Effect of Nbc Content on Ti(C ,N) Based Cermet Coating

Effect of Nbc Content on Ti(C ,N) Based Cermet Coating

• Admin
• 2024/4/17

ABSTRACT Ti (C, N) -based cermets with different Nbc contents (0% 、3% 、6% 、9% 、12% ) were prepared by powder metallurgy method, and then the TiAlCrSiN coating was coated on the surface of the prepared cermet matrix by physical vapor deposition (PVD). The microstructure of the cermets coated with TiAlCrSiN coating was observed and analyzed bySEM and EDS. The microstructure and mechanical properties of the matrix were studied. Meanwhile, the adhesion and friction properties of the coating were investigated. The results show that when the Nbc addition amount is 6% , the comprehensive mechanical properties of the substrate of the samples are the best. With more addition of Nbc, matrix grains were refined and the distribution of the bonding phase was more uniform. However, the adhesion of the coating was weakened and the frictional coefficient decreased gradually. When the TiAlCrSiN coating is deposited by PVD, the elements between the coating and the substrate migrate mutually, but the amount of migration is relatively small due to the relatively low temperature during PVD deposition.

In the field of metal material processing, cutting is the most commonly used and most important processing method, which directly affects production costs and production efficiency. As the hardness of the processed materials becomes higher and higher, the requirements for surface quality become more and more strict, and the requirements for processing efficiency increase, it is more and more difficult for traditional cutting tools to meet the requirements. In the past, carbide tools were often used to process these materials and achieved certain results, but carbide tools consumed a large amount of strategic precious metals such as W and Co. As these resources become more and more scarce, there is an urgent need for a cutting tool with good machinability and low cost. Ti(C, N) based metal ceramics have been widely used in cutting tools due to their high hardness, wear resistance, chemical stability, extremely low coefficient of friction with metals, excellent high-temperature mechanical properties and low density. And wear-resistant parts. Generally, the cutting speed of cermet cutting tools is 3 to 5 times higher than that of ordinary cemented carbide inserts, which can greatly improve production efficiency. At the same time, due to the characteristics of high hardness, good wear resistance, excellent high-temperature mechanical properties and not easy to bond with metals, cermet tools are widely used in cutting difficult-to-machine materials, and in high-speed cutting, ultra-high-speed cutting, and There is a lot of application space in the cutting of superhard materials. In the past, TaC was usually added to the cermet to improve the toughness and plastic deformation ability of the cermet tool, but F.Qi, S. Kang and SunYongAhn and other studies found that the role of Nbc is similar to TaC, adding an appropriate amount of Nbc can improve the plastic deformation resistance And red hardness, while Nbc can also reduce the sintering temperature. Since Nbc is much cheaper than TaC, and the roles of the two are similar, the cheaper Nbc can be used to replace the higher-priced TaC to achieve cost savings.

Because the coated cutter has strong oxidation resistance and anti-adhesion performance, it has good wear resistance and resistance to crescent crater wear and can obtain a high quality of the processed surface. The low coefficient of friction of the coating can effectively reduce the cutting force and cutting temperature during cutting, which can greatly improve the durability of the tool. The temperature of PVD coating is relatively low during the deposition of the coating, it will not damage the base material of the tool, and the thickness of the coating is thinner, which is more conducive to cutting, so it has been widely used.

Therefore, in view of the above purpose, this study adopts a mature cermet manufacturing process, replaces expensive TaC with cheaper and more abundant Nbc, and studies the content of Nbc on Ti(C, N)-based cermets coated with TiAlCrSiN. The influence of the microstructure, mechanical properties, bonding properties of the coating and the substrate and the friction properties of the substrate and coating before and after the layer.

1 Experimental Method

1.1 Preparation of Substrate

Experimental powders include Ti (C₀.₇, N₀.₃), WC, Co, Nbc, Mo₂C, Ni, where WC is produced by Xiamen Jinlu Special Alloy Co., Ltd., Co is provided by Nanjing Hanrui Cobalt Industry Co., Ltd., and Nbc is produced by Provided by Zhuzhou Cemented Carbide Group Co., Ltd. The characteristics of the original powder are listed in Table 1. See Table 2 for a sample composition. Mix the original powders of Ti(C₀.₇, N₀.₃), WC, Co, Nbc, Mo₂C, Ni according to the experimental formula and pour them into a ball mill tank with a mass ratio of 6:1, and a mass ratio of solid to liquid of 2 ：1 Add absolute ethanol, seal and place on a ball mill with a rotating speed of 68 r/min for 72h so that the original powder is fully uniform and refined. After ball milling, the slurry is dried and sieved. After sieving, SD rubber molding agent is added (10 mL of rubber and appropriate gasoline per 100 g of raw material powder), dried and granulated to improve the fluidity and filling performance of the mixture. Put the granulated powder into the mold and press it on a universal hydraulic press with a pressure of 500 MPa to form a rectangular compact with a size of 6.5 mm×5.25 mm×20 mm. The powder compact was placed in a ZYL-ZA150 vacuum sintering furnace, vacuum sintered at 1 310 ℃ for 0.5 h, and then sintered at 1 440 ℃ for 1 h under the protection of 5 MPa Ar. After sintering, the furnace was cooled for 10 h to obtain a cermet Sample.

1.2 Coating Deposition and Testing

The prepared cermet samples were polished and grinded on the P1 metallographic sample polishing machine to remove the contaminants on the surface, and then cleaned in an ultrasonic cleaning machine with absolute ethanol for 15 minutes, dried and placed in R2P- 800 type PVD coating machine. When the pressure in the coating chamber reaches 8.0×10-3 Pa and the temperature reaches 723 K, the substrate is preheated by ion bombardment for 60 minutes. Then, under a bias voltage of 200 V and a pulse voltage of -300 V, clean with pure Ar for 80 min to remove surface contaminants again. TiAlCrSiN coating was deposited by magnetron sputtering Ti-Al and Cr0.3Si0.1Al0.6 targets. Under a bias voltage of 823 K and 20 V, a mixture of Ar and N2 gas at a flow rate of 120 sccm was introduced into the cavity for 100 minutes and then cooled in the furnace for 90 minutes to obtain a coated sample.

SEM and EDS analysis uses field emission scanning electron microscope produced by Hitachi, Japan. The adhesion of TiAlCrSiN coating was tested with the WS-2005 testing machine (Lanzhou Zhongke Kaihua). When the loading speed is 100 N/mm, the applied load is gradually increased from 0 N to 100 N to evaluate the adhesion level of the coating on the ARK-600 hardness tester (Aakish City, Japan). Each series is tested 3 times, and then the average value is taken. The friction coefficient of the TiAlCrSiN coating is measured on the CFT-I material surface tester (Lanzhou Zhongke Kaihua). A 6 mm diameter Si3N4 ball with a loading force of 4 N was used to rub the surface of the sample with a distance of 5 mm each time. The friction coefficient was obtained by reciprocating friction for a total of 10 minutes. Each series was tested 3 times and the average was taken value.

2 Results and Discussion

2.1 the Microstructure and Performance of the Cermet Matrix

The microstructure of Ti(C, N)-Mo2C-WC-Ni-Co cermets with different Nbc content is shown in Fig. 1. It can be seen from the figure that the samples with different Nbc content all have the typical annular structure of Ti(C, N)-based cermet, that is, the core/shell structure, one is the classic black core-white inner shell-gray outer shell structure The other is a white core-gray shell structure, as shown in Figure 1 (e). This is also consistent with the addition of WC, TaC, Nbc, HfC, VC, AlN and other additives to Ti(C, N)-based cermets, or partial or full replacement of Ni with Co, which will not fundamentally change Ti(C, N) This kind of microstructure of base cermet [3]. It can be analyzed from these five figures that as the amount of Nbc added increases, the toroidal phase becomes more complete, especially the gray crust becomes thicker, the bonding phase is more evenly distributed, and the white core/black shell structure appears More and more. This is because Nbc has higher interface energy than Ti(C, N) in Ni-Co solution, and (Ti, Nb)(C, N) preferentially precipitates on the surface of undissolved Nbc to reduce the surface energy of the system. Secondly, with the increase of the amount of Nbc added, because the Nbc particles are relatively small and uniformly dispersed in the cermet matrix material, the number of crystal nuclei during the transition from liquid to solid during the vacuum sintering process increases, thereby making Ti( C, N)-based cermets cannot be reunited and refined, and the distribution is more uniform.

2.2 Coating Analysis

The morphology of the TiAlCrSiN coating is shown in Figure 3(a). The coating is in direct contact with the substrate and no transition layer is formed in the middle. The EDS of the coating and the substrate are shown in Figure 3(b) and Figure 3(c), and the element content of the coating and the cermet substrate are shown in Table 4. From the EDS diagrams of the substrate and coating, it can be found that there are no elements such as Al, Cr, and N in the original substrate, but after the TiAlCrSiN coating is applied, elements such as Al, Cr, and N are detected in the substrate, which is certain, However, some elements such as Al, Cr, and N in the coating migrate from the coating to the substrate during the coating process. Similarly, a small amount of Nb, Co, Mo and other elements in the matrix migrated into the coating. From S1 to S5 samples, the average thickness of the coating is 0.68, 1.25, 1.47, 0.73, 1.17 μm, respectively. It can be seen that when the Nbc addition amount is 6%, the thickness of the coating is the largest. Generally speaking, under the same deposition conditions, the thickness of the coating is closely related to its growth rate. According to different growth mechanisms, the growth rate of the coating on the ceramic phase is faster than the growth rate on the bonding phase [15]. The coating grows on the bonding phase by re-nucleation and has fine grains. On the ceramic phase, it is grown by epitaxial growth, and the crystal grains are coarse. With the increase of the amount of Nbc added, the total area of ​​the ceramic phase on the surface of the substrate increases after grain refinement, which provides more epitaxial growth points for the growth of the coating, thereby promoting the growth of the coating, especially in The best Nbc addition amount is 6%, when the coating is the thickest.

2.3 Coating Adhesion and Friction Coefficient

Through the scratch test on the samples S1 to S5 on the WS-2005 testing machine, the adhesion forces of the cermet substrate and coating of the samples S1 to S5 are measured as 39.33, 34.67, 19.50, 24.00, 19.67 N, respectively. It can be seen that as the amount of Nbc added increases, the adhesion of the coating begins to gradually decrease. When the amount of Nbc added is 6%, the adhesion of the coating has a minimum value of 19.5 N. Later, as the addition of Nbc continued to increase, the adhesion of the coating increased, but it gradually decreased after the addition of Nbc was 9%. Takadoum et al. found that in the scratch test when the strain energy of the coating exceeds the adhesion energy, the coating spalls. The total strain energy U[16] can be calculated by the formula (1):

In the formula, E is the elastic modulus of the coating, σ is the stress in the coating, and d is the thickness of the coating. From formula (1), we can see that as the thickness of the coating increases, the total strain energy becomes larger and the adhesion force decreases. However, the adhesion of the coating does not only depend on the thickness of the coating, the internal stress of the coating also affects the adhesion [17], so it appears that the S2 sample is thicker than the S5 sample, but the S2 test The adhesion of the sample is still greater than that of the S5 sample. This may be due to the mismatch of the thermal expansion coefficient between the coating and the cermet substrate, which caused the stress in the coating of the S2 sample to be greater than that in the S5 sample. Thermal stress can be calculated by the formula (2):

In the formula, σ is the thermal stress, E and ν are Young’s modulus and Poisson’s ratio of the coating, T is the temperature, and αs and αf are the thermal expansion coefficient of the cermet substrate and the coating. Due to the difference in thermal expansion coefficient between the coating film and the cermet substrate, cooling to room temperature after coating at a higher temperature will cause thermal stress to the coating film. The existence of internal stress will change the mechanical properties such as the hardness and elastic modulus of the coating film, as well as the bonding force between the coating film and the cermet substrate. Especially for the hard coating film, due to the huge internal stress, this internal stress will increase with the increase of the thickness of the coating film, so when the thickness of the coating increases, the bonding force with the substrate decreases.

Figure 4 shows the friction coefficients in the friction experiment after TiAlCrSiN coatings are deposited on the cermet substrates with different Nbc content. The friction coefficients of the S1 to S5 samples are 0.80, 0.77, 0.76, 0.67, 0.65, respectively. It can be seen that as the Nbc content increases, the friction coefficient of the coating gradually decreases, but the decrease is small. This is because Mo and Nb elements have a significant effect on reducing the coefficient of friction [20-21]. As the amount of Nbc in the substrate increases, the Nb element that migrates from the cermet substrate to the coating during the deposition of the coating also gradually increases. The more the friction coefficient is reduced, but as described above, since the temperature of the vapor deposition coating is not very high, the amount of Nb element migration is not much, so the friction coefficient is reduced less.

Conclusion

• When the addition amount of Nbc is 6%, the flexural strength and toughness of the cermet matrix appear maximum, and the overall performance is better. With the increase of the amount of NbC added, the hardness and relative density of the cermet matrix increased, and the crack propagation gradually changed from transgranular + intergranular fracture to intergranular fracture.

• During physical vapor deposition of TiAlCrSiN coating of cermet, Al, Cr, N and other elements will migrate from the coating to the cermet substrate, and the elements such as Mo and Nb in the substrate will also migrate to the coating. When the amount of Nbc added is 6%, the coating is the thickest and the adhesion is the lowest.

• As the amount of Nbc added to the cermet substrate increases, the adhesion of the coating decreases and the friction coefficient of the coating gradually decreases.

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