1.Preface

        Cermet is a heterogeneous material composed of one or several metal components and ceramic materials, with ceramic components accounting for approximately 15-85 vol%. Ti (C, N) based ceramic composite materials emerged in the 1970s. Due to their high hardness, ideal high-temperature performance and wear resistance, good chemical stability, as well as high resistance to crescent pit wear and oxidation resistance, they are widely used in cutting tools, wear-resistant materials and other fields. The abundance of Ti resources in the Earth's crust has made it one of the candidate materials to replace scarce WC based hard alloy materials. Therefore, in recent years, Ti (C, N) based ceramic composite materials have become a hot research direction. Ti (C, N) based ceramic materials are a composite material made by adding Ti (C, N) as the basic hard phase (mainly by directly adding Ti (C, N) or adding TiC and TiN to synthesize Ti (C, N), which has economic advantages), using metals or alloys such as Ni, Co, or (and) Mo as the bonding phase, and adding other refractory nitrides or carbides (such as SiN?, AIN, WC, MoC, VC, etc.) as the reinforcing phase according to the working conditions. In this experiment, Ti (C, N) based ceramic composites were synthesized using TC and TiN as raw materials, with carbides WC, MoC, CrCa, and TaC as reinforcing phases and Ni and Co as composite metal bonding phases. Ti (C, N) based ceramic composites were prepared through hot pressing sintering process.

2 Experimental process

2.1 Experimental raw materials

2.2 Experimental process

        Weigh the proportion of raw materials used according to Table 2, then put them into a ball mill and add an appropriate amount of anhydrous ethanol for wet mixing. The ball to material ratio is 5:1 (mass ratio, tungsten carbide hard alloy ball). After ball milling for a certain period of time, the slurry passes through a 325 mesh sieve. The screened slurry is dried in an oven, cooled, and passed through a 200 mesh sieve for later use. Weigh the required powder according to the set size of the sample, and then place it into a graphite mold (for the convenience of demolding the sample after sintering, the graphite mold needs to be pre treated, such as coating boron nitride powder on the inner surface of the mold, and then padding graphite paper with boron nitride powder on the upper and lower graphite indenters). Put it into a hot press sintering furnace, and set the firing process parameters: temperature 1500 ℃, insulation for 30 minutes, pressure 25 MPa, and then cool naturally after completion.

2.3 Performance testing

        The volume density of sintered samples is tested according to the Archimedean principle; The Vickers hardness test was conducted on the Wolbert 401MVA Vickers hardness tester, with a loading time of 5 seconds and a loading load of 500g; The bending strength test was conducted using the three-point method (Instron5566 universal material testing machine), with a loading rate of 0.05mm/min and a sample size of 4mm × 3mm × 40mm with a span of 30mm; The fracture toughness test was conducted using the single-sided notch beam method (Instron5566 universal material testing machine), with a loading rate of 0.05mm/min and a sample size of 6mm × 3mm × 40mm, notch size is Depth: 3mm, width: 0.2mm. Use field emission scanning electron microscopy (SEM, JSM-6700F type) to observe the surface and cross-sectional morphology of composite materials.

3 Results and Discussion

3.1 Microstructure and mechanical properties

Ti (C, N) based ceramic composites were prepared by hot pressing sintering process. The mechanical properties of T (C, N) metal ceramic composites were tested and their cross-sectional morphology was observed using scanning electron microscopy (SEM). The microstructure photo of the composite material is shown in Figure 1.

        Figures 1 (a), (b), and (c) show scanning electron microscopy (SEM) images of Ti (C, N) based ceramic composites at 5000 times, 10000 times, and 30000 times, respectively. From Figure 1 (a), it can be seen that the microstructure morphology of the fracture surface of the composite material is highly dense, with very few inclusions of closed pores. The grain boundaries are clear and the grain size is small. The fracture path of the cracks is clear and distributed in a spider like pattern, with undulating cross-sections and high concavity and convexity. There are also transgranular and transgranular fracture modes, obvious metal tearing edges, and large cleavage planes. In the photos taken at 10000 times in Figure 1 (b) and 30000 times in Figure 1 (c), it can be clearly seen that there are obvious ripple like patterns on the larger cleavage surface, indicating that there is still finer crack deflection in the transgranular fracture mode. At the same time, there is a sharp "V" - shaped fracture morphology in the cross-section (as shown in Figure 1 (c)), which is beneficial for improving performance. The various performance indicators of the sample reached: Vickers hardness 11.9GPa, bending strength 1255MPa, fracture toughness 8.3MPa · m1, and relative density 98.7%. The hot pressing sintering process is beneficial for the densification and uniformity of material microstructure, grain refinement, and diverse changes in fracture paths, all of which contribute to the improvement of the comprehensive properties of composite materials. This may be due to the characteristic of external forces introduced during the sintering process by the hot pressing sintering process. The high pressure and high temperature during the hot pressing sintering process are conducive to improving the viscous flow, mass transfer process, and diffusion and migration ability of ceramic materials, enabling them to achieve ideal performance indicators at lower temperatures and in a shorter time. However, at the same time, the hot pressing sintering process has disadvantages such as inability to prepare complex shaped products and low production capacity.

3.2 Antioxidant performance

        The antioxidant performance of Ti (C, N) based ceramic composites was studied. After grinding and polishing the sintered samples, static oxidation tests were conducted under pre-set oxidation conditions (holding at 750 ℃, 900 ℃, and 1150 ℃ for 2 hours respectively) and in an air atmosphere. The surface oxide film morphology (SEM) and cross-sectional morphology (BSE) of the samples were observed at different oxidation temperatures, and their oxidation behavior was analyzed. As shown in Figure 2.

        Figure 2 is a photo of the sample after being oxidized at different temperatures for 2 hours. From Figure 2 (a), it can be seen that there is no oxide film formed on the surface and there are many small particles, as well as pores and obvious cracks on the surface. Data studies have shown that MoO? At a temperature of 700 ℃, it exhibits sublimation characteristics, indicating that surface cracks may be MoO generated by MoC oxidation at high temperatures? Caused by the volatilization and escape of. Figure 2 (b) is a photo of 900 ℃ oxidation. It can be seen from the figure that the formed oxide film is relatively flat, but the structure is relatively loose, with small particle sizes on the surface becoming smaller, some small cracks and pores. Studies have shown that WO is formed at 850 ℃? It exhibits sublimation characteristics, and the escape of volatile substances can cause cracks and/or pores in the substrate. Oxygen can continue to oxidize the substrate by following defects such as cracks. The oxide film layer generated under this condition does not show any protective effect on the substrate. From Figure 2 (c), it can be seen that after 2 hours of oxidation at 1150 ℃, the oxide film formed on the surface is relatively dense and smooth, with almost no obvious defects found

Prove that the dense oxide film formed at this moment exhibits a protective effect on the substrate.

        Figure 3 is a BSE photo of the sample after oxidation. From the oxidation photo of the 750 ℃ fracture in Figure 3 (a), it can be seen that there is basically no formation of an oxide film layer under this oxidation condition, and there are obvious defects such as cracks and pores in the structure, indicating that gaseous substances (MoO?) do volatilize and escape during the oxidation process, which is consistent with the research results of the above literature. From the oxidation photo of the 900 ℃ fracture in Figure 3 (b), it can be seen that the thickness of the generated oxidation film layer at this time is approximately 24.34 μ m. It was found that there are many transverse and longitudinal cracks in the structure of the oxide film layer that penetrate deep into the interface of the substrate, indicating that volatile substances have escaped during the high-temperature oxidation process, resulting in defects such as cracks. At the same time, it also indicates that the oxide film layer cannot effectively block oxygen from continuing to oxidize the substrate along these defect channels, that is, the formed oxide film layer has no antioxidant effect. From the oxidation photo of the fracture surface at 1150 ℃ in Figure 3 (c), it can be seen that the overall cross-sectional structure is divided into an outer oxide layer, an intermediate transition layer, and an inner substrate from the outside to the inside. The outer oxide layer is relatively uniform and dense, with fewer defects such as cracks and inclusions. The thickness of the film layer is about 73.81 μ M. There is a clearly uniform dense transition layer between the outer oxide film layer and the inner substrate, with a thickness of approximately 18.57 μ M. The inner substrate is relatively dense, indicating that the oxide film layer generated under this condition has a protective effect on the substrate. At this time, the diffusion rate of oxygen in the formed oxide film and the dense intermediate transition layer determines the rate at which the substrate continues to be oxidized.

4 Conclusion

        The microstructure and microstructure of Ti (C, N) based ceramic composite materials prepared by hot pressing sintering process have high density, fewer pores, clear grain boundaries, and small grain size. There are obvious metal tearing edges, clear and undulating fracture paths, and transgranular and transgranular fracture modes. The mechanical properties are: bending strength of 1255MPa, fracture toughness of 8.3MPa · m, Vickers hardness of 11.9GPa, and relative density of 98.7%. After 2 hours of oxidation at the set temperature, no oxide film layer was formed during oxidation at 750 ℃; The oxide film formed on the surface of the sample during oxidation at 900 ℃ has no protective effect on the substrate; When oxidized at 1150 ℃, a dense oxide film and intermediate transition layer were formed on the surface of the sample, which had a protective effect on the substrate. At this time, the thickness of the oxide layer was approximately 73.81 μ m. And the thickness of the intermediate transition layer is about 18.57 μ M.