# Impact of Cryogenic Condition and Drill Diameter on Drilling Performance of CFRP
## 3 Results and Discussion
### Observation from the SEM and AFM  
Holes drilled using 50 m/min cutting speed, four different feed rates (0.075, 0.15, 0.225, 0.3 mm/rev) and a 4 mm drill under both dry and cryogenic conditions divided into two parts using a hand saw, and boreholes, were examined in detail with an atomic force microscope (AFM) and scanning electron microscope (SEM). In the AFM measurements, an area with the dimensions 30 \(\times\) 30 \(\upmu\)m was scanned with 60 \(\upmu\)/s scanning speed. The average surface roughness (Ra) and root mean square roughness (Rq) values obtained from the AFM are shown in Figure 7 and three-dimensional (3D) views are shown in Figure 8. Three measurements were taken from the borehole surface and their average was calculated for each hole. As clearly seen in Figures 7 and 8, as the feed rate increased, surface quality deteriorated. In addition, cryogenic conditions presented better surface quality than dry conditions in the drilling of CFRP. This case can be clarified in that the liquid nitrogen supplied both cooling and lubrication and thus temperature rise was prevented in the cutting zone. The main reason why higher forces occur in cryogenic machining is explained in the literature as follows: engineering materials show much more resistance to deformation at low temperatures than at high temperatures. Also, it was reported that the Young modulus and tensile strength of CFRP increase as the temperature of the material decreases. These variations in the material properties cause much higher thrust forces to be generated during machining. Although cryogenic machining increases thrust force, it enhances surface quality. Reducing temperature at the cutting zone prevents the occurrence of thermal damage on the borehole surface and alters the material properties from ductile to brittle, thus smoother surfaces can be obtained through cryogenic machining. Besides, it has been reported that cryogenic cooling generates higher quality holes by helping to prevent tool wear [17; 25; 26]. The results show that cryogenic machining both decrease cutting temperatures and increase thrust forces. When the overall process is evaluated, it is concluded that cutting temperature is a more effective factor on borehole surface quality than thrust force. In addition, a combination of lower cutting temperatures, lower thrust forces and lower feed rates presented better performance. Figure 6: The effect of hole numbers on the delamination factor and thrust force. Figure 8: _Cont._  
Figure 7: The average surface roughness and root mean square roughness under several machining conditions. Drilling in dry conditions generates higher temperatures and thus lower forces occur, but drilling with LN\({}_{2}\) reduces cutting zone temperature excessively, so higher forces occur. Low thrust forces and low cutting temperatures are desired so as to obtain better machining quality. Higher temperatures and larger cutting forces bring about undesirable defects such as delamination, fiber pull out and rough surface finish in the drilling of heterogeneous materials. The surface morphology of the drilled hole surface was studied using SEM imaging to identify the type of drilling-induced damage and defects. In this SEM study, fiber pullout, fiber breakout, matrix crack and matrix smearing were the primary surface defects or damages observed. A total of 324 holes were drilled on a CFRP material using a constant 50 m/min cutting speed and 0.15 mm/rev feed rate in dry and cryogenic conditions. The SEM images of the boreholes are demonstrated in Figure 9 for dry conditions and Figure 10 for cryogenic conditions. In Figure 9, it is shown that some partitions of carbon fibers were pulled away from the machined surface leaving cavities that caused a rough surface when dry drilling. In dry drilling, high cutting temperatures lead to the softening and decomposing of the matrix, resulting in poor support for fibers. This causes fibers to be pulled out and removed. Consequently, high surface roughness occurs due to cavities.