The analysis of tool wear mechanisms in the machining of pre-sintered zirconia dental crowns

Abstract

The growth of Digital Dentistry has opened new potentials in the dental restoration sector where personalised restorations could be made in a much shorter time than before. Zirconia has been widely used due to its excellent wear properties and biocompatibility. Zirconia is machined primarily in its pre-sintered state, using carbide tools. Even in its pre-sintered state, zirconia is abrasive and caused tool wear. Tool wear is affected by tool substrate used, cutting conditions used, machining method employed, etc. Tool substrates are such as tungsten carbide, steel, etc. Cutting conditions refer to speed, feed, depth of cut, etc. Machining methods refer to milling wet or dry. Understanding tool wear helps in better tool design. Tools that lasted longer would mean the end users i.e. dental technicians would have less unproductive downtime changing tools. In addition, worn tool would produce broken restorations, which is undesirable. With this in mind, the aim of this paper was to study tool wear by identifying the wear mechanism that occurred when carbide tools machined pre-sintered zirconia. Test was constructed using recommendations from ISO 8688:1989. Cutting conditions used were adapted from those used in desktop dental milling machines. Pre-defined stopping criterion was set and flank wear of tool was measured every 15 minutes using an optical microscope. When tools reached the stopping criterion, Scanning Electron Microscope (SEM) and Quadrant Backscatter Diffraction (QBSD) were used to analyse worn tool in details. Findings showed transgranular and intergranular fractures were the wear mechanisms on carbide tools when machining pre-sintered zirconia.