Thermal and Environmental Durability of Thin-Film Coatings
Why Durability Matters in Thin-Film Optical Coatings
Real world operating conditions for optical filters and mirrors require steady performance during normal aerospace and defense, LiDAR / ADAR, and IR system field use. Space transmissions and on-the-ground or sea defense systems must have reliability when it counts. This means there is little room for failure because of dust and sand abrasion, salty and humid corrosive conditions or aggressive temperature cycling when under use. In this article, we take you through how Omega’s engineered durability and testing make it possible to rely on your system performance.
Some of the life-saving applications that Omega supports regularly are in the following markets: aerospace, defense, automotive LiDAR, semiconductor tools, biomedical systems and outdoor sensing systems.
Thin-film optical coatings must maintain spectral performance while surviving thermal cycling, humidity exposure, vibration, abrasion, and environmental contamination.
Common Environmental Stressors that Omega Designs Around
Thermal Cycling and Coating Stress
When discussing space communications, we must include thermal cycling as a part of the initial conversation of your “can you make this” stage.
Our design team understands the critical factors in proper material selection for both coating and substrate glasses, ensuring alignment to minimize the expansion/contraction mismatches. Stress accumulation can be a factor that leads to spectral shift, cracking, delamination, and mounting failures.
Application Humidity and Moisture Exposure
When using coated optics in an open environment, the ocean can be one of the toughest on your equipment and instrumentation. Porous coating structures allow moisture and salt absorption, index changes, reduced transmission and eventually, adhesion loss.
Our sputtered coatings, paired with quality controls, listed MIL-Spec tests, and ISO 9001:20215 certification, allow your team to have full confidence in your coatings’ integrity. Omega tests, documents and certifies on request.
UV and Solar Exposure
Solarization and UV degradation severely impact an optic’s long-term stability. Many commonly used materials begin to absorb light at less than 400 nm.
Desired Performance in the UV
With our deep understanding of UV filter subtypes, including notch and specialty filters, we will select appropriate sputtered or evaporated coatings and materials to limit long-term throughput loss or failure.
Mechanical and Abrasive Exposure on the Ground
Rugged operating systems call for rugged optical coatings. Handling damage, field cleaning procedures, oil and jet fuel exposure, sand, and airborne particulates all add up to a tricky place to be an optic that must function perfectly.
Omega’s DLC coatings for IR optics create a layer of confidence between your optic and its working environment using an amorphous non-composite material, with low friction, high hardness and excellent corrosion resistance. The DLC coating was created for thermal cameras, optical windows, and IR optics that live on the outside and are exposed to the environment daily.
Vacuum and Space Environments
Outgassing, vacuum stability, atomic oxygen exposure and thermal extremes contribute to the lengthy list of concerns that our customers bring to the table. Omega’s experience with C-Band/SWIR filters, solar rejection filters for protection, lightweight imaging optics and pointing mirrors win with our standard testing, MIL-SPEC, ISO 9001:20215 certification. Our customers may also request additional MIL-SPEC tests and documentation depending on the specific need.
Our full suite of capabilities are trusted by leading global OEMs, as well as start-ups to deliver the photons to the right place, at the right time.
How Others Thin-Film Coatings Can Fail
| Failure | Typical Cause | Performance Impact |
| Delamination | Poor adhesion or thermal mismatch | Catastrophic failure |
| Spectral shift | Temperature variation or stress | Wavelength drift |
| Cracking | Excess compressive/tensile stress | Reduced durability |
| Moisture intrusion | Porous film structure | Reduced transmission, wavelength shift |
Engineering Durable Coatings for Harsh Environments
Omega’s design authority and 50+ years of Space Flight Heritage have advanced the field of space optics. By using Collaborative Engineering, Rapid Prototyping, and Balancing Tolerances, our expertise manufacturing NIR bandpass filters, diamond-turned reflectors and specialty coatings supports aerospace endeavors launching now and designed-in for future missions.
The specialty capability to create meter-class reflective coatings or DLC coatings for IR substrates combines high uniformity, custom engineering, clever cost controls, and stress balancing.
Environmental Testing Methods for Optical Coatings
Omega’s in-house metrology and environmental testing options are many.
- humidity testing
- atomic force microscopy
- FTIR spectrophotometry
- surface characterization (up to 14” flats)
- stray light measurement
- salt fog testing
- abrasion/adhesion testing
- thermal cycling
- thermal vacuum testing
- MIL-SPEC selections
- ISO testing
Your coatings can also benefit from including:
- witness samples (on request)
- standard spectral validation (Standard T%, R%, and/or OD in the desired wavelength range at your desired angle)
Common durability tests for thin-film coatings include:
– Thermal cycling
– Humidity exposure
– Adhesion testing
– Abrasion resistance testing
– Salt fog exposure
– Thermal vacuum cycling
– Spectral stability validation
Why Coating Durability Should Be Considered Early in Optical Design
When you design for manufacturing (DFM) and partner with a customer’s engineering section with non-optic specialists, there is a long list of considerations that can change the entire application set up.
These considerations might include:
- geometry of the optic
- cost and timing of replacements
- coating stress
- volume manufacturability
- timetable to launch
- substrate selection
- environmental operating conditions
Environmental durability is not added after coating design. It must be engineered into the coating architecture, substrate selection, and deposition process right from the first conversation.
Need help engineering durable coatings for demanding environments? Omega Optical supports custom thin-film coating development for aerospace, defense, infrared, analytical, and advanced optical systems.
FAQs
What causes thin-film optical coatings to fail?
Thin-film coatings commonly fail due to thermal stress, humidity exposure, contamination, poor adhesion, or mechanical abrasion.
How does thermal cycling affect optical coatings?
Repeated temperature changes create expansion and contraction stresses that can lead to cracking, delamination, or spectral drift.
What are hard oxide coatings?
Hard oxide coatings are dense, environmentally stable thin films engineered for high durability and long operational life.
Are optical coatings tested for environmental durability?
Yes. High-performance coatings are commonly tested using thermal cycling, humidity exposure, adhesion testing, abrasion testing, and MIL-spec qualification methods.
Why are DLC coatings used on infrared optics?
Diamond-like carbon (DLC) coatings improve durability and environmental resistance on sensitive infrared materials such as germanium and chalcogenide substrates.
