Key Takeaways
- Borosilicate 3.3 provides a low CTE (3.3 × 10⁻⁶/K) and withstands thermal shock up to roughly 150°C, making it ideal for laboratory and industrial glass parts.
- Compared to soda-lime, borosilicate offers superior chemical durability and temperature resistance; compared to fused silica, it is more cost-effective and machinable.
- Anti-reflection (AR), ITO (conductive), and hydrophobic coatings can be applied to borosilicate parts to enhance optical, electrical, or easy-clean performance.
- Borosilicate 3.3 complies with RoHS and REACH, suitable for food-contact and medical applications where regulatory compliance is needed.
- Custom machined borosilicate parts can include features like holes, grooves, and complex geometries with tight tolerances, available on request.
Introduction: Why Glass Material and Coating Choices Determine Machined Part Success
When sourcing machined technical glass components, the base material and any surface treatments define nearly every performance parameter: thermal endurance, chemical inertness, light transmission, electrical conductivity, and mechanical durability. Borosilicate glass 3.3 is the most frequently specified material for demanding industrial, laboratory, and optics applications because it provides a practical balance of low thermal expansion, high chemical resistance, and reasonable cost. Choosing the right glass—and pairing it with the correct coating—early in the design phase avoids costly redesigns and field failures.
Available Glass Materials for Machined Components
Manufacturers of precision glass parts typically offer a range of materials. While borosilicate 3.3 is the focus here, understanding the alternatives helps clarify its position:
- Fused Silica (Quartz) – The highest-purity glass, with near-zero thermal expansion and excellent UV to IR transmission.
- Borosilicate Glass 3.3 – A silica-boron trioxide formulation (e.g., Schott Borofloat® 33, Corning 7740) known for a CTE of 3.3 × 10⁻⁶/K.
- Soda-Lime Glass – The most common flat glass; low cost but limited thermal and chemical resistance.
- Aluminosilicate Glass – Chemically strengthened glass with high hardness and scratch resistance (e.g., Gorilla® Glass).
- Sapphire – Single-crystal aluminum oxide; extremely hard and scratch-resistant, but costly.
- Optical Glass (e.g., BK7) – Formulations optimized for broad visible spectrum transmission in lenses and windows.
Properties and Trade-Offs at a Glance
Each material’s properties dictate its suitability:
Fused Silica: CTE ~0.55 × 10⁻⁶/K, usable up to ~1000°C, transmits deep UV. Trade-off: high material and machining cost, limited availability in large formats.
Borosilicate 3.3: CTE 3.3 × 10⁻⁶/K, thermal shock resistance up to ~150°C, excellent resistance to water, acids, and solvents. Transmits visible light well but absorbs UV below ~300 nm. Cost is moderate, and it is readily machined with diamond tools.
Soda-Lime: CTE ~9 × 10⁻⁶/K, poor thermal shock resistance, fair chemical durability. Easily scratched. Lowest cost.
Aluminosilicate: CTE ~7–9 × 10⁻⁶/K before strengthening; after ion exchange, high surface compression provides durability. Optical transmission similar to soda-lime.
Sapphire: CTE ~5–6 × 10⁻⁶/K, extremely hard (9 Mohs), transmits from UV to mid-IR. High cost restricts use to small windows or wear parts.
Optical Glass: Varies widely; BK7 has CTE ~7.1 × 10⁻⁶/K and high visible transmission. Many optical glasses have poor chemical durability compared to borosilicate.
Coating and Surface Treatment Options
Machined borosilicate parts can be further enhanced with coatings:
- Anti-Reflection (AR) Coatings – Multi-layer thin films that reduce reflection loss, boosting transmission to over 99% in the visible range. Essential for display windows and sensor covers.
- Mirror Coatings – Metallic (aluminum, silver) or dielectric layers applied to reflect specific wavelengths. Used in optical assemblies.
- ITO (Indium Tin Oxide) Coating – A transparent conductive oxide that adds electrical conductivity and EMI shielding. Often specified for touchscreens and heated windows. See our custom ITO coating glass options.
- Hydrophobic/Oleophobic Coatings – Nanometer-thick layers that repel water and oils, easing cleaning and reducing staining.
- Thermal Tempering – A heat treatment that introduces surface compressive stress, increasing strength and causing the glass to break into small, less hazardous pieces if fractured.
- Chemical Strengthening – Ion-exchange process (e.g., in potassium salt baths) that creates high surface compression, improving scratch resistance and edge strength.
Trade-offs: AR coatings may slightly reduce surface hardness; ITO coatings can cut visible light transmission by 10–15%; tempering can induce optical distortion in thick sections.
Matching Material and Coating to Your Application
Selecting the right combination depends on the environment and function:
- Laboratory glassware and microfluidic chips: Borosilicate 3.3 without coating, or with a hydrophobic coating to reduce sample carryover. For microfluidics, BF33 glass chips are a common starting point.
- High-temperature viewports: Borosilicate 3.3 or fused silica, depending on the maximum temperature. AR coating if optical clarity matters.
- Outdoor optical sensors: Borosilicate with a broadband AR coating and a hydrophobic top layer resists weather and reflection.
- Imaging system windows: Optical glass like BK7 with precise AR coatings; if thermal stability is needed, borosilicate can substitute.
- Touch panels and EMI shielding: Borosilicate or aluminosilicate with ITO coating. For high durability, chemically strengthened aluminosilicate.
Compliance and Environmental Considerations
Most borosilicate 3.3 glasses, such as Borofloat 33 glass wafers and similar formulations, comply with RoHS (Restriction of Hazardous Substances) and REACH regulations as they contain no intentionally added lead, cadmium, or mercury. For food-contact or medical applications, this material is generally regarded as safe, though final compliance depends on the specific coating and processing aids used. Always request the manufacturer’s Declaration of Conformity for your particular part configuration.
Customization of Materials and Coatings on Request
While borosilicate 3.3 is a standard offering, most precision glass manufacturers can source and machine other materials, including fused silica, sapphire, or specialized optical glasses. Coatings are typically applied after machining and polishing. Custom specifications such as non-standard thickness, tight parallelism (<1 arc minute), or specified surface roughness (Ra < 1 nm) can be accommodated. Discuss your exact requirements with the engineering team early in the development cycle.
Material and Coating Selection Summary
| Material | Typical CTE (×10⁻⁶/K) | Key Trade-Off | Typical Application |
|---|---|---|---|
| Fused Silica | ~0.55 | Highest cost, best UV transmission | Laser optics, semiconductor |
| Borosilicate 3.3 | 3.3 | Best cost-performance balance for thermal and chemical resistance | Labware, sensor windows |
| Soda-Lime | ~9 | Low cost, poor thermal shock resistance | Consumer-level transparent parts |
| Aluminosilicate | ~7–9 | High strength after chemical tempering, moderate temperature limit | Touchscreens, aircraft windows |
| Sapphire | ~5–6 | Extreme hardness, very high material and machining cost | Watch crystals, spectrometer windows |
| Optical Glass (BK7) | ~7.1 | Excellent visible transmission, limited thermal/chemical resistance | Lenses, prisms |
Request a Material Recommendation
Choosing the optimal glass and coating can be straightforward when you provide a few details about the operating environment, mechanical loads, optical requirements, and budget. Share your specifications, and a materials engineer will provide a data-backed recommendation at no cost. Contact us to start the conversation.
Frequently Asked Questions
What does the "3.3" in borosilicate glass 3.3 signify?
It refers to the coefficient of thermal expansion (CTE) being approximately 3.3 × 10⁻⁶/K. This low expansion rate gives the glass its exceptional thermal shock resistance.
How does machined borosilicate glass compare to quartz?
Borosilicate 3.3 is more cost-effective and easier to machine than fused quartz, but quartz has higher UV transmission and thermal limits. Choose based on temperature and optical requirements.
Can borosilicate glass 3.3 parts be used in food or medical applications?
Yes, borosilicate 3.3 is generally considered food-safe and is widely used in laboratory and medical devices. It meets common regulatory standards like RoHS and REACH.
What coatings can be applied to borosilicate glass for optical applications?
Anti-reflection (AR) coatings improve visible light transmission; ITO coatings add conductivity; mirror coatings provide reflectivity; hydrophobic coatings reduce streaking.
What tolerances are achievable when machining borosilicate glass?
Tight tolerances such as ±0.05 mm or better are typically achievable, depending on part geometry and feature size. Complex shapes like microfluidic channels can be precision-machined.