A method for directly detecting silicon nitride grain boundary phase, replica film and application

Abstract

This invention provides a method, replica film, and application for directly detecting silicon nitride grain boundary phases. The method includes the following steps: S1, etching a polished silicon nitride ceramic sample with hydrofluoric acid solution; S2, replicating the etched polished surface with a first solvent and a cellulose acetate film to obtain a primary replica; S3, vacuum-depositing a carbon film onto the primary replica to obtain a secondary replica; S4, cutting and attaching the secondary replica film to low-temperature paraffin wax, then dissolving the cellulose acetate in a second solvent; S5, transferring the dissolved secondary replica film to a third solvent, and after the secondary replica film has fully expanded, retrieving it with a copper mesh, drying it, and observing it under an electron microscope. The method provided by this invention can separate the grain boundary phase from the silicon nitride main phase, obtaining the solid grain boundary phase while retaining the morphological outline of the silicon nitride main phase. This method is simple to operate, low in cost, and provides great convenience for studying the influence of grain boundary phases on the mechanical and optical properties of silicon nitride separately.

Description

Technical Field

[0001] This invention belongs to the field of phase testing technology, specifically relating to a method for directly detecting silicon nitride grain boundary phases, a replica film, and its application. Background Technology

[0002] Silicon nitride ceramics are a traditional structural ceramic material, widely used in cutting tools, refractory materials, high-end watch cases and bracelets, high-grade buttons, and mobile phone casings due to their excellent mechanical properties, vibrant and elegant colors, metallic luster, and environmental friendliness (non-toxic and non-allergenic). However, the high covalent bond nature of silicon nitride dictates that it can only be densified through liquid-phase sintering. This requires the addition of sintering aids to react with the oxygen-rich layer on the surface of the silicon nitride powder at high temperatures to generate a (nitrogen)oxide liquid phase. This liquid phase is then sintered into a dense body through particle rearrangement, dissolution-precipitation, and grain growth. During cooling, the liquid phase transforms into a solid phase, remaining at the grain boundaries as a glassy phase or / and crystalline phase. The mechanical properties (especially high-temperature properties) and optical properties (especially color rendering properties) of silicon nitride ceramics largely depend on the grain boundary phase.

[0003] Because the content of the grain boundary phase in silicon nitride is much smaller than that of the main silicon nitride phase, the study of the grain boundary phase is significantly affected by the main silicon nitride phase. For example, in phase analysis of silicon nitride, the strong peaks of the main silicon nitride phase may mask the weaker diffraction peaks of the grain boundary crystalline phase. When observing the grain boundary crystalline phase using an electron microscope, the small size of the grain boundary precipitates and the abundant distribution of the main silicon nitride phase make the search time-consuming and laborious. If a method could be developed to separate the grain boundary phase from the main silicon nitride phase, obtaining the solid grain boundary phase while preserving the morphology and outline of the main silicon nitride phase, it would greatly benefit the independent study of the grain boundary phase in silicon nitride ceramics, reducing the interference of the main silicon nitride phase. Summary of the Invention

[0004] The first objective of this invention is to provide a method for directly detecting silicon nitride grain boundary phases, which aims to separate the grain boundary phases from the silicon nitride main phase, obtain the physical grain boundary phases while retaining the morphology and outline of the silicon nitride main phase, and reduce interference from the silicon nitride main phase.

[0005] Therefore, the above-mentioned objective of the present invention is achieved through the following technical solution:

[0006] A method for detecting silicon nitride grain boundary phases, characterized by comprising the following steps:

[0007] S1. Place the polished silicon nitride ceramic sample into a hydrofluoric acid solution for a period of time to etch it.

[0008] S2. Apply an appropriate amount of the first solvent to the polished surface after etching in step S1, and attach a blue cellulose acetate membrane of similar size to the sample for replication for a period of time. After the cellulose acetate membrane dries completely, carefully peel it off. This cellulose acetate membrane is the required plastic primary replica, which can be used directly for the detection of silicon nitride grain boundary phase.

[0009] S3. Vacuum carbon film is deposited on the first-level replica obtained in step S2 to obtain a second-level replica;

[0010] S4. Cut the secondary replica film obtained in step S3 into small pieces smaller than the sample copper mesh, with the carbon-sprayed side attached to the low-temperature paraffin on the heated small glass slide. After the glass slide cools and the paraffin solidifies, place it in a container of the second solvent.

[0011] S5. After the cellulose acetate in step S4 has completely dissolved, transfer the secondary replica membrane to the third solvent. After the secondary replica membrane has fully expanded, retrieve it with a copper mesh, dry it, and observe it under an electron microscope.

[0012] While adopting the above technical solutions, the present invention may also adopt or combine the following technical solutions:

[0013] As a preferred technical solution of the present invention: In step S1, the silicon nitride ceramic sample is made by uniformly mixing silicon nitride powder, sintering aid, etc. in a ball mill, then dry pressing, debinding, and sintering in a pressure furnace (sintering atmosphere: nitrogen or 5% hydrogen-95% nitrogen; sintering temperature: 1800~1900℃; sintering time: 2~4h; gas pressure: 0.1~0.7MPa).

[0014] As a preferred embodiment of the present invention: in step S1, the concentration of the hydrofluoric acid solution is 5% to 40%, for example: 5%, 10%, 20%, 30%, or 40%. Since silicon nitride itself is a covalent compound with strong bonding, and grain boundaries are usually bonded by ionic bonds and intermolecular forces, selecting an HF solution concentration in the range of 5% to 40% ensures that while just partially etching away the grain boundary glass phase to reveal the crystalline phase present in the glass phase, the silicon nitride phase is not destroyed.

[0015] The etching time of hydrofluoric acid solution is 1 to 30 minutes, for example: 1, 5, 10, 15, 20, 30 minutes. This ensures that after etching away part of the grain boundary glass phase, the crystalline phase existing in the glass phase will be revealed, while not destroying the silicon nitride main phase.

[0016] As a preferred embodiment of the present invention: In step S2, the first solvent is acetone. The amount of acetone added to the polished sample surface should be appropriate, usually just enough to cover the sample surface without overflowing. If there is too much acetone, the AC paper (cellulose acetate membrane) will completely dissolve; if there is too little acetone, the AC paper will not make good contact with the surface, and the purpose of replica extraction will not be achieved.

[0017] As a preferred technical solution of the present invention: in step S2, the thickness of the cellulose acetate film is 0.03 to 0.10 mm. If the AC paper is too thin, it is easy to break during the replication process; if it is too thick, the replication contrast is low.

[0018] The replication time is 2 to 10 minutes. If the time is too short, the AC paper will not be completely dry and will easily tear when peeled off. If the time is too long, it will be difficult to peel the AC paper off the sample.

[0019] As a preferred technical solution of the present invention: in step S2, multiple layers of primary replica cellulose acetate membranes used for phase detection are stacked to enhance the test signal.

[0020] As a preferred technical solution of the present invention: In step S3, when vacuum depositing carbon film, the thickness of carbon film is estimated by the color change of the surface of the milky white ceramic sheet placed in the vacuum depositing device, and it is generally advisable when it turns into light brown.

[0021] As a preferred technical solution of the present invention: in step S4, the cutting size of the cellulose acetate-carbon composite membrane (secondary replica membrane) is 2×2mm.

[0022] As a preferred technical solution of the present invention: in step S4, the second solvent is acetone or methyl acetate.

[0023] As a preferred technical solution of the present invention: In step S5, the method for determining whether cellulose acetate is completely dissolved is that the blue color of AC paper completely fades and becomes transparent, then the cellulose acetate is completely dissolved.

[0024] As a preferred technical solution of the present invention: In step S5, the third solvent is an acetone aqueous solution, and the volume ratio of acetone to water is (50-5):(50-95), for example: (50:50), (20:80), (10:90), (5:95), etc., which can ensure that the secondary replica film is fully opened and does not break. The concentration of the acetone aqueous solution is adjusted according to the degree of opening of the replica film. If the film does not open or is not fully opened, distilled water needs to be added; if the film is over-opened and breaks, acetone needs to be added.

[0025] As a preferred technical solution of the present invention: in step S5, the electron microscope includes SEM (scanning electron microscope), TEM (transmission electron microscope), etc.

[0026] The second objective of this invention is to provide a primary replica membrane and a secondary replica membrane prepared by the method described above.

[0027] Another objective of this invention is to provide the application of the primary and secondary replica films, as described above, in the direct detection of grain boundary phases in silicon nitride ceramics.

[0028] This invention provides a method, a replica film, and applications for directly detecting the grain boundary phase of silicon nitride. Since silicon nitride is a covalent compound, it requires the addition of sintering aids for liquid-phase sintering. During cooling, the liquid phase transforms into a solid state, remaining at the grain boundaries as a glassy phase or / and crystalline phase. The mechanical properties (especially high-temperature properties) and optical properties (especially color development properties) of silicon nitride largely depend on the grain boundary phase. However, because the content of the silicon nitride grain boundary phase is much smaller than that of the main silicon nitride phase, the study of the grain boundary phase is severely affected by the main silicon nitride phase. The method provided by this invention can separate the grain boundary phase from the main silicon nitride phase, obtaining the solid grain boundary phase while retaining the morphological outline of the main silicon nitride phase. This method is simple to operate, low in cost, and provides significant convenience for studying the influence of the grain boundary phase on the mechanical and optical properties of silicon nitride independently.

[0029] Specifically, compared with the prior art, the present invention has at least the following beneficial effects.

[0030] 1) The method provided by this invention is simple, fast, and has a high success rate in sample preparation.

[0031] 2) The method provided by this invention has a low cost and can prepare multiple samples at the same time for observation under different electron microscopes. In particular, samples for transmission electron microscopy observation do not need to be further processed into ultrathin sections or ion thinning.

[0032] 3) The method, primary replica film and secondary replica film provided by this invention can be used to study the silicon nitride grain boundary phase independently, with little or no interference from the silicon nitride main phase. Attached Figure Description

[0033] Figure 1a This is a scanning electron microscope image of the secondary replica membrane in Example 1; Figure 1b The images shown are transmission electron microscope (TEM) images of the secondary replica film in Example 1 and electron diffraction patterns of the crystal structure at the grain boundaries.

[0034] Figure 2a This is a scanning electron microscope image of the secondary replica membrane in Example 2; Figure 2b The images shown are transmission electron microscope (TEM) images of the secondary replica film in Example 2 and electron diffraction patterns of the crystal structure at the grain boundaries.

[0035] Figure 3a The X-ray diffraction pattern of the silicon nitride bulk sample in Comparative Example 1 is shown. Figure 3bThe image shows a scanning electron microscope (SEM) image of the bulk silicon nitride sample in Comparative Example 1. Figure 3c The images show transmission electron microscopy (TEM) images of the silicon nitride bulk sample in Comparative Example 1 and electron diffraction patterns of the crystal structure at the grain boundaries.

[0036] Figure 4a The X-ray diffraction pattern of the silicon nitride powder sample in Comparative Example 2; Figure 4b The images show transmission electron microscopy (TEM) images of silicon nitride powder samples in Comparative Example 2 and electron diffraction patterns of the crystal structure at the grain boundaries.

[0037] Figure 5a The X-ray diffraction pattern of the etched silicon nitride bulk sample in Comparative Example 3 is shown. Figure 5b The image shows a scanning electron microscope (SEM) image of the etched silicon nitride bulk sample in Comparative Example 3. Figure 5c The images show transmission electron microscopy (TEM) images of the etched silicon nitride bulk sample in Comparative Example 3 and electron diffraction patterns of the crystal structure at the grain boundaries. Detailed Implementation

[0038] This invention provides a method for directly detecting silicon nitride grain boundary phases, comprising the following steps:

[0039] S1. The silicon nitride ceramic sample polished to a mirror surface is placed in an HF solution with a concentration of 5% to 40% (preferably 10%) for etching for 1 to 30 minutes (preferably 5 minutes);

[0040] S2. Apply 1-2 drops of acetone to the polished surface of the above-etched sample, attach a piece of blue AC paper similar in size to the sample for 2-10 minutes (preferably 6 minutes), and carefully peel off the AC paper with reverse tweezers after it has dried completely. This AC paper is the required plastic primary replica. Stack two layers of replica AC paper for phase detection.

[0041] S3. Place the first-level AC paper copy face up on the adhesive tape backed with the sheet, and put the fixed copy into the vacuum coating device. Stop coating when the color of the milky white ceramic sheet placed in the vacuum coating device turns light brown. This AC paper is a second-level copy.

[0042] S4. Cut the secondary composite AC paper into small squares of 2mm×2mm. Place the carbon-sprayed side onto the low-temperature paraffin on the heated small glass plate. After the glass plate cools and the paraffin solidifies, place it in a covered container of methyl acetate.

[0043] S5. After the cellulose acetate dissolves, transfer the secondary replica to an acetone aqueous solution (the volume ratio of acetone to water is preferably 10:90). After the secondary replica membrane is fully expanded, use a copper mesh to remove it, absorb water and dry it before placing it in an electron microscope for observation.

[0044] The present invention will now be described in detail with reference to specific embodiments:

[0045] Example 1

[0046] 57.8g of silicon nitride powder, 0.92g of magnesium oxide, and 8.2g of YAG:Ce were mixed. 3+ Phosphor powder and 3.08g europium oxide were mixed evenly, dry-pressed, and slowly debonded. The mixture was then sintered in a carbon tube furnace under a nitrogen atmosphere at a pressure of 0.6MPa, with a heating rate of 5℃ / min to reach 1850℃, and held for 2 hours. The sintered sample was processed into 3mm×4mm×36mm strips. The polished strips were then etched in a 10% HF solution for 5 minutes. After etching, the sample was soaked in clean water for 1 hour. After drying, 1-2 drops of acetone were applied to the surface, and a piece of AC paper of similar size to the sample was attached for replication for 6 minutes. Once the AC paper was completely dry, it was carefully peeled off; this AC paper is a first-order replica. Different treatments can be applied to the primary replica for phase detection and electron microscopy observation: 1) Phase detection: The reverse side of the primary replica is used to replicate another test strip to enhance the test signal; 2) Electron microscopy observation: The primary replica is flatly attached to adhesive tape lined with paper. The fixed replica is placed in a vacuum coating apparatus. Coating is stopped when the color of the milky white ceramic sheet placed in the vacuum coating apparatus turns light brown. This AC paper is the secondary replica. The secondary replica AC paper is cut into 2mm×2mm squares. The carbon-sprayed side is attached to low-temperature paraffin on a heated small glass slide. After the glass slide cools and the paraffin solidifies, it is placed in a covered container of methyl acetate to dissolve the cellulose acetate and paraffin. Then it is transferred to a 10% acetone aqueous solution. After the secondary replica film is fully expanded, it is retrieved with a copper mesh, dried, and then placed in an electron microscope for observation.

[0047] Using an 18kW rotating target X-ray diffractometer (D / MAX 2550V), crystalline phases such as Eu2Si2O7 and SiO2 were found at the grain boundaries of silicon nitride. Figure 1a As shown, a darker silicon nitride morphology and white crystalline grains were observed using a FEI field emission scanning electron microscope (Magellan 400). Figure 1b As shown, crystalline particles were observed using a 120KV transmission electron microscope (JEM-1400). The electron diffraction pattern of the crystalline phase is a ring symbolizing polycrystalline, and the high-resolution image shows lattice fringes of crystalline phase particles embedded in an amorphous state.

[0048] Example 2

[0049] 57.8g of silicon nitride powder, 0.92g of magnesium oxide, and 8.2g of YAG:Ce were mixed. 3+Phosphor powder and 3.08g of ytterbium oxide were mixed evenly, dry-pressed, and slowly debonded. The mixture was then sintered in a carbon tube furnace under a nitrogen atmosphere at a pressure of 0.6MPa, with a heating rate of 5℃ / min to reach 1850℃, and held for 2 hours. The sintered samples were processed into 3mm×4mm×36mm strips. The polished strips were then etched in a 10% HF solution for 5 minutes. After etching, the samples were soaked in water for 1 hour, dried, and then 1-2 drops of acetone were applied to the surface. A piece of AC paper, similar in size to the sample, was then attached for 6 minutes for replication. Once the AC paper was completely dry, it was carefully peeled off; this AC paper is a first-order replica. Different treatments can be applied to the primary replica for phase detection and electron microscopy observation: 1) Phase detection: The reverse side of the primary replica is used to replicate another test strip to enhance the test signal; 2) Electron microscopy observation: The primary replica is flatly attached to adhesive tape lined with paper. The fixed replica is placed in a vacuum coating apparatus. Coating is stopped when the color of the milky white ceramic sheet placed in the vacuum coating apparatus turns light brown. This AC paper is the secondary replica. The secondary replica AC paper is cut into 2mm×2mm squares. The carbon-sprayed side is attached to low-temperature paraffin on a heated small glass slide. After the glass slide cools and the paraffin solidifies, it is placed in a covered container of methyl acetate to dissolve the cellulose acetate and paraffin. Then it is transferred to a 10% acetone aqueous solution. After the secondary replica film is fully expanded, it is retrieved with a copper mesh, dried, and then placed in an electron microscope for observation.

[0050] Using an 18kW rotating target X-ray diffractometer (D / MAX 2550V), Yb₂Si₂O₇ and SiO₂ crystalline phases were found at the grain boundaries of silicon nitride. Figure 2a As shown, a darker silicon nitride morphology and white crystalline grains were observed using a FEI field emission scanning electron microscope (Magellan 400). Figure 2b As shown, crystalline particles were observed using a 120KV transmission electron microscope (JEM-1400). The electron diffraction pattern of the crystalline phase is a ring symbolizing polycrystalline, and the high-resolution image shows lattice fringes of crystalline phase particles embedded in an amorphous state.

[0051] Comparative Example 1

[0052] 57.8g of silicon nitride powder, 0.92g of magnesium oxide, and 8.2g of YAG:Ce were mixed. 3+Phosphor and 3.08 g of europium oxide were mixed evenly, dry-pressed, and slowly debonded. The mixture was then sintered in a carbon tube furnace under a nitrogen atmosphere at a pressure of 0.6 MPa, with a heating rate of 5 °C / min to reach 1850 °C, and held for 2 hours. The sintered samples were processed into 3 mm × 4 mm × 36 mm strips and small discs with a diameter of 3 mm and a thickness of 0.3 μm. One side of the strips was polished to a mirror finish for phase detection and scanning electron microscopy (SEM) observation of the grain boundary phase. The small discs were ion-thinned and carbon-sprayed for transmission electron microscopy (TEM) observation of the grain boundary phase.

[0053] like Figure 3a As shown, using an 18KW rotating target X-ray diffractometer (D / MAX 2550V), the phase composition of the silicon nitride bulk sample was found to contain only the β-Si3N4 phase, with no other second phase present. Figure 3b As shown, FEI field emission scanning electron microscopy (Magellan 400) revealed that the bulk silicon nitride sample contained only two phases: silicon nitride particles and a grain boundary liquid phase. The sintering aids were almost entirely present in the grain boundary liquid phase. Figure 3c As shown, a 120KV transmission electron microscope (JEM-1400) was used to observe that the bulk silicon nitride sample contained only silicon nitride particles and grain boundary liquid phase. The electron diffraction pattern of the grain boundary phase was a diffuse central spot symbolizing the amorphous state.

[0054] Comparative Example 2

[0055] 57.8g of silicon nitride powder, 0.92g of magnesium oxide, and 8.2g of YAG:Ce were mixed. 3+ Phosphor and 3.08 g of europium oxide were mixed evenly, dry-pressed, and slowly debonded. The mixture was then sintered in a carbon tube furnace under a nitrogen atmosphere at a pressure of 0.6 MPa, with a heating rate of 5 °C / min to reach 1850 °C, and held at that temperature for 2 hours. The sintered sample was then ground into powder for direct phase analysis and electron microscopy observation.

[0056] like Figure 4a As shown, X-ray diffraction of the silicon nitride powder sample using an 18kW rotating target X-ray diffractometer (D / MAX 2550V) revealed the presence of two phases: β-Si3N4 and SiO2. The SiO2 was primarily introduced by the quartz mortar used in grinding the silicon nitride bulk into powder. FEI field emission scanning electron microscopy (Magellan 400) observed only two phases in the silicon nitride powder sample: silicon nitride particles and a grain boundary liquid phase. The sintering aids were almost entirely present in the grain boundary liquid phase. Figure 4b As shown, a 120KV transmission electron microscope (JEM-1400) was used to observe that the silicon nitride powder sample contained only silicon nitride particles and grain boundary liquid phase. The electron diffraction pattern of the grain boundary phase was a diffuse central spot symbolizing the amorphous state.

[0057] Comparative Example 3

[0058] 57.8g of silicon nitride powder, 0.92g of magnesium oxide, and 8.2g of YAG:Ce were mixed. 3+ Phosphor and 3.08g europium oxide were mixed evenly, dry-pressed, and slowly debonded. The mixture was then sintered in a carbon tube furnace under a nitrogen atmosphere at a pressure of 0.6MPa, with a heating rate of 5℃ / min to reach 1850℃, and held for 2 hours. The sintered samples were processed into 3mm×4mm×36mm strips and small discs with a diameter of 3mm and a thickness of 0.3μm. The discs and mirror-polished strips were etched in a 10% HF solution for 5 minutes. The etched samples were then soaked in water for 1 hour. The dried strips were directly used for phase analysis and scanning electron microscopy (SEM) observation of grain boundary phases. The discs, after ion thinning and carbon spraying, were used for transmission electron microscopy (TEM) observation of grain boundary phases.

[0059] like Figure 5a As shown, using an 18KW rotating target X-ray diffractometer (D / MAX 2550V), the phase composition of the silicon nitride bulk sample was found to contain only the β-Si3N4 phase, with no other second phase present. Figure 5b As shown, FEI field emission scanning electron microscopy (Magellan 400) revealed that the bulk silicon nitride sample contained only two phases: silicon nitride particles and a grain boundary liquid phase. The sintering aids were almost entirely present in the grain boundary liquid phase. Figure 5c As shown, a 120KV transmission electron microscope (JEM-1400) was used to observe that in addition to silicon nitride particles, there were also some crystalline phases in the grain boundary liquid phase of the silicon nitride bulk sample. The electron diffraction pattern of the crystalline phase was a ring symbolizing polycrystalline.

[0060] Tables 1-3 show that conventional methods, i.e., Comparative Examples 1-3, struggle to determine the phase structure of the grain boundary crystalline phase due to interference from the silicon nitride main phase. However, the extraction replication detection method provided by this invention can separate the silicon nitride main phase from the grain boundary phase, allowing for independent study of the silicon nitride grain boundary phase with little or no interference from the silicon nitride main phase. Furthermore, after silicon nitride powder is mixed with sintering aids such as rare earth metal oxides, magnesium oxide, and YAG phosphor, a crystalline phase appears at the grain boundaries. This crystalline phase has a significant impact on the mechanical properties and color development of silicon nitride.

[0061] Table 1. Raw materials and their amounts used in each embodiment / comparative example.

[0062]

[0063] Table 2. Sample preparation methods used in each example / comparative example.

[0064] Examples / Comparative Examples Sample preparation method Example 1 HF etching, polishing, AC paper extraction and replication Example 2 HF etching, polishing, AC paper extraction and replication Comparative Example 1 Uncorroded, polished, ion-thinned Comparative Example 2 Uncorroded, ground into powder Comparative Example 3 HF etching, polishing, ion thinning

[0065] Table 3. Detection results of each example / comparative example.

[0066]

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for detecting silicon nitride grain boundary phases, characterized in that: Includes the following steps: S1. Place the polished silicon nitride ceramic sample into a hydrofluoric acid solution for a period of time to etch it. S2. Apply an appropriate amount of the first solvent to the polished surface after etching in step S1, and attach a blue cellulose acetate membrane of similar size to the sample for replication for a period of time. After the cellulose acetate membrane dries completely, carefully peel it off. This cellulose acetate membrane is the required plastic primary replica, which can be used directly for the detection of silicon nitride grain boundary phase. S3. Vacuum carbon film is deposited on the first-level replica obtained in step S2 to obtain a second-level replica; S4. Cut the secondary replica film obtained in step S3 into small pieces smaller than the sample copper mesh, with the carbon-sprayed side attached to the low-temperature paraffin on the heated small glass slide. After the glass slide cools and the paraffin solidifies, place it in a container of the second solvent. S5. After the cellulose acetate in step S4 has completely dissolved, transfer the secondary replica membrane to the third solvent. After the secondary replica membrane has fully expanded, retrieve it with a copper mesh, dry it, and observe it under an electron microscope.

2. The method according to claim 1, characterized in that: In step S1, the concentration of the hydrofluoric acid solution is 5% to 40%, and the corrosion time of the hydrofluoric acid solution is 1 to 30 minutes.

3. The method according to claim 1, characterized in that: In step S2, the first solvent is acetone.

4. The method according to claim 1, characterized in that: In step S2, the thickness of the cellulose acetate membrane is 0.03–0.10 mm; the replication time is 2–10 min.

5. The method according to claim 1, characterized in that: In step S2, multiple layers of primary replica cellulose acetate membranes are used for phase detection.

6. The method according to claim 1, characterized in that: In step S3, during vacuum carbon deposition, the carbon film thickness is estimated based on the color change of the surface of the milky white ceramic sheet placed simultaneously in the vacuum deposition apparatus.

7. The method according to claim 1, characterized in that: In step S4, the second solvent is acetone or methyl acetate.

8. The method according to claim 1, characterized in that: In step S5, the third solvent is an aqueous acetone solution, with a volume ratio of acetone to water of (50-5):(50-95).

9. The primary replica membrane and the secondary replica membrane prepared by the method according to any one of claims 1-8.

10. The application of the primary replica film and the secondary replica film according to claim 9 in the direct detection of grain boundary phases in silicon nitride ceramics.

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