PSI - Issue 42

1526 Tsanka Dikova et al. / Procedia Structural Integrity 42 (2022) 1520–1528 Tsanka Dikova, Angel Anchev, Vladimir Dunchev, Dzhendo Dzhendov, Yavor Gagov / Structural Integrity Procedia 00 (2019) 000 – 000 7 4. Discussion The modulus of elasticity of the Ti6Al4V alloy, produced by SLM and heat treated under the porcelain firing regimes (125 GPa), is slightly higher than the maximum given by the manufacturer, 115 Gpa (Technical data sheet, 2019). The most likely reasons are the difference in the determination methods and the additional heat treatment. For the milled alloy, the modulus of elasticity is about 25% higher than the data in the literature (136 GPa) (Antanasova et al. (2018)). Considering the position of the specimens in the milling disc (Fig. 1a), this difference is most likely due to anisotropy of the mechanical properties of the blank. Due to the higher modulus of elasticity of the milled Ti6Al4V alloy, the porcelain debonds from the metal surface at higher forces and lower deformations compared to the SLM alloy (Fig. 3 and Fig. 4). The adhesion strength of the coating depends not only on the applied force, but also on the geometric characteristics of the metal substrate, which is taken into account in formula (1) by the coefficient k (ISO 9693:2019). As a result, the adhesion strength values of the porcelain coating are comparable for the two groups of specimens produced in different ways (Fig. 5). These results are consistent with those obtained by Antanasova et al. (2020), according to which, in CAD/CAM technologies, the production method of the metal substrate does not have a great influence on the adhesion strength of the ceramic. In the milled samples, sandblasting leads to an increase in the adhesion strength of the porcelain coating to 30.89 MPa (Fig. 5), which confirms the data obtained by Al Hussaini and Al Wazzan (2005), Mohsen (212) and Antanasova et al. (2020). The application only of a bonding agent strongly decreases the adhesion strength and it falls below the minimum required by the standard. The combined surface treatment, consisting of sandblasting and bonding agent, increases the adhesion strength to 27.96 MPa. The last two results contradict the data of Al Hussaini and Al Wazzan (2005) and Antanasova et al. (2020), according to which the application of bonding agent on the surface of milled and cast specimens increases the adhesion strength of the porcelain, and the combined surface treatment leads to its maximum values. Only in the research of Dérand and Herø (1992) there is a significant drop in the adhesion strength observed in cast titanium samples whose surfaces are treated with a Gold bonding agent. In the SLM samples, the highest adhesion strength (31.04 Mpa) is of the control group (Fig. 5). All types of the applied surface treatments - sandblasting, bonding agent and combined lead to a decrease in its value, as in the last subgroup it falls below the required minimum of 25 MPa. This is in contrast to the results of Antanasova et al. (2020), according to which the surface treatments used increase the adhesion strength of the porcelain coating. The adhesion of the ceramics to the metal substrate consists of two main components – mechanical and chemical (Mohsen (2012), Anusavice et al. (2012), Dikova (2014), Antanasova et al. (2020)). Sandblasting the surface of cast and milled dental alloys leads to an increase in their roughness. This, on the one hand, creates more irregularities and, therefore, opportunities for increase of the mechanical adhesion of the porcelain, and on the other hand, a larger area for chemical interaction. Our previous research (Dikova et al. (2022)) shows that the roughness of the SLM samples is several times higher than the milled ones: Ra=6.7 µm and Ra=0.86 µm respectively. Sandblasting increases the roughness of the milled surfaces (Ra=1.38 µm) and decreases it in the SLM samples (Ra=6.12 µm). Furthermore, the surface morphology of the SLM samples is characterized by numerous partially melted particles, which defines roughness peaks with larger radii of roundness. After sandblasting, the peaks on the surface of both groups become sharper-edged, and the surface profile of the SLM samples consists mainly of valleys. All this predetermines the changes in the adhesion strength of the porcelain coating to the corresponding surface. After sandblasting, it increases in the milled samples and decreases in the SLM ones. In the samples with combined treatment (sandblasting and bonding agent), the adhesion strength in the milled samples is higher than the control group, and in the SLM alloy it is the lowest. Therefore, the applied bonding agent has an influence here. According to Al Hussaini and Al Wazzan (2005), Zinelis et al. (2010), Antanasova et al. (2020), the use of a bonding agent significantly increases the adhesion between ceramic and titanium. It has been found that the application of bonding agent prevents the formation of a thick oxide layer, loosely bonded to the metal substrate, which usually occurs when titanium is heat treated at high temperatures (Al Hussaini and Al Wazzan (2005), Antanasova et al. (2020)). The fine titanium particles in the bonding agent attract oxygen and thus slow down the growth of the oxide layer with each firing cycle (Al Hussaini and Al Wazzan (2005), Chakmakchi et al. (2009)). In addition, the bonding agents contain a mixture of titanium and ceramic particles, which may reduce the difference in the coefficients of thermal expansion of the metal and porcelain. When working with titanium and its alloys, low-temperature ceramics