JP5527949B2 - Silicon nitride sintered body - Google Patents

Description

  The present invention relates to a silicon nitride sintered body having high wear resistance.

  Silicon nitride-based sintered bodies are excellent in wear resistance and oxidation resistance due to their high hardness and stability at high temperatures, and are used as heat engine parts such as gas turbines and turbo rotors and cutting tools. .

In such a silicon nitride sintered body, attempts have been made to improve the characteristics by modifying the surface of the sintered body. For example, in Patent Document 1, the sintering aid component concentration on the surface of the sintered body is increased from the inside by applying a slurry having a high content ratio of the sintering aid component to the surface of the molded body and firing the slurry. Further, it is disclosed that the surface of a silicon nitride-based sintered body is smoothed. Moreover, in patent document 2, while making the amount of oxygen of a surface layer part smaller than the amount of internal oxygen, the grain boundary phase component of a surface layer part is crystallized without crystallizing an internal grain boundary phase so much, It is described that only a surface layer portion is advantageous at high temperatures, and that a sintered body excellent in strength can be produced up to an intermediate temperature range in terms of thickness. Further, Patent Document 3 discloses that a silicon nitride sintered body in which the grain boundary phase is crystallized in the surface portion and the ratio of silicon nitride particles in the surface portion is lower than that in the inside is used as a cutting tool. .
JP-A-7-309664 JP-A-6-191947 Japanese Patent Laid-Open No. 2-275863

  However, the silicon nitride sintered body having a large content of the sintering aid component such as the silicon nitride sintered bodies described in Patent Documents 1 to 3 described above has a limit to the wear resistance on the surface of the sintered body. was there. Further, simply reducing the amount of the sintering aid has a problem that the sinterability of the sintered body is lowered and the fracture resistance is lowered.

  Accordingly, an object of the present invention is to provide a silicon nitride sintered body having high wear resistance and improved fracture resistance.

The silicon nitride sintered material of the present invention, viewed including the silicofluoride-containing nitride than 93 wt%, 0.1 to 4.5 wt% of RE elements oxides terms _{of RE} 2 _{O 3,} magnesium oxide in terms of MgO 0 0.3 to 2.5% by mass, aluminum oxide in terms of Al ₂ O _{3 0} to 0.6% by mass, and the remaining oxygen in terms of silica (SiO ₂ ) 0.1 to 4.5% by mass, periodic table The group 6 element silicide is contained in a ratio of 0 to 2% by mass , and the cross-sectional polished surface including the surface of the sintered body is etched with a hydrofluoric acid aqueous solution, and the cross-sectional polished surface is in a range of 35 μm × 32 μm. About the structure | tissue photograph, the porosity in the surface area | region containing the surface of the said sintered compact is characterized by being smaller than the porosity in the inside of the said sintered compact.

  Here, in the structure described above, with respect to the structure photograph of the cross-sectional polished surface after the etching treatment, an average particle diameter of the silicon nitride crystal in the surface region is 0.35 to 0.55 μm, and the surface region It is desirable that the average grain size of the silicon nitride crystal is smaller than the average grain size of the silicon nitride crystal in the interior.

  Further, in the above structure, in the structure photograph of the cross-sectional polished surface after the etching treatment, the existence ratio of silicon nitride crystals observed with a minor axis diameter of 1 μm or more and an aspect ratio of 5 or more is more than the surface region than the inside. It is desirable that there be a small amount.

Furthermore, in the said structure, it is desirable to contain silicon nitride by the ratio of 94.5-99.5 mass% in the whole composition of the said silicon nitride sintered body.

  According to the silicon nitride sintered body of the present invention, the content ratio of silicon nitride crystals is high, and the porosity is smaller than the internal porosity even when the surface of the sintered body is etched with a hydrofluoric acid aqueous solution. The presence of the surface region can increase the hardness on the surface of the sintered body and increase the fracture resistance of the sintered body.

  Here, regarding the structure photograph of the cross-sectional polished surface after the etching treatment, the average particle diameter of the silicon nitride crystal in the surface region is 0.35 to 0.55 μm, and the silicon nitride crystal in the surface region is It is desirable in that the average particle size is smaller than the average particle size of the silicon nitride crystal in the interior, thereby increasing the toughness on the surface of the sintered body and further improving the fracture resistance.

  Further, in the structure photograph of the cross-sectional polished surface after the etching treatment, the existence ratio of silicon nitride crystals observed with a minor axis diameter of 1 μm or more and an aspect ratio of 5 or more is smaller in the surface region than in the interior. In addition, the chipping resistance in the surface region is high, and the thermal conductivity of the sintered body is improved and the thermal shock resistance is improved.

Further, the overall composition of the silicon nitride-based sintered body is 94.5 to 99.5% by mass of silicon nitride, 0.1 to 4.5% by mass of RE element oxide in terms of RE ₂ O ₃ , magnesium oxide. 0.1 to 4.5 mass 0.3 to 2.5 mass% in terms of MgO, an oxide of aluminum in terms _{of Al} 2 _{O 3} in 0 to 0.6 wt%, the residual oxygen in the silica (SiO ₂₎ in terms It is desirable that the periodic table group 6 element silicide is 0 to 2% by mass in that the hardness and high-temperature strength of the sintered body can be increased and the wear resistance of the sintered body can be increased. In particular, it is desirable that the content ratio of the magnesium element on the surface is 0.1 to 1.0% by mass because the wear resistance on the surface of the sintered body can be improved.

  For the silicon nitride sintered body of the present invention (hereinafter simply referred to as a sintered body), the surface of the cross-sectional polished surface after etching the cross-sectional polished surface including the surface of the sintered body with a hydrofluoric acid aqueous solution. Description will be made based on FIG. 1 which is a structure photograph in the region A and FIG. 2 which is a structure photograph in the inside B.

  Sintered body 1 contains 93% by mass or more of silicon nitride crystal 2, and in addition, RE element compound (wherein RE is at least one element of yttrium and rare earth elements), magnesium compound, aluminum compound, silica (oxidation) It contains a sintering aid component such as silicon. The content ratio of the silicon nitride crystal 2 can be estimated as the silicon nitride crystal 2 by determining the content of the sintering aid component by composition analysis (ICP analysis and oxygen analysis) of the sintered body 1. .

  And according to this invention, in FIG. 1 which is a structure | tissue photograph in the surface area | region A of the cross-section polishing surface after etching the cross-section polishing surface containing the surface of the sintered compact 1 with hydrofluoric acid aqueous solution, and the inside B As shown in FIG. 2, which is a structure photograph, voids 3 are formed in the sintered body 1 of the structure photograph, and the porosity in the surface region A of the sintered body 1 is the void in the interior B of the sintered body 1. It is a big feature that it is smaller than the rate. Thereby, the hardness in the surface region A of the sintered body 1 is high, and the fracture resistance of the sintered body 1 can be increased.

  Here, in the structure photograph of FIG. 1, the average grain size of the silicon nitride crystal in the surface region A is 0.35 to 0.55 μm, and as is clear from FIGS. It is desirable that the average grain size of the crystal 2 is smaller than the average grain size of the silicon nitride crystal 2 in the interior B, thereby increasing the toughness on the surface of the sintered body 1 and further improving the fracture resistance. In FIGS. 1 and 2, the grain boundary phase is present in the whitened region on the outer periphery of the silicon nitride crystal, and a part is eluted by the etching process.

  In addition, in the structure photographs of FIGS. 1 and 2, the presence of the silicon nitride crystal 2 observed at a minor axis diameter of 1 μm or more and an aspect ratio of 5 or more is smaller in the surface region A than in the interior B. There is an effect that the thermal conductivity of the body 1 is improved and the temperature of only the processed portion is prevented from increasing, and the thermal shock resistance is improved.

The surface area A of the sintered body 1 in the present invention refers to a range from the surface of the sintered body 1 to a depth area of 35 μm , and the porosity including the surface area A is a void inside the sintered body. It is desirable that the range smaller than the rate exists over a depth of 100 to 500 μm from the surface in terms of enhancing the fracture resistance while maintaining the wear resistance on the surface of the sintered body 1. Further, the inside B is a region excluding a range in which the porosity including the surface region A of the sintered body 1 is smaller than the porosity in the inside of the sintered body , and when observing the structure of the inside B in the present invention. is measured for a region in the thickness direction 35 [mu] m × width 32 mu m in the vicinity of the center portion of the sintered body 1.

Here, the overall composition of the sintered body 1 includes 93% by mass or more of silicon nitride , 0.1 to 4.5% by mass of RE element oxide in terms of RE ₂ O ₃ , and 0.3% of magnesium oxide in terms of MgO. 3 to 2.5% by mass, 0 to 0.6% by mass of aluminum oxide in terms of Al ₂ O ₃ , 0.1 to 4.5% by mass of residual oxygen in terms of silica (SiO ₂ ), 6th periodic table Although the group element silicide is contained in a ratio of 0 to 2% by mass, the desirable overall composition is 94.5 to 99.5% by mass of silicon nitride and 0.1 to 0.1% of RE element oxide in terms of RE ₂ O _3. 4.5 wt%, magnesium oxide 0.3 to 2.5 wt% in terms of MgO, 0 to 0.6 wt% of aluminum oxide in terms _{of Al} 2 _{O 3,} the residual oxygen in the silica (SiO ₂₎ in terms 0.1 to 4.5% by mass, consisting of 0 to 2% by mass of the periodic table group 6 element silicide, It is desirable in that the hardness and high-temperature strength of the sintered body 1 can be increased to increase the wear resistance of the sintered body 1. Note that the RE element compound, the magnesium compound, and the aluminum compound are all preferably present as oxides in that the silicon nitride crystal 2 can be firmly bonded with a small amount and the content ratio of the silicon nitride crystal 2 can be increased.

Furthermore, the content of the RE element oxide is 0.5 to 4.5% by mass in terms of RE ₂ O ₃ and further 1 to 2.5% by mass for densification of the sintered body 1. It is desirable to be. The content of magnesium oxide is 0.35 to 2.0% by mass in terms of MgO in order to densify the sintered body 1 at a lower temperature by lowering the liquid phase generation temperature of the sintering aid, Furthermore, it is desirable that it is 0.4-1.0 mass%. The content of aluminum oxide is reduced in terms of Al ₂ O ₃ in order to suppress lowering of the liquid phase generation temperature of the sintering aid, densification of the sintered body 2 and lowering of wear resistance due to lowering of oxidation resistance. It is desirable that it is 0.3-0.5 mass%. The remaining oxygen is considered to be present as an impurity of silicon nitride and as silica (SiO ₂ ), but its content is reduced in the liquid phase formation temperature of the sintering aid, the denseness of the sintered body In order to realize the sintered body 2 that keeps the temperature and improves the oxidation resistance and wear resistance, it is preferably 0.1 to 4.5% by mass in terms of SiO ₂ , especially 1.5 to 2.%. It is desirable to set it as 2 mass%.

In addition, when lanthanum (La) is contained as an essential element as the RE element, the sintered body can be densified at a lower temperature than when lanthanum (La) is not included, so that crystals in the sintered body grow abnormally. Crystal without atomization. For example, the average major axis of six silicon nitrides is suppressed to 10 μm or less from the larger major axis with a relative density of 99% or more and a field of view of 0.015 mm ² by atmospheric pressure firing at 1730 to 1830 ° C. Is possible. As a result, the hardness and strength of the sintered body can be improved.

Here, the RE compound, the magnesium compound, the aluminum compound, and silica (SiO ₂ ) form a grain boundary phase. The grain boundary phase may have a structure in which a part of the grain boundary phase is precipitated, but the hardness of the sintered body 1 can be reduced by reducing the abundance ratio of the grain boundary phase itself to 7% by mass or less, particularly 5% by mass or less. Since the high-temperature strength is increased, it is desirable that the grain boundary phase is present in an amorphous state in order to increase the bondability of the silicon nitride crystal while reducing the absolute amount of the grain boundary phase.

  The silicon nitride crystal 2 exists as a main crystal. The silicon nitride crystal 2 is mainly composed of β-silicon nitride crystal, and if desired, a part thereof contains a little aluminum to form β-sialon. It is desirable that Further, a part of the β-silicon nitride crystal may be an α-silicon nitride crystal, but it is desirable not to include the α-silicon nitride crystal in order to increase hardness and strength.

  Further, the Group 6 element silicide of the periodic table can suppress a decrease in high-temperature strength and can blacken the color of the sintered body 1. Examples of the Periodic Table Group 6 element silicide include chromium silicide, molybdenum silicide, and tungsten silicide. However, tungsten silicide is used because it can be present as fine particles in the fired body 1 using a fine oxide raw material. It is desirable to use The periodic table group 6 element silicide particles are dispersed in the grain boundary phase of the sintered body 1.

  Next, the manufacturing method of the sintered body described above will be described.

First, as starting materials, for example, silicon nitride (Si ₃ N ₄ ) powder, hydroxide (RE (OH) ₂ ) or oxide (RE ₂ O ₃ ), and aluminum oxide (Al ₂ O ₃ ) of RE element Magnesium hydroxide (Mg (OH) ₂ ) is prepared. If necessary, silicon dioxide (SiO ₂ ) and periodic table group 6 element silicide powders are 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. Inevitable oxygen exists as silicon oxide in the silicon nitride raw material. Therefore, the composition is adjusted on the assumption that the oxide present in the silicon nitride raw material is present as silica (SiO ₂ ). When the oxygen content is insufficient, silica (SiO ₂ ) powder is added.

Oxide powder may be used as a raw material for the RE element. For example, in the case of lanthanum (La), lanthanum hydroxide (La (OH) ₂ ) has high hygroscopicity because lanthanum oxide (La ₂ O ₃ ) has high hygroscopicity. It is preferable to use a compound having a low water absorption and changing to lanthanum oxide (La ₂ O ₃ ) during the firing process. Magnesium oxide (MgO) or magnesium carbonate (MgCO ₃ ) may be used as the magnesium (Mg) raw material, but magnesium oxide (MgO) has high water absorption, and magnesium carbonate (MgCO ₃ ) generates carbon dioxide gas. Therefore, it is preferable to use a compound such as magnesium hydroxide (Mg (OH) ₂ ) that has low water absorption, does not generate carbon dioxide, and changes to magnesium oxide (MgO) during the firing process.

  The raw material for forming the periodic table group 6 element silicide may be any of the oxides, carbides, silicides, nitrides, etc. of the periodic table group 6 element, but it is oxidized because it is inexpensive and easily obtains a fine powder. It is desirable to use a product.

  Next, a binder or a solvent is appropriately added to the mixed powder obtained by weighing these raw materials, mixed and pulverized, and dried and granulated by a spray drying method or the like. And this granulated powder is shape | molded by press molding in arbitrary shapes. According to the present invention, the molding pressure during press molding needs to be a molding pressure of 120 MPa or more. By molding at such a high molding pressure, a density difference between the surface and the inside of the molded body can be produced, and a difference between the surface and the inside can exist even after firing.

  Thereafter, firing is performed at a temperature of 1650 to 1830 ° C. by a normal pressure firing method, a gas pressure firing method, a hot press method, or the like, for example, in a nitrogen atmosphere.

  The specific conditions for this firing are that the molded body is placed in a firing bowl made of a silicon nitride sintered body, and Si and Mg components are placed in the firing bowl so that the sealing state of the firing bowl is so high. Close and set in the firing furnace. Then, after replacing the inside of the firing furnace with 1 atm of nitrogen, the temperature increase was started at 5 to 15 ° C./min, and the temperature increase rate in the temperature range of 1400 to 1500 ° C. was changed to 1 to 5 ° C./min. The heating rate from 1500 ° C. to the firing temperature of 1730 to 1830 ° C. is changed again to 5 to 15 ° C./min. And it is set as the conditions which hold | maintain at the baking temperature of 1730-1830 degreeC for 5 to 10 hours, and cool. Further, during the firing, the atmosphere in the furnace is adjusted to be maintained at 1 atm of nitrogen by degassing or additionally introducing nitrogen gas. As a result, magnesium volatilizes moderately on the surface and RE element does not volatilize, densifying the sintered body and obtaining a sintered body having a predetermined surface and internal structure.

Incidentally, Si and Mg components put together molded body during the firing bowl, Si powder, SiO ₂ powder, _Si 3 _{N 4} powder, MgO powder, include a method of putting in Mg _{(OH) 2} powder state, these powders Is placed around the molded body, spread on the lower surface of the molded body, or fired in a state where the molded body is embedded in the powder, thereby generating SiO gas and MgO gas in the firing atmosphere to sinter. The method of promoting is mentioned.

  Further, as described above, in order to obtain a difference in structure between the surface and the inside of the sintered body 1, it is necessary to sufficiently raise the temperature to the inside of the sintered body 1 in a temperature range where element diffusion starts. It becomes possible by setting the temperature rising rate between ˜1500 ° C. to 1-5 ° C./min. In addition, after firing at 1730 ° C. to 1830 ° C. in a nitrogen atmosphere, hot isostatic firing is performed at 9.8 MPa to 294 MPa and 1500 to 1700 ° C. to obtain dense and abnormal grain growth of the silicon nitride crystal 2 This is desirable in that a sintered body 1 with improved chipping resistance with reduced resistance can be obtained.

Moreover, although the sintered body mentioned above is ground according to the target performance, as long as it grinds in the state where the surface area | region prescribed | regulated by this invention remains, the effect of this invention is not lost. And since the said sintered compact is excellent in hardness and toughness, it can be adapted for structural members, but it is particularly suitably used as a cutting tool that requires wear resistance and fracture resistance. It may be subjected to hard coating layer of TiN and _Al 2 _O 3, TiAlN or the like on the surface of the sintered body.

As starting materials, silicon nitride (Si ₃ N ₄ ) powder having an average particle size of 0.3 μm, RE element compound (lanthanum hydroxide (La (OH) ₂ ), yttrium oxide (Y ₂ O) having an average particle size of 1.2 μm ₃ ), ytterbium oxide (Yb ₂ O ₃ ), erbium oxide (Er ₂ O ₃ ), cerium oxide (Ce ₂ O ₃ ) powder, and aluminum oxide (Al ₂ O ₃ ) having an average particle size of 0.7 μm ) After preparing the powder and magnesium hydroxide (Mg (OH) ₂ ) powder having an average particle diameter of 2.5 μm so that the total composition of the sintered body is the ratio shown in Table 2 , and adding the binder and the solvent Then, it was pulverized and mixed in an attritor mill for 70 hours. Thereafter, the solvent was removed by drying to prepare a granulated powder, and this granulated powder was press-molded into a SNGN120212 cutting tool shape with the molding pressure shown in Table 1.

When this molded body is set in a firing pot, it is set in a state where a mixed powder of at least one of Si ₃ N ₄ powder, Si powder, and SiO ₂ powder and Mg (OH) ₂ powder is filled in the firing pot. The lid was put on and placed in a firing furnace in a state where it was placed in a carbon cylinder. Then, the inside of the firing furnace was replaced with 1 atm of nitrogen, and after degreasing, the temperature was raised to 1400 ° C. at a rate of temperature rise of 10 ° C./min. Thereafter, the temperature was raised and fired under the conditions shown in Table 1. The atmosphere during firing was controlled to 1 atm of nitrogen. Thereafter, hot isostatic pressing (HIP) is performed at 1600 ° C. for 2 hours and 196 MPa, and the surface of the sintered body is further subjected to 0.3 mm thickness grinding (double-head processing and peripheral processing) to sinter silicon nitride. Got the body.

  About the obtained silicon nitride sintered body, the cross section including the surface of the sintered body was polished into a mirror surface state. Next, this sintered body including the mirror surface was immersed in a hydrofluoric acid aqueous solution having a concentration of 23 vol% at 50 ° C. for 2.5 hours for etching treatment, then pulled up from the hydrofluoric acid aqueous solution, washed with water, washed and dried. And about the mirror surface of this etched sintered compact, the structure | tissue photograph was each taken in the range of 35 micrometers x 32 micrometers in the surface area | region and the inside of a sintered compact, and the porosity in the surface area | region and the inside was calculated by image analysis from each photograph. . Moreover, using this structure photograph, the number of silicon nitride crystals observed with a minor axis diameter of 1 μm or more and an aspect ratio of 5 or more among the crystals constituting the structure was confirmed. Furthermore, the average particle size of these crystals was determined by image analysis. Furthermore, the width of the surface region was specified from the low-magnification photograph of this mirror surface. 1 and 2 are sample Nos. It is a structure photograph of 1.

  Moreover, content of each component contained in a sintered compact was computed by making a sintered compact into a powder form. The results are shown in Table 2.

Furthermore, cutting performance was evaluated under the following conditions using a cutting tool made of the obtained silicon nitride sintered body.
Cutting method: Milling material to be cut: FCD-450 Block material Cutting speed with 4 holes: 500 m / min Feed amount: 0.2 mm / rev
Cutting depth: 2.0mm
Cutting conditions: Dry cutting evaluation items: After 10 passes, the flank wear amount and chipping state of the cutting edge were confirmed by microscopic observation.
The results are shown in Table 3.

  According to the results shown in Tables 1 to 3, sample Nos. Within the scope of the present invention. Nos. 1 to 5 all showed cutting performance with a small amount of wear and little chipping of the blade edge.

In contrast, Sample No. with a silicon nitride content of less than 93% by mass. No. 6 had poor wear resistance. Sample No. 1 having a heating rate at 1400 to 1500 ° C. faster than 5 ° C./min. In No. 7, the porosity in the interior after the etching treatment was smaller than the porosity on the surface, and the wear resistance was lowered. Furthermore, sample No. 1 whose temperature increase rate in the temperature range of 1500 ° C. or higher is slower than 10 ° C./min. In No. 9, the porosity in the interior after the etching treatment was as small as the porosity on the surface, but the chipping frequently occurred and the wear easily progressed. Further, Sample No. with a molding pressure of 98 MPa (1 tonf / cm ² ) was used. In No. 8, the surface was etched as well as the inside, causing abnormal wear.

It is a structure | tissue photograph in the surface area | region A of the cross-section grinding | polishing surface after etching the cross-section grinding | polishing surface containing the surface of a sintered compact with a hydrofluoric acid aqueous solution about an example of the nitride-based elementary sintered body of this invention. It is a structure | tissue photograph in the inside B of the cross-section grinding | polishing surface of FIG.

Explanation of symbols

1 Nitride-based sintered body (sintered body)
2 Silicon nitride crystal 3 Void

Claims (4)

Nitride silicofluoride-containing more than 93 wt% seen including, 0.1 to 4.5 wt% of RE elements oxides terms _{of RE} 2 _{O 3,} 0.3 to 2.5 wt% of magnesium oxide in terms of MgO, aluminum oxide 0 to 0.6% by mass in terms of Al ₂ O ₃ , 0.1 to 4.5% by mass of residual oxygen in terms of silica (SiO ₂ ), and 0 to 2% by mass of Group 6 element silicides in the periodic table A structure in a range of 35 μm × 32 μm of the cross-sectional polished surface after etching the cross-sectional polished surface including the surface of the sintered compact with a hydrofluoric acid aqueous solution. A silicon nitride sintered body characterized in that the porosity in a surface region including the surface of the sintered body is smaller than the porosity in the inside of the sintered body.   Regarding the structure photograph of the cross-sectional polished surface after the etching treatment, the average particle size of the silicon nitride crystal in the surface region is 0.35 to 0.55 μm, and the average particle size of the silicon nitride crystal in the surface region is The silicon nitride based sintered body according to claim 1, wherein the silicon nitride sintered body is smaller than an average particle diameter of the silicon nitride crystal in the inside.   Regarding the structure photograph of the cross-section polished surface after the etching treatment, the existence ratio of silicon nitride crystals observed at a minor axis diameter of 1 μm or more and an aspect ratio of 5 or more is smaller in the surface region than in the interior. The silicon nitride based sintered body according to claim 1 or 2. The silicon nitride-based sintered body according to any one of claims 1 to 3, wherein the silicon nitride is contained in a ratio of 94.5 to 99.5 mass% in the overall composition of the silicon nitride-based sintered body. Union.