to the development of new and often more conservative esthetic restorative treatment procedures. The unique combination of properties offered by ceramics with respect to biocompatibility, stability, durability, and optical qualities makes them attractive for numerous restorative applications where metals and resin composites have been proven to be inadequate or undesirable. The increased application of ceramics for restorative procedures and the demand for improved clinical performance have led to the development and introduction of several new ceramic restorative materials and techniques. One of the primary focuses of dental ceramic developers and engineers has been improving the strength characteristics of the materials. While it is clear that the longterm clinical performance of both conventional ceramic and bonded ceramic restorations will depend on a number of factors, the ability of ceramic materials to withstand fracture is of significant interest. The fracture of glasses and ceramics is initiated by tensile stresses and can often be traced to the propagation of surface flaws through the bulk material (Kingery et at., 1975). F u n d a m e n t a l l y , in order for the fracture strength of a material to be evaluated, it is necessary to have a means of relating the load on a specimen, at the instant of fracture, to the stress at the fracture location. A direct tensile test - wherein a rod is subjected to uniformly distributed, axial end loads - is the principal tool for evaluating the strength of m a n y materials. Because ceramics lack any appreciable plastic deformation, it is virtually impossi-
ble to apply a tensile load uniformly over the end of a rod, making this approach unrealistic in practice (Mordfin and Kerper, 1969). Because of the problems associated with direct tension testing of brittle materials, bending or flexure tests have been the most widely promoted in the ceramics industry and are generally considered to be the most satisfactory means of assessing strength (Duckworth, 1951; Mordfin and Kerper, 1969; Newnham, 1975). The three- and four-point bending tests are the most commonly used methods of testing the strength of glass and ceramic materials. At present, ASTM recommends threepoint loading for the strength testing of concrete and electronic-grade ceramics. This technique has been previously utilized by a number of dental researchers to assess the strength of dental ceramics (Southan, 1970; Jones et al., 1972; Sherrill and O'Brien, 1974; Jones, 1983). Currently, the International Standards Organization (ISO 6872, 1984) supports the use of the simple threepoint bending test as a means of evaluating the strength of dental porcelains. While much work has been carried out over the years to assess the strength of the more traditional dental porcelain materials, very little inf o r m a t i o n has been r e p o r t e d concerning the relative strength of many of the newer ceramic materials already in clinical use. The purpose of this i n v e s t i g a t i o n is to evaluate the relative flexural strength of several new dental restorative ceramics. It is the intention of this work to provide both researchers and clinicians with information which needs to be considered in the design and selection of restorative ceramic materials and techniques. MATERIALS AND METHODS The flexural strengths of ten commercially available dental restorative ceramics were evaluated in this
Dental Materials~July 1990 181
study. The materials tested are listed in Table 1. The materials include four conventional low-fusing feldspathic porcelains, two alumina-reinforced porcelains, two silica-based glass-ceramics, a ceramic-fiber-reinforced silica-based ceramic, and an empirically derived strengthened ceramic of unknown structure. Mechanical testing was carried out by m e t h o d s similar to those described in the ISO 6872 standard for dental ceramic materials. All test specimens were fabricated to the final dimensions of approximately 6 mm x 1 mm x 20 ram. Ceramic materials supplied in powder form were fabricated in a polysiloxane mold (6.0 mm x 25.0 ram). Approximately 0.5 g of powder was placed in the mold and dry-compressed onto a glass slab with an acrylic plunger. The plunger was removed from the mold and enough water added to wet the entire mass of material. The wet powder was again compressed with the plunger and by a light tapping force from a mallet. Excess moisture was removed from the ceramic mass with absorbant tissue and the material removed e n bloc. The block samples were dried slowly and fired according to the manufacturers' recommendations. The fired block samples were ground flat on one side, glued to a glass slide, and sectioned with a slow-speed diamond wheel saw
(Isomet TM fluid, Buehler Ltd., Lake Bluff, IL 60044) under lubrication (Isocut® fluid, Buehler Ltd.) to a uniform thickness of approximately 1 mm, by utilization of a petrographic thin-section a t t a c h m e n t (Buehler Ltd.). The specimens were removed from the glass slide and cleaned ultrasonically in alcohol before being fired to a final glaze. The sectioned-core-type materials were fired on a fiat refractory tray to the manufacturers' recommended firing schedule for dentin ceramics before being submitted to the final glaze firing. The castable glass-ceramic (Dicor, Dentsply International, Inc., York, PA 17405) specimens were formed from acrylic patterns having dimensions close to those of the fired block-porcelain samples. The cast samples were sectioned on the diamond saw in a similar fashion and wet-ground with 1000-grit silicon carbide abrasive. Four layers of shading porcelain (Dentsply International, Inc., York, PA 17405) were fired onto the surface of the ceramic so that a final glaze comparable to that of the other samples would be achieved. The testing procedure consisted of a three-point bending test. The rectangular specimens were supported on fLxed, hardened steel, cylindrical supports. Prior to the fracture test, all specimens were measured with a micrometer. A 13-
TABLE 1 PRODUCT INFORMATION
Vitadur-N® (dentin) Vita VMK 68~
Excelco International, Inc. Santurce, PR 00908 Ceramco, Inc. E. Windsor, NJ 08520 Vita Zahnfabrik Sackingen, West Germany Vita Zahnfabrik Sackingen, West Germany Vita Zahnfabrik Sackingen, West Germany Vita Zahnfabrik Sackingen, West Germany Dentsply International, Inc. York, PA 17405-0872 Myron International, Inc. Kansas City, KS 66117 Den-Mat Corp. Santa Maria, CA 93455 Jeneric/Pentron, Inc. Wallingford, CT 06492
Vitadur-N® (core) HiOeram~ (core) Dicor~
Optec H . S . P .
SEGHI et aL/FLEXURAL S T R E N G T H OF D E N T A L CERAMICS
unreinforced (feldspathic porcelain) unreinforced (feldspathic porcelain) unreinforced (fetdspathic porcelain) unreinforced (feldspathic porcelain) Alumma Alumina Tetrasilicic fluormica Ceramic whiskers (zirconia) Unknown Leucite
mm test span was used for all samples to ensure a span-to-thickness ratio of greater than 10:1, as recommended by Jones et al. (1972). The specimens were loaded at a position directly centered between the two support cylinders. The testing was done on a universal testing machine (Instron Corp., Canton, MA 02021) at a crosshead speed of 0.025 mm/ min. The failure load was recorded in Newtons and the fiexural strength or modulus of rupture (MOR) calculated by the following formula, which has been derived from elementary strength-of-materials theory (Timoshenko and Young, 1962): MOR -
3Wl 2 b d2
where W is the breaking load in newtons, l is the test span in millimeters, b is the width of the specimen in millimeters, and d is the thickness of the specimen in millimeters. An analysis, of variance and Tukey's multiple comparisons test were performed on the transformed data (Box and Cox, 1964). Differences were reported at the 0.05 level of significance. RESULTS
The number of specimens tested and the means and standard deviations of the final sample dimensions and resulting MOR values are reported in Table 2 for each of the ceramic material groups. Materials VNC, DI, and HC resulted in the g r e a t e s t fracture strengths, while materials EX, CII, and VNB resulted in the lowest. The results of the one-way analysis of variance indicated that significant differences (p < 0.001) existed among the modulus-of-rupture values determined for the different ceramic material groups. The resalts of the multiple comparisons test are also summarized in Table 2. The vertical lines connect the group means which were found not to be significantly different at the 0.05 confidence level. All of the .reinforced dental ceramic materials produced mean flexural strength values significantly higher than those of the conventional feldspathic porcelains.
Material MI, however, was found to be significantly stronger than only one of the conventional porcelain groups tested. DISCUSSION The bend test is a simple, reliable, and sensitive method for testing the relative strength of dental ceramic materials. Elementary theory predicts that the sample will fail as a result of maximum tensile stresses acting on the lower curved surface of the loaded beam. As a result, these methods are sensitive to the presence of flaws or defects which are present at the surfaces of the materials. Dental researchers have supported the use of these methods, since failure in ceramic crowns is generally in tension from surface defects. It is generally accepted that the actual tensile strength of the material being tested is considerably lower than the modulus of rupture. Karpilovskii and Letskaya (1979) have found an approximate linear relationship among tensile, compressive, and flexural s t r e n g t h s for electrical porcelains. Generally, the values s u g g e s t t h a t the flexural strength is about double the tensile strength in brittle materials. A l t h o u g h the c r o s s - b r e a k i n g stl"ength measurement is simple in application and gives reliable results, comparisons of the results obrained by different investigators can be confusing. Determination of the strengths of glassy structures utilizing measurements involving specimen breakdown depends upon the sizes and shapes of the specimens, the method used to manufacture them, and other test conditions. Of significant importance is the span-todepth ratio, which has been investigated by a number of researchers (Milligan, 1953; Shevlin and Lindenthal, 1959; Binns, 1965; Jones, 1971; Jones et at., 1972). When the span used in the bending test is short relative to the thickness (depth) of the bar, the results tend to be higher, and correction factors need to be used in the formula. There is considerable agreement among these investigators that consistent flexm-al strength values are obtained at ratios greater than ten to one. The flexm-al strength values obtained in this investigation
TABLE2 NUMBEROF SPECIMENS,DIMENSIONS,MODULUSOF RUPTURE,AND STATISTICALANALYSIS
EX Cll VND VMK MI CE OP VNC DI HC
10 10 10 10 20 20 22 22 22 22
Specimen Dimensions(mm) b (width) d (thickness) 5.98 5.91 5.95 5.83 6.06 5.97 5.65 5.92 5.37 6.21
± 0.13 ± 0.19 ± 0.13 ± 0.15 ± 0.25 _+ 0.09 ± 0.19 ± 0.14 -- 0.46 _+ 0.42
1.08 1.03 0.99 1.00 1.00 0.99 1.10 0.96 0.98 0.96
± ± ± ± ± ± ± ± ± ±
0.13 0.07 0.04 0.08 0.05 0.06 0.08 0.06 0.11 0.08
Modulus of Rupture (MPa) 55.18 61.37 62.49 65.54 70.25 94.80 103.84 123.49 124.71 139.30
± 3.44 _+ 5.02 _+ 8.94 ± 5.20 -~- 8.68 ± 11.96 ± 13.10 ± 13.90 - 18.66 ± 21.70
Results of analysis of variance (P < 0.01). Verticle lines connect values which are not significantly different (Tukey test: alpha = 0.05).
were based on samples which were very similar in size and shape. The small variations found in the dimensions of the specimens do not significantly alter the span-to-depth ratio and are considered insignificant. The dental ceramics available for use by the clinician include a wide range of compositions and microstructures. In general, all the mat e r i a l s e v a l u a t e d consist of a substantially glassy matrix with various amounts of dispersed crystalline phases and the possible presence of random fabrication defects. It is primarily the glassy phase which is involved in the failure mechanism of the materials. It has been shown in a number of experiments that the actual strength of a glassy material is often on the order of 100 times less than the theoretically derived or expected strength of the material. The most widely accepted explanation for the inherent weakness of glasses was proposed by Griffith (1920), who suggested that the flaws at the surface of the material act as stress concentrators and that the separation of the surfaces in fracture takes place sequentially r a t h e r than simultaneously across the cross-section (Kingery et aL, 1975). Regardless of the precise fabrication technique used, the random presence of surface defects causes even materials of identical composition and thermal and mechanical histories to exhibit appreciable variations in strength. All dental ceramic materials tend to fail at the same critical strain (Jones, 1983); therefore, increases in strength can be achieved only by an increase in the elastic modulus. This
has been demonstrated by Spinner and Tefft (1961), who showed that the inclusion of particles of polycrystalline alumina into a feldspathic glassy matrix led to an increase in the elastic m o d u l u s and consequently an increase in the strength of the composite material. It was further shown by Frey and Mackenzie (1967) that the elastic modulus and t h e r e f o r e the flexural s t r e n g t h of the glass/crystalline composite system was influenced by the elastic properties and amount of dispersed phase present. The method of dispersion-strengthening by incorporation of crystalline-reinforcing components within a glass matrix was developed and first applied as a dental ceramic technique by McLean and Hughes (1965). Their system involved the use of a core material containing a high proportion of alumina particles embedded in a feldspathic glass m a t r i x and until recently has been the most commonly used method of producing an all-ceramic crown. While the alumina-containing ceramic materials (VNC and HC) produced the highest strength values, the opaque nature of their optical properties has precluded their use in full thickness as a restorative material. It can be seen from the formula used to calculate modulus of rupture that the bending strength of a brittle material is inversely proportional to the square of the sample thickness. This suggests that if the material is used in thinner cross-sections, as is the case with conventional alumina-reinforced crown fabrication techniques, we can expect the s t r e n g t h of the final
Dental Materials/J~dy 1990
structure to be only moderately increased over that of feldspathic porcelains. This has b e e n verified experimentally (Southan, 1987) and makes direct comparisons between s t r e n g t h values of alumina core porcelains and other restorative ceramics subject to careful interpretation. In recent years, the idea of utilizing controlled crystallization as a means of strengthening dental ceramics has been introduced (Chu, 1977). At least two of the materials t e s t e d are known to utilize this process as a means of strengthening the ceramic. Both materials DI and OP showed flexural strengths significantly improved over those of conventional dental porcelains. Material CE, which also showed improved strength over feldspathic porcelain, is believed to utilize the process of controlled crystallization during the fritting stages, but the exact process is still unclear (Chadwick, 1989). Since the optical properties of these ceramics are such that they can be used in full thickness as a restorative material, their use may be preferred for some restorative situations. The selection of an esthetic restorative material must be based on a number of properties, including biocompatibility, durability, strength, color, translucency, w e a r resistance, and thermal expansion. AIthough strength properties may not be the most important criterion upon which clinical performance can be predicted, a high-strength material is desirable so that catastrophic failures will be minimized. New, highers t r e n g t h ceramic m a t e r i a l s are available which can be expected to improve the clinical performance of
bonded ceramic intracoronal and extracoronal restorations. REFERENCES BINNS, D.B. (1965): The Testing of Alumina Ceramic for Engineering Applications, J B r Ceram Soc 2: 294-308. Box, G.E. and Cox, D.R. (1964): An Analysis of Transformation, J R Statist Soc 26: 211-243. CHADWICK,W. (1988): Personal communication (Research Chemist, Den-Mat Corp., Santa Maria, CA 93456). CHU, G.P.K. (1977): Glass-ceramics as a New Dental Porcelain-Its Properties and Potential Applications. In: Dental Porcelain: State of the Art, H.N. Yamada, Ed., Los Angeles: University of Southern California, pp. 35-40. DUCKWORTH, W.H. (1951): Precise Tensile Properties of Ceramic Bodies, J A m Ceram Soc 34: 1-9. FREY, W.J. and MACKENZIE,J.D. (1967): Mechanical Properties of Selected Glass Crystal Composites, J M a t Sci 2: 124130. GRIFFITH, A.A. (1920): The Phenomenon of Rupture and Flow in Solids, Phil Trans R Soc A221: 163. ISO 6872 (1984): Dental Ceramic, 1st ed., International Organization for Standardization, pp. 1-14. JONES, D.W. (1971): Factors Influencing the Test Methods and Strength of Dental Porcelain, Trans and J B r Cer a m Soc 70: 124-127. JONES, D.W. (1983): The Strength and Strengthening Mechanisms of Dental Ceramics. In: Dental Ceramics, Proceedings of the First International Symposium on Ceramics, J.W. McLean, Ed., Chicago: Quintessence Publishing Co., Inc., pp. 83-141. JONES, D.W.; JONES, P.A.; and WILSON, H.J. (1972): The Relationship Between Transverse Strength and Testing Methods for Dental Ceramics, J Dent 1: 85-91. KARPILOVSKII, L.P. and LETSKAYA,N.V. (1979): Comparative Assessment of Methods of Determining the Strength
184 SEGHI et al./FLEXURAL STRENGTH OF DENTAL CERAMICS
of Electrical Porcelain, Glass and Ceramics 35: 553-555.
KINGERY, W.D.; BOWEN, H.K.; and UHLMANN,D.R. (1975): Introduction to Ceramics, 2nd ed., New York: John Wiley and Sons, pp. 91-216, 768-815. McLEAN, J.W. and HUGHES, H. (1965): The Reinforcement of Dental Porcelain With Ceramic Oxides, B r Dent J 119:251-267. MILLIGAN, L.H. (1953): Note on the Modulus of Rupture of Cylindrical Ceramic Rods When Tested on a Short Span, J A m Ceram Soc 36: 159-160. MORDFIN, L. and KERPER, M.J. (1969): Strength Testing of C e r a m i c s - A Survey, in Mechanical and Thermal Properties of Ceramics, J.B. Wachtman, Ed., NBS Special Publication 303, pp. 243-262. NEWNHAM, R.C. (1975): Strength Tests for Brittle Materials. In: Mechanical Properties of Ceramic Materials, Vol. 25, R.W. Davidge, Ed., Stoke-onTrent, England: British Ceramics Society, pp. 281-293. SHERRILL, C.A. and O'BRIEN, W.J. (1974): Transverse Strength of Aluminous and Feldspathic Porcelain, J Dent Res 53: 683-690. SHEVLIN, T.S. and LINDENTHAL, J.W. (1959): Modulus of Rupture versus Rate of Loading, A m Ceram Soc Bull 38: 491-497. SOUTHAN, D.E. (1970): Strengthening Modern Dental Porcelain by Ion Exchange, AuNt Dent J 15: 507-510. SOUTHAN, D.E. (1987): Laminate Strength of Dental Porcelain, Quint Int 18: 357-359. SPINNER, S. and TEFFT, W.E. (1961): Method of Determining Mechanical Resonance Frequencies and for Calculating Elastic Moduli from these Frequencies, A m Soc Test & Mater Proc 61: 1221-1238. TIMOSHENKO, S.P. and YOUNG, D.H. (1962): Elements of Strength of Materials, 4th ed. New York: D. Van Nostrand Co., Inc., 377 pp.