Effects of Tin Plating onBase Metal Alloy-Ceramic

Bond StrengthSahire Deger^M. Babiir Caniklioglu^

Purpose: This study investigated the metal-ceramic bonding of treated metal surfaces. Materialsand Methods: The study was divided into two parts. In Part I, the depth of tin diffusion from a tin-plated bone metal alloy surface was measured using an energy-dispersive spectrometer. In Part IIthe metal-ceramic bond strength was determined using a shear test. Results: The weakest bondingwas observed in the directly tin-plated group, and the strongest metai-ceramic bonding wasmaintained in the tin-diffused group. A controlled oxidation produced the greatest bond strengths.Conclusions: Wilh the base metal alloys tested, diffusion under the argon environment wasconducive to a stronger metal-ceramic bond. The metal oxidation rate should approximate theceramic vitrification rate, and the diffusion rate of the metal elements should be slower than thevitrification rate to obtain the strongest metal-ceramic bond, ini J Prosthodortl 1998:11:165-172.

Metal ceramic restorations are widely used inrestorative dentistry. The goal of metal ce-

ramic restorations is to combine the adaptationand strength of cast restorations with the estheticproperties of ceramic.'"^

The nature of the metal-ceramic bond has beenthe target of much research, and countless researchreports are available on this subject.®''" The term"oxide bonding" was first used in a study of Van derWaal's forces at the end of the 1960s. Since thenoxide bonding has been accepted as the most stabieand important force that influences the metal-ce-ramic interface.' In 1962, Shell and Nielsen bondedceramic material to gold using metal oxides formedby elements such as tin, accarding to tbe report of

'Research Assistant, Department of Fixed Prosthodontics, Facuity ofDentistry, University of Istanbul, Turkey.

'•Professor, Department of Fixed Prosthodontics, Faculty of

Dentistry. University of Istanbui, Turitey.

Reprint requests: Dr Sabire Deger, Research Assistant, Department

of Fixed Prosthodontics, Facuity of Dentistry, University of Istanbul.

Istanbui, Turiiey.

Presented in a preliminary form af the tsth Annuai Conference of

EPA, 7-9 September 1993.

Tuccillo and Cascone." However, the bondingmechanism betweert the base metal alloy and ce-ramic has yet to be well defined.^

Research on every phase of metal-ceramicbonding indicates that this is a complex mecha-nism.•'•^'''''^''^ Ceramic materials heated to theproper high temperatures bond tn the metal frame-work. During this bonding process metal ions dif-fuse into ceramic. The metal-ceramic boundarylayer is the key for successful bonding. The resultsof previous Studies have shown that different ele-ments in the alloy diffuse into the ceramic, andthe elements of the ceramic diffuse into thealloy.6,10-12 |p 3 5j|yip|e metal oxide formation, sat-uration is preserved if the diffusion rate of ceramicis less than the melting rate of oxide at the inter-face.** According to Pask and Tomsia," the phasesat an interface between a metal and ceramic mustbe in stable thermodynamic equilibrium for anelectronic structure across an interface to be real-ized. In the presence of a distinct oxide layer instable equilibrium with the metal, the metal-oxideinterface is saturated with the oxide. This condi-tion is maintained as the oxide thickness de-creases as a result of its dissolution by the porce-lain, because the diffusion rate is slower than the

N.imber2, 1 The Irternalioiial Journal of Prosthodoniics

Tin Plating and Alioy.Ceramic Bond Slrength Dcgor/CanikhoBlu

Tabie 1 EDS Measurement Results of Tin DiffusionFrom Wiron 99 and Bondi-Loy Aiioy Surfaces

Diffusiontime (min)

Diffusion depth (|jm)

15

30

60

Wiron 99

30(dense)

70(dense)

Bondi-Loy

45(dense)

75

actual solution rate at the interface. If the dissolu-tion process is not carried to complete solution ofthe oxide layer, then the properties of the remain-ing distinct oxide layers affect the physical proper-ties of the overall assembly. If the heating is car-ried beyond tbis point, tben saturation at themetal-glass interface is lost.

Base metal alloys are complex systems becausethey may contain numerous elements that maybave oxidation potential.^' Microstructural differ-ences between base metal alloys add more compli-cations.'^ Although base metal alloys have similarcompositions and mechanical properties as agroup, important differences are observed whenthey are cast and when bonded to ceramic.'^ Asolution to control the rate of oxidation betweenbase metal and ceramic is yet to be found, andresearch concerning excess oxidation is still inprogress."''''*'

The beneficial effect of tin in noble alloys is wellknown,'''^•''"^•'^ Tbe purpose of this study was totin-plate base metal alloys to determine (1) thedensity of tin diffusion into the alloy and the con-tent of the ceramic powder, and (2) the effect oftin plating and alloy thermal preparation on tbeceramic-metal bond strength.

Materials and Methods

Evaluation of tiie Deptit of Tin Diffusion From theTin-Plated Base Metai Ailoy Surface

Eighteen wax patterns 1 x 1 X 2 mm were pre-pared. Nine were cast using Wiron 99 {Begol, andthe other nine were cast using Bondi-Loy alloy(Krupp). Castings were cooled at room temperatureand then alloy surfaces were airborne particleabraded using 125 pm A l ^ ^ . Cast alloy specimenswere cleaned in an ultrasonic bath (Schütz-Dental)for 10 minutes. After this procedure, alloy surfaces

were plated with a tin layer approximately 1 \imthick using an electrochemical tin-plating unit (ILMLabor).

The tin-platod specimens were divided into threegroups, each containing three specimens of Wiron99 and three of Bondi-Loy. The samples in the firstgroup were heated from room temperature to1,000°C in an argon environment (Strohlein),maintained at tbis temperature for 15 minutes, andcooled in the furnace. The second sample groupsfor each alloy were again heated from room tem-perature to 1,000°C, but were maintained at thistemperature for 30 minutes, after which they werecooled in the furnace. The same procedure wasfollowed for a third group of samples which wereretained in the furnace for 60 minutes and cooled.

All tin-plated and diffused samples were embed-ded in Bakaiite (Epofix, Struers). Bakalite surfaceswere trimmed and polished in a special polishingapparatus (Polishing Machine, Scheuers). Surfacesof the prepared samples were gold plated prior toexamination using an electron microscope (Jeol733, Superprob). The diffusion depths of all sam-ples were measured using an energy dispersivespectrometer (EDS, Tracor) and recorded (Table 1).

EDS Analysis of Ceramic Powder Usedin this Study

VMK 68 ceramic powder (Vita. Metal-Keramik)was chosen to evaluate the function of tin in metal-ceramic bonding as this product has been shownto have a low tin content. An EDS analysis of VMK68 opaque 88 and dentin 548 was conducted.

Investigation of Metal-Ceramic Bond Strength

Three base metal alloys were investigated: a Co-Cr-based alloy (Bondi-Loy, Krupp) for metal ceramicstudies, a Ni-Cr-based alloy (Wiron 99, Bego) formetal ceramic studies, and a Ni-Cr-based alloy(Wirolloy, Bego) for metal-resin restorations. Theporcelain investigated was VMK 68 for metal ce-ramic studies.

Wax patterns were prepared from a stainlesssteel mold ÍEig 1). One hundred twenty wax pat-terns were fabricated, eacb measuring 0.5 cm indiameter and 5 cm in lengtb. Forty Bondi-Loy, 40Wiron 99, and 40 Wirolloy rods were cast fromthese wax patterns. Metal rods were trimmed usingtungsten carbide burs in one direction to eliminateany changes in the diameter.

The metal rods were divided into five groups fordifferent surface treatments before ceramic firing.In each group there were 24 specimens, eight from

The Internalional Journal of Prostliodonlics 1 6 6 Volume 11, Number 2, 1998

DcÊer/Caniiiiioglij TinPiatingandAiioy-Cc : Simd Strength

Fig 1 Stainiess steel moid in which wax patterns were fabri-cated. Fig 2 Expérimentai sampie placed on tbe adaptation device.

each alloy group. Metal rods in groups 1, 2, 3, and4 were plated with tin 1 im thick using the electro-chemical method. The samples of group 1 werethen put aside for porcelain firing. Specimens ofgroups 2, 3. and 4 were heated from room temper-ature to 1,000°C for the diffusion of tin from thealloy surface in an argon environment. Sampleswere kept at this temperature for 30 minutes andthen left to cool. Samples of group 3 were oxidizedunder 720 mm Hg vacuum for 1 minute al 900°Cin the ceramic furnace. Croup 4 test samples wereoxidized under atmospheric pressure for 1 minuteat 900°C. The surfaces of samples in group 5 weretreated according to the manufacturer's recom-mendations. Ceramic was then fired on all of thesamples.

The first and second opaque layers were fired 1cm in length and 0.3 cm below the tip of the metalrods. A silicone pattern mold was prepared to facil-itate standard dimension of porcelain onto therods. The metal sample was a rod with a diameterof 0.5 cm and a length of 4.5 cm, and having ametal ring with 1.5 cm minor diameter, 2 cm majordiameter, and that was 0.4 cm high al its top. Thecompleted sample was prepared in a lathe. Themetal piece was screwed vertically to the surfacefrom below, and a silicone mold that left the supe-rior surface of the ring open was placed. The sur-face of the samples on which the opaque porcelainwas fired was placed on the mold, and a slurry ofporcelain mixed with distilled water was vibratedinto the mold. Excess liquid was removed using tis-sue paper. The specimens were subjected to thefiring cycle prescribed by the porcelain manufac-turer and cooled to room temperature under aglass cover. Test specimens were fastened to thetest machine using the assembly designed byKayadeniz (Department of Metallurgy, Faculty of

Fig 3 Expérimentai sampie piaced on the adaptation devicean(i localized on the testing device.

Chemistry, University of Istanbul) and Caniklioglu.The function of this assembly, the adaptation de-vice, ensured the fixation of the test samples be-tween the parts of tbe testing machine(Losenhausen Machinenbau) and applied shearforce to the metal-ceramic interface (Fig 2).

A rubber ring was placed between the ceramicring and the adaptation device in every test sam-ple (Fig 3) before the shear-pull lest to ensure thatthe applied force was distributed uniformly overihe ceramic surface.

The loads at failure were recorded (Table 2) andthe fracture surfaces were inspected by the un-aided eye and recorded according to O'Brien'smetal ceramic failures classification.'^ Four failureloads were evaluated using analysis of variance(ANOVA), and groups that had significant differ-ences were determined by the Duncan test with asignificance level of 0.05.

11 Njmher2, I99e 1 6 7 The Intern a tion, i oí Prostliodontics

TinPlatinRandAliov-Ce : Bond Strength Deger/Canikiioglu

Fig 4a Element peaks ol VMK 68 opaque powder. Fig 4b Element peaits of VMK 68 dentin powder.

Table 2 MGan Bonding Strengths (± Standard Deviations) of Test GroupsGroupno.

123

4

5

Procedure

Tin platingTin plating and dillusionTin plating and diffusion andpreoxidafion in vactjumTin plating and diffusion andpreoxidation in atmosptiericConventional mettiod

Bondi-Loy

17.5 ± 5.1219.7 ± 97.6168.4 ± 106.9

82 ± 37.5pressure

48.8 ± 25.7

Wiron 99

35.4 ±10182.8 ±65.9140.8 ±64.1

109.1 ± 76.8

133.2 ±73.1

Wirolloy

66.5 + 38.2

120.4-1-45.7

56.1 ±41.8

141 3 ±61.9

Results

The results of the elemental analysis of the VMK 68opaque and dentin porcelain art; recorded ¡n Table3, and the element peaks are graphed in Figs 4aand 4b.

According to EDS results, the deepest diffusiondistance of tin from the Wiron 99 surface was ob-served at 1,000°C under argon environment withsamples kept under these conditions for 60 min-utes, and in Bondi-Loy samples that were kept for30 minutes. However, Wiron 99 samples kept for30 minutes at 1,000°C under argon environmenthave also shown dense diffusion areas (see Table1). The SnO^ content of ceramic powders wasrecorded as follows: 7,8% in VMK 68 opaque 88and 0.0% in dentin 548 {Table 3).

The lowest mean bond strength values were ob-tained in the first group, whereas the highest meanbonding strength values were found in the secondgroup for Co-Cr-basod Bondi-Loy and Ni-Cr-basedWiron 99 alloys that were manufactured for metalceramic studies (Fig 5). However, Ni-Cr-basedWirolloy alloy manufactured for use in metal-resinrestorations showed the lowest mean bond strengthvalues in the fourth group, and the highest meanbond strength values in the fifth group.

Table 3 Analysis Results of Hemicountable Elementsfor Ceramic PowdGr Samples Used in the Study

Opaque Dentin

Na^OMgO

Sib,CaO

TiO,KpFeOMnOSnO,BaOTotal (%)

4.30.0

14.141.9

1 913.511.1

0.70.37.84.3

99.9

4.70.5

15-658.0

2.20.8

11.80.00.30.06.1

100.0

Four different types of fracture surfaces were ob-tained: (1) adhesive failure between the metal andthe oxide layer (Fig 6a); (2j cohesive failure withinthe oxide layer (Fig 6b); (.3) adhesive failure be-tween the oxide and ceramic layers (Fig 6c); (4)and cohesive failure within the ceramic iayer (Fig6d). Cohesive failure in the ceramic layer was usu-ally seen in group 1. Croup 2 had different typesof failure surfaces. In groups 3, 4, and 5, cohesive

The international iQjrnai of Proslhodontii 168 Volumen,Number; , 1998

Dcger/Canikliogiu Tir Plating and Alloy-reriimic Bond Slrcrgtli

Fig 5 Column graph represent-ing mean bonding strength val- 350-

ÄT 200-

~ 150-

3tf

V, 100-

o"^ 50-

219.7

65.5

i75ñ~^B

1

132.81

'114.8

•^ 1 ^

2

S8 4

140.s11120.4•••_

3Groups

n Bondi-Loy

• Wiron 99

1 Wirolloy

141.3133.2 H ^

109.1

821 1

1P ^ 48.8•r

—•_!_4

••1•

5

Fig 6a Fracture surfaces showing adhesive failure between Fig 6b Fracture surfaces shewing cohesive failure in themetal alloy and the oxide layer. metal oxide layer.

Fig 6c Fracture surfaces showing adhesive tailure between Fig 6d Fracture surfaces showing cohesive failure in the ce-oxide and oeramic layer. ramie layer.

jme 11, Number 2, 1998 ' | ( , 9 The International Journal of Proslfiodontics

and Alloy-Ce : lîond Strtngth Dcgor/Canilflioglii

Table 4 Sample Numbers Determined Accordingto Failure Types Seen on the Sample Surfaces UsingShear-Pull Test

Group

1

2

3

4

5

Alloys

Bondi-LoyWiron 99WirolloyBondi-LoyWiron 99WirolloyBondi-LoyWiron 99WirolloyBondi-LoyWiron 99WirotloyBondi-LoyWiron 99Wirolloy

Type of bond failure

M-O

322 ..,,8

41

0

32645a77665

0-C

1

C

S66

42133

11223

Failures seen were as follows' M-0 - between metal-oxide tayer; O •=ir Ihe OKide layer; 0-C = between cerarric and oxide iayer; and O = ince ramio layer.

at 0.05 significance level using the Duncan test aregiven in Table 6. The bond strength values of Bondi-Loy and ceramic were higher wben the methodsused for groups 2 and 3 were compared with themethods of groups 1, 4, and 5. Significantly lowerbond strength values were found for Wiron 99 withthe method of the first group when compared withOther groups. However, thi; method used for thesecond group displayed significantly higher bondstrength values compared with the fourth group.Wirolloy had significantly low bond strength valuesin groups 1 and 4 compared with other groups.

Cracks tbat appeared after Ihe firing of porcelainwere another finding that had to be addressed. Ingroups 1, 2, 3, and 4, cracks were seen in the ce-ramic ring after porcelain firing. In the first groupthroe additional firings were needed to eliminatetbe cracks, and in the second, third, and fourthgroups, two additional firings were used. Thecracks were filled with the ceramic material antjthen refired.

Table S One-Way Analysis of Variance Method andAlloys

Degrees of Meanfreedom (n -1 ) square

SignificanceF oí F

Main etfects 6Metal alloys 2

44198.t1 12.339 0.0004266.824 1.191 0.308

Table 6 Significant Differences Accordingto Duncan Test

Alloys Group

Bondi-Loy

Wirolloy

•Indicates grojps that stiowed signiticant difterences at 0.05 teuel cfsignificance.

failures in the oxide layer were most frequent(Table 4),

Analysis of variance (ANOVA) showed that therewas a correlation between the differences in bondstrength values and the methods used in the groups(Table 5). The groups that were significantly different

Discussion

The lowest bond strength value was obtained in thefirst test group, and the highest bond strength valuewas obtained in the second test group (see Fig 5) forBondi-Loy and Wiron <99 alloys manufactured formetal ceramic use. However, Wirolloy manufac-tured for use in metal-resin restorations yielded thelowest bond strengths in the fourth group and thehighest bond strengths in the fifth group. Table 4shows tbat cohesive failures in tbe ceramic layerwere present in all three alloys of the first group. Inthe second group, failures were found only in someof the samples and at higher strength values. Theonly difference between the first and second testgroups was the thickness of tbe tin layer, withgroup 1 samples having thicker tin surface layers.Tbe thinner tin layer in group 2 was the result ofdiffusing the tin in an argon environment. From thisresult it is concluded that the presence of a tin layeron the alloy surface that exceeds a certain thicknessweakens the VMK 68 ceramic structure. It may bethat the increased amount of tin leads to devitrifica-tion, thus weakening the ceramic. The cracks seenafter porcelain firing of the test specimens in thefirst, second, third, and fourth groups is anotherfinding that supports this result. As stated previ-ously, the cracks seen during ceramic firing in thesecond, third, and fourth groups were eliminatedwith a second firing; for the samples of the firstgroup a third firing was neces5ar>' for crack elimina-tion. The presence of these cracks is a sign of ce-ramic contraction. These findings are similar to

logrral of Proslticdontics 170 Vclumeil,Number 2, 1998

Tinl=Lilin¡5.intlAliny-Cc iHondSlicngth

those of McLean's study,^" in which the effect oftin-layer thickness on the bond strength betweenthe ceramic and noble alloy interfaces was investi-gated.

Table 7 shows there are only minor differencesbetween Wiron 99 and Wirolloy. Cobalt is presentin Bondi-Loy instead of Ni, and tbere are minordifferences in other components. The studyshowed that different bond strength values werepresent for three alloys in the same group, indicat-ing that minor differences in alloy compositionmay lead to difference in bond strengths, althoughthe difference may nol be found to be statisticaiiysigniffcant. Altbougb Ni-Cr and Co-Cr alloys havea fixed ratio in the alloy, differences in the ratio ofany other minor element could significantly affectthe oxidation rate of the alloy and its reaction withh i ^ " ' " ' ' '

Table 7 Composition of the Alloys Used in this Study

Bondi-Loy Wiron 99 Wiroiloy

Table 4 shows an increasing number of cohesivefailures in the oxide layer from group 2 to 3 to 4,while the bond strength ol these three groups de-creased in that order. The only difference betweenthese test groups was the oxidation potential. Ac-cording to Baran's study,-' an oxide layer formedunder atmospheric pressure was three times thickerthan an oxide layer formed under vacuum. Theseresults are in accordance witb those uf McLean,"who stated that as the tbickness of the oxide layer,which functions as the sandwich layer, increases,the possibility of failure at the oxide layer will alsoincrease. This study showed that as the alloy oxidelayer increases, bond strength decreases. Thisfinding is similar to many other researchers'

Baran,^' Bertolotti,^ McLean,'' Yamamoto,' andPask and Tomsia" report that an oxide layer mustbe present that bonds with both the metal and theceramic, and tbat this is the transition betweenmetal and ceramic. Pask and Tomsia" suggest thatoxides forming at tbe interface have a certain diffu-sion rate into the ceramic and that the ceramic hasa vitrification speed. Mean bond strength valuesand failure surfaces obtained from the presentstudy support this conclusion. This was bestdemonstrated by the Bondi-Loy alloy in the secondtest group, wherein the oxidation rate of the alloyexceeded the vitrification speed of the ceramic.The oxide layer forming on the alloy surface re-acted with the ceramic and the ceramic vitrifiedbefore devitrification of the free melal ions. That is,the vitrification speed of the ceramic was greaterthan the diffusion rate of the metal ions. Al the endof the reaction the metal alloy and an oxide layerbond to the ceramic between the metal and ce-ramic interface. It is assumed that if the oxide layer

NiCr 27Mo 5CeFeMnSiCe 66Total (%) 98

'Indícales minor element.

642410-••

63233

9

98

forming on the alloy surface were thicker, then thebond would be stronger, and fracture within theceramic (cohesive fracture) would be more fre-quent. In the first sample group cohesive failuresoccurred under low forces. The authors concludethat ceramic must vitrify before the metal ions dif-fuse and devitrification takes place. That is, the vit-rification rate of the ceramic is greater than themetal ion diffusion rate.

One of the most surprising restjits was that whenthe conventional method was used, liondi-Loyalloy had low bond strengths whereas Wirolloyalloy, an alloy for resin restorations, had the high-est bond strength (see Fig 5).

It has been stated that failure in the ceramic layerwas a sign of a desirable metal-ceramic bonding.^Conclusions based on examination of failure sur-faces and bond strength values obtained i'rom thepresent study were partially in accordance with thisstatement. For example, in the second group, sam-ples were tin plated and diffused and the ceramicwas then fired, and the bond strength value ofBondi-Loy was found to be 219.7 kg/cm-. In thethird group, samples that were tin plated, diffused,preoxidized in a vacuum environment and thenfired were found to have a bond strength of 168.4kg/cm-; in tbe fourth group the samples were tinplated, diffused, and then preoxidized in mean at-mospheric pressure, and the bond strength valuewas 82 kg/cm^. The bond strength values of Wiron99 were 182.8 kg/cm- in the second group, 140.8kg/cm^ in the third group, and 109.1 kg/cm^ in thefourth group, whereas the bond strength values ofWirolloy were found to be 114.8 kg/cm- in the sec-ond group, 120.4 kg/cm^ in the third group, and56.1 kg/cm- in the fourth group. The only increasewas found in the third group of Wirolloy whencompared with the second group, with an averageincrease of 5.5 kg/cm^. FHowever, the bond strengthvalues of the second, third, and fourth groups for

I'imber2, 1 171 The International Journal oí Pnjslhoduntii

nü Aiioy-Ceramic Bond Slrenglii Oeger/Cariii<iioglu

Bondi-Loy and Wiron 99 were in decreasing order.When the failure surface (see Table 4) was exam-ined, it was found that for all three alloys less fail-ure within the oxide layer was seen in the secondgroup compared to the third and fourth groups.Nearly all samples of the fourth group for the threealloys displayed failure in the oxide layer. The mostsignificant difference in the third group was the oxi-dation potential of the medium. As the oxidationpotential of the media increased, tbe bond strengthbetween the metal and ceramic decreased. Thisseems to be a statement that can only be made byexamining the failure surfaces; however, when thefailure surfaces of the samples of the first groupwere examined, cohesive fractures were observedin the ceramic layer, but the bond strength valuesbetween tbe metal and ceramic were not high. Forexample, in tbe first group the bond strength valueswere 17.5 kg/cm^ for Bondi-Loy, 35.4 kg/cm^ forWiron 99, and 66.5 kg/cm^ for Wirolloy. For thethree alloys the failure surfaces were mostly withinthe ceramic layer. In spite of this fact, the bondstrength values of the third group was 168.4 kg/cm^for Bondi-Loy, 140.8 kg/cm^ for Wiron 99, and120.4 kg/cm^ for Wirolloy, and the failure surfaceswere mostly within the oxidation layer, unlike thefirst group. Based on these findings it would seemto be unwise to evalúale the metal-ceramic bondstrength by only examining failure surfaces. Bondstrength values must aiways be considered with thefailure surfaces.

Conclusions

The diffusion of tin into two base metal alloys andthe bond strength of tin-plaled specimens of threebase metal alloys was investigated. The three al-loys were prepared under five conditions andbonded to opaque and dentin porcelain. Withinthe limitations of sample size and the materialsused, the following conclusions may be made:

1. Bondi-Loy alloy had the greatest bond strengthwith VMK 68 ceramic when plaled with tin anddiffused in an argon environment.

2. Wirolloy alloy had the highest bond slrengthwith VMK 6Ö ceramic when prepared using theconventional method.

3. Fxamination of failure surfaces to assess bondstrength can be misleading.

4. The factors that affect the transition layer be-tween the ceramic and the melal alloy are themetal alloy oxidation rate, the ceramic vitrifica-tion rate, and the metai ion diffusion rate.

5. The oxidation rate of the metal alloy should ap-proximate the ceramic vitrification rate, and thediffusion rate of the metal ions should be lessthan the vitrification rate to obtain an optimummetal-ceramic bond.

Acknowledgment

The authors wish to aci<nowledge the assistance and supportprovided hy Professor Dr Ilker Kayadeniz, Department ofMetallurgy, Faculty of Chemistry, University of Istanbul.

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The International lournal oí Prosthodonlii 172 Volumen, Numlier2, 1998