JP2771335B2 - Silicon nitride sintered body for cutting tools - Google Patents

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 body for a cutting tool having high hardness, high toughness, excellent wear resistance and chipping resistance, and particularly excellent cutting performance of a cast material.

[0002]

2. Description of the Related Art Conventionally, a silicon nitride sintered body has been known to have heat resistance,
Because of its excellent thermal shock resistance and abrasion resistance, application as a cutting tool material in addition to various structural materials for heat engines has been promoted.

Recently, yttrium oxide (Y ₂ O ₃ ) has been used as a sintering aid in order to produce a high-density silicon nitride sintered body.
Japanese Patent Application Laid-Open No. 55-32785 proposes a silicon nitride sintered body for a cutting tool which is obtained by adding an oxide such as above and firing by hot pressing.

As another method, Y _{2 is} used as a sintering aid.
O ₃ and aluminum oxide (Al ₂ O ₃ ) are added,
Japanese Patent Application Laid-Open No. 56-73670 proposes obtaining a tool material by baking at normal pressure in nitrogen gas.

[0005]

THE INVENTION Problems to be Solved] However, in the case of using Y ₂ O ₃ as a sintering aid is insufficient strength to a high temperature during cutting from room reached a practical level In addition, spots called spots are generated in the sintered body, and there is a problem that when a cutting tool having poor fracture resistance and a complicated shape is produced, an expensive hot press method must be used.

According to the method proposed in Japanese Patent Application Laid-Open No. 56-73670, complicated shaped articles can be produced at low cost by baking at normal pressure in nitrogen gas. A system using Y ₂ O ₃ and Al ₂ O ₃ also has a disadvantage that the fracture toughness of the sintered body is lowered, so that the fracture resistance is also poor.

[0007]

[Means for Solving the Problems] The present inventors have studied the above problems and found that a specific rare earth element oxide other than Y ₂ O ₃ was used as a sintering aid for silicon nitride. By adding and adjusting the amount of excess oxygen contained in the sintered body, and by including these sintering aids in a specific ratio, densification by normal pressure firing is possible, and hardness, It has been found that a sintered body suitable for a cutting tool having excellent strength, wear resistance and fracture resistance can be obtained.

That is, the silicon nitride sintered body for a cutting tool of the present invention contains 85 to 99 mol% of silicon nitride, Er, Yb, S
A sintered body comprising at least one rare earth element selected from c, Dy and Ho in a proportion of 0.5 to 5 mol% in terms of oxide and excess oxygen in a proportion of 10 mol% or less as SiO _2. The molar ratio of the excess oxygen to the amount of the rare earth element in terms of oxide is 0.5 to 2.5, and the Vickers hardness is 14.5 GPa or more.

According to the present invention, first, the composition of the sintered body is
85-99 mol% of silicon nitride, especially 88-97 mol%,
At least one selected from Er, Yb, Sc, Dy and Ho is selected as the rare earth element, and the oxide equivalent amount of this element (sometimes simply referred to as RE ₂ O ₃ ) is 0.5 to 0.5.
5 mol%, especially 2-4 mol%, and excess oxygen of SiO
It is important that the ₂ conversion amount (sometimes simply referred to as SiO ₂ ) be 10 mol% or less.

[0010] The excess oxygen is the amount of oxygen remaining after removing the oxygen contained in the component added to silicon nitride as a sintering aid excluding SiO ₂ from the total oxygen content contained in the entire sintered body. Specifically, it is composed of impurity oxygen inevitably contained in the silicon nitride powder or oxygen added as SiO ₂ .

The reason why the composition of the sintered body is set in the above range is that the hardness of the sintered body is reduced when the content of silicon nitride is less than 85% by mole or the amount of rare earth element in terms of oxide is more than 5% by mole. If the wear resistance as a tool deteriorates, the content of silicon nitride is more than 99 mol%, or the amount of rare earth element as oxide is less than 0.5 mol%, a dense body cannot be obtained, and the strength of the sintered body is not obtained. This is because the material itself is reduced. On the other hand, if the excess oxygen amount is more than 10 mol%, the toughness is reduced and the fracture resistance as a tool is deteriorated.

Further, according to the present invention, in the above-mentioned composition, (SiO ₂ / RE ₂ O ₃ )
It is also important that the molar ratio represented by is 0.5 to 2.5, especially 1 to 1.8. This is because if the molar ratio is less than 0.5, a dense sintered body cannot be obtained, and if the molar ratio is more than 2.5, toughness is reduced and fracture resistance is deteriorated.

The sintered body of the present invention is made of Al ₂ O ₃ or M _{2 in} consideration of wear resistance and fracture resistance as a cutting tool.
It is desirable that oxides such as gO do not exist as much as possible. Specifically, it is desirable to control the content to 1% by weight or less.

Next, the method for producing the silicon nitride sintered body of the present invention will be described. First, silicon nitride powder and Er ₂ O ₃ , Yb ₂ O ₃ , Sc ₂ O are used as raw material powders.
_3, at least one oxide powders selected from Dy ₂ O ₃ and Ho ₂ O _3, to prepare a SiO ₂ powder as the case may be.

[0015] It should be noted that two types of silicon nitride powder, α-type and β-type, are known, but α-S
i ₃ N ₄ is desirably 90% or more, the impurity oxygen amount is 0.8 to 1.5% by weight, and the BET specific surface area is 3 to 2%.
It is preferably 0 m ² .

Next, these raw material powders are weighed and mixed so as to have the above-described composition, a binder is appropriately added thereto, and the mixture is granulated. Then, a known molding method such as a press molding method, an extrusion molding method, or an injection molding method is used. Alternatively, it is formed into a desired shape by a molding method such as a casting method, and then fired.

As the firing means, a known method is employed. One feature of the present invention is that the composition as described above allows the atmosphere having a nitrogen gas pressure of 1 to 100 atm without mechanical pressure. 1700-2000 ℃
By firing at the firing temperature described above, a dense sintered body having a theoretical density ratio of 95% or more can be obtained. Thereby, it can be manufactured at a lower cost as compared with the hot press method generally used until now. Further, in some cases, it is possible to further densify by hot isostatic pressing after the above-mentioned firing.

[0018]

EXAMPLE As a raw material powder, silicon nitride powder (BET specific surface area 15 m ² / g, pregelatinization rate 95%, impurity oxygen amount 0.9
% By weight) and the respective rare earth oxide powders and silicon oxide powders shown in Table 1 were mixed and mixed with a binder so that the final sintered body composition became the composition shown in Table 1, and the pressure was 1 t / cm ² . Press molding was performed to obtain a molded body for a tool shape SNGN120412.

After degreased, the compact was fired at 1900 ° C. for 3 hours under a nitrogen gas pressure of 30 atm. The density of the obtained sintered body was measured by Archimedes method, and the Vickers hardness and IF were measured. The fracture toughness was determined by the method.

Further, after polishing to a tool shape, the following conditions were applied. Work material FC-25 Cutting speed 400 m / min Feed 0.2 mm / rev Depth of cut 2.0 mm A cutting test was performed to reduce the flank wear of the cutting edge to 0.2 mm. The time until the time was reached was measured, and the results are shown in Table 1.

[0021]

[Table 1]

According to Table 1, all of the samples of the present invention exhibited excellent properties of hardness of 14.5 GPa and toughness of 6.9 MPa · m ^1/2 or more, and also exhibited excellent cutting performance in a cutting test.

On the other hand, the sample No. using Y ₂ O ₃
Samples Nos. 10 and 11 showed high hardness and toughness values, but showed abnormal wear due to chipping in the cutting test. Further, Sample No. 12 in which the amount of rare earth element oxide exceeds 5 mol%, and Sample No. 12 in which the excess oxygen amount exceeds 10 mol%.
13. Samples No. 14 and 14 in which the ratio between the rare earth oxide and excess oxygen is smaller than 0.5 and samples No. and 14 in which the ratio exceeds 2.5.
No. 15 was lower in hardness and toughness as compared with the product of the present invention, and was inferior in the cutting test.

[0024]

As described above in detail, the silicon nitride sintered body for a cutting tool of the present invention contains a specific rare earth element with respect to silicon nitride so as to be composed of excess oxygen and a specific ratio. Since excellent hardness and toughness, which have never been achieved before, can be imparted, cutting characteristics can be improved as a cutting tool and the life of the tool can be extended.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C04B 35/584 B23B 27/14

Claims (1)

(57) [Claims] 1. The method according to claim 1, wherein 85 to 99 mol% of silicon nitride and Er, Y
A sintered body in which at least one rare earth element selected from b, Sc, Dy and Ho has a ratio of 0.5 to 5 mol% in terms of oxide and excess oxygen has a ratio of 10 mol% or less in terms of SiO _2. Wherein the molar ratio of the excess oxygen to the oxide equivalent of the rare earth element is 0.5 to 2.5, and the Vickers hardness is 14.5 GPa or more. Sintered body.