Sapphire Windows for High-Pressure, High-Temperature & Laser Applications
Single-Crystal Al₂O₃ · High-Pressure · High-Temperature · Laser
Sapphire Windows for High-Pressure, High-Temperature & Laser Applications
Single-crystal sapphire is the right window material when fused quartz can’t survive the environment. Mohs 9 hardness, ~2 GPa compressive strength, transmission from 185 nm to 5000 nm, working temperature to 1500 °C, and inert to almost everything except hot phosphoric acid. This guide covers crystal orientations, mechanical limits, application examples for CVD/PVD viewports, high-pressure cells, Ti:Sapphire and Nd:YAG laser optics, IR domes, and the custom geometry MachinedQuartz routinely makes for OEM partners.
185–5000 nm
Transmission Range
Mohs 9
Hardness
2 GPa
Compressive Strength
2 pcs
Custom MOQ
Single-crystal sapphire is one of three materials we routinely supply as optical windows; the other two are fused quartz and calcium fluoride (covered in our broader optical windows selection guide). What makes sapphire worth its 3–10x premium over quartz is a specific combination of properties that no other commonly available optical material matches: hardness second only to diamond, compressive strength of 2 GPa, transmission from 185 nm in the deep-UV through 5 µm in the mid-IR, working temperature up to 1500 °C in inert atmosphere, and inertness to most acids and process plasmas.
This guide is the deep-dive companion to our broader windows comparison. If your application needs a window that survives a hostile mechanical environment (high pressure, mechanical impact, abrasive flow), pushes into the mid-IR beyond what quartz can do, dissipates absorbed laser power without thermal lensing, or operates above 800 °C in air, sapphire is likely the right material. The 100-SKU MachinedQuartz sapphire range covers stock dimensions from Ø 1.5 mm to Ø 100 mm; custom geometry, crystal orientation, and AR coating are routine.
What MachinedQuartz wants you to know upfront. We routinely make custom sapphire windows from 2 pieces — you do not need a 100-piece commitment to get a custom diameter, custom thickness, or wedged geometry. The catalog covers the standard stock formats; custom is the default for OEM work, and the only adder is the engineering review (free) and the production lead time (12–25 working days for most custom geometry within standard machining envelope). Send a sketch and quantity; we respond within one business day.
1. Why sapphire — four standout properties
Sapphire is single-crystal aluminium oxide (Al₂O₃), grown synthetically by Czochralski, Verneuil, EFG (edge-defined film-fed growth), or HEM (heat-exchanger method) processes at temperatures above 2000 °C. The resulting boule is sliced, lapped, and polished into windows, lenses, substrates, and rods. Four properties separate sapphire from amorphous fused quartz and other transparent materials.
1. Mechanical hardness (Mohs 9, second only to diamond)
Sapphire is one of the hardest commercial materials. Vickers hardness is 1900 HV (vs ~600 for fused quartz). In practice this means sapphire windows are essentially scratch-proof during routine handling, survive abrasive process flows that would etch or pit quartz, and tolerate aggressive cleaning protocols (acid soak, ultrasonic, mechanical wipe) without surface degradation. For viewports in plasma-etch chambers, sand-blasted environments, or anywhere mechanical wear is a real concern, sapphire is the only choice.
2. Compressive strength ~ 2 GPa
Sapphire’s compressive strength is roughly 10x that of fused quartz. In a pressure-vessel viewport, this directly translates to safe operating pressure: a 25 mm diameter sapphire window 5 mm thick handles 200+ bar of pressure differential without fracture risk; the same geometry in fused quartz fails at about 20 bar. For oil & gas downhole pressure cells, supercritical CO₂ reactors, hydrothermal synthesis, and high-pressure spectroscopy, sapphire is the practical material.
3. High thermal conductivity (40 W/m·K)
Sapphire conducts heat about 28x better than fused quartz (1.4 W/m·K). For laser optics, this matters when the window absorbs a small fraction of the laser power: heat dissipates into the surrounding mount instead of building up in the window and causing thermal lensing or fracture. Sapphire output couplers in high-power Nd:YAG and Ti:Sapphire systems use this property to handle 50+ W of average laser power.
4. Wide transmission window (185–5000 nm)
Sapphire transmits from 185 nm in the deep-UV (slightly better than fused quartz at 190 nm) through visible and into the mid-IR up to 5 µm (where fused quartz cuts off at 2.5 µm). The combination of UV and mid-IR transmission in one material is rare; CaF₂ reaches further in both directions but lacks sapphire’s mechanical properties. For applications that span UV and IR (polarised pyrometers, dual-band sensors, broadband process monitors), sapphire is often the only single-material solution.
What sapphire is NOT for. If your application is purely visible-light at modest pressure and temperature, fused quartz at 1/3 to 1/10 the cost is the right choice. If your application needs deep-UV below 185 nm or mid-IR beyond 5 µm, CaF₂ is the right choice. Sapphire’s premium is justified only when the mechanical, thermal, or wide-spectral combination is binding.
2. Sapphire fundamentals — growth methods and grades
“Synthetic sapphire” covers single-crystal Al₂O₃ produced by four major growth methods. Each method produces material with slightly different defect density, optical clarity, and cost. For optical-window applications, all four are acceptable; the differences matter mostly for laser-grade and substrate-grade work.
Growth methods
| Method | Boule size | Optical quality | Cost | Best for |
|---|---|---|---|---|
| Czochralski (CZ) | up to Ø 200 mm | excellent, low defect | $$$$ | Laser-grade, HEMT substrates |
| Heat Exchanger (HEM) | up to 340 mm cube | excellent, large area | $$$ | Large windows, IR domes |
| Verneuil (flame fusion) | up to Ø 50 mm | good, some inclusions | $$ | Routine optical windows, jewelry |
| EFG (edge-defined) | shaped (rod, tube, ribbon) | good | $$ | Direct-grown rods, tubes, ribbons |
For optical windows, MachinedQuartz sources from CZ and HEM-grown boules; the resulting windows are essentially defect-free at the level visible in optical microscopy. Verneuil material is sometimes substituted for cost-sensitive applications where surface finish matters more than internal optical clarity.
Grades
- Optical grade: standard for windows; transparent across 185–5000 nm with no visible inclusions; the default specification
- Laser grade: tighter optical specifications (lower scatter, lower absorption coefficient); used in laser cavity optics where absorbed power and intracavity scatter matter; ~ 50% premium over optical grade
- Substrate grade: specific orientation and surface finish for crystal-epitaxy work (LED, HEMT); thicker (typically 0.43 mm or 0.65 mm) and polished to atomic flatness; not the same product as optical windows
Plates
Custom plates
square / rectangular · custom orientation · AR coating available
View product gallery →3. Crystal orientation — c-cut, a-cut, r-cut
Sapphire is a trigonal single-crystal material. The c-axis is the unique high-symmetry axis; standard window orientations are defined by the angle between this axis and the polished window face. For 95% of optical-window applications, the c-cut (c-axis perpendicular to the face) is the correct choice and what we ship by default.
c-cut (0001 or “c-plane”) — default
c-axis perpendicular to the window face. At normal incidence, the window is effectively isotropic — no birefringence, no polarisation rotation, no walk-off. Used for: standard optical windows, viewports, pressure cells, laser pump windows, IR domes. For unpolarised optical windows, c-cut behaves as effectively isotropic at normal incidence and is the conventional default across the optical industry.
a-cut (11−20 or “a-plane”)
c-axis parallel to the window face. The window is birefringent at normal incidence: incident light splits into ordinary and extraordinary rays with different refractive indices (1.768 vs 1.760 at 588 nm). Used for: polarisers, waveplates, polarisation-sensitive optical components, crystal-axis reference standards. Specify only when birefringence is the desired effect.
r-cut (1−102 or “r-plane”)
c-axis at 57.6° to the window face. Standard substrate for epitaxial crystal growth: GaN-based LEDs, HEMTs, and high-frequency power transistors. Different surface preparation (atomic-flat polish, stricter dimensional tolerance) than optical windows; usually a separate product family. Available on request for OEM substrate orders.
If your application is orientation-sensitive (polarised laser cavity, waveplate, polarisation-sensitive measurement, crystal-epitaxy substrate), tell us at quote time. Orientation-controlled material is sourced through a separate supply chain from standard optical-grade sapphire and requires explicit specification. For routine optical-window applications (most viewports, pressure cells, IR domes), orientation typically is not the binding constraint.
4. Mechanical properties — the hardness story
Sapphire’s mechanical edge over fused quartz is what makes it worth the cost premium for hostile-environment applications.
Hardness
- Mohs scale: 9 (vs 5.5–6.5 for fused quartz, 6 for BK7, 10 for diamond)
- Vickers: 1900 HV (vs 600 for quartz)
- Knoop: 1370 (vs 540 for quartz)
In practice, sapphire windows survive routine mechanical contact — tweezers, lens cleaning swabs, accidental tool contact during assembly — that would scratch fused quartz. For viewports inside abrasive process flows (sand-blasting equipment, slurry pump observation, tribology testing), sapphire’s hardness is a practical requirement.
Strength under load
- Compressive strength: ~2 GPa parallel to c-axis; ~3 GPa perpendicular
- Flexural (modulus of rupture): 350–500 MPa depending on orientation and surface finish
- Tensile strength: 350–700 MPa (similar to engineering ceramics)
- Young’s modulus: 345 GPa (vs 73 for quartz, 81 for BK7)
Pressure-vessel sizing
For a circular sapphire window in a pressure vessel, the safe-pressure formula (with safety factor 4) is approximately:
P_safe (bar) ≈ 1100 × (t / D)²
Where t is window thickness in mm and D is the unsupported diameter (clear aperture) in mm. Examples: a 25 mm aperture window 5 mm thick is safe to 44 bar; the same aperture 10 mm thick is safe to 176 bar. For 200 bar and above, double-window construction (two thinner windows separated by an oil-filled chamber) is standard practice. For pressure-rated viewports, always design with a 4× safety factor and perform a destructive proof-pressure test on a sample.
Custom pressure-cell windows. If you are designing a high-pressure cell and need a quote on a specific dimension, we provide free pressure-rating analysis as part of the quote review. Send the unsupported aperture, working pressure, and safety factor; we recommend the appropriate window thickness in c-cut sapphire.
5. Thermal properties & chemical resistance
Thermal performance
- Working temperature: 1500–1800 °C in inert atmosphere; 800 °C in air (above this, slow surface oxidation begins; alpha-Al₂O₃ is fully thermodynamically stable but practical use degrades)
- Thermal conductivity: 40 W/m·K (28x fused quartz at 1.4)
- Coefficient of thermal expansion: 5.6 × 10⁻⁶ /K parallel to c-axis; 6.6 × 10⁻⁶ /K perpendicular (vs 0.55 for fused quartz)
- Thermal shock resistance: moderate. Survives ~150 °C/min ramp without fracture; not as robust as fused quartz under thermal shock because of higher CTE
- Specific heat: 0.76 J/g·K
Chemical resistance
Sapphire is essentially inert to almost everything at room and modest temperature. The exceptions:
- Hot phosphoric acid (H₃PO₄) above 200 °C: slowly etches sapphire
- Molten alkali metals (Na, K) and molten alkali fluorides: attack sapphire
- HF at high temperature: some attack; cold HF is essentially inert (unlike quartz which dissolves)
- Hydrogen + oxygen plasma at high power: slow erosion; usually not a concern for normal CVD/PVD plasma exposures
For everything else — mineral acids (HCl, HNO₃, H₂SO₄ at room temperature), bases (NaOH, KOH at moderate temp), organic solvents, halogen process gases, fluorocarbon plasma, oxygen plasma — sapphire is the right choice. CVD process viewports and plasma-etch chamber windows routinely log thousands of hours of plasma exposure without measurable surface degradation.
Thermal-shock caution. Sapphire’s CTE is ~10x that of fused quartz, so sudden temperature changes (e.g., dropping a hot window into cold solvent during cleaning) can cause cracking. Allow temperature changes < 150 °C/min during heating and cooling; for cleaning hot windows, let them cool to ambient first.
6. Optical properties — transmission, refractive index, birefringence
Transmission
185 nm UV cutoff to 5000 nm (5 µm) IR cutoff with greater than 80% transmission for a 1 mm thick uncoated window across most of the range. The 185 nm cutoff is slightly better than fused quartz at 190 nm; the 5 µm IR cutoff is significantly better than quartz at 2.5 µm. AR coatings (V-coat single-line, broadband, dual-band) push peak transmission to 99%+ at the design wavelength.
Refractive index and dispersion
Sapphire is birefringent (uniaxial positive). At 588 nm:
- Ordinary ray (perpendicular to c-axis): n_o = 1.768
- Extraordinary ray (parallel to c-axis): n_e = 1.760
- Birefringence: Δn = n_o − n_e = 0.008
For c-cut windows at normal incidence, the optical path is along the c-axis and only the ordinary ray is excited — effectively unpolarised behavior. For a-cut windows or off-normal incidence on c-cut, the birefringence shows up as polarisation-dependent transmission, walk-off (rays of different polarisation diverge slightly), and possible interference effects. For laser cavity work where polarisation matters, specify the orientation precisely.
Surface specifications
| Spec | Routine | Precision | Laser-grade |
|---|---|---|---|
| Surface flatness | λ/4 (155 nm) | λ/8 (75 nm) | λ/10 (60 nm) or better |
| Scratch-dig | 40-20 | 20-10 | 10-5 or 5-2 |
| Parallelism | < 1 arcmin | < 10 arcsec | < 5 arcsec |
| Surface roughness Ra | < 2 nm | < 1 nm | < 0.5 nm |
| AR coating | uncoated typical | broadband AR | V-coat, IBS-coated |
7. High-pressure cell viewports
Sapphire’s compressive strength is what makes it the standard window material for pressure vessels above ~10 bar.
Oil & gas downhole
Downhole logging tools
500–1000 bar borehole pressure, 150–200 °C ambient. Sapphire viewports for optical sensors (UV fluorescence, laser-induced breakdown spectroscopy, borehole imaging). Window thickness 5–10 mm, diameter 10–40 mm typical.
Hydrothermal synthesis
Supercritical reactors
Supercritical water (220 bar, 374 °C critical point), supercritical CO₂ (74 bar, 31 °C critical), and high-pressure hydrothermal reactor viewports. In-situ Raman spectroscopy and visual observation of phase transitions during synthesis.
Pharma reactor
High-pressure pharma reactors
50–200 bar process cells for hydrogenation, supercritical fluid extraction, high-pressure crystallisation. Sapphire viewports for in-situ FTIR or Raman process monitoring.
Diamond anvil cell
Diamond anvil cell adjuncts
Although the anvils themselves are diamond, the surrounding optical access windows in DAC equipment use sapphire for the combination of pressure tolerance, optical clarity, and precision dimensions. Typically Ø 10–25 mm, thickness 2–5 mm.
Pressure-cell sizing summary
Use the formula in Section 4: P_safe = 1100 × (t/D)² bar with t in mm and D the clear aperture in mm. Always design with a 4× safety factor and proof-test a sample to 1.5x working pressure before commissioning the cell. For very high pressure (> 1 kbar), double-window construction or sapphire-on-sapphire stacks are standard.
Custom sapphire dimensions?
Send geometry + quantity (from 2 pieces). We respond within one business day with quote, lead time, and free CAD review.
8. High-temperature applications
CVD / PVD
Process chamber viewports
CVD reactors (300–800 °C wall, 1000–1100 °C substrate), PVD chambers, MOCVD, ALD. Sapphire viewports for in-situ pyrometry, plasma-emission monitoring, end-point detection. CTE-matched mounting (Kovar or Inconel flange) for thermal cycling.
Semiconductor RTP
Rapid thermal processing
Rapid thermal anneal (RTA), rapid thermal oxidation (RTO), and laser spike anneal chambers. Wafer surface heated to 1000–1300 °C in seconds; chamber-wall pyrometer needs sapphire window for 0.9–1.6 µm IR transmission and resistance to halogen process gases.
Industrial pyrometers
Steel mill / glass furnace
High-temperature pyrometers viewing molten metal, glass furnace ports (1400–1600 °C target). Sapphire window for the combination of mechanical robustness (mechanical impact from flying material), optical clarity at NIR pyrometer wavelengths (0.9–1.6 µm), and inertness to flue gases.
Plasma ignition
Plasma-arc / spark observation
Spectroscopic observation of arc plasmas, ignition spark studies, plasma-cutter optics. Sapphire’s combination of broadband transparency and resistance to sputter erosion makes it ideal for direct exposure to plasma flux.
9. Laser optics — substrates and output couplers
Ti:Sapphire laser substrates
Titanium-doped sapphire (Ti:Al₂O₃) is the laser gain medium itself in tunable solid-state lasers (700–1100 nm). The laser rod is single-crystal sapphire with controlled Ti dopant; the surrounding optical windows (pump-input window, output coupler, back mirror substrate) are also sapphire for the matched CTE and high thermal conductivity. Specify a-cut or c-cut depending on cavity polarisation requirements.
Nd:YAG and fibre laser windows
High-power Nd:YAG (1064 nm) and fibre-laser systems use sapphire substrates for output coupler windows where the substrate must dissipate ~1% absorbed laser power without thermal lensing. Sapphire’s 40 W/m·K thermal conductivity (vs 1.4 for quartz) is the deciding factor at average powers above ~50 W.
Ultrafast laser optics
Femtosecond Ti:Sapphire amplifier output windows, regenerative amplifier output couplers, OPA pump windows. Ultrafast lasers have peak power densities that can damage soft optical materials; sapphire’s hardness and laser-induced damage threshold (typically > 5 J/cm² at 800 nm, 100 fs pulses) make it the right substrate.
Excimer laser windows
193 nm ArF and 248 nm KrF excimer lasers can use sapphire (with deep-UV-grade material) as output windows. CaF₂ is more common for the very-deep-UV but sapphire serves well for 248 nm and 308 nm where its 185 nm cutoff is comfortably below the working wavelength.
Custom laser-grade sapphire. For high-power laser optics, specify: working wavelength, average power, peak power density, AR coating design, polarisation, surface flatness target, and orientation. Laser-grade sapphire (lower scatter, higher LIDT) is a 50–100% premium over optical-grade and worth it for cavity work; for routine output coupling, optical-grade is sufficient. We help spec the right grade as part of the quote review.
10. Defense, aerospace & ruggedized optics
IR domes and missile windows
Sapphire’s combination of mechanical robustness, broadband transmission (visible through 5 µm), and resistance to rain erosion and sand abrasion makes it the standard material for IR dome windows on high-speed aircraft, missile guidance systems, and forward-looking IR (FLIR) systems. Typical geometry: hemispherical dome 25–100 mm radius; or flat plate windows 25–75 mm.
Laser-protection eyewear and viewports
Some defense laser systems use sapphire windows to protect sensors from mechanical damage (debris from explosive ordnance, environmental exposure) while passing the laser wavelength. AR coatings tuned to the specific laser wavelength.
UAV and satellite optics
Outboard-mounted optical sensors on UAVs, satellites, and aerospace platforms use sapphire windows for protection against bird strike, meteoroid impact, and atmospheric debris. Stricter dimensional and surface specifications than industrial work because the window cannot be replaced in service.
ITAR / export control note. Sapphire windows for defense applications may be subject to ITAR or local export-control restrictions in some jurisdictions. We sell to commercial OEM partners; defense-specific applications are the responsibility of the customer’s compliance team. Verify ITAR / export status before specifying a sapphire window for a regulated application.
11. Mounting & sealing — CTE mismatch is the gotcha
Sapphire’s coefficient of thermal expansion (5.6–6.6 × 10⁻⁶ /K) is similar to typical metal flanges (Kovar 5.5, 304 stainless 17, copper 17) but very different from fused quartz (0.55) or borosilicate glass (3.3). For windows mounted in metal flanges that see thermal cycling, this matters.
Standard mounting options
- O-ring face seal: the easiest mount. Viton or EPDM O-ring compresses the window against a flange. Tolerates thermal cycling (the O-ring absorbs CTE mismatch) but is limited to vacuum levels above 10⁻⁶ mbar and temperatures below the elastomer max (Viton ~200 °C, EPDM ~150 °C).
- Threaded retaining ring: standard for laser optics and instrument viewports. Window seats in a recess; threaded ring compresses against an O-ring or rubber gasket. Quick replacement, minimal mechanical stress.
- Epoxy bonding: for permanent installations. Optical-grade epoxy (Norland 61, EPO-TEK 301) bonds window to flange. Cured assembly is gas-tight to ~10⁻⁶ mbar, low outgassing.
- Brazing (Au-Si, Cu-Ag eutectic): for ultra-high-vacuum and high-temperature applications. Sapphire is metallised (Mo-Mn ink fired at 1500 °C, then nickel-plated) and brazed to a Kovar or molybdenum flange. The result tolerates 10⁻⁹ mbar vacuum and 450 °C bake-out.
- Glass-to-metal seal (CTE-matched): sapphire fused to Kovar flange via direct bonding or intermediate solder glass. UHV-rated, high-temperature compatible.
Thermal cycling considerations
For windows that thermal-cycle > 100 °C, the CTE mismatch between sapphire and the surrounding metal causes radial stress on the window edge. Options to manage:
- Use Kovar flange (CTE 5.5) instead of stainless steel (CTE 17) — nearly perfect CTE match to sapphire
- Use a soft-metal seal (indium, copper) that absorbs CTE mismatch by plastic deformation
- For very high cycle counts, use a sapphire-to-sapphire seal with a sapphire support ring
- Or accept the stress and design with a 4× safety factor on flexural strength
12. Custom geometry — what MachinedQuartz makes
The standard catalog covers stock dimensions and shapes. For OEM partners and specialty applications, custom geometry is the norm rather than the exception. We routinely make:
Standard custom variations (no tooling fee, in-envelope)
- Custom diameters: any Ø from 1 mm to 200 mm (routine); larger up to ~300 mm on request
- Custom thicknesses: 0.1 to 20 mm; thinner or thicker on request with longer lead time
- Square / rectangular plates: any L × W up to 200 × 200 mm
- Wedged windows: 1–5 arcmin wedge angle for laser cavity etalon suppression
- Brewster-angle windows: for polarised laser optics
- Bevelled or chamfered edges: for sealing applications and stress reduction
- Drilled windows: through-holes for cable, gas, or fibre feedthrough with optical viewing
- AR coatings: single-line V-coat, broadband, or custom multi-band; outsourced to qualified vacuum-deposition partners
- Pair-matched sets: two windows matched within tight transmission tolerance for double-beam systems
Specialty custom (one-time engineering)
For specialty needs beyond the standard machining envelope, we work with partners on:
- Hemispherical IR domes: 25–100 mm radius for missile and FLIR optics
- Cylindrical lens windows: for line-illumination applications
- Stacked / sandwich assemblies: two sapphire windows with a sapphire spacer for double-window pressure cells
- Metallised and brazed assemblies: sapphire fused to Kovar flange for UHV applications
Custom MOQ = 2 pieces. Most specialty Hellma / Crystran / Meller-tier sapphire suppliers want 25–100 piece commitments before they will quote a custom geometry. We routinely make custom sapphire from 2 pieces — the only adders are the engineering review (free) and the production lead time (12–25 working days for most custom geometry within standard machining envelope). Send a sketch and quantity; we respond within one business day. This is the same commercial program that applies to our other product lines — see our bulk OEM page for the full pricing tiers and payment terms.
13. Catalog & ordering
The MachinedQuartz sapphire catalog has 100 stock SKUs covering Ø 1.5 mm to Ø 100 mm rounds with thickness 0.2–5 mm. All stock SKUs are optical-grade single-crystal sapphire with 185–5000 nm transmission. Custom geometry is the default for OEM work.
Stock dimensional range
| Format | Diameter | Thickness | Material grade |
|---|---|---|---|
| Round disc (small) | Ø 1.5–5 mm | 0.2–0.5 mm | optical grade single-crystal sapphire |
| Round disc (medium) | Ø 5–25 mm | 0.5–2 mm | optical grade single-crystal sapphire |
| Round disc (large) | Ø 25–100 mm | 1–5 mm | optical grade single-crystal sapphire |
| Custom rectangular | up to 100 × 100 mm | 0.5–5 mm | optical grade; orientation-specific on request |
| Custom shapes | per drawing | per drawing | per drawing |
Need a sapphire window quote?
Send the diameter (or shape), thickness, surface specs (or “routine”), AR coating requirement (or none), quantity, and any orientation requirement (only if your application is polarisation-sensitive). We respond within one business day with quote, lead time, and free CAD review on custom geometry. 2-piece MOQ on custom.
Request quote →Browse 100 stock SKUs →Browse the catalog
14. Frequently asked questions
What is the difference between c-cut, a-cut, and r-cut sapphire?
C-cut (0001) has the c-axis perpendicular to the window face — at normal incidence the window is effectively isotropic with no birefringence. This is the conventional default across the optical industry for unpolarised viewport applications. A-cut (11-20) has the c-axis parallel to the face and is birefringent at normal incidence (used for polarisers, waveplates). R-cut (1-102) has the c-axis at 57.6 degrees to the face and is used for crystal-epitaxy substrates (LED, HEMT). For most viewport, pressure-cell, and IR-dome applications, orientation is not specified explicitly. For polarisation-sensitive laser optics or substrate work, specify orientation at quote time and we will source through the appropriate supply chain.
What pressure can a 25 mm diameter sapphire window handle?
For a 25 mm clear aperture window 5 mm thick with safety factor 4, the safe operating pressure is approximately 44 bar; with 10 mm thickness, 176 bar. The formula is P_safe (bar) ≈ 1100 times (thickness/diameter) squared with both in mm. Always design with a 4x safety factor on the calculated burst pressure and proof-test a sample to 1.5 times working pressure before commissioning. For above 1 kbar, double-window construction with a buffer chamber is standard.
Can sapphire windows be AR coated?
Yes. Anti-reflection coatings are applied by qualified vacuum deposition partners (we outsource the coating step). Single-line V-coat for one wavelength achieves transmission greater than 99 percent at the design wavelength; broadband AR coatings cover ranges like 400 to 700 nm or 1000 to 1600 nm at 95 to 98 percent transmission. Specify the working wavelength when ordering AR-coated windows. Lead time is 25 to 35 working days for AR-coated parts versus 7 to 14 days for uncoated.
Why does sapphire cost so much more than fused quartz?
Single crystal growth is expensive. Synthetic sapphire is grown from molten Al2O3 by Czochralski, Verneuil, EFG, or HEM methods at temperatures over 2000 degrees Celsius in expensive iridium or molybdenum crucibles. Each boule takes days to weeks to grow. Fused quartz is amorphous, made by melting natural quartz sand or chemically depositing SiO2 from gas phase — much faster and cheaper per kilogram. Single crystal growth costs are inherent to sapphire; expect 3 to 10 times the per-window cost of equivalent fused quartz.
Does sapphire transmit in the mid-IR?
Yes, up to about 5 micrometres (5000 nm). Above 5 micrometres the Al-O lattice vibrations (multiphonon absorption) make the material rapidly opaque; transmission drops from 80 percent at 5 micrometres to under 10 percent at 6 micrometres. For mid-IR work above 5 micrometres, calcium fluoride (CaF2) is the right material — covers up to 9 micrometres. For routine UV-Vis-NIR through 5 micrometres, sapphire is excellent.
Can I use a sapphire window in an HF environment?
Cold HF: yes, sapphire is essentially inert. Hot HF (above about 200 degrees Celsius) or hot phosphoric acid: sapphire is slowly attacked over weeks-months timescale. Fluorine plasma at typical etch chamber conditions: sapphire is resistant. By contrast fused quartz is rapidly dissolved by HF at any temperature — sapphire is the right choice for any HF or fluoride-plasma environment. CaF2 is also resistant but lacks sapphire’s mechanical robustness.
What is the smallest custom sapphire diameter MachinedQuartz can make?
Routine custom diameters down to 1 mm. Below 1 mm becomes specialty work because handling and dimensional tolerance get harder; we have made down to 0.5 mm on request. The standard catalog starts at 1.5 mm. For very small custom sizes, expect a longer quote review and possibly a one-time engineering charge if the size is below the standard machining envelope.
What is the largest sapphire window MachinedQuartz can supply?
Routine custom up to 200 mm diameter. Above 200 mm becomes specialty because the boule size limits the window diameter (largest commercial Czochralski boules are 200 to 340 mm). We have supplied up to 250 mm on special-order from HEM-grown boules. For very large windows (above 300 mm), the supply chain narrows significantly and lead times extend to 8 to 12 weeks; ask for a specific quote.
Can sapphire be brazed to metal for UHV applications?
Yes. Standard process: sapphire is metallised by firing a Mo-Mn ink at 1500 degrees Celsius, which forms an adherent metal layer chemically bonded to the sapphire. The metallised surface is then nickel-plated and brazed (typically Au-Si or Cu-Ag eutectic) to a Kovar, Inconel, or molybdenum flange. The result tolerates 10 to the minus 10 mbar vacuum and 450 degrees Celsius bake-out. We do not braze in-house; we partner with qualified ceramic-metal bonding shops. Minimum order for brazed assemblies is typically 5 pieces because of setup costs.
How does MachinedQuartz handle thermal cycling for sapphire viewports?
Three approaches depending on the cycle severity. For modest cycling (less than 100 degrees Celsius range, infrequent), an O-ring seal absorbs the CTE mismatch and works fine. For higher cycling (100 to 400 degrees Celsius), CTE-matched flange material (Kovar at CTE 5.5 matches sapphire at 5.6 to 6.6) is the standard solution. For aggressive cycling (large delta T, high cycle count), brazed sapphire-to-Kovar assemblies handle it with the metal absorbing residual mismatch through plastic deformation. Tell us your cycling profile when requesting a quote.
15. Disclaimer & notes
Specifications on this page are typical values for commercial-grade single-crystal sapphire produced by the Czochralski, HEM, Verneuil, or EFG processes. Specific material properties (transmission, defect density, surface finish) depend on the supplier, growth method, and lot. For binding specifications, refer to the certificate of analysis supplied with each shipment.
Pressure-vessel sizing. Formulas in Section 4 and Section 7 are simplified guidelines for first-pass sizing. Final pressure ratings depend on full geometry, edge conditions, mounting design, dynamic vs static loading, and intended safety factor. For commissioned pressure cells, perform a destructive proof test on a sample at 1.5x working pressure and consult a mechanical engineer for high-pressure designs. MachinedQuartz does not certify pressure ratings; we supply the windows.
Custom geometry and brazed assemblies. Standard custom geometry within our machining envelope ships in 12–25 working days with no tooling fee. Specialty assemblies (brazed, hemispherical domes, large diameter, very tight tolerance) may require one-time engineering charges and longer lead times. Quoted as part of the quote review.
Trademark notice. Czochralski, HEM, Verneuil, EFG are crystal-growth method designations. Norland 61, EPO-TEK 301, Hellma, Crystran, Meller, Kovar, Inconel are trademarks of their respective owners. References are for technical comparison and supply-chain context only.
ITAR / export control: sapphire windows for defense applications may be subject to ITAR or local export-control restrictions in some jurisdictions. We supply commercial OEM partners; defense-specific applications are the responsibility of the customer’s compliance team.
Information currency: last reviewed May 2026. Catalog and SKU range subject to change.
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