PSI - Issue 13

Tsanka Dikova / Procedia Structural Integrity 13 (2018) 461–468

462

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Tsanka Dikova / Structural Integrity Procedia 00 (2018) 000–000

their high strength, high wear- and corrosion resistance and a relatively low price (Anusavice (2013), Lu et al. (2015). The lost wax casting process is the main technology for their production. In recent years, additive technologies (AT) that are part of the production module of CAD/CAM systems have rapidly developed. Using AT the object is built layer by layer by polymerization, material deposition, sintering, melting etc. (Van Noort (2012), Dikova et al. (2015 2), Minev R. and Minev E. (2016). In treatment with fixed partial dentures (FPD), AT can be applied for production of temporary crowns and bridges by 3D printing, casting of metal frameworks with 3D printed patterns and direct manufacturing by selective laser melting (SLM) (Dikova (2018). Co-Cr dental alloys, produced by SLM, are characterized with specific microstructure, which defines high hardness and mechanical properties (Lu Y. et al. (2015), Barucca et al. (2015), Mengucci et al. (2016), Mergulhão et al. (2017), Mergulhão et al. (2018). The position of the object towards the building direction and the laser scanning strategy can cause anisotropy of the mechanical properties (Takaichi et al. (2013), Kajima et al. (2016), Hong et al. (2017). The strength characteristics of dental materials are studied mainly on samples with a standardized shape (Takaichi et al. (2013), Al Jabbari (2014), Choi et al. (2014), Kajima et al. (2016), Hong et al. (2017). The bending strength is determined by the standardized procedures of three-point and four-point or bi-axial bending tests (Wang et al. (2003). In investigation of dental alloys, most authors use a three-point bending test (Mengucci et al. (2016), Mergulhão et al. (2018), Qian (2014). It has been found that the flexural strength of SLM Co-Cr-Mo alloys is higher than that of the cast ones. Its values reach 873±38 МРа (Qian (2014) - 2501.2±9.7 МРа (Mergulhão et al. (2018), while in alloying with W up to 2700±25 МРа (Mengucci et al. (2016). The big difference in the results obtained is due not only to the scale factor and chemical composition, but also to the different conditions (building direction) and technological parameters (laser power, speed and scanning strategy) when manufacturing the samples. Since FPDs – dental bridges and crowns have various and complex shapes, it is necessary that the strength properties of the materials from which they are made, to be studied under conditions of the experiment, closest to those of the operation. When examining metal-ceramic and all-ceramic constructions, most authors conduct bending test of three-part bridges from different parts of the dentition using both anatomically shaped teeth and simplified specimens (Seo et al. (2006), Yoon et al. (2010). According to the loading distribution on Filtchev and Kalachev (2008), the four-part dental bridges from the first pre-molar to the second molar are the most loaded, consequently for these constructions the bending strength is of particular importance for their durability. There is lack of data about conducting this kind of experiments, probably because of the complexity of test conditions. The difficulties in bending tests of four-part bridges are determined by the complex geometry of the teeth surface, the simultaneous action of two forces and the way the load is distributed. On the other hand, the mechanical properties of the newly developed dental alloys or manufactured by different technologies are examined mainly by tensile tests. Although, in fact, FPDs are cyclically loaded mostly by bending, data for such experiments or pure bending as well as the mode of rupture are rare in the literature. Therefore, the aim of the present article is to investigate the bending fracture of four-part dental bridges from 1-st premolar to 2-nd molar, produced from Co Cr alloys by additive technologies - casting with 3D printed patterns and selective laser melting, and to perform a comparative analysis with conventional lost wax casting. 2. Materials and methods 2.1. Materials and technologies for samples fabrication Four-point bending test (Fig. 1-а, b) of three groups of samples (four-part Co-Cr dental bridges from 1-st premolar to 2-nd molar) was done. The first group was produced by conventional centrifugal casting of wax patterns, handmade in a special mold. The second group was cast using patterns, 3D printed of special wax-like polymer by SolidScape R66 + ( SolidScape ) printer. The third group was directly fabricated from the virtual 3D model by SLM using SLM 125 machine ( SLM Solutions), as the samples were positioned with the teeth axes parallel to the building direction. Bridges of the first two groups were cast of Biosil-F ( Degudent ) alloy (Co-64.8; Cr-28.5; Mo-5.3; Si-0.5; Mn-0.5; C-0.4 wt.