590 Chiang Mai J. Sci. 2011; 38(4)

Chiang Mai J. Sci. 2011; 38(4) : 590-600http://it.science.cmu.ac.th/ejournal/Contributed Paper

Phase Formation, Microstructure and MechanicalProperties of ZrO2 Modified-Dental PorcelainCeramicsAttavit Pisitanusorn [a], Rattikorn Yimnirun [b], and Supon Ananta [c]*[a] Department of Prosthodontics, Faculty of Dentistry, Chiang Mai University, Chiang Mai 50200, Thailand.[b] School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000,

Thailand.[c] Department of Physics and Materials Science, Faculty of Science, Chiang Mai University,

Chiang Mai 50200, Thailand.*Author for correspondence; e-mail: suponananta@yahoo.com

Recieved: 2 May 2011Accepted: 21 June 2011

ABSTRACTThe potential of a combination between single-step sintering and ZrO2 additive as a

simple ceramic fabrication method to obtain dental porcelain-based ceramic-nanocompositeswith good mechanical properties was demonstrated. Influences of sintering condition onphase formation, densification, microstructure and mechanical properties of these porcelainceramics were examined via XRD, SEM-EDX and Vickers indentation. It has been foundthat the choice of both sintering temperature and dwell time is one of the key factors controllingdensification and leucite crystallization on the zirconia surface in dental porcelain ceramics.

Keywords: dental porcelain ceramics, leucite, sintering, strength.

1. INTRODUCTIONLeucite glass-ceramics are one of the

significant restorative materials for dentalapplications owing to their suitable biocom-patibility, wear resistance and esthetics.However, similar to other glass-ceramics, amajor disadvantage to dentistry of porcelain-based ceramics is their brittleness, which oftendeteriorates their utilization in fields where highstrength and toughness are required [1,2].Several approaches have been introduced tominimize this limitation and improve themechanical properties including a method ofreinforcing the glassy matrix with high strength

crystalline phases such as alumina (Al2O3),zirconia (ZrO2) or leucite (KAlSi2O6) [2-4].A significant milestone in this respect wasestablished in the development of dentalceramics containing leucite crystalline phasebecause of their excellent thermal compatibilitywhen bonding to metals and also a reinforcingagent in all ceramic restorations [5-7]. Ingeneral, most commercial porcelains for dentalrestorations are partially crystallized feldspathicglasses that consist of tetragonal-leucitecrystals embedded in a glassy matrix. It hasbeen reported that leucite glass-ceramics can

Chiang Mai J. Sci. 2011; 38(4) 591

be fabricated by controlled crystallization ofa glass via leucite additive, heat treatment andetc. [8-9]. In general, commercial dentalporcelain ceramics contain some amount ofleucite crystals sized ~5-10 μm causingextensive microcracking around these non-uniformed leucite crystals [8]. To overcomethese problems, several efforts have beenmade to tailor the leucite morphology, volumefraction and distribution which is thought toaffect the mechanical properties of theseporcelain ceramics via heat treatment [4,9] orheat pressing [10].

According to a study on the effects ofmultiple firing and isothermal anneals on theleucite volume fraction reported by Mackertand Evan [11], it is established that measurablechanges in the leucite concentration existupon a variety of porcelain heat treatments.Although the study on the formation of leucitecrystalline in porcelains has been reported byseveral researchers [7-9]; so far, attentionsdirectly paid on the Zr-modified porcelain-based ceramics are very few. In connectionwith this, our earlier works [12] havedemonstrated that the two-step sinteringtechnique together with 20-wt% ZrO2 additivehas considerable potential for the productionof leucite/porcelain ceramic-nanocomposites.It was also found that the tempering time is akey factor to controlling dendritic morphologyof leucite crystallization behavior andmicrostructural arrangement in ZrO2-modified dental porcelain ceramics. However,no information regarding the mechanicalproperties of these materials is available.Moreover, effects of single-step sintering forthe production of ZrO2-modified dentalporcelains have not yet been adequatelycharacterized. Thus, in the present study, dentalporcelain ceramics were fabricated by usingboth a single-step sintering and 20 wt% ZrO2

additive techniques. Their mechanicalproperties with respect to both firing

temperature and dwell time were investigated,compared and explained on the basis of theircomposition, densification, and microstructuralevolution.

2. MATERIALS AND METHODSThe starting materials were commercially

available dental porcelain powders (Vita-VMK95, Vita Zahnfabrik, Bad S ckingen,Germany), which had particle size range of~1.5-2.5 μm. Porcelain powders were mixedwith 20 wt% of zirconia powders (Sigma-Aldrich, purity > 99%) by using a rapidvibro-milling for 30 min [2,12]. The methodsof mixing, drying, grinding and sieving of theproducts were similar to those employed inthe preparation of the alumina-modifieddental porcelain powders as describedpreviously [2, 12]. Green samples wereobtained by mixing powders with polyvinylalcohol binder (PVA) via a slip-castingtechnique as recommended by the manufacturer[13], and then poured into a standard stainlesssteel mould with a normal-sized cavity of 30mm × 6 mm × 2 mm, reproducing the desireddimensions and shapes [2, 12]. After moulding,the ceramics were fabricated by employingdifferent firing schemes with heating rate of25oC/min in a vacuum furnace (MultimatTouch & Press, Germany), with details listedin Table 1. During heating, the temperaturewas maintained at 500oC for 1 h to burn outthe PVA binder. The dental porcelain, i.e. thecontrol group (D0), was sintered at 1,060oCfor 5 min. Further increase in the soaking timelonger than 5 min results in melted samplesprobably due to a liquid-phase formationduring the sintering process of dentalporcelains [14]. The other groups of samples(coded D1-D9) were sintered by employingvarious firing temperature and dwell time. Incontrast to those adopted by Chayada et al.[12], in this work, no second firing-step andquenching processes are required. Densities of

592 Chiang Mai J. Sci. 2011; 38(4)

the final sintered products were determinedby using the Archimedes principle. Phaseformation and microstructural were examinedby room temperature X-ray diffractionanalysis (XRD; X’pert MPD, Philips Corp,Japan) using Ni-filtered Cu Kα radiation andfield emission scanning electron microscopy(SEM; JSM-840A 6335 F, Jeol, Japan),respectively. The mean crystallite size wasdetermined using the diffraction peak (400)of the leucite pattern by using Scherrerequation [15]. Lattice parameters of the leucitephase were determined from the d spacingfor the (400) and (004) peaks for the tetragonalphase [4,12]. The grain size and morphologiesof leucite phase in the sintered samples weredetermined from SEM micrographs. Thechemical composition of the phase formedwas also elucidated by an energy dispersiveX-ray (EDX) analyzer with an ultra-thinwindow. EDX spectra were quantified withthe virtual standard peaks supplied with theOxford Instruments eXL software. In orderto evaluate the mechanical properties ofthese samples, the characterization methodspreviously described in our earlier works [2]were performed. The uniaxial flexural strengthwas measured with the three-point bendingtechnique and calculated as recommended byISO 6872. Hardness and Young’s moduluswere determined and calculated for allmaterials by the Vickers microhardness testingmachine, and were measured as recommendedby the ASTM C 1327-99 and C 1259-01,respectively. Fracture toughness was assessedusing the indentation strength method.Hardness and elastic modulus are parametersof the equation used for calculation of thefracture toughness. The fracture toughness wascalculated with the equation proposed byFischer and Mark [16].

3. RESULTS AND DISCUSSIONThe densification, leucite crystallization,

microstructural evolution and mechanicalbehavior of the Zr-modified dental porcelainceramics subjected to various sinteringconditions are given in Table 1. In the sinteringtemperature range of 1,060-1,140oC, thedensity increases with increasing temperature.For a given sintering temperature, similar trendof the density is also observed with theextending of dwell time from 20 to 30 min.However, it can be seen that further increasein the dwell time to 40 min leads to thedecrease of the density. This feature creates amaximum density value of about 2.88 g/cm3

which is comparable to the values reportedby Chayada et al. (~2.76 g/cm3 [12]) wherethe two-step sintering technique was used.The increasing density with rising dwell timeup to 30 min may be explained by theenhanced densification related to the effect ofZrO2 [3, 12]. Further increase in the dwell timecauses a decrease in density values. This maybe attributed to the suppression of atomicdiffusion at probably too long soaking timesresulting in an incomplete densification [12,17]. Similar behavior is also reported byChayada et al. [12] for zirconia-modifieddental porcelains derived from the two-stepsintering technique. As is well known, thedifference in thermal expansion between thetetragonal leucite crystals and the glassy matrixalso provides tangential compressive stressesaround the crystals that are thought responsiblefor significant strengthening in dental porcelain[4, 9]. The crystallization of a high expansiontetragonal leucite phase into a base glass isadvantageous to increase its expansioncoefficient to a level sufficient to allow efficientbonding to metals [18]. More recently, leucitehas been used in all-ceramic materials, not forthermal compatibility, but as a reinforcingmaterial for all-ceramic restorations [4, 9].

To study the crystalline phase developmentwith different sintering conditions in eachporcelain-based sample, the specimens were

Chiang Mai J. Sci. 2011; 38(4) 593

Tab

le 1

. Th

e sin

terin

g co

nditi

ons

empl

oyed

for t

he p

rodu

ctio

n of

sam

ples

and

their

phy

sical

prop

ertie

s.

Sin

teri

ng

con

dit

ion

:D

ensi

tyL

euci

te L

euci

teL

euci

te p

arti

cle

Fle

xura

lF

ract

ure

Har

dnes

s

Mat

eria

lsT

emp

erat

ure

/D

wel

l tim

e(g

/d

m3 )

cont

ent

crys

talli

ne

size

size

ran

gest

reng

thto

ughn

ess

(GP

a)

(o C/m

in)

(wt%

)( ±±±± ±

2 n

m)

( ±±±± ± 3

0 n

m)

(MP

a)(M

Pa⋅

m1/

2 )

D0

1060

/52.

4326

.14

15.7

680

-200

83.

4 (±

8.3)

1.01

(±0.

10)

3.21

(±0.

19)

D1

1060

/20

2.66

23.5

27.

7435

-150

154.

6 (±1

5.5)

2.03

(±0.

11)

7.20

(±1.

01)

D2

1060

/30

2.71

23.5

57.

8545

-155

170.

5 (±1

5.4)

2.20

(±0.

10)

7.41

(±0.

68)

D3

1060

/40

2.69

23.7

48.

3252

-161

169.

2 (±1

6.7)

2.18

(±0.

09)

7.33

(±0.

64)

D4

1100

/20

2.71

23.1

38.

9650

-180

172.

1 (±1

3.2)

2.24

(±0.

09)

7.55

(±0.

69)

D5

1100

/30

2.78

24.7

011

.26

60-1

9018

4.3 (±1

4.7)

2.39

(±0.

10)

7.98

(±0.

43)

D6

1100

/40

2.70

25.3

511

.75

60-1

9017

5.3 (±1

8.9)

2.36

(±0.

13)

7.83

(±0.

70)

D7

1140

/20

2.86

23.0

912

.18

60-2

2018

5.7 (±1

5.5)

2.40

(±0.

12)

8.01

(±0.

58)

D8

1140

/30

2.88

24.2

913

.32

70-2

4519

2.8 (±1

9.8)

2.50

(±0.

16)

8.58

(±0.

75)

D9

1140

/40

2.87

24.7

715

.87

70-2

5018

6.1 (±1

4.3)

2.48

(±0.

11)

8.43

(±0.

67)

594 Chiang Mai J. Sci. 2011; 38(4)

Figure 1. X-ray diffraction patterns of non-sintered (a) and sintered (b) dental ceramics,non-sintered (c) and sintered (d) D1 (e) D2, (f) D3, (g) D4, (h) D5, (i) D6, (j) D7, (k) D8and (l) D9 samples.

heat treated at various conditions (Table 1),followed by phase analysis using XRDtechnique. The X-ray diffraction patterns ofboth unfired and sintered dental porcelainceramics in this study are given in Figure 1(a and b), indicating a background ofamorphous glassy phase and evidence of theformation of only leucite crystalline phase,which could be matched with JCPDS file no.

15-47, in agreement with other works [12, 18].To a first approximation, this crystalline phasehas a tetragonal leucite-type structure in spacegroup I41/a (no. 88) with cell parameters a =1,306 pm and c = 1,375 pm [19], consistentwith previous work reported by Mackert et al.[20] where the lattice parameters for tetragonalleucite at room temperature were a = 13.197± 0.013 and c = 13.819 ± 0.016 .

Chiang Mai J. Sci. 2011; 38(4) 595

As demonstrated in Figure 1(c-l), it canbe seen that all samples showed almostidentical XRD patterns. The strongestreflection in the majority of XRD tracesderived from all 20-wt% ZrO2-modifiedporcelain groups indicated a combination ofmonoclinic-zirconia which could be matchwith JCPDS file no. 36-420 [21] andtetragonal-leucite phase. In addition, it shouldbe noted that XRD peaks of leucite (323) andzirconia ( 11) at 2θ ~28.5o are superimposed,in agreement with previous studies [12]. Inthis study, no phase transformation of leucitecan be detected at 1,060-1,140oC and the XRDpatterns of the samples subjected to thesintering process for various dwell time from20 to 40 min did not reveal the formation ofany additional crystalline phases. This probablyindicates the effectiveness of Zr as the leucitestabilizer, in analogous with those found for

other similar systems [12,19].In order to evaluate the relative amounts

of tetragonal leucite phase in each group, theleucite peak at the (004) and (400) reflectionsand the zirconia peak at the ( 11) reflectionwere the peaks of interest (Figure 2). Assuggested by Ong et al. [22], powder X-raydiffraction method can be used to approximatethe amount of leucite phase in porcelainceramics. It should be noted that for a givensintering temperature, some relationship wasobserved between the employed dwell timesof 20-40 min and the concentration of leucitephase in the samples (Table 1). In this work,the amount of leucite phase in the ZrO2-modified dental porcelain subjected to thesintering was found to slightly increase as afunction of dwell time, consistent with thosereported by Cheung and Darvell [23] fordental porcelains.

Figure 2. Enlarged X-ray diffraction patterns of all 20 wt% ZrO2 modified-dental porcelainceramics sintered at various conditions.

596 Chiang Mai J. Sci. 2011; 38(4)

It has been found that by extending dwelltime, some diffraction lines gradually sharpene.g. (004) and (400) peaks, an indication of acontinuous increase in crystalline size and ofthe reduction of lattice strain. Theseobservations point out that the prolongedheat treatment affects the leucite crystallite size.

Additionally, the crystallite size of leucitephase was estimated from these XRD patternsas also given in Table 1. The calculatedcrystallite size value was also found to increasewith increasing both firing temperature anddwell time. Though, the relative intensities ofthe Bragg peaks exhibit independent oftempering time, it is well documented that, asScherer’s analysis provides only a measurement

of the extension of the coherently diffractingdomains, the crystallite sizes determined bythis method can be significantly underestimated [15]. In addition to strain, factorssuch as defects, homogeneity of materials, thecomplex nature of the background due toamorphous matrix, processing variables andinstrument effect can attribute to peak shape,making it almost impossible to extract areliable crystallite size solely from XRD [15].In this connection, SEM technique was alsoutilized for grain size measurement (Table 1and Figure 3).

As shown in Figure 3 for the case of all20-wt% Zr modified-dental porcelainceramics, it can be seen that some leucite

Figure 3. SEM micrographs of all 20 wt% ZrO2 modified-dental porcelain ceramics sinteredat various conditions.

Chiang Mai J. Sci. 2011; 38(4) 597

particulates which vary between 35 and 250nm in size and are clustered together alongthe glassy matrix (closer observation is alsogiven in Figure 4(c) indicating a “nanocom-posite” structure [2, 12]) were initiated fromthe surface of zirconia grains. On the basisof this, the governing mechanism for theappearance of leucite phase in the glassy matrixis surface crystallization [11,24]. All sinteredsamples showed two (or more) distinct phasestructures with a glassy matrix phasereinforcing crystalline phase dispersed in theglassy matrix. In general, they have a very finemicrostructure with agglomerates. It shouldbe noted that dendritic-like leucite phasepreviously reported by several workers [12,25] are also found here, especially for thesamples subjected to high firing temperatureor prolong dwell time [D5-D9]. Theseobservations may be attributed to theinfluence of ZrO2 additives as nucleatingagents for leucite crystallization behavior,

similar to those found in other similar glass-ceramic systems [12, 26].

Microstructural features of dentalporcelain ceramics sintered at 1,060oC for 5min (D0) are shown in Figure 4(a). It is seenthat a smooth surface of typical porcelainglass-ceramics is observed, in agreement withthose reported earlier [9, 12, 24]. SEM-EDXanalysis of the dental porcelain ceramics(Figure 4(b)) marked as “ ” reveals the strongpresence of silica and oxygen indicating thecomposition of glass matrix (Figure 4(d)). Asshown in Figure 4(e) and (f), EDX spectraobtained from area “ ” confirm the existenceof all key elements related to the compositionof leucite (KAlSi2O6) [19]. While EDX analysisof the large grain marked as “ ” shows onlyzirconia rich phase together with spectra ofoxygen, indicating the existence of ZrO2

additive. The results of SEM-EDX measure-ment supported the XRD observation discussedearlier (Figures 1 and 2).

Figure 4. Representative SEM micrographs of (a) dental porcelain, (b) D8 sample and (c)enlarged of (b) and their corresponding EDX analysis, indicating the chemical compositionsof (d) glassy matrix, (e) leucite and (f) ZrO2 phases, respectively.

598 Chiang Mai J. Sci. 2011; 38(4)

Furthermore, there is no evidence ofcrack formation in the matrix or within theleucite crystals. From Figure 3, signs of leuciteparticle coalescence were visible in allZr-modified porcelain samples which mayhave been driven by a reduction in interfacialenergy when larger particles grow at theexpense of smaller ones (Ostwald ripening)[27]. In this work, it is believed that apart fromthe sintering temperature affects, the prolongedsoaking times may have allowed sufficienttime for small particles below a critical size todissolve and feed larger particles via a diffusiondown the concentration gradient.

Impression of a Vickers indenter producingtypical symmetry cracks, where the crackspropagate along the glassy phase of the matrixis illustrated in Figure 5(a,b), consistent withother similar porcelain-based ceramics [2,18].

While SEM image for ZrO2-modifiedporcelains (D8) with maximum densityvalue exhibits evidence of crack deflection(non-direct line) with the crystalline reinforcedphase as shown in Figure 5(c,d), in agreementwith literature [28]. The variations of mechanicalproperties with sintering conditions arealso given in Table 1. For all ZrO2-modifiedsamples in the present work, the values offlexural strength, fracture toughness andhardness are slightly greater than that of dentalporcelains (D0). Similar results have also beenreported for other porcelain-base systems[2,28]. The results in Table 1 clearly indicatethat the mechanical properties of the ZrO2-modified dental porcelain ceramics dependon the densification derived from the sinteringconditions. As listed in Table 1, maximumvalues of flexural strength, fracture toughness

Figure 5. (a) Optical micrograph of dental porcelain (D0), (b) SEM micrograph of D8sample, and typical crack paths after Vickers indentation for (c) D0 and (d) D8 samples.

Chiang Mai J. Sci. 2011; 38(4) 599

and hardness increase from 154.6 MPa, 2.03MPa⋅m1/2 and 7.20 GPa to 185.7 MPa, 2.40MPa⋅m1/2 and 8.01 GPa as the sinteringtemperature increases from 1,060-1,140oC,with constant dwell time of 20 min. As alsolisted in Table 1, the similar tendency is alsoobserved for the influence of dwell time from20-30 min at a given sintering temperature.However, it is seen that the increase in thesoaking time to 40 min results in a drop in thevalues of all mechanical properties, consistentwith the densification results discussed earlier.This observation may be attributed tomicrostructural inhomogeneity of theporcelains as a result of leucite crystallizationand/or agglomeration [14]. Consequently,the experimental work carried out heresuggests that other factors such as the sinteringconditions, the ZrO2 additive, the leucitecrystallization and microstructural arrangementclearly have some influence on the mechanicalproperties of the dental porcelain ceramics.Additional studies are essential to fullyunderstand the mechanism of leucitecrystallization and how this mechanism canbe exploited in future to enhance or tailor thekey mechanical properties of dental porcelain-based materials.

4. CONCLUSIONSResults indicate that using different firing

conditions (i.e. temperature and dwell time)to produce ZrO2-modified dental porcelainceramics greatly influences leucite crystalliza-tion, microstructure and mechanical propertiesof the resulting products. It has beenconcluded that using commercially availabledental porcelain powders as the startingmaterials, a combination of the optimizedfiring condition and the addition of 20-wt%ZrO2 has considerable potential for theproduction of leucite/porcelain ceramic-nanocomposites. It was also found that boththe sintering temperature and dwell time

are key factors in controlling dendriticmorphology of leucite crystallizationbehavior and microstructural arrangement inZr-modified dental porcelain ceramics.

ACKNOWLEDGEMENTSThe authors would like to acknowledge

the Thailand Research Fund (TRF), Faculty ofDentistry and Faculty of Science, Chiang MaiUniversity for financial support.

REFERENCES

[1] Kelly J.R., Nishimura I. and CampbellS.D., Ceramics in Dentistry: HistoricalRoots and Current Perspectives, J.Prosthet. Dent., 1996; 75: 18-32.

[2] Pisitanusorn A., Schulle W., Thiansem S.and Ananta S., The Influence of CrystallinePhase Additions on the MechanicalProperties of Dental Ceramic Materials,Part 1.2: Ceramic Materials Reinforcedwith Alumina-based Nanocomposites,Interceram, 2006; 55: 328-333.

[3] Morena R., Lockwood P.E., Evans A.L.and Fairhurt C.W., Toughening of DentalPorcelain by Tetragonal ZrO2 Additions,J. Am. Ceram. Soc., 1986; 69: C75-C77.

[4] Cattell M.J., Chadwick T.C., KnowlesJ.C., Clarke R.L. and SamarawickramaD.Y.D., The Nucleation and Crystalliza-tion of Fine Grained Leucite Glass-Ceramics for Dental Applications, Dent.Mater., 2006; 22: 925-933.

[5] Cattell M.J., Chadwick T.C., KnowlesJ.C., Clarke R.L. and Lynch E., FlexuralStrength Optimization of a LeuciteReinforced Glass Ceramic, Dent. Mater.,2001; 17: 21-33.

[6] Mackert J.R. and Russell C.M., LeuciteCrystallization During Processing of aHeat-Pressed Dental Ceramic, Int. J.Prosthodont., 1996; 9: 261-265.

600 Chiang Mai J. Sci. 2011; 38(4)

[7] Mackert J.R., Butts M.B. and FairhurstC.W., The Effect of The LeuciteTransformation on Dental PorcelainExpansion, Dent. Mater., 1986; 2: 32-36.

[8] Mackert Jr. J.R. and Williams A.L.,Microcracks in Dental Porcelain andTheir Behavior During Multiple Firing,J. Dent. Res., 1996; 75: 1884-1890.

[9] Cattell M.J., Chadwick T.C., Knowles J.C.and Clarke R.L., The Crystallization ofan Aluminosilicate Glass in the K2O-Al2O3-SiO2 System, Dent. Mater, 2005; 21:811-822.

[10] Rheinberger V., Perspectives in DentalCeramics, Glastech. Ber. Glass. Sci. Technol.,1997; 70: 339-440.

[11] Mackert Jr. J.R. and Evans A.L.Quantitative X-ray Diffraction Determi-nation of Leucite Thermal Instability inDental Porcelain, J. Am. Ceram. Soc., 1991;74: 450-453.

[12] Chayada S., Pisitanusorn A. and AnantaS., Effects of Tempering time on PhaseFormation and Microstructural Evolutionof Zirconia Modified-Dental PorcelainCeramics, Chiang Mai J. Sci., 2011; 38(2):176-178.

[13] International Organization for standardi-zation. ISO 6872: 1995(E): Dental ceramics,2nd Edn., Geneva, Switzerland, 1995.

[14] German R.M., Sintering Theory and Practice,Wiley, New York, 1996.

[15] Klug H. and Alexander L.E., X-rayDiffraction Procedures for Polycrystalline andAmorphous Materials, 2nd Edn., Wiley,New York, 1974.

[16] Fischer H. and Mark R., FractureToughness of Dental Ceramics:Comparison of Bending and IndentationMethod, Dent. Mater. 2002; 18: 12-19.

[17] Chen I.W. and Wang X.H., SinteringDense Nanocrystalline Ceramics WithoutFinal-Stage Grain Growth, Nature, 2000;404: 168-171.

[18] Denry I.L., Mackert Jr. J.R., Holloway J.A.and Rosenstiel S.F., Effect of CubicLeucite Stabilization on The Flexural

Strength of Feldspathic Dental Porcelain,J. Den. Res., 1996; 75: 1928-1935.

[19] JCPDS-ICDD card no. 15-47, Inter-national Centre for Diffraction Data,Newtown Square, PA 2000.

[20] Mackert Jr. J.R., Butts M.B. and FairhurstC.W., The Effect of the Leucite Trans-formation on Dental Porcelain Expansion,Den. Mater., 1986a; 2: 32-36.

[21] JCPDS-ICDD card no. 36-420, Inter-national Centre for Diffraction Data,Newtown Square, PA 2000.

[22] Ong J.L., Farley D.W. and Norling B.K.,Quantification of Leucite ConcentrationUsing X-ray Diffraction, Den. Mater.,2000; 16: 20-25.

[23] Cheung K.C. and Darvell B.W., Sinteringof Dental Porcelain: Effect of the Timeand Temperature on Appearance andPorosity, Den. Mater., 2002; 18: 163-173.

[24] Pisitanusorn A., Ananta S., Yimnirun R.and Thiansem S., Fabrication andCharacterization of High Strength DentalPorcelain Nanocomposite from Thailand’s Raw Materials, Chiang Mai J. Sci., 2005;32: 549-553.

[25] Alizadeh P. and Marghussian V.K., Effectof Nucleating Agents on the Crystalli-zation Behaviour and Microstructure ofSiO2-CaO-MgO(Na2O) Glass-Ceramics,J. Eur. Ceram. Soc. 2000; 20: 775-782.

[26] Barreiro M.M., Riesgo O. and VincenteE.E., Phase Identification in dentalPorcelains for Ceramo-Metallic Restora-tions, Dent. Mater. 1989; 5: 51-57.

[27] M ller R., Abu-Hilal L.A., Reinsch S. andH land W., Coarsening of Needle-like-shaped Apatite Crystals in SiO2-Al2O3-Na2O-K2O-CaO-P2O5-F Glass, J. Mater.Sci. 1999; 34: 65-69.

[28] Pisitanusorn A., Schulle W., Yimnirun R.and Ananta S., The Influence of CrystallinePhase Additions on the MechanicalProperties of Dental Ceramic Materials,Part 2.2: Ceramic Materials Reinforcedwith Alumina Oxide-based Nanocom-posites, Interceram, 2007; 56: 4-8.