Glass-Ceramics
A glass-ceramic is melted and formed as a glass, then heat-treated so that 50–90 % of its volume crystallizes into a fine, controlled phase. The crystals give properties no glass has: in the lithium-aluminosilicate family the negative expansion of the crystals cancels the positive expansion of the residual glass, so ZERODUR holds 0 ± 0.02 ppm/K and Robax and Ceran survive a flame on one face and a wet cloth on the other. Mica-based grades (Macor, Mycalex) crystallize into interlocking platelets that let the material be machined with carbide tools.
Popular glass-ceramic grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| ZERODUR | 60 | 30 | 2.53 | Poor | Not recommended | Zero-expansion mirror blanks and metrology |
| Robax | 90 | 35 | 2.55 | Not recommended | Not recommended | Transparent windows for fire and heat to 700 °C |
| Ceran | — | 30 | 2.55 | Not recommended | — | Opaque cooktop and heater panels |
| Neoceram | — | 50 | 2.51 | Not recommended | — | Transparent high-temperature glass-ceramic (NEG) |
| Pyroceram 9606 | — | 120 | 2.6 | Not recommended | — | Opaque radomes and hot structural covers |
| Mycalex | — | 45 | 3.4 | Very good | — | Glass-bonded mica insulators and arc barriers |
Choose glass-ceramics for
Mirror blanks, stages and reference frames that must hold size to nanometres; transparent windows for fireplaces, stoves, ovens, furnaces and infrared heaters; cooktop and heater panels; radomes and hot structural covers; and precision insulators and vacuum fixtures that are machined rather than fired to shape.
Look elsewhere when
The window is optical or must transmit UV (Fused Silica or N-BK7 — glass-ceramics scatter and most have a yellow tint), the panel needs to be cheap, large and tough (tempered Soda-Lime; glass-ceramics cannot be tempered), the service temperature is above about 1,000 °C or in molten metal (Alumina 99.5, Silicon Carbide, Boron Nitride), the part must be scratch-proof (Sapphire), or the fixture needs the strength of a technical ceramic rather than the 90–100 MPa of Macor.
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Glass-ceramic by application
- Metrology and mirror substrates
- ZERODUR in expansion class 0 (±0.02 ppm/K) or better for telescope mirrors, lithography stages and interferometer frames; lightweighted blanks are CNC-machined from solid and polished to λ/50.
- Fire and furnace viewing
- Robax or Neoceram sheet 3–5 mm for stove doors, oven windows and viewports to about 700 °C, cut and edge-ground to size; Pyroceram 9606 where the part is opaque and structural.
- Cooktop and heater panels
- Ceran panels in black, white or patterned finishes, transmitting the infrared of the burner while blocking visible light; printed, edge-finished and bonded into the frame with silicone.
- Machined insulators and fixtures
- Macor for vacuum feedthroughs, spacers, coil formers and optical mounts to ±0.013 mm; Mycalex for switchgear barriers, terminal boards and heater insulators in larger, less precise shapes.
Working with glass-ceramic
- Melting & forming
- The base glass is melted and cast, rolled or pressed like any glass, then ceramized in a two-stage heat treatment — nucleation around 700 °C and crystal growth at 800–1,000 °C over hours to days — with a small, predictable shrinkage. After ceramization the material behaves as a ceramic: it cannot be bent, slumped or tempered, so all shaping of sheet is by cutting and grinding.
- Grinding & lapping
- Robax and Ceran sheet are cut by waterjet or diamond saw and edge-ground; holes are core-drilled. ZERODUR is CNC-ground into lightweight structures and polished to λ/50 with roughness below 1 nm, grinding slightly slower than fused silica. Macor and Mycalex are turned, milled, drilled and tapped with carbide at cast-iron speeds, dry or with water, to ±0.01 mm.
- Joining & sealing
- ZERODUR is joined by hydroxide-catalysis or silicate bonding, epoxy or low-expansion adhesives, and to Invar or titanium fittings with compliant flexures. Robax and Ceran are gasketed with silicone or fibre seals into steel frames, never bolted rigidly. Macor is metallized and brazed, epoxy-bonded or glass-frit sealed, and takes tapped threads directly.
- Tolerances & design rules
- Cut sheet holds ±0.5 mm and ground edges ±0.1 mm; machined Macor ±0.01 mm; ZERODUR figure and expansion are specified by class. Design for the expansion of the grade — 0 for ZERODUR, near 0 for Robax, 9 ppm/K for Macor — and keep tensile stress low (design around 10 MPa for Robax, 20 MPa for Macor). Keep ZERODUR below about 130 °C where its expansion is certified; Robax and Ceran to 700 °C, Macor to 800 °C.
Other glass series
Glass-Ceramics — frequently asked
What is glass-ceramic?
A glass-ceramic is melted and formed as a glass, then heat-treated so that 50–90 % of its volume crystallizes into a fine, controlled phase. The crystals give properties no glass has: in the lithium-aluminosilicate family the negative expansion of the crystals cancels the positive expansion of the residual glass, so ZERODUR holds 0 ± 0.02 ppm/K and Robax and Ceran survive a flame on one face and a wet cloth on the other. Mica-based grades (Macor, Mycalex) crystallize into interlocking platelets that let the material be machined with carbide tools.
Which grade of glass-ceramic should I use?
ZERODUR for anything where dimensions must not move — mirror blanks, lithography and metrology stages, laser-gyro bodies, reference frames — ordered in the expansion class the tolerance demands. Robax when the part must be looked through at high temperature: fireplace and stove windows, oven and furnace viewports, heater covers to about 700 °C in continuous service and higher for short periods; Neoceram is the equivalent from Japan. Ceran for opaque cooktop and heater panels where colour and infrared transmission are tuned for the burner underneath. Pyroceram 9606 for opaque, strong (about 200 MPa) radomes and structural parts to about 1,000 °C. Macor when the fixture, insulator or vacuum part has to be machined to size in the shop — 800 °C continuous, zero porosity — and Mycalex for lower-cost glass-bonded-mica insulators and arc barriers to 350–650 °C depending on grade.
Which grade of glass-ceramic is the strongest?
Robax has the highest yield strength in the series at 90 MPa; ZERODUR follows at 60 MPa. Values are for each grade's default condition.
Which grade of glass-ceramic is easiest to machine?
Mycalex (machinability very good, 75/100), then ZERODUR (20/100).
Robax or tempered glass for a stove or oven door?
Robax (or Neoceram) is a transparent lithium-aluminosilicate glass-ceramic with near-zero expansion: it takes 700 °C continuously, tolerates a flame on one face and a cold splash on the other, and does not lose strength with age. Tempered Soda-Lime is fine for household oven doors that stay below about 250 °C behind an air gap, and costs a fraction as much, but it relaxes its temper above that and cracks at a thermal step of about 200 °C. Wood stoves, fireplaces, industrial ovens and furnaces need Robax; a domestic electric oven does not.
ZERODUR or fused silica for a stable substrate?
ZERODUR expands 0 ± 0.02 ppm/K (class 0; tighter classes are available) between 0 and 50 °C, against 0.5 ppm/K for fused silica, so a 500 mm ZERODUR frame moves less than 10 nm per degree where fused silica moves 250 nm. ZERODUR is also stiffer (90 versus 72 GPa) and its long-term dimensional drift is characterized. Fused silica wins when the substrate must transmit UV, be polished to the very lowest roughness or cost less; it is also the choice when the optic is small enough that 0.5 ppm/K does not matter.