Understanding the friction and wear of Zircaloy-4 during pilgering process

In the cold pilgering process, maintaining efficient lubrication regime in the inner surface of the tube is a major problem and has been reported to cause undesirable surface damage and increase in roughness. The efficiency of the lubrication is also jeopardized by the high contact pressures, which are of the order of GPa. In the present study, an investigation of inner surface topography of partly pilgered zircaloy tubes has been carried out in terms of wear mechanisms. SEM observations of worn surfaces indicate adhesion, delamination and abrasion as major wear mechanisms. With progress in pilgering and possible degradation of the lubricant, wear mechanism changed from adhesive wear to abrasive wear and delamination. Cross-section imaging indicated that the 'damages' were primarily in the form of delamination/chipping due to initiation and propagation of sub-surface cracks during linear reciprocating motion at such high contact pressures (and not truly a surface phenomena), which got suppressed in the presence of graphene-based lubricant/coating (probably due to 'buffering'/'accommodation' of the stresses by sliding of graphene layers)