Alumina waterproof thermal barrier coating and its preparation method and application

Abstract

A waterproof and heat-insulating alumina coating, its preparation method, and its application are disclosed, relating to the field of coating materials technology. This invention solves the problem that existing protective coating materials cannot meet the interfacial bonding strength and thermomechanical stability requirements of high-precision chips for dense coatings. The coating comprises a loose, porous alumina paste A and a relatively dense alumina paste B. A printing process is used to print alumina paste A onto the surface of a chip preform and then dry it; alumina paste A is the first layer, sintered at high temperature. Alumina paste B is then printed onto the surface of the chip preform with the alumina coating A and dried; alumina paste B is the second layer, sintered at low temperature. This invention can be applied to the protection of zirconia ceramic chips in nitrogen and oxygen sensors and has broad application prospects.

Description

Technical Field

[0001] This invention relates to the field of coating materials technology, specifically to an alumina waterproof and heat-insulating coating, its preparation method, and its application. Background Technology

[0002] NOx is one of the main components of air pollution, and its main source is the gas emitted from the internal combustion engine of automobiles. In order to solve the NOx emission problem, nitrogen oxide sensors are usually used to measure the concentration of nitrogen oxides (NOx) in the emissions of internal combustion engines.

[0003] The core component of the nitrogen oxide sensor is a zirconia ceramic chip. Pt electrodes are printed on both sides of a cured ZrO2 green ceramic sheet. Under high-temperature operating conditions, when a specific voltage is applied to the chip electrodes, NOx and O2 enter the chamber through the gas channel, forming a current in the closed circuit through redox reactions. The nitrogen oxide content is then measured by detecting the current signal.

[0004] However, during operation, the high-velocity gases and water vapor in the exhaust gas can impact the chip surface, causing the chip to overheat too quickly, leading to cracks and failure. Furthermore, condensation and oil residue adhering to the chip tip during non-operational periods can also trigger thermal shock, causing cracks and ultimately malfunctioning the nitrogen-oxygen sensor. Therefore, improving the reliability of chip sensors hinges on developing protective coating technologies that combine waterproofing and thermal insulation.

[0005] Currently, common coating preparation processes mainly include plasma spraying and dip coating. However, both have certain drawbacks. Plasma spraying requires melting at temperatures above 2000℃ before spraying the coating at high speed. Extreme temperatures and spraying pressures can easily lead to thermal stress concentration in ceramic chips, causing structural fracture. Dip coating suffers from uneven thickness and excessive porosity during film formation. The loose, porous structure generates stress concentration under thermal cycling loads, leading to coating cracking and failure, and a significant decrease in thermal shock resistance. In summary, there is an urgent need to develop new film formation technologies that combine waterproofing and thermal insulation functions while meeting the requirements of high-precision chips for the interfacial bonding strength and thermomechanical stability of dense coatings. Summary of the Invention

[0006] To address the issue that existing protective coating materials cannot meet the requirements of high-precision chips for interfacial bonding strength and thermomechanical stability of dense coatings, this invention proposes an alumina waterproof and heat-insulating coating, its preparation method, and its application.

[0007] The specific technical solution of the present invention is as follows:

[0008] A method for preparing an alumina waterproof and heat-insulating coating includes the following steps:

[0009] S1, Preparation of alumina slurry A:

[0010] S11. Terpineol, butyl acetate, isophorone, and ethyl cellulose are thoroughly mixed under sealed conditions at 80°C to form a homogeneous organic system.

[0011] S12. Activated carbon powder, nano-zirconia powder and nano-alumina powder are thoroughly mixed using a V mixer to obtain a mixed powder;

[0012] S13. Mix the organic system with the mixed powder, and after mixing evenly, grind and disperse it using a ceramic three-roll mill, vacuum stir, and then centrifuge to remove bubbles;

[0013] S2. Print alumina paste A onto the surface of the chip blank and dry it. Then, heat it to 1450℃~1500℃ for high-temperature sintering and then hold it at that temperature.

[0014] Preparation of S3, Alumina Slurry B:

[0015] S31. Terpineol, butyl acetate, isophorone, and ethyl cellulose are thoroughly mixed under sealed conditions at 80°C to form a homogeneous organic system.

[0016] S32. The magnesium aluminum spinel powder and nano alumina powder are thoroughly mixed using a V mixer to obtain a mixed powder;

[0017] S33. Mix the organic system with the mixed powder, stir evenly, grind and disperse using a ceramic three-roll mill, vacuum stir, and then centrifuge to remove bubbles;

[0018] S4. Print the alumina paste B onto the surface of the chip green plate with the alumina coating A obtained in step S2 and dry it. Then, heat it to 1100℃~1200℃ for low-temperature sintering and then keep it at the temperature to obtain the alumina waterproof and heat-insulating coating.

[0019] Preferably, in step S12, the activated carbon powder has a particle size of 10-50 nm; the nano-alumina powder has a particle size of 50-500 nm; and the nano-zirconia powder has a particle size of 50-500 nm.

[0020] The magnesium aluminum spinel powder in step S32 has a particle size of 20~80nm; the nano alumina powder has a particle size of 20~100nm.

[0021] Preferably, the mass ratio of activated carbon powder, nano-zirconia powder, nano-alumina powder, terpineol, butyl acetate, isophorone, and ethyl cellulose in step S1 is 10:0.3~0.35:40~41:2:6~6.5:1.5:7~8;

[0022] The mass ratio of magnesium aluminum spinel powder, nano alumina powder, terpineol, butyl acetate, isophorone, and ethyl cellulose in step S3 is 1~1.1:38.5~40:2~3:5.6~6.5:1.5:6.5~7.5.

[0023] Preferably, the stirring speed in steps S11, S13, S31 and S33 is 40 r / min and the stirring time is 1 h.

[0024] Preferably, the stirring rate of the V-type stirrer in steps S12 and S32 is 300 Hz / min, and the stirring time is 3 h.

[0025] Preferably, the grinding rate of the ceramic three-roll mill in steps S13 and S33 is 20 Hz / min, the grinding time is 30 min, and the ceramic roller is made of zirconium oxide.

[0026] Preferably, the vacuum stirring and centrifugal degassing in steps S13 and S33 are performed using a non-contact planetary stirring vacuum degassing integrated machine with a vacuum degree of -0.095MPa and a degassing time of 1 hour.

[0027] Preferably, the drying temperature in steps S2 and S4 is 70~150℃, the drying time is 30min, the heating rate is 0.5~1℃ / min, and the holding time is 2~5h.

[0028] The present invention also provides an alumina waterproof and heat-insulating coating, which is prepared by the above-described preparation method.

[0029] The present invention also provides an application of the above-mentioned alumina waterproof and heat-insulating coating, specifically for the protection of the zirconium oxide ceramic chip in a nitrogen and oxygen sensor.

[0030] Compared with the prior art, the specific beneficial effects of the present invention are as follows:

[0031] This invention employs a differentiated porosity matching strategy to construct a composite protection system consisting of a high-porosity alumina coating A and a low-porosity alumina coating B. Coating A, serving as the basic support layer, is sintered simultaneously during the chip fabrication stage. Its porous structure not only achieves mechanical interlocking anchoring with the substrate but also forms a micron-level ventilation network, without affecting chip performance and significantly improving interfacial bonding strength while ensuring heat dissipation efficiency. The dense surface coating B utilizes a low-temperature secondary sintering process to achieve surface densification while avoiding thermal damage, forming a continuous and complete physical barrier layer. The synergistic effect of these two coatings constructs a gradient protection system that combines stress buffering and dielectric barrier functions.

[0032] Experiments have verified that the dual-layer gradient pore structure designed in this invention effectively disperses thermal stress, thereby improving the thermal shock resistance of the coating; the synergistic effect of the dual-layer interface also significantly improves the coating bonding strength, eliminating interlayer cracking caused by thermal cycling; the printing process characteristics ensure that the coating thickness fluctuation is controlled within a reasonable range, enabling the chip to maintain performance stability under special operating conditions. Detailed Implementation

[0033] First, the current mainstream coating preparation processes include plasma spraying and dip coating.

[0034] Plasma spraying is a type of thermal spraying process. Its working principle involves using a plasma jet to heat and accelerate metal or ceramic powder in a short time. The powder, in a molten or semi-molten state, impacts the substrate at high speed, forming a layered coating that adheres firmly to the substrate surface. However, this process is affected by factors such as the powder flow rate, spray gun structure, operating current and voltage, and working gas flow rate. If the powder flow rate is too high, the powder cannot be fully melted, resulting in more powder inclusions, higher porosity, and a looser structure, reducing the bonding strength between coating layers. If the powder flow rate is too low, the powder deposition efficiency is low, increasing the spraying time to achieve a certain coating thickness, causing overheating and deformation of the substrate, and reducing spraying efficiency. The substrate material for nitrogen-oxygen sensors is zirconium oxide. Zirconia transforms from a tetragonal phase to a cubic phase above 2300℃, indirectly affecting product performance. Therefore, plasma spraying is not suitable for preparing the thermal insulation and waterproof layer of nitrogen-oxygen sensors.

[0035] The dip-coating method involves vertically immersing the substrate in an alumina composite solution / sol. The dip-coating process utilizes the interaction between the viscosity, surface tension, and gravity of the alumina composite solution. The process must ensure a vibration-free liquid surface and that the substrate rises vertically, at a uniform speed, smoothly, and continuously to guarantee the formation of a continuous and uniform oxide film on the substrate surface. However, this method requires numerous repeated dip-coating cycles, resulting in a long process cycle and uncontrollable thickness. Therefore, the dip-coating method is not suitable for preparing the thermal insulation and waterproof layer for nitrogen and oxygen sensors.

[0036] Therefore, this invention provides a novel alumina waterproof and heat-insulating coating that enables the preparation of a heat-insulating and waterproof layer for nitrogen and oxygen sensors, and exhibits excellent performance.

[0037] To make the technical solutions of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. It should be noted that the following embodiments are only used to better understand the technical solutions of the present invention and should not be construed as limiting the present invention.

[0038] Alumina waterproof coating for nitrogen and oxygen sensors was prepared strictly according to the different formulation ratios in Table 1 (alumina slurry A ingredient list) and Table 2 (alumina slurry B ingredient list). The coating includes alumina slurry A and alumina slurry B. Alumina slurry A is the first layer sintered at high temperature, and alumina slurry B is the second layer sintered at low temperature.

[0039] I. Preparation of Alumina Slurry A:

[0040] Step 1: Prepare the organic system: Thoroughly mix terpineol, butyl acetate, isophorone, and ethyl cellulose under sealed conditions at 80°C to form a homogeneous organic system;

[0041] Step 2, Dispersion of porous alumina slurry: Activated carbon powder, nano-zirconia powder and nano-alumina powder are thoroughly mixed using a V mixer;

[0042] The third step is to strictly follow the formula ratio to mix the organic system with the mixed powders of activated carbon powder, nano zirconium oxide powder and nano alumina powder. After mixing evenly, the mixture is ground and dispersed using a ceramic three-roll mill, followed by vacuum stirring, centrifugation and degassing. The ceramic rollers are made of zirconium oxide.

[0043] A. The particle size of the highly active carbon powder used is 10-50nm;

[0044] B. The nano-alumina powder used is 50-500nm;

[0045] C. The nano-zirconia powder used is 50-500nm.

[0046] Alumina paste A is printed onto the surface of a chip preform using a printing process and dried at 120°C for 30 minutes. Then, the temperature is increased to 1450°C~1500°C at a heating rate of <1°C / min for high-temperature sintering, and then held at that temperature for 3 hours to obtain a loose and porous alumina coating A.

[0047] In steps one and three, the mixer speed is 40 r / min and the mixing time is 1 h.

[0048] In step two, the stirring speed of the V-type agitator is 300 Hz / min, and the mixing time is 3 hours.

[0049] In step three, the three-roll mill speed is 20 Hz / min, and the mixing time is 30 min;

[0050] The vacuum degree of the non-contact planetary stirring vacuum degassing machine is -0.095MPa, and the degassing time is 1 hour.

[0051] II. Preparation of Alumina Slurry B:

[0052] Step 1: Prepare the organic system: First, mix terpineol, butyl acetate, isophorone, and ethyl cellulose thoroughly under sealed conditions at 80°C to form a homogeneous organic system.

[0053] Step 2, Dispersion of porous alumina slurry: Magnesium aluminum spinel powder and nano alumina powder are thoroughly mixed using a V mixer;

[0054] The third step involves strictly following the formula ratio to mix the organic system with the mixed powders of magnesium aluminum spinel powder and nano alumina powder. After stirring evenly, the mixture is ground and dispersed using a ceramic three-roll mill. The process involves non-contact planetary vacuum stirring and centrifugal degassing. The ceramic rollers are made of zirconium oxide.

[0055] A. The particle size of the magnesium aluminum spinel glass system powder used is 20-80nm;

[0056] B. The nano-alumina powder used is 20-100nm;

[0057] Alumina paste B is printed onto the surface of a chip preform with an alumina coating A using a printing process. The preform is then dried at 120°C for 30 minutes, and then sintered at a low temperature of 1100°C to 1200°C at a heating rate of <1°C / min. After holding at this temperature for 3 hours, a chip with an alumina waterproof and heat-insulating coating is obtained.

[0058] Table 1

[0059]

[0060] Table 2

[0061]

[0062] Example of results.

[0063] The coated chip elements prepared in Examples 1 to 5 above were subjected to performance tests, including thermal shock resistance tests, cold and hot start-stop tests, and service life tests. The test results are shown in Table 3.

[0064] Table 3

[0065]

[0066] Based on the above test results, the nitrogen and oxygen sensor chip operated at 840℃ for 20,000 hours under vibration. The coatings and sensor elements in Examples 1 to 5 showed no obvious damage or cracks and were able to work normally.

[0067] When the nitrogen and oxygen sensor chip operates at 840°C, it undergoes 10,000 cold and hot start-stop cycles. The coatings and sensor elements in Examples 1-5 show no obvious damage or cracks and can function normally.

[0068] The nitrogen oxide sensor chip was tested and operated continuously for 20,000 hours in an automotive exhaust environment at 840℃. The chip's appearance, size, resistance value, and electrical performance still met the product specifications after the test.

[0069] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing an alumina waterproof and heat-insulating coating, characterized in that, The alumina waterproof and heat-insulating coating includes alumina slurry A and alumina slurry B. Alumina slurry A is obtained by sintering the first layer, and alumina slurry B is obtained by sintering the second layer. The preparation method includes the following steps: S1, Preparation of alumina slurry A: S11. Terpineol, butyl acetate, isophorone, and ethyl cellulose are thoroughly mixed under sealed conditions at 80°C to form a homogeneous organic system. S12. Activated carbon powder, nano-zirconia powder and nano-alumina powder are thoroughly mixed using a V mixer to obtain a mixed powder; S13. Mix the organic system with the mixed powder, and after mixing evenly, grind and disperse it using a ceramic three-roll mill, vacuum stir, and then centrifuge to remove bubbles; S2. Print alumina paste A onto the surface of the chip preform and dry it. Then sinter it at 1450℃~1500℃ and hold it at that temperature. Preparation of S3, Alumina Slurry B: S31. Terpineol, butyl acetate, isophorone, and ethyl cellulose are thoroughly mixed under sealed conditions at 80°C to form a homogeneous organic system. S32. The magnesium aluminum spinel powder and nano alumina powder are thoroughly mixed using a V mixer to obtain a mixed powder; S33. Mix the organic system with the mixed powder, stir evenly, grind and disperse using a ceramic three-roll mill, vacuum stir, and then centrifuge to remove bubbles; S4. Print the alumina paste B onto the surface of the chip blank with the alumina coating A obtained in step S2 and dry it. Then heat it to 1100℃~1200℃ for sintering and then keep it at the temperature to obtain the alumina waterproof and heat-insulating coating. The mass ratio of activated carbon powder, nano-zirconia powder, nano-alumina powder, terpineol, butyl acetate, isophorone, and ethyl cellulose in step S1 is 10:0.3~0.35:40~41:2:6~6.5:1.5:7~8; The mass ratio of magnesium aluminum spinel powder, nano alumina powder, terpineol, butyl acetate, isophorone, and ethyl cellulose in step S3 is 1~1.1:38.5~40:2~3:5.6~6.5:1.5:6.5~7.

5.

2. The method for preparing the alumina waterproof and heat-insulating coating according to claim 1, characterized in that, The activated carbon powder in step S12 has a particle size of 10~50nm; the nano alumina powder has a particle size of 50~500nm; and the nano zirconium oxide powder has a particle size of 50~500nm. The magnesium aluminum spinel powder in step S32 has a particle size of 20~80nm; the nano alumina powder has a particle size of 20~100nm.

3. The method for preparing the alumina waterproof and heat-insulating coating according to claim 1, characterized in that, The stirring speed in steps S11, S13, S31 and S33 is 40 r / min and the stirring time is 1 h.

4. The method for preparing the alumina waterproof and heat-insulating coating according to claim 1, characterized in that, The stirring rate of the V-type stirrer in steps S12 and S32 is 300 Hz / min, and the stirring time is 3 h.

5. The method for preparing the alumina waterproof and heat-insulating coating according to claim 1, characterized in that, The grinding rate of the ceramic three-roll mill described in steps S13 and S33 is 20 Hz / min, the grinding time is 30 min, and the ceramic roller is made of zirconium oxide.

6. The method for preparing the alumina waterproof and heat-insulating coating according to claim 1, characterized in that, The vacuum stirring and centrifugal degassing described in steps S13 and S33 are performed using a non-contact planetary stirring vacuum degassing integrated machine with a vacuum degree of -0.095MPa and a degassing time of 1 hour.

7. The method for preparing the alumina waterproof and heat-insulating coating according to claim 1, characterized in that, The drying temperature in steps S2 and S4 is 70~150℃, the drying time is 30min, the heating rate is 0.5~1℃ / min, and the holding time is 2~5h.

8. An alumina waterproof and heat-insulating coating, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 7.

9. An application of the alumina waterproof and heat-insulating coating according to claim 8, characterized in that, Application in the protection of zirconium oxide ceramic chips in nitrogen and oxygen sensors.

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