Cutting performance and antifriction mechanism of Al2O3/TiC/TiB2/h-BN@Al2O3 self-lubricating ceramic tool

The machinability and wear reduction mechanism of self-repairing and self-lubricating ceramic tools sintered by vacuum hot-pressing method in the dry turning of 40Cr hardened steel was studied. By comparing the cutting performance and wear morphology of AT (Al2O3/TiC) ceramic tools under different cutting parameters, it was found that AT10B@5 (Al2O3/TiC/10 vol% TiB2/5 vol% h-BN@Al2O3) tool has a longer service life and better machining quality. Owing to the precipitation of solid lubricant during the cutting of AT10B@5 ceramic tool, the friction force during the cutting is reduced, thus decreasing the cutting force and cutting temperature of AT10B@5 ceramic tool during the cutting. The main cutting force decreased by 20.8%; the cutting temperature decreased by 22.2%; and the friction coefficient of front tool face decreased by 11.6% compared with AT tool. This effectively improved the surface quality of working parts, reduced the tool wear, increased the processing quality of work piece, and prolonged the tool life.


Introduction
Dry cutting [1][2][3], a processing technology that can effectively reduce environmental pollution, has been widely studied in the world. However, in the processing of difficult-toprocess materials, because of the low thermal conductivity of the difficult-to-process material itself, it is difficult to remove cutting heat through the chip, and dry cutting tool and work piece will have higher friction and adhesion, making the cutting temperature very high, aggravating tool wear, and shortening the life of the tool. Therefore, the manufacturing of new cutting ceramic tools that satisfy the processing conditions and effectively reduce the friction and increase the life has become the research focus.
A large number of scholars studied the wear resistance of new ceramic tools. Huang et al. [4,5] prepared ceramic tools with high bending strength and fracture toughness using the SiC whisker toughening method, resulting in a higher cutting wear resistance, but still could not solve the problems of high friction and high cutting temperature in dry cutting. Deng et al. [6] carried out dry high-speed machining experiments on hardened steel with Al 2 O 3 /TiB 2 -based ceramic tool and found that the lubrication film reduced the bonding between the tool and working parts and between the tool and chip, thus reducing tool wear. He et al. [7] studied the wear morphology of high-hardness alloy steel 20CrMo with coated cutting tool under dry cutting condition and pointed out that due to the good chemical stability and wear resistance of coating, the tool wear rate was greatly reduced. Xing et al. [8] added solid lubricant to Al 2 O 3 /TiC-based ceramic tools. The cutting force, cutting temperature, friction coefficient, and tool wear of the self-lubricating tool substantially reduced compared with those of a traditional tool. Deng et al. [9] determined that the mechanism was the formation of self-lubricating film on the tool-chip interface. However, owing to the low performance of solid lubricants, they cannot be applied well to cutting high-hardness working parts.
Considering the friction reduction and mechanical properties, SiC-coated h-BN powders ((h-BN)/SiC) [10] or Al 2 O 3 -coated h-BN powders ((h-BN)/Al 2 O 3 ) [11] were used to substitute h-BN as solid lubricant. The cutting performance and wear resistance significantly improved compared with those of ceramic tools with direct lubricant, thus avoiding the adverse effects of direct addition of h-BN. Zhang et al. [12] used Al 2 O 3 -coated CaF 2 powders prepare ceramic tool materials to perform dry cutting experiments on 40Cr hardened steel. The results showed that the addition of CaF 2 @Al 2 O 3 particle effectively reduced the adverse effect of directly adding CaF 2 particle on ceramic tool.
A ceramic tool is brittle [13][14][15]. During cutting, defect such as microcracks can easily occur. Thus, the mechanical properties of the tool are reduced. If not handled timely, the tool may even fail. Lange and Gupta [16] reported crack healing during heat treatment. Self-healing ceramic tools can effectively utilize the cutting heat, generate a liquid phase, fill cracks, and achieve self-healing of cracks. The self-healing of ceramic cracks [17][18][19] can not only help to recover the material strength, but even improve the mechanical properties of the tool, important for improving the reliability of ceramic materials. Zhai et al. [20] studied the surface stress crack healing behavior of nickel oxide aluminum bronze (NAB)/Ti 3 SiC 2 nanocrystalline composites that can simultaneously decompose and heal. Crack healing induced by wear was achieved by the decomposition and oxidation of Ti 3 SiC 2 on the surface. The improvement in friction performance was related to self-healing. In [21,22], the selfhealing effect of SiC is studied, and it is found that surface crack closure can be attributed to the formation and volume expansion of oxidation product layer. However, oxide evaporation and gas formation are harmful to crack repair ability [23]. Nguyen et al. [24] studied the self-healing behavior and strength recovery of YB 2 Si 2 O 7 -YB 2 SiO 5 -SiC ceramics reinforced by silicon carbide nanofillers and solved the problem that the volatilization of silica in the reinforcement material reduces the self-healing effect of ceramics. However, there are few reports on the self-healing phenomenon of cutting tools in the cutting process.
In this study, a new type of Al 2 O 3 /TiC/TiB 2 /h-BN@Al 2 O 3 self-lubricating ceramic tool with self-repairing ability was prepared using vacuum hot-pressing method. TiC is the reinforcing phase, TiB 2 is the repairing phase, and h-BN@Al 2 O 3 is the lubricant. In order to show that the self-lubricating ceramic tool with self-healing function can obtain better surface machining quality when dry cutting 40Cr hardened steel. The cutting performance of the tool was studied by measuring the cutting force, cutting temperature, and surface roughness under different cutting parameters. The tool surface was characterized by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS), and the effects of tool self-lubrication and self-healing on tool wear were studied.

Preparation of ceramic tool
The raw materials required for the preparation of ceramic tools are shown in Table 1. Al 2 O 3 /TiC/10 vo l%TiB 2 /5 vo l%h-BN@Al 2 O 3 and Al 2 O 3 /TiC ceramic cutting tools were prepared using vacuum hot-pressing sintering. The sintering temperature was 1650 °C; the holding time was 20 min; and the hot-pressing pressure was 32 MPa. The dry cutting tests of ceramic tools with TiB 2 particles and coated solid lubricant h-BN@Al 2 O 3 particles were compared with those of Al 2 O 3 /TiC ceramic tools.
For the convenience of description, the ceramic tool with 10 vo l%TiB 2 particles and 5 vo l%h-BN@Al 2 O 3 particles is denoted as AT10B@5, and the Al 2 O 3 /TiC ceramic tool is denoted as AT. Table 2 shows the content and mechanical properties of the components in the tools. Table 2 shows the mechanical properties of the two materials. The addition of h-BN@Al 2 O 3 particles reduces the mechanical properties of the material. Compared with AT materials, the bending strength decreased by 2.24%, and the fracture toughness and Vickers hardness did not decrease.
The comprehensive mechanical properties of ceramic embryos prepared by vacuum hot-pressing sintering were tested. First, the material pretreatment including slicing, rough grinding, fine grinding, grinding, and polishing was conducted. The material was processed into standard strip samples with a surface roughness Ra of less than 0.1 μm and size of 3 mm × 4 mm × 35 mm. To eliminate the effect of stress concentration, each edge of the spline was chamfered. In this study, the flexural strength of ceramic tool samples was tested using a three-point flexural method. A WDW-50E microcomputer-controlled electronic universal testing machine manufactured by Jinan Group Co., LTD. was used. An hV-120 Vickers hardness tester was used to measure the hardness of ceramic tool samples prepared using indentation method. In this study, the fracture toughness of ceramic materials was measured using indentation method. The elemental distribution of the material was analyzed by energy-dispersive X-ray spectroscopy (EDS).

Cutting experiment
The cutting workpiece was made of 40Cr hardened steel with a material hardness of 48-50 HRC. Geometric parameters of ceramic tool: The tool size was 12.7 mm × 12.7 mm × 7.9 mm (Table 3).
A CDE6140A lathe produced by Dalian Machine Tool Group was used as the machine tool, and the tool holder was Kenner GSSN R/L 2525M12-Mn7. The wear degree of the front and rear surface of the tool was observed using a PXS-1020 tool microscope and SUPRATM 55 thermal field-emission scanning electron microscope. The tool failure standard was VB = 0.3 mm. When the tool failed, the total cutting distance was used as the basis for evaluating the cutting performance of the tool. The roughness of the workpiece surface was measured using a TR200 hand-held roughness meter. The cutting temperature was measured using a FLAR-A320 infrared thermal imager. The cutting force was measured using a Kistler 9129A dynamometer.

Cutting performance of tool
The effects of different cutting speeds (v), feed rates (f), and depths of cut (a p ) on the wear of tool material and the surface roughness of workpiece were studied to determine the cutting performance of ceramic tool in cutting 40Cr hardened steel.   Figure 1 shows the changes between the cutting distance S and (a) wear amount of back tool surface VB and (b) workpiece surface roughness Ra at different cutting speeds (f = 0.102 mm/r, a p = 0.2 mm) when the AT10B@5 and AT tools cut the workpiece 40Cr hardened steel.
As shown in Fig. 1a, the cutting speed of the tool substantially affects the life of tool. When the cutting speed was 100 m/min, the effective cutting distance of AT10B@5 and AT tools was greater than 6000 m. The tool wear was small, and the wear rate was slow. With the increase in cutting speed, when the cutting speed increased to 300 m/ min, the wear of back tool face increased rapidly with the increase in cutting distance, and the wear state of back tool face showed a linear growth state. When the effective cutting distance was about 5500 m, the wear state of back tool face reached the failure standard. The wear of back tool face of AT10B@5 tool showed a slow increasing trend, belonging to the stable wear state. The effective cutting distance was more than 6000 m, and the back tool face did not reach the wear state and can be continued to use. By comparing AT10B @ 5, similar knife surface wear rules can be found for the cutting tool. The effective cutting distance of AT10B @ 5 knives is longer compared with the AT tool, mainly because of the precipitation of a solid lubricant in high-speed cutting. This increased the lubrication effect of cutting tools, reduced the friction resistance between the cutting tool workpiece, and decreased the blade wear. Figure 1b shows that with the increase in the cutting speed of the two tools, the machining surface roughness of the workpiece had an overall downward trend. This shows that high-speed cutting can obtain better surface finishing quality. The surface roughness of workpiece after machining by AT tool was generally between 1 and 3.5. The surface roughness was large when the cutting speed was 100 m/min, and the surface roughness reached 3.3 when the cutting distance reached 6000 m. The workpiece surface roughness of AT10B@5 tool after machining was maintained in the range of 0.1-1. The overall surface roughness was reduced compared with AT tool. Under the same cutting conditions, the surface roughness of AT10B@5 tool was smaller. This is because the addition of h-BN@Al 2 O 3 substantially increased the lubrication performance of the tool. During cutting, the solid lubricant h-BN@Al 2 O 3 precipitated on the surface of the tool and dragged to form a film, thus reducing the friction between the tool and workpiece and improving the workpiece surface processing quality. At the same time, it was observed that the roughness of workpiece decreased with the increase in the cutting distance of AT10B@5 tool to 3500 m and then continued to increase slowly. This is because with the increase in cutting distance, a large amount of cutting heat was generated on the tool surface. This led to the oxidation reaction of TiB 2 in the matrix, generating a large amount of molten TiO 2 with lubrication effect and thus increasing the lubrication performance of the tool and improving the workpiece surface processing quality. Figure 2 shows the changes between the cutting distance S and (a) wear amount of tool surface VB and (b) workpiece surface roughness Ra when AT10B@5 and AT tools cut the workpiece 40Cr hardened steel at different feeding rates (v = 300 m/min, a p = 0.2 mm). Figure 2a shows that with the increase in feed, the cutting performance of the two tools decreased due to the increased cutting resistance between the tool and workpiece due to a larger cutting feed, leading to severe tool wear. By comparing AT and AT10B@5 tools, it was found that the effective cutting distance of AT tool was 5500 m when the feed a Relationship between the wear amount of workpiece rear tool surface and cutting distance under different cutting material feeding b Relationship between workpiece surface roughness and cutting distance under different cutting material feeding speed was 0.102 mm/r. The cutting distance of AT10B@5 tool was greater than 6000 m with a long service life. When the feed rate increased to 0.198 mm/r, with the increase in cutting distance, the wear of the two tools became severe. The effective cutting distance was less than 5000 m, and the wear of back tool face reached 0.3 mm, satisfying the failure standard of the tool. Figure 2b shows that with the increase in the cutting feed of the two tools, the machining surface roughness of the workpiece increased, thus reducing the machining quality. By comparing the workpiece processed by AT and AT10B@5 tools, it was found that under the same cutting conditions, the AT tool had a higher machining surface roughness, and the roughness increased rapidly when the feed rate was 0.198. This is because the larger the feed, the greater the cutting force of the tool in cutting the workpiece, resulting in more heat. The resulting chip did not take away the heat in time, increasing the tool temperature and aggravating the tool damage. With the addition of h-BN@Al 2 O 3 , the surface roughness of AT10B@5 tool material decreased. With the increase in cutting distance, the roughness changed slightly, and a stable workpiece machining surface quality was maintained. Figure 3 shows the changes between the cutting distance S and (a) wear amount of tool surface VB and (b) workpiece surface roughness Ra when AT10B@5 and AT tools cut the workpiece 40Cr hardened steel at different cutting speeds (v = 300 m/min, f = 0.102 mm/r). Figure 3 shows that with the increase in the amount of cutting back, the wear degree of back surface of the two tools increased, making them more prone to failure. This is mainly because the back cutting of the tool-workpiecemachine tool system increased the vibration. At the same time, because the tool's main cutting edge and workpiece material contacted sidewise, the cutting force increased; a lot of cutting heat was generated; and the surface temperature of the tool increased, thus decreasing the wear resistance of the tool. By comparing the curves of the two tools AT10B@5 and AT in Fig. 3a with the same cutting amount on the back, it was observed that when the depth of cut was 0.1 mm, the effective cutting distance of the two tool materials AT10B@5 and AT was more than 6000 m, and the wear amount of back tool surface VB did not reach 0.3. When the depth of cut was 0.2 mm, the effective cutting distance of AT tool was about 5500 m, and the cutting distance of AT10B@5 tool material was more than 6000 m. Its wear resistance is stronger than AT tool. Figure 3b shows that both AT10B@5 and AT tools wear faster with the increase in back bite during cutting, seriously affecting the surface quality of the processed workpiece. When the depth of cut was 0.1 mm and 0.2 mm, the surface roughness of the workpiece processed by AT tool had a small difference with the increase in cutting distance, exhibiting a trend of slow growth. As the depth of cut increased to 0.3 mm, the roughness of workpiece increased substantially, especially when the cutting distance increased to 4000 m, the roughness increased sharply, because the wear of the tool back face reached 0.3. However, under the same cutting conditions, the surface roughness of workpiece processed by AT10B@5 tool changed slightly with the increase in back cutting amount. When the depth of cut was 0.3 mm, the maximum surface roughness within the effective cutting distance was maintained within 1, and the surface machining quality was good.
The main parameters of 40 Cr hardened steel including the cutting speed (v), feed rate (f), depth of cut (ap) of cutting tools, surface roughness of machining workpiece, and surface wear were studied. The results show that under the same cutting conditions, the cutting tools exhibited serious wear and tear with the increase in cutting distance, and the service life of the cutter decreased. The surface roughness of workpiece increased sharply with the increase in cutting distance, severely reducing the machined surface quality. The surface wear of AT10B@5 tool increased with the increase in cutting distance, and its effective cutting distance was longer than AT tool with a longer cutting life. The workpiece surface roughness was small and maintained within 1 with stable processing quality, satisfying the needs of finishing and semifinishing. Cutting force is the force when the tool deforms the workpiece surface and produces chips during the cutting. The cutting force is mainly composed of the main cutting force (circumferential force Fz), depth of cut resistance (radial force Fx), and feed force (axial force Fy). The cutting force can be used to calculate the cutting power and analyze the stable state of the cutting tool. It is also an important basis to analyze the workpiece surface quality, cutting temperature, tool friction coefficient, and wear amount during cutting. Figure 4 shows a cutting force comparison diagram of different tools under v = 300 m/min, a p = 0.2 mm, and f = 0.102 mm/r. The main cutting force Fz, radial force Fy, and axial force Fx of AT10B@5 tool were all less than those of the AT tool, in which the main cutting force decreased by 20.8%. This is because the addition of a solid lubricant made the cutting system of the tool more stable, and the impact force of the tool tip decreased, thus decreasing the amount of the main cutting force. According to the main cutting force Fz and radial force Fy measured during the cutting of the tool, the friction coefficient of front tool face can be calculated using Eq. 1: where μ is the friction coefficient of the tool front face, and 0 is the tool front angle.
Using Eq. (1), the friction coefficient of AT tool's front face was determined as 0.6, and that of AT10B@5 tool's front face was determined as 0.53, which was reduced by 11.6%. Under the same cutting conditions, the AT10B@5 tool has a lower cutting force and friction coefficient of front tool surface, thus effectively reducing the damage of the tool during the cutting and prolonging the service life of the tool.

Analysis of tool wear patterns
The tool wear morphology of ceramic tool after dry cutting 40Cr hardened steel was studied (v = 300 m/min, ap = 0.2 mm, f = 0.102 mm/r). Figure 5 shows the micromorphology of rake face wear of different tool materials. Figure 5a shows a semicircular peeling area on the rake face, forming a crescent sag wear morphology. This is caused by the large impact force on the tool nose. As shown in Fig. 5b, the rake face of the AT10B@5 tool is worn evenly and smoothly with good wear resistance, and no obvious fracture is found. This is due to the precipitation of solid lubricant during the cutting process, which reduces the friction between the chip and the rake face; the good lubricating performance accelerates the heat release of the cutting and reduces the cutting temperature. It can be seen from Fig. 5c and d that a large amount of h-BN and TiO 2 are precipitated from the tool rake face, which reduces the Fe element adhesion on the workpiece [6,25]. Figure 6a shows that the AT tool has obvious mechanical furrow scratches in the flank face. As shown in the region magnification (Fig. 6b), under a large amount of cutting heat, the tool flank face has partial bonding. Thus, adhesive wear was formed. Due to the friction between the tool and the workpiece, the temperature of the tool increases and the surface softens, and the components of the workpiece are more likely to diffuse to the tool surface, reducing the performance of the tool surface. As shown in Fig. 6d and e, when the tool comes into contact with the hard particles of the workpiece, long scratches are formed on the tool surface. When a large number of scratches accumulate, a narrow broken band is formed under the action of cutting force, cutting temperature, and diffusion, resulting in abrasive wear. Shown in Fig. 6f, g, h and i is the process of adhesive wear. Under the action of cutting heat and extrusion friction, part of the workpiece is attached to the tool, and the material on the tool will produce microcracks under the influence of shear stress, cutting temperature and diffusion, and then fall off, and finally form adhesive wear. Abrasive wear Figure 7a and b show that the overall wear area and wear depth of AT10B@5 tool flank face are relatively small, and no obvious edge collapse occurred, and there are partial oxidation wear and slight abrasive wear on the flank face. As can be seen from Fi. 7c, the distribution of N, B, and Ti has obvious boundaries with that of Fe, indicating that h-BN and TiO 2 have good lubrication performance and reduce the adhesion of Fe on the flank face. Diagram of crack healing and surface lubrication mechanism is reported in Fig. 7d, e, f and g. It can be seen that during the cutting process, the tool is squeezed by the workpiece to precipitate the lubricant, which reduces the adhesion particles on the surface of the tool. With the increase of cutting temperature, TiB 2 and part of h-BN are oxidized. Amorphous B 2 O 3 and TiO 2 have good crack repairing effect [26,27]. The cracks were partially repaired by filling with B 2 O 3 and TiO 2 . Self-healing prevents the accumulation of micro-cracks in the cutting process of ceramic tool materials and reduces the risk of brittle fracture. The final wear level of AT10B@5 ceramic tools is lower than that of AT ceramic tools (compare Figs. 6i and 7g). As can be seen from Fig. 7h and i, part of the crack area has been filled with TiO 2 and B 2 O 3 .
According to the surface machining quality and flank surface wear of different tools shown in Fig. 8, AT10B@5 has better surface machining quality and tool life. Figure 8 shows the temperature test when the cutting distance reached 1000 m during the cutting test. At this time, the cutting was relatively stable, and a stable temperature distribution could   Fig. 8, it was found that the cutting temperature of AT10B@5 tools with TiB 2 and h-BN@Al 2 O 3 was 22.2% lower than that of conventional AT tools. This is because the addition of a solid lubricant formed a lubricating film on the tool surface (Fig. 7c). The lubricant made the tool chip smoother, reduced the attachment of chip on the surface of the tool, thus removing a lot of heat and avoiding chip tumors and other adverse phenomena. Tool self-lubrication reduces cutting temperature, and self-healing reduces the accumulation of microcracks in the combination of tool self-healing and self-lubrication to ensure the processing quality and tool life.

Conclusions
In this study, the cutting performance of self-lubricating ceramic tools with repair ability was studied by adding TiB 2 and h-BN@Al 2 O 3 . The effects of different cutting speed (v), feed rate (f), and depth of cut (a p ) on the wear amount of tool material, surface roughness of machined workpiece, and effective cutting distance were analyzed by conducting cutting tests. The effects of the addition of TiB 2 and h-BN@ Al 2 O 3 on the cutting force and cutting temperature during the cutting were studied. The wear morphology of cutting tool was analyzed.
The main conclusions are as follows: 1. The wear of AT and AT10B@5 ceramic tools increased with the increase in cutting distance, exhibiting an increasing trend with the increase in cutting speed, back bite, and feed. However, the effective cutting distance of AT10B@5 tool is longer; it can significantly improve the workpiece processing surface quality and decrease the workpiece surface roughness with the increase in cutting speed. The overall roughness is less than 1; the degree of change is small, with stable surface processing quality. 2. Compared with AT tool, the cutting force and cutting temperature of AT10B@5 ceramic tool material decreased significantly during the cutting. The main cutting force decreased by 20.8%, the cutting temperature decreased by 22.2%, and the friction coefficient of the front tool face decreased by 11.6%, thus effectively improving the surface quality of workpiece and reducing tool wear and extending the service life of the tool. 3. During cutting, the main wear of AT tool front face is bond wear, accompanied by micro edge collapse, and the main wear of back face is abrasive wear. In AT10B@5, the wear of tool front surface is bond wear, while the wear of tool back surface is shallow and relatively small. The main wear is bond wear, oxidation wear, and slight abrasive wear, among which oxidation wear is beneficial to tool crack repair.