Novel speed sintered zirconia by microwave technology

Abstract

Objective: Continuous efforts have been made to hasten the zirconia densification process without compromising properties. This study evaluated the long-term structural durability of microwave speed-sintered zirconia (MWZ) relative to a conventionally sintered zirconia (CZ).

Methods: As-machined dental 3Y-TZP discs (Ø12 × 1.2 mm) were speed sintered at 1450 °C for 15 min using an industrial microwave oven, while conventional sintering was conducted in a standard dental furnace at 1530 °C for 2 h. Both were followed by natural cooling. The total sintering time was 105 min for MWZ and 600 min for CZ. Groups were compared regarding density, grain size, phase composition, and fracture resistance. Structural durability was investigated employing two fatigue protocols, step-stress and dynamic fatigue.

Results: Compared to CZ, MWZ exhibited a slightly lower density (MWZ = 5.98 g/cm³, CZ = 6.03 g/cm³), but significantly smaller grain sizes (MWZ = 0.53 ± 0.09 μm, CZ = 0.89 ± 0.10 μm), lower cubic-zirconia contents (MWZ = 15.3%, CZ = 22.7%), and poorer translucency properties (TP) (MWZ = 13 ± 1, CZ = 29 ± 0.8). However, the two materials showed similar flexural strength (MWZ = 978 ± 112 MPa, CZ = 1044 ± 161 MPa). Additionally, step-stress testing failed to capture the fatigue effect in 3Y-TZP, whereas dynamic fatigue revealed structural degradation due to moisture-assisted slow-crack-growth (SCG). Finally, MWZ possessed a slightly higher Weibull modulus (MWZ = 7.9, CZ = 6.7) but similar resistance to SCG (MWZ = 27.5, CZ = 24.1) relative to CZ.

Significance: Dental 3Y-TZP with similar structural durability can be fabricated six-times faster by microwave than conventional sintering.

Keywords: Dynamic fatigue; Flexural strength; Microstructure; Microwave sintering; Step-stress fatigue; Translucency; Yttria-stabilized tetragonal zirconia.

Figure 1:

Characterization results for microwave- and conventionally-sintered zirconias. (a) Micrographs produced by SEM, (b) XRD spectra showing the monoclinic peaks with a * symbol and the cubic peak with a ▼ symbol, and (c) digital photograph illustrating the materials’ translucency.

Figure 2:

Illustrations of the load (or stress) increase with number of cycles (or test durations) in each step-stress fatigue profiles: aggressive (Agg), moderate (Mod), mild (Mil), and ultra-mild (U-Mil). The load range where most of the samples fractured is depicted by the shaded band. The 5 stress rates of the dynamic test are also included for comparison: 1, 10⁻¹, 10⁻², 10⁻³, and 10⁻⁴ mm/min, indicated by grey lines labelled as I, II, III, IV, V, respectively.

Figure 3:

Flexural strength versus stressing rate for (a) MWZ and (b) CZ under dynamic fatigue. Solid lines are logarithmic regression fits to raw data, in accordance with slow-crack-growth (SCG) analysis. Grey curves represent 95% confidence bounds.

Figure 4:

(a) Replot of dynamic fatigue data (Dyn, open symbols) in Figure 3 as strength versus test duration. Step-stress strength data (SS, grey filled symbols) are also plotted for comparison. (b) Weibull plots for dynamic strength data of both zirconia materials. Data were reduced to fast fracture inert strength using Equation 7.