Silicon nitride sintered compact, tool for friction stir welding, and method for manufacturing tool for friction stir welding

Abstract

A silicon nitride sintered compact according to an embodiment contains 15-20 mass% of a grain boundary phase containing a lanthanoid element, and in a diffraction pattern when a prescribed cross-section is subjected to X-ray diffraction (XRD) measurements, the silicon nitride sintered compact has, in addition to an α-silicon nitride peak and a β-silicon nitride peak, four peaks resulted from a grain boundary crystal phase and an α-sialon phase in a diffraction angle range from 29° to 35°. In the silicon nitride sintered compact, the four peaks are positioned within a range of ±0.2° of each of the diffraction angles of 29.6°, 31.0°, 31.3°, and 34.4°, and when A1 to A4 are respectively the four peak areas for the four peaks, and A5 is the integrated intensity of the peak detected at the diffraction angle of 27.0°, it is preferable that 1.0 ≤ (A1+A2+A3+A4) / A5 ≤ 3.5.

Description

Silicon nitride sintered body, friction stir welding tool, and method for manufacturing friction stir welding tool

[0001] The embodiments generally relate to a silicon nitride sintered body, a friction stir welding tool, and a method for manufacturing a friction stir welding tool.

[0002] Friction stir welding (FSW) is a welding method in which a welding tool including a probe (hereinafter referred to as a "friction stir welding tool" or simply a "welding tool") is rotated while the probe is pressed against the workpieces to be welded, utilizing frictional heat to integrate the workpieces. The frictional heat softens the workpieces, and the rotational force of the probe induces plastic flow around the welded area, integrating the workpieces. Therefore, friction stir welding is a type of solid-state welding, and it has the advantage that the welded workpieces can be made of the same material or different materials, such as iron and aluminum. Furthermore, friction stir welding is a solid-state welding. Therefore, friction stir welding reduces heat input to the welded area, minimizing thermal damage to the workpieces, thereby minimizing strength loss due to welding. Furthermore, friction stir welding eliminates the need for brazing filler metals, which is expected to reduce costs. Friction stir welding tools are required to have wear resistance to withstand high-speed rotation and heat resistance to withstand the frictional heat generated.

[0003] As a conventional joining tool, International Publication No. 2018 / 030308 (Patent Document 1) discloses a friction stir welding tool made of a silicon nitride sintered body. According to Patent Document 1, by incorporating both β-silicon nitride crystal grains and α-sialon crystal grains into the sintered body, it is possible to provide a material with wear resistance and oxidation resistance. It also states that the total amount of sintering aid used in this case should not exceed 15 mass%.

[0004] The joining tool made of sintered silicon nitride described in Patent Document 1 shows some improvements in wear resistance and heat resistance. However, when using this joining tool to join high-melting-point materials such as iron or titanium, the temperature at the joining point can exceed 1000°C, resulting in a decrease in strength due to the brittleness of the grain boundary phase and changes in shape caused by wear. Therefore, there is a need for a joining tool that can withstand such high temperatures and has both strength (heat resistance) and wear resistance.

[0005] International Publication No. 2018 / 030308

[0006] The joining tool made of sintered silicon nitride described in Patent Document 1 shows some improvements in wear resistance and heat resistance. However, when using this joining tool to perform friction stir welding of high-melting-point metal materials such as iron and titanium, the temperature of the joining area can exceed 1000°C, and the tool shape can change due to wear. At temperatures exceeding 1000°C, problems arise, such as grain shedding due to softening of the grain boundary phase and adhesion of softened grain boundary phase components, which can affect friction characteristics. Furthermore, significant shape change can lead to stress concentration areas, which can cause a decrease in strength. For more stable and long-term use, materials with higher heat resistance and wear resistance are desired. Heat resistance here refers to maintaining high strength even at high temperatures exceeding 1000°C.

[0007] It has been found that in order to further improve heat resistance and wear resistance, it is more effective to form a hard α-sialon phase in the silicon nitride matrix portion of the silicon nitride member, increase the amount of grain boundary phase, and increase the degree of crystallization of the grain boundary phase.

[0008] In order to solve the above problems, the silicon nitride sintered body according to the embodiment contains 15% by mass or more and 20% by mass or less of a grain boundary phase containing a lanthanoid element, and in the diffraction pattern obtained by XRD (X-ray Diffraction) measurement of a predetermined cross section, the diffraction pattern has at least four peaks other than the peak of α-silicon nitride and the peak of β-silicon nitride in the diffraction angle range of 29° or more and 35° or less.

[0009] The present invention relates to a silicon nitride sintered body and a friction stir welding tool, and the present invention relates to a silicon nitride sintered body and a friction stir welding tool. Embodiment

[0010] The silicon nitride sintered body according to the embodiment contains 15% by mass or more and 20% by mass or less of a grain boundary phase containing a lanthanoid element, and in the diffraction pattern obtained by XRD measurement of a predetermined cross section, there are at least four peaks in addition to the α-silicon nitride peak and the β-silicon nitride peak in the diffraction angle range of 29° or more and 35° or less. FIG. 1 shows an example of a diffraction pattern obtained by XRD measurement of a predetermined cross section of the silicon nitride sintered body according to the embodiment. The horizontal axis of FIG. 1 is the diffraction angle (2θ), and the vertical axis is the intensity (cps). FIG. 1 shows an example of a diffraction angle (2θ) of 25° to 35°.

[0011] Also, Fig. 2 shows a silicon nitride sintered body according to an embodiment. Fig. 2(A) shows a side view of the silicon nitride sintered body according to an embodiment, and Fig. 2(B) shows a top view of the silicon nitride sintered body according to an embodiment. Furthermore, Fig. 3 shows a friction stir welding tool according to an embodiment. Fig. 3(A) shows a side view of the friction stir welding tool according to an embodiment, and Fig. 3(B) shows a top view of the friction stir welding tool according to an embodiment.

[0012] 2 and 3 , reference numeral 1 denotes a friction stir welding tool, reference numeral 2 denotes a friction surface, reference numeral 3 denotes a shoulder portion, reference numeral 4 denotes a probe portion, reference numeral 5 denotes a base portion, and reference numeral 10 denotes a silicon nitride sintered body. The friction stir welding tool 1 includes a probe portion 4 and a base portion 5. The probe portion 4 and the base portion 5 are preferably formed from an integrated silicon nitride sintered body 10. The end face of the probe portion 4, which faces the contact surface with the base portion 5, defines a friction surface 2. The friction surface 2 may be flat or curved. The side surface of the probe portion 4 may have a curved surface or a thread groove structure. The portion of the surface of the base portion 5 on which the probe portion 4 is provided and which does not contact the probe portion 4 defines a shoulder portion 3. The shoulder portion 3 may have an inclined surface. The end face of the base portion 5, which faces the contact surface with the probe portion 4, may have a curved shape. Friction stir welding is a method of joining workpieces by pressing the friction surface 2 of the probe part 4 against the workpieces while rotating the friction stir welding tool 1 at high speed. At this time, the temperature of the joint may reach a high temperature of around 1000°C.

[0013] The silicon nitride material of this embodiment has excellent strength characteristics, particularly at high temperatures, and has high strength and heat resistance that can withstand temperatures exceeding 1000° C. Therefore, it is suitable for use as a material for the friction stir welding tool 1.

[0014] On the other hand, in friction stir welding, in addition to heat resistance, hardness at high temperatures also affects material wear. Higher hardness can delay wear. In other words, wear is less likely to occur up to higher temperatures. By improving heat resistance and achieving a Vickers hardness HV of over 1600 at room temperature, high hardness can be maintained even at temperatures exceeding 1000°C, reducing wear and allowing for a longer service life. The silicon nitride sintered body 10 according to the embodiment can be used for purposes other than the friction stir welding tool 1. For example, application examples of the silicon nitride sintered body 10 include, in addition to the friction stir welding tool 1, bearing balls, roll members, engine parts, and molten metal members. Another application example of the silicon nitride sintered body 10 is a backing material for placing workpieces during friction stir welding. The silicon nitride sintered body 10 according to the embodiment is provided with a shape appropriate for each application.

[0015] Here, the predetermined cross section for performing XRD measurement includes a cross section from a cross section that bisects the silicon nitride sintered body 10 to a cross section that bisects the silicon nitride sintered body 10 in thirds. For example, in the case of a silicon nitride sintered body 10 for a bearing ball, the predetermined cross section includes a cross section that passes through the center of the substantially spherical silicon nitride sintered body 10 and bisects the silicon nitride sintered body 10, as well as a cross section that does not pass through the center and bisects the silicon nitride sintered body 10 in thirds. Furthermore, in the case of a silicon nitride sintered body 10 for a joining tool 1, the predetermined cross section includes a cross section that bisects the entire silicon nitride sintered body 10 (for example, a cross section that bisects the entire silicon nitride sintered body 10 shown in FIG. 2 into left and right halves) and a cross section that bisects the entire silicon nitride sintered body 10 in thirds (for example, a cross section that is parallel to the cross section that bisects the entire silicon nitride sintered body 10 shown in FIG. 2 into left and right halves and bisects the entire silicon nitride sintered body 10 in thirds). Furthermore, for example, in the case of a silicon nitride sintered body 10 for a joining tool 1, the specified cross section includes a cross section that bisects a portion of the silicon nitride sintered body 10 (a portion of the probe portion) (for example, a cross section that bisects the probe portion of the silicon nitride sintered body 10 shown in Figure 2 into upper and lower halves), and a cross section that bisects the portion of the silicon nitride sintered body 10 into thirds (for example, a cross section that bisects the portion of the silicon nitride sintered body 10 shown in Figure 2 into upper and lower halves and that bisects the portion of the silicon nitride sintered body 10 into thirds into upper and lower halves).

[0016] The XRD measurement of the silicon nitride sintered body 10 is performed using a Cu target (Cu-Kα) with a tube voltage of 40 kV, a tube current of 40 mA, a scan speed of 2.0° / min, a slit (RS) of 0.15 mm, and a diffraction angle (2θ) in the scanning range of 10° to 60°.

[0017] The silicon nitride sintered body 10 according to the embodiment is characterized in that, when a predetermined cross section is subjected to XRD measurement, it has at least four peaks P1 to P4 in addition to the α-silicon nitride peak Pα and the β-silicon nitride peak Pβ in the diffraction angle range of 29° to 35°. In FIG. 1, six peaks Pα, Pβ, and P1 to P4 are shown within the diffraction angle range of 29° to 35°. Among these, peak Pα is a peak based on α-silicon nitride, and peak Pβ is a β-silicon nitride peak. In FIG. 1, four peaks A1 to A4 are shown in addition to the α-silicon nitride peak Pα and the β-silicon nitride peak Pβ. Note that, in the silicon nitride sintered body 10 according to the embodiment, FIG. 1 describes a case in which four peaks other than the α-silicon nitride peak Pα and the β-silicon nitride peak Pβ are detected, but the present invention is not limited to this case. For example, in the silicon nitride sintered body 10, five or more peaks other than the peak Pα of α-silicon nitride and the peak Pβ of β-silicon nitride may be detected.

[0018] Furthermore, the grain boundary phase of the silicon nitride sintered body 10 is in the range of 15% by mass to 20% by mass. If the grain boundary phase is less than 15% by mass, the heat resistance at temperatures above 1000°C is poor, which is undesirable. By making the grain boundary phase 15% by mass or more, it is possible to achieve both heat resistance above 1000°C and good wear resistance. On the other hand, if it is more than 20% by mass, the strength of the material itself decreases. Therefore, it is preferable that the amount of grain boundary phase added be in the range of 15% by mass to 20% by mass, or even 15% by mass to 18% by mass.

[0019] The proportion (mass %) of the grain boundary phase in the silicon nitride sintered body 10 is determined by ICP analysis. When the relative density of the silicon nitride sintered body 10 is 98% or more, the mass % of the grain boundary phase may be calculated by subtracting the proportion of silicon nitride from 100%.

[0020] Furthermore, it is preferable that the four peaks P1 to P4 are peaks located within a range of ±0.2° of the diffraction angles of 29.6°, 31.0°, 31.3°, and 34.4°, respectively. The peak located within the diffraction angle range of 29.6° ±0.2° is referred to as the "29.6° peak" P1. The peak located within the diffraction angle range of 31.0° ±0.2° is referred to as the "31.0° peak" P2. The peak located within the diffraction angle range of 31.3° ±0.2° is referred to as the "31.3° peak" P3. The peak located within the diffraction angle range of 34.4° ±0.2° is referred to as the "34.4° peak" P4.

[0021] The four peaks P1 to P4 may represent either α-sialon or the grain boundary crystalline phase. Preferably, peak P2 at 31.0° and peak P4 at 34.4° are primarily α-sialon, and peak P1 at 29.6° and peak P3 at 31.3° are primarily the grain boundary crystalline phase of the material. However, the peaks containing these phases may be outside the diffraction angle range of 29° to 35°.

[0022] For example, the peak P1 at 29.6° is a peak containing a lanthanoid element-Hf—O based crystalline compound. Examples of the lanthanoid element-Hf—O based crystalline compound include YbHf ２ O ７ This peak P1 at 29.6° is detected even when no Hf component is added, and may therefore overlap with the peak of α-sialon. If the peaks overlap, qualitative analysis may be used.

[0023] The amount of wear of materials in friction stir welding is closely related to their hardness. The higher the hardness, the less wear there is. In other words, α-sialon has a fine grain size and is hard, so increasing its hardness is effective in increasing wear resistance, and the crystallization of the grain boundary phase increases its heat resistance. By increasing the quantity of these phases, it is possible to achieve both hardness and heat resistance.

[0024] Integrated intensities are used to represent the quantitative presence of these phases. The integrated intensity A1 for peak P1 at 29.6° is determined, the integrated intensity A2 corresponding to peak P2 at 31.0° is determined, the integrated intensity A3 corresponding to peak P3 at 31.3° is determined, and the integrated intensity A4 corresponding to peak P4 at 34.4° is determined. Furthermore, the integrated intensity A5 corresponding to peak P5 detected at a diffraction angle of 27.0°±0.2°, which is attributable to β-silicon nitride crystal grains, is determined. Then, it is preferable that the integrated intensity ratio calculated from the integrated intensities A1 to A5 using the following formula (1) satisfies the following formula (2): Integrated intensity ratio = (A1 + A2 + A3 + A4) / A5 (1) 1.0 ≦ Integrated intensity ratio ≦ 3.5 (2)

[0025] Here, the integrated intensities A1 to A1 are the areas of the regions (dashed line regions shown in Figure 1) enclosed by two dividing lines drawn perpendicularly from the peak start and peak end of the peak to the baseline, the peak waveform, and the baseline. The peak start and peak end can be determined, for example, by a tangent method based on the point where the tangents to the inflection points of the waveforms on the left and right of the peak intersect with the baseline. Alternatively, the peak start and peak end can also be determined by a half-width method, which determines the position where the peak height is half. The baseline is determined by background fitting and removal processing. The integrated intensities may also be determined using an analysis function provided in the analyzer.

[0026] Furthermore, when two adjacent peaks overlap, the integrated intensity for each peak is calculated by dividing the boundary line drawn perpendicular to the baseline from the point where the data between the two adjacent peaks is smallest. Satisfying the above formula (1) means that a large amount of highly crystalline phase is present in the matrix and grain boundary phase of silicon nitride, thereby improving both heat resistance and wear resistance. The α-sialon formed as part of silicon nitride is formed by the interstitial solid solution of alkaline earth and rare earth elements in silicon nitride. Unlike β-silicon nitride, the grain morphology is minimal, and the particles are equiaxed with a small aspect ratio. Therefore, α-sialon is part of the matrix. In other words, α-sialon is not included in the mass % of the grain boundary phase.

[0027] Furthermore, α-sialon is a hard, fine-grained material, which can increase hardness. However, if more than necessary is formed, fracture toughness decreases, making the material more brittle. On the other hand, the more crystalline phase resulting from the grain boundary phase there is, the better the heat resistance becomes. However, if there is an excessive amount, the strength of the sintered body itself may decrease. Increasing the amount of α-sialon and grain boundary crystalline phase within an appropriate range leads to improved heat resistance and increased strength and hardness, making it possible to provide a well-balanced material that combines both properties.

[0028] The four peaks P1 to P4 are one or more types selected from a lanthanoid-Hf—O based crystalline compound, a lanthanoid-Al—O based crystalline compound, a grain boundary crystalline phase containing a lanthanoid-Al—Hf—O based crystalline compound, and a phase containing an α-sialon phase. Note that the sample may also contain crystalline phases other than these four. Furthermore, the grain boundary phase may contain an amorphous phase.

[0029] Here, the lanthanoid element is at least one selected from Yb (ytterbium), Er (erbium), Lu (lutetium), and Ce (cerium). In the case of a lanthanoid element, it is preferable to add it in the form of a lanthanoid oxide. Yb is preferably Yb ２ O ３ , Er is Er ２ O ３ , Lu is Lu ２ O ３ , Ce is CeO ２ These oxides are silicon oxide (SiO ) contained in the oxides and silicon nitrides added at the same time. ２ ), etc., to generate compounds with higher heat resistance, and also form grain boundary phases with higher crystallinity. This improves the wear resistance during friction stir welding and extends the life. Silicon oxide (SiO ２ ) is used to react with the sintering aid, so the amount of sintering aid added and the content of the grain boundary phase may differ.

[0030] The lanthanoid element content is preferably in the range of 1% by mass or more and 15% by mass or less in terms of oxide. If the lanthanoid element content is less than 1% by mass in terms of oxide, the presence of a compound phase that improves heat resistance will be reduced, resulting in insufficient wear resistance. Furthermore, the formation of columnar silicon nitride particles, which is unique to the addition of lanthanoid elements, will be insufficient, preventing the benefits of silicon nitride's mechanical properties from being utilized. Furthermore, if the content exceeds 15% by mass, the grain boundary phase will be excessive, potentially reducing the strength of the sintered body. Lanthanoid elements are components that easily form crystalline compounds with Hf compounds and Al compounds. When a lanthanoid element and a Hf compound are added as grain boundary phases, the lanthanoid element-Hf—O crystalline compounds will predominate. Furthermore, when a lanthanoid element and an Al compound are added, the lanthanoid element-Al—O crystalline compounds will predominate. Furthermore, when a lanthanoid element and a Hf compound and an Al compound are added, the lanthanoid element-Hf—Al—O crystalline compounds will predominate. These are crystalline compounds with high melting points, which improves heat resistance.

[0031] The bending strength of the silicon nitride sintered body 10 at 1200°C is 900 MPa or more. The bending strength was evaluated by processing a sample into a size of 4 mm x 3 mm x 40 mm, chamfering the edges, and performing a three-point bending test in accordance with JIS_R_1604 (2008). JIS_R_1604 corresponds to ISO_17565.

[0032] The silicon nitride sintered body 10 has a Vickers hardness HV of 1600 or more. The Vickers hardness test was carried out by polishing the sample to a mirror finish and then pressing a diamond indenter into the sample with a load of 20 kgf for 30 seconds, and then evaluating the hardness based on the size of the indentation in accordance with JIS_R_1610 (2003). JIS_R_1610 corresponds to ISO_14705.

[0033] Next, a method for manufacturing the silicon nitride sintered body 10 will be described. As long as the silicon nitride sintered body 10 has the above-mentioned structure, the manufacturing method is not particularly limited, but the method for obtaining the silicon nitride sintered body 10 with a good yield is as follows. The method for manufacturing the silicon nitride sintered body 10 is to add AlN (aluminum nitride) as a sintering aid to silicon nitride powder. ２ O ３(aluminum oxide) was added, and the mass ratio of Al ２ O ３ In addition to AlN, a small amount of Al / AlN may be used. ２ O ３ Adding Al improves the appearance and stabilizes the properties. "Good appearance" here means that no clusters of microvoids are formed in the silicon nitride sintered body 10. Clusters of microvoids can also form white spots. Since the silicon nitride sintered body 10 has a gray appearance, the formation of white spots can cause defects in appearance. In addition, a small amount of Al ２ O ３ The addition of Al has the effect of improving strength and hardness. ２ O ３ If the addition ratio of Al is increased, voids are likely to be formed and the strength may decrease. ２ O ３ / AlN is preferably 0.4 or less.

[0034] In addition, elements such as Ti and Hf may be added in the form of at least one of oxide, nitride, and carbide. ２ (titanium oxide), Hf is HfO ２ It is preferable to add hafnium oxide (HfO). These can be present in the grain boundary phase as nitrides, oxides, or carbides. ２ By reacting with lanthanoid elements to form crystalline compounds, such as those mentioned above, the grain boundary phase can be strengthened.

[0035] Furthermore, Mg (magnesium) may be added as needed. Mg may be added in the form of oxide, nitride, or oxynitride. ２ O ３ Spinel MgO.Al compound ２ O ３ Mg forms a liquid phase at a lower temperature and contributes to improving sinterability.

[0036] Next, we will explain the method for manufacturing silicon nitride sintered body 10. The method for manufacturing silicon nitride sintered body 10 includes a raw material preparation step of adding 1 to 15 mass % of a lanthanoid element, calculated as an oxide, and 1 to 10 mass % of Al, calculated as an oxide, to silicon nitride powder, a molding step of molding the mixed powder to obtain a molded body, a degreasing step of heating the obtained molded body to remove organic matter, and a sintering step of sintering in a nitrogen atmosphere.

[0037] First, the silicon nitride powder used in the raw material preparation step will be described. The silicon nitride powder may be synthesized by either direct nitridation or imide pyrolysis, and there are no particular restrictions on the manufacturing method. It is preferable that the oxygen content be 1.5 mass% or less, that the α-type silicon nitride content be 80 mass% or more, and that the average particle size be 1 μm or less.

[0038] The α-type silicon nitride powder transforms into β-type silicon nitride crystal particles and undergoes grain growth during the sintering process, thereby producing a silicon nitride sintered body with excellent wear resistance.

[0039] The sintering aid contains 1 to 15 mass % of a lanthanoid element component, calculated as oxide, and 1 to 10 mass % of an Al component, calculated as oxide. The lanthanoid element is at least one element selected from Yb, Er, Lu, and Ce. These lanthanoid elements tend to facilitate crystallization of the grain boundary phase, making it easy to form a grain boundary phase with high heat resistance.

[0040] The Al component of the additive is AlN and Al ２ O ３ When MgO is added, it is preferable to add both Al ２ O ３ is spinel MgO・Al ２ O ３ It may be added at ２ O ３ Addition of Al can promote densification. In addition, it delays the α to β transition of the silicon nitride sintered body 10, and the hardness can be increased by a fine grain size. In the friction stir welding tool 1, hardness is effective in improving wear resistance, and increasing the hardness can improve the tool life. In addition, Al ２ O ３The amount of Al added is the ratio of Al to the amount of AlN added. ２ O ３ It is preferable that the ratio of MgO.Al / AlN is 0.4 or less. By doing so, it is possible to suppress the formation of voids. ２ O ３ When added as MgO / Al, the ratio of the amount added is ２ O ３ The ratio of the amount of addition of Al ２ O ３ Although the lower limit of the ratio AlN is not particularly limited, it is preferably 0.01 or more. ２ O ３ If the ratio of AlN to AlN is less than 0.01, the effect of adding both may be insufficient.

[0041] It is preferable to add the above sintering aid components so that their total amount is 15% by mass or more and 20% by mass or less. This makes it easier to control the amount of grain boundary phase. If it is less than 15% by mass, the grain boundary layer will have poor durability above 1000°C, and if it exceeds 20% by mass, the proportion of the grain boundary phase will be excessive, resulting in a decrease in strength. Furthermore, within this range, a liquid phase will be formed abundantly, which will facilitate particle rearrangement and densification. When added as a sintering aid, they may be added in the form of oxides, nitrides, or carbides.

[0042] In the case of lanthanide elements, lanthanide oxides are preferred. ２ O ３ , Er is Er ２ O ３ , Lu is Lu ２ O ３ , Ce is CeO ２ is preferred.

[0043] Furthermore, an element selected from Ti and Hf may be added to the above. These elements are added in the form of oxide, nitride or carbide. Ti is TiO ２ , Hf is HfO ２It is preferable to add these elements as Ti and Hf. These elements have the effect of lowering the liquid phase formation temperature and improving sinterability. By improving sinterability, it is possible to obtain a densified silicon nitride sintered body 10. These elements also form a highly heat-resistant grain boundary phase with lanthanoid elements, so their effect is enhanced by uniformly dispersing them in the sintered body. Furthermore, it is preferable that one or both of Ti and Hf be added in a total amount within the range of 0.1 mass% to 5 mass%. When added as a compound such as an oxide, the mass of the compound is counted. Furthermore, the amount of each sintering aid added is counted with the total of the silicon nitride powder and sintering aid powder as 100 mass%.

[0044] When Mg is added, MgO or MgO.Al ２ O ３ Spinel may also be added.

[0045] By selecting the sintering aid component from the above combination, it is possible to prepare a silicon nitride material that places importance on heat resistance and hardness.

[0046] Furthermore, since the grain boundary phase increases, the dispersion state of each component is important, and a mixing process that takes into consideration particle crushing is necessary.

[0047] Next, a dispersion medium is added to the silicon nitride powder and sintering aid, and the mixture is mixed, for example, in a ball mill to produce a slurry. Organic solvents such as ethanol, butanol, and toluene can be used as the dispersion medium. Any method that can uniformly mix the powders is not limited to ball milling. While the silicon nitride powder and sintering aid powder can be mixed simultaneously, it is preferable to first mix only the sintering aid powder for at least 8 hours. Next, the silicon nitride powder is added, and mixing is continued for at least 10 hours in a ball mill. An organic binder is then added, mixed, dried, and granulated to synthesize a granulated powder.

[0048] Next, in the molding step, the granulated powder is molded using a mold, and then, CIP (cold isostatic pressing) or the like may be used.

[0049] Next, in the degreasing step, the obtained compact is degreased by heating it to 400 to 600° C. in a nitrogen atmosphere.

[0050] Next, in the sintering process, the degreased body is sintered. The sintering process is preferably performed at 1600 to 1850°C, more preferably 1600 to 1750°C. Sintering is performed in a nitrogen gas atmosphere. Sintering may be atmospheric sintering, pressure sintering, or HIP (hot isostatic pressing), or a combination of multiple sintering methods. Silicon nitride powder or a powder mixture of silicon nitride and MgO may be used as the packing powder. These packing powders suppress the volatilization of gases such as SiO during sintering, facilitating densification. After sintering once, a second sintering using HIP is preferably performed. HIP sintering reduces voids, thereby improving densification and reducing strength variations, thereby improving reliability.

[0051] The silicon nitride sintered body 10 obtained by sintering is processed into a friction stir welding tool 1. The processing is performed using a diamond grindstone and an NC processing machine. Alternatively, electrical discharge machining or the like may be combined.

[0052] (Examples) (Examples 1 to 9 and Comparative Examples 1 to 2) Alpha-type silicon nitride powder and the sintering aid powder shown in Table 1 were prepared as silicon nitride powder. First, in Examples 1 to 9 and Comparative Example 2, ball mill mixing was carried out using only the sintering aid for 12 hours. Next, silicon nitride powder was added, and ball mill mixing was carried out for a further 96 hours. The slurry was dried and granulated to produce granulated powder. This powder was placed in a mold and molded to produce a rod-shaped compact. Next, the compact was CIP molded, and then heated to 500°C for a degreasing treatment to remove organic binders and the like. The degreasing compact was placed in a sintering furnace and sintered in a nitrogen atmosphere at 1650°C to 1750°C for 4 hours. Then, HIP sintering was carried out at 1700°C for 2 hours. The ratio of Al to AlN was 1.06. ２ O ３ A sample (Example 9) was also prepared in which the amount of sintering aid added was greater than 0.4. In Comparative Example 1, ball mill mixing was not performed using only the sintering aid.

[0053] Table 1 also shows the conditions for preparing the samples.

[0054]

[0055] In Examples 1 to 9, samples were prepared in which the total amount of sintering aids was 15% by mass or more and 20% by mass or less. After the sintering process, the amount of grain boundary phase in the silicon nitride sintered bodies 10 according to Examples 1 to 9 was within the range of 15% by mass or more and 20% by mass or less.

[0056] In Comparative Examples 1 and 2, samples with a total sintering aid content of less than 15% by mass and more than 20% by mass were prepared. After the sintering process, the amount of grain boundary phase in the silicon nitride sintered body of Comparative Example 1 was less than 15% by mass. In addition, the amount of grain boundary phase in the silicon nitride sintered body of Comparative Example 2 was more than 20% by mass.

[0057] XRD measurement was performed on the silicon nitride sintered bodies 10 according to Examples 1 to 9. According to the diffraction patterns obtained by XRD measurement (see, for example, FIG. 1), a peak Pα due to α-silicon nitride, a peak Pβ due to β-silicon nitride, four peaks P1 to P4 located within a range of ±0.2° at diffraction angles of 29.6°, 31.0°, 31.3°, and 34.4°, respectively, and a peak P5 at 27.0°±0.2° derived from β-silicon nitride crystal particles were detected. Then, the integrated intensity ratio ((A1 + A2 + A3 + A4) / A5) obtained by the above formula (1) was calculated from peaks P1 to P5.

[0058] The XRD measurement was performed using a Cu target (Cu-Kα), a tube voltage of 40 kV, a tube current of 40 A, a scan speed of 2.0° / min, a slit (RS) of 0.15 mm, and a diffraction angle (2θ) scan range of 10° to 60°. For example, the diffraction pattern shown in FIG. 1 is a portion of the diffraction pattern of Example 1 extracted from the entire diffraction pattern at a diffraction angle of 25° to 35°. According to FIG. 1, the integrated intensity A1 is 205, the integrated intensity A2 is 192, the integrated intensity A3 is 214, the integrated intensity A4 is 214, and the integrated intensity A5 is 427, and the integrated intensity ratio based on the formula (1) was 1.93. Similarly, the integrated intensity ratios calculated using the formula (1) based on the diffraction patterns obtained by XRD measurement of the silicon nitride sintered bodies 10 according to Examples 2 to 9 and the silicon nitride sintered body according to the comparative example are also shown in Table 2.

[0059]

[0060] In the silicon nitride sintered bodies 10 according to Examples 1 to 9 and the silicon nitride sintered body according to the comparative example, four peaks P1 to P4 were detected, each located within a range of ±0.2° at diffraction angles of 29.6°, 31.0°, 31.3°, and 34.4°. In all of the silicon nitride sintered bodies 10 according to Examples 1 to 9, the integrated intensity ratio satisfied the formula (2) and was within the range of 1.0 to 3.5. In contrast, in the silicon nitride sintered bodies according to Comparative Examples 1 and 2, the integrated intensity ratio did not satisfy the formula (2) and was outside the range of 1.0 to 3.5.

[0061] Table 3 shows the Vickers hardness and the results of a three-point bending strength test at 1200°C for the silicon nitride sintered bodies 10 according to Examples 1 to 9 and the silicon nitride sintered bodies according to Comparative Examples 1 and 2. The Vickers hardness test was performed by pressing a sample under a load of 20 kgf for 30 seconds and calculating the size of the indentation based on JIS R 1610. The three-point bending strength test was performed by processing the sample into a size of 4 mm x 3 mm x 40 mm, chamfering the edges, and evaluating the sample based on JIS R 1604 (2008).

[0062]

[0063] Thus, for all of the silicon nitride sintered bodies according to Examples 1 to 8, the integrated intensity ratio calculated by the formula (1) due to the grain boundary crystal phase and the α-sialon crystal phase satisfied the formula (2) and was within the range of 1.0 to 3.5. All of these silicon nitride sintered bodies according to Examples 1 to 8 exhibited a hardness of HV 1600 or more and a three-point bending strength of 900 MPa or more at 1200°C. Those whose integrated intensity ratio satisfies the formula (2) have both the heat resistance and hardness required for friction stir welding, which is expected to exceed 1000°C. Furthermore, for the silicon nitride sintered body 10 according to Example 9, although the integrated intensity ratio satisfied the formula (2), the addition ratio Al ２ O ３ Since the ratio of AlN to Cr was more than 0.4, the heat resistance and hardness were slightly reduced.

[0064] On the other hand, in the silicon nitride sintered body of Comparative Example 1, in which the total amount of sintering aid added was small, this value was less than 1.0 and the strength at 1200° C. was also low. In the silicon nitride sintered body of Comparative Example 2, the amount of sintering aid was increased too much, which increased the proportion of the grain boundary phase and reduced the strength.

[0065] Next, a friction stir welding tool 1 was fabricated using an NC lathe for the silicon nitride sintered bodies 10 according to Examples 1 to 9 and the silicon nitride sintered bodies according to Comparative Examples 1 and 2. The welding tool 1 consisted of a shoulder portion 3 with a concave surface and a cylindrical probe portion 4 with a diameter of 6 mm and a protrusion length of 1.9 mm. The friction stir welding tool 1 used had a spherical crown friction surface 2. A SUS304 stainless steel plate was used as the specimen for the welding tool 1. The welding tool 1 had a forward angle of 3° and a welding load of 3,500 kg. A linear welding test was performed with the welding tool 1 at a rotation speed of 550 rpm and a travel speed of 600 mm / min. After a 5-m welding test, the wear rate was calculated by dividing the total wear of the welding tool 1 by the welding distance. The wear of the welding tool 1 was evaluated using a three-dimensional shape measuring device to determine the difference in cross-sectional area between the tool before and after the test.

[0066] In addition, the temperature of the boundary region between the joining tool 1 and the workpiece during friction stir welding was measured with a radiation thermometer. The joining temperature and wear rate are shown in Table 4. The joining temperature under these joining conditions exceeded 1000°C in all cases, but it was found that the wear rate could be reduced to 0.7 to 1.0 in the joining tools 1 of Examples 1 to 8 in particular, and chipping and breakage did not occur. This makes it possible to extend the life of the joining tool 1. In contrast, the wear rate of the joining tool of Comparative Example 1 was higher than that of the joining tools 1 of Examples 1 to 9. Furthermore, chipping was observed in the joining tool of Comparative Example 2. On the other hand, Al ２ O ３ In the joining tool 1 according to Example 9, in which a large amount of Al was added, the integrated intensity ratio calculated by the above formula (1) was greater than 1.0, as in the silicon nitride sintered body 10, but the wear of the joining tool 1 according to Example 9 progressed somewhat faster. ２ O ３ The amount of addition is preferably 0.4 or less.

[0067]

[0068] As can be seen from Table 4, in the joining tools 1 according to Examples 1 to 9, even when the joining temperature reached 1000°C or higher, the wear rate was 1.3 mm ２ On the other hand, the wear rate of the welding tool according to Comparative Example 1 was 1.6 mm ２ / m, which was slightly worse. In addition, chipping occurred in the joining tool according to Comparative Example 2.

[0069] According to at least one of the embodiments described above, it is possible to improve the heat resistance and wear resistance of the silicon nitride sintered body 10 and the joining tool 1 obtained by polishing the silicon nitride sintered body 10.

[0070] Although several embodiments of the present invention have been described above, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. Furthermore, the above-described embodiments can be implemented in combination with each other.

Claims

1. A silicon nitride sintered body, comprising a grain boundary phase containing a lanthanoid element in an amount of 15% by mass or more and 20% by mass or less, and characterized in that in a diffraction pattern obtained by XRD (X-ray Diffraction) measurement of a specified cross section, the diffraction pattern has at least four peaks other than the peaks of α-silicon nitride and β-silicon nitride within a diffraction angle range of 29° to 35°.

2. The silicon nitride sintered body according to claim 1, characterized in that the four peaks are located within a range of ±0.2° of diffraction angles of 29.6°, 31.0°, 31.3°, and 34.4°, respectively, and when the four integrated intensities for the four peaks are A1, A2, A3, and A4, and the integrated intensity of the peak detected at a diffraction angle of 27.0°±0.2° is A5, then 1.0≦(A1+A2+A3+A4) / A5≦3.

5.

3. The silicon nitride sintered body according to claim 1 or 2, characterized in that the four peaks are one or more selected from a grain boundary crystal phase containing a lanthanoid element-Hf-O based crystalline compound, a lanthanoid element-Al-O based crystalline compound, and a lanthanoid element-Al-Hf-O based crystalline compound, and a phase containing an α-sialon phase.

4. The silicon nitride sintered body according to claim 1 or 2, characterized in that the lanthanoid element is at least one selected from the group consisting of Yb, Er, Lu and Ce.

5. The silicon nitride sintered body according to claim 3, characterized in that the lanthanoid element is at least one selected from the group consisting of Yb, Er, Lu and Ce.

6. The silicon nitride sintered body according to claim 1 or 2, characterized in that the bending strength of the silicon nitride sintered body at 1200°C is 900 MPa or more.

7. The silicon nitride sintered body according to claim 5, characterized in that the bending strength of said silicon nitride sintered body at 1200°C is 900 MPa or more.

8. The silicon nitride sintered body according to claim 1 or 2, characterized in that the silicon nitride sintered body has a Vickers hardness HV of 1600 or more.

9. The silicon nitride sintered body according to claim 7, characterized in that the Vickers hardness HV of the silicon nitride sintered body is 1600 or more.

10. A friction stir welding tool comprising the silicon nitride sintered body according to claim 1 or 2.

11. A friction stir welding tool comprising the silicon nitride sintered body according to claim 4.

12. A friction stir welding tool comprising the silicon nitride sintered body according to claim 8.

13. A friction stir welding tool comprising the silicon nitride sintered body according to claim 9.

14. The method for producing a silicon nitride sintered body according to claim 1 or 2, comprising adding AlN and Al as sintering aids to silicon nitride powder. ２ O ３ Addition of Al ２ O ３ 3. The method for producing a silicon nitride sintered body according to claim 1, further comprising a raw material mixing step in which the mass ratio of AlN to AlN is 0.4 or less.

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