The paper investigates the mechanisms governing the wear of pcBN tool materials when high speed machining superalloy Inconel 718. The approach includes application of diffusion couples that were exposed to high temperature (1250 ºC) and high pressure (2.5 GPa). Two grades of pcBN tool material were selected for the analysis: binderless materials and grade with low cBN content and TiC-based binder. Results indicate that the interaction proceeds with intensive formation of Nb2N, Mo2B and NbB2 for binderless grade and less intensive reaction also including formation of Nb2C, TiB, and TiN for the grade with TiC binder.