As machining technology has advanced, difficult-to-cut materials -which is literally very difficult to be machined by conventional tools due to superior hardness and high strengthare widely utilized in industries. One of the difficult-to-cut material widely used in die mold and micro-fluidic devices is tungsten carbide (WC) [1] . This type of application requires nano scale surface integrity and form accuracy. One of the possible ways to fabricate them is ultra-precision cutting of WC workpiece with single crystalline diamond (SCD) tool or polycrystalline diamond (PCD) tool which would result in achieving tight tolerance and good surface integrity in nano scale. Many researchers have studied the machining of WC with SCD or PCD tools. They machined WC molds with diamond tools and achieved the surface quality of the mold less than 2 nm in Ra [2] [3] [4] . However, the friction which is inevitable in the tool-work interface generates heat, thereby increasing the temperature introduce shortening the tool life [5] .
Supplying metal cutting fluid is a general method to cool down the machined area in cutting. Metal cutting fluid dissipates heat in the machining area as well as decreases the friction coefficient in the tool-work interface. As the good heat dissipation and the low friction prevent the temperature of the tool from increasing quickly, tool life can be prolonged. However, the fluid hampers the environment and human health [6] . Most metal cutting fluid has harmful chemical components. In addition, the liquid is hard to perform as lubricant or coolant because the liquid cannot soak into the tool-work interface in ultra-precision machining. Therefore, cutting without metal cutting fluid becomes important [7] .
Dry cutting has advantages on the environment and cost, but it scarifies the tool life and poor surface integrity of a machined surface caused by high temperature. One of the available technologies to reduce the high temperature between a toolwork interface is surface texturing. Engraving patterns on a metal surface improves the tribological property. The linear, square, circle, or dot patterns were engraved on a WC tool and tested in the machining of aluminum and steel. The patterns
As machining technology has advanced, difficult-to-cut materials -which is literally very difficult to be machined by conventional tools due to superior hardness and high strengthare widely utilized in industries. One of the difficult-to-cut material widely used in die mold and micro-fluidic devices is tungsten carbide (WC) [1] . This type of application requires nano scale surface integrity and form accuracy. One of the possible ways to fabricate them is ultra-precision cutting of WC workpiece with single crystalline diamond (SCD) tool or polycrystalline diamond (PCD) tool which would result in achieving tight tolerance and good surface integrity in nano scale. Many researchers have studied the machining of WC with SCD or PCD tools. They machined WC molds with diamond tools and achieved the surface quality of the mold less than 2 nm in Ra [2] [3] [4] . However, the friction which is inevitable in the tool-work interface generates heat, thereby increasing the temperature introduce shortening the tool life [5] .
Supplying metal cutting fluid is a general method to cool down the machined area in cutting. Metal cutting fluid dissipates heat in the machining area as well as decreases the friction coefficient in the tool-work interface. As the good heat dissipation and the low friction prevent the temperature of the tool from increasing quickly, tool life can be prolonged. However, the fluid hampers the environment and human health [6] . Most metal cutting fluid has harmful chemical components. In addition, the liquid is hard to perform as lubricant or coolant because the liquid cannot soak into the tool-work interface in ultra-precision machining. Therefore, cutting without metal cutting fluid becomes important [7] .
Dry cutting has advantages on the environment and cost, but it scarifies the tool life and poor surface integrity of a machined surface caused by high temperature. One of the available technologies to reduce the high temperature between a toolwork interface is surface texturing. Engraving patterns on a metal surface improves the tribological property. The linear, square, circle, or dot patterns were engraved on a WC tool and tested in the machining of aluminum and steel. The patterns 
Texturing on the rake face of the tool without the metal working fluid is a potential method to reduce the friction coefficient in ultra-precision machining of tungsten carbide (WC) with a diamond tool. Low friction coefficient of the patterned tool would generate less heat in the tool-work interface and improve the tool life. This paper studies the effect of texturing on the cutting performance in ultra-precision machining of WC with a nano-polycrystalline diamond (PCD) tool. Orthogonal cutting experiments were conducted to verify the cutting performance of the textured diamond tool in machining of WC. The linear patterns parallel and perpendicular to the cutting direction were fabricated on the rake face of the nano-PCD tool by using the focused ion beam process. The experimental result showed that the friction coefficient of the perpendicular patterned tool decreased by 10%. WC surface machined with the patterned tool and non-patterned tool was also analyzed and compared. Material separation at grain boundary between tungsten grains and cobalt and full detachment of tungsten grains dominantly affected the surface quality of the machined WC surface.
As machining technology has advanced, difficult-to-cut materials -which is literally very difficult to be machined by conventional tools due to superior hardness and high strengthare widely utilized in industries. One of the difficult-to-cut material widely used in die mold and micro-fluidic devices is tungsten carbide (WC) [1] . This type of application requires nano scale surface integrity and form accuracy. One of the possible ways to fabricate them is ultra-precision cutting of WC workpiece with single crystalline diamond (SCD) tool or polycrystalline diamond (PCD) tool which would result in achieving tight tolerance and good surface integrity in nano scale. Many researchers have studied the machining of WC with SCD or PCD tools. They machined WC molds with diamond tools and achieved the surface quality of the mold less than 2 nm in Ra [2] [3] [4] . However, the friction which is inevitable in the tool-work interface generates heat, thereby increasing the temperature introduce shortening the tool life [5] .
Supplying metal cutting fluid is a general method to cool down the machined area in cutting. Metal cutting fluid dissipates heat in the machining area as well as decreases the friction coefficient in the tool-work interface. As the good heat dissipation and the low friction prevent the temperature of the tool from increasing quickly, tool life can be prolonged. However, the fluid hampers the environment and human health [6] . Most metal cutting fluid has harmful chemical components. In addition, the liquid is hard to perform as lubricant or coolant because the liquid cannot soak into the tool-work interface in ultra-precision machining. Therefore, cutting without metal cutting fluid becomes important [7] .
Dry cutting has advantages on the environment and cost, but it scarifies the tool life and poor surface integrity of a machined surface caused by high temperature. One of the available technologies to reduce the high temperature between a toolwork interface is surface texturing. Engraving patterns on a metal surface improves the tribological property. The linear, square, circle, or dot patterns were engraved on a WC tool and tested in the machining of aluminum and steel. The patterns
As machining technology has advanced, difficult-to-cut materials -which is literally very difficult to be machined by conventional tools due to superior hardness and high strengthare widely utilized in industries. One of the difficult-to-cut material widely used in die mold and micro-fluidic devices is tungsten carbide (WC) [1] . This type of application requires nano scale surface integrity and form accuracy. One of the possible ways to fabricate them is ultra-precision cutting of WC workpiece with single crystalline diamond (SCD) tool or polycrystalline diamond (PCD) tool which would result in achieving tight tolerance and good surface integrity in nano scale. Many researchers have studied the machining of WC with SCD or PCD tools. They machined WC molds with diamond tools and achieved the surface quality of the mold less than 2 nm in Ra [2] [3] [4] . However, the friction which is inevitable in the tool-work interface generates heat, thereby increasing the temperature introduce shortening the tool life [5] .
Supplying metal cutting fluid is a general method to cool down the machined area in cutting. Metal cutting fluid dissipates heat in the machining area as well as decreases the friction coefficient in the tool-work interface. As the good heat dissipation and the low friction prevent the temperature of the tool from increasing quickly, tool life can be prolonged. However, the fluid hampers the environment and human health [6] . Most metal cutting fluid has harmful chemical components. In addition, the liquid is hard to perform as lubricant or coolant because the liquid cannot soak into the tool-work interface in ultra-precision machining. Therefore, cutting without metal cutting fluid becomes important [7] .
Dry cutting has advantages on the environment and cost, but it scarifies the tool life and poor surface integrity of a machined surface caused by high temperature. One of the available technologies to reduce the high temperature between a toolwork interface is surface texturing. Engraving patterns on a metal surface improves the tribological property. The linear, square, circle, or dot patterns were engraved on a WC tool and tested in the machining of aluminum and steel. The patterns 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to significantly reduced the cutting force and friction coefficient [5, [8] [9] [10] [11] [12] [13] . In order to machine hard materials, many researchers have created patterns on diamond tools and cubic boron nitride (CBN) tools. Kawasegi et al. [14] fabricated linear patterns on a SCD tool using a focused ion beam (FIB) machining process and machined a NiP workpiece which is used to avoid a direct contact of a diamond tool with tool steel substrate. The cutting with linear patterns perpendicular to cutting direction showed good improvement in reducing friction coefficient by 18 %. Hao et al. [15] tested a PCD tool with linear patterns in the machining of Ti-6Al-4V. They observed that the tool wear and tool adhesion were alleviated and the friction coefficient decreased by 11 %. Kim et al. [16] created a CBN tool with micro-textures on the rake face and tested the tool on bearing steel. They reported that the friction coefficient of the textured CBN tool decreased by 28% than a non-textured tool. The previous studies have fabricated patterns on several types of tools and tested the tools on aluminum, steel, titanium alloy, and nickel alloy. However, the cutting of the WC workpiece with a textured tool has not been reported yet.
In the study, a nano polycrystalline diamond (nPCD) tool with linear patterns parallel and perpendicular to cutting direction was used in ultra-precision machining of WC workpiece in order to improve cutting tool performance. The linear patterns were engraved with FIB process. Experiments with the nPCD tool with and without the linear patterns were conducted to compare the cutting performance of the textured tool. The machined surface of WC was analyzed to investigate the effect of the patterned tool on surface quality.
A nPCD tool (UXR00202, A.L.M.T. tool, Japan) was used to machine WC. Fig. 1 (a) shows a SEM image of the rake face and flank face of the diamond tool. The diamond tool was delicately ground to achieve 203 nm in the waviness of the cutting edge. The sharpness of cutting edge can be evaluated with an edge radius which is the transition area between the rake face and the flank face of the tool [17] . As cutting edge affects the cutting force and surface integrity [18] , the edge radius should be controlled and smaller than the actual uncut chip thickness. The tool used in the paper has an edge radius of 350 nm and the sharpness was measured with an atomic force microscope (Catalyst AFM, Bruker, USA) as shown in Fig. 1  (b) .
Linear patterns parallel and perpendicular to the cutting direction were fabricated on the rake face of the nPCD tool with the focused ion beam (FIB) process. The fabrication conditions are listed in Table 1 . The perpendicular patterns have 8 lines with a width of 0.8 μm, depth of 1 μm, pitch of 1 μm, and length varied with respect to tool nose radius as shown in Fig. 1 (c) . The maximum length is 100 μm and the pattern starts at 2 μm from the cutting edge. The parallel patterns were engraved with a width of 0.8 μm, depth of 1 μm, and pitch of 1 μm. The pattern starts at 2 μm from cutting edge and ends 8 μm from the cutting edge. The engraved areas of the parallel and perpendicular patterns are equal. The pattern length in the cutting direction should be equal to or less than the tool-chip contact length. According to the literature [19] , the contact length can be calculated from the machining parameters and measured force as follows: (1) Where ϕ, t, α, and β are the shear angle, uncut chip thickness, rake angle, and friction angle, respectively. The shear angle can be obtained from Merchant's equation; ϕ=π/4 -β/2 + α/2. The friction angle can be calculated from tan -1 (μ). The friction coefficient, μ is calculated from the cutting force and thrust force follows: (2) Where Fc and Ft are cutting force, and thrust force, respectively. The estimated cutting length at uncut chip thickness of 5.5 μm is 7.8 μm which is shorter than the pattern length in the cutting direction. 
Orthogonal cutting of the WC workpiece was tested on an ultra-precision machine tool (ROBONANO α-0iB, FANUC Corporation, Japan). The ultra-precision machine tool can have positioning stages in X-, Y-, and Z-axis with single nanometer command resolution and rotate in B-and C-axis with single micro degree command resolution. WC workpiece was installed on a dynamometer (9119AA1, Kistler, USA) which could measure the cutting force in x-, y-, and z-axis simultaneously and a microscope was utilized to measure a distance between the nPCD tool and the WC workpiece as shown in Fig. 2 . The orthogonal cutting experiments were conducted with machining conditions listed in Table 2 . The uncut chip thickness varied from 0.5 to 5.5 μm in an increment of 1.0 μm. As waviness and an edge radius of the tool are less than 500 nm, the effect of the waviness and edge radius at uncut chip thickness more than 500 nm on cutting force and surface integrity would be negligible. The tool engaged and disengaged with a slope of 1/50 and 1/100 as shown in Fig. 3 , respectively. The experiment without metal cutting fluid was carried out to observe the tribological benefit of the patterns on the rake face. The tested surface of the WC workpiece was prepared with a commercial grinding tool. The prepared surface had 52.3 nm in Ra with 14.8 nm standard deviation. The surface quality of 10 arbitrary areas on the prepared WC surface was measured with a x20 objective lens in a white light interferometer (Zygo New view 6300, USA). The plane filter with 1/mm low frequency was applied to compensate the slope in the measurement. Fig. 4 (a) shows cutting force and thrust force in the machining of WC with a nPCD tool with and without patterns with respect to uncut chip thickness. Cutting force and thrust force is linearly proportional to the uncut chip thickness. As the uncut chip thickness increases, the cutting force with and without pattern similarly increase within 3% discrepancy but thrust force with the patterned tool is lower up to 12.6% than the non-patterned tool. The trust force with the perpendicular patterns is sequentially reduced by 4.7, 7.4, 8.8, 11.8, 10.4, and 12 .6%, and the thrust force with the parallel patterns decreases by -1.1, 3.6, 9.1, 7.2, 9.9, and 12.1% rather than the nonpatterned tool. Fig. 4 (c) shows that the tribological characteristic after texturing on the rake face of the tool is improved. The friction coefficient with parallel and perpendicular patterns decreased by 7% and 10%, respectively.
The basic mechanisms to reduce the friction coefficient could be chip entrapment and contact area reduction. Once microchips are generated, the small and hard particle would introduce abrasive and adhesive action between the tool-chip interface. The wear behavior increases friction force and degrades tool condition. The linear pattern can trap the particles and prevent the tool from being worn out [20] . Reducing the contact area between chip and tool would also decrease the probability for chips to cling the rake face of the tool. As linear patterns perpendicular to cutting direction could effectively trap the chip and prevent the small chip from abrasion and adhesion rather than parallel patterns, it is assumed that the friction coefficient with perpendicular patterns shows better performance.
Surface integrity in ultra-precision machining is one of the important outcomes. According to the literature [21] , the patterning on the tool would negatively affect surface integrity because the micro pattern could leave pattern marks on the surface of a workpiece. However, contrary to this statement, many studies have reported that the machined surface with the patterned tool was improved since cutting and thrust force was effectively decreased and then chattering in machining was reduced.
To verify the effect of the patterns on the surface integrity in ultra-precision machining, the surface quality of WC machined with the pattered tool was measured and compared with the non-patterned tool. The machined surfaces were randomly measured three times as the same conditions in the measurement of the prepared WC surface. Fig. 5 illustrates the surface roughness of WC machined with the patterned tool and non-patterned tool in Ra. As an uncut chip thickness increases, the surface roughness is degraded regardless of the patterned or non-patterned tool. Chattering in machining at high uncut chip thickness would affect the degradation [22] . Improvement of surface quality machined with the patterned tool cannot be observed. Hypothetically, the above-mentioned advantage and disadvantage of the patterned tool may cancel the effect. Fig. 6 shows the machined surface of WC measured with a microscope and a SEM. Micro cracks and pits are clearly observed in Fig. 6 (b) and (c). It is assumed that a part of tungsten grain would cut off or a tungsten grain would be totally detached from the surface in machining. Once a tungsten grain is totally detached because the binding force of cobalt around the tungsten grain is weak, the pits may be created. On the other hand, it is considered that material separation at the grain boundary between tungsten grains and cobalt by cutting force would introduce the micro cracks. The size of the micro cracks and pits is approximately 0.2 to 3 μm. 
This study investigated the effect of the patterned tool on cutting performance in the ultra-precision machining of WC. Linear patterns in parallel and perpendicular to the cutting direction on the rake face of the tool were fabricated with the FIB process. It was found through orthogonal cutting experiments that the friction coefficient of the tool after texturing linear patterns parallel and perpendicular to cutting direction is improved by 7 % and 10 %, respectively. The patterned tool did not show significant improvement in surface quality. The pits and micro cracks observed regardless of patterns in the machining of WC were dominantly affecting the surface quality.
In the next study, the parametric study will be carried out with different machining conditions such as feedrate, rake angle, and grades of WC. Furthermore, an analytical model to predict the friction coefficient in the machining of WC will be built in terms of width, length, depth, and shape of the pattern for the optimization of pattern design.