JP2009173508A - Silicon nitride sintered compact, cutting tool, device for cutting work and cutting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a silicon nitride sintered compact where wear resistance and chipping resistance are improved, a cutting tool, a device for cutting works and a cutting method. <P>SOLUTION: The silicon nitride sintered compact has a crystal phase being mainly silicon nitride crystals and an amorphous grain boundary phase containing lanthanum, aluminum, magnesium, silicon and oxygen and existing in the grain boundary of the silicon nitride crystals. The content of magnesium in a face at a depth of 0.3 mm from the surface of the sintered compact is 1.3 mass% or less in terms of an oxide. The content of magnesium in a face at a depth of 0.3 mm from the surface of the sintered compact is less than that in a face at a depth of 1 mm from the surface of the sintered compact. The difference of the content of magnesium between in a face at a depth of 0.3 mm and in a face at a depth of 1 mm from the surface of the sintered compact is 0.5 mass% or less in terms of an oxide. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a silicon nitride sintered body, a cutting tool, a cutting apparatus, and a cutting method that have improved wear resistance and chipping resistance.

  Silicon nitride and sialon, known as engineering ceramics, have high strength, high temperature, high strength, and high toughness, as well as excellent heat resistance, thermal shock resistance, wear resistance, and oxidation resistance. Applications are being promoted as heat engine parts such as rotors and cutting tools.

  Among such parts, particularly in a cutting tool, a strong impact may be applied, so a material having improved impact resistance and chipping resistance is required.

  To date, in order to improve the mechanical and thermal characteristics of the silicon nitride sintered body, the grain boundaries of the silicon nitride sintered body are changed to include rare earth elements, silicon, aluminum, a small amount of magnesium, and oxygen as metal components. Attempts have been made to achieve the above-mentioned object by constituting the amorphous grain boundary phase composed of nitrogen and controlling the grain boundary phase components in a specific composition range (see, for example, Patent Document 1). ).

In addition, the thing using yttrium as a rare earth element which comprises the grain boundary of a silicon nitride crystal grain is known (refer to patent documents 2).
Japanese Patent No. 3454993 Japanese Patent No. 2997899

  However, in the silicon nitride-based sintered bodies described in Patent Documents 1 and 2, the difference in magnesium content between the sintered body surface layer and the inside of the sintered body is large. There was a problem that characteristic differences occurred and the wear resistance and chipping resistance were still inferior.

  The reason why the difference in magnesium content between the surface layer of the sintered body and the inside of the sintered body becomes large is not clear, but the present inventors volatilize magnesium during firing and escape between the silicon nitride powders to the outside. Therefore, it is considered that the difference in magnesium content between the sintered body surface layer and the inside of the sintered body becomes large.

  An object of the present invention is to provide a silicon nitride sintered body, a cutting tool, a cutting apparatus, and a cutting method that have improved wear resistance and chipping resistance.

  As a result of intensive studies on the above problems, the present inventors, by combining lanthanum, aluminum, magnesium, and silicon, generate a liquid phase at a temperature lower than the temperature at which MgO volatilization becomes significant during firing, Reduction of open pores due to volume shrinkage in the rearrangement process due to liquid phase generation can prevent MgO from escaping to the outside, and a surface having a depth of 0.3 mm from the surface of the sintered body (hereinafter sometimes referred to as the surface layer). And the present inventors have found that the difference in magnesium content between the sintered body surface and a surface having a depth of 1 mm (hereinafter sometimes referred to as “inside”) can be reduced.

  The silicon nitride sintered body of the present invention includes a crystal phase mainly composed of a silicon nitride crystal, and an amorphous grain boundary phase at the grain boundary of the silicon nitride crystal, containing lanthanum, aluminum, magnesium, silicon and oxygen. A magnesium content in a surface having a depth of 0.3 mm from the surface of the silicon nitride-based sintered body is 1.3% by mass or less in terms of oxide, and The magnesium content in the surface having a depth of 0.3 mm from the surface of the silicon nitride-based sintered body is less than the magnesium content in the surface having a depth of 1 mm from the surface of the silicon nitride-based sintered body, and the silicon nitride The difference in magnesium content between the surface having a depth of 0.3 mm and the surface having a depth of 1 mm from the surface of the sintered material is 0.5% by mass or less in terms of oxide.

  In the silicon nitride-based sintered body of the present invention, Mg in the grain boundary phase is present more uniformly in the depth direction of the sintered body than in the past, and the characteristics in the surface layer and inside of the sintered body Differences are less likely to occur, and wear resistance and chipping resistance can be improved.

Although the reason why the difference in magnesium content between the surface having a depth of 0.3 mm and the surface having a depth of 1 mm from the surface of the sintered body is 0.5% by mass or less in terms of oxide is not clear, the present inventor Among the rare earth element (Group 3 element) oxides, lanthanum oxide has a lower eutectic temperature with silica (SiO ₂ ) than other eutectic temperatures with other rare earth element oxides and silica. In addition, by combining aluminum, magnesium, and silicon, it becomes possible to lower the liquidus temperature of the sintering aid than the temperature at which MgO volatilization becomes significant, and MgO volatilization becomes significant at the firing stage. The liquid phase is generated at a low temperature before it becomes low, and densification starts at a low temperature due to the reduction of open pores due to volume contraction in the rearrangement process by the liquid phase generation. This Can be suppressed, believes Accordingly density difference of the surface layer and interior of magnesium (as oxide) is reduced, more homogeneous than the conventional dense sintered body can be produced.

  In the silicon nitride sintered body of the present invention, the sum of the lanthanum oxide equivalent, the aluminum oxide equivalent, and the magnesium oxide equivalent is 3. It is characterized by being 5% by mass or less. As described above, by combining lanthanum, aluminum, magnesium, and silicon, even when the amount of the auxiliary agent is small, it can be densified at low temperature, and wear resistance and chipping resistance are improved. A silicon nitride sintered body can be provided.

  Further, the silicon nitride sintered body of the present invention is characterized in that the surface of the silicon nitride sintered body is a fired surface (as fired). Although it is known that MgO is most volatilized on the grilled skin surface, in the present invention, the difference in concentration between the grilled skin surface and the internal magnesium (as oxide) can be reduced.

  Further, the silicon nitride based sintered body of the present invention has a grain boundary phase on a surface having a depth of 0.3 mm from the surface of the silicon nitride based sintered body and a surface having a depth of 1 mm from the surface of the silicon nitride based sintered body. Each area ratio is 10% or less. In the silicon nitride-based sintered body of the present invention, the grain boundary phase in the sintered body is small, and the difference in composition of the grain boundary phase is also small, so that the chipping resistance can be improved.

  The cutting tool of this invention consists of said silicon nitride sintered body, It is characterized by the above-mentioned. Such a cutting tool can provide a cutting tool having excellent wear resistance and chipping resistance and a long life.

  A cutting apparatus according to the present invention includes the above-described cutting tool and a holding table for holding a workpiece to be processed by the cutting tool. In such a cutting apparatus, cutting can be performed without replacing the cutting tool for a long period of time.

  The cutting method of this invention cuts a to-be-cut material using said cutting tool, It is characterized by the above-mentioned. In such a cutting method, along with improvement in wear resistance and chipping resistance of the cutting tool, it is possible to improve the dimensional accuracy of the material to be cut and to perform cutting without replacing the cutting tool for a long period of time.

  In the silicon nitride-based sintered body of the present invention, Mg in the grain boundary phase is present more uniformly in the depth direction of the sintered body than in the past, and the characteristics in the surface layer and inside of the sintered body Differences are less likely to occur, and wear resistance and chipping resistance can be improved.

  In the cutting tool of the present invention, since the cutting tool is made of the above-described silicon nitride sintered body, it is possible to provide a cutting tool having excellent wear resistance and chipping resistance and having a long life.

  In the cutting apparatus of the present invention, since the cutting apparatus includes the above cutting tool, cutting can be performed without replacing the cutting tool for a long period of time.

  In the cutting method of the present invention, since the workpiece is cut using the above-described cutting tool, the dimensional accuracy of the workpiece can be improved along with the improved wear resistance and chipping resistance of the cutting tool, Cutting can be performed without changing the cutting tool for a long time.

  The silicon nitride sintered body of this embodiment has a structure having a silicon nitride crystal as a main crystal and a grain boundary phase, and the silicon nitride is mainly composed of a β-silicon nitride crystal phase. Note that β-sialon may be formed by slightly dissolving aluminum in the β-silicon nitride crystal phase. In addition to silicon nitride, at least one of nitrides such as Ti, Hf, and Zr, carbides, carbonitrides, and silicides such as W and Mo may exist. The silicon nitride crystal is preferably contained in the total amount of 94.5% by mass or more, more preferably 96.5% by mass or more. Thereby, the excellent characteristics of the silicon nitride crystal can be sufficiently exhibited.

[Composition]
According to this embodiment, the grain boundary phase contains lanthanum, aluminum, magnesium, silicon, and oxygen. The composition of the elements constituting the grain boundary phase is such that the lanthanum oxide (La ₂ O ₃ ) conversion amount is 0.1 mass% or more and the aluminum oxide (Al ₂ O ₃ ) conversion amount in the total amount of the sintered body. Is 0.05 to 0.6 mass%, magnesium oxide (MgO) equivalent is 0.3 mass% or more, oxygen is 2.5 mass% or less, lanthanum oxide equivalent, aluminum oxide The total of the conversion amount and the magnesium oxide conversion amount is preferably 3.5% by mass or less.

  The oxide equivalent amount of lanthanum is preferably 0.5% by mass or more, more preferably 1% by mass or more, in order to densify the sintered body. The oxide equivalent amount of aluminum contains 0.2% by mass or more particularly for the purpose of lowering the liquid phase generation temperature of the sintering aid and densifying the sintered body, and lowering the oxidation resistance of the sintered body. In particular, 0.2 to 0.55% by mass, and more preferably 0.3 to 0.5% by mass is desirable in order to suppress a decrease in wear resistance due to. The oxide equivalent amount of magnesium may include 0.35% by mass or more, more preferably 0.4% by mass or more, in order to lower the liquid phase generation temperature of the sintering aid and to densify the sintered body. desirable.

  The amount of oxygen contained in the silicon nitride sintered body keeps the liquid phase formation temperature of the sintering aid low, keeps the sintered body dense, and improves the oxidation resistance and wear resistance. In order to realize the combined body, 2.5% by mass or less is preferable. In particular, it is desirable that the content be 2.2% by mass or less, and further 2% by mass or less.

By setting it as such a composition, even if it is a case where the quantity of a sintering auxiliary agent is small, a silicon nitride sintered compact can be densified at low temperature. Among group 3 element oxides, lanthanum oxide (La ₂ O ₃ ) has a eutectic temperature with silica (SiO ₂ ), and other group 3 elements (eg, erbium, yttrium) oxide and silica eutectic. Lower than temperature. Incidentally, the eutectic temperature of lanthanum oxide and silica is 1625 ° C., whereas the eutectic temperature of erbium oxide (Er ₂ O ₃ ) and silica is 1680 ° C., and yttrium oxide (Y ₂ O ₃ ) and silica are mixed. Is 1660 ° C., and the eutectic temperature of lanthanum oxide and silica is low. Silica is contained in the silicon nitride raw material.

  By further adding magnesium oxide and aluminum oxide to the combination of lanthanum oxide having a low eutectic temperature and silica, the liquid phase generation temperature can be lowered to 1400 ° C. or lower. As a result, the silicon nitride starts to be densified at a low temperature, and the microstructure can be refined. Moreover, abnormal grain growth of silicon nitride crystal grains can be suppressed. Therefore, a silicon nitride sintered body with improved wear resistance and chipping resistance is obtained.

When lanthanum, silicon, aluminum, and magnesium are used as sintering aids, the average major axis of silicon nitride crystal particles (six silicon nitride particles from the larger major axis with a visual field of 0.015 mm ² ) is 10 μm or less. It becomes possible to suppress it. As a result, in addition to wear resistance, chipping resistance can be improved.

  On the other hand, when rare earth elements other than lanthanum are added, only those having poor chipping resistance can be obtained.

  Note that other rare earth elements such as Ce, Pr, and Nd may be contained as lanthanum impurities in total of 1% by mass or less in terms of oxides. That is, other rare earth elements such as Ce, Pr, and Nd other than lanthanum may be contained in the total amount of the sintered body in an amount of 0.03% by mass or less. Although rare earth elements other than lanthanum may be contained as impurities, even if the amount of oxide is about 0.03% by mass in the total amount of the sintered body, the combination of lanthanum, silicon, aluminum and magnesium can improve the characteristics. Can fully demonstrate.

  In addition, when the silicon nitride sintered body of the present embodiment contains the Group 6 element silicide particles of the periodic table at a ratio of 0.1 to 2% by mass, it is possible to suppress a decrease in high temperature strength. Further, the wear resistance of the silicon nitride sintered body can be further improved. The periodic table group 6 element silicide particles are present dispersed in the grain boundary phase of the silicon nitride sintered body.

  Further, as the Group 6 element silicide of the periodic table, chromium silicide, molybdenum silicide, and tungsten silicide can be exemplified, but tungsten silicide is used for the reason that fine particles can be formed during firing using a fine oxide raw material. It is desirable to use it.

  Moreover, it is preferable that the relative density of the silicon nitride sintered body of this embodiment is 99% or more. When the relative density is 99% or more, there are almost no voids in the sintered body, and the wear resistance is further improved. Furthermore, since there are almost no voids in the sintered body, stress concentration is concentrated on the voids during chipping, such as milling, so that the chipping phenomenon is suppressed and chipping resistance is improved. .

  The silicon nitride-based sintered body of the present embodiment can be densified to a relative density of 99% or higher even under normal pressure firing. For example, by suppressing the abnormal grain growth of crystal grains by performing pressure firing, The relative density can also be increased.

[About the organization]
In the silicon nitride-based sintered body of the present embodiment, the magnesium content in a surface (hereinafter sometimes referred to as a surface layer) having a depth of 0.3 mm from the surface of the sintered body is 1.3% by mass or less in terms of oxide. In addition, the magnesium content on the surface having a depth of 0.3 mm from the surface of the sintered body is less than the magnesium content on the surface having a depth of 1 mm from the surface of the sintered body (hereinafter sometimes referred to as the inside), and The difference in magnesium content between the surface having a depth of 0.3 mm and the surface having a depth of 1 mm from the bonded surface is 0.5% by mass or less in terms of oxide (MgO).

  By adopting such a configuration, the components in the grain boundary phase exist more uniformly than in the past in the depth direction of the sintered body, resulting in a characteristic difference between the surface layer and the inside of the sintered body. It becomes difficult to improve wear resistance and chipping resistance.

  Although the reason why the difference in magnesium content between the surface having a depth of 0.3 mm and the surface having a depth of 1 mm from the surface of the sintered body is 0.5% by mass or less in terms of oxide is not clear, the present inventor Thinks as follows.

  That is, among the Group 3 elements of the periodic table, lanthanum has a lower eutectic temperature with silica than the eutectic temperature of other Group 3 elements with silica, so that lanthanum, silicon, By using a combination of Mg and aluminum, it becomes possible to lower the liquid phase generation temperature of the sintering aid than the temperature at which MgO volatilization becomes significant, and further from the liquid phase generation temperature of the sintering aid. In addition, by maintaining at a slightly higher temperature for a certain period of time, a liquid phase can be sufficiently generated at a low temperature before MgO volatilization becomes significant in the firing stage.

  Thereby, in the rearrangement process by liquid phase generation, volume shrinkage and open pores are reduced, so that MgO can be prevented from leaking to the outside even if MgO is volatilized. It is believed that the difference in concentration of magnesium (as oxide) becomes smaller and a more homogeneous and dense sintered body can be produced.

  The difference in magnesium content between the surface layer and the inside is preferably 0.4% by mass or less in terms of oxide (MgO).

  It is known that MgO is most volatilized on the surface of the grilled surface, but if the above configuration is used, the difference in concentration between the surface of the grilled surface and the internal magnesium (as oxide) can be reduced. In addition, a baked skin surface means the surface which has not performed processing, such as grinding, after baking. The total of the oxide equivalent of lanthanum, the oxide equivalent of aluminum, and the oxide equivalent of magnesium is 3.5% by mass or less.

  In the present embodiment, by setting the magnesium content of the surface layer to 1.3% by mass or less in terms of oxide, voids in the surface layer can be reduced and cutting performance can be improved. The magnesium content in the surface layer is preferably 0.3 to 0.8% by mass in terms of oxide.

  In addition, the silicon nitride sintered body of the present embodiment has a relative density of 99% or more, and grain boundaries on a surface having a depth of 0.3 mm from the surface of the sintered body and a surface having a depth of 1 mm from the surface of the sintered body. The area ratio of the phases is preferably 10% or less.

  The area ratio of the grain boundary phase on the surface having a depth of 0.3 mm from the surface of the sintered body and the surface having a depth of 1 mm from the surface of the sintered body is determined from the surface having the depth of 0.3 mm from the surface of the sintered body and the surface of the sintered body. Each surface with a depth of 1 mm is mirror-finished, and the mirror-finished surface is immersed in a liquid in a container composed of 23% by mass hydrofluoric acid and 77% by mass water, and placed in a water bath set to 50 ° C. for each container. It is allowed to stand for 3 hours, and the grain boundary phase portion is etched, the micrograph of the surface is subjected to image analysis, and the area ratio of voids to a certain area of the micrograph is obtained.

  The area ratio of the grain boundary phase on the surface with a depth of 0.3 mm from the surface of the sintered body and the surface with a depth of 1 mm from the surface of the sintered body is 10% or less, respectively, so there are few grain boundary phases in the sintered body Thus, the compositional difference of the grain boundary phase is also reduced, so that the chipping resistance can be improved.

  As described above, the liquid phase generation temperature of the grain boundary phase can be significantly lowered by the combination of lanthanum, silicon, aluminum, and magnesium. Thereby, firing at a low temperature becomes possible, abnormal grain growth in the length direction of the silicon nitride crystal grains can be suppressed, and refinement of the structure can be achieved. As a result, a silicon nitride sintered body with improved wear resistance and chipping resistance is obtained.

[About manufacturing method]
Below, the manufacturing method of the silicon nitride sintered compact of this embodiment is demonstrated. First, as a starting material, for example, silicon nitride powder, lanthanum hydroxide (La (OH) ₃ ), aluminum oxide (Al ₂ O ₃ ), and magnesium hydroxide (Mg (OH) ₂ ) are prepared. Moreover, if necessary, powder of silicon dioxide (SiO ₂ ) and periodic table group 6 element silicide is prepared.

  As the silicon nitride raw material, any of α-silicon nitride powder, β-silicon nitride powder, or a mixture thereof can be used. These particle sizes are preferably 1 μm or less, particularly 0.5 μm or less. As the lanthanum raw material, lanthanum oxide powder may be used. However, since lanthanum oxide has high hygroscopicity, a compound that has low water absorption and becomes lanthanum oxide in the firing process such as lanthanum hydroxide is used. preferable. In the lanthanum raw material, impurities of other rare earth elements of 1% by mass or less exist.

  Magnesium oxide or magnesium carbonate may be used as the magnesium raw material, but magnesium oxide has high water absorption, and magnesium carbonate generates carbon dioxide gas, so that water absorption is low like magnesium hydroxide. It is preferable to use a compound that does not generate carbon dioxide and becomes magnesium oxide in the firing process.

  Further, as the silicon raw material, one contained in the silicon nitride raw material is used. The composition is adjusted on the assumption that oxygen contained in the silicon nitride powder exists as silicon dioxide. If the silicon raw material is insufficient, silicon dioxide powder is added.

  As the aluminum raw material, aluminum oxide powder is used.

  Moreover, it is desirable to use tungsten silicide for the periodic table group 6 element silicide contained in the silicon nitride sintered body. As tungsten silicide raw material powder, any of tungsten oxide, carbide, silicide, nitride, etc. However, it is desirable to use an oxide because it is inexpensive and easily obtains a fine powder. In this way, even when raw materials other than silicide are used, the tungsten compound can be easily changed to tungsten silicide in the composition region of the silicon nitride-based sintered body of the present embodiment. Note that chromium or molybdenum can be used instead of tungsten.

  These raw materials are weighed, mixed and pulverized using a known pulverizing means such as a ball mill. A binder or a solvent is appropriately added to the mixed powder, and granulated by a spray drying method or the like.

  Then, the granulated powder obtained by granulating the mixed powder adjusted at a predetermined ratio is formed into an arbitrary shape by a known molding means such as die press molding, casting molding, extrusion molding, injection molding, cold isostatic pressing, etc. To form. The obtained molded body is fired at a temperature of 1650 to 1850 ° C. by a known firing means, for example, a normal pressure firing method in a nitrogen atmosphere, a gas pressure firing method, a hot press method, and the like, and then cooled and cooled. A silicon nitride-based sintered body can be obtained.

  And while magnesium content in the 0.3-mm depth surface from a sintered compact surface is 1.3 mass% or less in conversion of an oxide, magnesium content in a surface layer is less than internal magnesium content, and In order to make the difference in magnesium content between the surface having a depth of 0.3 mm and the surface having a depth of 1 mm from the surface of the sintered body 0.5 mass% or less in terms of oxide (MgO), a molded body is known. When firing by a firing means, for example, a normal pressure firing method in a nitrogen atmosphere, a gas pressure firing method, a hot press method, etc., hold at 1325 to 1375 ° C. for a predetermined time, then fire at a temperature of 1650 to 1850 ° C. and cool Thus, the silicon nitride sintered body of the present invention can be obtained.

  Although the reason why the difference in magnesium content between the surface having a depth of 0.3 mm and the surface having a depth of 1 mm from the surface of the sintered body is 0.5% by mass or less in terms of oxide is not clear, the present inventor Among the Group 3 elements, lanthanum is sintered at a temperature higher than the temperature at which MgO volatilization becomes remarkable because the eutectic temperature with silica is lower than the eutectic temperature between other Group 3 elements and silica. It becomes possible to lower the liquid phase generation temperature of the auxiliary agent, and by holding it at 1325 to 1375 ° C. for a predetermined time, the liquid phase can be sufficiently generated at a low temperature before MgO volatilization becomes remarkable in the firing stage. Even if MgO volatilizes due to the reduction of open pores due to volume shrinkage in the rearrangement process due to phase formation, it is possible to suppress the leakage of MgO to the outside, and this makes it possible to reduce the surface layer and internal magnesium (as oxide) Small density difference No longer homogeneous than conventional dense sintered body is considered to be produced.

  It goes without saying that the atmosphere used for the firing is mainly composed of nitrogen and may contain a small amount of oxygen as long as the silicon nitride sintered body is not oxidized in the firing step. Further, it may contain a component such as SiO or MgO that evaporates from a silicon nitride-based molded body or a so-called material.

  In particular, it is desirable to positively contain oxygen, silicon, and magnesium in a nitrogen atmosphere and fire. Examples of such an atmosphere containing oxygen, silicon, and magnesium include an atmosphere containing SiO and MgO.

Of the components added as sintering aids, lanthanum and aluminum oxide components are less likely to volatilize during firing, while magnesium and silicon oxide components are more likely to volatilize as SiO and MgO components. For this reason, by actively containing components such as SiO and MgO in the atmosphere and firing, decomposition of SiO ₂ added as a sintering aid or SiO ₂ contained in the silicon nitride raw material is suppressed, and MgO Volatilization is suppressed.

As a result, it becomes easy to produce a dense sintered body even with a small amount of added sintering aid. Such an atmosphere is obtained by, for example, mixing Si, SiO ₂ and MgO powder, or a mixed powder of silicon nitride powder and MgO powder in a firing pot together with a molded body that becomes the silicon nitride sintered body of the present embodiment by firing. Placed on the floor, spread or placed around the molded body that becomes the silicon nitride sintered body of the present embodiment by firing, and then fired together with the molded body that becomes the silicon nitride sintered body of the present embodiment by firing. It can be realized by evaporating Si or Mg.

  Furthermore, as described above, in order to reduce the grain size of silicon nitride crystal particles and suppress abnormal grain growth of silicon nitride crystal grains, it is necessary to start densification from a low temperature and suppress grain growth at a high temperature. In order to increase the relative density, it is important to promote the densification at a low temperature at which grain growth is not remarkable, and then raise the temperature and sufficiently densify. It is desirable that after firing at 1650 ° C. to 1850 ° C. in a nitrogen atmosphere, hot isostatic firing is performed at 9.8 MPa to 294 MPa and 1500 to 1700 ° C.

  As a result, a dense silicon nitride sintered body with improved chipping resistance in which abnormal grain growth of silicon nitride crystal grains is suppressed can be obtained.

[About cutting tools]
In addition, when used as a tool, the silicon nitride-based sintered body of the present embodiment is a tool with improved wear resistance and chipping resistance, and in particular, cutting that requires wear resistance and chipping resistance. It is suitably used as a tool.

  In addition, as shown in FIG. 1, when using as the cutting tool 11, the square plate-shaped form provided with the cutting blade 13 in the corner | angular part and a conventionally well-known form are applicable.

  The manufacturing method of the cutting tool is the above-described manufacturing method of the silicon nitride sintered body. By forming the cutting tool into a desired cutting tool shape, the cutting tool made of the silicon nitride sintered body of the present embodiment can be easily obtained. Can be produced. Moreover, it cannot be overemphasized that you may process into a cutting tool shape by cutting, grinding | polishing, etc. after baking. Furthermore, a coating such as TiN or TiAlN may be applied to the surface of the cutting edge of the cutting tool made of the silicon nitride sintered body of the present embodiment.

  In addition, when cutting a workpiece using the above cutting tool, the cutting tool can improve wear resistance and chipping resistance, thus improving the dimensional accuracy of the workpiece and improving the durability of the cutting tool. Therefore, cutting can be performed without replacing the cutting tool for a long period of time, and the replacement time of the cutting tool can be reduced.

  Furthermore, although not shown in figure, a cutting apparatus can be comprised including at least the said cutting tool and the holding stand for hold | maintaining the to-be-cut material processed with this cutting tool. Examples of the cutting device include a turning device such as a lathe and a milling device such as a machining center. In this case, cutting can be performed without replacing the cutting tool for a long period of time, and the number of replacements of the cutting tool is reduced, thereby reducing the cost.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further in detail, this invention is not limited to a following example.

As starting materials, silicon nitride powder with an average particle size of 0.3 μm, lanthanum hydroxide (La (OH) ₃ ) with an average particle size of 1.2 μm containing Ce, Pr, Nd as impurities in an amount of 1% by mass or less, average particles An aluminum oxide having a diameter of 0.7 μm and a magnesium hydroxide having an average particle diameter of 2.5 μm were prepared.

  These raw material powders were mixed together with a binder and a solvent, and then pulverized and mixed in a ball mill for 72 hours. Thereafter, the solvent was removed by drying to produce a granulated powder, and this granulated powder was press-molded at a pressure of 98 MPa to obtain a molded product for the cutting tool shape SNGN120212.

The molded body was degreased, held at 1350 ° C. for the time shown in Table 1 (3 to 6 hours), fired at 1750 ° C. for 5 hours, and N ₂ atmosphere at normal pressure, and then 1600 ° C., 2 hours, 196 MPa. The sintered body was obtained by hot isostatic firing (HIP) under the conditions of

  When the density of the sintered body was measured by the Archimedes method for the obtained silicon nitride-based sintered body and converted to the relative density, the relative density was 100% for all samples.

  Further, the oxygen content of the obtained silicon nitride sintered body was measured by an infrared absorption method after pulverizing the silicon nitride sintered body into a powder form. The outermost layer (baked skin surface) to 0.3 mm was collected as the surface layer, and the 0.3 to 0.7 mm portion was sampled and measured inside.

Further, the surface of the obtained silicon nitride-based sintered body was subjected to surface grinding by 0.3 mm and 1.0 mm, and composition analysis was performed on each surface using fluorescent X-ray analysis. Table 1 shows the amounts of MgO, rare earth elements Re ₂ O ₃ and Al ₂ O ₃ , and MgO concentration difference (internal MgO amount−surface layer MgO amount). The total amount of the calculated periodic table group 3 element oxides Re ₂ O ₃ , Al ₂ O _{3 and} MgO was determined. Rare earth elements other than lanthanum were 0.03% by mass or less in the total amount in terms of oxides.

Each surface is mirror-finished, and the mirror-finished surface is immersed in a liquid in a container composed of 23% by mass hydrofluoric acid and 77% by mass water, and placed in a water bath set to 50 ° C. for each container. Then, leave it for 3 hours to etch the grain boundary phase part, take a 0.015 mm ² field of view of the surface with a scanning electron micrograph (1000 times), and use the image analyzer to determine the ratio of the porosity in the field The area ratio of the grain boundary phase was determined and the value is shown in Table 2.

  Further, using the obtained silicon nitride sintered body, a cutting tool as shown in FIG. 1 is manufactured, and a workpiece: FCD-450, a cutting speed: 500 m / min, a feed amount: 0.2 mm / rev, Turning test under conditions of cutting depth: 2.0 mm, cutting time: 120 sec, material to be cut: FCD-450, cutting speed: 500 m / min, feeding amount: 0.5 mm / tooth, cutting amount: 2.0 mm, pass Number of times: Milling test was performed under the condition of 10 passes. In the evaluation, the flank wear amount of the blade edge was calculated by taking a photograph using a microscope with a length measuring device after the turning test and measuring the average value of the wear amount. Further, the chipping of the blade edge was measured by observing the blade edge with a microscope and taking a photograph after the milling test to determine the area of the chipping region. These results are shown in Table 2.

Sample No. 12 uses Y ₂ O ₃ as the rare earth element oxide (Re ₂ O ₃ ). In No. 13, Er ₂ O ₃ was used as a rare earth element oxide (Re ₂ O ₃ ).

Sample No. No. 14 is a case where MgO powder is disposed around the molded body and fired simultaneously with the molded body. These results are also shown in Table 2.

  According to the results shown in Tables 1 and 2, sample Nos. Within the scope of the present invention. All of Nos. 1 to 9 showed cutting performance with a small amount of wear and little chipping of the blade edge. In contrast, Sample No. Nos. 10 and 11 showed increased wear in the cutting test. In addition, comparative sample No. 1 using Y or Er as the rare earth element was used. 12 and 13 have a large amount of wear and a large chipping area.

It is a perspective view which shows an example of the cutting tool of this embodiment.

11 ... Cutting tool 13 ... Cutting blade

Claims (7)

  A silicon nitride-based sintered body having a crystal phase mainly composed of a silicon nitride crystal and an amorphous grain boundary phase containing lanthanum, aluminum, magnesium, silicon and oxygen and present at the grain boundary of the silicon nitride crystal. In addition, the magnesium content in the plane having a depth of 0.3 mm from the surface of the silicon nitride sintered body is 1.3 mass% or less in terms of oxide, and the depth from the surface of the silicon nitride sintered body is The magnesium content in the 0.3 mm plane is less than the magnesium content in the 1 mm depth from the surface of the silicon nitride sintered body, and the depth from the surface of the silicon nitride sintered body is 0. A silicon nitride sintered body characterized in that a difference in magnesium content between a 3 mm surface and a 1 mm deep surface is 0.5% by mass or less in terms of oxide.   The total of the oxide equivalent amount of the lanthanum, the oxide equivalent amount of the aluminum, and the oxide equivalent amount of the magnesium is 3.5% by mass or less in the total amount of the silicon nitride sintered body. Item 2. A silicon nitride sintered body according to Item 1.   The silicon nitride based sintered body according to claim 1 or 2, wherein the surface of the silicon nitride based sintered body is a burnt surface.   The area ratio of the grain boundary phases on the surface having a depth of 0.3 mm from the surface of the silicon nitride-based sintered body and the surface having a depth of 1 mm from the surface of the silicon nitride-based sintered body is 10% or less, respectively. The silicon nitride based sintered body according to any one of claims 1 to 3.   A cutting tool comprising the silicon nitride sintered body according to any one of claims 1 to 4.   A cutting apparatus comprising: the cutting tool according to claim 5; and a holding base for holding a workpiece to be processed by the cutting tool.   A cutting method, comprising: cutting a workpiece using the cutting tool according to claim 5.

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