Anisotropies in wear of sapphire single crystal have been periodically studied since the early '60's. It was noted that wear rates for c-cut samples with small degrees of tilt from surface normal showed a large degree of variability.

MRF is an excellent process for studying polishing anisotropies in optical materials. By examining a series of c-cut sapphire plates using different MR fluids, we demonstrated that the difference in material removal rates for polishing "up" or "down" the basal planes could be significantly reduced by the choice of polishing abrasive.

I. A. Kozhinova, S. R. Arrasmith, J. C. Lambropoulos, S. D. Jacobs, and H. J. Romanofsky, “Exploring Anisotropy in Removal Rate for Single Crystal Sapphire Using MRF,” in Optical Manufacturing and Testing IV, edited by H. P. Stahl (SPIE, Bellingham, WA, 2001), Vol. 4451, pp. 277–285.

Anisotropy is easily seen with numerous MR spots. Eighteen 3-minute spots placed around the part in different directions with respect to C-axis tilt and the center of the part. Spots 1 and 11 show the same amount of removal, as indicated by the data and the similar coloration of inside-spot area. Adjacent spots 5 (dark blue color) and 6 (light yellow) show a dramatic difference in spot depth for the same spotting time.