SURFTERM - Thermal stability of white layers intended as process-induced functional surfaces

Purpose and goal

The project aimed to determine if mechanically induced white layers introduced during hard turning can be used as process-induced functional coatings in applications were elevated temperatures are to be expected. In-situ TEM investigations showed that the grain structure is stable up to 435°C. The subgrain structure changed at higher temperatures, but no recrystallization was observed. At atomic level, however, changes have occurred already towards formation of an equilibrium microstructure.

Expected results and effects

It could be confirmed that mechanically induced white layers are stable beyond temperatures usually reached in bearing applications. Aim is now to determine fatigue properties and to find a process window as large as possible by using improved cooling strategies with modified cutting tools and process settings.

Planned approach and implementation

The in-situ TEM investigations were decisive for the understanding of what happens in the mechanically induced white layers when exposed to higher temperature. Hardness and atom probe measurements gave valuable input for the understanding. The results were partly unexpected but support the general idea of making use of mechanically induced white layers as functional coatings.