MIL-C-48497A in Plain English: What These Durability Tests Prove?

If you work with optical coatings for military or demanding applications, you have probably seen MIL-C-48497A on legacy drawings. But what does it actually mean, what do the required tests prove, and where does this legacy spec fit in a modern optics workflow?

MIL-C-48497A is a legacy U.S. military specification that sets minimum durability requirements for both single-layer and multilayer interference coatings (such as anti-reflection, bandpass, and edge stacks) on optical components. It was originally intended for optics used inside sealed instruments, not for exposed outdoor environments. The focus is on ensuring the coating will not fail during reasonable military use.

MIL-C-48497A mandates a series of durability tests, typically performed on witness samples or coated parts. The main tests (with pass/fail criteria) include:

• Adhesion ('tape snap test'): A piece of 1/2-inch cellophane tape (per L-T-90) is pressed on the coating and snapped off quickly. Pass: No damage or removal of coating.
• Humidity soak: 24 hours at 120°F (about 49°C) and 95–100% relative humidity. The coating must still meet cosmetic and physical inspection requirements after this treatment.
• Moderate abrasion: 50 strokes with a cheesecloth pad under about 1 lb of force. No scratches or streaks allowed on the coated surface.
• Thermal cycling: Cycling from –80°F to +160°F (about –62°C to +71°C), with 2 hours at each extreme. After completion, the coating is inspected for damage or film failure.
• Solvent exposure: Sequence of 10 minutes each in trichloroethylene, acetone, and ethyl alcohol. After air drying between soaks, there must be no coating deterioration.

• Severe abrasion (eraser test): 20 strokes using a MIL-E-12397 eraser at 2–2.5 lbs force; no visible coating damage allowed (common when coatings are exposed).
• Salt solubility: 24 hours in salt solution, then pass re-inspection for film integrity.
• Water solubility: 24 hours in distilled water, then pass re-inspection for damage.
• Surface quality: Passes the scratch-dig inspection per the classical MIL-O-13830 method.

• Prove the coating can withstand moderate humidity, handling, solvent cleaning, and temperature swings typical of fielded military equipment.
• Confirm that the coating is well adhered and does not flake or delaminate during normal use or cleaning.
• Verify that the surface cosmetic quality is maintained per military standards.

• Performance for long-term, fully exposed, or outdoor use; MIL-C-48497A targets sealed systems, not constant weathering.
• Decades of aging resistance or harsh cleaning except as described.
• Suitability for all possible chemicals or severe abrasion; only the named solvents and tests are required.
• “Premium” durability—modern specifications (such as ISO 9211-4) may require stricter or broader testing.

MIL-C-48497A is now Inactive for New Design (officially since 1997), but it is still referenced for legacy equipment or contracts. Modern programs often replace it with ISO 9211-4 (abrasion, adhesion, water) and ISO 9022 (humidity, temperature, etc.) for similar or stricter tests.

• MIL-C-675C: Older AR coating spec; covers similar durability tests.
• MIL-M-13508C: Protected aluminum mirror coatings; durability for metallic mirrors.
• ISO 9211-4/9022: Modern international standards for optics now preferred for new projects.

• If a customer specifies MIL-C-48497A, you must pass the required test set—usually adhesion, humidity, moderate abrasion, with optional thermal, solvents, and severe abrasion depending on the note.
• Always verify if tests must be performed sequentially on a single witness sample and if optical performance re-testing is needed after each durability step.
• For new projects, recommend referencing ISO methods using the same (or stricter) levels, which are recognized internationally.

• MIL-C-48497A is a sensible “minimum bar” for durability in sealed military optics, but not a guarantee of outdoor, long-term, or ultra-rugged use. Use ISO standards for new projects, and always check the specific durability tests your customer expects.