Photo: Nacera Zirconia (Nacera.US), ceraMotion® (Dentaurum), by Carlo Paoletti, Odt
For dental purposes, the consistency of zirconia and its strength have made it a reliable and highly popular material for fixed prosthetics. Fortunately, we’re able to mask the high value nature of sintered zirconia with coloring liquids to make it more esthetic. However, some have begun to ask if the penetrating stains will weaken the structure of zirconia, and if so, how much and in which ways.
It has been reported that stains for ceramics rely on a multitude of metal ions for colorations. Theil and Stephan explained the following color sources in a patent application: Iron (Fe) for brown, erbium (Er) for light violet, neodynium (Nd) for light pink, cerium (Ce) for cream or orange, terbium (Tb) only for light orange, manganese (Mn) for black, and praseodymium (Pr) for dark yellow. Unfortunately, coloring ions have the potential to become impurities in zirconia that can adversely affect its properties. Some studies have reported a decrease in flexural strength, while others have reported no effects. The different results on strength, however, might be due not on whether or not stain has been applied, but instead, its concentration, as reported by Sutter et al. An alternative type of esthetic stain, reported by Holand et al in 2012, avoids metal ions by adding metal oxides to zirconia powder prior to its pressing. Unfortunately, oxides have proven to be sometimes unreliable when used for darker shades, and can lead to surface pitting.
Testing conducted at Kagoshima and Aichi Gajuin Universities provided mixed answers to how coloring affects zirconia strength. Researchers at the Universities tested four zirconia brands: “P-NANOZR (ceria-stabilized TZP and alumina nanocomposite (30 vol.% alumina)) . Cercon, ZENOSTAR, and Zirkonzahn Prettau are Y-TZPs.” Test samples were made by dipping the zirconia into “six kinds of coloring liquid for 30 min at room temperature and dried in air. The immersed plates were sintered for 2 h at 1350 ºC for Cercon, at 1450 ºC for P-NANOZR and ZENOSTAR, and at 1600 ºC for Zirkonzahn Prettau…Three-point flexural strengths were determined at a span length of 16 mm. Fracture toughness was determined by an indentation method in terms of Palmqvist cracks.”
Test results showed flexural strength and fracture toughness were largely unaffected by the test stains, the only exception being stains with Er ions. There were no unusual differences in x-ray diffraction patterns “with and without coloring except those for Er,” which was associated with a phase change to cubic zirconia, while the other samples remained as tetragonal zirconia. For example, a SEM of ZENOSTAR stained with Er showed a shift to cubic on its surface. Also noticed was that “the concentrations of Er and Nd increased in the large grains and that of Zr decreased.” The investigators concluded that the surface structure of these particular samples were likely cubic, which does not undergo phase toughening in the presence of micro fractures.
The content of Fe2O3 and CoO ions in the sintered zirconia after firing was too small to be detected by x-ray diffraction. Only a small amount of these ions was required for the coloration of zirconia. According to analytical results of the coloring liquids’ baking powders, the three kinds of ivory liquid mainly consist of Fe ions and a small number of Cr ions, the two violet liquids mainly consist of Co ions and small amounts of Ca, Y, Mn, and P.
The final results showed through X-ray diffraction that a phase shift to cubic structures was associated with stains containing Er. “The formation of the cubic phase resulted in a reduction of the flexural strength and fracture toughness. It is well known that cubic zirconia is weaker than tetragonal zirconia, because, as shown by Sutter et al, the stable cubic zirconia is impossible to be strengthened by the stress- induced transformation.”
According the scholars from Japan, “It is known that both Er and Nd act as stabilizers for cubic zirconia, creating a large strain in the crystal lattice due to their substitutions because the ionic radii,” facts substantiated by Katamura and his team in 1995. The findings indicate that stains “containing Er and/or Nd should be avoided. Furthermore, coloring with Fe and Co showed no remarkable property changes, indicating little reaction with zirconia and the formation of each oxide at grain boundaries. In other words, coloration with Fe and Co ions does not appear to affect the crystalline phase or mechanical properties of the final product, as previously found by Denry and Kelly.”
We question the results found in dental laboratories. What if zirconia is exposed to these coloring liquids for far less than 30 minutes? Regardless, there have been more reports of fractured cubic zirconia in the mouth than tetragonal zirconia. However, it is our believe that this problem is more related to mismanagement of the material than the type of coloring liquids.