Thermal Shock Resistance Testing of Cold-Sprayed Copper Coatings

Thermal shock resistance testing of cold-sprayed copper coatings determines how these solid-state metal deposits withstand rapid, repeated temperature excursions without cracking, delamination, or loss of adhesion. This article explains the test principle, sample preparation, evaluation items, testing methods based on standards such as ISO 13123 and ASTM C633, and typical applications ranging from rocket combustion chamber liners to electrical conductors and heat exchangers.

Technical Overview

Cold spray, also known as cold gas dynamic spraying or kinetic spraying, is a solid-state coating process in which helium or nitrogen is heated to approximately 300–800 °C—far below the 1,083 °C melting point of copper—and accelerated through a de Laval nozzle to supersonic speed. Copper powder particles measuring roughly 15–45 μm strike the substrate at 500–1,000 m/s, exceeding the critical velocity for metallic bonding, and adhere through severe plastic deformation. Because no bulk melting occurs, cold-sprayed copper coatings feature low oxygen content (typically below 0.2 wt.%), porosity below 1%, compressive residual stress, and retained electrical conductivity commonly reaching 80–90% IACS in the as-sprayed condition.

Thermal shock testing subjects these coatings to rapid cycling between temperature extremes. The dominant damage driver is the mismatch in coefficients of thermal expansion: copper expands at roughly 16.5–17.7 × 10⁻⁶/K, compared with about 23 × 10⁻⁶/K for aluminum alloys, 11–13 × 10⁻⁶/K for steels, and 8.6–16 × 10⁻⁶/K for titanium and nickel alloys. Each thermal transient generates interfacial shear stress proportional to the elastic modulus, the expansion mismatch, and the temperature differential. Stress concentrations at coating edges, pores, and embedded grit amplify this effect, and repeated cycling promotes crack initiation, oxide wedging at interfacial defects, and ultimately spallation.

A second phenomenon specific to cold-sprayed copper is thermal annealing of the heavily work-hardened deposit. Recrystallization begins at approximately 200–300 °C, reducing microhardness from a typical as-sprayed value of 120–160 HV to roughly 50–70 HV while increasing ductility and conductivity. A well-designed thermal shock program therefore tracks both failure events and the evolution of mechanical and physical properties across the cycle history.

Testing Samples

Flat Coupon Specimens

Flat coupons are the most common test articles, typically 50 mm × 50 mm squares or 25.4 mm diameter discs with 3–6 mm thick substrates. Representative substrate materials include 6061 and 7075 aluminum alloys, 304 and 316L stainless steels, carbon steel, Ti-6Al-4V, GRCop-type copper alloys, and nickel superalloys. Substrates are degreased and grit-blasted with alumina to a surface roughness of approximately Ra 5–20 μm before deposition. Coating thickness is usually 0.5–3 mm, and specimen edges are chamfered or radiused to at least 1 mm to reduce the edge stress singularity that can cause premature, non-representative failure.

Cylindrical and Tubular Specimens

Tubular samples, such as liner segments with a 25–50 mm outer diameter and 2–5 mm wall thickness, are used when the coating is applied to inner or outer cylindrical surfaces. These specimens reproduce the hoop stresses and curvature effects found in real hardware, and their ends are capped or sealed to prevent quench water from entering the bore and creating uncontrolled cooling conditions.

Full-Scale Components and Mock-Ups

Actual components, including combustion chamber liner sections, busbar joints, mold inserts, and cold-plate elements, may be tested directly in burner-rig or gradient-cycling setups when qualification must reflect real geometry, coating access, and heat-flow conditions rather than coupon-level behavior.

Baseline Characterization Before Cycling

• Coating thickness by eddy-current probe, micrometer, or metallographic cross-section.
• Tensile adhesion strength per ASTM C633 using 25.4 mm loading fixtures; cold-sprayed copper commonly achieves 30–60 MPa and frequently exceeds the strength of the adhesive itself.
• Porosity by image analysis per ASTM E2109, typically below 1% for well-optimized cold spray parameters.
• Microhardness per ASTM E384 at HV0.1–HV0.5 loads to profile the deposit through its thickness.
• Electrical conductivity in % IACS by the eddy-current method per ASTM E1004.
• Initial surface condition documented by photography, roughness measurement, and dye-penetrant inspection per ASTM E1417.

Testing Items

The thermal shock program quantifies coating durability through the following measurables:

• Number of cycles to the first visible crack and to the defined failure criterion.
• Critical temperature differential (ΔTc) that the coating survives without damage.
• Adhesion strength retention, measured by ASTM C633 or ASTM D4541 pull-off tests on cycled versus uncycled coupons.
• Crack density (total crack length per unit area), maximum crack length, and crack orientation mapped after each inspection interval.
• Spallation or blister area, together with specimen mass change, quantified by analytical balance weighing.
• Microhardness evolution to capture annealing and recovery of the deposit.
• Porosity change and interfacial microstructure assessed on cross-sections prepared per ASTM E3.
• Electrical and thermal conductivity retention relevant to functional performance.
• Hidden delamination detected by ultrasonic C-scanning or flash thermography where surface inspection is inconclusive.

Acceptance criteria are set by the client specification. A representative requirement is no visible spallation and no through-thickness cracks after 100 water-quench cycles from 500 °C, with post-test adhesion retaining at least 80% of the baseline value and crack density below 0.5 mm/mm².

Testing Methods

Furnace Water-Quench Method

Based on the cyclic heating principle of ISO 13123, the specimen is heated in a calibrated muffle furnace to the target temperature—commonly 300–650 °C for copper systems, with probes up to 950 °C for substrate-limited cases—held for 10–20 minutes to achieve thermal equilibrium, transferred to the quench bath in under 10 seconds, and immersed in water at 20–25 °C. The cycle is repeated under programmable control. Specimens are inspected visually and at 10×–30× magnification every 5–10 cycles, weighed at defined intervals, and subjected to penetrant or ultrasonic checks periodically. Testing terminates upon failure or upon completing the specified cycle count. Water quenching imposes the most severe quench rate and is the standard screening method.

Two-Chamber Air-Cooling Method

In the two-chamber configuration, specimens shuttle between a hot oven and a cold chamber maintained at ambient temperature, −40 °C, or liquid-nitrogen temperature of −196 °C for cryogenic duty. Forced convection limits quench rates to roughly 10–50 °C/s, producing gentler, more service-representative transients suitable for electrical and thermal-management hardware. Automated equipment allows evaluation over hundreds to thousands of cycles.

Burner-Rig and Gradient Cycling

For combustion chamber liners and similar components, an oxy-acetylene or kerosene burner, or induction heating, applies heat to the coated face while coolant flows through internal channels on the reverse side. This reproduces the through-wall thermal gradient of rocket-engine service, with hot-face temperatures of 500–700 °C and coolant temperatures spanning −180 °C to +50 °C. Heating segments of 60–120 seconds alternate with cooling segments of 30–60 seconds. Infrared cameras and embedded thermocouples record the thermal history, and failure is detected by coolant pressure rise, hot spots in thermal images, or visible cracking.

Post-Test Evaluation

• Visual, stereomicroscopic, and dye-penetrant examination per ASTM E1417 to map surface cracking.
• Adhesion pull-off testing per ASTM C633 or ASTM D4541 on specimens cut from cycled coupons.
• Metallographic cross-sections prepared per ASTM E3, with porosity quantified per ASTM E2109 and interfacial oxides characterized by scanning electron microscopy with energy-dispersive spectroscopy.
• Microhardness per ASTM E384 and conductivity per ASTM E1004 to document property evolution.
• Ultrasonic C-scan or active thermography to locate subsurface delamination.

All measurements should be performed by ISO/IEC 17025-accredited third-party laboratories or qualified in-house facilities. Reporting includes cycle-by-cycle temperature logs, photographic documentation, dimensional and mass data, quantitative metallography, and an explicit pass/fail determination against the acceptance criteria.

Applications

• Rocket-engine combustion chamber liners and nozzle extensions, where cold-sprayed copper alloys must survive thousands of severe thermal gradients between hot gases and cryogenic propellants.
• Electrical infrastructure, including busbars, high-current contacts, connectors, and electromagnetic-shielding enclosures exposed to load-cycling temperature swings.
• Heat exchangers, vapor chambers, and cold plates combining copper coatings on aluminum or steel structures for lightweight thermal management.
• Repair and refurbishment of continuous-casting mold plates, copper crucibles, and machined copper components subjected to repeated heating and quenching in service.
• Sputtering targets, radiation shields, and beam-stopping components in scientific and semiconductor equipment.
• Qualification of additively manufactured copper structures and hybrid cold-spray builds used in aerospace and energy systems.

FAQ

What temperature range is used for thermal shock testing of cold-sprayed copper coatings?

The upper temperature depends on the substrate and application. Copper on aluminum is typically cycled up to 300–350 °C to avoid substrate softening, copper on steel up to 500–650 °C, and gradient tests for propulsion hardware reach hot-face values of 700 °C. The lower extreme is room-temperature water for quench methods, or −40 °C to −196 °C in two-chamber or burner-rig setups for cryogenic service simulation.

How many thermal shock cycles are usually performed?

Water-quench programs commonly specify 30–100 cycles, while forced-air and gradient cycling may extend to several hundred or thousands of cycles. The number is defined by the product specification and should reflect the expected service life with margin.

What constitutes failure in the test?

Failure is defined by the acceptance criteria and typically includes any visible spallation or blistering, a through-thickness crack detected by penetrant testing, spallation area exceeding a stated limit such as 5 mm², adhesion strength dropping more than 20% from baseline, or coolant leakage in gradient tests.

Which standards govern the test methods?

ISO 13123 covers the cyclic-heating thermal shock method for metallic and other inorganic coatings. Supporting methods include ASTM C633 and ASTM D4541 for adhesion, ASTM E1417 for penetrant inspection, ASTM E2109 for porosity, ASTM E384 for microhardness, ASTM E1004 for conductivity, ASTM E3 for metallography, and ISO 14917 for thermal spray terminology and classification.

How does cold-sprayed copper perform compared with plasma-sprayed or HVOF copper under thermal shock?

Cold-sprayed copper generally shows superior thermal shock resistance because its low oxide content, low porosity, and compressive residual stress improve cohesion and interfacial integrity. Thermal cycling also anneals the work-hardened deposit, increasing ductility, which further delays crack propagation; the practical design consideration is the accompanying hardness decrease, which must be acceptable for the application.

Can testing be performed on actual components instead of coupons?

Yes. Burner-rig and gradient-cycling setups accommodate real hardware such as liner segments and mock-ups, with in-situ infrared imaging and embedded thermocouples providing the thermal history needed to interpret results against coupon-level data.

How many specimens are required, and what is the typical project duration?

A statistically meaningful program normally uses three to five specimens per condition, including uncycled baseline coupons. Depending on the cycle count and the depth of post-test metallography, projects typically run two to four weeks; accredited third-party laboratories can provide formal quotations and turnkey reporting for specific coating and substrate combinations.