Tribochemical Wear of Tool Steels against Unfilled Polymers

Sammanfattning

The prevalence of polymeric materials in products and engineering applications is steadily increasing, and polymer working is thus becoming more and more important for global manufacturing. The materials in contact with the polymer during manufacturing are most often made from tool steel that should not be altered by sliding contact against the much softer polymer. However, practical industry experience show that the tools are worn and must regularly be replaced, even in cases where the polymer is devoid of abrasive additives. The wear mechanism that degrades the tool steel is believed to be tribochemical as wear occurs in cases with no mechanical wear. The purpose of this work is to study the wear of tool steels in sliding contact with unfilled polymer. A test rig where a tool steel cylinder was fed axially along a rotating polymeric cylinder in a crossed cylinder configuration under an applied load was used to mimic tool usage. Unimax branded tool steel was tested in as-delivered and polished states against PVC, PTFE and POM. Orvar Supreme and Vancron 50 tool steels were additionally tested against PVC. The resulting wear marks on the tool steel were analysed with scanning electron microscopy (SEM), optical profilometry and X-ray photoelectron spectroscopy (XPS). The steel surface was found to be significantly chemically worn by the sliding contact by PVC and somewhat chemically worn by PTFE, whereas POM had no apparent effect on the surface. It is suspected that PVC wears the steel through active iron oxidation in the contact through chlorine gas evolution from the polymer degradation. The asperities of the steel surface were sheared in the sliding direction by contact with PVC. The wear rate of tool steel is estimated to be 1 nm/m against PVC.