Gradient CrN / Cr composite coating structure for nuclear fuel cladding and preparation method thereof

Abstract

The present invention mainly provides a gradient CrN / Cr composite coating structure and a preparation method for a nuclear fuel cladding, including: providing a zirconium alloy substrate; forming a ZrN diffusion barrier layer on the surface of the zirconium alloy substrate; forming a Cr gradient transition layer on the ZrN layer; and alternately depositing a CrN / Cr multi-layer structure on the gradient transition layer. The technical solution of the present invention can effectively inhibit the Zr-Cr interdiffusion, avoid the formation of brittle ZrCr2 phase at the substrate / coating interface, and at the same time, the CrN transition layer further strengthens the diffusion barrier effect through chemical stability, improving the structural integrity of the coating in a high-temperature environment; by adopting the design of periodically introducing the CrN layer, the continuous growth of the coarse columnar crystals of the Cr layer is interrupted by introducing new interfaces, forming a densified structure; the wear resistance and crack resistance of the coating are improved through the high hardness characteristics of CrN. At the same time, the high-power pulsed magnetron sputtering technology combined with automatic control can accurately control the coating thickness and structure, ensuring the process repeatability.

Description

Technical Field

[0001] The present invention belongs to the technical field of surface treatment for coating preparation, and particularly relates to a gradient CrN / Cr composite coating structure formed by ion implantation and high-power pulsed magnetron sputtering on a Zr alloy substrate, and a preparation method therefor. Background Art

[0002] At present, due to the advantages of zirconium alloys such as small thermal neutron absorption cross-section, good thermal conductivity, good irradiation stability, water-side corrosion resistance, good mechanical properties, and easy processing, they are widely used in the field of nuclear fuel cladding in water-cooled reactors. However, under the condition of a reactor loss-of-coolant accident, zirconium alloys will undergo a violent oxidation reaction with high-temperature steam, releasing a large amount of heat and hydrogen, triggering an explosion, resulting in a large-scale release of radioactive substances and causing catastrophic consequences. Therefore, it is very necessary to improve the ability of nuclear fuel cladding to resist reactor accidents, that is, to enhance its high-temperature steam oxidation resistance.

[0003] Preparing an antioxidant coating on the surface of commercial mature zirconium alloy cladding has the advantages of short R & D cycle and low cost, showing great potential for engineering applications. Metal Cr can form a relatively dense and well-protective Cr2O3 film layer in an oxidizing medium. At the same time, due to its good compatibility with the zirconium alloy matrix, it has become one of the preferred materials for the cladding surface coating. However, there are some problems that need to be optimized. One is that interdiffusion is likely to occur at the Zr-Cr interface. Especially in a high-temperature environment, a ZrCr2 interdiffusion layer is easily formed in the coating-substrate interface region. ZrCr2 is a brittle Laves phase, which has a great negative impact on the mechanical properties of the coating material. At the same time, it will also cause the loss of the Cr coating material, resulting in a reduction or even failure of the coating protection effect.

[0004] On the other hand, as a protective coating for nuclear fuel accident-tolerant cladding, there are certain requirements for the thickness, generally about 15 μm, which is a relatively thick coating for magnetron sputtering technology. The metal pure Cr coating prepared by magnetron sputtering usually shows a columnar crystal growth morphology. The continuous grain boundaries of thick columnar crystals are a kind of defect, which is prone to cause cracking and peeling of the coating under external force. In addition, such grain boundaries are prone to allow oxygen elements to diffuse to the substrate, triggering the failure of the coating. To obtain a dense coating, if only the substrate bias voltage is increased, the internal stress of the coating will be greatly increased, and the coating is prone to adhesion failure and peeling. Summary of the Invention

[0005] In order to solve the above problems, the present invention provides a gradient CrN / Cr composite coating structure for nuclear fuel cladding, including:

[0006] A zirconium alloy substrate;

[0007] A ZrN diffusion barrier layer is provided on the surface of the zirconium alloy substrate, wherein the thickness of the ZrN diffusion barrier layer is 50-200 nm;

[0008] A Cr gradient transition layer located on the ZrN layer, wherein the gradient transition layer has a total thickness of 100-200 nm;

[0009] A CrN / Cr multilayer structure is alternately deposited on the gradient transition layer, wherein the thickness of a single CrN layer is 50-100 nm, and the thickness of a single Cr layer is 300-500 nm.

[0010] Furthermore, the Cr gradient transition layer is Cr (x-1) N x -CrN-Cr x N (1-x) Transition layer, where 0<X≤1.0.

[0011] Furthermore, the nitrogen atomic concentration in the ZrN diffusion barrier layer is distributed in a gradient, with the nitrogen content on the surface being 15-25 at.%, and the nitrogen content at the interface being 5-10 at.%.

[0012] Furthermore, in the CrN / Cr multilayer structure, the thickness ratio of the Cr layer to the CrN layer is 4:1 to 6:1.

[0013] The present invention also provides a method for preparing the composite coating structure, comprising the following steps:

[0014] Step 1: performing ion implantation on the zirconium alloy substrate, wherein the implanted element is N, so as to form a ZrN diffusion barrier layer on the surface of the zirconium alloy;

[0015] Step 2: CrN with a gradient of N content is plated on the surface of the ZrN diffusion barrier layer by high-power pulsed magnetron sputtering to form Cr (x-1) N x Gradient transition layer;

[0016] Step 3: Periodically adjust the nitrogen flow rate to alternately deposit CrN layers and Cr layers, with the amount of N gradient changing the Cr (x-1) N x A CrN / Cr multilayer structure is formed on the transition layer.

[0017] Furthermore, in step 1, the ion implantation energy is 50-100 keV and the dose is 1×10¹ 7 -5×10¹ 7 ions / cm².

[0018] Furthermore, in step 2, high pulse magnetron sputtering is carried out at a background vacuum of ≤5×10 -5Under Pa conditions, the N2 flow rate is regulated to form a Cr transition layer with a gradient change in N content on the surface of the ZrN diffusion barrier layer. (x-1) N x transition layer.

[0019] Furthermore, in step 3, when depositing the CrN layer and the Cr layer, the substrate bias voltage is -50V to -150V, and the deposition temperature is 200 - 400°C.

[0020] Therefore, the beneficial effects of the present invention are as follows: effectively suppressing the Zr-Cr interdiffusion, forming a ZrN layer on the substrate surface by ion implantation of N element, and combining with the gradient design of the CrN transition layer to significantly slow down the interfacial interdiffusion between Zr and Cr at high temperatures. The ZrN layer can hinder the migration of Cr atoms to the substrate and avoid the formation of brittle ZrCr2 phase. At the same time, the CrN transition layer further strengthens the diffusion barrier effect through chemical stability and improves the structural integrity of the coating in high-temperature environments.

[0021] Optimizing the microstructure and mechanical properties of the coating, adopting the design of periodically introducing the CrN layer (thickness 50 - 100 nm), interrupting the continuous growth of the coarse columnar crystals of the traditional Cr coating, and forming a densified structure. This method not only reduces grain boundary defects and inhibits oxygen diffusion channels, but also improves the wear resistance and crack resistance of the coating through the high hardness characteristics of CrN.

[0022] Improving the bonding strength and process stability: The CrN-CrN-CrN gradient transition layer realizes the composition gradient through reactive sputtering, enhances the bonding force between the coating and the substrate, and avoids the peeling problem caused by internal stress concentration. At the same time, the high-power pulsed magnetron sputtering technology combined with automated control can accurately regulate the coating thickness and structure, ensuring the process repeatability. x N-CrN-Cr x N gradient transition layer realizes the composition gradient through reactive sputtering, enhances the bonding force between the coating and the substrate, and avoids the peeling problem caused by internal stress concentration. At the same time, the high-power pulsed magnetron sputtering technology combined with automated control can accurately regulate the coating thickness and structure, ensuring the process repeatability. Description of the Drawings

[0023] Figure 1 It is a surface view of the composite coating structure of the present invention.

[0024] Figure 2 It is a cross-sectional view of the composite coating structure of the present invention.

[0025] Figure 3 It is a cross-sectional view of the composite coating structure of the present invention after high-temperature steam oxidation test.

[0026] Figure 4 It is a comparison diagram after scratch test. Detailed Embodiments

[0027] To make the technical problems, technical solutions and beneficial effects to be solved by the present invention clearer and more understandable, please refer to the accompanying drawings and embodiments for a further detailed description of the present invention. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. Example

[0028] First, a Zr-4 zirconium alloy plate with dimensions of 30×30×2 mm was selected as the substrate. After mechanical polishing to make the surface roughness Ra≤0.1 μm, acetone ultrasonic cleaning and nitrogen drying treatments were carried out in sequence. The pretreated zirconium alloy substrate was placed in an ion implantation device, and under a vacuum of 5×10 -5 Pa, nitrogen ions with an energy of 80 keV and a dose of 3×10 17 ions / cm² were implanted. The substrate temperature was controlled at 250 °C to form a ZrN diffusion barrier layer with a thickness of 100 nm on the surface, where the nitrogen atom content in the surface layer was 20.5 at.%, and the nitrogen content gradient in the interface transition region decreased to 8.2 at.%.

[0029] Subsequently, high-power pulsed magnetron sputtering (HiPIMS) was used to deposit a gradient transition layer. In a working chamber with a base vacuum of 5×10 -5 Pa, the Cr x N phase transformation was controlled by dynamically regulating the nitrogen flow rate: in the initial stage, the nitrogen flow rate was linearly increased from 0 sccm to 40 sccm to deposit a 75 nm thick Cr (x-1) N x layer, where 0 < X ≤ 1.0; in the second stage, the nitrogen flow rate was continuously linearly decreased from 40 sccm to 0 sccm to form a 75 nm thick Cr x N (1-x) layer (0 < X ≤ 1.0); finally, a Cr (x-1) N x -CrN-Cr x N (1-x) gradient transition layer was formed, and the total thickness of this gradient transition layer was 150 nm.

[0030] During the alternating deposition phase, the substrate bias voltage was set at -100V and the temperature was 300°C. The nitrogen supply was periodically switched to achieve layered growth of Cr and CrN. During Cr deposition, nitrogen was turned off, and the argon flow rate was maintained at 80 sccm, resulting in a single layer thickness of 400nm. During CrN deposition, 40 sccm of nitrogen and 60 sccm of argon were used, with a single layer thickness of 80nm. After 12 co-deposition cycles, a multilayer structure with a total thickness of 5.76μm was obtained, with a strictly controlled Cr to CrN thickness ratio of 5:1. Transmission electron microscopy (TEM) observations revealed uniform grain size within the coating (30-50nm), with no brittle ZrCr2 phase detected at the interface. Scratch testing revealed a critical bond load of 42N, a 180% increase compared to untreated zirconium alloy. After high-temperature steam oxidation at 1200°C for 600 seconds, the oxidation weight gain was only 2.3mg / cm², demonstrating an 89% improvement in oxidation resistance. Example

[0031] A tubular zirconium alloy fuel cladding (Φ10mm×0.5mm) was selected as the substrate, and after electrolytic polishing, the surface roughness Ra≤0.05μm. A dual-energy injection strategy was used in the ion implantation process: first, 1×10¹ was injected at 50keV energy. 7 ions / cm² dose of nitrogen ions, followed by an additional implantation of 5×10¹ 6 ions / cm², forming a gradient ZrN layer with a thickness of about 60nm on the surface. The peak nitrogen concentration in the near-surface area reaches 18.7at.%, and the concentration gradient extends to a depth of 15μm inside the substrate.

[0032] During the magnetron sputtering deposition stage, the thinning design was achieved by optimizing the process parameters. The transition layer was introduced using the x linear increase and decrease nitrogen flow method: the nitrogen flow rate was linearly increased from 0 sccm to 40 sccm in the initial stage, and a 75nm thick Cr layer was deposited. (x-1) N x layer (0<X≤1.0); in the second stage, the nitrogen flow rate is further reduced linearly from 40sccm to 0sccm to form a 75nm thick Cr x N (1-x) layer (0<X≤1.0); finally forming a 150nn thick Cr (x-1) N x -CrN-Cr x N (1-x) The gradient transition layer is formed. During the alternating deposition, the thickness of the Cr layer was reduced to 300nm (deposition rate 18nm / min), and the thickness of the CrN layer was controlled to 60nm (deposition rate 6nm / min). After 15 cycles of deposition, the total thickness reached 5.4μm. Specifically, a trace amount of nitrogen (0.5sccm) was introduced during the deposition of the last three layers, increasing the nitrogen content of the surface Cr phase to 3at.%, forming a surface hardened layer.

[0033] The grain size inside the coating is controlled within the range of 20 - 50 nm. A high-temperature steam oxidation experiment is carried out at 1200 °C for 1 h. Only a 3.05-μm-thick Cr2O3 oxide layer is formed on the coating surface, and no peeling phenomenon occurs. The additional thermal neutron absorption cross-section introduced by the coating is less than 0.1 barn, meeting the operating requirements of nuclear reactors.

[0034] The technical solution of the present invention has the following advantages:

[0035] 1. Breakthrough improvement in high-temperature oxidation resistance

[0036] By constructing a gradient CrN / Cr composite coating structure, the present invention demonstrates excellent protection ability under the simulated reactor loss-of-coolant accident conditions (1200 °C high-temperature steam environment). The data of Example 1 show that the oxidation weight gain of the coating specimen is only 2.3 mg / cm², which is 87.6% lower than that of the uncoated zirconium alloy (18.6 mg / cm²), fundamentally suppressing the violent reaction between zirconium alloy and high-temperature steam. This protective effect stems from the synergistic effect of the gradient transition layer: the ZrN diffusion barrier layer (50 - 200 nm) suppresses the oxygen diffusion rate to the order of 1×10⁻¹ 6 m² / s, two orders of magnitude lower than that of traditional Cr coatings; while the CrN x -CrN gradient transition layer (X = 0~1) blocks the rapid penetration path of the oxidation medium along the grain boundaries through the stepwise change of the nitrogen content. During the 600-second accident tolerance test, the coating integrity retention rate reaches 98%, providing a critical time window for nuclear safety emergency response.

[0037] 2. Dual optimization of the coating structure design

[0038] Aiming at the failure mechanism of zirconium alloy cladding, the present invention innovatively adopts a multi-layer composite structure design: in terms of mutual diffusion inhibition, the gradient ZrN layer formed by ion implantation (surface nitrogen content 15 - 25 at.%, interface 5 - 10 at.%) makes the thickness of the Zr-Cr mutual diffusion layer less than 5 nm, reducing by 90% compared with conventional coatings, effectively avoiding the formation of brittle ZrCr2 phase; in terms of crystal defect control, the CrN / Cr alternating deposition layer (thickness ratio 4:1 - 6:1) refines the columnar crystal size to 30 - 50 nm through periodic interface insertion, and the coating density reaches 99.2%, with the porosity lower than 0.5%. This structure reduces the crack growth rate of the coating under thermal cyclic loading to 1×10⁻ 8 m / cycle, reaching the protection level of coatings for aero-engine turbine blades.

[0039] 3. Synergistic enhancement of mechanical properties

[0040] The coating system of the present invention has achieved remarkable breakthroughs in toughening: in terms of interface bonding strength, ion implantation pretreatment has increased the critical bonding load of the coating to 42 - 45 N, which is 180% - 200% higher than that of untreated zirconium alloy (15 N), meeting the first-level requirements in the nuclear power structural material standards; in terms of toughness matching, the alternating combination of the nanocrystalline Cr layer (hardness 9 - 10 GPa) prepared by HiPIMS process and the CrN hard phase (hardness 22 - 25 GPa) has made the hardness of the composite coating reach 18 - 19 GPa (nanoindentation), and the fracture toughness KIC value reach 4.5 MPa·m¹ / ², which is 120% higher than that of a single Cr coating; in terms of thermal shock resistance, through the regulation of the vacuum annealing process at 400 - 550 °C, the residual stress of the coating has been reduced from -3.5 GPa to -1.2 GPa, and it still remains intact after 2000 water quench cycles between 500 °C and room temperature, meeting the mechanical properties and thermal shock resistance of nuclear fuel assemblies under normal conditions.

[0041] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A gradient CrN / Cr composite coating structure for nuclear fuel cladding, characterized in that, Comprising: A zirconium alloy substrate; A ZrN diffusion barrier layer disposed on the surface of the zirconium alloy substrate, the thickness of the ZrN diffusion barrier layer being 50 - 200 nm; A Cr gradient transition layer located above the ZrN layer, the total thickness of the gradient transition layer being 100 - 200 nm; A CrN / Cr multilayer structure alternately deposited on the gradient transition layer, wherein the single-layer thickness of the CrN layer is 50 - 100 nm and the single-layer thickness of the Cr layer is 300 - 500 nm; The Cr gradient transition layer is deposited by high-power pulsed magnetron sputtering to achieve phase change control by dynamically regulating the nitrogen flow rate: in the initial stage, the nitrogen flow rate linearly increases from 0 sccm to 40 sccm, and in the second stage, the nitrogen flow rate continues to linearly decrease from 40 sccm to 0 sccm, finally forming a gradient transition layer.

2. The gradient CrN / Cr composite coating structure for nuclear fuel cladding according to claim 1, wherein In the ZrN diffusion barrier layer, there is a nitrogen atom concentration gradient distribution, the surface nitrogen content being 15 - 25 at.%, and the nitrogen content at the interface being 5 - 10 at.%.

3. A gradient CrN / Cr composite coating structure for nuclear fuel cladding according to claim 1, characterized in that, In the CrN / Cr multilayer structure, the thickness ratio of the Cr layer to the CrN layer is 4:1 to 6:

1.

4. A method for preparing a CrN / Cr composite coating structure according to any one of claims 1-3, characterized in that, Including the following steps: Step 1: Perform ion implantation treatment on the zirconium alloy substrate, the implanted element being N, to form a ZrN diffusion barrier layer on the surface of the zirconium alloy; Step 2: Dynamically regulate the nitrogen flow rate to achieve a gradient change in N content, and deposit CrN with a gradient change in N content on the surface of the ZrN diffusion barrier layer by high-power pulsed magnetron sputtering. In the initial stage, the nitrogen flow rate linearly increases from 0 sccm to 40 sccm, and in the second stage, the nitrogen flow rate continues to linearly decrease from 40 sccm to 0 sccm, finally forming a gradient transition layer; Step 3: Periodically adjust the nitrogen flow rate to alternately deposit the CrN layer and the Cr layer to form a CrN / Cr multilayer structure on the Cr gradient transition layer.

5. The preparation method according to claim 4, wherein In the step 1, the ion implantation energy is 50 - 100 keV, and the dose is 1×10¹ 7 -5×10¹ 7 ions / cm².

6. The preparation method according to claim 4, characterized in that, In the step 2, high-power pulsed magnetron sputtering is carried out under the condition of background vacuum degree ≤ 5×10 -5 Pa, regulating the flow rates of Ar and N2, maintaining the working pressure at 0.5 Pa, so as to form a gradient transition layer on the surface of the ZrN diffusion barrier layer.

7. The preparation method according to claim 4, characterized in that, In Step 3, when depositing the CrN layer and the Cr layer, the substrate bias voltage is -50 V to -150 V, and the deposition temperature is 200 - 400 °C.

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