cold-spraying

name	cold-spraying
description	Cold spray coating — kinetic energy deposition, critical velocity, bonding mechanism, powder materials (Cu/Ti/Al/MCrAlY), coating properties, process parameters, HVAF comparison, SAE AMS2440.
metadata	{"priority":7,"promptSignals":{"phrases":["cold spray","cold spraying","kinetic spray","cold gas dynamic spray","cold spray coating"],"minScore":3}}

Cold Spray Coating — Complete Skill

Process Fundamentals

Cold spray: supersonic particles (300–1200 m/s) impact substrate at solid state (no melting) → bonding through adiabatic shear instability and plastic deformation Temperature: particles < melting point; typically 100–600°C gas temperature (vs. 2000–3500°C for thermal spray) Pressure: 20–70 bar driving gas (N₂ or He)

No oxidation: particles bond without melting → minimal oxide formation Compressive residual stress: particle impact → compressive stress (unlike tensile in APS) No phase transformation: feedstock retained (WC, cermets, reactive metals)

Critical Velocity

Critical particle velocity for bonding: v_crit = √[4σ_f (1 + C_p T_p / (σ_f/ρ))] / √ρ [m/s; simplified]

Empirical model (Assadi et al.): v_crit = -0.25 (T_m - T_i) + 667/(ρ_p) - 14 (σ_UTS/ρ) + 0.08 T_m + 7 c_p

Practical critical velocities:

Material	v_crit [m/s]
Copper	570–600
Aluminum	680–720
Titanium	700–900
Steel	550–650
MCrAlY	750–850
Ni	620–650

Above v_crit: adiabatic shear at particle-substrate interface → intimate bonding Below v_crit: particles bounce off (erosion of substrate); no coating builds

Particle velocity from nozzle: v_p = v_gas × [1 - exp(-π c_D ρ_gas r_nozzle² L / (4 m_p))] [approximate] Or: use isentropic flow + drag equations; typically v_p = 0.85–0.95 × v_gas

Gas velocity (converging-diverging nozzle): At exit: v_gas = M_exit × a_exit [M = exit Mach from nozzle design; a = sound speed at T_exit]

Gas Selection

Nitrogen (N₂): Lower molecular weight (28 vs. He 4) → lower particle velocity P = 20–70 bar; T = 300–800°C Lower cost; most common for Cu, Al, Ni v_gas ≈ 500–700 m/s at nozzle exit

Helium (He): 4× higher sound speed → much higher particle velocity v_gas ≈ 1200–1500 m/s; much higher v_p possible Use for: high-strength alloys requiring high v_crit (Ti, steel, ceramics); expensive (~10× N₂) Often He at 5–15 bar gives same v_p as N₂ at 70 bar

Powder Materials and Properties

Copper (Cu)

Applications: electrical conductivity (additive repair, bus bar, electronics); corrosion barrier Process: N₂ at 30 bar, 500°C; v_p = 600–800 m/s; easily exceeds v_crit Coating properties: Porosity: < 0.5%; electrical conductivity: 85–90% IACS Hardness: 120–150 HV (work-hardened vs. 70–80 HV annealed Cu) Thermal conductivity: 340–380 W/(m·K)

Titanium (Ti, CP Ti)

Applications: aircraft repair, corrosion protection, bone implant coating Process: He or high-P N₂; T = 600–700°C; v_p = 800–1000 m/s Properties: Porosity: 1–3% (N₂ spray); < 0.5% (He spray) Oxygen content: 0.5–0.9% (vs. 0.15% for wrought; some oxidation even without melting) Adhesion: > 50 MPa (cold spray); requires bonding temperature

Aluminum (Al)

Applications: aircraft structure repair, galvanic protection, thermal management Process: N₂ at 25–40 bar, 300–500°C Properties: Porosity: 0.5–2% Electrical conductivity: 30–40 MS/m (vs. 37 MS/m for wrought) Adhesion: 40–70 MPa

MCrAlY (Bond Coat Alternative)

Cold spray MCrAlY: preserves chemistry; no Y₂O₃ oxidation (critical for TBC adhesion) Process: He + high P; T = 700–900°C gas T Properties: Lower oxidation than HVOF MCrAlY → better TBC life Porosity 1–3%; densification by HIP improves properties

Process Parameters

Parameter	Effect
Gas pressure ↑	v_p ↑ → better bonding; less porosity
Gas temperature ↑	v_gas ↑ (sound speed ↑); material softer → lower v_crit
He vs. N₂	He: much higher v_p; much higher cost
Particle size ↓	v_p ↑; too small → poor inertia; optimal 10–45 μm
Stand-off distance	Optimal typically 15–40 mm
Spray angle	Best at 90°; < 45° → porosity increases significantly

Deposition efficiency (DE): DE = mass deposited / mass fed × 100% [typically 50–80% above v_crit; strongly material-dependent]

Comparison to HVAF/HVOF

Feature	Cold spray	HVOF	HVAF
Temperature	Below melting	Near-melting	Very low
Oxidation	Minimal	Moderate	Low
Phase preservation	Excellent	Good	Excellent
Velocity	300–1200 m/s	600–900 m/s	> 900 m/s
Residual stress	Compressive	Compressive	Compressive
Porosity	< 0.5–3%	0.5–2%	0.5–2%
Best for	Reactive metals, thick deposits, repair	WC-Co, MCrAlY	WC-Co

Standards

Standard	Scope
SAE AMS2440	Cold spray metallic coatings
ASTM B532	Porosity measurement for coatings
AMS2449	HVOF (for comparison; cold spray being added)
MIL-STD-1687	Thermal spray coatings (being updated for cold spray)

Output

Provide: powder material and size [μm], driving gas (N₂/He) and pressure [bar], gas temperature [°C], nozzle type (de Laval), estimated particle velocity v_p [m/s] vs. v_crit [m/s], coating thickness [μm] per pass and total, porosity [%], hardness [HV], adhesion strength [MPa], residual stress (compressive), deposition efficiency [%], application (repair/protection/additive), and applicable standard (SAE AMS2440).