CrsiC composite coating with high temperature oxidation resistance and structural thermal stability, preparation method and application

Abstract

The present application relates to the field of protective coating, in particular to a CrSiC composite coating with high-temperature oxidation resistance and structural thermal stability, a preparation method and application. The CrSiC composite coating shows amorphous structure in the deposited state, and after high-temperature oxidation at 1200 DEG C, the coating structure evolves into a double-oxidation layer of Cr2O3+SiO2 in the outermost layer, a ZrC layer formed in situ at the coating / substrate interface, and a Cr3Si layer and a Cr3Si+Cr7C3 double-phase nanocrystalline layer doped with amorphous SiO2 in the residual coating. The overall structure of the CrSiC composite coating is stable after high-temperature oxidation, without defects such as cracks and bulges. The multilayer structure formed in situ at high temperature promotes the coating to exhibit excellent oxidation resistance and comprehensive mechanical properties, and can keep the zirconium alloy substrate from being oxidized in a 1200 DEG C water vapor environment for 2h and from undergoing eutectic reaction with the coating and being oxidized in a 1400 DEG C steam environment for 600s.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of alloy protective coating, in particular to a CrSiC composite coating with high-temperature oxidation resistance and structural thermal stability, a preparation method and a nuclear fuel cladding surface application. BACKGROUND

[0002] The traditional cladding material zirconium alloy will quickly oxidize and fail in the high-temperature steam environment under accident conditions, and the hydrogen produced has the risk of explosion. In order to improve the safety of nuclear reactors, a coating is prepared on the surface of the zirconium alloy to improve its oxidation resistance, which is an important development direction of ATF cladding. Cr coating, MAX phase coating, carbide coating, nitride coating and FeCrAl alloy coating all have certain accident tolerance capability. Relatively speaking, the Cr coating has a simple preparation process, can quickly form a dense Cr2O3 protective film on the surface, and has good thermal matching with the zirconium matrix, high melting point and excellent corrosion resistance, which can cope with complex accident environments. However, recent research has highlighted the potential problems of Cr coating. When the environmental temperature rises above 1000℃, Zr diffuses into the Cr coating and precipitates ZrO2 along the Cr grain boundary, which provides a fast channel for the diffusion of oxygen elements and reduces the high-temperature oxidation resistance of the coating. The oxidation-reduction reaction of zirconium diffusing below the chromium oxide layer will reduce the adhesion and density of the oxidation layer. In addition, if a super-design basis accident (>1332℃) occurs, ZrCr2 at the coating / substrate interface will eutectic melt, resulting in coating collapse, strong acceleration of oxidation kinetics and faster degradation of the zirconium alloy cladding. Therefore, how to control the interdiffusion of Cr-Zr interface is a key problem for optimizing the Cr coating.

[0003] Researchers have explored various methods to improve the oxidation resistance and stability of Cr coating, such as multilayer design and the addition of a diffusion barrier layer to inhibit Cr-Zr interdiffusion. Studies have shown that pre-deposited Mo can replace the ZrCr2 intermediate layer. In 1100℃ air, the coating consumption rate is significantly lower than that of pure chromium coating. The number and growth rate of cohen-dal pores at the interface are also significantly reduced. The ZrN layer formed in situ at the interface between the CrN coating material and the substrate can also act as a barrier to Cr-Zr interdiffusion, enhancing the oxidation resistance and mechanical properties of the original Cr coating. Yang et al. studies have shown that the in-situ formed ZrN layer can form within 10min in a 1200℃ steam environment and inhibit Cr-Zr interdiffusion, but loses its effect at 1400℃. In addition, CrN will decompose to produce N2 at temperatures above 650℃, and N2 is also produced by the oxidation of CrN. The pores formed by decomposition or oxidation may damage the dense structure of the coating, resulting in a loss of coating protection.

[0004] The alloying design of the coating can significantly improve the performance of the Cr coating. The addition of an appropriate amount of Al or Si in the Cr coating can form a double-oxidation layer structure of an outer Cr2O3 layer and an inner Al2O3 or SiO2 layer in a high-temperature environment, thereby inhibiting the internal diffusion of O and improving the oxidation resistance of the coating. However, due to the continuous diffusion of Al and Si in the residual coating to the Zry-4 substrate during the oxidation process, it is insufficient to form a continuous layered oxidation structure on the surface. In addition, blindly increasing the Al content will result in poor compactness of the oxidation layer on the surface of the Cr coating, and there are gaps that provide channels for the penetration of oxygen. Moreover, a-Al2O3 is prone to cracking and peeling, and after water corrosion, it is easy to form high-temperature soluble AlOOH, which leads to the hydrolysis and peeling of the coating. The CrSi coating with high Si content will produce stress due to the mismatch of the coefficient of thermal expansion (CTE) between the Cr2O3 layer, the coating and the Zry-4 alloy substrate, which will cause cracks or peeling of the oxidation layer. These phenomena make the composition design factors of the coating complex, and limit the further improvement of the oxidation resistance of the coating.

[0005] It can be seen that the modification of the Cr coating still has no optimal solution. The simultaneous satisfaction of the basic conditions of mechanical properties, oxidation resistance, interface stability and water chemical stability at high temperature greatly limits the use of elements. The most commonly used method for preparing Cr coating is physical vapor deposition, such as the PVD preparation technology of ATF zirconium cladding surface Cr coating by AREVAN in France and Nuclear Power Research and Design Institute (NPIC) in China. However, the PVD method for depositing Cr coating is prone to introduce columnar crystals, which also restricts the service performance of the Cr coating. SUMMARY

[0006] In view of the failure problem of the current Cr coating, the main purpose of the present application is to provide a CrSiC composite coating with high-temperature oxidation resistance and structural thermal stability, a preparation method and a nuclear fuel cladding surface application. The present application prepares a CrSiC composite coating with uniform element distribution, compactness, no defects and tight bonding with the substrate on the Zry-4 alloy substrate by a co-sputtering method of magnetron sputtering. The coating has the following structural characteristics:

[0007] (1) The CrSiC composite coating is in a dense amorphous structure in the deposited state, and can maintain a stable amorphous structure below 800℃. Under normal working conditions, the coating has excellent corrosion resistance.

[0008] (2) After the CrSiC composite coating is oxidized at 1200 DEG C, the coating structure evolves into a continuous Cr2O3+SiO2 double oxidation layer including an outermost layer, an in-situ formed ZrC layer at the coating / substrate interface, and a Cr3Si layer and a Cr3Si+Cr7C3 double-phase nanocrystalline layer doped with amorphous SiO2 in the residual coating. The evolved coating structure is stable and can maintain the excellent long-term oxidation resistance of the CrSiC composite coating under the condition of a loss of coolant accident.

[0009] Another object of the present application is to provide a preparation method of the CrSiC composite coating with high-temperature oxidation resistance and structural thermal stability. The introduced C element preferentially diffuses to the substrate at high temperature to form a continuous and dense ZrC diffusion barrier layer in-situ, effectively inhibiting the element interdiffusion between Cr, Si and the Zry-4 substrate, and the coating has excellent high-temperature oxidation resistance and mechanical properties.

[0010] To solve the above technical problems, the technical scheme adopted by the present application is as follows:

[0011] A CrSiC composite coating with high-temperature oxidation resistance and structural thermal stability is prepared by using a magnetron sputtering technique to prepare a Si and C co-doped modified CrSiC composite coating on the surface of a Zry-4 alloy substrate. The CrSiC composite coating in the deposited state exhibits an amorphous structure, with elements uniformly distributed and the coating being tightly combined with the Zry-4 alloy substrate. The content of Si in the CrSiC composite coating is controlled at 8.0-10.0 at.%, and the content of C is controlled at 4.0-10.0 at.%.

[0012] Further, the thickness of the CrSiC composite coating with high-temperature oxidation resistance and structural thermal stability is 3.8-4.3 microns.

[0013] The present application also provides a preparation method of the above CrSiC composite coating with high-temperature oxidation resistance and structural thermal stability, which is performed according to the following steps:

[0014] (1) After the Zry-4 alloy substrate is subjected to acid pickling, silicon carbide sandpaper polishing and ultrasonic cleaning, it is dried with nitrogen and is ready for use;

[0015] (2) The Zry-4 alloy substrate obtained in step (1) is attached to a substrate base plate and is placed in a rotating heating table in a magnetron sputtering chamber;

[0016] (3) The Si target and the C target are connected to a radio frequency power source, and the Cr target is connected to a direct current power source. The preparation parameters are adjusted, and a CrSiC composite coating is prepared on the substrate base plate obtained in step (2) by using a co-sputtering method of magnetron sputtering.

[0017] Further, the acid pickling solvent in step (1) is a mixed solution composed of 10% HNO3, 10% HF and 10% H2O2 in volume concentration; the granularity of the silicon carbide sandpaper is 100#, 300#, 600#, 1000# and 1500# in sequence; the ultrasonic cleaning method is that the Zry-4 alloy substrate is sequentially placed in acetone, anhydrous ethanol and deionized water for ultrasonic cleaning for 20 minutes.

[0018] Further, the target base distance is adjusted to 15 cm in step (2).

[0019] Further, the thickness of the Si target material, the C target material and the Cr target material in step (3) is 7 mm, the diameter is 3 inches, and the purity of the target material is 99.999%.

[0020] Further, the preparation parameters in step (3) are as follows: the vacuum degree is 6.0*10 -4 pa, the deposition temperature is 200-300 DEG C, the substrate bias is -50 to -100 V, the working gas pressure is 0.5-0.6 pa, and the deposition time is 1-2 h; wherein the sputtering power of the Cr target material is fixed at 300 W, the sputtering power of the Si target material is 100-200 W, and the sputtering power of the C target material is 100-300 W. The content of Si is controlled to be 8.0-10.0 at.%, and the content of C is controlled to be 4.0-10.0 at.%.

[0021] The application further provides application of the CrSiC composite coating with high-temperature oxidation resistance and structural thermal stability.

[0022] Further, after high-temperature oxidation at 1200 DEG C, the coating structure of the CrSiC composite coating with high-temperature oxidation resistance and structural thermal stability evolves into an outermost continuous Cr2O3+SiO2 double oxidation layer, a ZrC layer formed in situ at the coating / substrate interface, a Cr3Si layer in the residual coating and a Cr3Si+Cr7C3 double-phase nanocrystalline layer doped with amorphous SiO2; in the coating structure after high-temperature oxidation, the ZrC layer formed in situ preferentially effectively inhibits the element interdiffusion between Cr, Si and the Zry-4 substrate; the continuous Cr2O3+SiO2 double oxidation layer and the Cr3Si layer structure effectively inhibit the internal diffusion of O, the coating has excellent high-temperature oxidation resistance, and the Cr3Si+Cr7C3 double-phase nanocrystalline layer doped with amorphous SiO2 makes the coating have excellent thermal stability and mechanical properties.

[0023] Further, the in-situ formed multi-layer structure of the CrSiC composite coating with high-temperature oxidation resistance and structural thermal stability promotes the coating to exhibit excellent oxidation resistance and comprehensive mechanical properties, and the zirconium alloy substrate can be kept from being oxidized within 2h at 1200℃ in a water vapor environment and from being oxidized and from having eutectic reaction with the coating within 600s at 1400℃ in a steam environment.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] (1) The co-sputtering technology of the magnetic control sputtering is used to introduce the two beneficial elements of Si and C into the Cr coating, and the coating thickness and element content can be flexibly adjusted through the sputtering process parameters, so that the preparation process has flexibility. The doping of Si and C can also destroy the columnar growth mode of the Cr crystal, induce the coating to change into a dense amorphous structure, avoid the generation of corrosion channels such as grain boundaries, and thus improve the excellent corrosion resistance of the coating under normal operating conditions.

[0026] (2) Under the simulated loss of water accident environment, the CrSiC composite coating of the present application will generate a 1-2μm ZrC diffusion barrier layer on the substrate side in-situ, and the Si and Cr elements in the coating are effectively retained in the coating without mutual diffusion with the Zry-4 substrate, so as to ensure the stability of the coating structure. The eutectic temperature of ZrC itself and with Cr and Zr is very high (>1800℃), which can prevent the eutectic reaction under the super-accident standard condition (>1200℃), and can improve the accident safety of the coating.

[0027] (3) Due to the ZrC as a diffusion barrier layer, the effective oxidation resistance elements Cr and Si in the coating are hindered from diffusing to the substrate, and a continuous and dense Cr2O3 and amorphous SiO2 double oxidation layer is formed on the surface of the coating. The formation of the continuous amorphous SiO2 layer can further hinder the diffusion of O to the inside of the coating and the diffusion of Cr to the surface of the coating, inhibit the thickening of the chromium oxide layer to reduce the growth stress, improve the adhesion of the Cr2O3 layer and the residual coating, and play a role in buffering the growth stress of the oxidation layer, and can also hinder the reduction of Zr to the Cr2O3 layer to form holes. These effects avoid the bulging and peeling phenomenon caused by the over-thickening of the oxidation layer, and the rapid consumption of the coating due to oxidation, so that the coating can have excellent long-term oxidation resistance. The survival time of the about 4.3μm CrSiC composite coating under the simulated loss of water accident condition (1200℃ steam) is more than 2h.

[0028] (4) The CrSiC composite coating of the present application forms a surface Cr3Si layer inside, a dual-phase nanocrystalline structure of Cr3Si+Cr7C3 inside, and SiO2 nanoparticles precipitated and passivated at the grain boundaries of the residual coating, thereby inhibiting the rapid diffusion of elements such as O and Zr along the grain boundaries. The growth of SiO2 particles is limited by the O content diffusing into the coating and the Cr7C3 grains, so it is difficult to grow and is dispersedly distributed at the grain boundaries inside the coating, further improving the thermal stability of the dual-phase nanocrystals inside the coating. The synergistic strengthening mechanism of the dual-phase equiaxed nanocrystalline structure inside the coating, the stacking faults universally present in the Cr7C3 grains, and the pinning of the grain boundaries by SiO2 particles makes the Cr 80 Si 10 C 10 The coating shows excellent strength-plasticity balance and high thermal stability. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The cross-sectional morphology and composition analysis of the as-deposited coating prepared in the present application comparative example 1 and examples 1-3;

[0030] Figure 2 The XRD test spectrum of the as-deposited coating prepared in the present application comparative example 1 and examples 1-3;

[0031] Figure 3 The weight gain curve of the coating prepared in the present application comparative example 1 and examples 1-3 oxidized in a 1200℃ water vapor environment for 0-2h;

[0032] Figure 4 The surface macroscopic morphology and SEM image of the coating prepared in the present application comparative example 1 and examples 1-3 oxidized in a 1200℃ water vapor environment for 60min;

[0033] Figure 5 The cross-sectional morphology and EDS area scan of the coating prepared in the present application comparative example 1 oxidized in a 1200℃ water vapor environment for 60min;

[0034] Figure 6 The cross-sectional morphology and EDS area scan of the coating prepared in the present application example 1 oxidized in a 1200℃ water vapor environment for 60min;

[0035] Figure 7 The cross-sectional morphology and EDS area scan of the coating prepared in the present application example 2 oxidized in a 1200℃ water vapor environment for 60min;

[0036] Figure 8 The cross-sectional morphology and EDS area scan of the coating prepared in the present application example 3 oxidized in a 1200℃ water vapor environment for 60min;

[0037] Figure 9 Cross-sectional morphology and EDS area scan of the coating prepared in Example 3 of the present application after oxidation in 1400℃ steam environment for 10min;

[0038] Figure 10 Average force-displacement curve and morphology of the indentation position obtained by micro-indentation test on the surface of the coating prepared in Comparative Example 1 and Example 3 of the present application after oxidation in 1200℃ steam environment for 60min;

[0039] Figure 11 TEM analysis of the cross-section of the coating prepared in Example 3 of the present application after oxidation in 1200℃ steam environment for 60min: (a) Cr 80 Si 10 C 10 Cross-sectional overview of the coating after steam oxidation at 1200℃ for 60min: (b) STEM image and corresponding element distribution map of the interface between Cr2O3 / residual coating; (c) STEM image and corresponding element distribution map of the residual coating; (d) STEM image and corresponding element distribution map of the ZrC layer; (e) Average grain size distribution map of the Cr3Si layer; (f) Average grain size distribution map of the bi-phase nanocrystalline grain layer. DETAILED DESCRIPTION

[0040] The present application will be further described in conjunction with specific examples. It should be understood that the following examples are only used to illustrate the present application and not intended to limit the scope of the present application, and those skilled in the art can make some non-essential improvements and adjustments according to the content of the above-mentioned application.

[0041] Comparative Example 1

[0042] In this comparative example, a pure Cr coating was prepared on a Zry-4 alloy substrate, and the preparation method was as follows:

[0043] (1) Pretreatment of zirconium alloy substrate: the Zry-4 alloy substrate was pickled with a mixed solution of 10% HNO3, 10% HF and 10% H2O2 by volume concentration. The surface of the Zr-4 alloy substrate was polished with 100#, 300#, 600#, 1000# and 1500# silicon carbide sandpaper, respectively, and then ultrasonically cleaned in acetone, anhydrous ethanol and deionized water for 20min, respectively, and dried with nitrogen for standby; the treated Zr-4 alloy substrate was attached to a substrate plate and placed on a rotating heating table in a magnetron sputtering chamber, and the target-substrate distance was adjusted to 15cm;

[0044] (2) Cr coating preparation: the Cr target is connected with a direct current (DC) power supply, the thickness of the Cr target is 7 mm, the diameter is 3 inches, and the purity of the target is 99.999%. The preparation parameters are adjusted, the vacuum degree is 6.0*10 -4 pa, the deposition temperature is 300℃, the substrate bias is-50~ -100V, the working pressure is 0.5~0.6pa, and the deposition time is 1.2h, wherein the sputtering power of the Cr target is fixed at 350w.

[0045] Figure 1 a is the cross-sectional morphology and element content of the as-deposited Cr coating, it can be seen that the Cr coating is a dense columnar crystal structure without any cracks, holes and other defects, the coating is well combined with the Zry-4 alloy substrate, and the thickness is 4.0μm. The XRD spectrum of the as-deposited coating shows that the Cr coating has a preferred orientation of (100). Figure 2

[0046] Example 1

[0047] In this embodiment, a CrSiC composite coating is prepared on a Zry-4 alloy substrate, and the preparation method is as follows:

[0048] (1) Zirconium alloy substrate pretreatment: the Zry-4 alloy substrate is pickled with a mixed solution of 10% HNO3, 10% HF and 10% H2O2 by volume concentration. The surface of the Zry-4 alloy substrate is polished with 100#, 300#, 600#, 1000# and 1500# silicon carbide sandpaper in turn, and then ultrasonic cleaned in acetone, anhydrous ethanol and deionized water for 20min respectively, and dried with nitrogen. The treated Zry-4 alloy substrate is placed on a substrate substrate and placed on a rotating heating table in a magnetron sputtering chamber, and the target-substrate distance is adjusted to 15cm;

[0049] (2) CrSiC composite coating preparation: Si target, C target and Cr target are connected with radio frequency (RF) power supply respectively, the thickness of Si target, C target and Cr target is 7mm, the diameter is 3 inches, and the purity of the target is 99.999%. The preparation parameters are adjusted, the vacuum degree is 6.0*10 -4 pa, the deposition temperature is 250℃, the substrate bias is-50~ -100V, the working pressure is 0.5~0.6pa, and the deposition time is 1.2h; wherein the sputtering power of the Cr target is fixed at 300w, the sputtering power of the Si target is 100w, and the sputtering power of the C target is 100w. The content of Si is controlled at 8.0at.%, and the content of C is controlled at 4.0at.%, which is recorded as Cr 88 Si8C4 coating.

[0050] ​Figure 1 b is as-deposited Cr 88 The cross-sectional morphology and element content of the Si8C4 coating can be seen that, Cr 88 The Si8C4 coating is a dense columnar crystal structure without any defects such as cracks and holes, and the coating is well combined with the Zry-4 alloy substrate, with a thickness of 3.8 μm. The XRD spectrum of the as-deposited coating can be seen that, Cr Figure 2 The cross-sectional morphology and element content of the Si8C4 coating can be seen that, Cr 88 The relatively wider diffraction peak of the Si8C4 coating than the Cr(110) crystal plane in the Cr coating is observed, and no other intermetallic compound is observed, indicating that the grain size of the coating is refined.

[0051] Example 2

[0052] In this example, a CrSiC composite coating is prepared on a Zry-4 alloy substrate, and the preparation method is as follows:

[0053] (1) Zirconium alloy substrate pretreatment: The Zry-4 alloy substrate is pickled with a mixed solution of 10% HNO3, 10% HF and 10% H2O2 by volume concentration. The surface of the Zry-4 alloy substrate is polished with 100#, 300#, 600#, 1000# and 1500# silicon carbide sandpaper, and then ultrasonically cleaned in acetone, anhydrous ethanol and deionized water for 20 min, respectively, and dried with nitrogen. The treated Zry-4 alloy substrate is placed on a substrate substrate and placed on a rotating heating table in a magnetron sputtering chamber, and the target-substrate distance is adjusted to 15 cm;

[0054] (2) Preparation of CrSiC composite coating: Si target, C target and radio frequency (RF) power source are connected respectively, Cr target and direct current (DC) power source are connected, thickness of Si target, C target and Cr target is 7 mm, diameter is 3 inches, purity of target material is 99.999%. The preparation parameters are adjusted, vacuum degree is 6.0×10 -4 pa, deposition temperature is 250℃, substrate bias is -50 to -100V, working pressure is 0.5 to 0.6pa, deposition time is 1.2h; wherein the sputtering power of Cr target is fixed at 300w, the sputtering power of Si target is 100w, and the sputtering power of C target is 200w. The content of Si is controlled at 8.0at.%, and the content of C is controlled at 6.0at.%, which is recorded as Cr 86 Si8C6 coating.

[0055] Figure 1 c is as-deposited Cr 86 The cross-sectional morphology and element content of the Si8C4 coating can be seen that, Cr 86Si8C6coating has no obvious columnar structure, and no defects such as cracks and holes, and the coating is well combined with the Zry-4 alloy substrate, with a thickness of 4.0 μm. Figure 2 The XRD spectrum of the as-deposited coating can be seen that Cr 86 The diffraction peak corresponding to the Cr(110) crystal plane in the Si8C6coating is wider, and no other intermetallic compound is observed, indicating that the Cr 86 The Si8C6coating is gradually amorphized.

[0056] Example 3

[0057] In this example, a CrSiC composite coating is prepared on a Zry-4 alloy substrate, and the preparation method is as follows:

[0058] (1) Zirconium alloy substrate pretreatment: The Zry-4 alloy substrate is pickled with a mixed solution of 10% HNO3, 10% HF and 10% H2O2 by volume concentration. The surface of the Zry-4 alloy substrate is polished with 100#, 300#, 600#, 1000# and 1500# silicon carbide sandpaper, and then ultrasonically cleaned in acetone, anhydrous ethanol and deionized water for 20 min, respectively, and dried with nitrogen for standby. The treated Zry-4 alloy substrate is placed on the substrate substrate and placed on the rotating heating table in the magnetron sputtering chamber, and the target-substrate distance is adjusted to 15 cm;

[0059] (2) Preparation of CrSiC composite coating: Si target, C target and RF power source are connected respectively, Cr target and DC power source are connected, thickness of Si target, C target and Cr target is 7 mm, diameter is 3 inches, purity of target material is 99.999%. The preparation parameters are adjusted, vacuum degree is 6.0×10 -4 pa, deposition temperature is 250℃, substrate bias is-50 to-100 V, working pressure is 0.5 to 0.6 pa, deposition time is 1.2 h; the sputtering power of Cr target is fixed at 300 W, the sputtering power of Si target is 150 W, and the sputtering power of C target is 300 W. The content of Si is controlled at 10.0 at.%, and the content of C is controlled at 10.0 at.%, which is recorded as Cr 80 Si 10 C 10 coating.

[0060] Figure 1 d is the cross-sectional morphology and element content of the as-deposited Cr 80 Si 10 C 10 coating, it can be seen that Cr 80 Si 10 C 10The coating has no obvious preferred orientation, and has no any defects such as cracks, holes and the like, the coating is well combined with the Zry 4 alloy substrate, and the thickness is 4.3 μm. The coating is combined Figure 2 The XRD spectrum of the deposition state coating can be seen that the Cr 80 Si 10 C 10 The coating is completely amorphous.

[0061] From the structure comparison of the deposition state coatings prepared in Comparative Example 1 and Examples 1-3, it can be seen that after adding Si and C elements in the Cr coating, the preferred columnar crystal orientation along the Cr (100) crystal plane is gradually destroyed, and with the increase of Si and C elements, the structure of the coating gradually changes from columnar crystal structure to amorphous structure. The amorphous structure helps to improve the excellent corrosion resistance of the CrSiC coating under normal operating conditions.

[0062] Performance test:

[0063] 1. High temperature oxidation resistance:

[0064] In order to more accurately evaluate the life of the coated Zry-4 alloy in the loss of water accident environment, the above coating was subjected to a 1200℃ / 0-2h steam oxidation test. The TGA analysis was used to simulate the oxidation kinetics of the coating. Figure 3 The weight gain curves of the Cr coating, Cr 88 Si8C4, Cr 86 Si8C6, Cr 80 Si 10 C 10 Coating and bare zirconium alloy are shown. The results show that the mass gain of the Cr coating shows a relatively slow parabolic form before 20 min, but the mass gain increases rapidly after 20 min and tends to the mass gain of the bare zirconium alloy, indicating that the Cr coating loses the protection effect; the Cr 88 Si8C4, Cr 86 Si8C6 coating has a good protective effect before 70 min and 80 min respectively, and the mass increases slowly, but then the mass increase rate of the coating increases rapidly, indicating that the coating loses the protection effect. The Cr 80 Si 10 C 10 Coating shows the lowest mass gain, and the growth rate curve remains a slow parabolic form within 2h, indicating that the Cr 80 Si 10 C 10 Coating has the best oxidation resistance.

[0065] Figure 4The surface SEM morphology and composition analysis of various coatings after 1200℃ oxidation for 60min in steam environment are shown. After steam oxidation, the macroscopic surface of other coatings containing Si and C remains certain integrity, without obvious peeling or cracking, except for Cr coating. EDS point analysis shows that the oxidation product on the surface of the coating is mainly Cr203, and no Si or Zr-containing oxide is found. Cr 88 Si8C4 coating has large area of oxidation layer peeling and cracking on the surface, and the peeled oxidation layer is exposed to the size of Cr203 particles, which shows that peeling mainly occurs during high temperature oxidation, and Cr 86 Si8C6 coating has no obvious oxidation layer peeling, but part of the oxidation layer appears bulging and the bulging part has cracks. In comparison, Cr 80 Si 10 C 10 coating surface is more flat, and has no obvious cracks, and the oxidation layer is dense, showing better oxidation resistance.

[0066] 2、CrSiC composite coating and elemental interdiffusion performance of zirconium alloy

[0067] Figure 5 The cross-sectional morphology and element distribution of Cr coating after 1200℃ steam oxidation for 60min are shown. In Cr coating sample, interdiffusion between Cr coating and Zry-4 substrate can be observed, and the surface Cr203 layer is reduced by Zr diffused along the coating grain boundary, resulting in the decrease of Cr203 layer thickness. O diffuses to the substrate through the ZrO2 network in the coating, resulting in the oxidation of the substrate and a large number of cracks.

[0068] Figure 6 The cross-sectional morphology and element distribution of Cr 88 Si8C4 coating after 1200℃ steam oxidation for 60min are shown. In Cr 88 Si8C4 coating, a discontinuous SiO2 layer is formed below the Cr203 layer on the surface of the coating, which is mainly because a large amount of Si in the coating diffuses to the Zry-4 substrate to form a discontinuous ZrSi phase and is consumed. Due to the formation of ZrSi diffusion layer on the substrate side, the diffusion of Cr into the substrate is inhibited to some extent, but there is still a part of Cr diffusion area above the ZrSi layer.

[0069] Figure 7 The cross-sectional morphology and element distribution of Cr 86 Si8C6 coating after 1200℃ steam oxidation for 60min are shown. In Cr 86A thin ZrC layer was formed on the substrate side of the Si8C6 coating, which hindered the interdiffusion of Cr, Si and Zr substrate. This was well demonstrated by the EDS elemental mapping. A thick but discontinuous SiO2 layer was formed under the Cr2O3 layer. The discontinuous SiO2 layer could not completely inhibit the outward diffusion of Cr and the inward diffusion of O, resulting in the continuous growth of the Cr2O3 layer with a relatively thick thickness. Some large SiO2 phases were formed in the coating, which may be due to the oxidation of silicon in the residual coating.

[0070] Figure 8 For Cr 80 Si 10 C 10 Cross-sectional morphology and elemental distribution of the coating after steam oxidation at 1200°C for 60 min. Cr 80 Si 10 C 10 A continuous ZrC layer was also formed on the substrate side of the coating. The Cr and Si elements in the coating did not diffuse to the substrate, and there was no diffusion of Zr into the coating. Due to the higher Si content, a continuous and dense Cr2O3 and SiO2 double oxidation layer structure was formed in the coating, which helped to inhibit the inward diffusion of O and the outward diffusion of Cr. The thickness of the Cr2O3 layer was relatively smaller than that of the Cr 86 Si8C6 coating, indicating an improvement in the oxidation resistance of the coating. In the Cr 80 Si 10 C 10 Some smaller SiO2 particles were dispersedly distributed in the residual coating, which may be due to the extremely small amount of O diffusing into the coating.

[0071] Figure 9 For Cr 80 Si 10 C 10 Cross-sectional morphology and elemental distribution of the coating after steam oxidation at 1400°C for 10 min. The in-situ formed ZrC layer on the substrate side can prevent the eutectic reaction between the coating and the substrate and the interdiffusion of Cr and Si in the coating and the substrate, and the coating structure is complete, indicating that the coating still has excellent oxidation resistance in this environment.

[0072] In summary, when the C content in the coating is ≥6%, a 1-2 μm ZrC diffusion barrier layer is formed in-situ on the substrate side at high temperature. The Si and Cr elements in the coating are effectively retained in the coating and do not interdiffuse with the Zry-4 substrate, ensuring the stability of the coating structure. Cr 80 Si 10 C 10The coating surface forms a continuous and dense Cr2O3 and amorphous SiO2 double oxide layer. The formation of the continuous amorphous SiO2 layer can further hinder the diffusion of O to the inside of the coating and the diffusion of Cr to the surface of the coating, inhibit the thickening of the chromium oxide layer to reduce the growth stress, while improving the adhesion of the Cr2O3 layer to the residual coating and playing a role in buffering the growth stress of the oxide layer, and can also hinder the reduction of the Cr2O3 layer by Zr to produce pores. Since the eutectic temperature of ZrC itself and with Cr and Zr is very high (>1800℃), no eutectic reaction occurs under super-accident criteria conditions (>1200℃), which can improve the accident safety of the coating.

[0073] 3. Mechanical property test

[0074] The micro indentation technique was used to measure the Cr coating and Cr 80 Si 10 C 10 The loading and unloading characteristics of the coating sample were observed under the condition that the maximum load was 0.8N and the indenter was withdrawn from the sample within 20 seconds. From the results of the micro indentation test, it can be seen that the Cr Figure 10 80 Si 10 C 10 The coating has higher hardness and elastic modulus, and the loading curve is relatively smooth, indicating that the coating did not crack during loading and has good fracture toughness. However, the Cr coating has a large plastic deformation, and the loading curve is irregular, indicating that a large number of cracks are generated during loading, because the columnar grains of the Cr coating after oxidation are relatively thick and easy to crack.

[0075] 4. Thermal stability of coating structure

[0076] Figure 11 Cr 80 Si 10 C 10 ​TEM image of the cross section of the coating after oxidation at 1200℃ for 60min. In addition to the Zr-4 alloy substrate, a 5-layer structure can be observed. Combined with the corresponding EDS element distribution and point measurement (Table 1) analysis, the five layers from top to bottom are: Cr2O3 layer, SiO2 layer, Cr3Si layer, Cr3Si+Cr7C3 dual-phase nanocrystalline layer, and ZrC diffusion layer. Among them, the Cr3Si layer and the Cr3Si+Cr7C3 dual-phase nanocrystalline layer both belong to the residual coating, but the grain size in the Cr3Si layer is larger than that in the dual-phase nanocrystalline layer. The average grain size of the Cr3Si layer is 400nm, and the average grain size of the dual-phase nanocrystalline layer is 160nm. This is because in the dual-phase grain layer, Cr3Si and Cr7C3 grains can inhibit the growth of each other, keeping the nanocrystalline size stable. SiO2 nanoparticles will precipitate at the grain boundaries of the residual coating and passivate the grain boundaries, thereby inhibiting the rapid diffusion of O, Zr and other elements along the grain boundaries. The growth of SiO2 particles is limited by the O content diffusing into the coating and the Cr7C3 grains, making it difficult to grow and disperse at the grain boundaries inside the coating, further improving the thermal stability of the dual-phase nanocrystalline inside the coating. The synergistic strengthening mechanism of the dual-phase equiaxed nanocrystalline structure inside the coating, the stacking faults commonly present in the Cr7C3 grains, and the SiO2 particles pinning the grain boundaries makes the Cr 80 Si 10 C 10 The coating shows excellent strength-plasticity balance performance and high thermal stability.

[0077] Table 1 EDS point measurement analysis of the cross section of the Cr 80 Si 10 C 10 EDS point measurement analysis of the cross section of the coating

[0078]

[0079]

[0080] The CrSiC composite coating prepared by the present application is stable in structure as a whole after high temperature oxidation, without cracks, bulges and other defects. The multi-layer structure formed in situ at high temperature promotes the coating to exhibit excellent oxidation resistance and comprehensive mechanical properties, enabling the zirconium alloy substrate to be not oxidized within 2h at 1200℃ in a steam environment and not to have eutectic reaction with the coating within 600s at 1400℃ in a steam environment and not to be oxidized.

[0081] The above shows and describes the basic principles and main features of the present application and the advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above-mentioned embodiments, and the above-mentioned embodiments and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A CrSiC composite coating that combines high-temperature oxidation resistance and structural thermal stability, characterized in that, A Si and C co-doped modified CrSiC composite coating was prepared on the surface of Zry-4 alloy substrate by magnetron sputtering. The CrSiC composite coating exhibited an amorphous structure with uniform element distribution and was tightly bonded to the Zry-4 alloy substrate. The Si content in the CrSiC composite coating is controlled at 8.0~10.0 at.%, and the C content is controlled at 4.0~10.0 at.%. The method for preparing the CrSiC composite coating, which combines high-temperature oxidation resistance and structural thermal stability, includes the following steps: (1) After pickling, polishing with silicon carbide sandpaper and ultrasonic cleaning, the Zry-4 alloy substrate is dried with nitrogen and set aside for later use. (2) The Zry-4 alloy substrate obtained in step (1) is attached to the substrate and placed in the rotating heating stage of the magnetron sputtering chamber. (3) Connect the Si target and C target to the RF power supply respectively, and connect the Cr target to the DC power supply. Adjust the preparation parameters and use the co-sputtering method of magnetron sputtering to prepare the CrSiC composite coating on the substrate obtained in step (2). The CrSiC composite coating, which combines high-temperature oxidation resistance and structural thermal stability, evolved into a structure including an outermost continuous Cr2O3+SiO2 double oxide layer, a ZrC layer formed in situ at the coating / substrate interface, and a Cr3Si layer and a Cr3Si+Cr7C3 dual-phase nanocrystalline layer doped with amorphous SiO2 within the residual coating. The multi-layer structure formed in situ at high temperature by the CrSiC composite coating, which combines high-temperature oxidation resistance and structural thermal stability, enables the coating to exhibit excellent oxidation resistance and comprehensive mechanical properties. It can prevent the zirconium alloy substrate from being oxidized within 2 hours in a 1200℃ water vapor environment and from undergoing a eutectic reaction with the coating and being oxidized within 600 seconds in a 1400℃ steam environment.

2. The CrSiC composite coating with both high-temperature oxidation resistance and structural thermal stability according to claim 1, characterized in that: The thickness of the CrSiC composite coating is 3.8~4.3μm.

3. The method for preparing the CrSiC composite coating with both high-temperature oxidation resistance and structural thermal stability according to claim 1 or 2, characterized in that, Includes the following steps: (1) After pickling, polishing with silicon carbide sandpaper and ultrasonic cleaning, the Zry-4 alloy substrate is dried with nitrogen and set aside for later use. (2) The Zry-4 alloy substrate obtained in step (1) is attached to the substrate and placed in the rotating heating stage of the magnetron sputtering chamber. (3) Connect the Si target and C target to the RF power supply respectively, and connect the Cr target to the DC power supply. Adjust the preparation parameters and use the co-sputtering method of magnetron sputtering to prepare the CrSiC composite coating on the substrate obtained in step (2).

4. The method for preparing the CrSiC composite coating according to claim 3, characterized in that: The solvent for pickling in step (1) is a mixed solution consisting of 10% HNO3, 10% HF and 10% H2O2 by volume; the silicon carbide sandpaper has grit sizes of 100#, 300#, 600#, 1000# and 1500# respectively; the ultrasonic cleaning method is as follows: the Zry-4 alloy substrate is placed in acetone, anhydrous ethanol and deionized water for ultrasonic cleaning for 20 min respectively.

5. The method for preparing the CrSiC composite coating according to claim 3, characterized in that: In step (2), the target-base distance is adjusted to 15cm.

6. The method for preparing the CrSiC composite coating according to claim 3, characterized in that, The Si, C, and Cr targets in step (3) are all 7 mm thick, 3 inches in diameter, and 99.999% pure.

7. The method for preparing the CrSiC composite coating according to claim 3, characterized in that, The preparation parameters in step (3) are: vacuum degree of 6.0 × 10⁻⁶. -4 The deposition temperature is 200℃~300℃, the substrate bias voltage is -50~-100V, the working gas pressure is 0.5~0.6pa, and the deposition time is 1~2h; the sputtering power of Cr target is fixed at 300w, the sputtering power of Si target is 100~200w, and the sputtering power of C target is 100~300w.

8. The application of the CrSiC composite coating with both high-temperature oxidation resistance and structural thermal stability as described in claim 1 or 2, characterized in that: The CrSiC composite coating is used as an anti-oxidation and corrosion coating for the protection of nuclear fuel cladding materials.

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