Determination of the Friction Coefficient of Cold-Sprayed Copper Coatings: Standards, Methods, and A

The friction coefficient of cold-sprayed copper coatings is a critical performance parameter for components operating under sliding contact, including electrical contacts, bearing surfaces, and repaired copper parts. Accurate determination of this coefficient requires standardized tribological testing methods such as pin-on-disk, reciprocating ball-on-flat, and block-on-ring configurations performed by accredited third-party laboratories. This article presents a comprehensive technical overview of friction coefficient testing for cold-sprayed copper coatings, covering sample requirements, testing standards, measurement procedures, and typical applications.

Technical Overview

Cold spray, also known as cold gas dynamic spraying or kinetic spraying, is a solid-state coating process in which fine copper powder is accelerated to supersonic velocities (typically 500–1200 m/s) by a heated high-pressure gas stream through a de Laval nozzle. Upon impact with the substrate, the particles undergo severe plastic deformation and bond metallurgically without melting. The resulting cold-sprayed copper coating exhibits low oxide content, high electrical and thermal conductivity, and significantly higher hardness than annealed bulk copper due to extensive work hardening.

The coefficient of friction (COF, denoted as μ) is defined as the ratio of the tangential friction force (Ft) to the applied normal load (Fn) between two surfaces in relative sliding motion. For cold-sprayed copper coatings sliding against hardened steel or ceramic counterfaces under dry conditions, the COF typically falls within the range of approximately 0.3 to 0.8, depending strongly on hardness, porosity, oxide content, surface roughness, counterface material, normal load, sliding velocity, and environmental conditions. Because the friction behavior of cold-sprayed copper differs from that of wrought or electroplated copper, standardized measurement is essential for material qualification, design verification, and quality control in engineering applications.

Friction coefficient determination for these coatings is generally performed in accordance with internationally recognized standards, including ASTM G115, ASTM G99, ASTM G133, ASTM G77, ISO 20808, and the GB/T 12444 series, ensuring reproducible and comparable results across testing facilities.

Testing Samples

Flat Disc or Coupon Samples for Pin-on-Disk Testing

Rotating disc samples, typically 30–60 mm in diameter, or square coupons of at least 20 mm × 20 mm are used for pin-on-disk testing. The cold-sprayed copper coating should be deposited to a minimum thickness of 0.3–1.0 mm to ensure the wear track remains entirely within the coating layer during the test. After deposition, the surface is ground and polished to a controlled finish, commonly Ra ≤ 0.1–0.4 μm, to minimize the influence of as-sprayed roughness on friction measurement.

Plate Samples for Reciprocating Ball-on-Flat Testing

Rectangular plate samples with a minimum flat area of approximately 30 mm × 30 mm are used for linearly reciprocating tests. The coating surface must be flat within tight tolerances to maintain uniform contact pressure throughout the stroke, and edges should be chamfered to avoid plowing artifacts at reversal points.

Block Samples for Block-on-Ring Testing

For block-on-ring configurations, the cold-sprayed copper coating is applied to the contact face of a rectangular block, which is pressed against a rotating ring of hardened steel or another specified counterface material. This configuration is particularly suitable for evaluating friction behavior under higher contact stresses.

Sample Preparation and Conditioning

Prior to testing, samples should be ultrasonically cleaned in acetone or ethanol and dried to remove machining residues, oils, and particulate contamination. Conditioning should be performed in a controlled laboratory atmosphere, typically 23 ± 2 °C and 50 ± 5% relative humidity, and the coating hardness, thickness, porosity, and surface roughness should be documented as part of the test record since these parameters directly influence friction behavior.

Testing Items

Friction coefficient determination for cold-sprayed copper coatings typically encompasses the following measurable quantities:

• Static (breakaway) coefficient of friction at the onset of relative sliding
• Kinetic (dynamic) coefficient of friction under continuous sliding
• Running-in behavior and transition to steady-state friction
• Average steady-state COF with standard deviation from replicate tests
• Friction force trace stability and fluctuation amplitude over the test duration
• COF as a function of normal load and sliding speed matrices
• COF under dry sliding and lubricated (oil or grease) conditions
• COF at elevated temperatures or under controlled humidity environments
• Comparative evaluation of as-sprayed, machined, polished, and heat-treated (annealed) coating conditions

Testing Methods

ASTM G115 provides the overarching guidance for measuring and reporting friction coefficients, specifying required documentation of material pair, geometry, load, velocity, environment, and data reduction methodology. Building on this framework, several standardized test configurations are routinely applied to cold-sprayed copper coatings.

Pin-on-Disk Method (ASTM G99 / ISO 20808)

A stationary ball or pin, commonly a 3–10 mm diameter sphere of hardened bearing steel (SAE 52100 / GCr15), alumina, or silicon nitride, is pressed against the rotating coated disc under a normal load of typically 1–20 N. The tangential friction force is measured continuously by a calibrated force transducer, and the instantaneous COF is calculated as μ = Ft / Fn. Typical sliding speeds range from 0.01 to 1.0 m/s with defined track radii and total sliding distances. The average COF is determined from the steady-state portion of the friction trace, excluding the running-in segment.

Linearly Reciprocating Ball-on-Flat Method (ASTM G133 / ASTM G181)

A ball counterface oscillates against the flat coated surface along a linear track of defined stroke length (commonly 5–10 mm) and frequency. This method is well suited to reciprocating service conditions and to evaluating friction behavior in narrow contact zones. Both the forward and reverse stroke friction forces are recorded, and the kinetic COF is averaged over the steady-state cycles.

Block-on-Ring Method (ASTM G77 / GB/T 12444 Series)

The coated block is loaded against a rotating ring under a specified force, and the friction torque is converted to a COF value. This method accommodates higher loads and line-contact geometries, making it appropriate for applications involving concentrated contact stresses.

Data Processing and Reporting

For all methods, a minimum of three replicate tests under identical conditions is recommended to establish statistical confidence. The test report should include the counterface material and its roughness, normal load, sliding speed, total sliding distance, environmental conditions, calculated average COF with standard deviation, and representative friction traces. When coupled with post-test examination of wear tracks by optical or electron microscopy, the results support correlation between friction behavior and coating microstructural features such as porosity and oxide distribution.

Applications

Friction coefficient data for cold-sprayed copper coatings directly supports engineering decisions across multiple industries:

• Sliding electrical contacts, including pantograph contact strips, slip rings, and current-carrying guides where stable low friction must coexist with high conductivity
• Bearing and bushing surfaces in which copper coatings provide embedability and thermal dissipation
• Qualification of cold spray additive-manufactured copper components for tribological service
• Repair and dimensional restoration of copper alloy components such as molds, heat exchanger parts, and furnace tuyeres, where friction behavior must match the base material
• Thermal management and electromagnetic shielding surfaces in which handling and assembly friction characteristics are relevant
• Rail transit, marine, and power generation equipment requiring documented friction performance of sprayed copper surfaces

FAQ

What is a typical friction coefficient value for cold-sprayed copper coatings?

Under dry sliding against hardened steel or ceramic counterfaces at moderate loads, cold-sprayed copper coatings typically exhibit COF values of approximately 0.3 to 0.8. As-sprayed, work-hardened coatings generally show higher friction than polished or annealed coatings, and values must always be interpreted in the context of the specific load, speed, and environment used in testing.

Which testing standard is most commonly used?

ASTM G99 pin-on-disk testing is the most widely applied method for determining the friction coefficient of metallic coatings, supplemented by ASTM G133 for reciprocating conditions and ASTM G115 for reporting guidance. ISO 20808 and the GB/T 12444 series provide equivalent international and Chinese national frameworks.

How does porosity affect the measured friction coefficient?

Higher porosity in cold-sprayed copper can increase friction fluctuation, promote counterface abrasion by debris entrapment, and alter wear mechanism from adhesive to abrasive-dominated. Porosity levels should therefore be documented alongside COF results, and dense coatings above 99% theoretical density are preferred for tribological applications.

Does heat treatment change the friction behavior?

Yes. Annealing relieves work hardening and reduces the hardness of cold-sprayed copper, typically lowering the dry sliding COF and stabilizing the friction trace. Comparative testing of as-sprayed and annealed conditions is frequently requested for components that undergo post-spray heat treatment.

Can friction testing be performed under lubricated or high-temperature conditions?

Yes. Accredited third-party laboratories can conduct friction coefficient measurements under oil or grease lubrication, elevated temperatures, or controlled humidity by adapting the standard test configurations with environmental chambers and lubricant delivery systems.

What coating thickness is required for a valid friction test?

A minimum coating thickness of approximately 0.3 mm after surface finishing is recommended so that the contact zone and any resulting wear scar remain fully within the copper layer, preventing substrate influence on the measured friction coefficient.