Effect of Cu-Fe Bond Coat Composition on the Microstructure and Adhesion of Cold Sprayed Copper Coatings on Steel Substrates

Cold spray enables deposition of dense metallic coatings below melting temperature; however, achieving strong adhesion is challenging when soft metals are deposited onto hard substrates like copper onto steel. This limitation is often attributed to hardness mismatch and insufficient substrate plastic deformation. Previous strategies have investigated laser-assisted cold spray, in which a laser softens the substrate prior to deposition. Another approach is the use of composite bond coats combining soft and hard powders, where harder powders are expected to bond more effectively to the substrate, with higher hard powder content improving coating adhesion.

In our prior work, thin copper coatings were deposited onto steel using copper-iron (Cu-Fe) composite bond coats with varying feedstock ratios. The average microhardness of copper particles, iron particles, and the substrate remained unchanged with feedstock composition, indicating that additional iron did not enhance substrate plastic deformation and that deformation was primarily accommodated by the deposited particles. Scratch testing revealed improved adhesion with increasing iron content: critical scratch adhesion load increased from 0.1 N for pure copper bond coats to 0.2 N at 25% iron and reached 0.4 N at 50% iron, perhaps due to the formation of an interconnected Cu–Fe network that enhances bonding at the coating–substrate interface.

Building on these results, ongoing work aims to further quantify and optimize this bonding strategy. Thicker coatings will be produced to enable quantitative adhesion measurements using pull-off testing. Microstructural features and the extent of particle and substrate plastic deformation will be analyzed using scanning electron microscopy and microhardness measurements. Multiple coating configurations will be fabricated, including pure copper and Cu–Fe composite bond coat systems, with and without laser assistance. This will allow evaluation of bond coat performance relative to laser-assisted deposition, while also assessing potential synergistic effects when the two strategies are combined.