Synthetic Sapphire Fabrication for High Performance Applications

This paper explains the crystallographic plane orientation and capability of synthetic sapphire in two distinct high-performance application spaces: Life Science and Medical markets. Additionally, we will explore the manufacturing and assembly orientation utilizing synthetic sapphire material, and touch on defects that can adversely affect material performance in applications that use pistons/rods as sealing surfaces. Typical uses for these components require deliberate design choices and tightly toleranced product for fail safe applications.

Understanding Synthetic Sapphire

Synthetic sapphire is a robust, versatile material that can withstand harsh conditions and is suitable for a wide variety of applications. Since synthetic sapphire is an aluminum oxide material grown in hexagonal crystalline form, it is hard and has a high resistance to heat and extreme chemistries. Due to its crystalline structure, material and optical characteristics can change depending on the orientation of the crystallographic planes to the designer's placement. The material can be milled, drilled, cut, lapped, and polished into many sizes, shapes, and surface finishes while maintaining lattice orientation thorough out the material processing stages. These finished pieces can be used in aggressive environments where reliability, optical transmission properties and hardness are required.

Sapphire is the hardest of all known oxide crystals with a hardness of 9 on the Mohs scale. Synthetic sapphire is second in hardness only to diamond, and retains its high strength at high temperatures. Sapphire has good thermal properties, with excellent electrical and dielectric properties. Higher grades of synthetic sapphire combine zero porosity with near total resistance to acids and alkaline substances. Synthetic sapphire is insoluble in water, and only reacts with hydrofluoric acid, phosphoric acid and potassium hydroxide at high temperature above 300 degree Celsius.

Since Synthetic sapphire is an anisotropic hexagonal crystal, its properties depend on crystallographic direction (relative to the optical C-axis). When a synthetic sapphire part is produced, the orientation of the part may affect the performance of that part. “Orientation” refers to the angle of the sapphire crystal from its optical axis, which is the C-axis. The C-axis of the sapphire crystal is the most commonly specified crystal orientation for sapphire windows. The primary reason for this is that sapphire is naturally slightly birefringent in all other axis, while C-axis (or 0 degree) cut sapphire windows eliminate the inherent birefringent properties of the crystal. The C-axis is also the strongest and most mechanically symmetric orientation when forces are applied parallel to the C axis, sometimes as high as 20% stronger.

If no orientation is specified, then the orientation can be what is called “random”. Random orientation means that the part was cut with no regard to the crystal orientation. Verification of the crystal orientation adds cost due to the process of continuously tracking the orientation during cutting, grinding and polishing. Random orientation is common because of lower costs and is an acceptable choice if ideal optical or mechanical qualities are not required: note that the easiest orientation to cut is ~60° from the C-axis. Because the mechanical performance of sapphire will vary depending on orientation, random orientation allows for unpredictable variability in the strength of the sapphire part to part, and across a given part. Other orientations that are less frequently requested include M-Plane, R-Plane, and A-Plane.

Typical Choices for Part Orientation:

1. Zero Degree: The direction of view is parallel to the optical axis of the crystal, i.e the C vector, perpendicular to the C-plane. This provides the highest strength and lowest birefringence.

2. C-Axis: In a rod, the direction along its length: perpendicular to the C-plane. In a window, the direction perpendicular to the face: perpendicular to the C-plane.

3. 90 Degree: The direction of view is perpendicular to the optical axis, i.e. the direction of view is parallel to the C-plane axis of the crystal.

4. M-Plane: The plane containing the optic axis (C) and inclined 30 degrees to the A-axis.

5. A-Plane: The plane that is perpendicular to the A-axis, containing the C-axis.

6. R-Plane: A plane inclined 57.5667 degrees to the optic axis in the same zone as the M-plane.

7. Random: There is no specified relationship between the part and the crystalline orientation.

8. The part is manufactured without concern about orientation.