Amorphous Alloy Thermal Spray Coating: Advanced Processing Techniques And Industrial Applications

Fundamental Principles Of Amorphous Alloy Thermal Spray Coating Formation

The formation of amorphous alloy thermal spray coating hinges on achieving critical cooling rates that suppress crystallization during solidification. Unlike conventional thermal spray coatings that exhibit polycrystalline structures with inherent defects, amorphous coatings maintain a disordered atomic arrangement throughout their thickness3. This non-equilibrium state is achieved when molten alloy particles are cooled at rates exceeding 10⁵–10⁶ K/s, preventing atomic diffusion and long-range ordering9.

The glass-forming ability (GFA) of an alloy determines its suitability for thermal spray applications. High-GFA alloys—such as Fe-based (Fe₄₈Cr₁₅Mo₁₄Y₂C₁₅B₆), Zr-based, and Ni-based compositions—can tolerate slower cooling rates while retaining amorphous structure, making them compatible with industrial thermal spray equipment14. The critical casting thickness for bulk metallic glasses typically ranges from 1–20 mm depending on composition, but thermal spray processes impose additional constraints due to substrate heat extraction and particle size effects3.

Particle Size Optimization And Cooling Rate Control

Particle size optimization (PSO) is essential for ensuring complete amorphization during thermal spraying9. Smaller particles (10–100 μm diameter) exhibit higher surface-area-to-volume ratios, facilitating rapid heat dissipation upon impact with the substrate or cooling medium17. When particle diameter exceeds the critical threshold for a given alloy composition, the core region may retain crystalline phases due to insufficient cooling rates, compromising coating performance9.

Research demonstrates that Fe-based amorphous alloy powders with particle sizes of 15–45 μm achieve >90% amorphous phase retention when sprayed using high-velocity oxygen fuel (HVOF) or plasma spray systems with auxiliary cooling15. The cooling rate experienced by individual particles can be expressed as:

Cooling Rate (K/s) = λ × ΔT / (ρ × Cp × d²)

where λ is thermal conductivity of the substrate (80–350 W/m·K for typical metals), ΔT is the temperature difference between molten particle (~1300–1400°C) and substrate (~90–120°C), ρ is particle density, Cp is specific heat capacity, and d is particle diameter11.

Auxiliary Cooling Techniques For Enhanced Amorphization

To overcome the thermal limitations of conventional spray systems, auxiliary cooling methods have been developed7. These include:

• Radial gas injection: Inert gases (Ar, N₂) or reducing atmospheres (H₂-containing mixtures) are injected perpendicular to the spray plume from the outer periphery toward the centerline, creating a cooling envelope around molten particles17. This approach increases cooling rates by 30–50% compared to standard spraying while preventing oxidation7.

• Liquid mist cooling: Fine water or cryogenic liquid droplets are introduced into the spray zone, leveraging evaporative cooling to achieve localized cooling rates exceeding 10⁶ K/s7. This method is particularly effective for high-melting-point alloys (>1500°C) that require extreme quenching.

• Substrate pre-cooling: Maintaining substrate temperatures at 90–120°C using embedded cooling channels or cryogenic backing plates enhances heat extraction from impacting particles, promoting amorphous structure retention in the first 50–100 μm of coating thickness11.

Experimental data from Fe-Cr-based amorphous coatings show that combining radial gas injection (flow rate: 40–60 L/min) with substrate cooling reduces crystalline phase content from 25% to <5%, as confirmed by X-ray diffraction analysis showing a single broad hump at 2θ = 40–50° rather than sharp crystalline peaks712.

Material Compositions And Alloy Design For Thermal Spray Amorphous Coatings

Iron-Based Amorphous Alloys

Iron-based amorphous alloys dominate thermal spray coating applications due to their combination of high saturation magnetization (1.2–1.8 T), excellent corrosion resistance, and cost-effectiveness compared to precious-metal-containing glasses1115. Typical compositions include:

• Fe₄₈Cr₁₅Mo₁₄Y₂C₁₅B₆: This alloy exhibits a critical cooling rate of ~10³ K/s and forms fully amorphous coatings when sprayed with particle sizes <30 μm14. The addition of 2 at% Y enhances glass-forming ability by increasing liquid viscosity and suppressing nucleation kinetics. Coatings demonstrate hardness of 950–1100 HV₀.₃ and corrosion current densities <1 μA/cm² in 3.5 wt% NaCl solution14.

• Fe₅₄.₆₁Mo₁₆.₈Cr₂₅.₈C₂.₄₄Si₀.₃₅: Used in cold spray applications for Cu-based composite coatings, this composition provides wear resistance enhancement while maintaining electrical conductivity (>40% IACS)8. The high Mo and Cr content (42.6 at% combined) stabilizes the amorphous phase against crystallization up to 550°C8.

• Fe-Ni-Cr-P-C systems: Alloys containing 5–12 at% Ni, 5–25% Cr, 0.3–5.0% Mo, 8–13% P, and 7–15% C form amorphous flakes (0.5–5 μm thickness, 5–500 μm lateral dimensions) suitable for corrosion-resistant pigments and thermal spray feedstocks17. The phosphorus content is critical for glass formation, as it increases the complexity of the liquid structure and reduces crystal growth rates17.

Nickel-Based And Multi-Component Amorphous Alloys

Nickel-based amorphous alloys offer superior high-temperature stability and oxidation resistance compared to Fe-based systems213. Key compositions include:

• Ni-(Nb,Ta) systems: Alloys with 40–60 at% (Nb+Ta) and balance Ni exhibit glass-forming ability suitable for plasma spray coating in semiconductor chamber applications2. These coatings maintain amorphous structure up to 600°C and show etching rates <0.5 nm/min in Cl₂ plasma environments, outperforming crystalline Al₂O₃ and Y₂O₃ coatings by factors of 5–1013.

• Ni-Cr-P ternary systems: Compositions with 5–40 at% Cr and 15–25% P form amorphous coatings with exceptional corrosion resistance in acidic and alkaline media17. The Cr content provides passivation capability, while P disrupts crystalline ordering17.

Aluminum-Based Mechanically Alloyed Thermal Spray Materials

While not fully amorphous, mechanically alloyed Al-based powders containing transition metals (Mo, Cr, Zr, Ti) exhibit nano-scale mixing that approaches amorphous-like behavior during thermal spraying5. The mechanical alloying process involves high-energy ball milling of pure Al powder (≥98% purity) with 5–15 wt% transition metal powders for 20–50 hours under inert atmosphere5. This creates core-shell structures where transition metals are intimately bonded to Al particle surfaces at the atomic scale5.

Upon thermal spraying (plasma or HVOF), the mechanically alloyed particles undergo further solid-state alloying, producing coatings with:

• Hardness: 180–250 HV₀.₃ (compared to 60–80 HV₀.₃ for pure Al coatings)5
• Corrosion potential: -0.65 to -0.55 V vs. SCE in 3.5% NaCl (150–200 mV nobler than atomized Al-alloy coatings)5
• Wear rate: 2–5 × 10⁻⁶ mm³/N·m under dry sliding conditions (load: 10 N, speed: 0.5 m/s)5

The enhanced properties arise from nano-scale dispersion of intermetallic phases (Al₃Mo, Al₇Cr, Al₃Zr) within the Al matrix, which act as barriers to dislocation motion and corrosive species penetration5.

Thermal Spray Processing Methods For Amorphous Alloy Coatings

High-Velocity Oxygen Fuel (HVOF) Spraying

HVOF spraying is the preferred method for depositing dense, well-adhered amorphous coatings due to its combination of high particle velocities (400–800 m/s) and moderate flame temperatures (2500–3000°C)39. The high kinetic energy of particles upon impact promotes mechanical interlocking and reduces porosity to <1%, while the relatively short dwell time in the flame (1–3 ms) minimizes oxidation and overheating3.

Critical HVOF parameters for amorphous coating formation include:

• Oxygen-to-fuel ratio: 4.0–5.5 for kerosene-based systems; lower ratios produce reducing atmospheres that prevent oxide formation on particle surfaces3
• Spray distance: 250–350 mm; shorter distances increase particle temperature and reduce amorphous retention, while longer distances cause excessive cooling and poor adhesion9
• Powder feed rate: 30–60 g/min; higher rates can overload the flame and reduce particle heating uniformity9
• Carrier gas flow: 8–12 L/min (typically Ar or N₂); optimized to ensure stable powder injection without flame disruption9

Coatings produced via optimized HVOF exhibit bond strengths of 55–75 MPa (ASTM C633 tensile adhesion test) and surface roughness (Ra) of 3–6 μm as-sprayed, suitable for precision bearing and seal applications39.

Plasma Spraying With Controlled Atmosphere

Atmospheric plasma spraying (APS) and controlled-atmosphere plasma spraying (CAPS) enable deposition of amorphous coatings from high-melting-point alloys (>1600°C) that are difficult to process via HVOF27. Plasma torches generate temperatures of 8000–15,000 K, fully melting refractory alloy particles, but require careful control to prevent crystallization during cooling7.

Key innovations for plasma-sprayed amorphous coatings include:

• Shrouded plasma guns: Inert gas shrouds (Ar, He) surrounding the plasma jet reduce oxygen pickup and provide auxiliary cooling, increasing amorphous phase retention from 60–70% (standard APS) to >85% (shrouded APS)27
• Pulsed plasma operation: Modulating arc current at 50–200 Hz creates periodic cooling intervals that enhance quenching rates without reducing overall deposition efficiency7
• Substrate rotation and cooling: Rotating substrates at 60–120 rpm while maintaining backside cooling (water-cooled fixtures) prevents heat accumulation and thermal cycling damage2

Plasma-sprayed Fe-Cr-based amorphous coatings on steel substrates demonstrate corrosion rates of 0.02–0.05 mm/year in 10% H₂SO₄ solution at 60°C, comparable to Hastelloy C-276 and superior to 316L stainless steel (0.15–0.25 mm/year)7.

Cold Spray Technology For Amorphous-Crystalline Composites

Cold spray (CS) is a solid-state deposition process where particles (typically 5–50 μm) are accelerated to 300–1200 m/s using supersonic gas jets (N₂, He) at temperatures below the melting point of the feedstock material8. Upon impact, particles undergo severe plastic deformation and adiabatic shear instability, bonding to the substrate and previously deposited layers through mechanical interlocking and localized metallurgical bonding8.

For amorphous alloy applications, cold spray offers unique advantages:

• No thermal degradation: Since particles remain solid throughout the process, pre-existing amorphous structure in the feedstock powder is preserved without risk of crystallization8
• Compressive residual stress: CS coatings exhibit residual stresses of -200 to -600 MPa, enhancing fatigue resistance and crack closure8
• Low oxidation: Minimal exposure to oxygen during deposition results in oxide content <0.5 wt% in as-sprayed coatings8

Recent work on Cu-based composites reinforced with Fe₅₄.₆₁Mo₁₆.₈Cr₂₅.₈C₂.₄₄Si₀.₃₅ amorphous alloy particles demonstrates:

• Bonding strength: 45–60 MPa (higher than thermal-sprayed Cu coatings at 30–40 MPa)8
• Porosity: <2% (measured via image analysis of polished cross-sections)8
• Wear resistance: 3× improvement over pure Cu coatings under reciprocating sliding (load: 5 N, frequency: 10 Hz, stroke: 5 mm)8
• Electrical conductivity: 42–48% IACS (compared to 58% IACS for pure Cu), acceptable for electrical contact applications8

The composite microstructure consists of ductile Cu matrix with dispersed amorphous alloy particles (5–15 vol%) that act as load-bearing reinforcements and barriers to crack propagation8.

Flame Spraying With Rapid Quenching

Conventional flame spraying (oxy-acetylene or oxy-propane torches) produces relatively low particle velocities (40–150 m/s) and flame temperatures (2500–3200°C), making it challenging to achieve fully amorphous coatings16. However, modified flame spray systems incorporating rapid quenching mechanisms have demonstrated success with certain alloy compositions16.

The process involves:

1. Powder injection: Alloy powder (10–100 μm) is fed into the flame using carrier gas (flow rate: 5–10 L/min)1
2. Melting zone: Particles are heated to 1300–1500°C and fully melted within 10–20 mm of the nozzle exit1
3. Quenching zone: Before particles reach the substrate (spray distance: 150–250 mm), a cooling fluid (inert gas at 100–200 L/min or liquid mist at 0.5–2 L/min) is injected radially from multiple ports surrounding the flame17
4. Deposition: Quenched particles impact the substrate at 140–180°C, solidifying into amorphous splats with minimal reheating from subsequent layers1

This approach has been successfully applied to Fe-Cr magnetic alloys, producing coatings with 75–85% amorphous content and coercivity values of 8–15 A/m, suitable for electromagnetic shielding applications7. The lower equipment cost compared to HVOF or plasma systems makes flame spraying attractive for large-area coating applications where moderate amorphous content is acceptable1.

Microstructural Characteristics And Phase Analysis Of Amorphous Thermal Spray Coatings

X-Ray Diffraction And Amorphous Phase Quantification

X-ray diffraction (XRD) is the primary technique for confirming amorphous structure in thermal spray coatings1214. Fully amorphous materials produce diffraction patterns characterized by:

• Broad diffuse halo: A single broad peak centered at 2θ = 40–50° (for Cu Kα radiation) with full-width-at-half-maximum (FWHM) of 5–10°, indicating short-