JPH06247773A - Silicon nitride based sintered body and engine member - Google Patents

Abstract

(57) [Summary] [Purpose] Not only static strength, but also dynamic strength, especially excellent fatigue strength characteristics, making it a sliding member for various equipment, especially exhaust valves for automobile engines. Provided is a silicon nitride-based sintered body useful as an engine member. [Constitution] α-silicon nitride (α-Si ₃ N ₄ ) and β'-sialon are present, and at least one crystalline phase containing silicon is present, and the crystalline phase containing silicon is the main component in X-ray diffraction. The diffraction intensity of the peak is (210) of α-silicon nitride.
Plane and β'-sialon (210) plane diffraction intensity in the range of 0.5 to 5%, the Ono formula has a rotational bending fatigue limit of 600 MPa or more, and a CO ₂ gas atmosphere at 900 ° C. Weight change rate after leaving for 1000 hours is ± 3%
A silicon nitride-based sintered body that is within.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride-based sintered body which is particularly excellent in durability and fatigue strength characteristics, and an engine member made of this silicon nitride-based sintered body.

[0002]

2. Description of the Related Art In recent years, as pollution of the global environment due to exhaust gas and the like has become a problem, in an automobile-related field, an engine such as a valve train is used for the purpose of weight reduction and high efficiency of the engine in order to improve fuel efficiency. Attempts have been made to use ceramics as component materials. In particular, the silicon nitride-based sintered body is the most promising material because of its light weight, high strength, high toughness, excellent thermal shock resistance, and high Young's modulus.

Examples of engine parts that have been put into practical use by using such a silicon nitride sintered body include turbocharger rotors and exhaust valves. However, these utilize the lightness and wear resistance of the silicon nitride-based sintered body, and in order to use the silicon nitride-based sintered body as an engine part such as a valve train, its strength characteristics, particularly durability At present, it cannot be said that the fatigue strength characteristics are sufficient.

Of the engine parts, particularly when used in a normal reciprocating engine or the like, an exhaust valve or the like is repeatedly subjected to compressive-tension stress in a corrosive atmosphere such as CO ₂ gas at a high temperature of about 1000 ° C. in a steady operation state. . Therefore, considering the number of fatigue caused by long-distance running,
In order to make engine parts such as exhaust valves into ceramics, it is desired to develop a silicon nitride-based sintered body that is further excellent in corrosion resistance while further improving durability fatigue strength.

[0005]

SUMMARY OF THE INVENTION In view of the above conventional circumstances, the present invention provides not only static strength but also dynamic strength,
It is an object of the present invention to provide a silicon nitride-based sintered body which is particularly excellent in durability and fatigue strength characteristics and is useful as a sliding member such as an engine member such as an exhaust valve of an automobile engine and a bearing for various devices.

[0006]

To achieve the above object, a silicon nitride-based sintered body provided by the present invention contains α-silicon nitride (α-Si ₃ N ₄ ) and β'-sialon, There is at least one other crystalline phase containing silicon, and the diffraction intensity of the main peak in X-ray diffraction of the crystalline phase containing silicon is (210) plane of α-silicon nitride and (210) of β′-sialon. 0.5 to 5% of the sum of the diffraction intensity of the surface
It is in the range of.

Further, the silicon nitride-based sintered body of the present invention is excellent in static strength and has a rotational bending fatigue limit of 600.
The durability fatigue strength is significantly improved as compared with the conventional silicon nitride-based sintered body, and it is 9 MPa in a CO ₂ gas atmosphere.
It has excellent corrosion resistance, with a weight increase / decrease rate of 3% or less after being left at 00 ° C for 1000 hours, so it can be used as an engine component such as exhaust valves, cam followers, piston heads, piston pins, etc. that compose engines for automobiles, etc. Are suitable.

[0008]

In the silicon nitride sintered body of the present invention, α-
By including silicon nitride (Si ₃ N ₄ ) and β′-sialon, a structure where equiaxed and fine α-Si ₃ N ₄ crystal grains exist between columnar β′-sialon crystal grains is formed. As a result, the packing density of the crystal grains is improved, the thickness of the grain boundary phase between the crystal grains is thinned, and the corrosion resistance of the grain boundary phase, which has low crystallinity and is easily corroded and deteriorated, improves the defect The improvement of the propagation resistance improves and improves the strength, particularly the fatigue strength. still,
β′-sialon has the general formula Si _6-Z Al _Z O _Z N _8-Z (0
It is represented by <Z <4.2).

The following two points are also important as conditions for improving the packing density of the crystal grains of α-Si ₃ N ₄ and β'-sialon in the silicon nitride sintered body. As a first point, the peak intensity in X-ray diffraction of each crystal phase is 0 <α-Si ₃ N ₄ ≦ 30% when the sum of the peak intensities of both is 100%.
And 70% ≦ β′-sialon <100%. If the precipitation value of α-Si ₃ N ₄ exceeds 30%, the sinterability deteriorates, and the densification due to sintering often does not proceed, and the packing density of the crystal grains decreases and the grain boundary phase This is because the corrosion resistance and fatigue strength of the sintered body decrease because the thickness increases.

Secondly, the average crystal grain size of α-Si ₃ N _{4 in} the sintered body is 0.5 μm or less, and the average crystal grain size of β'-sialon is 1.0 to 1.0 in the major axis direction. It is preferably 5.0 μm and 0.5 μm or less in the minor axis direction. If any of the average crystal grain diameters exceeds the respective upper limit values, the packing density is lowered, the thickness of the grain boundary phase is increased, and the corrosion resistance and fatigue strength of the sintered body are lowered. In particular, when the sintered body of the present invention is used as an engine member or the like, by controlling the average crystal grain size within the above range, the dynamic friction coefficient with a sliding metal counterpart member is reduced, and energy loss due to friction work is lost. And the effect of improving the seizure load resistance with the mating member can be expected.

Further, the silicon nitride-based sintered body of the present invention has α
Besides the -Si ₃ N ₄ beta .'- sialon, it is necessary to include at least one crystalline phase containing silicon (Si). The Si-containing crystal phase includes a silicate or a silicide compound containing a metal element added as a sintering aid, an apatato phase or a melilite phase which is a compound of an oxide of these metal elements and Si ₃ N ₄ . The conventional grain boundary phase was present as a glass phase, but the presence of these crystal phases in the grain boundary phase of the sintered body of the present invention improves the corrosion resistance of the sintered body, and particularly strength. Fatigue strength is improved.

However, the diffraction intensity of the main peak in the X-ray diffraction of the crystal phase containing Si is (21) of α-Si ₃ N ₄ (21
If it is less than 0.5% with respect to the sum of the diffraction intensity of the (0) plane and the (210) plane of β'-sialon, the existence ratio of the glass phase in the grain boundary phase becomes high, so that the corrosion resistance decreases. Conversely, 5%
If it exceeds, α-Si ₃ N _{4 which is} originally superior in corrosion resistance to the grain boundary phase
The ratio of β'-sialon and β'-sialon is low, so it cannot be said to be sufficient in terms of corrosion resistance, and the increase in the grain boundary phase with low cleavage strength decreases the resistance to the development of defects at fracture, resulting in a decrease in strength, especially fatigue strength. Cause a decrease in

The porosity of the silicon nitride sintered body is preferably 2% or less. This is because if the porosity exceeds 2%, when the sintered body is used as an engine member or the like in a corrosive gas atmosphere, the corrosive gas penetrates into the pores and promotes the corrosion of the grain boundary phase and the like.

In the production of the silicon nitride-based sintered body of the present invention, both α-Si ₃ N ₄ and β'-sialon crystal phases are precipitated in the sintered body and the packing density of both crystal grains is improved. In order to achieve this, the particle size of the raw material powder and the amount of impurities are adjusted, and 1300 to 1 in a nitrogen gas atmosphere at or near atmospheric pressure is used.
Sinter at a temperature of 750 ° C. As the raw material powder, for example, α-Si ₃ N ₄ is 80% by weight or more and the average particle size is 0.7 μm.
Hereinafter, it is preferable to use a silicon nitride powder containing 0.2% by weight or less of transition metal impurities.

As the sintering aid, Y ₂ O ₃ which mainly causes a low melting point liquid phase reaction with SiO _{2 on the} surface of the silicon nitride powder,
Oxides such as MgO, CaO and TiO ₂ are used. or,
An aluminum compound is added for the production of β'-sialon, but the general formula of β'-sialon Si ₆ -Z Al _Z O
An aluminum compound such as Al ₂ O ₃ is added so that the Z value of _Z N _8-Z is preferably 0 <Z <1.0.

In order to precipitate a crystal phase containing Si other than α-Si ₃ N ₄ and β'-sialon in the sintered body, the sintered body is further pressurized after sintering and an inert gas such as nitrogen is added. A method of heat-treating at 800 to 1700 ° C. for 1 hour or more in an atmosphere, or a raw material powder of silicon nitride is 600
There is a method of increasing the amount of oxygen, such as an oxidation treatment at up to 1000 ° C. to generate an oxide on the powder surface. Further, in order to suppress the porosity of the sintered body to 2% or less, for example, the conditions of sintering or heat treatment are set to 1 in a nitrogen gas atmosphere of 10 atm or more.
It is preferably set to 450 to 1700 ° C.

The silicon nitride-based sintered body of the present invention is a fine α-S equiaxed with the above-mentioned columnar β'-sialon crystal grains.
Due to the improvement of the packing density due to the i ₃ N ₄ crystal grains and the crystal phase containing Si existing as the grain boundary phase, not only the static strength is excellent but also the fatigue strength is particularly improved, and at the same time, the corrosion resistance is improved. Specifically, the rotational bending fatigue limit is 600 MPa or more, which is much higher than that of the conventional silicon nitride-based sintered body, and the weight increase / decrease rate after standing for 1000 hours at 900 ° C. in a CO ₂ gas atmosphere is 3%. Within

The silicon nitride-based sintered body having such excellent fatigue strength and corrosion resistance is used as a sliding member for various devices, particularly engine parts of automobiles such as exhaust valves, cam followers, pistons. It is suitable as an engine member such as a head and a piston pin. In the case of engine parts, especially when the above-mentioned weight increase / decrease rate exceeds 5%, the strength of the sintered body deteriorates due to corrosion of the grain boundary phase, and wear resistance due to precipitation of surface products due to chemical changes on the surface of the sintered body. This is not preferable because it causes problems such as deterioration of sex.

In the case of various machine parts such as engine members, the machined surface preferably has a ten-point average roughness (R _Z ) of 1.2 μm or less. This roughness is 1.2
This is because if the thickness exceeds μm, the groove on the surface acts as a defect, which reduces the fatigue strength of the component and abnormally wears the sliding mating member. In particular, this abnormal wear and strength deterioration pose a problem when used as an engine member or the like used in a corrosive gas atmosphere.

[0020]

Example 1 92% by weight of commercially available Si ₃ N ₄ powder (90% α conversion, average particle size 0.6 μm) and 5% by weight Y ₂ O ₃ as a sintering aid.
Powder, 2 wt% Al ₂ O ₃ powder, and 1 wt% MgO
1 powder in ethanol with a nylon ball mill in ethanol
After wet mixing for 00 hours, the resulting slurry was 20 μm
And screened with a nylon mesh. An organic binder was added to this slurry in an amount of 3% by weight based on the total amount of the powder, and the mixture was mixed with a stirrer for 2 hours, and then granulated and dried at 90 ° C.

The dried granulated powder was sieved with a nylon mesh of 150 μm, and the obtained powder was 3000 kg each.
CIP molding at a pressure of 1 / cm < ² >, each molded product is treated in the air at 600 [deg.] C. for 20 hours to remove the organic binder, and then 1500 to 1 in a nitrogen gas atmosphere of 1 to 50 atm.
Sintered at a temperature of 800 ° C. for 2-8 hours. Further, each of the obtained sintered bodies was subjected to 1200 in a nitrogen gas atmosphere of 1 to 5 atm.
Heat treatment at a temperature of ˜1650 ° C. for 2 to 10 hours,
The respective sintered bodies of Samples 1 to 6 were obtained.

When each of the obtained sintered bodies was analyzed by an X-ray diffraction method, yttrium silicate was detected as a crystal phase containing Si other than α-Si ₃ N ₄ and β'-sialon. In addition, α-Si ₃ N ₄ in X-ray diffraction
And β'-sialon peak intensity ratio, and α-of the main peak intensity of yttrium silicate which is another crystal phase
(210) plane of Si ₃ N ₄ and (21) of β'-sialon
The intensity ratio to the sum of the diffraction intensities of the 0) plane was obtained. Furthermore,
For each sintered body, the porosity was calculated by backcalculating the relative specific gravity in the same manner as in Example 1. The results of these analyzes are shown in Table 1.

[0023]

[Table 1] (Note) Samples marked with * in the table are comparative examples.

For each of the sintered bodies of Samples 1 to 6, JI
A three-point bending strength test based on S R1601 and a Charpy impact test using the same test piece were performed. Further, each sintered body was ground and finished into a shape of a test piece 1 for Ono-type rotary bending fatigue test shown in FIG. 1 by using a diamond grindstone, and both ends of the test piece 1 were rotated by Ono-type rotary bending shown in FIG. The test piece 1 is fixed to the sample fixing part 2 of the fatigue testing machine, the sample fixing part 2 is rotated by a motor to give a rotational motion to the test piece 1, and at the same time, a bending moment is applied to the test piece 1 by the weight 3 via the sample fixing part 2. An endurance fatigue test was given. By this endurance fatigue test, the fatigue limit due to rotary bending at which the test piece breaks was measured.

On the other hand, in order to test practical characteristics, an exhaust valve of a commercially available 6-cylinder direct-drive gasoline engine with a displacement of 2000 cc was produced from each of the above-mentioned sintered bodies, and the gasoline engine equipped with this exhaust valve was used. , 400
A practical endurance fatigue test was carried out by scanning from 0 to 7,400 rpm at intervals of about 20 seconds, and the endurance time until the exhaust valve was broken was determined by the integrated time. The cumulative time was set to 500 hours as the test termination time, and the test was terminated for the samples that were not destroyed even after this time elapsed.

Further, each of the sintered bodies of Samples 1 to 6 was allowed to stand in an atmosphere having a CO ₂ gas concentration of 75% or more at 900 ° C. for 1000 hours, and the weight change rate of each sintered body before and after this test was measured. The results of these tests are shown in Table 2. In addition, the weight increase / decrease rate of the sintered body is expressed as an absolute value.

[0027]

[Table 2]Strength test results Exhaustion test results Weight Bending strength Charpy impact value Fatigue limit durability time Increase / decrease ratesample (MPa) (kgm / cm ² ) (MPa) (hr) (%)  1 1760 0.22 860 500hr censored 0.4 2 1820 0.24 950 500hr censored 0.5 3 1520 0.20 730 420hr 0.5 4 * 840 0.05 310 2hr 3.8 5 * 965 0.03 460 36hr 2.2 6 * 660 0.03 220 0.5hr 4.6 (Note) * in the table The attached sample is a comparative example.

Example 2 Commercially available Si ₃ N ₄ powder (alpha conversion rate 90%, average particle size 0.6 μm)
m) was added with Y ₂ O ₃ powder, Al ₂ O ₃ powder and MgO powder as sintering aids in the proportions shown in Table 3 below, and mixed, granulated and molded in the same manner as in Example 1. After doing
Each compact was sintered at 1550 ° C. for 5 hours in a nitrogen gas atmosphere of 2 atm. Then, each sintered body obtained was
HIP for 1 hour at 1600 ° C in a nitrogen gas atmosphere at atmospheric pressure
It processed and obtained each sintered compact of samples 7-12.

[0029]

[Table 3] (Note) Samples marked with * in the table are comparative examples.

Each of the obtained sintered bodies was analyzed by an X-ray diffraction method, and a crystal phase containing α-Si ₃ N ₄ and β'-sialon, and other detected Si was determined as Example 1. It evaluated similarly. Further, the sample surface cut out from each sintered body was mirror-finished so that the ten-point average roughness (R _Z ) was 0.1 μm or less, and then the surface was treated with an etching solution of HF: HNO ₃ = 1: 2. It was used for etching at 60 ° C. for 2 hours.
This etched surface was observed with a SEM at a magnification of 5000, and 100 samples of each crystal grain were selected from the observation photograph.
The average crystal grain sizes of the short axis and the long axis of Si ₃ N ₄ and β′-sialon were obtained. Further, the porosity of each sintered body was calculated in the same manner as in Example 1. The results of these analyzes are shown in Table 4.

[0031]

[Table 4] αSi ₃ N ₄ : β'sialon Other Si-containing crystal phase Porosity grain size (μm) Sample peak intensity ratio (%) to α + β 'ratio (%) (%) α β' (long / Short) 7 22: 78 0.6 0.8 0.3 1.8 / 0.2 8 15: 85 1.4 0.2 0.4 2.2 / 0.3 9 9: 1 1.6 0.2 0.5 2.8 / 0.3 10 3: 97 2.8 0.5 0.5 3.5 / 0.4 11 * 0: 100 6.5 1.2 − 7.6 / 1.2 12 * 35: 65 None 5.3 0.3 2.1 / 0.3 (Note) Samples marked with * in the table are comparative examples.

Further, for each of the sintered bodies of Samples 7 to 12, the fatigue limit due to rotary bending was measured using an Ono-type rotary bending fatigue tester in the same manner as in Example 1, and the exhaust valve produced from each sintered body was measured. The durability time until the exhaust valve is broken is obtained by the practical durability fatigue test of
The weight increase / decrease rate (absolute value) of the sintered body before and after the test in which it was left in ₂ gases was measured. The results are shown in Table 5.

[0033]

[Table 5] (Note) Samples marked with * in the table are comparative examples.

Example 3 With respect to the sintered body of sample 2 of Example 1, using test pieces and exhaust valves in which the ten-point average roughness (R _Z ) of the machined surface was variously changed, Example 1 Similar to the Ono-type rotary bending fatigue test, the fatigue limit of rotary bending, the exhaust valve practical durability fatigue test, the endurance time until the exhaust valve is destroyed, and the exhaust valve Ono after the test of leaving it in heated CO ₂ gas The fatigue limit of rotary bending was measured by the rotary bending fatigue test, and the results are shown in Table 6.

[0035]

[Table 6] (Note) Samples marked with * in the table are comparative examples.

[0036]

According to the present invention, the packing density is improved by the columnar β'-sialon crystal grains and the equiaxed and fine α-Si ₃ N ₄ crystal grains, and Si existing as the grain boundary phase is eliminated. By including the crystal phase, it is possible to provide a silicon nitride-based sintered body that is excellent in static strength, particularly improved in fatigue strength as compared with conventional ones, and at the same time improved in corrosion resistance.

Since the silicon nitride sintered body of the present invention has excellent fatigue strength and corrosion resistance, it can be used as a sliding member for various devices, especially engine parts of automobiles such as exhaust valves, cam followers, Applications as engine members such as piston heads and piston pins are expected.

[Brief description of drawings]

FIG. 1 is a side view showing the shape and dimensions of a test piece used for an Ono-type rotary bending fatigue test in Examples.

FIG. 2 is a partially cutaway conceptual view for explaining an outline of an Ono-type rotary bending fatigue tester used in Examples.

[Explanation of symbols]

 1 Test piece 2 Sample fixing part 3 Weight

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akira Yamakawa 1-1-1 Kunyokita, Itami City, Hyogo Prefecture Sumitomo Electric Industries, Ltd. Itami Works

Claims (8)

[Claims] 1. At least one crystal phase containing α-silicon nitride and β′-sialon and containing silicon is present, and the diffraction intensity of the main peak in the X-ray diffraction of the crystal phase containing silicon is: α-silicon nitride (210) plane and β '
-A silicon nitride-based sintered body characterized by being in the range of 0.5 to 5% with respect to the sum of the diffraction intensity of the (210) plane of sialon. 2. The peak intensity in the X-ray diffraction of each crystal phase of α-silicon nitride and β′-sialon in the sintered body is 0 <α-nitriding, when the sum of the peak intensities of the two is 100%. Silicon ≦ 30% and 70% ≦ β′-sialon <10
The silicon nitride-based sintered body according to claim 1, wherein the relationship is 0%. 3. The average crystal grain size of α-silicon nitride in the sintered body is 0.5 μm or less, and the average crystal grain size of β′-sialon is 1.0 to 5.0 μm and short in the major axis direction. The silicon nitride based sintered body according to claim 1 or 2, wherein the axial direction is 0.5 µm or less. 4. The silicon nitride sintered body according to claim 1, wherein the sintered body has a porosity of 2% or less. 5. The rotary bending fatigue limit of the sintered body is 600MP.
a or more and 1 at 900 ° C. in a CO ₂ gas atmosphere
The silicon nitride-based sintered body according to any one of claims 1 to 4, characterized in that the rate of change in weight after being left for 000 hours is within 3%. 6. A crystalline phase containing α-silicon nitride and β′-sialon, and at least one other crystalline phase containing silicon is present, and the diffraction intensity of the main peak in X-ray diffraction of the crystalline phase containing silicon is: α-silicon nitride (210) plane and β '
-It is in the range of 0.5 to 5% with respect to the sum of the diffraction intensity of the (210) plane of Sialon, and the rotational bending fatigue limit is 60.
0 MPa or more and 900 in a CO ₂ gas atmosphere
An engine member comprising a silicon nitride-based sintered body having a weight change rate of 3% or less after being left at 1000C for 1000 hours. 7. The average crystal grain size of α-silicon nitride in the sintered body is 0.5 μm or less, and the average crystal grain size of β′-sialon is 1.0 to 5.0 μm in the major axis direction. The engine member according to claim 6, wherein the length is 0.5 μm or less in the minor axis direction. 8. The engine member according to claim 6, wherein the machined surface has a ten-point average roughness (R _Z ) of 1.2 μm or less.