- THE MAGAZINE
Microgrinding is used in a variety of advanced ceramics applications to generate fine particle sizes and homogeneous dispersions. However, the conventional ceramic beads typically used in this operation often wear quickly and also cause excessive mill wear.
Recently, a new generation of high-density ceramic grinding beads* has been introduced that promises to overcome these drawbacks. Made from yttria-stabilized zirconia powder, the new beads are designed to provide both increased productivity and longer media life in a variety of microgrinding operations.
The Advantages of Yttria-ZirconiaIn the field of engineered ceramics, yttria-zirconia is well known for its excellent toughness, hardness, strength and density—all of which are crucial for a high-performance bead. The new beads have a density of 5.9, which is 55 percent higher than typical ceramic beads. They are produced by sintering fine, uniform-sized mineral oxides into a homogeneous and highly cohesive bead structure. Their crystallographic and chemical composition (93 percent zirconium oxide stabilized by 5 percent yttria) provides them with extremely high wear resistance, and their high mechanical resistance enables them to successfully grind even highly abrasive products, such as ceramic pastes.
The new beads can be used in both horizontal and high-energy mills. Because of their low wear and abrasiveness on mill parts, the beads do not contaminate the suspensions, which allows them to be used in the grinding of ceramic powders for structural and electronic applications.
Testing the New BeadsTo evaluate the performance of the new microgrinding beads, two different formulations were ground using both the new beads and conventional ceramic beads. One formulation was an aqueous suspension with 60 wt. percent monoclinic zirconia powder (CC10) (d50 ~ 4 µm), while the other was an aqueous suspension with 50 wt. percent yttria-stabilized zirconia powder (YZ) (d50 ~ 5 µm). Both formulations were abrasive enough to create significant wear in a short time and were representative of different situations encountered (hardness, toughness, etc.) when grinding powders for advanced ceramic markets. However, the YZ powder was much more abrasive than the CC10 powder due to its properties and morphology.
The tests were carried out in a Netzsch LME 1.2 liter horizontal mill with a water-cooled stainless-steel chamber and a shaft composed of nine discs of hardened steel set off-center. A dynamic slot of 0.2 mm was used in the mill’s separation system.
Both formulations were ground in one pass with a premix flow rate of 6 liters per hour, a disc tip speed of 10 meters per second and a bead fill ratio of 83 percent. The less abrasive CC10 formulation was ground for one hour and 30 minutes, and the YZ formulation was ground for five hours.
Improved MicrogrindingTo determine the wear rate of the beads, the weight loss of the washed and dried beads and mill discs was measured both before and after milling. The results, which are shown in Table 1, illustrate that the wear rate of the new beads is extremely low compared to traditional ceramic beads. The performance ratio between the conventional beads and the new beads is further increased when microgrinding very abrasive powders, such as a YZ suspension.
Even though the density of the new beads is considerably greater than that of the conventional beads, the smooth surface and very fine structure of the new beads prevented them from causing increased disc wear. In fact, the disc wear with the new beads was even lower than conventional beads when microgrinding the YZ formulation. Because the new beads were more efficient than the conventional beads, they more easily decreased the size of the YZ powder, which lowered the powder’s abrasiveness.
The very low wear of the new yttria-zirconia beads, combined with their low abrasiveness on the mill, enables them to be used successfully for a variety of advanced ceramics applications.