JP2742621B2 - High toughness silicon nitride sintered body - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a silicon nitride sintered body having a high toughness, and more specifically, a material suitable for various machine parts and structural parts including tools. About.

(Prior Art) A silicon nitride sintered body has been conventionally applied to various mechanical parts because of its excellent strength, hardness and thermal and chemical stability.

However, the silicon nitride-based sintered body has a problem that the toughness is low although the bending strength is excellent.

Therefore, conventionally, attempts have been made to further increase the toughness value of this silicon nitride-based sintered body and further expand its application field.

In increasing the toughness of a silicon nitride based sintered body, Lange et al.
It has been reported that the more α-Si ₃ N ₄ in the primary material, the higher the fracture toughness value (Am, Ceram, Soc, Bull., 62, 1369, 1
983). Faber et al. Report that the toughness value increases as the aspect ratio of silicon nitride crystal grains increases (Acta, Metall., 31, 565-576, 577-584, 1983).

Another method is to add silicon nitride to Periodic Table IVa, V
A method is adopted in which hard particles such as carbides, silicides or borides of Group a and VIa are dispersed in a sintered body, and the clutch is deflected by the hard particles.

(Problems to be Solved by the Invention) However, in the method of increasing the toughness by making the silicon nitride particles themselves acicular, the effect of improving the toughness is limited,
In addition, although the improvement in toughness is certainly recognized as compared with the hard particle dispersion system not mixed at all, the toughness value is greatly affected by the grain boundary phase of the silicon nitride particles as the base material, so depending on the manufacturing conditions, There is a problem that the toughness value tends to fluctuate and the original effect of adding hard particles is not sufficiently exhibited.

(Objects of the Invention) Accordingly, the present invention has a main object of solving the above problems, and specifically, to improve the properties of the base material and exhibit the effect of improving the inherent toughness of the hard particle dispersion system. It is an object of the present invention to provide a high toughness silicon nitride sintered body that can be obtained.

(Means for Solving the Problems) As a result of repeated studies on the above problems, the present inventors have determined that the toughness value of the sintered body is determined by the compressive stress applied to the silicon nitride crystal grains, and the compressive stress is dispersed. The present invention has been found to be greatly affected by the thermal expansion coefficient of the hard particles and the composition of the grain boundaries of silicon nitride.

That is, the present invention mainly comprises silicon nitride crystal particles,
It is composed of hard particles having a coefficient of thermal expansion of at least twice the coefficient of thermal expansion of silicon nitride by volume and a grain boundary phase composed of silicon-oxygen-nitrogen-rare earth element, and acts on the silicon nitride crystal particles. The compressive stress is 40MPa or more, the bending strength at room temperature is 800MPa or more, and the toughness is 8MPa ・ m.
^An object of the present invention is to provide a high-toughness silicon nitride-based sintered body characterized by being ^1/2 or more.

 The present invention will be described in more detail.

The silicon nitride-based sintered body of the present invention basically has a structure in which hard particles are dispersed in a base material composed mainly of silicon nitride crystal grains and a grain boundary phase.

The hard particles have a coefficient of thermal expansion of at least twice the coefficient of thermal expansion of silicon nitride.
× As 10 ^-6 / ° C., using a 6 × 10 ^-6 / ° C. or more hard particles. Further, it is necessary that the hard particles do not melt during firing at a high temperature, and their melting points are required to be higher than the firing temperature. Such hard particles include NbSi ₂ , TaSi ₂ , WS
_{i 2, MoSi 2, Nb 4} Si, Ta 5 Si 3, Ta 2 Si, but _{Mo 3 Si 2, Cr 3 C} 2 and the like, this is commonly SiC, known as hard particles smaller coefficient of thermal expansion Although unsuitable in the invention, it may be used in combination with the hard particles. Further, ZrSi _{2 and the} like have a melting point of about 1500 ° C., which is also unsuitable for the present invention.

As described above, the hard particles having a higher thermal expansion coefficient than silicon nitride and having a high melting point are used, and by firing the particles, a compressive stress acts on the silicon nitride particles due to a difference in thermal expansion between the silicon nitride and the hard particles. Thereby, the toughness value of the sintered body can be increased. The larger the difference between the thermal expansion of the hard particles and silicon nitride, the greater the compressive stress. However, since the silicon nitride particles need to withstand the compressive stress sufficiently, the thermal expansion coefficient of the hard particles is particularly high. Is preferably 2 to 5 times.

It is desirable that the hard particles occupy 5 to 30% by volume, especially 5 to 20% by volume in the sintered body. If this ratio is less than 5% by volume, the compressive stress acting on silicon nitride is small and the toughness is low. When the content exceeds 30% by volume, the toughness is improved, but the bending strength tends to decrease.

Further, according to the present invention, the grain boundary phase in the base material is silicon-
It is also important to use oxygen-nitrogen-rare earth elements to exert a large compressive stress on silicon nitride particles. Due to the high melting point of the grain boundary phase made of such a specific element, stress acts on the silicon nitride particles from a high temperature, whereby the stress on the silicon nitride particles can be increased. Specifically, it is preferable that the grain boundary phase, that is, the molar ratio of SiO ₂ / rare earth element oxide of the sintering aid is 1 or more. Here, the amount of SiO ₂ is the total amount of the amount of impurity oxygen in silicon nitride in terms of SiO ₂ and the SiO ₂ powder externally added as required.

On the other hand, conventionally used Al ₂ O ₃ , MgO, Y ₂ O ₃
-If Al ₂ O ₃ or the like is used as an auxiliary, these oxides remain at the grain boundaries and greatly reduce the melting point of the grain boundaries themselves, so that a large compressive stress cannot be applied to the silicon nitride particles. Is desirably substantially absent except when unavoidably mixed as an impurity, and the total amount is desirably 0.5% or less.

In addition, Y is generally used as the rare earth element used in the present invention, but in the present invention, heavy rare earth elements such as Yb, Er, Ho, and Dy are most preferable in terms of increasing the strength of the sintered body itself.

Such a grain boundary phase accounts for 1 to 30% by volume, particularly 1 to 30% by volume in the sintered body.
Desirably, it is present at a ratio of 20% by volume, and even if the ratio of the grain boundary phase is less than 1% by volume or exceeds 30% by volume, the stress acting on the silicon nitride particles tends to be small, and improvement in toughness tends not to be expected.

According to the present invention, a large compressive stress is applied to silicon nitride particles by selecting the specific hard particles and the specific grain boundary phase described above.
A silicon nitride-based sintered body having a stable and high toughness value can be obtained by setting it to at least a, particularly at least 80 MPa.

When producing the silicon nitride sintered body of the present invention, in addition to the rare earth element oxide powder described above as an auxiliary component (grain boundary phase forming component) and silicon dioxide as an auxiliary component (silicon nitride) as a base material, in some cases, SiO ₂ may be used.
The powder is prepared so that the ratio of the grain boundary phase in the final sintered body becomes the above-mentioned ratio. On the other hand, hard particles having a coefficient of thermal expansion of at least twice that of silicon nitride are mixed with the base material component.

The silicon nitride powder used at this time may be any of α-type, β-type or a mixture thereof, and the particle size is preferably 0.2 to 1 μm from the viewpoint of sinterability.

It is desirable to use hard particles having a particle size in the range of 0.5 to 10 μm from the viewpoints of applying stress to silicon nitride and strength.

The mixture prepared as described above is a desired molding method,
For example, after molding by means such as press molding, extrusion molding, injection molding, and casting molding, firing is performed.

Firing is performed at 1600 to 1850 in a non-oxidizing atmosphere containing nitrogen gas.
C. is appropriate, and if it exceeds 1850.degree. C., the strength decreases, and a strength of 800 MPa or more cannot be obtained. The nitrogen gas is set to a pressure equal to or higher than the equilibrium decomposition pressure of silicon nitride at the firing temperature. As a specific firing method, a known method such as a normal pressure firing method, a hot press method, a gas pressure firing method, and a hot isostatic pressure firing method can be employed.

Further, the sintered body after firing is heat-treated again to precipitate a crystal phase composed of silicon-oxygen-nitrogen-rare earth element in the grain boundary phase, so that a larger stress can be applied to the silicon nitride particles.

 Hereinafter, the present invention will be described with reference to the following examples.

(Example) A commercially available silicon nitride powder having an average particle diameter of 0.6 μm, an impurity oxygen amount of 1% by weight, and an α ratio of 92% was used. Yb having an average particle diameter of 0.4 to 0.8 μm was used as a rare earth element oxide. ₂ O ₃ ,
Using Er ₂ O ₃ , Y ₂ O ₃ , and Al ₂ O ₃ , and further adding SiO ₂ powder as needed, the molar ratio of the rare earth element oxide to SiO ₂ (including impurity oxygen in the silicon nitride powder) is 2 Set to.

NbSi with an average particle size of 2.0 to 5.0 μm as hard particles
₂ , TaSi ₂ , WSi ₂ , MoSi ₂ , ZrSi ₂ , and SiC powders were weighed and mixed in proportions shown in Table 1.

Next, this mixed powder was placed in a graphite mold and hot-pressed at a temperature of Table 1 at a pressure of 30 GPa.

The obtained sintered body was ground into a shape of 3 × 4 × 45 mm, and the residual stress acting on the silicon nitride crystal particles was measured by an X-ray residual stress measuring device.

Further, the fracture toughness value and the bending strength at room temperature were measured by the SEBP method.

Further, the average aspect ratio of the silicon nitride crystal particles was measured from an electron micrograph.

 The results are shown in Table 1.

According to Table 1, Sample No. 1 containing no hard particles has a compressive stress of about 10 MPa and a low toughness value of about 7 MPa. Sample N containing a small amount of hard particles
In o.2, the compressive stress was small and no effect of improving toughness was seen. Conversely, Sample No. 8, which has a large amount of compounding, showed an improvement in toughness but a decrease in bending strength.

In sample No. 9 having a low firing temperature, the generation of compressive stress due to the dispersion of the hard particles in the present invention is small, and the toughness and the bending strength are low. Conversely, sample No. 12 having a high firing temperature has a low bending strength.

Sample N of hard particles whose melting point is lower than the firing temperature
o.16 and coefficient of thermal expansion less than twice that of silicon nitride
In Sample No. 17 using SiC, the compressive stress on silicon nitride was small, and the object of the present invention was not achieved. Further, even in Sample No. 19 containing Al ₂ O ₃ as a sintering aid, the compressive stress on silicon nitride was small.

In contrast, the samples of the present invention all have excellent toughness and strength, and have a toughness value (K _1c ) of 8 MPa ·
m ^1/2 or more and room temperature flexural strength of 800 MPa or more were achieved.

(Effects of the Invention) As described in detail above, the silicon nitride-based sintered body of the present invention disperses specific hard particles and configures a grain boundary phase from a specific composition to apply compressive stress to silicon nitride crystal particles. Let
By setting the stress to 40MPa or more, toughness,
A sintered body having excellent properties in both bending strength can be obtained.

As a result, it is possible to further expand the field of use such as mechanical parts of the silicon nitride sintered body.

Claims (1)

(57) [Claims] (1) Mainly silicon nitride crystal particles, 5 to 30% by volume
Hard particles having a coefficient of thermal expansion of at least twice the coefficient of thermal expansion of silicon nitride and a grain boundary phase composed of silicon-oxygen-nitrogen-rare earth element, and acting on the silicon nitride crystal particles. A high-toughness silicon nitride-based sintered body having a compressive stress of 40 MPa or more, a bending strength at room temperature of 800 MPa or more, and a toughness of 8 MPa · m ^1/2 or more.