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Dental alloys and structural ceramics
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New materials for the load-bearing component of dental prostheses come on to the market all the time. Strong ceramics based on zirconia (ZrO2) and new, strong cobalt-chromium alloys with varying amounts of iron are being currently in focus. Both these materials are veneered with porcelain to give both appearance and function similar to the teeth they replace. A lasting bond between these strong skeleton materials and the veneer is critical to the performance of the dental crown or bridge. NIOM is conducting several investigations into factors that cause failure of the bond and ways that this can be tested.
One method being evaluated measures the stresses that develop in the bonding layer during firing. Porcelain can to a degree adapt to the difference between its own thermal contraction and that of the framework metal or ceramic. Failure to adapt leaves high residual stresses across the bonding layer. When later subject to additional stresses during fitting or functioning of the prosthesis the veneer may de-bond.
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The stiffness of dental casting alloys governs their suitability for dental bridges with many units. It is also an important parameter in evaluating test results for the veneer bond described above. NIOM is investigating means of improving the measurement precision of elastic modulus data for alloys especially the newer, stiffer products.
Laser welding is used increasing to join parts of large prostheses instead of traditional brazing. The suitability of an alloy for welding needs to be ascertained by investigating standardised, typical joints. NIOM is conducting experimental studies into methods for investigating the effects of laser welding both on alloys and on the strength of the joint.
The lifetime of dental materials is a quality issue especially for components that are designed to carry the major loads from chewing. Many newer materials gain their strength from an internal structure not previously utilized in dentistry. In other contexts, like the transport sector, such materials are avoided because they are prone to fatigue failure. It is therefore important for NIOM to develop protocols for testing whether materials and bonded constructions withstand repeated loading.
Tooth wear is difficult to measure. On a micrometer scale no part of a tooth is immune from wear, erosion or chemical breakdown. NIOM together with SINTEF ICT has developed an apparatus based on structured illumination to measure the topography of individual teeth directly from an impression. With a resolution of 25 µm laterally and 2.5 µm in depth over a depth of field of 20 mm, the apparatus is capable of discerning the margins of inlays and relative changes in the restorative material and surrounding tooth substance.
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