thermal-spray

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.