Porcelain Fused To Pure Titanium And Effect Of Ti Surface Modification On Ti/Porcelain Strength

Porcelain fused to metal (PFM) is prosthesis with composite structure, which is applied low-melting dental ceramic on metal substrate surface, and then sintered in a vacuum. Its essence is the joining of metal and dental ceramic. PFM is a kind of permanent prosthesis for missing teeth or dentition due to it having the strength, toughness and shock resistance of metal, and the abrasion resistance, corrosion resistance, realistic appearance, color stability and smooth surface of porcelain. The PFM has been widely used in the field of prosthodontics.Titanium has excellent biocompatibility and corrosion resistance, high specific strength, and low density and thermal conductivity. Compared with other metals and alloys, it has better protection of dental pulp to avoid hot or cold stimulation, and is considered to be the ideal metal material of PFM. However, porcelain fused to Ti has not achieved clinical application until today. The main problem is the bonding strength of Ti/porcelain can not meet requirements of performance. Therefore, it is of great theoretical significance and practical values to study the porcelain fused to pure Ti and effect of Ti surface modification on Ti/porcelain strength, and reveal the microstructure features, response mechanisms and influence factors of Ti/porcelain interface so as to promote the clinical application of porcelain fused to Ti.In this paper, the microstructure and reaction mechanism of Ti/porcelain interface have been investigated. Results show that the interface of Ti/porcelain is composed of oxide layer formed on Ti and reaction layer near the porcelain side. The formation of oxide layer changed the connection of Ti/porcelain interface into Ti/oxide layer/porcelain interface. At a high temperature, Ti4+ and O2-in TiO2 could form quadridentate structure [TiO4], which then connected with [SiO4] through non-oxo bridge to become glass network SiO2·TiO2. It connected Ti with porcelain, which are two different materials in structure and nature, and given a certain bonding strength of interface.The technology parameters (firing temperature, firing time and cooling rate) have obvious effects on the microstructure and bonding strength of Ti/porcelain interface. With the increase of firing temperature and firing time, the thickness of the oxide layer and reaction layer increased. The bonding strength achieved the highest value (23.5MPa) when the firing temperature and firing time were 800℃and 1 min, rspectively. Further increasing the firing temperature and firing time, the bonding strength sharply lowered due to Ti oxide layer with excessive thickness and loose structure. The effect of cooling rate on the bonding strength of Ti/porcelain is due to the change of heat stress at Ti/porcelain interface. Rapid cooling easily leads to cracks at Ti/porcelain interface, accordingly reduces the bonding strength of Ti/porcelain. Therefore, it is favorable to select slow cooling for improving the bonding strength of Ti/porcelain.In order to improve the bonding strength of Ti/porcelain, the microstructure and bonding strength were studied after different methods of Ti surface modification was applied. Experimental results show that the methods of Ti surface modification had an obvious influence on the microstructure and bonding strength of Ti/porcelain. Sandblasting on Ti surface prior to sintering did not have a significant impact on improving the bonding strength, but it could restrain the propagation of cracks at Ti/porcelain interface. The pre-oxidation treatment of Ti could form a uniform film of oxide on surface, so the content of TiO2 which was participate in the interfacial reaction was increased, and resulted in the thickening of reaction layer. The temperature of pre-oxidation had a great effect on the bonding strength of Ti/porcelain. The bonding strength achieved the highest value (28.9MPa) under the condition of pre-oxidation temperature of 750℃, and increased by 17% compared with non pre-oxidation. The nitriding on Ti surface could restrain the formation of oxide layer to a certain extent, and the change of N2/Ar had an obvious effect on bonding strength. When the N2/Ar was 1/2, the bonding strength achieved the highest value (28.9MPa) and increased by 17% compared with sandblasted. Magnetron sputtering Zr middle layer on Ti surface could decrease the thickness of oxide layer effectively, and the sputtering time had a great influence on bonding strength of Ti/porcelain. When the sputtering time was 1 hour, the bonding strength achieved the highest value (29.7MPa) and increased by 20.2% compared with no middle layer being applied. The use of ZrN middle layer on Ti surface was also effective to control the thickness of oxide layer. Different N2/Ar resulted in different influences on bonding strength of Ti/porcelain. The bonding strength achieved the highest value (29.2MPa) under the condition of N2/Ar of 1/4, and increased by 18.2% compared with sandblasted.The porcelain had a high susceptibility for cracks and gas pores during the process of porcelain fused to Ti due to physical and chemical properties of porcelain and Ti. According to the characteristic of crack distribution, the cracks could be classified as the crack on porcelain surface, Ti/porcelain interface crack, crack in porcelain and crack at porcelain edge angle. The formation of crack on porcelain surface, Ti/porcelain interface crack and crack at porcelain edge angle were due to the fact that the solidification characteristic of porcelain and there were thermal stress caused by the mismatch of thermal expansion coefficients between Ti and porcelain. The crack in porcelain was mainly resulted from defects such as inclusions and incomplete sintering of the porcelain. The crack distribution had important influences on the bonding strength of Ti/porcelain and the strength of the porcelain itself. It was effective to avoid improper operation and control the sintering parameters such as sintering frequency, heating rate and cooling rate for improving the cracking susceptibility of porcelain fused to Ti. The pores could be divided into the surface pore, isolated pore, porosity and pore space. They were produced from residual gases in crystallization process of ceramic particle and water vapor in skeleton growth process. Regular and globular shape pore might float out of porcelain until grown up to a certain extent. However, pore space could not float due to its complex shape, and could just change in shape and size according to the transfer of grain boundary. The low density and high viscosity of melting porcelain caused the decreased rising velocity of pores, and the quick heating and cooling rates led to the high gas pore susceptibility of porcelain fused to Ti.The high residual stresses existed in the interface of Ti/porcelain specimen according to the finite model of transient heat-stress. Porcelain surface tended to produce cracks due to the tensile stress parallel to the surface. Stress concentration in the rim of interface of Ti/porcelain made it a vulnerable area of the bonding and usually led to porcelain collapse or crack, especially at the corner angle of Z-orientation edge. The degree of stress concentration in the interface increased when a certain thickness of Zr interlayer was sprayed on the Ti surface. As the thickness of the Zr interlayer was increased from 1um to 5um, the residual stress in Ti/porcelain interface increased obviously, thus affected the bonding strength accordingly. However, the interlayer of Zr was oxidized to form ZrO2 during sintering when the layer was thin. ZrO2 could efficiently alleviate distortion mismatch during cooling and lower the degree of stress concentration in the interface, which was favourable to improve the bonding strength of Ti/porcelain and reduce the cracking tendency of porcelain. If the thickness of interlayer increased, only a small amount of Zr was to be oxidized, which was not favourable to alleviate distortion mismatch and would reduce the bonding strength. When the sintering was with different cooling rates, fast cooling resulted in the maximum temperature gradient, then the normal cooling and slow cooling. During cooling, the temperature gradient was the maximal at a high temperature and become lower with the decrease of temperature. Large temperature gradient led to larger distortion of Ti/porcelain during cooling, which would affect the final residual stress. Therefore, equable heating and slow cooling should be followed under the satisfaction of sintering requirements. Further understanding has been obtained for the process of porcelain fused to pure Ti and the method of improving the bonding strength by the above research. Detailed experimental data and theories are supplied for the process of Ti/porcelain restorations.