Wear mechanisms of cubic boron nitride tools in precision turning of hardened steels
Two types of cubic boron nitride tools, BZN6000 (high CBN content) and BZN8100 (low CBN content), have been tested. Overall, BZN8100 shows better performance in finish turning of hardened steel than BZN6000. The results suggest that BZN8100 has greater wear resistance under higher cutting speeds than BZN6000 because of favorable chemical characteristics. In addition, BZN8100 seems to have the potential to generate finer surface finish with smaller depths of cut. On the other hand, BZN6000 shows no improvement on surface finish in finish cutting, even though flank wear and wear rate reduce significantly. Scanning electron microscopy (SEM) photographs and X-ray photoelectron spectroscopy (XPS) analysis indicate that piled-up layers on the flank wear show different structure and composition between different CBN tools, and have significant effects on the tool wear process. The piled-up layer on BZN6000 shows a rough, groove structure, and, in addition to iron oxide/carbide, has a significant amount of silicon dioxide. The piled-up layer on BZN8100 shows a smooth, flake-type structure, with iron oxide/carbide as the major component. The segregation of cobalt binder on the worn surface of BZN6000 and shallow CBN holes on BZN8100 indicate that adhesive wear is an important wear mechanism of CBN tools. However, the piled-up layer (metal oxide layer) seems to have a strong interaction with the tool surface during cutting. Therefore, adhesive wear associated with the tribochemical process should be considered as the dominant wear mechanism for CBN tools. The chemical characteristics of binder materials affect the structure of piled-up layer, and in turn the tool wear. The cobalt binder in the BZN6000 tends to form strong bonds with the piled-up layer, and hence results in more severe adhesive wear. On the other hand, the titanium nitride in BZN8100 prevents strong bonds with the piled-up layer, thus alleviating adhesive wear.
Barash, Purdue University.
Industrial engineering|Materials science|Mechanical engineering
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