# Study on Burr Formation and Tool Wear in Drilling CFRP and Its Hybrid Composites
###### Abstract
Metallic, polymer and ceramic materials are commonly used as the matrices, while particles or crystal filaments in the form of fiber are commonly used as the reinforcements. Various chemical compositions and microstructural arrangements are possible in each matrix category. In general, fibers are the principal load-carrying members, while the surrounding matrix keeps them in the desired location and orientation [1; 4]. This kind of composite material, formed by high-strength fibers and moduli embedded in or bonded to a matrix with distinct interfaces, is called a fiber-reinforced composite (FRC). FRC can be classified into four groups, according to their matrices--metal matrix composites (MMCs), ceramic matrix composites (CMCs), carbon/carbon composites (C/C), and polymer matrix composites (PMCs) or polymeric composites [6]. Among these materials, fiber-reinforced plastics/polymers (FRP) are either comparable to or better than many traditional metallic materials (e.g., steel and aluminum alloys), due to their high strength-to-weight ratio, stiffness-to-weight ratio, fracture toughness and corrosion resistance [7; 8]. Three common FRP mainly include carbon FRP (CFRP), glass FRP (GFRP), and aramid FRP (AFRP). FRP is widely used in the construction, military, marine, aerospace, and automotive industries, and other fields based on their advantages, such as low density, high damping ability, high strength, high thermal resistance, low thermal expansion, stability, good corrosion resistance, etc. [8; 9; 10]. Therefore, the use of lightweight and elevated strength composites will become the mainstream of industrial design and manufacturing in the future. Especially in the field of aerospace and the automobile industry, the use of lightweight and high-strength materials has greatly help to reduce fuel consumption and improve safety. However, FRP materials have various machining failures, including delamination (separation of laminated layers), burrs, fiber pull out, fiber break, uncut fibers, melting of the matrix, adhesion of materials to drill, and sub-surface failures, due to the non-homogeneity, multi-phase structure and anisotropic nature of the composites [11; 12; 13]. The common problems of finish machining and surface integrity often occur when using conventional processing tools, such as drilling, cutting and milling to FRP. This leads to the scrapping of a large number of workpieces, which is also one of the main reasons for the high machining cost of CFR. Among various machining operations, drilling is the most frequently used process in industrial operation for polymer composite laminates, owing to the need for joining structures in various industries, such as the automotive, aerospace, marine and oil industries. For example, the Airbus A350 aircraft (large-size CFRP composite components) needs about 55,000 holes in the manufacturing process [13]. Some drilling damage, such as delamination, has a negative impact on the load-carrying capacity of the structure, which directly leads to the decrease in the durability of the material [14]. In addition, the cutting tool is also very easy to wear in the drilling of FRP materials, because FRP is a difficult-to-cut material, due to the non-homogenous and anisotropic properties of carbon fiber [15; 16; 17]. The machinability of FPR materials mainly depends on the fiber matrix interface interaction, fiber orientations, cutting directions and tool wear. Moreover, different machining parameters, cutting tools, tool coatings, and lubrication modes have different damage degrees on tool wear and composite laminates [18; 19; 20; 21; 22]. Therefore, many researchers, in general, have sought to determine the optimum cutting parameters to avoid the above failures [23; 24]. [PERSON] et al. [25] found that delamination in fabric woven CFRP laminates is observed by SEM images, which is closely related to the condition of the diamond coating. They also pointed out that increasing the feed rate and rotational speed can protect diamond-coated carbide bits and improve hole quality.