Dental alloy with lower the cofficient of thermal expansion and contradiction
Answers
Metal-ceramic compatibility is an important factor in manufacturing metal-ceramic restorations. The coefficient of linear thermal expansion (CTE), thermal conductivity and the nature and strength of the bond are all factors that influence the porcelain's capacity to resist fracture during clinical use of the restoration11,21.
In agreement with many authors, the difference in CTEs of porcelain and metal has been recognized as a major parameter in predicting compatibility1,4,12,24. The general consensus is that the alloy should have higher CTE than the porcelain (a positive expansion coefficient mismatch) in order to produce compressive stress in the porcelain when cooling11,24. Usually, a variation ranging from 0.5 to 1.0 × 10-6 °C-1 between the CTEs of the alloy and ceramic is considered adequate when the metal coefficient is higher than that of the ceramic. It keeps the ceramic compressed; increasing the lifetime of the restoration3. Some authors reported that metal-ceramic specimens with a negative CTE difference failed at significantly lower flexural loads than specimens did with positive CTE difference2,10.
Various tests have been designed and selected by researchers to evaluate metal-ceramic bond strength and the minimal value recommended by the ISO standard 969315 for metal-ceramic dental restorative systems is 25 MPa for 3-point bending test. However, it can be affirmed that this value is related more to the flexure strength of the metallic substrate than to the metal-ceramic bond strength12,13,18, causing difficulty when comparing different metallic substrates.
Noble metal alloys had been widely used for porcelain veneering in dentistry. However, with the continuous price fluctuations of noble metals, more attention has been given to alternative alloys. Ni-Cr alloys have good mechanical properties, such as high degree of hardness, low density and high tensile strength. Also, the low cost and easy fabrication of Ni-Cr alloys have caused them to be widely used in dental fixed prosthesis for quite some time14.
The addition of Be in these Ni-Cr alloys promoted castability improvement of these alloys and enhanced bonding strength between porcelain and metal3,6,14,17,20,23. However, in addition to beryllium, the presence of other potentially pathogenic metallic components in Ni-Cr alloys has contributed for reinsertion of Pd-Ag in the market5,8. Porcelains suitable for Pd-Ag alloys were then developed, which avoid wearing of the ceramic coverage by Ag diffusion in ceramics7,9 evidencing that this alloy system is safer than alloys with Ni 7.
High palladium alloys were introduced in the early 1980s and are currently widely used in metal-ceramic restorations, even though they have not yet been scientifically investigated to the extent that their widespread use requires10,13,14,17,18. This type of alloy presents characteristics that, in spite of not interfering with the porcelain-to-metal bond, must be observed. Moreover, there are elements in the alloy composition, such as silver and tin, which are susceptible to oxidation. Thus, previous oxidation is an important procedure that could increase the bond strength of such metal-ceramic interface1,16,19. Furthermore, the Pd-Ag alloy (Pors-on 4) presents much smaller grains than other Pd-Ag alloys that provide better mechanical qualities, besides promoting larger external contact surface14.
Another characteristic of the palladium alloy is its high sensitivity to the presence of carbon during casting procedures. This could generate problems such as ceramic porosities and carbon contamination in the alloy6,15,17,18. The carbon promotes a formation of carbon monoxide during the ceramic baking, creating bubbles and porosities, which could be partly responsible for undesirable outcomes, such as cracks or fractures.
The purpose of this study was to evaluate the metal-ceramic bond strength (MCBS) of 6 metal-ceramic pairs (2 Ni-Cr alloys with/without Be and 1 Pd-Ag alloy with 2 dental ceramics) and correlate the MCBS values with the differences between the CTEs of the metals and ceramics.