E ect of Face Milling Parameters of Carbon Fiber Reinforced Plastics Composites on Surface Properties

This paper analyses the infl uence of face milling process parameters on the surface properties of carbon fi bre reinforced polymer. The infl uence of milling speed and feed per tooth on the surface properties was determined. The infl uence of cutting speed and feed per tooth on surface energy properties was determined. The object of research was a carbon fi ber reinforced plastics (CFRP) composites plate made of carbon fi bre in epoxy matrix. The too l used in the study was a double-edged end mill. The machining parameters used were variable: cutting speeds of 100 m·min-1, 120 m·min-1, 140 m·min-1 and 160 m·min-1, and feeds per tooth of 0.015 mm/tooth, 0.020 mm/tooth, 0.025 mm/tooth and 0.03 mm/tooth. The axial depth of cut and radial depth of cut was a constant parameter. After milling, tests were carried out on the surface contact angle, which was used to determine the surface free energy. Based on the contact angle measurements carried out with the sitting-drop method and the calculation of the surface free energy with the Owens-Wendt model, it was observed, that the increase in the value of the surface free energy is signifi cantly infl uenced by the increase in the cutting speed.


INTRODUCTION
Polymer matrix composites have been of immense interest to engineers and researchers alike for the past decade and this interest is still increasing. A carbon fi bre reinforced plastic composites consist of carbon fi bres as reinforcement and a matrix phase in the form of a polymer resin [1,2]. The fi bres are load-bearing and the bonds and protects the carbon fi bres with the matrix [3][4][5][6]. CFRP composites have excellent properties such as a high stiff ness-to-weight ratio and low density, or fatigue and wear resistance. They are also characterised by high corrosion resistance, dimensional stability, and low coeffi cient of friction and low electrical conductivity, as well as low thermal expansion [7][8][9][10]. Fiber reinforced composite materials are used in industry sectors, e.g.: aerospace, yachting, shipbuilding, automotive, sports, medicine as well as biomedical applications [11][12][13][14]. The production of the Airbus A350 XWB aircraft is an example of the use of CFRP composites. In this case, CFRP accounts for up to 50% of the total material, and components made from CFRP included wing spars and parts of the fuselage. The unpressurised fuselage, landing gear as well as the trailing edge on commercial aircraft are also produced from CFRP composites [13].
Polymer composites, which are a relatively new type of engineering materials, were subjected to numerous studies. These works mainly focus on the description of their surface quality after machining operations in terms of geometry and surface topography. One of the papers concerning milling of this type of materials was published by K. Ciecieląg [15], in which the influence of face milling parameters on the surface roughness of glass and carbon fibre reinforced plastics was determined. R. Teti, in his work [16] presents a general description of composites with polymer, metal or ceramic matrices, where he also described the turning, cutting, milling and drilling process of each of these materials.
When machining fibre reinforced composite materials, it is important to use tools with the suitable geometry, which are made of wear-resistant materials [17]. In this study, the materials were prepared by face milling. Polymeric composites are subjected to face milling in many cases, including as a means of surface preparation for adhesive processes, as well as a finishing treatment to remove the allowance created during manufacturing, or just to improve the surface quality [18][19][20]. The selection of the milling parameters should take into account in particular the structure of the material and the orientation and type of fibres [21]. By reviewing previously published research papers on the milling process of carbon fibre reinforced plastics composites [15,19,[22][23][24][25][26] it was observed that the cutting speeds used by researchers ranged from 20 to 250 m/min, the feed per tooth in the range of 0.01 to 0.5 mm/blade and the depth of cut from 0.1 to 4 mm. Analysing the research results presented in the above-mentioned works, it can be observed a variety of results and a changeable influence of the applied processing parameters on the surface quality of the obtained surface. However, taking into account the adhesive or energetic properties of the surface, in the current review of literature there were no papers describing the influence of the face milling process of carbon fibre reinforced plastics composites on the value of the surface contact angle and the surface free energy.
Studies of the properties of CFRP composites, including surface energetics, wettability and surface free energy, may prove to be important elements in the assessment of surface properties.
Moreover, as A. Rudawska presents in paper [27], the observation and analysis of contact angle values and surface free energy is also important when processing the tested composite materials by adhesion-based processes. Similar observations in other papers are presented by Kłonica and Kuczmaszewski [28,29] that the energy state of structural materials is particularly important in technologies where adhesion is crucial for process performance. This makes it possible to analyse the adhesive properties of surfaces that are applied to processes such as painting, coating or bonding. Therefore, the aim of this study is to analyse the influence of cutting parameters such as face cutting speed and feed per tooth on the surface energy properties of carbon fibre reinforced plastics composites determined by the contact angle and surface free energy.

Materials used in the study
The material used in the study was CFRP plates. The plates were made using vacuum bag technology with a vacuum pressure of 0.9 bar. The moulds used were two steel plates with a polished surface treated with Frekote 770NC liquid release agent from Loctite Company (Hemel Hempstead, UK) placed in a plastic bag and then electrically resistance welded. The board layers were impregnated layer by layer using the wet method and applying successive layers by hand. The prepared material was cured in an oven at 50° for 4 hours, followed by an 8-hour heat treatment at 110°C. The material was then left to cool down spontaneously. After the plates were de-moulded, their edges were CNC machined to obtain the final dimension. The final thickness of the CFRP sheets was 9.5 mm.
A carbon fibre fabric in an epoxy matrix was used to manufacture the CFRP composites The outer layers of the board were two layers of 2X2 twill CF fabric by 245 g/sq. 3K threads. The stacking sequence of the exterior layers was [0/90/±45]. In interior of the CFRP plates was used ±45 biaxial CF by 300 g/sq, type CF-BI-300-127. The stacking sequence of the layers was [±45]. MGS® RS-L 285 epoxy laminating resin and Hardeners RS-H 286 catalyst from Lange-Ritter GmbH (Gerlingen, Germany), were used as matrices. The mixing ratio of resin and catalyst was 100:40 parts in weight ratio. The stacking sequence of the exterior layers was [0/90/±45]. In interior of the CFRP plates was used ±45 biaxial CF by 300 g/sq, type CF-BI-300-127. A schematic layout of the material layers in the plate structure is shown in Figure 1.

Test stand and tools
The machining of CFRP was carried out on an Avia VMC 800HS vertical machining centre. Specimens were clamped in a vice on the machine's milling table. A view of the station and specimen clamping is shown in Figure 2.
The tool used in this study to machine the CFRP surface was an end milling cutter dedicated to machining composites (shown in Figure 3). The Seco cutter has a diameter of ø12mm and is fitted with two teeth and is commercially available as EDP 65056.
During the concurrent face milling of the CFRP composites, the constant parameters were the axial depth of cut and radial depth of cut. The variable parameters were cutting speed and feed per tooth. The machining parameters used in the study are presented in Table 1.

Test stations and computational methods
After the face milling process contact angles were measured, which were then used to determine the surface free energy (SFE), which was determined using the Owens-Wendt model. The Owens-Wendt model is based on the direct measurement of the contact angle using two measuring fluids (non-polar -diiodomethane CH 2 J 2 and polar -distilled water), whose value is known for the polar and dispersive components of the SFE, which are presented in Table 2. On the basis of the obtained angle values, the polar and dispersive components of the SFE were determined, according to the equation (1) [30][31][32].
The components γ s d and γ s p of the SFE of the interfaces can be calculated from the equations (2) i (3): -polar component of the SFE of water, θ d -the value of the contact angle measured with diiodomethane, θ w -the value of the contact angle measured with distilled water.
The droplet volume of the measuring liquids was about 2 μl. Contact angle measurements were carried out at 21±1°C and 30±1% air humidity. PGX pocket goniometer was used for the measurements. Ten repetitions of the measurement were carried out on each sample with each measuring liquid. All results of contact angle measurements, both for water and for diiodomethane, were statistically processed, which became the basis for the determination of the SFE by the Owens-Wendt model.
After the face milling process, control images of the surface structure were also taken with an Alicona InfiniteFocusG5 3D microscope (Raaba, Graz, Austria).

RESULTS AND DISCUSSION
On the basis of the research carried out, values of the contact angle were obtained for the surfaces machined by face milling using the parameters in Table 1. The obtained results were statistically processed and the average values were determined, and the standard deviation was determined. The results are presented in Table 3.
Based on the results presented in Table 3, it can be seen that the contact angle value for water was higher for lower cutting speeds. Similarly for the contact angles measured with diiodomethane, but in this case the range of scatter of the results was not so high. The standard deviation of the obtained results was within the following ranges: for water: 8.3-17.7%, for diiodomethane: 5.1-14.2%. These standard deviations may be due to defects on the processed surface caused by fibres being pulled out  Table 4. It also shows the droplet obtained during contact angle measurements for a set of technological parameters of face milling of surfaces, for which minimum and maximum of contact angles were obtained. Analysing the images shown, it can be seen that the machined surfaces exhibited localised pulling out of the fi bres of the outer layer of the CFRP composites. In the case of surfaces where  On the basis of the graphs presented, it can be seen that the value of the SFE increases with an increase in the cutting speed from 100 m·min -1 to 160 m·min -1 . There is also an increase in the polar component of the SFE, while the dispersion component decreases or remains at a similar level. In each case, regardless of the parameters of machining the  For a more detailed comparison, the eff ect of changing the feed per tooth on the value of SFE was also evaluated. This comparison is shown in Figures 8-10. However, in order to accurately analyse the results obtained, it is necessary to perform a The assumption of normality of distribution and equality of variance was fulfi lled at the assumed signifi cance level α = 0.05. At fi rst, a correlation study between factors was carried out to determine the infl uence of machining parameters on the value of SFE of the CFRP composites after face milling. Table 5 summarises the results obtained.
Analysing the results of the correlation tests presented in Table 5, it can be observed that in case of cutting speed, the Pearson correlation coeffi cient is 0.80, which indicates a strong linear dependence of cutting speed on SFE. The coeffi cient of determination is 0.64, which means that the change in SFE is explained 64% by the change in cutting speed. The signifi cance level p for the t statistic is less than 0.05, which means that the correlation coeffi cient is signifi cantly diff erent from 0. When analysing the feed rate per tooth, it can be seen that the correlation coeffi cient is  where: r (X,Y) -Pearson correlation coeffi cients, r2 -coeffi cient of determination, t -value of the t statistic testing the signifi cance of the correlation coeffi cient, p -the calculated signifi cance level for the t-test. only 0.10, so it can be concluded that there is no relationship between the feed rate per tooth and the value of SFE. In order to illustrate the change in SFE as a function of the change in face milling process parameters, scatter diagrams of the results obtained are shown in Figure 12.
The next step was to evaluate the diff erences between the obtained the SFE results. In order to fi nd unequivocally which parameters allow to obtain a CFRP composites surface with the lowest SFE, i.e. characterised by hydrophobic properties, an ANOVA analysis was performed. The Tukey test of homogeneous   Table 6. When machining CFRP composites, it is desirable to obtain the highest possible value of the polar liquid contact angle and, at the same time, the lowest value of the SFE when considering the acquisition of hydrophobic properties. When face milling of a composites is used as a surface preparation treatment for further operations, it is expected to increase the surface wettability [27]. According to the results presented in Table 6, it can be observed that lower cutting speeds produce a surface characterised by a lower value of SFE. The results obtained at cutting speeds of 100 m/min and 120 m/min place them in one homogeneous group. The last homogeneous group contains the results obtained at the highest of the used cutting speeds, 140 m/min and 160 m/min.

CONCLUSIONS
Carbon fibre reinforced plastic composites, are characterised by excellent properties and therefore attract the attention of researchers and are very popular in numerous engineering applications such as automotive, aerospace, biomedical applications, sports items, robot cells, etc. The knowledge of the surface properties of engineering materials is very important for the preparation of the surfaces of components, for further operation, as they influence all the operations involved in the joining, painting or coating process, as well as the resistance to external factors.
In this paper, the influence of cutting parameters such as cutting speed and feed per tooth on the surface energy properties of CFRP was evaluated. For this purpose, the contact angle values were measured with two measuring liquids: polar -distilled water, and dispersive -diiodomethane. On the basis of the obtained values, the SFE was calculated using the Owens-Wendt model. On the basis of the results obtained, the following conclusions were formed. The contact angle values for both measuring liquids were higher for lower cutting speeds. In the case of angle measurements with a dispersion liquid, the scatter of results was lower. For both water and diiodomethane contact angle measurements, the standard deviation did not exceed 15%. Differences between contact angle measurements within one surface could be due to surface defects caused by fibres being pulled out of the CFRP plate structure during milling. On surfaces with less surface defects, the values of the contact angles measured with the polar liquid were higher, which may consequently result in less wetting of the surface by the measuring liquid. Such surfaces show hydrophobic properties. As the cutting speed increases, the value of the surface free energy increases. There is also an increase in the polar component of the surface free energy, while the dispersion component decreases or remains at a similar level. Regardless of the CFRP composites surface machining parameters, the dispersion component was significantly higher than the polar component (more than 70-90 % of the total SEP value).
There is a strong linear correlation between the cutting speed and the surface free energy, as shown by the Pearson correlation coefficient of 0.80, while there is no correlation between the feed per tooth and the surface free energy, with a correlation coefficient of only 0.10. Lower cutting speeds on the CFRP composites allow surfaces with lower surface free energy to be obtained, and the results obtained at cutting speeds of 100 m·min -1 and 120 m·min -1 fall into one homogeneous group. These parameters should be used when the machining objective is to obtain hydrophobic properties of the surface. Higher cutting speeds for face milling of the CFRP composites produce surfaces characterised by higher surface free energy values. The results obtained at the highest of the applied cutting speeds, 140 m·min -1 and 160 m·min -1 , fall into one homogeneous group. These parameters should be used when the machining objective is to increase the surface wettability and improve the surface adhesion properties.
It can be seen that the surfaces tested in this study, irrespective of the face milling parameters, are characterized by a rather significant value of the SFE, being in the range of 34.7-49.4 mJ/ m 2 . Since the SFE of polymeric plastics according to various sources is usually between 20 and 56 mJ/m 2 these are therefore average and one of the higher values of the SFE, which means in case of lower values better hydrophobic properties, and in case of higher values better adhesive properties. The information presented can have a significant impact on the planning of the surface machining of CFRP composites. Further research is planned to compare the value of surface free energy with the parameters of roughness and geometric structure of the surface after face milling. It is also planned to check the correlation of face milling process parameters on surface energy properties, using other end milling cutter.