Laser assisted cold spray for improved adhesion of soft materials to hard substrates: Case study for copper coatings on steel

In cold spray, high adhesion of soft materials on hard substrates is commonly achieved by using helium as the propelling gas. This is the case of copper coatings on steel where adhesion may reach values as high as 60 to 80 MPa (glue failure); however, helium is a limited, expensive natural resource, and the use of more abundant nitrogen gas is preferred in the industry. Unfortunately, when using nitrogen gas, little to no adhesion is obtained. In order to eliminate the use of helium gas we studied laser assisted cold spray as a strategy for adhesion enhancement. In this work, several laser parameters (e.g., power and spot size) and process parameters (traverse speed, relative position laser spot vs. gas jet) were varied at a coupon level. Upon optimization, an equivalent adhesion to the coatings prepared with helium was obtained. Furthermore, the cross section of the coatings showed that the copper particles penetrated the steel, similar to what is observed when using helium gas. Optimization of these parameters for application to large diameter (600 mm) pipes was also performed. A discussion on the mechanisms which contribute to achieving high adhesion considering the use of helium versus laser assistance is provided.