protective coatings for optics - deposition sciences, inc

LOCKHEED MARTIN PROPRIETARY INFORMATION / EXPORT CONTROLLED 1

Protective Coatings for Optics

Eric Kurman Chief Technical Officer

Santa Rosa, California

October 10, 2018

Outline

• Protective coatings in general and for optics in particular

• Application-specific requirements for optics

• Testing methods and Challenges

– The nature of challenges to optical surfaces

– “Always test to the requirement”

• Selected Cases

– Nanoindentation of sputtered multilayers at DSI

– “Diamondlike carbon” (DLC) on Ge and Si infrared optics

– Rain erosion coatings especially on ZnS / ZnSe optics

– Oleophobic / Hydrophobic surface layers

• Conclusions

“God created the bulk; the surface was invented by the devil.”

– Wolfgang Pauli

Protective Coatings Most Widespread

• The broadest and oldest field of application for coatings – not simply optical coatings – is in surface protection and surface modification.

• Ranging from historical methods such as paints, varnishes, case hardening, rust inhibition, all the way up to modern techniques such as nitriding, chemical strengthening of glass, and ion implantation.

• Durability will depend on… – Specific application (above all)

– Hardness of substrate

– Tribology / Lubricity

– Chemical Environment

– Hardness of coating materials

Factors Involved in Durability

Special Consideration for Optics

• For optical coatings… • The protective layer (or layers) must fulfill an

optical design function. • Must function within the design (multilayer or single

layer) • Optical thickness must be within design tolerances • Optical properties (n and k) must fall within

tolerance limits

Or… • The protective layer must be optically inactive.

• This means it must be so thin that it has little to no optical effect.

• This limits the type and degree of protection the layer can offer.

Bulk Protection of Optics

• An additional protective scheme exists for the optics themselves: modification of the outer layer of the optic.

• Chemically strengthened glass • Ion-exchange layers creating surface compressive stress

• Ion implantation

• Toughened surface layers • ZnS/ZnSe “laminates” or “clad” optics

• These schemes can be effective but they leave off protection for the coating – the outermost layers.

Bulk Durability Methods

http://www.hoyaoptics.com/img/dcg-1/ion_exchange.jpg

Ion Implantation

https://www.idonus.com/images/ion/ECR_ion_implantation.jpg

• The earliest “antireflection” coating was “glass bloom”. This is a surface corrosion layer that is rather fragile.

• Which optics need surface protection? Most optical materials are rather robust.

• Mechanically fragile materials, e.g. many infrared materials • Materials undergoing unusual service conditions, e.g.,

marine environment, aircraft, exposure to mud or sand

• Mechanisms of surface (or bulk!) damage could include:

• Chemical: corrosion, undesired deposition, residue • Mechanical: wear, abrasion, impact, pressure, rupture • Biological: corrosion, fungus, biofilm formation

Protection of Optics

Test Methods for

Protective Coatings

• Traditional tests for hardness and durability of optical coatings were developed in the 1950s and 1960s.

• MIL-C-675C, MIL-F-48497, MIL-F-48616 • Have been obsoleted and reinstated multiple times

(see DOD ASSIST for history of these and others)

• Abrasion methods • “Moderate abrasion” using cheesecloth rub

• This test is done using a “crockmeter” that is adapted from fabric durability testing

• “Crocking” is the transfer of dye between two samples of fabric • “Severe abrasion” using pencil eraser

• Pencil eraser compound is a natural latex rubber formulated with adhesive particles.

• Typically feldspar (Bon-Ami™) is dispersed in the rubber.

Traditional Optical Coating Hardness Tests

Taber Model 418 Crockmeter

• Cheesecloth pad moves in linear strokes across sample surface • Can be used to test “moderate abrasion” for optical coatings • Available from multiple manufacturers

(https://www.taberindustries.com/crockmeter)

Manual MIL Specification Test Methods

https://www.optical-cement.com/cements/lens_testing/military1.jpeg

Typical test kit containing: eraser rub tester cheesecloth rub tester tape

• Origin of these tests were in a time when coatings were much less durable than nowadays.

• They are not very stringent and “designed to be passed”.

• Traditional tests will usually only reveal a gross failure.

• Improper or missed cleaning; poor substrate preparation • Poor base pressure in the coating process; air or water leak

in deposition chamber • Incorrect deposition recipe or equipment / process fault

• Absence of a failure gives no indication of how close the system might be to a failure condition.

Why Traditional Durability Tests Are Not So Good

• Testing to failure allows one to assess and compare from batch to batch and over the history of a product line.

• Samples that show either decreased or increased durability uniquely identify process or material conditions that can be investigated.

• Testing can assess number of cycles to failure; or, degree of failure induced by a certain level of challenge.

• Retained samples demonstrate that the test was performed.

• In traditional testing, “passed” samples may be indistinguishable from “untested” samples.

• Drawback: these methods are destructive and thus biased toward use on a witness rather than on a part.

Why Overstress Testing Is Valuable

• Historically, hardness assessment methods have been based on indentation, mass loss, or appearance.

• Indentation methods (Rockwell, Brinell, Knoop) sample a section of surface thickness much greater than the thickness of optical coatings.

• Micro-indentation and nano-indentation methods have more recently come into use to confine the sample volume to the coating layer.

• These benefit from actuators and transducers developed for applications such as Atomic Force Microscopy

Methods to Assess Hardness

Indentation Techniques

Vickers Hardness Test

136° Square Pyramidal Indenter

Knoop Hardness Test

Rhombic Pyramidal Indenter – makes elongated indentation

Brinell Hardness Test

5mm or 10mm Carbide Ball Indenter

Rockwell Hardness Test

Diamond Cone or Steel Ball Indenter

complex test that covers a wide range of hardness

30 separate Rockwell Scales

By Vickers-path.svg: Original uploader was User A1 at en.wikipedia(Original

text : User A1 (talk))derivative work: Nerdture (talk) - Vickers-path.svg, CC

BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6531452

Vickers Geometry

Micro- and Nano-Indentation Testing

https://www.nanoscience.com/wp-content/uploads/2018/06/Nanoindenation-Graphic.png

Measure or estimate material properties for a very shallow surface layer

• There is no such thing as a generic surface protection layer.

• Each surface protection scheme must be matched to its unique application needs.

• Testing may be expensive and quite time-consuming.

• Each candidate protection system must be tested against the requirements of the application

Always Design and Test to the Requirements

• Tests must replicate the challenges seen by an optic including known service conditions.

• Soils and contaminants

• Silica

• Biologic / Organic matter

• Greases, oils, lubricants

• Polishing compounds and abrasives

• Cleaning residues (often, mineral or solute from cleaning fluids such as water or IPA)

• Cleaning solutions, techniques, and materials • Corrosion or abrasion challenges

Defining Test Requirements

Selected Cases

• Nano-indentation testing of sputtered multilayers at DSI

• Single-layer and multilayer “hard carbon” (DLC) on Ge or Si: particulate challenge, rain erosion challenge

• Systems to protect ZnS and ZnSe: rain erosion / rain impact challenge

• Hydrophobic / oleophobic treatments on glass: soiling and corrosion challenge

Selected Cases

• Nano-indentation testing was performed at DSI to characterize uncoated and coated substrates.

• In no case are coatings harder than the substrates they cover.

• Some coatings have lower hardness than the substrate.

Nano-Indentation Testing at DSI

Depth Effect on Uncoated Substrates

sapphire has slight variation with depth; silica does not

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

indentation depth [μm]

sapphire fused silica

Nano-Indentation of Coated Sapphire

one coating has equivalent hardness to sapphire while the other is softer

0 0.05 0.1 0.15 0.2 0.25 0.3

sapphire sapphire with AR 1 sapphire with AR 2

Nano-Indentation of Coated Silica

silica is fairly robust and coatings neither enhance nor degrade hardness

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

fused silica fused silica with DSI AR borofloat with DSI AR

• Both Ge and Si are excellent infrared substrates. • Low absorption (in selected bands); mechanically durable;

readily fabricated with a variety of techniques; good (Ge) to excellent (Si) price/availability.

• High refractive index of both materials requires AR treatment for adequate transmittance

• DLC has a fruitful history with both Si and Ge • Large variety of suitable deposition methods • Some methods are “conformal” and readily adapted to

figured or unusually-shaped optics • Refractive index of ~2 gives excellent (Ge) and reasonable (Si)

performance when deposited as a single-layer AR.

Diamondlike “Hard” Carbon (DLC) in IR

• Single-layer and multilayer DLC structures pass the “wiper test” or “Grittington test” originally defined by UK MOD TS 1888

• Typically 5000 rotations of a wiper blade segment in contact with the substrate and a sand/water slurry.

DLC Passes Severe Challenges

https://www.jenoptik.com/-

/media/websiteimages/optics/optics/4x3/coating-dlc-

wipertest.jpg

Silicon substrate

without DLC with DLC

• Many workers have become interested in hard and durable coating materials and attempted to use them for rain erosion protection.

• Generally these attempts have been unsuccessful.

• Considering rain erosion gives good insight into mechanical surface protection in general.

Rain Erosion / Rain Impact

• Typical standard conditions for rain erosion testing

• 1 mm drop diameter

• Rainfall rate is 1 inch per hour (moderate to severe weather).

• Airspeed through the rain field 450 knots (231 m/sec).

• When an individual rain drop strikes the optic surface, it sets up a series of stress waves:

• compression followed by tension

• most damage occurs during the “tensile rebound”

Physics of Rain Erosion

Coad, E.J. et al

Proc. Roy. Soc. A (1998) 454, 213-218

• The compressive stress wave following impact of a 1 mm rain drop extends some 30-50 µm into the substrate, leading to two main consequences:

• The depth of the compressive stress wave is much thicker than any typical optical coating. The coating is “along for the ride” of substrate excursion.

• Rebound of the compressed substrate material creates a tensile stress wave. The optical material is typically strong in compression but weak in tension.

Physics of Rain Erosion (2)

Coad, E.J. et al

Proc. Roy. Soc. A (1998) 454, 213-218

speed 300 m/s; jet diameter 0.8 mm

• Standard rain erosion materials and coatings are compliant, elastomeric substances that “spread” the impact over a broad area.

• Urethane and neoprene resins

• Surface protection as a coating or as an applique

• Used on: • Rotary-wing aircraft leading edges of rotors

• Canopy coatings for high-speed fixed-wing aircraft

• Rotors for wind power turbines

Rain Erosion Materials

• Prevalence of touch screen devices (smartphones, tablets, kiosks) creates a need for materials that will…

• Resist corrosion by materials deposited from skin contact

• Resist soiling by skin oils, perspiration, cosmetics, etc.

• Allow easy cleaning of common soils

• Most commonly silicones and fluorocarbons.

• Applied by spin/spray or, vapor deposition.

Hydrophobic / Oleophobic Treatments

• Work with material and technology providers to craft an approach specific to the application.

• Use existing methods whenever possible. It is likely they grew out of a long and expensive development effort.

• Create a testing program that matches the challenges the parts will see in service.

• Use overstress testing to evaluate the robustness of your approach.

Conclusions

Thank you

Eric Kurman

Deposition Sciences, Inc.

3300 Coffey Lane

Santa Rosa, CA 95403

Deposition Science, Inc.

www.depsci.com 707-573-6700

protective coatings for optics - deposition sciences, inc

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