Wear at elevated temperatures – microstructural investigations of tool materials after high temperature sliding wear

Abstract

The presented study investigates the hardness and sliding wear behavior of cemented carbides and high-speed tool steels at elevated temperatures. Therefore, experimental analysis focus on the behavior and the microstructural changes of the aforementioned materials during sliding wear at room temperature, 400°C, and 600°C. Investigations are performed with respect to materials mechanical properties, which are represented by high temperature hardness measurements. As a result a main output of this study is an overview about the interdependencies of the microstructure and the high temperature hardness of tool materials (cemented carbides, high-speed steels). Results show that the absolute value of the room temperature and the high temperature hardness of cemented carbides are connected to the WC-carbide volume fraction. In contrast, the development of the hardness with increasing temperature is mainly influenced by the volume fraction and the characteristics of the binder matrix. In addition to the investigation of cemented carbides, the hardness of HS 6-5-3 steel grade at elevated temperatures was investigated. The value of the high temperature hardness of HS 6-5-3 high-speed steel is comparable to the hardness of cemented carbide materials, having higher binder content of 30 vol.-%.The absolute value and the temperature dependent course of the high temperature hardness of the high-speed steel are further influenced by the applied heat-treatment. Moreover, detailed wear experiments and microscopic analyses of the worn surfaces of selected materials are presented. These investigations show the main differences between room temperature sliding wear and high temperature sliding wear. In the systems studied, an increase in testing temperature leads to a rapid decrease of friction and wear rate. Microscopic investigations reveal that, the reason for this effect is the formation of a tribochemical wear layer at the surface of the wear bodies. This layer suppresses direct metallic contact and changes the characteristics of the tribological system. Discussed issues of high temperature sliding wear are the formation and stability of tribochemical wear layers, their connection to and support by the bulk material as well as the fracturing and damage of the layer-bulk-material compound.