Mechanical properties of commercial high strength ceramic core materials

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1 Dental Materials (2004) 20, Mechanical properties of commercial high strength ceramic core materials A.S. Rizkalla a, *, D.W. Jones b a Division of Biomaterials Science, School of Dentistry, Faculty of Medicine and Dentistry, The University of Western Ontario, London, Ont., Canada N6A 5C1 b Division of Biomaterials, Dalhousie University, Halifax, NS, Canada Received 11 November 2002; accepted 21 January 2003 KEYWORDS Dental ceramics; Flexural strength; Dynamic elastic modulus; True hardness; Glass-ceramic Summary Objective. The objective of the present study is to evaluate and compare the flexural strength, dynamic elastic moduli and true hardness ðh o Þ values of commercial Vita In-Ceram w alumina core and Vita In-Ceram w matrix glass with the standard aluminous porcelain (Hi-Ceram w and Vitadur w ), Vitadur N w and Dicor w glass and glass-ceramic. Methods. The flexural strength was evaluated ðn ¼ 5Þ using 3-point loading and a servo hydraulic Instron testing machine at a cross head speed of 0.5 mm/min. The density of the specimens ðn ¼ 3Þ was measured by means of the water displacement technique. Dynamic Young s shear and bulk moduli and Poisson s ratio ðn ¼ 3Þ were measured using a non-destructive ultrasonic technique using 10 MHz lithium niobate crystals. The true hardness ðn ¼ 3Þ was measured using a Knoop indenter and the fracture toughness ðn ¼ 3Þ was determined using a Vickers indenter and a Tukon hardness tester. Statistical analysis of the data was conducted using ANOVA and a Student Newman Keuls (SNK) rank order multiple comparative test. Results. The SNK rank order test analysis of the mean flexural strength was able to separate five commercial core materials into three significant groups at p ¼ 0:05: Vita In-Ceram w alumina and IPS Empress w 2 exhibited significantly higher flexural strength than aluminous porcelains and IPS Empress w at p ¼ 0:05: The dynamic elastic moduli and true hardness of Vita In-Ceram w alumina core were significantly higher than the rest of the commercial ceramic core materials at p ¼ 0:05: Significance. The ultrasonic test method is a valuable mechanical characterization tool and was able to statistically discriminate between the chemical and structural differences within dental ceramic materials. Significant correlation was obtained between 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 *Corresponding author. Tel.: þ x86086; fax: þ address: arizkalla@eng.uwo.ca In the early 1950s, the ceramics employed in the conventional porcelain jacket crown were medium to high fusing feldspathic porcelains. Due /$ see front matter Q 2003 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi: /s (03)

2 208 A.S. Rizkalla, D.W. Jones to the relatively low strength of this type of porcelain, an alumina-reinforced porcelain core material was developed by McLean 1 for the fabrication of porcelain jacket crowns. These aluminareinforced crowns were regarded as providing better esthetics for anterior teeth than metalceramic crowns, but they exhibited a lower flexural strength, which limited their use for posterior teeth. The brittle nature of ceramics demands a greater margin of safety in strength than with metals. All dental ceramics tend to fail at the same critical strain 2 of the order of 0.1%. For this reason any increase in strength and toughness can only be achieved by an increase in the elastic modulus. The high-strength all ceramic biomaterials that are currently used in dentistry consists of alumina, zirconia, pressed, castable or machinable glass ceramics. Several developments have taken place in these areas resulting in the production of ceramic materials for clinical use: These include the aluminous porcelain crown Vitadur w, Hi-Ceram w ), the non shrink ceramic crown (Cerestore w ), the castable mica glass-ceramic crown (Dicor w ), 3 the machinable glass ceramic known as Dicor w MGC 4 and Vitabloc w as well as the pressed leucite glass ceramics IPS Empress w All these ceramic systems exhibit low flexural strengths ( MPa) that are considerably below the yield point of the gold alloys used for ceramic-to-metal bonding and therefore are at risk of failure when used for the construction of either molar crowns bridges or fixed partial dentures. The In-Ceram technique has been developed 5 using an aluminum oxide slip casting technique used to build the framework, which is then fired to an open-pore microstructure. The material gains its strength by infiltration of the open-pore In-Ceram w alumina microstructure with lanthanum glass. The high flexural strength of the glass-infiltrated In- Ceram w alumina material ( MPa) depends on the strength of the fired bond between the aluminum oxide particles and the complete wetting Figure 1 The Student Newman Keuls rank order test comparing the flexural strength of five commercial all of the open-pore microstructure by lanthanum glass infiltration. 6 The mechanical properties of In- Ceram w alumina can be improved by adding zirconium oxide. 7 The addition of 33 wt% of zirconium oxide led to an increase of the flexural strength up to 750 MPa while fracture toughness is doubled. 7 A later version of the In-Ceram system uses a crystalline spinel in place of the alumina, presumably to increase the translucency, while at the same time sacrificing strength. A pressed lithium disilicate glass-ceramic has been developed 8 10 and is known as IPS Empress w 2. Table 1 Material Commercial ceramic core used for this study. Supplier Vita In-Ceram w alumina core Vita In-Ceram w matrix glass Hi-Ceram w core Vitadur w core Vitadur N w dentine Vitadur N w enamel Dicor w as cast Dicor w cerammed Vident Dentsply Dentsply Figure 2 The Student Newman Keuls rank order test comparing the density of 8 commercial all ceramic core materials.

3 Mechanical properties of commercial high strength ceramic core materials 209 Figure 3 The Student Newman Keuls rank order test comparing Poisson s ratio of 8 commercial all ceramic core materials. It exhibits superior mechanical properties to IPS Empress w.höland et al. 11 reported that the flexural strength and fracture toughness of IPS Empress w 2 are 440 þ 0.40 MPa and 3.3 þ 0.3 MPa m 0.5, respectively. These values are comparable to In- Ceram w alumina. The aim of the present study is to evaluate and compare the flexural strength, dynamic elastic moduli and true hardness ðh o Þ values of commercial Vita In-Ceram w alumina core and Vita In-Ceram w matrix glass with the standard aluminous porcelain Figure 5 The Student Newman Keuls rank order test comparing the dynamic shear moduli of 8 commercial all (Hi-Ceram w and Vitadur w ), Vitadur N w and Dicor w glass and glass-ceramic. Materials and methods Seven commercial ceramic materials were used in this study. Information about the materials is given in Table 1. Figure 4 The Student Newman Keuls rank order test comparing the dynamic Young s moduli of commercial all Figure 6 The Student Newman Keuls rank order test comparing the dynamic bulk moduli of 8 commercial all

4 210 A.S. Rizkalla, D.W. Jones Table 2 Length of Knoop indentations vs. square root of indentation load for all Square root of Knoop indentation load (N) 1/2 Slope(mm/N 0.5 ) p, 0:001 (SD) 1.4 (SD) 2.21 (SD) 2.62 (SD) 2.8 (SD) 2.97 (SD) 3.13 (SD) Knoop indentation length (mm) Vita In-Ceram w (0.29) (0.95) (1.35) 97.6 (1.20) (1.09) (1.46) (0.84) alumina core Vita In-Ceram w (1.48) (1.43) (0.72) (0.62) (0.58) (0.31) (0.88) matrix glass Hi-Ceram w core (0.42) 98.8 (2.33) (0.99) (0.62) (2.10) (3.05) (1.81) Vitadur w core (0.52) (0.75) (3.84) (3.86) (1.74) (0.96) (0.63) Vitadur N w dentine (0.71) (1.11) (1.28) (0.53) (0.49) (198) (0.84) Vitadur N w enamel (0.35) 95.5 (1.28) (0.01) (1.57) (1.46) (1.60) (0.90) Dicor w as cast (0.12) (1.13) (0.23) (0.14) (0.05) (0.21) (0.14) Dicor w cerammed (0.05) (0.21) (0.60) (0.13) (0.09) (0.10) (0.04) For flexural strength evaluation, specimens (n ¼ 5) having a rectangular cross section (1 5 mm) and 3-point loading over a span of 11.5 mm. Testing was carried out in air using a servo hydraulic Instron testing machine at a cross head speed of 0.5 mm/min. The formula 12 used to calculate the bend strength is given below s 3PT ¼ PL 2bd 2 where s 3PT ¼ flexural strength (MPa), P ¼ applied load (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 the elastic moduli evaluation, specimens ðn ¼ 3Þ of dimensions 6.4 ^ 0.1 mm (diameter) and 5.0 ^ 0.1 mm (length) were prepared for each material. The end surfaces of each specimen were ground flat and parallel using a technique described elsewhere. 13,14 The dynamic Young s shear and bulk moduli of these materials were evaluated using an ultrasonic method. For true hardness evaluation, the cylindrical specimens were sectioned in half, embedded in resin and repolished. The true hardness was calculated using Knoop indentations and the method developed by Li et al. 13,21 A series of 6, crack free Knoop indentations were performed at different loads ranging from 1.96 to 9.80 N. Indentation length was plotted vs. the square root of the load values. The true hardness ðh o Þ was calculated from the slope of the regression line. the bar diagram in Fig. 1. These values ranged from MPa for IPS Empress w to MPa for Vita In-Ceram w alumina. A Student Newman Keuls (SNK) rank order test separated the means into three significant groups at p ¼ 0:05: There was no significant difference in the flexural strength values between Vitadur w, Hi-Ceram w and IPS Empress w core materials. The density values of the commercial ceramic core materials are shown in the bar diagram in Fig. 2. These values ranged from 2.56 g/cc for Dicor w as cast to 3.86 g/cc for Vita In-Ceram alumina core. The density values for Vitadur N w dentin and Vitadur N w enamel were 2.32 ^ 0.02 and 2.31 ^ 0.01 g/cc, respectively. The SNK rank order test separated eight materials into seven significant groups at p ¼ 0:05: The Poisson s ratio data are shown in Fig. 3. The mean values ranged from for Hi-Ceram w core to for Vita In-Ceram matrix glass. The SNK Results and discussion The flexural strength values of five high strength commercial ceramic core materials are shown in Figure 7 Length of Knoop indentation vs. the square root of load for Vita In-ceram w alumina and Dicor w cerammed.

5 Mechanical properties of commercial high strength ceramic core materials 211 Figure 8 The Student Newman Keuls rank order test comparing the true hardness of 8 commercial all ceramic core materials. rank order test separated eight materials into six significant groups at p ¼ 0:05: The dynamic Young s modulus ðeþ results are shown in Fig. 4. The mean values for the core materials ranged from GPa for Dicor w to GPa for Vita In-Ceram w alumina core and The E values for Vitadur N emamel and Vitadur N dentine were ^ 1.36 and ^ 0.85, respectively. The SNK rank order test separate eight different materials into five significant group at p ¼ There was no significant difference between The E value of Vitadur N w enamel, dentine and Dicor w as cast and cerammed at p ¼ 0:05: The dynamic shear modulus ðgþ results are shown in Fig. 5. The mean values for the core materials ranged from GPa for Dicor w cerammed to GPa for Vita In-Ceram w alumina core. The SNK rank order test separated eight different materials into six significant groups at p ¼ 0:05: There was no significant difference between The E value of Vitadur N w enamel, dentine and Dicor w as cast and cerammed at p ¼ 0:05: The dynamic bulk modulus ðkþ results are shown in Fig. 6. The mean values for core materials ranged from GPa for Dicor w as cast for Vitadur N to GPa for Vita In-Ceram w alumina. The SNK rank order test separated eight different materials into seven significant groups at p ¼ 0:05: There was no significant difference between The E value of Vitadur N w enamel, dentine and Dicor w as cast at p ¼ 0:05: For the true hardness evaluation, the mean length of the Knoop indentation at each of the applied load for the different dental porcelain materials is given in Table 2. Three linear regression analyses were conducted for each material. Significant correlations were obtained at p, 0:001: The mean of the slopes of the linear regressions for each material is also shown in Table 2. As example, the relationship between the length of the Knoop indentation vs. the square root of the applied load is displayed in the scattergram shown in Fig. 7 for Dicor w and Vita In-Ceram w alumina. The true hardness was calculated from the slope of the regression line as described by Li et al. The true hardness ðh o Þ values are displayed in the bar diagram in Fig. 8. These values ranged from 3.38 GPa for Dicor w cerammed to GPa for Figure 9 A scattegram showing the relationship between the dynamic Young s modulus and true hardness for all commercial

6 212 A.S. Rizkalla, D.W. Jones Vita In-Ceram w alumina. The same multiple comparative test separated the eight (8) mean values into five groups p ¼ 0:05: There was no significant difference between the true hardness of Vita In- Ceram w matrix glass and Vitadur w core. Similarly, there was no significant difference between Vitadur N w enamel and dentine. p ¼ 0:05: Linear regression analysis was conducted between the dynamic elastic moduli and the true hardness for the different commercial materials. A significant correlation was obtained ðr ¼ 0:984Þ as displayed in Fig. 9. Conclusions 1. The mechanical properties of Vita In-Ceram w alumina core were significantly higher than the seven commercial ceramic materials in this present study p ¼ 0:05: 2. The flexural strength of Vita In-Ceram w alumina is comparable to IPS Empress w There was no significant difference in the mechanical properties between Dicor w as cast, Dicor w cerammed, Vitadur N w enamel and dentine, p ¼ 0:05: 4. Significant correlation was obtained between the Dynamic Young s modulus and the true hardness of the eight commercial all ceramic core materials, r ¼ 0:984: References 1. McLean JW, The science and art of dental ceramics, vol. 1. Chicago: Quintessence Publication Company; Jones DW. The strength and strengthening mechanisms of dental ceramics. In: McLean JW, editor. Dental ceramic: proceeding of the first international symposium on ceramics p Adair PJ, Grossman D. The castable ceramic crown. Int J Periodontics Restor Dent 1984; Mörmann WH, Branderstini M, Lutz F. Das Cerec w -system, computergestützte herstellung direkter. Konservierende Zahnheilkunde 1987;3: Sadoun M. All ceramic bridges with the slip casting technique. Presented at the Seventh International Symposium on Ceramics, Paris, September Hornberger H, Marquis PM, Christiansen S, Stunk HP. Microstructure of a high strength alumina glass composite. J Mater Res 1996;11: Sadoun M. In-ceram: zukünftige entwicklung mit in-ceram. In: Kappert HF, editor. Volkeramik, werkstoffkunde-zahntechmik klinche erfrahrung. Berlin: Quintessenz; p Frank M, Schweiger M, Rheinberger V, Höland W. Highstrength translucent sintered glass-ceramic for dental restorations. Glastech Ber Glass Sci Technol 1998;71C: Schweiger M, Höland W, Frank M, Drescher H, Rheinberger V. IPA Empress w 2: a new pressable high strength glass-ceramic for esthetic all ceramic restorations. Quint Dent Tech 1999; 22: Höland W. Materials science fundamentals of IPS Empress w 2 glass-ceramics. Ivoclar Vivadent Report 1998;12: Höland W, Schweiger M, Frank M, Rheinberger V. A comparison of the microstructure and properties of IPS Empress w 2 and IPS Empress w glass-ceramics. J Biomed Mater Res (Appl Biomater) 2000;53: Richerson DW. Mechanical properties and their measurements. Modern ceramic engineering: properties, processing and use in design. New York: Marcel Dekker; Rizkalla AS, Jones DW. Indentation fracture toughness and dynamic elastic moduli for commercial felspathic dental porcelain materials. Dental Materials 2003 PII:S (03) Jones DW, Rizkalla AS, Sutow EJ, King HW. Indentation fracture toughness and dynamic young s modulus of ceramic biomaterials. Third International Conference on the Science of Hard Material. Material Science and Engineering 1988; A105/106: Jones DW, Rizkalla AS, Johnson JA, Sutow EJ. Effects of composition on selected physical properties of SiO 2 K 2 O Na 2 O glasses. J Mater Sci 1991;26: Jones DW, Rizkalla AS, King HW, Sutow EJ. Fracture toughness, and dynamic modulus of a tetrasilicic-micaglass-ceramic (K 2 O MgF 2 MgO SiO 2 ). J Can Ceram Soc 1988;57: Jones DW, Rizkalla AS. Fracture toughness of bioglasse/ ceramic systems. In: Rusin RP, Fishman GS, editors. Bioceramic materials and applications, 2nd ed. Ceramic transactions, vol p Rizkalla AS, Jones DW, Miller RP. Parameters controlling the indentation fracture toughness values for Na 2 O K 2 O SiO 2 glasses. Br Ceram Trans 1996;95(6): Rizkalla AS, Jones DW, Archibalt T, Hall GC, Langman M. Elastic modulus of experimental bioactive glass composites. Bioceramics, vol. 12. World Science Publication Company; p Rizkalla AS, Jones DW, Sutow EJ. Effect of nonbridging oxygens on the fracture toughness of synthesized glasses. Br Ceram Trans J 1992;91: Li Z, Ghosh A, Kobayashi AS, Bradt RC. Indentation fracture toughness of sintered carbide in the Palmqvist crack regime. J Am Ceram Soc 1989;72(6):