Friction stir processing (FSP) of ferrous alloys is possible using polycrystalline cubic boron nitride (PCBN) as the tool material. FSP tool wear characteristics are dependent on the grade of PCBN and the ferrous alloy being processed. Several grades of PCBN were evaluated in machining wear tests using 304L and AL-6XN stainless steels. Mechanical wear, chemical wear, and diffusion wear were the failure modes identified by these tests. These results were correlated with failure modes identified in PCBN tools used to friction stir process 304L and AL-6XN.