Basic Study On Joining Technology Of Ni-Cr Alloy And Dental Porcelain

Porcelain fused to metal restorations is sintered with dental porcelain material (mixing multiple porcelain into porcelain slurry) on the metal or alloy matrix under the vacuum condition. Virtually, it is a material bonded by metal and porcelain. Ni-Cr alloy can make the alloy crown thinner by its lower price, high elasticity and strength as well as reducing natural teeth abrasion when repairing absent or defect teeth. So Ni-Cr alloy is popular with doctors and sufferers and takes a main position in domestic metal of porcelain firing restorations. There are much difference in the performance parameters between Ni-Cr alloy and porcelain, these are: different chemical bond and it is difficult to bond better with metallurgy combination or bonding; Different thermal expansion coefficient and there are larger residual stress easily producing at the interface; Lower thermal shock resistance property of porcelain, etc. and these are taking more difficulties to making Ni-Cr alloy restorations. Therefore, it has a deeply theoretical value on discovering the mechanism of action between Ni-Cr alloy and the oxide of dental porcelain, studying the effects of interface products category, quantity and distribution on the bonding strength of metal/porcelain interface, lowing porcelain bursting and cracking due to the higher residual stress at the interface resulted in by the different thermal expansion coefficient between metal and porcelain, exploring the new restoration making technology and improving the development of crossing subjects.This article studies the critical desiderating problems to solve about the Ni-Cr alloy restorations which taking a main place in domestic dental repairing area through investigating the situations in several domestic dental hospitals and reading relating reference documents. Appearance characters, cause of formation and harm of the defects of Ni-Cr alloy restorations have been studied through analyzing the made Ni-Cr alloy restorations and the collected failed restorations from clinic. The defects, such as impurity, shrinkage cavity, pores, cracks, etc., generally exit in Ni-Cr alloy restorations, the processing measures have been put forward to reduce the defects inner Ni-Cr alloy restorations and offer the technical basis to improve the quality of Ni-Cr alloy restorations.Mechanical properties and changing rules of Ni-Cr/porcelain have been studied through changing processing parameters. The results show that the interface reaction is much complicated and the new generating phase at interface is SnCr0.14OX. In definite range, as the increasing of the firing temperature and extending of the firing time, the quantity of inter-dissolving and inter-diffusing between Ni-Cr alloy and porcelain increase and the reaction layer widens, the type of metal/porcelain interface changes from diffusing type to mixed type of diffusing and compounding which has an important effect on the bonding strength of metal/porcelain interface. When firing temperature is T=990℃and firing time is t=2.5min, the shear strength is 43.8 MPa of Ni-Cr/porcelain interface and the three point bending bonding strength is 40.2 MPa.Reaction mechanism of Ni-Cr/porcelain interface is discovered after thermal theoretical calculation as well as test results. The reaction at Ni-Cr/porcelain interface is a composite oxide producing course by NiO, Cr2O3, etc. produced from oxidizing reaction at Ni-Cr alloy surface and the oxides in porcelain. The composite oxide mainly is SnCr0.14OX in interfacial reaction layer. There is less difference between the electronegativity, so composite oxides SnCr0.14OX can be looked as an aggregation by two kinds of metallic ions and oxyanion and it qualifies with metallicity and porcelain character, which can react as a transition layer between Ni-Cr alloy and porcelain. The composite oxides mixing with other oxides at the interface forms a Ni-Cr/porcelain interface reaction layer and realize to bond Ni-Cr alloy with porcelain.Systemic study has been done on the influence of Ti, Zr, Au and TiN interlayer to the microstructure and property of Ni-Cr alloy/porcelain interface. The design for interlayer is extremely important. Under the effect of interlayer, the bonding of Ni-Cr /porcelain interface translates into the bonding of Ni-Cr/interlayer/porcelain interface. Selection of interlayer should consider the products formed from interaction between Ni-Cr alloy and porcelain to be good to interface bonding, improve anti-corrosion property of Ni-Cr alloy, adjust the thermal action of Ni-Cr alloy and porcelain, ensure the instant thermal stress and residual stress being lower and qualifying with suitable direction during the course of cooling and avoid immediate or overdue porcelain bursting and cracking.When using Ti interlayer, Ni-Cr/Ti/porcelain interface bond compactly without cracks and holes, etc.. The reaction is complicated at Ni-Cr/Ti/porcelain interface, and the new forming phases at interface are SnCr0.14OX, NiCr2O4, Cr2O3, TiO2, AlTi3 and Ti2Ni. When firing temperature is T=990℃, firing time is t=2.5min and thickness of Ti interlayer is 3μm, bonding strength of Ni-Cr/Ti/porcelain interface is 48.4 MPa, which is improved 20.4% compared with Ni-Cr/porcelain interface.Pores, cracks, etc. exit on near porcelain side of Ni-Cr/Zr/porcelain interface with Zr interlayer. Products of reaction at Ni-Cr/Zr/porcelain interface are SnCr0.14OX, ZrO2 and Al3Zr. When firing temperature is T=990℃, firing time is t=2.5min and thickness of Zr interlayer is 3μm, the bonding strength of Ni-Cr/Zr/porcelain interface is 41.5 MPa, so there is a little bonding strength improving with Zr interlayer.When using Au interlayer, Ni-Cr/Au/porcelain interface bond compactly, porcelain embeds in dimples on Ni-Cr matrix caused by sandblasting and both interlock together. Products of reaction at Ni-Cr/Au/porcelain interface are SnCr0.14OX and Cr2O3 and there is no Au compound being detected. When firing temperature is T=990℃, firing time is t=2.5min and thickness of Au interlayer is 1μm, the bonding strength of Ni-Cr/Au/porcelain reaches 51.1 MPa. The bonding strength increases 25.2% compared with Ni-Cr/porcelain interface.When using TiN interlayer, the Ni-Cr/TiN/porcelain is clear and bond compactly, and there are no defects of cracks, gaps, etc. and the N2/Ar flow ratio has less effect on interface microstructure. Products of reaction at Ni-Cr/TiN/porcelain interface are SnCr0.14OX, TiO2 and Cr2O3; Interface bonding strength is different with different N2/Ar flow ratio. When N2/Ar flow ratio is 1/3, the bonding strength of Ni-Cr/TiN/porcelain reaches 47.8 MPa. The bonding strength increases 18.9% compared with Ni-Cr/porcelain interface.The sequence of interface bonding strength from low to high is Zr