JPH0848563A - Molybdenum disilicide-based composite ceramics and its production - Google Patents

Abstract

PURPOSE:To produce molybdenum disilicide-based composite ceramics having satisfactory deformation resistance at a high temp. of >=1,000 deg.C and excellent in oxidation resistance. CONSTITUTION:An MoSi2 matrix is combined with 3-40vol.% SiC whiskers having 0.2-1.0mum average diameter and 2-50mum average length to obtain the objective molybdenum disilicide-based ceramics. In this ceramics, >=50wt.% of the SiC whiskers are made of 1st SiC having an alpha-form crystal structure and/or 2nd SiC having a beta-form crystal structure and a peak at 33.6 deg. (2theta) and no peak at 41.4 deg. (2theta) in its X-ray diffraction pattern.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molybdenum disilicide-based composite ceramic having improved mechanical strength and excellent oxidation resistance and a method for producing the same, and more particularly to improved mechanical strength and fracture at room temperature. The present invention relates to a molybdenum disilicide-based composite ceramic reinforced with silicon carbide whiskers having toughness and excellent deformation resistance at high temperatures, and a method for producing the same.

[0002]

2. Description of the Related Art Molybdenum disilicide exhibits excellent oxidation resistance due to its high ability to form a silica protective film in the atmosphere.
Used for heating up to 750 ° C. Since molybdenum disilicide has excellent high-temperature characteristics as described above, it is attracting attention as a high-temperature structural material for gas turbine members and the like, and is being evaluated.

However, since molybdenum disilicide is brittle at room temperature and has low strength at high temperatures, it cannot be used as a structural material subjected to a mechanical load.

Therefore, attempts have been made to combine various second phases for the purpose of improving the toughness of molybdenum disilicide and the strength at high temperatures. In the old days, it began with a system combining borides as seen in JP-A-60-195061, and as disclosed in JP-A-1-115876, boride of Mo and W was used as a second phase. 1 depending on the combined system
Sufficient oxidation resistance is achieved even at 600 ° C. or higher.

DH Carter et al., Ceramic Engineering & S
cience Proceedings Vol. 10, No. 9-10, pp. 1121-112
In 9 and 1989, a material obtained by pressure-sintering molybdenum disilicide composited with silicon carbide whiskers has a room temperature bending strength of 15%.
It reports that the bending strength at 1300 ° C. is improved from 0 MPa to about 300 MPa from 70 MPa to 170 MPa. Here, the silicon carbide whiskers are Los Alamos Na
V made at the National Laboratory, Los Alamos, NM, USA
APOR-LIQUID-SOLID (VLS) silicon carbide whisker,
Alternatively, Fuber Corporation's VAPOR-SOLID (VS)
Among silicon carbide whiskers, VLS and SiC whiskers were β-type crystals having a diameter of 5 μm and a length of 100 to 200 μm, and VS SiC whiskers had a diameter of 0.1 μm and a length of 1 to 5 μm.

[0006]

Although the mechanical strength of molybdenum disilicide composited with silicon carbide whiskers has been greatly improved as compared with the mechanical strength of the matrix alone, it is still practically used as a structural member. The reality is that the absolute strength is insufficient and not enough.

Accordingly, an object of the present invention is to provide a molybdenum disilicide composite material having sufficient mechanical strength and fracture toughness at room temperature, sufficient deformation resistance at a high temperature of 1000 ° C. or more, and excellent oxidation resistance. An object of the present invention is to provide a ceramic and a method for producing the ceramic.

[0008]

A molybdenum disilicide-based composite ceramic according to the present invention is a composite ceramic consisting essentially of MoSi2 and SiC whiskers, wherein the MoSi2 matrix has an average diameter and an average length of 0.2. 3 to 40% by volume of SiC whiskers of 1 to 1.0 μm and 2 to 50 μm, and 50% by weight or more of the SiC whiskers have a first SiC or β type crystal structure having an α type crystal structure. 2C = 33.6 ° in X-ray diffraction pattern
And one or two types of second SiC in which a peak at 2θ = 41.4 ° does not appear.

The molybdenum disilicide-based composite ceramics according to a preferred embodiment of the present invention comprises: (a) an SiC whisker having an average diameter and an average length of 0.4 to 0.9 μm, respectively.
and (b) at least 80% by weight of the SiC whiskers are one or two of the first SiC or the second SiC, and (c) the MoSi2 matrix further includes Si3 N4, TiC, ZrC, HfC, T
iB, TiB2, ZrB2, HfB2, ZrO2, Hf
O2, SiC, MoB, Mo2B, MoB2, Mo2B
5, one or two or more compounds selected from the group consisting of WB, W2B, WB2, and W2B5, and (d) combining the one or more compounds with the total capacity of MoSi2 and SiC. Combine 30% by volume or less as 100% by volume,
(E) the one or more compounds are MoB or WB, and (f) the one or more compounds are
The present invention relates to a molybdenum disilicide-based composite ceramic having a composite of 0 vol% or less.

The method for producing a molybdenum disilicide-based composite ceramic according to the present invention comprises the steps of:
oSi2 powder and (b) SiC having an average diameter and an average length of 0.2 to 1.0 .mu.m and 2 to 50 .mu.m, respectively.
Whiskers, 50% by weight of the SiC whiskers
The above is the first SiC having the α-type crystal structure or the SiC having the β-type crystal structure, and 2θ =
A whisker, which is one or two kinds of second SiC in which a peak at 33.6 ° appears and a peak at 2θ = 41.4 ° does not appear, is used at 1400 to 1850 ° C.
This method is characterized in that pressure sintering is performed at a pressure of 50 to 500 kg / cm 2 for 10 minutes to 5 hours.

Hereinafter, the present invention will be described in detail. In the present invention, the average diameter is 0.2 to 1.0 μm and the average length is 2 to 5 to strengthen the whisker of the MoSi 2 matrix.
A 0 μm SiC whisker is used. If the average diameter of the SiC whiskers is less than 0.2 μm, the toughness and strength are not sufficiently improved, and if the SiC whiskers are larger than 1.0 μm, the density of introduced defects becomes high and the strength becomes poor. Further, if the average length of the SiC whiskers, that is, the average length of each whisker for all whiskers, is less than 2 μm, the strength improvement by the whiskers is not sufficient. On the other hand, SiC whiskers having an average length exceeding 50 μm increase the probability of introducing defects into the sintered body, and as a result, the strength of the composite decreases. Preferably, a SiC whisker having an average diameter of about 0.4 to 0.9 μm and an average length of about 10 to 40 μm is used.

Further, in the present invention, it is necessary that at least 50% by weight or more of the SiC whiskers used for reinforcement is the first and / or second whiskers. First
A whisker is SiC having an α-type crystal structure, that is, a hexagonal structure. The second whisker is cubic SiC, and 2θ = 33.6 in the CuKα-X-ray diffraction pattern.
It is assumed that the crystal has a crystal structure in which a peak at ° appears and a peak at 2θ = 41.4 ° does not appear. 1st SiC and 2nd S
Examples of the X-ray diffraction pattern of iC are shown in FIGS. 1 and 2, respectively, and the X-ray diffraction pattern of ordinary βSiC whiskers is shown in FIG. The diffraction condition was a speed of 4 ° / min with CuKα radiation of 40 KVA-30 mA.

If the first and / or second whiskers are less than 50% by weight, no improvement in the toughness and strength of the composite ceramic is observed. Preferably, 80% by weight or more of the SiC whiskers are used as the first and / or second whiskers.

In the present invention, the MoSi 2 matrix is added to the MoS 2 matrix as long as the effect of the SiC whisker is not impaired.
Si3 N4, TiC, ZrC, HfC, TiB, TiB
2, ZrB2, HfB2, ZrO2, HfO2, Si
C, MoB, Mo2 B, MoB2, Mo2 B5, WB,
1 selected from the group consisting of W2B, WB2, and W2B5
Species or two or more compounds can be combined. Borides in these compounds improve oxidation resistance, and
B or WB has a remarkable effect. It is preferable that one or more of these compounds is combined with 30% by volume or less, particularly 10% by volume or less, with the total volume of MoSi2 and SiC being 100%.

When a raw material powder having a size of 2 μm or more is used as the above compound, the composite ceramics grows grains before being densified by sintering, resulting in poor mechanical strength. Preferably, a raw material powder having an average particle diameter of 1 .mu.m or less is used, especially for MoSi2. The composition of each component is as follows. That is, Mo
The sum of Si2 ceramics and SiC whiskers is 1
3% to 40% by volume of SiC, with the balance being MoSi
2 series ceramics. 3% SiC whiskers
If it is less than 10, the toughness and strength of the composite ceramics are not sufficiently improved. On the other hand, when the SiC whisker is added in an amount exceeding 40% by volume, defects are introduced into the sintered body and the strength is reduced. Preferably, it is 10 to 30% by volume. The relative density of the sintered body is preferably 92% or more, particularly 97% or more, from the viewpoint of mechanical strength.

Next, a method for producing the MoSi 2 —SiC whisker composite ceramics will be described. First, when MoSi2 as a raw material and the second to n-th phases are simultaneously composited, Si3N4, TiC, ZrC, HfC, TiB, T
iB2, ZrB2, HfB2, ZrO2, HfO2, S
iC, MoB, Mo2 B, MoB2, Mo2 B5, W
Particles such as B, W2 B, WB2, W2 B5
A predetermined amount of the SiC whiskers having the structure is weighed and mixed. This mixing is preferably performed sufficiently by a ball mill or the like, for example, for 48 hours or more. When starting from raw material powders having an average particle size of 2 μm or more, these raw material powders may be first sufficiently pulverized with an attritor or the like, and then SiC whiskers may be added and mixed with a ball mill.

After the obtained mixed powder is dried, it is placed in a mold having a desired shape, and is subjected to pressure sintering at 1400 to 1850 ° C. for 10 minutes to 5 hours while applying a pressure of 50 to 500 kg / cm 2. Here, if the pressure during sintering is less than 50 kg / cm 2, the sintered body will not be sufficiently densified and improved mechanical strength cannot be obtained. Further, even if the pressure exceeds 500 kg / cm2, there is no difference in the effect, so the upper limit is set to 500 kg / cm2.
And On the other hand, if the sintering temperature is lower than 1400 ° C., densification of the composite ceramics cannot be achieved. When sintering at a temperature exceeding 1850 ° C., MoSi 2
Reacts with inevitable impurities to lower the melting point, causing a bubbling phenomenon and making it impossible to obtain a dense sintered body. On the other hand, if the sintering time is less than 10 minutes, densification of the composite ceramics cannot be achieved. On the other hand, since the sintering time exceeding 5 hours is not practical, the upper limit is set to 5 hours.

[0018]

[Function] PF Becher et al. Described the Journal of American Ceramic Society, Vo.
1, No. 12, pp. 1051-1061, 1988, the increase in fracture toughness ΔK of whisker reinforced ceramics (composite) is generally ΔK = σf [Vf · r · Ec · Gm / 6. (L-ν2) E
w · Gi] 1/2. Where σf is the whisker breaking strength, Vf is the whisker volume fraction, r is the whisker radius, Ec is the composite Young's modulus, Gm is the matrix strain energy release rate, v is the Poisson's ratio of the composite, Ew is the whisker's Young's modulus, and Gi is the whisker / matrix interface strain energy release rate.

According to this equation, if the mixing amount of the whiskers is the same in volume ratio, the fracture strength of the whiskers is high, and the fracture toughness of the ceramic composite is improved as the whisker diameter increases. However, in practice, defects may exist inside whiskers, so as described in A. Kelly's "Strong Solids", Oxford University Press, 1966, whiskers with smaller diameters have more defects inside. Is expected to be small, and therefore the strength is also expected to be large. Also, as noted above, high strength whiskers have an essentially smooth surface. Comparing the two crystal forms of α-type (hexagonal) and β-type (cubic) in SiC whiskers, α-type whiskers growing in the c-axis direction tend to have a smoother surface. Also, the β-type has many stacking faults (irregularities) in the growth direction (2θ = 41.appearing in a normal β-type crystal in an X-ray diffraction pattern).
(The peak at 4 ° does not appear but the peak at 2θ = 33.6 ° appears), and it has been observed that the straightness is excellent and the surface becomes smooth. Such whiskers have higher strength than whiskers having a normal β-type crystal structure.

As a result of intensive studies in consideration of the above points, the present inventors have found that molybdenum disilicide-based ceramics
A system into which C whiskers are introduced is selected. As the SiC whiskers, those having high fracture strength of the SiC whiskers from the viewpoint of strengthening the whiskers, that is, basically those of α-type crystals or many mismatches even in β-type The whisker diameter was also selected from the viewpoint of improving the fracture toughness and the probability of internal defects from 0.2 to 1.0 μm.

Further, in the production of molybdenum disilicide ceramics, if the sintering conditions are precisely controlled to obtain a dense ceramic structure, it is possible to obtain a molybdenum disilicide composite ceramic having excellent mechanical strength. They discovered what they could do and came up with the method of the present invention. The present invention will be described in more detail with reference to the following specific examples.

[0022]

【Example】

Example 1 (jl) MoSi2 powder having a purity of 99% and an average particle diameter of 0.8 μm was 7
5% by volume, average diameter 0.5 μm, average length 30 μm, α
Si with a type crystal ratio (α / (α + β)) of 85% by weight
The mixed powder mixed so that C whisker (SiCw) became 25% by volume was wet-mixed with a ball mill for 72 hours and dried.

Using the obtained mixed powder, at a temperature of 1700 ° C. while applying a pressure of 300 kg / cm 2 in an argon gas stream.
For 1 hour to obtain a sintered body of 50φ × 5 mm.
From the obtained sintered body, a test piece of 3 × 4 × 40 mm and a test piece of 1.5 × 4 × 25 mm were collected, and 3 × 4 × 40 mm.
mm test piece, a three-point bending test (span 30 mm, crosshead speed 0.5 mm / min) and a fracture toughness test (indentation method according to Niihara's formula) were performed at 1.5 ×
For a 4 × 25 mm specimen, a 4-point bending test with a crosshead speed of 0.03 mm / min, an upper span of 10 mm and a lower span of 20 mm was performed at room temperature, 1000 ° C., and 1300 ° C. using a strain gauge capable of measuring up to 0.1 μm. And load /
The displacement and the Young's modulus at a load of 5 N were measured from the displacement curve. Table 1 shows the results.

[0024]

[Table 1] Example 3 points Fracture Displacement Young's modulus (J) Bending strength Toughness (Load / 5N) GPa μm MPa MPam1 / 2 RT 1000 ℃ 1300 ℃ RT 1000 ℃ 1300 ℃ Jl. MoSi2 670 4.8 2.0 2.8 6.3 338 196 104 / 25SiCw

Example 2 (J2) The average diameter was 0.8 μm and the average length was 20 μm, and a peak at 2θ = 33.6 ° appeared in the X-ray diffraction pattern.
Except for using a SiC whisker (second SiC) having a β-type crystal structure in which a peak at θ = 41.4 ° does not appear,
A composite ceramic was produced in the same manner as in Example 1. The strength, fracture toughness, displacement, and Young's modulus of this composite ceramic were measured in the same manner as in Example 1. Table 2 shows the results
Shown in

[0026]

[Table 2] Example 3 points Fracture Displacement Young's modulus Bending strength Toughness (Load / 5N) GPa μm MPa MPam1 / 2 RT 1000 ℃ 1300 ℃ RT 1000 ℃ 1300 ℃ J2. MoSi2 590 4.5 1.8 3.0 5.8 356 210 112 / 25SiCw

Comparative Example 1 (Hl) For comparison, MoS having a purity of 99% and an average particle diameter of 0.8 μm was used.
A sintered body was prepared using i2 powder alone in the same manner as in Example 1, and the strength, fracture toughness, displacement, and Young's modulus were measured. Table 3 shows the results.

[0028]

[Table 3] Comparative example 3 points Fracture Displacement Young's modulus (H) Bending strength Toughness (Load / 5N) GPa μm MPa MPam1 / 2 RT 1000 ℃ 1300 ℃ RT 1000 ℃ 1300 ℃ Hl.MoSi2 240 1.7 2.1 3.9 13.5 290 146 38

Examples 3-4 (J3-J4) and Comparative Examples 2-3
(H2-H3 ) MoB2 powder having a purity of 98% and an average particle size of 1.2 μm, or a MoSi2 matrix having a particle dispersion strengthened by using a WB powder having a purity of 99% and an average particle size of 0.9 μm,
In the same manner as in Example 1, a SiC whisker-reinforced composite ceramic was produced. Here, the content of MoB or WB was 10% by volume, and the content of SiC whiskers was 25% by volume. Therefore, MoSi2 is 65% by volume. As for the characteristic values, the strength, fracture toughness, displacement, and Young's modulus were measured as in Example 1. For comparison, MoSi2 composite ceramics not containing SiC whiskers and strengthened in particle dispersion were similarly prepared and subjected to the same property evaluation. Table 4 shows the results.

[0030]

[Table 4] Example, 3 points Breaking Displacement Young's modulus Comparative example Bending strength Toughness (Load / 5N) GPa μm MPa MPam1 / 2 RT 1000 ° C 1300 ° C RT 1000 ° C 1300 ° C J3. MoSi2 700 5.5 1.9 2.4 3.6 370 225 126 126 / MoBi / SiCw J4. MoSi2 650 5.2 2.0 2.5 3.9 410 240 135 / WB / SiCw H2. MoSi2 530 3.9 1.8 2.9 10.2 375 204 54 / MoB H3. MoSi2 490 3.5 2.1 3.1 9.5 403 210 69 / WB

Examples 5 to 7 (J5 to J7) and Comparative Examples 4 to 9
(H4 to H9) MoSi2 / MoB / Si in the same manner as in Example 3 except that the average diameter of SiC whiskers, the ratio of crystal form, and the blending amount of SiC whiskers were as shown in Table 5.
C whisker composite ceramics were produced.

[0032]

[Table 5] Example MoSi2 MoB SiC whisker blending blending blending average diameter α / (α + β) (volume%) (volume%) (volume%) (μm) (%) J5 80 10 10 0.5 85 J6 70 10 10 20 0.5 85 J7 60 10 30 0.5 85 H4 88 10 20.5 85 H5 45 10 45 0.5 85 H6 65 10 25 1.5 90 H7 65 10 25 0.60 H8 65 10 25 1.8 0 H9 65 10 25 0.4 30

The obtained composite ceramics were subjected to the three-point bending test and the fracture toughness test of Example 1. Table 6 shows the results.

[0034]

[Table 6] Example 3-point bending strength Fracture toughness Remarks MPa MPam1 / 2 J5 584 5.1-J6 650 5.4-J7 720 5.8-H4 512 4.0 Low SiC content H5 550 6.0 SiC content H6 560 6.2 SiC has a large average diameter of SiC H7 620 4.6 SiC has only β structure H8 490 4.8 SiC has only β structure H9 605 4.2 High β ratio, low α ratio

Examples 8 to 10 (J8 to J10) and Comparative Example 1
0 to 15 (H10 to H15) A composite ceramic was produced under the same composition and conditions as in Example 1 except that pressure sintering was performed under the conditions shown in Table 7. The composite ceramic thus obtained was subjected to the three-point bending test and the fracture toughness test of Example 1. The results are shown in Table 7.

[0036]

[Table 7] Example Mechanical properties of pressure sintering Temperature Pressure time Three-point bending strength Fracture toughness Remarks (° C) (kg / cm2) (hour) MPa MPam1 / 2 J8 1450 450 1595 4.9-J9 1600 200 1650 5.1-J10 1800 450 1 686 4.7-H10 1350 450 1 220 1.5 Low temperature sintering H11 1900 450 1 137 1.1 High temperature sintering H12 1700 450 0.05 380 3.9 Short time firing Conclusion H13 1700 450 6 574 4.8 Long time sintering H14 1700 30 1 290 3.4 Low pressure sintering H15 1700 600 1 699 5.0 High pressure sintering

[0037]

As described above, the molybdenum disilicide-based composite ceramic according to the present invention has excellent mechanical strength at room temperature and deformation resistance at high temperature, and can be used as a high-temperature structural member.

[Brief description of the drawings]

FIG. 1 is an X-ray diffraction pattern diagram of SiC having an α-type crystal structure.

FIG. 2 shows a peak at 2θ = 33.6 °, and 2θ = 4
FIG. 7 is an X-ray diffraction pattern diagram of SiC having a β crystal structure in which a peak at 1.4 ° does not appear.

FIG. 3 is an X-ray diffraction pattern diagram of SiC having a normal β-type crystal structure.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hiroshi Yamashita 4-14-1, Suehiro, Kumagaya-shi, Saitama Pref.

Claims (9)

[Claims] 1. A composite ceramic consisting essentially of MoSi2 and SiC whiskers, wherein the MoSi2 matrix has an average diameter and an average length of 0.2 to 2.0, respectively.
3 to 40% by volume of SiC whiskers of 1.0 μm and 2 to 50 μm, and at least 50% by weight of the first SiC whiskers have an α-type crystal structure.
C or SiC having a β-type crystal structure, and a peak at 2θ = 33.6 ° appears in the X-ray diffraction pattern, and 2θ = 4
A molybdenum disilicide-based composite ceramic, which is one or two types of second SiC in which a peak at 1.4 ° does not appear. 2. The SiC whiskers have an average diameter and an average length of 0.4 to 0.9 μm and 10 to 40, respectively.
The molybdenum disilicide-based composite ceramic according to claim 1, wherein the thickness of the composite ceramic is μm. 3. The molybdenum disilicide-based composite ceramic according to claim 1, wherein 80% by weight or more of the SiC whisker is one or two of the first SiC and the second SiC. 4. The MoSi2 matrix further comprises S
i3 N4, TiC, ZrC, HfC, TiB, TiB2
, ZrB2, HfB2, ZrO2, HfO2, SiC,
MoB, Mo2 B, MoB2, Mo2 B5, WB, W2
The molybdenum disilicide-based composite ceramics according to any one of claims 1 to 3, wherein one or more compounds selected from the group consisting of B, WB2, and W2B5 are compounded. 5. The total of MoSi2 and SiC is 10
Assuming that 0% by volume, the one or more compounds are used in 30
The molybdenum disilicide-based composite ceramic according to claim 4, wherein the composite is not more than a volume%. 6. The method according to claim 1, wherein the one or more compounds are MoB.
6. The molybdenum disilicide-based composite ceramic according to claim 5, wherein the ceramic is WB. 7. The molybdenum disilicide-based composite ceramic according to claim 5, wherein the one or more compounds are compounded in an amount of 10% by volume or less. 8. (a) MoSi2 having an average particle size of 2 μm or less.
Powder and (b) the average diameter and average length are each 0.2
-1.0 μm and 2-50 μm, wherein at least 50% by weight of the SiC whiskers is α.
X-ray diffraction pattern 2θ = 33.6 °, which is first SiC having a type crystal structure or SiC having a β type crystal structure.
And a whisker which is one or two types of second SiC in which a peak of 2θ = 41.4 ° does not appear and a peak of 2θ = 41.4 ° is used.
A method for producing a molybdenum disilicide-based composite ceramics, wherein pressure sintering is performed at a pressure of 00 kg / cm2 for 10 minutes to 5 hours. 9. The raw material powder further comprises (c) Si3 N4
, TiC, ZrC, HfC, TiB, TiB2, Zr
B2, HfB2, ZrO2, HfO2, SiC, Mo
B, Mo2 B, MoB2, Mo2 B5, WB, W2 B, W
9. The method for producing a molybdenum disilicide-based composite ceramic according to claim 8, comprising one or more selected from the group consisting of B2 and W2 B5.