Mohammad Albakry

Fracture toughness and hardness evaluation of three pressable all-ceramic dental materials

OBJECTIVES This study evaluates the fracture toughness and hardness of three pressable all-ceramic materials: IPS-Empress, Empress 2 and an experimental ceramic material. METHODS Fifteen discs and 15 bars per material were prepared. Fracture toughness was measured with two different techniques: indentation fracture and indentation strength. During the indentation fracture tests the hardness of each material was also measured. Statistical significance among groups of population was studied using one-way Anova and Tukeys multiple comparison tests. RESULTS Fracture toughness results using the indentation strength technique (with three-point bending and biaxial flexure tests) were: IPS-Empress (1.39 (SD 0.3) and 1.32 (SD 0.3)); Empress 2 (3.14 (SD 0.5) and 2.50 (SD 0.3)) MPa x m(1/2); and the experimental ceramic (3.32 (SD 0.6) and 2.43 (SD 0.3)) MPa x m(1/2). The indentation fracture technique generated orthogonal cracks of different lengths for Empress 2 and the experimental ceramic, whether perpendicular or parallel to the lithium disilicate elongated crystals. Thus, two values were reported: Empress 2 (1.5 (SD 0.2) and 1.16 (SD 0.2)) MPa x am(1/2) and the experimental ceramic (1.67 (SD 0.3) and 1.15 (SD 0.15)) MPa x m(1/2). The IPS-Empress indentation fracture result was 1.26 (SD 0.1). The hardness results were: 6.6, 5.3 and 5.5 GPa for IPS-Empress, Empress 2 and the experimental ceramic, respectively. CONCLUSIONS No significant differences in fracture toughness and hardness results were found between Empress 2 and the experimental ceramic (P>0.05 ANOVA). Both materials exhibited fracture toughness anisotropy following pressing. They demonstrated improved fracture toughness and reduced hardness compared with IPS-Empress P<0.05(ANOVA), which should be beneficial for clinical applications.

Microstructure of Alumina- and Alumina/Zirconia-Glass Infiltrated Dental Ceramics

The microstructure and the fracture behavior of four dental cerami cs In-Ceram Alumina and In-Ceram Zirconia (slip and block) were investigated with SEM in order to study the differences existing among the four materials and correlate them with a pparently contradictory previous mechanical studies. Microstructural observations indicate, In-Ceram Alu ina slip and block to be two different materials and a dissimilar mechanical behavior is also expected. It is postulated that the presence of a low modulus glassy phase and a significant porosity, counteract the transformation toughening potential of the tetragonal zirconia in In-Ceram Zirconia. Introduction In-Ceram Alumina (IA) (Vita Zahnfabrik, Bad Säckingen, and Germa ny) is a partially-sintered dental ceramic, which achieves its ultimate strength after infiltra t on with lanthanum silicate glass. The ceramic is partially sintered to avoid undesirable shrinkage and the molt n glass fills the pores resulting from the incomplete sintering process. In order to improve the mechani cal properties of IA, Vita has developed a new ceramic by adding 33vol% of CeO 2-PSZ to the original material. The new compound is known as In-Ceram Zirconia (IZ). Both materials are commer cially available as an aqueous based slip, which is used to shape the dental coping and then sintered by the dental technicia n, or alternatively as block already sintered which is milled before glass-infiltration. Processing of the block by the manufacturer should result in a mor e homogeneous material with better mechanical properties. Besides, IZ should be significantly stronger and tougher than IA due to the tetragonal to monoclinic phase transformation ( m-t) of Zirconia. However, studies regarding the mechanical properties of these materials do not consistently support such assumptions, suggesting that some other factors are also important in determining the performance of IA and Z [1-3]. The aim of this study was to investigate the microstructure of IA and IZ (slip and block) and find a rationale to this apparent incongruence between theoretical expectation and r eported values. Materials and Methods Several specimens for each material were polished to 1 μm and carbon coated for SEM observation (XL 30, Philips, Windhaven, Holland). Some other samples were polished to 1 μm, etched with ions beam coated with platinum and observed with Field Emission SEM (JSM 6000 FSEM, Joel, Tokyo, Japan). Indentations with a Vickers indenter were also made to inves tigat the crack propagation. Image analysis of SEM micrographs was used to appraise the volume fraction of each phase and porosity. Shape of the grains particle size and features related to the crack propagation were also studied. Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 879-882 doi:10.4028/www.scientific.net/KEM.240-242.879

Fracture Toughness, Microstructure and Toughening Mechanism of Leucite and Lithium Disilicate Glass Ceramics

Introduction In recent years the use of all-ceramic materials for dental prosthesis has increased rapidly. All ceramic materials offer better optical properties and biocompati bility as compared with the traditional porcelain-fused-to metallic-substrates. The main dra wback of these materials has been their brittle nature and the extreme sensitivity to flaws and d efects [1]. One class among these systems requires hot pressing by means of a special furnace to produce the required shape (pressable materials). Empress 1 (E1) (leucite reinforced glass-ceramic) and Empress 2 (E2) (lithium disilicate glass-ceramic) (Ivocalr-Vivadent, Schaan, Liechtenstein) are well known pressable materials, which have generated considerable interest b cause of their ease of fabrication (lost waxing technique) and excellent esthetic features. The fi rst material, E1, is recommended for restoring single units, and preferably in low stress bearing are as. E2 was developed in 1998s for the purpose of restoring 3-unit fixed partial dentures up to the second pre molar. Another pressable material (experimental) (E3) (also a lithium disilicate gl ass-ceramic) is about to be introduced into the market by the same manufacturer. This material is cla med to offer better mechanical and optical properties than Empress 2 and is also suitable for the fabri cation of 3-unit fixed partial dentures. The aim of the present study was to evaluate the fract ure toughness of the 3 abovementioned all-ceramic materials, using the indentation fracture (IF) and t he indentation strength (IS) techniques, and to elucidate the toughening mechanism involved in each system.