Achieving Mil Spec standards with optical coatings begins at the optical design stage and follows through to the end of the manufacturing cycle.
What is Mil Spec?
Mil Spec is shorthand for Military Specification. If you work within a military environment or create products and materials that must function within challenging situations regularly, you will see this term used frequently. There are numerous ways that an optic must conform and perform to meet the multiple rigorous standards for defense, aerospace, and safety requirements. Understanding Mil Spec testing for durability in environments or exposure to specific chemicals will help you to seek out and request the appropriate tests for your optic’s final use before sending in that first order.
In optics manufacturing, we use selective, identical test protocols that mimic and push the limit of durability properties for humidity, temperature shock, salt solubility, and fogging. Frequently, we chemical test for durability on contact, using chemicals such as insect repellent, antifreeze, or rifle cleaning compound to name a few. Fully testing a design before final approval will allow the project to stay on time when it is scaled up for high volume manufacturing, without any surprises.
Why is ‘Mil Spec’ Crucial With Respect to Optical Coatings?
Knowing that a part will not fail under regular heavy use is essential when systems operate under the most demanding situations. Abrasion that removes a functioning coating will cause failure in the field and may not be repairable or replaceable. Commonly, mirror coatings and interference coatings such as AR or IR coatings will require at least one Mil Spec test for verification, depending upon the operational requirements. Optics on board a ship will require salt and humidity testing for salt-ion corrosion and will have a determined life span requirement as well.
At the AccuCoat facility, we have specially designed equipment and dedicated floor space for testing coatings in challenging environmental conditions, including abrasion, adhesion, humidity, and temperature cycling testing.
- MIL-M-13508C - Aluminum mirror coatings with protective coatings
- MIL-PRF-13830B - Surface quality and allowed level of imperfections
- MIL-C-14806A - Reflection reducing coatings applied to substrates
- MIL-C-48497A - Durability for single or multilayer interference coatings
- MIL-C-675C - Minimum requirements for anti-reflection coatings for glass optics
- MIL-F-48616 - Performance requirements for thin-film coatings designed for 0.7 um - 50.0 um
- ISO 9211-3 - Characteristics for surface treatments of optics with a standard form and specification.
Creating Optical Coatings for Military Usage
Typically with a non-military optic, tests are first run on the prototype design as part of the R&D process, and once the first optic passes all needed testing, the design is determined to be complete.
When Mil Spec optics are involved, a witness sample is run through the initial gamut of Mil Spec testing in order to prove that the design can withstand the requirements before the project is approved to run as a full batch. The military may require testing for each batch. Depending on the final use case, actual parts to be tested and logged for traceability. Your team will need to know which requirements are needed and request those tests.
Documentation of batch and part testing are important and necessary when creating parts for field use. Once your optics pass the initial testing, traceability allows for each step of the material batch, design, cleaning, and coating to be documented. AccuCoat maintains standard traceability records for coatings for five years and can be extended even further if required, up to ten years.
In addition to any Mil Spec testing, separate standard quality tests for performance, internal quality controls, and monitoring are performed regularly at AccuCoat to ensure consistency for each coating run, A witness sample from every run is tested for spectral performance with a final batch inspection before packing and shipping to the customer.
ITAR Compliance is a major factor in the selection of a coating house that does military work, as it requires the whole of the supply chain to be compliant. These regulations are updated frequently for international and security reasons; as well as technological advances and apply to any manufacturer shipping worldwide from the USA. AccuCoat has been ITAR compliant since 2008 and is expertly familiar with the strict requirements for documentation and information control.
At AccuCoat, we design for manufacturability, keeping your order on time and on budget. Our entire workforce understands the requirements for quality and documentation. By treating the initial design and testing with the same rigor as a high volume run, we ensure that all demands of the optics that we provide to our military and security perform at the highest levels.
Operating environment tests:
- 24-hour humidity exposure
- 10-day humidity exposure
- Temperature cycling: -62°C to +85°C
- Temperature shock: +23°C/-57°C/+71°C/+23°C
- Moderate abrasion/severe abrasion
- Salt solubility (not appropriate for metals)
- Salt Fog (not appropriate for metals)
Chemical exposure tests:
- Insect Repellent
- Penetrating oil
- Carbon removing compound
- Lubricating oil
- Rifle bore cleaning compound
- Vacuum pump oil
[*Note: Not all tests will be passed by all coatings on all substrates. Please consult an AccuCoat engineer to discuss your specific requirements.]