Characterization of functionally graded TİCN and TİBCN coating of cutting tools

Shortening the processing time and increasing the efficiency have been a need in machining at all times. The search for improving the properties of cutting tools in order to fulfill this need continues today. In addition to continuous developments in materials of cutting tools provided by the search every passing day, coating the surfaces of the existing cutting tools has been evaluated as more economical and more efficient solution. The purpose of coating the cutting tools is to improve the surface properties of the substrate material. Coating material must be compatible with substrate material and it must have a better high temperature, wear and friction resistance than substrate material. Thus, both the tool life is extended and the efficiency is increased. Studies deal with coating of cutting tools was started in the 1970s and coated tools were recognized as an important development in machining. Titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN), cubic boron nitride (CBN), chromium nitride (CrN) and aluminum oxide (Al2O3) are among the most widely used coating materials. Coating studies focused on especially high-speed steel and sintered carbide cutting tools as they have wide application area. In traditional coating studies, ceramic based materials are coated in the form of layers on metal based substrate materials. In this type of coatings, having different thermal expansion coefficients of the layers is an important problem. As a result of occurring high temperatures during metal cutting, the different thermal expansions take place between the layers. Different expansions cause micro-cracks and this reduces the coating life. As a result of efforts to prevent the formation of the micro cracks, functional graded coatings (FGC) has been developed. These kinds of coatings are non-layered and properties of coating material vary functionally along the coating thickness. The different thermal expansions in layered coatings are not seen in non-layered coatings therefore micro-cracks caused by thermal expansion differences are not observed. As a result of literature reviews, it has been considered that there is not enough research on the functional graded (FG) TiCN and TiBCN coatings of cutting tools. In this study, two different substrate materials, which are high-speed steel and sintered carbide, to be coated as functionally graded with titanium carbonitride (TiCN) and titanium boron carbonitride (TiBCN) has been aimed. It has been seen that, in the literature research and search of commercial use of cutting tools the thicknesses of coating were in the range of 3-9 µm. Before coating of main substrate materials, the coating conditions needed for targeted thickness and structure have been determined by means of pre-experimental studies. Coatings have been made by magnetic sputtering method. Firstly, scanning electron microscopy (SEM) images have been taken and linear EDS analyses have been realized for the characterization of the coatings. As a result of the images and linear analyses, it has been seen that both titanium carbonitride (TiCN) and titanium boron carbonitride (TiBCN) coatings had targeted structure (FD) and thickness of 3-9 µm. Then, in order to determine characterization of coatings, XRD analysis, scratch test, wear test and hardness measurement have been performed. After the characterization of the coatings, machining lifes of high speed steel and sintered carbide tools have been tested without using any cutting fluid. Due to structural differences, usage areas and operational parameters of high speed steel and sintered carbide tools also differ from each other. Hence, performing of machining life tests of two different tools using two different materials have been preferred. The workpieces to be machined for both tools have also been noted to depict commonly using materials in manufacturing. AISI 1040 has been machined by using high speed steel, while AISI 4140 has been machined by using sinter carbide tools. For both tools, depth of cut (ap) of 2.5 mm, feed rate (?) of 0.25 mm/rev have been determined. Cutting speed for high-speed steel and sintered carbide tools have been selected to be 40 m/min and 100 m/min, respectively. ?Uncoated?, ?laminar TiBCN coated? and ?functionally graded TiBCN coated? high-speed steel and sinter carbide tools have been in machining. Thus, tool lifes of both ?uncoated? and ?coated? and ?laminar coated? and ?functionally graded coated? tools have been compared. At the end of machining, it has been seen that functionally graded coated tools have longer life than laminar coated tools. In addition, ?Wear ? Time? graphs have been drawn for high speed steel and sinter carbide tools. The effects of worn tools on the workpiece surface roughness have also been investigated along with tool life determination. The roughness of the surface machined with functionally graded coated tools has been less than that of machined with laminar coated tools. ?Roughness-Time? graphs have been drawn for AISI 1040 and AISI 4140 workpieces.