Effects of the chromium to aluminum content on the tribology in dry machining using (Cr,Al)N coated tools

The present study investigates the potentials of newly developed (Cr,Al)N coatings compared to uncoated tools. The aim of using chromium-based coatings is to create a thermal barrier and thus redirecting the heat from work piece and tool into the chip. Due to their high resistance for oxidation, CrxAlyN coatings with different chromium to aluminum contents were deposited onto indexed carbide inserts. In order to find improved coatings for dry machining operations both tribological and wear tests were performed. These tests include fundamental properties such as thickness, hardness, and critical load. With Glow Discharge Optical Emission Spectroscopy (GDOES) depth profile analyses were conducted. In reciprocating sliding tests, using an SRV III apparatus from Optimol, friction and wear has been studied in a ball-on-disc arrangement. Longitudinal turning tests were performed to characterize the dry machining characteristics. As described above, the coatings' characteristics such as hardness and critical load as well as the results from the reciprocating sliding test correlate with the dry turning tests. The best coatings in the tests have similar chromium and aluminum contents in the region of 15 at.%, the nitrogen content was about 60 at.%, respectively. An increase of the Cr content in the coating deteriorates the performance of the coated carbide beneath the level of the uncoated tool. An adhesion phenomenon that leads to a formation of an adherent layer on the cutting edge is attributed to the completely unlimited mutual solid solubility of Cr and Fe at a temperature beyond 800 °C.