Mechanical properties of commercial highstrength ceramic core materials

A.S. Rizkallaa,*, D.W. Jonesb

aDivision of Biomaterials Science, School of Dentistry, Faculty of Medicine and Dentistry,The University of Western Ontario, London, Ont., Canada N6A 5C1bDivision of Biomaterials, Dalhousie University, Halifax, NS, Canada

Received 11 November 2002; accepted 21 January 2003

KEYWORDSDental ceramics; Flexural

strength; Dynamic elastic

modulus; True hardness;

Glass-ceramic

Summary Objective. The objective of the present study is to evaluate and comparethe flexural strength, dynamic elastic moduli and true hardness ðHoÞ values ofcommercial Vita In-Ceramw alumina core and Vita In-Ceramw matrix glass with thestandard aluminous porcelain (Hi-Ceramw and Vitadurw), Vitadur Nw and Dicorw glassand glass-ceramic.

Methods. The flexural strength was evaluated ðn ¼ 5Þ using 3-point loading and aservo hydraulic Instron testing machine at a cross head speed of 0.5 mm/min. Thedensity of the specimens ðn ¼ 3Þ was measured by means of the water displacementtechnique. Dynamic Young’s shear and bulk moduli and Poisson’s ratio ðn ¼ 3Þ weremeasured using a non-destructive ultrasonic technique using 10 MHz lithium niobatecrystals. The true hardness ðn ¼ 3Þ was measured using a Knoop indenter and thefracture toughness ðn ¼ 3Þ was determined using a Vickers indenter and a Tukonhardness tester. Statistical analysis of the data was conducted using ANOVA and aStudent–Newman–Keuls (SNK) rank order multiple comparative test.

Results. The SNK rank order test analysis of the mean flexural strength was able toseparate five commercial core materials into three significant groups at p ¼ 0:05: VitaIn-Ceramw alumina and IPS Empressw 2 exhibited significantly higher flexural strengththan aluminous porcelains and IPS Empressw at p ¼ 0:05: The dynamic elastic moduliand true hardness of Vita In-Ceramw alumina core were significantly higher than therest of the commercial ceramic core materials at p ¼ 0:05:

Significance. The ultrasonic test method is a valuable mechanical characterizationtool and was able to statistically discriminate between the chemical and structuraldifferences within dental ceramic materials. Significant correlation was obtainedbetween the dynamic Young’s modulus and true hardness, p ¼ 0:05:Q 2003 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Introduction

In the early 1950s, the ceramics employed in theconventional porcelain jacket crown weremedium to high fusing feldspathic porcelains. Due

0109-5641/$30.00 - see front matter Q 2003 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.doi:10.1016/S0109-5641(03)00093-9

Dental Materials (2004) 20, 207–212

http://www.intl.elsevierhealth.com/journals/dema

*Corresponding author. Tel.: þ1-519-661-2111x86086; fax:þ1-519-661-3416.

E-mail address: arizkalla@eng.uwo.ca

to the relatively low strength of this type ofporcelain, an alumina-reinforced porcelain corematerial was developed by McLean1 for the fabrica-tion of porcelain jacket crowns. These alumina-reinforced crowns were regarded as providingbetter esthetics for anterior teeth than metal-ceramic crowns, but they exhibited a lower flexuralstrength, which limited their use for posteriorteeth.

The brittle nature of ceramics demands a greatermargin of safety in strength than with metals. Alldental ceramics tend to fail at the same criticalstrain2 of the order of 0.1%. For this reason anyincrease in strength and toughness can only beachieved by an increase in the elastic modulus.

The high-strength all ceramic biomaterials thatare currently used in dentistry consists of alumina,zirconia, pressed, castable or machinable glassceramics. Several developments have taken placein these areas resulting in the production of ceramicmaterials for clinical use: These include thealuminous porcelain crown Vitadurw, Hi-Ceramw),the non shrink ceramic crown (Cerestorew), thecastable mica glass-ceramic crown (Dicorw),3 themachinable glass ceramic known as Dicorw MGC4

and Vitablocw as well as the pressed leucite glassceramics IPS Empressw All these ceramic systemsexhibit low flexural strengths (100–150 MPa) thatare considerably below the yield point of the goldalloys used for ceramic-to-metal bonding andtherefore are at risk of failure when used for theconstruction of either molar crowns bridges or fixedpartial dentures.

The In-Ceram technique has been developed5

using an aluminum oxide slip casting technique usedto build the framework, which is then fired to anopen-pore microstructure. The material gains itsstrength by infiltration of the open-pore In-Ceramw

alumina microstructure with lanthanum glass. Thehigh flexural strength of the glass-infiltrated In-Ceramw alumina material (400–605 MPa) dependson the strength of the fired bond between thealuminum oxide particles and the complete wetting

of the open-pore microstructure by lanthanum glassinfiltration.6 The mechanical properties of In-Ceramw alumina can be improved by addingzirconium oxide.7 The addition of 33 wt% ofzirconium oxide led to an increase of the flexuralstrength up to 750 MPa while fracture toughness isdoubled.7 A later version of the In-Ceram systemuses a crystalline spinel in place of the alumina,presumably to increase the translucency, while atthe same time sacrificing strength.

A pressed lithium disilicate glass-ceramic hasbeen developed8–10 and is known as IPS Empressw 2.

Figure 2 The Student–Newman–Keuls rank order testcomparing the density of 8 commercial all ceramic corematerials.

Figure 1 The Student–Newman–Keuls rank order testcomparing the flexural strength of five commercial allceramic core materials.

Table 1 Commercial ceramic core used for this study.

Material Supplier

Vita In-Ceramw alumina core Vita ZahnfabrikVita In-Ceramw matrix glass Vita ZahnfabrikHi-Ceramw core VidentVitadurw core Vita ZahnfabrikVitadur Nw dentine Vita ZahnfabrikVitadur Nw enamel Vita ZahnfabrikDicorw as cast DentsplyDicorw cerammed Dentsply

A.S. Rizkalla, D.W. Jones208

It exhibits superior mechanical properties to IPSEmpressw. Holand et al.11 reported that the flexuralstrength and fracture toughness of IPS Empressw 2are 440 þ 0.40 MPa and 3.3 þ 0.3 MPa m0.5,respectively. These values are comparable to In-Ceramw alumina.

The aim of the present study is to evaluate andcompare the flexural strength, dynamic elasticmoduli and true hardness ðHoÞ values of commercialVita In-Ceramw alumina core and Vita In-Ceramw

matrix glass with the standard aluminous porcelain

(Hi-Ceramw and Vitadurw), Vitadur Nw and Dicorw

glass and glass-ceramic.

Materials and methods

Seven commercial ceramic materials were used inthis study. Information about the materials is givenin Table 1.

Figure 6 The Student–Newman–Keuls rank order testcomparing the dynamic bulk moduli of 8 commercial allceramic core materials.

Figure 5 The Student–Newman–Keuls rank order testcomparing the dynamic shear moduli of 8 commercial allceramic core materials.

Figure 4 The Student–Newman–Keuls rank order testcomparing the dynamic Young’s moduli of commercial allceramic core materials.

Figure 3 The Student–Newman–Keuls rank order testcomparing Poisson’s ratio of 8 commercial all ceramiccore materials.

Mechanical properties of commercial high strength ceramic core materials 209

For flexural strength evaluation, specimens(n ¼ 5) having a rectangular cross section(1 £ 5 mm) and 3-point loading over a span of11.5 mm. Testing was carried out in air using aservo hydraulic Instron testing machine at a crosshead speed of 0.5 mm/min. The formula12 used tocalculate the bend strength is given below

s3PT ¼PL

2bd2

where s3PT ¼ flexural strength (MPa), P ¼ appliedload (N), L ¼ distance between the two supports(mm), d ¼ thickness of the rectangular bar (mm),b ¼ width of the rectangular bar (mm). For theelastic moduli evaluation, specimens ðn ¼ 3Þ ofdimensions 6.4 ^ 0.1 mm (diameter) and5.0 ^ 0.1 mm (length) were prepared for eachmaterial. The end surfaces of each specimen wereground flat and parallel using a technique describedelsewhere.13,14 The dynamic Young’s shear and bulkmoduli of these materials were evaluated using anultrasonic method.13 –20

For true hardness evaluation, the cylindricalspecimens were sectioned in half, embedded inresin and repolished. The true hardness was calcu-lated using Knoop indentations and the methoddeveloped by Li et al.13,21 A series of 6, crack freeKnoop indentations were performed at differentloads ranging from 1.96 to9.80 N. Indentation lengthwas plotted vs. the square root of the load values.The true hardness ðHoÞwas calculated from the slopeof the regression line.

Results and discussion

The flexural strength values of five high strengthcommercial ceramic core materials are shown in

the bar diagram in Fig. 1. These values ranged from102.33 MPa for IPS Empressw to 547.53 MPa for VitaIn-Ceramw alumina. A Student–Newman–Keuls(SNK) rank order test separated the means intothree significant groups at p ¼ 0:05: There was nosignificant difference in the flexural strength valuesbetween Vitadurw, Hi-Ceramw and IPS Empressw

core materials.The density values of the commercial ceramic

core materials are shown in the bar diagram inFig. 2. These values ranged from 2.56 g/cc forDicorw as cast to 3.86 g/cc for Vita In-Ceramalumina core. The density values for Vitadur Nw

dentin and Vitadur Nw enamel were 2.32 ^ 0.02 and2.31 ^ 0.01 g/cc, respectively. The SNK rank ordertest separated eight materials into seven significantgroups at p ¼ 0:05:

The Poisson’s ratio data are shown in Fig. 3. Themean values ranged from 0.166 for Hi-Ceramw coreto 0.288 for Vita In-Ceram matrix glass. The SNK

Table 2 Length of Knoop indentations vs. square root of indentation load for all ceramic core materials.

Square root of Knoop indentation load (N)1/2 Slope(mm/N0.5)p , 0:001 (SD)

1.4 (SD) 2.21 (SD) 2.62 (SD) 2.8 (SD) 2.97 (SD) 3.13 (SD)

Knoop indentationlength (mm)Vita In-Ceramw

alumina core48.23 (0.29) 78.26 (0.95) 93.66 (1.35) 97.6 (1.20) 106.18 (1.09) 113.95 (1.46) 37.16 (0.84)

Vita In-Ceramw

matrix glass67.84 (1.48) 111.85 (1.43) 132.8 (0.72) 142.66 (0.62) 152.06 (0.58) 161.38 (0.31) 53.72 (0.88)

Hi-Ceram w core 61.22 (0.42) 98.8 (2.33) 118.25 (0.99) 125.32 (0.62) 133.12 (2.10) 141.26 (3.05) 46.02 (1.81)Vitadur w core 66.47 (0.52) 107.87 (0.75) 126.05 (3.84) 138.03 (3.86) 148.63 (1.74) 158.3 (0.96) 52.44 (0.63)Vitadur Nw dentine 75.41 (0.71) 94.74 (1.11) 112.7 (1.28) 126.75 (0.53) 139.29 (0.49) 150.32 (198) 62.66 (0.84)Vitadur Nw enamel 76.71 (0.35) 95.5 (1.28) 112.28 (0.01) 128.11 (1.57) 140.34 (1.46) 152.54 (1.60) 61.82 (0.90)Dicorw as cast 68.28 (0.12) 117.53 (1.13) 141.2 (0.23) 150.73 (0.14) 161.18 (0.05) 170.59 (0.21) 58.89 (0.14)Dicorw cerammed 78.67 (0.05) 131.69 (0.21) 155.53 (0.60) 170.71 (0.13) 180.74 (0.09) 191.73 (0.10) 65.07 (0.04)

Figure 7 Length of Knoop indentation vs. the squareroot of load for Vita In-ceramw alumina and Dicorw

cerammed.

A.S. Rizkalla, D.W. Jones210

rank order test separated eight materials into sixsignificant groups at p ¼ 0:05:

The dynamic Young’s modulus ðEÞ results areshown in Fig. 4. The mean values for the corematerials ranged from 62.47 GPa for Dicorw to271.32 GPa for Vita In-Ceramw alumina core andThe E values for Vitadur N emamel and Vitadur Ndentine were 64.75 ^ 1.36 and 64.37 ^ 0.85,respectively. The SNK rank order test separateeight different materials into five significant groupat p ¼ 0.05. There was no significant differencebetween The E value of Vitadur Nw enamel, dentineand Dicorw as cast and cerammed at p ¼ 0:05:

The dynamic shear modulus ðGÞ results are shownin Fig. 5. The mean values for the core materialsranged from 25.48 GPa for Dicorw cerammed to113.39 GPa for Vita In-Ceramw alumina core. TheSNK rank order test separated eight differentmaterials into six significant groups at p ¼ 0:05:There was no significant difference between The Evalue of Vitadur Nw enamel, dentine and Dicorw ascast and cerammed at p ¼ 0:05:

The dynamic bulk modulus ðKÞ results are shownin Fig. 6. The mean values for core materials rangedfrom 37.72 GPa for Dicorw as cast for Vitadur N to149.163 GPa for Vita In-Ceramw alumina. The SNKrank order test separated eight different materialsinto seven significant groups at p ¼ 0:05: There wasno significant difference between The E value ofVitadur Nw enamel, dentine and Dicorw as cast atp ¼ 0:05:

For the true hardness evaluation, the meanlength of the Knoop indentation at each of theapplied load for the different dental porcelainmaterials is given in Table 2. Three linear regressionanalyses were conducted for each material. Signifi-cant correlations were obtained at p , 0:001: Themean of the slopes of the linear regressions for eachmaterial is also shown in Table 2. As example, therelationship between the length of the Knoopindentation vs. the square root of the applied loadis displayed in the scattergram shown in Fig. 7 forDicorw and Vita In-Ceramw alumina. The truehardness was calculated from the slope of theregression line as described by Li et al. The truehardness ðHoÞ values are displayed in the bardiagram in Fig. 8. These values ranged from3.38 GPa for Dicorw cerammed to 10.79 GPa for

Figure 8 The Student–Newman–Keuls rank order testcomparing the true hardness of 8 commercial all ceramiccore materials.

Figure 9 A scattegram showing the relationship between the dynamic Young’s modulus and true hardness for allcommercial ceramic core materials.

Mechanical properties of commercial high strength ceramic core materials 211

Vita In-Ceramw alumina. The same multiple com-parative test separated the eight (8) mean valuesinto five groups p ¼ 0:05: There was no significantdifference between the true hardness of Vita In-Ceramw matrix glass and Vitadurw core. Similarly,there was no significant difference between VitadurNw enamel and dentine. p ¼ 0:05:

Linear regression analysis was conductedbetween the dynamic elastic moduli and the truehardness for the different commercial materials. Asignificant correlation was obtained ðr ¼ 0:984Þ asdisplayed in Fig. 9.

Conclusions

1. The mechanical properties of Vita In-Ceramw

alumina core were significantly higher than theseven commercial ceramic materials in thispresent study p ¼ 0:05:

2. The flexural strength of Vita In-Ceramw aluminais comparable to IPS Empressw 2.

3. There was no significant difference in themechanical properties between Dicorw as cast,Dicorw cerammed, Vitadur Nw enamel anddentine, p ¼ 0:05:

4. Significant correlation was obtained between theDynamic Young’s modulus and the true hardnessof the eight commercial all ceramic corematerials, r ¼ 0:984:

References

1. McLean JW, The science and art of dental ceramics, vol. 1.Chicago: Quintessence Publication Company; 1980.

2. Jones DW. The strength and strengthening mechanisms ofdental ceramics. In: McLean JW, editor. Dental ceramic:proceeding of the first international symposium on ceramics.1983. p. 83—141.

3. Adair PJ, Grossman D. The castable ceramic crown. Int JPeriodontics Restor Dent 1984;33—45.

4. Mormann WH, Branderstini M, Lutz F. Das Cerecw-system,computergestutzte herstellung direkter. KonservierendeZahnheilkunde 1987;3:1—14.

5. Sadoun M. All ceramic bridges with the slip castingtechnique. Presented at the Seventh International Sym-posium on Ceramics, Paris, September 1988.

6. Hornberger H, Marquis PM, Christiansen S, Stunk HP.Microstructure of a high strength alumina glass composite.J Mater Res 1996;11:855—8.

7. Sadoun M. In-ceram: zukunftige entwicklung mit in-ceram. In:Kappert HF, editor. Volkeramik, werkstoffkunde-zahntech-mik klinche erfrahrung. Berlin: Quintessenz; 1996. p. 227—9.

8. Frank M, Schweiger M, Rheinberger V, Holand W. High-strength translucent sintered glass-ceramic for dental res-torations. Glastech Ber Glass Sci Technol 1998;71C:345—8.

9. Schweiger M, Holand W, Frank M, Drescher H, Rheinberger V.IPA Empressw2: a new pressable high strength glass-ceramicfor esthetic all ceramic restorations. Quint Dent Tech 1999;22:143—52.

10. Holand W. Materials science fundamentals of IPS Empressw 2glass-ceramics. Ivoclar Vivadent Report 1998;12:3—10.

11. Holand W, Schweiger M, Frank M, Rheinberger V. Acomparison of the microstructure and properties of IPSEmpressw 2 and IPS Empressw glass-ceramics. J BiomedMater Res (Appl Biomater) 2000;53:297—303.

12. Richerson DW. Mechanical properties and their measure-ments. Modern ceramic engineering: properties, processingand use in design. New York: Marcel Dekker; 1982.

13. Rizkalla AS, Jones DW. Indentation fracture toughness anddynamic elastic moduli for commercial felspathic dentalporcelain materials. Dental Materials 2003 PII:S0109-5641(03)00092—7.

14. Jones DW, Rizkalla AS, Sutow EJ, King HW. Indentationfracture toughness and dynamic young’s modulus of ceramicbiomaterials. Third International Conference on the Scienceof Hard Material. Material Science and Engineering 1988;A105/106:207—13.

15. Jones DW, Rizkalla AS, Johnson JA, Sutow EJ. Effects ofcomposition on selected physical properties of SiO2—K2O—Na2O glasses. J Mater Sci 1991;26:6501—6.

16. Jones DW, Rizkalla AS, King HW, Sutow EJ. Fracturetoughness, and dynamic modulus of a tetrasilicic-mica-glass-ceramic (K2O—MgF2—MgO—SiO2). J Can Ceram Soc1988;57:39—46.

17. Jones DW, Rizkalla AS. Fracture toughness of bioglasse/ceramic systems. In: Rusin RP, Fishman GS, editors.Bioceramic materials and applications, 2nd ed. Ceramictransactions, vol. 63. 1995. p. 87—91.

18. Rizkalla AS, Jones DW, Miller RP. Parameters controlling theindentation fracture toughness values for Na2O—K2O—SiO2

glasses. Br Ceram Trans 1996;95(6):250—3.19. Rizkalla AS, Jones DW, Archibalt T, Hall GC, Langman M.

Elastic modulus of experimental bioactive glass composites.Bioceramics, vol. 12. World Science Publication Company;1999. p. 445—48.

20. Rizkalla AS, Jones DW, Sutow EJ. Effect of nonbridgingoxygens on the fracture toughness of synthesized glasses. BrCeram Trans J 1992;91:12—15.

21. Li Z, Ghosh A, Kobayashi AS, Bradt RC. Indentation fracturetoughness of sintered carbide in the Palmqvist crack regime.J Am Ceram Soc 1989;72(6):904—11.

A.S. Rizkalla, D.W. Jones212