New multi-layered zirconias: Composition, microstructure and translucency

Abstract

Objectives: To fully realize the range of indication and clinical advantages of the new multi-layered zirconias, a comprehensive understanding of their chemical composition, microstructure, low temperature degradation (LTD) resistance, and translucency properties is paramount.

Methods: A zirconia system (Katana, Kuraray Noritake), including 3 distinct grades of multi-layered zirconias, was selected for study: Ultra Translucent Multi-layered zirconia (UTML), Super Translucent Multi-layered zirconia (STML), and Multi-layered zirconia (ML). For different materials and their individual layers, the chemical composition, zirconia phase fractions, and microstructure were determined by X-ray fluorescence (XRF), X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM). Also, their resistance to LTD and translucency properties were characterized.

Results: Our findings revealed no major differences amongst layers, but the 3 materials were very distinct-UTML: 5Y-PSZ (5 mol% yttria-partially-stabilized zirconia) with ˜75 wt% cubic content and a 4.05 (±0.85) μm average grain size, STML: 4Y-PSZ with ˜65 wt% cubic content and a 2.81 (±0.17) μm average grain size, and ML: 3Y-PSZ with <50 wt% cubic content and a 0.63 (±0.03) μm average grain size. After water aging at 120 °C for 12 h, greater monoclinic content was found in ML. UTML and STML did not show detectable monoclinic phase. The translucency was similar among layers, and also between UTML and STML, which were superior to ML.

Significance: For each multi-layered zirconia grades, the layers are only differed in pigment types and contents, which yield remarkably natural shade gradients. Also, despite the significant compositional difference between STML and UTML, both materials showed similar translucencies.

Keywords: Chemical composition; Dental ceramics; Low temperature degradation; Microstructure; Multi-layered zirconia; Phase content; Translucency properties.

Figure 1.

(a) Schematic of the multi-layer CAD/CAM zirconia pucks provided by the manufacturer and (b) the monolayer specimens prepared in this study.

Figure 2.

XRD patterns of each layer for the 3 zirconia grades investigated. First row shows peaks at 35° (2θ), and second row at 74° (2θ). Cubic (c) and tetragonal (t) peaks are identified in ML spectra, since for UTML and STML the peaks are more superimposed.

Figure 3.

Representative FE-SEM micrographs for UTML, STML, and ML.

Figure 4.

XRD patterns of the enamel and dentin layers of each zirconia grade after hydrothermal aging at 120°C for 12 h. For ML, arrows indicate the monoclinic diffraction peaks at 2θ = 28° and 32°.

Figure 5.

(a) Translucency parameter (average and standard deviation) for each of the clinical indications (thicknesses), comparing various zirconia grades, as well as the enamel and dentin layers for each material. Lines link statistically similar groups. (b) Digital photographs of the enamel and dentin plate-specimens on white and black backgrounds for each of the 3 thicknesses.