The providers of CAD/CAM systems for the production of full-ceramic restorations based on zirconia (ZrO2) use a sintering furnace in addition to a 3D scanner and a cutting/grinding machine.
The requirements of processing such materials have increased continually, along with the growing diversity of the zirconia blanks and colour liquids. In addition to materials characteristics that are to ensure the longest possible service life of the work, the visual impression of crowns, bridges, abutments, supraconstructions, etc., plays an essential role.
Translucent, highly translucent and fully coloured round blanks have been developed in addition to the white blanks in the course of the last years to improve this impression. Additionally, the market offers a growing number of colour liquids from various providers to adjust the colour of the zirconia to the patient's tooth colour. This has clearly changed the requirements in particular to sintering furnaces in the last years. Apart from this, the temperature range is growing and the ZrO2-works are sintered faster and faster. Temperatures for zirconia ceramics from below 1400°C to above 1600°C may be possible in future. The duration of the sintering cycle ranges from less than 15 minutes for individual crowns to overnight firings for complete restorations. The materials that are installed in the furnaces are exposed to extreme temperature changes for years.
It is often said that zirconia works suffer discolouration from molybdenum(VI) oxide (MoO3) which can be released by molybdenum disilicide heating elements. This is not accurate. MoO3 is released by oxidation of MoSi2 at temperatures of 400°C to 600°C. Usually, the MoSi2 heating elements form a protective SiO2 layer on the surface that prevents oxidation of the basic material. The MoO3 can be released when the SiO2 layer grows too thick and spalling results, or the layer is destroyed by corrosive materials.
Internal examinations [chemical analyses, colour analyses by RGB 24 bit colour meter, secondary ion mass spectrometry (SIMS)] have documented that the MoO3 is not what causes the yellowing of zirconia in molybdenum disilicide heating elements, but the contamination of the heating elements by iron oxide (Fe2O3).
This yellowing occurs when heating elements of standard MoSi2 qualities are used in zirconia furnaces. MolyCom®-Hyper 1800 heating elements are high-purity heating elements that have a much lower degree of contamination. If higher demands must be met, MolyCom®-Hyper 1800 Super Clean elements can be used.
MolyCom®-Hyper 1800 and MolyCom®-Hyper 1800 Super Clean heating elements are produced with the purest raw materials. This reduces contamination from iron oxide to a minimum, so that discolouration of zirconia ceramics is nearly excluded.