A method for preparing an in-situ self-generated gradient coating on a zirconium alloy surface by carbonitridation treatment

Abstract

The application is a preparation method of preparing in-situ gradient coating on the surface of zirconium alloy by carbon-nitrogen-oxygen co-permeation treatment. The method is as follows: the cut and polished zirconium alloy is placed in a graphite crucible, and is embedded with a carburizing agent, the embedding depth is 20-30mm, then the crucible is placed in a vacuum tube furnace, and is heated, under the nitrogen atmosphere, to 800-1100 DEG C, and is kept for 4-8 hours; then it is naturally cooled to room temperature, and the zirconium alloy treated by carbon-nitrogen-oxygen is obtained. The coating obtained by the application is a double-layer multi-element in-situ coating with an inner ZrO2 layer and an outer ZrN-ZrC-ZrO2 multi-element composite layer; the coating has excellent corrosion resistance, wear resistance and mechanical properties, and the heat treatment process is simple, efficient and low in cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of zirconium alloy, and particularly relates to a method for preparing in-situ self-grown gradient layer on the surface of zirconium alloy by carbonitridation and nitridation. BACKGROUND

[0002] Zirconium (Zr) is an excellent corrosion-resistant structural material for chemical industry, which is silver-white or light gray, belongs to refractory metal (1850 DEG C), has good plasticity and strength, and is an important structural material for atomic energy industry and nuclear industry. Zirconium has good corrosion resistance, small thermal neutron absorption area, high plasticity and strength, high melting point, moderate density and other advantages. However, Zr and its alloys have poor wear resistance, high adhesion and high friction coefficient. In nuclear applications, Zr and its alloys are inevitably corroded by environmental media, resulting in pitting phenomenon. In order to prolong the service life of Zr alloy and fully exert its application potential, the corrosion resistance, wear resistance and mechanical properties of Zr alloy must be greatly improved.

[0003] In order to improve the performance of zirconium alloy, surface treatment technology is a key process for improving corrosion resistance and wear resistance, including vapor phase deposition (VPD) technology, surface laser treatment technology, electron beam surface treatment technology, ion implantation technology, thermal oxidation and nitriding technology and the like. Among the many surface treatment processes, the coating generated in-situ by C, N, O and other inorganic non-metallic elements on the surface of zirconium alloy has excellent corrosion resistance, wear resistance and mechanical properties.

[0004] Among the many surface treatment processes, the coating generated in-situ by C, N, O and other inorganic non-metallic elements on the surface of zirconium alloy has excellent corrosion resistance, wear resistance and mechanical properties. ZrN thin film has high density, high melting point and high hardness. ZrC coating has high elastic modulus, high bonding strength, high hardness and good wear resistance. ZrO2 coating is more widely used. ZrO2 coating has low density, good biocompatibility, smooth and dense surface, which makes it have excellent corrosion resistance and wear resistance. These coatings have been studied by predecessors.

[0005] To meet the needs of higher service conditions, the theory of multilayer coating is proposed. Under the premise of ensuring good bonding force of multilayer structure, the multilayer structure has better performance than single layer structure. Zirconium alloy surface coating with mechanical, wear-resistant and corrosion-resistant properties can be prepared by combining the properties of ZrC, ZrN and ZrO2. In the patent CN113981361A (a method for simultaneously improving the corrosion resistance and wear resistance of zirconium alloy surface by nitriding treatment), a layer of ZrN with a thickness of about 10 μm is prepared on the surface of zirconium niobium alloy by heat treatment method and reverse nitriding treatment in a tube furnace. Due to the high density and high hardness of ZrN layer, the coating has good corrosion resistance and mechanical properties, the pitting potential is increased to about 0.5 V, and the hardness is about 900 HV0.2. However, due to the hardness of ZrN coating is 14.2-19.6 GPa, which is lower than that of ZrC coating (28-35 GPa), and the ZrN coating structure is single, it cannot meet the higher service conditions of wear-resistant devices, and its wear resistance and mechanical properties have room for further improvement. And scholars have little theory on using heat treatment method to prepare multilayer coating on the surface of alloy. SUMMARY

[0006] The purpose of the present application is to provide a method for simultaneously improving the mechanical, corrosion-resistant and wear-resistant properties of zirconium alloy surface by carbon-nitrogen-oxygen treatment, which solves the problems existing in the prior art. The method uses a heat treatment furnace to increase the carburizing treatment in a nitrogen environment on the surface of zirconium alloy. By taking advantage of the different reaction temperatures of carbon, nitrogen and oxygen with zirconium element, the reaction with oxygen is carried out first, followed by reaction with nitrogen, and finally reaction with carbon, thereby generating a gradient multi-element multilayer in-situ self-grown coating. The coating obtained by the present application is a double-layer multi-element in-situ self-grown coating with an inner ZrO2 layer and an outer ZrN-ZrC-ZrO2 multi-element composite layer. The coating has excellent corrosion resistance, wear resistance and mechanical properties, and the heat treatment process is simple, efficient and low in cost.

[0007] The technical scheme of the present application is as follows:

[0008] A method for simultaneously improving the corrosion resistance and wear resistance of zirconium alloy surface by nitriding treatment, which comprises the following steps:

[0009] First step: wire cutting processing:

[0010] The zirconium alloy is wire cut according to the required size;

[0011] The zirconium alloy is R60702, R60705, R60706 or R60700.

[0012] Second step: polishing treatment:

[0013] Use 150 # , 800 # , 1000 #SiC sand paper of 2000 # SiC sand paper of 3000 # SiC sand paper of 5000 # The zirconium alloy after cutting is polished by SiC sand paper until the sample surface has no obvious scratch, then is polished by polishing paste, and is immersed in alcohol for ultrasonic cleaning for 10-20 min, and is dried for use;

[0014] Third step: heat carbon-nitrogen-oxidation treatment:

[0015] The polished zirconium alloy is placed in a graphite crucible, and is embedded with carburizing agent, the embedding depth is 20-30 mm, then the crucible is placed in a vacuum tube furnace, is heated, is heated to 800-1100 DEG C under nitrogen atmosphere, and is kept for 4-8 hours, then is naturally cooled to room temperature, and the zirconium alloy treated by carbon-nitrogen-oxidation is obtained.

[0016] The nitrogen is nitrogen with a purity of 99.9%; the pressure of the nitrogen atmosphere is 0.101-0.141 MPa.

[0017] The heating rate in the third step is 5-6 DEG C / min.

[0018] The carburizing agent is 90% active carbon powder, 5% CaCO3 powder and 5% BaCO3 powder.

[0019] The substantial features of the application are:

[0020] In the current surface treatment technology of zirconium alloy, the zirconium alloy is treated by using air or oxygen, and the heat oxidation technology is used to form a passivation oxide film on the surface of the alloy to protect the metal; or the carburizing technology is used for carburizing treatment, or the surface is treated by nitriding. The multilayer is generated by using the technology such as magnetron sputtering.

[0021] The application improves the wear resistance, corrosion resistance and mechanical properties of the zirconium alloy by embedding carburizing treatment on the basis of nitriding, and realizes the effect gain of 1+1>2.

[0022] The inventors find that due to the inevitable participation of oxygen element in the reaction, ZrO2 is generated, and a multilayer coating containing ZrO2, ZrC and ZrN is obtained; and the temperature conditions of the reactions of C, N and O elements with Zr element are different, the reaction temperature of ZrO2 is about 200 DEG C, the reaction temperature of ZrN is about 700 DEG C, and the reaction condition of ZrC is higher, which creates conditions for the formation of the multilayer gradient structure.

[0023] But in this process, if the temperature is too low, the carbon and nitrogen elements cannot be fully reacted, the prepared carbon and nitrogen is discontinuous, or the multi-layer structure cannot be generated, which cannot improve the mechanical properties of the substrate, and cannot significantly improve the corrosion and wear resistance; if the temperature is too high, the excessive carbon and nitrogen oxidation changes the nature of the alloy material itself, which causes cracks, holes and other defects in the coating, reduces the bonding strength, and is easy to peel off. The penetration of oxygen elements is inevitable, which also creates conditions for the formation of the oxide layer.

[0024] Therefore, the present application uses the different reaction enthalpy principles of C, N and O elements and Zr to prepare a multi-element gradient in-situ self-grown coating with a thickness of about 10 microns, an inner layer of ZrO2 and an outer layer of ZrO2-ZrN-ZrC composite layer through the method of heat treatment, embedding carbonization treatment in a nitrogen atmosphere.

[0025] The present application has the following advantages:

[0026] (1) The nitrided layer prepared on the surface of the zirconium alloy has a wear depth of about 0.08 mm under the conditions of 40N pressure, 2HZ frequency reciprocating wear for 900s, while the present study achieves a wear pit depth of about 0.01 mm under the conditions of 40N, 2.5HZ, running for 900s, which reduces the wear amount of the substrate by 98%.

[0027] (2) Compared with the zirconium substrate, the product obtained after carbonitridation is more resistant to pitting corrosion than the raw material. The pitting corrosion potential is improved, and the pitting corrosion pit is not easy to form. The pitting corrosion potential of the substrate is 0.17V, the pitting corrosion potential in the patent CN113981361A (a method for improving the corrosion and wear resistance of the surface of zirconium alloy by nitriding treatment) is increased to about 0.5V, and in the present study, the pitting corrosion potential is almost disappeared, and the pitting corrosion phenomenon is almost eliminated.

[0028] (3) The hardness of the zirconium substrate is about 150HV0.2, and in the patent CN113981361A, the hardness is increased to about 900HV0.2, and in the present study, the hardness is increased to about 1100HV0.2, which is six times higher than the substrate.

[0029] (4) The coating presents a double-layer multi-element structure, the outer layer is ZrO2-ZrC-ZrN, and the inner layer is ZrO2 double-layer multi-element in-situ gradient structure. The preparation of multi-layer structure is mostly used high-cost equipment such as magnetron sputtering or spraying. Compared with the preparation of multi-layer film by magnetron sputtering method and the present application by tube furnace, only from the cost of the instrument, the magnetron sputtering coating machine is worth hundreds of thousands to millions, while a vacuum tube furnace only needs tens of thousands of yuan, and the former has a very strict size limit on the sample, which needs to be monitored at all times, and the labor cost is also high, and it cannot be mass-produced, therefore, the present application has the advantages of low production cost, simple processing process, high efficiency and the like. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 Flow chart of heat treatment process for Examples 1-3;

[0031] Figure 2 Cross-section scanning topography of zirconium alloy surface coating for Examples 1-3;

[0032] Figure 3 Electron probe composition scanning graph of zirconium alloy surface coating for Example 2;

[0033] Figure 4 Comparison graph of polarization curves of zirconium alloy substrate and Examples 1-3;

[0034] Figure 5 Comparison graph of wear topography of zirconium alloy substrate and Examples 1-3; DETAILED DESCRIPTION

[0035] Example 1

[0036] The wire-cut 10mm x 10mm x 10mm square zirconium alloy (i.e. industrial grade R60702 (Zr702)) was polished with 150 # , 800 # , 1000 # , 2000 # , 3000 # , 5000 # SiC sandpaper in sequence until the sample surface had no obvious scratches, then polished with polishing paste 1.0 for 30 min, then immersed in alcohol for ultrasonic cleaning for 15 min, and dried for use.

[0037] The polished 4 zirconium alloys were respectively placed in a cylindrical graphite crucible with an internal depth of 40 mm and a diameter of 30 mm, embedded with a carburizing agent with a mass of about 11 g and a composition (all in mass percentage) of 90% active carbon powder, 5% CaCO3 and 5% BaCO3 powder, with an embedding depth of 20 mm, and then placed in a vacuum tube furnace after covering the lid. After sealing the tube furnace, 3 times of gas washing were performed and the nitriding program was set. The heating rate adopted during heating was 6℃ / min. From room temperature to 800℃, nitrogen gas was introduced throughout the process, and the pressure in the tube was maintained at 0.141 MPa, with an atmosphere of nitrogen gas with a purity of 99.9%. After 4h of heat preservation, it began to decrease to room temperature, and then the sample was taken out, immersed in alcohol for ultrasonic cleaning, and dried for use. The flow chart is shown in Figure 1 .

[0038] Example 2

[0039] The wire-cut 10mm x 10mm x 10mm square Zr-2.5Nb alloy (Zr60705) sample was polished with 150# , 800 # , 1000 # , 2000 # , 3000 # , 5000 # SiC sandpaper, until the sample surface without obvious scratches, then polished for 30 min, and immersed in alcohol for ultrasonic cleaning 15 min, drying for use.

[0040] The four zirconium alloys after polishing were placed in a cylindrical graphite crucible with an internal depth of 40 mm and a diameter of 30 mm, and a carburizing agent with a mass of about 11 g and a composition of 90% active carbon powder, 5% CaCO3 and 5% BaCO3 powder was embedded, with an embedding depth of 20 mm. After covering the lid, the crucible was placed in a vacuum tube furnace, and after sealing the tube furnace, three times of gas washing and nitrogenation program were set. The heating rate was 6°C / min. From room temperature to 1000°C, nitrogen was passed through the whole process, and the pressure in the tube was maintained at 0.141 MPa. The atmosphere was 99.9% pure nitrogen. After 4h of holding, it began to cool to room temperature. Then the sample was taken out, immersed in alcohol for ultrasonic cleaning, and dried for use.

[0041] Example 3

[0042] The six faces of the 10mm x 10mm x 10mm square Zr-2.5Nb alloy (Zr60705) sample cut by wire were polished with 150 # , 800 # , 1000 # , 2000 # , 3000 # , 5000 # SiC sandpaper, until the sample surface without obvious scratches, then polished for 30 min, and immersed in alcohol for ultrasonic cleaning 15 min, drying for use.

[0043] The four zirconium alloys after polishing were placed in a cylindrical graphite crucible with an internal depth of 40 mm and a diameter of 30 mm, and a carburizing agent with a mass of about 11 g and a composition of 90% active carbon powder, 5% CaCO3 and 5% BaCO3 powder was embedded, with an embedding depth of 20 mm. After covering the lid, the crucible was placed in a vacuum tube furnace, and after sealing the tube furnace, three times of gas washing and nitrogenation program were set. The heating rate was 6°C / min. From room temperature to 1100°C, nitrogen was passed through the whole process, and the pressure in the tube was maintained at 0.141 MPa. The atmosphere was 99.9% pure nitrogen. After 4h of holding, it began to cool to room temperature. Then the sample was taken out, immersed in alcohol for ultrasonic cleaning, and dried for use.

[0044] Composition structure:

[0045] The cross-sectional morphology of the carbon-nitrogen-oxygen coating and the carbon-nitrogen-oxygen film thickness were observed by scanning electron microscopy (SEM, JSM-7100), as shown in Figure 2 The thickness was about 10 μm, and a uniform carbon-nitrogen-oxygen layer was formed on the surface. The element composition distribution and change of the sample coating were analyzed by electron probe microscopy (EPMA, JXA-8530F), as shown in Figure 3 The outer layer was carbon-nitrogen and a small amount of oxygen elements, and the inner layer was oxygen elements, forming a double-layer multi-element in-situ self-grown gradient structure of a ZrO2-ZrC-ZrN layer with a thickness of about 3 μm in the outer layer and a ZrO2 layer with a thickness of about 6 μm in the inner layer.

[0046] Performance test:

[0047] Table 1: Hardness test results of examples 1-3 of the present application

[0048]

[0049] The average Vickers hardness test of examples 1-3 is shown in Table 1. Hardness refers to the ability of the surface of a material to resist deformation or indentation, and can be used to evaluate the mechanical properties of the surface of the material. The Vickers hardness of the zirconium alloy samples before and after carbon-nitrogen-oxygen coating was measured by using a JMHV-1000AT precision microhardness tester, in order to study the influence of different coating processes on the hardness of the sample. The applied load was 1.96 N, and the pressure holding time was 15 s. Ten points were taken on the surface of the sample at different positions, and the hardness was measured and the average value was taken as the test value. With the increase of temperature, the increase of carbon-nitrogen element content led to the further increase of hardness. Compared with the substrate zirconium alloy, the hardness of the product in the present application was increased by about 600% compared with the comparative material.

[0050] Table 2: Electrochemical test results of examples 1-3 of the present application

[0051]

[0052] The polarization curve of examples 1-3 alloy is shown in Figure 4As shown, all examples were tested in 3.5% NaCl solution using a CHI660E electrochemical test system. The data obtained from the tests were fitted to the polarization curves of Examples 1-3 using C-View software. Table 2 shows the results of the corrosion performance tests of Examples 1-3, where the corrosion current density indicates the rate at which the alloy corrodes, and the smaller the corrosion current density, the slower the corrosion rate of the alloy and the more corrosion resistant the alloy. The greater the corrosion potential indicates that the alloy is more resistant to corrosion. The higher the pitting potential indicates that the alloy is more resistant to pitting corrosion. As can be seen from Table 2, the corrosion current density of Example 1 decreased and the corrosion potential increased, indicating that the corrosion resistance of Example 1 increased. Although the corrosion current density of Examples 2 and 3 increased, there was no significant pitting potential, indicating that the pitting corrosion resistance of Examples 2 and 3 increased.

[0053]

[0054] The wear morphology of Examples 1-3 is shown in Figure 5 As shown, the wear tests were performed using a wear and friction tester (Bruker UMT-3) to evaluate the wear resistance of the samples. The dry friction coefficient was estimated using a 4-7 mm diameter silicon carbide ball as the counter ball at room temperature. The friction conditions were a load of 40 N, a stroke of 5 mm, and a frequency of 2.5 Hz, and each sample was tested for 900 s. The wear morphology was observed using a super-zoom digital microscope (Smart zoom 5) and the wear volume was calculated. As can be seen from Table 3, the wear resistance of the zirconium alloy subjected to heat treatment was significantly improved, and the wear amount was significantly reduced, with an increase in wear resistance of about 93%.

[0055] In order to facilitate laboratory coating preparation and performance testing, the present application is based on zirconium alloy particles. Since the experiment can be scaled up, the experimental conditions (atmosphere and carburizing agent) are relatively easy to meet, and this preparation method can be adapted to the preparation of alloy surface coatings of various shapes such as plates and spheres in industrial production and application. When treating plates, as long as there is sufficient atmosphere in the furnace and a carburizing agent with a thickness of 20-30 mm is obtained, a zirconium alloy coating with excellent wear and corrosion resistance and mechanical properties can be obtained.

[0056] Comparative Example 1

[0057] The other steps are the same as in Example 1, except that the temperature is raised to 700°C under a nitrogen atmosphere for 6 hours, and the coating obtained has an oxide layer with very little carbon and nitrogen content, indicating that the alloy first reacts with oxygen and is difficult to react with carbon and nitrogen at low temperatures, and thus cannot achieve good wear resistance and surface mechanical properties.

[0058] Comparative Example 2

[0059] Other steps are the same as example 2, the difference is that the temperature is raised to 1000 DEG C under the condition of nitrogen throughout, and the coating thickness is still about 10 microns, and there is no obvious thickness change, which shows that the pre-formed oxide layer hinders the subsequent carbon and nitrogen element penetration, and cannot further significantly diffuse the coating thickness.

[0060] The application is described by examples, but does not limit the application, and other changes in the disclosed examples are easily guessed by researchers in the field of titanium alloy and zirconium alloy, and the changes should belong to the range defined in the application patent.

[0061] The remaining matters of the application are the known technology.

Claims

1. A method for simultaneously improving corrosion resistance and wear resistance of a zirconium alloy surface by nitriding treatment, characterized in that the method comprises the following steps: Step 1: wire cutting processing; Step 2: polishing processing; and Step 3: hot carbonitridation processing. In Step 1, the zirconium alloy is wire cut to a desired size. In Step 2, the polished zirconium alloy is placed in a graphite crucible and embedded with a carburizing agent to a depth of 20-30 mm, and then the crucible is placed in a vacuum tube furnace and heated to 800-1100°C under a nitrogen atmosphere for 4-8 hours, and then naturally cooled to room temperature to obtain a carbonitrided zirconium alloy. The carburizing agent comprises 90% active carbon powder, 5% CaCO3 and 5% BaCO3 powder. In Step 3, the nitrogen gas has a purity of 99.9% and the nitrogen atmosphere has a pressure of 0.101-0.141 MPa.

3. The method for simultaneously improving corrosion resistance and wear resistance of a zirconium alloy surface by nitriding treatment according to claim 1, characterized in that the zirconium alloy is R60702, R60705, R60706 or R60700.

4. The method for simultaneously improving corrosion resistance and wear resistance of a zirconium alloy surface by nitriding treatment according to claim 1, characterized in that the heating rate in Step 3 is 5-6°C / min. ​ 2. The method for improving the corrosion and wear resistance of zirconium alloy surface by nitriding treatment according to claim 1, wherein the polishing in the second step is performed by using SiC sandpaper with grits of 150 # # # # # # in sequence until no obvious scratches are left on the surface of the sample, and then polishing with polishing paste and ultrasonic cleaning in alcohol for 10-20 min, and drying for use.​​​​​​ ​ ​

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