MACOR glass ceramic
Specify Corning MACOR by finished-part requirements; interpret temperature, mechanical and electrical data with their test conditions and qualify machining and joints.
Tags: materials, ceramics, glass-ceramic, macor, machining
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Corning lists a mean thermal expansion coefficient of 9.0 × 10⁻⁶/°C from 25 to 300°C for MACOR. A free 100 mm length heated uniformly across that interval expands by 0.2475 mm (100 × 9.0 × 10⁻⁶ × 275). The brochure lists 800°C continuous operation and 1000°C maximum without load. Assess restraint, temperature gradients and preload for a loaded joint. Corning MACOR thermal table, p. 4
Material identity and useful applications
MACOR® is Corning's machinable glass ceramic, supplied as slab and rod. Its structure contains approximately 55% fluorophlogopite mica and 45% borosilicate glass. It is a specific material rather than a generic name for all machinable ceramics. Corning MACOR brochure, p. 2
Morgan identifies electrical insulation, zero nominal porosity and fabrication with ordinary metalworking tools among its useful characteristics. No post-firing step is required after machining. These attributes make it worth considering when a custom insulating component needs complicated features without a subsequent firing allowance. They do not establish that a particular design meets its service requirements. Morgan MACOR material guidance
Begin selection with the actual function: insulating spacer, instrument support, fixture, feedthrough component or another defined part. Record loads, mounting restraint, operating atmosphere, temperature history, electrical duty and critical dimensions. Compare candidate materials against those requirements rather than selecting from maximum temperature alone. Do not transfer another machinable ceramic's data to MACOR merely because both can be cut with similar equipment.
Read properties with their qualifications
Corning publishes the following MACOR data:
| Quantity | Published value and condition |
|---|---|
| Density | 2.52 g/cm³ |
| Young's modulus | 66.9 GPa at 25°C |
| Flexural strength | 94 MPa at 25°C; minimum specified average |
| Thermal conductivity | 1.46 W/(m·°C) at 25°C |
| Mean expansion coefficient | 9.0 × 10⁻⁶/°C, 25–300°C |
| Mean expansion coefficient | 12.3 × 10⁻⁶/°C, 25–800°C |
The brochure notes that production batches may vary and that general properties come from sample testing. These data are not finished-part design allowables. Corning property table and qualification, pp. 4–5
Use the statistical basis of each property to set individual acceptance limits and derive design allowables. Assess bending, threads and thin ligaments around holes with evidence for the actual geometry, surface, temperature and lifetime.
Temperature ratings are not loaded-part ratings
Corning lists 800°C continuous operation and 1000°C maximum no-load temperature. Keep the no-load qualification attached to the latter. Corning thermal table, p. 4
An assembly assessment still needs heating and cooling rates, gradients, supports, fastener preload and exposure duration. Define acceptable dimensional change and insulation performance at the working temperature. A room-temperature strength or stiffness value cannot establish hot-part capacity. Similarly, the catalog temperature does not authorize every joint, adhesive, coating or adjacent metal at that temperature.
The free-expansion example gives an estimated hot length of 100.2475 mm. Use the coefficient for the intended temperature interval, especially when the part approaches 800°C.
Machining and feature design
Corning's machining sheet recommends carbide for tool life while allowing high-speed steel, and emphasizes suitable speeds and coolant. Its guidance calls for coolant intended for glass or ceramics. For drilling, it specifies at least 0.050 in. extra material at the back for breakout, removable afterward. Corning machining guidance, ©2013
Use the supplier's operation-specific guidance as a starting point for a controlled machining trial. Qualify the combination of tool, machine, workholding, stock condition and feature geometry. Inspect the first parts before assuming that a speed/feed combination transfers to a smaller hole, thinner wall or less-supported edge. Leave access for inspection and define where minor edge defects are acceptable.
For tapping, Corning advises larger hole clearance than metal practice, chamfering both ends, rotating the tap in one direction and continuously flushing chips. Reversing the tap back and forth can cause chipping. Corning tapping instructions
Specify thread form, engagement, assembly torque and inspection instead of accepting a thread solely because the tap passed through it. Plan support at exits and intersections. Establish rejection criteria for chips and cracks near loaded or electrically critical features. The absence of post-firing does not remove the need for final dimensional and surface inspection.
Electrical service, vacuum and joining
Morgan lists electrical properties with temperature and frequency context and describes metallization, soldering, brazing, epoxy bonding, glass sealing and mechanical joining as possible routes. These are process options, not interchangeable joint qualifications. Morgan electrical and joining guidance
For an insulator, define waveform, voltage, frequency, temperature, geometry, creepage/clearance and environmental contamination. Require verification appropriate to the completed assembly; a bulk dielectric-strength figure cannot be multiplied by arbitrary wall thickness to promise a system voltage rating. Keep room-temperature measurements separate from hot-service acceptance.
For vacuum use, distinguish the substrate from the complete cleaned and joined assembly. Specify cleaning, bakeout, joint leak requirements and any outgassing evidence needed for the application. Material porosity does not determine whether a joint leaks or whether machining residues and attached materials suit the vacuum process.
For joining, identify surface preparation, metallization or interlayer, thermal cycle and inspection. Assess thermal expansion mismatch and restraint using the actual temperature interval. Qualify the joint under representative load and environment rather than assigning it parent-material strength.
Purchase and release requirements
Order the named material with traceable stock identity and a drawing that controls critical dimensions, edge condition, surface requirements and inspection. Agree any lot-specific certification or testing with the supplier. Record machining and cleaning processes where they affect acceptance, and protect finished surfaces during packaging and assembly. Morgan notes that machining dust can be irritating; use appropriate machining controls and the supplied safety documentation. Morgan material and machining notes
Sources reviewed November 1, 2026. The Corning brochure prints ©2012 and its machining sheet ©2013. The Morgan web page does not establish a technical revision date; its website copyright is not treated as one.