Kovar Alloy Glass Expansion Analysis Testing: Thermal Expansion Characterization for Glass-to-Metal

Kovar alloy glass expansion analysis testing is a specialized thermal characterization service that measures the coefficient of thermal expansion (CTE) of Kovar (iron-nickel-cobalt sealing alloy) and its matched sealing glasses to verify compatibility in hermetic glass-to-metal seal assemblies. Using push-rod dilatometry and thermomechanical analysis in accordance with ASTM E228, ASTM E831, ASTM F15, and related ISO and GB standards, accredited third-party laboratories deliver precise expansion curves, mean CTE values, and inflection temperature data that ensure reliable long-term sealing performance.

Technical Overview

Kovar is a vacuum-melted iron-nickel-cobalt sealing alloy covered by ASTM F15, with a nominal composition of approximately 29% nickel, 17% cobalt, and the balance iron. Its defining characteristic is a controlled coefficient of thermal expansion (CTE) that closely matches that of borosilicate glasses over the temperature range in which glass-to-metal seals are formed and cooled. Because the alloy exhibits a low-expansion behavior below approximately 430°C and a rapidly increasing expansion rate above its Curie point, precise measurement of both the mean and instantaneous CTE is essential for seal design and quality control.

Kovar alloy glass expansion analysis testing quantifies the dimensional change of the alloy and the sealing glass as a function of temperature, typically from room temperature to 500–800°C. By superimposing the expansion curves of the metal and the glass, engineers can verify that the differential expansion during cooling remains within the tolerance that the compressive seal can accommodate. Excessive mismatch generates residual stress at the interface, which may cause glass cracking, seal leakage, or catastrophic failure during thermal cycling. Accredited third-party laboratories perform this testing using calibrated dilatometers, providing traceable data that supports material qualification, incoming inspection, and failure analysis.

Testing Samples

Kovar Alloy Specimens

Test specimens are machined from the product form of interest, including rod, bar, strip, sheet, wire, and drawn tubing. For push-rod dilatometry, a typical specimen is a cylindrical or rectangular bar measuring 25 mm to 50 mm in length with flat, parallel, and polished ends. Specimens must be representative of the delivered mill condition (annealed, deep-drawn, or stress-relieved), since cold work and heat treatment history significantly influence the expansion behavior of the alloy.

Glass Specimens

Matching sealing glasses, such as borosilicate and aluminosilicate compositions, are prepared as rods or coupons with the same nominal length as the metal reference specimens. Glass samples are annealed prior to testing to remove residual stress, and their expansion behavior is characterized across the full range through the glass transition temperature and dilatometric softening point to capture the complete sealing and annealing window.

Seal Assemblies and Finished Components

Completed glass-to-metal seal components, including hermetic packages, feedthrough terminals, connector pins, and optical windows, may be submitted for expansion-matching verification and residual stress evaluation. Sectioned coupons from production lots allow verification that seal geometry, oxide pre-oxidation treatment, and processing conditions have not altered the effective expansion compatibility between the alloy and the glass.

Plated and Coated Materials

Nickel-plated, gold-plated, or pre-oxidized Kovar materials used in final assembly can also be tested, with surface conditions documented so that plating thickness and interfacial oxides are considered when interpreting expansion data against the uncoated baseline.

Testing Items

The core scope of a Kovar alloy glass expansion analysis test program typically includes the following measured and derived parameters:

• Mean coefficient of thermal expansion (CTE) over defined intervals, such as α(20–100°C), α(20–300°C), α(20–400°C), α(20–450°C), and α(20–500°C)
• Instantaneous (tangent) coefficient of thermal expansion as a function of temperature
• Complete linear thermal expansion curves (ΔL/L₀ versus temperature) for both alloy and glass
• Curie point / expansion inflection temperature of the Kovar alloy
• Glass transition temperature (Tg) and dilatometric softening point of the sealing glass
• Strain point and annealing point of the glass, where applicable to seal processing
• Differential expansion between metal and glass across the sealing and cooling temperature range
• Thermal expansion hysteresis behavior during heating and cooling cycles
• Chemical composition verification of the alloy against ASTM F15 limits, where required for full material qualification

Testing Methods

Push-Rod Dilatometry (ASTM E228)

The reference method for metallic specimens is horizontal or vertical push-rod dilatometry per ASTM E228. The specimen is placed in a controlled furnace and contacted by a low-expansion push rod made of fused silica or a ceramic such as sintered alumina. Dimensional change is transmitted to a precision displacement transducer, while temperature is monitored with a calibrated thermocouple in close proximity to the specimen. Tests are performed under a flowing inert atmosphere, typically argon or high-purity nitrogen, to prevent oxidation of the iron-based alloy at elevated temperature. Standard heating rates of 3°C/min to 5°C/min are used, and a minimum of two thermal cycles is recommended, with data from the second cycle reported to eliminate the effect of low-temperature stabilization of the specimen.

Thermomechanical Analysis (ASTM E831)

For smaller specimens, thin sections, or localized verification, thermomechanical analysis per ASTM E831 is applied. A quartz probe applies a small static force to the specimen surface while the furnace ramps at a controlled rate, and the probe displacement records the expansion. TMA is particularly useful for screening plating effects, evaluating small production samples, and cross-checking dilatometer results.

Glass Expansion Testing (ISO 7991)

Glass specimens are tested using comparable dilatometric techniques, following ISO 7991 or equivalent national standards. The glass expansion curve is recorded through the transition region so that the strain point, annealing point, and softening point can be determined from the same run. Because glass results are sensitive to thermal history, annealing schedules are documented and standardized across comparative test campaigns.

Calibration, Correction, and Data Analysis

• Instrument baseline correction is performed using a certified reference material such as platinum, tungsten, or a standard reference silicate glass to remove the contribution of the sample holder and push rod.
• Temperature calibration is verified against the Curie transition of a certified reference alloy or the melting points of pure metals.
• Mean CTE is calculated from the slope of the expansion curve between the specified temperature limits, with measurement uncertainty typically reported at the 95% confidence level.
• The alloy and glass expansion curves are superimposed on a common axis to evaluate the differential expansion through the glass setting temperature down to room temperature, which governs residual seal stress.

Acceptance Criteria

For ASTM F15 Kovar intended for borosilicate glass sealing, the mean CTE in the 25°C to 300°C range is typically specified at approximately 4.6 to 5.2 × 10⁻⁶/°C, with tighter windows defined by the customer drawing or the sealing process. The glass must remain in slight compression after sealing, which requires careful evaluation of the crossover point where the glass expansion curve intersects the alloy curve during cooling.

Applications

Kovar alloy glass expansion analysis testing supports a wide range of high-reliability industries and products:

• Hermetic electronic packages and transistor outlines (TO headers and lids) for semiconductor devices
• Vacuum interrupter housings, vacuum tube envelopes, and microwave device components
• Hermetic connectors, feedthroughs, and pin assemblies for aerospace and defense electronics
• Fiber optic feedthroughs, optical windows, and sensor housings requiring gas-tight seals
• Battery and electrolytic capacitor hermetic terminals
• Medical implantable device enclosures and implantable feedthroughs
• Lithium niobate and crystal oscillator packages for telecommunications
• Material qualification for alloy mills, glass suppliers, and seal assemblers, including incoming inspection and supplier change control
• Failure analysis of cracked seals, leaking packages, and components that fail thermal cycling or thermal shock screening

FAQ

Why must the CTE of Kovar be matched to the sealing glass?

During cooling after seal formation, any difference between the contraction of the metal and the glass produces residual stress at the interface. If the differential is too large, the glass experiences tensile stress and cracks or develops microleakage paths. Matching the expansion curves ensures that the seal remains under mild compressive stress, which glass tolerates well, thereby preserving hermeticity over the service life of the component.

What temperature range should the test cover?

The test should at minimum span from room temperature to beyond the glass setting temperature, typically 400°C to 500°C for borosilicate sealing systems. Extending the run to 800°C or higher can provide additional information on the alloy behavior well above the Curie point and on the glass softening region, which is useful for process development and annealing schedule design.

Why does the CTE of Kovar increase sharply above approximately 430°C?

Kovar loses its ferromagnetic ordering at its Curie point, around 430°C. Below this temperature, magnetostriction-related volume effects, known as the Invar effect, suppress the normal lattice expansion. Above the Curie point, the alloy expands at a rate comparable to ordinary austenitic alloys, which is why the usable low-expansion matching window is limited to the sub-Curie temperature range.

How are heating rate and atmosphere controlled?

Heating rates of 3°C/min to 5°C/min are standard for push-rod dilatometry to avoid thermal gradients between the specimen and the thermocouple. Testing is conducted under flowing argon or nitrogen to prevent oxidation, which would alter the surface expansion behavior and contaminate the dilatometer. Controlled cooling segments are also recorded to capture expansion hysteresis.

Can finished sealed assemblies be tested instead of raw material coupons?

Yes. Sectioned or whole assemblies can be evaluated to verify seal compatibility, and complementary techniques such as polarized stress analysis of the glass and helium leak testing can be combined with expansion data to give a complete picture of seal integrity and residual stress state.

What standards govern this testing?

The principal standards include ASTM E228 for push-rod dilatometry of solid materials, ASTM E831 for thermomechanical analysis, ASTM F15 for the chemical and expansion requirements of the iron-nickel-cobalt sealing alloy, and ISO 7991 for the determination of the mean linear thermal expansion of glass. Equivalent national standards, including GB/T series methods for metallic expansion measurement, may be specified for projects in the Chinese market, and accredited third-party laboratories can report results under any of these frameworks.

How long does a typical test take?

A single dilatometer run covering two thermal cycles from room temperature to 500°C generally requires several hours, including furnace loading, atmosphere purge stabilization, and controlled heating and cooling. Including specimen preparation, calibration verification, and data reduction, standard turnaround is typically three to five working days, with expedited service available for urgent engineering or production-hold situations.