| name | thermal-spraying |
| description | Thermal spray coatings — HVOF (high velocity oxygen fuel), plasma spray (APS, VPS), cold spray, flame spray, arc spray; splat formation; coating porosity, hardness, bond strength; feedstock (powder, wire); substrate preparation (grit blast Sa 3); coating thickness and layer count; thermal barrier coatings (TBC: YSZ, MCrAlY bond coat); wear coatings (WC-Co, Cr₂C₃); ASTM C633 bond strength; Scoprega test; residual stress; and aerospace/industrial applications. |
Thermal Spray Coatings — Complete Skill
Thermal Spray Processes Overview
Process Classification
Thermal spray: coating material heated to molten/semi-molten state → propelled toward substrate → splats form → build coating
Classification by heat source and particle velocity:
| Process | Flame T [°C] | Particle v [m/s] | Porosity [%] | Bond Strength [MPa] | Key Application |
|---|
| Flame spray | 3,000 | 50–100 | 10–20 | 10–20 | Low-cost wear; dimensional restoration |
| Arc spray | ~4,000 | 100–200 | 10–15 | 15–25 | Corrosion; structural steel |
| Atmospheric Plasma Spray (APS) | 10,000–15,000 | 200–400 | 3–15 | 20–40 | TBC; ceramics |
| Vacuum Plasma Spray (VPS) | 10,000–15,000 | 400–600 | < 1 | > 50 | Aerospace bond coats |
| HVOF | 2,500–3,000 | 600–1,000 | 0.5–2 | 50–100 | WC-Co; dense metals |
| HVAF | 1,800–2,000 | 800–1,200 | 0.1–1 | > 70 | Metal alloys; cermet |
| Cold spray | 100–900 | 500–1,200 | < 0.5 | > 60 | Cu; Al; Ti; metal repair |
Splat Formation and Coating Microstructure
Particle Impact Mechanics
Weber number:
We = ρ × v² × d / σ [ρ = density; v = velocity; d = particle diameter; σ = surface tension]
We >> 1 → splat spreads flat; disk diameter D_splat = d × √(We/4) (ideal spreading)
Typical: We = 10,000–100,000 → disk-like splat
Splat diameter:
D_splat = d × We^0.25 [empirical; d = particle diameter; D_splat ≈ 3–5× d for metals]
Splat thickness:
h_splat = d / (3 × (D_splat/d)²) × 4/3 [volume conservation assuming disk shape]
Typical: h_splat = 1–5 μm; D_splat = 20–100 μm
Cooling rate after impact:
Q̇ = k_substrate × (T_melt − T_substrate) / h_splat → R_cool = Q̇ / (ρ × cₚ × h_splat × ΔT) ≈ 10⁶–10⁸ K/s
Rapid solidification → non-equilibrium phases; high dislocation density; fine grain
Microstructure Features
Porosity: inter-splat voids + unbonded regions; connected porosity = corrosion path
HVOF: 0.5–2%; APS: 5–15% (intentional in TBC top coat for strain tolerance)
Measured: image analysis (metallographic cross-section); mercury intrusion porosimetry; gas permeability
Oxides: atmospheric oxidation of liquid droplets in flight → oxide stringers in coating
APS in air: 2–10% oxide content; VPS: < 0.5%
Residual stress: quenching stress (tensile, from splat cooling) + peening effect (HVOF compressive)
APS: tensile residual stress (σ ≈ +50 to +200 MPa); HVOF: compressive (σ ≈ −200 to −500 MPa)
Compressive → better fatigue and corrosion resistance
Substrate Preparation
Critical for adhesion — ASTM C633 bond strength depends on substrate prep:
- Cleaning: degrease with acetone/MEK; remove oils, oxides
- Grit blasting: angular Al₂O₃ or chilled iron grit (80–120 grit); pressure 0.4–0.7 MPa
- Target: surface roughness Ra = 4–8 μm; Rz = 30–60 μm (mechanical interlocking anchor)
- ISO 8501-1 cleanliness: Sa 2.5 minimum; Sa 3.0 preferred for aerospace
- Spray within 2 hours of blast (avoid re-oxidation)
Bond strength vs. surface prep:
Poorly prepared (Ra = 1 μm): ASTM C633 bond strength 10–20 MPa
Well-prepared (Ra = 6 μm): 40–70 MPa for HVOF WC-Co
HVOF Process
Operating Principles
HVOF (High Velocity Oxygen Fuel):
Fuel + oxygen → combustion in water-cooled combustion chamber → supersonic nozzle → particle injection
Fuels: kerosene (liquid; JP-5/Jet-A), propylene, H₂, natural gas
Chamber pressure: 0.5–1.0 MPa; particle velocity 600–1,000 m/s
Particle temperature: below most metal melting points for some systems → semi-molten/ductile impact → dense, low-oxide coating
Dwell time in flame: 1–5 ms → limited oxidation compared to APS
HVOF vs. APS for WC-Co:
APS WC-Co: high porosity (5–10%); W₂C decomposition → reduced hardness (600 HV) vs. sintered (1,200 HV)
HVOF WC-Co: porosity 0.5–2%; minimal decomposition; hardness 900–1,100 HV → preferred for wear applications
HVOF Coating Properties (WC-12%Co)
Hardness: 1,000–1,100 HV300 [Vickers; cross-section; 5 measurements; ASTM C1327]
Porosity: 0.5–1.5% [image analysis; ASTM E2109]
Bond strength (ASTM C633): 60–80 MPa [glue-on fixture; tension test]
Wear rate (ASTM G65 dry sand): 2–8 × 10⁻⁶ mm³/(N·m) [orders better than steel]
Coating thickness: 0.15–0.5 mm typical; up to 2 mm for heavy wear
Plasma Spray (APS)
APS Operating Principles
Direct current plasma arc: gas (Ar + H₂ or N₂) ionized between cathode and anode → plasma jet 10,000–15,000 K
Particles injected into plasma → melt → accelerate → 200–400 m/s at substrate
Standoff distance: 80–120 mm typical
Powder feeding: carrier gas (Ar, 2–5 L/min) conveys powder from hopper to gun
Powder size: 15–75 μm for metals; 10–60 μm for ceramics
APS advantages over HVOF: higher temperature → melts ceramics (Al₂O₃, YSZ, TiO₂); wider range of materials
APS disadvantage: higher porosity; more oxides; tensile residual stress
Thermal Barrier Coating (TBC) System
TBC stack (gas turbine blades/vanes):
- Superalloy substrate (Ni-based; René 142, CMSX-4; T_blade ~ 900°C)
- MCrAlY bond coat (80–150 μm; APS or VPS; NiCoCrAlY): oxidation resistance; anchors TBC
- TGO (Thermally Grown Oxide): Al₂O₃ layer grows on bond coat during service (1–10 μm); critical for adhesion
- YSZ top coat (125–250 μm; APS; 6–8 wt% Y₂O₃ stabilized ZrO₂): thermal insulation
YSZ properties:
k_YSZ = 0.8–2.2 W/(m·K) [vs. Ni superalloy k = 12 W/(m·K) → 5–6× insulation]
CTE_YSZ = 10–11 ppm/K [matches Ni at TBC deposition T; mismatch at operating T → cracking risk]
Phase stability: tetragonal t' phase stable to 1,200°C; above → monoclinic transformation (volume change → spallation)
Temperature drop across TBC:
ΔT_TBC = Q_flux × L_TBC / k_YSZ [L_TBC = 250 μm; k ≈ 1.5 W/(m·K); Q_flux typical 1–2 MW/m²]
ΔT = 1,500,000 × 0.00025 / 1.5 = 250°C → blade surface temperature reduced by 250°C
Allows higher turbine inlet temperature → improved η_cycle
TBC failure modes:
TGO thickening → stored elastic energy → delamination at YSZ/TGO interface (TGO > 7 μm = spallation risk)
CMAS (Calcium-Magnesium-Aluminosilicate) attack: molten at > 1,240°C → dissolves YSZ → accelerated spallation
Sintering: APS columnar structure densifies → higher k; lower strain tolerance → cracking
EB-PVD TBC: electron beam physical vapor deposition → columnar structure → superior strain tolerance; lower TGO thickening rate; used for rotating blades; more expensive
Cold Spray
Process and Principles
Cold spray: particles accelerated to supersonic velocity (500–1,200 m/s) by pre-heated gas below melting point
Bonding by adiabatic shear instability at particle-substrate interface: kinetic energy → local plastic deformation → clean metallic bond
No melting → no oxides; no phase changes; compressive residual stress
Critical velocity v_cr: minimum velocity for bonding (particle fails to bond below this)
v_cr = √(4 × σ_y × cₚ × T_m / (ρ × (T_m − T_spray))) + additional empirical correction
Typical v_cr: Al = 620 m/s; Cu = 500 m/s; Ti = 650 m/s; Ni = 550 m/s
Cold spray advantages:
No oxidation (no melting); oxygen-sensitive materials (Ti, Cu, Al); additive manufacturing potential
Repair: restore dimensions without heat damage (thin-wall aerospace structures)
Cold spray limitations:
Hard ceramics cannot be sprayed (no plastic deformation); WC-Co ineffective without process optimization
Porosity can be higher than HVOF for same material if v < v_cr
Wear and Corrosion Coatings
Common Coating Systems
WC-Co (HVOF): tungsten carbide cermet; hardness 900–1,100 HV; excellent abrasion resistance
Replace hard chrome plating in aerospace (hexavalent Cr regulatory elimination); REACH/EPA
Application: landing gear, hydraulic cylinders, pump shafts
Cr₂C₃-NiCr (HVOF/APS): chromium carbide; superior oxidation to 870°C; used for hot-section wear
Hardness 600–900 HV; lower than WC-Co but stable at high T
Al₂O₃-TiO₂ (APS): electrical insulation + wear resistance; dielectric; sliding wear
Hardness 700–900 HV; porosity 3–8%
NiAl / NiCrAlY (HVOF/APS): bond coats; oxidation resistant; MCrAlY for TBC systems
Zn/Al arc spray: corrosion protection for steel bridges, offshore; sacrificial; 100–300 μm
Testing and Characterization
Bond strength: ASTM C633; glue coating to fixture; tension test; failure mode: adhesive (substrate/coating) vs. cohesive (within coating)
Hardness: Vickers HV300 on polished cross-section; minimum 5 readings; ASTM C1327
Porosity: image analysis per ASTM E2109 (metallographic; 10 fields; 500× magnification); or mercury intrusion
Thickness: eddy current (ISO 2360); magnetic (ISO 2178); metallographic cross-section
Residual stress: X-ray diffraction (sin²ψ); incremental hole drilling; beam deflection (curvature method)
Wear testing: ASTM G65 (dry sand/rubber wheel); G99 (pin-on-disk); G132 (abrasion)
Standards and References
| Standard | Scope |
|---|
| ASTM C633 | Bond strength — thermal spray coatings |
| ASTM C1327 | Vickers hardness of advanced ceramics |
| ASTM E2109 | Porosity by image analysis |
| AWS C2.18 | Guide for thermal spray operator qualification |
| AMS 2447 | Thermal spray powder coating — aerospace |
| SAE AMS 7879 | WC-Co HVOF coating specification |
| ISO 14917 | Thermal spraying — terminology |
Output
Provide: component (material: substrate alloy; geometry; surface area [cm²]; dimensional tolerance after coating), service conditions (temperature [°C]; wear mechanism: abrasion/erosion/sliding; corrosion: aqueous/high-T oxidation; contact stress [MPa]), coating system (process: HVOF/APS/cold spray; material: WC-Co/YSZ/MCrAlY/other; layer sequence: bond coat + top coat thicknesses [μm]), feedstock (powder type; size distribution d10/d50/d90 [μm]; manufacturer/grade), process parameters (standoff distance [mm]; gun traverse speed [mm/s]; powder feed rate [g/min]; carrier gas flow [L/min]; expected deposition rate [kg/h]), expected properties (hardness HV [value]; porosity [%]; bond strength [MPa] from ASTM C633; residual stress: tensile/compressive; oxide content [%]), substrate preparation (grit blast: grit type; pressure [MPa]; angle [°]; target Ra [μm]; cleanliness grade Sa), post-spray (grinding/honing to final dimension; heat treatment for bond coat diffusion; sealing with polymer for porosity), quality checks (thickness measurement: eddy current or cross-section; hardness per C1327; porosity per E2109; pull-off test per C633 from lot), and applicable standard (AMS 2447; SAE AMS 7879; AWS C2.18; ISO 14917).
