Surface Roughness Testing of Cold-Sprayed Copper Components: Methods, Standards, and Technical Pract

Surface roughness testing of cold-sprayed copper components is a critical quality-control step that quantifies the texture of copper deposits produced by cold gas dynamic spraying, a solid-state additive coating process widely used in aerospace, electronics, and power engineering. This article explains the relevant measurement standards, sampling requirements, profile and areal roughness parameters, stylus and optical measurement techniques, and best practices for obtaining reliable results in accredited third-party laboratories and in-house inspection environments.

Technical Overview

Cold gas dynamic spraying, commonly known as cold spray or kinetic spraying, is a solid-state deposition process in which copper powder particles are accelerated to supersonic velocities, typically 500 to 1,000 m/s, by a heated high-pressure nitrogen or helium jet. Upon impact with the substrate, the particles undergo severe plastic deformation, establish metallurgical and mechanical interlocking bonds, and build up a dense copper deposit without bulk melting. Because the process operates far below the melting point of copper, oxidation and thermal distortion are minimized, but the stacked, un-melted particle splats produce a characteristically irregular as-sprayed surface.

The as-sprayed surface of cold-sprayed copper typically exhibits amplitude parameters an order of magnitude higher than machined surfaces, because the topography is defined by individual deformed particles, partially embedded fragments, and open porosity. Surface roughness measurement therefore serves several engineering purposes: it verifies that spray parameters such as powder size distribution, gas temperature and pressure, standoff distance, and traverse speed are under control; it defines the machining allowance required to reach the final dimensional and finish specification; and it governs functional performance in service, including electrical and thermal contact resistance, adhesive or metallurgical bonding of subsequent layers, sealing behavior, fatigue response, and corrosion initiation sites.

Measurement practice is anchored in international and national standards. The ISO 21920 series defines profile-method surface texture terminology, parameters, and specification operators, superseding and harmonizing the long-established ISO 4287 and ISO 4288. ISO 3274 and ISO 12179 govern the nominal characteristics and calibration of contact stylus instruments, ISO 5436-1 defines material measures used for verification, and the ISO 25178 series covers areal (three-dimensional) measurement methods. In the Chinese national standard system, GB/T 1031, GB/T 3505, GB/T 10610, and GB/T 6062 are the corresponding profile-method documents. Accredited third-party laboratories and in-house metrology departments routinely apply these standards to cold-sprayed copper components.

• As-sprayed cold-sprayed copper (15 to 53 micrometer powder): typical Ra of 6 to 25 micrometers, Rz of 40 to 150 micrometers.
• As-sprayed copper with fine powder (5 to 25 micrometers): typical Ra of 3 to 10 micrometers.
• After turning or milling: typical Ra of 0.8 to 3.2 micrometers.
• After precision grinding or single-point diamond machining: typical Ra of 0.2 to 0.8 micrometers.
• After lapping or polishing (for optical or electrical contact surfaces): Ra below 0.1 micrometer.

Testing Samples

Flat Witness Coupons and Coated Substrates

The most common test article is a flat witness coupon, for example 50 mm by 50 mm or 100 mm by 50 mm, prepared from aluminum, steel, or copper substrate that has been grit-blasted to provide an anchor tooth texture. The coupon is sprayed at the same time and with the same parameters as the production part, so that its surface texture statistically represents the component. Coupons must be sufficiently flat and rigid to allow stable datum support during tracing; coating thickness is normally kept between 0.5 and 3 mm to ensure the measured topography reflects the steady-state deposit rather than the substrate profile.

Machined Deposits and Near-Net-Shape Additive Parts

For cold spray used as an additive manufacturing route, samples are cut from finished or semi-finished copper deposits, including freeform blocks, flanges, bosses, and repair welds on castings. When the purpose of testing is to verify a final surface finish after machining, sampling must avoid burrs, feed marks from tool runout, and edges where the tool entered or exited the material, since these locally distort the roughness evaluation.

Complex Geometry Components

Curved surfaces such as cylindrical busbars, slip-ring tracks, tubular current leads, inner bores of heat exchanger components, and rails for electromagnetic launchers require special fixturing. For cylindrical outer surfaces, the trace is taken along the generator line and the curvature is removed mathematically by least-squares form extraction. For inner diameters, dedicated bore gauges or right-angle stylus attachments are used. Measurement zones should be marked relative to the spray gun traverse direction, because cold spray deposits often show directionality between the parallel and perpendicular directions.

Sample Preparation and Conditioning

Prior to measurement, samples are cleaned with suitable solvents such as acetone or isopropanol to remove oil, dust, and loose particles. Mechanical brushing, blasting, or polishing must never be applied, because it permanently alters the surface under evaluation. Any loosely adherent particles detected after cleaning are documented as surface defects rather than texture. Samples are conditioned in the metrology laboratory, ideally at the standard reference temperature of 20 degrees Celsius defined in ISO 1, so that thermal expansion does not influence the measurement geometry.

Testing Items

The test report for a cold-sprayed copper component normally specifies the surface texture parameters listed below, together with the measurement direction, filter cut-off, evaluation length, and instrument type. Selection of parameters depends on the function of the surface: amplitude parameters quantify general height irregularities, spacing and hybrid parameters describe the texture character, and material ratio parameters link topography to functional behavior such as contact area and wear-in.

Profile Amplitude Parameters

• Ra, the arithmetic mean deviation of the assessed profile, is the default acceptance parameter for most drawings and specifications.
• Rq, the root-mean-square deviation, is more sensitive to deep valleys associated with open porosity.
• Rz, the maximum height of the profile within one sampling length, and Rt, the total height within the evaluation length, capture extreme peaks and valleys that govern sealing and fatigue concerns.
• Rp and Rv, the maximum profile peak height and maximum profile valley depth, are reported when machining allowance or debris-trapping capacity must be controlled.

Spacing, Hybrid, and Statistical Parameters

• RSm, the mean spacing of profile irregularities, reflects the effective particle or splat spacing on as-sprayed surfaces.
• Rsk, profile skewness, distinguishes spiky surfaces (positive skew) from porous, valley-dominated surfaces (negative skew); strongly negative skew on cold-sprayed copper often indicates interconnected open porosity.
• Rku, profile kurtosis, indicates the sharpness of the height distribution and helps identify isolated defects.
• Rdq, the root-mean-square slope, correlates with friction and lubricant retention behavior.

Areal Parameters

• Sa and Sq, the areal equivalents of Ra and Rq, are reported for optical three-dimensional measurements over a defined area.
• Sz, the maximum height of the scale-limited surface, replaces Rz when full-field data are available.
• Ssk and Sku provide areal skewness and kurtosis statistics over the entire measured zone.
• Str, texture direction ratio, and Std, texture direction, quantify whether the deposit is isotropic or shows lay induced by the raster spray pattern.
• Sdr, the developed interfacial area ratio, is relevant for electrochemical and thermal contact applications.

Material Ratio and Functional Parameters

The material ratio curve parameters Rmr(c), Rk, Rpk, and Rvk describe the core roughness, the reduced peak height, and the reduced valley depth of the profile. These are particularly informative for cold-sprayed copper sliding or contact surfaces, because they separate the load-bearing core from the initial wear-in peaks and the lubricant-retaining valleys. Waviness parameters such as Wt are occasionally reported when spray pass overlap or robot path errors introduce long-wavelength undulation on large-area deposits.

Testing Methods

Contact Stylus Profilometry

Stylus profilometry remains the reference method for cold-sprayed copper and is performed in accordance with ISO 21920 or the legacy ISO 4287 and ISO 4288 procedures using instruments conforming to ISO 3274 or GB/T 6062. A diamond stylus with a tip radius of 2 or 5 micrometers is traversed across the surface at constant speed, and the vertical displacement is filtered with a Gaussian filter to separate roughness from waviness. Because copper is a soft metal, the static measuring force must be minimized, typically to 0.75 to 1 millinewton, to prevent the stylus from plowing the surface. A practical verification is to repeat the trace along the same path; a genuine profile reproduces itself, whereas a scratched surface shows a deepening groove. A 5-micrometer tip radius is preferred for as-sprayed deposits because the larger contact area reduces contact pressure while the deep inter-particle valleys are still adequately resolved.

The cut-off wavelength is selected from the tables in ISO 4288 or ISO 21920-2 according to the expected roughness. For as-sprayed cold-sprayed copper with Ra between 2 and 10 micrometers, a cut-off of 2.5 mm with an evaluation length of 12.5 mm is typical; for Ra above 10 micrometers, a cut-off of 8 mm with an evaluation length of 40 mm is used. For machined surfaces with Ra between 0.1 and 2 micrometers, a cut-off of 0.8 mm applies. A minimum of five sampling lengths is evaluated at each position, and at least five positions distributed over the functional surface, including both parallel and perpendicular orientations relative to the spray raster direction, are measured to characterize the deposit adequately.

Non-Contact Optical Areal Measurement

Optical instruments classified under ISO 25178, including focus variation microscopy, laser scanning confocal microscopy, chromatic confocal sensors, and coherence scanning interferometry, capture a full three-dimensional height map of the surface without mechanical contact. This is advantageous for cold-sprayed copper because it eliminates stylus scratching on soft surfaces and images open pores without stylus-tip radius limitation. Typical measurements use a lateral sampling interval of 1 to 5 micrometers over a field of several square millimeters, with stitching when larger areas are required. Two material-specific cautions apply: the high reflectivity and steep local slopes of copper splats can cause signal loss in deep valleys, so instrument settings must be optimized, and data dropout points must be treated according to the filling rules of ISO 25178-2 rather than silently interpolated. Optical results are frequently cross-verified against stylus measurements during method validation.

Replica and Shop-Floor Comparison Techniques

For on-site inspection of installed components or large structures where direct instrument access is impossible, replica materials can transfer the surface texture to a portable copy that is subsequently measured in the laboratory. Shop-floor roughness comparison specimens, in the spirit of the comparator approach standardized in ISO 8503 for blast-cleaned steel, provide a rapid tactile and visual estimate of as-sprayed texture. These methods are suitable for process monitoring but are not acceptable substitutes for instrumented measurement when formal acceptance against a numeric drawing limit is required.

Instrument Verification and Traceability

All measurements must be traceable to national metrology institutes. Contact instruments are calibrated and periodically verified per ISO 12179 using material measures defined in ISO 5436-1, such as single-groove depth standards and irregularity specimens. Areal instruments are verified using artifact types described in the ISO 25178-70 series. Daily checks before measurement campaigns, environmental records, and intermediate checks are documented in the laboratory quality system, which is a mandatory element for laboratories holding accreditation to ISO/IEC 17025.

Data Evaluation and Reporting

The measured primary profile is leveled, form is removed, and the profile is filtered into roughness and waviness components using the specified cut-off. Reported values include the mean and the range or standard deviation of all measurement positions, the maximum value when a drawing invokes a maximum-material limit, and full metadata: parameter names and values, cut-off and filter type, evaluation length, stylus tip radius and force or optical method class, measurement direction relative to the spray raster, ambient temperature, and instrument identification. Outliers caused by recognized surface defects such as spitting, embedded unmelted particles, or cracks are annotated separately rather than being averaged into the texture result.

Applications

Surface roughness testing of cold-sprayed copper supports design, process qualification, and final release across a broad range of industries:

• Aerospace: conductive copper deposits on composite and metallic structures, lightning-strike protection layers, electromagnetic interference shielding, and heat-exchanger surfaces.
• Electric power and grid equipment: busbars, high-current connectors, switchgear contact surfaces, and electromagnetic launcher rails, where contact resistance is directly influenced by texture.
• Electronics and semiconductor equipment: cold-sprayed copper heat spreaders, sputtering target blanks, and RF cavity preforms requiring controlled finish after machining.
• Additive manufacturing: qualification of near-net-shape copper blocks and repair deposits before finish machining, and verification of post-machined surface quality.
• Maintenance and repair: restoration of worn copper alloy bearing seats, valve sealing faces, and mold surfaces, where roughness confirms both the as-sprayed build and the final restored geometry.
• Marine and corrosion engineering: copper-based anti-fouling and corrosion barrier coatings whose roughness affects flow resistance and fouling behavior.
• Research and development: comparison of nitrogen versus helium processes, powder morphology studies, and parameter-window mapping, where roughness serves as a fast statistical indicator of particle deposition behavior.

FAQ

What surface roughness should I expect from an as-sprayed cold-sprayed copper coating?

Typical as-sprayed values fall between Ra 6 and 25 micrometers, depending mainly on powder particle size, gas pressure, and standoff distance. Finer powder fractions and optimized parameters can reduce Ra to 3 to 10 micrometers. If the component requires a functional finish, machining to Ra 0.8 or better is normal, and the as-sprayed measurement is used to set the machining allowance.

Which standards are applied for the measurement?

The current governing standards are the ISO 21920 series for profile surface texture and the ISO 25178 series for areal measurement. Many legacy specifications still reference ISO 4287, ISO 4288, and ISO 3274, and Chinese national drawings commonly invoke GB/T 1031, GB/T 3505, GB/T 10610, and GB/T 6062, which are technically aligned with the corresponding ISO documents.

Can the stylus scratch the copper and distort the result?

Yes, copper is soft enough that a standard measuring force can plow the surface. Laboratories therefore use reduced stylus force, typically below 1 millinewton, and verify repeatability by retracing the same profile. Optical methods under ISO 25178 avoid this issue entirely and are recommended when the surface must remain untouched.

How many measurement positions are required?

A minimum of five evaluation positions is standard, each comprising at least five sampling lengths. For large or functionally critical surfaces, ten or more positions distributed across the area, and in both directions relative to the spray raster, are measured so that the reported statistics capture directional texture and local non-uniformity.

What cut-off should be selected for rough as-sprayed deposits?

Following the selection tables in ISO 4288 and ISO 21920-2, an Ra between 2 and 10 micrometers calls for a 2.5 mm cut-off, while an Ra above 10 micrometers calls for an 8 mm cut-off. Choosing too short a cut-off filters real surface structure into waviness and underreports roughness.

Does roughness tell me anything about coating quality beyond texture?

It provides indirect evidence. Abnormally high Rz, strongly negative Rsk, or inconsistent results between positions frequently correlate with excessive porosity, poorly bonded particle clusters, or unstable spray parameters, and typically triggers supplementary testing such as metallographic porosity determination or adhesion testing to confirm.

Should the measurement be done before or after machining?

Both stages are commonly tested: the as-sprayed measurement verifies process stability and sets stock removal, while the post-machining measurement verifies compliance with the drawing finish specification. Test plans should clearly state which condition each requirement applies to.