This guide compares fused quartz (fused silica) and borosilicate glass so you can choose the right material for optical, thermal-process and laboratory applications. It covers composition, optical transmission, thermal performance (softening, thermal shock), mechanical/abrasion behavior, chemical resistance, manufacturing/processing, cost and typical industry use cases.
1. What are these materials？
Fused Quartz / Fused Silica: a nearly pure, non-crystalline silicon dioxide (SiO₂) produced by melting high-purity silica feedstock (electrically fused or flame fused). Extremely low impurity content, excellent optical transmission from deep-UV to IR, very low thermal expansion, high softening/annealing temperatures. Often marketed as “quartz” or “UV-grade silica” depending on OH content and process.
Borosilicate Glass: a silicate glass with significant boron oxide (B₂O₃) content (typical commercial examples: Pyrex®, Borofloat®). It has much better thermal shock resistance than soda-lime glass, good chemical resistance, and is inexpensive and easy to manufacture in many shapes and sizes. Optical performance and thermal limits are lower than fused silica.
2. Key comparative properties
3. Optical performance: when optics matter
- Deep-UV applications (photolithography, UV lamps): fused silica is the clear choice. Borosilicate absorbs strongly below ~300–350 nm and cannot be used where deep-UV transmission is required.
- Visible & NIR optics: both materials transmit well in the visible; fused silica may still be preferred where low absorption and minimal dispersion are required.
- Surface finish & polish: both can be polished to optical quality, but achieving very tight optical flatness and scratch-dig specifications is easier and more repeatable from fused silica blanks in high-precision optics.
4. Thermal performance: temperature and thermal shock
- High continuous temperature/bakeout: fused quartz tolerates much higher continuous temperatures and repeated thermal cycles (softening point ~1700 °C; workable temps much lower, but still far above borosilicate). For furnace tubes, process windows, and high-temperature viewports, fused silica/silica tubes are standard.
- Thermal shock: Although fused silica has a very low coefficient of thermal expansion (CTE), its thermal shock resistance depends on geometry and mounting. Thin fused-silica elements often survive rapid changes better than borosilicate of the same thickness. Borosilicate is superior to soda-lime glass for shock but inferior to fused silica in high-temperature gradients.
- Bakeout with seals: if using organic seals (Viton), consider the seal temperature limit; metal seals may be needed for high bakeouts even if the window can tolerate higher temperatures.
5. Mechanical, abrasion and impact considerations
Neither material is ductile — both are brittle. Fused silica offers marginally better stiffness and abrasion resistance in some contexts; sapphire and other crystals are harder. For scratch-critical surfaces and where mechanical durability is crucial, consider sapphire (if budget allows) or protective coatings/guard windows for both fused silica and borosilicate.
6. Chemical resistance and compatibility
Acids and most corrosive atmospheres: fused silica shows superior chemical inertness at moderate temperatures. Borosilicate is resistant to many chemicals but can be attacked by strong alkalis and hydrofluoric acid. Select based on process chemistry and temperature.
If your process involves hot alkaline cleaning, fused silica tends to perform better.
7. Manufacturing, tolerances, and lead time
Fused silica fabrication is more specialized — grinding, polishing, and annealing to meet optical flatness and low internal stress require experienced shops. Custom diameters, flanges and drilled ports increase lead time and cost.
Borosilicate parts are easier to produce in standard sizes and shapes, enabling shorter lead times and lower unit cost for commodity sight glasses and labware.
8. Cost and procurement considerations
Material cost: Fused silica is more expensive; machining/polishing costs are also higher. Use fused silica only where its performance advantages (optical, thermal, chemical) are required.
Volume & variants: for commodity sight glasses in many standard sizes, borosilicate gives the best cost/performance. For specialized optics, furnace tubes, or deep-UV applications, fused silica justifies the extra cost.
9. Practical selection checklist (use this to pick material)
What is the operating wavelength?
- < 300 nm (deep UV) → Fused silica.
- Visible/NIR and no deep-UV requirement → Either, check other constraints.
Maximum continuous temperature or required bakeout?
- 600 °C, repeated cycles → Fused silica.
- < 500 °C → borosilicate may suffice (if other factors allow).
Thermal shock environment?
- Very rapid and large temperature swings at high temperatures → Fused silica (thin geometry preferred).
Chemical exposure (hot strong alkali / HF)?
- HF or hot alkali → Fused silica (but HF attacks silica — for HF both are problematic; use compatible materials).
Optical flatness / low scatter required?
- High precision optics → Fused silica.
Budget & lead time constraints?
- Tight budget, fast delivery → Borosilicate for standard parts.
Mechanical abrasion concerns?
- Consider protective layers, guard windows, or alternate materials (sapphire) if abrasion is a primary failure mode.
10. Application recommendations
- Semiconductor furnace tubes/process windows: fused silica (for temperature and contamination control).
- UV lamp sleeves & photochemical reactors: fused silica (low OH variants for deep UV).
- General laboratory sight glass, cooling jackets, and low-temp viewports: borosilicate (economical and adequate).
- High-temperature optical diagnostics or laser coupling in vacuum/plasma systems: fused silica or sapphire (depends on mechanical demands).
- Disposable or low-budget viewing windows for non-critical optical use: borosilicate.
11. Typical specifications to request from a supplier
When you request a quote, ask for:
- Exact material grade (e.g., UV-grade fused silica with OH content specified).
- Optical specs: transmission curve, refractive index at target wavelength, scratch-dig, surface flatness (λ/4, λ/10), and AR coating options.
- Thermal specs: maximum recommended continuous temperature, allowable bakeout temp, CTE.
- Mechanical specs: thickness tolerances, edge treatment, and any strength testing.
- Chemical compatibility and provided test reports (e.g., impurity analysis, outgassing data for vacuum use).
- Lead time, MOQ, packing, and shipping conditions (to avoid thermal/mechanical damage).
12. Frequently asked questions
Q: Is fused silica always better than borosilicate?
A: No. Fused silica outperforms borosilicate in deep-UV transmission and high-temperature stability, but borosilicate is more economical and adequate for many visible/NIR, low-to-moderate temperature applications.
Q: Can HF be used with fused silica?
A: Hydrofluoric acid attacks silica strongly; neither material is HF-compatible. Use appropriate fluorinated polymers or specialty materials for HF service.
Q: When should I choose sapphire instead?
A: Choose sapphire when extreme hardness, scratch resistance, and very high mechanical strength are required (e.g., heavy abrasion). Sapphire is more expensive but often used for rugged viewports and device covers.
Conclusion
Selecting between fused quartz (fused silica) and borosilicate should be driven by wavelength needs, temperature and thermal cycling, chemical exposure and budget. For deep-UV optics, high-temperature process tubes and demanding semiconductor applications, fused silica is usually the correct choice. For many laboratory and general industrial sight glass needs, borosilicate offers an economical and effective solution. Use the selection checklist above when specifying materials and require supplier test data where performance is mission-critical.
