Effects of Crystal Orientation in Deterministic Microgrinding of Sapphire

A new high precision optical manufacturing process for nearly hemispherical sapphire missile domes has been established at the Center for Optics Manufacturing. During development of this process, an interesting lobing phenomenon was observed, and is the subject of this paper. The substrate material used for the guided missile domes was Crystal System’s highest quality HEMEX grade sapphire. The dome was cut normal to the c (0001) orientation. When cutting a steep spherical surface in sapphire, normal to the c plane, the machining process is influenced by the change in material properties as other crystal orientations are intersected (Figure 1). The work described here is intended to explain the reason for the lobing effect through a combination of activities including: machining of sapphire domes; loose abrasive lapping of various crystal orientations of sapphire; and microhardness testing of various crystal orientations of sapphire.