| name | thermal-spray |
| description | Thermal spray coatings — HVOF, plasma spray, cold spray, wire arc, flame spray; coating properties (porosity, bond strength, hardness); applications, substrate prep, quality control. |
| metadata | {"priority":7,"promptSignals":{"phrases":["thermal spray","HVOF coating","plasma spray","cold spray","wire arc spray","thermal spray coating"],"minScore":3}} |
Thermal Spray Coatings — Complete Skill
Process Overview
Thermal spray: feedstock (powder, wire, rod) heated → propelled at substrate → forms coating by impact/splat/solidification
Coating formation: flattened splats layer on layer; lamellar structure; inter-splat oxide stringers
Process Types
HVOF (High Velocity Oxy-Fuel)
Gas: propylene, propane, hydrogen, or HPPS fuel + oxygen → combustion chamber → supersonic nozzle
Particle velocity: 400–800 m/s; temperature: 2500–3000°C (brief; particle cools in flight)
Properties:
Dense coatings: porosity 0.5–2%
High bond strength: > 70 MPa (WC-Co)
Low oxide content: < 1% (fast deposition → minimal oxidation in flight)
Compressive residual stress → good fatigue performance
Best for: WC-Co (wear + erosion), WC-CrC-Ni, CrC-NiCr, MCrAlY bond coats
HVAF (Air instead of O₂): lower temp → less decarburization of WC; slightly less dense
Plasma Spray (APS — Atmospheric Plasma Spray)
Gas: Ar + H₂ (or Ar + He) plasma torch; T = 10,000–15,000 K
Particle velocity: 100–400 m/s
Properties:
Porosity: 5–15% (higher than HVOF; intentional for TBC)
Bond strength: 20–50 MPa (lower than HVOF)
Oxide stringers: higher content due to oxidizing atmosphere
Best for: ceramic coatings: 8YSZ TBC, Al₂O₃, Cr₂O₃, MCrAlY, bioceramic HA (hydroxyapatite)
TBC: 8YSZ at 100–300 μm; engineered porosity 15–25% for low thermal conductivity
VPS/LPPS (Vacuum): plasma spray in reduced pressure; cleaner; used for MCrAlY bond coats in aerospace
Cold Spray
Gas: N₂ or He; pressure 1.5–5 MPa; temperature 300–1000°C (below melting point)
Particle velocity: 600–1200 m/s (kinetic energy → plastic deformation at impact)
Deposition mechanism: adiabatic shear instability at interface → bonding without melting
Properties:
Very dense: porosity < 1%
Compressive residual stress (tensile in other spray processes)
No oxidation, no phase change, no thermal distortion
Bond strength: > 50 MPa (Cu, Al, Ti, Ni)
Limitations: cannot spray brittle ceramics (requires plastic deformation); high capital cost (He gas)
Best for: Repair and restoration: Cu electrical contacts, Al structural repair, Ti aerospace
Dimensional restoration of worn parts; additive for near-net-shape components
Wire Arc Spray
Process: two wires form arc; compressed air atomizes and propels droplets
Inexpensive: no expensive gases; high deposition rate
Properties:
Porosity: 5–15%; lower bond strength; high oxide content
Suitable for: structural steel protection (Zn, Al, ZnAl); building cladding; boiler tubes
Flame Spray
Process: fuel-oxygen flame; powder or wire feedstock; lowest velocity (50–150 m/s)
Properties:
Porosity: 10–20%; lowest cost; lowest performance
Used for: dimensional restoration; sacrificial coatings; OEM replacement coatings
Key Properties
| Process | Porosity | Bond strength [MPa] | Oxide [%] | Residual stress |
|---|
| Cold spray | < 0.5% | > 50 | < 0.1 | Compressive |
| HVOF | 0.5–2% | 50–80 | < 1 | Compressive |
| APS (ceramic) | 5–15% | 20–50 | 5–15 | Tensile |
| Wire arc | 5–15% | 15–30 | 5–20 | Tensile |
| Flame | 10–20% | 7–20 | 10–25 | Tensile |
Substrate Preparation
Critical: surface cleanliness and roughness determine bond strength
Grit blast: 20–40 mesh alumina or steel grit to Ra = 4–8 μm (anchor profile)
Cleanliness: degrease; no contamination; blast within 4 hrs before spray
Bond coat: MCrAlY (for TBC) or Ni-Al (general) applied first for better adhesion
Quality Control
Porosity: image analysis of metallographic cross-sections (ASTM E2109)
Bond strength: tensile adhesion test (ASTM C633); pull bar epoxied to coating; tensile load to failure
Hardness: Vickers microhardness on cross-section; compare to specification
Thickness: eddy current gauge (conductive substrate); micrometer on pulled-off coating
Phase analysis: XRD to confirm no decarburization (WC-Co) or phase change
Common Applications
| Application | Coating | Process |
|---|
| Turbine blade TBC | 8YSZ | APS or EB-PVD |
| Turbine blade bond coat | MCrAlY | VPS or HVOF |
| Pump impeller (erosion) | WC-Co | HVOF |
| Roll (oil/paper industry) | Cr₂O₃ | APS |
| Landing gear (hard chrome replacement) | WC-CrC-Ni | HVOF |
| Steel bridge (corrosion) | Zn or Al | Wire arc |
| Orthopedic (bioactive) | HA | APS |
| Electrical contacts | Cu | Cold spray |
Output
Provide: process selection (HVOF/APS/cold spray/wire arc) with justification, coating material, thickness [μm], expected porosity [%], bond strength [MPa], hardness [HV], substrate prep specification, QC testing requirements.
