protective coatings for optics - deposition sciences, inc
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LOCKHEED MARTIN PROPRIETARY INFORMATION / EXPORT CONTROLLED 1
Protective Coatings for Optics
Eric Kurman Chief Technical Officer
Santa Rosa, California
October 10, 2018
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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
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“God created the bulk; the surface was invented by the devil.”
– Wolfgang Pauli
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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.
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• Durability will depend on… – Specific application (above all)
– Hardness of substrate
– Tribology / Lubricity
– Chemical Environment
– Hardness of coating materials
Factors Involved in Durability
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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.
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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.
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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
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• 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
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Test Methods for
Protective Coatings
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• 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
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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)
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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
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• 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
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• 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
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• 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
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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
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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
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• 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
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• 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
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Selected Cases
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• 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
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• 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
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Depth Effect on Uncoated Substrates
sapphire has slight variation with depth; silica does not
0
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0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Vic
kers
har
dn
ess
[VH
N]
indentation depth [μm]
sapphire fused silica
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Nano-Indentation of Coated Sapphire
one coating has equivalent hardness to sapphire while the other is softer
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0 0.05 0.1 0.15 0.2 0.25 0.3
Vic
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[VH
N]
indentation depth [μm]
sapphire sapphire with AR 1 sapphire with AR 2
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Nano-Indentation of Coated Silica
silica is fairly robust and coatings neither enhance nor degrade hardness
0
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0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Vic
kers
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dn
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[VH
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indentation depth [μm]
fused silica fused silica with DSI AR borofloat with DSI AR
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• 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
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• 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
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• 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
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• 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
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• 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
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• 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
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• 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
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• 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
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Thank you
Eric Kurman
Deposition Sciences, Inc.
3300 Coffey Lane
Santa Rosa, CA 95403
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Deposition Science, Inc.
www.depsci.com 707-573-6700