Speed sintering preserves key properties of low-alumina zirconia

A new in vitro study has investigated how peak temperature and dwell time during speed sintering affect the optical and mechanical properties of low-alumina 3Y-TZP zirconia.

The study shows that zirconia can be sintered in less than one hour while maintaining near-complete densification, optical transmittance and flexural strength. Sintering at 1500°C with a 10-minute dwell time provided the most favourable balance of properties.

The study was carried out by Senior Scientist Siamak Eqtesadi, Scientist Asbjørn Slagtern Fjellvåg and Engineers André Konarboland and Magnus Johansen Anderson at NIOM, together with NIOM:s CEO professor Per Vult von Steyern and researchers at Malmö University.

What is this about?

Yttria-stabilised zirconia (3Y-TZP) is widely used in restorative dentistry due to its high strength and biocompatibility. Conventional sintering, however, often takes several hours, which is poorly suited to chairside workflows where restorations are made and placed in a single visit.

Speed-sintering protocols shorten processing time, but they can affect densification, grain growth and phase composition, and thereby the material's optical and mechanical performance. Previous studies have mainly compared predefined protocols. This study systematically varied peak temperature and dwell time to clarify their individual effects.

What did the researchers find out?

Researchers sintered disc-shaped specimens of low-alumina 3Y-TZP (Zpex) at 1500 or 1550°C with dwell times of 0, 5, 10 and 20 minutes, using fixed heating and cooling rates.

All protocols produced relative densities above 99.5%, indicating near-complete densification even without dwell time. Increased thermal exposure was mainly associated with changes in phase composition and grain size. At 1550°C, grain size increased markedly with longer dwell times, while it remained nearly constant at 1500°C.

Total forward transmittance increased with thermal exposure and remained close to the manufacturer-reported value for conventionally sintered Zpex. Indentation fracture resistance showed limited variation, and biaxial flexural strength was approximately 1100 MPa for both protocols tested.

What are the key results?

The findings indicate that a modified speed-sintering protocol can reduce processing time without compromising the properties of low-alumina 3Y-TZP relevant to clinical performance.

Sintering at 1500°C for 10 minutes provided the most favourable balance between densification, optical transmittance and mechanical performance, and showed more consistent strength than sintering at 1550°C for 5 minutes. The researchers note that optimised protocols should be established separately for different zirconia compositions, and that further studies on ageing and cyclic loading are needed.