Tribological behaviour of Zn coated UHSS sliding against hot-work tool steel at high temperatures

Abstract

The increasing demands for light-weight components in vehicles contribute to the global expansion of hot sheet metal forming technologies. Structural components are typically produced using hot stamping of ultra-high strength steel (UHSS). This process allows forming of complex shapes whilst enabling control of the mechanical properties of the end product. Interest in zinc coated UHSS has increased in recent years in view of the corrosion protection it provides to the final components. There is a need for increased understanding of its tribological behaviour during the interaction with tool steel at elevated temperatures. In this work, tribological studies have been carried out in a novel hot strip tribometer. The aim was to study the effect of different operating conditions on the tribological behaviour of zinc coated UHSS sliding against a hot-work tool steel under un-lubricated conditions. The parameters studied in this work were; temperature, ranging from 400°C to 700°C; and contact pressure, from 5 to 30MPa. The UHSS was initially heated up to austenitising temperature (840°C) and then cooled down to the testing temperature. Upon stabilisation of temperature, the load was applied and sliding was carried out for a total of 1500mm at 100mm/s. The results showed a trend towards decreasing average coefficient of friction as temperature and contact pressure increased. Unstable friction behaviour was observed at low temperature (400°C) and high contact pressure (30MPa) whilst higher temperatures (600°C) facilitated the development of a low and stable friction behaviour. It is proposed that the friction behaviour is controlled by the properties of the zinc phases in the coating developed during heating of the UHSS. The combination of high temperature and sliding conditions result in the removal of the uppermost oxide layer and the phases beneath control the friction behaviour.