# Reconditioning of Diamond Coated Tools and Its Impact on Cutting Performance for CFRP Laminates
## 3 Results and Discussion
### Hole Quality Evaluation  
Drilling CFRPs can induce damage surrounding the hole circumferences or inside the hole wall surfaces, including burrs, tearing, delamination, and matrix thermal damage [31]. These defects affect the quality of drilled holes and may risk rejection of final composite  
Figure 13: SEM images of the TLC0 (**a–c**), the TLC1 (**d–f**), and the TLC2 (**g–i**) after drilling 200 holes.  
structures. Moreover, delamination during drilling CFRP components has been recognized as the main irreparable failure that frequently occurs at the hole entry and exit sides [32]. It is defined as the separation of adjacent composite plies and is characterized by interlaminar cracks in the material. The damage that occurs on the hole entrance is usually called peel-up delamination, while the hole exit damage is called push-out delamination. Peel-up delamination is a consequence of the cutting force pushing the separated and cut materials to the tool flute surface. In comparison, push-out delamination is caused by the thrust force bending the residual laminas shortly before the exit.  
In this study, both types of CFRP delamination induced by the new and reconditioned drills were evaluated through the frequently used method proposed by [PERSON] [33]. Delamination factor \(F_{\mathrm{d}}\) is defined as the ratio of the maximum diameter of a damaged zone (\(D_{\mathrm{max}}\), mm) to the nominal diameter of the hole (\(D\), mm), and two circles are concentric. Then, the value of \(F_{\mathrm{d}}\) can be expressed as:  
\[F_{\mathrm{d}}=D_{\mathrm{max}}/D. \tag{2}\]  
The maximal diameter of the damaged zone around each hole was measured using a digital microscope (VHX-1000 SP, Keyence, Osaka, Japan). For both delamination cases, as shown in Figures 14 and 15, the curves of the delamination induced by the TLC0 and the TLC1 showed similar behaviors and correlated with the drill flank wear data. The tool wear evolution generated an increase in the torque and the thrust force, which led to the CFRP delamination appearance. Indeed, when the cutting forces value exceeded the interlaminar fracture toughness of the laminates, delamination occurred.  
Figure 14: Variations of the delamination factor on the entry hole side as a function of the drilled holes number.  
Figure 15: Variations of the delamination factor on the exit hole side as a function of the drilled holes number.  
Meanwhile, the TLC2 induced some delamination since the first hole was drilled, as shown in Figures 14 and 15. Further, the peel-up and push-out delamination curves showed gradually increased trends. Such curves behaviors can be explained as the effect of the non-uniform globular formation of the diamond film along the drill cutting edges. These formations generated varied edge geometries with different cutting angles that may lead to an unstable cutting process through the vibration with changing of the cutting forces. Subsequently, abrasion led to a smoothing of the diamond film, and the delamination curves correlated with the corresponding flank wear curve.  
Figures 16 and 17 show images of holes entries and exits for the first and final holes drilled by the tested drills with the corresponding delamination factor values. The images presented the minor delamination of the last holes and the absence of cutting-induced breakout damages, such as burrs and splintering.  
Regarding the holes' size comparison, the drilled hole sizes were measured with a gauge plug having a nominal diameter of 6.36 mm. The gauge passed through all holes drilled by new and all reconditioned drills. Therefore, it is possible to conclude that the tested drills kept the drilled holes in the tolerance zone with the required size.  
Based on the above evaluations, it was suggested that holes produced by the TLC0, the TLC1, and the TLC2 had comparable qualities in terms of the hole size and the delamination.