JP2004149395A - Method for producing surface coated silicon nitride sintered compact - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a surface coated silicon nitride sintered compact for which a coated layer which is precise has a large adhesion force and a small adhesion force variation is easily formed. <P>SOLUTION: A molten liquid is formed by melting an RE<SB>2</SB>Si<SB>2</SB>O<SB>7</SB>powder (RE is at least one element of the group 3a elements in the periodic table) or a mixed powder of an RE<SB>2</SB>O<SB>3</SB>powder with an SiO<SB>2</SB>powder and covered on a substrate surface consisting of a silicon nitride sintered compact in a nitrogen atmosphere and then the molten liquid is crystallized and the coated layer whose main crystal is RE<SB>2</SB>Si<SB>2</SB>O<SB>7</SB>is formed on the substrate surface. The molar ratio of Si to RE in the coated layer is 1.9-3 in terms of the oxides ratio, SiO<SB>2</SB>/RE<SB>2</SB>O<SB>3</SB>. RE contained in the coated layer is preferably at least one of Lu, Yb and Er. <P>COPYRIGHT: (C)2004,JPO

Description

【００４３】
本発明の試料Ｎｏ．１〜１６、１８〜２１は、付着力が２００ＭＰａでばらつきが３０ＭＰａ以内、破断界面がエポキシ樹脂と表面被覆層界面であり、耐食性は減肉量が１μｍ以下であった。
【００４４】
一方、被覆層を形成する際溶融していない試料Ｎｏ．１７は付着力が１８０ＭＰａでばらつきが７０ＭＰａ、破断界面が表面被覆層と窒化珪素焼結体基材界面であった。さらに表面被覆層がモノシリケートやダイシリケート以外のもので形成されているＮｏ．２２〜２６は、耐食性が１００μｍ以上と大きく、耐腐食性が著しく劣っている。
【００４５】
【発明の効果】
本発明の表面被覆窒化珪素質焼結体の製造方法により、窒化珪素焼結体基材と表面被覆層の付着力が高い、高耐食性の表面被覆窒化珪素質焼結体を得ることができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention has excellent strength properties from room temperature to high temperature, and also has excellent fracture toughness and oxidation resistance, and especially automotive parts such as pistons, cylinders, valves, cam rollers, rocker arms, piston rings, piston pins, and glow plugs. For producing surface-coated silicon nitride sintered body suitably used for gas turbine engine parts such as turbine rotor, turbine blade, nozzle, combustor, scroll, nozzle support, seal ring, spring ring, diffuser, duct, shroud, etc. About.
[0002]
[Prior art]
Conventionally, silicon nitride sintered bodies have been applied as engineering ceramics, particularly as heat engine structural materials, because of their excellent strength, hardness, and thermal and chemical stability.
[0003]
A silicon nitride sintered body is generally manufactured by adding a sintering aid such as Y ₂ O ₃ , Al ₂ O _3, or MgO to silicon nitride powder and firing it. It has been reported that high-density and high-strength properties are obtained by firing using an auxiliary agent.
[0004]
Such a silicon nitride sintered body is used, for example, as an engine component. However, as the operating conditions of the engine component increase, the strength and oxidation resistance of the silicon nitride sintered body at high temperatures are further improved. Is required.
[0005]
In response to such demands, improvements have been made with a focus on improvement of sintering aids, grain boundary phases, firing conditions, and the like, and formation of an oxide protective film on the surface of a sintered body.
[0006]
Therefore, the present applicant first forms a coating layer having a crystalline phase such as disilicate or monosilicate having excellent oxidation resistance on the surface of the silicon nitride sintered body, thereby causing peeling or cracking of the coating layer. It is suggested that can be prevented.
[0007]
For example, a predetermined amount of RE ₂ O ₃ powder and SiO ₂ powder are mixed, a binder is added, and a slurry is applied to the surface of the base material or sprayed by spraying or the like, and then baked at a high temperature of 1000 to 1800 ° C. or mixed. Japanese Patent Application Laid-Open No. 7-172958 discloses that a compound is synthesized by previously treating a powder at a temperature of 1000 to 1800 ° C. and then pulverized to form a coating layer by a similar method. And in Example, it baked at 1400 degreeC and Ar atmosphere for 1 hour, and formed the coating layer.
[0008]
In JP-A-2001-130983, a slurry was applied to the surface of a sintered body by spraying, and heat treatment was performed at 1700 ° C. to form a coating layer.
[0009]
[Problems to be solved by the invention]
However, in the surface coating method described in JP-A-7-172958 and JP-A-2001-130983, a dense and high-adhesion coating layer can be obtained, but some of the coating layers have extremely low adhesion. And there was a problem that the dispersion of the adhesive force was large.
[0010]
Accordingly, an object of the present invention is to provide a method for producing a surface-coated silicon nitride sintered body that easily produces a coating layer that is dense, has high adhesion, and has little variation in adhesion.
[0011]
[Means for Solving the Problems]
According to the present invention, by crystallizing RE ₂ Si ₂ O ₇ from a melt, it is possible to form a coating layer having a high adhesive force to a substrate while suppressing decomposition of the silicon nitride surface, and further, to crystallize from the melt. Therefore, it is based on the finding that the dispersion of the adhesive force is small, and in particular, by controlling the pressure and temperature when producing the melt, the decomposition of silicon nitride is suppressed, and the base material and the coating layer Can be effectively prevented from being generated at the interface of the melt, and by controlling the viscosity of the melt, the bubbles inside the melt can be easily removed and the outflow of the melt can be suppressed.
[0012]
That is, the method for producing a surface-coated silicon nitride sintered body of the present invention provides a method for producing a RE ₂ Si ₂ O ₇ powder (RE is a group 3a element of the periodic table) provided on the surface of a substrate made of a silicon nitride sintered body. (At least one) or a mixed powder of RE ₂ O ₃ powder and SiO ₂ powder is heated and melted in a nitrogen atmosphere to form a melt, and the melt is crystallized to form RE ₂ Si ₂ O ₇ crystals. The present invention is characterized in that a coating layer serving as a main component is formed on the surface of the base material.
[0013]
In particular, it is preferable that the oxide / equivalent molar ratio SiO ₂ / RE ₂ O _{3 of} Si and RE contained in the coating layer is 1.9 to ₃ . Thereby, the corrosion resistance can be increased, and the variation in the corrosion resistance can be effectively reduced.
[0014]
Preferably, the RE contained in the coating layer is at least one of Lu, Yb, and Er. Thereby, the difference in thermal expansion from silicon nitride can be reduced, and generation of cracks can be suppressed.
[0015]
Further, it is preferable that the thickness of the coating layer is 3 to 100 μm. Thereby, generation of cracks due to a difference in thermal expansion coefficient can be suppressed.
[0016]
Further, the pressure of the nitrogen atmosphere is preferably 0.1 to 0.9 MPa. Thereby, decomposition of silicon nitride can be suppressed, so that generation of bubbles can be effectively prevented, and a more dense coating layer can be formed.
[0017]
Furthermore, the temperature for forming the melt _for the melting point of _{RE 2 Si 2 O 7 (T} M), is preferably _{(T M -250 ℃) ~ (} T M + 100 ℃). As a result, a melt having an appropriate viscosity can be generated, air bubbles contained in the melt can be easily removed, and outflow and volatilization of the melt can be suppressed.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
The surface-coated silicon nitride sintered body of the present invention has a crystal phase of a Group 3a element (RE) of the periodic table, ie, RE, on a surface of a substrate made of a silicon nitride sintered body such as a silicon nitride based sintered body. _{The present} invention relates to a method for producing a surface-coated silicon nitride sintered body for forming a crystal phase of a compound represented by ₂ Si ₂ O ₇ (die silicate).
[0019]
According to the present invention, first, a silicon nitride sintered body is prepared as a base material. This substrate is not particularly limited as long as it is a silicon nitride sintered body, and may be any composition system, or may be a silicon nitride sintered body produced by any manufacturing method. .
[0020]
Also, a raw material powder for forming a coating layer is prepared. Here, a case will be described in which RE ₂ Si ₂ O ₇ (disilicate) powder is synthesized from SiO ₂ powder and RE ₂ O ₃ powder and used.
[0021]
If the raw material powder to be prepared contains a large amount of impurities, the impurities segregate at the crystal grain boundaries, preferentially corrode, and lower the corrosion resistance.
[0022]
According to the present invention, the RE ₂ O ₃ powder is a Group 3a element of the periodic table, and specific examples thereof include Y, La, Ce, Lu, Yb, Er, Dy, Sm, Sc, and Gd. . Among these, Lu, Yb, and Er are preferred, and the disilicate phase and the monosilicate phase of Lu, Yb, and Er all have a high melting point, and thus can provide excellent high-temperature strength and oxidation resistance. Further, Yb, Lu and Er are advantageous in that they are easy to sinter and have high strength, can reduce the difference in coefficient of thermal expansion from silicon nitride, and effectively suppress the generation of cracks.
[0023]
Next, the SiO ₂ powder and the RE ₂ O ₃ powder, which are the raw material powders, are mixed at a predetermined ratio to prepare a composite mixed powder. This composite mixed powder is subjected to a heat treatment in advance, and the SiO ₂ powder and the RE ₂ O ₃ powder are reacted to synthesize RE ₂ Si ₂ O ₇ . The calcined RE ₂ Si ₂ O ₇ thus obtained is pulverized to produce a powder having a desired particle size.
[0024]
According to the present invention, since the crystal phase of RE ₂ Si ₂ O ₇ (disilicate) or RE ₂ SiO ₅ (monosilicate) itself has very excellent corrosion resistance, the coating layer has excellent corrosion resistance. However, even if the corrosion resistance of such a crystal phase is good, this coating layer is made of a polycrystalline material, and its crystal grain boundaries are present, and this crystal grain boundary is formed of not only disilicate but also YAM or apatite. In some cases, a nitrogen-containing crystal phase precipitates, causing variations in the characteristics of the coating layer. When impurity oxygen (excess oxygen) is present as SiO _{2 at} the grain boundaries, the excess oxygen reacts with water or the like. And it is washed away by the combustion gas. As a result, the grain boundary phase falls off, the coating layer is easily broken, and the silicon nitride-based sintered body is easily corroded through the grain boundary phase which has become void. .
[0025]
Therefore, it is preferable to control the composition of the coating layer. That is, by setting the SiO ₂ / RE ₂ O ₃ molar ratio to 1.9 to 3.0, particularly 2.0 to 2.5, and more preferably to 2.1 to 2.3, the excess SiO ₂ of the coating layer is obtained. Can be reduced, the corrosion resistance can be increased, and a coating layer with less corrosion resistance variation can be realized.
[0026]
When the coating layer contains RE ₂ SiO ₅ (monosilicate), the SiO ₂ / RE ₂ O ₃ ratio (molar ratio) is 0.9 to 2.3, particularly 1.0 to 1.5. It is preferably within the range.
[0027]
By such composition control, SiO ₂ contained in the grain boundary of the coating layer can be reduced, and generation of such a grain boundary phase can be suppressed. As a result, diffusion of oxygen or water vapor through the crystal grain boundary in the coating layer can be prevented. And a coating layer having higher corrosion resistance can be realized.
[0028]
Next, a predetermined solvent and a binder are added to the obtained RE ₂ Si ₂ O ₇ powder to prepare a slurry. This slurry is applied to the surface of a silicon nitride sintered body as a base material. As a coating method, a CVD method, a thermal spraying method, or a printing method, which is excellent in uniformity, can be used. However, a slurry dipping method or a slurry spraying method is preferable because it can be more easily formed. Thereby, the RE ₂ Si ₂ O ₇ powder can be uniformly applied to the substrate surface.
[0029]
The coating amount of the slurry, that is, its thickness affects the thickness of the coating layer after the heat treatment, but the thickness of the coating layer sufficiently plays a role as a protective layer, and considering the service life, the internal stress is large. The thickness is preferably 3 to 100 μm, particularly 5 to 70 μm, and more preferably 10 to 50 μm, in order to prevent the film from being easily peeled off, and it is preferable to adjust the amount of the slurry applied so as to have such a thickness.
[0030]
Next, the base material having the surface coated with the RE ₂ Si ₂ O ₇ powder is subjected to a heat treatment, whereby a coating layer made of a desired crystal phase can be formed.
[0031]
This heat treatment is preferably performed in an atmosphere containing at least nitrogen at a pressure of 0.1 to 0.9 MPa. By containing nitrogen, decomposition on the surface of the silicon nitride sintered body can be suppressed, and by setting the pressure within the above range, the effect is enhanced, and the coating layer is formed by bubbles generated by decomposition of silicon nitride. Can be effectively suppressed from decreasing.
[0032]
In particular, the lower limit of the atmospheric pressure is preferably 0.2 MPa, more preferably 0.3 MPa. When the atmosphere is pressurized with nitrogen, the effect of the atmosphere can be further enhanced.
[0033]
The temperature for producing a melt may be a lower limit to the _RE 2 _Si 2 melting point of _{O 7 (T M)} than 250 ° C. lower temperature _(T M -250 ℃), low especially 210 ° C. than the melting point temperature _(T M -210), even lower temperatures _(T M -150) is 0.99 ° C., more preferably 100 ° C. lower temperature _(T M -100 ℃) are preferred. The reason why the melt is generated at a temperature lower than the melting point is that the surface coating component reacts with silicon nitride of the substrate or nitrogen in the atmosphere to lower the melting point. On the other hand, the upper limit is preferably a temperature 100 ° C. higher than the melting point (T _M + 100 ° C.), particularly preferably (T _M + 80 ° C.), and further preferably (T _M + 50 ° C.).
[0034]
As described above, by setting the heat treatment temperature in the above range, a melt having an appropriate viscosity can be generated, so that bubbles contained in the melt can be easily removed, and the melt can be removed from the surface of the base material. Since the flow-down can be effectively suppressed, it is possible to form a coating layer which is dense and has few residual pores, and can prevent outflow and volatilization of the melt. In particular, when heat treatment is performed in a pressurized atmosphere, decomposition of the silicon nitride surface can be suppressed, and a coating layer having higher adhesion to a substrate can be formed. In general, although it depends on the type of RE, the temperature is preferably from 1500 to 1900 ° C, particularly preferably from 1600 to 1850 ° C.
[0035]
Although the heat treatment atmosphere is preferable in terms of suppressing the volatilization of the composition under a pressurized state, it further suppresses the volatilization of SiO ₂ , prevents the composition fluctuation of the coating layer, and stably obtains excellent characteristics. It is preferable that the Si / SiO ₂ mixed powder be placed in a heat treatment furnace to generate the powder.
[0036]
The coating layer thus obtained can easily have a porosity of 2% or less, particularly 1% or less, more preferably 0.5% or less, and more preferably 0.3% or less. The mechanical strength of the layer can be further improved and exhibit better corrosion resistance.
[0037]
That is, a coating layer made of RE ₂ Si ₂ O ₇ (disilicate) crystal is formed on the surface-coated silicon nitride sintered body to protect conventional SiO ₂ , Al ₂ O ₃ , mullite, cordierite, YAG, and the like. Compared with the film, it has a higher adhesive force and is very stable even in a high-temperature oxidizing atmosphere, so that excellent corrosion resistance is exhibited. Further, since the melting point is as high as 1550 to 1800 ° C., the heat resistance is excellent, and the life at a high temperature is long.
[0038]
【Example】
Various mixed powders of SiO ₂ and RE ₂ O ₃ are mixed at a ratio shown in Table 1, and the obtained mixed powder is applied to a silicon nitride sintered body as a base material. Was melted to form a surface coating layer. The sample No. Sample No. 10 was calcined at 1400 ° C. before coating to form a calcined body. This was pulverized and applied to a substrate as a mixed powder, and then a surface coating layer was formed in the same manner.
[0039]
The mixed powder was analyzed by X-ray fluorescence to measure RE and Si. The oxide molar ratio SiO ₂ / RE ₂ O ₃ was calculated from the obtained values and is shown in Table 1 as the Si / RE ratio.
[0040]
The obtained surface-coated silicon nitride sintered body sample was cut, the cross section was mirror-finished, and the surface coating thickness was measured by SEM observation. Further, a stainless steel rod having a diameter of 25 mm was fixed to the front and back of the obtained surface coating sample of 25 mm square with an epoxy resin, and a tensile test was performed. The adhesive force at this time was measured, and the average value and the variation were calculated. Further, the sample after the tensile test was cut, and the cross section thereof was observed to determine a peeled portion. And, the case where it was peeled off from the interface between the epoxy resin and the surface coating layer was indicated as “surface”, and the case where it was peeled off at the interface between the surface coating layer and the substrate was indicated as “inside”.
[0041]
An exposure test was performed on the obtained sample in a combustion gas atmosphere to evaluate corrosion resistance. The combustion gas conditions were a pressure of 0.4 MPa, a flow rate of 100 m / s, and a partial pressure of steam of 30 kPa. As for the corrosion resistance, the weight loss and the density after the exposure test for 100 hours were measured, and the thinning amount was calculated from these data, and displayed as the corrosion resistance. The results are shown in Table 1.
[0042]
[Table 1]

[0043]
Sample No. of the present invention In Nos. 1 to 16 and 18 to 21, the adhesive force was 200 MPa, the variation was within 30 MPa, the fracture interface was the interface between the epoxy resin and the surface coating layer, and the corrosion resistance was 1 μm or less in wall thickness.
[0044]
On the other hand, in the case of Sample No. In No. 17, the adhesive force was 180 MPa, the variation was 70 MPa, and the fracture interface was the interface between the surface coating layer and the silicon nitride sintered body substrate. Further, No. 1 in which the surface coating layer was formed of a material other than monosilicate and disilicate. Samples Nos. 22 to 26 have a large corrosion resistance of 100 μm or more, and have extremely poor corrosion resistance.
[0045]
【The invention's effect】
According to the method for producing a surface-coated silicon nitride sintered body of the present invention, a highly corrosion-resistant surface-coated silicon nitride sintered body having high adhesion between the silicon nitride sintered body substrate and the surface coating layer can be obtained.

Claims (6)

RE ₂ Si ₂ O ₇ powder (RE is at least one element from Group 3a of the periodic table) or a mixture of RE ₂ O ₃ powder and SiO ₂ powder provided on the surface of a substrate made of a silicon nitride sintered body After the powder is heated and melted in a nitrogen atmosphere to form a melt, the melt is crystallized to form a coating layer mainly composed of RE ₂ Si ₂ O ₇ crystals on the surface of the base material. For producing a surface-coated silicon nitride sintered body. _{2. The} method for producing a surface-coated silicon nitride sintered body according to claim 1, wherein a molar ratio SiO ₂ / RE ₂ O ₃ of oxides of Si and RE contained in the coating layer is 1.9 to _{3. 3.} . The method for producing a surface-coated silicon nitride sintered body according to claim 1 or 2, wherein RE contained in the coating layer is at least one of Lu, Yb, and Er. The method for producing a surface-coated silicon nitride sintered body according to any one of claims 1 to 3, wherein the thickness of the coating layer is 3 to 100 µm. The method for producing a surface-coated silicon nitride sintered body according to any one of claims 1 to 4, wherein the pressure of the nitrogen atmosphere is 0.1 to 0.9 MPa. Temperature to form a _melt, RE respect 2 _Si 2 _{O 7} melting point _{(T M),} according to claim 1 to 5, characterized in that a _{(T M -250 ℃) ~ (} T M + 100 ℃) The method for producing a surface-coated silicon nitride sintered body according to any one of the above.