ABSTRACT
The milling process of zirconia (ZrO2) aqueous suspensions has been studied and a comparison between QBZ-58A 1.6-1.8 mm beads and electrofused Zr-Si beads of similar size has been conducted.
QBZ-58A beads are a new grade of "Generation II" QBZ-64 beads of improved wear resistance. Both types of QBZ beads are zircon (zirconium silicate).
In an earlier report, a comparison was made between 0.6-0.9 mm "Generation II" QBZ-64 and electrofused Zr-Si beads of a similar size. It was found that at this size range and when zirconia is milled, "Generation II" QBZ-64 is superior to electrofused Zr-Si with respect to disk and bead wear. Based upon these results, the milling process of QBZ-58A beads and electrofused Zr-Si beads of a similar size range has been studied.
It was found that at this size range and when a hard material is milled, QBZ-58A beads are superior to electrofused Zr-Si with respect to bead wear. Both bead types exhibit the same milling efficiency.
Investigation of the bead
microstructure
by Scanning Electron Microscopy (SEM) suggests an explanation of the
lower
wear rates encountered in QBZ-58A beads when hard materials are milled.
EXPERIMENTAL PROCEDURE
The study was conducted using a horizontal continuous bead mill with a volume of 1.4 liters. It was equipped with 4 stainless steel disks of 70 mm diameter.
The milled material was an aqueous suspension of 70% by weight zirconia (ZrO2), with initial particle average diameter of 4.8 micrometers.
The experimental parameters were:
Rheological measurements of the suspension were made by means of a rotation viscometer (Model DVII Brookfield Engineering Labs., Inc.)
The microstructure of the bead surface before and after the milling was examined by Scanning Electron Microscope (JOEL, JSM 5400)
EXPERIMENTAL RESULTS
The wear results of the beads are
described
in Table 1.
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Bead Wear (% per hour) |
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From Table 1 it can be clearly seen that QBZ-58A beads exhibit significantly lower bead wear (by a factor of 2.3).
Milling efficiency of both bead types is the same. The final average diameter of the zirconia particles is 1.6 micrometers. (Starting average diameter was 4.8 micrometers).
MICROSTRUCTURAL INVESTIGATION
The surface of QBZ-58A beads before and after the milling process can be seen in Figures 1 and 2. It can be seen that before milling, the surface is very smooth. After milling with zirconia, the surface becomes rougher, the roughness is homogeneous. No large defects can be seen in the bead’s surface.
The surface of the electrofused Zr-Si beads before milling is also rather smooth (Figure 3). As a result of milling with a hard material the surface becomes rougher (Figure 4), but the roughness of the electrofused Zr-Si beads is not homogeneous and large defects can be seen (marked by arrows in Figure 4) in contrast to QBZ-58A beads. The origin of this phenomenon seems to be the non-homogeneous structure of SEPR’s electrofused Zr-Si beads (a dendritic structure of zirconia and silica). It is assumed that the hard milled material causes an enhanced wear of the amorphous silica, while the wear resistant dendritic zirconia remains. EDS analyses taken from the electrofused Zr-Si beads before and after the milling process as shown in Figure 5, and the reduction in the silica content after milling can be seen. As a consequence of this phenomenon, free dendrites of zirconia appear in the surface (marked by arrows in Figure 4) and these regions are subjected to high wear rates because of their rough nature and high porosity.
QBZ-58A beads are macroscopically homogeneous, therefore, their wear resistance is superior to SEPR’s electrofused Zr-Si beads when milling hard materials, as can be seen in this report, since the beads are "polished" homogeneously.
SUMMARY AND CONCLUSIONS
