Optical Coating Adhesion Problems: Causes, Testing, and Solutions
Optical coating adhesion problems can lead to delamination, blistering, micro-cracks, coating loss, and long-term reliability issues in aerospace, defense, ADAS, IR, and other high-performance optical systems. This Omega Optical article explains the most common causes of coating adhesion failure, including contamination, poor substrate preparation, film stress, thermal expansion mismatch, and geometry-related defects, along with how adhesion is evaluated through process controls and MIL-spec durability testing.
Adhesion in the context of optical coatings at Omega Optical
longside durability, optical performance, density, scattering, and even electrical properties, by pairing our specific deposition technologies with qualified material sets and proprietary deposition parameters.
We also emphasize scale and repeatability: 66 coating chambers across 5 sites, including specialty, extra-large format capacity for telescope and space applications, and coating on 3-meter diameter substrates in our largest chamber.
Understanding the pairing of the correct substrate material and coating is at the core of avoiding this issue. Our highly experienced coating designers and material experts help our customers avoid the following issues right from the start.
Potential causes of adhesion problems in optics
- Process parameter issues or poor equipment configuration – are root cause categories of adhesion issues.
- Contamination & inadequate substrate preparation – not properly cleaned or outgassed, resulting in trapped solvents, liquids, or air, which creates defects that can weaken the coating-substrate bond.
- Film stress & delamination behavior – Hard coatings have the potential to delaminate rather than deform under residual stress, due to low ductility. Some oxides, nitrides or highly dense thin-film stacks exhibit low fracture strain and high modulus film, which can result in peeling at the edges, blistering or spalling, or micro-cracks that lead to failure during temperature cycling.
- Thermal expansion mismatch between the substrate and coating is a primary driver of residual stress, which often leads to adhesion failure. This is avoided by first understanding the application needs for the correct material/substrate during the coating design phase.
- Geometry/orientation factors (part orientation and “top-down” coating susceptibility are mentioned in a defect formation context)
You may also see catchall references tied to other defect modes such as pinholes, non-uniformity, or stress.
Why coating adhesion is critical for successful optical applications
In our market of aerospace, defense, ADAS, and IR systems, this type of failure is unacceptable, as it may cause optical performance drift, increased scatter, environmental fails, coating loss, or difficult-to-pin system reliability issues. When in the field, air, or space, your ability to rely on a system that uses Omega Optical-coated optics should be 100%.
How Omega talks about testing and verification
Our Metrology & Quality Assurance page is very direct about the qualification frameworks that we offer. Standard offering MIL-Spec durability is shown below, unless the customer states otherwise during the quote phase of the project.
- Surface durability/quality evaluated to MIL-C-48497A
- Environmental humidity durability to MIL-STD-810 Method 507.3
- Temperature/solubility testing to MIL-PRF-13830B
- Documentation/certification available upon request; sampling per MIL-STD-105E when applicable
In plain language, MIL-C-48497 includes a classic tape adhesion requirement (press tape to coated surface and remove quickly; coating should not lift).
Delivering stable precision surfaces and coatings – run after run
PARMS (Plasma-Assisted Reactive Magnetron Sputtering), Ion-Assisted, E-beam, PVD, and PE-CVD technologies may all be used to produce your custom optical coatings, depending on the required specifications and the manufacturing location involved in your project.
Each coating run is carefully controlled and calibrated for optimal transmission, reflection, absorption, scattering, adhesion and quantity of parts per run.
Omega’s dedication and experience focus on process design choices to prevent adhesion failures rather than relying upon patch fixes after the fact.
- PE-CVD with “Coat Up” gas flow addresses common defect drivers via advantageous gas flow and strong prep practices to virtually eliminate pinholes
- Lower process temperature during the PE-CVD process to reduce residual stress risk
- Our proprietary PE-CVD process “virtually eliminates” typical defects and ensures that coatings adhere to multiple MIL durability standards.
- For harder substrate/coating pairings (such as chalcogenides using DLC), we acknowledge these are more challenging and historically plagued by adhesion problems, so we rely on proven processes, commitment to finding the right solution, and being capable of doing what other engineers cannot.
Our supporting capabilities for a successful coating on any material type include:
Having stress, adhesion, or durability issues? Let’s Talk.
