The art of combining two zirconium oxide materials

Welcome to the fascinating subject of sintered ceramics. Ivoclar Vivadent has spent many years studying the sintering behaviour of ceramics, with special emphasis on zirconium oxide. The findings from these research efforts are used to improve the company’s existing products and develop new ones. In some cases these findings help to make the impossible possible: IPS e.max ZirCAD Prime is a case in point.The new all-ceramic solution is comprised of the two zirconium oxide raw materials 3Y-TZP^[1] (high-strength zirconium oxide) and 5Y-TZP^[2] (highly translucent zirconium oxide). Due to their different physical properties, these two raw materials could not be successfully combined in the past to produce structural elements without any stress or distortion during pre-sintering and dense sintering. Ivoclar Vivadent, however, has developed a special manufacturing method to adjust the compression curves of the two materials (patent pending). This process is an integral part of what is known as Gradient Technology (GT), which is at the heart of IPS e.max ZirCAD Prime.

Introducing Gradient Technology

Gradient Technology comprises three manufacturing steps which combine 3Y-TZP and 5Y-TZP to produce the exceptional properties of IPS e.max ZirCAD Prime.

1. Sophisticated powder conditioning ensures optimum matching of the raw material in terms of their compression behaviour and taking into account their different colour compositions. This ensures a high accuracy of fit of the resulting restorations.
2. The innovative filling technology used during manufacturing produces a seamless and layer-free progression of the shade and translucency. This in turn allows you to fabricate highly esthetic, layerless restorations that mimic the different levels of translucency exhibited by the dentin and incisal areas of the natural tooth.
3. The next part manufacturing step is called Cold Isostatic Pressing (CIP). It improves the homogeneity of the material’s microstructure and enhances its translucency. Furthermore, it allows the material to be sintered at shorter intervals (2 h 26 min for single crowns in the Programat S1 1600).

Influences on the sintering properties

The sintering properties of ceramics are influenced by numerous factors. The materials-dependent parameters include, for example, the chemical composition and the press compaction. The most important factor, however, involves the size, distribution, structure and shape of powder particles. For instance, the pigmentation of a powder changes the material’s composition and sintering properties, since most of the pigmenting ions act as a sintering activator. As a result, the developers of this multilayer zirconium oxide material, in other words, an ingot featuring various gradations of colour, were confronted with the challenge of effectively matching the sintering properties of the different colour layers. This would prevent distortion and the build-up of stress in the material during the pre-sintering and dense sintering phases, and therefore ensure the accuracy of fit of the restoration. Therefore, different raw materials per se are not the only reason for having to adjust the sintering kinetics.

The challenge

IPS e.max ZirCAD Prime is composed of more than 60 per cent of the clinically proven, high-strength 3Y-TZP. This material typically demonstrates strength values of 1200 MPa and a fracture toughness of 5 MPa m^1/2. In the new manufacturing process of Gradient Technology, it is effectively combined with the highly translucent zirconium oxide raw material 5Y-TZP.

Successfully combining two raw materials always represents a major challenge. If the two materials demonstrate different sintering kinetics and physical properties, undesirable effects may result:

• Even though the materials may show a good bond, distortion can occur during thermal compression.
• The desired degree of compaction is not achieved in a particular layer due to distortion, which negatively influences the resulting properties of this layer.
• During and after sintering the thermophysical properties of the individual layers differ to such a degree that flaws occur and stresses build up between the layers, which may lead to delamination.
• After cooling, residual thermal stresses remain within the Multi material due to thermophysical incompatibility. 

However, if the raw materials are suitably adjusted to show the same sintering properties and if their thermophysical properties are properly matched, the sintering process will produce a homogenous result: The material will behave like one solid piece.

The solution

Adding a dopant to the raw material can influence its sintering properties. Nevertheless, a material containing a low content of yttrium oxide does not necessarily show enhanced sintering kinetics compared with a material that has a higher content. As previously mentioned, a far bigger number of factors play an influencing role, for example, the primary and secondary particle size, the degree of compaction and the chemical composition. Generally, 3Y-TZP particles are finer and demonstrate a larger specific surface area compared with 5Y-TZP particles. During the sintering process, the high surface energy is reduced: the larger the specific surface area, the higher the surface energy. Consequently, raw materials with a large surface area and a low content of yttrium oxide sinter more rapidly. By adding further components, the sintering process can be selectively controlled, in other words, accelerated or delayed as required. The differences in the shrinkage properties of the two zirconium dioxide raw materials of IPS e.max ZirCAD Prime are thus balanced out in a process that adjusts the compression curves of the components (Ivoclar Vivadent patent pending). Due to this technology, the progression of the sintering kinetics curves of the two powders is virtually identical throughout the entire sintering process. Within the resulting product, the raw materials merely differ in their final colour and mechanical properties. As a result of this sophisticated production process in which the sintering kinetics of the raw materials are selectively controlled, restorations made of IPS e.max ZirCAD Prime exhibit outstanding accuracy of fit after sintering.

What makes IPS e.max ZirCAD Prime so special?

Zirconium oxide ingots composed of only one raw material have their mechanical (high translucency but low strength) and esthetic limitations (high strength but low translucency). IPS e.max ZirCAD Prime, however, offers an ideal combination of both these attributes: The desired properties are located in the restoration exactly where they are needed – high-strength, conditioned 3Y-TZP in the dentin part and highly translucent, conditioned 5Y-TZP in the incisal part. Furthermore, the low strength of 650 MPa is nevertheless higher than that of lithium disilicate ceramics, which have shown excellent clinical performance for more than a decade (for single tooth restorations in the anterior and posterior region). IPS e.max ZirCAD Prime can be used to fabricate full-contour and (partially) cut-back three to four-unit bridges in the anterior and posterior region. The new all-ceramic solution is strong enough to withstand maximum loading in the lower jaw and in pontics. Nevertheless, the minimum connector dimensions must be located within the high-strength dentin zone.

IPS e.max ZirCAD Prime is a veritable One Disc Solution for a wide variety of clinical situations and manufacturing techniques. It combines high-end esthetics with high strength and flexibility in one disc. The material covers a large indication spectrum and offers maximum potential in terms of processing possibilities.