JPH06344203A - Cutting tool made of composite ceramic and its manufacture - Google Patents

Abstract

PURPOSE:To provide a cutting tool made of the composite ceramics of silicon nitride and silicon carbide which can be used as a cutting tool such as a throw away tip for lathing and milling, a drill and an end mill. CONSTITUTION:A cutting tool is provided made of the composite ceramics of silicon nitride and silicon carbide which is the composite sintered compact of silicon nitride and silicon carbide having a fine structure where particles of silicon carbide of <=1mum in the average grain size are dispersed in the grain boundary and silicon carbide of several nm to several hundred nm in size are dispersed in the particle of silicon nitride, and where the oxide layer containing silica on the surface of the sintered compact and/or the layer with different hue from that of the inside part is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride-silicon carbide composite cutting tool and a method for producing the same, which can be used as a cutting tool such as a throw-away tip for turning and milling, a drill, an end mill and the like. A composite ceramic cutting tool made of silicon.

[0002]

2. Description of the Prior Art Ceramic cutting tools made of silicon nitride, which have higher strength and fracture toughness, or heat resistance and thermal shock resistance as compared with alumina cutting tools, have recently begun to be used especially as high speed cutting tools for cast iron. . However, as the cutting speed increases, the mechanical and thermal shock increases,
Since conventional silicon nitride cannot obtain a sufficient life due to wear or chipping, various improvements have been made.

As one of the methods, it is known that a single layer or a multi-layer coating layer of alumina or titanium is provided on the surface of silicon nitride to improve fracture toughness and wear resistance. However, this method has drawbacks in that it does not exhibit sufficient fracture resistance and abrasion resistance due to peeling of the coating layer, and the coating process causes an increase in cost of the cutting tool.

As another method, a material having strength, fracture toughness, fracture resistance and wear resistance superior to those of conventional silicon nitride has been developed.

For example, Japanese Patent Laid-Open No. 63-159256 discloses a silicon nitride-silicon carbide composite sintered body obtained from an amorphous composite powder in which silicon carbide particles having an average particle size of 1 μm or less are dispersed at grain boundaries. It has been shown that silicon carbide particles of several nm to several hundred nm have a unique microstructure in which they are dispersed in silicon nitride particles, and both room temperature strength and fracture toughness values are superior to those of silicon nitride.

Further, in the method for producing the above-mentioned sintered body, after the molding and sintering, by heat treatment at 800 to 1600 ° C. in an oxidizing atmosphere at atmospheric pressure or under pressure, it has more excellent strength characteristics, It is disclosed in JP-A-2-218183 that this can be used as a high temperature member such as a gas turbine or an engine, or as a wear resistant material or a cutting tool.

[0007]

However, although the silicon nitride-silicon carbide composite sintered body according to the above-mentioned invention is characterized by high strength, variations in strength due to roughness of heat-treated surface and fracture toughness are not sufficient. I had a problem. Therefore, when this material is used as a cutting tool, the variation in strength is small and the fracture toughness,
Further improvements in wear resistance and fracture resistance have been demanded.

[0008]

SUMMARY OF THE INVENTION The present invention is directed to silicon nitride-
It has been found that, by subjecting the silicon carbide composite sintered body to heat treatment and then grinding the surface, variation in strength can be suppressed and the fracture toughness of the surface can be improved.

That is, according to the present invention, silicon carbide particles having an average particle size of 1 μm or less are dispersed at the grain boundaries, and a few nm to a few hundreds n.
A silicon nitride-silicon carbide composite sintered body having a fine structure in which silicon carbide particles of m are dispersed in the silicon nitride particles, and an oxide layer containing silica and / or a layer having a different hue from the inside is formed on the surface of the sintered body. The present invention relates to a silicon nitride-silicon carbide composite ceramic cutting tool characterized by having.

Further, according to the present invention, silicon carbide particles having an average particle size of 1 μm or less are dispersed in the grain boundaries, and several nm to several hundreds nm.
Of silicon carbide particles having a fine structure in which the silicon carbide particles are dispersed in the silicon nitride particles, heat treatment in an oxidizing atmosphere at 1300 ~ 1600 ℃, the surface after 5-2
The present invention relates to a method for producing a composite ceramic cutting tool made of silicon nitride-silicon carbide, which is characterized by removing 000 μm.

According to the present invention, the variation in strength of the cutting tool can be suppressed by the simple heat treatment of the surface and the subsequent grinding, and the fracture toughness of the surface can be improved and the same effect as the coating can be obtained. And a composite ceramic cutting tool having wear resistance can be obtained. As a result, the cutting tool according to the present invention can be used not only as a long-lived high-speed cutting tool such as cast iron, but also as a cutting tool such as a drill and an end mill.

In the silicon nitride-silicon carbide composite sintered body used in the present invention, silicon carbide particles having an average particle size of 1 μm or less are dispersed at grain boundaries, and silicon carbide particles having a particle size of several nm to several hundred nm are silicon nitride particles. It consists of microstructures dispersed within.

As a raw material for obtaining a silicon nitride-silicon carbide composite sintered body having such a microstructure, for example, silicon carbide powder and silicon nitride powder which are present at 0.5 μm or less at the time of sintering are used. . More preferable raw materials include amorphous silicon, carbon, nitrogen and oxygen powders obtained by the method disclosed in JP-A-60-235707, or powders obtained by crystallizing a part or all of them. Is used.

These powders are well mixed with a sintering aid that produces a liquid phase during sintering and, if necessary, a molding binder and the like. As the sintering aid, any of those conventionally used for silicon nitride and silicon carbide can be used.For example, B, C, MgO, Al ₂ O ₃ , Y ₂ O ₃ , AlN, ZrO ₂ , S
Examples thereof include iO ₂ , CrO ₂ , Sc ₂ O ₃ , HfO ₂ , and other lanthanum-based oxides. These can be added not only as a simple substance but also as a carbonate, a nitrate, an alkoxide, or an amorphous state. At least one of these sintering aids is mixed with the above-mentioned raw material powder, and the amount used is usually 0.1 to 20 weight.
It is in the range of%.

Among these sintering aids, Y ₂ O ₃ is effective in enhancing the strength characteristics of the sintered body, and in the present invention,
It is preferred to add 0.5-15% by weight, preferably 2-10% by weight. During the heat treatment process of the present invention, this Y ₂ O ₃ moves to the surface of the sintered body by diffusion, forms a solid solution with the oxidation product on the surface of the composite sintered body, and also forms grain boundaries inside the sintered body. It forms a crystal phase with high heat resistance.

The mixing method may be any conventionally used dry method or wet mixing method. In the case of hot press sintering, this mixed powder is directly filled in a die to be sintered, and in other cases, it is molded by a method such as die molding or injection molding, and the next sintering step is performed.
As the sintering method, a normal atmospheric pressure sintering, a gas pressure sintering, or a conventionally practiced method such as hot pressing or HIP can be applied as it is.

The silicon nitride-silicon carbide composite sintered body obtained as described above is heat treated according to the method of the present invention,
After that, the surface is removed by grinding. That is, the above-mentioned silicon nitride-silicon carbide composite sintered body was heat-treated at 1300 to 1600 ° C. in an oxidizing atmosphere, and then the surface was treated with 5-200
Remove 0 μm.

The heat treatment in the present invention is carried out in an oxidizing gas atmosphere such as oxygen or air, or in a gas stream. The heat treatment conditions should be appropriately selected depending on the size of the sintered body to be treated, the sintering aid used, or the treatment atmosphere, but generally the treatment temperature is 1300 to
1600 ° C, preferably 1400 to 1550 ° C, treatment time 1 to 48
Time is chosen. This heat treatment may be carried out either under normal pressure or under pressure.

When heat treatment is performed according to the present invention, the following changes occur in the sintered body. That is, inside the sintered body, a part of the sintering aid component existing in the grain boundary layer changes to a crystal phase having high heat resistance. In addition, a layer having a hue different from that of the inside is formed on the surface of the sintered body. This hue is greenish gray inside, whereas the surface is dark greenish gray or black. It is speculated that this is because silica is formed on the surface by oxidation, or the sintering aid component accompanying diffusion reacts with silica and silicon nitride components to form a complex oxide layer of crystalline or vitreous material. .

For example, when a silicon nitride-silicon carbide composite sintered body obtained by using Y ₂ O ₃ as a sintering aid is heat-treated in accordance with the present invention, the surface of the sintered body exhibits a black color and yttrium ions diffuse. As a result, crystalline yttrium disilicate (Y ₂ O ₃ 2SiO ₂ ) is produced, or crystalline cristobalite (S
A glass phase containing iO ₂ ) and silica is formed.

The thickness of this oxide layer is usually 5 μm or more, though it depends on the heat treatment conditions.
After heat treatment for a long time, the thickness of the oxide layer reaches about 10 μm. At this time, in a part of the surface of the sintered body, the oxidation progresses rapidly due to the non-uniform oxidation, and the surface is roughened to cause variations in strength.

Therefore, in the present invention, the surface of the heat-treated sintered body is ground and removed. The oxidation product layer formed on the surface by this grinding is gradually removed, but in the present invention, it is not preferable to completely remove the oxidation product layer. That is, the degree of removal of the surface layer is determined by the required surface roughness and the size of the cutting tool.
It is in the range of 2000 μm, preferably 10-200 μm.

By this grinding, the surface roughened portion is first removed, and the variation in strength can be reduced. On the other hand, the fracture toughness of the surface of the sintered body decreases as the grinding amount increases, and finally reaches the internal fracture toughness value. With this change, the color of the surface of the sintered body gradually changes from black to dark green-grey, and finally becomes green-grey which is the color inside the sintered body.

Among the sintered bodies obtained by the method of the present invention as described above, those more suitable for a cutting tool include an oxide layer containing silica on the surface of the sintered body and / or a layer having a hue different from that of the inside. It is a composite sintered body that has. This sintered body can be obtained by carrying out the heat treatment conditions and the grinding amount described above according to the present invention. The thus obtained sintered body has a small variation in strength and a higher fracture toughness than the inside. As a cutting tool, it shows more excellent fracture resistance and wear resistance.

As a specific method for removing the surface of the sintered body, when the member is a flat surface, a surface grinding machine is used.
When the shape is complicated, NC lathe or barrel polishing is used.

The following examples show one example of the present invention, and are not limited to these as long as they do not exceed the gist of the present invention.

In the present invention, the room temperature and high temperature strength test pieces were tested with a size of 3 × 4 ×> 36 mm, and the strength test was performed with a 4-point bending strength, a span of 30 mm and a crosshead speed of 0.5 mm / min. . The fracture toughness of the surface of the sintered body is
After the surface of the sintered body is mirror-polished, the indenter press-fitting method (load 2
It was measured at 0 kg and a holding time of 20 seconds).

Examples 1 and 2 and Comparative Example 1 Y ₂ O ₃ 8 was added to an amorphous powder containing 8.5% by weight of carbon and containing silicon, carbon, nitrogen and oxygen and having an average particle size of 1 μm or less.
wt% was added as a sintering aid, wet-mixed in ethanol and dried, then 1750 in nitrogen gas at a pressure of 350 kg / cm ^2.
Hot press sintering was performed at 4 ° C. for 4 hours. The obtained sintered body was heat-treated in air at 1500 ° C. for 2 hours to obtain two types of test pieces whose surfaces were ground to 20 μm and 50 μm. Further, similar hot press sintering was performed, and grinding was performed without heat treatment to obtain a predetermined test piece.

The surface of the heat-treated sintered body has a dark dark greenish gray color, and X-ray diffraction shows that in addition to silicon nitride and silicon carbide, yttrium disilicate (Y ₂ O ₃ 2SiO ₂ ) and cristobalite (SiO ₂ The crystal phase of) was strongly recognized.

The inside of this test piece had the same greenish gray color as that of the non-heat-treated sintered body, and a small amount of crystal phase similar to that of the non-heat-treated sintered body was observed by X-ray diffraction. As a result of observing the microstructure inside the sintered body with an electron microscope, the sintered body was composed of equiaxed particles of 1 μm or less and columnar particles of silicon nitride having a length of 5 μm or less. In addition, silicon carbide particles of 1 μm or less are dispersed in the grain boundary, and are several nm to several hundred nm.
The silicon carbide particles of No. 3 were dispersed in the silicon nitride particles.

On the other hand, the surface of the non-heat-treated sintered body is greenish gray, and in X-ray diffraction, in addition to silicon nitride and silicon carbide, Si ₃ Y ₃ O which is a reaction product of yttrium oxide and silica or silicon nitride. ₃ N ₄ , Si ₃ NY ₅ O ₁₂ , Y ₂ SiO ₅ , Y ₂ S
A small amount of i ₂ O ₇ etc. was observed.

Specimen 1 of the obtained three kinds of sintered bodies
The average bending strength at room temperature by the measurement of 5 pieces, the Weibull coefficient (m) as an index of the variation in strength, and the fracture toughness of the surface of the sintered body were obtained. The results are shown in Table 1.

[0033]

[Table 1] Table 1 ─────────────────────────── Example 1 Example 2 Comparative Example 1 ───────── ─────────────────── With or without heat treatment Grinding amount (μm) 20 50 − Room temperature average bending strength (MPa) 1320 1230 1100 Weibull coefficient 15.4 16.7 9.8 Fracture toughness 8.5 7.9 6.3 (MPa ・ m ^1/2 ) ────────────────────────────

Examples 3 and 4 and Comparative Examples 2 and 3 Sintered bodies similar to those of Examples 1 and 2 were subjected to heat treatment shown in Table 2 in air, and two types, one having a surface of about 50 μm ground and the other not ground. A test piece was prepared. The room temperature bending strength and the strength at 1400 ° C. were measured for each of 15 test pieces, and the average strength and the Weibull coefficient were determined. The results shown in Table 2 were obtained. (The following margin)

[0035]

[Table 2] Table 2 ─────────────────────────────── Example 3 Comparative Example 2 Example 4 Comparative Example 3 ────────────────────────────── Heat treatment temperature 1550 1550 1450 1450 (° C) Time (h) 1 1 5 5 Grinding amount ( μm) 50 0 50 0 Average bending strength (MPa) Room temperature 1210 950 1300 1040 1400 ° C 970 870 1000 900 Weibull coefficient Room temperature 14.8 8.7 18.9 11.0 1400 ° C 16.5 11.2 23.5 14.6 ─────────────── ────────────────

Examples 5 and 6 and Comparative Examples 4 and 5 Using the sintered bodies obtained in Examples 1 and 2 before heat treatment, the following two types of cutting chips were produced. That is, the sintered body was heat treated according to Example 1 and then ground by up to 50%.
A sintered body (Examples 5 and 6) which was a throwaway chip in the shape of SNGN 120408 except for the μm surface, and S
After processing into an NGN 120408 shape throw-away tip, the same heat treatment as in Example 1 was performed to prepare a surface not ground (Comparative Example 4). In addition, a continuous turning test was performed under the following conditions, including a commercially available silicon nitride cutting tip of the same shape (Comparative Example 5), and the flank wear width (VB) and the life were compared. The results are shown in Table 3.

Work Material: FC30 Cutting Speed: 400 m / min, 180 m / min Cutting Depth: 1.5 mm Feed: 0.3 mm / rev. Cutting Time: <40 min

[0038]

[Table 3] Table 3 ───────────────────────────── Example 5 Example 6 Comparative Example 4 Comparative Example 5 ─── ─────────────────────────── Cutting speed 180 400 400 400 (m / min) VB (mm) 0.21 0.52 − − Life (min) > 40> 40 35 12 Remark Not missing Not missing Not missing ────────────────────────────────

[0039]

EFFECTS OF THE INVENTION The silicon nitride-silicon carbide composite cutting tool obtained by the present invention has a small variation while maintaining excellent strength and is excellent in fracture toughness. For this reason, it is possible to use not only cast iron but also a wide range of materials as a cutting tool having a longer life than conventional silicon nitride tools.

Claims (3)

[Claims] 1. A silicon nitride-silicon carbide composite having a microstructure in which silicon carbide particles having an average particle diameter of 1 μm or less are dispersed in a grain boundary, and silicon carbide particles having a diameter of several nm to several hundreds nm are dispersed in the silicon nitride particles. A silicon nitride-silicon carbide composite ceramics cutting tool, which is a sintered body and has an oxide layer containing silica and / or a layer having a hue different from that of the inside of the sintered body. 2. Yttrium oxide is 2 as a sintering aid.
The silicon nitride-silicon carbide composite ceramic cutting tool according to claim 1, wherein the cutting tool is added in an amount of not less than wt%. 3. A silicon nitride-silicon carbide composite having a fine structure in which silicon carbide particles having an average particle diameter of 1 μm or less are dispersed in a grain boundary, and silicon carbide particles having a diameter of several nm to several hundreds nm are dispersed in the silicon nitride particles. Sinter the sintered body in an oxidizing atmosphere at 1300 ~
A method for manufacturing a silicon nitride-silicon carbide composite ceramic cutting tool, which comprises heat-treating at 1600 ° C. and then removing the surface by 5-2000 μm.