Coated member and method of manufacturing the same

Abstract

Provided are a coated member in which damage of a coating film can be suppressed in a high temperature environment and the coating may be performed at low cost, and a method of manufacturing the same. A coated member includes a bond coat and a top coat sequentially laminated on a substrate made of a Si-based ceramic or a SiC fiber-reinforced SiC matrix composite, wherein the top coat includes a layer composed of a mixed phase of a (Y.sub.1-aLn.sub.1a).sub.2Si.sub.2O.sub.7 solid solution (here, Ln.sub.1 is any one of Nd, Sm, Eu, and Gd) and Y.sub.2SiO.sub.5 or a (Y.sub.1-bLn.sub.1′.sub.b).sub.2SiO.sub.5 solid solution (here, Ln.sub.1′ is any one of Nd, Sm, Eu, and Gd), or a mixed phase of a (Y.sub.1-cLn.sub.2c).sub.2Si.sub.2O.sub.7 solid solution (here, Ln.sub.2 is any one of Sc, Yb, and Lu) and Y.sub.2SiO.sub.5 or a (Y.sub.1-dLn.sub.2′.sub.d).sub.2SiO.sub.5 solid solution (here, Ln.sub.2′ is any one of Sc, Yb, and Lu).

Claims

1. A coated member comprising: a bond coat and a top coat sequentially laminated on a substrate made of a silicon (Si)-based ceramic or a SiC fiber-reinforced SiC matrix composite, wherein the bond coat is consist of at least one of silicide or mullite, the top coat includes a first layer composed of a mixed phase of a rare earth disilicate and a rare earth monosilicate, the rare earth disilicate being a (Y.sub.1-aLn.sub.1a).sub.2Si.sub.2O.sub.7 solid solution (here, Ln.sub.1 is any one of Nd, Sm, Eu, and Gd, a is 0.1 or more and 0.5 or less when Ln.sub.1 is Nd, Sm, or Eu, and a is 0.2 or more and 0.5 or less when Ln.sub.1 is Gd), and the rare earth monosilicate being Y.sub.2SiO.sub.5 or a (Y.sub.1-bLn.sub.1′b).sub.2SiO.sub.5 solid solution (here, Ln.sub.1′ is any one of Nd, Sm, Eu, and Gd, and b is more than 0 and equal to or less than 0.5).

2. A coated member comprising: a bond coat and a top coat sequentially laminated on a substrate made of a Si-based ceramic or a SiC fiber-reinforced SiC matrix composite, wherein the bond coat is consist of at least one of silicide or mullite, the top coat includes a first layer composed of a mixed phase of a rare earth disilicate and a rare earth monosilicate, the rare earth disilicate being a (Y.sub.1-cLn.sub.2c).sub.2Si.sub.2O.sub.7 solid solution (here, Ln.sub.2 is any one of Sc, Yb, and Lu, when Ln.sub.2 is Sc, c is 0.05 or more and 0.5 or less, and when Ln.sub.2 is Yb or Lu, c is 0.1 or more and 0.5 or less), and the rare earth monosilicate being Y.sub.2SiO.sub.5 or a (Y.sub.1-dLn.sub.2′d).sub.2SiO.sub.5 solid solution (here, Ln.sub.2′ is any one of Sc, Yb, and Lu, and d is more than 0 and equal to or less than 0.5).

3. The coated member of claim 1, wherein the top coat has a second layer on the first layer, and the second layer is made of Re.sub.2SiO.sub.5 (here, Re is a rare earth element).

4. The coated member of claim 1, wherein a difference between a thermal expansion coefficient of the substrate on which the bond coat is laminated and a thermal expansion coefficient of the first layer is 3×10.sup.−6/K or less.

5. The coated member of claim 3, wherein a difference between a thermal expansion coefficient of the first layer and a thermal expansion coefficient of the second layer is 3×10.sup.−6/K or less.

6. The coated member of claim 2, wherein the top coat has a second layer on the first layer, and the second layer is made of Re.sub.2SiO.sub.5 (here, Re is a rare earth element).

7. The coated member of claim 2, wherein a difference between a thermal expansion coefficient of the substrate on which the bond coat is laminated and a thermal expansion coefficient of the first layer is 3×10.sup.−6/K or less.

8. The coated member of claim 4, wherein a difference between a thermal expansion coefficient of the first layer and a thermal expansion coefficient of the second layer is 3×10.sup.−6/K or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic cross-sectional view of a coated member according to a first embodiment.

(2) FIG. 2 is a graph illustrating a relationship between a temperature and a crystal structure of a rare earth disilicate.

(3) FIG. 3 is a view illustrating a change in crystal structure of a Y.sub.2Si.sub.2O.sub.7 spray coating film by heat-treatment.

(4) FIG. 4 is a view illustrating a change in crystal structure of a (Y.sub.2.8Gd.sub.0.2).sub.2Si.sub.2O.sub.7 solid solution spray coating film by heat-treatment.

(5) FIG. 5 is a view illustrating a change in crystal structure of a (Y.sub.2.8Yb.sub.0.2).sub.2Si.sub.2O.sub.7 solid solution spray coating film by heat-treatment.

(6) FIG. 6 is a graph illustrating a relationship between a mixing ratio of a rare earth disilicate and a rare earth monosilicate and a thermal expansion coefficient.

(7) FIG. 7 is a schematic cross-sectional view of a coated member according to a second embodiment.

DESCRIPTION OF EMBODIMENTS

First Embodiment

(8) FIG. 1 is a schematic cross-sectional view of a coated member according to a first embodiment. A coated member 100 can be formed by sequentially laminating a bond coat 102 and a top coat 103 on a substrate 101.

(9) The substrate 101 is a turbine member of an aircraft engine, or a gas turbine member for power generation such as a shroud, a combustion liner, or the like. The substrate 101 is made of a silicon (Si)-based ceramic or a SiC fiber-reinforced SiC matrix composite (CMC). The Si-based ceramic is a ceramic containing Si such as SiC, Si.sub.3N.sub.4, or the like. The SiC fiber-reinforced SiC matrix composite is a matrix composite in which SiC fiber is used as reinforcing fiber and SiC is used as a matrix.

(10) The bond coat 102 is formed on a surface of the substrate 101. The bond coat 102 is to secure good adhesion between the substrate 101 and the top coat 103. The bond coat 102 is made of silicide such as Si, MoSi.sub.2, LuSi.sub.2, or the like, mullite (3Al.sub.2O.sub.3-2SiO.sub.2), barium strontium aluminosilicate (BSAS, (Ba.sub.1-xSr.sub.x)O—Al.sub.2O.sub.3—SiO.sub.2), or the like. The bond coat 102 can be made of one of the above-mentioned materials or formed by laminating a plurality of materials. A thickness of the bond coat 102 is 20 μm or more and 200 μm or less.

(11) The bond coat 102 is formed by a spray method, a sintering method, or the like.

(12) The top coat 103 is formed on the bond coat 102. The top coat 103 according to the present embodiment is composed of a mixed phase of a rare earth disilicate and a rare earth monosilicate. A thickness of the top coat 103 is 20 μm or more and 400 μm or less.

(13) FIG. 2 is a graph illustrating a relationship between a temperature and a crystal structure of a rare earth disilicate (Reference of the graph: A. J. F. Carrion et al., “Structural and kinetic study of phase transitions in LaYSi.sub.2O.sub.7”, Journal of the European Ceramic Society, Vol. 32 (2012) PP. 2477-2486, a boundary line of the crystal structure is added by the present inventor). In FIG. 2, a horizontal axis indicates an ionic radius of a rare earth element, and a vertical axis indicates a temperature.

(14) An ionic radius of Y.sup.3+ is 0.90 Å, and referring to FIG. 2, phase transition (γ phase.fwdarw.β phase) occurs at about 1280° C. That is, when the coated member is used in a temperature environment higher than 1300° C., phase transformation accompanied with a volume change is generated by repetitive cooling and heating. Therefore, cracks occur in the top coat 103 due to phase transformation of Y.sub.2SiO.sub.7.

(15) In the case in which a rare earth element has an ionic radius larger than that of Y, there is a boundary between an a phase and a γ phase as illustrated in FIG. 2. Referring to FIG. 2, an ionic radius on a boundary line between the α phase and the γ phase at 1300° C. is 0.905 Å. That is, when the ionic radius of the rare earth element is 0.905 Å or more, crystal stability of the rare earth disilicate can be secured up to 1300° C. An ionic radius on a boundary line between the α phase and the γ phase at 1400° C. is 0.91 Å. That is, when the ionic radius of the rare earth element is 0.91 Å or more, crystal stability of the rare earth disilicate can be secured up to 1400° C.

(16) In order to allow an average ionic radius to be larger than that of Y, Y is substituted with a rare earth element having an ionic radius larger than that of Y. As a difference in ionic radius between a rare earth element and Y is increased, an effect of changing an average ionic radius of the rare earth elements is increased. Therefore, it is advantageous to select an element having an ionic radius larger than that of Gd in FIG. 2. Meanwhile, Pr, Ce, and La have high reactivity with steam, such that steam resistance of the coating film is deteriorated. Therefore, Gd, Eu, Sm, and Nd are suitable as a substitution element of Y.

(17) Table 1 illustrates a substitution amount of a substitution element (Gd, Eu, Sm, or Nd) in the (Y.sub.1-aLn.sub.1).sub.2Si.sub.2O.sub.7 solid solution and an average ionic radius of the rare earth elements.

(18) TABLE-US-00001 TABLE 1 Substitution Element a Nd Sm Eu Gd 0 0.9 0.9 0.9 0.9 0.1 0.908 0.906 0.905 0.9038 0.2 0.917 0.912 0.909 0.9076 0.3 0.925 0.917 0.914 0.9114 0.4 0.933 0.923 0.919 0.9152 0.5 0.942 0.929 0.924 0.919

(19) According to Table 1, the average ionic radius is to be 0.905 Å or more if a is 0.1 or more for Nd, Sm, or Eu, and 0.2 or more for Gd. That is, when a is equal to or higher than the above-mentioned range, the coated member can withstand an operation temperature up to 1300° C.

(20) Further, according to Table 1, the average ionic radius is to be 0.91 Å or more if a is 0.2 or more for Nd or Sm, and 0.3 or more for Eu or Gd. That is, when a is equal to or higher than the above-mentioned range, the coated member can withstand an operation temperature up to 1400° C.

(21) Meanwhile, if a is large, an amount of the substitution element is increased. Therefore, in consideration of raw material cost, cost for substitution is increased by using Nd, Sm, Eu, and Gd, which are expensive elements. Therefore, there is an upper limit in the substitution amount of Nd, Sm, Eu, or Gd. In detail, it is preferable that an upper limit of a is 0.5.

(22) Referring to FIG. 2, an ionic radius on a boundary line between the β phase and the γ phase at 1300° C. is 0.897 Å. That is, when the ionic radius of the rare earth element is 0.897 Å or less, crystal stability of the rare earth disilicate can be secured up to 1300° C. An ionic radius on a boundary line between the β phase and the γ phase at 1400° C. is 0.885 Å. That is, when the ionic radius of the rare earth element is 0.885 Å or less, crystal stability of the rare earth disilicate can be secured up to 1400° C.

(23) In the (Y.sub.1-cLn.sub.2c).sub.2Si.sub.2O.sub.7 solid solution in which Y is substituted with a separate rare earth element, an average ionic radius of rare earth elements (Y and Ln.sub.2) is changed depending on a substitution amount. In order to allow the average ionic radius to be smaller than that of Y, Y is substituted with a rare earth element having an ionic radius smaller than that of Y. Referring to FIG. 2, the elements having an ionic radius smaller than that of Y are Sc, Yb, Lu, Tm, and Er. Particularly, in the cases of Sc, Yb, and Lu, the ionic radius is small as compared to Y, and a β phase of disilicate is stably present up to a high temperature.

(24) Table 2 illustrates a substitution amount of a substitution element (Sc, Yb, or Lu) in the (Y.sub.1-cLn.sub.2c).sub.2Si.sub.2O.sub.7 solid solution and an average ionic radius of the rare earth elements.

(25) TABLE-US-00002 TABLE 2 Substitution Element c Sc Yb Lu 0 0.9 0.9 0.9 0.05 0.892 0.898 0.898 0.1 0.885 0.897 0.897 0.2 0.869 0.894 0.894 0.3 0.854 0.89 0.89 0.4 0.838 0.887 0.887 0.5 0.823 0.884 0.884

(26) According to Table 2, the average ionic radius is to be 0.897 Å or less if c is 0.05 or more for Sc, and 0.1 or more for Yb or Lu. That is, when c is equal to or higher than the above-mentioned range, the coated member can withstand an operation temperature of 1300° C.

(27) Further, according to Table 2, the average ionic radius is to be 0.885 Å or less if c is 0.1 or more for Sc, and 0.5 or more for Yb or Lu. That is, when c is equal to or higher than the above-mentioned range, the coated member can withstand an operation temperature of 1400° C. or more.

(28) Meanwhile, if c is large, an amount of the substitution element is increased. Therefore, in consideration of raw material cost, cost for substitution is increased by using Sc, Yb, and Lu, which are expensive elements. Therefore, there is an upper limit in the substitution amount of Sc, Yb, or Lu. In detail, it is preferable that an upper limit of c is 0.5.

(29) When the rare earth disilicate is (Y.sub.1-aLn.sub.1a).sub.2Si.sub.2O.sub.7, the rare earth monosilicate according to the present embodiment is Y.sub.2SiO.sub.5 or a (Y.sub.1-bLn.sub.1′.sub.b).sub.2SiO.sub.5 solid solution (here, b>0). It is preferable that the substitution element Ln.sub.1′ of Y is the same as the substitution element of the rare earth disilicate. In detail, Ln.sub.1′ is any one of Nd, Sm, Eu, and Gd.

(30) As the rare earth monosilicate, Y.sub.2SiO.sub.5 (substitution amount b=0) can also be used, but in order to prevent concentrations of the rare earth element from being changed by interdiffusion with the rare earth disilicate, (Y.sub.1-aLn.sub.1a).sub.2Si.sub.2O.sub.7, coexisting in the mixed phase, it is preferable that the substitution amount b is the same as the substitution amount a of the rare earth disilicate. Therefore, the upper limit of the substitution amount b is 0.5. It is preferable that when the substitution element is Nd, Sm, or Eu, a lower limit of the substitution amount b is 0.1, and when the substitution element is Gd, the lower limit of the substitution amount b is 0.2.

(31) When the rare earth disilicate is (Y.sub.1-cLn.sub.2c).sub.2Si.sub.2O.sub.7, the rare earth monosilicate is Y.sub.2SiO.sub.5 or a (Y.sub.1-dLn.sub.2′.sub.d).sub.2SiO.sub.5 solid solution (here, d>0). It is preferable that the substitution element Ln.sub.2′ of Y is the same as the substitution element of the rare earth disilicate. In detail, Ln.sub.2′ is any one of Yb, Lu, and Sc.

(32) As the rare earth monosilicate, Y.sub.2SiO.sub.5 (substitution amount d=0) can also be used, but in order to prevent concentrations of the rare earth elements from being changed by interdiffusion with the rare earth disilicate, (Y.sub.1-cLn.sub.2c).sub.2Si.sub.2O.sub.7, coexisting in the mixed phase, it is preferable that the substitution amount d is the same as the substitution amount c of the rare earth disilicate. Therefore, the upper limit of the substitution amount d is 0.5. It is preferable that when the substitution element is Sc, a lower limit of the substitution amount d is 0.05, and when the substitution element is Yb or Lu, the lower limit of the substitution amount d is 0.1.

(33) The top coat 103 is formed by a spray method. In the present embodiment, as spray particles, particles made of a mixture of rare earth disilicate powder and rare earth monosilicate powder are used.

(34) The spray particles are prepared by the following method. First, particles of the rare earth disilicate solid solution having the above mentioned composition are prepared. As raw material powders, SiO.sub.2 powder, Y.sub.2O.sub.3 powder, [Ln.sub.1].sub.2O.sub.3 powder (here, Ln.sub.1 is any one of Nd, Sm, Eu, and Gd), [Ln.sub.2].sub.2O.sub.3 powder (here, Ln.sub.2 is any one of Sc, Yb, and Lu) are weighed and mixed with each other so as to have a predetermined composition. Alternatively, Y.sub.2Si.sub.2O.sub.7 powder and Ln.sub.2Si.sub.2O.sub.7 powder (here, Ln is any one of Sc, Yb, and Lu or any one of Nd, Sm, Eu, and Gd) are weighed and mixed with each other so as to have a predetermined composition.

(35) Solid solution formation by heat-treatment can be promoted by using fine powders having a particle size of 1 μm or less as the raw material powders. Therefore, it is possible to decrease a heat-treatment time while removing unreacted particles.

(36) A solid solution powder is obtained by performing heat-treatment on the mixed powders. As a method of preparing the solid solution powder, there is a method of performing heat-treatment at 1300° C. or more using an electric furnace, a method of performing plasma heat-treatment, a method of melting the raw material powders and grinding the obtained melt, or the like.

(37) The rare earth monosilicates represented by Y.sub.2SiO.sub.5, (Y.sub.1-bLn.sub.1′.sub.b).sub.2SiO.sub.5 (solid solution), and (Y.sub.1-eLn.sub.2′.sub.d).sub.2SiO.sub.5 (solid solution) are also prepared by the same method as described above.

(38) The rare earth disilicate powder and the rare earth monosilicate powder prepared by the above-mentioned method are weighed and granulated so as to have a predetermined composition ratio. The particles obtained as described above are classified, and particles having a size of 10 μm to 200 μm are used as the spray particles.

(39) Spray powders of Y.sub.2Si.sub.2O.sub.7, a (Y.sub.0.8Gd.sub.0.2).sub.2Si.sub.2O.sub.7 solid solution, a (Y.sub.0.8Yb.sub.0.2).sub.2Si.sub.2O.sub.7 solid solution were prepared. Further, a spray coating film was manufactured using these powders.

(40) The manufactured spray coating film was heat treated at 1300° C. for 100 hours and at 1400° C. for 100 hours, and a change in crystal phase was determined by X-ray diffraction (XRD). As illustrated in FIG. 3, in the case of Y.sub.2Si.sub.2O.sub.7, a coating film was amorphous in the as-sprayed state, and after heat-treatment at 1300° C. for 100 hours, the coating film was composed of a β-Y.sub.2Si.sub.2O.sub.7 phase and an X.sub.2-Y.sub.2SiO.sub.5 phase. After heat-treatment at 1400° C. for 100 hours, the coating film was composed of two phases, that is, a γ-Y.sub.2Si.sub.2O.sub.7 phase and the X.sub.2-Y.sub.2SiO.sub.5 phase. Therefore, it may be appreciated that at 1300° C. or more, phase transformation from β-Y.sub.2Si.sub.2O.sub.7 to γ-Y.sub.2Si.sub.2O.sub.7 occurred.

(41) Meanwhile, FIG. 4 illustrates the results obtained by heat-treating a spray coating film of the (Y.sub.0.8Gd.sub.0.2).sub.2Si.sub.2O.sub.7 solid solution in which Y was partially substituted with Gd at 1300° C. for 100 hours and at 1400° C. for 100 hours. It may be appreciated that in the cases of heat-treatment at 1300° C. for 100 hours and at 1400° C. for 100 hours, there was almost no change in a diffraction peak, and thus, phase transformation was suppressed.

(42) Meanwhile, FIG. 5 illustrates the results obtained by heat-treating a spray coating film of the (Y.sub.0.8Yb.sub.0.2).sub.2Si.sub.2O.sub.7 solid solution in which Y was partially substituted with Yb at 1300° C. for 100 hours and at 1400° C. for 100 hours. It may be appreciated that in the cases of heat-treatment at 1300° C. for 100 hours and at 1400° C. for 100 hours, there was almost no change in a diffraction peak, and thus, in this case, phase transformation was suppressed.

(43) The mixing ratio of the rare earth disilicate powder and the rare earth monosilicate powder is determined in consideration of a thermal expansion coefficient of the top coat. In the coated member 100 according to the present embodiment corresponding to a laminate, when a difference in thermal expansion coefficient between the top coat 103 and a base of the top coat 103 is large, thermal stress is generated in the top coat 103. Cracks, or the like, can occur in the top coat 103 due to thermal stress generated in the top coat 103 by repeating operation and stop of equipment (aircraft engine or gas turbine).

(44) In order to alleviate thermal stress in the top coat 103, it is preferable that the difference between the thermal expansion coefficient of the top coat 103 and the thermal expansion coefficient of the substrate 101 including the bond coat 102, which is the base of the top coat 103, is 3×10.sup.−6/K or less (at room temperature to 1200° C.). In order to allow the difference in the thermal expansion coefficient to be in the above-mentioned range, the mixing ratio of the rare earth disilicate and the rare earth monosilicate is determined depending on the kind of substrate, the kind of rare earth disilicate, and the kind of rare earth monosilicate. At the time of preparing the spray powder, the raw material powders are weighed and mixed with each other so as to have the mixing ratio.

(45) FIG. 6 is a graph illustrating a relationship between a mixing ratio of a rare earth disilicate and a rare earth monosilicate and a thermal expansion coefficient. FIG. 6 illustrates an example in which (Y.sub.0.8Yb.sub.0.2).sub.2Si.sub.2O.sub.7 is used as the rare earth disilicate, and Y.sub.2SiO.sub.5 is used as the rare earth monosilicate. In FIG. 6, a horizontal axis indicates a ratio of (Y.sub.0.8Yb.sub.0.2).sub.2Si.sub.2O.sub.7 in a solid solution ((Y.sub.0.8Yb.sub.0.2).sub.2Si.sub.2O.sub.7+Y.sub.2SiO.sub.5), and a vertical axis indicates a thermal expansion coefficient of the solid solution (at room temperature to 1200° C.)

(46) The thermal expansion coefficient of the rare earth monosilicate is larger than that of the rare earth disilicate. Therefore, as illustrated in FIG. 6, as the mixing ratio of the rare earth disilicate is increased, the thermal expansion coefficient is decreased. Since a thermal expansion coefficient of a SiC fiber-reinforced SiC composite member including a bond coat (Si) is 4×10.sup.−6/K (at room temperature to 1200° C.), when the ratio of the rare earth disilicate is in a range of 0.05 to 0.85, the difference in thermal expansion coefficient can become 3×10.sup.−6/K or less.

(47) A top coat 103 having excellent steam resistance and thermal cycle durability can be obtained by mixing the rare earth disilicate having a thermal expansion coefficient relatively close to that of the substrate and the rare earth monosilicate having excellent steam resistance to form the solid solution as described above. A composition of the top coat 103 can be uniform by forming the top coat 103 using the spray particles made of the solid solution in advance. Since in the top coat 103, the rare earth disilicate does not remain in an un-reacted state, excellent steam resistance can be secured.

Second Embodiment

(48) FIG. 7 is a schematic cross-sectional view of a coated member according to a second embodiment. In a coated member 200 according to the second embodiment, a bond coat 202 and a top coat 203 are sequentially laminated on a substrate 201, wherein the top coat 203 is composed of two layers.

(49) The substrate 201 and the bond coat 202 are made of the same materials as those in the first embodiments. A first layer 204 of the top coat 203 is made of the same material as that of the top coat 103 according to the first embodiment, and has the same film thickness as that of the top coat 103 according to the first embodiment.

(50) The second layer 205 of the top coat 203 is made of a rare earth monosilicate represented by Re.sub.2SiO.sub.5. Re is one of rare earth elements or a plurality of rare earth elements can be selected. For example, the second layer 205 can be made of Y.sub.2SiO.sub.5, Yb.sub.2SiO.sub.5, Lu.sub.2SiO.sub.5, (Y, Yb).sub.2SiO.sub.5, (Y, Lu).sub.2SiO.sub.5, or the like. Considering raw material cost, it is preferable that the second layer 205 is made of Y.sub.2SiO.sub.5 or a composite oxide, (Y, Re′).sub.2SiO.sub.5 (here, Re′ is another rare earth element except for Y) in which Y is partially substituted with another rare earth element. In view of preventing concentrations of rare earth elements from being changed by interdiffusion with the first layer 204, it is preferable that a substitution element Re′ is selected from Yb, Lu, Sc, Nd, Sm, Eu, and Gd. Particularly, it is preferable that the substitution element Re′ is the same material as the rare earth element contained in the first layer 204. Considering raw material cost, it is preferable that a substitution amount of Re′ is 0.5 or less.

(51) The second layer 205 is formed by a spray method similarly to the first layer 204. A thickness of the second layer 205 is 50 μm or more and 300 μm or less.

(52) In the case of applying the composite oxide containing a rare earth element selected from the plurality of rare earth elements to the second layer 205, particles solid-solubilized by heat-treatment after weighing and mixing raw material powders so as to have a predetermined substitution ratio can be used as the spray particles. In this way, uniformity of the composition in the second layer 205 is secured.

(53) In the present embodiment, it is preferable that a difference in thermal expansion coefficient between the first layer 204 and a base (the substrate 201 including the bond coat 202) is 3×10.sup.−6/K (at room temperature to 1200° C.). It is preferable that a difference in thermal expansion coefficient between the first layer 204 and the second layer 205 is 3×10.sup.−6/K (at room temperature to 1200° C.)

(54) As described in the first embodiment, the material of the substrate 201 (and the bond coat 202), the kinds of rare earth monosilicate and rare earth disilicate and a mixing ratio therebetween in the first layer 204, and the material of the second layer 205 are selected so that the respective layers can satisfy the differences in thermal expansion coefficient.

(55) Damage by steam erosion under a high temperature environment can be prevented by allowing the top coat 203 to be composed of two layers and forming a layer of the rare earth monosilicate having excellent steam resistance in an outermost layer as in the present embodiment.

(56) {Example}

(57) Tables 3 and 4 illustrate examples of coated members using a SiC fiber-reinforced SiC matrix composite (Tyrannohex, Ube Industries, Ltd., thermal expansion coefficient: 4×10.sup.−6/K (at room temperature to 1200° C.) as a substrate. Further, a difference in thermal expansion coefficient between a first layer and a base (a substrate including a bond coat) (at room temperature to 1200° C.), and a difference in thermal expansion coefficient between first and second layers (at room temperature to 1200° C.) are illustrated in Tables 3 and 4.

(58) TABLE-US-00003 TABLE 3 Example 1 Example 2 Example 3 Example 4 Example 5 Top Coat Second Y.sub.2SiO.sub.5 Y.sub.2SiO.sub.5 Y.sub.2SiO.sub.5 Y.sub.2SiO.sub.5 — Layer First 0.2(Y.sub.0.8Yb.sub.0.2).sub.2Si.sub.2O.sub.7 + 0.5(Y.sub.0.8Yb.sub.0.2).sub.2Si.sub.2O.sub.7 + 0.2(Y.sub.0.8Yb.sub.0.2).sub.2Si.sub.2O.sub.7 + 0.5(Y.sub.0.8Yb.sub.0.2).sub.2Si.sub.2O.sub.7 + 0.5(Y.sub.0.8Yb.sub.0.2).sub.2Si.sub.2O.sub.7 + Layer 0.8Y.sub.2SiO.sub.5 0.5Y.sub.2SiO.sub.5 0.8Y.sub.2SiO.sub.5 0.5Y.sub.2SiO.sub.5 0.5Y.sub.2SiO.sub.5 (Molar Ratio) Bond Coat Si Si Si Si Si Difference in 1.3 2.5  1 2.2  — Thermal Expansion Coefficient between First and Second Layers (×10.sup.−6/K) Difference in 2   0.86 2 0.86 0.86 Thermal Expansion Coefficient between First and Base Layers (×10.sup.−6/K)

(59) TABLE-US-00004 TABLE 4 Comparative Comparative Example 6 Example 7 Example 8 Example 1 Example 2 Top Coat Second Y.sub.2SiO.sub.5 Y.sub.2SiO.sub.5 Gd.sub.2SiO.sub.5 Y.sub.2SiO.sub.5 — Layer First 0.2(Y.sub.0.8Gd.sub.0.2).sub.2Si.sub.2O.sub.7 + 0.5(Y.sub.0.8Gd.sub.0.2).sub.2Si.sub.2O.sub.7 + 0.2(Y.sub.0.8Gd.sub.0.2).sub.2Si.sub.2O.sub.7 + Y.sub.2Si.sub.2O.sub.7 Y.sub.2SiO.sub.5 Layer 0.8(Y.sub.0.8Gd.sub.0.2).sub.2SiO.sub.5 0.5(Y.sub.0.8Gd.sub.0.2).sub.2SiO.sub.5 0.8(Y.sub.0.8Gd.sub.0.2).sub.2SiO.sub.5 (Molar Ratio) Bond Coat Si Si Si Si Si Difference in 1.3 2.6 2.6 3.7 — Thermal Expansion Coefficient between First and Second Layers (×10.sup.−6/K) Difference in 2.2 0.9 2.2 0.3 3.4 Thermal Expansion Coefficient between First Layer and Base (×10.sup.−6/K)

(60) As illustrated in Tables 3 and 4, since in Examples 1 to 8, differences in thermal expansion coefficients between the respective layers were 3×10.sup.−6/K or less, thermal stress in the top coat was alleviated. Therefore, thermal cycle durability can be improved as compared in Comparative Examples 1 and 2 in which the difference in thermal expansion coefficient was more than 3×10.sup.−6/K.