Bulk Metallic Glass Thermal Spray Coating: Advanced Manufacturing, Properties, And Industrial Applications

Fundamental Principles And Structural Characteristics Of Bulk Metallic Glass Thermal Spray Coating

Bulk metallic glass thermal spray coating technology exploits the unique thermophysical properties of amorphous metallic alloys to create high-performance surface layers through controlled rapid solidification. The process fundamentally relies on heating metallic glass powder particles to their supercooled liquid state—a metastable temperature region between the glass transition temperature (Tg) and crystallization temperature (Tx)—and subsequently depositing them onto substrate surfaces at high velocities to achieve rapid quenching and amorphous phase retention 125.

The supercooled liquid temperature range ΔTx (defined as Tx - Tg) serves as a critical parameter governing the processability and coating quality of bulk metallic glass thermal spray systems. Research demonstrates that metallic glasses with ΔTx ≥ 30°C exhibit superior thermal spray characteristics, enabling sufficient processing windows for particle heating without triggering crystallization 1257. The glass transition temperature Tg typically remains below 500°C for optimal thermal spray applications, facilitating energy-efficient processing while maintaining substrate integrity 1.

During thermal spray deposition, metallic glass powder particles undergo partial melting with only surface regions reaching the supercooled liquid state while particle cores remain solid, a phenomenon that prevents excessive heat accumulation and subsequent devitrification 14. The particles impact the substrate at velocities exceeding 300 m/s, resulting in rapid flattening and solidification with cooling rates in the range of 10⁴ to 10⁵ K/s during initial contact, followed by secondary cooling at 50-200 K/s 6. This two-stage cooling profile necessitates careful control of coating thickness and deposition rate to prevent thermal mass accumulation that could induce crystallization in previously deposited layers 6.

The resulting thermal spray coatings exhibit distinctive microstructural features:

• Amorphous phase purity: Properly processed coatings maintain fully amorphous structures without detectable crystalline phases, as confirmed by X-ray diffraction analysis 5713
• Porosity control: Advanced processing techniques achieve porosity levels ≤ 2%, with complete elimination of continuous through-thickness pinholes that would compromise barrier properties 1257
• Interfacial bonding: Mechanical interlocking combined with localized metallurgical bonding at the coating-substrate interface provides adhesion strengths suitable for demanding service environments 15
• Compositional flexibility: Bulk metallic glass thermal spray coatings can be formulated from multiple alloy systems including Zr-based, Ni-based, Al-based, Pd-based, Mg-based, Fe-based, Co-based, and Ti-based compositions, each offering distinct property profiles 7917

The amorphous atomic structure inherent to these coatings eliminates grain boundaries, dislocations, and other crystalline defects that typically serve as preferential sites for corrosion initiation and crack propagation in conventional metallic coatings. This structural homogeneity translates directly into enhanced functional performance across multiple property domains 57.

Thermal Spray Processing Methods And Parameter Optimization For Bulk Metallic Glass Coatings

The successful deposition of bulk metallic glass thermal spray coatings requires precise control over multiple interdependent processing parameters to maintain amorphous phase stability while achieving adequate coating density and adhesion. Two primary thermal spray techniques dominate current industrial and research applications: plasma spraying and flame spraying, each offering distinct advantages for specific coating requirements 145.

Plasma Spraying Process Parameters

Plasma spraying utilizes an electric arc to generate high-temperature ionized gas streams (10,000-15,000 K) that melt and accelerate metallic glass powder particles toward the substrate. For bulk metallic glass applications, the process must be carefully controlled to heat only the particle surfaces to the supercooled liquid region while maintaining solid cores 413. Key operational parameters include:

• Powder particle size distribution: Mean particle diameters of 5-100 μm optimize the balance between heating efficiency and cooling rate, with finer particles (30 μm average) enabling more uniform melting and reduced porosity in the final coating 10
• Plasma gas composition and flow rate: Inert atmospheres (argon or nitrogen) prevent oxidation of reactive metallic glass compositions during flight and deposition 917
• Standoff distance: The separation between the plasma gun nozzle and substrate surface critically influences particle temperature and velocity at impact, requiring optimization for each specific metallic glass composition
• Power input: Plasma gun power levels must provide sufficient thermal energy to reach the supercooled liquid state without exceeding the crystallization temperature of the metallic glass

Flame Spraying And High-Velocity Techniques

Flame spraying methods, including high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) processes, offer alternative approaches for bulk metallic glass coating deposition with distinct thermal and kinetic characteristics 411. These techniques generate lower peak temperatures compared to plasma spraying but compensate through higher particle velocities (up to 1000 m/s or greater), promoting enhanced coating density through kinetic compaction 15.

The HVOF process has demonstrated particular effectiveness for depositing metallic glass coatings on thin substrates where thermal damage must be minimized. Research shows that HVOF-deposited coatings can achieve particle velocities exceeding 300 m/s while maintaining substrate temperatures below critical thresholds, enabling coating of resin substrates as thin as 30 μm without perforation or thermal degradation 4.

Substrate Temperature Management

Substrate preheating represents a critical process variable that significantly influences coating adhesion and residual stress development. Thermomechanical analysis (TMA) of metallic glass materials reveals inflection points in linear thermal expansion behavior that define optimal substrate temperature ranges 2. Maintaining substrate temperatures between 100°C and the TMA inflection point (typically below the glass transition temperature) during deposition enhances coating adhesion while preventing substrate damage and excessive thermal stress accumulation 2.

For extremely thin substrates (metal sheets 5-300 μm thick or resin films 30 μm-1 mm thick), specialized thermal management strategies prevent substrate failure during coating deposition. These include:

• Controlled heat input through reduced spray gun power or increased standoff distance 14
• Intermittent deposition with cooling intervals to limit cumulative thermal exposure
• Substrate backing or fixturing to provide mechanical support during thermal cycling
• Post-deposition pressurization treatments in the supercooled liquid region to improve coating density and adhesion without substrate damage 1

Atmospheric Control And Oxidation Prevention

Thermal spraying in controlled atmospheres substantially improves the quality of bulk metallic glass coatings by preventing oxidation of reactive alloying elements during particle flight and deposition 917. Inert gas chambers or localized shrouding with argon or nitrogen atmospheres maintain oxygen levels below critical thresholds, particularly important for aluminum-containing, magnesium-containing, and titanium-containing metallic glass compositions that exhibit high oxidation susceptibility at elevated temperatures.

Compositional Design And Alloy Systems For Bulk Metallic Glass Thermal Spray Coatings

The selection of metallic glass composition fundamentally determines the functional properties, processing characteristics, and application suitability of thermal spray coatings. Modern bulk metallic glass alloy systems span multiple base metal families, each offering distinct advantages for specific engineering requirements 7917.

Zirconium-Based Metallic Glass Systems

Zirconium-based bulk metallic glasses represent one of the most extensively studied alloy families for thermal spray applications, typically formulated with additions of nickel, copper, aluminum, and titanium to achieve large supercooled liquid regions and excellent glass-forming ability. These alloys exhibit ΔTx values frequently exceeding 50°C, providing robust processing windows for thermal spray deposition 57. Zr-based coatings demonstrate exceptional corrosion resistance in acidic and chloride-containing environments, making them particularly suitable for chemical processing equipment and marine applications.

Iron-Based And Cobalt-Based Metallic Glass Compositions

Iron-based and cobalt-based metallic glasses offer cost advantages compared to precious metal-containing systems while delivering excellent magnetic properties and wear resistance 713. These alloys typically incorporate metalloid elements such as boron, phosphorus, and silicon to promote glass formation, along with chromium additions to enhance corrosion resistance. Fe-based metallic glass thermal spray coatings find applications in electromagnetic shielding, soft magnetic components, and wear-resistant surfaces for industrial machinery.

Nickel-Based And Copper-Based Systems

Nickel-based bulk metallic glasses, often alloyed with phosphorus, boron, and silicon, provide excellent thermal stability and oxidation resistance at elevated temperatures 7. These compositions suit high-temperature applications including turbine components and exhaust system parts. Copper-based metallic glasses offer superior thermal and electrical conductivity compared to other amorphous alloy families, enabling specialized applications in thermal management and electrical contact surfaces.

Aluminum-Based And Magnesium-Based Lightweight Alloys

Aluminum-based and magnesium-based metallic glasses deliver exceptional specific strength (strength-to-weight ratio) and corrosion resistance, particularly valuable for aerospace and automotive lightweighting initiatives 7917. However, these reactive metal systems require stringent atmospheric control during thermal spraying to prevent oxidation. Al-Mg alloys processed through plasma spraying with inert gas shielding produce coatings with densities below 3 g/cm³ while maintaining amorphous structure and corrosion resistance superior to conventional aluminum alloys 18.

Multi-Component Alloy Design Strategies

Advanced bulk metallic glass compositions for thermal spray applications increasingly employ multi-component alloying strategies (five or more principal elements) to simultaneously optimize glass-forming ability, mechanical properties, and functional characteristics. The confusion principle—whereby increased compositional complexity suppresses crystallization kinetics—enables the design of alloys with expanded supercooled liquid regions and enhanced thermal stability during spray deposition. Research demonstrates that carefully balanced multi-component systems can achieve ΔTx values exceeding 70°C while maintaining glass transition temperatures compatible with thermal spray processing constraints 57.

Microstructural Characteristics And Quality Control Of Bulk Metallic Glass Thermal Spray Coatings

The microstructural features of bulk metallic glass thermal spray coatings directly govern their functional performance and service reliability. Comprehensive characterization and quality control protocols ensure that deposited coatings meet stringent specifications for industrial applications 1257.

Amorphous Phase Verification And Crystallinity Assessment

X-ray diffraction (XRD) analysis serves as the primary technique for confirming amorphous phase retention in thermal spray coatings, with properly processed bulk metallic glass coatings exhibiting broad diffuse scattering maxima characteristic of non-crystalline atomic arrangements rather than sharp Bragg peaks indicative of crystalline phases 5713. Differential scanning calorimetry (DSC) provides complementary information by measuring the glass transition temperature and crystallization enthalpy, enabling quantitative assessment of amorphous fraction and thermal stability of as-deposited coatings.

Transmission electron microscopy (TEM) offers nanoscale resolution for detecting incipient crystallization or nanocrystalline phases that may form during thermal spray processing, particularly at splat boundaries where cooling rates may locally deviate from optimal values. High-resolution TEM imaging combined with selected area electron diffraction (SAED) can identify crystalline regions as small as 2-5 nm, providing sensitivity beyond conventional XRD techniques 5.

Porosity Characterization And Density Measurement

Porosity represents a critical quality parameter for bulk metallic glass thermal spray coatings, with specifications typically requiring porosity levels ≤ 2% and complete absence of continuous through-thickness pinholes 1257. Multiple complementary techniques assess coating porosity:

• Optical microscopy of polished cross-sections: Quantitative image analysis of metallographically prepared specimens enables statistical measurement of pore area fraction, size distribution, and spatial arrangement
• Helium pycnometry: Gas displacement measurements determine true coating density, which can be compared to theoretical amorphous phase density to calculate total porosity
• Helium leak testing: Pressurized helium permeation tests detect continuous porosity networks, with acceptable coatings exhibiting leak rates below 6×10⁻⁶ standard cubic centimeters per second 16
• Mercury intrusion porosimetry: This technique characterizes pore size distribution and interconnectivity, providing insights into coating permeability and barrier properties

Research demonstrates that optimized thermal spray processing of bulk metallic glass powders can achieve coating porosities as low as 0.5-1.0%, approaching the density of bulk cast metallic glass materials 57. Such low-porosity coatings eliminate preferential corrosion pathways and maximize mechanical load-bearing capacity.

Surface Roughness And Coating Thickness Control

As-sprayed bulk metallic glass coatings typically exhibit surface roughness (Ra) values in the range of 3-10 μm, depending on powder particle size distribution and spray parameters 4. Applications requiring smooth surfaces may employ post-deposition finishing operations including grinding, polishing, or superplastic forming in the supercooled liquid region to achieve Ra values below 1 μm while maintaining amorphous structure 14.

Coating thickness capabilities span a wide range from thin films (10 μm) suitable for electronic applications to thick structural layers (500 μm or greater) for wear and corrosion protection 145. Ultrasonic thickness gauging and eddy current measurements provide non-destructive thickness verification, while metallographic cross-sectioning enables detailed assessment of coating uniformity and interface quality.

Adhesion Strength Evaluation

The bond strength between bulk metallic glass thermal spray coatings and substrates critically determines coating durability under mechanical and thermal loading. Standardized adhesion testing methods include:

• ASTM C633 tensile adhesion test: Coated specimens are bonded to loading fixtures with high-strength epoxy, then pulled in tension until coating failure occurs, with acceptable bulk metallic glass coatings typically exhibiting adhesion strengths exceeding 40 MPa 5
• Scratch testing: A diamond stylus is drawn across the coating surface under progressively increasing load until coating delamination occurs, defining the critical load for adhesion failure
• Indentation testing: Controlled indentation with subsequent examination for coating delamination or cracking provides qualitative assessment of coating-substrate interfacial integrity

Post-deposition pressurization treatments in the supercooled liquid region can substantially enhance adhesion by promoting interfacial diffusion and reducing residual stresses, with reported adhesion strength improvements of 30-50% compared to as-sprayed conditions 1.

Mechanical Properties And Tribological Performance Of Bulk Metallic Glass Thermal Spray Coatings

Bulk metallic glass thermal spray coatings exhibit exceptional mechanical properties that derive from their amorphous atomic structure, including high hardness, elastic strain limits, and wear resistance that frequently surpass conventional crystalline metallic coatings 5713.

Hardness And Elastic Modulus

Nanoindentation testing reveals that bulk metallic glass thermal spray coatings achieve Vickers hardness values typically ranging from 500-1200 HV, depending on alloy composition, with Zr-based and Fe-based systems generally exhibiting hardness in the 600-900 HV range 57. This hardness level exceeds most conventional thermal spray coatings including stainless steel (200-350 HV), nickel-chromium alloys (300-500 HV), and many cermet coatings.

The elastic modulus of bulk metallic glass coatings spans 80-150 GPa for most alloy systems, providing a favorable combination of stiffness and elastic deformation capacity 5. The absence of grain boundaries and crystalline defects enables elastic strain limits of 2-3%, substantially higher than the 0.2-0.5% typical of crystalline metals, allowing bulk metallic glass coatings to accommodate greater mechanical deformation before yielding.

Wear Resistance And Friction Characteristics

Tribological testing under various sliding and abrasive wear conditions demonstrates that bulk metallic glass thermal spray coatings deliver wear resistance superior to conventional metallic coatings and competitive with ceramic thermal spray systems 57. Pin-on-disk testing at contact pressures of 5-10 MPa reveals specific wear rates in the range of 10⁻⁶ to 10⁻⁷ mm³