JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS 4 (2011) 1291–1298 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jmbbm Research paper Crack tip fracture toughness of base glasses for dental restoration glass-ceramics using crack opening displacements J. Deubener a,∗ , M. Höland b , W. Höland c , N. Janakiraman a , V.M. Rheinberger c a Institute of Non-Metallic Materials, Clausthal University of Technology, D-38678 Clausthal-Zellerfeld, Germany b Institute for Micro- and Nanotechnology, Interstate University of Applied Sciences, CH- 9470 Buchs, Switzerland c Ivoclar-Vivadent AG, Li-9494 Schaan, Principality of Liechtenstein ARTICLE INFO Article history: Received 5 January 2011 Received in revised form 15 April 2011 Accepted 18 April 2011 Published online 28 April 2011 Keywords: Glasses Vickers indentation Crack opening displacement Crack tip toughness ABSTRACT The critical stress intensity factor, also known as the crack tip toughness K tip , was determined for three base glasses, which are used in the manufacture of glass-ceramics. The glasses included the base glass for a lithium disilicate glass-ceramic, the base glass for a fluoroapatite glass-ceramic and the base glass for a leucite glass-ceramic. These glass- ceramic are extensively used in the form of biomaterials in restorative dental medicine. The crack tip toughness was established by using crack opening displacement profiles under experimental conditions. The crack was produced by Vickers indentation. The crack tip toughness parameters determined for the three glass-ceramics differed quite significantly. The crack tip parameters of the lithium disilicate base glass and the leucite base glass were higher than that of the fluoroapatite base glass. This last material showed glass-in-glass phase separation. The discussion of the results clearly shows that the droplet glass phase is softer than the glass matrix. Therefore, the authors conclude that a direct relationship exists between the chemical nature of the glasses and the crack tip parameter. c ⃝ 2011 Elsevier Ltd. All rights reserved. 1. Introduction The fracture toughness or the critical stress intensity factor K C is a crucial parameter required for the prediction of the mechanical performance of technical glasses, i.e. tagging critical failure sizes and dimensioning products for designated stress regimes. Among the various experimental methods for measuring the fracture toughness, fracturing by surface indentation is widely used, because this method is quicker and costs less than alternative techniques (Vullo ∗ Corresponding author. E-mail address: joachim.deubener@tu-clausthal.de (J. Deubener). and Davis, 2004). Due to their brittle nature, glasses are susceptible to localized surface damage in the form of cracking when subjected to indents by sharp objects such as Vickers diamonds. The mechanical behaviour of glasses under Vickers indenters changes considerably, depending on the intrinsic properties of the glasses and on their environment (Lawn et al., 1980, 1983). Both responses, plastic flow and densification, are initiated by the high stresses during indentation and lead to a remnant plastic impression in the glass surface. The residual stress components arising 1751-6161/$ - see front matter c ⃝ 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jmbbm.2011.04.016