Hertzian contact response and damage tolerance of dental ceramics.

Abstract

To determine the contact response and damage tolerance or strength degradation of a range of dental CAD/CAM ceramic materials including novel polymer-infiltrated-ceramic-network (PICN) materials by means of spherical indentations at various loads and indenter radii. The seven tested materials included Mark II, PICN test materials 1 and 2, In-Ceram Alumina, VM 9, In-Ceram YZ (Vita Zahnfabrik, Bad Saeckingen, Germany) and IPS e.max CAD, (Ivoclar Vivadent, Schaan, Liechtenstein). To evaluate the damage tolerance and role of indenter size, indentations with tungsten carbide spheres (0.5mm and 1.25mm radius) were placed on bending bars with varying loads (1.96-1000N). The indented bending bars were subsequently loaded to fracture in three-point bending. The contact induced damage was analyzed by light microscopy (LM) and SEM. The spherical contact response was measured on polished surfaces. The initial strengths for the individual materials were found to reduce above specific indentation loads, which were a function of the indenter radius. Employing a 0.5mm radius sphere resulted in the following strength degrading loads and ordering of materials: VM9 (98N)<MarkII-PICN1 (147N)<ICAlumina-e.maxCAD (300N)<PICN2-YTZP (500N). For the materials indented with the 1.25mm sphere, higher loads were required for the onset of strength degradation: VM9 (190N)<MarkII (300N)<PICN1 (400N)<e.maxCAD (500N)<ICAlumina (700N)<PICN2 (1000N)<YTZP (above 1000N). Two different damage modes were observed by SEM and LM - brittle cone cracking and plastic deformation. The PICN materials exhibited elastic-plastic behavior with creep. In contrast YTZP showed entirely elastic behavior upon loading with both spheres. This study aims to emulate the likely clinical behavior of contact loading by opposing cusps to dental restorative ceramic materials by utilizing spherical indentations at various loads and sphere diameters.