New Approach for Evaluating Metal-Porcelain Interfacial Bonding Napa Suansuwan, BSc, DDS, Grad Dip Clin Se, MSc' Michael Vincent Swain, BSc (Hons), PhD'' Purpose: The purpose of this study was to evaluate the honding characteristics of porcelain-fused-to-metal (PFM) systems hy determining the strain energy release rate associated with interface fracture of porcelain and metals. Materials and Methods: Porcelain-veneered metal plates cast from commercially pure titanium and 3 metal alloys (gold, palladium, and nickel-chromium alloys) were made to dimensions of 25 mm x 8 mm X 2,5 mm with comparable thicknesses of porcelain and melal. The porcelain side of the specimens was notched to the interface with a thin diamond saw, and a small precrack was initiated at the metal-porcelain interface. The samples were subjected to a limited number (typically less than 4) of load-unload cycles under 4-point bending at a crosshead speed of 0,1 mm/min. The loading and unloading force displacements associated with stable crack extension were recorded. The strain energy release rate was calculated. The interfacial area was also examined under scanning electron microscope (SEMI after the test. Results: The mean strain energy release rates were 72.7 ± 10,0 |/m-, 58,5 ±13,5 l/m-, 39,4 ±4,3 J/m^ and I6,6± 2,5 |/m^ tor the samples of gold, palladium, nickel-chromium alloys, and titanium, respectively. The SEM photographs showed that the crack occurred in the porcelain layer close to the interface. Conclusion: The bonding characteristics of PFM systems were determined with .^i types of metal alloys and commercially pure titanium by a fracture mechanics approach. The gold alloy and titanium are considered to obtain the greatest and least adhesion, respectively. The test system has proven to be a simple and reliable approach lo determine the bonding in bimaterial systems. Int j Prosthodont 1999:12:547-552. G old alloys have been used in dentistry as both cast and porcelain-fused-to-metal (PFM) restora- tions for decades because of their accuracy in the casting process and durability and corrosion resis- ^Assistant Professor, Department of Prosthodontics, Facuity of Dentistry, Khan Kaen University, Thailand. "Professor, Department of Mechanics and Mechatronics, Facuity of Engineering: and Department of Dentai Materials Science, Faculty of Dentistry, The University of Sydney, Austraiia. Reprint requests: Prof Dr Michaei Vincent Swam. Biomateriais Science Research Unit. Faculty of Dentistry, The University of Sydney, Suite Cli, National Innovation Centre, Australian Technology Park, Eveleigh, NSW 1430, Australia. Fax: + 61-2- 9351 !8IS. e-maii mswain<Smail.usyd edu.ou tance in the aggressive conditions of the mouth,' However, their disadvantages, ie, high density, low elastic modulus, poor sag resistance at porcelain fir- ing temperature, and relatively high cost, especially of high-gold alloys used in PFM restorations, stimu- lated a search for alternatives.^'- Many types of den- tal aiioys, such as Pd, Co-Cr, and Ni-Cr alloys, have been developed as economical alternatives that also have a lower density and higher elastic modulus,'-^ Recently, commercially pure titanium and titanium alloys have been introduced to replace these metal- lic alloys because of their mechanical and corrosion resistance properties compared to those of noble- metal alloys. They are also lightweight, inexpensive, and biocompatible."' • 12, Number 6, 547 The InternaiionsI lournal of Proslhodontk
Transcript
New Approach for EvaluatingMetal-Porcelain Interfacial Bonding
Napa Suansuwan, BSc, DDS, Grad Dip Clin Se,MSc'
Michael Vincent Swain, BSc (Hons), PhD''
Purpose: The purpose of this study was to evaluate the honding characteristics ofporcelain-fused-to-metal (PFM) systems hy determining the strain energy release rateassociated with interface fracture of porcelain and metals. Materials and Methods:Porcelain-veneered metal plates cast from commercially pure titanium and 3 metal alloys(gold, palladium, and nickel-chromium alloys) were made to dimensions of 25 mm x 8mm X 2,5 mm with comparable thicknesses of porcelain and melal. The porcelain sideof the specimens was notched to the interface with a thin diamond saw, and a smallprecrack was initiated at the metal-porcelain interface. The samples were subjected to alimited number (typically less than 4) of load-unload cycles under 4-point bending at acrosshead speed of 0,1 mm/min. The loading and unloading force displacementsassociated with stable crack extension were recorded. The strain energy release rate wascalculated. The interfacial area was also examined under scanning electron microscope(SEMI after the test. Results: The mean strain energy release rates were 72.7 ± 10,0 |/m-,58,5 ±13,5 l/m-, 39,4 ±4,3 J/m^ and I 6 , 6 ± 2,5 |/m^ tor the samples of gold, palladium,nickel-chromium alloys, and titanium, respectively. The SEM photographs showed thatthe crack occurred in the porcelain layer close to the interface. Conclusion: The bondingcharacteristics of PFM systems were determined with . i types of metal alloys andcommercially pure titanium by a fracture mechanics approach. The gold alloy andtitanium are considered to obtain the greatest and least adhesion, respectively. The testsystem has proven to be a simple and reliable approach lo determine the bonding inbimaterial systems. Int j Prosthodont 1999:12:547-552.
Gold alloys have been used in dentistry as bothcast and porcelain-fused-to-metal (PFM) restora-
tions for decades because of their accuracy in thecasting process and durability and corrosion resis-
^Assistant Professor, Department of Prosthodontics, Facuity ofDentistry, Khan Kaen University, Thailand."Professor, Department of Mechanics and Mechatronics, Facuityof Engineering: and Department of Dentai Materials Science,Faculty of Dentistry, The University of Sydney, Austraiia.
Reprint requests: Prof Dr Michaei Vincent Swam. BiomateriaisScience Research Unit. Faculty of Dentistry, The University ofSydney, Suite Cli, National Innovation Centre, AustralianTechnology Park, Eveleigh, NSW 1430, Australia. Fax: + 61-2-9351 !8IS. e-maii mswain<Smail.usyd edu.ou
tance in the aggressive conditions of the mouth,'However, their disadvantages, ie, high density, lowelastic modulus, poor sag resistance at porcelain fir-ing temperature, and relatively high cost, especiallyof high-gold alloys used in PFM restorations, stimu-lated a search for alternatives.^'- Many types of den-tal aiioys, such as Pd, Co-Cr, and Ni-Cr alloys, havebeen developed as economical alternatives that alsohave a lower density and higher elastic modulus,'-^Recently, commercially pure titanium and titaniumalloys have been introduced to replace these metal-lic alloys because of their mechanical and corrosionresistance properties compared to those of noble-metal alloys. They are also lightweight, inexpensive,and biocompatible."'
• 12, Number 6, 5 4 7 The InternaiionsI lournal of Proslhodontk
New Approach for Evalualing Metül-PorcelLiin Bonding Su snsu wan/Swain
Apart from these mechanical properties of Ihe metalalloys required for PFM restorations, the alloys mustform a strong bond with the compatible porcelain.Shear tests, which are simple to perform, are the mostpopular, and several metal-ceramic bond tests havebeen done in this mode with different test configura-tion designs.^"-' The planar shear test is claimed to bea well-suited design for evaluation of metal-ceramic.'-Other studies use a flexural mode, ie, 3- and 4-pointbending tests. In this mode, porcelain was bonded tothe tension surface of a metal bar only in the middlepart.--'--" However, the result in all of these studies isquoted as the critical stress at bond failure. Some re-searchers have calculated a "bond strength ratio" bydividing the maximum stress required to break thebond by the elastic modulus ofthe metal substrate tominimize the influence of elastic modulus on the frac-ture strength. "*' ^ However, neither bond strength norbond strength ratio represent the energy or work re-quired to separate porcelain from metal. Rather, theymeasure only the comparative stresses to initiate cat-astrophic breaking ofthe specimen.
As a result of the inherent limitations of shear tests,and also in an attempt to compare the actual charac-teristics ofthe stress distribution in different bond tests,finite element stress analysis has been reported. - "These stress distribution characteristics have been com-pared with the experimental observations of fracture lo-cation and type of failure.^'••'' In the 3- and 4-pointbending tests there is a severe stress concentration at theporcelain endpoints.' ' However, the stress distributionin shear tests shown in the finite element stress analy-sis resulted from a force ideally applied at the metal-porcelain interface, which does not occur experimen-tally. Eor this reason, the failure of the bonded systemmay possibly be initiated within the porcelain, close tothe interface but not on it, in which case the failure iscaused by tensile stresses. ' ^ The studies also showedthat the bond strength could change with specimengeometry, loading configurations, or material stiffnessas these give rise to different stress distributions at thebonded interface.^" It is also important to remember thatalloy with an elevated elastic modulus will resist bend-ing to a greater extent, thus creating the impression ofahigherbond.^^Therefore, the traditional bond strengthdata based on static load-to-failure tests should not beused to characterize the bond. Another approach,which is proposed as an alternative to the IntemationalStandards Organization 9693 test, considers the 3-point flexure of a metallic strip partly coafed with ce-ramic. Although in this test Young's modulus and thethickness ofthe metal substrate are taken into account,this is also a bond strength measurement test. ^
Recently, there has been an upsurge of interest inthe characterization of interfacial fracture toughness
of dissimilar materials.^* There have also been somestudies determining the fracture resistance of bimate-rial interfaces, including metal-ceramu" inierfaces,based on the concepts of fracture mech.mics.-' " A4-point bending test has been used to demonstrate thecharacteristics of crack propagation and u¡ show thesensitivity ofthe fracture resistance to moisture througha stress corrosion mechanism,^^'*" The test method hasbeen recently applied to evaluate the interfacial frac-ture toughness of a titanium-porcelain bonding sys-tem.^^ From these studies, it has been suggested thatthe bimaterial bend test geometry combined with in-terfacial fracture toughness measurement is an ap-propriate alternative method for studying the bondingcharacteristics in a PFM system. In the present studythe strain energy release rate at the metal-porcelain in-terface of 3 types of dental alloys and commerciallypure titanium bonded to compatible porcelain was de-termined using a 4-point bending test.
Materials and Methods
Twelve rectangular plates (8 mm x 25 mm x 1.4 mm)were cast for each group of gold, palladium, andnickel-chromium alloys and for titanium using a cen-trifugal technique, except for titanium. An argon-pressure, one-chamber casting machine (AutocastHC III, CC) was used to cast the titanium. The goldalloy (KIK, Ishifugu Metal) is classified as Au-Pd-Pt^and the palladium alloys (KIK Wing, Ishifugu) as Pd-Sb-ln. The nickel-chromium alloy (UniMetai, Shofu)contains 77.0% nickel, 14.9% chromium, and 8.1%other elements. The titanium used in this study iscommercially pure titanium (ASTM Ti Grade 2, T-Alloy M, CC).
The cast metal plates were ground with silicon car-bide paper to achieve flat and smooth surfaces. Oneof the 8 mm x 25 mm surfaces was grit blasted with50-|jm AUO^. The specimens were cleaned in an ul-trasonic bath before bonding to porcelain. VintageHalo porcelain (Shofu) was used for the gold, palla-dium, and nickel-chromium alloys and VitaTitankeramik porcelain (Vita) was used with titanium.The porcelain was applied onto the grit blasted surfaceofthe metal plates and baked layer by layer atthe tem-peratures recommended by the manufacturers. Thepaste bonder of Vita Titankeramik was applied to tita-nium plates as the first layer, followed by the normalsteps of opaque-and body-porcelain application.
After firing, both 8 mm x 25 mm surfaces of thespecimens were ground flat and smooth but still re-tained an even layer of each material. The edges ofthe specimens were also ground at right angles to theprevious surfaces. Notching across the width andentirely through the depth ofthe porcelain layer at the
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Suansuwan/Swairi New Approach br Ev^lualing Metdi-Pomeiain Bonding
Fig 1 (left) Initial inlerfaciai crack (precrack) af the bottom offhe notch.
Fig 2 (beiowj Tesfing configuration and relative dimensions ofsystem. P= ioad necessary lo sfably propagate crack; ü = spec-imen width, h = tolal specinnen fhicitness; Í = moment arm or dis-fance between inner and outer load iines (rollers) on the same side.
middle of fhe specimen was performed using a rotarydiamond cutting blade with kerosene lubricant,which gave an accurate notch width of 0.4 mm. Thenotch was filled with kerosene when a precrack wascreated in a bending jig in which the applied load iscontrolled by a screw knob and the specimen is sup-ported completely on rubber. The precrack startedfrom the base of the notch and extended along the in-terface with a total length of 2.0 mm (Fig 1). Withoutthe presence of a small precrack, it was found that thespecimen had to be overloaded to initiate crackgrowth, often resulting in complete debonding of theporcelain from the metal substrate.
The precracked specimen was then placed on a 4-point bending jig mounted in a universal testing ma-chine (Autograph AG-50kNE, Shimadzu). The speci-men was subjected to a limited number of load andpartial unload cycles (typically less than 4) at acrossheadspeedofO.i mm/min until the crack reachedthe inner rollers. During crack extension, the cracklength was estimated using a measuring gauge with atraveling light microscope located in front of the jig. Foraclear view ofcrack length, one side of the specimenwas thinly painted with white correction liquid(Coverup, Marbig Rexel), which readily revealed thecrack beneath during loading. The load and crossheaddisplacement data were collected for calculation.
The testing configuration shown in Fig 2 illustratesthe relationship between the sample and the rollersofthe4-point bending jig. The strain energy release
rate, G, is given by^^
where Pis the load necessary to stably propagate thecrack, / is the moment arm or distance between innerand outer load lines (rollers) on the same side, v^ andE arethe Poisson ratio and elastic modulus of metalsubstrate, respectively, and band /i are the specimen
width and total thickness, respectively. The nondi-mensional parameter T| is calculated by
/here
1
/]„
h
),
+ * h + 3* h ¡\tl * h
£J1-V)
V and E are the Poisson ratio and elastic modulusor porcelain, respectively, and h and b^ are thethickness of porcelain and metal layers, respectively.As T) increases with increase in relative specimenthickness,-*^ ''p-Ai/ ='" specimens in this study have aralio of thickness of approximately 1 ;1 .
A feature of the expression relating the strain en-ergy release rate, C, (equation 1 ) to measured para-meters is that it is independent of interface crackiength, provided that the crack lies within the innerrollers. In a previous study it was shown that thevalue of C calculated from equation 1 was in goodagreement with a simple estimation of the fracture en-ergy per unit area of crack surface generated, as de-termined from the area under the force displacementcurve and fracture surface created.-"
Results
A typical load-displacement curve demonstrating theload-unload cycles is shown in Fig 3. The specimenstiffness (F/Ô) is initially constant up to a critical loadforthe onset of stable crack extension, whereupon, asanticipated from equation 1, the load remains almostconstant, although the stiffness decreases with crack ex-tension. The mean load at the plateau area of eachcuive, which is associated with stable crack extension,was used to calculate fhe value of C; this is presentedwith the Poisson ratio and elastic modulus values of tbe
Volume12, Number 6, 1999 549 The lnlernalion.il lournai of Prosthodortics
New Approach lor Evaiuating Metsi-Porcelain Bonding Suanîu wan/Swam
Table 1 Characteristics of 4 Metal Aiioys Bonded toPorcelain
G= resultant strain energy release rate^ E= elastic modulus, w = Poisscn
Û 0.02 0.04 0 Oe 0.08 0,10 0.12
Dispiacemant (mm)
Fig 3 Load-dis placement curve illustrates the plateau duringcrack extension.
Fig 4 Intertaciai area ot goid (Au) bonded to porceiain (P)shows ttie reaction zone (R) and the crack ¡Cj.
.* ^ • • • ¿
Fig 5 Intertaciai zone ot paliadium (Pd) bonded to porcelain (Pjshows the reaction zone (R) and the crack (C).
metal substrates in Table 1. The difference between themeans of the C values of each group was significant{P<0.05).
Scanning electron microscope (5EM) photographs ofcross-sectioned samples showed the location of thecrack and the unique characteristics of the reactionzone at the interfacial area of the gold and palladiumspecimens (Figs 4 and 5). Similar images ofthe inter-face region and cracking for nickel-chromium alloy andtitanium are shown in Figs 6 and 7. The crack in allgroups was located at the interfacial area and wasmore likely to be in the porcelain. From observation,the crack in the palladium and gold sampies was moreirregular than in the nickel-chromium and titaniumsamples. There were some small cracks extending fromthe interface into the inner layer of porcelain in allgroupsexcepttitanium. A reaction zone was noticeablein the gold and palladium samples but was hardlyseen in the nickel-chromium and titanium groups.
Discussion
The present technique has a number of advantagesover previous estimations of bond strength (bothtensile and shear). Recent finite element analysis ofshear bond strength tests indicated that a very com-plex stress situation exists about the interface, whichmay result in crack initiation within or propagatingthrough the porcelain rather than along the metal-porcelain interface.^"'•^^ In the present study, thespecimen geometry and the precrack made beforetesting contributed to the observed stability of crackextension along the Interface. It prevented high stressconcentration and specimen overload from occur-ring at the initiation of the crack growth. Such fea-tures could be misleading because the force em-ployed for breaking the bond would be higher thanit should be, and tbat fracture tended to be very un-stable.
Tlie Inlernational lournal of Pro5tliodontf< 550 Volume 12, Number 6, t
Suaiisuwan/Swain New Approach for Evaiuating Melal-Porcclain Bonding
Fig 6 Interfaciai area of nickel-chromium (Nij bonded to porce-lain IP) shows the crack (C) located ciose to the interface.
Fig 7 Interfaciai area of tuanium (Til bonded to porceiam fP)shows tiie crack (C) iocated at the interface.
One ofthe advantages of using this 4-point bend-ing technique is that it is simple to perform experi-mentally. The interfacial strain energy release rate, C,is also a simple function of the critical load observedduring crack propagation; other parameters remainconstant for a given specimen dimension and elasticproperties, Tbe test has also been found to be highlyreproducible, even with a 20% to 30% variation inthe ratio of h :h^ in the test geometry, provided thatallowance for the r\ dependence of this ratio is in-corporated into the analysis. Therefore, this test canbe used for almost any bimaterial system.
It has been suggested that aqueous environments candecrease the interfacial fracture resistance or increasethe rate of slow crack growth by the mechanism ofstress corrosion,^ ' ® The present study minimized tbeinfluence of moisture by using kerosene to fill the notchand crack during testing. This was considered to be sim-pler and more economical than putting the entire sys-tem in an H^O-free environment, even though it maynot entirely prevent such reactions from occurring.
The gold-porcelain system in this study gave fhehighest value of strain energy release rate, whereaspalladium-, nickel-chromium-, and titanium-porce-lain systems presented weaker bonds, respectively.The previously reported bond strength data varywidely. Some studies on shear mode^''^ and flexuralmode" have suggested that the bond strength ofnonprecious alloy-porcelain is higher than that ofnoble alloy-porcelain, while in others the oppositehas been reported,^!^ It has been reported that thebanding of titanium to porcelain is much weakerthan that of palladium-copper alloy to porcelainwben the load at tensile bond failure from the 3-point bending test was measured,"" However, another
5tudy using a shear test based on a torsional loadshowed that the bonding of titanium to porcelainwas better than that of the gold-porcelain system.-•'These results obviously vary and are noncomparable.
The SEM photographs showed a reaction zone inthe gold and palladium specimens, which occurreddur ing firing of the porcelain. The reaction zone con-tained meta I oxides from the bonded metal alloy andporcelain. This zone appeared to be brittle and al-lowed the crack to pass through it in some areas.Although the reaction zone in nickel-chromium andtitanium samples is not clearly seen in the SEM pho-tographs, their weak bond allowed the crack to prop-agate mostly along the interface.
Conclusion
The bonding characteristics of 4 dental alloys bondedto porcelain were determined in terms of the strain en-ergy release rate, G. The gold-porcelain samples gavethe highest value of C, while palladium, nickel-chromium, and titanium samples exhibited lower val-ues, respectively. The SEM photographs ofthe cross-sectioned interfacial area revealed that the crack waslocated mostly in porcelain, close to the interface, andalso showed the wide reaction zone between metaland porcelain in the gold and palladium samples.This interfacial 4-point bending test provides a moreintrinsic estimation of interface properties resultingfrom the stable crack extension along the interface.The present measurements show a high degree of re-peatability of the value determined for the interfacetoughrtess when allowing for the specimen dimen-sions. The test has also proven to be a simple approachto determine the bonding in bimaterial systems.
12, Number 6, 1 551 The International lournal of Proithodontii
New Approath for ng Metal-Porcelain Bonding Suansu wan/Swain
Acknowledgments
Theauthoiswish to iicknovvleciiie ihe support provided by IshifuguMetai industry, Tokyo, |apan, tor the loan of the gold and palla-dium alloys. We also appreciate the generosity of Shofu, Kyoto,Japan, tor providing the nickei-chromium alloy and dental porce-lain; and Vita Zahnfabrik, Bad Säckingen, Germany, for the tita-nium porcelain used in ihis study.
References
1, Brown D, Curtis RV. Alternatives to gold. Dent Update1992;19:325-326, 328-330,
2, Anusavice KJ, Noble metal alloys for metal-ceramic restora-tions, DentClin Norlh Am 19S5;29:789-e0J.
3, Craig RC, Restorative Dental Materials, ed 9, St l.ouisi Mosby,1993:491-501,
4, Lautenschiager EP, Monaglian P. Titanium and titanium alloysas dental materials, int DentJ 1993;43:245-253,
5, Mofla ]P, Lugassy AA, Cuckes AD, Gettleman L, An evaluationof nonprecious alloys for use with porcelain veneers. Part 1,Piiysicai properties, J Prosthet Dent 1973;30:424-431.
(i. Wight TA, Bauman JC, Pelleu GB. An evaluation of four variablesaffecting the bond strengtii of porcelain to nonprecious alloy. IProsthet Dentl977;37:57Ü-577,
7, Lubovich RP, Coodfcind RJ. Bond strength studies of precious,semiprecious, and nonprecious ceramic-metal alloys with hvoporcelains. J Prosthet Dent l977;37;38B-299.
12, tdammad lA, Cocdkind R|, Gerberich WV\' A shear test fot thebond strength of ceramometals. I Prosthet Dent 1987:56:431^37.
13. Uusalo EK, Lassila VP, Yli-Urpo AU, Bonding of dental porce-lain to ceranijc-metal aiioys. J Prosthet Dent 19B7;57:26-29.
14. Murai<ami I, Vaidyanathan J, Vaidyanatban TK, Schjiman A,interactive effeas of etching and pre-oxidation on porcelainadherence (o non-precious alloys: A guided planar shear teststudy. DentMater1990;6:217-222.
15, Hammad lA, Stein RS. A qualitative study for the bond andcolorofceramometals. Parti.J Prosthet Dent 1990;63:643-653,
16, Lund PS, Davis PW, Shear bond strength of textured opaqueportelain, Int | Proslhodont 1992;5:505-509,
17, Inoue K, Murakami T, Terada Y. The bond strength of porcelainto Ni-Cr alloy—The influence of tin or chromium plaling, int |Prosthodont 1992;5:262-260,
18, Wagner w e , Asgar K, BigelowWC, Elinn RA. Effect of intcria-ciai variabies on metal-porceiain bonding. J Biompd Mater Res1993;27:531-537.
19. Hurzeier MB, Kohal R, EiscberC, Eischer], Strub |R. in vitro andin vivo evaluation of a new opaque system for metal-ceramicrestorations, int | Prosthodont 1995:8:142-149,
20, Persson M, Bergman M, M eta I .-ceramic bond strength. ActaOdontol Scand 1996;54:16O-165.
2 1 , Deger S, Canikliogiu MB, Effects of lin pialin.- '!•> base metalalloy.-ceramic bond strengtb. int| ProsthodonL I'l ^ ' ' . i ' ;l 65-172,
22, Hammad [A, Taiic VF, Designs of bond strengli i ü sts for metal-ceramic complexes: Review of tbe lileraturr. I Prosthet Dent1996:75:602-608,
23, CaputoAA, Dunn B, RetsbickMH, A flpxural mctbod for evalu-ation of metal-ceramic bond strengths, i Dent Res 1 977;56:1,501-1,506.
24, Scbaffer SP, An approacb to determining the bond strengtb ofceramometal systems. J Prosthet Dent 1982;4e:282-284.
25, |ocbenDG,CaputoAA,MatyasJ. Effect of metal surface treatmenton ceramic bond strength, | Prosthet Dent 19B6;55:186-188.
26, Jochen DC, CaputoAA, Matyas], Eftect of opaque porcelain ap-plication on strength of bond to silver-paiiadium alloys. J ProstbetDent1990;63:414.-4l8,
27, Wu Ï , Moser JB, lameson LM, Malone WFP, The effect of oxi-dation heat treatment on porcelain bond strength in selected basemetal alloys, ) Prosthel Dent 1991 ;66:439^44.
28, Rake PC, Coodacre CJ, Moore K, Munoz CA. Effect of twoopaquing techniques and two metal surface conditions on metal-ceramic bond strength, ) Prosthet Dent 1995:74:8-17,
29, Anusavice K|, DeiHoff PH, Eairburst CW. Comparative evalua-tion of ceramic-metal bond tests using finite element stressanalysis. J Dent Res 1960:59:608-613.
30, Van Noort R, Noroozi S, Howard IC, Cardew C, A critique ofbond strengtb measurements, J Dent 1989;17:61-67,
31, DeHoffPH, Anusavice KJ,HathcockPW. An evaluation of tbefour-point flexural test foi metal-ceramic bond strengtb. | DentRes 1982:61:1,066-1,069.
32, DeiHûff PH, Anusavice K|, Wang Z, Three-dimensional finite el-ement analysis of the shear bond test. Dent Mater 1995;11:126-131,
33, Len; J, Schwarz S, Schvvíickeratb I I , Sperner E, Schafpr A, Bondstrength of metal-ceramic systems in three-point flexure bondtest, J Appl Bromater 1995:6:55-64.
34, Mencik J. Mechanics of Components witb Treated or CoatedSurfaces. Dordrecbl, The Netberlands: Kluwer Academic, 1996:92-176.
35, Charaldmbides PG, Lund J, Evans AG, McMeeking RM, A testspecimen for determining the fractLjre resistance ot bimaterial in-terfaces, J Appl Mech 1989:56:77-82,
36, Cao l i e , Evans AG, An experimental study of tbe fracture resis-tance of bimaterial interfaces, Mecb Mater 1989;7:295-304,
37 Reimanis E, Dalgleisb B|, Evans AG, The fracture resistance ot amodel metai/ceramif interface. Acta Metali Mater 1991:39:3,133-3,141,
38. Cazzato A, Eaber KT, Fracture eneigy of glass-alumina inter-faces via tbe himaterial bend test, J Am Ceram Soc 1997:80:181-188,
39. Chung HG, Swain MV, Mori T. Evaluation ofthe strain energyrelease rate for the fraclure of titanium-porcelain interfacialbonding, Biomaterials 1997;18:1,553-1,557.
40. Pang I, Gilbert |L, Chai J, Lautcnschlager EP. Bonding charac-teristicsof low-fusing porcelain bonded to pure titanium and pal-ladium-copper alloy, J Prosthet Dent 1995:73:17-25,
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