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The effect of using recast metal on the bond between porcelain and a gold-palladium alloy
ADHESIVE
COMPOSITES
Table I, Limiting
WITH
CURING
strength
CONTRACTION
REFERENCES
values for Silar MPa
Tensile strength at 45 min Volumetric contraction stress Adhesion to etched enamel Adhesion to Scotchbond-treated
dentin
30 32 15 7
Light-cured composites attain 80% of the bond strength and the shrinkage stress within the first 20 seconds; chemically initiated composites reach this stage only after 10 to 15 minutes.3 These data suggest that the incremental technique is advantageous with light-cured composites.
SUMMARY Adhesive dentistry is dependent on new materials, improved cavity form, and technique of application.
1. Bowen RL, Cobb EN: A method for bonding 10 tltwtin and enamel. J Am Dent Assoc 107:734, 1983. 2. Bausch JR, De Lange C, Davidson CL, Peters A, De (;cc. i1.J: Clinical significance of polymerization shrinkaqr 01 uImposit( resins. J PROSTHET DENT 48:59, 1982. 3. Davidson CL, De Gee AJ, Feilzer A: The competition IXYM.CL’II the composite-dentin bond strength .rntl the polytncl,iz;ltior1 contraction stress. J Dent Res 63:1396, 1984. 4. Davidson CL, De Gee AJ: Relaxation of polymeriLaticm wntr,tc’tion stresses by Ilow in dental composites. J Drnt Rrs 63:146, 1984. 5. Porte A, Lutz F, Lund MR, Schwartz ML, (:ochran MA: Cavity designs for composite resins. Oper Dent 9:50, 1984. ll%/wml ?-lY/lw>l\ ICI.. DR. CAREL L. DAVIDSON UNIVERSITY OF AMSTERDAM SCHOOL OF DENTISTRY L~UWESWEG 1, 1066 EA AMSTERDAM THE NETHERLANDS
The e#fect of using recast metal on the bond between porcelain and a gold-palladium alloy S. T. Rasmussen, Ph.D.,* and A. A. Doukoudakis, Case Western Reserve University,
School of Dentistry,
R
esearch, matching the thermal expansion coefficients of metal and porcelain and determining the effect of metal oxides on the bond between metal and porcelain, has produced successful metal-ceramic restorations.‘” Since particular metal oxides are critical for adequate adhesion of dental porcelain to metal copings,4z5it is possible that repeated casting of metals could be detrimental, McLean’ has suggested that at least 50% new metal be included in copings for metal-ceramic restorations and most manufacturers concur. However, there is sparse experimental justification for the 50% rule.6 This investigation evaluated the effect of various percentages of recast metal on bonding with a dental porcelain. The introduction of a fracture technique coupled with a fractographic study proved useful for investigation of a metal-porcelain system.’ This apThis investigation was supported in part by a biomedical research support grant given to the School of Dentistry, Case Western Reserve University. *Associate Professor, Oral Biology. **Associate Professor, Fixed Prcsthodontics. THE JOURNAL
OF PROSTHETIC
DENTISTRY
D.D.S., M.S.*”
Cleveland, Ohio
preach was adapted because it allowed the inspection of minute specimens likely to lose cohesion.‘BB Fracture studies might also offer improvement for the dental adhesive systems.* The methods and principles of fracture mechanics have been applied sparingly to dental metal-ceramic systems. However, a fracture technique has been applied to a gold alloy-porcelain system and a fracture toughness test to a nickel alloy-alumina combination used for implants.’ In research in dental adhesives the most common procedure adopted from fracture mechanics is the determination of the path of fracture. In the cohesive plateau theory of porcelain-alloy metal bonding, the types of failure for the metal-porcelain system have previously been specified.”
METHODS AND MATERIAL The gold alloy (Olympia; Jelenko, Dental Health Products, Armonk, N.Y.) was 51.5% gold, 38.5% palladium, 8.5% indium, and 1.5% gallium. The recast metal was collected after the alloy was cast into investment (Ceramigold; Whip Mix Corp., Louisville, Ky.) maids with a natural gas/oxygen multiorifice torch. The mold was formulated from No. 10 gauge round wax with a 447
RASMUSSEN
I I Water
Metal
Porcelain
1 (t) interface
b
(t)
between
\ metal and porcelain
,
Fractured Surfaces
Fig. 1. Schematic drawing of (a) test configuration and (b) fractured specimen. a, Three-point loading until fracture by metal edge engages specimen along upper surface at center of specimen. b, Fractured specimen with triangular fractured surface.
high surface area mold space (area/v01 = 1.3/mm). Manufacturer’s recommendations were followed for the investment and casting of the metal and the recovered castings were scrubbed and sandblasted. Examination by stereo microscope revealed minute amounts of investment in the castings. This investment was not removed since it was unlikely that buttons would be cleaner at a dental laboratory. The “recast” metal was cast and cleaned three more times for a total of four castings. Wax forms 2 by 3 mm in cross section by 6 to 8 mm in length were the molds for the metal halves of the specimens. Twelve specimens were cast simultaneously with a predetermined percentage of new and recast metal. The percentages of new and recast metal for each specimen group are listed in Table I. The specimens were scrubbed and sandblasted after casting. The specimens were secured in a jig so one end of each specimen could be sanded flat with an 83-degree angle (0) to the long axis of the specimens (Fig. 1). The sanding was achieved with 600-grit wet/dry silicon carbide paper. The specimens were degassedand opaque porcelain (Ceramco Al, Opaque (C); Ceramic Inc., Johnson and Johnson Co., E. Windsor, N.J.) was applied to the finished flat end. The specimens were ground to have two parallel sides.” ” The angle,@ varied 448
DOUKOUDAKIS
Table I. The percentage of new plus the percentage recast alloy that composed the metal half of each specimen in a group and the number of specimens in a group .----_______ No. of 7’~New p/rRecast Specimen
Load
Metal edge
AND
group
alloy
A K c
100 50 25
D
15
E
0
specimens
alloy
6 4 6 7 8
0 SO 75 85 100
from specimen to specimen because of the final finishing but was less than 90 degrees. The specimens were grooved at the interface7r1’ to confine the fracture path to the adhesive interface (Fig. 1). A number of specimens from each group were fractured initially during grooving while only a few specimens fractured during the final procedure. The specimens were grooved with partly a rubberbonded abrasive cutoff wheel (7.6 cm in diameter by 0.38 mm; Allison-Campbell Division of ACCCJ, Shelton, Conn.). The final grooving was accomplished with approximately 2.5 cm diameter disks, formed from larger: wfteels, B@ a slow-speed handpiece. The grooved specimens are shown in Fig. 2, a, b, e, and& Group sizes were initi&y 12 but the final group size is listed in Table I. The specimens were loaded to failure at 0.05 cm/mm with an Instron (Instron Corp., Canton, Mass.) universal testing machine with a 23 kg compression load cell. The specimens, including those broken during preparation, were inspected with a light microscope. A limiter number of specimens, selected becauseof an interesting feature in the light microscope, were coated (Polaron Instruments, Inc., Harfield, Pa.) with 10 nm of pa&&iurn and observed with a scanning electron microscope (SEM). The light microscope and SEM fractographs show both halves of the specimen (Figs. 2 through 4). The upper fractographs in each figure are the metal halves of the specimen while directly below are the matehmg regions of the porcelain halves. RESULTS Light microscope and SEM examin&on of the fractured surfaces revealed three types of &aaured -su;rfiws. The predominant failure mode was cohe&e witb&n the porcelain, and a type III classilication by .@%ien.‘” Fig. 2, d and h demonstrates a specimen with nearly l&$41 cohesive failure with a few voids. The specimens ex&bited cohesive failure over approxizmnel~ 84% of their fractured surfaces. Thirty specimeno.d 32 ~~~~~~ evidence of the other two failure m&!s, The ol;ht?f two failure modes were indistinguishable with a light microAPRIL
1986
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PORCELAIN-GOLD-PALLADIUM
ALLOY
BOND
Fig. 2. a and e present central view of notched area of entire specimen (a) and fractured specimen fe). b and f exhibit top of a whole specimen (b) and a broken one (f). c and g are light microscopic fractographs displaying 75% recast metal half of specimen (c) and matching porcelain half (g) from same specimen. d and h are SEM fractographs presenting metal (75% recast) half (d) and matching porcelain half (h).
scope at x40. The dark areas in Fig. 2, c represent the other two failure modes located in the metal half of a specimen. The metal oxide layer is responsible for the dark appearance. The second mode of failure involved fracture at voids located close to the interface. Two types of voids were observed with SEM. One was hemispheric with the flat side of the void against the metal noted at region A in Fig. 3, a and the curved side in the porcelain was observed at region A in Fig. 3, d. The metal side of this void was covered with a thin layer of porcelain and referred to as a “hemispheric” void. The other void was Sat with minimal porcelain on the metal half located at region B in Fig. 3, b and region A in Fig. 3, c. The matching region on the porcelain half of a specimen had the appearance of glazed porcelain noted at region A in Fig. 3, j and 4, c and region B in Fig. 3, e which indicated that the porcelain was not in contact with the metal during firing. This type of defect was labeled a “flat” void. The third mode of failure appeared “adhesive” with a light microscope at x40 but was not pure adhesive failure as confirmed from SEM fractographs. For example; if the lower portion of the triangle in Fig. 4, b were observed with a light microscope it would appear as an adhesive failure, but patches of porcelain are present with regions of metal/metal oxide showing finish lines THE JOURNAL
OF PROSTHETIC
DENTISTRY
emphasized with arrow A. On the porcelain half, straight lines occur in some regions highlighted by arrow B in Fig. 4, d indicating close porcelain adaptation to the finished metal surface. Approximately 60% of the specimens showed “adhesive” failure at the beginning of fracture i.e., the tip of the triangle indicated by arrow A in Figs. 2, c and e and 4,b. Approximately 50% of the tested specimens showed “adhesive” failure on the same edge of each specimen. The location of “adhesive” failure could not be linked to a specimen group. The hemispheric voids were found in all specimens but the largest and greatest number of voids occurred on specimens containing 85% or more of recast metal. Fractographs shown in Figs. 2, c, d, g, and h and 4, b and dare representative of specimens containing 75% or less recast metal. The flat voids shown in Fig. 4, a and c were found primarily in specimens containing 85% or more recast metal. Flat voids were not found in the SEM fractographs of two specimens containing 75% recast metal or specimens without recast metal. Other specimens were examined with a light microscope. While flat voids were harder to distinguish with a light microscope than with SEM, voids were discovered in one specimen containing 75% recast metal. Fig. 2, c shows “adhesive” failure along the right edge indicated by arrow B. The porcelain 449
Fig. 3. SEM fractographs (a, b, and c) of metal halves of three specimens while dire&: below are matching porcelain halves (d, e, and f). A 100% recast metal specimen is showr; in a and d while remaining fractographs are two specimens with 85% recast metai.
half of the specimen noted by arrow B and particularly below the arrow in Fig. 2, g showed evidence of a Rat void. The frequency and size of interfacial voids increased for specimens containing 85% or more recast metal. Exact calculations were not performed because of the limited scope of this investigation. However, the limited SEM work and the light microscopic examination of specimens, including those fractured during preparation, showed that the frequency per unit area and average diameter of hemjspheric voids increased for specimens containing 85% recast metal. SEM fractographs revealed that the size and frequency of “flat” voids also increased with specimens containing 85% or more recast metal. However, not all specimens that had 85% or more recast metal had more and larger voids. Some specimens had hemispheric voids or flat voids while others appeared similar to specimens containing less than 85% recast metal.
Ink&da1
\toids
Brittle and semibrittle materials fail under load because of stress concentration about defects or voids. 450
Adhesive joints involving at least one brittle mater&&&o fail because of stress concentration about an inter&c&l void. On loading, an interfacial void could preeip&e failure in either the adhesive or cohesive mode? Forcohesive failure, stress concentration at an interfacial void could initiate failure but the mode would be cohesive. Large interfacial voids at a region of high stress concentration could be responsible for “cohesive” failure without apparent cause. The most significant observation of this investigation is that the size and frequency of interfacial porosity increased with 85% or more recast metal. An interfacial void could seriously weaken a metal-ceramic restoration if the void was in an area of high interfacial stress Em loading. An increase in frequency and size makes the void a more effective stress concentrator. Consequently, use of 85% or more recast metal for a rnetal~~~c restoration increases the probability of failure. The .#?4 rule’ for the maximum amount of recast metal is, an excellent guideline. The results suggested that ‘lY% recast metal could be used but recast metal must be clean. The hemispheric voids occurred clot to the pore&&nmetal interface. The evidence was fourfold: (1) with a APRIL
1986
VOLUME
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METAL
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PORCELAIN-GOLD-PALLADIUM
ALLOY
BOND
Fig. 4. SEM fractographs with (a) exhibiting
“flat” void from region A in Fig. 3, c at higher magnification (c) illustrates matching area from Fig. 3, f. b, metal half of specimen depicts “adhesive” failure on lower third of triangle. d, shows matching porcelain half of specimen in b. Specimen in b and d contained 100% new metal. light microscope the dark metal oxide showed through the porcelain on the flat side of the hemispheric voids suggesting a thin porcelain layer; (2) the metal side of the voids were flat; (3) when the larger voids on the metal halves of the specimens were tested with an ohm meter and a probe, light pressure on the probe gave zero meter reading without resistance suggesting that the porcelain was thin, and easily fractured to give metalto-metal contact; and (4) Fig. 3, a indicates that the voids were primarily near the porcelain-metal interface. Note the high region running vertically through the triangle with the large void near the center. This region is relatively free of voids, particularly hemispheric voids, while many voids occur in the regions closer to the metal surface on the left and right of the high region. When hemispheric voids appear they result from the release of absorbed gases during firing. The flat voids showed metal-metal oxide on the metal halves of specimen Fig. 4, a without porcelain-to-metal adaptation over the corresponding area on the porcelain halves (Fig. 4, c). The Aat voids probably indicate that the porcelain did not wet the metal in this area during firing. The flat voids were the largest defects and seriously limited the strength of a restoration. THE JOURNAL
OF PROSTHETIC
DENTISTRY
Adhesive and cohesive failure This fractographic investigation indicated that the bonding between this particular porcelain and alloy was adequate and recast alloy did not seriously alter the bonding. Three subjects must be discussed to understand this finding. First, the specimens were designed so that the tensile stresses induced by loading would be greater at the interface than in the porcelain.’ This is important when concluding that cohesive failure implies that the adhesive bond is sufficient.8 For example, cohesive failure could be induced by higher stressesin the porcelain than at the interface even if the adhesive bond was inadequate. Because of the experimental design, failure of tested specimens was likely initiated close to or at the sharp apex of the specimens. This region of the specimen is indicated by the arrow labeled “interface between metal and porcelain” in Fig. 1, a and by arrow A in Fig. 2, a, c, and g. Second, the adhesive failure was probably not pure interfacial failure. Diffusion is likely between the various phases without a clear interfacial boundary between the porcelain and metallic oxide as noted by Anusavice et a1.12However, it is appropriate to identify the various 451
RASMUSSEN
failure modes as closely as the results will allow. The region identified as “adhesive” failure with a light microscope was shown, with SEM, to consist of two different types of failure (Fig. 4, b and d). Limited failures occurring at the metal-oxide porcelain interface can be described as adhesive failure or class II, according to O’Brien.” Interspersed among the regions of adhesive failure were patches of porcelain indicating a cohesive class III’O failure within the porcelain. The composition of this porcelain could have been altered by diffusion of the metal-oxide layer. Third, fracture data were obtained for all specimen groups. Analysis indicated that the specimens may have been precracked during preparation. Consequently, the W, data are not reported. The possibility of precracked specimens would not have altered our conclusion concerning interfacial defects because defects occurred during firing of the porcelain. However, it cannot be concluded that the “adhesive” failure, as identified by light microscopy, occurred during testing. Regardless of how the “adhesive” failure was initiated, failure resulted from mechanical stress. There was no relationship between the occurrence, amount, or location of “adhesive” failure, and the specimen’s content of recast alloy. If the bonding between porcelain and alloy were altered seriously by recast alloy, interfacial precracking would have been more extensive and evident in the fractographs as an increase in “adhesive” failures. Similarly, if the failures occurred during testing the fractographs would have indicated that the bonding was seriously altered by the recast metal. The strongest evidence for adequate adhesive bonding was the characteristics of the region identified as “adhesive” failure by light microscopy. This region on the metal halves revealed portions of metal oxide surface covered with patches of porcelain. If the adhesive bond was weak in the porcelain, more metal-oxide surface would have been evident whether “adhesive” failure was induced during testing or specimen preparation. If the porcelain close to the metal-oxide surface was weakened by diffusion relative to the porcelain farther from the interface, “adhesive” failure would have predominated. This is because the specimens were designed to produce the highest stresses at the interface. However, cohesive failure was the predominant mode of failure. The SEM fractographs indicated that the adhesive bond between the porcelain and alloy was adequate.
CONCLUSfON Fracture and fractographic techniques were used to investigate the adhesion of a dental porcelain to a recast gold-palladium alloy. A coping of 50% new metal for a porcelain-metal restoration was an adequate safety margin. However, there were no adverse effects noted with 452
AND
DOUKOCQRY~F
75% recast metal but serious changes were ;ounti .:cai’ 85% or more recast metal. Poor cleaning prctceduiw resulted in a more contaminated alloy so it was suqgesd that the 50% guideline be followed and the recast metals be cleaned thoroughly. The use of a recast metal totaling 85% 0; more increased the frequency and size of interfacia! voids. This was serious because failures initiated at a void are enhanced by stress concentration and the increased frequency of voids raises the chance of a defect being in an area of high stress. An enlargement in the size of a failure-inducing void reduces the load required for failure. Consequently, the probability of failure is greater for a porcelain-metal restoration with 85% or more recast metal. The predominant mode of failure at catastrophic failure rates was cohesive. Some specimens had limited “adhesive” failure. However, adhesive failure could not be linked to the percentage of recast metal. SEM fractographs indicated that the adhesive bond was adequate and there was no evidence that recast metal altered the bonding. We acknowledge the assistance of J, F. Jelenko and Co.. Armorrk, N. Y. for supplying the Olympia metal; Mr. Paul J. (:ascone, hIanager. Research and Development at Jelenko, for his advice and assistance; and Mr. John Lavicka, Dental Ceramics, Garlield Heights, Ohio.
REFERENCES I.
2. 3.
4. 5.
6.
McLean JW: The science and art of dental ceramics. III The Nature of Dental Ceramics and Their Clinical Use. Chicago, 1979, Quintessence Publishing Co Inc, vol 1. pp 55-95. Shell TS, Nielsen JP: Study of the bond between gold alioys and porcelain. J Dent Res 41:1424, 1462. McLean JW, Seed IR: Bonding of dental porcelain to metal. 1. The gold alloy/porcelain bond. Trans Br Ceran SOC 72~229, 1973. Knapp JF, Ryge G: Study of bond strength of dental porcelain fused IO metal. J Dent Res 45~1047, 1966. Anthony DS, Burnett AP, Smith DL, Brooks MS: Shear test for measuring bonding in cast gold alloy-porcelain composites. J Dent Res 49:27, 1970. Heshy DA, Kobes P, Garver DG, Pelieu GB Jr: Physical properties of a repeatedly used nonprecious metal alloy. J PROSTHET DENT 44:291,
1980.
Rasmussen ST: Fracture studies of adhesion. J Dent Res 57:11, 1978. 8. O’Brien WJ, Rasmussen ST: A critical appraisal of dental adhesion testing. In Mittal KL, editor: Adhesive Joints: Formation: Characteristics, and Testing New York, 1984, Plenum Publishing Carp, pp 289-305. 9 Elssner G, Barrish WF, Pabst R: Fracture toughness of motatto-ceramic ,joints as a function of environment. In Hastings GW, Williams DF, editors: Mechanical Properties of Biornattrials. New York, 1980, John Wiley & Sons Inc,.pp 265-274. IO. O’Brien WJ: Cohesive plateau theory of por&Gn-&y bond@g.. In Yamada HN, editor: Dental Porcelaiti The State of the Art-1977. Los Angeles, 1977, University of Sou&wn ‘GaGorma, School of Dentistry, pp 137-141. 7.
APRIL
1986
VOLUME
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NUMBER
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RECAST
METAL
AND
PORCELAIN-GOLD-PALLADIUM
ALLOY
BOND
/?eprm1mqueJlsto:
11. Rasmussen ST, Patchin RE, Scott DB, Heuer AH: Fracture properties of human enamel and dentin. J Dent Res 55:154, 1976. 12. Anusavice KJ, Ringle RD, Fairhurst CW: Identification of fracture zones in porcelain-veneered-to-metal bond test specimens by ESCA. J PROSTHET DENT 42417, 1979.
DR. ASTERIOS DOUKOUDAKI~ CASE W&TERN RESERVE UNIVERSITY SCHOLL OF DENTISTRY CLEVELAND, OH 44106
Role ef mmganese in alloy-porcelain
bonding
K. Bruggers, B.S.,* C. Corcoran,* E. E. Jeansonne, D.D.S.,*. and N. K. Sarkar, M.Sc., Ph.D.*** Louisiana
State University,
School of Dentistry,
New Orleans,
P
revious studies have shown that chemical bonding of dental porcelain to nickel-chromium ceramic alloys was dependent on oxidation-reduction reactions.‘-’ These reactions involved the major elements Ni and Cr, as well as minor elements such as B, Be, Al, MO, Si, Fe, etc., that may be present in this complex alloy system. Past research has emphasized the role of major elements in porcelain-alloy chemical adherence with sparse attention directed to the minor elements. However, some of these elements may be crucial to the formation of a strong porcelain-alloy bond. Manganese is a minor constituent of many nickelchromium casting alloys and the subject of the present study. Manganese has a highly oxidizing nature. One of its oxidation products (MnO) has the ability to form limited solid solutions with SiO*, which is a major component of dental porcelain.5-* The objective of this study was to confirm that Mn performed a significant role in alloy-porcelain chemical interaction.
MATERIAL
AND METHODS
The samples consisted of the following material: (1) Mn-containing nickel-chromium alloy (Wiron-S; Williams Gold Refining Co., Buffalo, N.Y.) with Ni, 70.5%; Cr, 16%; MO, 4%; Al, 3.4%; Mn, 3.5%; Si, 1.3%; Co, 1.2%; and Fe, 0.1% and (2) a commercial dental porcelain (Biobond; Dentsply International Co., York, Presented in part at the American Association for Dental Research, Los Angeles, Calif. Supported by BRSG grant No. RR 050704 from the National Institute of Dental Research, Bethesda, Md. *Student Assistant. **Dean Emeritus. ***Professor and Head, Department of Biomaterials. THE JOURNAL
OF PROSTHETIC
DENTISTRY
La.
Pa.). The alloy has demonstrated excellent adherence to porcelain.9 Two specimens of the following samples were prepared according to the manufacturer’s recommendations: (1) as cast flat (2 by 4 by 8 mm) sample of the alloy finished with 600-grit Sic (Buehler Ltd., Lake Bluff, Ill.) paper, (2) sample No. 1 vacuum degassedat 940’ C for 2 minutes, and (3) alloy-porcelain couple made by fusing porcelain on sample No. 2 with sample No. 1 as the control. Scanning electron microscopy (SEM) (Model 1000; Advanced Metals Research, Burlington, Mass.) and energy-dispersive x-ray analysis (EDXA) (PGT 1000; Princeton Gamma-Tech, Princeton, N.J.) were used to characterize the surface morphology and chemistry of samples 1 and 2 and the interface morphology of a polished section (0.5 pm) of sample 3. After the debonding of sample 3 by three-point bending, the debonded alloys and porcelain were also examined by SEM and EDXA to ascertain the bonding mechanism.
RESULTS The surface morphology of the alloy, before and after degassing, is shown in the SEMs in Fig. 1, A through C. The scratch marks from grinding were apparent on both surfaces. Moreover, the degassed surface was characterized by the presence of a localized growth above a thin, uniform deposit. Typical x-ray analyses from the control and degassed surfaces are displayed in Fig. 1, D and E. The degassedsurface indicated a higher concentration of Cr and Mn than did the control surface. X-ray analysis demonstrated that the major Mn line (Mn-K,) occurred in conjunction with the Cr-K, line. Therefore the Mn-kB line was used to identify Mn in this study. When the 453
COMPOSITES
Table I, Limiting
WITH
CURING
strength
CONTRACTION
REFERENCES
values for Silar MPa
Tensile strength at 45 min Volumetric contraction stress Adhesion to etched enamel Adhesion to Scotchbond-treated
dentin
30 32 15 7
Light-cured composites attain 80% of the bond strength and the shrinkage stress within the first 20 seconds; chemically initiated composites reach this stage only after 10 to 15 minutes.3 These data suggest that the incremental technique is advantageous with light-cured composites.
SUMMARY Adhesive dentistry is dependent on new materials, improved cavity form, and technique of application.
1. Bowen RL, Cobb EN: A method for bonding 10 tltwtin and enamel. J Am Dent Assoc 107:734, 1983. 2. Bausch JR, De Lange C, Davidson CL, Peters A, De (;cc. i1.J: Clinical significance of polymerization shrinkaqr 01 uImposit( resins. J PROSTHET DENT 48:59, 1982. 3. Davidson CL, De Gee AJ, Feilzer A: The competition IXYM.CL’II the composite-dentin bond strength .rntl the polytncl,iz;ltior1 contraction stress. J Dent Res 63:1396, 1984. 4. Davidson CL, De Gee AJ: Relaxation of polymeriLaticm wntr,tc’tion stresses by Ilow in dental composites. J Drnt Rrs 63:146, 1984. 5. Porte A, Lutz F, Lund MR, Schwartz ML, (:ochran MA: Cavity designs for composite resins. Oper Dent 9:50, 1984. ll%/wml ?-lY/lw>l\ ICI.. DR. CAREL L. DAVIDSON UNIVERSITY OF AMSTERDAM SCHOOL OF DENTISTRY L~UWESWEG 1, 1066 EA AMSTERDAM THE NETHERLANDS
The e#fect of using recast metal on the bond between porcelain and a gold-palladium alloy S. T. Rasmussen, Ph.D.,* and A. A. Doukoudakis, Case Western Reserve University,
School of Dentistry,
R
esearch, matching the thermal expansion coefficients of metal and porcelain and determining the effect of metal oxides on the bond between metal and porcelain, has produced successful metal-ceramic restorations.‘” Since particular metal oxides are critical for adequate adhesion of dental porcelain to metal copings,4z5it is possible that repeated casting of metals could be detrimental, McLean’ has suggested that at least 50% new metal be included in copings for metal-ceramic restorations and most manufacturers concur. However, there is sparse experimental justification for the 50% rule.6 This investigation evaluated the effect of various percentages of recast metal on bonding with a dental porcelain. The introduction of a fracture technique coupled with a fractographic study proved useful for investigation of a metal-porcelain system.’ This apThis investigation was supported in part by a biomedical research support grant given to the School of Dentistry, Case Western Reserve University. *Associate Professor, Oral Biology. **Associate Professor, Fixed Prcsthodontics. THE JOURNAL
OF PROSTHETIC
DENTISTRY
D.D.S., M.S.*”
Cleveland, Ohio
preach was adapted because it allowed the inspection of minute specimens likely to lose cohesion.‘BB Fracture studies might also offer improvement for the dental adhesive systems.* The methods and principles of fracture mechanics have been applied sparingly to dental metal-ceramic systems. However, a fracture technique has been applied to a gold alloy-porcelain system and a fracture toughness test to a nickel alloy-alumina combination used for implants.’ In research in dental adhesives the most common procedure adopted from fracture mechanics is the determination of the path of fracture. In the cohesive plateau theory of porcelain-alloy metal bonding, the types of failure for the metal-porcelain system have previously been specified.”
METHODS AND MATERIAL The gold alloy (Olympia; Jelenko, Dental Health Products, Armonk, N.Y.) was 51.5% gold, 38.5% palladium, 8.5% indium, and 1.5% gallium. The recast metal was collected after the alloy was cast into investment (Ceramigold; Whip Mix Corp., Louisville, Ky.) maids with a natural gas/oxygen multiorifice torch. The mold was formulated from No. 10 gauge round wax with a 447
RASMUSSEN
I I Water
Metal
Porcelain
1 (t) interface
b
(t)
between
\ metal and porcelain
,
Fractured Surfaces
Fig. 1. Schematic drawing of (a) test configuration and (b) fractured specimen. a, Three-point loading until fracture by metal edge engages specimen along upper surface at center of specimen. b, Fractured specimen with triangular fractured surface.
high surface area mold space (area/v01 = 1.3/mm). Manufacturer’s recommendations were followed for the investment and casting of the metal and the recovered castings were scrubbed and sandblasted. Examination by stereo microscope revealed minute amounts of investment in the castings. This investment was not removed since it was unlikely that buttons would be cleaner at a dental laboratory. The “recast” metal was cast and cleaned three more times for a total of four castings. Wax forms 2 by 3 mm in cross section by 6 to 8 mm in length were the molds for the metal halves of the specimens. Twelve specimens were cast simultaneously with a predetermined percentage of new and recast metal. The percentages of new and recast metal for each specimen group are listed in Table I. The specimens were scrubbed and sandblasted after casting. The specimens were secured in a jig so one end of each specimen could be sanded flat with an 83-degree angle (0) to the long axis of the specimens (Fig. 1). The sanding was achieved with 600-grit wet/dry silicon carbide paper. The specimens were degassedand opaque porcelain (Ceramco Al, Opaque (C); Ceramic Inc., Johnson and Johnson Co., E. Windsor, N.J.) was applied to the finished flat end. The specimens were ground to have two parallel sides.” ” The angle,@ varied 448
DOUKOUDAKIS
Table I. The percentage of new plus the percentage recast alloy that composed the metal half of each specimen in a group and the number of specimens in a group .----_______ No. of 7’~New p/rRecast Specimen
Load
Metal edge
AND
group
alloy
A K c
100 50 25
D
15
E
0
specimens
alloy
6 4 6 7 8
0 SO 75 85 100
from specimen to specimen because of the final finishing but was less than 90 degrees. The specimens were grooved at the interface7r1’ to confine the fracture path to the adhesive interface (Fig. 1). A number of specimens from each group were fractured initially during grooving while only a few specimens fractured during the final procedure. The specimens were grooved with partly a rubberbonded abrasive cutoff wheel (7.6 cm in diameter by 0.38 mm; Allison-Campbell Division of ACCCJ, Shelton, Conn.). The final grooving was accomplished with approximately 2.5 cm diameter disks, formed from larger: wfteels, B@ a slow-speed handpiece. The grooved specimens are shown in Fig. 2, a, b, e, and& Group sizes were initi&y 12 but the final group size is listed in Table I. The specimens were loaded to failure at 0.05 cm/mm with an Instron (Instron Corp., Canton, Mass.) universal testing machine with a 23 kg compression load cell. The specimens, including those broken during preparation, were inspected with a light microscope. A limiter number of specimens, selected becauseof an interesting feature in the light microscope, were coated (Polaron Instruments, Inc., Harfield, Pa.) with 10 nm of pa&&iurn and observed with a scanning electron microscope (SEM). The light microscope and SEM fractographs show both halves of the specimen (Figs. 2 through 4). The upper fractographs in each figure are the metal halves of the specimen while directly below are the matehmg regions of the porcelain halves. RESULTS Light microscope and SEM examin&on of the fractured surfaces revealed three types of &aaured -su;rfiws. The predominant failure mode was cohe&e witb&n the porcelain, and a type III classilication by .@%ien.‘” Fig. 2, d and h demonstrates a specimen with nearly l&$41 cohesive failure with a few voids. The specimens ex&bited cohesive failure over approxizmnel~ 84% of their fractured surfaces. Thirty specimeno.d 32 ~~~~~~ evidence of the other two failure m&!s, The ol;ht?f two failure modes were indistinguishable with a light microAPRIL
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Fig. 2. a and e present central view of notched area of entire specimen (a) and fractured specimen fe). b and f exhibit top of a whole specimen (b) and a broken one (f). c and g are light microscopic fractographs displaying 75% recast metal half of specimen (c) and matching porcelain half (g) from same specimen. d and h are SEM fractographs presenting metal (75% recast) half (d) and matching porcelain half (h).
scope at x40. The dark areas in Fig. 2, c represent the other two failure modes located in the metal half of a specimen. The metal oxide layer is responsible for the dark appearance. The second mode of failure involved fracture at voids located close to the interface. Two types of voids were observed with SEM. One was hemispheric with the flat side of the void against the metal noted at region A in Fig. 3, a and the curved side in the porcelain was observed at region A in Fig. 3, d. The metal side of this void was covered with a thin layer of porcelain and referred to as a “hemispheric” void. The other void was Sat with minimal porcelain on the metal half located at region B in Fig. 3, b and region A in Fig. 3, c. The matching region on the porcelain half of a specimen had the appearance of glazed porcelain noted at region A in Fig. 3, j and 4, c and region B in Fig. 3, e which indicated that the porcelain was not in contact with the metal during firing. This type of defect was labeled a “flat” void. The third mode of failure appeared “adhesive” with a light microscope at x40 but was not pure adhesive failure as confirmed from SEM fractographs. For example; if the lower portion of the triangle in Fig. 4, b were observed with a light microscope it would appear as an adhesive failure, but patches of porcelain are present with regions of metal/metal oxide showing finish lines THE JOURNAL
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emphasized with arrow A. On the porcelain half, straight lines occur in some regions highlighted by arrow B in Fig. 4, d indicating close porcelain adaptation to the finished metal surface. Approximately 60% of the specimens showed “adhesive” failure at the beginning of fracture i.e., the tip of the triangle indicated by arrow A in Figs. 2, c and e and 4,b. Approximately 50% of the tested specimens showed “adhesive” failure on the same edge of each specimen. The location of “adhesive” failure could not be linked to a specimen group. The hemispheric voids were found in all specimens but the largest and greatest number of voids occurred on specimens containing 85% or more of recast metal. Fractographs shown in Figs. 2, c, d, g, and h and 4, b and dare representative of specimens containing 75% or less recast metal. The flat voids shown in Fig. 4, a and c were found primarily in specimens containing 85% or more recast metal. Flat voids were not found in the SEM fractographs of two specimens containing 75% recast metal or specimens without recast metal. Other specimens were examined with a light microscope. While flat voids were harder to distinguish with a light microscope than with SEM, voids were discovered in one specimen containing 75% recast metal. Fig. 2, c shows “adhesive” failure along the right edge indicated by arrow B. The porcelain 449
Fig. 3. SEM fractographs (a, b, and c) of metal halves of three specimens while dire&: below are matching porcelain halves (d, e, and f). A 100% recast metal specimen is showr; in a and d while remaining fractographs are two specimens with 85% recast metai.
half of the specimen noted by arrow B and particularly below the arrow in Fig. 2, g showed evidence of a Rat void. The frequency and size of interfacial voids increased for specimens containing 85% or more recast metal. Exact calculations were not performed because of the limited scope of this investigation. However, the limited SEM work and the light microscopic examination of specimens, including those fractured during preparation, showed that the frequency per unit area and average diameter of hemjspheric voids increased for specimens containing 85% recast metal. SEM fractographs revealed that the size and frequency of “flat” voids also increased with specimens containing 85% or more recast metal. However, not all specimens that had 85% or more recast metal had more and larger voids. Some specimens had hemispheric voids or flat voids while others appeared similar to specimens containing less than 85% recast metal.
Ink&da1
\toids
Brittle and semibrittle materials fail under load because of stress concentration about defects or voids. 450
Adhesive joints involving at least one brittle mater&&&o fail because of stress concentration about an inter&c&l void. On loading, an interfacial void could preeip&e failure in either the adhesive or cohesive mode? Forcohesive failure, stress concentration at an interfacial void could initiate failure but the mode would be cohesive. Large interfacial voids at a region of high stress concentration could be responsible for “cohesive” failure without apparent cause. The most significant observation of this investigation is that the size and frequency of interfacial porosity increased with 85% or more recast metal. An interfacial void could seriously weaken a metal-ceramic restoration if the void was in an area of high interfacial stress Em loading. An increase in frequency and size makes the void a more effective stress concentrator. Consequently, use of 85% or more recast metal for a rnetal~~~c restoration increases the probability of failure. The .#?4 rule’ for the maximum amount of recast metal is, an excellent guideline. The results suggested that ‘lY% recast metal could be used but recast metal must be clean. The hemispheric voids occurred clot to the pore&&nmetal interface. The evidence was fourfold: (1) with a APRIL
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Fig. 4. SEM fractographs with (a) exhibiting
“flat” void from region A in Fig. 3, c at higher magnification (c) illustrates matching area from Fig. 3, f. b, metal half of specimen depicts “adhesive” failure on lower third of triangle. d, shows matching porcelain half of specimen in b. Specimen in b and d contained 100% new metal. light microscope the dark metal oxide showed through the porcelain on the flat side of the hemispheric voids suggesting a thin porcelain layer; (2) the metal side of the voids were flat; (3) when the larger voids on the metal halves of the specimens were tested with an ohm meter and a probe, light pressure on the probe gave zero meter reading without resistance suggesting that the porcelain was thin, and easily fractured to give metalto-metal contact; and (4) Fig. 3, a indicates that the voids were primarily near the porcelain-metal interface. Note the high region running vertically through the triangle with the large void near the center. This region is relatively free of voids, particularly hemispheric voids, while many voids occur in the regions closer to the metal surface on the left and right of the high region. When hemispheric voids appear they result from the release of absorbed gases during firing. The flat voids showed metal-metal oxide on the metal halves of specimen Fig. 4, a without porcelain-to-metal adaptation over the corresponding area on the porcelain halves (Fig. 4, c). The Aat voids probably indicate that the porcelain did not wet the metal in this area during firing. The flat voids were the largest defects and seriously limited the strength of a restoration. THE JOURNAL
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Adhesive and cohesive failure This fractographic investigation indicated that the bonding between this particular porcelain and alloy was adequate and recast alloy did not seriously alter the bonding. Three subjects must be discussed to understand this finding. First, the specimens were designed so that the tensile stresses induced by loading would be greater at the interface than in the porcelain.’ This is important when concluding that cohesive failure implies that the adhesive bond is sufficient.8 For example, cohesive failure could be induced by higher stressesin the porcelain than at the interface even if the adhesive bond was inadequate. Because of the experimental design, failure of tested specimens was likely initiated close to or at the sharp apex of the specimens. This region of the specimen is indicated by the arrow labeled “interface between metal and porcelain” in Fig. 1, a and by arrow A in Fig. 2, a, c, and g. Second, the adhesive failure was probably not pure interfacial failure. Diffusion is likely between the various phases without a clear interfacial boundary between the porcelain and metallic oxide as noted by Anusavice et a1.12However, it is appropriate to identify the various 451
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failure modes as closely as the results will allow. The region identified as “adhesive” failure with a light microscope was shown, with SEM, to consist of two different types of failure (Fig. 4, b and d). Limited failures occurring at the metal-oxide porcelain interface can be described as adhesive failure or class II, according to O’Brien.” Interspersed among the regions of adhesive failure were patches of porcelain indicating a cohesive class III’O failure within the porcelain. The composition of this porcelain could have been altered by diffusion of the metal-oxide layer. Third, fracture data were obtained for all specimen groups. Analysis indicated that the specimens may have been precracked during preparation. Consequently, the W, data are not reported. The possibility of precracked specimens would not have altered our conclusion concerning interfacial defects because defects occurred during firing of the porcelain. However, it cannot be concluded that the “adhesive” failure, as identified by light microscopy, occurred during testing. Regardless of how the “adhesive” failure was initiated, failure resulted from mechanical stress. There was no relationship between the occurrence, amount, or location of “adhesive” failure, and the specimen’s content of recast alloy. If the bonding between porcelain and alloy were altered seriously by recast alloy, interfacial precracking would have been more extensive and evident in the fractographs as an increase in “adhesive” failures. Similarly, if the failures occurred during testing the fractographs would have indicated that the bonding was seriously altered by the recast metal. The strongest evidence for adequate adhesive bonding was the characteristics of the region identified as “adhesive” failure by light microscopy. This region on the metal halves revealed portions of metal oxide surface covered with patches of porcelain. If the adhesive bond was weak in the porcelain, more metal-oxide surface would have been evident whether “adhesive” failure was induced during testing or specimen preparation. If the porcelain close to the metal-oxide surface was weakened by diffusion relative to the porcelain farther from the interface, “adhesive” failure would have predominated. This is because the specimens were designed to produce the highest stresses at the interface. However, cohesive failure was the predominant mode of failure. The SEM fractographs indicated that the adhesive bond between the porcelain and alloy was adequate.
CONCLUSfON Fracture and fractographic techniques were used to investigate the adhesion of a dental porcelain to a recast gold-palladium alloy. A coping of 50% new metal for a porcelain-metal restoration was an adequate safety margin. However, there were no adverse effects noted with 452
AND
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75% recast metal but serious changes were ;ounti .:cai’ 85% or more recast metal. Poor cleaning prctceduiw resulted in a more contaminated alloy so it was suqgesd that the 50% guideline be followed and the recast metals be cleaned thoroughly. The use of a recast metal totaling 85% 0; more increased the frequency and size of interfacia! voids. This was serious because failures initiated at a void are enhanced by stress concentration and the increased frequency of voids raises the chance of a defect being in an area of high stress. An enlargement in the size of a failure-inducing void reduces the load required for failure. Consequently, the probability of failure is greater for a porcelain-metal restoration with 85% or more recast metal. The predominant mode of failure at catastrophic failure rates was cohesive. Some specimens had limited “adhesive” failure. However, adhesive failure could not be linked to the percentage of recast metal. SEM fractographs indicated that the adhesive bond was adequate and there was no evidence that recast metal altered the bonding. We acknowledge the assistance of J, F. Jelenko and Co.. Armorrk, N. Y. for supplying the Olympia metal; Mr. Paul J. (:ascone, hIanager. Research and Development at Jelenko, for his advice and assistance; and Mr. John Lavicka, Dental Ceramics, Garlield Heights, Ohio.
REFERENCES I.
2. 3.
4. 5.
6.
McLean JW: The science and art of dental ceramics. III The Nature of Dental Ceramics and Their Clinical Use. Chicago, 1979, Quintessence Publishing Co Inc, vol 1. pp 55-95. Shell TS, Nielsen JP: Study of the bond between gold alioys and porcelain. J Dent Res 41:1424, 1462. McLean JW, Seed IR: Bonding of dental porcelain to metal. 1. The gold alloy/porcelain bond. Trans Br Ceran SOC 72~229, 1973. Knapp JF, Ryge G: Study of bond strength of dental porcelain fused IO metal. J Dent Res 45~1047, 1966. Anthony DS, Burnett AP, Smith DL, Brooks MS: Shear test for measuring bonding in cast gold alloy-porcelain composites. J Dent Res 49:27, 1970. Heshy DA, Kobes P, Garver DG, Pelieu GB Jr: Physical properties of a repeatedly used nonprecious metal alloy. J PROSTHET DENT 44:291,
1980.
Rasmussen ST: Fracture studies of adhesion. J Dent Res 57:11, 1978. 8. O’Brien WJ, Rasmussen ST: A critical appraisal of dental adhesion testing. In Mittal KL, editor: Adhesive Joints: Formation: Characteristics, and Testing New York, 1984, Plenum Publishing Carp, pp 289-305. 9 Elssner G, Barrish WF, Pabst R: Fracture toughness of motatto-ceramic ,joints as a function of environment. In Hastings GW, Williams DF, editors: Mechanical Properties of Biornattrials. New York, 1980, John Wiley & Sons Inc,.pp 265-274. IO. O’Brien WJ: Cohesive plateau theory of por&Gn-&y bond@g.. In Yamada HN, editor: Dental Porcelaiti The State of the Art-1977. Los Angeles, 1977, University of Sou&wn ‘GaGorma, School of Dentistry, pp 137-141. 7.
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11. Rasmussen ST, Patchin RE, Scott DB, Heuer AH: Fracture properties of human enamel and dentin. J Dent Res 55:154, 1976. 12. Anusavice KJ, Ringle RD, Fairhurst CW: Identification of fracture zones in porcelain-veneered-to-metal bond test specimens by ESCA. J PROSTHET DENT 42417, 1979.
DR. ASTERIOS DOUKOUDAKI~ CASE W&TERN RESERVE UNIVERSITY SCHOLL OF DENTISTRY CLEVELAND, OH 44106
Role ef mmganese in alloy-porcelain
bonding
K. Bruggers, B.S.,* C. Corcoran,* E. E. Jeansonne, D.D.S.,*. and N. K. Sarkar, M.Sc., Ph.D.*** Louisiana
State University,
School of Dentistry,
New Orleans,
P
revious studies have shown that chemical bonding of dental porcelain to nickel-chromium ceramic alloys was dependent on oxidation-reduction reactions.‘-’ These reactions involved the major elements Ni and Cr, as well as minor elements such as B, Be, Al, MO, Si, Fe, etc., that may be present in this complex alloy system. Past research has emphasized the role of major elements in porcelain-alloy chemical adherence with sparse attention directed to the minor elements. However, some of these elements may be crucial to the formation of a strong porcelain-alloy bond. Manganese is a minor constituent of many nickelchromium casting alloys and the subject of the present study. Manganese has a highly oxidizing nature. One of its oxidation products (MnO) has the ability to form limited solid solutions with SiO*, which is a major component of dental porcelain.5-* The objective of this study was to confirm that Mn performed a significant role in alloy-porcelain chemical interaction.
MATERIAL
AND METHODS
The samples consisted of the following material: (1) Mn-containing nickel-chromium alloy (Wiron-S; Williams Gold Refining Co., Buffalo, N.Y.) with Ni, 70.5%; Cr, 16%; MO, 4%; Al, 3.4%; Mn, 3.5%; Si, 1.3%; Co, 1.2%; and Fe, 0.1% and (2) a commercial dental porcelain (Biobond; Dentsply International Co., York, Presented in part at the American Association for Dental Research, Los Angeles, Calif. Supported by BRSG grant No. RR 050704 from the National Institute of Dental Research, Bethesda, Md. *Student Assistant. **Dean Emeritus. ***Professor and Head, Department of Biomaterials. THE JOURNAL
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La.
Pa.). The alloy has demonstrated excellent adherence to porcelain.9 Two specimens of the following samples were prepared according to the manufacturer’s recommendations: (1) as cast flat (2 by 4 by 8 mm) sample of the alloy finished with 600-grit Sic (Buehler Ltd., Lake Bluff, Ill.) paper, (2) sample No. 1 vacuum degassedat 940’ C for 2 minutes, and (3) alloy-porcelain couple made by fusing porcelain on sample No. 2 with sample No. 1 as the control. Scanning electron microscopy (SEM) (Model 1000; Advanced Metals Research, Burlington, Mass.) and energy-dispersive x-ray analysis (EDXA) (PGT 1000; Princeton Gamma-Tech, Princeton, N.J.) were used to characterize the surface morphology and chemistry of samples 1 and 2 and the interface morphology of a polished section (0.5 pm) of sample 3. After the debonding of sample 3 by three-point bending, the debonded alloys and porcelain were also examined by SEM and EDXA to ascertain the bonding mechanism.
RESULTS The surface morphology of the alloy, before and after degassing, is shown in the SEMs in Fig. 1, A through C. The scratch marks from grinding were apparent on both surfaces. Moreover, the degassed surface was characterized by the presence of a localized growth above a thin, uniform deposit. Typical x-ray analyses from the control and degassed surfaces are displayed in Fig. 1, D and E. The degassedsurface indicated a higher concentration of Cr and Mn than did the control surface. X-ray analysis demonstrated that the major Mn line (Mn-K,) occurred in conjunction with the Cr-K, line. Therefore the Mn-kB line was used to identify Mn in this study. When the 453