Why porosity forms: bonding mechanics and void origins in cold spray copper

Residual porosity in cold spray copper deposits is a direct consequence of insufficient plastic deformation at particle-particle interfaces during impact. In cold spray, solid copper particles are accelerated to supersonic velocities by a heated propellant gas, and bonding is driven entirely by kinetic energy converted to plastic deformation upon impact — not by melting. As established by Helmut Schmidt University (2009), the thermo-mechanical deformation at impact must exceed a material-specific critical velocity to trigger adiabatic shear instability and metallurgical bonding; below this threshold, particles rebound or adhere weakly, leaving interparticle pores.

ARCI (2019) confirms that plastic deformation-induced adiabatic shear instability — governed by adiabatic temperature rise, interfacial plastic strain, and flow stress collapse — is the governing bonding process in cold spray, fundamentally distinguishing it from thermal spray. This mechanism is the reason cold spray avoids oxidation and phase changes inherent in fusion-based processes, making it a preferred deposition route for copper thermal spreaders.

The spatial distribution of porosity is non-uniform and carries direct implications for heat flow. Research from INSA de Lyon and MATEIS CNRS (2018), using three-dimensional X-ray micro-tomography on cold spray coatings, demonstrated that cold spray deposits produce non-connected, micron-sized pores with a gradient spatial distribution — with near-surface and inter-track zones being most vulnerable. For a copper thermal spreader layer conducting heat away from a high-flux chip, these inter-track void concentrations represent the weakest thermal links in the deposit.

Helmut-Schmidt-University (2021) further confirmed that non-bonded interfaces and work-hardening-induced dislocations accumulate during deposition, reducing ductility and leaving microvoid networks. The work-hardening effect means that even particles that nominally bond can leave a surrounding zone of elevated dislocation density that, under thermal cycling in service, can evolve into crack initiation sites — a critical reliability concern for electronics packaging, as noted by standards bodies including IEEE.

Process parameter optimization: gas conditions, standoff distance, and nozzle trajectory

The most direct lever for reducing porosity in cold spray copper is elevating particle impact velocity and temperature to ensure all impacting particles exceed critical velocity. ENEA Italy (2021) explicitly reports that minimum-porosity copper coatings were achieved at process conditions of 400 °C and 30 bar for Cu powders, with the resulting low-porosity structure directly yielding hardness of 1.5–2 GPa and superior fretting wear resistance. This combination of elevated gas temperature and pressure is the most widely validated route to dense copper deposits and represents the baseline from which other optimizations build.

Nozzle trajectory and scanning strategy represent an equally important but often overlooked process variable. Lyon University (2019) demonstrated that low nozzle traverse speeds produce thick, quasi-triangular track cross-sections where particle impact angles on the track periphery increase substantially, resulting in insufficient deformation and elevated local porosity at the track edges. Increasing traverse speed improved particle deformation conditions across the full track width and measurably increased coating density. For thermal spreader applications — where large-area, uniform copper deposits are needed — this finding implies that optimised raster scan speed and track overlap ratios are as critical as gas pressure and temperature settings.

“Low nozzle traverse speeds create quasi-triangular track profiles with elevated peripheral impact angles — a geometry that concentrates porosity precisely at the inter-track boundaries most critical to lateral heat conduction.”

Standoff distance (SoD) similarly affects particle velocity at the point of impact and thus porosity. Research from Jan Kochanowski University (2021, 2022) — though performed on titanium — establishes the general principle that SoD profoundly affects deposition efficiency and coating microstructure, with an optimal window beyond which particles decelerate in ambient gas before impact. Microhardness peaks at an SoD of 70 mm, reflecting denser deposit microstructure at the optimal stand-off. Both principles apply directly to copper spraying: SoD must be tuned to the specific nozzle and gas combination to avoid premature deceleration. The intersection of these three variables — gas conditions, traverse speed, and SoD — creates a multi-dimensional optimisation space that process engineers must navigate systematically, a task well suited to patent landscape analysis tools such as PatSnap Eureka.

A Korean patent by Cyanos Co. (2024, active) specifically claims that forming a coating layer by the cold spray coating method yields high coating density, low porosity within the coating, and reduced outgassing — directly targeting electronics applications where thermally conductive, defect-minimal copper layers are required for semiconductor device reliability.

Powder feedstock engineering: plasticity, composite additions, and nanostructured variants

The physical and mechanical state of the copper powder feedstock has a decisive influence on achievable deposit density, independent of spray process settings. National Cheng Kung University (2022) compared three copper feedstock types — electrolytic (EP), gas-assisted water atomized (WA), and inert gas atomized (GA) — and found that pre-existing plastic microstrain within particles, measured by EBSD, directly governs bonding quality upon impact. Annealing powder feedstocks at 500 °C for 30 minutes released these microstrains, improving plastic deformation response at impact and consequently coating density. This is a feedstock-level intervention that can supplement process parameter optimisation without requiring capital investment in higher-specification spray equipment.

Composite powder additions exploit the peening effect — also called the hammer or tamping effect — whereby hard secondary particles embedded in or mixed with the copper feedstock compressively deform already-deposited copper splats upon their own impact, collapsing interparticle voids in-situ. Jiangxi University of Science and Technology (2023) identifies the peening mechanism as a key route to denser Cu-composite microstructures in a comprehensive review of copper-based composite coatings. Inner Mongolia Enterprise Key Laboratory (2022) reports that all prepared Cu-Zn-Al composite coatings presented a dense microstructure through this hammer effect, with hardness and coating thickness showing a negative correlation — confirming progressive in-situ densification as secondary particles compact the growing deposit.

For electronics thermal spreaders, Cu-ceramic composite formulations must balance the conductivity penalty of non-metallic additions against the densification benefit. Ceramic secondary particles improve deposit density through peening but reduce bulk thermal conductivity proportionally to their volume fraction — a trade-off that requires careful optimisation. Nanostructured copper feedstocks produced by spray-drying represent a further avenue: Worcester Polytechnic Institute (2020) characterised spray-dried nanostructured copper versus conventionally gas-atomized powder and found that the nanostructured variant produces coatings with distinct microstructural features relevant to densification, including higher degrees of high-strain-rate deformation within deposits. Research published by Nature on nanostructured metal deposition further contextualises how grain boundary density in feedstocks affects plastic flow behaviour at high strain rates.

Post-spray treatments that close residual voids in cold spray copper thermal spreaders

Even under optimised spray conditions, as-sprayed cold spray copper coatings retain some residual porosity at interparticle boundaries, making post-spray treatments essential for thermal spreader applications where any connected void network impedes heat flow. Rolls-Royce@NTU / Nanyang Technological University (2020) provides a comprehensive map of post-treatment options including heat treatment, hot isostatic pressing (HIP), friction stir processing, laser treatment, and shot peening — with thermal treatments specifically targeting non-bonded interparticle interfaces and work-hardened zones.

Thermal annealing is the most widely applied post-spray method. Helmut-Schmidt-University (2021) demonstrates that annealing simultaneously reduces non-bonded interfaces and work-hardening dislocations, causing annealed samples to exhibit higher fracture strain while maintaining strength. NHK Spring (2014) found that annealing at appropriate temperatures improved electrical conductivity and bending strength of cold-sprayed Cu-Al composites, consistent with pore closure and enhanced interparticle bonding — a result directly transferable to copper thermal spreader performance, where electrical and thermal conductivity are closely correlated properties governed by the same interparticle interface quality. Standards for thermal management materials in electronics are tracked by IEEE and by the IEC.

Rapid induction heating offers a localized, time-efficient alternative to furnace annealing. Nanyang Technological University (2022) showed that porosity levels in cold-sprayed coatings reduced significantly after only 10 minutes of induction heating at 800–1100 °C, compared to conventional furnace treatments requiring hours. This is particularly attractive for electronics manufacturing, where thermal spreaders may be attached to heat-sensitive substrates and localized heating prevents collateral damage to surrounding components.

Mechanical post-processing through static compression achieves significant porosity reduction without any thermal input. Poznan University of Technology (2023) demonstrated 90% reduction in porosity after static loading and 86% reduction after dynamic loading of sprayed metallic coatings, accompanied by increased microhardness. The cold pressure sealing (CPS) approach documented by Guizhou Power Grid (2023) similarly reduced coating porosity to 2% through cold-pressing, providing a scalable, heat-free densification route particularly suited to electronics manufacturing environments where thermal budgets are tightly constrained.

Impact Innovations GmbH (2022) demonstrates that through optimised process design, bulk-like ductility approaching that of wrought copper can be achieved even without post-heat treatment — pointing toward next-generation spray equipment capable of intrinsically low-porosity deposition as the ultimate target for thermal spreader applications. This finding, corroborated by WIPO-registered patents on advanced cold spray systems, suggests that the equipment frontier is advancing toward eliminating the post-spray treatment requirement entirely.

Key institutions, active patents, and the convergence toward hybrid strategies

Analysis of the available data reveals a clear cluster of leading institutions and assignees driving innovation in cold spray copper porosity reduction. Lyon University and MATEIS CNRS (France) have established quantitative methods for tracking porosity in cold spray copper deposits through X-ray tomography, providing the measurement infrastructure that underpins all other optimisation work. Nanyang Technological University and Rolls-Royce@NTU (Singapore) lead on post-spray treatment reviews and rapid induction heating, with strong focus on industrial aerospace and electronics repair applications. Helmut-Schmidt-University Hamburg (Germany) provides fundamental mechanistic insight into how porosity connects to mechanical failure and ductility in cold spray copper.

Impact Innovations GmbH (Germany) occupies a distinctive position as an industrial process developer demonstrating bulk-like copper properties in the as-sprayed state — pointing toward next-generation equipment capable of intrinsically low-porosity deposition. Jiangxi University of Science and Technology and Inner Mongolia Enterprise Key Laboratory (China) have produced comprehensive reviews and experimental work on Cu-based composite coatings, including peening mechanisms for in-situ densification. ARCI Hyderabad (India) situates porosity within the broader adiabatic shear instability framework, while National Cheng Kung University (Taiwan) has contributed detailed feedstock plasticity studies connecting powder pre-treatment to improved interparticle bonding. Patent activity from Cyanos Co. (South Korea, 2024) signals active commercial interest in low-porosity cold spray coatings specifically for semiconductor and electronics applications — a sector where WIPO patent filings in thermal management materials have grown substantially over the past decade.

The most significant strategic insight from this body of evidence is the convergence toward hybrid strategies combining optimised spray parameters with targeted post-spray treatment. Neither process optimisation alone nor post-spray treatment alone is sufficient for the sub-1% porosity targets implied by high-flux thermal spreader performance requirements. The data suggests a three-stage approach: (1) maximise as-sprayed density through elevated gas conditions (400 °C, 30 bar), optimised traverse speed, and SoD; (2) use annealed feedstock or composite additions to address residual interparticle voids through peening; and (3) apply rapid induction heating or cold pressing as a final densification step to close any remaining void network. This pipeline approach is consistent with the direction of active patent claims from both academic and industrial assignees tracked in the PatSnap Analytics platform.

“Neither process optimisation alone nor post-spray treatment alone is sufficient for the sub-1% porosity targets implied by high-flux thermal spreader performance requirements — the evidence converges on hybrid, three-stage strategies.”