HTS Coatings — machining & grinding

Nickel Graphite

75% Ni, 25% Graphite

  • Plasma

Nickel graphite — 75% nickel, 25% graphite — is the coating that is supposed to wear. That's the point. Cross-reference on the print: Oerlikon Metco 307NS. It's a plasma-sprayed, engineered-porous composite that lines a gas turbine compressor casing, across from the rotor blade tips, and lets those tips cut a custom-fit groove into it without blade damage. The tighter the blade-to-casing clearance, the more efficient the engine — and nickel-graphite is what makes the tightest clearance safe.

The chemistry does two jobs. The nickel matrix is the skeleton: it holds the deposit together to ~450-480°C and resists corrosion. The graphite is the self-lubricating, easy-release phase — 15-25% by weight — giving the blade tip something to carve through without welding or heating the rotor. Porosity, tuned by APS parameters between 15% and 25%, is the abradability dial.

HTS applies this by atmospheric plasma spray over an 80Ni/20Al or 96NiCr/4Al bond coat. Top-coat thickness is 40-100 mils, drawing-driven. We finish with carbide tools, never grinding — grinding compresses the porosity and destroys the whole reason the coating exists.

This is an abradable, not a wear coating. Service ceiling ~480°C (graphite oxidation limit). If the part needs to resist wear, pick chrome carbide or tungsten carbide. If it needs to be cut INTO by a rotating blade, nickel-graphite is the answer.

Technical data

Bond strength
~1,500-3,000 psi (10-20 MPa) typical for the abradable top layer alone — intentionally lower than a wear coating because the composite is porous and graphite-loaded. The bond-coat interface (NiAl or NiCr-Al underneath) carries the real adhesion load; published 85/15 nickel-graphite systems spec ~12,000 psi minimum average through the bond-coat / top-coat stack.
Max service temp
~840-900°F (450-480°C) practical service ceiling per Oerlikon Metco datasheets and published APS research. Above ~500°C the graphite phase begins to oxidize and the abradable character degrades — specify NiCrAl-bentonite or a hBN-based top coat for hotter compressor stages.
Max service temp
450-480°C service ceiling (graphite oxidation limit). The nickel matrix holds structural integrity well beyond this, but without intact graphite the coating loses its self-lubricating, easy-release behavior and can damage blade tips.
As-sprayed porosity
15-25% typical for compressor-seal abradables; research-grade optimizations have demonstrated deposits as porous as 46-56% (trading erosion resistance for extreme abradability). Porosity is tuned deliberately via APS hydrogen flow, spray distance, and graphite fraction. Graphite burn-off during post-spray heat treatment further increases open porosity.
Typical thickness
40-100 mils (1-2.5 mm) typical for compressor shroud abradable seal service; specific OEM drawings govern. Unlike wear coatings, thickness is driven by allowable rub depth over the life of the engine, not by substrate protection. Coatings are NOT ground — machining is done with carbide tools to avoid compressing the porous structure.

Where it earns its keep

  • Engineered softness — the coating is SUPPOSED to be abraded. The blade tip cuts a custom groove on first rub without damage, giving the engine the tightest possible running clearance.
  • Graphite provides solid-film self-lubrication at the rub interface, minimizing blade-tip heating and preventing welding/smearing events during transient rubs
  • Nickel matrix holds the structure together at moderate compressor temperatures (to ~450-480°C) and resists corrosion better than aluminum-polymer grades
  • Porosity is a feature, not a defect — tunable 15-25% (and higher in research grades) lets HTS dial in abradability vs erosion resistance for a specific engine duty
  • Flight-qualified chemistry with decades of aerospace service history — Metco 307NS and its variants are OEM-approved at every major engine builder
  • Application gain of 1-3% fuel efficiency from tightening compressor clearance is documented across aero and industrial gas turbine fleets

Where it doesn't

  • THIS IS AN ABRADABLE, NOT A WEAR COATING. If the part needs to resist abrasion, erosion, or sliding wear, pick chromium-carbide NiCr, tungsten carbide, or aluminum oxide — not nickel-graphite.
  • Service-temperature ceiling is ~450-480°C — graphite oxidation above this kills the self-lubricating phase. For hotter compressor stages specify NiCrAl-bentonite (to ~500°C) or an hBN-based abradable (to ~600-700°C).
  • DO NOT GRIND. Grinding compresses the porous structure and destroys abradability. Finish with carbide tooling only, per Oerlikon Metco guidance.
  • Requires a NiAl or NiCr-Al bond coat (3-6 mils) first — the nickel-graphite top coat alone has low adhesion and must key into a dense metallic anchor layer.
  • Particle erosion from ingested debris is the dominant in-service degradation mode — balance abradability (softer/more porous) against erosion life (harder/denser) with the OEM
  • Aerospace qualification is almost always OEM-proprietary (PWA, B50, MSRR, Safran spec). Public AMS numbers alone are rarely sufficient for flight hardware — expect drawing-specific process approval.

Typical applications

  • Aerospace gas turbine high-pressure compressor shroud abradable seals (the reference application)
  • Aerospace gas turbine low-pressure and intermediate-pressure compressor casing seals (fan-exit and booster stages)
  • Aircraft engine front start-up box forward case assembly clearance control
  • Industrial gas turbine (land-based power and mechanical drive) compressor clearance-control seals
  • Small turboprop and APU compressor stage abradables
  • Steam turbine and centrifugal compressor labyrinth and shroud seal replacements
  • Repair and overhaul (MRO) re-coating of worn compressor casing segments and ring seals
  • Turbine stationary sealing surfaces where intermittent blade tip rub is a design-intent event

Wear modes addressed

  • Blade-tip rub (design-intent) — the rotor cuts its own clearance into the soft coating without damaging the tip
  • Graphite oxidation above ~500°C (progressive loss of the self-lubricating phase)
  • Particle erosion from ingested debris and scale (the main in-service degradation mode)
  • Thermal cycling and cracking (less severe than on wear coatings — the porous matrix absorbs strain)
  • Hot-gas path corrosion and salt-spray attack (why NiAl bond coat outperforms NiCr-Al)
  • Densification under abusive grinding (closes porosity, kills abradability — a processing failure, not service wear)

Industries

  • Aerospace engine OEMs and MRO (Pratt & Whitney, GE Aviation, Rolls-Royce, Safran, Honeywell)
  • Land-based industrial gas turbine power generation
  • Oil and gas mechanical-drive turbines (pipeline, LNG)
  • Defense propulsion (military turbofans and turboshafts)
  • Rotating equipment repair shops servicing compressors, steam turbines, and centrifugal fans
  • Auxiliary power unit (APU) and small turboprop manufacturing

Substrates

  • Nickel-base superalloy compressor casings and shroud segments (Inconel 718, Waspaloy)
  • Stainless steel shroud rings and casing segments (410, 17-4 PH)
  • Titanium compressor casings on low-pressure stages (Ti-6Al-4V)
  • Low-alloy steel industrial gas turbine casings with NiAl bond coat
  • Cobalt-base superalloy seal hardware in hotter stages (used with caution near the 480°C ceiling)

Sources

Data points on this page draw on the following published references. Nothing here replaces a material-specific review by our process engineers — but it's the working starting point.

Material data on this page is provided as a general reference and can vary by lot, substrate, and application. Contact HTS to confirm the right material and specification for your specific part.

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