Philip S. Baker, DDSAssistant Professor

Compositional Influenceon the Strength of

Dental Porcelain

Arthur E. Clark, r, PhD. DMDProfessor and Chairman

Department of ProsthodonticsCollege of DentistryUniversity of Florida

This work compared the tensile strength of two glasscompositions with published values of currently availabledental porcelains. The influence of several variables, such asair and vacuum firing, firing temperature, surface roughness,immersion in water, and the inclusion of filler particles, onthe inherent strength of the glass compositions is alsodiscussed. The authors developed a glass frit having a tensilestrength of 102.4 MPa as determined by the biaxial flexuretechnique. This value compares favorably with the reportedstrength of commercial brands of feldspathic porcelains. Thestrength of this glass was not significantly reduced as a resultof surface abrasion or immersion in distilled water. Int IProsthodont 1993:6:291-297.

D ental ceramics are generally recognized as be-ing esthetically superior to other restorativedental materials. They exhibit excellent tissue com-palibility, extreme chemical durability, and lowthermal conductivity. Unfortunately, dental ce-ramics are fragile and can fail when exposed tomasticatory stresses. This results from their brittle-ness as weli as their low tensile and shear strength.Numerous investigators have attempted to evaluateand improve the strength of dental ceramics withvarying degrees of success.'"" These techniquesfiave included modification of current products aswell as development of new materials to attempt toreplace porcelain.

One approach toward strengthening dental por-celain involves an ion exchange technique. If alarge ion replaces a smaller one, compressive stressis produced in the exchanged region. A number ofstudies on strengthening dentai porcelain by usingion exchange have been reported,' ' and all dem-onstrated a significant increase in the modulus ofrupture. Potential limitations to this technique are

Reprint requests: Dr Philip S. Baker, Department of Prosthodon-tics, College of Dentistry, Unn/ersity of florida. Box W045,Gainesville, Florida 32610-0435.

the thinness of the strengthened layer and thedeleterious effects of scratches on treated surfaces.Penetration or scoring of the layer can result in adramatic weakening." Hence, further investigationinto alternative strengthening methods is war-ranted.

Another method to improve the mechanicalproperties of dentai porcelain is bulk strengthen-ing, ie, producing a porcelain with a uniform in-crease in strength. Dental porcelains are basicallyaluminosilicate glasses with residual quartz crystalsand a small percentage of pigmenting agents. In acomposite system with a continuous glass matrix,crystalline dispersions limit the size of the Griffithflaws and strengthen the system.''For crystals to actas effective strengthening agents, however, theyshould possess a thermal expansion similar to thematrix phase to prevent the introduction of stress atthe matrix-crystal interface. In dental porcelains thecooling shrinkage of quartz inversion at 573^ canbe so great that the quartz particles separate fromthe glass matrix. McLean and Hughes' reported onreplacing the quartz crystalline phase with aluminaand observed a significant improvement instrength. This resulted in part from the lower ex-pansion of alumina particles which enabled them toremain in intimate contact with the glass matrix.

6, Number 3,1993 291 The [nternatiorai loutnal of Prosltiodontii

Compositional Inlluence on Dentai Potceiain

Unfortunately, although alumina particlesstrengthen porcelain, they also increase its opacity,

Southan' has classified modern dental porcelainsinto two groups: one group having an intact vitre-ous matrix and the other having a disrupted vitre-ous matrix. The first group includes aluminous por-celains and any feldspathic porcelains havingmatrices that are sufficiently plastic at 573''C toaccommodate the volumetric shrinkage of thequartz inversion. The second group includes feld-spathic porcelains having matrices that are rigid at573^, When porcelains in this group are cooled,the glass matrix is stressed to the point wheremicrocracks are formed, Southan's data indicatedthat aluminous porcelains were not significantlystronger than matrix-intact feldspaf hie porcelains.

The implication of this concept is that strength-ening may be achieved with quartz filler particles ifthe viscosity or rigidity of the glass matrix is suchthat the volumetric contraction of the quartz parti-cles does not set up stresses at the quartz-glassinterface.

Although the presence of crystals, whetherquartz or alumina, can strengthen a dental porce-lain under proper conditions, fracture may alsopropagate through the glass in the regions betweencrystalline particles. Therefore, an inquiry intomethods of strengthening the glass matrix is war-ranted.

The purpose of this study was to measure thetensile strength of two glass compositions devel-oped by the authors and to compare these materi-als with currently available dental porcelains. Theinfluence of firing temperature, surface roughness,water immersion, and inclusion of filler particles onthe new glasses was also evaluated.

Materials and Methods

The glasses were prepared from reagent gradesodium carbonate, calcium carbonate, potassiumcarbonate, boric anhydride, aluminum oxide(Fisher Scientific, Pittsburgh, PA), and S-jxm silica(Min-U-Sil 5|j,m, Pennsylvania Class Sand Corp,Pittsburgh, PA). The two glass compositions re-ported in this paper are:

1, L-1, composed of 65% SiO,, 5% AljO,, 4% K,0,16% Na,0,10% CaO, and Q% B,0,

2. A-2, composed of 40% SiO,, 30% AI^Oj, 2% K,0,8% Na,0,10% CaO, and 10% B,0,

The authors formulated 200-g batches of eachcomposition by weight using an electronic balance(Mettler Flectronic Balance Model BB 240, Mettler

Fig 1 Diagram of biaxial fiexure test device.

Instrument Corp, Hightstown, N|) with an accuracyof 0.1 g. The mixtures were melted in platinumcrucibles in a temperature range from 1,300C to1,450''C for 24 hours in a high-temperature ceramicfurnace (Model LCH, Burrell Corp, Pittsburgh, PA).The molten glass was quenched once in water pro-ducing the frit. The fritted materials were ballmilled using a jar mill (US Stoneware, Akron, OH)and pelleted milling medium (Burundum GrindingMedium, Fisher Scientific) and sieved to -100 and-325 mesh powders. No colorants were added.Discs were prepared by dry powder compaction of4 g of powder in 28.6-mm-diameter dies {Fred S.Carver, Inc, Menomonee Falls, W!), An appliedload of 103.4 MPa was used (Carver LaboratoryPress, Fred S. Carver, lnc. Pressed specimens werefired in sets of 10 in a porcelain oven (Jelenko Tru-Fire VPF, elenko Dental Health Products, Armonk,NY) by gradual introduction into the oven muffle at600^C and firing to final temperatures at a heatingrate of 50C per minute. The discs were fired for 1hour at the final temperatures, then air cooled toroom temperature (22.5X] on the firing tray placedupon the porcelain oven stage.

Three groups of samples were fired. Croup 1consisted of 50 samples of composition L-1 and10 samples of 80% L-1 combined with 20% AkO,,fired under different conditions, with selectedsamples then being air abraded (Abrasive Blaster,50-|j.m aluminum oxide. Belle de St Claire,Chatsworth, CA) while others were immersed indistilled water for 24 hours; group 2 consisted ofsamples of composition L-1 sieved to two differentparticle sizes; and group 3 consisted of 10 samplesof composition A-2 and 10 samples of 60% A-2combined with 40% A1,O.

The internationai Joumai of Protiiodontics 292 Volume 6,

Fig 2 Mean tensiie breakingstress measures for six series otsampies ot composition L-1.

Compositional Iniluenceon Denial Porcelain

140

120

100

80

60

40

20

0Sample Series

Comp L-1

A Air Fired 700C 1 hB Air Fired 75O'C 1 hC Air Fired 700^C 1 h

Surface AbradedD Vacuum Fired 700C 1 hE 80% L-1 + 20% AljC,

Air Fired/oO'C 1 hF Air Fired 70DC 1 h

immersed

Samples were then subject to mechanical stressvia the biaxial flexure method developed by Wacht-man et al." This method entails use of a thin circu-lar disc placed on three equally spaced ball bear-ings with the specimen loaded at tbe center of thesupport circle (Figi).

The apparatus was placed in a universal testingmachine (Instron Model 1125, Instron Corp, Can-ton, MA) and loaded at a rate of 1.27 mm/min. Thetensile stress developed in the center of the lowersurface of the plate can be calculated as follows."

3P (XY)

where

^ ^ (1 - [TD

II]

12]

(3)

where |v] = Poisson's ratio, = radius of loadedarea, A = radius of support circle, C = radius ofspecimen, P = applied load, and D = thickness.

An advantage of this test is that slightly warpedspecimens may be used without affecting theresults. The effects of edge flaws seen with rectan-gular bar specimens are minimized." McLean and

Hughes' have made use of a related technique andnoted a similarity to stresses that are likely to occurin a thin section such as a jacket crown.

Results

Figure 2 presents the mean tensile breakingstressforthe tests of composition L-1 (group 1]. Sixconditions were tested and the results of each con-dition represent the average of ten samples. Thepowder size for all six series was - 325 mesh or iessthan 44 |i.m. The mean tensile strength for composi-tion L-1 when air fired at 700C for 1 hour and aircooled is 86.8 13.2 MPa (series A). Raising thefiring temperature SOX resulted in a mean break-ing stress of 102.4 16.0 MPa (series B]. Series Crepresents the mean strength of composition L-1when fired at 700C and surface abraded prior tomechanical testing. The mean breaking stress was81.6 18.5MPa. When vacuum fired for 1 hour, themean flexure strength of composition L-1 was 77.5 19.6 MPa (Series D]. Series E shows the meanflexure strength for a mixture of 80 wt% L-1 powderand 20 wt% ALO,. When air fired for 1 hour at 75O''Cthe mean was 102.8 17.0 MPa. A series of L-1samples was air fired at 700C for 1 hour and thenimmersed in distilled water for 24 hours prior tomechanical testing series F]. The mean breakingstress was 88.7 27.4 MPa.

ume 5, Number 3,1993 293

Compositional Inllucrceon Dental Porcelain

Table 1 One-way ANOVA Resulfs

Group 1 Group 2 Group 3

Source

DFSSMSFP

Betweengroups

55563.4111268

3 04< 0 5

Withingroups

5419767,69

366,07

Total

5925331.09

Betweengroups

34024.31341.43

3,75

Cumpositiorai Influence on Dental Porceii

Fig 3 Mean flexure stress mea-sures for two particie size rangesof air- and vacuum-tired composi-tion L-1,

140 r

120

100

80

60

40

20

Sample Series

Comp L-1

A Air Fired 700C 1 h-325 Mesh Frit

B Vacuum Fired 700C 11n-325 Mesh Frit

C Air Fired 700C 1 h-100 Mesh Frit

D Vacuum Fired 700C 1 h-100 Mesh Frrf

Fig 4 Mean fiexure stress mea-sures tcr compositions A-2, A-2pius 40% A IA . L-1. ahd L-1 plus20% ALO,.

140

120'S%. ""^

S 80

60

40

20

O

w

Sample Series

A Comp A-2 Air Firedaooc 1 h

B 60% A-2 + 40% AljOAir Fired 1000C 1 h

C Comp L-1 Air Fired750^ 1 h

D 80% L-1 + 20%Ai;O3Air Fired 75QC 1 h

MPa. The average breaking strength for composi-tion L-1 was 86,8 MPa when air fired at 700C and aircooled (Fig2, series A), The mean breaking strengthof series C in Fig 2 (81,6 MPa) indicates that thestrength of series A is not associated with the for-mation of a surface compressive layer during therapid air cooling period. This is in agreement withthe findings of Corbitt et al,-' in that the strength ofdental porcelain seems more dependent upon in-fernal configuration than surface flaws. However,the coefficient of variation did increase from 15,2%to 22,7%.

Vacuum firing glass L-1 at 700C led to a meanflexure strength of 77,5 MPa and a rather largecoefficient of variation of 25%, Microscopic exami-nation of fractured specimens of glass L-1 whichhad been air fired and vacuum fired indicated thatthe vacuum-fired specimens contained largerpores than the air-fired samples. The presence ofthese defects could account for the greater datadispersion of the specimens fired under vacuum.

Immersion of specimens of glass L-1 in distilledwater (Fig 2, series F) 24 hours prior to mechanicaltesting did not produce any notable weakening

re6, Nurrber3,1 295 The International lour

effect (86.8 MPa mean not immersed vs 88.7 MPamean immersed). However, longer periods of im-mersion may lower the tensile strength of dentalporcelains by reducing the energy required forpropagation at the crack tips of surface flaws.- Thiseffect, termed static fatigue, would not be antici-pated after the brief exposure to water in this inves-tigation.

Microscopic examination of the L-1 glass fired at7(iOC revealed that the particles were still evidentafter firing. Incomplete sintering at the boundarieswould produce a situation in which stress transferbetween particles would not be possible. Interest-ingly, air firing glass L-1 at 700^ with two particlesize ranges produced a mean strength change from86.8 MPa for the smaller size to 100.943 MPa for theglass containing the larger particles. However, vac-uum firing at 700C produced very similar means forboth particle size ranges. Large voids were presentin the vacuum-fired specimens of both particle sizeranges. Their presence could explain the similarstrengths and rather large coefficients of variationin each (22% and 25%). If the 700G air-firing tem-perature does lead to incomplete sintering andfailure at the particle boundaries, then the in-creased strength observed with the L-1 glass con-taining larger particles may be attributed to thepresence of fewer potential faiiure sites and indi-cates that the material should be fired at highertemperatures. Raising the firing temperature ofglass L-1 from ZOOT to 750T increased the meanflexure strength from 86.8 to 102.4 MPa. This in-crease is probably associated with more completefusion of the glass particles during sintering.

Series F in Fig 2 shows that the addition of20wt%alumina to the L-1 glass frit had no effect on themean breaking strength. This may be attributed tothe low firing temperature (750G), resulting in in-complete sintering between the glass particles andthe alumina particles. Firing at temperatures near1,000G to enhance sintering would be impracticalwith glass L-1 because of extreme softening of theglassy matrix.

Binns-' listed the compositions of several com-mercial dental porcelains. The alkali (Na.O andK.O) concentration of glass L-1 is 20%, whereas thecommercial brands contain approximately 12%.The difference was usually replaced by B^ O^ addi-tions. The extra alkali content of L-1 should have theeffect of reducing the viscosity at a given tempera-ture." At the onset of this study it was felt thatincreasing the alkali content to reduce viscositywould lower the strength concurrently. The factthat the strength as determined in this study seemsequal or superior to commercially available porce-

lains is most encouraging to further research. Eval-uation of larger sample groups with different testparameters, as well as an examination of the effectsof colorants on the breaking strength of the newporcelains, should provide information on the fea-sibility of such compositions for commercial use.

Composition A-2 contains only 40 wt% silica and30 wt% alumina. Its mean breaking strength whenfired at 800X for 1 hour was significantly iower thanthat of glass L-1 fired at 75O'G (74.5 MPa vs 102.4MPa). As with glass L-1, the incorporation of alu-mina particles with the glass frit had no significanteffect on the mean hreaking stress. However, thecoefficient of variation was reduced from 26% to14%. It should be noted that Jones and Wilson"found that the inclusion of small amounts of alu-mina (5% to 20%) to veneer porcelains did not act asan effective strengthening phase.

Conclusions

A glass frit has been developed which has a ten-sile strength of 102.4 MPa as determined by thebiaxial flexure technique. This value compares fa-vorably with the reported strength of commercialbrands of feldspathic porcelains. The results of thisstudy suggest the foliowing conclusions.

1. Air firing glass composition L-1 at 75O''C pro-duced a significantly stronger porcelain thanfiring itat700C.

2. The addition of 20 wt% A[,0, to composition L-1air fired at 750G had no significant effect onbreaking strength.

3. When fired at700C, no significant difference inbreaking strength was noted for conditions ofair firing, vacuum firing, air abrasion, or short-term water immersion for glass composition L-1.

4. Air-fired -100 mesh particle size glass L-1 wassignificantly stronger Ihan vacuum-fired -100mesh, or air-or vacuum-fired -325 mesh size atthe 700C firing temperature.

5. Glass compositions L-1 and 80% L-1 plus 20%AljOj, fired at 7.WC, were significantly strongerthan glass A-2 (40% silica and 30% Al ,Oj) air firedat 800C, or 60% A-2 plus 40% alumina particlesfired at1,000C.

References

1. Kistler SS. Stre55e5 in giass produced by noniiniform ex-change of monovalenl ions. | Am Ceram Soc 1962;45:59-68.

2. Ward IB, Sugarman B and Symmers C. Siudies on the chemi-cal strengthening ol soda-lime-siiita glass. Glass Technol1%5;6;90-97.

3. Soulhan DE. Slrengtiiening modern dentai porcciain by ionexchange. Aiist Dent 11970;15:507-510.

The Internationa I ot Proslhodonlics 296

Compositional Influence on Denial Prtela

4, Dunn B, Reisbick MH, Strengthening of dental ceramics byion exchange (paper S03I, Presented at the lADR 54th Gen-eral Session, March 1976.

5, Fine C|, Danielsori PS. Chemicai strengthening by ion ex-change of lithium for sodium. Phys Chem Glasses1988:29:134-140,

6, Seghi RR, Crispin BC, Mito W, The effect of ion exchange onthe flexurai strength of feldspathic porcelains, int I Prostho-dcnt 1990:3:130-134,

7, Pettrow |N, Practical factors in building and firing charactef-istics of dental porcelain. J Prosthet Dent 1961:11:334-44,

8, McLean |W, A higher strength porcelain fot crown andbridge wori