JP2892248B2 - Silicon nitride based composite ceramics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to fracture toughness, fracture resistance,
And a composite ceramic sintered body having excellent high-temperature fracture characteristics.

[0002]

2. Description of the Related Art Silicon nitride ceramics are expected to be applied to structural members for gas turbines, automobile engines, etc. because of their high breaking strength at high temperatures and excellent thermal shock resistance. However, it also has the brittleness characteristic of ceramics, so it improves the fracture toughness value, which is the critical stress intensity factor when a crack occurs, and the fracture resistance that prevents the growth of a crack once it has occurred. Is an issue in the field. In particular, it is desired to improve the fracture toughness value without impairing the fracture strength.

[0003] With respect to the improvement of the toughness value, the silicon nitride particles are grown in a fibrous form by promoting the change from the α phase to the β phase.
2. Description of the Related Art It has been practiced to produce a composite material characteristic by generating an intermeshing effect between particles. One of the methods is a gas pressure sintering method at a temperature exceeding the decomposition temperature of silicon nitride. Table 1 shows the mechanical properties of the samples produced by this method.

[0004]

[Table 1]

[0005] In the samples shown in Table 1, 95% or more of α
Type silicon nitride powder as a starting material, Y ₂ O ₃
-Al ₂ O ₃ -based sintering additive is added,
It was produced by a hot press sintering method in an N ₂ gas atmosphere of atm. As shown in Table 1, the fracture toughness value K _IC (according to the SEPB method) is improved as the sintering time is increased. It is thought to be due to the meshing effect.

[0006] As another means for improving the toughness value, it has been widely practiced to add a whisker into a composite material. This is to improve the characteristics by the effect of crosslinking and pulling out the whiskers near the crack tip, and as shown in FIG. 2, the breaking energy increases as the added amount of the whiskers increases. In addition to increasing the fracture energy, when whiskers are added, the effect of suppressing the growth of cracks in the sintered body as shown in FIG. 3 and the improvement of high-temperature strength by suppressing grain boundary sliding at high temperatures The effect of doing it appears. Attempts have also been made to strengthen the composite of one-dimensional particles such as whiskers by plate-like particles that are two-dimensional particles. Plate-like particles have a greater effect of increasing the breaking energy and breaking resistance than whiskers, and as shown in FIG. 4, the effect increases with the addition amount.

[0007]

As described above, various techniques have been used to improve the fracture toughness value of ceramics. However, when the phase change is promoted by a gas pressure sintering method, As shown in Table 1, the strength decreases with an increase in the sintering time.
Even when whiskers are blended, if the base material particles grow into needles more than or equal to the whiskers, the additive effect is small as shown in FIG. Further, when plate-like particles are blended, there is a problem that the fracture strength of ceramics is greatly reduced as shown in FIG.

[0008] Regardless of whether whiskers or plate-like particles are used, if the compounding ratio is 30% by weight or more of the sintered body, it becomes difficult to obtain a dense sintered body, and local particles are not obtained. Agglomeration of the particles degrades the fracture characteristics.

As described above, in the case of a material based on silicon nitride, even when whiskers or plate-like particles are added to form a composite material, their additive effects are satisfied in that both strength, toughness and high-temperature properties are satisfied. Not enough.

Accordingly, an object of the present invention is to provide a silicon nitride-based composite ceramic having high fracture toughness and fracture resistance and maintaining high strength and creep resistance even at high temperatures.

[0011]

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention is characterized in that it contains silicon nitride, silicon carbide whiskers, and silicon carbide plate-like particles and is manufactured by a gas pressure sintering method. To provide a silicon nitride-based composite ceramic.

[0012]

The silicon nitride composite ceramic of the present invention contains silicon carbide whiskers and silicon carbide plate-like particles. By including these one-dimensional particles (whiskers) and two-dimensional particles (plate-like particles), the ceramic sintered body has a higher-order composite structure. That is, the whiskers suppress the extreme grain growth of the silicon nitride particles as the base material to suppress the decrease in strength, and, together with the plate-like particles, act as a resistance to grain boundary slip caused by grain boundary softening at high temperatures. . In addition, this complexing complicates the crack propagation path and suppresses a decrease in strength at room temperature.
Furthermore, by performing gas pressure sintering to express an interlocking effect, it has become possible to improve fracture characteristics and fracture resistance.

The following was suggested as a combined effect when whiskers were added. Table 2 shows the mechanical properties of a composite material obtained by hot-press sintering a silicon nitride powder containing 20% by weight of silicon carbide whiskers under the same conditions as when the samples shown in Table 1 were prepared. . At this time, 95% or more of the silicon nitride as a starting material was α-type. Comparison with the results shown in Table 1 shows that although the fracture toughness value is not improved, the decrease in fracture strength due to the increase in sintering time due to the whisker composite effect is small.

[0014]

[Table 2]

As described above, the whisker improves the toughness by suppressing a decrease in strength due to the gas pressure sintering method. In the present invention, a high fracture toughness value, high fracture resistance and high high-temperature strength can be imparted to the silicon nitride ceramics by further adding plate-like particles.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for producing a silicon nitride-based composite ceramic of the present invention will be described in the order of steps.

The raw material used for producing the silicon nitride-based composite ceramic of the present invention is a mixed powder obtained by mixing silicon carbide whisker and silicon carbide plate-like particles with silicon nitride powder and adding a sintering aid. As silicon nitride powder,
Although any of α-type and β-type may be used, the maximum particle diameter is preferably 10 μm or less and the aspect ratio is preferably 20 or less when the sintered body was prepared, and the diameter is preferably 5 μm or less and the aspect ratio is 10 or less. More preferably, there is.

The silicon carbide whiskers have a diameter of 0.5 to 5
μm and an aspect ratio of 10 or more, more preferably 1 to 2 μm in diameter and an aspect ratio of 20 or more. The amount of the whisker is preferably less than 30% by weight, more preferably 5 to 20% by weight of the whole mixed powder. When the content is 30% by weight or more, it becomes difficult to obtain a dense sintered body, and the fracture characteristics are deteriorated due to local aggregation of particles and the like.

The silicon carbide plate-like particles have a diameter of 50 μm.
m and a thickness of preferably 0.1 to 5 μm, more preferably a diameter of 20 μm or less and a thickness of 1 to 2 μm. The amount of the plate-like particles is preferably less than 30% by weight, more preferably 5 to 20% by weight of the whole mixed powder for the same reason as in the case of the whisker.

As the sintering aid, Y ₂ O ₃ , Al
A multi-component oxide containing at least _two such as ₂ O ₃ , MgAl ₂ O ₄ , MgO, and TiO ₂ can be used. Among them, Y ₂ O ₃ —Al ₂ O ₃ or Y ₂ O ₃ −
MgAl ₂ O ₄ is preferred. The compounding amount of the auxiliary agent is preferably less than 20% by weight of the whole mixed powder, more preferably 1 to 10% by weight. The above powders are uniformly wet-mixed with a ball mill, then dried and sieved to prepare a mixed powder.

The mixed powder is placed in a mold and cold-pressed to form a preform. In this case, the mechanical properties in the direction perpendicular to the orientation plane can be further enhanced by orienting the whiskers and plate-like particles using, for example, an extrusion molding method.

Thereafter, hot press sintering is performed in an N ₂ atmosphere to obtain a silicon nitride-based composite ceramic. In addition, hot press sintering is performed in a temperature range of 1800 to 2300 ° C, preferably 1800 to 2000 ° C.
The pressure is increased to 100 atm, preferably 5 to 9.9 atm. If it is less than 2 atm, at a temperature exceeding 1900 ° C., silicon nitride is decomposed according to the following formula (1). Si ₃ N ₄ (s) → 3Si (l) + 2N ₂ (g) (1) Hereinafter, specific examples and comparative examples of the present invention will be shown, and effects of the present invention will be specifically described.

First, a submicron-sized α-type or β-type
A silicon carbide whisker and a silicon carbide plate-like particle having a compounding amount shown in Table 3 below are mixed with any one of the silicon nitride powders of the mold, and a sintering aid is further added thereto, followed by mixing, drying and sieving. And three types of mixed powders were prepared. The diameter and aspect ratio of the blended whiskers were about 1 μm and 30 to 40, respectively, and the diameter and thickness of the plate-like particles were 30 μm or less and 1 to 2 μm, respectively. In addition, as a sintering aid, 5% by weight of Y ₂ O ₃ + 2% by weight of Al ₂ O
Either ₃ or 5 wt% Y ₂ O ₃ +5 wt% MgAl ₂ O ₄ was used.

[0024] After these mixed powder was cold pressed molding placed in a mold, N ₂ atmosphere of 8 atm, 2000
The sintered bodies obtained by hot press sintering at 2 ° C. for 2 hours were designated as Samples A, B and C, respectively.

As a comparative example, a sample was prepared by hot press sintering only silicon nitride powder without adding any of whiskers and plate-like particles. A sample obtained by subjecting this powder to hot press sintering under the same conditions as described above was used as sample D.
Sample E was obtained by hot press sintering at 70 ° C. in a normal pressure N ₂ gas atmosphere. Furthermore, the sample was subjected to hot press sintering at 1770 ° C. under a normal pressure of N ₂ gas atmosphere using mixed powder having the same compounding ratio as that of samples A to C (samples F to H)
Was also prepared. Samples E to H sintered at 1770 ° C
The sintering time was 1 hour.

[0026]

[Table 3]

Each sintered body was processed into a test piece of 4 mm × 3 mm × 40 mm, and subjected to a bending fracture test in accordance with JIS R1601, SEPB (Single Edge Pre-c).
A fracture toughness test by a racked beam method and a measurement of fracture energy by a CN (Chevron Notch) method were performed. The results are shown in Table 4 together with the density of each sample.

[0028]

[Table 4]

From Table 4, it can be seen that all of the prepared samples are sufficiently densified. Although the breaking strength of the sample of the comparative example tends to decrease as the added amount of the whiskers and plate-like particles increases, the ratio of the sample of the present invention is small. This is probably because the whisker addition described above suppressed the grain growth of silicon nitride.
The strength of the ceramic sintered body (samples A to C) of the present invention is as follows.
Silicon nitride single material sample sintered for 5 hours under gas pressure (Table 1)
Medium, about the same as sample 2), but the fracture toughness value was increased. This is the silicon nitride particles that are the base material,
It is thought to be due to the synergistic effect of addition of whiskers and plate-like particles. Also, a large increase in the breaking energy was observed according to the amount of the composite particles added. Further, as a result of observing the fracture surface, a large deflection of the crack propagation path was confirmed as compared with the conventional material, and it was found that the addition of the composite particles increased the fracture resistance. On the other hand, in the case of the sample to which whiskers and plate-like particles were not added, the fracture energy was inferior in the sample D subjected to gas pressure sintering, the sintering temperature and sintering time were reduced, and sintering was performed at normal pressure. In sample E, the fracture toughness value and the fracture energy are reduced. Further, Samples F and G sintered under the same conditions as Sample E are:
The fracture toughness value and the fracture energy are reduced, and in Sample H, the bending strength is significantly reduced.

Next, the sintering temperature was lowered by 100 ° C. to 1900
A sample (C-2) was prepared under the same conditions as in Sample C, except that the sintering time was changed to 2 hours, and the SEPB fracture toughness test was performed. As a result, 11.1 MPam ^{1/2 which} is equivalent to the fracture toughness value of Sample C sintered at 2000 ° C. Was obtained.

FIG. 1 shows the ISB (Ind) of the sample (C-2).
entation Strength in Bend
2) shows the evaluation results (curve a) of the crack growth resistance (R curve) by the ing method. FIG. 1 also shows a crack growth resistance curve (curve b) of a silicon nitride single material (sample No. 3 in Table 1) gas-sintered at 2000 ° C. for 10 hours. From FIG. 1, the crack growth resistance of these gas-pressure sintered materials is shown in FIG.
It is understood that the result is sufficiently larger than the result of the conventional material shown in FIG.

That is, the material of the present invention is a highly reliable material that is less sensitive to the presence of cracks, and even if the sintering temperature is lowered and the sintering time is shortened. It is clear that an effect equal to or greater than that of a material grown by giant particles can be expected.

The sample (C-2) was measured in air
As a result of conducting a bending fracture test at 1250 ° C.,
A three-point bending strength of Pa was obtained. From these results, it is considered that the silicon nitride particles, whiskers, and plate-like particles that have grown greatly play a role in suppressing the slip due to the softening of the grain boundary phase at high temperatures, and not only improve the high temperature strength but also improve the high temperature creep characteristics. Can also be expected.

Further, a composite prepared under the same sintering conditions as the sample (C-2), except that the sample (C-2) and a silicon carbide whisker whose surface is coated with TiO ₂ in the order of several tens of angstroms were used. Sintered body (sample (C-3))
With respect to and, the room temperature strength and the initial fracture resistance value (toughness value determined by the SENB method) were measured. The results obtained are shown in Table 5 below.

[0035]

[Table 5]

As shown in Table 5, the room temperature strength and the initial fracture resistance of the sample (C-2) were 757 MPa, respectively.
And 8.8MPam ^1/2 Whereas the sample (C-
The room temperature strength and the initial fracture resistance of 3) were 789, respectively.
MPa and 9.2MPam ^1/2 Met. As described above, when the surface of the silicon carbide whisker was coated with TiO ₂ , both the strength and the initial fracture resistance were improved. Note that the breaking resistance of the sample (C-3) by the SENB method was 10.7 MPam ^{1/2, which} is equivalent to the value of the sample (C-2).
Met. The improvement in mechanical properties as described above is due to Ti
It is considered that silicide is formed at the interface with the silicon carbide reinforcing fiber to form a strong bond.

Elements having the same effect include:
Cr, Mo, Ni, W, Al and the like.
172735). Further, by increasing the thickness of the interface layer, further improvement in strength and initial fracture resistance can be expected.

On the other hand, by aligning the orientation of the whiskers, the strength and toughness in a certain direction can be increased. In the present material system, whiskers and plates can be formed at the stage of the molding process, for example, by extrusion / sheet lamination. The particles can be oriented to further improve the properties.

[0039]

As described in detail above, according to the present invention,
Fracture toughness due to the interlocking effect between silicon nitride particles grown by gas pressure sintering and the effect of blocking stress near crack edges by whiskers, one-dimensional particles, and plate-like particles, two-dimensional particles It has become possible to obtain a silicon nitride-based composite ceramic having improved values and fracture resistance during crack propagation.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing crack propagation resistance of a silicon nitride-based composite ceramic according to the present invention.

FIG. 2 is a characteristic diagram showing the fracture energy of a whisker-reinforced silicon nitride-based composite ceramic as a conventional example.

FIG. 3 is a characteristic diagram showing crack propagation resistance of a whisker-reinforced silicon nitride-based composite ceramic as a conventional example.

FIG. 4 is a characteristic diagram showing the fracture energy of a conventional plate-like particle-reinforced silicon nitride-based composite ceramic.

FIG. 5 is a characteristic diagram showing a fracture toughness value of a conventional whisker-reinforced silicon nitride-based composite ceramic.

FIG. 6 is a characteristic diagram showing the fracture strength of a conventional plate-like particle-reinforced silicon nitride-based composite ceramic.

[Explanation of symbols]

 a: Characteristic curve of crack growth resistance of Sample C-2 of the present invention, b: Characteristic curve of crack growth resistance of Sample 3 of the conventional example.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C04B 35/584 C04B 35/80

Claims (1)

(57) [Claims] 1. A silicon nitride-based composite ceramic containing silicon nitride, silicon carbide whiskers, and silicon carbide plate-like particles and produced by a gas pressure sintering method.