Non-monotonic grain size gradient composite coating as well as preparation method and application thereof

Abstract

The invention relates to a non-monotonic grain size gradient composite coating and a preparation method and application thereof.The non-monotonic grain size gradient composite coating comprises a fine grain layer, a coarse grain layer and a transition layer which are sequentially arranged on a substrate from bottom to top, grains in the fine grain layer, the coarse grain layer and the transition layer are columnar crystals, and the radial average size of the grains in the fine grain layer is 1t; the average radial size of crystal grains in the coarse crystal layer is 220-500 nm, and the average radial size of crystal grains in the transition layer is 50-200 nm. A fine-coarse-transition non-monotonic gradient crystal region structure is formed, a fine grain region of the composite coating provides high hardness, a coarse grain region of the composite coating improves toughness, a transition grain region simulates a mineralized gradient interface, stress dispersion is achieved, and the obtained composite coating shows more excellent comprehensive performance under complex working conditions and has high hardness, toughness and wear resistance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of coating technology, in particular to a non-monotonic grain size gradient composite coating and a preparation method and application thereof. BACKGROUND

[0002] Grain size is one of the important parameters in the microstructure of materials, which can directly affect the mechanical properties of materials. In the design of many high-performance materials, precise control of grain size, especially the grain structure in the interface region of the coating, has become a key factor to improve the overall performance. The distribution and change pattern of grain size play a decisive role in the key performance of the coating, such as hardness, wear resistance, corrosion resistance, etc.

[0003] In traditional coating materials, a single grain size structure is usually adopted. Although the performance of the coating can be improved by fine-tuning the grain size, due to the single grain structure and linear variation of the size, these coatings often cannot take into account all the properties of the material when dealing with complex working environments. For example, the grain size monotonically increases or decreases along the thickness direction of the coating. This inherent structure has the performance defects that are difficult to overcome: in the field of mechanical processing, the coating with monotonically increasing grain size has insufficient surface hardness, and is prone to plastic deformation under high-strength load, resulting in a decrease in processing precision and a shortening of the service life of the coating; the structure with monotonically decreasing grain size will cause stress concentration in the coating, and under alternating load, the toughness is significantly reduced, resulting in peeling, cracking and other failure phenomena of the coating. SUMMARY

[0004] The purpose of the present application is to overcome the above technical deficiencies, provide a non-monotonic grain size gradient composite coating and a preparation method and application thereof, and solve the technical problems that the grain size of the coating in the prior art leads to insufficient surface hardness or easy stress concentration due to monotonic variation.

[0005] To achieve the above technical purpose, the technical solution provided by the present application is: In a first aspect, the present application provides a non-monotonic grain size gradient composite coating, which comprises a fine-grained layer, a coarse-grained layer and a transition layer arranged in order from bottom to top on a substrate. The grains in the fine-grained layer, the coarse-grained layer and the transition layer are columnar grains, and the radial average size of the grains in the fine-grained layer is <100 nm, the radial average size of the grains in the coarse-grained layer is 220-500 nm, and the radial average size of the grains in the transition layer is 50-200 nm.

[0006] In a second aspect, the present application provides a preparation method of a non-monotonic grain size gradient composite coating, comprising the following steps: S1, depositing a film on a substrate by a high-power pulsed magnetron sputtering method to form a fine-grained layer; wherein the conditions of the high-power pulsed magnetron sputtering method include: a substrate temperature of 260-340℃, a target-substrate distance of 28-32mm, a sputtering power of 9-11kW, and a sputtering bias voltage of-380--420V; S2, adjusting the substrate temperature to 90-500℃, the target-substrate distance to 8-12mm, the sputtering power to 9-22kW, and the sputtering bias voltage to-40--60V, and depositing a film on the fine-grained layer to form a coarse-grained layer; S3, adjusting the substrate temperature to 280-520℃, the target-substrate distance to 28-32mm, the sputtering power to 9-22kW, and the sputtering bias voltage to-100--420V, and depositing a film on the coarse-grained layer to form a transition layer, thereby obtaining the non-monotonic grain size gradient composite coating.

[0007] In a third aspect, the present application provides an application of the non-monotonic grain size gradient composite coating in the preparation of a tool coating or a biomedical coating.

[0008] Compared with the prior art, the present application has the following beneficial effects: The present application arranges the fine-grained layer, the coarse-grained layer and the transition layer in sequence along the thickness direction of the coating, corresponding to the fine-grained region, the coarse-grained region and the transition region, to form a non-monotonic gradient crystal region structure of "fine-coarse-transition"; wherein the fine-grained region of the composite coating provides high hardness, the coarse-grained region improves toughness, and the transition region simulates a mineralization gradient interface to achieve stress dispersion, so that the obtained composite coating exhibits more excellent comprehensive performance under complex working conditions, with high hardness, toughness and wear resistance. BRIEF DESCRIPTION OF DRAWINGS

[0009] Figure 1 SEM images of the surfaces of the coatings prepared in Example 1 and Comparative Example 1 of the present application, (a) Example 1, (b) Comparative Example 1; Figure 2 SEM images of the cross sections of the coatings prepared in Example 1 and Comparative Example 1 of the present application, (a) Example 1, (b) Comparative Example 1; Figure 3 SEM enlarged image of the cross section of the coating prepared in Example 1 of the present application; Figure 4 Average friction coefficient images of the friction and wear of the coatings prepared in Example 1-3 and Comparative Example 1 of the present application; Figure 5 Coating hardness and modulus images of the coatings prepared in Example 1-3 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0010] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and not to limit the present application.

[0011] In view of the defects that the grain size of the current coating leads to insufficient surface hardness or easy stress concentration due to monotonic change, the present application provides a non-monotonic grain size gradient composite coating and a preparation method and application thereof, and the non-monotonic grain size gradient composite coating is prepared based on multi-parameter synergistic control to realize performance breakthrough through simulating the non-monotonic gradient of biological materials. The biological materials with non-monotonic gradient, such as tooth enamel (nanoscale apatite crystal, hardness 300 HV) and dentin (micron-scale collagen fiber, fracture toughness 12 MPa·m 1 / 2) of teeth, disperse chewing stress through a mineralization gradient interface (DEJ), and the crack deflection rate reaches 85%. The outer hard scale (highly mineralized hydroxyapatite) and the inner soft collagen of fish scales are connected through a fiber bridging gradient, and the impact energy absorption efficiency is improved by 60%.

[0012] Therefore, the present application simulates the layer-by-layer growth mechanism and stress buffering principle of biological mineralization through high-power pulse magnetron sputtering technology (HiPIMS), and provides experimental basis and theoretical support for the biomimetic design of high-performance coatings. The non-monotonic grain size gradient composite coating of the present application can realize the cross-scale precise control of grain size, phase composition and interface structure, and becomes the core direction to break through the upper limit of the performance of traditional coatings.

[0013] In a first aspect, the present application provides a non-monotonic grain size gradient composite coating, which comprises a fine-grained layer, a coarse-grained layer and a transition layer arranged in sequence from bottom to top on a substrate. The grains in the fine-grained layer, the coarse-grained layer and the transition layer are all columnar grains, and the radial average size of the grains in the fine-grained layer is <100 nm, the radial average size of the grains in the coarse-grained layer is 220-500 nm, and the radial average size of the grains in the transition layer is 50-200 nm.

[0014] In the present application, the fine-grained layer, the coarse-grained layer and the transition layer are arranged in sequence along the thickness direction of the coating, corresponding to a fine-grained zone, a coarse-grained zone and a transition zone, forming a non-monotonic gradient of "fine-coarse-transition". The fine-grained zone of the coating provides high hardness; the coarse-grained zone improves toughness; and the transition zone simulates a mineralization gradient interface (DEJ) to realize stress dispersion. Through the non-linear change of the grain size, the present application realizes the synergistic optimization of multiple performances such as strength (hardness is improved by 8.60%-27.61%) and wear resistance (COF is reduced by 7.07%-12.74%), and solves the problems of stress concentration, grain boundary oxidation and interface failure of traditional coatings in extreme environments.

[0015] It can be understood that the radial average size of the grain corresponds to the thickness of the columnar crystal, and the grain size in the following text refers to the radial average size.

[0016] Preferably, the radial average size of the grain in the fine-grain layer is 30-80 nm, the average size of the grain in the coarse-grain layer is 240-480 nm, and the average size of the grain in the transition layer is 50-180 nm.

[0017] Preferably, the substrate is stainless steel or a single crystal silicon wafer.

[0018] Preferably, the material of the fine-grain layer, the coarse-grain layer and the transition layer includes CrN.

[0019] In a second aspect, the present application provides a preparation method of a non-monotonic grain size gradient composite coating, comprising the following steps: S1, depositing a film on the substrate by high-power pulsed magnetron sputtering to form a fine-grain layer; wherein the conditions of the high-power pulsed magnetron sputtering include: a substrate temperature of 260-340℃, a target-substrate distance of 28-32mm, a sputtering power of 9-11kW, and a sputtering bias voltage of -380 to -420V; S2, adjusting the substrate temperature to 90-500℃, the target-substrate distance to 8-12mm, the sputtering power to 9-22kW, and the sputtering bias voltage to -40 to -60V, depositing a film on the fine-grain layer to form a coarse-grain layer; S3, adjusting the substrate temperature to 280-520℃, the target-substrate distance to 28-32mm, the sputtering power to 9-22kW, and the sputtering bias voltage to -100 to -420V, depositing a film on the coarse-grain layer to form a transition layer, thereby obtaining a non-monotonic grain size gradient composite coating.

[0020] The present application can precisely control the size of the grain size by adjusting the combination of the substrate temperature, the target-substrate distance, the sputtering power and the sputtering bias voltage of each layer through multi-parameter coordination; the present application can accurately adjust the grain size of each layer according to different working conditions and application requirements in the same coating, thereby optimizing multiple performance indicators of the coating, and having flexibility and multifunctionality.

[0021] Preferably, the conditions of the high-power pulsed magnetron sputtering further include: a pulse frequency of 850-950Hz, a pulse duration of 95-105μs, a discharge voltage of 1.0-1.2kV, a peak current of 180-200A, a working gas pressure of 0.35-0.65Pa, and N2 / Ar flow rates of 40-60sccm each.

[0022] When depositing the film, a relatively low gas pressure is required to prolong the free path of the sputtered target material particles, thereby ensuring efficient directional deposition of the coating.

[0023] Further preferably, the conditions of the high power pulsed magnetron sputtering method include: a pulse frequency of 900 Hz, a pulse duration of 100 mu s, a discharge voltage of 1.1 kV, a peak current of 190 A, a working gas pressure of 0.5 Pa, and N2 / Ar flow rates of 50 sccm each.

[0024] Preferably, before the deposition of the film in step S1, the substrate is subjected to ultrasonic cleaning and plasma glow cleaning; the ultrasonic cleaning is to sequentially place the substrate in acetone and anhydrous ethanol for ultrasonic treatment; the plasma glow cleaning is to place the substrate after ultrasonic cleaning on the substrate table of the high power pulsed magnetron sputtering device, the working gas is 100 sccm of argon, the gas pressure is 2.5-3.0 Pa, the sputtering bias is -600 V, and the cleaning time is 1200 s.

[0025] During the process of the glow cleaning, if the gas pressure is too low, the plasma density is not enough, and the cleaning effect cannot be achieved, therefore, the gas pressure should be higher than the gas pressure of the deposition of the film so as to successfully generate the glow under the condition of the bias voltage.

[0026] The present application removes the oil and impurities through ultrasonic treatment, and removes the surface oxide through plasma glow cleaning.

[0027] Preferably, in step S1, the target material for the deposition of the film is a Cr target; before the deposition of the film, the target material is subjected to plasma glow cleaning and pre-sputtering treatment, the conditions of the plasma glow cleaning include: the working gas is 100 sccm of argon, the gas pressure is 2.5-3.0 Pa, and the cleaning time is 600 s; the conditions of the pre-sputtering treatment include: the working gas is 100 sccm of argon, the reaction gas is 50 sccm of nitrogen, the pre-sputtering power is 10 kW, the sputtering bias is -100 V, and the time is 20 min.

[0028] The present application removes the surface oxide layer through the pre-sputtering of the target material.

[0029] Preferably, in step S3, the rate of the temperature rise of the substrate is 5-8 ℃ / min.

[0030] Preferably, in step S3, the adjustment rate of the sputtering bias is 8-12 V / min.

[0031] Further preferably, the temperature of the substrate is gradually increased from 100 ℃ to 500 ℃ at a rate of 5 ℃ / min, the sputtering bias is increased from -50 V to -400 V at a rate of 10 V / min, and the target-substrate distance is gradually increased from 10 mm to 30 mm through the mechanical adjustment device, and the total time of the temperature rise and the parameter adjustment is 80 min, so as to realize the continuous gradient of the grain size.

[0032] In a third aspect, the present application provides a use of the non-monotonic grain size gradient composite coating in the preparation of a tool coating or a biomedical coating.

[0033] The structure design of "fine grain-coarse grain-transition grain" is based on the comprehensive optimization of the structural continuity, interface bonding force and stress buffering performance of the multi-layer coating system. By placing fine grains on the surface of the substrate, the lattice matching and bonding performance of the coating and the substrate can be effectively improved, laying a solid foundation for the entire coating and reducing the interface defects between the substrate and the coating. The coarse grains in the middle layer can provide structural support for the surface transition grains and buffer thermal stress and mechanical stress during service, avoiding stress concentration caused by performance between layers. The transition grains can ensure the hardness of the coating while transitioning with the performance of the middle coarse grains, ultimately meeting the service requirements of a firm interface, stable surface performance and overall structural stability.

[0034] The application will be further described in detail through specific examples. In the examples, 304 stainless steel is selected as the substrate material, and a CrN composite coating is prepared thereon. Before preparing the CrN composite coating, the substrate and the target material are pretreated and cleaned respectively: (1) The substrate is placed in acetone and anhydrous ethanol in turn, and each is subjected to ultrasonic treatment for 15 min to remove various solid particles attached to the surface of the substrate. After cleaning, the residual water stains and impurities on the surface are blown off by a N2 air gun; (2) The Cr target is loaded in the deposition cavity of the HiPIMS, and the cleaned substrate is placed on the substrate table of the high-power pulsed laser deposition instrument. The cavity is subjected to vacuum treatment; (3) The substrate is subjected to plasma glow cleaning to remove the oxides on the surface. The process conditions are as follows: 100 sccm of argon gas is introduced as the working gas, the gas pressure in the vacuum cavity during work is 2.5 Pa, the sputtering bias is -600 V, and the cleaning time is 1200 s; (4) The target material is subjected to plasma glow cleaning to remove the oxides on the surface. The process conditions are as follows: 100 sccm of argon gas is introduced as the working gas, the gas pressure in the vacuum cavity during work is 2.5 Pa, and the cleaning time is 600 s.

[0035] Example 1 A preparation method of a non-monotonic grain size gradient composite coating, comprising the following steps: S1. A high-power pulsed magnetron sputtering method is used to deposit a film on the cleaned substrate. The basic conditions of the high-power pulsed magnetron sputtering method (the same as those in subsequent steps) include: rotation speed of 10 r / min, chamber pressure of 0.5 Pa, nitrogen and argon working gases with a flow rate of 50 sccm each, pulse frequency (f) of 900 Hz, pulse duration of 100 μs, discharge voltage (Vp) of 1.1 kV, and peak current (Ip) of 190 A. The variable conditions include: substrate temperature of 300℃, target-substrate distance of 30 mm, sputtering power of 10 kW, and sputtering bias of -400 V. The deposition time is set to 3600 s. After the deposition is completed, the temperature in the chamber is allowed to drop to 40℃ to obtain a fine-grained layer with fine grains (50 nm) on the surface. S2, adjust the substrate temperature to 100℃, the target-substrate distance to 10mm, the sputtering power to 10kW, and the sputtering bias voltage to -50V; set the deposition time to 3600s, and prepare a coarse-grained layer with coarse grains on the surface by sputtering on the fine-grained layer. S3, adjust the substrate temperature to 500℃, the target-substrate distance to 30mm, the sputtering power to 10kW, and the sputtering bias to -300V; set the deposition time to 3600s, and prepare a transition layer with transition grains on the surface by sputtering on the coarse grain layer, thus obtaining a non-monotonic grain size gradient composite coating.

[0036] like Figure 1 (a) and Figure 2 As shown in (a), the average size of the fine grains in the fine grain layer obtained in this embodiment is about 50 nm, the average size of the coarse grains in the coarse grain layer is about 360 nm, and the average size of the transition grains in the transition layer is about 129 nm.

[0037] Example 2 Compared with Example 1, the only difference is that the parameters of steps S2 and S3 are different; the other steps and conditions are the same as in Example 1. S2, with the substrate temperature set to 500℃, the target-substrate distance set to 10 mm, the sputtering power set to 10kW, the sputtering bias set to -50 V, and the deposition time set to 3600s, a coating structure with coarse grains on the surface was prepared by sputtering. S3, repeat step S1 of Example 1 to prepare a coating structure with small grains on the surface by sputtering.

[0038] In this embodiment, the average size of the fine grains in the fine-grained layer is about 50 nm, the average size of the coarse grains in the coarse-grained layer is about 270 nm, and the average size of the transition grains in the transition layer is about 50 nm.

[0039] Example 3 Compared with Example 1, the only difference is that the parameters of step S2 and step S3 are different, other steps and conditions are the same as Example 1, the difference is that: The sputtering power in step S2 is set to 20 kW; the sputtering bias in step S3 is set to -100 V.

[0040] The average size of fine grains in the fine grain layer prepared in this example is about 50 nm, the average size of coarse grains in the coarse grain layer is about 460 nm, and the average size of transition grains in the transition layer is about 168 nm.

[0041] Comparative Example 1 Compared with Example 1, the only difference is that steps S2-S3 are removed, the substrate temperature in step S1 is set to 300℃, the deposition time is set to 10800 s, and after the deposition is completed, the temperature in the chamber is waited to drop to 40℃ before taking out; other conditions are the same as Example 1.

[0042] As Figure 1 (b) and Figure 2 As shown in (b) and (b) of the embodiment, the CrN coating prepared in this comparative example has uniform grains on the surface, and the size is about 25 nm.

[0043] From the grain size of Example 1-3 and Comparative Example 1, and Figure 1 and Figure 2 It can be seen from the characterization that the coating surface prepared by the present application has grains of different sizes, and the cross sections of Example 1-3 are non-monotonic changes in grain size, from Figure 3 (Example 1 coating cross section SEM magnification) can be more obviously seen, which presents a non-monotonic grain size gradient (fine grain zone→large grain zone→transition grain zone); while Comparative Example 1 is uniform columnar crystal growth, and the gradient interface of Example 1-3 is obviously better than the single structure of Comparative Example 1.

[0044] And compared with Example 1, Comparative Example 1 uses long-time continuous treatment, and under long-time continuous bias, there is a back sputtering effect to inhibit the growth of grains, so that the grain size is reduced.

[0045] Figure 4 The average friction coefficient (COF) of Example 1-3 and Comparative Example 1 can be seen, the COF of Example 1-3 is significantly lower than that of Comparative Example 1, and is stably in the range of 0.65-0.69, with small fluctuation amplitude; the COF of Comparative Example 1 is maintained at 0.75, and the fluctuation is frequent.

[0046] If the surface layer is a low-hardness large-grain region, it is prone to plastic deformation and material removal during frictional contact, and the surface microprotrusions are easily sheared off during sliding, leading to an increase in wear rate. At the same time, the large-grain surface has a higher surface roughness, which is not conducive to the establishment of a stable friction interface, and can cause friction coefficient fluctuations and heat accumulation to intensify. The system of the present application can effectively reduce and stabilize the friction coefficient, and has better tribological performance. The friction coefficient of Examples 1-3 is reduced by 7.07%-12.74% compared with Comparative Example 1, indicating that the non-monotonic gradient structure can significantly improve the friction stability, and the mechanism is related to the elastic-plastic deformation coordination ability of the transition grain region.

[0047] Figure 5 It can be seen that due to the difference in grain size, the hardness and modulus of Examples 1-3 show a large change. Specifically, the hardness (H) and elastic modulus (E) of Examples 1-3 and Comparative Example 1 generally show a positive correlation trend. The H and E of Example 1 are relatively high and stable, being 23 GPa and 342 GPa, respectively; the H and E of Examples 2-3 gradually decrease, but the H and E of Comparative Example 1 are significantly lower than those of Example 1, and the mechanical properties are obviously deteriorated, reflecting that different examples have different effects on the mechanical properties of the material. In the structure of Example 1, the hardness is 23 GPa, and the transition grain is used as the surface layer, which can provide a certain flexibility and buffering capacity without significantly sacrificing the hardness. Compared with the large-grain layer, the transition grain has a moderate size, and the number of grain boundaries increases, which can effectively limit the dislocation movement, thereby maintaining a high surface hardness; at the same time, the elastic modulus is similar to that of the middle large-grain layer, which is helpful to realize a continuous and gradual stress transmission path, and avoid interface stress concentration caused by modulus mutation. The middle large-grain layer plays a major role in mechanical bearing and energy absorption, providing strength support for the overall structure. This makes the friction coefficient stable at 0.65-0.69, which is reduced by 7.07%-12.74% compared with Comparative Example 1 (0.75), which is similar to the stress dispersion mechanism of the enamel-dentin interface, indicating that the structure of the present application overcomes the performance defects caused by the monotonous change of grain size in traditional coating technology.

[0048] In summary, the present application simulates the gradient transition mechanism of the enamel-dentin interface and the multi-layer structure design of the exoskeleton of crustaceans, and uses the high-power pulsed magnetron sputtering technology (HiPIMS) to cooperatively control the parameters such as substrate temperature, sputtering bias, sputtering power and target-substrate distance to control the grain size, realize the non-monotonic gradient grain structure of the coating along the thickness direction, and realize the optimization of the performance between different levels of the coating through this non-monotonic grain size change mode, avoiding the performance defects of traditional coatings in high strength, high temperature and strong corrosion environment. The obtained composite coating has more excellent comprehensive performance under complex working conditions, and has high hardness, toughness and wear resistance.

[0049] The above description of the specific embodiments of the present application is not intended to limit the scope of the present application. Various other corresponding changes and modifications according to the technical concept of the present application should be included in the scope of the present application claimed.

Claims

1. A non-monotonic grain size gradient composite coating, characterized in that, The material comprises a fine-grained layer, a coarse-grained layer, and a transition layer disposed sequentially from bottom to top on a substrate. The grains in the fine-grained layer, the coarse-grained layer, and the transition layer are all columnar crystals. The average radial size of the grains in the fine-grained layer is <100 nm, the average radial size of the grains in the coarse-grained layer is 220–500 nm, and the average radial size of the grains in the transition layer is 50 nm–200 nm.

2. The non-monotonic grain size gradient composite coating according to claim 1, characterized in that, The substrate is stainless steel or a monocrystalline silicon wafer.

3. The non-monotonic grain size gradient composite coating according to claim 1, characterized in that, The fine-grained layer, coarse-grained layer, and transition layer are made of CrN.

4. The method for preparing a non-monotonic grain size gradient composite coating as described in any one of claims 1-3, characterized in that, Includes the following steps: S1, a fine-grained layer is formed by depositing a film on the substrate using high-power pulsed magnetron sputtering; wherein, the conditions of high-power pulsed magnetron sputtering include: substrate temperature of 260~340℃, target-substrate distance of 28~32mm, sputtering power of 9~11kW, and sputtering bias voltage of -380~-420V; S2, adjust the substrate temperature to 90~500℃, the target-substrate distance to 8~12mm, the sputtering power to 9~22kW, and the sputtering bias voltage to -40~-60V, deposit a film on the fine grain layer to form a coarse grain layer; S3, adjust the substrate temperature to 280~520℃, the target-substrate distance to 28~32mm, the sputtering power to 9~22kW, and the sputtering bias voltage to -100~-420V, deposit a film on the coarse grain layer to form a transition layer, and obtain a non-monotonic grain size gradient composite coating.

5. The method for preparing a non-monotonic grain size gradient composite coating according to claim 4, characterized in that, The conditions for high-power pulsed magnetron sputtering also include: pulse frequency of 850–950 Hz, pulse duration of 95–105 μs, discharge voltage of 1.0–1.2 kV, peak current of 180–200 A, working gas pressure of 0.35–0.65 Pa, and N2 / Ar flow rates of 40–60 sccm each.

6. The method for preparing a non-monotonic grain size gradient composite coating according to claim 4, characterized in that, In step S1, before deposition, the substrate undergoes ultrasonic cleaning and plasma glow discharge cleaning. Ultrasonic cleaning involves placing the substrate in acetone and anhydrous ethanol sequentially for ultrasonic treatment. Plasma glow discharge cleaning involves placing the ultrasonically cleaned substrate on the substrate stage of a high-power pulsed magnetron sputtering device. In plasma glow discharge cleaning, the working gas is argon at 100 sccm, the gas pressure is 2.5–3.0 Pa, the sputtering bias is -600 V, and the cleaning time is 1200 s.

7. The method for preparing a non-monotonic grain size gradient composite coating according to claim 4, characterized in that, In step S1, the target material for deposition is a Cr target. Before deposition, the target material undergoes plasma glow discharge cleaning and pre-sputtering treatment. The conditions for plasma glow discharge cleaning of the target material include: working gas is argon at 100 sccm, gas pressure is 2.5-3.0 Pa, and cleaning time is 600 s. The conditions for pre-sputtering treatment include: working gas is argon at 100 sccm, reaction gas is nitrogen at 50 sccm, pre-sputtering power is 10 kW, sputtering bias is -100 V, and time is 20 min.

8. The method for preparing a non-monotonic grain size gradient composite coating according to claim 4, characterized in that, In step S3, the substrate is heated at a rate of 5–8 °C / min.

9. The method for preparing a non-monotonic grain size gradient composite coating according to claim 4, characterized in that, In step S3, the sputtering bias voltage is adjusted at a rate of 8–12 V / min.

10. The application of the non-monotonic grain size gradient composite coating as described in any one of claims 1-3 in the preparation of tool coatings or biomedical coatings.

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