N-BK7 vs Si (optical grade)
Glass · grade comparison
N-BK7 and Si (optical grade) are both glass grades. Si (optical grade) is the stronger of the two: tensile strength 124 MPa (18 ksi) against 50 MPa (7.25 ksi) for N-BK7, about 148% more. Si (optical grade) is harder — 1,100 HV against 610 HV for N-BK7 — which matters for wear surfaces and threads. Si (optical grade) is lighter (2.33 g/cm³ vs 2.51 g/cm³ for N-BK7) and Si (optical grade) is stiffer (130 GPa vs 82 GPa). On the shop floor, N-BK7 sits in a lower cost tier (standard, $$) than Si (optical grade) (high, $$$$).
N-BK7Choose it when the part sees heat, or when cost is the deciding factor.
Si (optical grade)Choose it when strength drives the design, or when surface hardness matters.
N-BK7 vs Si (optical grade): propertiesHow to read these bars
- One bar per material. A full bar is the highest value among all glass grades in the FabDigit library; each bar is drawn as a share of that.
- The better value (by the usual engineering direction) is in bold. Rows with no direction — density, thermal conductivity — have no bold.
- Values are typical room-temperature figures for the condition named on each card.
- Confirm against the certified mill or resin datasheet before design release.
How to read these bars
- One bar per material. A full bar is the highest value among all glass grades in the FabDigit library; each bar is drawn as a share of that.
- The better value (by the usual engineering direction) is in bold. Rows with no direction — density, thermal conductivity — have no bold.
- Values are typical room-temperature figures for the condition named on each card.
- Confirm against the certified mill or resin datasheet before design release.
Mechanical
Strength, stiffness, ductility and hardness at room temperature.
Elastic (Young's) Modulus
Shear Modulus
Bulk Modulus
Poisson's Ratio
Tensile Strength (Ultimate)
Compressive Strength
Flexural Strength
Shear Strength
Fracture Toughness (K_IC)
Hardness, Vickers
Hardness, Knoop
Hardness, Mohs
Same in both: Elongation at Break 0%
Physical
Density and transition temperatures.
Density
Refractive Index
Light Transmission
Melting Point (Solidus)
Same in both: Porosity 0%
Thermal
How each material carries heat and how hot it can run.
Thermal Conductivity
Specific Heat Capacity
Thermal Expansion (CTE, 20–100 °C)
Max Service Temperature (continuous)
Min Service Temperature
Electrical
Conductivity and insulation.
Electrical Resistivity
Dielectric Strength
Dielectric Constant (1 MHz)
Volume Resistivity
Dissipation Factor (1 MHz)
Chemical & Environmental
Corrosion and environmental resistance (0–100 scores).
Corrosion Resistance
Manufacturability
FabDigit workshop ratings (0–100 scores) — how each material behaves on the machine.
Machinability
Same in both: Polishability 95 / 100
Sustainability
Primary-production footprint per kilogram.
Embodied Energy (primary production)
Embodied Carbon (primary production)
Embodied Water
Typical Recycled Content
Common Calculations
Ratios derived from the values above.
Specific Strength (UTS / density)
Specific Stiffness (E / density)
Thermal Diffusivity
Thermal Shock Resistance Index
Strength versus weight
Yield strength against density for every glass grade in the FabDigit library; the compared grades are highlighted. Up and left is stronger for its weight.
Manufacturing and finishing
| Capability | N-BK7 | Si (optical grade) |
|---|---|---|
| Default condition | Annealed Glass | — |
| Relative cost | $$ Standard | $$$$ High |
| Min. machined wall | — | — |
| Best machined surface | — | — |
N-BK7 vs Si (optical grade) — frequently asked
Is Si (optical grade) stronger than N-BK7?
Yes. Si (optical grade) has a typical tensile strength of 124 MPa (18 ksi) against 50 MPa (7.25 ksi) for N-BK7 — about 148% more.
Which is cheaper, N-BK7 or Si (optical grade)?
N-BK7 is the lower-cost material: it sits in the standard tier ($$) while Si (optical grade) is high ($$$$). Part price depends far more on geometry, tolerance and quantity than on raw stock — upload a CAD file for a live quote.
When should I choose N-BK7 over Si (optical grade)?
Pick N-BK7 when the part sees heat, or when cost is the deciding factor. Pick Si (optical grade) when strength drives the design, or when surface hardness matters.
Ready to make it?
Upload a CAD file and FabDigit prices the part in N-BK7 and Si (optical grade) side by side — material, machining time, tolerance and finish in one quote. Certified material data ships with the order on request.
Typical handbook values for the conditions named above, normalised for comparison. Hardness on non-Brinell scales is converted approximately for charting only. Request certified data with your order for design-critical properties.