JP2005008489A - Silicon nitride ceramic having rare earth titanate high temperature corrosion resistant layer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a silicon nitride ceramic having a high temperature corrosion resistant coating film and its manufacturing method. <P>SOLUTION: The silicon nitride ceramic has a rare earth titanate as the high temperature corrosion resistant layer on the surface thereof in which a rare earth silicate is contained by volume ratio of 1:(0-0.7) as a material for forming a multi-phase and has a rare earth silicate as a bonding layer for the silicon nitride ceramic and the high temperature corrosion resistant layer. As a result, the silicon nitride ceramic has excellent high temperature steam corrosion resistance at a high temperature of ≥1,100°C. The high temperature corrosion resistant layer is durable for a long time even when the film is thin because there is no wear of the coating film due to corrosion and exhibits self-repairing function even if crack is produced on the coating film by accident by giving a rare earth enriched rare earth titanate composition to the coating film. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-temperature corrosion-resistant layer for coating on silicon nitride ceramics, and more specifically, at a high temperature of 1100 to 1600 ° C., under a partial pressure of water vapor of 10 to 30%, and an alkali component of 3% or less. The present invention relates to a silicon nitride ceramics having a novel high-temperature corrosion-resistant layer capable of suppressing the promotion of corrosion caused by water vapor corrosion or alkali at high temperatures even in the presence of.
The present invention is a high temperature corrosion when applying silicon nitride as a gas turbine member in a combustion field where a high water vapor partial pressure of about 20% and an alkali component of several percent or less exist by burning fossil fuel. This is useful for providing a silicon nitride ceramic material capable of significantly suppressing the above.
[0002]
[Prior art]
In general, silicon nitride ceramics are oxidized at a high temperature up to 1600 ° C. to produce silica on the surface, and at a higher temperature, vapor phase SiO is produced, and the SiO sublimates and wears while thinning. In an environment where water vapor exists at high temperature, corrosion due to water vapor occurs in addition to oxidation, and wear is accelerated. For example, silicon nitride ceramics as a gas turbine member in a combustion field where the actual water vapor partial pressure reaches about 20% When applying, it is necessary to coat a layer that inhibits water vapor corrosion at high temperatures. Furthermore, the combustion field of the gas turbine contains several percent of alkali components, and these alkali components are easily taken into the silica phase of silicon nitride and rapidly lower the melting point of the grain boundary phase, resulting in wear and tear. Becomes more intense. Therefore, when silicon nitride ceramics is applied as a gas turbine member, it is necessary to form a corrosion-resistant layer that prevents oxidation at high temperatures, water vapor corrosion, solid solution and diffusion of alkali components, and the surface of the corrosion layer has a glass phase. It is necessary to coat a silica-free phase that does not cause the formation of a corrosion resistant layer.
[0003]
With respect to silicon nitride ceramics having excellent oxidation resistance at high temperatures, as described in prior art documents (for example, see Patent Documents 1, 2, and 3), a rare earth oxide is added as a sintering aid, and the compound A mechanism for improving the oxidation resistance by forming a metal on the surface has been proposed. When sintering difficult-to-sinter silicon nitride, rare earth silicate compounds produced by reaction of rare earth oxide added as a sintering aid with silica are excellent in oxidation resistance and steam corrosion resistance. Therefore, application research and development for making these rare earth silicate phases oxidation-resistant and steam-corrosion-resistant films at high temperatures is being studied.
[0004]
The strength of silicon nitride ceramics is known to improve as the rare earth oxide added as a sintering aid becomes heavy rare earth, and Lu has the highest strength.₂  O₃It is believed that silicon nitride ceramics using as a sintering aid are advantageous for application to high temperature structural members. This Lu₂  O₃  When added as a sintering aid, Lu₂  O₃  Reacts with silica formed by oxidation of silicon nitride, and Lu₂  SiO₅And Lu₂  Si₂  O₇  Of the silicate is formed on a part of the surface, of which Lu₂  Si₂  O₇  Was found to be close to the thermal expansion coefficient of silicon nitride and excellent in oxidation resistance.₂Si₂  O₇Layer-coated oxidation / steam corrosion-resistant silicon nitride ceramics are being developed.
[0005]
However, in an actual fossil fuel combustion field, there are some alkali components such as sodium and potassium together with water vapor. When a double oxide containing silica as a component is used as a corrosion-resistant layer, the alkali component is first rich in silica. In the gas turbine combustion field exposed to high-speed air flow, the wear and tear due to the drop of crystal grains is drastically reduced. Is also expected to become intense. Therefore, the conditions of the anticorrosion film required when using silicon nitride ceramics as a gas turbine member are: (1) the crystal grain boundary does not contain a glass phase of silica component, and (2) water vapor absorption / desorption. It is required from the viewpoint of chemical stability of the corrosion-resistant film at high temperatures that the components contained in the corrosion-resistant layer are not easily detached by the reaction.
[0006]
Furthermore, the thermal expansion coefficient of silicon nitride ceramics is 4 × 10.^-6Since it has a low thermal expansion of about / ° C., it is necessary to select a material system in which the thermal expansion coefficient of the corrosion-resistant layer is the same as or close to that of silicon nitride in consideration of adhesion to the substrate. Gas turbine components must not cause chemical changes or property changes for a long time of 8000 hours in a high-temperature combustion field, and a system that suppresses the generation of new phases as much as possible by the chemical reaction between the coating and the substrate is required. It becomes.
[0007]
Since oxides or double oxides not containing a silica component generally have a thermal expansion coefficient larger than that of silicon nitride ceramics, there has been almost no investigation as a corrosion-resistant film of silicon nitride ceramics. Examples of the development of a corrosion-resistant layer having a function of suppressing corrosion and suppressing the formation of a new phase even when used for a long time in a high-temperature combustion field where alkali is present and the silicon nitride ceramics having the same are disclosed in the above patent document There are no reports other than those described in.
[0008]
[Patent Document 1]
JP-A-6-32658
[Patent Document 2]
JP-A-5-221728
[Patent Document 3]
Japanese Patent Laid-Open No. 5-208870
[0009]
[Problems to be solved by the invention]
Under such circumstances, the present inventors have developed a new high-temperature corrosion-resistant layer of silicon nitride ceramics that makes it possible to drastically solve the problems in the prior art in view of the prior art. As a result of intensive research aimed at achieving this goal, rare earth silicates with the same thermal expansion coefficient as silicon nitride ceramics are used in the bond coat, and the rare earth titanate has the same crystal structure as the rare earth silicate and has the same crystal lattice constant. By using a corrosion-resistant layer that is a composite of silicate and rare earth silicate, steam corrosion at high temperatures is suppressed to a low level, and the reaction between the substrate and coating layer is suppressed even during long-term use at high temperatures, and corrosion by alkali is promoted. It has been found that it is possible to produce silicon nitride ceramics having a high-temperature corrosion-resistant coating that can suppress It was.
[0010]
That is, the present invention is (1) high temperature by making a rare earth silicate having the same thermal expansion coefficient as that of the silicon nitride ceramic used as a base material and a rare earth titanate having the same crystal structure as the rare earth silicate to form a corrosion resistant phase. Suppresses the mass loss due to water vapor corrosion in the steel and solves various problems such as adhesion failure caused by the difference in thermal expansion with the base material. (2) Since titania and silica are not solid-solved with each other, the temperature is high. The rare earth silicate and rare earth titanate, which are stable phases, do not react and can coexist even at high temperatures, so that it is possible to form a chemically stable phase even at high temperatures for long periods of time. (3) Since the rare earth composition has a wide non-stoichiometric range and the original crystal structure can be retained even with an excess of rare earth, the corrosion-resistant layer is temporarily cracked Even if the silicon nitride of the base material is oxidized at the same time, the rare earth component is supplied from the rare earth titanate phase and reacts with the silica component generated by the oxidation of silicon nitride to form a rare earth silicate phase, thereby providing self-repairing ability. An object of the present invention is to provide a silicon nitride ceramic having a high-temperature corrosion-resistant film capable of realizing the above and a method for producing the same.
[0011]
[Means for Solving the Problems]
The present invention for solving the above-described problems comprises the following technical means.
(1) A silicon nitride ceramic having a rare earth titanate as a high-temperature corrosion-resistant layer on the surface thereof, characterized in that it contains rare earth silicate in a volume ratio of 1: 0 to 0.7 as a multiphase material of the high-temperature corrosion-resistant layer. A silicon nitride ceramic having a high temperature corrosion resistant layer.
(2) The silicon nitride ceramic according to (1), wherein the rare earth is any one of Yb and Lu, or a combination of two kinds.
(3) The composition of rare earth titanate is rare earth oxide (Ln₂  O₃  ) And titania (TiO)₂  ) Is a silicon nitride ceramic according to (1), wherein the molar ratio is 1: 0.8-2.
(4) The composition of the rare earth silicate is rare earth oxide (Ln₂  O₃  ) And silica (SiO₂) Is a silicon nitride ceramic according to (1), wherein the molar ratio is 1: 1 to 2.
(5) The silicon nitride ceramic according to (1), wherein the silicon nitride ceramic has a rare earth silicate as a bond layer between the silicon nitride ceramic and the high temperature corrosion resistant layer.
(6) The rare earth silicate of the bond layer is a dense layer, having a thickness of 0 to 10 microns, and a rare earth oxide (Ln₂  O₃  ) And silica (SiO₂  ) Is a silicon nitride ceramic according to (5) above, wherein the molar ratio is 1: 1 to 2.
(7) The silicon nitride ceramic according to (5), wherein the dense bond layer is formed by sputtering, laser ablation, sol-gel, or a combination of two or more methods.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be described in more detail.
The present invention employs rare earth titanate as a corrosion-resistant layer, and in order to minimize the difference in thermal expansion coefficient between the base material and the corrosion-resistant layer, it is combined with a rare earth silicate having a smaller thermal expansion coefficient, and the film is cracked. In order to make the self-healing ability possible even if the generation of selenium occurs, the ratio of the rare earth element to titanium is made rich in rare earth to the extent that the same structure can be maintained in the composition of the rare earth titanate, and the silica generated as a result of oxidation of the substrate is absorbed. In order to suppress the reaction between the oxide component contained in the substrate and the corrosion-resistant layer, if necessary, manufacture a corrosion-resistant film with a dense rare earth silicate thin film bond-coated on the substrate. And providing a silicon nitride ceramic having a corrosion-resistant layer.
[0013]
Rare earth silicate and rare earth titanate are both high temperature stable phases and have the same crystal structure. Since silica and titania do not completely dissolve, rare earth silicate and rare earth titanate can coexist in two phases even for annealing at high temperature for a long time once a multiphase structure is formed. By forming a fine structure of the two phases that do not form a solid solution with each other, it is possible to suppress the occurrence of cracks in the film or improve the apparent toughness, and to make the part that causes self-healing ability finer Therefore, the reliability with respect to long-term use of the film can be improved.
[0014]
Corrosion-resistant layers having a fine multiphase structure can be formed by using a fine starting material, a pre-synthesized rare earth silicate and a rare earth titanate fine powder, a gas phase method, or an amorphous coating by a sol-gel method. Although there are a method of heat treatment for a short time at a high temperature and a method of rapid cooling, the multiphase structure of the present invention is predicted to be approximately the same as the size and width of a crack generated due to a difference in thermal expansion coefficient. Any structure may be used as long as the structure exhibits a certain degree of structure, and there is no restriction on the size and formation method of the multiphase structure.
[0015]
The rare earth titanate has a meteorite structure and exhibits a wide non-stoichiometry on the rare earth oxide side in the phase diagram. As the ratio of rare earth to titanium approaches 1: 1 in terms of atomic ratio, the structure changes from an irregular structure to an ordered structure, but neither structure has a significant change in the lattice constant or thermal expansion coefficient. By utilizing this non-stoichiometry, that is, a composition in which the rare earth is rich, cracks are generated in the film, and the silicon nitride ceramics of the base material are oxidized to produce silica. Can be absorbed and rare earth silicate can be generated, and a self-healing function can be provided.
(Generation of rare earth silicate)
Ln_{2 + x}  Ti₂O₇  + YSiO₂  → Ln_{2 + xy}  + 1 / 2Ln₂  Si₂  O₇
[0016]
When silica is present at the grain boundaries of the corrosive layer or at the surface of the corrosive layer, the alkali component readily dissolves in the silica component and dramatically lowers the melting point of the grain boundary phase or the surface silica phase, accelerating steam corrosion. In addition, when the member is exposed to a high-speed air stream, wear such as dropping of crystal grains accompanying melting of the grain boundary phase is accelerated. Therefore, a mechanism to suppress the silica component of the grain boundary phase in the film as much as possible is necessary. However, the above reaction formula can effectively generate a rare earth silicate having excellent steam corrosion resistance even if the silica component is generated. It can be a mechanism that effectively suppresses acceleration of corrosion due to the presence of alkali components.
[0017]
In order to minimize the generation of silica-rich grain boundary phases in the corrosion-resistant layer, it is desirable not to include a rare-earth silicate phase in the corrosion-resistant layer, and for applications where high thermal stress is not applied, rare earth titanate and rare earth The mixing ratio of silicate may be 1: 0. Rare earth titanates and rare earth silicates are used to reduce the thermal expansion coefficient difference in applications where cracks are a problem due to a slight difference in thermal expansion coefficient between the substrate and the coating due to large thermal stress. In this case, the rare earth silicate is a double oxide containing a silica component, although the crystal itself is excellent in water vapor corrosion resistance. Since solid solution is allowed and corrosion by water vapor is accelerated, it is advantageous that the mixing ratio of the rare earth silicate is as small as possible. Since the silica component of the grain boundary phase associated with the rare earth silicate phase can be effectively removed by making a double phase with the rare earth-rich rare earth titanate phase, the volume fraction of the rare earth titanate and the rare earth silicate is about 1: 1. That is, it is possible to obtain a corrosion resistant layer that does not lose the high corrosion resistance of the rare earth titanate to a fraction of about 1: 0.7 in molar ratio.
[0018]
Since silicon nitride ceramics are difficult to sinter, generally a sintering aid is added to obtain a sintered body. Considering application as a high-temperature structural material, silica and rare earth oxides are generally added as sintering aids as sintering aids. Development of sintered silicon nitride ceramics using lutecia or ytterbia as a binder has been widely promoted. Accordingly, since silica and lutetia or ytterbia are present in the grain boundary phase in the silicon nitride ceramic used as the base material of the present invention, the type of rare earth component contained in the corrosion-resistant layer formed thereon is lutetium or ytterbium. It is limited to. Which is used depends on the kind of rare earth oxide added to the substrate.
[0019]
When silicon nitride is developed as a high-temperature structural material, the sintering aid added to obtain the sintered body is generally limited to the above-mentioned aid, but further improvement in strength on the low temperature side is possible. For the purpose, it may be added other than the above sintering aid such as alumina. When a sintering aid other than the above additives is added, a reaction between the components such as alumina present in the grain boundary phase of the silicon nitride substrate and the corrosion-resistant layer occurs, and a thermal expansion coefficient is formed by forming a new phase. As a result, it is predicted that defects in film performance will occur due to the formation of a new layer, such as a large difference, and sintering aids other than rare earth oxides and silica were added to the sintered silicon nitride ceramics. In some cases, a mechanism for suppressing the reaction between the component of the base material and the corrosive layer at a high temperature may be required. For example, when alumina is contained in a silicon nitride sintered body, the rare earth titanate, which is a corrosion-resistant layer, reacts with alumina to produce aluminum titanate having a large anisotropy in the thermal expansion coefficient. This will increase the stress.
[0020]
In the present invention, when a sintering aid other than the rare earth oxide and silica is added to the silicon nitride ceramic sintered body, the base material is used to suppress the reaction between the base material and the corrosion-resistant layer at a high temperature. A dense rare earth silicate layer is provided on the substrate by a vapor phase method such as a sputtering method or a laser ablation method or a solution method such as a sol-gel method. A method for forming this layer is not particularly limited as long as a dense layer can be formed. By providing the rare earth silicate phase, even if the rare earth silicate and alumina produce mullite by the high temperature reaction, the amount is small, and the thermal expansion coefficient of mullite is small. The increase is slight and can be ignored.
[0021]
In addition, rare earth silicate and rare earth titanate have the same crystal structure and can also serve as a bonding layer. The thickness of the dense layer is designed according to parameters such as temperature and time for using the member. However, a thin film is advantageous for forming a dense layer without cracks. The thickness of such layers is preferably 0 to 10 microns thick. When the silicon nitride ceramic sintered body does not contain a sintering aid such as alumina, this layer is not necessary, and the range relating to the thickness of this layer includes zero.
[0022]
As described above, by using rare earth titanate as the corrosion resistant layer, the silicon nitride of the substrate exhibits excellent steam corrosion resistance even in long-term use at high temperatures in the presence of an alkali component. In order to reduce the difference in thermal expansion, it is easy to stabilize the structure at a high temperature by forming a multiphase with a rare earth silicate, and by selecting a rare earth-rich composition, cracks and the like are generated in the film. Even if the silicon nitride of the base material is oxidized to produce a silica component, it is possible to produce a silicon nitride ceramic having a water vapor corrosion resistant layer that can absorb the silica component and change it into a rare earth silicate. Furthermore, when a sintering aid other than the rare earth oxide and the silica component is present in the silicon nitride sintered body, the components contained in the rare earth titanate and the silicon nitride sintered body by the dense rare earth silicate formed on the silicon nitride. This makes it possible to produce silicon nitride ceramics having a water vapor corrosion resistant layer that can effectively block the reaction with the high temperature and long time annealing.
[0023]
The conceptual diagram of the silicon nitride ceramics which has a high temperature corrosion-resistant layer of this invention is shown in FIG. In the present invention, the rare earth titanate is coated as a corrosion resistant material on the silicon nitride ceramic substrate. In this case, the film thickness is up to 200 microns. In order to reduce the difference between the thermal expansion coefficient of the silicon nitride ceramic substrate and the thermal expansion coefficient of the corrosion resistant layer, the basic structure is the same as that of the rare earth titanate and the thermal expansion coefficient is small depending on the type of silicon nitride ceramics. In addition, a composite with a rare earth silicate excellent in steam corrosion resistance at high temperatures is intended. In this case, the thickness is 0 to 10 microns. The compounding with the rare earth silicate is not always necessary when the thickness of the layer can be reduced, and the mixing ratio of the rare earth silicate to the rare earth titanate includes 0. Further, the thickness of the thick film can be appropriately designed according to parameters such as temperature, time and reliability at which the member is used.
[0024]
Since silicon nitride is a difficult-to-sinter material, a sintering aid is added, but the sintering aid is mainly a rare earth oxide and silica. The kind of rare earths of the rare earth titanate and the rare earth silicate is adapted to the rare earth of the sintering aid. A nitride ceramic sintered body for the purpose of developing a high-temperature structural member may be sintered by adding alumina or the like in addition to rare earth oxide and silica. When alumina is contained in the crystal grain boundary of the silicon nitride sintered body, when a rare earth titanate is coated thereon, the titania component and alumina may react at a high temperature to generate aluminum titanate. Since aluminum titanate is a substance having a strong thermal expansion coefficient anisotropy, there is a possibility of creating a large stress in the film, and thus a mechanism for suppressing the formation of an aluminum titanate phase is required. In the present invention, a dense rare earth silicate film is produced on silicon nitride ceramics for the purpose of suppressing the formation of a new phase due to the reaction between the components contained in the silicon nitride sintered body and the film, such as the formation of aluminum titanate. To do. This layer is not always necessary when the silicon nitride sintered body has no sintering aid other than the rare earth oxide and silica or can be ignored, and the film thickness includes zero. Even when the rare earth silicate reacts with aluminum, mullite is generated as a new phase. However, since the thermal expansion coefficient of mullite is small, an increase in stress in the film accompanying the generation of the phase can be ignored.
[0025]
As shown in FIG. 2, the rare earth titanate has a single crystal structure and a wide non-stoichiometry to the rare earth side. By using rare earth-rich rare earth titanate as a coating material, even if cracks occur in the coating and the silicon nitride ceramics of the substrate are oxidized, the silica generated by the oxidation of the substrate reacts with the rare earth-rich rare earth titanate. Since the rare earth silicate is produced, as a result, self-repairing ability can be imparted. Silica rapidly lowers the melting point due to the solid solution of the alkali component in the presence of the alkali component and accelerates corrosion due to water vapor, but if a double oxide is formed like the rare earth silicate, the water vapor corrosion of the rare earth silicate crystal phase itself. Therefore, it is possible to effectively suppress steam corrosion at high temperatures. In the present invention, the composition of the rare earth titanate is a rare earth oxide (Ln₂  O₃  ) And titania (TiO)₂  ) Ratio in the molar ratio of 1: 0.8 to 2, that is, a region indicated by hatching in FIG.
[0026]
【Example】
Next, the present invention will be specifically described based on examples, but the present invention is not limited to the following examples.
Example
In this example, the results of conducting a steam corrosion test at 1500 ° C. and demonstrating excellent corrosion resistance of the coating material of the present invention will be described. In the present invention, Yb and Lu are preferably used as the rare earth, but here, the results of demonstrating that the corrosion resistance is excellent by using Lu as the rare earth are shown. For the purpose of comparing the effects of steam corrosion, the corrosion test was also performed under the same conditions for titania and lutecia, which are the edge members of the phase diagram of FIG. The composition of the lutetium titanate tested here is Lu₂  Ti₂O₇  And Lu₂TiO_x  It is. Lu₂  O₃  , Lu₂  Ti₂  O₇  , Lu₂TiO_x  TiO₂  Of these, TiO₂  There are reports on the water vapor corrosivity of, and it is well known.
[0027]
(1) Production of sintered body
In order to clarify the difference in corrosion mechanism due to the presence of the silica component, high purity TiO having a purity of 99.9% was used as a starting material for the sintered body.₂  And Lu₂O₃Was used. In addition, the purity of the starting material also needs to be high in coating silicon nitride ceramics. After starting materials were weighed so as to have a predetermined molar ratio, the powders were put in a polyester pot, and ultrasonically stirred in pure water for 1 hour together with a monoball. Polyvinyl alcohol was used as the binder. After the obtained slurry was dried, it was press-molded and reacted and sintered at 1600 ° C. for 12 hours in the atmosphere.₂O₃  , Lu₂  Ti₂  O₇  , Lu₂  TiO_xTiO₂A sintered body was obtained.
[0028]
(2) Corrosion test
The high temperature steam corrosion test was performed under the following conditions. Temperature: 1500 ° C., ascending / descending speed: 250 ° C./hour, time: 100 hours, water vapor amount relative to air: 30 wt%, flow rate: 175 ml / min. In order to remove the contribution of corrosion due to the low temperature side corrosion mechanism occurring at 300 ° C to 500 ° C, after the sample temperature reaches 1500 ° C, water vapor is introduced to completely remove water and hydroxyl groups adhering to the surface. For the purpose of removing and clarifying the corrosion effect at high temperature, the introduction of water vapor was stopped when 100 hours had passed. In order to clarify the difference between the corrosion mechanism due to the presence of the alkali component and the pure steam corrosion mechanism, the sample was placed on an alumina sintered plate containing an alkali component of 1% or less and 0.1% or more. By testing by placing a bulk sample on an alumina plate containing an alkali component, the effect of adding the contribution of the alkali component to the corrosion mechanism by water vapor can be confirmed at the portion in contact with the alumina plate, Since the upper part of the bulk away from the alumina plate has a corrosion mechanism caused only by corrosion due to water vapor, it is possible to clarify the contribution of alkali components in water vapor corrosion by analyzing the corrosion conditions above and below the bulk.
[0029]
(3) Test results
All the test samples had no significant change in appearance before and after the test, and all showed good steam corrosion resistance. The weight change of each sample accompanying said high temperature steam corrosion test is shown in FIG. With TiO2, the weight decreases and Lu₂  O₃  , Lu₂  Ti₂  O₇  , Lu₂TiO_xThen, the weight increased slightly. In general, when water vapor is present at high temperature, oxides are worn out due to desorption of component elements as water molecules are absorbed and desorbed, and the weight decreases. Further, when an alkali component is present, it is generally considered that the melting point is lowered by the solid solution of the alkali component, and the diffusion of water molecules into the substance is accelerated, so that corrosion is accelerated.
[0030]
As a result of the above corrosion test, Lu₂  O₃, Lu₂  Ti₂  O₇  , Lu₂  TiO_x  However, since the weight is slightly increased, the mechanism of desorption of the component elements does not work with the absorption and desorption of water molecules, and although water vapor diffuses into the crystal layer, corrosion due to desorption of the component elements It was found that wear was suppressed. Although changes in material properties such as a decrease in the mechanical strength of the material due to the incorporation of water molecules into the crystal phase are expected, these materials are not suitable for high-temperature steam corrosion phases. Therefore, it can be said that it is an excellent material as a corrosion resistant material at high temperatures. However, Lu₂  O₃  The coefficient of thermal expansion of is about twice that of silicon nitride ceramics, so Lu₂O₃Is excluded from the corrosion resistant coating material of silicon nitride ceramics.
[0031]
FIG. 4 shows the result of infrared spectrum obtained from the surface of titania after the corrosion test. It can be seen that OH groups and free water molecules are not adsorbed on the surface. Water molecules or OH groups adsorbed on the surface of a substance are generally skipped at a temperature of 500 ° C. or higher. In the above corrosion test, the introduction of water vapor was stopped at 1500 ° C. at the time when the test was completed for 100 hours in order to eliminate the effect of corrosion at low temperature, so that water molecules or OH groups adsorbed on the surface were completely blown off. It can be said that The same is true for other Lu₂  O₃  , Lu₂  Ti₂  O₇  , Lu₂  TiO_x  But I can say, Lu₂  O₃  , Lu₂  Ti₂  O₇  , Lu₂  TiO_x  Then, the weight after the test slightly increases because the water molecule is Lu.₂  O₃  , Lu₂  Ti₂  O₇  , Lu₂  TiO_xIt can be seen that this is because it is dissolved in the crystal lattice.
[0032]
FIG. 5 shows a photograph of the surface of the sample after the test and the back surface of the sample in contact with the alumina plate containing the alkali component. There is no difference in the appearance of the front and back surfaces, and it can be seen that acceleration of corrosion due to the solid solution of the alkali component is suppressed in this material system. Furthermore, since rare earth titanate does not observe substrate wear due to pure steam corrosion, that is, weight loss, when used as a high temperature corrosion resistant coating, the film thickness may be thin. This facilitates member design.
[0033]
From the above, TiO₂  -Lu₂  O₃  It was proved that the steam corrosion resistance at high temperature of the system was excellent, but Lu₂  O₃  Shows excellent water vapor corrosivity even in the presence of an alkali component, but its thermal expansion coefficient is about twice that of silicon nitride ceramics, which is also excluded as a corrosion-resistant coating material. Rare earth titanate has a relatively small thermal expansion coefficient and has the same crystal structure as rare earth silicate. Therefore, the composite phase is very useful as a corrosion resistant coating material for silicon nitride ceramics. Furthermore, as shown in the above reaction formula, the composition of the film is made of a rare earth-rich composition, so that it is expected to exhibit a self-repair function even if the film breaks due to an accident and a crack occurs. Therefore, it can be said that the film can be thin, and can have excellent performance in terms of film design, member design, and safety for long-term use.
[0034]
【The invention's effect】
As described above in detail, the present invention relates to silicon nitride ceramics having a rare earth titanate high temperature corrosion resistant layer. According to the present invention, 1) the high temperature steam resistant corrosion layer of the present invention is at a high temperature of 1100 ° C. or higher. It is possible to provide a silicon nitride ceramic having a corrosion-resistant film layer and a corrosion-resistant layer that can withstand long-term use even with a thin film thickness because it has excellent high-temperature steam corrosion resistance and does not wear the film due to corrosion. 2) To provide a silicon nitride ceramic having a corrosion-resistant coating layer that exhibits a self-healing function even if cracks or the like occur in the coating due to an unexpected accident by making the coating composition a rare earth-rich rare earth titanate composition And the like.
[Brief description of the drawings]
FIG. 1 shows a conceptual diagram of silicon nitride ceramics having a high temperature corrosion resistant layer.
FIG. 2 shows the composition range of rare earth titanates.
FIG. 3 shows a change in weight due to a corrosion test.
FIG. 4 shows an infrared spectrum obtained from a titania surface after a corrosion test.
FIG. 5 shows the appearance of the front and back surfaces after a corrosion test.

Claims (7)

A silicon nitride ceramic having a rare earth titanate as a high temperature corrosion resistant layer on its surface, the rare earth silicate being contained in a volume ratio of 1: 0 to 0.7 as a multiphase material of the high temperature corrosion resistant layer. Silicon nitride ceramics with a corrosive layer. The silicon nitride ceramic according to claim 1, wherein the rare earth is any one of Yb and Lu, or a combination of two kinds. The silicon nitride ceramics according to claim 1, wherein the composition of the rare earth titanate is such that the molar ratio of the rare earth oxide (Ln ₂ O ₃ ) and titania (TiO ₂ ) is 1: 0.8 to 2. The silicon nitride ceramics according to claim 1, wherein the composition of the rare earth silicate is such that the molar ratio of the rare earth oxide (Ln ₂ O ₃ ) to silica (SiO ₂ ) is 1: 1 to 2. 2. The silicon nitride ceramic according to claim 1, wherein the silicon nitride ceramic has a rare earth silicate as a bond layer between the silicon nitride ceramic and the high temperature corrosion resistant layer. The rare earth silicate of the bond layer is a dense layer, has a thickness of 0 to 10 microns, and the ratio of the rare earth oxide (Ln ₂ O ₃ ) to silica (SiO ₂ ) is 1: 1 to 2 in molar ratio. Item 6. The silicon nitride ceramics according to Item 5. The silicon nitride ceramic according to claim 5, wherein the dense bond layer is formed by sputtering, laser ablation, sol-gel, or a combination of two or more methods.

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