Technology - Microwave sintering applied to zirconia-toughened alumina

Contributors : Nouhaila KHALILE, Clémence PETIT, Christophe MEUNIER, François VALDIVIESO

Contact : clemence.petit@emse.fr 

Institution(s) : Mines Saint-Etienne, LGF CNRS UMR 5307

Topic : Additive manufacturing for bioceramics; Sintering

Microwave sintering is a rapid densification technique that allows heating of materials by absorbing the electromagnetic wave and then converting it into heat. It is a volumetric heating, with low thermal gradients. This enables to sinter ceramics with higher heating rates and shorter processing times. It leads to dense materials with fine grains. Microwave sintering is particularly interesting for bioceramics due to the possibility to tailor the microstructures. The reduction of processing times is also important to facilitate production of custom-shaped parts and prototypes. The heating capability by microwave depends on its own dielectric, electrical and magnetic properties. Most technical ceramics are dielectric and mainly interact with the electric field. In this case, the microwave/material interactions highly depend on the dielectric loss factor of the material, i.e., the ratio ε’’/ε’ (imaginary and real part of the complex dielectric permittivity). For most ceramics, the dielectric loss factor is low at room temperature and increases when temperature increases. To initiate heating of such materials, a susceptor with high dielectric loss factor is generally used. The sample then couples with microwave at higher temperatures. In this case, the sample is heated by radiative heating by the susceptor and direct microwave/material interaction. This is called hydrid microwave heating. But there are still questions about the exact nature of microwave/material interactions, especially about the influence of the material’s characteristics (e.g., crystalline structure or chemical composition). To answer these questions, Nouhaila Khalile studied during her PhD microwave sintering of Zirconia-Toughened Alumina (ZTA). This material is interesting because alumina and zirconia have different dielectric properties. Alumina is considered as transparent to microwave whereas zirconia (especially yttria-doped zirconia) becomes absorbent when temperature increases. In this PhD thesis, various ZTA samples (with different contents and types of zirconia, i.e., undoped, yttria-doped and ceria-doped zirconia) were sintered with and without a SiC susceptor. The effect of the material was visible during sintering without susceptor, especially in the initial and intermediate stages of sintering [1].

A high influence of the material’s composition was also observed when the samples were heated by application of a constant microwave power. This effect depended on the volume fraction of zirconia and on the doping of zirconia. For this last point, a link with the concentration of point defects has been suggested (excess of point defects in yttria-doped zirconia and not in undoped and ceria-doped zirconia). However, few effects of the material were noted in the properties of sintered materials. It was linked to the influence of the sintering cell, i.e., the insulation box used to sinter the material in the microwave cavity. Different sintering cells were tested with alumina and yttria-doped zirconia. It confirmed a susceptor effect of all the elements of the sintering cell [2].

Finally, the influence of the architecture of the samples was studied. Different yttria-doped zirconia shaped by Digital Light Processing were sintered. Solid and architectured parts were densified with fine microstructures. Different behaviours between these parts were observed on the evolution of microwave powers during sintering [3].

References :

[1] N. Khalile, C. Petit, C. Meunier, F. Valdivieso, Hybrid microwave sintering of Zirconia Toughened Alumina in a multimode cavity - Influence of the content of 3 mol% yttria stabilized zirconia and sintering configurations, Open Ceram. 14 (2023) 100371. DOI: 10.1016/j.oceram.2023.100371.

https://www.sciencedirect.com/science/article/pii/S2666539523000433 

[2] N. Khalile, C. Petit, C. Meunier, F. Valdivieso, Hybrid microwave sintering of alumina and 3mol% Y2O3-stabilized zirconia in a multimode cavity – Influence of the sintering cell, Ceram. Inter. 48 [13] (2022) 18143-18150. DOI: 10.1016/j.ceramint.2022.03.072.

https://www.sciencedirect.com/science/article/pii/S0272884222008483 

[3] N. Khalile, C. Meunier, C. Petit, F. Valdivieso, B. Coppola, P. Palmero, Microwave sintering of dense and lattice 3Y-TZP samples shaped by Digital Light Processing, Ceram. Inter. 49 [5] (2023) 7350-7358. DOI: 10.1016/j.ceramint.2022.10.194.

https://www.sciencedirect.com/science/article/pii/S0272884222037762