A method for preparing an alumina ceramic coating on a surface of a guide roll of a paper machine

Abstract

This invention discloses a method for preparing an alumina ceramic coating on the surface of a paper machine guide roll, including substrate pretreatment, preparation of a gradient composite transition layer, ceramic surface coating, and post-treatment; the gradient composite transition layer is sequentially coated with the following three layers or forms a continuous gradient: (1) the first transition sublayer uses 100wt% NiCrAl alloy powder, with a coating thickness of 30-50μm; (2) the second transition sublayer uses a mixture of 70wt% NiCrAl and 30wt% Al2O3 powder, with a coating thickness of 20-40μm; (3) the third transition sublayer uses a mixture of 50wt% NiCrAl and 50wt% Al2O3 powder, with a coating thickness of 20-40μm; the total thickness of the gradient composite transition layer is controlled at 80-120μm. The service life of the paper machine guide roll using this invention is extended by 30%-50% compared to conventional double-layer ceramic coated guide rolls.

Description

Technical Field

[0001] This invention belongs to the field of metal surface treatment technology, specifically relating to a method for preparing an alumina ceramic coating on the surface of a paper machine guide roller, applicable to surface strengthening of guide rollers in various sections of a paper machine such as forming section, pressing section, and drying section. Background Technology

[0002] In papermaking, guide rollers are core components controlling paper transport, forming, and drying. Their surface performance directly determines the paper's basis weight stability, surface smoothness, and production continuity. Currently, the industry commonly uses a rubber-coating process to treat the guide roller surface. This process attaches a rubber layer to the steel substrate surface through vulcanization. While it offers advantages such as low initial cost and ease of construction, it suffers from unavoidable key drawbacks in practical applications: (1) Short service life and frequent maintenance: The rubber material has low hardness (Shore A 60-80), and after long-term friction with paper and felt, it is easy to wear, wrinkle or even peel off. The average service life is only 3-6 months. Each time the rubber coating layer is replaced, the machine needs to be stopped for 8-12 hours, and the annual cumulative downtime exceeds 100 hours, which seriously disrupts the continuity of production. In addition, each replacement requires labor and rubber material costs, and the annual maintenance cost exceeds 30,000 yuan per guide roller.

[0003] (2) Poor corrosion resistance and limited adaptability to working conditions: In the papermaking process, pulp often contains acidic (pH 3-4) or alkaline (pH 9-10) additives. In the drying section, bleaching agents such as sodium hypochlorite are also required. Rubber is easily corroded and swollen in this environment. The roller surface will be deformed after 1-2 months, resulting in uneven force during paper transfer, expanding the basis weight fluctuation range to ±5%, and significantly increasing the scrap rate.

[0004] (3) Insufficient thermal conductivity, which easily leads to paper defects: The thermal conductivity of rubber is only 0.15 to 0.2 W / (m·K). The frictional heat generated during the operation of the guide roller cannot be dissipated quickly, which easily forms local high temperature (above 120°C) on the roller surface, resulting in uneven paper drying and scorch spots. This problem is more prominent in high-speed paper production lines (speed > 1000 m / min), reducing the rate of high-quality products by 15% to 20%.

[0005] In existing technologies, although some studies have attempted to improve performance by modifying rubber (such as adding wear-resistant particles) or increasing rubber thickness, the corrosion resistance of the modified rubber still cannot adapt to strong acid and alkali conditions, and increasing the thickness further reduces thermal conductivity. The balance between wear resistance, corrosion resistance, and thermal conductivity has not yet been achieved, failing to meet the paper industry's requirements for the long-term stable operation of guide rollers. Therefore, a guide roller surface treatment process that can fundamentally solve the above-mentioned defects is urgently needed.

[0006] To improve the wear resistance and crack resistance of rolling mill guide rolls, plasma cladding technology is usually used to clad an Al2O3-doped Fe-Ni-based high-temperature wear-resistant alloy coating on the surface of 45 steel. The process flow is as follows: 45 round steel blank → guide roll blank → plasma cladding → guide roll surface grinding treatment → plasma alloy guide roll.

[0007] Patent document CN114774829A discloses an insulating ceramic coating for a roller used in an electrolytic copper foil surface treatment machine and its manufacturing method. The insulating ceramic coating comprises, from bottom to top, a base layer, an intermediate layer, and a top layer. The base layer is formed using Hastelloy C276 alloy powder via supersonic flame spraying, with a thickness of 50–100 µm. The intermediate layer is formed using a mixture of Hastelloy C276 and alumina-titanium oxide powder via plasma spraying, with a thickness of 100–200 µm. The top layer is formed using alumina powder via plasma spraying, with a thickness of 500–2000 µm. The primary objective of this document is electrical insulation, and all designs (thick coating, sealing treatment) serve this purpose.

[0008] Patent document CN111254380A discloses a coating process for the surface of rollers used for aluminum-silicon plating on steel plates. The coating includes a base layer and a top layer resistant to aluminum melt corrosion. The powder raw material of the base layer is a nickel-chromium alloy (NiCr) and molybdenum boride (MoB) metal alloy powder; the composition content is MoB: 60%–80%, NiCr: 20%–40%, formed by mechanical mixing or agglomeration sintering; the top layer is pure alumina powder with an alumina content ≥99%, formed by agglomeration sintering. However, the main failure mode of this document is severe corrosion and aluminum adhesion from high-temperature molten aluminum. It is a dedicated coating system designed to solve the single core problem of extreme high-temperature melt corrosion (especially the introduction of MoB).

[0009] Patent document CN102211484A discloses a process for spraying a ceramic coating onto the surface of a printing roller. It uses a high-energy plasma device to spray alumina ceramic powder to a thickness of 0.30 mm, followed by grinding. However, this document describes a simple single-layer ceramic coating application with relatively simple operating conditions. It primarily focuses on wear resistance and surface precision, without considering multi-layer gradient design or optimization for complex corrosion and thermal fatigue conditions. Summary of the Invention

[0010] To address the shortcomings of existing rubber coating processes, such as short lifespan, poor corrosion resistance, insufficient thermal conductivity, and the tendency of existing ceramic coatings to crack and peel due to mismatched coefficients of thermal expansion, this invention provides a method for preparing an alumina ceramic coating on the surface of paper machine guide rollers. By constructing a gradient layer with a continuously transitioning coefficient of thermal expansion, the thermal shock resistance and bonding strength of the coating system are significantly improved, thereby meeting the requirements for long-term stable operation of guide rollers under complex papermaking conditions (long lifespan, high corrosion resistance, strong thermal conductivity, and low maintenance).

[0011] To achieve the above objectives, the present invention adopts the following specific technical solution: The present invention discloses a method for preparing an alumina ceramic coating on the surface of a paper machine guide roller, comprising the steps of substrate (45# steel or stainless steel) pretreatment, gradient composite transition layer preparation, ceramic surface layer spraying and post-treatment; The gradient composite transition layer is applied using plasma spraying technology. At the same work station, the following three layers are sprayed sequentially by continuously changing the powder feeding ratio, or a continuous gradient is formed: (1) The first transition sublayer uses 100wt% NiCrAl alloy powder, with a spraying thickness of 30-50μm; (2) The second transition sublayer uses a mixture of 70wt% NiCrAl and 30wt% Al2O3 powder, with a spraying thickness of 20-40μm; (3) The third transition sublayer uses a mixture of 50wt% NiCrAl and 50wt% Al2O3 powder, with a spraying thickness of 20-40μm; The total thickness of the gradient composite transition layer is controlled at 80-120μm. The ceramic surface layer is coated with supersonic plasma spraying. The spraying material is Al2O3 powder or Al2O3-TiO2 composite powder, with a thickness of 200-500 μm and a porosity of ≤3%.

[0012] The NiCrAl alloy powder has the following composition: Ni 70%–75%, Cr 15%–20%, Al 5%–10%, and unavoidable impurities, with a particle size of 20–50 μm; the coefficient of thermal expansion of this alloy powder material is 10–11 × 10⁻⁶. -6 / ℃) is between ceramic (7~9×10) -6 / ℃) and steel substrate (12~14×10 -6 The temperature range is between 10°C and 10°C, which can buffer the stress of thermal cycling; the purity of the Al2O3 powder is ≥99.5%, and the particle size is 15~45μm; the mass fraction of TiO2 in the Al2O3-TiO2 composite powder is 10%~20%.

[0013] Preferably, the spraying parameters for the gradient composite transition layer are: Ar / H2 plasma gas flow rate 40–60 L / min, current 280–450 A, voltage 75–85 V, powder feed rate 30–60 g / min, spraying distance 100–150 mm, and spray gun moving speed 30–60 mm / s. The three-layer spraying of the gradient composite transition layer is performed continuously. The mixing ratio of NiCrAl powder and Al2O3 powder is adjusted in real time by a dual-path powder feeder to achieve a smooth transition of components, with no obvious abrupt changes in composition between adjacent sublayers.

[0014] Preferably, the spraying parameters for the ceramic surface coating are: spraying pressure 0.8-1.0 MPa, spraying distance 120-150 mm, spray gun moving speed 50-80 mm / s, spraying in 2-3 coats, single coat thickness 100-200 μm, final coating thickness 200-500 μm, and porosity ≤3% to avoid the penetration of pulp impurities.

[0015] The post-processing involves using diamond grinding wheels for fine grinding and alumina polishing paste for polishing, so that the roundness error of the guide roller is ≤0.01 mm and the surface finish of the roller is Ra0.8~1.6μm.

[0016] Preferably, the matrix pretreatment includes: (1) Degreasing: Soak or wipe the surface of the guide roller with acetone or ethanol to remove oil stains; (2) Sandblasting roughening: Use brown corundum abrasive with a particle size of 0.5-1mm, sandblasting pressure of 0.6-0.8MPa, and spraying angle of 45-60° to make the surface roughness of the substrate reach Ra 5-8μm, forming a uniform texture, providing sufficient "interlocking points" for the transition layer, and improving the coating adhesion.

[0017] The post-processing includes: (1) fine grinding: using a diamond grinding wheel with a particle size of 800-1000 mesh, grinding the coating surface at a speed of 300-500 r / min to correct the small deviations in the spraying process, so that the roundness error of the guide roller is ≤0.01mm; (2) polishing: using alumina polishing paste with a particle size of 1500-2000 mesh, polishing pressure of 0.1-0.2MPa, so that the surface finish Ra of the roller reaches 0.8-1.6μm.

[0018] The present invention also provides a paper machine guide roll prepared by the method described above for preparing an alumina ceramic coating on the surface of a paper machine guide roll. The surface of the guide roll, from the inside out, consists of: a substrate, a gradient composite transition layer, and an alumina ceramic surface layer. The gradient composite transition layer transitions from NiCrAl to NiCrAl-Al2O3, with a total thickness of 80–120 μm. The ceramic surface layer has a thickness of 200–500 μm, a porosity of ≤3%, a roll surface roundness error of ≤0.01 mm, and a surface finish Ra of 0.8–1.6 μm.

[0019] Compared with the prior art, the present invention has the following significant advantages: (1) Eliminating abrupt changes in the coefficient of thermal expansion, significantly improving thermal shock resistance: This invention improves thermal shock resistance by designing a material from NiCrAl (with a coefficient of thermal expansion of approximately 12 × 10⁻⁶). -6 (at / ℃) transitions through a gradient layer to a ceramic surface layer (thermal expansion coefficient approximately 8×10⁻⁶). -6The compositional gradient (at / ℃) eliminates the abrupt interface present in traditional double-layer structures (metal bottom layer + ceramic top layer), reducing residual thermal stress by more than 50%. Thermal shock tests (alternating water cooling at 300℃ and 25℃) show that the gradient composite transition layer of this invention can withstand more than 50 cycles without cracking, while conventional NiCrAl single-layer transition layers peel off within 20 cycles.

[0020] (2) Significantly improved bonding strength: The continuous gradient structure improves the bonding between the coating and the substrate, and between the coating layers, from "mechanical interlocking" to "metallurgical-mechanical composite bonding". The measured bonding strength is ≥70MPa, which is better than the 50-60MPa of conventional double-layer structure, ensuring that the coating does not peel off under high speed, high humidity and impact conditions.

[0021] (3) Good process compatibility and controllable cost: The gradient transition layer of the present invention can be directly implemented on existing plasma spraying equipment by adjusting the powder feeding system. No additional workstations or complex tooling are required. Only dual-path powder feeders or premixed powders are needed. The process changes are small and the cost increase is limited, making it suitable for industrial promotion.

[0022] (4) Extend the service life of the guide roller and reduce the total life cycle cost: The paper machine guide roller of the present invention is continuously tested under simulated papermaking conditions (including acid and alkali pulp, filler wear and temperature cycle). The service life is 3 to 8 times longer than that of traditional rubber-coated guide rollers and 30% to 50% longer than that of conventional double-layer ceramic-coated guide rollers. The annual downtime maintenance time is reduced to less than 10 hours and the total life cycle cost is reduced by more than 60%. Detailed Implementation

[0023] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1 A method for preparing an alumina ceramic coating on the surface of a paper machine guide roller includes the following steps: substrate (45# steel) pretreatment, gradient composite transition layer preparation, ceramic surface layer spraying, and post-treatment. S1, Matrix pretreatment: (1) Degreasing: Use acetone or ethanol to soak or wipe the surface of the guide roller to remove oil stains that adhered during production and storage, so as to avoid poor bonding between the subsequent coating and the substrate; (2) Sandblasting roughening: Brown corundum abrasive with a particle size of 0.8mm is used, sandblasting pressure is 0.7MPa, and spraying angle is 60° to thoroughly remove oxide scale and rust. At the same time, the sandblasting parameters are controlled to make the surface roughness of the substrate reach Ra 7μm, forming a uniform texture, providing sufficient "interlocking points" for the transition layer, and improving the coating adhesion. S2, Preparation of gradient composite transition layer: The gradient composite transition layer adopts plasma spraying technology. The following three layers are sprayed sequentially or a continuous gradient is formed by continuously changing the powder feeding ratio at the same station; (1) The first transition sublayer uses 100wt% NiCrAl alloy powder with a spraying thickness of 40μm; (2) The second transition sublayer uses a mixture of 70wt% NiCrAl and 30wt% Al2O3 powder with a spraying thickness of 20μm; (3) The third transition sublayer uses a mixture of 50wt% NiCrAl and 50wt% Al2O3 powder with a spraying thickness of 30μm; The total thickness of the gradient composite transition layer is controlled at 90μm; The mass composition of NiCrAl alloy powder is: Ni 73%, Cr 18%, Al 9% and unavoidable impurities, with an average particle size of 34μm; the purity of Al2O3 powder is ≥99.5%, with an average particle size of 30μm; the mass fraction of TiO2 in Al2O3-TiO2 composite powder is 15%.

[0025] The spraying parameters for the gradient composite transition layer are: plasma gas Ar / H2 flow rate 50L / min, current 300A, voltage 80V, powder feed rate 57g / min, spraying distance 120mm, and spray gun moving speed 45mm / s. The three-layer spraying of the gradient composite transition layer is carried out continuously. The mixing ratio of NiCrAl powder and Al2O3 powder is adjusted in real time by a dual-path powder feeder to achieve a smooth transition of components, with no obvious abrupt change in components between adjacent sublayers. S3, Ceramic Surface Coating: Supersonic plasma spraying is used, and the spraying material is Al2O3 powder. The spraying parameters for the ceramic surface coating are: spraying pressure 0.9MPa, spraying distance 130mm, spray gun moving speed 60 mm / s, sprayed in 2 coats, with a single coat thickness of 175μm and a final thickness of 350μm. The porosity is 2.7%, which avoids the penetration of pulp impurities. S4, Post-processing: (1) Fine grinding: Diamond grinding wheels with a particle size of 800 mesh and 1000 mesh are used to grind the coating surface at a speed of 500 r / min and 300 r / min respectively to correct the small deviations in the spraying process and make the roundness error of the guide roller ≤0.01mm; (2) Polishing: Alumina polishing paste with a particle size of 1800 mesh and a polishing pressure of 0.15MPa is used to make the surface finish Ra of the roller reach 1.2μm.

[0026] Example 2 A method for preparing an alumina ceramic coating on the surface of a paper machine guide roller includes the following steps: substrate (45# steel) pretreatment, gradient composite transition layer preparation, ceramic surface layer spraying, and post-treatment. S1, Matrix pretreatment: (1) Degreasing: Use acetone or ethanol to soak or wipe the surface of the guide roller to remove oil stains that adhered during production and storage, so as to avoid poor bonding between the subsequent coating and the substrate; (2) Sandblasting roughening: Brown corundum abrasive with a particle size of 1.0 mm is used, sandblasting pressure is 0.6 MPa, and spraying angle is 45° to thoroughly remove oxide scale and rust. At the same time, the sandblasting parameters are controlled to make the surface roughness of the substrate reach Ra 5.5 μm, forming a uniform texture, providing sufficient "interlocking points" for the transition layer, and improving the coating adhesion. S2, Preparation of gradient composite transition layer: The gradient composite transition layer adopts plasma spraying technology. The following three layers are sprayed sequentially or a continuous gradient is formed by continuously changing the powder feeding ratio at the same station; (1) The first transition sublayer uses 100wt% NiCrAl alloy powder and the spraying thickness is 30μm; (2) The second transition sublayer uses a mixture of 70wt% NiCrAl and 30wt% Al2O3 powder and the spraying thickness is 40μm; (3) The third transition sublayer uses a mixture of 50wt% NiCrAl and 50wt% Al2O3 powder and the spraying thickness is 30μm; The total thickness of the gradient composite transition layer is controlled at 100μm; The mass composition of NiCrAl alloy powder is: Ni 74%, Cr 16%, Al 10% and unavoidable impurities, with an average particle size of 22μm; the purity of Al2O3 powder is ≥99.5%, and the average particle size is 16μm. The spraying parameters for the gradient composite transition layer are: plasma gas Ar / H2 flow rate 58L / min, current 400A, voltage 75V, powder feed rate 35g / min, spraying distance 100mm, and spray gun moving speed 35mm / s. The three-layer spraying of the gradient composite transition layer is carried out continuously. The mixing ratio of NiCrAl powder and Al2O3 powder is adjusted in real time by a dual-path powder feeder to achieve a smooth transition of components, with no obvious abrupt change in components between adjacent sublayers. S3, Ceramic Surface Coating: Supersonic plasma spraying is used, and the spraying material is Al2O3-TiO2 composite powder, in which the mass fraction of TiO2 in the Al2O3-TiO2 composite powder is 18%; the spraying parameters for ceramic surface coating are: spraying pressure 1.0MPa, spraying distance 140mm, spray gun moving speed 75mm / s, sprayed in 3 coats, with a single coat thickness of 120μm, and a final coating thickness of 360μm and a porosity of 2.6%, to avoid the penetration of pulp impurities; S4, Post-processing: (1) Fine grinding: Use diamond grinding wheels with a particle size of 800 mesh and 1000 mesh respectively to grind the coating surface at a speed of 400 r / min to correct the small deviations in the spraying process and make the roundness error of the guide roller ≤0.01mm; (2) Polishing: Use alumina polishing paste with a particle size of 2000 mesh and a polishing pressure of 0.18MPa to make the surface finish Ra of the roller reach 1.0μm.

[0027] Example 3 A method for preparing an alumina ceramic coating on the surface of a paper machine guide roller includes the following steps: substrate (45# steel) pretreatment, gradient composite transition layer preparation, ceramic surface layer spraying, and post-treatment. S1, Matrix pretreatment: (1) Degreasing: Use acetone or ethanol to soak or wipe the surface of the guide roller to remove oil stains that adhered during production and storage, so as to avoid poor bonding between the subsequent coating and the substrate; (2) Sandblasting roughening: Use brown corundum abrasive with a particle size of 0.5-1mm, sandblasting pressure of 0.6-0.8MPa, and spraying angle of 45-60° to thoroughly remove oxide scale and rust. At the same time, control the sandblasting parameters to make the surface roughness of the substrate reach Ra 5-8μm, forming a uniform texture, providing sufficient "interlocking points" for the transition layer, and improving the coating adhesion. S2, Preparation of gradient composite transition layer: The gradient composite transition layer adopts plasma spraying technology. The following three layers are sprayed sequentially or a continuous gradient is formed by continuously changing the powder feeding ratio at the same station; (1) The first transition sublayer uses 100wt% NiCrAl alloy powder with a spraying thickness of 50μm; (2) The second transition sublayer uses a mixture of 70wt% NiCrAl and 30wt% Al2O3 powder with a spraying thickness of 20μm; (3) The third transition sublayer uses a mixture of 50wt% NiCrAl and 50wt% Al2O3 powder with a spraying thickness of 20μm; The total thickness of the gradient composite transition layer is controlled at 90μm; The mass composition of NiCrAl alloy powder is: Ni 73%, Cr 18%, Al 9% and unavoidable impurities, with a particle size of 20-50 μm; the purity of Al2O3 powder is ≥99.5%, with a particle size of 15-45 μm; the mass fraction of TiO2 in Al2O3-TiO2 composite powder is 15%.

[0028] The spraying parameters for the gradient composite transition layer are: plasma gas Ar / H2 flow rate 40L / min, current 280A, voltage 85V, powder feed rate 60g / min, spraying distance 145mm, and spray gun moving speed 55mm / s. The three-layer spraying of the gradient composite transition layer is carried out continuously. The mixing ratio of NiCrAl powder and Al2O3 powder is adjusted in real time by a dual-path powder feeder to achieve a smooth transition of components, with no obvious abrupt change in components between adjacent sublayers. S3, Ceramic Surface Coating: Supersonic plasma spraying is used, and the spraying material is Al2O3 powder. The spraying parameters for the ceramic surface coating are: spraying pressure 0.8MPa, spraying distance 120mm, spray gun moving speed 50 mm / s, sprayed in 3 coats, with a single coat thickness of 150μm, and a final coating thickness of 450μm and a porosity of 2.4%, to prevent pulp impurities from seeping in. S4, Post-processing: (1) Fine grinding: Use a diamond grinding wheel with a particle size of 800-1000 mesh to grind the coating surface at a speed of 300-500 r / min to correct the small deviations in the spraying process and make the roundness error of the guide roller ≤0.01mm; (2) Polishing: Use alumina polishing paste with a particle size of 1800 mesh and a polishing pressure of 0.15MPa to make the surface finish Ra of the roller reach 1.2μm.

[0029] Comparative Example 1 Compared with Example 1, Comparative Example 1 omits the gradient composite transition layer and only sets a single-layer NiCrAl alloy underlayer. The coating structure is: substrate → NiCrAl alloy underlayer → alumina ceramic surface layer.

[0030] The specific process is as follows: S1. The matrix pretreatment steps are the same as in Example 1; S2. Without performing gradient composite transition layer spraying, directly spray a single layer of NiCrAl alloy substrate with a thickness of 50μm onto the pretreated substrate surface. S3, the ceramic surface coating material, spraying parameters, thickness, and S4 post-treatment process are all completely consistent with those in Example 1.

[0031] Comparative Example 2 Compared to Example 1, Comparative Example 2 uses a homogeneous mixed transition layer without compositional gradient changes. The coating structure is: substrate → homogeneous mixed transition layer → alumina ceramic surface layer. The specific process is as follows: S1. The matrix pretreatment steps are the same as in Example 1; S2. The transition layer uses a single ratio of 50wt% NiCrAl + 50wt% Al2O3 mixed powder, which is sprayed in one go to form a homogeneous transition layer with a thickness of 90μm without gradient adjustment. S3, the ceramic surface coating material, spraying parameters, thickness, and S4 post-treatment process are all completely consistent with those in Example 1.

[0032] Test Experiment Example The alumina ceramic coatings on the guide rollers of papermaking machines in Examples 1-3 and Comparative Examples 1-2 were tested, and the results are listed in Table 1.

[0033] The residual thermal stress was determined according to the standard GB / T 31310-2014 Determination of Residual Stress in Metallic Materials by Drilling Strain Method. Three test points were selected for each sample, and the blind hole method was used for testing. The average value was taken as the final test value. There should be no stress concentration during the test process. The residual thermal stress should be ≤45MPa.

[0034] The method for determining thermal shock resistance (300℃ and water cooling alternation) is as follows: the sample is kept at 300℃ for 30 minutes and then quickly immersed in 25℃ room temperature water for 10 minutes, which is one cycle; the counting is stopped when the coating cracks and peels off; the number of cycles should be ≥50, and the coating should not crack, peel off, or bulge.

[0035] The coating bond strength was determined by tensile method according to the standard GB / T 8642-2002 Thermal spraying - Determination of tensile bond strength. Each sample was tested once, and the coating was observed to be free of peeling or breakage after the test. The required bond strength is ≥70MPa, and there should be no peeling or breakage of the coating after the test.

[0036] Table 1 Performance test results of Examples 1-3 and Comparative Examples 1-2 As can be seen from Table 1, the present invention achieves a continuous and smooth transition of the coefficient of thermal expansion from the substrate to the ceramic surface layer through the composite of NiCrAl-NiCrAl / Al2O3 gradient composite transition layer and ceramic surface layer, which significantly reduces the residual stress at the interface, greatly improves the coating bonding strength and thermal shock resistance, and significantly extends the service life of the paper machine guide roller under complex working conditions and significantly improves its stability.

[0037] Comparative Example 1 uses a traditional metal base layer + ceramic surface layer structure. Due to the large difference in thermal expansion coefficients, the interface stress concentration is severe, the thermal shock resistance is poor, the coating is prone to cracking and peeling, and the service life is short.

[0038] Comparative Example 2 uses a homogeneous mixed transition layer, which alleviates stress to some extent. However, the transition layer has a single composition and does not achieve a continuous gradient change in composition. As a result, there are still problems such as weak interfacial bonding and insufficient stress release, and the performance improvement is limited.

Claims

1. A method for preparing an alumina ceramic coating on the surface of a paper machine guide roll, comprising substrate pretreatment, preparation of a gradient composite transition layer, ceramic surface coating, and post-treatment; characterized in that, The gradient composite transition layer is applied using plasma spraying technology. At the same work station, the following three layers are sprayed sequentially by continuously changing the powder feeding ratio, or a continuous gradient is formed: (1) The first transition sublayer uses 100wt% NiCrAl alloy powder, with a spraying thickness of 30-50μm; (2) The second transition sublayer uses a mixture of 70wt% NiCrAl and 30wt% Al2O3 powder, with a spraying thickness of 20-40μm; (3) The third transition sublayer uses a mixture of 50wt% NiCrAl and 50wt% Al2O3 powder, with a spraying thickness of 20-40μm; The total thickness of the gradient composite transition layer is controlled at 80-120μm. The ceramic surface layer is coated with supersonic plasma spraying. The spraying material is Al2O3 powder or Al2O3-TiO2 composite powder, with a thickness of 200-500 μm and a porosity of ≤3%.

2. The method for preparing the alumina ceramic coating on the surface of the paper machine guide roller according to claim 1, characterized in that, The NiCrAl alloy powder has the following composition by mass: Ni 70%–75%, Cr 15%–20%, Al 5%–10%, and unavoidable impurities, with a particle size of 20–50 μm; the Al2O3 powder has a purity of ≥99.5% and a particle size of 15–45 μm; the Al2O3-TiO2 composite powder has a TiO2 mass fraction of 10%–20%.

3. The method for preparing the alumina ceramic coating on the surface of the paper machine guide roller according to claim 1, characterized in that, The spraying parameters for the gradient composite transition layer are as follows: flow rate of plasma gas Ar / H2 40-60 L / min, current 280-450 A, voltage 75-85 V, powder feed rate 30-60 g / min, spraying distance 100-150 mm, and spray gun moving speed 30-60 mm / s.

4. The method for preparing the alumina ceramic coating on the surface of the paper machine guide roller according to claim 1, characterized in that, The three-layer spraying of the gradient composite transition layer is carried out continuously. The mixing ratio of NiCrAl powder and Al2O3 powder is adjusted in real time by a dual-path powder feeder to achieve a smooth transition of components, with no obvious abrupt change in components between adjacent sublayers.

5. The method for preparing the alumina ceramic coating on the surface of the paper machine guide roller according to claim 1, characterized in that, The spraying parameters for the ceramic surface coating are as follows: spraying pressure 0.8-1.0 MPa, spraying distance 120-150 mm, spray gun moving speed 50-80 mm / s, spraying in 2-3 coats, single coat thickness 100-200 μm, final coating thickness 200-500 μm, and porosity ≤3%.

6. The method for preparing the alumina ceramic coating on the surface of the paper machine guide roller according to claim 1, characterized in that, The post-processing involves precision grinding with diamond grinding wheels and polishing with alumina polishing paste to ensure that the roundness error of the guide roller is ≤0.01mm and the surface finish Ra is 0.8~1.6μm.

7. The method for preparing the alumina ceramic coating on the surface of the paper machine guide roller according to claim 1, characterized in that, The matrix pretreatment includes: (1) Degreasing: Soak or wipe the surface of the guide roller with acetone or ethanol to remove oil stains; (2) Sandblasting roughening: Brown corundum abrasive with a particle size of 0.5-1mm is used, the sandblasting pressure is 0.6-0.8MPa, and the spraying angle is 45-60°, so that the surface roughness of the substrate reaches Ra 5-8μm.

8. The method for preparing the alumina ceramic coating on the surface of the paper machine guide roller according to claim 1, characterized in that, The post-processing includes: (1) Fine grinding: Use a diamond grinding wheel with a grit size of 800 to 1000 mesh to grind the coating surface at a speed of 300 to 500 r / min to make the roundness error of the guide roller ≤ 0.01 mm; (2) Polishing: Alumina polishing paste with a particle size of 1500-2000 mesh and a polishing pressure of 0.1-0.2 MPa is used to make the surface finish Ra of the roller reach 0.8-1.6 μm.

9. The paper machine guide roller prepared by the method for preparing the alumina ceramic coating on the surface of the paper machine guide roller according to claim 1, characterized in that, The surface of the guide roller consists of, from the inside out, a substrate, a gradient composite transition layer, and an alumina ceramic surface layer. The gradient composite transition layer transitions from NiCrAl to NiCrAl-Al2O3 with a total thickness of 80–120 μm. The ceramic surface layer has a thickness of 200–500 μm, a porosity of ≤3%, a roller surface roundness error of ≤0.01 mm, and a surface finish Ra of 0.8–1.6 μm.

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