Mechanical characterisation of dental crown restoration materials subjected to artificial ageing through nanoindentation tests.

Nowadays, in the field of dental prosthetics, numerous solutions can be adopted in terms of implant design, and it is of fundamental importance to be aware of their mechanical properties in order to assess their clinical reliability. In the present study, a characterisation of the mechanical properties of different dental resins was carried out, including ceramic-polymeric hybrid, 3D-printed, flowable and ceramic materials. Nanoindentation tests were performed before and after artificial ageing processes (to which all samples were subjected). Ceramic materials were sintered separately at 1500, 1550 and 1600°C, in two different furnaces, individually or in groups and the influence of these parameters were mechanically characterised. Five different null-hypotheses were then advanced to evaluate the obtained results: • Mechanical parameters do not change between pre and post treatment; • Parameters do not change depending on the type of sintering oven used; • Parameter do not change by sintering several samples together or a single one; • Parameters do not change with sintering temperature; • There is no difference between 3D printed resins and CAD/CAM resins. A total of 148 samples were analysed. Among them: ceramic (Zirconia, IPS e.max CAD, Initial Lisi Block), hybrid (Vita Enamic, Cerasmart), 3D-printed (Irix Max, Irix Plus) and flowable materials (Clearfill majesty ES, Filtek Bulk Fill, G-aenial Universal Injectable, Venus Bulk Flow ONE). Nanoindentation tests were performed in order to estimate Elastic modulus, Martens hardness, Vickers hardness and Plasticity index before and after a thermocycling process from 5 to 55°C for a total of 10000 cycles to which all samples were subjected, and an artificial ageing process, to which only the flowables were submitted, which consisted in their immersion in coffee at 37°C for 7 days. In addition, this same type of analysis was also carried out specifically on ceramic specimens to assess the consequences in mechanical terms of the different sintering parameters adopted. All the results were then subjected to a one-way Kruskal-Wallis ANOVA statistical test followed by a Dunn’s post-hoc test. Statistical analyses show that the type of used furnace, as well as the number of specimens sintered at once, has no significant influence on the structural properties of the specimen, in contrast to the sintering temperature; as the sintering temperature of the Zirconia increases up to 1600°C, the mechanical properties of the samples appear to improve. The mechanical parameters then, declined as a result of the thermocycling process in all materials due to the occurrence of microstructural changes such as grain boundary or phase transformations that weaken the material, except for 3D-printed and flowable materials where the opposite process took place, potentially due to post-curing phenomena, relaxation of internal stresses, restabilisation of the microfiller and evaporation of absorbed water by the material. Finally, from the observation of the results, it is evident that the mechanical properties of the 3D-printed materials differ from all other materials produced with CAD/CAM techniques. From the analysis of the results, it can therefore be concluded that the second and third null-hypotheses were accepted, while the fourth and fifth were rejected, as was the first, although with a reduction for ceramic and hybrid materials and an increase on 3D-printed and flow materials.