Stirred media wet milling process is used in a large variety of industrial sectors such as pigments, pharmaceutical, electronics etc... During this process, ceramic beads are put in motion in a grinding chamber where collisions between them will reduce granulometry of a slurry. Due to the excellent mechanical properties, wear and corrosion resistance of zirconia, ceramic beads, most often made of yttria stabilized zirconia (Y-TZP) are widely used in milling and dispersion of different materials to achieve maximum milling results at low product contamination rates.
           During milling process, the large amount of shocks and frictions that occur can damage Y-TZP ceramic beads [1]. Furthermore, contacts between beads generate heating of the suspension [2] which can be critical for the material to be milled but also for the degradation of Y-TZP beads through Low Temperature Degradation (LTD) [3].  In fact, in Y-TZP ceramics, the stabilized tetragonal phase (t) transformed into monoclinic (m) when exposed to moist environment. [4] Kinetic of this process is accelerated when temperature increases. Within this context, our work aims to better describe and understand wear mechanisms of zirconia beads during wet milling.
            In this presentation, the impact of the temperature of the suspension (from 30°C to 55°C) and the nature of lining of the chamber (Polyurethane PU or Silicon Carbide SiC) on the beads wear will be discussed. Milling experiments were carried out using a stirred media mill equipment (LabStar, Netzsch, Germany), 3%mol of Y2O3 yttria stabilized zirconia beads (Zirmil-Y, Saint-Gobain, France) and monoclinic zirconia suspension (CC10, Saint-Gobain, France). To control the temperature of the suspension external chillers were used. Several milling tests were carried out and operational parameters such as power consumption and temperature were monitored and recorded. Grinding beads wear was estimated from mass measurements taken before and after each test. The surface of the beads at different stages of milling was characterized through X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and local roughness measurements.
           The results show that a moderate increase in temperature from 30 to 55°C leads to an increase in the wear rate of Y-TZP beads. However, XRD analyses indicated no phase (t and m) difference from one milling treatment to another. This result could suggest either a very low level of zirconia aging with grain removal or a possible removal of the transformed zones during milling. Finally, the wear comportment of the beads was different when using PU or SiC lining of the chamber since SiC lining generated less beads wear than PU even though SiC has higher hardness. This behavior was also correlated to the power consumption of the motor which is higher with PU than with SiC lining.

[1] Juhnke, Martin et al. (2012). Eur J Pharm Biopharm 81(1) : 214-222

[2] Guner, Seetharaman et al. (2022), International Journal of Pharmaceutics : 122020.

[3] Ohnishi, H et al. (2009). International Journal of Applied Ceramic Technology.

[4] Chevalier, Cales et al. (1999), Journal of the American Ceramic Society 82(8) : 2150-2154