Influence of Heat-Treatment Cycles on the Microstructure, Mechanical Properties, and Corrosion Resistance of Co-Cr Dental Alloys Fabricated by Selective Laser Melting

Abstract

The effect of repeated heat-treatment (HT) cycles, as precisely applied in the regular dental practice to produce metal-ceramic restorations, on microstructure, mechanical properties, and corrosion resistance of CoCrMo dental alloys, fabricated via the advanced digital technique of selective laser melting (SLM) (with the aid of CAD technology), is thoroughly investigated. Fifteen samples were subjected to four well-defined firing cycles. Crystallographic (by x-ray diffraction) and microstructure analyses (by SEM/BSE and EDS), measurements of mechanical properties (modulus of elasticity, Vickers micro-hardness, and yield strength), and corrosion resistance tests (by potentiodynamic polarization measurements in artificial saliva-simulating solution at 37 °C) along with the evaluation of the resultant microstructure after the corrosion tests were conducted in all prepared samples in order to shed light on the influence of the HT on the above properties. Compared to the as-prepared samples (15 samples), the heat-treated alloy presents a markedly refined microstructure with a notable carbide/Laves phase dissolution (which are present in as-prepared alloy) and extinction (to a large extent) of the dendritic/cellular morphology (which characterizes the as-prepared alloy). The mixed γ and e composition persists; nevertheless, the amount of γ-phase increases. A small reduction in the values of the mechanical properties was recorded (i.e., the modulus of elasticity was reduced from 240 to 210 GPa, Vickers microhardness from 280 to 250 GPa, and yield strength from 841 to 752 MPa). However, the HT improved the resistance of the samples toward general corrosion, which is mainly ascribed to the stress relief and refined/uniform microstructure occurring due to the HT (ascribed to the intergranular coarse carbide/Laves phase dissolution). The alterations that were caused to the investigated samples owing to the HT were relatively small, suggesting that the Co-Cr dental alloys produced by SLM technique qualify for safe use in preparing metallic substrates of metal-ceramic dental restorations.