Thermal Spray Coatings for Aerospace: Processes, Specs and Callouts
Thermal spray builds a coating from molten or softened particles driven onto a prepared surface. Aerospace work is governed by process specifications such as AMS 2437 for plasma spray, AMS 2447 for HVOF, AMS 2448 for tungsten carbide on ultrahigh strength steel and MIL-STD-3021 for cold spray, with bond strength tested per ASTM C633 and processors usually Nadcap accredited.
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The processes and where each fits
The process is chosen by the coating material and the property you need, mainly density, bond strength, heat input to the part and thickness. Particle velocity and temperature are the two levers. Faster particles give denser, better bonded coatings, and hotter particles melt ceramics that slower, cooler processes cannot.
| Process | How it works | Typical aerospace use | Governing document |
|---|---|---|---|
| Air plasma spray (APS) | Powder injected into a plasma jet, melts and impacts the part | Ceramic thermal barrier top coats, abradables, oxide and metallic coatings | AMS 2437 (revision E, December 2024) |
| Low pressure or vacuum plasma | Plasma spray in a reduced pressure chamber to limit oxidation | MCrAlY bond coats under thermal barrier coatings | Engine maker and program specs |
| HVOF | Combustion of fuel and oxygen accelerates powder to high velocity at moderate temperature | WC-Co and WC-CoCr wear coatings replacing hard chrome, carbide coatings on landing gear and actuators | AMS 2447 (revision E, July 2026); AMS 2448 for tungsten carbide on steels of 220 ksi and above |
| Detonation gun | Pulsed combustion drives powder in bursts | Dense carbide and oxide wear coatings | Processor and engine maker specs |
| Flame and electric arc wire | Combustion flame or an arc melts wire or powder feedstock | Corrosion coatings, dimensional restoration, lower cost metallic coatings | Processor and program specs |
| Cold spray | Preheated, pressurized gas expands through a nozzle and accelerates solid powder (5 to 100 µm) to bond without melting | Repair and dimensional restoration, corrosion and wear protection, including magnesium components | MIL-STD-3021 (Change 2, March 2015) |
SAE's own summary of AMS 2447 notes HVOF is typically chosen for lower porosity and higher adhesive or cohesive strength than typical plasma spray can reach. AMS 2447 does not cover duplex coatings, meaning a coating sprayed over another coating, so a bond coat plus top coat system needs its own callout.
Wear coatings and the move away from hard chrome
HVOF tungsten carbide has replaced electrolytic hard chrome on many military landing gear, actuator and engine parts. The driver was hexavalent chromium regulation from OSHA and EPA, and much of the qualification work was done by the Hard Chrome Alternatives Team (HCAT), a group of the military services, engine and airframe makers and industry, with Naval Research Laboratory reports published through DTIC.
- Coatings evaluated. The main candidates were HVOF WC/17Co (83 percent tungsten carbide, 17 percent cobalt) and WC/10Co4Cr (tungsten carbide with 10 percent cobalt and 4 percent chromium).
- Fatigue as the gate. The question for each component was whether the HVOF coated part matched or beat the chrome plated part in fatigue, wear and corrosion testing, as the NRL landing gear validation report set out to show.
- Engine results. In SERDP and ESTCP project WP-200023, seven TF33 engine components normally hard chromed were coated with HVOF WC/17Co and run in an accelerated mission test. Post test inspection found performance better than would have been expected for hard chrome, and the project reports the Navy implemented HVOF on the TF34 engine.
WC-Co is not a drop-in swap. The coating is sprayed oversize and ground, and AMS 2448 notes that its procedures apply only to the properties of the as-deposited coating, so grinding needs its own controls. Ask the processor how grinding heat is controlled and how the finished surface is inspected for cracks. Chrome carbide in a nickel chrome matrix is the usual carbide choice where wear happens at higher temperature than WC-Co is suited for; check the temperature limit on the powder maker's data sheet for your application. If the alternative being weighed is a plated coating, compare against the plating and cadmium alternatives page.
Have a part like this?Send it in under a minute. A person replies within 1 business day.Thermal barrier coatings
A thermal barrier coating (TBC) is a two layer system: a metallic bond coat that resists oxidation and anchors the ceramic, and a low conductivity ceramic top coat. It lets hot section parts in turbines run hotter than the metal alone could tolerate.
The ceramic is almost always zirconia stabilized with about 7 weight percent yttria. A 2017 NASA Glenn technical memorandum (NASA/TM-2017-219452) traces that composition to a 1978 NASA study of 6 to 8 weight percent yttria stabilized zirconia plasma sprayed coatings, and notes that 7YSZ was at one point used in every new aircraft and ground power turbine engine produced. Its toughness comes from a metastable tetragonal phase that switches orientation under stress.
Bond coats are typically MCrAlY alloys, where M is nickel, cobalt or both, applied by low pressure plasma spray, or diffusion aluminides. NASA Glenn cyclic tests of plasma sprayed zirconia top coats over low pressure plasma sprayed MCrAlY on single crystal superalloy, cycling at 1,163 °C, saw top coat lives ranging from tens to hundreds of cycles depending on composition and spray parameters. That spread is the reason TBC processes are locked down by engine makers and changed only with requalification.
Top coats are applied by air plasma spray, which gives a layered splat structure, or by electron beam physical vapor deposition, which gives a columnar structure. The two are not interchangeable without requalification. Substrate guidance is on the nickel superalloys page.
Abradable seals
An abradable is a deliberately soft, porous coating on a static shroud that the rotating blade tips cut into, so the engine runs with tighter tip clearance without the blades wearing. Abradability, erosion resistance and temperature capability pull against each other, so the material is matched to the stage and the blade alloy.
- Aluminum silicon with polyester is a long used compressor abradable. The polyester is a fugitive phase that is burned out or left to create porosity, and patent literature notes these polymer systems have limited temperature capability.
- Nickel graphite is used where some self lubrication is wanted, with porosity set by the spray parameters, and requires a bond coat.
- Higher temperature stages move to metallic abradables with solid lubricant fillers and, in the turbine, porous ceramics.
An abradable that sprays too hard wears the blade tips, and one that sprays too soft erodes. Acceptance criteria come from the engine maker and are measured on coupons sprayed with the parts. Use the engine maker's specification number and the powder product name on the PO, since generic callouts do not exist for most abradables.
Cold spray for repair and restoration
Cold spray builds metal deposits without melting the powder, so there is no oxidation, phase change or grain growth in the deposit and little heat goes into the part. That makes it attractive for restoring worn or corroded aluminum and magnesium parts that cannot tolerate welding or plating.
MIL-STD-3021, Materials Deposition, Cold Spray, was developed under Army Research Laboratory leadership and received the Defense Standardization Program Award in 2008. The Army reported that Sikorsky and the UH-60 program office approved cold spray as a repair for a magnesium component, after roughly six years of qualification work. The standard sets process controls so manual or automated operations are repeatable, and for structural or safety critical applications it requires the engineering authority to provide a qualification plan per its Appendix A, specified in the contract or purchase order.
Read that last point carefully when buying. A processor can be capable of cold spray and still not be qualified for your part. If the repair is on a structural or flight critical component, the qualification plan has to come from the design authority before any parts are sprayed.
Bond strength and lab evaluation
Bond strength is tested per ASTM C633, which pulls a coated fixture in tension normal to the coating. The result is either adhesive failure at the substrate or cohesive failure within the coating, and the report should say which.
- The coating is sprayed on the face of a fixture, bonded to a loading fixture with an adhesive, and pulled. The adhesive must be stronger than the coating or the test only measures the glue.
- The 2013 edition, reapproved 2021, says coatings must be thicker than 0.015 in (0.38 mm) because adhesive can penetrate thinner coatings and invalidate the result. The active edition is C633-24, so confirm the current wording.
- ASTM states the method is for quality control, acceptance, process development and comparing similar coatings, not a design value. Residual stress makes real bond strength depend on part shape.
Bond strength sits alongside hardness and metallography for porosity, oxide content, unmelted particles and interface contamination. Nadcap treats this lab work as its own checklist, AC7109/5, and accreditation scopes note it is required for coatings audits unless the supplier uses a Nadcap approved AC7109/5 lab. Specimens should be sprayed in the same run as the parts, with the same fixturing, gun distance and grit blast. Coating thickness on finished parts is often checked mechanically or by eddy current; methods are on the nondestructive testing page.
Nadcap coatings accreditation
Thermal spray is audited under the Nadcap coatings checklists, not chemical processing. The basic coatings checklist is AC7109 (revision G for audits on or after 13 February 2022) and thermal spray is AC7109/1 (revision F for audits on or after 3 May 2020). Vapor deposition is AC7109/2 and coating laboratory evaluation is AC7109/5.
Read the processor's scope of accreditation, not just the certificate. Published scopes list each process separately, for example HVOF, high velocity air fuel, plasma, detonation gun and electric arc wire, and add items such as mechanical thickness measurement, sealing, stripping and residual stress. A shop accredited for plasma spray is not automatically accredited for HVOF. Primes also keep their own approved processor lists, and a Nadcap scope does not replace that approval. The same advice applies to heat treat, which is covered on the heat treatment page.
What to call out on the drawing or PO
- The process specification and revision (AMS 2437, AMS 2447, AMS 2448, MIL-STD-3021 or the engine or program spec), plus the coating material by powder specification or qualified product.
- Bond coat and top coat as separate callouts with separate thickness ranges, since AMS 2447 excludes duplex coatings.
- As-sprayed and finished thickness, the grinding or finishing specification and final surface finish, and which surfaces are masked.
- Grit blast media and any shot peening the design requires before coating.
- Test coupons: bond strength per ASTM C633 with failure mode reported, hardness, and metallography for porosity and oxides, run with each lot.
- Nadcap AC7109/1 accreditation for the specific process, and any prime approval required.
- For cold spray on structural parts, reference the design authority's qualification plan.
Related coatings: dry film lubricants for lighter sliding loads, spacecraft thermal control coatings, and the coatings hub. Common finish callouts are on the surface finishing page.
Get thermal spray sourced with the right approvals
If you need HVOF carbide, plasma spray, a thermal barrier system or a cold spray repair, send the drawing and coating callout. We source the work from processors whose accreditation scope covers your process, flow down the specification, coupon testing and grinding requirements, check bond strength and metallography results before parts ship, and a person replies within one business day. Machining, peening, spray and grind can go on one purchase order. We sign a mutual NDA before files, and controlled drawings follow the process on controlled programs.
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Questions
What is the difference between HVOF and plasma spray?
Plasma spray melts powder in a very hot plasma jet, which suits ceramics such as thermal barrier top coats. HVOF uses combustion to drive powder at much higher velocity and lower temperature, which suits carbides and metals. SAE's summary of AMS 2447 notes HVOF is typically chosen for lower porosity and higher bond strength than typical plasma spray. AMS 2437 governs plasma spray and AMS 2447 governs HVOF.
Can HVOF tungsten carbide replace hard chrome on landing gear?
On many military programs it already has. The Hard Chrome Alternatives Team validated HVOF WC-Co and WC-CoCr coatings against chrome plated parts in fatigue, wear and corrosion testing. AMS 2448 covers tungsten carbide by HVOF on steels of 220 ksi and above. The change still needs design authority approval, a grinding procedure and qualification data for your part.
How is thermal spray bond strength tested?
ASTM C633 is the standard method. A coated fixture is bonded with adhesive to a loading fixture and pulled in tension normal to the coating, and the report gives the strength and whether failure was adhesive or cohesive. The 2013 edition requires coatings thicker than 0.015 in because thin coatings let adhesive penetrate. ASTM says the result is for quality control and comparison, not a design value.
Which Nadcap checklist covers thermal spray?
Thermal spray is audited under AC7109/1, which sits under the basic coatings checklist AC7109. Coating laboratory evaluation is AC7109/5, and vapor deposition is AC7109/2. Published accreditation scopes list each spray process separately, such as HVOF, plasma, detonation gun and electric arc wire, so confirm the processor's scope includes the exact process your part needs.
Is cold spray accepted for structural repairs?
It can be, but only with a qualification plan. MIL-STD-3021 requires the cognizant engineering authority to provide a qualification plan per its Appendix A for structural or critical applications, specified in the contract or PO. The Army reported Sikorsky and the UH-60 program office approved cold spray repair of a magnesium component after about six years of qualification work.
What is a thermal barrier coating made of?
A metallic bond coat, typically an MCrAlY alloy or a diffusion aluminide, under a ceramic top coat of zirconia stabilized with about 7 weight percent yttria. NASA Glenn notes the 7YSZ composition traces to a 1978 NASA study and became the standard across turbine engines. Top coats are applied by air plasma spray or electron beam physical vapor deposition, depending on the part.
Related
Sources
- SAE AMS2447E: Coating, Thermal Spray, High Velocity Oxygen/Fuel Process
- SAE AMS2437E: Coating, Plasma Spray Deposition
- DIN Media: SAE AMS2448D tungsten carbide coatings on ultrahigh strength steels, HVOF
- EverySpec: MIL-STD-3021 Change 2, Materials Deposition, Cold Spray
- U.S. Army: Army wins prestigious awards (cold spray, MIL-STD-3021)
- ASTM C633: adhesion or cohesion strength of thermal spray coatings
- SERDP and ESTCP WP-200023: chromium replacement on gas turbine engine components
- DTIC: NRL validation of HVOF WC/Co as a hard chrome replacement on landing gear
- NASA/TM-2017-219452: thermal barrier coating composition history (NTRS)
- NASA NTRS: cyclic testing of plasma sprayed TBCs over MCrAlY
- Linde AMT: Nadcap coatings scope of accreditation (AC7109, AC7109/1, AC7109/5)
- Justia Patents: thermal spray compositions for abradable seals
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