TiN / TiSiN gradient multilayer coating mechanical property and stress distribution prediction method

Abstract

The invention relates to a method for predicting mechanical properties and stress distribution of a TiN / TiSiN gradient multilayer coating. The method comprises the following specific steps: 1) carrying out three-dimensional solid modeling by utilizing finite element software; (2) meshing is conducted on the geometric models of the pressure head, the gradient multi-layer coating and the base body; 3) endowing the geometric model with material attributes; 4) assembling the geometric model; 5) determining a loading mode and boundary conditions for the model; (6) carrying out simulation calculation by utilizing finite element software, and calculating the hardness and effective elastic modulus of the gradient coating; and 7) predicting the internal intrinsic stress of the gradient multilayer coating. According to the method, the plastic parameters of the TiN layer and the TiSiN layer are obtained by adopting an iterative calculation method, so that the result obtained when the performance data of the gradient multi-layer coating is predicted by adopting a finite element analysis method is closer to reality. The high-performance TiN / TiSiN gradient multi-layer coating is screened in an auxiliary mode based on the prediction method, and the method has the advantages of being high in efficiency and low in cost.

Description

Technical Field

[0001] The invention belongs to the technical field of design, verification or simulation using finite element method, and specifically relates to a method for predicting mechanical properties and stress distribution of a TiN / TiSiN gradient multilayer coating. Background Art

[0002] Nitride hard coatings are widely used in the field of metal processing due to their high hardness. Binary coatings such as TiN and ternary coatings such as TiSiN have received widespread attention. With the improvement of cutting processing requirements, single-layer hard coatings can no longer meet the requirements of toughening and crack resistance while improving hardness. Therefore, researchers pay more attention to multi-layer hard coatings and improve the toughness of hard coatings through gradient multi-layer structure design. Since different coating structures will have different effects on the internal stress distribution, which in turn affects the macroscopic mechanical properties, the design of gradient multi-layer coating structures based on internal stress distribution is particularly important. However, the multi-layer coatings in most current technologies use two materials to be deposited alternately, and the modulation period or modulation ratio is changed to obtain better mechanical properties. The internal stress, a coating damage factor, is not considered when designing the coating structure, resulting in a deviation between the test results of the gradient multi-layer coating material and the actual application effect.

[0003] In addition, the preparation of gradient multilayer coatings is complicated and time-consuming, and thickness errors may occur, which may cause the test samples to fail to meet the preset ideal conditions and result deviations. Based on this, the present invention uses the finite element method (FEM) to predict the mechanical properties and stress distribution of gradient multilayer coatings. Through the optimization scheme, the prediction method is closer to the actual situation, which makes the performance prediction and screening of gradient multilayer coating materials a feasible way, and can serve as a reference for actual experimental tests. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for predicting the mechanical properties and stress distribution of a TiN / TiSiN gradient multilayer coating in view of the deficiencies in the prior art. The prediction method is optimized by an algorithm, and the prediction of the gradient multilayer coating is closer to the actual situation.

[0005] The present invention provides a method for predicting mechanical properties and stress distribution of a TiN / TiSiN gradient multilayer coating.

[0006] Here are the steps:

[0007] 1) 3D solid modeling was performed using ABAQUS finite element software, and the geometric models of the indenter, gradient multilayer coating, and substrate were established according to the parameters in the Vickers nanoindentation experiment of the TiN / TiSiN gradient multilayer coating, and the coating structure was set in the component module;

[0008] 2) meshing the geometric models of the indenter, the gradient multilayer coating and the substrate described in step 1) respectively;

[0009] 3) assigning material properties to the geometric models of the indenter, gradient multilayer coating and substrate described in step 2);

[0010] 4) assembling the geometric model of the indenter, the gradient multilayer coating and the substrate described in step 3);

[0011] 5) determining the loading mode and boundary conditions for the three-dimensional finite element model obtained in step 4);

[0012] 6) Using ABAQUS finite element software to simulate the three-dimensional finite element model described in step 5), the stress distribution and maximum stress value in the gradient coating are obtained through the simulation results, and the indentation depth-load force relationship curve is obtained by using the post-processing function of ABAQUS finite element software to calculate the hardness (H) and effective elastic modulus (E) of the gradient coating. r );

[0013] 7) Modify the assembly method, material properties and loading method of the three-dimensional finite element model described in step 4) to predict the internal intrinsic stress of the gradient multilayer coating without external load.

[0014] According to the above scheme, the TiN / TiSiN gradient multilayer coating is obtained by alternately compounding multiple layers of TiN layers and TiSiN layers on the surface of a Ti-6Al-4V alloy substrate, and a transition layer is provided between the Ti-6Al-4V alloy substrate and the TiN / TiSiN gradient multilayer coating, the transition layer is a Ti layer, the surface of the gradient multilayer coating is a TiSiN layer, and the coating in contact with the transition layer is a TiN layer.

[0015] According to the above scheme, the thickness of the Ti layer is set to 0.5 μm, the total thickness of the TiN / TiSiN gradient multilayer coating and the transition layer is 4.5 μm, the thickness of the Ti–6Al–4V alloy substrate is 5.5 μm, and the Ti–6Al–4V alloy substrate and the TiN / TiSiN gradient multilayer coating models are both cylinders with a radius of 10 μm, and the modulation period is set to decrease linearly, and the modulation ratio is 1:1.

[0016] According to the above scheme, step 1) sets the indenter height to 1.5 μm, the diamond indenter geometric model is modeled according to the Vickers tetrahedron, the opposite angle of the tetrahedron is 136°, and a stretching operation is used to form a three-dimensional entity. Since all geometric models have four-fold symmetry, a quarter of the part is cut for modeling.

[0017] According to the above scheme, step 2) the geometric models of the indenter, the gradient multilayer coating and the substrate are meshed respectively, and the specific method is as follows: the meshing mode of the indenter model is set to free, the unit shape is a tetrahedron, and the seeds are arranged for the edges by a unidirectional offset method, and the density is gradually increased from the bottom to the top of the cone, and the unit type is C3D10; the meshing mode of the gradient multilayer coating and the substrate geometric model is set to sweep, the unit shape is a hexahedron, and the seeds are arranged for the straight edges by a unidirectional offset method, and the density is gradually increased from the upper vertex of the quarter cylinder to the edge. The minimum grid size is 12.5nm, the number of units is 37370, and the unit type is C3D8.

[0018] According to the above scheme, step 3) assigns material properties to the geometric models of the indenter, the gradient multilayer coating and the substrate, specifically: the indenter and the substrate are set as elastic materials, and the Ti layer, the TiN layer and the TiSiN layer are set as elastic-plastic materials, wherein the indentation test curves of the single-layer coatings of the TiN layer and the TiSiN layer are inverted to obtain the elastic-plastic properties, specifically as follows:

[0019] ① Single-layer TiN layer and TiSiN coating were prepared on two hard Ti-6Al-4V alloy substrates by multi-arc ion plating method, and the coatings were subjected to nanoindentation test;

[0020] ② The power law model is used to characterize the elastic-plastic properties of the TiN layer and the TiSiN layer. The yield strength and strain hardening exponent of the single-layer coating are preset in a range of 0.01 to 1, and is temporarily set to 0.5. The corresponding relationship between plastic strain and stress is listed in a table, and the strain increment is 5%. The indentation test model of the single-layer coating is established using ABAQUS finite element software. The coating setting thickness is consistent with the actual sample thickness. After the setting is completed, the above strain-stress relationship is imported into the material properties of the single-layer coating. The simulated indentation depth is the same as the indentation depth in the actual test. The maximum load force obtained is calculated by finite element method. Compared with the maximum load force in the test, if the difference is greater than 5%, the strain hardening exponent is kept unchanged and the yield strength is iterated, the plastic strain-stress relationship is rearranged and brought into the calculation until the difference is less than 5%; the stiffness of the unloading section curve is obtained from the nanoindentation curve of the actual test, the yield strength is kept unchanged and the strain hardening exponent is iterated according to the stiffness error value, the plastic strain-stress relationship is rearranged and brought into the calculation until the stiffness difference of the unloading curve is less than 5%; the simulated indentation curve obtained after multiple iterations is basically consistent with the real indentation curve, and the yield strength and strain hardening exponent of the TiSiN layer and the TiN layer are obtained.

[0021] According to the above scheme, step 4) assembles the geometric models of the indenter, the gradient multilayer coating and the substrate, specifically as follows: assemble the geometric models of the indenter, the gradient multilayer coating and the substrate from top to bottom, apply rigid constraints to the indenter, apply binding constraints between the gradient multilayer coating and the substrate, set surface-to-surface contact and frictionless on the top of the indenter and the gradient multilayer coating, and set all models to circular symmetry.

[0022] According to the above scheme, step 5) determines the loading mode and boundary conditions for the three-dimensional finite element model, as follows:

[0023] A displacement loading mode was adopted with a maximum penetration depth of 200 nm. The static general analysis mode was selected, the bottom of the substrate was set to be completely fixed, the substrate and the gradient multilayer coating were set to deform only along the downward pressure direction of the indenter, and the loading-holding-unloading time was set in proportion to 5s:2s:5s according to the actual test.

[0024] According to the above scheme, step 7) predicts the internal intrinsic stress of the gradient multilayer coating without external load, as follows: delete the indenter model, reassemble the gradient multilayer coating and substrate model; add the thermal expansion coefficient to each material property, and the thermal expansion coefficients of the Ti layer, TiN layer, TiSiN layer and substrate are 8.35×10 -6 / K, 9.35×10 -6 / K, 8.9×10 -6 / K, 5.71×10 -6 / K; apply binding constraints to the TiN / TiSiN gradient multilayer coating and substrate model; delete the original indenter displacement load, apply a cooling load to all models from 350℃ to 20℃, and then simulate and calculate the three-dimensional finite element model, use the post-processing function of the finite element software to analyze the thermal stress distribution, and extract the maximum compressive stress (i.e., intrinsic internal stress).

[0025] The present invention also includes a screening and design method for high-performance TiN / TiSiN gradient multilayer coatings based on the above prediction method, and the specific steps are: designing a variety of TiN / TiSiN gradient multilayer coatings with different gradient structures and modulation periods, using the above TiN / TiSiN gradient multilayer coating mechanical properties and stress distribution prediction method to calculate the hardness, elastic modulus and intrinsic internal stress of each gradient multilayer coating, and compare them to select samples with high hardness, high elastic modulus and low intrinsic internal stress, which are the TiN / TiSiN gradient multilayer coatings with the best comprehensive performance.

[0026] The beneficial effect of the present invention is that the present invention adopts an iterative calculation method to obtain the yield strength, strain hardening index and other plastic parameters of the TiN layer and the TiSiN layer, so that the results obtained when the finite element analysis method is used to predict the hardness and elastic modulus of the gradient multilayer coating are closer to reality. Based on the prediction method, the high-performance TiN / TiSiN gradient multilayer coating is assisted to be screened, which has the advantages of high efficiency and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a flow chart of a method for predicting mechanical properties and stress distribution of a TiN / TiSiN gradient multilayer coating according to Example 1 of the present invention;

[0028] Figure 2 Schematic diagram of the Ti–6Al–4V alloy substrate and TiN / TiSiN gradient multilayer coating model structure;

[0029] Figure 3 This is a comparison diagram of the simulated indentation curve and the actual indentation curve after iterative calculation of the TiSiN layer in step 3) of Example 1;

[0030] Figure 4 The three-dimensional finite element model diagram after meshing and assembly in step 4) of Example 1;

[0031] Figure 5 This is the stress distribution diagram of step 6) of Example 1;

[0032] Figure 6 Compressive stress cloud diagram of step 7) in Example 1;

[0033] Figure 7 This is a comparison diagram of H and Er obtained by simulation calculation of the multi-layer coating model with different structures and different modulation periods in Example 3;

[0034] Figure 8 This is a comparison chart of the maximum compressive stresses obtained by simulation calculation of the gradient multilayer coating models with different structures and different modulation periods in Example 3. DETAILED DESCRIPTION

[0035] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below using embodiments in conjunction with the accompanying drawings.

[0036] Example 1

[0037] A method for predicting mechanical properties and stress distribution of TiN / TiSiN gradient multilayer coatings, the flow chart is as follows Figure 1As shown in the figure, the designed TiN / TiSiN gradient multilayer coating is built on the surface of the hard Ti–6Al–4V alloy substrate, and a transition layer is provided between the Ti–6Al–4V alloy substrate and the TiN / TiSiN gradient multilayer coating.

[0038] Layer (thickness 0.5 μm, the total thickness of the TiN / TiSiN gradient multilayer coating and the transition layer is 4.5 μm, the thickness of the Ti–6Al–4V alloy substrate is 5.5 μm, the Ti–6Al–4V alloy substrate and the TiN / TiSiN gradient multilayer coating models are both cylinders with a radius of 10 μm, the structural schematic diagram is shown in Figure 2 As shown in II, the surface of the gradient multilayer coating is a TiSiN layer, the coating in contact with the transition layer is a TiN layer, and the modulation period is set to decrease linearly, the number of periods is 4, the modulation ratio is 1:1, and the total number of layers of the gradient multilayer coating is designed to be 8. Each modulation period represents the total thickness of adjacent TiN layers and TiSiN layers, and the thickness of the TiN layer and the TiSiN layer are the same in each modulation period. The specific steps of the prediction method are as follows:

[0039] 1) ABAQUS finite element software was used for three-dimensional solid modeling. According to the parameters in the Vickers nanoindentation experiment of TiN / TiSiN gradient multilayer coating, the geometric models of the indenter (indenter height 1.5 μm), gradient multilayer coating and substrate were established. The coating structure was set in the component module. The diamond indenter geometric model was modeled according to the Vickers tetrahedron. The opposite angle of the tetrahedron was 136°. The three-dimensional solid was formed by stretching operation. Since all geometric models had four-fold symmetry, one-quarter of the part was cut for modeling.

[0040] 2) The geometric models of the indenter, the gradient multilayer coating and the substrate described in step 1) are meshed respectively, as follows:

[0041] The meshing mode of the indenter model is set to free, the unit shape is tetrahedron, and the seeds are arranged for the edges by a one-way offset method, gradually increasing from the bottom to the top of the cone, and the unit type is C3D10; the meshing mode of the gradient multilayer coating and substrate geometry model is set to sweep, the unit shape is hexahedron, and the seeds are arranged for the straight edges by a one-way offset method, gradually becoming sparse from the upper corner of the quarter cylinder to the edge, with a minimum grid size of 12.5nm, 37370 units, and the unit type is C3D8;

[0042] 3) Assigning material properties to the geometric models of the indenter, gradient multilayer coating and substrate described in step 2), setting the diamond indenter and alloy substrate as elastic materials, and setting the elastic modulus and Poisson's ratio of the two. The specific material properties are shown in Table 1.

[0043] Table 1

[0044]

[0045] In Table 1, A represents the initial yield stress, B represents the hardening modulus, n represents the strain hardening exponent, and c represents the strain rate constant.

[0046] The Ti layer, TiN layer and TiSiN layer are set as elastic-plastic materials, where the plastic properties of Ti are constructed according to the Johnson-Cook (JC) model. The parameters required in the model are listed in Table 1. The indentation test curves of the single-layer coatings of the TiN layer and the TiSiN layer are inverted to obtain the elastic-plastic properties, as follows:

[0047] ① The single-layer TiN layer and TiSiN coating were prepared on two hard Ti-6Al-4V alloy substrates by multi-arc ion plating method, with a thickness of about 2 μm. The preparation process is as follows: Ti target and Ti 75 Si 25 The target is used to deposit two coatings in a multi-arc ion plating equipment. The Ti target or Ti 75 Si 25 The target was placed in a rectangular vacuum chamber; all substrates were ultrasonically cleaned in alcohol for 15 minutes and placed on the equipment turntable; before deposition, they were etched with Ar ions for 20 minutes at a pressure of 1.9 Pa and a negative bias of 600 to 1000 V; during the deposition process, the pressure of argon and nitrogen (purity of 99.99%) was 1.3 Pa, and the deposition temperature was 350°C; during deposition, the bias was set to -150 V, and Ti 75 Si 25 The target and Ti target currents are 110 and 120A respectively; the distance between the target and the substrate is fixed at 240mm. After deposition, the coating is subjected to nanoindentation test, and the indentation depth is 1 / 10 of the film thickness; taking the TiSiN layer as an example, the obtained nanoindentation curve is as follows Figure 3 As shown (EXP);

[0048] ② The power law model is used to characterize the elastic-plastic properties of the TiN layer and the TiSiN layer (listed in Table 1). The yield strength and strain hardening exponent of the TiSiN layer are preset in a range of 0.01 to 1, and are temporarily set to 0.5. The corresponding relationship between plastic strain and stress is listed in a table, and the strain increment is 5%. The ABAQUS finite element software is used to establish an indentation test model of a TiSiN single-layer coating. The establishment method is the same as before. The coating thickness is consistent with the sample thickness. The material properties of the indenter and the substrate are set according to Table 1. After the setting is completed, the above strain-stress relationship is imported into the material properties of TiSiN to simulate the indentation depth. The degree is the same as the indentation depth in the actual test. The maximum load force obtained is compared with the maximum load force in the test through finite element calculation. If the difference is greater than 5%, the strain hardening exponent is kept unchanged and the yield strength is iterated. The plastic strain-stress relationship is rearranged and brought into the calculation until the difference is less than 5%. The stiffness of the unloading curve is obtained from the nanoindentation curve of the actual test. The yield strength is kept unchanged according to the stiffness error value, the strain hardening exponent is iterated, the plastic strain-stress relationship is rearranged and brought into the calculation until the difference in the stiffness of the unloading curve is less than 5%. The simulated indentation curve (FEM) obtained after 20 iterations is compared with the real indentation curve (EXP). Figure 3 As shown, the two curves are basically consistent. At this time, the yield strength of the TiSiN layer is 34.3 GPa, and the strain hardening exponent is 0.2. The yield strength of the TiN layer is 12 GPa and the strain hardening exponent is 0.1 obtained by the same method.

[0049] 4) Assembling the geometric models of the indenter, gradient multilayer coating and substrate described in step 3), specifically as follows: assembling the geometric models of the indenter, gradient multilayer coating and substrate from top to bottom, applying rigid constraints to the indenter, applying binding constraints between the gradient multilayer coating and the substrate, setting surface-to-surface contact and friction-free at the top of the indenter and the gradient multilayer coating, setting cyclic symmetry for all models, and meshing and assembling the three-dimensional finite element model as shown in FIG. Figure 4 As shown;

[0050] 5) Determine the loading mode and boundary conditions for the three-dimensional finite element model obtained in step 4), as follows:

[0051] The displacement loading mode was adopted, the maximum indentation depth was 200nm, the static general analysis mode was selected, the bottom of the substrate was set to be completely fixed, the substrate and the gradient multilayer coating were set to deform only along the pressing direction of the indenter, and the loading-holding-unloading time was set in proportion to 5s:2s:5s according to the actual test;

[0052] 6) Using ABAQUS finite element software to simulate the three-dimensional finite element model described in step 5), the stress distribution and maximum stress value in the gradient coating are obtained through the simulation results, and the indentation depth-load force relationship curve is obtained by using the post-processing function of ABAQUS finite element software to calculate the hardness (H) and effective elastic modulus (E) of the gradient coating. r ) ; To reduce the error, calculate H and E r The projected contact area (A c ) is calculated as follows:

[0053] Ac=24.5h c 2 +C1h c +C2h c 1 / 2 +C3h c 1 / 4

[0054] h c is the contact depth, C1 to C3 are the parameters to be fitted; h of the nanoindentation test results of TiN layer and TiSiN layer single layer coating is c and A c Substituting the above formula, the values ​​of C1 to C3 are 3694.1, -96025 and 197868.4 respectively after fitting with MATLAB software. The stress distribution results are as follows: Figure 5 As shown;

[0055] 7) Modify the assembly method, material properties and loading method of the three-dimensional finite element model described in step 4) to predict the internal intrinsic stress of the gradient multilayer coating without external load, as follows:

[0056] Delete the indenter model, reassemble the gradient multilayer coating and substrate models; add the thermal expansion coefficient to each material property. The thermal expansion coefficients of the Ti layer, TiN layer, TiSiN layer and substrate are 8.35×10 -6 / K, 9.35×10 -6 / K, 8.9×10 -6 / K, 5.71×10 -6 / K; apply binding constraints to the TiN / TiSiN gradient multilayer coating and substrate model; delete the original indenter displacement load, apply a cooling load to all models, from 350℃ to 20℃, and then simulate and calculate the three-dimensional finite element model, use the post-processing function of the finite element software to analyze the thermal stress distribution, extract the maximum compressive stress, and the compressive stress cloud diagram is as follows Figure 6 shown.

[0057] Example 2

[0058] A method for predicting mechanical properties and stress distribution of a TiN / TiSiN gradient multilayer coating, wherein the remaining steps are similar to those of Example 1, except that the number of modulation cycles of the gradient multilayer coating is increased from 4 to 20 and the total thickness remains unchanged ( Figure 2 and structure II in Table 2), the thickness of each period still decreases linearly from bottom to top, and the mechanical properties and thermal stress simulation calculations are performed according to the method of Example 1.

[0059] According to the comparison of the results of Example 1 and Example 2, for the TiN / TiSiN gradient multilayer coating with a modulation period decreasing from bottom to top, the number of modulation periods increases from 4 to 20, the hardness decreases from 57 GPa to 37 GPa, and the elastic modulus decreases from 472 GPa to 412 GPa; as the number of periods increases, the ratio of hardness to elastic modulus decreases from 0.12 to 0.09, and the coating's deformation resistance (toughness) decays; the intrinsic maximum compressive stress decreases from -158 MPa to -177 MPa.

[0060] Example 3

[0061] A screening method for high-strength and high-toughness TiN / TiSiN gradient multilayer coatings was designed, and 7 gradient multilayer coatings were designed. The schematic diagram of their structures is shown in Figure 2 As shown, the total thickness of the gradient multilayer coating remains unchanged, all 4μm. A series of models are designed for each structure to change the modulation period. The modulation period change trend and the number of modulation periods are shown in Table 2 below (the thickness change of each layer is uniformly increasing or decreasing):

[0062] Table 2

[0063]

[0064] All models were subjected to simulation calculations of mechanical properties and thermal stresses according to the method of Example 1.

[0065] Comparing the simulation results of the above different structures and different modulation period models, the comparison results of H and Er are as follows: Figure 7 The maximum compressive stress comparison results are shown in Figure 8 As shown. According to the comparison results, the hardness of structures III, V, and VII is generally higher than that of other structures with similar number of cycles, and the elastic modulus is significantly higher when the number of cycles is small, and slightly lower than that of other structures when the number of cycles increases; the maximum compressive stress of structures II, V, and VII changes more slowly with the number of cycles compared with other structures, and the value is lower. After the above analysis, structures V and VII are selected as gradient structures with better mechanical properties and internal stress distribution.

Claims

1. A method for predicting mechanical properties and stress distribution of TiN / TiSiN gradient multilayer coatings, characterized in that: The specific steps are as follows: 1) 3D solid modeling was performed using ABAQUS finite element software, and the geometric models of the indenter, gradient multilayer coating, and substrate were established according to the parameters in the Vickers nanoindentation experiment of the TiN / TiSiN gradient multilayer coating, and the coating structure was set in the component module; 2) meshing the geometric models of the indenter, the gradient multilayer coating and the substrate described in step 1) respectively; 3) assigning material properties to the geometric models of the indenter, gradient multilayer coating and substrate described in step 2); 4) assembling the geometric model of the indenter, the gradient multilayer coating and the substrate described in step 3); 5) determining the loading mode and boundary conditions for the three-dimensional finite element model obtained in step 4); 6) using ABAQUS finite element software to simulate the three-dimensional finite element model described in step 5), and obtaining the stress distribution and maximum stress value in the gradient coating through the simulation calculation results, and using the post-processing function of ABAQUS finite element software to obtain the indentation depth-load force relationship curve, thereby calculating the hardness and effective elastic modulus of the gradient coating; 7) Modify the assembly method, material properties and loading method of the three-dimensional finite element model described in step 4) to predict the internal intrinsic stress of the gradient multilayer coating without external load.

2. The method for predicting mechanical properties and stress distribution of TiN / TiSiN gradient multilayer coatings according to claim 1, characterized in that: The TiN / TiSiN gradient multilayer coating is obtained by alternately compounding a TiN layer and a TiSiN layer on the surface of a Ti-6Al-4V alloy substrate, and a transition layer is provided between the Ti-6Al-4V alloy substrate and the TiN / TiSiN gradient multilayer coating, the transition layer is a Ti layer, the surface of the gradient multilayer coating is a TiSiN layer, and the coating in contact with the transition layer is a TiN layer.

3. The method for predicting mechanical properties and stress distribution of TiN / TiSiN gradient multilayer coating according to claim 1, characterized in that: The thickness of the Ti layer is set to 0.5 μm, the total thickness of the TiN / TiSiN gradient multilayer coating and the transition layer is 4.5 μm, the thickness of the Ti-6Al-4V alloy substrate is 5.5 μm, the Ti-6Al-4V alloy substrate and the TiN / TiSiN gradient multilayer coating models are both cylinders with a radius of 10 μm, and the modulation period is set to decrease linearly, and the modulation ratio is 1:

1.

4. The method for predicting mechanical properties and stress distribution of TiN / TiSiN gradient multilayer coating according to claim 1, characterized in that: Step 1) The indenter height is set to 1.5 μm. The diamond indenter geometry model is modeled according to a Vickers pyramid, the opposite angle of the pyramid is 136°, and a three-dimensional entity is formed by stretching. Since all geometric models have four-fold symmetry, a quarter of the part is cut for modeling.

5. The method for predicting mechanical properties and stress distribution of TiN / TiSiN gradient multilayer coating according to claim 1, characterized in that: Step 2) The geometric models of the indenter, the gradient multilayer coating and the substrate are meshed respectively, and the specific method is as follows: the meshing mode of the indenter model is set to free, the unit shape is a tetrahedron, and the seeds are arranged for the edges by a unidirectional offset method, and the density is gradually increased from the bottom to the top of the cone. The unit type is C3D10; the meshing mode of the gradient multilayer coating and the substrate geometric model is set to sweep, the unit shape is a hexahedron, and the seeds are arranged for the straight edges by a unidirectional offset method, and the density is gradually increased from the upper vertex of the quarter cylinder to the edge. The minimum grid size is 12.5nm, the number of units is 37370, and the unit type is C3D8.

6. The method for predicting mechanical properties and stress distribution of TiN / TiSiN gradient multilayer coating according to claim 1, characterized in that: Step 3) assigning material properties to the geometric models of the indenter, the gradient multilayer coating and the substrate, specifically: setting the indenter and the substrate as elastic materials, setting the Ti layer, the TiN layer and the TiSiN layer as elastic-plastic materials, wherein the indentation test curves of the single-layer coatings of the TiN layer and the TiSiN layer are inverted to obtain the elastic-plastic properties, specifically as follows: ① Single-layer TiN layer and TiSiN coating were prepared on two hard Ti-6Al-4V alloy substrates by multi-arc ion plating method, and the coatings were subjected to nanoindentation test; ② The power law model is used to characterize the elastic-plastic properties of the TiN layer and the TiSiN layer. The yield strength and strain hardening index of the single-layer coating are preset in a preset range of 0.01 to 1, and are temporarily set to 0.

5. The corresponding relationship between plastic strain and stress is listed in a table, and the strain increment is 5%. The indentation test model of the single-layer coating is established using ABAQUS finite element software. The coating setting thickness is consistent with the actual sample thickness. After the setting is completed, the above strain-stress relationship is imported into the material properties of the single-layer coating. The simulated indentation depth is the same as the indentation depth in the actual test. The maximum load force obtained is compared with the maximum load force in the test through finite element calculation. If the difference is greater than 5%, the strain hardening index is kept unchanged and the yield strength is iterated. The plastic strain-stress relationship is rearranged and brought into the calculation until the difference is less than 5%; the unloading segment curve stiffness is obtained from the nanoindentation curve of the actual test. The yield strength is kept unchanged and the strain hardening index is iterated according to the stiffness error value. The plastic strain-stress relationship is rearranged and brought into the calculation until the unloading curve stiffness difference is less than 5%; The simulated indentation curve obtained after multiple iterations is basically consistent with the real indentation curve, and the yield strength and strain hardening exponent of the TiSiN layer and TiN layer are obtained.

7. The method for predicting mechanical properties and stress distribution of TiN / TiSiN gradient multilayer coating according to claim 1, characterized in that: Step 4) Assemble the geometric models of the indenter, the gradient multilayer coating and the substrate, as follows: assemble the geometric models of the indenter, the gradient multilayer coating and the substrate from top to bottom, apply a rigid constraint to the indenter, apply a binding constraint between the gradient multilayer coating and the substrate, set surface-to-surface contact and frictionless on the top of the indenter and the gradient multilayer coating, and set all models to be cyclically symmetric.

8. The method for predicting mechanical properties and stress distribution of TiN / TiSiN gradient multilayer coating according to claim 1, characterized in that: Step 5) Determine the loading method and boundary conditions for the three-dimensional finite element model, as follows: adopt a displacement loading mode, the maximum indentation depth is 200nm, select a static general analysis mode, set the bottom of the substrate to be completely fixed, and set the substrate and the gradient multilayer coating to be deformable only along the direction of the indenter's downward pressure. The loading-holding-unloading time in the actual test is set in a ratio of 5s:2s:5s.

9. The method for predicting mechanical properties and stress distribution of TiN / TiSiN gradient multilayer coating according to claim 1, characterized in that: Step 7) predict the internal intrinsic stress of the gradient multilayer coating without external load, as follows: delete the indenter model, reassemble the gradient multilayer coating and substrate models; add the thermal expansion coefficient to each material property, and the thermal expansion coefficients of the Ti layer, TiN layer, TiSiN layer and substrate are 8.35×10 -6 / K, 9.35×10 -6 / K, 8.9×10 -6 / K, 5.71×10 -6 / K; apply binding constraints to the TiN / TiSiN gradient multilayer coating and substrate model; delete the original indenter displacement load, apply a cooling load to all models from 350℃ to 20℃, and then simulate and calculate the three-dimensional finite element model, use the post-processing function of the finite element software to analyze the thermal stress distribution, and extract the maximum compressive stress.

10. A screening and design method for high-performance TiN / TiSiN gradient multilayer coatings based on the prediction method according to any one of claims 1 to 9, characterized in that: The specific steps are: design a variety of TiN / TiSiN gradient multilayer coatings with different gradient structures and modulation periods, use the above-mentioned TiN / TiSiN gradient multilayer coating mechanical properties and stress distribution prediction method to calculate the hardness, elastic modulus and intrinsic internal stress of each gradient multilayer coating, and compare them to select samples with high hardness, high elastic modulus and low intrinsic internal stress, which are the TiN / TiSiN gradient multilayer coatings with the best comprehensive performance.

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