Surface-coated silicon nitride sintered body

Abstract

To provide a surface-coated silicon nitride sintered body with excellent water vapor resistance. [Solution] The surface-coated silicon nitride sintered body comprises a substrate mainly composed of silicon nitride crystals and a coating layer located on the substrate. The coating layer contains at least one of RE2Si2O7 and RE2SiO5 as a crystalline phase, where RE is the rare earth element. The substrate and the coating layer contain Al. The substrate contains 0.5 mol% or more of Al in terms of oxide.

Description

Technical Field

[0001] The present disclosure relates to a surface-coated silicon nitride sintered body.

Background Art

[0002] Conventionally, silicon nitride sintered bodies are excellent in strength, hardness, and thermal and chemical stability. However, in terms of oxidation resistance, their use conditions are becoming more severe and the atmosphere is becoming highly oxidizing, so sufficient durability has not been achieved. As a method for imparting such oxidation resistance, there is a sintered body in which the surface of silicon nitride is coated.

[0003] For example, in Patent Documents 1 to 3, sintered bodies having sufficient oxidation resistance by coating the surface of silicon nitride with rare earth elements are disclosed. The sintered body produced in this way is excellent in strength, hardness, and thermal and chemical stability, and thus, for example, as engineering ceramics such as parts for semiconductor manufacturing equipment, parts for liquid crystal manufacturing equipment, machine tools, combustion device components, parts for optical fiber manufacturing equipment, etc., its use as a material for thermal engine structures in particular is being studied.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the above-mentioned sintered body, for example, from the viewpoint of water vapor resistance, there is room for improvement in optimizing the composition of the coating layer and the silicon nitride substrate constituting the sintered body.

[0006] The purpose of this disclosure is to provide a surface-coated silicon nitride sintered body having excellent water vapor resistance. [Means for solving the problem]

[0007] A surface-coated silicon nitride sintered body according to one embodiment comprises a substrate containing silicon nitride crystals as the main component and a coating layer located on the substrate. The coating layer contains at least one of RE2Si2O7 and RE2SiO5 as a crystalline phase, where RE is a rare earth element. The substrate and the coating layer contain Al. The substrate contains 0.5 mol% or more of Al in terms of oxide. [Effects of the Invention]

[0008] According to this disclosure, it is possible to provide a surface-coated silicon nitride sintered body having excellent water vapor resistance. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a cross-sectional view showing an example of a surface-coated silicon nitride sintered body according to an embodiment. [Figure 2] Figure 2 is a cross-sectional view showing another example of a surface-coated silicon nitride sintered body according to the embodiment. [Modes for carrying out the invention]

[0010] The embodiments of the surface-coated silicon nitride sintered body disclosed in this application will be described in detail below. However, this disclosure is not limited to the embodiments described below.

[0011] Figure 1 is a cross-sectional view showing an example of a surface-coated silicon nitride sintered body according to an embodiment. As shown in Figure 1, the surface-coated silicon nitride sintered body 10 according to this embodiment has a substrate 12 and a coating layer 14.

[0012] The substrate 12 mainly contains silicon nitride crystals. Specifically, the substrate 12 may contain silicon nitride in a range of 70 mol% to 99 mol%, particularly 85 mol% to 99 mol%, in order to sufficiently exhibit high-temperature strength. The silicon nitride crystals may include, for example, Si3N4, Si2N2O, etc. The substrate 12 may also contain rare earth elements (RE) in an oxide equivalent range of 0.5 mol% to 10 mol%. The rare earth element content of the substrate 12 is particularly preferably in the range of 1 mol% to 10 mol%. For example, if the rare earth element content is within the above range, a substrate made of a dense silicon nitride sintered body with excellent sinterability can be easily obtained. Furthermore, a substrate with excellent high-temperature strength and high-temperature creep characteristics can be easily obtained.

[0013] The coating layer 14 is located on the substrate 12. The coating layer 14 contains rare earth elements. When the rare earth element is denoted as RE, the coating layer 14 contains at least one of RE2Si2O7 and RE2SiO5 as a crystalline phase. The coating layer 14 may also contain at least one of Yb, Y, Dy, Ho, Er, Tm, and Lu as the rare earth element.

[0014] Furthermore, the coating layer 14 may have grain boundaries containing SiO2. In such cases, the amount of SiO2 contained in the coating layer 14 may be 10 mol% or less. Note that grain boundaries containing SiO2 mean that the grain boundaries contain at least Si and O, and do not necessarily contain a crystalline phase of SiO2.

[0015] The porosity of the coating layer 14 may be greater than that of the substrate 12. In this case, the porosity of the coating layer 14 may be between 5% and 30%.

[0016] As the coating layer 14 containing RE2Si2O7 and RE2SiO5, for example, a mixture of rare earth element oxides (RE oxides) and silicon oxides (Si oxides) in a molar ratio of (Si oxide) / (RE oxide) = 1.5 to 3.5, followed by firing, may be used.

[0017] Further, the substrate 12 and the coating layer 14 contain Al. The substrate 12 may contain 0.5 mol% or more, particularly 0.5 mol% or more and 1 mol% or less of Al in terms of oxide conversion.

[0018] In the surface-coated silicon nitride sintered body 10 having such a configuration, even when thermally expanded by a high-temperature environment, for example, the coating layer 14 is less likely to peel off from the substrate 12. As a result, since the coating layer 14 acts as a corrosion-resistant layer, corrosion of the substrate 12 is also less likely to occur. Thereby, a surface-coated silicon nitride sintered body 10 having excellent water vapor resistance can be obtained.

[0019] In the surface-coated silicon nitride sintered body 10 of the present disclosure, the coating layer 14 may have a grain boundary phase located between crystal phases. The grain boundary phase may contain Al. When the grain boundary phase of the coating layer 14 contains Al, for example, rare earth elements contained in the coating layer 14 are likely to be substituted with Al, and the bond between the substrate 12 and the coating layer 14 becomes stronger. As a result, the coating layer 14 is even less likely to peel off from the substrate 12, and corrosion of the substrate 12 is even less likely to occur.

[0020] Furthermore, when the grain boundary phase of the coating layer 14 contains Al, the thermal expansion difference between the coating layer 14 and the substrate 12 can be reduced when used in a high-temperature environment. As a result, the coating layer 14 is even less likely to peel off from the substrate 12, and corrosion of the substrate 12 is even less likely to occur.

[0021] Also, in the surface-coated silicon nitride sintered body 10 of the present disclosure, the thickness of the coating layer 14 may be 1 μm or more and 400 μm or less, particularly 3 μm or more and 300 μm or less, 5 μm or more and 200 μm or less, and further 10 μm or more and 150 μm or less. Thereby, for example, even when thermally expanded by a high-temperature environment, the coating layer 14 is less likely to peel off from the substrate 12, and corrosion of the substrate 12 is less likely to occur. For this reason, a surface-coated silicon nitride sintered body 10 having excellent water vapor resistance can be obtained.

[0022] Furthermore, the surface-coated silicon nitride sintered body 10 of this disclosure may contain Si in the grain boundaries of the coating layer 14. In this case, the Si contained in the grain boundaries of the coating layer 14 may be contained as SiO2.

[0023] For example, if the grain boundaries of the coating layer 14 contain SiO2, then, for example, RE2SiO5 and SiO2 in the grain boundary phase can maintain a state of phase equilibrium and exist stably, making both reactions and diffusion less likely to occur. As a result, even when the surface-coated silicon nitride sintered body 10 is exposed to an oxidizing atmosphere for a long time, phase changes at the grain boundaries of the coating layer 14 are less likely to occur, and the coating layer 14 becomes even less likely to peel off from the substrate 12. As a result, the surface-coated silicon nitride sintered body 10 of this disclosure can have excellent water vapor resistance.

[0024] Furthermore, the surface-coated silicon nitride sintered body 10 of this disclosure may have a chemical bond between the coating layer 14 and the substrate 12. For example, the grain boundary phase in the substrate 12 may have a SiAlON structure or a similar structure. Also, if Al is contained in the coating layer 14, the crystalline phase of the coating layer 14 may have, for example, an Al-O-Si bond. As a result, chemical bonding becomes easier at the interface between the substrate 12 and the coating layer 14, and the bond between the substrate 12 and the coating layer 14 becomes stronger. This makes it less likely for the coating layer 14 to peel off from the substrate 12 even in high-temperature environments, and also reduces corrosion of the substrate 12. Therefore, a surface-coated silicon nitride sintered body 10 with excellent water vapor resistance can be obtained.

[0025] In Figure 1, a surface-coated silicon nitride sintered body 10 is shown having a coating layer 14 located on the first surface 12a of the substrate 12. However, the coating layer 14 may be positioned to cover the entire surface of the substrate 12.

[0026] Figure 2 is a cross-sectional view showing another example of a surface-coated silicon nitride sintered body according to the embodiment. As shown in Figure 2, the surface-coated silicon nitride sintered body 10 according to this embodiment has a base body 12, a coating layer 14, and an intermediate layer 16.

[0027] The intermediate layer 16 is located between the coating layer 14 and the substrate 12. The intermediate layer 16 may have an Al content that is lower than that of the substrate 12 and higher than that of the coating layer 14. Alternatively, the intermediate layer 16 may have a rare earth element content that is higher than that of the substrate 12 and lower than that of the coating layer 14. In this case, the thermal expansion coefficient of the intermediate layer 16 is considered to be between that of the coating layer 14 and the substrate 12, which makes it easier to reduce the stress associated with the difference in thermal expansion between the coating layer 14 and the substrate 12. As a result, the coating layer 14 becomes less likely to peel off from the substrate 12 even in high-temperature environments, and corrosion of the substrate 12 becomes even less likely. Therefore, it becomes easier to obtain a surface-coated silicon nitride sintered body 10 with excellent water vapor resistance.

[0028] Here, the Al content of the intermediate layer 16 may be, for example, 0.5 mol% to 5 mol% in terms of Al2O3. Also, the rare earth element content of the intermediate layer 16 may be, for example, 0.5 mol% to 6 mol% in terms of oxides.

[0029] Furthermore, the surface-coated silicon nitride sintered body 10 of this disclosure may have a chemical bond between the coating layer 14 and the intermediate layer 16. The surface-coated silicon nitride sintered body 10 of this disclosure may also have a chemical bond between the intermediate layer 16 and the substrate 12. When chemical bonding occurs at the interface between the coating layer 14 and the intermediate layer 16, and at the interface between the intermediate layer 16 and the substrate 12, the bonding between the coating layer 14 and the intermediate layer 16, and the bonding between the intermediate layer 16 and the substrate 12 becomes stronger. As a result, the coating layer 14 becomes less likely to peel off from the substrate 12 even in high-temperature environments, and corrosion of the substrate 12 becomes even less likely. Therefore, a surface-coated silicon nitride sintered body 10 with excellent water vapor resistance can be obtained.

[0030] The location and extent of each part of the surface-coated silicon nitride sintered body 10 of this disclosure, as well as the type and content ratio of each metal element contained in each part, can be confirmed by elemental analysis of the cross-section of the surface-coated silicon nitride sintered body 10 using, for example, an electron probe microanalyzer (EPMA) or wavelength-dispersive X-ray spectroscopy (WDS), such as mapping of specific elements. Alternatively, the composition of the coating layer 14 may be confirmed and identified by analyzing the surface using, for example, X-ray diffraction (XRD). The content of each metal element obtained by measurement is converted to metal oxides to obtain the content rate of each metal element. Specifically, for example, Al is converted to Al2O3, Si to SiO2, Yb to Yb2O3, and Y to Y2O3. If the surface-coated silicon nitride sintered body 10 contains other rare earth elements, it may be converted to representative oxides of each rare earth element.

[0031] Next, an example of a method for manufacturing the surface-coated silicon nitride sintered body 10 of the present disclosure will be described.

[0032] A coating slurry is applied to the surface of a substrate 12 containing 70 mol% or more of silicon nitride by methods such as spraying it with a spray gun, applying it with a brush, or immersing the substrate 12 in the slurry and then removing it, and then drying. After drying, a heat treatment is performed at a temperature of 1400°C to 1700°C for 0.5 hours to 5 hours to form a coating layer 14.

[0033] The heat treatment temperature may be, for example, 1500°C to 1650°C, or more precisely, 1530°C to 1600°C. Furthermore, during heat treatment, there may be movement of Al-containing elements between the substrate 12 and the coating layer 14.

[0034] Furthermore, as shown in Figure 2, the surface-coated silicon nitride sintered body 10 having an intermediate layer 16 may, for example, have the intermediate layer 16 material applied to the surface of the substrate 12, and then the coating layer 14 formed. Alternatively, the coating layer 14 material may be applied to the surface of the substrate 12, and then the intermediate layer 16 may be formed by heat treatment. In this case, elemental migration may occur between the substrate 12 and the intermediate layer 16, and between the intermediate layer 16 and the coating layer 14.

[0035] Furthermore, for the production of the surface-coated silicon nitride sintered body 10 of this disclosure, for example, film formation by coating using a coating slurry method by spray coating and dipping followed by heat treatment, atmospheric plasma spraying, reduced-pressure plasma spraying, CVD, aerosol deposition method, cold spray method, etc. may be used. [Examples]

[0036] Substrates with different compositions were prepared, and coating layers with different compositions were applied to the surfaces of these substrates to create samples of surface-coated silicon nitride sintered bodies with different configurations. Furthermore, steam corrosion tests and thermal cycling tests were performed on these samples to evaluate their performance.

[0037] (Substrate preparation) A slurry was obtained by mixing a silicon nitride ceramic powder with an average particle size of 0.4 μm with sintering aids such as yttrium oxide, aluminum oxide, and silicon dioxide, as well as other components as needed, and then adding a binder, solvent, etc. After drying the obtained slurry, a flat plate was formed using press molding and fired at a temperature of 1850°C for 8 hours to obtain substrate A containing Al. The composition of substrate A was 85 mol% silicon nitride, 9 mol% yttria (in terms of oxide), and 0.7 mol% aluminum (in terms of oxide).

[0038] Furthermore, ceramic powders such as silicon nitride, silicon carbide, and aluminum nitride with an average particle size of 0.4 μm were mixed with sintering aids such as lutetium oxide and other components as needed, and then a binder, solvent, etc. were added and mixed to obtain a slurry. After drying the obtained slurry, a flat plate was formed using press molding, and substrate B was obtained by firing at a temperature of 1900°C for 10 hours. The composition of substrate B was 90 mol% silicon nitride, 3 mol% lutetium (in terms of oxide), and 0.01 mol% aluminum (in terms of oxide).

[0039] (Preparation of raw material powder and surface-coated silicon nitride sintered body) Ytterbium oxide powder and silicon oxide powder were mixed in a molar ratio of SiO2 / Yb2O3 = 2.1 and calcined at 1400°C for 3 hours to obtain a powder. This powder was then pulverized to an average particle size of approximately 1 μm to obtain the raw material powder for the coating layer. Water and polyvinyl alcohol (PVA) were added to the obtained powder to prepare a slurry in which the powder was dissolved. The ceramic particle content was 25% by volume relative to the total volume of the slurry.

[0040] The obtained slurry was sprayed onto the surface of the Al-containing substrate A using a spray gun to coat it with powder. Then, the mixture was heat-treated at 1540°C for 100 minutes to dissolve the powder and produce a surface-coated silicon nitride sintered body.

[0041] Furthermore, powder was applied to the surface of substrate B using the same method as for substrate A. Then, the substrate was heat-treated at 1630°C for 100 minutes to dissolve the powder, thereby producing a surface-coated silicon nitride sintered body.

[0042] (Steam corrosion test) A sample (Sample No. 2-1) measuring 4 mm × 3 mm × 20 mm was prepared by forming a coating layer over the entire surface of substrate A. The thickness of the coating layer was 30 μm. Similarly, a sample (Sample No. 2-2) measuring 4 mm × 3 mm × 20 mm was prepared by forming a coating layer over the entire surface of substrate B.

[0043] The prepared samples (Samples No. 2-1 and 2-2) were placed in a pressure vessel, and the surface area S (cm²) of the samples was measured. 2 ) and the volume V (cm³) of distilled water 3 Distilled water was added to the pressure vessel so that the relationship with (V / S) was V / S = 10. The pressure vessel was sealed and heated and maintained at 200°C for 100 hours, and the mass loss per unit area (mg / cm²) was measured. 2 The value was calculated. The equilibrium water vapor partial pressure at this time was adjusted to 1.55 MPa.

[0044] In addition, samples without a coating layer on the surface of substrate A (sample No. 1-1) and samples without a coating layer on the surface of substrate B (sample No. 1-2) were prepared, and the same tests as those performed on samples No. 2-1 and 2-2 were carried out.

[0045] Three samples were prepared and tested for each type of sample, and the average of these results was used as the mass loss per unit area for each sample. The results are shown in Table 1.

[0046] [Table 1]

[0047] As shown in Table 1, when comparing No. 1-1 and No. 1-2, which do not have a coating layer, No. 1-1 has a greater mass loss. However, when comparing No. 2-1 and No. 2-2, which have a coating layer, No. 2-1 has a smaller mass loss. This is thought to be because the Al in the coating layer reduces the water vapor resistance.

[0048] Furthermore, samples without a coating layer (samples No. 1-1 and 1-2) showed a decrease in mass as a result of the steam corrosion test. This is thought to be because the SiO2 generated on the substrate surface reacted with water, and the resulting Si(OH)4 was removed.

[0049] In contrast, samples (samples No. 2-1 and 2-2) having a coating layer 14 corresponding to the surface-coated silicon nitride sintered body 10 according to the embodiment showed a reduced mass loss per unit area compared to samples No. 1-1 and 1-2. This revealed that samples No. 2-1 and 2-2 have superior water vapor resistance compared to samples No. 1-1 and 1-2. Furthermore, sample No. 2-1, with an Al content of 0.7 mol%, showed less mass loss than sample No. 2-2, with an Al content of 0.01 mol%. This is thought to be because optimizing the Al content prevented peeling of the coating layer, improving water vapor resistance and resulting in reduced mass loss.

[0050] (Thermal cycle test) A sample (Sample No. 3) measuring 30 mm × 30 mm × 10 mm was prepared, with a coating layer formed only on the first surface of substrate A. Sample No. 3 had a coating layer with an average thickness of 30 μm. In addition, there was an intermediate layer between the substrate and the coating layer with an average thickness of 200 μm.

[0051] The above samples were exposed to constant temperature chambers set to 700°C and 1100°C for 300 cycles with a 3-minute period, and then the presence or absence of peeling of the coating layer was confirmed by SEM observation of the surface of the coating layer.

[0052] Furthermore, samples with different average coating layer thicknesses (samples No. 4 and 5) and samples with different average coating layer thicknesses on the surface of substrate B (samples No. 6, 7, and 8) were prepared. Samples No. 4 and 5 each had an intermediate layer with an average thickness of 200 μm between the substrate and the coating layer. Samples No. 6, 7, and 8 did not have an intermediate layer. Samples No. 4 to 8 were subjected to the same tests as sample No. 3. The results are shown in Table 2.

[0053] [Table 2]

[0054] As shown in Table 2, no delamination of the coating layer was observed in the samples with average coating layer thicknesses of 30 μm and 150 μm (Samples No. 3 and 4), demonstrating excellent durability. On the other hand, in the sample with an average coating layer thickness of 400 μm (Sample No. 5), the coating layer delaminated as a result of the cycle test.

[0055] Similarly, in samples (Samples No. 6 and 7) where substrate B was used to form coating layers with average thicknesses of 30 μm and 200 μm, no peeling of the coating layer was observed, demonstrating excellent durability. On the other hand, in sample (Sample No. 8) with an average coating layer thickness of 800 μm, the coating layer peeled off as a result of the cycle test.

[0056] However, when observed after 100 alternating exposure cycles, no peeling was observed in any of the samples.

[0057] Therefore, it was confirmed that the surface-coated silicon nitride sintered body according to the embodiment has the same peel resistance as conventional bodies under a thermal cycling environment.

[0058] Although the present disclosure has been described in detail above, this disclosure is not limited to the embodiments described above, and various modifications and improvements are possible without departing from the gist of this disclosure.

[0059] Further effects and modifications can be readily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the specific details and representative embodiments expressed and described above. Accordingly, various modifications are possible without departing from the spirit or scope of the overall concept of the invention as defined by the appended claims and their equivalents. [Explanation of symbols]

[0060] 10 Surface-coated silicon nitride sintered body 12 Base 14 Covering layer 16. Middle Class

Claims

1. A substrate containing silicon nitride crystals as the main component, The coating layer located on the substrate and It has, The coating layer, when RE is a rare earth element, ２ Si ２ O ７ and RE ２ SiO ５ It contains at least one of the following as a crystalline phase, The substrate and the coating layer contain Al, The aforementioned substrate contains 0.5 mol% or more of Al on an oxide basis. Surface-coated silicon nitride sintered body.

2. The coating layer has grain boundary phases located between the crystalline phases, The grain boundary phase contains Al. The surface-coated silicon nitride sintered body according to claim 1.

3. It has an intermediate layer located between the coating layer and the substrate, The intermediate layer has an Al content that is lower than that of the substrate and higher than that of the coating layer, and an RE content that is higher than that of the substrate and lower than that of the coating layer. A surface-coated silicon nitride sintered body according to claim 1 or 2.

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