CN113430491B

CN113430491B - Surface oxidation-resistant coating, preparation method thereof and surface modified titanium alloy

Info

Publication number: CN113430491B
Application number: CN202110722849.XA
Authority: CN
Inventors: 李凤吉; 余修晗; 张善勇; 王锦标
Original assignee: Southwest University
Current assignee: Southwest University
Priority date: 2021-06-29
Filing date: 2021-06-29
Publication date: 2022-06-17
Anticipated expiration: 2041-06-29
Also published as: CN113430491A

Abstract

The invention relates to a technology for modifying the surface of a materialThe technical field specifically discloses a surface oxidation resistant coating, a preparation method thereof and a surface modified titanium alloy, wherein the surface oxidation resistant coating adopts Si/MoSi₂Composite coating of MoSi₂The phase structure of (A) is C11b type MoSi₂Excellent high-temperature stability; the Si element exists in a simple substance form and can make up for MoSi₂The diffusion speed of the medium Si element is slow, which is beneficial to quickly forming SiO on the surface of the coating₂A protective layer; at the same time, Si and MoSi are controlled₂The ratio of the amount of the substance is (1-1.5): 1, and sufficient Si element and O can be ensured₂Reaction to promote continuous and dense SiO₂Formation of protective layer, preventing O₂Diffuse to the inside to cause M_OO₃The generation of an oxidation phase is effectively inhibited, thereby effectively inhibiting MoSi₂The coating is oxidized at 500-800 ℃ to cause the phenomenon of PESTING pulverization.

Description

Surface oxidation-resistant coating, preparation method thereof and surface modified titanium alloy

Technical Field

The invention relates to the technical field of surface modification, in particular to a surface oxidation resistant coating, a preparation method thereof and a surface modified titanium alloy.

Background

The titanium alloy has the characteristics of high specific strength, low density, high heat resistance and the like, and has good application prospect in high-temperature parts such as aerospace engine parts, parts of ship internal combustion engines and the like. Below 500 deg.c, titanium will produce oxidation reaction in the presence of oxygen to form one layer of well combined and compact oxide film on the surface of titanium alloy to prevent further oxidation of titanium alloy. However, as the temperature continues to rise, the oxide film gradually becomes porous and cannot suppress the diffusion of oxygen into the titanium alloy, resulting in a decrease in the oxidation resistance of the titanium alloy. Therefore, the temperature of the working environment of the titanium alloy is generally limited to be within 500 ℃.

In order to enable the titanium alloy to meet the service condition of a high-temperature part at the temperature of more than 500 ℃, researchers generally adopt a surface modification technology to prepare an antioxidant coating on the surface of the titanium alloy, and improve the oxidation resistance of the titanium alloy at high temperature on the premise of not changing the overall performance of the titanium alloy. The commonly used surface modification techniques are mainly by ion implantation, vapor deposition,And forming a coating on the surface of the titanium alloy by using methods such as laser cladding, plasma spraying and the like. In a high-temperature oxidation environment, the coating can form a continuous and compact oxide film (such as SiO) on the surface of the titanium alloy₂、TiO₂、Al₂O₃Etc.), further oxidation of the coating surface can be prevented. Wherein MoSi is generated under high temperature condition₂Can form a layer of dense SiO on the surface₂The protective layer, therefore, has very excellent high temperature oxidation resistance and is considered to be one of the most suitable high temperature coating materials for engineering applications.

Patent document No. CN109321865B discloses a method for forming MoSi on the surface of titanium alloy₂The method for preparing the anti-oxidation coating comprises the steps of firstly forming a Mo coating of 40-70 mu m on the surface of a titanium alloy by adopting a plasma spraying technology, then carrying out electrodeposition in a molten salt system by using a Si sheet as an auxiliary electrode, a Pt sheet as a reference electrode and a titanium alloy sheet with the Mo coating on the surface as a working electrode, and forming MoSi with the thickness of about 3-10 mu m on the surface of the titanium alloy by electrodeposition₂And (4) coating. By forming MoSi on the surface of titanium alloy₂The protective coating solves the problem that the titanium alloy has poor oxidation resistance at the temperature of over 600 ℃.

As is well known, MoSi₂The coating has a PESTING pulverization phenomenon in a temperature range of 500-800 ℃, namely MoO formed by Mo oxidation₃The oxidation phase volatilizes and destroys the SiO formed on the surface of the coating₂The continuity of the protective film leads to the increased oxidation, and MoSi is formed on the surface of the titanium alloy provided by the above patent document₂The method for oxidation resistant coating is characterized in that the titanium alloy matrix and MoSi are coated₂Mo coatings are formed among the coatings, and are more easily oxidized at the temperature range of 500-800 ℃ to generate MoO₃An oxidized phase, increasing the likelihood of further deterioration of the "permanent" phenomenon.

Disclosure of Invention

MoSi for improving surface oxidation resistant coating₂The invention provides a surface oxidation resistant coating and a preparation method thereof, and the surface oxidation resistant coating is applied to titanium alloy to carry out PESTING pulverization phenomenon on the surface of the titanium alloy in a temperature range of 500-800 DEG CModifying to improve the oxidation resistance of the titanium alloy, especially the oxidation resistance in a middle temperature range of 500-800 ℃.

In a first aspect, the invention provides a surface oxidation resistant coating, which is realized by adopting the following technical scheme:

the key point of the surface oxidation resistant coating is as follows: the surface oxidation resistant coating is Si/MoSi₂Composite coating of Si and MoSi₂The amount ratio of the Si is (1-1.5): 1, the Si is a simple substance Si crystal phase, and the MoSi is₂Is MoSi of C11b type₂A crystalline phase.

The surface oxidation resistant coating adopts Si/MoSi₂Composite structure of which MoSi₂The phase structure of (A) is C11b type MoSi₂Has excellent high-temperature stability, and Si exists in the form of simple substance Si crystal phase and can compensate C11b type MoSi in the composite coating₂The diffusion speed of the medium Si element is slow, which is beneficial to forming SiO on the surface of the coating in time₂And a protective layer. At the same time, Si and MoSi are controlled₂The amount of the substance(s) is (1-1.5): 1, ensuring sufficient Si element and O₂Reaction to promote continuous and dense SiO₂Formation of protective layer, preventing O₂Diffuse to the inside to cause M_OO₃The generation of oxidation phase is effectively inhibited, and MoSi is effectively inhibited₂A 'PESTING' powdering phenomenon at 500-800 ℃.

Preferably, said Si/MoSi₂The crystal grain structure of the composite coating is equiaxial crystal, the average crystal grain size is 30-40 nm, and the equiaxial crystal is in close packing arrangement.

Compared with MoSi₂Bulk flake deposition, Si/MoSi, inside the coating₂The close-packed equiaxed crystals in the composite coating can prevent O₂Diffusing into the coating. At the same time, MoSi is initially present due to the reduction of the initial oxygen partial pressure₂MoO formed by the reaction₃Reduce and reduce MoO₃Volatile destruction of SiO₂The possibility of continuity of the protective layer.

Preferably, the MoSi type C11b₂Having a preferred orientation in the (002) direction.

Si/MoSi₂C11b type MoSi in composite coating₂The orientation in the (002) direction is enhanced, so that the MoSi of C11b type is obtained₂The high temperature stability of (2) is further improved.

Preferably, the oxidation curve of the surface oxidation-resistant coating is linearly fitted with the equation that t is 0.011x after being oxidized for 60 hours under the air environment condition at 700 DEG C^1/2+1.510, growth rate constant of 0.011 μm²h^-1T is the thickness of the surface oxidation resistant layer in microns, x is time, and time is hours.

Oxidizing for 60 hours under the air environment condition of 700 ℃, compared with MoSi₂Oxidation weighted growth rate constant of coating, Si/MoSi₂The composite coating consists of 0.034 mu m²h^-1Down to 0.011 μm²h^-1The oxidation resistance is obviously improved.

Preferably, the surface oxidation-resistant coating is oxidized for 60 hours under the air environment condition of 700 ℃, a continuous and compact protective layer is formed on the surface of the surface oxidation-resistant coating, and the thickness of the protective layer is 100 nm.

Compared with MoSi₂Coating of Si/MoSi₂The composite coating can generate a durable and continuous compact protective layer on the surface.

In a second aspect, the invention provides a method for preparing a surface oxidation-resistant coating, which comprises the following steps:

step 1, by adopting MoSi₂Co-sputtering the compound target and the simple substance Si target to deposit Si/MoSi on the surface of the substrate₂Co-depositing a layer, controlling the elementary Si target and the MoSi during sputtering₂The deposition rate ratio of the compound target is (1-1.5): 1.

Step 2, the Si/MoSi is carried out₂Annealing the codeposition layer to prepare Si/MoSi₂And (4) composite coating.

By adopting the magnetron sputtering process, the Si/MoSi is enabled₂Co-deposition occurs with the two in a state of uniform mixing. With MoSi₂Taking a high-temperature stable phase as a main antioxidant component and Si as a synergistic antioxidant component, and converting an amorphous state into a crystalline state through subsequent annealing treatment to prepare the productSi/MoSi₂The composite coating improves the oxidation resistance.

Preferably, the MoSi is₂The power of the compound target is 100-130W, and the power of the Si target is 30-120W.

By controlling MoSi₂Control of MoSi by sputtering power of compound target and Si target₂The ratio of the deposition rate of Si to the deposition rate of Si, thereby precisely controlling the Si/MoSi ratio₂Si and MoSi in composite coating₂The amount of the substance(s) of (c).

Preferably, the MoSi is₂The sputtering power of the compound target is 110-120W, and the sputtering power of the Si target is 50-100W.

By mixing MoSi₂The sputtering power of the compound target and the Si target is controlled in the above range, so that the finally prepared Si/MoSi₂Si and MoSi in composite coating₂The amount ratio of the substances is controlled to be (1-1.5): 1, so that a continuous and compact protective layer can be continuously formed on the surface of the oxidation resistant coating. Excessive amount of Si substance, Si/MoSi₂The bonding force between the composite coating and the substrate is weakened, and the coating is easy to fall off. The amount of Si species is too low to ensure a durable anti-oxidation effect of the protective layer.

Preferably, in the step 2, the annealing temperature is 950 ℃ to 1000 ℃, and the annealing time is 0.5 to 1 hour.

By adopting the process, the co-deposition layer obtained by magnetron sputtering can be subjected to crystallization transformation, and the crystal particles are prevented from growing up.

In a third aspect, the present application provides a surface modified titanium alloy, which is characterized in that: the titanium alloy comprises a titanium alloy substrate and the surface oxidation resistant coating formed on the titanium alloy substrate, wherein the thickness of the surface oxidation resistant coating is 1-2 mu m.

The surface oxidation-resistant coating is formed on the titanium alloy substrate, so that the oxidation resistance of the titanium alloy is improved, and particularly the oxidation resistance in a medium-temperature region of 500-800 ℃. At the same time, the properties of the titanium alloy substrate itself can be maintained.

In conclusion, the invention has the following beneficial effects:

1. the invention provides a surfaceThe oxidation resistant coating adopts Si/MoSi₂Composite coating of MoSi inhibition₂The continuous and compact protective layer can be continuously formed on the surface of the coating by the PESTING pulverization phenomenon at the temperature range of 500-800 ℃.

2. According to the preparation method of the surface antioxidant coating, the prepared surface antioxidant coating is well combined with a substrate, and Si and MoSi are adopted₂The components are uniformly mixed, so that the synergistic effect between the components is promoted, and the oxidation resistance is improved.

3. The surface modified titanium alloy provided by the invention has excellent oxidation resistance and can maintain the performance of the titanium alloy.

Drawings

Fig. 1 is an XRD pattern of the oxidation-resistant coating of different examples and comparative examples of the present invention. Wherein the curve No. 0 is an XRD pattern of the substrate which is not annealed after the substrate of the comparative example 1 is subjected to magnetron sputtering to form a codeposition layer; 1.

curves

2, 3 and 4 correspond to XRD patterns of comparative example 1, example 2 and comparative example 2.

Fig. 2 is an oxidation curve of the oxidation resistant coating of different examples of the present invention and comparative examples under 700 ℃ air condition, and the curves No. 1, No. 2, No. 3, No. 4 correspond to the oxidation curves of comparative example 1, example 2, comparative example 2, respectively.

Fig. 3 is a linear fit equation of the oxidation curves of comparative example 1 and example 2.

Fig. 4 is a FESEM image and an EDS elemental analysis map of the oxidation resistant coatings of comparative example 1 and example 2.

Fig. 5 is a surface FESEM image of the oxidation-resistant coating of comparative example 1 oxidized for 1 hour under 700 ℃ air condition.

Fig. 6 is a surface FESEM image of the oxidation resistant coating of example 2 oxidized for 1 hour at 700 ℃ under air conditions.

Fig. 7 is a cross-sectional FESEM image of the oxidation resistant coatings of comparative example 1 and example 2 oxidized for 1 hour under 700 ℃ air condition. Fig. 7(a) is an oxidation-resistant coating of comparative example 1, and fig. 7(b) is an oxidation-resistant coating of example 2.

Fig. 8 is a surface FESEM image of the oxidation resistant coatings of comparative example 1 and example 2 oxidized for 9 hours at 700 ℃ under air conditions. Fig. 8(a) is the oxidation-resistant coating of comparative example 1, and fig. 8(b) is the oxidation-resistant coating of example 2.

Fig. 9 is a cross-sectional FESEM image of the oxidation resistant coatings of comparative example 1 and example 2 oxidized for 9 hours at 700 ℃ under air conditions. Fig. 9(a) is an oxidation resistant coating of comparative example 1, and fig. 9(b) is an oxidation resistant coating of example 2;

fig. 10 is a surface FESEM image of the oxidation resistant coatings of comparative example 1 and example 2 oxidized for 60 hours under 700 ℃ air condition. Fig. 10(a) is an oxidation-resistant coating of comparative example 1, and fig. 10(b) is an oxidation-resistant coating of example 2.

Fig. 11 is a cross-sectional FESEM image of the oxidation resistant coatings of comparative example 1 and example 2 oxidized for 60 hours under 700 ℃ air condition. Fig. 11(a) is an oxidation-resistant coating of comparative example 1, and fig. 11(b) is an oxidation-resistant coating of example 2.

Fig. 12 is oxidation weight gain curves of the titanium alloy substrate with the modified surface of the oxidation-resistant coating and the unmodified titanium alloy substrate of comparative example 1, example 2 and comparative example 2 at 700 ℃ in air, the comparative example 1 is a curve corresponding to magnetron sputtering power of 0W, the example 1 is a curve corresponding to magnetron sputtering power of 50W, the example 2 is a curve corresponding to magnetron sputtering power of 100W, the comparative example 2 is a curve corresponding to magnetron sputtering power of 150W, and TC4 is a curve corresponding to the unmodified titanium alloy.

Detailed Description

Compared with the nickel-based high-temperature alloy which is applied more in the field of aviation at present, the titanium alloy has lower density, can be used as parts such as blades and machine discs of an aeroengine to reduce the weight of the aeroengine, improves the thrust-weight ratio of an airplane, and further expands the application of the titanium alloy in the field of aerospace. However, titanium alloys are susceptible to oxidation at high temperatures due to the strong affinity of titanium and oxygen. Researchers mainly modify the surface of the titanium alloy to form an antioxidant coating on the surface of the titanium alloy, so that the oxidation resistance of the titanium alloy at high temperature is improved. MoSi₂Has high melting point, low density and excellent high-temperature oxidation resistance, and is the second generation of high-temperature titanium-aluminum alloy after nickel-based high-temperature alloyThe third generation of ultra-high temperature structural materials. MoSi is formed on the surface of titanium alloy₂The anti-oxidation coating solves the problem that the titanium alloy has weak anti-oxidation performance in a high-temperature environment of more than 500 ℃, and widens the application range of the titanium alloy.

However, MoSi₂The coating can generate PESTING pulverization phenomenon in a medium temperature region of 500-800 ℃. Researchers believe that the reaction to form SiO with oxygen occurs primarily due to the rate of oxygen in-diffusion compared to Si out-diffusion₂The nucleation/growth rate of the protective layer is high, resulting in oxygen diffusion into the coating and oxidation of Mo to MoO₃. And MoO within the temperature range of 500-800 DEG C₃Has extremely strong volatility. MoO₃Escape of (2) destroys SiO₂The continuity of the protective layer forms pores, which provide a passage for oxygen to enter the coating and increase the oxidation degree of the coating.

The invention develops a surface oxidation resistant coating, namely Si/MoSi₂Composite coating of MoSi₂The phase structure of (A) is C11b type MoSi₂And has excellent high-temperature stability. Si exists in the form of simple substance Si phase, and can compensate C11b type MoSi in the composite coating₂The diffusion speed of the medium Si element is slow, which is beneficial to forming SiO on the surface of the coating in time₂Protective layer, inhibit O₂Diffuse into the coating and react with Mo to form MoO₃And (4) volatilizing the phase. At the same time, Si and MoSi are controlled₂The amount of the substance(s) is (1-1.5): 1, ensuring that sufficient Si is supplied as a reserve with O₂And (4) reacting. Even MoO₃Volatilization of the oxidized phase destroys the SiO₂The continuity of the protective layer can also form SiO in time₂Repair is carried out, and SiO is ensured₂Continuity of the protective layer, effective suppression of MoSi₂A 'PESTING' phenomenon in a temperature range of 500-800 ℃. The present invention has been made on the basis of this study.

The present invention will be described in detail with reference to examples.

Example 1

The method comprises the steps of cutting a TC4 titanium alloy into a substrate with the diameter of 30 mm multiplied by 10 mm, sequentially polishing the substrate by using SiC metallographic abrasive paper of 80 meshes, 120 meshes, 240 meshes, 400 meshes, 600 meshes, 800 meshes, 1000 meshes, 2000 meshes, 3000 meshes and 5000 meshes until the surface of the substrate is free from visible scratches, cleaning the substrate by using acetone, and drying the substrate for later use.

And (3) placing the substrate in a magnetron sputtering system for coating. The target material used for magnetron sputtering is MoSi₂Compound target and Si target. MoSi₂Purity of compound target 99.95 wt.%, purity of Si target 99.95 wt.%, MoSi₂The compound target and the Si target each had a diameter of 101 mm and a thickness of 5 mm.

The background vacuum degree of the magnetron sputtering chamber is 5.0 multiplied by 10^-6And (5) Torr. Before coating, the substrate needs to be cleaned in a bias way in order to eliminate residual pollutants and native oxide layers on the surface of the substrate. The substrate was not heated during the cleaning process, the bias power was set to RF100W, the argon flow was 50sccm, and the gas pressure was 3.9X 10^-3Torr, and a cleaning time was 10 minutes.

After the bias cleaning was completed, the chamber was maintained at a pressure of 3.9X 10^-3Torr, co-sputtering, MoSi₂The DC sputtering power of the compound target was 120W, the DC sputtering power of the Si target was 50W, and the distance between the sputtering target and the substrate was 150 mm. The substrate was not heated during sputtering and rotated at 25rpm during deposition to achieve Si and MoSi₂Uniformity of co-deposited films. The codeposition is completed by 5 steps, the deposition time of each step is 30min, and the deposition interval of each step is 10min, so that the target material is cooled. After the sputtering is finished, taking out the substrate to obtain Si/MoSi₂And (6) co-depositing the layers. Subsequently, the substrate is placed in a tube furnace for annealing to prepare crystalline Si/MoSi₂And (4) composite coating. The annealing temperature is 950 ℃, argon protective atmosphere is introduced into the tubular furnace, the flow of the argon is 100sccm, and the temperature is maintained for 0.5 hour. Preparing the obtained Si/MoSi₂The thickness of the composite coating was 1.27 μm.

Example 2

The difference from example 1 is that: in the magnetron sputtering, the DC sputtering power of the Si target was 100W. Preparing the obtained Si/MoSi₂The thickness of the composite coating was 1.49 μm.

Comparative example 1:

the difference from example 1 is that: in the magnetron sputtering, the DC sputtering power of the Si target was 0W. Preparing the obtained Si/MoSi₂The thickness of the composite coating was 1.42 μm.

Comparative example 2:

the difference from example 1 is that: in the magnetron sputtering, the DC sputtering power of the Si target was 150W. Preparing the obtained Si/MoSi₂The thickness of the composite coating was 1.58 μm.

Performance detection

And performing phase analysis on the prepared surface oxidation resistant coating by using an X-ray diffractometer (XRD). The radiation source of the X-ray diffractometer is CuK alpha, the working voltage is 35kV, the current is 40mA, the scanning speed is 2 DEG/min, and the scanning range is 10-80 deg. The results of the detection are shown in FIG. 1.

As shown in fig. 1, curve 0 is the non-annealed XRD pattern after completion of magnetron sputtering of the substrate of comparative example 1, and the XRD test result of the non-annealed coating after completion of sputtering shows an amorphous coating;

curves

2 and 3 are XRD test results of examples 1 and 2, respectively, and curves 1 and 4 are XRD test results of comparative examples 1 and 2, respectively. In example 1, Si/MoSi was measured at a sputtering power of 50W for the Si target₂The phase structure of the composite coating is C11b type MoSi₂And an elemental Si crystal phase, and MoSi type C11b₂Has a preferred orientation in the (002) crystal direction, and the simple substance Si has a characteristic peak in the (111) crystal direction. In example 2, Si/MoSi when the sputtering power of the Si target was increased to 100W₂The phase structure of the composite coating is C11b type MoSi₂And elemental Si crystalline phase, MoSi type C11b₂The preferred orientation in the (002) crystal direction is further enhanced, and the characteristic peak intensity of the simple substance Si in the (111) crystal direction is increased. In comparative example 1, when the sputtering power of the Si target was 0W, only MoSi type C11b was detected in the coating₂. Comparative example 2 Si/MoSi with continued increase of the sputtering power of the Si target to 150W₂The phase structure of the composite coating is C11b type MoSi₂And elemental Si phase, MoSi type C11b₂The intensity of the characteristic peak in the (002) crystal direction is weaker than that in example 2, and the intensity of the characteristic peak in the (111) crystal direction of elemental Si is enhanced as compared with that in example 2.

To further investigate the antioxidant coatings obtainedThe grain size was calculated using Scherrer formula D ═ K λ/Bcos θ. Wherein D is the average thickness of crystal grains vertical to the crystal plane direction, namely the average crystal grain size; b is the half-height width of the diffraction peak of the actually measured sample; theta is a Bragg diffraction angle; λ is the X-ray wavelength and λ is

K is 0.89; the average grain size was calculated by taking the three-intensity diffraction peaks, and the grain parameters of examples 1 and 2 are shown in table 1, and the grain parameters of comparative examples 1 and 2 are shown in table 2:

TABLE 1 Crystal grain parameters of examples 1 and 2

TABLE 2 grain parameters of comparative examples 1 and 2

As can be seen from tables 1 and 2, the C11b type MoSi was observed as the sputtering power of the Si target was increased during the magnetron sputtering process₂The crystal grains have a tendency to decrease gradually, and when the sputtering power of the Si target in comparative example 1 is 0W, that is, only MoSi is turned on₂Sputtering a compound target to prepare the obtained MoSi₂The average grain size of the coating is 34-50 nm; in example 1, Si/MoSi was prepared with a sputtering power of 50W for the Si target₂In the composite coating, MoSi type C11b₂The average grain size of the crystal grains is 34-48 nm; in example 2, when the sputtering power of the Si target is 100W, the obtained Si/MoSi is prepared₂In the composite coating, MoSi type C11b₂The average grain size of (a) is 34 to 37 nm; in comparative example 2, when the sputtering power of the Si target is 150W, the obtained Si/MoSi is prepared₂In the composite coating, MoSi type C11b₂Has an average grain size of 27 to37nm。

And (3) selecting 3 points on the anti-oxidation layer on the surface of the substrate to perform Mo and Si element analysis respectively by combining an EDS element analysis method, and averaging to obtain at% (Mo) and at% (Si). Since in C11b type MoSi₂In (1), at.% (Si) — 2 × at.% (Mo), so Si/MoSi₂MoSi in composite coatings₂The medium Si content is 2 × at.% (Mo), while the free state at.% (Si) — at.% (Si) -2 × at.% (Mo).

According to the EDS elemental analysis results, the atomic ratios of Mo/Si of comparative example 1, example 2 and comparative example 2 are 1: 2. 1: 3. 1: 3.5, 1: 4. according to the method, Si and MoSi in comparative example 1, example 2 and comparative example 2 are calculated₂The material ratios are respectively as follows: 0. 1: 1. 1.5: 1. 2: 1.

from the above results, it can be seen that Si/MoSi₂C11b type MoSi in composite coating₂Is that MoSi is reduced during sputtering₂MoSi when compound target and Si target are co-sputtered₂The compound and the Si simple substance are subjected to codeposition on the surface of the substrate, the compound and the Si simple substance are uniformly mixed and stacked, and the Si simple substance is distributed on MoSi₂Around, has an inhibition of MoSi₂The crystal grains grow greatly, the deposition rate of the Si simple substance is increased along with the increase of the sputtering power of the Si target, the total amount of the Si simple substance is increased, and the Si simple substance blocks MoSi₂The effect of grain growth is enhanced.

The substrates of examples 1 and 2 and comparative examples 1 and 2 on which the surface anti-oxidation layers were deposited were oxidized in an air atmosphere at 700 ℃ for 60 hours, and the coating thicknesses were measured for 1 hour and 9 hours, respectively, to obtain the early-stage conditions where the "pest" effect is most significant, and the coating thicknesses were measured every 12 hours, thereby obtaining the overall change in the oxidation of the coating. The thickness of the coating as a function of oxidation time is shown in Table 3.

TABLE 3 rule of coating thickness as a function of Oxidation time

Fig. 2 is an oxidation curve of the surface oxidation-resistant coatings of examples 1 and 2 and comparative examples 1 and 2, wherein curves No. 1, 2, 3 and 4 correspond to the oxidation curves of comparative example 1, example 2 and comparative example 2, respectively. As can be seen from fig. 2, the oxidation rate of the surface oxidation resistant layer can be characterized by the slope of the oxidation curve. In the early stage of oxidation, i.e., oxidation for 1h, the oxidation rates of example 1 and comparative example 2 were small; whereas the oxidation rates of comparative example 1 and example 2 were greater. Upon continuing the oxidation for 9h, the oxidation rates of example 1 and comparative example 2 remained almost unchanged, while the oxidation rates of comparative example 1 and example 2 decreased. Wherein the oxidation rate of comparative example 1 was still higher than the oxidation rates of example 1 and comparative example 2, while the oxidation rate of example 2 had been reduced to be lower than the oxidation rates of example 1 and comparative example 2. Further oxidation was carried out for 12h, and the oxidation rate of example 2 was further reduced to almost 0. Oxidation to 60h, the coating thickness of example 2 changed little with increasing oxidation time. While the oxidation rates of example 1, comparative example 1 and comparative example 2 also gradually decreased with increasing time. The oxidation rates of comparative examples 1, 2 are still greater than the oxidation rate of example 1. It can be seen that the surface antioxidation layer of example 2 has the best durability against oxidation at 700 ℃, and after oxidation for 12h, the antioxidation layer is not substantially oxidized any more. The oxidation durability of the surface oxidation resistant layer of example 1 at 700 ℃ is inferior, and the oxidation durability of the coatings of comparative examples 1 and 2 at 700 ℃ is inferior.

For better oxidation rates of comparative example 2 and comparative example 1, the linear fit equation of the oxidation curve is given in fig. 3, and the linear fit equation of example 2 is t 0.011x^1/2+1.510, growth rate constant of 0.011 μm²h^-1T is the thickness of the surface antioxidation layer, the unit of the thickness is micrometer, x is time, and the unit of the time is hour; the linear fit equation for comparative example 1 is t 0.035x^1/2+1.418, growth rate constant 0.035 μm²h^-1。

And (3) analyzing the surface appearance, the section appearance and the microstructure of the surface oxidation-resistant coating of the substrate by using a Field Emission Scanning Electron Microscope (FESEM), and performing element analysis by combining EDS (electron-beam spectroscopy) to further investigate the structure of the oxidation-resistant coating and the structural change generated in the oxidation process.

Fig. 4(a) is a FESEM surface view of the surface antioxidant coating of comparative example 1 before annealing, fig. 4(b) is a FESEM surface view after annealing, and fig. 4(c) is EDS elemental analysis result after annealing; fig. 4(d) is a FESEM surface view of the antioxidant coating of example 2 before annealing, fig. 4(e) is a FESEM surface view after annealing, fig. 4(f) is an EDS elemental analysis result after annealing, fig. 4(g) is a FESEM cross-sectional view of the surface antioxidant coating of comparative example 1 before annealing, fig. 4(h) is a FESEM cross-sectional view of the surface antioxidant coating of comparative example 1 after annealing, fig. 4(i) is a FESEM cross-sectional view of the surface antioxidant coating of example 2 before annealing, and fig. 4(j) is a FESEM cross-sectional view of the surface antioxidant coating of example 2 after annealing. As can be seen from the combination of FIGS. 4(a), (b), (d), and (e), the surface morphology of the coatings of comparative example 1 and example 2 did not change significantly before and after annealing. As is clear from FIG. 4(c), the atomic ratios of Mo/Si are 1: 2, calculating to obtain Si and MoSi in comparative example 1₂The ratios of the compounds are 0 respectively. As is clear from FIG. 4(f), the atomic ratios of Mo/Si are 1: 3.5, calculation of Si and MoSi in example 2₂The phase ratios of (A) to (B) are 1.5:1, respectively. As can be seen from FIGS. 4(g) and (h), MoSi of comparative example 1₂The coating and the substrate are well combined, no hollow hole or gap appears, and MoSi before and after annealing₂No significant grain structure was observed for the coating. As is clear from FIGS. 4(i) and (j), MoSi of example 2₂The coating has the same good combination with the substrate, no holes or gaps appear, and before annealing, the Si/MoSi₂No grain structure observed in the composite coating, and annealed Si/MoSi₂The composite coating has an equiaxed grain structure, the average grain size is 30-40 nm, and equiaxed grains are in close packing arrangement.

Fig. 5 is a surface morphology of the surface oxidation resistant coating of comparative example 1 after being oxidized at 700 c for 1 hour. MoSi₂A large number of bumps and pits are observed on the surface of the coating, obvious cracks are formed on the surface of the coating, EDS analysis is carried out on the bumps, the content of molybdenum and oxygen is obviously higher than that of the surrounding flat parts, and the fact that the content of molybdenum and oxygen is obviously higher than that of the surrounding flat parts can be judgedMoO is gathered at the bump₃. FIG. 6 is the surface topography of the surface oxidation resistant coating of example 2 after oxidation at 700 ℃ for 1 hour. Si/MoSi₂The surface of the composite coating still keeps a flat structure, no obvious crack is observed, particles in an aggregation state appear locally, and the size of the particles is about 0.5-0.6 mu m. The center of the particle has a bulge with the size of about 1.5 mu m, EDS elemental analysis shows that the content of molybdenum and oxygen at the bulge is obviously higher than that of the surrounding area, and MoO is presumed at the bulge₃Whereas no significant oxygen was detected in the aggregated particles.

Fig. 7(a) is a sectional morphology of the surface oxidation resistant coating of comparative example 1 after oxidation at 700 c for 1 hour. As can be seen from FIG. 7(a), MoSi of FIG. 4(h)₂In comparison with the original cross-sectional morphology of the coating, no significant grain structure was observed, and a bright zone with a thickness of about 400nm was observed near the surface. EDS element line scanning analysis shows that the diffusion degree of oxygen element to the inside of the coating is high, the content of Mo and Si is reduced at the position close to the surface, and the fact that a layer of Mo-Si-O structure is generated on the surface layer can be inferred. FIG. 7(b) is the cross-sectional morphology of the surface oxidation resistant coating of example 2 after oxidation at 700 ℃ for 1 hour. Comparing with FIG. 4(j), Si/MoSi₂The change of the section appearance of the composite coating after being oxidized for 1 hour is not obvious, the crystal grain structure is still maintained as isometric crystal, and the close packing arrangement is maintained. The particles near the surface have a slight enlargement or a phenomenon of flaky connection, and the thickness is about 100 nm. EDS element line scanning analysis shows that the content of Mo and Si at the near surface is obviously larger than that of MoSi of comparative example 1₂Mo, Si content detected in the coating. Example 2 the near-surface layer having a thickness of 100nm generated less Mo-Si-O compounds. The diffusion depth of oxygen element of comparative example 1 was about 400nm, and the diffusion depth of oxygen element of example 2 was about 57 nm. The diffusion depth of oxygen element of comparative example 1 is significantly larger than that of example 2.

Fig. 8(a) is a surface morphology of the surface oxidation resistant coating of comparative example 1 after being oxidized at 700 c for 9 hours. As can be seen from FIG. 8(a), MoSi₂Destructive swelling of the coating surface was observed. There was a bulge around the ridge similar to the 1 hour oxidation of comparative example 1 of fig. 5 (a). The bulge was centered on a line segment having a length of 20 μm, and the crack extended all around. In the process of crackingA large number of bumps were partially found. EDS elemental analysis showed that the molybdenum content in the bump was significantly lower than ambient and the oxygen and silicon content was significantly higher than ambient, so it was possible to judge that after 9 hours of oxidation, a large amount of MoO formed by previous oxidation₃Has escaped outwards and the remaining substance is SiO₂And a small amount of unoxidized MoSi₂. FIG. 8(b) is the surface topography of the surface oxidation resistant coating of example 2 after oxidation at 700 ℃ for 9 hours. Si/MoSi₂The same aggregated particles appeared on the surface of the composite coating as in example 2 of fig. 6(b) after 1 hour of oxidation. The aggregated particles are covered by a layer of transparent colloidal substance, and no obvious cracks still appear on the surface of the coating. EDS elemental analysis shows that the Mo content is obviously reduced and the oxygen content is obviously increased at the position covered by the transparent colloidal substance, so that the transparent substance can be inferred to be SiO₂The uncoated portion has a significantly higher Mo content than the coated portion and a lower oxygen content than the coated portion, and therefore, it is presumed that the uncoated portion has a SiO component₂And MoSi₂。

Fig. 9(a) is a sectional morphology of the surface oxidation-resistant coating of comparative example 1 after being oxidized at 700 ℃ for 9 hours. From the figure, it can be seen that the coating layer has obvious delamination phenomenon, and the thickness of the near surface layer is 200 nm-300 nm. EDS elemental analysis shows that the Mo and Si contents are obviously lower in the near-surface layer than in the lower region. The Mo element tends to increase from top to bottom. The oxygen element begins to accumulate near the surface. In combination with the surface morphology, holes appear near the surface, indicating that the Mo element begins to escape, and the oxygen element diffuses further inwards, and forms Mo-Si-O compounds in deeper areas. FIG. 9(b) is the cross-sectional morphology of the surface oxidation resistant coating of example 2 after oxidation at 700 ℃ for 9 hours, Si/MoSi₂The near-surface layer of the composite coating starts to become continuous and the enlarged particles start to link, forming a protective layer with a thickness of about 100 nm. EDS elemental analysis shows that the content distribution of Mo and Si elements in the coating is uniform, and oxygen elements are less diffused into the coating. According to the cross section morphology, the grains in the coating still keep the equiaxed crystal structure and are densely packed and arranged, no cavity appears, and the condition that MoO does not appear is shown₃Escape phenomenon.

Fig. 10(a) is a surface morphology of the surface oxidation resistant coating of comparative example 1 after oxidizing at 700 ℃ for 60 hours. As is clear from FIG. 10(a), MoSi₂The coating surface bulges and falls off, and obvious cracks still exist on the surface. FIG. 10(b) is the surface topography of the surface oxidation resistant coating of example 2 after oxidation at 700 ℃ for 60 hours. Si/MoSi₂Obvious filling marks appear on the surface of the composite coating, the filling is similar to a transparent colloidal substance oxidized for 9 hours, but the coating is thicker in thickness, larger in area and compact in surface structure. EDS element analysis shows that the content of Mo element in the filling area is obviously reduced compared with that in the surrounding area, and the content of oxygen element is higher. Therefore, it is presumed that SiO is a main component of the transparent colloidal substance₂. The filling area has a compact structure, and can effectively prevent oxygen from continuously entering.

Fig. 11(a) is a cross-sectional morphology of the surface oxidation resistant coating of comparative example 1 after oxidation at 700 ℃ for 60 hours. As can be seen from FIG. 11(a), MoSi₂The delamination of the coating is further aggravated, the thickness of the surface layer on the side far away from the substrate is increased to 300-400 nm, and obvious holes exist at the interface of the surface layer. The boundaries of the lamellar structure become blurred and bulky, possibly at high temperatures to promote their interconnection, compared to the unoxidized phase. EDS element analysis shows that the near-surface Mo and Si elements are low in content, the oxygen element content is high, and the diffusion depth of the oxygen element is about 600 nm. FIG. 11(b) is the cross-sectional morphology of the surface oxidation resistant coating of example 2 after oxidation at 700 ℃ for 60 hours. Si/MoSi₂The composite coating is also in a two-layer structure, the thickness of the surface layer far away from the substrate is about 100nm, the thickness is uniform and continuous and compact, no hole is formed in a layering interface, and the layer structure close to the substrate still keeps in equiaxial crystal dense packing arrangement. EDS elemental analysis shows that the content of Mo element is less in the near surface layer, the content of oxygen element is more in the surface layer far away from the substrate and less in the inner layer near the substrate, and the diffusion depth of the oxygen element is about 150 nm.

From the above analysis, it can be seen that the Si/MoSi in example 2₂In the composite coating, the main antioxidant component MoSi₂The Si element added in an auxiliary way is combined, so that the Si/MoSi is greatly improved₂Compactness of composite coating and Si elementSupplement velocity so that MoO formed due to the oxidation of Mo₃SiO generated during the process of volatile escape of the oxidation phase₂The continuous damage of the protective layer can be repaired in time, so that the Si/MoSi₂The composite coatings all showed an excess of MoSi after oxidation at 700 ℃ for 1 hour, 9 hours and 60 hours₂The oxidation resistance and the durability of the oxidation resistance of the coating.

To better explain the optimization of the surface oxidation resistance of the coating during oxidation, it is necessary to analyze the reactions occurring during oxidation, hereinafter the reactions occurring during oxidation of the coating.

When the temperature is 700 ℃:

5/7MoSi₂+O₂→1/7Mo₅Si₃+SiO₂ △G＝-685.3kj/mol (1)

2/7MoSi₂+O₂→2/7MoO₃+4/7SiO₂ △G＝-524.6kj/mol (2)

Si+O₂→SiO₂ △G＝-743.2kj/mol (3)

the Gibbs free energy of the three reactions is less than 0, so that the three reactions can spontaneously proceed at 700 ℃, and the equilibrium oxygen partial pressure formula lnP is used_O2＝△G⁰The lowest oxygen partial pressure for the three reactions can be calculated:

5/7MoSi₂+O₂→1/7Mo₅Si₃+SiO₂ lnP_O2＝0.919Pa (1)

2/7MoSi₂+O₂→2/7MoO₃+4/7SiO₂ lnP_O2＝0.937Pa (2)

Si+O₂→SiO₂ lnP_O2＝0.912Pa (3)

in the early stage of oxidation, complete and continuous SiO is not formed on the surface of the coating₂Protective layer of O at this time₂The transmittance is relatively high and thus the partial pressure of oxygen inside the coating is relatively high, at which the reaction (2) most easily occurs. As the reaction proceeds, SiO₂Gradually become more, O₂The transmittance gradually decreases, and the oxygen partial pressure in the coating begins to decrease, and the reactions (1), (2) and (3) are the sameA reaction takes place. Compared to MoSi in comparative example 1₂Coating, Si/MoSi prepared in example 2₂The composite coating, due to the presence of Si, also has a reaction (3). Thus, it generates SiO₂The efficiency of (2) is higher than the former. From the above oxidation morphology, it was found that MoSi in comparative example 1 was present in the initial stage₂The oxygen transmission rate of the coating was higher than that of Si/MoSi of example 2₂The coating is mainly formed on MoSi in the form of simple substance by adding Si₂Around, to some extent, block O₂Diffusion of (2). From the observation of the cross-sectional morphology, the coating of example 2 exhibited an equiaxed, close-packed structure, compared to MoSi in comparative example 1₂The large platelet particles of (a) are packed, and are significantly more dense, which results in a lower initial oxygen partial pressure for the coating of example 2 than for the coating of comparative example 1. Thus, in the initial stage, the reaction (1) proceeds to a greater extent in example 2 than in comparative example 1, resulting in MoO₃Then relatively less, continuously dense SiO₂The coating is more. This is why a transparent island-like structure appears after 9 hours of oxidation and the appearance of bumps is significantly smaller than in comparative example 1. The coating of example 2 has enough Si element as a reserve element and O₂The reaction can make up for the defect of slow diffusion speed of Si element in the coating. MoSi of comparative example 1₂The coating is not enough in Si element and O₂The reaction is carried out, and the speed of Si element in the coating diffusing and filling into the reaction layer is slow, which is not enough to completely compensate MoO₃The defect caused by the escape. At this time, O₂Will reach a steady rate, thus, MoSi₂The coating is gradually consumed and gradually replaced by the reaction layer, and the thickness becomes thicker. This also explains the cause of the oxidation curve of comparative example 1 in fig. 2.

Fig. 12 is a weight gain curve of the surface oxidation resistant coating modified titanium alloy substrate of comparative example 1, example 2, comparative example 2 and the unmodified titanium alloy substrate oxidized under 700 ℃ air condition for 60 hours. Wherein comparative example 1 is a curve corresponding to 0W, example 1 is a curve corresponding to 50W, example 2 is a curve corresponding to 100W, comparative example 2 is a curve corresponding to 150W, and TC4 is a curve corresponding to the unmodified titanium alloy. As can be seen in fig. 12, the unmodified titanium alloy oxidized weight gain rate was the fastest, and the oxidized weight gain continued to occur as oxidation time increased; the oxidation rates of comparative example 1, comparative example 2, example 1, and example 2 decreased in order, and after 12 hours of oxidation, the oxidation rates all tended to decrease, with the oxidation weight gain rate of example 2 being the slowest.

The above are all preferred embodiments of the present invention, and the protection scope of the present invention is not limited thereby, so: equivalent changes made according to the structure, shape and principle of the invention shall be covered by the protection scope of the invention.

Claims

1. A surface oxidation resistant coating is characterized in that: the surface antioxidant coating is applied to titanium alloy and is used for modifying the surface of the titanium alloy so as to improve the antioxidant performance of the titanium alloy in a medium temperature range of 500-800 ℃, and the surface antioxidant coating is Si/MoSi₂Composite coating of Si and MoSi₂The ratio of the amount of the Si is 1.5:1, the Si is a simple substance Si crystal phase, and the MoSi is₂Is MoSi type C11b₂Crystalline phase of said Si/MoSi₂The grain structure of the composite coating is isometric crystal, the average grain size is 30-40 nm, the isometric crystal is in close packing arrangement, the composite coating is oxidized for 60 hours at 700 ℃ in an air environment, a continuous and compact protective film is formed on the surface of the surface antioxidant coating, and the thickness of the protective film is 100 nm.

2. The surface oxidation-resistant coating according to claim 1, characterized in that: the C11b type MoSi₂The crystalline phase has a preferred orientation in the (002) direction.

3. The surface oxidation-resistant coating according to claim 1, characterized in that: oxidizing for 60 hours under the air environment condition at 700 ℃, wherein the linear fitting equation of the oxidation curve of the surface oxidation-resistant coating is t 0.011x^1/2+1.510, growth rate constant of 0.011 μm²h^-1T is the thickness of the surface anti-oxidation layer in microns, and x is timeThe time unit is hour.

4. A method for preparing a surface oxidation-resistant coating, for preparing the surface oxidation-resistant coating of any one of claims 1 to 3, characterized in that: the method comprises the following steps:

step 1, forming Si/MoSi on the surface of a substrate by adopting magnetron sputtering₂Co-deposition layer, magnetron sputtering and simultaneously adopting MoSi₂Co-sputtering a compound target and an elemental Si target, and controlling the elemental Si target and the MoSi during sputtering₂The ratio of deposition rates of the compound targets was 1.5: 1;

step 2, the Si/MoSi is carried out₂Annealing the codeposition layer to prepare Si/MoSi₂A composite coating;

in step 1, the MoSi₂The DC sputtering power of the compound target is 120W, the DC sputtering power of the Si target is 100W, the distance between the sputtering target and the substrate is 150mm, the substrate is not heated in the sputtering process, and the substrate rotates at 25rpm in the deposition process to realize Si and MoSi₂The uniformity of the co-deposition film is completed by 5 steps in total, the deposition time of each step is 30min, and the deposition interval of each step is 10min, so that the target material is cooled;

in the step 2, the annealing temperature is 950 ℃ to 1000 ℃, and the annealing time is 0.5 to 1 hour.

5. A surface modified titanium alloy, characterized in that: the titanium alloy comprises a titanium alloy substrate and the surface oxidation resistant coating formed on the titanium alloy substrate according to any one of claims 1-3, wherein the thickness of the surface oxidation resistant coating is 1-2 microns.