The response surfaces for delamination factor Fda can be seen in Fig. (b) and it is evident that the Fda response surfaces against feed rate and spindle speed are markedly different for the two composites. As shown in Fig. (b-d), Fda of CF/PEKK first declines and then grows with both feed rate and spindle speed, while that of CF/epoxy consistently increases with feed rate and is insensitive to spindle speed. This difference can be attributed to the different delamination damage mechanisms of CF/PEKK and CF/epoxy. Delamination damage of CF/PEKK shows typical thermal-mechanical interaction characteristics and the fact that its high drilling temperature approaching / exceeding the Tg can soften the thermoplastic PEKK matrix and weaken its resistance against deformation. Therefore, under low feed rate (Ff ≤ 0.05 mm/rev), even the thrust force is quite low, the high drilling temperature (see Fig. ) can deteriorate the PEKK matrix's stiffness and cause high Fda. At high feed rate (Ff ≥ 0.1 mm/rev), the high thrust force (see Fig. ) takes over the dominance on Fda due to Mode-I opening fracture. This can explain why Fda of CF/PEKK first decreased and then increased with feed rate. In contrast, the delamination damage of CF/epoxy is solely caused by the high thrust force and its mechanical properties are less dependent on temperature. CF/epoxy exhibits more substantial delamination than CF/PEKK, and the largest Fda value can double that of the CF/PEKK. This can be as a result of the extremely low interlaminar fracture toughness of CF/epoxy (0.277 N/mm vs. 1.564 N/mm of CF/PEKK) . The CF/epoxy delamination factor Fda results reported in this study shows good consistence with previous work by Davim et al.