Electrical Conductivity Evaluation of Cold-Sprayed Copper Parts: Standards, Methods, and Acceptance

Cold spray technology produces dense, low-oxide copper deposits and freestanding parts through supersonic solid-state particle bonding, making electrical conductivity the decisive quality metric for busbars, heat spreaders, and power-electronics components. This article explains how the conductivity of cold-sprayed copper parts is evaluated, covering % IACS benchmarks, governing standards such as ASTM B193, ASTM E1004, and IEC 60468, and the process variables that most strongly influence test results.

Technical Overview

Cold spray, also known as cold gas dynamic spraying (CGDS) or kinetic spraying, is a solid-state deposition process in which fine copper powder (typically 5–45 μm) is accelerated to supersonic velocities of roughly 500–1,000 m/s by a heated nitrogen or helium jet expanding through a de Laval nozzle. Because the process gas temperature remains well below the melting point of copper (approximately 1,085 °C), particles bond through high-strain-rate plastic deformation and adiabatic shear instabilities rather than melting. The result is a dense, low-oxide deposit that is ideally suited to electrical and thermal current paths.

Electrical conductivity is conventionally expressed as a percentage of the International Annealed Copper Standard (% IACS), where 100 % IACS corresponds to a volume resistivity of 0.017241 μΩ·m (conductivity 58.0 MS/m) at 20 °C. Depending on powder quality, propellant gas, and parameter optimization, as-sprayed nitrogen cold-sprayed copper typically achieves 70–90 % IACS, while helium-processed deposits and properly heat-treated parts can reach 95–100 % IACS.

The main microstructural reasons for the conductivity gap relative to wrought annealed copper are residual porosity, inter-particle oxide films that act as electron-scattering barriers, very high dislocation densities produced by severe plastic deformation, grain refinement, and surface roughness effects that disturb probe coupling during non-destructive measurement. A structured evaluation program therefore combines direct electrical measurements with porosity, oxygen, adhesion, and microstructural analysis.

Testing Samples

Flat Deposit Coupons

Rectangular coupons (commonly 50 mm × 50 mm or larger) deposited on grit-blasted aluminum, copper, or steel substrates to a thickness of 1–5 mm represent the baseline sample type. Coupons are characterized both in the as-sprayed and machined or polished conditions so that surface roughness effects can be isolated from true bulk conductivity.

Freestanding Parts

Thick deposits exceeding approximately 3 mm can be removed from the substrate and machined into strips or bars with defined gauge cross-sections for four-terminal resistivity measurement, density determination, and heat-treatment studies, eliminating any influence of the substrate material.

Coated Product Samples

Actual components such as copper-coated aluminum busbars, heat spreaders, or repaired induction coils are tested at defined locations using portable eddy current instrumentation, with conductivity mapping used to document uniformity across the entire deposit area.

Sample Preparation Requirements

• Surfaces for eddy current testing must be degreased and lightly polished; as-sprayed roughness (Ra typically 5–20 μm per ISO 4287) must be recorded because probe lift-off depresses apparent conductivity.
• Specimen temperature must be stabilized at 20 °C or corrected using the temperature coefficient of resistance of copper (approximately 0.0039 per °C).
• Metallographic cross-sections are prepared following ASTM E1920 guidelines to avoid smearing that could mask true porosity.
• Powder lot identity, gas type, stagnation pressure and temperature, standoff distance, and traverse parameters must accompany every sample for full traceability.

Testing Items

A complete conductivity evaluation of cold-sprayed copper parts normally covers the following items:

• Electrical conductivity in % IACS and volume resistivity in μΩ·m or μΩ·cm at 20 °C
• Sheet resistance and four-point-probe resistance mapping for deposit uniformity assessment
• Contact resistance at coated interfaces for busbar and connector applications
• Density and porosity by Archimedes and quantitative image analysis methods
• Oxygen content of powder and deposit by inert gas fusion analysis
• Adhesion and cohesion tensile strength per ASTM C633
• Microhardness per ASTM E384 and tensile properties of freestanding deposits per ASTM E8/E8M
• Thermal conductivity by laser flash diffusivity per ASTM E1461 as a cross-check through the Wiedemann–Franz relation
• Temperature coefficient of resistance for defined service-temperature exposure classes

Commonly specified acceptance criteria for power-electronics and traction applications are at least 85 % IACS in the as-sprayed condition and at least 95 % IACS after the specified stress-relief or annealing treatment, combined with porosity below 1–2 % and adhesion strength of 60 MPa or higher.

Testing Methods

DC Four-Terminal Resistivity Measurement

Direct current resistance is measured with separate current and potential leads in a Kelvin configuration on machined bars of known cross-section, following ASTM B193 and IEC 60468, with GB/T 351 serving as the corresponding Chinese national method. Volume resistivity is calculated from measured resistance and exact dimensions and then converted to % IACS. This is the reference or arbitration method, capable of accuracy better than ±1 %.

Eddy Current Conductivity Measurement

Non-destructive conductivity measurement with an electromagnetic conductivity meter per ASTM E1004 is the standard production control method. Deposit thickness must exceed approximately three to five skin depths—about 1 mm for copper at the common 60 kHz test frequency—and edge effects and probe lift-off must be controlled. Instruments are calibrated on certified conductivity reference blocks bracketing the expected range, for example 50–100 % IACS.

Density and Porosity Characterization

Bulk density is measured by the Archimedes method (ASTM B962) and converted to relative density against 8.96 g/cm³ for fully dense copper. Porosity distribution is quantified on polished cross-sections by image analysis per ASTM E2109; even 1 % porosity measurably depresses conductivity, so pore morphology (isolated versus interconnected) is reported alongside total porosity.

Adhesion and Mechanical Verification

Tensile bond testing per ASTM C633 confirms that the conductive deposit will survive assembly forces and thermal cycling. Well-optimized cold-sprayed copper frequently exceeds the epoxy failure limit of approximately 70 MPa, particularly after post-spray heat treatment.

Microstructure and Oxygen Analysis

Optical microscopy and SEM/EDS examination of cross-sections documents particle bonding quality, interfacial oxide layers, and residual prior-particle boundaries. Oxygen determination by inert gas fusion on both feedstock powder and deposit quantifies oxide pickup; deposits produced from low-oxygen powders (below roughly 200–300 ppm) retain markedly higher conductivity.

Thermal Conductivity Cross-Check

Laser flash diffusivity testing per ASTM E1461 yields thermal conductivity, which through the Wiedemann–Franz relation provides an independent consistency check on the electrical data, since both properties are governed by the same electron-scattering mechanisms.

Post-Deposition Heat Treatment Evaluation

Annealing studies, typically conducted between 200 °C and 600 °C for 0.5 to 4 hours, track conductivity recovery as dislocation density decreases and inter-particle oxides spheroidize. Conductivity gains of 5–15 percentage points IACS are commonly reported, and the selected schedule must be verified not to degrade adhesion strength or substrate properties.

Applications

Conductivity-qualified cold-sprayed copper is used across industries where dense, low-oxide current paths must be added to or restored on existing components:

• Copper metallization and busbar layers on aluminum heatsinks and power-electronics modules for SiC and IGBT packaging
• Electric-vehicle battery interconnects, cooling plates, and cell contactor surfaces
• Combustion-chamber liners and nozzle extensions for liquid rocket engines
• Repair and dimensional restoration of induction coils, commutators, and copper-alloy tooling
• Electromagnetic launcher rails, rail contacts, and high-current sliding contacts
• Waveguide and antenna conductive layers and EMI shielding enclosures
• Copper coatings on steel or titanium substrates to create solderable, corrosion-protected electrical interfaces

Independent verification by accredited third-party laboratories is routinely specified for aerospace, rail, and power-grid deliverables, with test certificates referencing the applicable ASTM, IEC, EN, or GB methods.

FAQ

What conductivity level can cold-sprayed copper actually reach?

As-sprayed nitrogen-processed deposits typically achieve 70–90 % IACS, optimized helium-processed deposits reach 90–95 % IACS or higher, and deposits given an appropriate annealing treatment can approach 95–100 % IACS, effectively matching wrought annealed copper.

Why is as-sprayed copper less conductive than wrought copper?

Residual porosity, thin oxide films at particle interfaces, and the extremely high dislocation density generated by severe plastic deformation all scatter conduction electrons. Post-spray heat treatment removes much of this deficit by promoting recovery and recrystallization.

Which method is more accurate: eddy current or four-terminal DC measurement?

The four-terminal DC method per ASTM B193 and IEC 60468 is the arbitration technique with uncertainty below about ±1 %. Eddy current testing per ASTM E1004 is faster and non-destructive, making it ideal for production screening and mapping, provided thickness and lift-off conditions are controlled.

What minimum deposit thickness is required for reliable eddy current readings?

The deposit should be at least three to five skin depths thick, which corresponds to roughly 1 mm for copper at the standard 60 kHz test frequency. Thinner deposits require higher-frequency probes or destructive four-point-probe measurement.

Does post-spray heat treatment always improve conductivity?

Recovery-annealing almost always raises conductivity, often by 5–15 percentage points IACS, but the schedule must be validated to confirm that adhesion strength, dimensional stability, and substrate temper are not adversely affected.

How does powder oxygen content influence the test results?

Oxygen forms resistive oxide barriers at particle boundaries. Selecting low-oxygen powder, storing it under inert conditions, and using nitrogen or helium propellant gas keeps oxide pickup low and preserves high deposit conductivity.

How frequently should conductivity be verified in production?

Conductivity should be verified for every powder lot and every significant process parameter change, on witness coupons from each production batch, with periodic audits and lot-release testing performed by accredited third-party laboratories for safety-critical applications.

Can cold-sprayed copper replace electroplated copper in electrical applications?

Yes. For thick current paths, cold spray offers higher deposition rates, lower oxide content, compressive residual stresses, and stronger adhesion than plating, while achieving comparable conductivity of 95 % IACS or more after heat treatment.