A thermally and mechanically dual-sensitive composite coating layer, a preparation method and application thereof

Abstract

This invention relates to the field of nondestructive testing technology for low-carbon steel, specifically disclosing a thermo-sensitive dual-sensitivity composite coating, its preparation method, and its application. The thermo-sensitive dual-sensitivity composite coating comprises a stress-sensitive fluorescent layer and a thermochromic layer. The stress-sensitive fluorescent layer uses core-shell hybrid particles with mechanoluminescence enhancement as key functional fillers, while the thermochromic layer uses an irreversible thermochromic material based on IPN as the core functional material. This invention uses novel self-made functional materials to prepare the stress-sensitive fluorescent layer and thermochromic layer, enabling the thermo-sensitive dual-sensitivity composite coating to possess high-sensitivity signal sensing, optical enhancement, and permanent recording functions. Coupled with load excitation and digital image analysis processes, it transforms the stress-temperature coupled field information excited by deep defects under load into a high-intensity, highly interference-resistant, and permanently recordable visual signal, achieving precise location, quantification, and classification of defects.

Description

Technical Field

[0001] This invention relates to the field of nondestructive testing technology for low-carbon steel, and in particular to a thermo-sensitive composite coating, its preparation method, and its application. Background Technology

[0002] During the manufacturing processes of low-carbon steel, such as continuous casting and hot rolling, fluctuations in the process (e.g., uneven cooling, changes in rolling force) or the release of gases (e.g., hydrogen, oxygen) from within the material can easily lead to a special type of defect on and beneath the surface. These defects include subcutaneous bubbles, deeply embedded inclusions, non-surface-opening microcracks (hairline cracks), and deep roll marks. These defects are typically located below the surface or have extremely fine openings, making them highly concealed and difficult to detect visually. These defects are not only potential fracture sources but also stress concentration points, causing localized temperature rises due to plastic deformation and internal friction under subsequent stress.

[0003] Currently used nondestructive testing methods, such as manual visual inspection, magnetic particle testing, or penetrant testing, are almost powerless against deep defects that are not exposed on the surface. Conventional machine vision systems heavily rely on surface optical features, and under the complex lighting conditions, oil stains, and oxide scale on steel billets in industrial environments, it is difficult to obtain stable and clear images, thus hindering the effective identification and localization of deep defects. Furthermore, existing stress fluorescent coatings generally suffer from weak signals and are prone to quenching, and traditional thermochromic materials are mostly reversible, unable to permanently record localized temperature changes experienced by the material.

[0004] Therefore, the industry (especially low-carbon steel) urgently needs a new detection technology that can actively enhance defect signals, suppress environmental interference, and simultaneously record stress concentration and historical temperature rise in the defect area with high signal-to-noise ratio and permanent visualization, so as to achieve efficient and accurate diagnosis of deep defects in low-carbon steel. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a thermo-sensitive composite coating, its preparation method, and its application. A stress-sensitive fluorescent layer and a thermochromic layer are prepared using novel self-made functional materials, enabling the thermo-sensitive composite coating to possess high-sensitivity signal sensing, optical enhancement, and permanent recording functions. Coupled with a load-excitation and digital image analysis process, the stress-temperature coupled field information excited by deep defects under load is transformed into a high-intensity, highly interference-resistant, and permanently recordable visual signal, achieving precise location, quantification, and classification of defects.

[0006] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows:

[0007] In a first aspect, the present invention provides a thermo-sensitive composite coating comprising a stress-sensitive fluorescent layer and a thermochromic layer, wherein the stress-sensitive fluorescent layer is located between a substrate and the thermochromic layer.

[0008] The stress-sensitive fluorescent layer comprises the following raw materials: a resin matrix, core-shell hybrid particles, and a first diluent; the core-shell hybrid particles have a mononuclear double-shell structure, the core material includes trivalent europium ions, the inner shell material includes mesoporous silica nanomaterials, and the outer shell material includes a polymer material obtained by polymerizing a tetraphenylvinyl unsaturated monomer and a borate ester unsaturated monomer;

[0009] The thermochromic layer comprises the following raw materials: a thermally reversible crosslinked network precursor, an aqueous crosslinked network precursor containing a leuco dye, a solid acid color developer, and a second diluent; the thermally reversible crosslinked network precursor is prepared by a Diels-Alder reaction of furan-modified epoxy resin and bismaleimide.

[0010] Compared to existing technologies, the thermo-sensitive composite coating provided by this invention employs core-shell hybrid particles with mechano-induced fluorescence enhancement as key functional fillers in the stress-sensitive fluorescent layer. This material, through its ingenious single-core, double-shell structure design, achieves a significant enhancement of fluorescence intensity under stress, fundamentally solving the problem of weak signals in traditional stress-sensitive fluorescent materials. The inherent high-intensity fluorescence of trivalent europium ions effectively penetrates common interferences in industrial production environments, such as oil stains and shallow oxide films, providing a stable and bright signal source for image acquisition. The thermochromic layer uses an irreversible thermochromic material based on interpenetrating polymer networks (IPNs) (a thermoreversible crosslinked network precursor and an aqueous crosslinked network precursor are cured to form an irreversible thermochromic material) as the core functional material. This material can achieve mechanical interlocking between the thermoreversible polymer network and the aqueous crosslinked network by setting the curing conditions, thereby achieving irreversible color changes at specific temperature thresholds. It can permanently record the historical temperature rise at defects, overcoming the signal fading problem of traditional reversible thermochromic materials.

[0011] When core-shell hybrid particles are subjected to force, the borate ester groups in the outer shell material undergo ring-opening reactions or bond angle twisting, thereby changing the state of the outer shell and enabling it to more effectively absorb excitation light energy and transfer it to the core material Eu. 3+ This improves luminescence efficiency. In the Diels-Alder reaction (DA reaction for short), a 4+2 cycloaddition reaction occurs between the furan group and the maleimide group to form a reversible DA adduct, thereby constructing a thermally reversible crosslinked network precursor.

[0012] Preferably, the resin matrix includes at least one of epoxy resin or polyurethane.

[0013] Preferably, the mass ratio of resin matrix to core-shell hybrid particles in the stress-sensitive fluorescent layer is 100:(5~30).

[0014] Through extensive testing, this invention has found that when the amount of core-shell hybrid particles is within the aforementioned range, the coating can simultaneously possess high fluorescence signal and good physical properties. Sufficient addition ensures signal strength and coverage uniformity; the upper limit prevents particle agglomeration, ensuring coating transparency, uniformity, and adhesion.

[0015] Preferably, the first diluent includes butanone.

[0016] Preferably, the method for preparing the core-shell hybrid particles includes the following steps:

[0017] S1. A template agent, a basic catalyst, a europium ion complex, and a silicon source are added to an alcohol solvent, and the reaction proceeds to obtain Eu. 3+ @MSNs;

[0018] S2. Under an inert atmosphere, add the Eu to a benzene-based solvent. 3+ @MSNs, a silyl compound containing an initiating group, and an acid-binding agent are subjected to a reflux reaction to obtain initiated functionalized Eu. 3+ @MSNs;

[0019] S3. Under an inert atmosphere, add the aforementioned initiating functionalized Eu to the mixed solvent. 3+ The core-shell hybrid particles are obtained by radical polymerization of MSNs, tetraphenylvinyl unsaturated monomers, borate ester unsaturated monomers, and coordination catalysts.

[0020] The method for preparing core-shell hybrid particles provided by this invention involves the hydrolysis of active groups in a silicon source and europium ion complex under the action of an alkaline catalyst to generate silanol groups and rare earth-silicon oxide species. Subsequently, through a co-condensation reaction, the Eu... 3+ Rare earth Eu is synthesized by embedding it into a growing silica network. 3+ In-situ doped mesoporous silica nanospheres (i.e., Eu) 3+ @MSNs). Fluorescent molecules, when directly exposed to the environment, readily react with oxygen, moisture, etc., leading to a decrease in fluorescence intensity. 3+ Protected within a mesoporous silica inner shell, it boasts a longer lifespan and better stability. Further, it utilizes silicides containing initiating groups to treat Eu... 3+ The surface of @MSNs is modified to covalently anchor initiating groups onto the surface of the nanospheres. Then, the initiating groups are homolytically cleaved under the action of a coordination catalyst to generate surface free radicals, which initiate free radical polymerization (SI-ATRP) of the carbon-carbon double bonds of unsaturated monomers. A polymer chain of a certain length grows at each initiation site, forming a "polymer brush" outer shell layer.

[0021] More preferably, in S1, the alcohol solvent includes water and ethanol in a volume ratio of (10~20):(2~5).

[0022] More preferably, in S1, the alcohol solvent includes water and ethanol in a volume ratio of (10~15):(3~5).

[0023] More preferably, in S1, the template agent comprises hexadecyltrimethylammonium bromide (CTAB).

[0024] More preferably, in S1, the alkaline catalyst comprises an aqueous ammonia solution with a mass concentration of 25% to 30%.

[0025] More preferably, in S1, the europium ion complex includes at least one of Eu(TTA)3 or Eu(TTA)3Phen.

[0026] In this invention, Eu(TTA)3 represents tri(thiophenecarboxyltrifluoroacetone)europium, and Eu(TTA)3Phen represents (1,10-phenanthroline)tri[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione]europium(III).

[0027] More preferably, in S1, the silicon source includes tetraethyl orthosilicate (TEOS).

[0028] More preferably, in S1, the molar ratio of the silicon source to the europium ion complex is 100:(0.5~5).

[0029] This invention, through extensive experimentation, reveals that the molar amount of europium ions is crucial for balancing fluorescence intensity and concentration quenching effects. Within this range, europium ions can be uniformly distributed within the mesopores, providing high-intensity background fluorescence and laying the foundation for signal enhancement under stress. If the amount of europium ions is too small, the background fluorescence intensity of the nuclear material is insufficient, resulting in a low absolute value of the final stress response signal, poor signal-to-noise ratio, and difficulty in camera capture. If the amount of europium ions is too large, the europium ions become too close together, easily leading to concentration quenching. This consumes energy through non-radiative transitions, causing the fluorescence intensity to decrease rather than increase. Furthermore, excessive europium doping may disrupt the regular structure of the mesoporous silica.

[0030] For example, in S1, to ensure uniform dispersion of the europium ion complex in the reaction system, the europium ion complex can be pre-dissolved in ethanol and added to the reaction system in solution form. This invention does not require a specific concentration of the europium ion complex solution; conventional procedures in the art can be used.

[0031] More preferably, in S1, the volume-to-mass ratio of the alcohol solvent, template agent, alkaline catalyst and silicon source is (130~200)mL:(0.5~2)g:(1~3)mL:(1.5~5)mL.

[0032] More preferably, in S1, the volume-to-mass ratio of the alcohol solvent, template agent, alkaline catalyst and silicon source is (150~200)mL:(0.8~1.5)g:(1.5~2.5)mL:(2~4)mL.

[0033] More preferably, in S1, the reaction temperature is 25℃~65℃ and the reaction time is 6h~24h.

[0034] More preferably, in S1, the reaction temperature is 30°C to 60°C, and the reaction time is 10h to 20h.

[0035] By limiting the temperature and time of the reaction in S1, this invention can ensure that the hydrolysis and condensation reactions are moderate, which is conducive to the formation of mesoporous nanospheres with uniform particle size and ordered structure.

[0036] In a further preferred embodiment, S1, after the reaction is complete, further includes: solid-liquid separation, washing with ethanol and water to remove the template agent, and drying at 60°C~80°C to obtain Eu. 3+ @MSNs.

[0037] Further preferably, in S1, the Eu 3+ The particle size of @MSNs ranges from 50nm to 200nm.

[0038] For example, in S2, Eu 3+ Before adding benzene-based solvents, @MSNs also undergo vacuum drying at 110℃~120℃ for 1.5h~2.5h to remove surface adsorbed water and chemically bound water.

[0039] More preferably, in S2, the benzene solvent includes toluene.

[0040] More preferably, in S2, the silyl compound containing the initiating group includes (3-(2-bromoisobutyryl)propyl)dimethoxysilane or (3-(2-bromoisobutyryl)propyl)trimethoxysilane.

[0041] In this invention, the silyl compound containing the initiating group can be denoted as BIBB-Si. The methoxysilane (-Si(OCH3)3) at one end of the BIBB-Si molecule hydrolyzes to generate silanol (-Si-OH) under heating and in the presence of trace amounts of water or an acid-binding agent, which then reacts with Eu. 3+ The silanol groups (Si-OH) on the surface of @MSNs nanospheres undergo a dehydration condensation reaction to form strong Si-O-Si covalent bonds, thereby covalently anchoring the 2-bromoisobutyryl (-COC(Br)(CH3)2) ATRP (atom transfer radical polymerization) initiating group at the other end to the nanosphere surface, facilitating subsequent grafting of shell materials.

[0042] More preferably, in S2, the acid-binding agent comprises triethylamine, used to neutralize the acid produced in the reaction.

[0043] More preferably, in S2, the benzene solvent, Eu 3+ The volume-to-mass ratio of @MSNs, silyl compounds containing initiating groups, and acid-binding agents is (50~80)mL:1g:(0.05~0.25)g:(0.15~0.4)mL.

[0044] More preferably, in S2, the benzene solvent, Eu 3+ The volume-to-mass ratio of @MSNs, silyl compounds containing initiating groups, and acid-binding agents is (50~80)mL:1g:(0.1~0.2)g:(0.2~0.4)mL.

[0045] More preferably, in S2, the reflux reaction temperature is 100℃~120℃, and the reaction time is 18h~36h.

[0046] For example, in S2, after the reflux reaction, the process further includes: solid-liquid separation, washing with toluene, acetone, or ethanol to remove physically adsorbed silicides containing initiating groups, and vacuum drying to obtain initiation-functionalized Eu. 3+ @MSNs.

[0047] More preferably, in S3, the mixed solvent comprises toluene and N,N-dimethylformamide (DMF) in a volume ratio of (1.5~2.5):1.

[0048] More preferably, in S3, the tetraphenylvinyl group unsaturated monomer includes 4-(1,2,2-triphenylvinyl)phenyl acrylate (abbreviated as Acrylate-TPE).

[0049] More preferably, in S3, the borate ester-containing unsaturated monomer includes (4-vinylphenyl)borate pinacol ester (St-BPin for short).

[0050] In this invention, the tetraphenylvinyl group has aggregation-induced emission properties, the styrene group can provide polymerization activity, the borate ester group serves as a mechanoresponsive unit, and the synergistic effect of each unsaturated monomer can ensure the realization of the mechanoluminescence enhancement effect.

[0051] More preferably, in S3, the molar ratio of the tetraphenylvinyl unsaturated monomer to the borate ester unsaturated monomer is (85~99):(15~1).

[0052] This invention, through extensive experimentation, has revealed that the molar amount of borate ester groups directly determines the sensitivity of the force response. Within this range, a sufficient number of force-responsive groups can induce effective chemical changes (such as ring opening) under stress, leading to significant alterations in the conformation or electronic structure of the polymer shell, thereby achieving a significant mechanosensitive fluorescence enhancement effect. If there are too few borate ester groups, the force-responsive units are insufficient, and stress cannot effectively trigger abrupt changes in shell properties, resulting in a weak fluorescence enhancement effect and poor contrast with the background. Excessive content of force-responsive groups may lead to excessively rigid polymer chains or abnormal crosslinking density, which in turn restricts chain segment movement, reduces the efficiency of stress transfer and conversion, decreases sensitivity, and may also affect the mechanical properties of the underlying coating.

[0053] More preferably, in S3, the coordination catalyst comprises CuBr and pentamethyldiethylenetriamine (PMDETA).

[0054] More preferably, in S3, the mixed solvent, the initiator functionalized Eu 3+ The volume-to-mass ratio of @MSNs, tetraphenylvinyl group unsaturated monomer, CuBr and pentamethyldiethylenetriamine is (30~50)mL:0.5g:(0.8~1.5)g:(0.01~0.03)g:(0.02~0.06)mL.

[0055] More preferably, in S3, the temperature of the free radical polymerization reaction is 70℃~90℃, and the reaction time is 24h~48h.

[0056] During the free radical polymerization reaction, the Cu(I) catalyst activates and initiates the functionalization of Eu. 3+ @MSNs surface -Br bonds generate free radicals, initiating free radical polymerization of unsaturated monomers to form covalently linked polymer brushes on the nanosphere surface. This invention, by limiting the temperature of the free radical polymerization reaction in S3, ensures good controllability of chain growth, which is beneficial for forming polymer brushes with a narrow molecular weight distribution. Furthermore, by limiting the time of the free radical polymerization reaction in S3, this invention can form a polymer brush outer shell layer with a certain grafting density and a thickness of 20nm~100nm, thus ensuring the response amplitude of the mechanoluminescence enhancement effect.

[0057] For example, in S3, after the free radical polymerization reaction is completed, the process also includes: solid-liquid separation, washing with tetrahydrofuran (THF) to remove homopolymer and coordination catalyst, and drying to obtain core-shell hybrid particles.

[0058] Preferably, the solid acid colorimetric agent includes bisphenol A.

[0059] Preferably, the mass ratio of the thermally reversible crosslinking network precursor, the aqueous crosslinking network precursor, and the solid acid color developer in the thermochromic layer is 100:(5~30):(0.5~5).

[0060] Preferably, the second diluent includes at least one of butanone, methyl isobutyl ketone (MIBK), ethyl acetate, butyl acetate, or propylene glycol methyl ether acetate (PMA).

[0061] Preferably, the crosslinking density of the thermally reversible crosslinked network precursor is 5% to 30%.

[0062] This invention, through extensive experimentation, reveals that crosslinking density directly affects the temperature and mechanical strength of the surface-layer thermally reversible "switch." Within a specific crosslinking density range, the thermally reversible polymer network is stable at room temperature and can effectively "melt" (resulting in a reverse DA reaction) within a set defect temperature rise range (e.g., 50℃~90℃), creating conditions for the destruction of the aqueous crosslinked network. If the crosslinking density is too low, the thermally reversible crosslinked network precursor has insufficient network structural strength and poor room temperature stability, potentially softening or flowing during storage or transportation, leading to false triggering or functional failure. If the crosslinking density is too high, the thermally reversible crosslinked network precursor is too rigid, requiring excessive energy (temperature) for the reverse DA reaction, exceeding the actual temperature rise range of the defect, preventing the "switch" from opening and the thermochromic reaction from initiating.

[0063] Preferably, the method for preparing the thermally reversible crosslinked network precursor includes the following steps:

[0064] S4. Add epoxy resin and phosphine catalyst to furan-methanol to carry out ring-opening addition reaction to obtain furan-modified epoxy resin solution.

[0065] S5. Add bismaleimide to the furan-modified epoxy resin solution and carry out the Diels-Alder reaction to obtain the thermally reversible crosslinked network precursor.

[0066] More preferably, in S4, the epoxy resin is a bisphenol A type epoxy resin.

[0067] For example, in S4, the epoxy resin is E-51.

[0068] More preferably, in S4, the phosphine catalyst comprises triphenylphosphine.

[0069] More preferably, in S4, the molar ratio of epoxy groups, phosphine catalyst, and furanol in the epoxy resin is 100:(1~3):(120~300).

[0070] More preferably, in S4, the temperature of the ring-opening addition reaction is 60℃~90℃, and the reaction time is 2h~6h.

[0071] In S4 of this invention, under specific temperature and catalyst conditions, the hydroxyl groups of furanol undergo a ring-opening addition reaction with the epoxy groups, thereby introducing the furan ring into the epoxy resin molecular chain. By controlling the amount of reactants and the reaction time, the number of furan groups grafted onto each epoxy resin molecule can be adjusted.

[0072] More preferably, in S5, the bismaleimide comprises 1,6-bismaleimide hexane.

[0073] More preferably, in S5, the molar ratio of furan groups in the furan-modified epoxy resin solution to maleimide groups in the bismaleimide is 1:(0.9~1.1).

[0074] By controlling the amounts of furan groups and maleimide groups to be close to equimolar levels, the integrity and reversibility of the crosslinked network can be ensured.

[0075] More preferably, in S5, the temperature of the Diels-Alder reaction is 70℃~100℃, and the reaction time is 4h~10h.

[0076] By controlling the temperature and time of the Diels-Alder reaction, this invention can preferably control the reaction rate and degree of reaction (crosslinking density) to form an effective crosslinking network.

[0077] Preferably, the method for preparing the aqueous crosslinked network precursor includes the following steps:

[0078] S6. Mix and melt the leuco dye and C16~C22 alkanes to form an oil phase;

[0079] A water-soluble vinyl monomer, crosslinking agent, photoinitiator, and emulsifier are added to water to form an aqueous phase;

[0080] S7. Mix the oil phase and the aqueous phase to obtain the aqueous crosslinked network precursor.

[0081] More preferably, in S6, the leuco dye includes crystal violet lactone (CVL).

[0082] More preferably, in S6, the C16~C22 alkane includes straight-chain saturated alkanes or branched-chain saturated alkanes.

[0083] More preferably, in S6, the mass ratio of the leuco dye, C16~C22 alkane and aqueous phase is (1~10):(5~20):(70~94).

[0084] This invention, through extensive experimentation, has discovered that the amount of leuco dye used can ensure a sufficiently high color difference (ΔE) and visual contrast in the color-changing reaction. Within this range, the leuco dye can be effectively encapsulated and, upon release, produces a vivid and easily identifiable color change. If the amount used is insufficient, the color after the color change will be too light, with a small ΔE value, making it difficult for cameras or the human eye to detect, leading to missed detections. If the amount of leuco dye is excessive, it may not be completely encapsulated, resulting in partial color development during the coating preparation stage, affecting the initial appearance and background consistency. It may also precipitate due to oversaturation, affecting the coating quality.

[0085] Furthermore, the content of C16-C22 alkanes as the solvent phase determines the migration ability and reaction efficiency of the leuco dye molecules. Within this range, it can be ensured that the leuco dye can effectively flow and contact the solid acid color developer after being released, completing a rapid and complete color development reaction.

[0086] More preferably, in S6, the water-soluble vinyl monomer includes acrylamide.

[0087] Waterborne crosslinked networks can be obtained by polymerization of water-soluble vinyl monomers such as acrylamide.

[0088] More preferably, in S6, the crosslinking agent includes methylenebisacrylamide (MBA).

[0089] More preferably, in S6, the photoinitiator includes 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP).

[0090] More preferably, in S6, the emulsifier is a nonionic surfactant.

[0091] More preferably, in S6, the emulsifier comprises sorbitan monooleate (Span 80) and polyoxyethylene sorbitan monooleate (Tween 80) in a mass ratio of 1:(2~4).

[0092] The emulsifier preferred in this invention has good emulsification effect on alkane oil phases, excellent biocompatibility, and minimal interference with the subsequent photocuring process.

[0093] More preferably, in S6, the mass ratio of water, water-soluble vinyl monomer, crosslinking agent, photoinitiator and emulsifier is (70~80):(18~22):(0.1~0.3):(0.8~1.2):(1~5).

[0094] Secondly, the present invention provides a method for preparing the aforementioned thermosensitive composite coating, comprising the following steps:

[0095] Step 1: Weigh the raw materials for the stress-sensitive fluorescent layer according to the design ratio, mix the resin matrix, core-shell hybrid particles and first diluent evenly to obtain the bottom layer slurry; coat the bottom layer slurry onto the substrate surface and cure to form the stress-sensitive fluorescent layer;

[0096] Step 2: Weigh the raw materials for the thermochromic layer according to the designed ratio, mix the thermally reversible crosslinking network precursor, the aqueous crosslinking network precursor containing leuco dye, the solid acid color developer, and the second diluent evenly to obtain the surface layer slurry; coat the surface layer slurry onto the surface of the stress-sensitive fluorescent layer, and perform ultraviolet curing and low-temperature curing in sequence. The low-temperature curing temperature is 20℃~60℃ to form the thermochromic layer.

[0097] Preferably, in step one, the viscosity of the bottom slurry is 200 mPa·s to 800 mPa·s.

[0098] Preferably, in step one, the substrate comprises low-carbon steel.

[0099] Preferably, in step one, the coating method includes high-pressure airless spraying.

[0100] Preferably, in step one, the curing temperature is 60℃~100℃ and the curing time is 1h~4h.

[0101] Preferably, in step one, the thickness of the stress-sensitive fluorescent layer is 50 μm to 150 μm.

[0102] Preferably, in step two, the viscosity of the surface slurry is 100 mPa·s to 400 mPa·s.

[0103] Preferably, in step two, the coating method includes spraying.

[0104] Preferably, in step two, the light intensity of the ultraviolet curing is 50 mW / cm². 2 ~500mW / cm 2 The UV curing time is 30s~300s.

[0105] Preferably, in step two, the low-temperature curing time is 2h~48h, and more preferably 20h~40h.

[0106] Preferably, in step two, the thickness of the thermochromic layer is 20μm~50μm.

[0107] Thirdly, the present invention provides an application of the aforementioned thermo-sensitive composite coating in the detection of surface defects in carbon steel.

[0108] Preferably, the carbon steel includes low-carbon steel.

[0109] Fourthly, the present invention provides a method for visualizing defects on a substrate surface, comprising the following steps:

[0110] Step a: Following the preparation method of the thermo-sensitive composite coating described above, form the thermo-sensitive composite coating on the substrate surface;

[0111] Step b: Apply a load to the thermosensitive composite coating, perform image detection and digital acquisition, and perform digital analysis and defect diagnosis based on the test results.

[0112] The present invention provides a method for visualizing and detecting surface defects in a substrate. Under load, the stress concentration area where a deep defect is located triggers a significant change in fluorescence intensity or wavelength in the stress-sensitive fluorescent layer (bottom layer) above it. Microscopic plastic deformation and internal friction at the defect site generate a localized instantaneous temperature rise (ranging from 10°C to 50°C), which is captured by the thermochromic layer (top layer) above, resulting in a color change. After unloading, a clear "dual-signal spectrum" remains on the workpiece surface: the high-contrast fluorescent pattern of the bottom layer marks the stress concentration area (i.e., the vertical projection area of ​​the deep defect), while the color change pattern of the top layer marks historically high-temperature areas. Then, a dual-mode, high-precision image acquisition method is used to capture the permanent optical signals left by the stress and temperature fields, respectively. The results are output and integrated into the system through image registration and preprocessing, feature extraction and quantization, fusion decision-making, and defect determination.

[0113] Preferably, in step b, the load includes a cyclic load with a frequency of 0.1Hz to 5Hz, a static tensile load in a specific direction, or a static bending load in a specific direction.

[0114] Preferably, in step b, the strength of the load is 30% to 70% of the yield strength of the matrix, and the holding time of the load is 30s to 300s.

[0115] The specific load provided by this invention can induce microscopic deformation in the workpiece, causing subcutaneous bubbles under the substrate surface to deform under pressure, deep-buried inclusions to debond from the substrate, and stress concentration at the tips of deep microcracks, but without causing macroscopic damage. Simultaneously, this load scheme aims to drive a low-viscosity stress-sensitive fluorescent coating or its microflow under stress to better fill and conform to the contours of deep defects opened by the load. Extensive testing has shown that load strength is the core of achieving "effective activation" and "non-destructive" results. This load ensures the induction of microscopic plastic deformation and internal friction at the deep defects (resulting in stress concentration and temperature rise), while avoiding macroscopic yielding or damage to the entire workpiece.

[0116] The present invention has the following beneficial effects:

[0117] The method for preparing a thermo-sensitive composite coating provided by this invention involves first curing the surface slurry with ultraviolet light to selectively cure an aqueous crosslinked network precursor. Under ultraviolet light irradiation, the photoinitiator in the aqueous phase decomposes to generate free radicals, which initiate the rapid polymerization and crosslinking of monomers such as acrylamide, forming an aqueous crosslinked network in situ within the coating. At the moment of formation, this network mechanically locks the dispersed "oil-phase microspheres" containing CVL and alkanes within the mesh. The ultraviolet curing step does not trigger the DA reaction of the thermally reversible polymer network; at this point, the surface coating is an incompletely cured composite material containing a cured aqueous crosslinked network encapsulating functional microspheres, dispersed within an uncrosslinked thermally reversible crosslinked network precursor system. Then, through low-temperature curing, the DA reaction proceeds slowly, and furan groups and maleimide groups form covalent crosslinks, ultimately constructing a complete thermally reversible polymer network. This network penetrates and encapsulates the pre-formed aqueous crosslinked network, thus forming an interwoven interpenetrating network (IPN) structure. The aqueous crosslinked network acts like a "bird's nest" to fix the thermochromic microspheres, while the thermally reversible polymer network provides a robust overall framework with a thermally responsive "switch". Simultaneously, the IPN matrix itself possesses moderate light scattering capabilities, modulating incident ambient light and emitted underlying fluorescence to suppress specular glare.

[0118] This invention employs the synergistic effect of a stress-sensitive fluorescent layer and a thermochromic layer to transform the stress-temperature coupled field information deep within the substrate into a high-contrast, permanent visual signal. This solves the problem of insufficient display of surface defects below the substrate surface or in extremely small openings by traditional images, and also addresses the issue that existing sensors cannot simultaneously and permanently record stress and temperature change information. Attached Figure Description

[0119] Figure 1 Eu in Embodiment 1 of the present invention 3+ TEM image by @MSNs-Br.

[0120] Figure 2 This is a morphological image of the surface of low-carbon steel without a thermo-sensitive composite coating in Application Example 1 of the present invention.

[0121] Figure 3 This is a morphological image of the low-carbon steel surface after being coated with a thermo-sensitive composite coating in Application Example 1 of the present invention.

[0122] Figure 4 This is a detection image of a low-carbon steel surface coated with a thermo-sensitive composite coating in Application Example 1 of this invention.

[0123] Figure 5 This is a detection image of a low-carbon steel surface coated with a thermo-sensitive composite coating in Application Example 2 of the present invention.

[0124] Figure 6 The image shown is a detection image of a low-carbon steel surface coated with a thermo-sensitive composite coating in Comparative Application Example 1 of this invention.

[0125] Figure 7 This is a detection image of a low-carbon steel surface coated with a thermo-sensitive composite coating in Application Example 3 of the present invention.

[0126] Figure 8 The image shown is a detection image of a low-carbon steel surface coated with a thermo-sensitive composite coating in Comparative Application Example 2 of this invention.

[0127] Figure 9 This is a detection image of a low-carbon steel surface coated with a thermo-sensitive composite coating in Application Example 1 of this invention.

[0128] Figure 10 The image shown is a detection image of a low-carbon steel surface coated with a thermo-sensitive composite coating in Comparative Application Example 3 of this invention.

[0129] Figure 5 , Figure 9 and Figure 10 The various symbols in the chart are corresponding markings based on the test results: 1 is mild, 2 is moderate, and 3 is severe. Detailed Implementation

[0130] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0131] Unless otherwise specified in the embodiments of this invention, all raw materials used in the preparation of this product are commercially available. In this application example, a Q235 low-carbon steel hot-rolled plate, known to contain subcutaneous bubbles and deeply embedded non-metallic inclusions, was selected as the substrate, with dimensions of 150mm × 100mm × 10mm. The substrate surface was uniformly ground using an angle grinder equipped with a wire brush wheel to remove oxide scale; subsequently, it was ultrasonically cleaned for 10 minutes using acetone and anhydrous ethanol in sequence to thoroughly remove oil and dust; finally, it was dried with dry compressed air to obtain a clean, roughened surface.

[0132] In this invention, the crosslinking density is calculated by measuring the storage modulus of the thermally reversible crosslinked network precursor at the rubbery plateau (i.e., above its glass transition temperature Tg). The method for testing the crosslinking density of the thermally reversible crosslinked network precursor specifically includes the following steps:

[0133] Step a: Prepare the fully cured thermally reversible crosslinked network precursor (sample) into standard dimensions (e.g., length × width × thickness of 20mm × 5mm × 0.5mm). Perform temperature scanning tests on a DMA instrument (heating from room temperature to 150℃ at a rate of 3℃ / min and a frequency of 1Hz), and record the curve of storage modulus (E') as a function of temperature.

[0134] Step b: In the rubbery plateau region (a stable region with a temperature approximately 50°C above Tg), according to the rubber elasticity theory, the crosslinking density ν e (Number of effective network segments per unit volume, unit: mol / m³) 3 The relationship between ν and the energy storage modulus E' (unit: Pa) is: e =E' / (3RT). Where R is the ideal gas constant (8.314 J / (mol·K)) and T is the absolute temperature (K) of the rubbery plateau region.

[0135] Step c: The obtained ν e This can be further converted into a more intuitive "crosslinking density percentage", which is defined as the ratio of the actual effective crosslinking point density of a thermally reversible crosslinked network precursor to its theoretical maximum possible crosslinking point density (assuming all functional groups react).

[0136] To better illustrate the present invention, further examples are provided below.

[0137] Example 1

[0138] This embodiment provides a thermosensitive composite coating, including a stress-sensitive fluorescent layer and a thermochromic layer, wherein the stress-sensitive fluorescent layer is located between the substrate and the thermochromic layer.

[0139] The stress-sensitive fluorescent layer comprises the following raw materials: epoxy resin and core-shell hybrid particles in a mass ratio of 100:30, and methyl ethyl ketone (MEK) for adjusting the viscosity of the slurry. The core-shell hybrid particles have a mononuclear double-shell structure. The core material includes trivalent europium ions, the inner shell material includes mesoporous silica nanomaterials, and the outer shell material includes a polymer material obtained by polymerizing Acrylate-TPE and St-BPin.

[0140] The thermochromic layer comprises the following raw materials: a thermally reversible crosslinked network precursor in a mass ratio of 100:15:2, an aqueous crosslinked network precursor containing a leuco dye, and bisphenol A, and a second diluent (butanone) for adjusting the viscosity of the slurry. The crosslinking density of the thermally reversible crosslinked network precursor is 20.3%.

[0141] The preparation method of core-shell hybrid particles includes the following steps:

[0142] S1. Add 1g of template agent (CTAB), 2mL of alkaline catalyst (28% ammonia solution), europium ion complex solution (Eu(TTA)3 dissolved in 10mL of anhydrous ethanol) to 180mL of alcohol solvent (water and anhydrous ethanol in a volume ratio of 15:3), and 3mL of TEOS. The molar ratio of TEOS to europium ion complex is 100:2. React at 60℃ for 12h. Separate the solid and liquid phases, wash with anhydrous ethanol and water, and dry to obtain Eu particles with a particle size of 50nm~200nm. 3+ @MSNs.

[0143] S2. Under an inert atmosphere, add Eu (after removing surface adsorbed water and chemically bound water) to toluene. 3+ @MSNs, BIBB-Si ((3-(2-bromoisobutyryl)propyl)dimethoxysilane) and triethylamine, toluene, Eu 3+ @MSNs, BIBB-Si, and triethylamine were reacted in a volume-to-mass ratio of 60 mL:1 g:0.15 g:0.3 mL at 110 °C under reflux for 24 h. The mixture was then separated into solid and liquid phases, washed with toluene, and vacuum dried to obtain the initiator-functionalized Eu. 3+ @MSNs (referred to as Eu) 3+ @MSNs-Br).

[0144] Eu prepared in this embodiment 3+ @MSNs-Br was scanned by transmission electron microscopy, and the results are as follows: Figure 1 As shown. From Figure 1 It can be seen from Eu 3+ @MSNs-Br has a smooth surface and good monodispersity, providing a rigid core basis for the subsequent shell.

[0145] S3. Under an inert atmosphere, add Eu to the mixed solvent (toluene and DMF in a volume ratio of 2:1). 3+ @MSNs-Br, Acrylate-TPE, St-BPin, CuBr and PMDETA, with a molar ratio of Acrylate-TPE to St-BPin of 90:10, mixed solvent, Eu 3+ The volume-to-mass ratio of @MSNs-Br, Acrylate-TPE, CuBr and PMDETA was 40 mL:0.5 g:1.2 g:0.02 g:0.04 mL. Free radical polymerization was carried out at 80 °C. After 36 h, solid-liquid separation was performed, and the mixture was washed with THF and dried to obtain core-shell hybrid particles.

[0146] The preparation method of the thermally reversible crosslinked network precursor includes the following steps:

[0147] S4. Epoxy resin E-51 and triphenylphosphine were added to furan-methanol. The molar ratio of epoxy groups, triphenylphosphine and furan-methanol in the epoxy resin was 100:2:150. The ring-opening addition reaction was carried out at 90℃. After 2 hours, a furan-modified epoxy resin solution was obtained.

[0148] S5. Add bismaleimide (1,6-bismaleimide hexane) to the furan-modified epoxy resin solution. The molar ratio of furan groups to maleimide groups is 1:1. Perform Diels-Alder reaction at 90℃. After 6 h, a thermally reversible crosslinked network precursor is obtained.

[0149] The preparation method of the aqueous crosslinked network precursor includes the following steps:

[0150] S6. Mix and melt the leuco dye (CVL) and C18 straight-chain saturated alkanes to form an oil phase.

[0151] Acrylamide, MBA, HMPP, and emulsifier (Span 80 and Tween 80 in a mass ratio of 1:3) were added to water, with the mass ratio of water, acrylamide, MBA, HMPP, and emulsifier being 75:20:0.2:1:3, to form an aqueous phase.

[0152] The mass ratio of the leuco dye, C18 straight-chain saturated alkane, and aqueous phase is 5:12:83.

[0153] S7. Mix the oil phase and the water phase to obtain an aqueous crosslinked network precursor.

[0154] This embodiment does not limit the order in which the core-shell hybrid particles, the thermally reversible crosslinked network precursor, and the aqueous crosslinked network precursor are prepared, that is, it does not limit the order between S1~S3, S4~S5, and S6~S7.

[0155] Example 2

[0156] This embodiment provides a thermosensitive composite coating, including a stress-sensitive fluorescent layer and a thermochromic layer, wherein the stress-sensitive fluorescent layer is located between the substrate and the thermochromic layer.

[0157] The stress-sensitive fluorescent layer comprises the following raw materials: epoxy resin and core-shell hybrid particles in a mass ratio of 100:20, and methyl ethyl ketone (MEK) for adjusting the viscosity of the slurry. The core-shell hybrid particles have a mononuclear double-shell structure. The core material includes trivalent europium ions, the inner shell material includes mesoporous silica nanomaterials, and the outer shell material includes a polymer material obtained by polymerizing Acrylate-TPE and St-BPin.

[0158] The thermochromic layer comprises the following raw materials: a thermally reversible crosslinked network precursor in a mass ratio of 100:20:4, an aqueous crosslinked network precursor containing a leuco dye, and bisphenol A, and a second diluent (butanone) for adjusting the viscosity of the slurry. The crosslinking density of the thermally reversible crosslinked network precursor is 15.8%.

[0159] The preparation method of core-shell hybrid particles includes the following steps:

[0160] S1. Dissolve the europium ion complex (Eu(TTA)3Phen) in 15 mL of anhydrous ethanol to form a europium ion complex solution. Add the template agent (CTAB), basic catalyst (25% ammonia solution), europium ion complex solution, and TEOS to an alcohol solvent (water and anhydrous ethanol in a volume ratio of 10:5). The molar ratio of TEOS to europium ion complex is 100:4. The volume-to-mass ratio of alcohol solvent, template agent, basic catalyst, and TEOS is 200 mL:0.8 g:1.5 mL:2 mL. React at 40 °C for 18 h. Separate the solid and liquid phases, wash with anhydrous ethanol and water, and dry to obtain Eu particles with a particle size of 50 nm to 200 nm. 3+ @MSNs.

[0161] S2. Under an inert atmosphere, add Eu (after removing surface adsorbed water and chemically bound water) to toluene. 3+ @MSNs, BIBB-Si ((3-(2-bromoisobutyryl)propyl)dimethoxysilane) and triethylamine, toluene, Eu 3+ @MSNs, BIBB-Si, and triethylamine were reacted in a volume-to-mass ratio of 80 mL:1 g:0.2 g:0.4 mL at 105 °C under reflux for 30 h. The mixture was then separated into solid and liquid phases, washed with acetone, and vacuum dried to obtain the initiator-functionalized Eu. 3+ @MSNs (referred to as Eu) 3+ @MSNs-Br).

[0162] S3. Under an inert atmosphere, add Eu to the mixed solvent (toluene and DMF in a volume ratio of 1.5:1). 3+ @MSNs-Br, Acrylate-TPE, St-BPin, CuBr and PMDETA, with a molar ratio of Acrylate-TPE to St-BPin of 95:5, mixed solvent, Eu 3+ The volume-to-mass ratio of @MSNs-Br, Acrylate-TPE, CuBr and PMDETA was 30 mL:0.5 g:1.5 g:0.03 g:0.05 mL. Free radical polymerization was carried out at 90 °C. After 24 h, solid-liquid separation was performed, and the mixture was washed with THF and dried to obtain core-shell hybrid particles.

[0163] The preparation method of the thermally reversible crosslinked network precursor includes the following steps:

[0164] S4. Epoxy resin E-51 and triphenylphosphine were added to furan-methanol. The molar ratio of epoxy groups, triphenylphosphine and furan-methanol in the epoxy resin was 100:3:200. The ring-opening addition reaction was carried out at 70℃. After 5 h, a furan-modified epoxy resin solution was obtained.

[0165] S5. Add bismaleimide (1,6-bismaleimide hexane) to the furan-modified epoxy resin solution. The molar ratio of furan groups to maleimide groups is 1:1. Perform Diels-Alder reaction at 100℃. After 4 hours, a thermally reversible crosslinked network precursor is obtained.

[0166] The preparation method of the aqueous crosslinked network precursor includes the following steps:

[0167] S6. Mix and melt the leuco dye (CVL) and C18 straight-chain saturated alkanes to form an oil phase.

[0168] Acrylamide, MBA, HMPP, and emulsifier (Span 80 and Tween 80 in a mass ratio of 1:2) were added to water, with the mass ratio of water, acrylamide, MBA, HMPP, and emulsifier being 70:18:0.1:0.8:1, to form an aqueous phase.

[0169] The mass ratio of the leuco dye, C18 straight-chain saturated alkane, and aqueous phase is 3:8:89.

[0170] S7. Mix the oil phase and the water phase to obtain an aqueous crosslinked network precursor.

[0171] This embodiment does not limit the order in which the core-shell hybrid particles, the thermally reversible crosslinked network precursor, and the aqueous crosslinked network precursor are prepared, that is, it does not limit the order between S1~S3, S4~S5, and S6~S7.

[0172] Example 3

[0173] This embodiment provides a thermosensitive composite coating, including a stress-sensitive fluorescent layer and a thermochromic layer, wherein the stress-sensitive fluorescent layer is located between the substrate and the thermochromic layer.

[0174] The stress-sensitive fluorescent layer comprises the following raw materials: epoxy resin and core-shell hybrid particles in a mass ratio of 100:10, and methyl ethyl ketone (MEK) for adjusting the viscosity of the slurry. The core-shell hybrid particles have a mononuclear double-shell structure. The core material includes trivalent europium ions, the inner shell material includes mesoporous silica nanomaterials, and the outer shell material includes a polymer material obtained by polymerizing Acrylate-TPE and St-BPin.

[0175] The thermochromic layer comprises the following raw materials: a thermally reversible crosslinking network precursor in a mass ratio of 100:10:1, an aqueous crosslinking network precursor containing a leuco dye, and bisphenol A, and a second diluent (ethyl acetate) for adjusting the viscosity of the slurry. The crosslinking density of the thermally reversible crosslinking network precursor is 18.1%.

[0176] The preparation method of core-shell hybrid particles includes the following steps:

[0177] S1. Dissolve the europium ion complex (Eu(TTA)3Phen) in 12 mL of anhydrous ethanol to form a europium ion complex solution. Add the template agent (CTAB), basic catalyst (30% ammonia solution), europium ion complex solution, and TEOS to an alcohol solvent (water and anhydrous ethanol in a volume ratio of 12:4). The molar ratio of TEOS to europium ion complex is 100:1. The volume-to-mass ratio of alcohol solvent, template agent, basic catalyst, and TEOS is 150 mL:1.5 g:2.5 mL:4 mL. React at 50 °C for 15 h. Separate the solid and liquid phases, wash with anhydrous ethanol and water, and dry to obtain Eu particles with a particle size of 50 nm to 200 nm. 3+ @MSNs.

[0178] S2. Under an inert atmosphere, add Eu (after removing surface adsorbed water and chemically bound water) to toluene. 3+ @MSNs, BIBB-Si ((3-(2-bromoisobutyryl)propyl)trimethoxysilane) and triethylamine, toluene, Eu 3+ @MSNs, BIBB-Si, and triethylamine were reacted in a volume-to-mass ratio of 50 mL:1 g:0.1 g:0.2 mL at 115 °C under reflux for 20 h. The mixture was then separated into solid and liquid phases, washed with anhydrous ethanol, and vacuum dried to obtain the initiator-functionalized Eu. 3+ @MSNs (referred to as Eu) 3+ @MSNs-Br).

[0179] S3. Under an inert atmosphere, add Eu to the mixed solvent (toluene and DMF in a volume ratio of 2.5:1). 3+ @MSNs-Br, Acrylate-TPE, St-BPin, CuBr and PMDETA, with a molar ratio of Acrylate-TPE to St-BPin of 88:12, mixed solvent, Eu 3+ The volume-to-mass ratio of @MSNs-Br, Acrylate-TPE, CuBr and PMDETA was 50 mL:0.5 g:0.8 g:0.01 g:0.02 mL. The mixture was subjected to free radical polymerization at 70 °C. After 45 h, the solid and liquid phases were separated, washed with THF, and dried to obtain core-shell hybrid particles.

[0180] The preparation method of the thermally reversible crosslinked network precursor includes the following steps:

[0181] S4. Epoxy resin E-51 and triphenylphosphine were added to furan-methanol. The molar ratio of epoxy groups, triphenylphosphine and furan-methanol in the epoxy resin was 100:1:150. The ring-opening addition reaction was carried out at 80℃. After 3 hours, a furan-modified epoxy resin solution was obtained.

[0182] S5. Add bismaleimide (1,6-bismaleimide hexane) to the furan-modified epoxy resin solution. The molar ratio of furan groups to maleimide groups is 1:1. Perform Diels-Alder reaction at 80℃. After 8 hours, a thermally reversible crosslinked network precursor is obtained.

[0183] The preparation method of the aqueous crosslinked network precursor includes the following steps:

[0184] S6. Mix and melt the leuco dye (CVL) and C20 branched saturated alkanes to form an oil phase.

[0185] Acrylamide, MBA, HMPP, and emulsifier (Span 80 and Tween 80 in a mass ratio of 1:4) were added to water, with the mass ratio of water, acrylamide, MBA, HMPP, and emulsifier being 80:22:0.3:1.2:5, to form an aqueous phase.

[0186] The mass ratio of the leuco dye, C20 branched saturated alkane, and aqueous phase is 8:18:74.

[0187] S7. Mix the oil phase and the water phase to obtain an aqueous crosslinked network precursor.

[0188] This embodiment does not limit the order in which the core-shell hybrid particles, the thermally reversible crosslinked network precursor, and the aqueous crosslinked network precursor are prepared, that is, it does not limit the order between S1~S3, S4~S5, and S6~S7.

[0189] Comparative Example 1

[0190] This comparative example provides a thermo-sensitive dual-sensitivity composite coating, similar to Example 2, except that the molar ratio of Acrylate-TPE to St-BPin in the shell material of the core-shell hybrid particles of the stress-sensitive fluorescent layer is 99.5:0.5. The remaining components and raw material ratios are the same as in Example 2 and will not be repeated.

[0191] Comparative Example 2

[0192] This comparative example provides a thermo-sensitive composite coating, similar to Example 3, except that the crosslinking density of the thermally reversible crosslinked network precursor is 34.5%.

[0193] The preparation method of the thermally reversible crosslinked network precursor includes the following steps:

[0194] S4. Epoxy resin E-51 and triphenylphosphine were added to furan-methanol. The molar ratio of epoxy groups, triphenylphosphine and furan-methanol in the epoxy resin was 100:1:300. The ring-opening addition reaction was carried out at 80℃. After 3 hours, a furan-modified epoxy resin solution was obtained.

[0195] S5. Add bismaleimide (1,6-bismaleimide hexane) to the furan-modified epoxy resin solution. The molar ratio of furan groups to maleimide groups is 1:1. Perform Diels-Alder reaction at 105℃. After 10 h, a thermally reversible crosslinked network precursor is obtained.

[0196] The remaining components and raw material ratios are the same as in Example 3, and will not be repeated here.

[0197] Application Example 1

[0198] This application example provides a method for visually detecting surface defects in low-carbon steel, including the following steps:

[0199] S8. Weigh the raw materials for the stress-sensitive fluorescent layer according to the design ratio of Example 1. Mix the resin matrix, core-shell hybrid particles and methyl ethyl ketone evenly to obtain a bottom layer slurry with a viscosity of 450 mPa·s. Coat the bottom layer slurry onto the substrate surface and cure at 90°C for 2 hours to form a 100 μm thick stress-sensitive fluorescent layer.

[0200] S9. Weigh the raw materials for the thermochromic layer according to the design ratio in Example 1. Mix the thermally reversible crosslinking network precursor, the aqueous crosslinking network precursor, bisphenol A, and the second diluent evenly to obtain a surface slurry with a viscosity of 300 mPa·s. Coat the surface slurry onto the surface of the stress-sensitive fluorescent layer at 300 mW / cm². 2 UV curing was performed, followed by curing at 50°C for 36 hours to form a 40μm thick thermochromic layer.

[0201] See the morphological images of the low-carbon steel surface before and after coating with a thermosensitive composite coating. Figures 2-3 ,Depend on Figures 2-3 It can be seen that there is little difference in the surface morphology of low-carbon steel before and after coating with a thermo-sensitive composite coating.

[0202] S10. Apply load to the thermosensitive composite coating, perform image detection and digital acquisition, and perform digital analysis and defect diagnosis based on the test results.

[0203] S10-1, Specific Load Excitation

[0204] The coated specimen is mounted on an electronic universal testing machine, and the loading program is set to: apply an amplitude equal to the specimen's yield strength (σ). s Apply 60% static tensile load and hold the load for 120 seconds, then unload.

[0205] S10-2, Image Detection and Digital Acquisition

[0206] After unloading, the sample was moved to the self-built dual-mode image acquisition platform.

[0207] (1) Fluorescence signal acquisition:

[0208] Turn off all ambient light sources and use a 365nm ultraviolet LED panel lamp to evenly irradiate the sample surface.

[0209] A 400nm long-pass filter was installed in front of the camera lens (MER-500-7UM CMOS camera).

[0210] Camera parameters were set as follows: exposure time 150ms, gain 1.0. Images were acquired and saved as 16-bit TIFF files. Bright blue-green fluorescence was observed at the locations corresponding to subcutaneous bubbles and inclusions (see...). Figure 4 This creates high-contrast spots.

[0211] (2) Acquisition of thermochromic signals:

[0212] Turn on the D65 standard white LED ring light source.

[0213] Remove the UV filter in front of the camera and perform white balance correction using a standard white board.

[0214] Camera parameters: exposure time 30ms, color saturation +5. Capture the image and save it as a RAW file in RGB format. It can be observed that at the same defect location, the surface layer permanently changes from black to colorless (revealing the underlying color), forming a clear area of ​​color difference.

[0215] S10-3, Digital Analysis and Defect Diagnosis

[0216] The two acquired images were imported into our self-developed intelligent defect diagnosis software for analysis.

[0217] (1) Image registration and preprocessing:

[0218] The software automatically uses the ORB feature matching algorithm to perform subpixel-level registration between the fluorescence image and the color image.

[0219] The fluorescence image was subjected to contrast-limited adaptive histogram equalization (CLAHE) with specific parameters: tile size of 8×8 and clip limit of 2.0, to enhance the visibility of weak stress signals.

[0220] Non-local mean denoising was performed on the color image with a search window size of 21×21 and a similarity window size of 7×7 to suppress noise.

[0221] (2) Feature extraction and quantization:

[0222] ① Generation of the stress distribution matrix (S-Matrix):

[0223] Extract the green channel from the registered fluorescence image (or use the grayscale value directly in the monochrome image) as the object of analysis.

[0224] Image thresholding is performed using the Otsu method, which automatically calculates the optimal threshold and initially separates the background and potential defect signal regions.

[0225] Morphological opening operations are applied, using circular structuring elements (radius 3 pixels) for erosion followed by dilation to eliminate tiny noise points and connect adjacent small regions.

[0226] For each identified connected component, the average gray value and pixel area within that region are calculated. This generates a stress distribution matrix (S-Matrix) corresponding to the image pixel location, the values ​​of which quantify the relative stress level at each location.

[0227] ② Generation of the temperature distribution matrix (T-Matrix):

[0228] The registered color image is converted from the RGB space to CIEL, which has a more uniform color perception. * a * b * Color space.

[0229] Using the average Lab value of the normal region as a reference, calculate the color difference ΔE between each pixel in the image and the reference. The formula is ΔE = [(ΔL * ) 2 +(Δa * ) 2 +(Δb * ) 2 ] 1 / 2 .

[0230] By setting a ΔE threshold (e.g., ΔE > 5), significant color change areas are identified, and a temperature distribution matrix (T-Matrix) is generated, the value of which represents the degree of color difference at that point due to historical temperature rise.

[0231] (3) Integration of decision-making and defect assessment:

[0232] ① Feature Fusion: The normalized stress distribution matrix (S_norm) and temperature distribution matrix (T_norm) are fused. This invention proposes two fusion strategies:

[0233] Strategy 1 (Weighted Linear Fusion): Construct a comprehensive defect risk index matrix (R), calculated as R = α × S_norm + β × T_norm. The weighting coefficients α and β can be determined experimentally (e.g., initially set α = 0.6, β = 0.4) to balance the contributions of the two signals to different types of defects.

[0234] Strategy Two (Deep Learning Fusion): The S_norm and T_norm are used as dual-channel images and input into a pre-trained Convolutional Neural Network (CNN). This CNN network structure includes: an input layer (dual-channel, same size as the image), three convolutional layers (3×3 kernel size, ReLU activation function), two pooling layers (max pooling, 2×2 pooling window), a fully connected layer, and an output layer (using the Sigmoid activation function to output defect probabilities). The network is trained using a large number of known defect samples, employing the cross-entropy loss function and the Adam optimizer. After training, the network directly outputs a Defect Probability Map, where each pixel value represents the probability of a defect existing at that location.

[0235] ②Defect identification and classification:

[0236] Threshold segmentation and localization: For the risk matrix or probability map obtained after fusion, an empirical judgment threshold is set (e.g., R > 0.7 or probability > 0.8). The system automatically marks all connected regions exceeding the threshold and outputs their center pixel coordinates and the surrounding rectangular region as the defect location.

[0237] Intelligent Classification: Based on a large sample library of known defect types (metallographically verified), multi-dimensional features of each defect region are extracted, including but not limited to: morphological features (area, perimeter, roundness, aspect ratio) and signal features (mean stress signal intensity S_mean, mean thermochromic signal intensity T_mean, S / T ratio). S_mean refers to the arithmetic mean of the grayscale values ​​or fluorescence intensities of all pixels within each connected region of the defect identified by the image processing algorithm on the stress distribution matrix (S-Matrix); T_mean refers to the arithmetic mean of the color difference (e.g., ΔE value) of all pixels within the same connected region of the defect on the temperature distribution matrix (T-Matrix); the S / T ratio, also called the stress-temperature signal ratio, refers to the ratio of the above two features, i.e., S_mean / T_mean. These features are used to train a Support Vector Machine (SVM) or Random Forest classifier. The system can automatically classify newly detected defect regions into categories such as "subcutaneous bubbles," "deeply buried inclusions," "microcracks," or "roll marks," and provide the corresponding confidence scores.

[0238] S10-4 Result Output

[0239] The system ultimately generates an inspection report and sends the defect coordinates (with the top left corner of the image as the origin) and a list of defects to the simulated PLC system via TCP / IP protocol. The report shows that two defects were successfully detected: one was a high-risk subcutaneous bubble, for which the system recommends "scrapping"; the other was a deeply embedded inclusion, for which the system recommends "focusing on subsequent processes." Subsequent wire cutting and metallographic examination of the sample confirmed the existence and type of the two defects, verifying the accuracy and reliability of the method.

[0240] The detection image in this application example is as follows: Figure 9 As shown.

[0241] Application Example 2

[0242] This application example provides a method for visually detecting surface defects in low-carbon steel, including the following steps:

[0243] S8. Weigh the raw materials for the stress-sensitive fluorescent layer according to the design ratio of Example 2, mix the resin matrix, core-shell hybrid particles and methyl ethyl ketone evenly to obtain a bottom layer slurry with a viscosity of 600 mPa·s; coat the bottom layer slurry onto the substrate surface and cure at 80°C for 2.5 h to form a stress-sensitive fluorescent layer with a thickness of 120 μm.

[0244] S9. Weigh the raw materials for the thermochromic layer according to the design ratio of Example 2. Mix the thermally reversible crosslinking network precursor, aqueous crosslinking network precursor, bisphenol A, and second diluent evenly to obtain a surface slurry with a viscosity of 400 mPa·s. Coat the surface slurry onto the surface of the stress-sensitive fluorescent layer at 100 mW / cm². 2 UV curing was performed, followed by curing at room temperature for 24 hours after 100 seconds to form a 50μm thick thermochromic layer.

[0245] S10. Apply a load to the thermo-sensitive composite coating, perform image detection and digital acquisition, and conduct digital analysis and defect diagnosis based on the test results. The specific steps are similar to those in Application Example 1 and will not be repeated here.

[0246] The detection image in this application example is as follows: Figure 5 As shown.

[0247] Comparative Application Example 1

[0248] This comparative application example provides a method for visually detecting surface defects in low-carbon steel, similar to Application Example 2, except that in steps S8-S9, the preparation of the base slurry and top slurry is based on the condition parameters of the thermo-sensitive composite coating in Comparative Example 1. The remaining steps and parameter conditions are the same as in Application Example 2 and will not be repeated here.

[0249] The application compares the detected images to a similar scale, such as... Figure 6 As shown. From Figures 5-6 As can be seen, the thermo-sensitive composite coating provided in Comparative Example 1 suffers from a severe lack of force response units, resulting in the polymer shell material being unable to effectively respond to mechanical stress. After undergoing the same load, the fluorescence enhancement factor (ΔI / I0) of the resulting coating is only about 1.5 times, far lower than that of the thermo-sensitive composite coating provided in Example 2 (more than 4 times). The results indicate that if the amount of St-BPin is too small, the stress-sensitive fluorescent layer will be insensitive to defect stress, failing to generate a sufficiently strong optical signal for detection, ultimately leading to a very high risk of missing even minute defects.

[0250] Application Example 3

[0251] This application example provides a method for visually detecting surface defects in low-carbon steel, including the following steps:

[0252] S8. Weigh the raw materials for the stress-sensitive fluorescent layer according to the design ratio of Example 2, mix the resin matrix, core-shell hybrid particles and methyl ethyl ketone evenly to obtain a bottom layer slurry with a viscosity of 350 mPa·s; coat the bottom layer slurry onto the substrate surface and cure at 70°C for 3 h to form an 80 μm thick stress-sensitive fluorescent layer.

[0253] S9. Weigh the raw materials for the thermochromic layer according to the design ratio of Example 2. Mix the thermally reversible crosslinking network precursor, aqueous crosslinking network precursor, bisphenol A, and second diluent evenly to obtain a surface slurry with a viscosity of 200 mPa·s. Coat the surface slurry onto the surface of the stress-sensitive fluorescent layer at 450 mW / cm². 2 UV curing was performed, followed by curing at 60°C for 300 seconds, and then at 60°C for 30 hours to form a 22μm thick thermochromic layer.

[0254] S10. Apply a load to the thermo-sensitive composite coating, perform image detection and digital acquisition, and conduct digital analysis and defect diagnosis based on the test results. The specific steps are similar to those in Application Example 1 and will not be repeated here.

[0255] The detection image in this application example is as follows: Figure 7 As shown.

[0256] Comparative Application Example 2

[0257] This comparative application example provides a method for visually detecting surface defects in low-carbon steel, similar to Application Example 3, except that in steps S8-S9, the preparation of the base layer slurry and the top layer slurry is based on the condition parameters of the thermo-sensitive composite coating in Comparative Example 2. The remaining steps and parameter conditions are the same as in Application Example 3 and will not be repeated here.

[0258] The application compares the detected images to a similar scale, such as... Figure 8 As shown. From Figures 7-8 As can be seen, the crosslinking density of the thermally reversible crosslinked network precursor in Comparative Example 2 is too high, making the thermally reversible polymer network too rigid, and its reverse DA reaction temperature rises to about 95°C. Under the same load conditions (local temperature rise at the defect is about 70°C), no visible color change occurred in the surface layer of Comparative Application Example 2, proving that under this parameter (crosslinking density of the thermally reversible crosslinked network precursor is 34.5%), the thermal triggering mechanism fails, and the surface layer completely loses its ability to record the local temperature rise at the defect.

[0259] Comparative Application Example 3

[0260] This comparative application example provides a method for visually detecting surface defects in low-carbon steel, similar to Application Example 1, except that in S10, the applied amplitude is equal to the sample's yield strength (σ). s Apply a static tensile load of 20%. The remaining steps and parameter conditions are the same as in Application Example 1, and will not be repeated here.

[0261] The application compares the detected images to a similar scale, such as... Figure 10 As shown. From Figures 9-10 As can be seen from the comparison with application example 3, the sample σ sA static tensile load level of 20% is insufficient to excite deep defects (such as deformation of subcutaneous bubbles or interfacial debonding of inclusions), thus preventing significant stress concentration and localized temperature rise in the defect area. After unloading, no effective signal corresponding to the known defect location was observed in either fluorescence or thermochromic images. This demonstrates that the load was too low, preventing the detection system from starting and rendering the entire method ineffective.

[0262] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A thermo-sensitive composite coating, characterized in that, It includes a stress-sensitive fluorescent layer and a thermochromic layer, wherein the stress-sensitive fluorescent layer is located between the substrate and the thermochromic layer; The stress-sensitive fluorescent layer comprises the following raw materials: a resin matrix, core-shell hybrid particles, and a first diluent; the core-shell hybrid particles have a mononuclear double-shell structure, the core material includes trivalent europium ions, the inner shell material includes mesoporous silica nanomaterials, and the outer shell material includes a polymer material obtained by polymerizing a tetraphenylvinyl unsaturated monomer and a borate ester unsaturated monomer; the molar ratio of the tetraphenylvinyl unsaturated monomer to the borate ester unsaturated monomer is (85~99):(15~1); The thermochromic layer comprises the following raw materials: a thermally reversible crosslinked network precursor, an aqueous crosslinked network precursor containing a leuco dye, a solid acid color developer, and a second diluent. The thermally reversible crosslinked network precursor is prepared by a Diels-Alder reaction of furan-modified epoxy resin and bismaleimide; the crosslinking density of the thermally reversible crosslinked network precursor is 5%~30%.

2. The thermosensitive composite coating as described in claim 1, characterized in that, The mass ratio of the resin matrix to the core-shell hybrid particles in the stress-sensitive fluorescent layer is 100:(5~30); The solid acid color developer includes bisphenol A; the mass ratio of the thermally reversible crosslinking network precursor, the aqueous crosslinking network precursor containing the leuco dye, and the solid acid color developer in the thermochromic layer is 100:(5~30):(0.5~5).

3. The thermosensitive composite coating as described in claim 1, characterized in that, The preparation method of the core-shell hybrid particles includes the following steps: S1. A template agent, a basic catalyst, a europium ion complex, and a silicon source are added to an alcohol solvent, and the reaction proceeds to obtain Eu. 3+ @MSNs; S2. Under an inert atmosphere, add the Eu to a benzene-based solvent. 3+ @MSNs, a silyl compound containing an initiating group, and an acid-binding agent are subjected to a reflux reaction to obtain initiated functionalized Eu. 3+ @MSNs; S3. Under an inert atmosphere, add the aforementioned initiating functionalized Eu to the mixed solvent. 3+ The core-shell hybrid particles are obtained by radical polymerization of MSNs, tetraphenylvinyl unsaturated monomers, borate ester unsaturated monomers, and coordination catalysts.

4. The thermo-sensitive composite coating as described in claim 3, characterized in that, In S1, the europium ion complex includes at least one of Eu(TTA)3 or Eu(TTA)3Phen; the silicon source includes tetraethyl orthosilicate; the molar ratio of the silicon source to the europium ion complex is 100:(0.5~5); the volume-to-mass ratio of the alcohol solvent, template agent, basic catalyst and silicon source is (130~200)mL:(0.5~2)g:(1~3)mL:(1.5~5)mL; In S1, the reaction temperature is 25℃~65℃, and the reaction time is 6h~24h; the Eu 3+ The particle size of @MSNs is 50nm~200nm; In S2, the silyl compound containing the initiating group includes (3-(2-bromoisobutyryl)propyl)dimethoxysilane or (3-(2-bromoisobutyryl)propyl)trimethoxysilane; the benzene solvent, Eu 3+ The volume-to-mass ratio of @MSNs, initiator-containing silyl compounds, and acid-binding agents is (50~80) mL: 1 g: (0.05~0.25) g: (0.15~0.4) mL; In S2, the reflux reaction temperature is 100℃~120℃, and the reaction time is 18h~36h; In S3, the tetraphenylvinyl group unsaturated monomer includes 4-(1,2,2-triphenylvinyl)phenyl acrylate; the borate ester group unsaturated monomer includes (4-vinylphenyl)boronic acid pinacol ester; In S3, the coordination catalyst comprises CuBr and pentamethyldiethylenetriamine; the mixed solvent and the initiator functionalized Eu 3 + The volume-to-mass ratio of @MSNs, tetraphenylvinyl group unsaturated monomer, CuBr and pentamethyldiethylenetriamine is (30~50)mL:0.5g:(0.8~1.5)g:(0.01~0.03)g:(0.02~0.06)mL; In S3, the temperature of the free radical polymerization reaction is 70℃~90℃, and the reaction time is 24h~48h.

5. The thermosensitive composite coating as described in claim 1 or 2, characterized in that, The method for preparing the thermally reversible crosslinked network precursor includes the following steps: S4. Add epoxy resin and phosphine catalyst to furan-methanol to carry out ring-opening addition reaction to obtain furan-modified epoxy resin solution. S5. Add bismaleimide to the furan-modified epoxy resin solution and carry out a Diels-Alder reaction to obtain the thermally reversible crosslinked network precursor. The method for preparing the aqueous crosslinked network precursor containing the leuco dye includes the following steps: S6. Mix and melt the leuco dye and C16~C22 alkanes to form an oil phase; A water-soluble vinyl monomer, crosslinking agent, photoinitiator, and emulsifier are added to water to form an aqueous phase; S7. Mix the oil phase and the aqueous phase to obtain the aqueous crosslinked network precursor containing the leuco dye.

6. The thermo-sensitive composite coating as described in claim 5, characterized in that, In S4, the epoxy resin is a bisphenol A type epoxy resin; the phosphine catalyst includes triphenylphosphine; the molar ratio of epoxy groups, phosphine catalyst and furanol in the epoxy resin is 100:(1~3):(120~300); In S4, the ring-opening addition reaction is carried out at a temperature of 60℃~90℃ for a reaction time of 2h~6h. In S5, the bismaleimide comprises 1,6-bismaleimide hexane; the molar ratio of furan groups in the furan-modified epoxy resin solution to maleimide groups in the bismaleimide is 1:(0.9~1.1); In S5, the temperature of the Diels-Alder reaction is 70℃~100℃, and the reaction time is 4h~10h.

7. The thermosensitive composite coating as described in claim 5, characterized in that, In S6, the leuco dye includes crystal violet lactone; the mass ratio of the leuco dye, C16~C22 alkane and aqueous phase is (1~10):(5~20):(70~94); the mass ratio of water, water-soluble vinyl monomer, crosslinking agent, photoinitiator and emulsifier is (70~80):(18~22):(0.1~0.3):(0.8~1.2):(1~5).

8. The method for preparing the thermo-sensitive composite coating according to any one of claims 1 to 7, characterized in that, Includes the following steps: Step 1: Weigh the raw materials for the stress-sensitive fluorescent layer according to the design ratio, mix the resin matrix, core-shell hybrid particles and first diluent evenly to obtain the bottom layer slurry; coat the bottom layer slurry onto the substrate surface and cure to form the stress-sensitive fluorescent layer; Step 2: Weigh the raw materials for the thermochromic layer according to the designed ratio, mix the thermally reversible crosslinking network precursor, the aqueous crosslinking network precursor containing leuco dye, the solid acid color developer, and the second diluent evenly to obtain the surface layer slurry; coat the surface layer slurry onto the surface of the stress-sensitive fluorescent layer, and perform ultraviolet curing and low-temperature curing in sequence. The low-temperature curing temperature is 20℃~60℃ to form the thermochromic layer.

9. The method for preparing the thermo-sensitive composite coating as described in claim 8, characterized in that, In step one, the viscosity of the bottom layer slurry is 200 mPa·s to 800 mPa·s; in step two, the viscosity of the top layer slurry is 100 mPa·s to 400 mPa·s. In step one, the curing temperature is 60℃~100℃, and the curing time is 1h~4h; in step two, the UV curing light intensity is 50mW / cm². 2 ~500mW / cm 2 The UV curing time is 30s~300s, and the low-temperature curing time is 2h~48h; In step one, the thickness of the stress-sensitive fluorescent layer is 50 μm to 150 μm; In step two, the thickness of the thermochromic layer is 20μm~50μm.

10. The application of the thermo-sensitive composite coating according to any one of claims 1 to 7, or the thermo-sensitive composite coating prepared by the preparation method of the thermo-sensitive composite coating according to claim 8 or 9, in the detection of surface defects in carbon steel.

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