Analysis of Functional Surface and Coating Properties

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Laser-induced degradation and damage of optical components is a primary factor limiting the performance and lifetime of high-power optical systems.
The laser stability of optical components is not only limited by the intrinsic absorption properties of the materials involved, but is also strongly influenced by extrinsic factors such as defects and impurities. In addition, the field distribution within the components plays an important role, particularly in multilayer coatings. All these factors lead to a strong wavelength dependence of the laser-induced damage threshold (LIDT) of optical components.
We provide LIDT testing across the full spectrum from the UV to the near-infrared. A distinctive capability is our integration of LIDT measurements with in-situ, highly sensitive angle-resolved light scattering analyses (ARS), allowing defect-specific probing of damage thresholds.

We offer:

• Spectral LIDT tests according to ISO 21254
• Various test methods (S-on-1 test/ R-on-1 test etc.)
• Linked analysis of defects, as well as light scattering and absorption properties

Parameters:

• Wavelengths: 1064 nm, 532 nm (Nd:YAG) and spectrally 200 nm - 1600 nm (OPO)
• Laser parameters: Nanosecond regime (5ns); repetition rate 20 Hz
• On- and offline monitoring of structural and material properties using angle-resolved light scattering

High abrasion resistance and low mechanical stresses are key quality features of thin coatings. Depending on the application, other properties are also relevant, such as scratch resistance or environmental stability at extreme temperatures.
Optimizing the fabrication processes therefore requires comprehensive characterization of these properties.

We offer:
Fraunhofer IOF has established an extensive portfolio for testing mechanical surface properties and durability.
We design and validate dedicated test protocols and offer tailored analysis programs aligned with the specific application. These procedures serve both to characterize our in-house optical coatings in detail and to support our customers’ development processes.

Test Methods and Parameters:
Mechanical Stability

• Chemo-mechanical abrasion test using ABREX® (including DIN EN 60068-2-70/ IEC 68-2-70)
• Abrasion test according to ISO 9211-4-01
• Abrasion test under application-specific conditions
• Adhesive strength of layers (tape test according to ISO)
• Scratch resistance and nanoscratching (1µN - 20N)
• Hardness measurement/ E-modulus determination using nanoindentation
• Stress measurement on individual layers on Si wafers, glasses and thin plastic substrates, also as a function of temperature using TENCOR® FLX2320

Environmental Stability:

• Standard-compliant low temperature and alternating climate test (ISO): (-70°C to +180°C, 10°C to 95°C at 10-98%rh)
• Temperature stability test up to 600°C
• Global radiation test under defined climatic conditions
• Hot air sterilization and autoclaving
• Solubility test

We also offer the analysis of additional properties, such as electrical coating properties (sheet resistance, specific resistance, charge carrier density, charge carrier mobility).

The wetting behavior of optical and technical surfaces often plays a decisive role in their functional performance. Typical examples are self-cleaning superhydrophobic optical surfaces (e.g. architectural glass) and technical surfaces engineered for optimal oleophilic wetting (e.g. highly stressed machine components).

Alongside surface chemistry, the wetting of a surface is essentially determined by its roughness. Achieving the desired functionality therefore requires a tailored surface topography. At Fraunhofer IOF we have developed a method that links roughness to wetting properties through detailed roughness analysis and data evaluation. Central to this approach is the novel wetting parameter κB, which reflects both the vertical and lateral expansion of stochastic roughness structures. Empirical correlations show, for instance, that a value κB > 0.3 - together with suitable surface chemistry – to achieve superhydrophobic behavior. For reference: the lotus leaf - as an example of superhydrophobicity par excellence - has a κB value of 0.7.

The structure-property correlation also showed that meaningful, prcticable wetting characterization is only achieved via dynamic contact angle measurement. We therefore use a modular contact-angle measurement system featuring automatic droplet dosing, sample tilting for roll-off and sliding tests, controlled heating of both surface and test liquid as well as comprehensive measurement data acquisition options for contact angle analysis.

We Offer:

• Surface structure analysis and derivation of the wetting parameter κB for the development and realization of tailored wetting properties
• Determination of hydrophobic, hydrophilic, oleophobic, oleophilic properties, fogging analysis
• Static and dynamic contact angle measurement in the test range between -20 °C and 400 °C
• Analysis of smallest sample surfaces using a nanoliter dosing system