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The development of new dental gold alloys

The development of new dental alloys with at least 75% au and pt-group metals. Study of properties, corrosion, functionality and biocompatibility and ease of manufacture as the most important selection criteria.

The project focuses on studying production techniques for new gold dental alloys for porcelain techniques and consequently their properties in view of the future role of these alloys in restorative and conservative dentistry. The inert nature of gold and its pliability make it a natural material for the oral environment. Because pure gold is very soft (HV 25), has a very low 0.2% proof stress (30 MPa) and large elongation (45%), it must be alloyed with the right metal elements. The new gold dental casting alloys proposed will be composed of more than 75 % wt. Au and Pt-group metals (high-carat dental casting alloys) and will contain five or more alloying elements. Because of the fact that hardness is closely related to other mechanical properties of alloy, the Au alloys will be subjected to heat treatment to improve them. Through the heat treatment, the hardness and mechanical strength of dental gold alloys will be changed by various mechanisms. One of these is the age-hardening mechanism, which is greatly affected by the element composition of alloy. This is the reason for the presence of base metals such as indium, tin and others in high gold alloys, even in small quantities. They are necessary for precipitation hardening as well as the ability to bond the porcelain to the metal surface. The oxides of the base metals are formed at the surface of the Au alloy during the firing cycle of porcelain. The required bond strength can be obtained by means of a very thin oxide layer of less than 1 micrometer on the surface of the alloy. Current research is looking to identify the most effective dispersoids for gold dental alloys and to develop the successful production technology. Unfortunately, the ideal candidates for dispersion strengthening of gold are practically insoluble in this metal. In the cast structure of the gold alloys, they occur as coarse inter-metallic precipitates that prevent a fine dispersion of oxide particles by internal oxidation. The inclusion of a small percentage of finely dispersed, incoherent, non-sharable dispersoids in a gold matrix is an efficient way of improving the mechanical properties. In such dispersion-strengthened gold alloys the strengthening effect is obtained by dispersoid-dislocation interaction, whereby the dispersoids impede the dislocation motion. The additional supplied external stress, which is required so that the dislocation avoids the incoherent particle and becomes free to move again, is associated with the energy for the climb process and the energy for subsequent detachment of the dislocation line from the particle. Another important property for porcelain-fused-to-metal alloys is the coefficient of thermal expansion (CTE), which must be compatible with that of the porcelain in order to avoid internal stresses in the porcelain veneer during cooling from the firing temperature. Porcelain can withstand higher compressive stresses than tensile stresses, and it is therefore desirable that the thermal expansion of the dental alloy should be slightly above that of the porcelain. In this way compressive stresses are generated in the porcelain shell during cooling. Thermal expansion of porcelain, in contrast to that of a metal or an alloy, is not independent of the cooling rate. Slow cooling rates after firing lead to a higher thermal expansion of porcelain, therefore higher alloy CTE can be compensated by slower cooling. We expect that our new Au dental alloys will have the ideally balanced CTE for high fusing porcelains. The other difficulties faced in the selection of dental materials are mostly concentrated in the area of biocompatibility which means a collection of phenomena that are involved with the interaction between dental materials and the tissues of the mouth. Phenomena can result in adverse and undesirable effects on either the material or the tissues or both, but ultimately lead to a failure of the dental material or device required for performing the function. It is well known that the tissue-mouth environment is very aggressive, and metallic corrosion and stress-mechanical behaviour are of great significance. Consequently our project study of the new Au dental alloys will contain research into the biological acceptability and biological performance of the afore-mentioned alloys used in dentistry. The biocompatibility will be balanced with the lack of significant interaction between materials and tissues. The dental alloys used in dentistry must also have high corrosion resistance, as they are placed in the oral cavity and can react with the environment and deteriorate. The two main forms of attack on dental metallic materials are sulphide tarnishing and chloride corrosion. The reactions involved in both processes are electrochemical in nature. Chloride corrosion causes deterioration of less noble metals; the attack is usually in the form of pitting and sometimes penetrates deep into the microstructures. The effect can range from degradation of appearance to loss of mechanical strength. Therefore special attention will be paid to enhanced corrosion effects of new Au dental alloys. Keywords: Au dental alloys, production, properties.
Acronym: 
DENT-MAT
Project ID: 
3 555
Start date: 
01-11-2004
Project Duration: 
35months
Project costs: 
950 000.00€
Technological Area: 
Metals and Alloys
Market Area: 
Therapeutic services

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