Kovar Alloy: Properties, Specifications & Sealing Applications

Kovar’s mean CTE is roughly 4.60–5.20 ×10⁻⁶/°C over 30–400°C and about 5.10–5.50 over 30–450°C (per producer acceptance data; the older 5.9 upper figure is corrected). A matched expansion coefficient reduces stress and enables a seal, but it does not guarantee freedom from cracking or leakage — compatibility with the specific glass or ceramic, joint geometry, surface preparation, residual stress and thermal-cycle qualification all matter, and the finished seal is qualified rather than assumed.

Kovar is matched to hard (borosilicate) glasses and alumina ceramics, but “borosilicate” and “alumina” are broad categories — the actual glass grade or ceramic composition and the relevant temperature interval must be named. Direct glass sealing and brazing to metallized ceramic are different systems with different interface requirements, and the glass/ceramic transition and joining temperatures come from that supplier’s data.

Preparation has separate stages: degassing/annealing, producing a thin adherent oxide (typically in air) for glass bonding, and any Ni/Au plating. These are not one combined step, and plating is not a universal substitute for the glass-bonding oxide — which areas need oxide and which need plating depends on the specified joining route. Wet-hydrogen treatment is used for degassing/decarburising, not as an automatic way to create the final sealing oxide.

A producer reference (NILO K) is nominally Ni 29, Co 17, Fe 53, with C ≤0.04, Mn ≤0.50, Si ≤0.20 and controlled Al, Cr, Mg, Zr, Ti, Cu and Mo. That is source-specific, not universal acceptance limits, and a tighter carbon limit (for example ≤0.02) applies to some producer material because carbon is critical for sealing. Nominal constituents are adjusted to meet expansion requirements and need not sum to 100%; the certificate governs.

Carefully, by status: ASTM F15 was withdrawn in 2024 and is kept only as an identified legacy purchase requirement; MIL-I-23011 was canceled in 1998 and superseded by SAE-AMS-I-23011 (AMSI23011C, stabilized), so the class and revision are stated explicitly. Current SAE routes are AMS 7726 (wire), AMS 7727 (bars/forgings) and AMS 7728 (sheet/strip/plate). The applicable edition and acceptance are confirmed per order, and machined/plated parts need their own drawing and finish requirements.

Kovar tube can be offered where supported, described by actual OD × wall, ID, tolerances, construction (seamless/welded), seam treatment and testing — not a stainless pipe schedule. We do not publish a generic schedule chart as a Kovar specification, a schedule is not a pressure rating, and unsupported tubular forms are marked “subject to technical review”. The verified range is mainly bar, strip and wire.

Provide the producer/designation, form and dimensions, condition, the glass or ceramic grade, the CTE interval and limit, the joining route, finish/plating, service cycle, the applicable drawing and documentation, and the acceptance testing (including any finished-seal leak test). Availability and the accepted specification route are confirmed with the quotation.