N-BK7 vs Sapphire
Glass · grade comparison
N-BK7 and Sapphire are both glass grades. Sapphire is the stronger of the two: tensile strength 400 MPa (58 ksi) against 50 MPa (7.25 ksi) for N-BK7, about 8.0 times as much. Sapphire is harder — 2,000 HV against 610 HV for N-BK7 — which matters for wear surfaces and threads. N-BK7 is lighter (2.51 g/cm³ vs 3.98 g/cm³ for Sapphire) and Sapphire is stiffer (400 GPa vs 82 GPa). On the shop floor, N-BK7 machines more easily (poor vs not recommended).
N-BK7Choose it when weight is the constraint, or when the part is machined in volume.
SapphireChoose it when strength drives the design, or when surface hardness matters.
N-BK7 vs Sapphire: 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
Polishability
Sustainability
Primary-production footprint per kilogram.
Embodied Energy (primary production)
Embodied Carbon (primary production)
Embodied Water
Same in both: Typical Recycled Content 5%
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 | Sapphire |
|---|---|---|
| Default condition | Annealed Glass | Annealed Glass |
| Relative cost | $$ Standard | $$ Standard |
| Min. machined wall | — | — |
| Best machined surface | — | — |
N-BK7 vs Sapphire — frequently asked
Is Sapphire stronger than N-BK7?
Yes. Sapphire has a typical tensile strength of 400 MPa (58 ksi) against 50 MPa (7.25 ksi) for N-BK7 — about 8.0 times as much.
Which is cheaper, N-BK7 or Sapphire?
Both sit in the same relative cost tier ($$, standard); the part price will be decided by machining time, tolerance and quantity rather than by the material.
Which is easier to machine, N-BK7 or Sapphire?
N-BK7. Its machinability rates poor (18/100) against not recommended (5/100) for Sapphire.
Ready to make it?
Upload a CAD file and FabDigit prices the part in N-BK7 and Sapphire 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.