Research paper
Phenomena and mechanisms of crack propagation in glass-ceramics

https://doi.org/10.1016/j.jmbbm.2007.11.005Get rights and content

Abstract

Lithium disilicate, leucite and apatite glass-ceramics have become state-of-the-art framework materials in the fabrication of all-ceramic dental restorative materials. The goal of this study was to examine the crack propagation behaviour of these three known glass-ceramic materials after they have been subjected to Vickers indentation and to characterize their crack opening profiles (δmeas vs. (ar)). For this purpose, various methods of optical examination were employed. Optical microscopy investigations were performed to examine the crack phenomena at a macroscopic level, while high-resolution techniques, such as scanning electron microscopy (SEM) and atomic force microscopy (AFM), were employed to investigate the crack phenomena at a microscopic level. The crack patterns of the three glass-ceramics vary from fairly straightforward to more complex, depending on the amount of residual glass matrix present in the material. The high-strength lithium disilicate crystals feature a high degree of crosslinking, thereby preventing crack propagation. In this material, the crack propagates only through the residual glass phase, which constitutes 30%–40% by volume. Having a high glass content of more than 65% by volume, the leucite and apatite glass-ceramics show far more complex crack patterns. Cracks in the leucite glass-ceramic propagate through both the glass and crystal phase. The apatite glass-ceramic shows a similar crack behaviour as an inorganic–organic composite material containing nanoscale fillers, which are pulled out in the surroundings of the crack tip. The observed crack behaviour and the calculated Ktip values of the three types of glass-ceramics were compared to the KIC values determined according to the SEVNB method.

Introduction

Glass-ceramics are multiphase materials consisting of at least one glass phase and one crystalline phase. Glass-ceramics are manufactured from base glasses, using the mechanisms of controlled nucleation and crystallization (Kingery et al., 1975, Höland and Beall, 2002 and 2006, Höland, 2006). Moreover, glass-ceramics exhibit special properties that are characteristic of both glass and ceramic materials. Consequently, special combinations of properties can be achieved in this group of materials. Furthermore, materials with new properties, known neither in glass nor in ceramic materials, can be designed. The mechanical properties of glass-ceramics have always played a major role, since they are particularly important to the application of these materials, for instance as biomaterials in restorative dentistry. The mechanical properties, such as flexural strength and toughness, have already been investigated elsewhere (Albakry et al., 2004, Hill et al., 2000, Kelly et al., 1989, Marshall et al., 1987, Mecholsky et al., 1976, Quinn and Bradt, 2007, Quinn and Lloyd, 2000, Soares and Lepienski, 2004). However, the relationship between crack propagation, crack branching and the microstructure has hardly been studied to date. The present study has been carried out to help close this gap in the relevant research.

The objective of this study was to initiate cracks in a controlled way of three different types of glass-ceramic materials (A: leucite-type, B: lithium disilicate-type, C: fluoroapatite-type) and to analyse and describe the resulting crack propagation. This analysis is used as a basis to draw comparisons with the microstructure of the glass-ceramic materials and to sum up the findings on the mechanism of crack propagation as well as to draw comparisons with the mechanical parameters established in previous studies.

The Vickers indentation method was used to apply a controlled damage to the glass-ceramic samples. However, this method does not always permit the critical stress intensity factor, KIC, to be accurately determined (Quinn and Bradt, 2007). Therefore, the KIC parameters of the three glass-ceramics were determined with the Single-Edge-V-Notched Beam (SEVNB) method and not by using the Vickers indentation method. KIC is the value of fracture toughness after loading a specimen with a crack until unstable crack propagation will become critical (Munz and Fett, 1999). On the other side, Ktip values were determined according to Fett et al. (2005) from the crack opening displacement created by Vickers indentation. Ktip represents the stress intensity factor at the crack tip at the onset and during crack extension. The crack propagation and crack peak phenomena of the three glass-ceramic materials were compared with each other and examined in comparison to the microstructure.

The three types of glass-ceramics selected for this study are particularly important to restorative dentistry. Glass-ceramic A contains leucite, KAlSi2O6, as the main crystal phase. Leucite glass-ceramic has been used as a model compound for IPS Empress®(Ivoclar Vivadent AG), a biomaterial which has been utilized since 1990 in more than 30 million dental restorations to create metal-free dental reconstructions, such as inlays, crowns and veneers. This glass-ceramic has been developed from the SiO2–Al2O3–K2O–Na2O chemical system. Its crystal content is approx. 25%–30% by volume.

Glass-ceramic B contains lithium disilicate crystals, Li2Si2O5, as the main crystal phase. Its crystal content is approx. 60% by volume. This glass-ceramic has been derived from the SiO2–Li2O–K2O–Al2O3–P2O5 chemical system (Apel et al., 2007, Höland et al., 2006). A glass-ceramic of a similar composition is utilized as metal-free framework material (IPS e.max®CAD, Ivoclar Vivadent AG) to construct dental crowns and three-unit bridges.

Glass-ceramic C is a model compound for a fluoroapatite-containing glass-ceramic (fluoroapatite: Ca5(PO4)3F). The base glass is derived from the SiO2–Al2O3–K2O–Na2O–CaO–P2O5–F system. The apatite containing glass-ceramic of this study was developed via monolithic glass processing and applying internal nucleation and crystallization mechanisms. But this base glass was also used to develop leucite- apatite glass-ceramics via powder compact processing by applying surface and internal mechanisms of nucleation and crystallization (see IPS d.SIGN®, (Höland et al., 2007)). Fluoroapatite is the main crystal phase of the glass-ceramic of the present study. Rhenanite, NaCaPO4 represents the secondary crystal phase of this material. The overall crystal content of the glass-ceramic is less than 10 vol%.

Section snippets

Experimental

The base glasses to manufacture the glass-ceramics were prepared via the molten-phase route. The base glass to form the leucite glass-ceramic (glass-ceramic A) was melted at 1600 C for 60 min. A glass frit was produced by quenching. This glass frit was milled to a grain size of <90 μm. This technique is necessary as leucite glass-ceramic forms by controlled surface nucleation and surface crystallization, favoured by tribochemical activation of the base glass (Höland et al., 1995). Nucleation

Results

The results obtained from analysing the surface damage caused by the Vickers indentation on the three different types of glass-ceramics are outlined below. The results are presented individually for each measuring method and for each material, the results obtained for the individual glass-ceramics are compared with each other.

Discussion

The three measuring methods, i.e. optical microscopy, scanning electron microscopy and atomic force microscopy, selected for this study enabled us to conduct a comprehensive investigation of the basic phenomena related to the crack propagation in glass-ceramics. The crack path of the leucite ceramic is characterized by discontinuities (Fig. 3(A)). The area of the pyramid indent contains numerous cracks. Cracks are also visible in the surroundings of the pyramid. The macroscopic heterogeneous

Conclusions

  • 1.

    In view of the specific crack paths of all three glass-ceramics, we conclude that the crack propagation mechanisms in the three materials differ from each other.

  • 2.

    Leucite glass-ceramic

    The complex crack pattern and the microbridging in the crack path are indicative of the fact that compressive stresses occurred at the interface between the crystals and residual glass matrix. These stresses have a strength-increasing effect. Nonetheless, the crack propagated through both the crystal and glass phase

Acknowledgements

The authors like to thank the European Union for sponsoring the project by the Efonga program and Dr. J.R. Kelly, USA for fundamental scientific discussions.

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