The Role of Ultra‐Thin Passive Films and Organic Adsorbates on Aluminum Chips for a Friction‐Induced Solid‐State Recycling Process

Friction‐induced recycling is a method to recycle aluminum with a lower energy footprint than conventional methods. However, the quality of a metal–metal joint is strongly influenced by surface contamination, in particular by naturally formed oxide layers. Octadecyl phosphonic acid and methyl diphosphonic acid self‐assembled monolayers (SAMs) are deposited on aluminum chips bound for recycling in order to improve their storage stability. In addition, chips were etched with NaOH for varying amounts of time to further increase hydrophobicity through increased roughness as well as to create more binding sites for the phosphonic acids. Adsorption is confirmed via FTIR and XPS. Water contact angle measurements show the highest hydrophobicity for ODPA‐coated chips, followed by the bare substrate and MDPA coatings. As a side effect, it is also found that ODPA coatings and MDPA coatings, to a lesser extent, reduce friction within the bulk material. However, the presence of ODPA, especially in combination with etching, deteriorates the mechanical performance of the recycled product, making the removal of the molecule prior to recycling vital. Other pretreatments have next to no influence on the mechanical properties of the recyclates.