SILICON NITRIDE SINTERED PRODUCT

Abstract: A silicon nitride sintered product comprises, in % by weight, 1 to 10% Mg, calculated as MgO, 1 to 10% Zr, calculated as ZrO₂, 0.1 to 2.0% of at least one of Aℓ, calculated as Aℓ₂O₃, Li, calculated as Li₂O, and Na, calculated as Na O, and the remainder being Si₃N₄ and unavoidable impurities.

Description: FIELD OF THE INVENTION This invention relates to silicon nitride sintered product, particularly to silicon nitride sintered product improved in resistances against chipping and wear. The silicon nitride sintered produce of this invention is suitably used for wear resistant industrial members such as cutting tools and the like. BACKGROUND OF THE INVENTION Known additive components suitably employed for cutting tools are MgO and ZrO₂ as disclosed in JP-B-60-16388 and 60-20346 (the term "JP-B" as used herein means an "examined Japanese patent publication"). Aℓ₂O₃, WC, Y₂O₃, etc., are also disclosed in JP-A-56-73670 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Further, though not being limited to cutting tools, addition of magnesia-alumina spinel and partially stabilized ZrO₂ is disclosed in JP-A-60-77174 to improve mechanical properties. The above-mentioned conventional techniques are useful for improving strength and toughness; in addition to such improvements, however, further reduction of wear is required to prolong a tool life. SUMMARY OF THE INVENTION The object of the present invention is to provide a silicon nitride sintered product improved in resistances against chipping and wear to meet the above requirements. According to the present invention, there is provided a silicon nitride sintered product comprising by weight, 1 to 10% Mg (calculated as MgO), 1 to 10% Zr (calculated as ZrO₂), 0.1 to 2.0% at least one of Aℓ (calculated as Aℓ₂O₃), Li (calculated as Li₂O) and Na (calculated as Na₂O) and the remainder being Si₃N₄ and unavoidable impurities. DETAILED DESCRIPTION OF THE INVENTION Components Mg, Zr, Aℓ, Li and Na contribute to product densification by forming liquid phase in the interstices between Si₃N₄ particles together with Si, N and O during the firing, and also bond Si₃N₄ particles by glassification during cooling. Especially, Aℓ, Li and Na enhance bonding strength of Si₃N₄ particles in the intergranular phase and avoid debonding of Si₃N₄ particles during abrasion. When the bonding strength is appropriate, the so-called "pull-out", i.e., a pulling out phenomenon of Si₃N₄ columnar crystal, occurs, so that stress concentration at the tip of the crack is considerably reduced to give high toughness to the sintered product. In this respect Aℓ is the most effective among Aℓ, Li and Na. Aℓ reinforces bonding of Si₃N₄particles, and avoids debonding of Si₃N₄ particles during abrasion. A part of the Zr is often incorporated as represented by ZrO x N y C z in the sintered product. The reason for limiting the respective contents of Mg and Zr as 1 to 10% by weight each (calculated as MgO and ZrO₂, respectively) is given below. When the amount is less than 1%, no densification is attained, whereas an amount exceeding 10% lead to an excess formation of intergranular phase so that neither high toughness nor high strength is achieved. The amount of at least one of Aℓ, Li and Na is limited to 0.1 to 2.0% by weight in oxide base, since when the addition is lower than said limit, debonding of Si₃N₄ particles occurs during abrasion due to insufficient adhesion strength between the Si₃N₄ particles, leading to deterioration of wear resistance. When an addition exceeds 2.0%, on the other hand, the bonding strength of the Si₃N₄ particles in the intergranular phase becomes too strong, thereby depressing the "pull-out" phenomenon and avoiding toughening thereof. The sintered product of the present invention can be obtained, for instance, by firing a mixture of Mg compounds, Zr compounds, and compounds of at least one of Aℓ, Li, and Na, which may give oxides when each is solely fired, wherein the mixture comprises, in an oxide base, by weight, 1 to 10% Mg, 1 to 10% Zr, and 0.1 to 2.0% of at least one of Aℓ, Li, and Na, and Si₃N₄ powder as the remainder. The mixture is molded and fired at 1,500 to 1,900°C under nitrogen or an inert gas atmosphere. Preferable is gas pressure sintering, but is not limited thereto. The gas-pressure sintering is carried out under overpressure of N₂ gas, typically in the range of 0,2-30 MPa (2 to 300 atm), more preferably 0,5-10 MPa (5 to 100 atm). Since the over-pressure of N₂ gas suppresses decomposition of Si₃N₄ at high temperature, in the gas-pressure sintering higher sintering temperature can be employed to form fully dense sintered products. As is clear from the above explanations, the silicon nitride sintered product of the present invention enables longer cutting tool life, maintaining the mechanical strength comparable to the conventional silicon nitride sintered products. The present invention is hereinafter described in greater detail with reference to examples, which are not to be construed as limiting the scope thereof. Unless otherwise indicated, all parts, percents, and ratios are by weight. EXAMPLES 1-5 AND COMPARATIVE EXAMPLES 6-9 Si₃N₄ powder having average particle size (diameter) of 0.7 µm and BET specific surface area of 10 m²/g, MgCO₃ powder having 20 m²/g, ZrO₂ powder having 20 m²/g, and Aℓ₂O₃ powder having 10 m²/g were weighed out, as shown in Table 1, subjected to wet mixing for 16 hours using a pot mill with balls made of Si₃N₄, dried, and granulated to an average diameter of 250 µm. The powder granules were molded in a metal mold under a pressure of 1.5 ton/cm², and fired under conditions as shown in Table 1 to give silicon nitride sintered products of Nos. 1 to 9. Flexural strength at room temperature according to Japanese Industrial Standard (JIS R1601), fracture toughness by IM method, and resistances against wear and chipping were measured on the sintered products of No. 1 to No. 9 and the results are shown in Table 1. Regarding cutting property in Table 1, a wear resistance is shown as a flank wear width V B , whcih was measured by using test piece formed by the method specified in JIS B4103, SNGN 432 with chamfer of 0.05 to machine a cutting material (JIS FC23) in 240 dia. x 200 length, conditions such as cutting speed of 180 m/min, depth of cut of 1.0 mm, feed of 0.15 mm/rev. and cutting length of 800 mm. A chipping resistance is shown as number of disks which were successfully cut without chipping, using a test piece formed by the method specified in JIS B4103, SNGN432 with chamfer of 0.05 to machine a cutting material JIS FC23 having black skin, outer falnk of the disk with an 11 mm thickness and an outer diameter of 200 mm in an axial direction under conditions of a cutting speed of 150 m/min, feed of 0.3 m/rev. and depth of cut of 2.0 mm. As is seen in Table 1, it is considered that sintered test piece No. 6 gave high wear due to Aℓ₂O₃ deficiency, No. 7 showed poor toughness due to excess Aℓ₂O₃, and test pieces No. 8 and No. 9 were not dense enough, due to the deficiency of Mg and Zr respectively, and were poor in overall properties. EXAMPLES 10-14 AND COMPARATIVE EXAMPLES 16-17 Li₂CO₃ powder having a specific surface area of 8 m²/g and Na₂CO₃ powder having 10 m²/g were additionally used, and the powder mixtures, in the ratio shown in Table 2, were processed according to the same process as in Example 3 to giave sintered products No. 10 to No. 17. The properties are measured and the results are shown in Table 2. As is seen in Table 2, both Li and Na had the same effect as Aℓ on the sintered products.

Claims: A silicon nitride sintered product comprising, in % by weight: 1 to 10% Mg, calculated as MgO; 1 to 10% Zr, calculated as ZrO₂; 0.1 to 2.0% of at least one of Al, calculated as Al₂O₃, Li, calculated as Li₂O, and Na, calculated as Na₂O; and the remainder being β-Si₃N₄ and unavoidable impurities. A silicon nitride as claimed in Claim 1, wherein Al is contained in an amount of 0.1 to 2.0% by weight, calculated as Al₂O₃.

Inventors: NAKANISHI, KENJI NGK SPARK PLUG CO., LTD., TAJIMA, YO NGK SPARK PLUG CO., LTD., URASHIMA, KAZUHIRO NGK SPARK PLUG CO., LTD., WATANABE, MASAKAZU NGK SPARK PLUG CO., LTD.
Applicants: NGK SPARK PLUG CO. LTD.
Application year: 1988
Main classification: C04B35
IPC codes: C04B35