Tribological performance of surface coated tool steel at elevated temperatures

In metal forming at elevated temperatures the tools are subjected to thermal cycling, increased oxidation and wear which will influence the lifetime of the tools and the quality of the produced parts. In addition to this, the frictional behaviour will also change with temperature and this can affect the performance of the forming operation itself. One way of controlling friction and reducing wear is to utilise the latest developments in surface engineering and modify or coat the tool surface with some thermally stable layer. However, in the open literature there are very few studies pertaining to the high temperature tribological properties of surface coatings applied on to the tool steels and sliding against actual workpiece material. This study thus aims at experimentally investigating the friction and wear characteristics of a surface coated tool steel during sliding against ultra high strength boron steel at elevated temperatures. The surface coatings applied on the tool steel in this work were of TiAlN and CrN respectively. The tribological studies were conducted by using both a reciprocating tribometer as well as a pin-on-disc machine at temperatures in the range from ambient to 800 ºC. A 3D optical surface profiler has been used for surface topography measurements and SEM/EDS have been used for investigating the resultant surface damage. The results have shown that the friction coefficient increases with temperature in unidirectional sliding. Wear of the coatings is negligible at room temperature but at 400 ºC the CrN disc is worn by abrading action of hard oxidised wear debris and the coating is removed to a large extent. In reciprocating sliding the friction also increases with temperature and the coatings are completely removed during sliding at 800 ºC.