Sapphire
Single-crystal alumina, transparent and very hard
Typical values for the temper in the title
Sapphire is alumina in single-crystal form. It is transparent over a wide range of wavelengths (from ultraviolet to infrared), very hard (about HV 2000), and resistant to scratching, heat and chemicals. Only diamond and a few materials are harder.
It is used for optical windows in harsh conditions (pressure, heat, abrasion), watch and camera covers, LED substrates, and windows in sensors and military systems. It is expensive and very hard to machine.
Mechanical and physical properties
| Property | Value | Note |
|---|---|---|
| Modulus of elasticity E | 345 GPa | |
| Density ρ | 3.98 g/cm³ | |
| Flexural strength | 400 MPa (flexural) | database value |
| Vickers hardness HV | 2000 | |
| Machinability | diamond/laser only | relative rating in the site database |
Structure and production
| Feature | Value | Meaning |
|---|---|---|
| Type | single-crystal alumina | transparency and hardness |
| Transparency | from ultraviolet to infrared | optical windows |
| Directionality | properties depend on crystal orientation |
Typical values per producer data.
Grades and production routes
Sapphire is grown as a single crystal from the melt, and cut to a defined crystal orientation. Orientation affects strength and optical properties.
| Version | Properties | When to choose it |
|---|---|---|
| Cut by orientation (C, A, R) | different properties by orientation | by the optical and mechanical requirement |
Applications
- Optical windows in harsh conditions
- Watch and camera covers
- LED substrates
- Windows in sensors and military systems
- Tiny precision bearings
Fabrication
Processing
Diamond and laser cutting, diamond grinding and polishing (database: "diamond / laser").
Brittleness
Very hard but brittle. A sapphire window breaks from a sharp impact or an edge flaw.
Directionality
Design the cut orientation for the application.
Compared with similar materials
| Material | Rm / Rp0.2 | Welding | When it is the better choice |
|---|---|---|---|
| Alumina (Al₂O₃ 99.5%) | 380 MPa | Not possible | polycrystalline alumina: opaque, cheap · material page |
| PMMA (Acrylic) | 72 MPa | Solvent / Ultrasonic | PMMA: transparent, soft and very cheap · material page |
| PC (Polycarbonate) | 65 MPa | Ultrasonic | PC: transparent and impact resistant, scratches · material page |
Common mistakes
Designing a sapphire window with unpolished sharp edges
Edge flaws initiate fracture.
Ignoring crystal orientation
Strength and optical properties depend on it.
Choosing sapphire where a transparent plastic is enough
Sapphire is very expensive.
Equivalents, standards and supply
| Standards | Producer specification |
|---|---|
| Forms | window · pipe · wafer |
| Producers | Kyocera · GT Advanced · Rubicon |
Learn more
Values are typical and for comparison only. In ceramics, strength is measured in bending and scatters widely (depending on flaws and finish), so design to producer data with suitable safety factors. Sources: ISO 513, ISO 13356, ASTM F603, producer data sheets, and the METAL HUB materials database.
Updated: October 31, 2026