Microstructural Observations of Laser-Sintered Specimens for Prosthodontic Applications
ABSTRACTObjectives: Laser sintering is a recently introduced technology that has been used to prepare dental restorations from a biocompatible Co-Cr base metal alloy. The objectives of this study were to obtain detailed information about dimensional accuracy available with laser sintering, along with the microstructure and Vickers hardness of the alloy.Methods: Representative maxillary coping and implant framework specimens were prepared from STL files by a commercial laser sintering apparatus (Phenix Systems), using the dedicated Co-Cr alloy. Compatible dental porcelain was bonded to the coping specimen (North Shore Dental Labs, Lynn, MA). Specimens were sectioned with a slow-speed diamond saw, prepared for metallographic examination by resin-mounting and polishing with a series of abrasives, and observed with a scanning electron microscope and optical microscope at a range of magnifications. Alloy compositions were obtained by x-ray energy-dispersive spectrometric analyses. A Co-Cr alloy implant framework prepared by conventional milling/polishing and a cast Ni-Cr alloy coping for a conventional metal-ceramic restoration with the dental porcelain served as controls. Microstructures of the laser-sintered and cast alloys were revealed by electrolytic etching. Values of Vickers hardness for the Co-Cr alloy in the laser-sintered implant framework and the coping for the metal-ceramic restoration were compared, along with the cast coping. Dimensions of the milled/polished and laser-sintered implant frameworks were measured microscopically and compared.Results: The laser-sintered Co-Cr alloy had a fine-grained microstructure, and there was some difficulty in revealing full details by electrolytic etching, whereas the cast Ni-Cr alloy had a well-defined dendritic microstructure. The laser-sintered Co-Co alloy coping had intimate interfacial attachment to dental porcelain. The Vickers hardness was slightly (< 10%, but statistically significant), higher for the laser-sintered Co-Cr implant framework, compared to the laser-sintered Co-Cr coping. The laser-sintered Co-Cr alloy was twice as hard as the cast Ni-Cr alloy. Fits on the original cast/die of the implant frameworks and metal-ceramic specimens were judged to be clinically acceptable by two prosthodontists. Dimensions of the laser-sintered and milled/polished implant frameworks were in excellent agreement for well-defined measurement locations.Conclusions: The fine-grained laser-sintered Co-Cr alloy should have high values of strength, indicated by the high measured Vickers hardness. More research is needed for the electrolytic etching process to reveal the laser-sintered microstructure in greater detail. Use of a nanoindenter is recommended to provide further information about any local hardness variations in the microstructure. The laser-sintering process produces representative prostheses with clinically acceptable accuracy, in agreement with previous publications from other research groups.
Dentistry, Laser sintering, Alloys, Prosthodontics, Hardness, Composition, Metals, Structure
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