Thermal Spray Coatings: HVOF, Plasma Spray, and Cold Spray Processes
Thermal spray is a family of surface engineering processes in which feedstock — powder, wire, or rod — is heated and accelerated towards a substrate to build up a coating layer by layer through the successive impact and solidification of individual particles. The resulting lamellar microstructure, and its consequences for porosity, bond strength, residual stress, and functional performance, are determined by particle velocity, temperature, and the chemistry of both feedstock and substrate. This article provides a rigorous examination of the four main thermal spray variants — flame spray, HVOF, atmospheric plasma spray (APS), and cold spray — together with vacuum plasma spray (VPS), covering process physics, process parameters, microstructure-property relationships, quality testing, and industrial applications across aerospace, power generation, oil and gas, and heavy engineering.
Key Takeaways
- Thermal spray encompasses a spectrum from low-velocity flame spray to supersonic cold spray; particle velocity and temperature at impact are the primary variables governing coating microstructure.
- HVOF produces the densest cermet coatings (porosity <2%, bond strength 70–90 MPa) by combining supersonic particle velocity with a sub-melting flame temperature that avoids decarburisation of WC-Co.
- Atmospheric plasma spray (APS) operates at 6,000–15,000 K, enabling deposition of refractory ceramics (ZrO₂, Al₂O₂, Cr₂O₂) that cannot be processed by HVOF or flame spray.
- Cold spray is the only thermal spray process that deposits material entirely in the solid state, producing coatings free of oxidation, phase transformation, and residual tensile stress — uniquely suited for oxygen-sensitive materials and additive repair.
- Thermal barrier coatings (TBCs) for gas turbine hot sections use a duplex architecture: MCrAlY bond coat (HVOF or VPS) + 7YSZ ceramic top coat (APS or EB-PVD), reducing metal temperature by 100–200 °C.
- All thermal spray coatings require grit-blasted substrate preparation to Ra 5–10 µm and are qualified by bond strength (ASTM C633), microhardness (ASTM E384), and porosity image analysis (ASTM E2109).
Fundamentals of Thermal Spray: Particle Dynamics and Splat Formation
Every thermal spray process involves three stages: heating (and in most cases partial or complete melting) of feedstock particles; acceleration towards the substrate through a gas jet or plasma flame; and impact, deformation, and solidification to form a splat. The lamellar coating is built up by successive splat layers, each typically 1–5 µm thick and 20–200 µm in diameter.
Impact and Spreading Dynamics
When a fully molten droplet strikes a solid substrate, spreading is governed by a competition between inertial forces (driving spreading) and surface tension forces (resisting spreading). The dimensionless Reynolds number (Re) and Weber number (We) characterise this competition:
Re = ρ · v · d / μ (inertia vs viscosity)
We = ρ · v² · d / σ (inertia vs surface tension)
Splat diameter D_s ≈ d · Re^0.2 (empirical, for We >> 1)
Where:
ρ = particle density (kg/m³)
v = impact velocity (m/s)
d = particle diameter (m)
μ = dynamic viscosity of molten particle (Pa·s)
σ = surface tension (N/m)At HVOF velocities (600–800 m/s), Re is typically 104–105, driving spreading ratios Ds/d of 3–6. The high velocity also promotes fragmentation and void elimination at the splat-substrate interface, directly explaining HVOF's lower porosity relative to APS where Re is 103–104.
Cooling Rate and Amorphous Phase Formation
Solidification of individual splats is extremely rapid — cooling rates of 106–108 K/s are typical. These rates are high enough to suppress equilibrium solidification in many alloy systems, producing metastable phases, retained austenite, or amorphous regions within individual splats. In cold spray, which deposits in the solid state, the adiabatic shear heating at the particle-substrate interface produces a nanoscale interfacial layer that facilitates bonding without bulk melting.
Flame Spray
The simplest and oldest thermal spray process, flame spray uses an oxy-fuel torch (acetylene/oxygen or propane/oxygen) to melt wire, rod, or powder feedstock in the combustion flame and project particles towards the substrate at 40–100 m/s using a compressed-air atomising jet. Flame temperatures reach 2,700–3,100 °C for acetylene/oxygen. The low particle velocity produces high porosity (10–20%), relatively poor bond strength (15–30 MPa by ASTM C633), and high oxide content in metallic deposits.
Despite these limitations, flame spray remains cost-effective for large-area zinc and aluminium cathodic protection coatings on structural steelwork, and for low-cost wear and corrosion restorative layers on shafts and bores. Wire flame spray in particular offers very high deposition rates (3–10 kg/h) and capital equipment costs one-tenth those of HVOF systems. For understanding of the corrosion mechanisms that drive selection of metallic thermal spray for structural protection, refer to the MetallurgyZone corrosion science section.
High-Velocity Oxy-Fuel (HVOF) Spray
Process Description
HVOF was developed in the 1980s to overcome the porosity and decarburisation limitations of flame spray for hard-metal cermet coatings. Fuel (hydrogen, propylene, propane, or kerosene) is combusted with oxygen at high pressure (0.4–0.8 MPa) in a water-cooled combustion chamber. The hot, high-pressure gas expands through a converging-diverging nozzle, achieving gas exit velocities of 1,500–2,000 m/s at gas temperatures of 2,500–3,100 °C. Powder feedstock injected axially or radially is accelerated to 600–800 m/s.
Critically, the relatively short dwell time and the subsonic-to-supersonic expansion mean that WC and Cr3C2 carbide particles reach the substrate in a semi-molten or solid-state condition — above the metal binder (Co, Ni) melting point but below the temperature at which WC decomposes extensively to W2C or tungsten metal. This distinguishes HVOF from APS, where plasma temperatures of 6,000–15,000 K cause significant WC decarburisation in a single pass.
Key Process Parameters
| Parameter | Typical Range | Effect on Coating |
|---|---|---|
| Fuel/oxygen ratio | 0.9–1.1 (stoichiometric) | Slightly fuel-rich reduces WC decarburisation; excess fuel increases carbon deposit risk |
| Chamber pressure | 0.4–0.8 MPa | Higher pressure increases gas velocity; exceeds supersonic transition at >0.5 MPa |
| Powder feed rate | 20–80 g/min | High feed rate reduces particle temperature; optimum balances deposition rate vs porosity |
| Stand-off distance | 200–380 mm | Short stand-off: hotter particles, higher velocity; too short causes substrate overheating |
| Spray angle | 60°–90° to substrate | Angles <60° increase shadowing porosity and reduce bond strength by up to 30% |
| Substrate temperature | <150 °C (interpass) | Overheating causes decarburisation, residual stress relief, and dimensional distortion |
WC-Co HVOF Coatings: The Industrial Workhorse
WC-12Co and WC-17Co (weight percent cobalt) are the most widely used HVOF feedstocks for sliding wear, erosion, and fretting applications. Hardness values of 1,100–1,400 HV0.3 are routinely achieved, with porosity below 1.5% and compressive residual stress of 50–200 MPa. The compressive stress state — resulting from the peening effect of high-velocity impact — directly improves fatigue performance of coated components, distinguishing HVOF from APS coatings which carry tensile residual stress.
Cr3C2-NiCr for Elevated-Temperature Wear
Chromium carbide (Cr3C2-25NiCr) is the preferred HVOF cermet for applications above 450 °C, where WC-Co undergoes accelerated cobalt binder oxidation and cobalt leaching in corrosive environments. Cr3C2-NiCr coatings maintain hardness of 800–1,100 HV to 870 °C due to the formation of a protective Cr2O3 layer on the NiCr binder. Typical applications include fan blade erosion shields, boiler tube waterwall protection, and exhaust valve faces.
Atmospheric Plasma Spray (APS)
Process Description
APS uses a DC electric arc struck between a thoriated-tungsten cathode and a water-cooled copper anode to ionise a primary gas (argon or nitrogen, 30–80 slm) and secondary gas (hydrogen or helium, 5–20 slm). Plasma jet temperatures of 6,000–15,000 K and gas velocities of 200–600 m/s produce a flame capable of melting any engineering ceramic. Powder feedstock is injected radially into the plasma downstream of the arc, transported in the plasma jet, and deposited at particle velocities of 150–350 m/s.
The high plasma temperature — enabling deposition of alumina (m.p. 2,072 °C), yttria-stabilised zirconia (m.p. 2,715 °C), and chromia (m.p. 2,435 °C) — is the principal advantage of APS. However, the relatively low particle velocity and strong tendency for in-flight oxidation are serious limitations for metallic and cermet deposits. Porosity of APS metallic coatings is typically 4–10%; APS ceramic coatings intentionally target 8–15% porosity to achieve the strain tolerance required for thermal cycling service.
Plasma Gas Selection
Hydrogen secondary gas significantly increases plasma enthalpy (thermal content per unit volume) due to hydrogen's high specific heat, raising achievable particle temperatures by 1,000–2,000 K compared to argon-only plasmas. Helium secondary gas provides high thermal conductivity, producing rapid particle heating, but at substantially higher operating cost. The power input (typically 30–80 kW in laboratory systems, up to 250 kW in high-power industrial systems) and gas composition together determine the plasma enthalpy available for particle melting.
Thermal Barrier Coatings (TBC) Architecture
The most technically demanding application of APS is the thermal barrier coating (TBC) system on gas turbine combustion liners, transition ducts, nozzle guide vanes, and first-stage blades. The duplex TBC architecture consists of:
- Bond coat (100–200 µm): MCrAlY alloy (M = Ni, Co, or NiCo; Cr 15–25 wt%, Al 8–12 wt%, Y 0.3–0.6 wt%) applied by HVOF or VPS to provide oxidation resistance and anchor the ceramic top coat. The bond coat develops a thin, adherent thermally grown oxide (TGO) layer of α-Al2O3 during service.
- 7YSZ top coat (150–300 µm): Yttria-stabilised zirconia (ZrO2 + 6–8 wt% Y2O3) applied by APS (columnar-free lamellar structure) or EB-PVD (columnar structure with superior strain tolerance). The metastable tetragonal t'-ZrO2 phase is preserved by the yttria addition, preventing the monoclinic transformation that would cause catastrophic volume change (ΔV +3.5%) on thermal cycling.
The thermal conductivity of APS 7YSZ is 0.8–1.2 W/m·K, compared to 2.0–2.5 W/m·K for dense ZrO2, because the lamellar splat boundaries and controlled porosity impede cross-coating heat flow. A 250 µm TBC reduces metal surface temperature by 100–200 °C, enabling turbine inlet temperatures above 1,400 °C — beyond the capability of the Ni superalloy substrate even with internal cooling channels. For a review of heat-affected zone microstructure in welded superalloy components that are subsequently coated, refer to the MetallurgyZone HAZ guide.
Vacuum Plasma Spray (VPS)
Vacuum plasma spray (VPS), also called low-pressure plasma spray (LPPS), operates in an evacuated, inert-atmosphere chamber (5–50 kPa Ar). The near-vacuum eliminates in-flight oxidation entirely, producing metallic coatings with oxide content below 0.1 vol% — compared with 1–5 vol% for HVOF and 5–15 vol% for APS. VPS also produces higher particle velocities (400–700 m/s) and lower porosity (0.5–2%) than APS. The process is the preferred method for MCrAlY bond coats on the most critical turbine components where TGO composition and thickness must be tightly controlled.
VPS capital and operating costs are substantially higher than APS or HVOF (inert atmosphere chamber, pump-down cycle, complex part fixturing), restricting its use to high-value aerospace components where the performance premium justifies the cost. Throughput is limited by the chamber volume and pump-down time — typically one batch per 2–4 hours for a production chamber.
Cold Spray
Process Principles
Cold spray (kinetic energy deposition, or KED) accelerates solid-state powder particles to supersonic velocities using a converging-diverging de Laval nozzle with preheated high-pressure gas (N2 or He at 1–5 MPa, 300–1,000 °C). Gas exit velocities of 500–1,500 m/s drive particles to 300–1,200 m/s — well above the critical velocity (vc) for each material at which adiabatic shear instability at the particle-substrate interface produces the jet of plastically deformed material that constitutes bonding.
Critical velocity (empirical, Schmidt et al.):
v_c = C1 · √(σ_UTS / ρ) · (1 − T_r / T_m)^0.25 · exp(−T_r / T_m)
Where:
C1 = empirical constant (~667 for metals)
σ_UTS = ultimate tensile strength (MPa)
ρ = density (kg/m³)
T_r = particle temperature (K)
T_m = melting temperature of particle (K)
T_r/T_m = homologous temperature ratio (0–1)Particles impacting below vc rebound elastically; those above vc undergo the adiabatic shear instability, producing bonding with no melting. Porosity below 0.5% and bond strength above 100 MPa (for Cu and Al) are achievable because the mechanical interlocking is augmented by true metallurgical bonding at the deformed interface.
Gas Selection: Helium vs. Nitrogen
Helium has a much lower molecular weight (4 g/mol) than nitrogen (28 g/mol), producing significantly higher gas exit velocity at the same temperature and pressure. For materials with high critical velocity (Ti, Ni, steel), helium enables deposition that is marginal or impossible with nitrogen. However, helium cost is 50–100 times that of nitrogen, making helium cold spray economically viable only for high-value components (aerospace repair, additive manufacture of reactive metals). Nitrogen cold spray is the industrial standard for copper, aluminium, and stainless steel deposits.
Applications: Repair and Additive Manufacture
Cold spray's solid-state nature makes it uniquely capable for dimensional restoration of high-value components. Turbine blade tip restoration (Ni-based superalloy or MCrAlY deposit), aircraft structural aluminium repair (AA7075 or AA2024 build-up), and magnesium gearbox housing repair are established applications. Unlike welding-based repair, cold spray produces no heat-affected zone, no distortion, and no solidification defects. For context on how hydrogen-induced cracking limits conventional weld repair of high-strength steels — and thus drives adoption of cold spray — refer to the MetallurgyZone HIC guide.
Surface Preparation
No thermal spray coating performs better than its substrate preparation. The standard sequence is:
- Degreasing: Solvent or alkaline cleaning to ISO 8502-6; any residual oil or grease on the grit-blasted surface will prevent bonding.
- Grit blasting: Alumina (Al2O3) or steel grit to ISO 8501-1 Sa 2.5 cleanliness and Ra 5–10 µm roughness. Blasting angle 60–90°, standoff 150–300 mm. For cold spray on aluminium, Ra 3–6 µm is preferred — coarser grit embedment can become stress concentration sites.
- Elapsed time: Spray within 4 hours of blasting to prevent oxide reformation. In high-humidity environments, coat within 2 hours.
- Masking: Remove fixtures and masks before spraying to prevent shadow porosity at mask edges.
Microstructure and Properties of Thermal Spray Coatings
Porosity
Porosity in thermal spray coatings originates from three sources: (1) incompletely filled inter-splat boundaries where surface tension arrested spreading before void closure; (2) gas entrapment during rapid solidification; (3) geometric shadowing by previously deposited surface roughness. Porosity is detrimental to wear coatings (subsurface crack initiation at pore walls), corrosion-barrier coatings (through-thickness permeability), and thermal conductivity-sensitive coatings (unless intentionally used to reduce k, as in TBCs).
Quantification is by image analysis of polished cross-sections per ASTM E2109 (minimum 10 fields, 200× magnification, threshold by grey-level segmentation) or by mercury intrusion porosimetry for closed-pore detection. Pore size distributions in HVOF WC-Co are typically log-normal with mean diameter 1–5 µm; APS ceramics show a bimodal distribution (inter-splat micro-cracks + spherical pores).
Hardness and Cohesive Strength
Vickers microhardness (HV0.3) measured on coating cross-sections per ASTM E384 is the standard production acceptance test. Expected ranges: WC-12Co HVOF 1,100–1,400 HV0.3; Cr3C2-NiCr HVOF 800–1,100 HV0.3; APS Al2O3 800–1,200 HV0.3; APS 7YSZ 450–650 HV0.3; cold spray Cu 80–160 HV0.3. Hardness measured on the top surface of as-sprayed coatings is generally 10–20% lower than cross-section values due to surface porosity and grit embedment effects — always specify measurement location.
Residual Stress
HVOF and cold spray coatings carry compressive residual stress (−50 to −300 MPa) because the high-velocity impact produces a peening effect that exceeds the quench-induced tensile stress from rapid splat solidification. APS coatings carry tensile residual stress (+50 to +200 MPa) because the quench stress dominates at the lower particle velocities. Compressive stress is beneficial for fatigue life; tensile stress promotes through-thickness cracking under thermal cycling, which is one reason APS TBCs fail preferentially at the TGO/bond coat interface rather than within the 7YSZ layer.
Residual stress measurement uses X-ray diffraction (sin2ψ method, ASTM E915) for surface layers or neutron diffraction for through-thickness profiling. The curvature method (Stoney equation applied to thin, thermally-sprayed plate specimens) is a production-friendly alternative for APS coatings:
Stoney equation (modified for coating-substrate):
σ_c = (E_s · t_s²) / (6 · t_c · (1 − ν_s)) · (1/R − 1/R₀)
Where:
σ_c = coating stress (MPa)
E_s = substrate Young's modulus (GPa)
t_s = substrate thickness (mm)
t_c = coating thickness (mm)
ν_s = substrate Poisson's ratio
R = radius of curvature after spraying (mm)
R₀ = initial substrate radius of curvature (mm)Comparison of Thermal Spray Processes
| Property | Flame Spray | HVOF | APS | VPS | Cold Spray |
|---|---|---|---|---|---|
| Particle velocity (m/s) | 40–100 | 600–800 | 150–350 | 400–700 | 300–1,200 |
| Flame/gas temperature (°C) | 2,700–3,100 | 2,500–3,100 | 6,000–15,000 | 6,000–15,000 | 200–1,000 |
| Porosity (%) | 10–20 | 1–4 | 4–15 | 0.5–2 | <1 |
| Bond strength (MPa, ASTM C633) | 15–30 | 70–90 | 30–50 | 50–80 | 80–150+ |
| Oxide content (vol%) | 5–15 | 1–5 | 5–15 | <0.1 | <0.1 |
| Residual stress | Tensile | Compressive | Tensile | Low tensile | Compressive |
| Ceramics capability | Limited (<2,000 °C) | No (oxidation) | Yes (all ceramics) | Yes | No |
| Deposition rate (kg/h) | 3–10 | 2–8 | 2–8 | 1–4 | 1–10 |
| Equipment cost (relative) | Low | Medium-High | High | Very High | High |
| Substrate heat input | Medium | Medium | High | High | Low |
Quality Testing and Inspection
Thermal spray coating qualification and production verification follows a hierarchy of tests specified by the coating procedure specification (CPS) and agreed with the customer or design authority:
Bond Strength — ASTM C633
Dogbone specimens (25.4 mm diameter) are sprayed, glued to matching fixtures with epoxy adhesive (FM1000 or equivalent, E > 70 MPa), and pulled in tension. The reported bond strength is the lower of the coating-to-substrate and cohesive within-coating failure load divided by cross-section area. Results below the adhesive strength of the glue (>70 MPa) are adhesive-limited and reported as "greater than 70 MPa" for HVOF coatings — in such cases, pin-on-disc or four-point bending shear tests provide discrimination.
Microhardness — ASTM E384
Vickers indentations at HV0.1–HV0.3 loads on metallographic cross-sections, minimum 10 indentations, mean and standard deviation reported. High standard deviation (>15% of mean) indicates process instability (variable particle temperature or velocity) and triggers process investigation. For the relationship between hardness and wear rate in WC-Co systems, the inverse Hall-Petch-type relationship between carbide grain size and hardness is relevant — finer WC grain sizes (0.4–1 µm) produce higher hardness and better sliding wear resistance than coarse-grained (3–5 µm) equivalents. For background on hardness testing methods and scale conversions, refer to the MetallurgyZone hardness testing guide.
Thickness — ASTM B499 / E376
Eddy-current gauges (for non-magnetic coatings on ferromagnetic substrates) or magnetic induction gauges (for non-magnetic coatings on non-magnetic substrates using a calibrated external field) provide non-destructive thickness measurement. Accuracy ±5% or ±5 µm (whichever is greater) per ASTM standards. Metallographic cross-section remains the reference method for calibration and dispute resolution.
Industrial Applications
Aerospace and Gas Turbines
TBC systems (APS 7YSZ on VPS or HVOF MCrAlY) are universal on first- and second-stage blades and vanes in modern industrial and aeroengine gas turbines. Compressor blade and blisk erosion protection uses HVOF Cr3C2-NiCr or APS Al2O3-TiO2. Landing gear components (undercarriage cylinders, actuator rods) use HVOF WC-CoCr as the standard hard chrome replacement — a critical environmental application as hexavalent chromium (Cr VI) electroplating is progressively restricted under REACH regulation. Cold spray is qualified for repair of aluminium and magnesium aircraft structural components under FAA-accepted data packages.
Oil and Gas
Gate valves, ball valves, and plug valves in sour service use HVOF WC-Co or WC-CoCr on sealing faces to achieve the combination of hardness (>1,100 HV), low porosity (<1.5%), and corrosion resistance in H2S/CO2 environments required by API 6A and NACE MR0175/ISO 15156. Pump sleeves, impellers, and casing wear rings in produced water service use HVOF WC-Co or APS Cr2O3 (hardness 1,400–2,000 HV) for combined abrasion and corrosion resistance. For engineers working with pitting corrosion in process equipment, the interaction between coating porosity and localised corrosion at pore bases is an important qualification consideration.
Power Generation
Superheater and reheater tube waterwall panels in coal-fired boilers suffer severe erosion-corrosion from fly ash (erosion) and molten salt (hot corrosion above 550 °C). HVOF Cr3C2-NiCr is the industry-standard coating for these surfaces, extending tube life from 3–5 years to 15+ years on replacement panels. Arc spray Zn-Al and HVOF Inconel 625 coatings protect transition pieces and combustion liners in CCGT plant against hot corrosion. For background on the iron-carbon phase diagram and phase stability considerations relevant to ferritic boiler tube alloys, refer to the MetallurgyZone fundamentals section.
Printing, Paper, and Textile
Anilox rolls in flexographic and gravure printing use APS Cr2O3 (hardness 1,500–2,000 HV) to provide cell-engraved surfaces of extreme hardness and chemical resistance to ink solvents. Paper machine rolls, dryer cylinders, and calendering rolls use APS or HVOF coatings for wear resistance and controlled surface release properties. These applications require the grain boundary structure and phase purity of the coating to be controlled, as secondary phases in APS Al2O3 (gamma-Al2O3 alongside alpha-Al2O3) reduce hardness and wear performance.
Environmental and Regulatory Considerations
Hexavalent chromium electroplating (EHC) has been the dominant hard coating for aerospace and hydraulic components for decades, but is subject to phased bans under REACH (EU) and US EPA regulations. HVOF WC-CoCr and WC-Co are the primary qualified replacements, certified under AMS2447 for aerospace applications. Cold spray copper and chromium deposits are also qualified for specific EHC replacement duties. The transition from EHC to HVOF represents one of the largest surface engineering technology shifts of the past two decades, requiring full re-qualification of coating procedures, inspection criteria, and fatigue lives of coated components.
HVOF process emissions include fuel combustion products (CO, NOx) and metallic particulate. Enclosed spray booths with filtered exhaust, nitrogen-purging when spraying reactive metals (Ti, Al), and respiratory protection (HEPA-filtered) are mandatory per OSHA 29 CFR 1910.1000 (TWA limits for Co: 0.02 mg/m³, Cr2O3: 0.5 mg/m³).
Frequently Asked Questions
What is the difference between HVOF and plasma spray?
What is the typical bond strength of HVOF coatings?
Can thermal spray coatings be applied to aluminium substrates?
What is a thermal barrier coating (TBC) and how is it applied?
What surface preparation is required before thermal spraying?
What is porosity in thermal spray coatings and why does it matter?
What is decarburisation in WC-Co HVOF coatings and how is it controlled?
How is cold spray different from other thermal spray processes?
What quality tests are specified for thermal spray coatings?
Recommended Reference Books
Handbook of Thermal Spray Technology — ASM International
The definitive ASM reference on thermal spray processes, materials, process parameters, microstructure, and industrial applications. Essential for process engineers and R&D.
View on AmazonThermal Spray Coatings: Research, Design and Applications — Berndt & Berndt
Comprehensive coverage of coating design, powder characterisation, microstructure analysis, and performance testing for all major thermal spray variants.
View on AmazonSurface Engineering for Corrosion and Wear Resistance — Davis (ASM)
Broad surface engineering reference covering thermal spray, PVD, CVD, electroplating, and diffusion treatments — essential for materials selection decisions across coating technologies.
View on AmazonHigh-Temperature Coatings — Goswami
Focused reference on thermal barrier coatings, MCrAlY bond coats, oxidation-resistant coatings, and hot corrosion mechanisms — the go-to text for turbine coating engineers.
View on AmazonFurther Reading and Related Topics
