In order to study and expand the application of Inconel718 alloy in high temperature environment, Co/TiN composite coating was prepared on its surface by laser cladding, and its tribological behavior at room temperature and 600 ℃ and its oxidation resistance at 800 ℃ were studied by combining XRD, SEM and EDS analysis methods. The results show that the hardness of the prepared coating is improved relative to the substrate, which is about 1.3~1.4 times that of the substrate. In addition, the phases in the coating are mainly solid solution and intermetallic compounds. The tribological properties of the coating were tested. When the TiN addition amount was 4% (mass fraction, the same below), the coating had the best friction reduction performance; when the addition amount was 6%, the coating had the best wear resistance, and the wear rate could be reduced by up to 90.02%. In addition, oxidation experiments show that the Co/TiN composite coating has certain anti-oxidation properties, and the oxidation rate is 8.7634mg’2·cm’-4·h’-1, which is not much different from the substrate. This shows that the composite coating can significantly reduce the wear rate at high temperature while retaining the oxidation resistance of the substrate, and the wear rate decreases with the increase of TiN. Through the wear mechanism analysis, it can be seen that all coatings undergo oxidation wear at 600 ℃, and the generation of surface oxide film also helps to reduce the wear rate to a certain extent.
Inconel718 is a new type of precipitation-strengthened nickel-chromium-iron-based high-temperature alloy, which is currently widely used in the industrial field. It has good tensile strength and other mechanical properties in the range of 650~1000 ℃. After years of research and practical application, it is found that this type of nickel-based high-temperature alloy is suitable for manufacturing important hot-end moving parts such as engine blades and turbine disks. However, these parts often fail due to wear, fatigue and other reasons after long-term service in harsh environments such as high temperature and high pressure, which affects the service life of the workpiece and increases the cost. Therefore, it is of great significance to improve the wear resistance and friction reduction performance of Inconel718 alloy under extreme working conditions.
Laser cladding technology usually refers to the process of cladding materials on the surface of the substrate under the action of a laser beam to prepare a coating with specific properties. It is currently widely used in various industrial fields to improve the friction and wear resistance, creep resistance, oxidation resistance and other properties of the substrate surface. Feng et al.’s research shows that compared with electric arc welding technology, the Inconel625 coating prepared by laser cladding has a finer microstructure and can reduce the segregation of Mo and Nb. Yu et al. used laser cladding technology to prepare NiAl coatings and studied their tribological properties under a wide temperature range. The results show that at a high temperature of 1000 ℃, a glaze layer composed of NiO, Ni2O3 and other phases is formed on the wear surface, which can be used as a solid lubricant and anti-wear material to improve the tribological properties of the NiAl coating at high temperatures.
Pure Co powder is a high-hardness metal powder. At present, many studies have used Co to prepare composite coatings to improve the tribological properties of stainless steel surfaces. Jin et al. used TC4 alloy as a substrate and clad Co/Ti3SiC2 composite coatings on its surface. The experiment showed that under the same conditions, the friction coefficient of each coating was lower than that of the substrate. Liang Weiyin et al. prepared WC/TiC/Co coatings on the surface of YG8 cemented carbide by laser cladding technology. The results showed that the hardness of the coatings was between 1700 and 1800 HV0. 5, which was higher than that of YG8 cemented carbide, and the wear resistance was also improved by 90.67% compared with YG8 alloy.
Nitride coatings are the earliest developed and used hard coatings, with many excellent properties such as good coating adhesion, good wear resistance, and high hardness. Among them, TiN is the most commonly used nitride with a hardness of about 2000HV. It has the characteristics of high strength, high hardness, and high temperature resistance. Li Zhiyuan et al. used laser cladding technology to add TiN to Co-based alloys. The results showed that TiN can improve the wear resistance of the composite coating. When 5% (mass fraction, the same below) TiN is added, the TiN/Co-based composite coating has the best performance. Wang Yonglin et al. prepared TiN coating with YG6 alloy as the substrate, and tested the friction and wear characteristics of the coating with load and speed as independent variables. The results showed that the friction coefficient of the TiN coating was 4%~21% lower than that of the substrate.
In recent years, there have also been studies on the preparation of coatings on the surface of Inconel718 to improve its antioxidant properties. Zhang et al. prepared Stellite3-Ti3SiC2 composite coatings with different mass ratios on the surface of Inconel718. The results showed that the microhardness of the coating generally reached 1.8~2.5 times that of the substrate, among which Stellite3-10%Ti3SiC2 coating had the best oxidation resistance. According to literature research, previous studies on the preparation of coatings on the surface of Inconel718 alloy mainly focused on improving strength and wear resistance, and rarely took into account its oxidation resistance. Since Co powder has good fluidity and wettability, it can prepare a smooth and dense coating on the surface of the substrate. Therefore, this experiment intends to use pure Co as the bonding phase of the coating, and use TiN with good wear resistance as the reinforcing phase to prepare Co/TiN composite coatings, explore its tribological properties at room temperature and 600 ℃ and its oxidation resistance at 800 ℃, and provide a new idea for improving the application ability of nickel-based high-temperature alloy Inconel718 at high temperatures.
1 Experimental materials and methods
The substrate used in the experiment is the nickel-based high-temperature alloy Inconel718, and its main chemical composition is shown in Table 1. In the experimental preparation stage, the sample size was first processed to 40 mm×20 mm×8 mm, and the surface of the pre-cladding coating was polished (equipment: MPD-2W metallographic grinding and polishing machine). The powder system of the coating was set to Co (N1), Co-4%TiN (N2), and Co-6%TiN (N3) according to the mass ratio. The powders were weighed with an electronic balance, and then mixed and dried. The powder morphology of Co, TiN and the mixture of the two is shown in Figure 1. As can be seen from Figure 1, the shape of cobalt powder is mostly round, while the particles of titanium nitride are irregular. After mixing, the two particles are evenly distributed without obvious agglomeration.
Since the synchronous powder feeding method has the advantages of uniform powder heating and strong laser absorption, the laser cladding experiment adopts this method to prepare the composite coating. The experimental equipment uses the YLS-3000 fiber laser (process parameters are shown in Table 2). After the experiment, the sample is ground and polished to prepare for the next experimental analysis.
X-ray diffraction (XRD, Smartlab SE) is used to characterize the phase of the composite coating. The coating sample is cut into 2 mm thick slices along the cross section, and the metallographic specimen is made of resin. It is ground and polished until the mirror surface is free of marks. Then the specimen is corroded with aqua regia (VHCl∶VHNO3=3∶1) for about 60 s. The distribution of elements in the coating is characterized by an energy dispersive spectrometer (EDS, Oxford Xplore 30. Aztec one), and the microstructure of different areas of the coating is observed by a scanning electron microscope (SEM, TESCAN MIRA 4).
The microhardness was measured along the depth direction of the coating cross section using a Vickers microhardness tester (HX-1000TM/LCD), and multiple sets of data were obtained and averaged, where the load was set to 4.9 N and the pressure duration was 15 s. The tribological experiment was carried out using a friction and wear tester (HT-1000), and the friction and wear parameters are shown in Table 3. Si3N4 ceramic balls with a diameter of 5 mm and a hardness of 1700 HV were used as counter grinding balls, and the experimental temperatures were room temperature (25 ℃) and 600 ℃, respectively. Finally, the MT-500 probe wear scar measuring instrument was used, and the wear rate was calculated using formula (1). Where: WR is the wear rate, mm’3/(N·m); V is the volume lost during wear, mm’3; D is the sliding distance, m; F is the applied load, N.
In order to explore the high-temperature oxidation resistance of the coating, a high-temperature oxidation experiment was carried out on the sample at 800 ℃ using a tubular furnace OTF-1200x. The sample was first cut into small pieces of 5 mm×5 mm×8 mm in size, polished and cleaned, and then loaded into the experimental furnace, the temperature was set to 800 ℃, and the heating rate was 10 ℃/min. During the constant temperature oxidation process of the experiment, the mass and weight changes of the sample were recorded using an electronic scale at 1, 4, 7, 10, 20, 30, 40 h and 50 h. After the oxidation experiment, the samples were analyzed by XRD, SEM and EDS.
2 Results and discussion
2.1 Phase and microstructure analysis
The XRD results of the three coatings are shown in Figure 2. It can be seen that the N1 coating contains γ-Co and (Fe, Ni) solid solutions, as well as intermetallic compounds FeNi3 and Cr3Ni2. With the addition of TiN, weak TiN diffraction peaks and intermetallic compounds Co2Ti were detected in the N2 and N3 composite coatings. According to the references, Co has an allotropic transformation phenomenon, that is, it will transform into α-Co phase below 417 °C, but due to the fast cooling rate of laser cladding, the γ-Co phase has no time to transform, so most of it is retained. In addition, during the cladding process, various metal elements combine in the molten pool to form various metal compounds, such as FeNi3, Co2Ti, etc. The formation of (Fe, Ni) solid solution is due to the diffusion of Fe and Ni elements in the matrix into the coating, most of which exist in the form of solid solution during the rapid cooling process.
Since the diffraction peaks of the phases in the coating may be close to each other, and the rapid solidification of laser cladding produces a non-equilibrium effect in the molten pool, which may cause lattice deformation, XRD cannot accurately identify all phases. Therefore, when analyzing the phases in the coating, it is necessary to combine EDS and other methods for comparison and analysis.
Because the microstructures of the three composite coatings are similar, the N3 coating with the highest TiN content is selected for analysis. The SEM images of different areas of the N3 coating are shown in Figure 3. The overall morphology of the coating is shown in Figure 3 (a). It can be seen from the figure that the thickness of the coating is about 1.12 mm, the surface is flat, and there are a small number of pores. From the EDS scan results, it can be seen that the elements in the coating are distributed evenly, and the presence of matrix elements such as Fe, Ni, and Cr is also observed, which indicates that during the laser cladding process, the elements in the matrix will diffuse into the coating.
The microstructures of the upper, middle, and lower parts of the coating are shown in Figure 3 (b) to (d), respectively. From the solidification theory, it can be seen that the evolution of the microstructure is determined by G/V, where G is the temperature gradient and V is the solidification rate. It is observed that the structure in the upper area is fine and dense, as shown in Figure 3 (b). This is because the temperature gradient G of the molten pool in the upper area is small, and it is cooled by the protective gas, and the cooling rate and solidification crystallization rate are fast, so the structure is finer and more uniform. In addition, TiN is easy to float up during the solidification of the molten pool due to its low density, which will also hinder the growth of dendrites and lead to the formation of a large number of cellular crystals. In the middle area of the coating, there are fine equiaxed crystals in a mesh or elliptical shape, with a small amount of hard granular structure distributed around them, as shown in Figure 3 (c); the G/V value reaches the maximum in the bonding area at the bottom of the coating, and the structure is mainly columnar crystals, as shown in Figure 3 (d). EDS analysis was performed on the middle area of the coating, and the results are shown in Table 4. The atomic fractions of Ti and N elements in A are 57.30% and 25.62%, respectively. Combined with XRD, it is speculated that this point may be unmelted TiN particles. The atomic fractions of Ni and Fe elements at the gray-white grain boundary point B are 12.73% and 4.81%, respectively, and it is speculated that FeNi3 exists here. In addition, it is found that the Co content at the gray area C is high, which should be mainly γ-Co, and the atomic fractions of Cr and Ni elements are 5.71% and 12.82%, respectively. There may also be intermetallic compounds such as Cr3Ni2.
2.2 Microhardness analysis
The average microhardness of the substrate and three coatings is shown in Figure 4. It can be seen from the figure that the average microhardness of the substrate coating, N1~N3 coatings are 279.3HV0.5, 300.4HV0.5, 370HV0.5 and 399.3HV0.5 respectively. HAZ (heat affected zone) is the heat affected zone, which refers to the area where the structure and performance of the substrate change significantly under the action of laser cladding. The hardness of the coatings is improved to varying degrees compared with the substrate. Among them, the hardness of the N1 coating with pure Co added is only 7.5% higher than that of the substrate. However, after adding TiN, the hardness of the N2 and N3 coatings is increased to 1.3~1.4 times that of the substrate, and the hardness of the coating increases with the increase of TiN addition. It can be attributed to the following aspects: First, the strong convection effect in the molten pool makes the intermetallic compounds evenly distributed, resulting in dispersion strengthening; second, the rapid solidification of laser cladding makes it impossible for alloy elements such as Fe and Co to fully react in the molten pool, thus forming a supersaturated solid solution in the cladding layer, causing lattice distortion and forming solid solution strengthening. At the same time, the hard phase TiN itself has a high hardness, so increasing its addition amount can effectively enhance the hardness of the coating; and it has the same FCC structure as the γ-Co solid solution, which is conducive to the non-uniform nucleation of γ-Co on the surface of TiN particles, thereby strengthening the organizational properties of the coating. Therefore, the N3 coating with the largest TiN addition has the highest microhardness.
2. 3 Tribological properties
The friction coefficient curves of the substrate and coating at room temperature and 600 ℃ are shown in Figure 5. It can be seen from the figure that the curve fluctuates greatly in the early stage of wear, which may be caused by the flaking of debris and the stratification of surface tissue, resulting in unstable friction coefficient. As the wear time increases, the fluctuation of the curve becomes smaller and the friction coefficient gradually stabilizes. The average friction coefficient of each sample within 15-30 min is shown in Figure 6. At room temperature, the average friction coefficients of Inconel718 and three coatings (N1-N3) are 0.71, 0.69, 0.65, and 0.68, respectively. Among them, the friction coefficient of N2 coating is the lowest, that is, it has the best friction reduction performance. At 600 ° C, the average friction coefficients of the coatings are 0.55, 0.47 and 0.58, respectively, which are lower than the substrate (0.82). Combined with Figure 6, it can be seen that under both temperature conditions, the friction coefficient of the N2 coating is the lowest, which is 8.45% and 42.68% lower than that of the substrate, which proves that the friction reduction performance of the coating is the best when 4% TiN is added to the cobalt powder. However, after continuing to add TiN to 6%, the friction coefficient of the coating increases instead. This may be due to the increase in hard phase particles in the coating, which are exposed on the surface of the coating during the wear process, making the contact surface rough, thereby increasing the friction coefficient.
The wear rates of the substrate and the three coatings at room temperature and 600 °C are shown in Figure 7. The wear rates at room temperature are 8.39×10′-5, 5.55×10-5, 1.80×10′-5, and 1.36×10′-5 mm’3/(N∙m), respectively. As can be seen from the figure, the wear rate of the N3 coating is the lowest, which is 83.79% lower than that of the substrate. At 600 °C, the wear rates of the substrate and each coating are 9.42×10′-5, 8.19×10′-5, 1.49×10′-5, and 0.94×10′-5 mm’3/(N∙m). This shows that with the increase of TiN content, the wear resistance of the composite coating gradually increases. When 6% TiN is added, the coating has the best wear resistance. Combined with the above microhardness analysis, the hard phase TiN can reduce the surface wear by increasing the hardness of the coating, and the high melting point TiN particles can serve as the core of non-uniform nucleation to produce dispersion strengthening, thereby improving the microstructure and properties of the coating.
2.4 Wear mechanism analysis
2.4.1 Wear mechanism analysis at room temperature
The wear scar morphology of the substrate and each coating is shown in Figure 8. It can be seen from the figure that the wear scar of the substrate is the widest. Figure 8 (b-1) and (c-1) are the wear debris morphology of some areas of the substrate wear scar surface, respectively. The surface mainly undergoes plastic deformation, and there are also furrows scratched by abrasive particles. The wear debris is mainly powdery and
blocky. Inference of substrate wear process: Due to its low hardness, the substrate is prone to plastic deformation when it contacts the silicon nitride ceramic ball during friction, resulting in the peeling of its surface material, and then becomes granular wear debris during the subsequent wear process, causing the surface to be rubbed and forming abrasive wear. In addition, combined with the EDS results in Table 5, it is inferred that metal oxides such as Cr2O3 and NiO exist in the wear debris. This indicates that during the experiment, a thin oxide film may be generated on the substrate surface. Under the action of stress, the oxide film is easily detached from the surface and mixed into the wear debris. In short, in addition to severe plastic deformation, there is also micro-oxidation and abrasive wear on the substrate surface.
From the wear morphology of the substrate and coating at room temperature in Figure 8 (a-1) ~ (a-4), it can be seen that the wear marks of the three coatings are significantly lighter than those of the substrate. Combined with microhardness and EDS analysis, the addition of hard phases such as Co and TiN can effectively improve the hardness of the composite coating, thereby inhibiting micro-cutting and plastic deformation during wear and reducing surface wear [18]. As shown in Figures 8 (a-2) and (b-2), the N1 coating wear surface has light plastic deformation and a small amount of flaking. It is speculated that microcracks are formed on the coating surface under the action of friction, causing the surface layer to be squeezed or even broken and detached. As shown in Figure 8 (c-2), the wear debris is in powder form and a small amount of block form. As shown in Figures 8 (a-3) and (c-3), pits appear on the wear surface of the N2 coating. Combined with the EDS results of the wear debris, it can be seen that the O content is high, indicating that there may be oxide film shedding, and as the friction process proceeds, a new thin oxide film will be generated on the surface. This cycle will cause the phenomenon of adhesion-stripping-re-adhesion to occur continuously, resulting in adhesive wear. As shown in Figures 8 (a-4) and (b-4), material wear and deformation occur on the surface of the N3 coating. The atomic fraction of O element in the wear debris is 20.3%, and the atomic fraction of Co is 67.3%, which is much higher than the content of other metal elements. It is speculated that micro-oxidation wear may have occurred on the surface of N3 coating, and the oxide in the wear debris is mainly CoO. In summary, N3 coating has plastic deformation and micro-oxidation wear. Overall, the wear of the three composite coatings is less than that of the substrate.
2.4.2 Wear mechanism analysis at 600℃
The wear morphology of the substrate at 600℃ is shown in Figure 9 (a-1) and (b-1). It is observed that the substrate surface has more serious plastic deformation, and there are some furrows and pits. This shows that the hardness of the substrate decreases during the heating process, which makes the Si3N4 ceramic ball wear the substrate more seriously and causes pits on the wear surface. Combined with the EDS analysis in Table 6, the atomic fraction of O in the grinding debris is 66%. In addition, there are other elements such as Cr (5.2%), Ni (14.1%) and Fe (8.2%). It is speculated that the white grinding debris produced is metal oxides such as Cr2O3, NiO and Fe2O3. According to the literature, different types of metal oxide films have different growth rates and expansion rates. The PBR value (Pilling-Bedworth ratio) is generally used to evaluate the expansion degree of metal oxide films. Only when 1<PBR<2, the oxide film is easily passivated and can form a good protective effect. The study found that the PBR values of Fe2O3 and Cr2O3 are 2.14 and 2.07 respectively, and the PBR value of NiO is 1.65, so the oxide film formed by Fe2O3 and Cr2O3 is loose and easy to peel off, but NiO can better adhere to the surface of the substrate to prevent the material from being further oxidized.
As can be seen from Figure 9 (a-2) to (c-2), the surface wear of the N1 coating is lighter than that of the substrate. The reason is speculated to be that the high hardness of cobalt improves the wear resistance of the coating, thereby reducing wear. From the enlarged view of the worn surface in Figure 9 (b-2), white wear debris appeared on the surface of the coating. Combined with EDS analysis, the atomic fractions of O and Co elements are 25% and 63.1%, respectively. It is speculated that the wear debris mainly contains phases such as CoO, that is, the N1 coating is mainly slightly oxidized and abrasive wear. It was observed that the surface of the N2 coating had flaking, and a small amount of cracks appeared on the surface. The wear debris was powdery particles and a small amount of blocks, as shown in Figure 9 (a-3) to (c-3). The surface wear morphology of the N3 coating is shown in Figure 9 (a-4) to (b-4). Obvious plowing grooves and plastic deformation are observed on the surface. It is speculated that when the wear surface is in contact with the Si3N4 grinding ball, stress concentration occurs at the contact point, resulting in microcracks, and the cracks expand under the action of stress, causing the surface material to peel off. From Figure 9 (c-4), it can be seen that the wear debris is mainly in powder form. Compared with the EDS results at room temperature, it can be seen that the oxygen content in the coating and substrate wear debris at 600 ℃ is higher than that at room temperature, indicating that the sample is more susceptible to oxidative wear at 600 ℃, and the oxide film is easy to fall off to form wear debris. At the same time, combined with the wear rate conclusion and wear morphology, it can be seen that the wear of the substrate and N1 coating at 600 ℃ is more serious than at room temperature, and the wear marks are more obvious, but the wear of N2 and N3 coatings at 600 ℃ is lower than that at room temperature, indicating that the pure Co coating can only relatively reduce the wear of the substrate, and the addition of TiN can further improve the wear resistance of the coating at 600 ℃.
2. 5 Analysis of anti-oxidation mechanism
Since Inconel718 alloy is often used in high-temperature environments, and the wear mechanism shows that the material surface is more susceptible to oxidation wear in high-temperature friction and wear experiments, it is necessary to explore the oxidation mechanism of the coating at high temperature. In the previous tribological experiment, the N3 coating has the best wear resistance among the designed coatings, so it was subjected to a constant temperature oxidation experiment at 800 ℃ for 50 h, and the substrate was used as a control. After 50 h, the weight gain per unit area (Δm) of the two materials is 56.55 mg/cm2 and 66.16 mg/cm2 respectively. The oxidation kinetic curve is plotted according to the weight gain as shown in Figure 10, and it is observed that the curve is a parabola. The literature shows that the law of the oxidation kinetic curve conforms to the characteristics of a parabola, indicating that the material has antioxidant properties.
According to Cui oxidation theory, the oxidation rate constant calculation formula (2) can be obtained: Δm’2 = Kp ⋅ t (2)
Where: Δm is the weight gain per unit area; t is the oxidation time; Kp is the oxidation rate constant, and the smaller its value is, the stronger the material’s antioxidant ability is. The oxidation rate (Kp) and square correlation coefficient (R2) of the substrate and coating at 800 °C are shown in Table 7. The closer R2 is to 1, the better the fitting accuracy of the experimental data and the fitting curve. It can be seen from the table that the oxidation rate of the N3 coating (8.7634 mg·cm-4·h-1) is slightly higher than that of the substrate, but not much different from that of the substrate.
The XRD spectra of the oxidized surface of the substrate and N3 coating are shown in Figure 11. Analysis of the XRD spectrum shows that the substrate surface is mainly composed of Cr, Fe and Ni oxides, including Fe2O3 and Cr2O3 metal oxides and NiCr2O4 spinel structure oxides. The spinel structure oxide is generated by the solid phase reaction of Cr2O3 and Ni oxides. Its structure is dense and helps to improve the oxidation resistance. The reaction formula is: Formula (3) – NiO + Cr2O3 = NiCr2O4 (3)
The oxidized surface of the N3 coating is mainly composed of Co and Ti oxides, including CoOx and TiO2. In addition, Fe oxide Fe2O3 is also detected.
The surface morphology of the oxide layer of the substrate and N3 coating is shown in Figure 12. It can be seen from Figure 12 (a) that the oxidized surface of the substrate is mainly gray block particles, and a small amount of cavities and pits appear. Combined with the EDS analysis in Table 8, the atomic fraction of Cr in the gray-white block material A is 37.82%, and the atomic fraction of O is 59.89%. It is speculated that Cr2O3 exists here. Point C is a pit structure, and the atomic fractions of Fe and Ni are 2.53% and 7.26% respectively. Combined with XRD, it is speculated that it may contain a small amount of Fe2O3 and NiCr2O4. Different from the substrate, the oxidized surface of the N3 coating is a gray-white mesh structure, and there are obvious cracks in some areas. It is speculated that the difference in thermal expansion coefficient between the substrate and the oxide (Inconel718: (13.0~14.2)×10’-6℃‘-1; Cr2O3: (7.2~7.8)×10’-6℃‘-1; TiO2: (8.5~9.0)×10’-6℃‘-1; CoO: (12.5~13.7)×10’-6℃‘-1; Fe2O3: (11.3~11.8)×10’-6℃‘-1) may have caused the cracks. Moreover, the sample underwent several hot and cold cycles from 800℃ to room temperature in the experiment. When the critical stress was exceeded, cracks also appeared on the surface. EDS shows that the Co content in the N3 coating is much greater than that of other metals. For example, at point D, in addition to Co (46.61%), other metal elements such as Cr (0.06%), Fe (0.80%), and Ni (0.37%) are present in low amounts, so CoOx is the main component. In addition, there is also Ti element (0.03%) at point E, and it is speculated that TiO2 is also present there. The above main reaction equations are shown in equations (4) to (7):
4Cr+3O2→2Cr2O3 (4)
4Fe+3O2→2Fe2O3 (5)
2Co+O2→2CoO (6)
Ti+O2→TiO2 (7)
3 Conclusions
(1) The main phases in the composite coating include solid solution γ-Co and (Fe, Ni), as well as FeNi3 and Cr3Ni2 formed between alloy elements in the molten pool. In addition, N2 and N3 coatings also contain Cr2Ti and TiN. The hardness of the coating is also improved relative to the substrate, reaching 1.3~1.4 times that of the substrate (262.7HV0.5).
(2) The N2 coating has a better friction reduction effect, while the N3 coating has the best wear resistance, indicating that the Co/TiN composite coating can effectively improve the tribological properties of the substrate. And with the increase of TiN content, the coating wears less and the wear resistance is better. From the analysis of the wear mechanism, it can be seen that an oxide film is generated on the coating surface at 600 ℃, which helps to reduce surface wear to a certain extent.
(3) At 800 ℃, the N3 coating has oxidation resistance, and the oxidation rate constant is 8. 7634 mg’2 ∙cm’-4 ∙h’-1, which is not much different from that of the substrate. This proves that the composite coating has a certain oxidation resistance and can significantly reduce the wear rate of the substrate at high temperature, improve the tribological performance, and extend the service life of parts, thereby reducing costs and making the Inconel718 alloy more widely used under high temperature extreme conditions.
