Two-stage laser compound polishing and repairing method for silicon carbide coated surface

Abstract

The application discloses a two-stage laser composite polishing and repairing method for a silicon carbide coating surface and belongs to the field of laser processing and surface engineering. The method takes repairing-polishing cooperation as the core and completes defect repairing and surface finishing in a single clamping thermal cycle. First, the coating morphology, cracks, internal pores and thermal physical parameters are characterized, and first-stage continuous / long-pulse laser large-spot remelting repairing parameters are determined. The remelting is carried out under a protective atmosphere, the melt fills the cracks under the surface tension and capillary action, and the interface is recombined, the open pore capillary is backfilled, and the closed pore is densified and healed by laser after the filler is pre-placed; and the temperature is controlled by infrared temperature measurement in a closed loop. After the threshold value is reached, short / ultrashort pulse fine repair is switched, and a smooth and dense surface is obtained. The application takes into account crack repairing, pore filling and low-damage fine repair, and has novelty and practical value compared with simple polishing.

Description

Technical Field

[0001] This invention relates to the fields of laser processing, surface engineering, and surface defect repair and precision finishing of hard and brittle materials. Specifically, it is a two-stage laser composite polishing and repair method for silicon carbide coating surfaces. This method is applicable to the repair of surface cracks, filling of internal pores, and surface leveling and finishing of silicon carbide (SiC) coatings prepared by chemical vapor deposition, physical vapor deposition, plasma spraying, and reaction sintering. It can be applied to the defect repair and functional manufacturing of hard and brittle coated parts such as aerospace thermal protection structural components, semiconductor precision tooling, high-temperature wear-resistant and corrosion-resistant components, and optical precision molds. Background Technology

[0002] Silicon carbide (SiC) coatings possess excellent properties such as high hardness, high temperature resistance, wear resistance, corrosion resistance, and high thermal conductivity, making them a core protective coating for high-temperature equipment and semiconductor devices. However, due to limitations in the deposition, spraying, and sintering mechanisms, raw SiC coatings not only exhibit a spherical accumulation morphology on the surface but also commonly contain bulk defects such as through-cracks, open and closed pores (gas holes), and inclusions in the surface and subsurface layers. Surface roughness is typically in the range of several to tens of micrometers. These defects exhibit a "surface-to-bulk" interconnected characteristic: surface cracks extend into the bulk, forming channels for stress corrosion and fracture propagation, while internal pores constitute bottlenecks in coating density and channels for medium penetration. Together, these form stress concentration sources, significantly reducing the aerodynamic performance, sealing performance, oxidation and corrosion resistance, and high-temperature service life of components. Traditional processes treat "polishing" and "repair" separately—polishing only eliminates surface undulations and is powerless against internal cracks and pores; while repair often relies on subsequent independent processes such as infiltration and hot isostatic pressing, which are difficult to coordinate with surface finishing, resulting in complex processes, weak interfacial bonding, and mismatch between the repaired area and the substrate properties. Therefore, there is an urgent need to develop a SiC coating treatment method that integrates defect repair and surface polishing in a co-current thermal cycle.

[0003] SiC is a typical hard and brittle material. Traditional mechanical grinding and polishing suffer from drawbacks such as high tool wear, extremely low processing efficiency, subsurface deep damage, and difficulty in machining complex curved surfaces. Furthermore, they lack the ability to repair internal pores and crack interfaces within the coating. Existing laser polishing technologies fall into two categories: continuous laser remelting polishing or long-pulse laser remelting polishing, which can melt surface protrusions to achieve surface reconstruction, but can only passively fill open cracks, failing to repair closed pores or re-bond crack interfaces, and easily leaving microcracks and re-condensing particles, making it difficult to reduce roughness to precision-grade standards; and short-pulse laser ablation polishing or ultrashort-pulse laser ablation polishing, which can remove a small amount of surface material to achieve high-precision leveling, but the removal amount per pass is small and inefficient, unable to fill internal through cracks and pores, and the scanning edges are prone to ablation steps, layered peeling, and graphitization discoloration. Both types of simple polishing technologies remain at the level of "surface morphology correction" and lack the ability to repair bulk defects.

[0004] Single-laser operating modes cannot simultaneously address the four requirements of in-situ crack repair, internal pore sealing, high-efficiency leveling, and low-thermal-damage precision polishing. Existing composite laser polishing processes lack phased threshold switching closed-loop control, quantitative back-calculation logic for parameters based on material thermal properties and defect distribution, real-time temperature control throughout the polishing process, and a repair mechanism designed for "crack interface re-bonding—pore backfilling and densification." This results in poor process consistency, uncontrollable surface defects, residual bulk defects, and large roughness fluctuations. Therefore, there is an urgent need to develop a novel two-stage laser composite treatment method for SiC coatings that features time-controllable, dual-laser mode collaboration, infrared temperature control and defect threshold determination closed-loop, and a "repair-polishing" synergy as its core. Summary of the Invention

[0005] To address the shortcomings of existing SiC coating treatment technologies, such as the separation of polishing and repair, the inability of simple polishing to repair bulk defects, the lack of closed-loop process control, the incompatibility between crack interface recombination and internal pore closure, and the difficulty in stably controlling the final surface roughness to Ra≤1.1μm with residual bulk defects, this invention aims to provide a two-stage laser composite polishing and repair method for silicon carbide coating surfaces. This method takes "repair-polishing" synergy as the main line, and through process timing, laser mode, and quantitative parameter control, achieves crack interface recombination, backfilling and densification of open and closed pores, and gradual surface leveling in the first stage of homogeneous thermal cycling. In the second stage, low-damage fine repair eliminates surface micro-defects. The entire process relies on infrared thermometry and online defect and roughness monitoring to achieve adaptive closed-loop parameter adjustment. It does not require a dedicated composite processing device and can be implemented on existing multi-axis laser processing platforms.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A two-stage laser composite polishing and repair method for silicon carbide coated surfaces includes the following steps: S1. Morphological observation and quantitative roughness detection are performed on the surface of the SiC coating to be treated. The size of spherical protrusions, distribution of through-cracks, location and size of open and closed pores, and initial roughness Ra are recorded. Thermophysical parameters of the SiC coating and substrate are also collected to establish the thermal boundary conditions of the SiC coating-substrate composite system. The laser processing effect (melt pool depth, heat-affected zone) strongly depends on the material's absorption of laser light and heat conduction behavior. Different deposition processes (CVD, PVD, plasma spraying) result in significant differences in porosity, grain size, and thermal conductivity of the coating. If the three-dimensional distribution of defects and the thermal boundary are not obtained beforehand, during remelting, either insufficient energy will prevent crack healing, or excessive energy will burn through the coating. Closed pores, being unconnected, cannot be filled by the melt itself; their coordinates must be pre-calibrated for subsequent point-to-point filling to avoid waste or contamination caused by blindly coating the entire surface. The "defect classification and characterization" strategy provides a basis for subsequent graded repair. The introduction of substrate thermophysical parameters solves the problem of thermal diffusivity matching in the coating-substrate composite system, prevents the molten pool from overheating and damaging the substrate, and makes the method adaptable to different substrates such as metals and ceramics, breaking through the dependence on specific coating preparation processes. S2. Based on the SiC coating surface condition, defect distribution, and thermophysical parameters obtained in step S1, determine the laser power, spot diameter, scanning speed, and scanning overlap rate for the continuous or long-pulse laser remelting repair stage. During the remelting stage, ensure the melt pool depth is between the height of the raised feature and the coating thickness, thus melting the raised surface to obtain a flat substrate while avoiding excessive melting depth that could lead to substrate melting or interface delamination. Based on the inverse calculation logic of heat transfer and melt pool kinetic formulas, the process parameters are directly derived from the material's thermophysical properties, eliminating the need for extensive pre-experiments. S3. Using a continuous-wave laser or a long-pulse laser with a millimeter-scale large spot size, the SiC coating is remelted and repaired under an inert gas or vacuum environment. This allows the molten material to spread and fill low-lying areas and through cracks, achieving crack interface rebonding. Simultaneously, capillary seepage fills openings and connecting pores, and pre-placed repair fillers are laser-densified to heal closed pores, forming a continuous remelted repair layer that seals defects. The laser power is controlled in a closed-loop manner using infrared thermometry. SiC melt has high surface tension and a large surface tension temperature coefficient. The Marangoni effect allows the melt to flow directionally to the low-temperature region; the narrow crack can generate capillary suction pressure of several megapascals, sufficient to draw the melt into the depths of the through crack. This achieves "crack interface rebonding" (not just physical filling)—the melt weeps into both walls of the crack and solidifies to form a continuous metallurgical bond, restoring the integrity of the coating structure and solving the defect that simple mechanical polishing cannot heal cracks. S4. The status of the remelted repair layer is detected using online morphology, roughness, and defect repair rate monitoring equipment. When the roughness or defect repair rate does not reach the preset threshold, the remelting process parameters are adjusted and step S3 is repeated. When both the roughness and defect repair rate reach the preset threshold, step S5 is initiated. A single fixed parameter is insufficient to address the actual conditions such as local porosity fluctuations and uneven crack density in the coating. An adaptive closed loop is needed through online detection, feedback, and iteration. The dual-index judgment stage switching of "roughness + defect repair rate" solves the problem of residual bulk defects (cracks / pores) caused by traditional roughness-only switching. S5. Switch to nanosecond pulsed laser, picosecond pulsed laser, femtosecond short pulsed laser, or ultrashort pulsed laser with micron-sized spot size to perform fine scanning on the remelted repair layer, removing surface micro-undulations, burrs, and molten re-condensed particles. Use 355nm ultraviolet short pulses, with photochemical ablation as the main method and thermal melting as a secondary method. The material is removed by jetting with minimal lateral heat conduction, and the heat-affected zone can be controlled at the submicron level. S6. After removal treatment, conduct final state testing to evaluate the SiC coating roughness, through crack filling, closure status, internal pore sealing rate, as well as ablation steps, layered peeling, and graphitization discoloration defects. The final quality must simultaneously meet the dual standards of "smooth surface + dense bulk phase," neither of which can be omitted.

[0007] Preferably, in step S1, the SiC coating is a chemical vapor deposition, physical vapor deposition, plasma spraying, or reaction sintering coating; morphology and defect observation are performed using a scanning electron microscope, optical microscope, micro-CT, or machine vision system; roughness and porosity are detected using a contact roughness meter, laser confocal profilometer, or micro-CT imaging method.

[0008] Preferably, in step S2, based on the SiC coating thermal conductivity, substrate material density, substrate material specific heat capacity at constant pressure, laser spot diameter, scanning speed, pulse width, effective absorptivity, and center-to-center spacing of the scanning channels obtained in step S1, the thermal diffusivity, equivalent laser action time, thermal penetration depth, molten pool depth, laser surface energy density, and scanning overlap rate of the composite system are calculated using the corresponding heat transfer and molten pool kinetics formulas. The remelting scanning process parameters for step S3 are then set based on these calculated results; wherein: thermal diffusivity... , The thermal diffusivity of the coating-substrate composite system is expressed in m² / s. k The value represents the thermal conductivity of the SiC coating, expressed in W / (m·K). Density of the matrix material, in kg / m³; The matrix material has a specific heat capacity under constant pressure, expressed in J / (kg·K), and the equivalent duration of continuous laser treatment is given. , The equivalent interaction time between the laser and the coating material is expressed in seconds.D The laser spot diameter is expressed in meters (m). v Laser scanning speed, in m / s; equivalent action time of long-pulse laser. , The pulse width of a long-pulse laser is measured in seconds (s), and the thermal penetration depth is also mentioned. The depth of the molten pool meets the requirements. , The depth of the laser remelting pool, in meters; The height of the spherical protrusions on the coating surface, in meters (m). SiC coating thickness, in meters; laser surface energy density. , Laser surface energy density, in J / m²; To determine the effective absorption rate of the SiC coating to laser light, based on the laser wavelength and the room temperature absorption coefficient of SiC, The value ranges from 0.6 to 0.8; Average power of continuous / long-pulse laser, in W; D is the scanning speed, in m / s; D is the laser spot diameter, in m; and the scanning overlap rate is... , s The distance between the centers of two adjacent laser scanning tracks is expressed in meters (m).

[0009] Preferably, in step S3, the melt surface tension driving force and the crack capillary filling pressure are respectively derived from the corresponding melt kinetic formulas. Calculations show that the backfilling of open and connected pores is characterized by a Washburn-type capillary flow relationship, and the healing of closed pores is achieved by pre-placed repair filler material through laser remelting and densification; among these, The driving force is the surface tension of the melt. γ is the capillary filling pressure of the crack; γ is the surface tension of the melt. This represents the rate of change of surface tension with temperature. For temperature gradient, Let be the equivalent radius of curvature of the crack.

[0010] Preferably, in step S5, the wavelengths of the short-pulse laser and the ultrashort-pulse laser are 355nm and 532nm, respectively. The single-photon energy, single-pulse energy, peak power, and pulse flux of the short-pulse laser and the ultrashort-pulse laser are calculated using the corresponding laser physics formulas; wherein, the single-photon energy... Single pulse energy Peak power Pulse flux ; h It is Planck's constant. c Where is the speed of light in a vacuum, and 'e' is the unit conversion factor. λ It is the wavelength of ultraviolet laser. The average power of the pulsed laser. f The pulse repetition frequency, The pulse width. D denoted as the diameter of the light spot.

[0011] Preferably, the qualification criteria for step S6 include: SiC coating roughness Ra≤1.1μm; through cracks in the detection area are effectively filled or closed, and the crack opening, length or depth reaches the preset control index; internal pore closure rate≥preset threshold; and no obvious ablation sinking steps, layered peeling and graphitization discoloration defects in the scanned area.

[0012] Preferably, steps S1 to S6 are completed on the same laser processing platform, and the workpiece is clamped only once without disassembly and transfer.

[0013] Preferably, the protective gas for the remelting repair scan in step S3 is argon or nitrogen.

[0014] Preferably, the roughness preset threshold in step S4 is Ra≤2μm and the defect repair rate reaches the preset index.

[0015] Compared with the prior art, the present invention has the following beneficial technical effects: 1. Repair and polishing work in synergy, completing bulk defect repair and surface finishing in the same thermal cycle. The first stage employs continuous or long-pulse laser large-spot remelting with "defect repair" as the primary goal: the surface melt spreads and fills through cracks and depressions under surface tension and capillary action, achieving re-bonding of the crack interface. Simultaneously, capillary backfilling seals open / connected pores, and pre-filling-laser densification heals closed pores, repairing the SiC coating with an initial Ra≈8.55μm, through cracks, and internal pores to Ra≈1.621μm, significantly improving pore sealing rate, and effectively filling or closing cracks. The second stage switches to 355nm ultraviolet short / ultra-short pulse laser small-spot fine finishing, using photochemical micro-ablation to remove micro-undulations, burrs, and re-condensed particles from the surface of the remelted layer. The final surface Ra is reduced to 1.048μm (meeting the qualified target of Ra≤1.1μm), and it is confirmed that there are no ablation steps, no layered peeling, and no obvious graphitization discoloration. The two stages are decoupled in terms of time sequence. "Remelting and repair" and "fine finishing and leveling" are optimized independently and do not hinder each other. This makes the method of this invention have a clear differentiation and innovation in the "repair dimension" compared with simple polishing technology: simple polishing only corrects the surface morphology and is powerless against internal cracks and pores. However, this invention simultaneously achieves crack interface healing and pore sealing, resulting in a complete functional surface with "smooth surface and dense bulk".

[0016] 2. Full-process quantitative parameter back-calculation and dual closed-loop control of "power closed loop + defect / switching threshold" ensure stable and reproducible process. Based on the SiC coating's thermal conductivity of 100–120 W / (m·K) and thermal diffusivity... Heat penetration depth The process parameters are directly calculated from the material's thermal properties and defect distribution, eliminating the need for extensive trial-and-error experiments to determine the process window. During the remelting and repair stage, infrared thermography and a PID closed-loop system (Kp=0.5~10 W / K, Ki=0.05~2 W / (K·s), Kd=0.01~0.5 W·s / K) are used to adjust the laser power in real time, stabilizing the molten pool temperature within the suitable melting and densification range for SiC. Stage switching is automatically determined online using two indicators: "roughness threshold Ra≤2μm + defect repair rate threshold," forming a dual closed-loop system of "power closed loop + switching threshold." This mechanism can automatically adapt to SiC coatings of different thicknesses, initial roughness (e.g., 8.55μm level), and porosities, resulting in good consistency between the final Ra and defect closure during batch processing.

[0017] 3. The entire process is completed in a single clamping, with highly integrated defect repair and polishing procedures. In traditional processes, remelting, polishing, inspection, and even infiltration repair require multiple machines to complete the process step by step. Multiple disassembly and assembly of the workpiece introduces positioning errors and repeated preheating energy consumption, and the waiting time between processes causes in-process time loss. Independent repair processes such as infiltration and hot isostatic pressing are difficult to coordinate with polishing, which easily leads to performance mismatch between the repaired area and the substrate. The method of this invention integrates the characterization of original surface and bulk defects, remelting repair (crack rebonding + pore backfilling), online morphology / defect monitoring, short-pulse fine repair, and final state detection into the same processing platform and completes them continuously in a single clamping, eliminating errors caused by inter-process transfer and secondary clamping. The parameters are calculated from the material properties and supplemented by dual closed-loop control, reducing trial-and-error waste and rework. The overall processing cycle, energy consumption, and repair quality are superior to the traditional "step-by-step polishing + independent repair" process.

[0018] 4. Adaptable to various types of SiC hard and brittle coatings and defect conditions, with a wide range of applications. The method of this invention does not rely on a specific coating preparation process and can be adapted to various SiC protective coatings such as CVD, PVD, plasma spraying, and reaction sintering. It is also compatible with planar surfaces, complex free-form surfaces, and components with localized damage repair containing surface cracks / internal pores. It is suitable for the mass precision manufacturing of high-temperature hot-end components of aero-engines, vacuum tooling for semiconductor manufacturing, precision wear-resistant molds, and parts with bulk defects that require "repair + finishing".

[0019] 5. The novel approach of "synergistic repair mechanism and polishing process" demonstrates significant practical value. This invention expands the "surface reconstruction" capability of laser remelting from simple morphological leveling to a bulk repair capability that combines "crack interface rebonding, capillary backfilling of open pores, and densification of closed-pore fillers." Furthermore, a second-stage fine-tuning stage connects to the polishing dimension, allowing the same heat source to serve both repair and polishing sequentially, thus resolving the industry contradiction of "crack repair inevitably leaving scars, and polishing being difficult to fill pores." Compared to existing simple laser polishing or independent impregnation repair solutions, this invention simultaneously improves three indicators: defect sealing rate, interface bonding strength, and surface integrity, providing a novel and practical process path for extending the lifespan and functionalizing SiC coated components. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of the present invention.

[0021] Figure 2 This is a schematic diagram of the original surface morphology of the SiC coating.

[0022] Figure 3 This is a SEM image of the surface after continuous laser remelting.

[0023] Figure 4 This is a SEM image of the surface after a single pulsed laser ablation.

[0024] Figure 5 This is a surface SEM image after multiple scans with low-power pulses.

[0025] Figure 6 This is a SEM image of the surface after two-stage laser composite polishing.

[0026] Figure 7 A bar chart comparing the surface roughness after polishing using various methods. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0028] like Figure 1 As shown, a two-stage laser composite polishing and repair method for silicon carbide coating surfaces includes the following steps: S1. Characterization of original surface and bulk defects of coating and acquisition of thermal conductivity boundary Morphological observation and quantitative roughness detection were performed on the SiC coating surface to be treated. The size of the spherical protrusions, the distribution morphology of the through cracks, the location, size, connectivity of open and closed pores, and the initial roughness Ra were recorded. Simultaneously, the coating thickness Hc, the thermal conductivity k of the SiC coating, the density ρ of the substrate material, and the specific heat capacity cp of the substrate material at constant pressure were collected to establish the thermal boundary conditions of the coating-substrate composite system. Based on the pore connectivity, the defects were divided into three categories: surface open cracks, internally connected pores, and closed pores (isolated pores), providing input for the subsequent graded repair process.

[0029] The equipment used for morphological observation and defect detection includes scanning electron microscopes, optical microscopes, micro-CT, or machine vision imaging systems to achieve three-dimensional characterization of surface cracks and internal pores; the equipment used for roughness detection includes contact roughness testers, laser confocal profilometers, or micro-CT imaging methods; this step is applicable to SiC coatings including chemical vapor deposition, physical vapor deposition, plasma spraying, and reaction sintering formed SiC protective coatings.

[0030] S2. Calculation of first-stage remelting laser process parameters based on SiC thermophysical properties. Based on the SiC coating and substrate thermophysical parameters obtained in step S1, and considering thermally conductive boundaries such as coating thickness, surface protrusion size, and crack distribution, the thermal diffusivity, laser equivalent interaction time, thermal penetration depth and molten pool control depth, surface energy density, and scanning overlap rate are calculated sequentially. Based on these parameters, the power for the continuous or long-pulse laser remelting stage is then determined. P Spot diameter D Scanning speed v and scan overlap rate .

[0031] First, based on the thermal conductivity of the SiC coating k Matrix density ρ Specific heat capacity of the matrix c p Calculate the thermal diffusivity of the coating-substrate composite system: Equation (1) In the formula: The thermal diffusivity of the coating-substrate composite system is expressed in m² / s. k The value represents the thermal conductivity of the SiC coating, expressed in W / (m·K). Density of the matrix material, in kg / m³; The specific heat capacity at constant pressure of the base material, expressed in J / (kg·K). When the coating thickness is less than or equal to the heat penetration depth... and Take the substrate parameters; otherwise, take the coating body parameters.

[0032] Subsequently, the equivalent laser action time was calculated based on the laser scanning method used.

[0033] For continuous laser scanning, the equivalent action time is calculated using the following formula: Equation (2) For long-pulse laser scanning, the equivalent action time is taken as the single-pulse width: Equation (3) In the formula: The equivalent interaction time between the laser and the coating material is expressed in seconds. D The laser spot diameter is expressed in meters (m). v Laser scanning speed, in m / s; The pulse width of a long-pulse laser is expressed in seconds (s).

[0034] Then, the thermal penetration depth under laser irradiation is calculated based on the thermal diffusivity α and the equivalent action time ti: Equation (4) In the formula: The thermal penetration depth of the coating under laser irradiation, in meters (m). Thermal diffusivity, in m² / s; The equivalent action time is expressed in seconds (s).

[0035] Then, the remelting pool depth is constrained based on the height of the surface spherical protrusions and the coating thickness: Equation (5) In the formula: The depth of the laser remelting pool, in meters; The height of the spherical protrusions on the coating surface, in meters (m). The value represents the SiC coating thickness, in meters (m). Meeting this constraint allows the main surface protrusions to enter the melting range and reduces the risk of burning through the coating and damaging the substrate.

[0036] Based on the constraints of molten pool depth, the laser surface energy density is calculated according to laser power, scanning speed, and spot size: Equation (6) In the formula: Laser surface energy density, in J / m²; To determine the effective absorption rate of the SiC coating to laser light, based on the laser wavelength and the room temperature absorption coefficient of SiC, The value ranges from 0.6 to 0.8; Average power of continuous / long-pulse laser, in W; D is the scanning speed, in m / s; D is the spot diameter, in m.

[0037] Finally, the scanning overlap rate is calculated based on the spot diameter D and the center-to-center distance s between adjacent scanning tracks: Equation (7) In the formula: For laser scanning overlap rate; s The distance between the centers of two adjacent laser scanning tracks, in meters; D The value is the diameter of the laser spot, in meters (m).

[0038] By combining the above calculation results with the risks of material melting, vaporization, and matrix damage, a process window is selected that allows the SiC surface layer to melt fully without significant vaporization and decomposition and prevents the coating from being burned through, thus determining the parameters for the first-stage large-spot remelting.

[0039] S3. First-stage continuous or long-pulse laser large-spot remelting defect repair treatment (crack recombination and pore backfilling). Using a 1064nm continuous pulse laser or a 1070nm long pulse laser, paired with a millimeter-scale large spot, the SiC coating is subjected to a full-coverage remelting scan based on the parameters calculated in step S2 under an inert gas environment of argon, nitrogen, or vacuum. This stage prioritizes "defect repair" and completes surface finishing and bulk phase repair simultaneously in the same thermal cycle.

[0040] During the remelting process, the spherical protrusions on the coating surface are completely melted by laser heating to form a liquid melt, which simultaneously completes crack repair and pore filling under the drive of the repair mechanism: (1) Surface crack repair: The melt flows and spreads to the low-lying area of ​​the coating and the interior of the through crack under the combined drive of Marangoni surface tension gradient and crack capillary pressure, and wets the two walls of the remelted crack so that the interface is remelted and bonded (crack interface rebonding). After cooling and solidification, a continuous and dense healing layer is formed, realizing in-situ filling of cracks and interface healing. (2) Internal pore repair and filling: For open pores connected to the surface and internal pores connected to the surface, the surface melt seeps back into the pore channels under capillary suction and seals the pore channels after solidification; For closed pores (isolated pores) not connected to the surface, nano-SiC repair slurry or Si-based melting agent is pre-placed at the characterization site in step S1 as repair filler, and then the filler is melted by laser remelting in this stage, reacts and combines with the pore wall in situ and densifies, so as to achieve in situ healing of closed pore defects; the pore backfill density is controlled by the synergistic control of the molten pool holding time and surface energy density.

[0041] The driving force for pore capillary seepage backfilling can be characterized by the Washburn-type capillary rise relationship: Equation (8) In the formula, L is the depth of melt penetration into the pores. For melt surface tension, The contact angle is denoted as _r_, where _r_ is the equivalent radius of the pore. The value is the melt viscosity, and t is the seepage time. This formula shows that small-diameter, highly interconnected pores are more easily backfilled by the melt, which guides the setting of the holding time and scanning speed in step S2.

[0042] Pre-filling of closed-pore repair fillers can be achieved through micro-injection, slurry brushing, or atmospheric pressure plasma spraying, filling the open ends of closed pores with nano-SiC repair slurry or Si-based infiltrating agents. During laser remelting, the filler reacts and bonds with the matrix in situ (e.g., Si infiltrating agents react with the C phase to form β-SiC or fill amorphous SiC), both sealing the pores and ensuring that the repair zone matches the matrix composition and thermal expansion, preventing delamination of the repair interface. This "pre-filling-laser densification" mechanism, together with the "capillary backfilling" mechanism for open pores, constitutes the internal pore repair and filling technology system.

[0043] Marangoni surface tension driving force calculation formula: Equation (9) In the formula: The driving force of melt surface tension, in Pa; The surface tension of SiC melt is expressed in N / m. T x represents the real-time temperature of the coating surface, in Kelvin; x represents the coordinates of the laser scanning direction, in meters. Formula for calculating capillary pressure in cracks: Equation (10) In the formula: Capillary filling pressure for cracks, in Pa; The equivalent radius of curvature of the through-crack is expressed in meters (m). Throughout the remelting scan, an infrared thermometer is used to collect the real-time temperature of the molten pool and the surface temperature of the newly solidified remelted layer. A PID closed-loop algorithm is used to dynamically adjust the laser output power, maintaining the molten pool temperature stably within the suitable melting range for SiC. The laser power closed-loop correction formula is as follows: Equation (11) In the formula: The laser correction power for the next scanning cycle, in W; The laser output power for the current cycle is expressed in W. The target melting temperature of SiC is given in Kelvin (K). The current real-time infrared temperature is in Kelvin (K). This is the proportional control coefficient, with a value range of 0.5~10 W / K; This is the integral control coefficient, with a value range of 0.05~2 W / (K·s); This is the differential control coefficient, with a value range of 0.01~0.5 W·s / K; The time interval for temperature measurement sampling is measured in seconds (s).

[0044] S4. Online threshold determination of defect repair rate and roughness of remelted repair layer and process iteration Using laser displacement sensing, structured light 3D measurement, machine vision morphology recognition, or micro-CT system, online roughness and defect repair rate detection are performed on the coating surface and bulk phase after remelting scanning. If the surface roughness Ra of the remelted repair layer is found to be greater than 2μm or the defect repair rate (crack filling rate + pore closure rate) does not meet the preset index, the repair and leveling effect is deemed unsatisfactory. The three parameters of laser power P, scanning speed v and scanning overlap rate in step S2 are finely adjusted, and step S3 remelted repair scanning is repeated. If the surface roughness Ra of the remelted repair layer is found to be ≤2μm and the defect repair rate meets the standard, the first stage of the remelting defect repair process is considered complete, and the process directly proceeds to the second stage of precision finishing.

[0045] S5. Second-stage short / ultrashort pulse laser small spot precision polishing (refinement dimension) Switch the laser output mode to nanosecond pulse laser, picosecond pulse laser, femtosecond short pulse laser or ultrashort pulse laser, and use a micron-level small spot to perform full-coverage fine scanning of the continuous remelted layer; The selectable wavelengths for short-pulse lasers and ultrashort-pulse lasers are 355nm and 532nm, respectively; when using a 355nm ultraviolet pulse laser, the formula for calculating the single-photon energy is as follows: Equation (12) In the formula: Laser single-photon energy, measured in eV; h is Planck's constant, in J·s; c is the speed of light in vacuum, in m / s; λ The wavelength is the ultraviolet laser wavelength, in meters (m); e is a unit conversion factor, e = 1.602 × 10⁻⁶. -19 J / eV is used to convert Joules to electron volts; the single-photon energy of a 355nm ultraviolet laser is about 3.49eV. The SiC coating has a higher absorption rate for ultraviolet lasers, and photochemical micro-ablation is the main process, with a very small heat-affected zone. This stage utilizes a trace material removal mechanism to eliminate residual micro-undulations, burrs, and re-condensed particles on the surface of the remelted layer, without causing deep thermal damage, ablation steps, or layered peeling defects. The formulas for the pulsed laser single-pulse energy, peak power, and pulse flux constraints are as follows: Single-pulse energy type: Equation (13) Peak power formula: Equation (14) Pulse flux type: Equation (15) In the formula: Single pulse energy, in J; The average power of the pulsed laser is expressed in W. f The pulse repetition frequency is expressed in Hz. Pulse width, in seconds; Peak power of the pulse, in W; F Laser pulse flux, in J / m²; S6. Detection of final surface quality and bulk defects in coating. Qualified workpieces must simultaneously meet the following quality requirements: the surface roughness Ra of the SiC coating is ≤1.1μm; the through cracks in the detection area are effectively filled or closed, and the crack opening, length or depth reaches the preset control index; the internal pore closure rate is ≥ the preset threshold (e.g., ≥95%), and there are no residual voids after the closed pores are densified by filler; there are no obvious ablation sinking steps, layered peeling, or graphitization discoloration caused by preferential volatilization of Si elements in the scanning area.

[0046] Furthermore, steps S1 to S6 of the method of the present invention are completed entirely on the same laser processing platform and in a single workpiece clamping state, without the need for secondary disassembly and transfer of the workpiece, thus eliminating repeated clamping and positioning errors and simplifying the processing flow.

[0047] Example Polishing and repair experiments were conducted on SiC-coated metal substrate samples prepared by plasma spraying. The original coating thickness of the samples was [not specified]. =120μm, height of spherical protrusions on the coating surface =8~12μm, the original surface roughness Ra=8.55μm was measured, the coating shows a large number of micron-sized through cracks, and the bulk porosity of the coating is about 8%~12%, with both open and closed pores; the matrix is ​​a high-temperature alloy with high thermal conductivity. =11.2W / (m·K), density ρ=8900kg / m³, specific heat capacity =420J / (kg·K), SiC coating thermal conductivity k=110W / (m·K).

[0048] Step 1: Characterize the original surface and bulk defects. Observe the spherical accumulation morphology, through crack distribution and internal pore (open / closed) three-dimensional morphology using scanning electron microscopy and micro-CT. The Ra=8.55μm and the bulk porosity is about 10%. The coordinates of the closed pores are calibrated as pre-filling sites. Collect the coating thickness and substrate thermal properties as inputs to the thermal conductivity boundary control system.

[0049] Step 2: Substitute into equation (1) to calculate the thermal diffusivity of the composite system. Combined with scanning speed v =0.08m / s, spot diameter D =0.003m, the equivalent action time is calculated using equation (2). ;Calculate the thermal penetration depth using formula (4) Constraining the depth of the molten pool =30~60μm, satisfying < < The surface energy density is defined by equations (6) and (7). Scanning overlap rate =0.7, thus determining the first-stage continuous laser power. P =420W.

[0050] Step 3: Pre-place nano-SiC repair slurry as filler at the closed pore sites; introduce high-purity argon as a protective atmosphere, and use a 1070nm continuous fiber laser with a 3mm large spot to perform remelting repair scanning; collect the coating surface temperature in real time with an infrared thermometer, and dynamically fine-tune the laser power based on the PID formula of formula (11) to stabilize the molten pool temperature at 2700~3000K (SiC melting and densification range); the molten SiC melt is driven by the surface tension and capillary pressure of formulas (8) and (9) to fill the surface cracks and wet and reconnect the crack interface, and the openings and connected pores are backfilled and sealed by capillary seepage, and the closed pores are healed by melting and densification of the pre-placed filler. After the scanning is completed, a continuous remelting repair layer is formed on the coating surface.

[0051] Step 4: Online detection of the roughness and defect repair rate of the remelted repair layer. The detection results show Ra=1.621μm, crack filling rate>95%, and pore closure rate>93%, which meet the requirements of Ra≤2μm and the defect repair rate switching threshold. The process can then proceed directly to the second stage of fine repair.

[0052] Step 5: Switch the optical path of the device to output a 355nm nanosecond ultraviolet pulse laser with a spot diameter of D=20μm. Based on equations (11) to (14), constrain the average pulse power, repetition frequency, pulse flux, and peak power, perform a full-coverage fine scan of the remelted repair layer surface to remove surface re-condensed particles and micro-undulations. After the second stage of fine repair, the final surface roughness Ra=1.048μm was re-measured, achieving the target of Ra≤1.1 μm.

[0053] Scanning electron microscopy and micro-CT showed that the main through cracks in the detection area had been filled and closed, the internal pore closure rate was >95%, and the repaired area was well bonded to the substrate without peeling.

[0054] Multiple sets of process comparison experiments Five control experiments were conducted using the same batch of SiC coated samples (original Ra=8.55μm, bulk porosity approximately 10%) with five different processes. Only objective test data were recorded for each group, and the corresponding subjective effects are described below: 1) such as Figure 2 The original untreated sample had spherical protrusions and dense through cracks on the surface, a bulk porosity of about 10%, and a roughness Ra=8.55μm, making it unsuitable for direct service. 2) such as Figure 3 1064nm continuous laser remelting: The surface is macroscopically leveled and the cracks are filled, but there are residual molten and re-condensed particles and unhealed closed pores on the surface. The roughness Ra=1.621μm, which cannot meet the precision finishing standard. 3) such as Figure 4 Single ablation with 1064nm pulsed laser: obvious scanning edge steps, debris accumulation, and localized layered cracking appeared on the surface, the bulk defects were not improved, the roughness Ra=2.537μm, and the surface defects were aggravated. 4) such as Figure 5 Multiple scans with a 1064nm low-power pulse: the surface micro-melting forms an island-like resolidification structure, the surface ripples are obvious, the pores and cracks are not repaired, the roughness Ra=3.887μm, and the polishing effect is deteriorated; 5) such as Figure 6 Dual-stage laser composite polishing and repair (this invention): First, a 1064nm continuous or long-pulse laser with a large spot remelting scan is used. After the surface spherical protrusions melt, they spread and fill the through cracks and depressions under the drive of surface tension and capillary action, and the crack interface is rejoined. Open / connected pores are backfilled and sealed by capillary seepage, and closed pores are healed by laser densification with pre-placed fillers. The temperature of the molten pool is stabilized in the suitable melting and densification range of SiC by infrared thermometry and PID closed loop. After online monitoring confirms that Ra≤2μm and the defect repair rate meets the standard, the process is switched to 355nm ultraviolet short / ultra-short pulse small spot fine scanning to remove surface micro-undulations, burrs and re-condensed particles. The final roughness Ra=1.048μm meets the qualified standard of Ra≤1.1μm. The cracks are effectively filled and closed, the bulk pore closure rate is >95%, and there are no ablation steps, layered peeling and graphitization discoloration defects on the surface.

[0055] Figure 7 The bar chart shows the surface roughness comparison after polishing for each method. The horizontal axis represents, in descending order: untreated original sample (Ra=8.55μm), 1064nm continuous laser remelting treatment (Ra=1.621μm), 1064nm pulsed laser single ablation (Ra=2.537μm), 1064nm low-power pulse multiple scans (Ra=3.887μm), and the present invention's two-stage laser composite polishing and repair method (Ra=1.048μm). The vertical axis represents the surface roughness Ra value (in μm).

[0056] As can be seen from the comparison of the bar charts, the final roughness of this invention (1.048 μm) is the lowest among all processes, reducing it by approximately 35.4% compared to simple continuous laser remelting, approximately 58.7% compared to single-pulse ablation, and approximately 73.0% compared to multiple-pulse scanning. This reduction is due to the first-stage remelting process using melt flow self-leveling to reduce Ra from 8.55 to 1.62 μm, and the second-stage ultraviolet short-pulse ablation using photochemical micro-ablation to further remove micro-undulations and re-condensed particles. In contrast, the single-pulse process results in an increase in roughness due to thermal damage (scratching / steps) or residual re-condensed particles. This bar chart visually demonstrates the significant advantages of the 'remelting repair + fine-tuning' two-stage synergistic strategy of this invention in improving surface integrity.

[0057] The present invention provides a phased, closed-loop temperature control and defect threshold composite repair-polishing and repair method, which can simultaneously improve the surface morphology defects and bulk porosity defects of the original coating, and has significant advantages over simple polishing in terms of repair dimension.

Claims

1. A two-stage laser composite polishing and repair method for silicon carbide coating surfaces, characterized in that, Includes the following steps: S1. Perform morphological observation and quantitative roughness detection on the surface of the SiC coating to be treated, record the size of the spherical protrusions, the distribution of through cracks, the location and size of open and closed pores and the initial roughness Ra, and collect the thermal property parameters of the SiC coating and the substrate to establish the thermal boundary conditions of the SiC coating and substrate composite system. S2. Based on the SiC coating surface condition, defect distribution and thermophysical parameters obtained in step S1, determine the laser power, spot diameter, scanning speed and scanning overlap rate for the continuous or long pulse laser remelting repair stage. S3. Using a continuous laser or a long-pulse laser with a large spot size in millimeters, the SiC coating is remelted and repaired under an inert gas or vacuum protection environment. The melt spreads and fills the low-lying areas and through cracks, and the crack interface is rebonded. At the same time, the openings and connecting pores are backfilled by capillary seepage, and the pre-placed repair filler is laser densified and healed to close the pores, forming a continuous remelted repair layer that seals the defects. The laser power is adjusted in a closed loop by infrared thermometry. S4. Use online morphology, roughness and defect repair rate monitoring equipment to detect the status of the remelted repair layer; when the roughness or defect repair rate does not reach the preset threshold, adjust the remelting process parameters and repeat step S3; when both the roughness and defect repair rate reach the preset threshold, proceed to step S5. S5. Switch to nanosecond pulse laser, picosecond pulse laser, femtosecond short pulse laser or ultrashort pulse laser and micron-sized spot to perform fine scanning on the remelted repair layer and remove surface micro-undulations, burrs and melt-re-condensed particles. S6. After removal treatment, conduct final state testing to evaluate the SiC coating roughness, through crack filling, closure state, internal pore sealing rate, as well as ablation steps, layered peeling, and graphitization discoloration defects.

2. The method for two-stage laser composite polishing and repair of silicon carbide coating surface according to claim 1, characterized in that: In step S1, the SiC coating is formed by chemical vapor deposition, physical vapor deposition, plasma spraying, or reaction sintering; morphology and defect observation are performed using scanning electron microscope, optical microscope, micro-CT, or machine vision system; roughness and porosity are detected using contact roughness meter, laser confocal profilometer, or micro-CT imaging method.

3. The method for two-stage laser composite polishing and repair of silicon carbide coating surface according to claim 1, characterized in that, In step S2, based on the SiC coating thermal conductivity, substrate material density, substrate material specific heat capacity at constant pressure, laser spot diameter, scanning speed, pulse width, effective absorptivity, and center-to-center spacing of the scanning channels obtained in step S1, the thermal diffusivity, equivalent laser action time, thermal penetration depth, molten pool depth, laser surface energy density, and scanning overlap rate of the composite system are calculated using the corresponding heat transfer and molten pool kinetic formulas. The remelting scanning process parameters for step S3 are then set based on these calculated results; where: thermal diffusivity... , The thermal diffusivity of the coating-substrate composite system is expressed in m² / s. k The value represents the thermal conductivity of the SiC coating, expressed in W / (m·K). Density of the matrix material, in kg / m³; The matrix material has a specific heat capacity at constant pressure, expressed in J / (kg·K), and the equivalent duration of continuous laser treatment is given. , The equivalent interaction time between the laser and the coating material is expressed in seconds. D The laser spot diameter is expressed in meters (m). v Laser scanning speed, in m / s; equivalent action time of long-pulse laser. , The pulse width of a long-pulse laser is measured in seconds (s), and the thermal penetration depth is also mentioned. The depth of the molten pool meets the requirements. , The depth of the laser remelting pool, in meters (m). The height of the spherical protrusions on the coating surface, in meters (m). SiC coating thickness, in meters; laser surface energy density. , Laser surface energy density, in J / m²; To determine the effective absorption rate of the SiC coating to laser light, based on the laser wavelength and the room temperature absorption coefficient of SiC, The value ranges from 0.6 to 0.8; Average power of continuous / long-pulse laser, in W; D is the scanning speed, in m / s; D is the laser spot diameter, in m; and the scanning overlap rate is... , s The distance between the centers of two adjacent laser scanning tracks is expressed in meters (m).

4. The method for two-stage laser composite polishing and repair of silicon carbide coating surface according to claim 1, characterized in that, In step S3, the melt surface tension driving force and the crack capillary filling pressure are respectively determined by the corresponding melt kinetic formulas. Calculations show that the backfilling of open and connected pores is characterized by a Washburn-type capillary flow relationship, and the healing of closed pores is achieved by pre-placed repair filler material through laser remelting and densification; among these, The driving force is the surface tension of the melt. γ is the capillary filling pressure of the crack; γ is the surface tension of the melt. This represents the rate of change of surface tension with temperature. For temperature gradient, Let be the equivalent radius of curvature of the crack.

5. The method for two-stage laser composite polishing and repair of silicon carbide coating surface according to claim 1, characterized in that, In step S5, the wavelengths of the short-pulse laser and the ultrashort-pulse laser are 355 nm and 532 nm, respectively. The single-photon energy, single-pulse energy, peak power, and pulse flux of the short-pulse laser and the ultrashort-pulse laser are calculated using the corresponding laser physics formulas; among them, the single-photon energy... Single pulse energy Peak power Pulse flux ; h It is Planck's constant. c Where is the speed of light in a vacuum, and 'e' is the unit conversion factor. λ It is the wavelength of ultraviolet laser. The average power of the pulsed laser. f The pulse repetition frequency, The pulse width. D denoted as the diameter of the light spot.

6. The method for two-stage laser composite polishing and repair of silicon carbide coating surface according to claim 1, characterized in that, The qualification criteria for step S6 include: SiC coating roughness Ra≤1.1μm; through cracks in the detection area are effectively filled or closed, and the crack opening, length or depth meets the preset control index; internal pore closure rate≥preset threshold; and no obvious ablation sinking steps, layered peeling and graphitization discoloration defects in the scanned area.

7. The method for two-stage laser composite polishing and repair of silicon carbide coating surface according to claim 1, characterized in that, Steps S1 to S6 are completed on the same laser processing platform, and the workpiece is clamped only once without disassembly and transfer.

8. The method for two-stage laser composite polishing and repair of silicon carbide coating surface according to claim 1, characterized in that, The protective gas for the remelting repair scan in step S3 is argon or nitrogen.

9. The method for two-stage laser composite polishing and repair of silicon carbide coating surface according to claim 1, characterized in that, The roughness preset threshold in step S4 is Ra≤2μm and the defect repair rate reaches the preset index.

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