PSI - Issue 42

Tsanka Dikova et al. / Procedia Structural Integrity 42 (2022) 1520–1528 2 Tsanka Dikova, Angel Anchev, Vladimir Dunchev, Dzhendo Dzhendov, Yavor Gagov / Structural Integrity Procedia 00 (2019) 000 – 000 1. Introduction In recent years, pure titanium and its alloys have been widely used in medicine mainly as implant materials due to their high corrosion resistance, biocompatibility, good mechanical properties and modulus of elasticity, which is closest to that of the bone tissue (Dikova (2012), Gagov et al. (2022)). Recently, due to the low relative mass, Ti and its alloys are also being used in prosthetic dentistry for fabrication of fixed and over-implant metal-ceramic prostheses (Mohsen (2012), Antanasova et al. (2018), Gagov et al. (2022)). The high melting temperature and reactivity of the titanium make it difficult to produce dental constructions by casting. CAD/CAM milling and selective laser melting (SLM) are successful alternatives to the conventional lost wax casting process (Dikova et al. (2015), Antanasova et al. (2018), Antanasova et al. (2020), Antanasova et al. (2021)). They belong to two main groups of technologies, which differ in the way of manufacturing the detail. CAD/CAM milling is part of subtractive production technologies, while SLM belongs to the group of additive technologies. In the first process, the detail is manufactured by milling from a solid blank with guaranteed mechanical properties. Therefore, it is not expected change of the mechanical properties of the construction. The surface characteristics in this case depend on the features of the cutting tool used, the parameters of the cutting process and the position of the detail relative to the blank. Selective laser melting is an innovative 3D printing technology where the objects are built layer by layer by melting of metal powder with a laser and welding of the new layer on the top of the previous volume (Dikova et al. (2015), Kazantseva (2018), Kazantseva et al. (2019)). In this technology, the properties and surface characteristics of the detail depend on several groups of factors related to the initial material, the process parameters, the construction of the object and its position relative to the built direction. The durability of a metal-ceramic dental prosthesis depends on the adhesion between the porcelain coating and the metal substructure. In order to increase the adhesion strength, various types of surface treatments of the alloy are applied: sandblasting, chemical etching, laser treatment, preliminary heat treatment - oxidation or application of bonding agent (Chakmakchi et al. (2009), Zinelis et al. (2010)). They are intended to influence both the mechanical and chemical components of the adhesion. The latter occurs due to the presence of an oxide layer on the surface of the metal substructure. As titanium is highly reactive element, at room temperature, it spontaneously reacts with the oxygen from the air and as a result, an oxide layer is created on its surface (Haag and Nilner (2010)). To ensure sufficiently strong adhesion, the oxide layer must be of a guaranteed thickness. It has been found that if the thickness of the oxide layer is too large, there is a high risk of fractures between the titanium and the oxide layer or within the oxide layer itself. When porcelain is fired in temperatures above 800 o C, Ti oxidizes very quickly and as a result a layer of titanium oxide with a high thickness and weak adhesion to the metal surface is obtained (Chakmakchi et al. (2009), Zinelis et al. (2010), Antanasova et al. (2018)). Pre-oxidation of the Ti6Al4V alloy surface shows an increase in the thickness of the titanium oxide layer with increasing the temperature (from 10 nm at room temperature to 300 nm at temperature of 800 o C) (Antanasova et al. (2021). In order to limit the increase in the thickness of the titanium oxide layer, it is recommended to apply a layer of bonding agent on the surface of the metal substructure before firing the porcelain (Chakmakchi et al. (2009), Zinelis et al. (2010), Antanasova et al. (2020)). The bonding agents consist mainly of different types of metal oxides with presence of titanium particles in some of them. The titanium particles absorb the oxygen and prevent excessive oxidation of the metal construction during porcelain firing. Additionally, the presence of titanium and ceramic particles in the bonding agent lowers the difference in the coefficients of thermal expansion of both materials and thus reduces stresses at the porcelain-metal interface. Therefore, the application of bonding agent can increase the adhesion strength between the titanium and ceramic (Al Hussaini and Al Wazzan (2005), Antanasova et al. (2020)). It should be noted that not all bonding agents lead to an increase in the adhesion strength (Dérand and Herø (1992), Zinelis et al. (2010)). It is found by Mohsen (2012) that the surface treatments such as sandblasting and silicon coating increase the adhesion strength of ceramics to Ti6Al4V alloy up to 33.14 MPa and 37.80 MPa, respectively. The adhesion strength of low-temperature ceramic to cast pure titanium increases to 25.60 MPa after sandblasting of its surface, to 24.65 MPa after using bonding agent, and the highest is after combined treatment (sandblasting and bonding agent) - 35.60 MPa (Al Hussaini and Al Wazzan (2005)). Treatment of the titanium surface, however, with a 10% hydrochloric acid solution leads to adhesion decrease, even with use of bonding agent (15.10 MPa and 24.65 MPa, respectively). Lubas et al. (2021) found that when treating the surface of pure titanium with different reagents, different values of adhesion strength are obtained, the highest being after sandblasting and chemical etching (39.16-48.77 MPa). 1521