Benchmark · 4 materials
Fused Silica vs Robax vs Borofloat 33 vs N-BK7
All glass grades — bands show the family's 10th–90th percentile. The best value in each row is in bold; metric first, imperial in gray.
Properties
| Property | Fused Silica | Robax | Borofloat 33 | N-BK7 |
|---|---|---|---|---|
| Yield strengthMPa | 90 MPa13 ksi | 90 MPa13 ksi | 64 MPa9 ksi | 60 MPa9 ksi |
| Tensile strengthMPa | 110 MPa16 ksi | 110 MPa16 ksi | 70 MPa10 ksi | 70 MPa10 ksi |
| Specific strengthMPa·cm³/g | 41 | 36 | 29 | 24 |
| Densityg/cm³ | 2.2 g/cm³0.079 lb/in³ | 2.5 g/cm³0.09 lb/in³ | 2.2 g/cm³0.079 lb/in³ | 2.51 g/cm³0.091 lb/in³ |
| Hardness (HB equiv.)HB | 590 HV≈ 561 HB | 700 HV≈ 665 HB | 480 HV≈ 456 HB | 610 HV≈ 580 HB |
| Max service temp°C | 1,100 °C2,012 °F | 760 °C1,400 °F | 450 °C842 °F | 500 °C932 °F |
| Min service temp°C | -200 °C-328 °F | -200 °C-328 °F | -200 °C-328 °F | -200 °C-328 °F |
| Electrical conductivity% IACS | < 0.001 % IACSinsulator | < 0.001 % IACSinsulator | < 0.001 % IACSinsulator | < 0.001 % IACSinsulator |
| Machinability/100 | Poor20 / 100 | Poor20 / 100 | Poor20 / 100 | Poor20 / 100 |
| Weldability/100 | Not recommended5 / 100 | Not recommended5 / 100 | Not recommended5 / 100 | Not recommended5 / 100 |
| Formability/100 | Not recommended5 / 100 | Not recommended5 / 100 | Not recommended5 / 100 | Not recommended5 / 100 |
| Relative cost | $$ Standard | $$ Standard | $$ Standard | $$ Standard |
| Processes | CNC, Sheet | CNC, Sheet | CNC, Sheet | CNC, Sheet |
Where each grade sits
One track per property; coloured markers are the compared grades, the shaded band is the glass family 10th–90th percentile and the tick its median.
Yield strength90 MPa
54 MPafamily median 77 MPa490 MPa
Tensile strength110 MPa
67 MPafamily median 90 MPa520 MPa
Density2.2 g/cm³
2.2 g/cm³family median 2.5 g/cm³2.96 g/cm³
Hardness (HB equiv.)561 HB
503 HBfamily median 585 HB1,093 HB
Max service temp1,100 °C
457 °Cfamily median 600 °C1,310 °C
Electrical conductivity< 0.001 % IACS
< 0.001 % IACSfamily median < 0.001 % IACS< 0.001 % IACS
Machinability20 / 100
20 / 100family median 20 / 10020 / 100
Relative cost35
35family median 3535
Fused SilicaRobaxBorofloat 33N-BK7
Strength versus weight
Manufacturing and finishing
| Capability | Fused Silica | Robax | Borofloat 33 | N-BK7 |
|---|---|---|---|---|
| CNC machining | Yes | Yes | Yes | Yes |
| Sheet metal | Yes | Yes | Yes | Yes |
| Default condition | Annealed Glass | Annealed Glass | Annealed Glass | Annealed Glass |
| Min. machined wall | — | — | — | — |
| Best machined surface | — | — | — | — |
| Heat treatments | — | — | — | — |
| Chemical Etching | Yes | Yes | Yes | Yes |
| Dot Peen Marking | Yes | Yes | Yes | Yes |
| Electro-chemical Etching | Yes | Yes | Yes | Yes |
| Embossing / Debossing | Yes | Yes | Yes | Yes |
| Ink Stamping | Yes | Yes | Yes | Yes |
| Laser Marking | Yes | Yes | Yes | Yes |
| Mechanical Engraving | Yes | Yes | Yes | Yes |
| Vibro Engraving / Scribing | Yes | Yes | Yes | Yes |
Typical handbook values for the default condition, normalised for comparison. Hardness on non-Brinell scales is converted approximately for charting only. Request certified data with your order for design-critical properties.