Physical Properties of Optical Thin Film Coatings

Overview

The physical properties of thin-film filters encompass two primary aspects: dimensional accuracy and ruggedness. While dimensional tolerances are straightforward mechanical specifications, the filter’s robustness against handling and environmental factors is more complex. This section focuses on the methods and specifications for assessing and ensuring the durability of thin-film coatings.

Ruggedness Assessment

The ruggedness of a filter is commonly evaluated through a combination of performance specifications and testing protocols. This involves subjecting the filter to controlled tests designed to simulate conditions it may encounter during use. Performance is then assessed based on the filter’s ability to withstand these tests, which helps avoid the ambiguity of setting up general performance specifications.

Key Differences in Testing

  • Optical Tests: Typically non-destructive (e.g., transmission, reflection, and spectral analysis), except in cases such as laser damage threshold testing.
  • Ruggedness Tests: Predominantly destructive, aiming to measure the extent or onset of damage under simulated stress conditions.

Due to the destructive nature of ruggedness tests, they are generally not conducted on every filter being supplied to a customer. Instead, a system of batch testing and type testing is employed.

Testing Approaches

  1. Batch Testing:
    • A representative sample (typically one or two filters) is selected from a production batch for rigorous testing.
    • If the sampled filters pass, the entire batch is deemed acceptable.
    • This method is standard across industries, not limited to thin-film coatings.
  2. Type Testing:
    • Conducted when large-scale production involves filters of identical type and characteristics.
    • Samples from multiple batches undergo extensive and severe tests to ensure reliability under extreme conditions.
    • Once a type passes this comprehensive testing, subsequent production batches are subjected to less intensive quality checks.
    • Critical Caveat: Any modification to the production process after a successful type test invalidates the test results, necessitating a repeat of the type testing.

Importance of Type Testing

Type testing is vital in certifying the long-term reliability of a filter design. It provides confidence that all filters of the same type, produced under identical conditions, will meet the required performance standards in real-world applications. This allows for streamlined production testing, reducing time and costs while maintaining quality.

By establishing clear and rigorous testing protocols, manufacturers can ensure that filters not only meet optical performance specifications but are also durable enough to handle real-world challenges. This process safeguards both the functionality and reliability of thin-film coatings.

1. Abrasion Resistance

Importance of Abrasion Resistance

Thin-film coatings on exposed optical surfaces, such as antireflection coatings on lenses, often require cleaning. Cleaning typically involves rubbing with a cloth or tissue, which may also include abrasive particles like dust or grit. This process can cause abrasion, making it essential to maximize the abrasion resistance of such coatings.

Challenges in Measuring Abrasion Resistance

Abrasion resistance is challenging to define in absolute terms. Instead, controlled tests simulate real-world abrasion under more severe conditions, providing a practical guide to the coating’s durability. The performance in such tests serves as an indicator of how the coating will perform in practice.

Testing Standards and Methods

1. Sira Institute’s Standard Test:

  • A standardized rubber pad loaded with emery powder is used.
  • The pad, under a precise load (e.g., 5 lb/in²), is drawn across the coating surface a specific number of times (typically 20 strokes).
  • This test, primarily for single-layer magnesium fluoride coatings, found that sufficiently rugged coatings show no damage under these conditions.

2. U.S. Military Specifications:

  • MIL-E-12397B: Specifies a pumice-loaded eraser for abrasion testing.
  • MIL-C-675C: Originally for magnesium fluoride coatings but often applied more broadly to multilayer coatings.
  • Variations in pad composition and test setups can lead to inconsistent results, requiring new standards for different coating types.

Improved Testing by Holland and van Dam

Holland and van Dam developed a test to quantify abrasion resistance through measurable damage:

– Setup:

  • A reciprocating arm holds a Sira-type abrasive pad (0.25-inch diameter) loaded with 5.5 lb.
  • The test sample is placed on a rotating table, which completes one rotation for every three pad strokes.
  • The pad creates a series of spirals over a 1.25-inch diameter area.

– Procedure:

  • The test continues until the central area of the coating is completely removed, while the edges remain intact.
  • The abrasion resistance is quantified using the formula:
    \[
    w = \left(\frac{d^2}{D^2}\right) \times 100\%
    \]
    where \(d\) is the diameter of the fully abraded area, and \(D\) is the total abraded area.

– Findings:

  • For magnesium fluoride coatings, typical values for \(w\) range between 2 and 5, depending on deposition conditions.
  • Optimal results were achieved when:
    – Substrate temperature: 300°C during evaporation.
    – Glow discharge cleaning: 10 minutes before coating.
  • Reductions in abrasion resistance were observed with lower temperatures (e.g., 260°C) or shorter cleaning durations (e.g., 5 minutes).
  • Post-deposition treatments like burnishing with Selvyt cloth or baking at 400°C improved abrasion resistance.
  • A critical angle of vapor incidence (~40° for thicknesses >300 nm) was identified, beyond which resistance drops rapidly.

Practical Application and Limitations

Although Holland and van Dam’s test provides detailed insights, it has not been widely adopted in industry specifications. However, its potential as a quality control tool is significant, enabling early detection of issues during manufacturing. Identifying reductions in coating quality before they fall below standard test thresholds allows for timely corrective actions, reducing waste and improving production efficiency.

This test is particularly beneficial for multilayer coatings, which are inherently more susceptible to abrasion than single-layer coatings, emphasizing its utility in modern optical manufacturing.

2. Adhesion

Adhesion Testing Overview

Adhesion testing evaluates how well a thin-film coating adheres to its substrate. Several methods exist, ranging from simple binary tests to more quantitative approaches. This section discusses the widely used tape test and alternative quantitative techniques, highlighting their applications and limitations.

The Tape Test

  1. Procedure:
    • A piece of adhesive tape is pressed onto the coating and then pulled off.
    • The result indicates whether the adhesion of the film to the substrate exceeds that of the tape to the film.
    • This go–no-go (binary) method is quick but requires consistent application for reliable results.
  2. Key Considerations:
    • Tape Properties:
      • Use tape with a consistent peel adhesion rating.
      • Peel adhesion is measured by sticking the tape to a clean surface (e.g., metal), then pulling it off at a steady angle, normal to the surface.
      • The force per unit tape width (e.g., grams per inch) represents the rating.
    • Test Consistency:
      • Use fresh tape for every test.
      • Apply uniform pressure when sticking the tape to the coating.
      • Remove the tape steadily, pulling at a right angle to the surface—never snatch it off.
      • Use tape of the same thickness, as thicker tape may produce less severe results despite having the same peel adhesion rating.
  3. Common Tape Test Rating:
    • A common rating is 1200 g/in width, and it can be verified with a spring balance.
  4. Limitations:
    • The test primarily identifies areas with virtually nonexistent adhesion.
    • The adhesive properties of the tape differ significantly from those of the film-to-substrate bond:
      • Tape adhesive forces are low but have a long range due to adhesive stretch.
      • Film-to-substrate forces are high but act over a short range.

Quantitative Adhesion Tests

  1. Direct-Pull Test:
    • A cylindrical pin is adhered to the coating, and the force required to pull it off is measured.
    • The adhesive force is calculated by dividing the measured force by the pin’s contact area.
    • Advantages:
      • Simple and straightforward.
    • Limitations:
      • Thin-film adhesive forces have a short range, making it difficult to avoid progressive fracturing of the bonds during the test.
      • Results typically underestimate the true adhesion strength, which is beneficial for quality assessment.
  2. Scratch Test:
    • A stylus is loaded and drawn across the coating with gradually increasing force.
    • The coating is inspected under a microscope after each stroke for signs of damage.
    • The load at which the coating is completely removed serves as the adhesion measure.
    • Goldstein and DeLong Technique:
      • Uses a microhardness tester as a scratch tester.

Summary

While the tape test provides a quick and practical method to assess adhesion, it is primarily qualitative and may not fully represent the true adhesion strength. Quantitative methods, like the direct-pull test and scratch test, offer more detailed insights but require specialized equipment and techniques. Each method has strengths and limitations, and their choice depends on the specific application and testing requirements.

3. Environmental Resistance

Importance of Environmental Resistance

The resistance of thin-film coatings to environmental conditions is a critical factor in their performance and longevity. While humidity resistance is universally important, other factors such as temperature, vibration, shock, and exposure to corrosive substances like saltwater may also need consideration, depending on the application.

Types of Specifications

  1. Operational Specifications:
    • The coating must perform adequately during environmental testing.
  2. Derangement Specifications:
    • The coating need not function perfectly during testing but must recover fully without permanent damage after the test.
    • Commonly used for interference filters, these specifications allow normal performance to be assessed outside the environmental test chamber.

It is essential for users to understand whether a coating’s specification is operational or derangement-based, as this affects expectations during testing.

Resistance to Atmospheric Moisture

Atmospheric moisture is one of the most destructive environmental factors for thin-film coatings. For most standard applications, the following test is sufficient:

  • Test Conditions:
    • Humidity: 98% ± 2% relative humidity.
    • Temperature: 50°C ± 2°C.
    • Duration: 24 hours.
  • Outcome:
    • Coatings may soften but should not detach or degrade permanently.
    • This test is often performed before adhesion or abrasion-resistance tests, which follow immediately.

Comprehensive Environmental Tests

Government standards and specifications outline rigorous environmental tests for equipment and components. Some prominent standards include:

  1. United Kingdom:
    • DEF133 and DTD1085 (aircraft equipment).
  2. United States:
    • MIL-C-675, MIL-C-14806, MIL-C-48497, and MIL-M-13508.

These specifications address the most extreme environmental conditions encountered in tropical and polar climates. Tests may include:

  • Humidity and Temperature Cycling:
    • Often over 28 days to simulate prolonged exposure.
  • Dust and Sand Exposure:
    • Tests simulate dust storms, but well-sealed instruments typically mitigate these risks.
    • External surfaces of exposed elements remain vulnerable.
  • Salt, Fog, and Water:
    • These are particularly challenging for thin-film coatings and are considered severe tests.
  • Fungus Attack:
    • While fungus does not typically harm coatings, it can damage certain glass substrates, indirectly affecting the coating’s performance.
  • Vibration and Shock:
    • Simulates mechanical stresses during operation or transport.

Application-Specific Considerations

Coatings may not always meet the most extreme tests outlined in these specifications. However, concessions are often granted if:

  • The coatings are used inside sealed instruments.
  • The environmental exposure is limited to conditions less severe than the specification standards.

Summary

Environmental resistance, particularly against humidity, is a vital attribute for thin-film coatings. While comprehensive government specifications provide benchmarks for extreme conditions, real-world requirements often necessitate balancing durability, application constraints, and cost. Proper understanding of operational versus derangement specifications and the environmental challenges likely to be encountered ensures optimal performance and longevity of thin-film coatings.

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