Microstructural and Optical Properties of Aged 80% Al₂O₃-20% ZrO₂Composites

Objectives: To develop an experimental zirconia-toughened alumina (ZTA) with 80%-Al₂O₃-20%-ZrO₂(ZTA-ZPEX) and to compare its optical properties and microstructure with a translucent zirconia (ZPEX) before and after aging.
Methods: Green disc-shaped specimens (12x1mm) were obtained by uniaxial and isostatic ceramic powders pressing technique using a 15mm-diameter matrix to compensate sintering shrinkage (n=20/group). Discs were sintered to 1,600°C for 1 hour with heating and cooling rate of 4°C/minute. Two-sided polishing was performed with diamond discs (220, 120, 90, 40, 25, 9, 6, and 1 μm) and diamond suspensions to 1 µm. An aging protocol using autoclave (134°C, 20 hours, 2.2 bar) was used to induce t-m phase transformation. Optical characterization comprised reflectance tests on white and black background to determine the Contrast Ratio (CR) and Translucency Parameter (TP) by color difference (ΔE). Scanning-electron-microscopy (SEM) and characterization of t-m phase transformation by X-ray diffraction (XRD) were performed.
Results: Opacity values (CR) before and after aging were: 0.99 (±0.002) and 0.99 (±0.005) for ZTA-ZPEX and 0.77 (±0.003) and 0.66 (±0.006) for ZPEX, with significant difference between groups (p=0.0001). ZTA-ZPEX (0.19, ±0.08 and 0.22, ±0.21) demonstrated significantly lower TP compared to ZPEX (10.21, ±0.11 and 15.51, ±0.31) before and after aging (p=0.000). While aging protocol did not affect the optical properties of ZTA-ZPEX, ZPEX depicted a significant decrease in opacity values along with a significant increase in PT values. ZPEX group demonstrated a higher increase of monoclinic peak (2θ=28° and 2θ=31.2°) compared to ZTA ZPEX 80/20 after aging. SEM depicted a dense surface for ZTA composites.
Conclusions: ZTA-ZPEX presented higher masking ability and lower translucency compared to translucent zirconia. ZTA ZPEX 80/20 was resistant to the accelerated aging protocol, while translucent zirconia presented a significant extent of tetragonal to monoclinic phase transformation.