Wear and corrosion of silicon nitride rolling tools in copper rolling

For hot rolling applications it is essential to have materials with excellent high-temperature properties. Silicon nitride combines the properties optimum relevant to such high demanding applications, including thermal shock resistance, low density, high elastic modulus and low coefficient of friction. However, its chemical stability and corrosion resistance when brought into contact with copper at high temperatures and pressures is still a topic of research. In this study the corrosion and wear behavior of silicon nitride applied in rolling copper wires is investigated. For this purpose laboratory-scale wire rolling experiments were carried out and a series of atomistic simulations based on density functional theory DFT calculations were performed. The goal here is to identify the factors leading to the corrosion of silicon nitride in copper wire rolling, and to show how silicon nitride rolls differ from conventional steel rolls in terms of adhesion with copper. The experimental and numerical results were then compared with failed silicon nitride specimens tested in an industrial wire-rolling mill. The experimental results showed no signs of corrosive pitting or fracture on the rolls, however, indicated remarkable tribochemical wear especially in the presence of a lubricant. The numerical computations showed that the affinity of silicon nitride to copper is low in comparison to the affinity of ferrous-based tools to copper. The DFT calculations also explained one of the major wear-assisting mechanisms in this process. Finally, the failure of the industrial ceramic rolls was clarified by relying on FIB-SEM and EDX analyses.