JPH0523921A - Silicone nitride basis sintered body for cutting tool - Google Patents

Abstract

PURPOSE:To enhance wear resistance and defection resistance, and improve cutting performance of casting material in the case of a silicon nitride basis sintered body containing the 3a group elements of the Periodic Table by forming the sintered body made of needle-shaped particles wherein the average short diameter and average aspect ratio of silicone nitride particles are prescribed. CONSTITUTION:A silicon nitride basis sintered body is formed with a chemical compound of the 3a group elements of the Periodic Table and silicon nitride as its nucleuses. These ingredients should preferably contain, for example, the 3a group elements of the Periodic Table in the ratio of about 0.5-5mol percentage when calculated in term of an oxide conversion (RE2O3), and contain excess oxygen in the ratio of not more than 10mol percentage when calculated in term of SiO, and, formation of these silicon nitride particles is set to needle- shaped particles whose average short diameter is not more than 1mum and average aspect ratio is not less than 3%, and the mol ratio shown by SiO2/RE3 should be preferably made in a range from 0.5 to 2.5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride sintered material for a cutting tool which has high hardness, high toughness, excellent wear resistance and fracture resistance, and particularly excellent cutting performance of a cast material.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have been
Due to its excellent thermal shock resistance and wear resistance, it is being applied as a cutting tool material in addition to various structural materials for heat engines.

Recently, yttrium oxide (Y ₂ O ₃ ) has been used as a sintering aid for producing a high-density silicon nitride sintered body.
JP-A-55-32785 proposes a silicon nitride-based sintered body for a cutting tool, which is obtained by adding an oxide such as the above and firing by hot pressing.

In another method, Y _{2 is} used as a sintering aid.
O ₃ and aluminum oxide (Al ₂ O ₃ ) are added,
JP-A-56-73670 proposes to obtain a tool material by firing at normal pressure in nitrogen gas.

[0005]

However, according to the conventional silicon nitride-based sintered body, when it is hot-press fired at a relatively low temperature, the structure is composed of fine crystal grains, so that the strength is improved. However, there is a problem that the toughness is low in the use as a cutting tool and the chip is likely to be broken.

Further, according to the system in which the sintering aid such as Al ₂ O ₃ or MgO is added, the melting point or the softening point of the grain boundary phase is low, and it becomes easy to react with the work material during cutting, and wear progresses. Then, there was a problem that the life of the tool was shortened.

[0007]

Means for Solving the Problems As a result of repeated studies on the above problems, the present inventors added an oxide of a Group 3a element of the periodic table as a sintering aid to silicon nitride. Suitable for cutting tools with excellent hardness, strength, wear resistance and chipping resistance by making the silicon nitride crystals that make up the sintered body acicular and controlling the particle size to a fine shape in the system It was found that a sintered body can be obtained.

That is, the silicon nitride sintered body for a cutting tool of the present invention is a silicon nitride sintered body containing at least a Group 3a element of the periodic table, and the silicon nitride particles in the sintered body have an average short length. It is characterized by being composed of fine acicular particles having a diameter of 1 μm or less and an average aspect ratio of 3 or more.

The silicon nitride sintered material used in the present invention is mainly composed of at least a Group 3a element compound of the periodic table and silicon nitride. In some cases, oxygen inevitably contained in the silicon nitride raw material or the like, or a component intentionally added as silicon oxide is contained therein, and this is hereinafter referred to as excess oxygen.

According to the present invention, it is desirable that these components are elements of Group 3a of the periodic table in oxide conversion (RE ₂ O ₃ ).
Is contained at a rate of 0.5 to 5 mol%, and the excess oxygen is SiO 2.
It may be contained at a ratio of 10 mol% or less in terms of ₂ . The excess oxygen is calculated as the remaining oxygen amount from the total oxygen amount in the sintered body, which is obtained by removing the oxygen component mixed by being added as a Group 3a element oxide of the periodic table.

Here, the composition of the sintered body is limited to the above range because the hardness of the sintered body is lowered when the oxide conversion amount of the Group 3a element of the periodic table is more than 5 mol%, and as a tool. Wear resistance is decreased, and if it is less than 0.5 mol%, dense silicon nitride cannot be obtained and the strength of the sintered body itself tends to be decreased. This is because the toughness of the body is lowered and the fracture resistance as a tool is deteriorated.

Further, according to the present invention, SiO ₂ / RE of the excess oxygen and the Group 3a element of the periodic table in the above composition.
_The molar ratio represented by ₂ O ₃ is 0.5 to 2.5, particularly 1 to
It is preferable that it is 1.8, and if this molar ratio is less than 0.5, a dense sintered body cannot be obtained, and if it exceeds 2.5, the toughness decreases and the fracture resistance deteriorates.

The sintered body of the present invention is made of Al ₂ O ₃ and Mg in consideration of wear resistance and fracture resistance as a cutting tool.
It is desirable that oxides such as O are not present as much as possible,
In particular, it is desirable to control it to 1% by weight or less.

The silicon nitride sintered material of the present invention is composed of silicon nitride crystal grains and a grain boundary phase existing between the grains because of its structure. According to the present invention, the silicon nitride crystal particles are present as fine acicular particles, and the acicular particles are fine particles having an average minor axis of 1 μm or less, particularly 0.7 μm or less, and the acicular particles have an average diameter of less than 0.7 μm. A major feature is that the particles have an aspect ratio (major axis / minor axis) of 3 or more, particularly 5 or more.

The shape of the acicular silicon nitride crystal particles is set in the above range because if the average minor axis is larger than 1 μm, if grain shedding occurs during cutting, the effect is large and abnormal wear occurs. If the aspect ratio is less than 3, the toughness of the sintered body will be low, and the crystal grains will be likely to be shed. As a result, in any case, the wear amount will increase and the tool life will be impaired. ..

On the other hand, the grain boundaries in the sintered body are 3a of the periodic table.
It is composed of a group element, nitrogen, oxygen, and silicon, and these elements form a glass phase or a crystal phase.

The Group 3a element of the periodic table used in the present invention includes at least one selected from Y, Er, Yb, Sc, Dy and Ho.
It is preferable to use other elements for Y, since stains called spots may occur and the fracture resistance may be reduced.

Next, the method for producing the silicon nitride sintered material of the present invention will be described. First, as the raw material powder, silicon nitride powder and a Group 3a element oxide of the periodic table, preferably Er ₂ O ₃ , Yb ₂ O ₃ , Sc ₂ O ₃ , Dy ₂ O ₃
And at least one oxide powder selected from Ho ₂ O ₃ and optionally SiO ₂ powder.

Two types of α-type and β-type are known as the silicon nitride powder, and 90% or more of α-Si ₃ N ₄ is contained in order to control the above-mentioned grain size and average aspect ratio. Desirably, the impurity oxygen amount is 0.8 to 1.5% by weight, and the BET specific surface area is 3 to ²⁰ m ² / g.

Next, these raw material powders are weighed and mixed at a predetermined ratio, and a binder is appropriately added to the raw material powder to granulate it, and then a known molding method such as press molding method, extrusion molding method, injection molding method or cast molding It is formed into a desired shape by a forming method such as a method and then fired.

Although a known method is adopted as the firing means, one of the features of the present invention is that the nitrogen gas pressure is 1 to 100 atm without mechanical pressure.
It is desirable to perform firing at a firing temperature of 0 to 2000 ° C.
Further, as described above, the particle size of the silicon nitride particles is reduced,
In order to increase the average aspect ratio, once baked in nitrogen at 1700 to 1850 ° C., then 100 to 300
It is desirable to carry out hot isostatic pressing at a temperature of 0 atm and 1500 to 1900 ° C.

[0022]

According to the present invention, the silicon nitride crystal particles of the silicon nitride-based sintered body to be used are present as fine needle-shaped particles, whereby the toughness of the sintered body itself is increased by the entanglement of the particles, and thereby the tool It is possible to improve the chipping resistance. Further, even when the crystal grains are shed during the cutting process, the grains are so fine that abnormal wear is not caused and excellent wear resistance can be obtained.

[0023]

EXAMPLES As the raw material powders, the samples No. 1 to 9 in Table 1 were silicon nitride powders having a BET specific surface area of 15 m ² / g, an α conversion of 95% and an impurity oxygen content of 0.9% by weight. N
o, 10 to 11 are BET specific surface area 14 m ² / g, alpha conversion rate 7
2% and the amount of impurity oxygen was 0.8% by weight.

Further, various periodic table 3a shown in Table 1
A mixture of a group element oxide and a silicon oxide powder is mixed and mixed with a binder so that the final sintered body has the composition shown in Table 1, press-molded at a pressure of 1 t / cm ² , and a tool shape SN
A molded product for GN120412 was obtained. After degreasing this molded body, it was fired under the conditions shown in Table 1 and hot isostatically fired to obtain a sintered body.

The density of each of the obtained sintered bodies was measured by the Archimedes method.
It was confirmed to be 5% or more. Further, the average minor axis of silicon nitride crystal grains and the average aspect ratio of the sintered body were determined by an electron micrograph.

After polishing into a tool shape, a cutting test was conducted under the following conditions: Work material FC-25 Cutting speed 400 m / min Feed 0.2 mm / rev Depth of cut 2.0 mm.
Was measured or the time until a defect was generated was measured, and the results are shown in Table 2.

[0027]

[Table 1]

[0028]

[Table 2]

According to Tables 1 and 2, all the samples of the present invention showed high toughness and excellent cutting performance. On the other hand, samples No, 7 to 9 having a high firing temperature and a particle size of more than 1 μm exhibited abnormal wear due to shedding in the cutting test. Further, Samples No. 10, 10 and 11 using a silicon nitride raw material having a low .alpha. Ratio also showed abnormal wear.

Sample No. 1 having a composition outside the scope of the present invention was used.
In Nos. 2 to 15, it was difficult to obtain a sintered body in which the minor axis of crystal grains and the aspect ratio were within the range of the present invention, and the cutting test was also inferior to the product of the present invention.

[0031]

As described above in detail, the silicon nitride sintered material for a cutting tool of the present invention has an unprecedented excellent toughness due to the presence of silicon nitride particles in the sintered material as fine particles. It is possible to add, and it is possible to prevent chipping and wear during cutting, improve the cutting characteristics, and extend the life of the tool.

Claims (1)

Claim: What is claimed is: 1. A silicon nitride sintered body containing at least a Group 3a element of the periodic table, wherein the silicon nitride particles have an average minor axis of 1.
A silicon nitride-based sintered body for a cutting tool, characterized in that it is composed of needle-shaped particles having an average aspect ratio of 3 or more and having a diameter of μm or less.