Low-power-consumption sensing coating material for monitoring service life of automobile calipers and application method

Abstract

The invention relates to the technical field of sensing materials, and discloses a low-power-consumption sensing coating material for monitoring the service life of automobile calipers and an application method of the low-power-consumption sensing coating material for monitoring the service life of the automobile calipers. 20 to 40 parts of crystalline flake graphite; 5 to 20 parts of nano silver particles; 5 to 20 parts of hexagonal boron nitride nanosheets; and 0.5-5 parts of a silane coupling agent. According to the invention, resistance temperature self-compensation is realized by compounding crystalline flake graphite and nano-silver particles; the piezoresistive effect is inhibited by using the hexagonal boron nitride nanosheets; and a silane coupling agent and a polyimide matrix are adopted to ensure strong binding force and environmental stability of the coating and a metal substrate. The coating is arranged in the abrasion area of the caliper, and the reduction amount of the thickness of the coating is monitored by measuring the change of a resistance value. The problems that an existing sensing coating signal is prone to being interfered by temperature and pressure and the environment stability is poor are solved, and accurate, reliable and long-acting monitoring on abrasion of the automobile calipers is achieved.

Description

Technical Field

[0001] This invention relates to the field of sensing materials technology, and in particular to low-power sensing coating materials and application methods for monitoring the lifespan of automotive calipers. Background Technology

[0002] The automotive braking system is a core component ensuring driving safety, and its performance reliability is paramount. As a key actuator in the braking system, the brake caliper will wear down due to friction during long-term use, affecting braking performance and even leading to safety hazards. Therefore, real-time and accurate monitoring of the wear condition of the brake caliper is of great significance for enabling predictive maintenance and improving driving safety.

[0003] Currently, using conductive polymer composite materials to create sensing coatings and characterizing wear by monitoring changes in their resistance is a promising technical approach. This method involves applying the sensing coating to the wear area of ​​the caliper; as wear occurs, the coating thickness decreases, and its resistance value changes accordingly, thus establishing a correlation between resistance change and wear amount.

[0004] However, conventional sensor coating materials still face significant challenges in practical applications, as their resistance values ​​often exhibit high sensitivity to operating temperature. Due to the inherent temperature effect of resistance in the material itself and the difference in thermal expansion coefficients between the substrate and the filler, drastic temperature changes during braking can cause significant drift in the resistance signal. This drift may be misinterpreted as a genuine wear signal, severely impacting monitoring accuracy. Furthermore, these materials typically exhibit a significant piezoresistive effect; the high pressure applied during braking can cause displacement or compaction of the conductive filler, leading to transient fluctuations in resistance values. This pressure-induced signal noise can interfere with the assessment of wear trends. In addition, brake calipers operate in extremely harsh environments, enduring long-term exposure to high temperatures, vibration, humidity, and salt spray. Existing coatings generally suffer from insufficient adhesion to the metal substrate and poor long-term environmental stability, making them prone to delamination, cracking, or interface corrosion during their service life, ultimately leading to premature sensor failure.

[0005] Therefore, the present invention provides a low-power sensing coating material and application method for monitoring the life of automotive calipers, in order to overcome the shortcomings of the prior art. Summary of the Invention

[0006] The purpose of this invention is to provide a low-power sensing coating material and application method for monitoring the life of automotive calipers, which solves the problem of signal instability caused by temperature changes, pressure fluctuations and environmental aging in existing automotive caliper wear monitoring technologies.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] In a first aspect, the present invention provides a low-power sensing coating material for monitoring the lifespan of automotive ignition points, employing the following technical solution:

[0009] A low-power sensing coating material for monitoring the lifespan of automotive calipers, made from raw materials comprising the following parts by weight:

[0010] Polyamic acid solids: 100 parts;

[0011] Flake graphite: 20-40 parts;

[0012] Nano silver particles: 5-20 parts;

[0013] Hexagonal boron nitride nanosheets: 5-20 parts;

[0014] Silane coupling agent: 0.5-5 parts.

[0015] By employing the above technical solution, this invention utilizes the synergistic effect of multiple functional fillers in a polymer matrix to construct a sensing system that maintains a highly stable resistance signal across a wide temperature range, high pressure, and humid and hot environments. The specific technical principle is explained below:

[0016] Resistance Temperature Self-Compensation Mechanism: The coating system incorporates two conductive fillers: flake graphite and nano-silver particles. Flake graphite exhibits a negative temperature coefficient (NTC), meaning its resistance decreases with increasing temperature; while nano-silver particles exhibit a positive temperature coefficient (PTC), meaning their resistance increases with increasing temperature. By combining these two fillers in a specific ratio, a composite conductive network with parallel and series connections is formed within the polyimide matrix. When the operating temperature changes, the decreasing resistance of the graphite conductive pathway cancels out the increasing resistance of the nano-silver conductive pathway, effectively compensating for the overall temperature coefficient of resistance (TCR) of the coating. This ensures that the coating's reference resistance remains essentially constant within the wide operating temperature range of 25-300℃ for automotive calipers, eliminating the interference of temperature drift on wear monitoring signals.

[0017] Piezoresistive effect suppression mechanism: Hexagonal boron nitride (h-BN) nanosheets are introduced into the coating system. h-BN is an electrically insulating two-dimensional material with high elastic modulus and low coefficient of friction. In the coating, these nanosheets are uniformly dispersed in the polyimide matrix and form a mechanical support and buffer network between the conductive filler particles. When the high pressure (up to 20 MPa) generated during braking is applied to the coating surface, the stress is mainly absorbed and dispersed by the h-BN network. The h-BN nanosheets dissipate mechanical energy through their own elastic deformation or interlayer slip, effectively preventing the conductive fillers (graphite, silver) from being compacted or displaced under pressure, thereby maintaining the stability of the conductive network topology. This makes the coating's resistance value insensitive to changes in external pressure, avoiding signal artifacts caused by braking pressure fluctuations.

[0018] Interface anchoring and long-term stability mechanism: A silane coupling agent, particularly (3-mercaptopropyl)trimethoxysilane (MPTMS), a bifunctional compound, was added to the coating system. This coupling agent plays a dual role in interface anchoring within the system:

[0019] Step 1 (Bonding with the Substrate): The trimethoxysilyl group (-Si(OCH3)3) in the MPTMS molecule hydrolyzes in the presence of trace amounts of moisture, generating highly reactive silanol groups (-Si(OH)3). These silanol groups can undergo condensation reactions with the hydroxyl groups (-OH) on the surface of the steel substrate to form stable -Si-O-Fe chemical bonds; on the other hand, the silanol groups can self-condense, forming a dense -Si-O-Si cross-linked network on the substrate surface. This chemically bonded interfacial layer greatly enhances the adhesion between the coating and the metal caliper substrate and acts as a physical barrier, effectively preventing moisture and corrosive media from penetrating to the interface.

[0020] Step Two (Binding with Filler): The thiol (-SH) group at the other end of the MPTMS molecule has a strong chemical affinity for the surface of the silver nanoparticles, enabling them to firmly anchor the silver nanoparticles within the polymer matrix network. Through this dual action, the conductive filler and the polymer matrix are firmly bonded to the substrate, effectively preventing coating delamination, cracking, or disruption of conductive pathways caused by interface failure and filler migration under long-term humid and hot aging and vibration environments, thus ensuring the long-term reliability of the sensing signal.

[0021] Preferably, the raw materials are in the following weight proportions: polyamic acid solid: 100 parts; flake graphite: 25-30 parts; silver nanoparticles: 10-15 parts; hexagonal boron nitride nanosheets: 10-15 parts; silane coupling agent: 1.0-2.5 parts.

[0022] By adopting the above technical solution, the proportions of each functional component are optimized and balanced, so that the resistance temperature compensation effect, piezoresistive suppression effect and interface stability are all at the best level.

[0023] Preferably, the median particle size of the flake graphite is 5-10 µm; the median particle size of the silver nanoparticles is 50-100 nm; and the lateral dimension of the hexagonal boron nitride nanosheets is 0.5-2 µm, and the thickness is 5-30 nm. By adopting the above technical solution, fillers of different sizes and morphologies can form an efficient and stable functional network structure at the microscale.

[0024] Preferably, the polyamic acid solid is polymerized from pyromellitic dianhydride and 4,4'-diaminodiphenyl ether, wherein the molar ratio of pyromellitic dianhydride to 4,4'-diaminodiphenyl ether is (0.98-1.02):1. By adopting the above technical solution, the obtained polyamic acid precursor has a high molecular weight and suitable viscosity. The polyimide matrix formed after curing exhibits excellent high-temperature resistance (glass transition temperature > 300℃) and mechanical properties, providing a fundamental guarantee for the structural integrity of the sensing coating under harsh operating conditions.

[0025] Preferably, the silane coupling agent is (3-mercaptopropyl)trimethoxysilane. By employing the above technical solution and utilizing the different functional groups at both ends of its molecule, simultaneous and efficient anchoring of metal substrates and noble metal conductive fillers is achieved.

[0026] Secondly, the present invention provides a method for preparing a low-power sensing coating material for monitoring the life of automotive calipers, employing the following technical solution:

[0027] A method for preparing the sensing coating material described in any one of the preceding claims includes the following steps: (1) mixing polyamic acid, silane coupling agent, flake graphite, hexagonal boron nitride nanosheets and silver nanoparticles in a solvent to obtain a coating slurry; (2) coating the coating slurry onto the surface of a substrate; (3) subjecting the coated substrate to gradient curing, wherein the curing process includes at least three heating and holding stages, and the highest curing temperature is 280-320℃.

[0028] By employing the above technical solutions, this invention ensures that the functional components are uniformly dispersed and form the desired microscopic functional structure through a specific preparation process. The dispersion process in step (1) ensures the uniformity of different fillers in the slurry, which is a prerequisite for achieving uniform and stable macroscopic performance. The gradient curing process in step (3) involves a staged temperature increase. First, at a lower temperature (e.g., 120°C), most of the solvent slowly evaporates to avoid bubbles or pinhole defects in the coating. Then, in the medium-temperature zone (e.g., 200°C) and the high-temperature zone (e.g., 300°C), the imidization reaction of polyamic acid is gradually completed, that is, the molecular chain dehydrates and closes the ring to form a structurally stable polyimide. This mild curing regime ensures the density, smoothness, and good adhesion of the final coating to the substrate, which is a necessary process guarantee for achieving the technical effects of the invention.

[0029] Preferably, before step (2), a pretreatment step of sandblasting and cleaning the substrate is included. By adopting the above technical solution, sandblasting creates a uniform roughness on the surface of the substrate, increasing the contact area between the coating and the substrate and forming mechanical locking points; the subsequent cleaning removes oil and impurities from the surface. This pretreatment significantly enhances the physical adhesion of the coating.

[0030] Preferably, in step (1), the mixing in the solvent is carried out by high-speed shear dispersion and ultrasonic dispersion. By adopting the above technical solution, high-speed shear can effectively break up the agglomerates of larger-sized fillers (such as flake graphite), while the high-energy shock waves generated by ultrasonic cavitation can further disperse the nanoscale fillers (nano silver, h-BN) uniformly, avoiding their secondary agglomeration, which is crucial for constructing a uniform conductive network and mechanical buffer network.

[0031] Preferably, in step (3), the gradient curing includes: holding at 100-140℃ for 20-40 minutes, holding at 180-220℃ for 20-40 minutes, and holding at 280-320℃ for 40-80 minutes. By adopting the above technical solution, sufficient time and suitable temperature are provided for each stage of solvent evaporation and imidization reaction, ensuring the complete progress of the curing reaction.

[0032] By adopting the above technical solution, the application principle of this invention is based on the following fact: since the resistance signal of the coating material is not affected by the operating temperature and braking pressure, its resistance value... With coating thickness (i.e., wear) has a clear functional relationship. ,in The electrode spacing, (This refers to the coating width). The thickness of the sensing coating increases as the caliper wears during braking. It will decrease uniformly, resulting in its resistance value It exhibits a predictable, monotonous increase. By continuously or periodically measuring this resistance value through an external circuit, the remaining thickness of the coating can be accurately deduced in real time, thereby enabling quantitative monitoring and early warning of caliper life.

[0033] This method simplifies complex wear condition monitoring into simple resistance measurement, offering advantages such as low power consumption, high reliability, and ease of integration.

[0034] In summary, the present invention has at least one of the following beneficial technical effects:

[0035] 1. The sensing coating material provided by this invention exhibits excellent resistance-temperature stability, with signal output minimally affected by operating temperature fluctuations. This is because this invention composites flake graphite with a negative temperature coefficient (NTC) and nano-silver particles with a positive temperature coefficient (PTC) into a polyimide matrix. By adjusting the ratio of these two components, a self-compensating conductive network with resistance-temperature response is constructed. Therefore, within the wide operating temperature range of automotive calipers, the coating's resistance base value remains highly stable, effectively eliminating the interference of temperature drift on wear monitoring signals and improving the accuracy of monitoring results.

[0036] 2. The sensing coating material provided by this invention exhibits significant piezoresistive suppression, resulting in low signal noise under braking pressure. This is because this invention introduces high-modulus, electrically insulating hexagonal boron nitride (h-BN) nanosheets as a piezoresistive inhibitor. These nanosheets construct a mechanical support structure within the conductive network, effectively absorbing and dispersing the high stress generated during braking and maintaining the stability of the relative positions between the conductive fillers. This design makes the coating's resistance insensitive to pressure changes, avoiding spurious signals caused by braking pressure fluctuations and ensuring a high signal-to-noise ratio for wear monitoring signals.

[0037] 3. The sensing coating material provided by this invention exhibits high bonding strength with the metal substrate and excellent long-term environmental stability. On one hand, this invention uses high-performance polyimide as the matrix, ensuring the structural integrity of the coating under high temperature and vibration conditions. On the other hand, by introducing a (3-mercaptopropyl)trimethoxysilane coupling agent, a stable chemical bonding interface is formed between the coating and the steel substrate, and between the filler and the matrix. This not only greatly improves the adhesion of the coating but also effectively prevents the penetration of moisture and corrosive media, ensuring the long-term reliability and service life of the sensing coating under harsh operating environments such as humid heat and salt spray. Detailed Implementation

[0038] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0039] Preparation example: Preparation of polyamic acid (PAA) precursor solution

[0040] In a 1000 mL four-necked flask equipped with a mechanical stirrer, nitrogen inlet tube, and thermometer, 502.08 parts by weight of N-methylpyrrolidone (NMP) were added. While stirring, 60.06 parts by weight of 4,4'-diaminodiphenyl ether (ODA) were added and stirred until completely dissolved. The flask was cooled in an ice-water bath at 0–5 °C. Under vigorous stirring and nitrogen protection, 65.46 parts by weight of pyromellitic dianhydride (PMDA) powder were slowly added in batches, controlling the addition rate to keep the system temperature below 10 °C. After the addition was complete, the ice-water bath was removed, and the reaction was continued at room temperature (20–25 °C) for 24 hours to obtain a clear, high-viscosity polyamic acid (PAA) precursor solution. The solids content of the PAA solution was determined to be 20.0% by drying. This solution was sealed and stored for use in subsequent examples and comparative examples.

[0041] Examples 1-3:

[0042] Example 1:

[0043] This embodiment provides a method for preparing a low-power sensing coating material for monitoring the life of automotive calipers, including the following steps:

[0044] (1) Slurry preparation: 500 parts by weight of the polyamic acid (PAA) precursor solution (containing 100 parts by weight of PAA solid and 400 parts by weight of NMP) prepared in Preparation Example 1 were placed in a stirred tank. While stirring, 1.0 part by weight of (3-mercaptopropyl)trimethoxysilane (MPTMS) was added, and the mixture was stirred at room temperature for 30 minutes. Subsequently, 30 parts by weight of flake graphite and 10 parts by weight of hexagonal boron nitride (h-BN) nanosheets were added sequentially, and the mixture was transferred to a high-speed shear disperser and sheared at 4000 rpm for 1 hour. Then, 10 parts by weight of silver nanoparticles were added. The final mixture was placed in an ultrasonic disperser and circulated at 20 kHz frequency and 500 W power for 45 minutes to obtain a uniform coating slurry A.

[0045] (2) Substrate pretreatment: Take a test sample of No. 45 steel (i.e., No. 45 steel in GB / T699 standard) and sandblast it with 100-mesh alumina abrasive at a spray pressure of 0.5 MPa. Then, place the sample in acetone and anhydrous ethanol for ultrasonic cleaning for 15 minutes in sequence, remove it and blow it dry with high-purity nitrogen, and place it in an oven at 120℃ for drying for 1 hour for later use.

[0046] (3) Coating application: The coating slurry A prepared in step (1) is uniformly sprayed onto the surface of the substrate treated in step (2) using an air-assisted spraying method. Spraying parameters: nozzle diameter 1.0 mm, spraying air pressure 0.3 MPa, spraying distance 20 cm.

[0047] (4) Gradient curing: Place the sprayed workpiece in a nitrogen-filled programmed temperature oven and perform heat treatment according to the following procedure: raise the temperature from room temperature to 120°C and hold for 30 minutes; then raise the temperature to 200°C at a rate of 5°C / min and hold for 30 minutes; then raise the temperature to 300°C at a rate of 3°C / min and hold for 60 minutes. After the process, allow the workpiece to cool naturally to room temperature.

[0048] (5) Electrode preparation: The electrode area was defined on the cured coating surface using polyimide tape, and conductive silver paste was printed on the designated area using a 200-mesh screen. The sample was then placed in a 160°C oven and heated for 30 minutes to cure the silver paste and form the electrode.

[0049] Example 2:

[0050] This embodiment provides a method for preparing a low-power sensing coating material for monitoring the life of automotive calipers. The steps are basically the same as those in Embodiment 1, with the main difference being the NTC / PTC filler ratio in the slurry preparation step (1).

[0051] (1) Slurry preparation: Take 500 parts by weight of the PAA precursor solution (containing 100 parts by weight of PAA solid) obtained in Preparation Example 1. Add 1.0 part by weight of MPTMS while stirring, and stir at room temperature for 30 minutes. Then, add 25 parts by weight of flake graphite and 10 parts by weight of h-BN nanosheets in sequence, and shear at high speed for 1 hour. Then, add 15 parts by weight of silver nanoparticles. Disperse ultrasonically for 45 minutes to obtain coating slurry B.

[0052] (2) Substrate pretreatment: Same as in Example 1.

[0053] (3) Coating application: Same as in Example 1, spray coating slurry B.

[0054] (4) Gradient curing: Same as Example 1.

[0055] (5) Electrode preparation: Same as in Example 1.

[0056] Example 3:

[0057] This embodiment provides a method for preparing a low-power sensing coating material for monitoring the life of automotive calipers. The steps are basically the same as those in Embodiment 1, with the main difference being the different ratio of piezoresistive inhibitor in the slurry preparation step (1).

[0058] (1) Slurry preparation: Take 500 parts by weight of the PAA precursor solution (containing 100 parts by weight of PAA solid) obtained in Preparation Example 1. Add 1.0 part by weight of MPTMS while stirring, and stir at room temperature for 30 minutes. Then, add 30 parts by weight of flake graphite and 15 parts by weight of h-BN nanosheets in sequence, and shear at high speed for 1 hour. Then, add 10 parts by weight of silver nanoparticles. Disperse ultrasonically for 45 minutes to obtain coating slurry C.

[0059] (2) Substrate pretreatment: Same as in Example 1.

[0060] (3) Coating application: Same as in Example 1, spray coating slurry C.

[0061] (4) Gradient curing: Same as Example 1.

[0062] (5) Electrode preparation: Same as in Example 1.

[0063] Comparative Examples 1-4:

[0064] Comparative Example 1: Compared with Example 1, the difference is that in the preparation of the slurry in step (1), only 40 parts by weight of flake graphite were added to 500 parts by weight of PAA precursor solution (containing 100 parts by weight of PAA solid), and no nano-silver particles, hexagonal boron nitride nanosheets, and (3-mercaptopropyl)trimethoxysilane were added. The remaining steps were the same.

[0065] Comparative Example 2: Compared with Example 1, the difference is that hexagonal boron nitride (h-BN) nanosheets and (3-S-propyl)trimethoxysilane (MPTMS) were not added in the slurry preparation of step (1). The remaining steps were the same.

[0066] Comparative Example 3: Compared with Example 1, the difference is that hexagonal boron nitride (h-BN) nanosheets were not added in the slurry preparation of step (1). The remaining steps are the same.

[0067] Comparative Example 4: Compared with Example 1, the difference is that (3-mercaptopropyl)trimethoxysilane (MPTMS) was not added in the slurry preparation of step (1). The remaining steps were the same.

[0068] Test Example 1-3:

[0069] Test Example 1: Resistance Temperature Stability (TCR Characteristic) Test

[0070] Experimental description:

[0071] The purpose of this test is to quantitatively evaluate the resistance stability of coatings with different formulations within a simulated caliper operating temperature range (room temperature to 300°C). The test measures the change in resistance with temperature and calculates the temperature coefficient of resistance (TCR) to verify the effect of the composite conductive network in this invention on suppressing temperature drift.

[0072] Experimental steps:

[0073] Sample preparation: Take three coating samples prepared in Examples 1-3 and Comparative Examples 1-2, and weld four high-temperature resistant wires onto the electrodes of each sample after curing.

[0074] Equipment Connection: Fix the sample with soldered wires onto the four-probe test fixture inside the high and low temperature test chamber (model: LHS-100CL). Connect the external wires to a Keithley 2400 digital source meter and configure it for four-wire (Kelvin) measurement mode.

[0075] Parameter settings: Set the digital source meter to apply a constant DC test current of 100µA. Set the test program for the high and low temperature test chamber: the test atmosphere is high-purity nitrogen (to prevent high-temperature oxidation), the starting temperature is 25℃, the ending temperature is 300℃, and the heating rate is 5℃ / min.

[0076] Data Acquisition: Start the test program. First, record the initial resistance value of the sample at 25℃. During the heating process, record the resistance value every 25℃ until reaching 300℃, then record the final resistance value.

[0077] Data processing: For each sample, calculate its average temperature coefficient of resistance. Average the data from three parallel samples.

[0078] Experimental data:

[0079] Table 1. Test results of resistance-temperature characteristics of each embodiment and comparative example

[0080] Sample number Initial resistance value (Ω) Final resistance value (Ω) Temperature coefficient of resistance (ppm / °C) Example 1 151.3 151.1 -4.8 Example 2 138.5 139.2 +18.5 Example 3 162.7 162.9 +4.5 Comparative Example 1 120.4 85.9 -1030.4 Comparative Example 2 145.8 131.5 -250.8

[0081] in conclusion:

[0082] Test data show that the absolute TCR values ​​of Examples 1-3 are all at extremely low levels (less than 20 ppm / ℃), indicating that their resistivity remains essentially constant over a wide temperature range of 25℃ to 300℃. This is attributed to the effective complementarity of electrical properties between the conductive network with a positive temperature coefficient (PTC) formed by the nano-silver particles and the conductive network with a negative temperature coefficient (NTC) formed by the flake graphite during the high-temperature curing process. This synergistic effect successfully suppresses the drastic resistance fluctuations caused by temperature changes in a single conductive filler.

[0083] In contrast, Comparative Example 1, containing only graphite as a conductive filler, exhibits inherent NTC characteristics that cause a significant decrease in resistance with increasing temperature, resulting in a TCR absolute value exceeding 1000 ppm / ℃ and severe temperature drift. Although Comparative Example 2 combines graphite with nano-silver, its TCR absolute value remains at a relatively high level of -250 ppm / ℃. This demonstrates that simply physically mixing NTC and PTC fillers cannot form a structurally stable and efficiently compensated composite conductive network; its resistance stability falls far short of the requirements for accurate sensing.

[0084] Therefore, the test results confirm that the multi-component composite coating constructed by the present invention through specific component ratios and preparation processes can effectively solve the problem of temperature drift of conductive coatings under high-temperature conditions, and provide a stable signal basis for achieving accurate wear monitoring.

[0085] Test Example 2: Resistance Pressure Response (Piezoresistive Effect) Test

[0086] Experimental description:

[0087] This test aims to evaluate the resistive response characteristics of the coating under vertical pressure, i.e., the piezoresistive effect. By simulating the pressure applied to the coating by the caliper during braking, the instantaneous change in the coating resistance is quantitatively analyzed, thereby verifying the effectiveness of hexagonal boron nitride (h-BN) as a piezoresistive inhibitor in this invention.

[0088] Experimental steps:

[0089] Sample preparation: Take three coating samples each from Examples 1 and 3 and Comparative Examples 2 and 3. Cover the upper and lower surfaces of each sample with a polytetrafluoroethylene (PTFE) insulating film with a thickness of 0.1 mm.

[0090] Equipment setup: Place the sample with the insulating film in the center of the upper and lower pressure plates of the INSTRON 5967 universal testing machine. Connect the sample electrodes to the Keithley 2400 digital source meter via the four-probe test fixture, configure it for four-wire measurement mode, and apply a constant test current of 100µA.

[0091] Pressure loading: Start the material testing machine and first apply a preload pressure of 0.1 MPa to the sample to ensure good contact at all interfaces. Record the initial resistance value at this pressure. Subsequently, increase the pressure uniformly to 20 MPa at a loading rate of 1 MPa / s.

[0092] Data acquisition: Once the pressure reaches 20MPa and stabilizes, immediately record the resistance value at this point.

[0093] Data processing: For each sample, calculate its rate of change of resistance using the formula for rate of change of resistance. Average the data from three parallel samples.

[0094] Experimental data:

[0095] Table 2. Resistance-pressure response test results for each embodiment and comparative example

[0096] Sample number Initial resistance (Ω) at 0.1 MPa Resistance value after pressure (Ω) at 20MPa Resistance change rate (%) Example 1 150.2 149.0 -0.80 Example 3 161.9 160.8 -0.68 Comparative Example 2 144.5 128.1 -11.35 Comparative Example 3 158.1 137.8 -12.84

[0097] in conclusion:

[0098] Test results show that in Examples 1 and 3, which contain hexagonal boron nitride (h-BN), the absolute value of the rate of change of resistivity is less than 1% when subjected to a pressure of 20 MPa. This result confirms the mechanism of action of h-BN nanosheets in this system. As a sheet-like filler with both electrical insulation and high elasticity, h-BN forms a mechanical buffer network in the coating. When external pressure is applied, the h-BN nanosheets undergo elastic deformation or interlayer slippage, effectively absorbing and dispersing most of the mechanical stress, thereby preventing the conductive fillers (graphite and nano-silver) from displacing or reducing their spacing due to pressure, and maintaining the stability of the conductive pathway topology.

[0099] In contrast, Comparative Examples 2 and 3, which do not contain h-BN, showed absolute values ​​of resistance change rates exceeding 10%. In these samples, external pressure acted directly on the conductive network composed of graphite and nano-silver, causing the conductive particles to be compacted, increasing the number of contact points between particles and making the contact tighter, thus forming new conductive pathways. Macroscopically, this resulted in a significant decrease in the total resistance of the coating.

[0100] This test clearly demonstrates that the introduction of h-BN is key to suppressing the piezoresistive effect of the coating. In practical applications, this pressure-insensitive resistive characteristic ensures that the sensing system can output a stable resistance reference signal under both braking (high pressure) and non-braking (normal pressure) conditions, avoiding signal artifacts caused by pressure fluctuations, thus attributing the resistance change solely to mechanical wear.

[0101] Test Example 3: Accelerated Aging and Electrochemical Stability Test

[0102] Experimental description:

[0103] This test, conducted under harsh conditions of high temperature and humidity, aims to evaluate the electrochemical stability and durability of the coating during long-term service. The core of the test is monitoring the drift in resistance values ​​before and after aging to verify the role of (3-mercaptopropyl)trimethoxysilane (MPTMS) as an interface anchoring agent in maintaining the structural integrity of the coating and the long-term reliability of signals.

[0104] Experimental steps:

[0105] Sample preparation: Take three coating samples prepared in Example 1 and Comparative Examples 2 and 4 respectively, and measure and record their initial resistance in a room temperature dry environment using the four-probe method.

[0106] Aging treatment: All test samples were placed in a constant temperature and humidity chamber (model: TH-80P), and the chamber environment was set and maintained at a temperature of 85℃ and a relative humidity of 85%RH.

[0107] Periodic measurement: The sample is aged continuously in the chamber for 500 hours. After aging, the sample is removed and placed in a dry environment at room temperature (25°C) and relative humidity below 40% to cool and equilibrate for 1 hour to eliminate the influence of residual moisture and temperature on the measurement.

[0108] Data acquisition: After the sample has stabilized, the same four-probe method as the initial measurement is used to measure its final resistance after aging.

[0109] Data processing: For each sample, calculate its resistivity drift. Average the data from three parallel samples.

[0110] Experimental data:

[0111] Table 3. Resistance stability test results of each embodiment and comparative example after damp heat aging.

[0112] Sample number Initial resistance (Ω) Final resistance (Ω) Resistance drift (%) Example 1 152.4 154.1 +1.11 Comparative Example 2 146.1 188.9 +29.30 Comparative Example 4 155.8 193.2 +24.01

[0113] in conclusion:

[0114] The test results clearly show that Example 1, which incorporates MPTMS, exhibits excellent long-term stability with an absolute resistance drift of less than 2% after 500 hours of damp heat aging. In contrast, Comparative Examples 4 and 2, which do not contain MPTMS, both showed significant positive resistance drift exceeding 20%.

[0115] The fundamental reason for this performance difference lies in the dual function of MPTMS in the coating system. First, the trimethoxysilyl groups in the MPTMS molecule hydrolyze during slurry preparation and curing, enabling them to chemically bond with the hydroxyl groups on the steel substrate surface, while simultaneously self-condensing to form a dense -Si-O-Si- crosslinked network layer. This interfacial layer greatly enhances the adhesion between the polymer coating and the metal substrate and effectively blocks the penetration of water molecules into the coating-substrate interface. Second, the thiol functional groups at the other end of MPTMS have a strong affinity for the surface of the silver nanoparticles, anchoring the conductive filler within the polyimide matrix.

[0116] In the comparative samples, due to the lack of this chemical anchoring and physical barrier, water molecules can more easily penetrate under high temperature and high humidity conditions. The penetration of water molecules may, on the one hand, cause microscopic displacement or aggregation of conductive filler particles, destroying the original conductive pathways; on the other hand, it may lead to micro-corrosion at the interface, the products of which will also affect the stability of electrical properties, ultimately manifesting as irreversible drift in the resistance value on a macroscopic scale.

[0117] Therefore, this test confirms that the addition of the MPTMS interface anchoring agent is necessary and effective in ensuring the signal reliability and durability of the sensing coating during long-term service.

Claims

1. A low-power sensing coating material for monitoring the lifespan of automotive calipers, characterized in that, Made from the following ingredients in parts by weight: Polyamic acid solids: 100 parts; Flake graphite: 20-40 parts; Nano silver particles: 5-20 parts; Hexagonal boron nitride nanosheets: 5-20 parts; Silane coupling agent: 0.5-5 parts.

2. The low-power sensing coating material for monitoring the lifespan of automotive calipers according to claim 1, characterized in that, The weight parts of the raw materials are: Polyamic acid solids: 100 parts; Flake graphite: 25-30 parts; Nano silver particles: 10-15 parts; Hexagonal boron nitride nanosheets: 10-15 parts; Silane coupling agent: 1.0-2.5 parts.

3. The low-power sensing coating material for monitoring the lifespan of automotive calipers according to claim 1, characterized in that, The median particle size of the flake graphite is 5-10µm; the median particle size of the nano-silver particles is 50-100nm; and the lateral dimension of the hexagonal boron nitride nanosheets is 0.5-2µm, and the thickness is 5-30nm.

4. The low-power sensing coating material for monitoring the lifespan of automotive calipers according to claim 1, characterized in that, The polyamic acid solid is polymerized from pyromellitic dianhydride and 4,4'-diaminodiphenyl ether, wherein the molar ratio of pyromellitic dianhydride to 4,4'-diaminodiphenyl ether is (0.98-1.02):

1.

5. The low-power sensing coating material for monitoring the lifespan of automotive calipers according to claim 1, characterized in that, The silane coupling agent is (3-mercaptopropyl)trimethoxysilane.

6. A method for preparing a low-power sensing coating material for monitoring the lifespan of automotive calipers, comprising the method for preparing the low-power sensing coating material for monitoring the lifespan of automotive calipers as described in any one of claims 1-5, characterized in that, Includes the following steps: (1) Polyamic acid, silane coupling agent, flake graphite, hexagonal boron nitride nanosheets and silver nanoparticles are mixed in a solvent to obtain a coating slurry; (2) The coating slurry is applied to the surface of the substrate; (3) Gradient curing is performed on the coated substrate. The curing process includes at least three heating and heat preservation stages, with the highest curing temperature being 280-320℃.

7. The method for preparing a low-power sensing coating material for monitoring the life of automotive calipers according to claim 6, characterized in that, Before step (2), a pretreatment step of sandblasting and cleaning the substrate is also included.

8. The method for preparing a low-power sensing coating material for monitoring the life of automotive calipers according to claim 6, characterized in that, In step (1), the mixing in the solvent is carried out by high-speed shear dispersion and ultrasonic dispersion to obtain the coating slurry.

9. The method for preparing a low-power sensing coating material for monitoring the life of automotive calipers according to claim 6, characterized in that, In step (3), the gradient curing includes: holding at 100-140℃ for 20-40 minutes, holding at 180-220℃ for 20-40 minutes, and holding at 280-320℃ for 40-80 minutes.

10. A method for applying a low-power sensing coating material for monitoring the lifespan of automotive calipers, wherein the material is applied to the low-power sensing coating material for monitoring the lifespan of automotive calipers as described in any one of claims 1-5, characterized in that... This includes applying the sensing coating material to the wear area of ​​the caliper and monitoring the reduction in coating thickness by measuring changes in resistance.