JP4013372B2 - Silicon nitride powder - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、高強度、高信頼性の窒化珪素質焼結体が安定して得られる易焼結性の窒化珪素粉末に関する。
構造用セラミックスである窒化珪素は、強度、靭性、耐熱衝撃性などの機械的特性や耐熱性、耐食性などの種々の特性に優れた材料として、切削チップ、エンジン部品、ガスタービン部材、鋳造用ノズル、軸受け部材等に利用されている。
【０００２】
【従来技術及びその問題点】
窒化珪素質焼結体の製造原料である窒化珪素粉末の製造法としては、（１）金属珪素の直接窒化法、（２）シリカ還元窒化法、（３）シリコンジイミドの熱分解法が知られている。これらの方法で製造される窒化珪素粉末は、焼結体の機械的特性や耐熱性などの特性を向上させるために、近年何れも高純度、超微粉、高α相含有等に改善され、金属不純物量や酸素含有量或いは粒子径・比表面積などの特性においては同程度の粉末が製造されるようになってきた。
【０００３】
一般に、窒化珪素質焼結体の機械的特性や耐熱性などの特性は、原料の窒化珪素粉末中に存在する粗大な一次粒子や凝集粒子及び／又は融着粒子等の粗大粒子の影響を大きく受けることが知られている。（１）の方法で製造された粉末は、直接窒化反応時に生成する粗大粒子の未粉砕粒子を多く含み、（２）の方法で製造された粉末は、原料のシリカ粉末の融着による粗大粒子を多く含み、（３）の方法で製造された粉末は、微粒子の凝集による粗大粒子を多く含んでいる。
【０００４】
この窒化珪素粉末中の粗大な一次粒子や凝集粒子及び／又は融着粒子等の粗大粒子は硬く、焼結体作製工程における原料調製時の乾式或いは湿式のボールミル混合粉砕では解砕或いは粉砕することが難しい。窒化珪素質焼結体は、焼結助剤を添加した混合粉末の成形体を焼結することにより得られている。窒化珪素粉末と焼結助剤との混合は、通常湿式ボールミル混合によって行なわれている。したがって窒化珪素粉末中の粗大粒子は焼結助剤との混合粉末中にそのまま残存する。残存する粗大粒子は、通常は焼結助剤混合スラリーから開孔径５０μｍ前後の篩を用いて取り除かれる。篩の開孔径は、５０μｍより小さい場合は篩の目詰まりが急激に増えることから小さくても２５μｍ程度である。通常は作業性を考慮して４５μｍ程度の篩が用いられている。
つまり、窒化珪素質焼結体の製造原料である窒化珪素粉末において、高強度で特性のバラツキの少ない焼結体が安定して得られる易焼結性の窒化珪素粉末を提供するためには、窒化珪素粉末中の５０μｍ以下の粗大な一次粒子や凝集粒子及び／又は融着粒子等の粗大粒子量を一定値以下に制御する必要がある。
【０００５】
この窒化珪素粉末中の粗大な一次粒子や凝集粒子及び／又は融着粒子等の粗大粒子の含有量は、走査型電子顕微鏡写真及び透過型電子顕微鏡写真により一次粒子の大きさ、状態を定性的に測定するか、或いは遠心沈降光透過法、Ｘ線透過法、光散乱法、レーザー回折法、コールターカウンター法等の粒度分布測定器による粒度分布測定等により定量されてきた。
しかしながら、走査型または透過型電子顕微鏡写真による粗大粒子の測定は、極微小部分の測定であり再現性に欠け、凝集粒子の粒子径分布の判定も困難で定量的な測定ができなかった。また、各粒度分布測定装置による粗大粒子の測定は、主に二次粒子の測定であり、しかも測定限界が％単位であることから、０．１％以下の少量の粗大粒子量を再現性良く測定することが困難であった。
このため、これらの粗大粒子量を低減して高強度で特性のバラツキの少ない焼結体が安定して得られる易焼結性の窒化珪素粉末を製造すること自体が検討できないという問題があった。
【０００６】
【発明の目的】
本発明は、上記課題を解決するために窒化珪素粉末中の粗大粒子量の測定法を検討し、この結果に基づき高強度で特性のバラツキの少ない焼結体が安定して得られる易焼結性の窒化珪素粉末を提供することを目的としている。
【０００７】
【問題点を解決するための手段】
本発明者等は、従来定量的な測定が困難であった５０μｍ以下の粗大な一次粒子や凝集粒子及び／又は融着粒子等の粗大粒子量の測定法として、高精度な篩分け法を用いた測定法を採用し、これにより、高強度で特性のバラツキの少ない焼結体を安定して製造することができる窒化珪素粉末を得ることができた。
【０００８】
即ち、本発明は、比表面積が４〜２５ｍ２／ｇで、酸素含有量が０．７〜２．０ｗｔ％である窒化珪素粉末であって、該窒化珪素粉末中に存在する、２５μｍ以上の粗大粒子並びに凝集粒子及び／又は融着粒子の重量分率が１０００ｐｐｍ以下であり、１０μｍ以上の粗大粒子並びに凝集粒子及び／又は融着粒子の重量分率が２０００ｐｐｍ以下であることを特徴とする窒化珪素粉末に関するものである。
【０００９】
本発明の窒化珪素粉末は、比表面積が４〜２５ｍ2 ／ｇである。比表面積が４ｍ2 ／ｇ未満の窒化珪素粉末中には、粒成長した粗大粒子の割合が増加し、比表面積が２５ｍ2 ／ｇ超の窒化珪素粉末には、微粒子の凝集、融着による粗大粒子が増加する。また、比表面積は、焼結性及び焼結体特性を支配する重要な因子であり、比表面積が４ｍ2 ／ｇ未満の場合には、緻密化が難しくなり、高密度な焼結体が得られない。比表面積が２５ｍ2 ／ｇを超えると、得られる窒化珪素質焼結体の強度のバラツキが増大して、信頼性が低下する。
さらに、本発明の窒化珪素粉末は、酸素含有量が０．７〜２．０ｗｔ％である。
比表面積と同様に、窒化珪素粉末の酸素含有量は、焼結性及び焼結体特性を支配する重要な因子であり、酸素含有量０．７ｗｔ％未満の場合には、緻密化が難しくなり、高密度な焼結体が得られない。酸素含有量が２．０ｗｔ％を超えると、得られる窒化珪素質焼結体の高温強度、破壊靭性が低下する。
【００１０】
また、本発明の窒化珪素粉末は、α分率が８５％以上であることが望ましい。α分率が８５％未満の高β分率の窒化珪素粉末は、結晶化温度が１５５０℃以上の高温に達する場合に生成し、この高温下では、窒化珪素粒子は粒成長、融着及び凝集し易くなり、粗大粒子が増加する。
【００１１】
さらに、本発明の窒化珪素粉末は、窒化珪素粉末中に存在する、２５μｍ以上の粗大な一次粒子並びに凝集粒子及び／又は融着粒子の重量分率が１０００ｐｐｍ以下、好ましくは８００ｐｐｍ以下である。
また、１０μｍ以上の粗大粒子並びに凝集粒子及び／又は融着粒子の重量分率が２０００ｐｐｍ以下、好ましくは１５００ｐｐｍ以下であり、さらには５μｍ以上の粗大粒子並びに凝集粒子及び／又は融着粒子の重量分率が３０００ｐｐｍ以下、好ましくは２１００ｐｐｍ以下であることが好ましい。
窒化珪素粉末中の２５μｍ以上の粗大粒子の重量分率が１０００ｐｐｍよりも多く存在すると、窒化珪素の焼結過程において窒化珪素粒子が焼結助剤等からなる粒界相に溶解する際の溶解速度のずれ、遅れを生じ、緻密化の進行を妨げる。また、緻密化の際に粗大粒子を核にした粒成長が進み、異常な粒成長部分を持つ焼結体組織となり、異常粒成長部分の周囲にマイクロクラックを生じて、焼結体の強度を低下させる。
窒化珪素粉末中の２５μｍ以上の粗大粒子の重量分率が１０００ｐｐｍ以下の場合は、粒界相への窒化珪素粒子の溶解速度が均一で速く、焼結体組織が均一となり、高強度、高信頼性な窒化珪素質焼結体となる。
【００１２】
一方、窒化珪素粉末は、どの製造法においても、電気炉中１４００〜１７００℃で焼成することにより製造されている。電気炉の炉材（ヒーター、断熱材、炉心管、炉床板等）としては、通常、炭素質物質を使用するため、得られる製品粉末中には、不可避的に炭素が混入してくる。この混入した炭素質物質は粗大な凝集体に成長するため、２５μｍ以上の粗大粒子並びに凝集粒子及び／又は融着粒子に含有される炭素量は数ｗｔ％と多くなる。このように炭素含有量が高濃度になると、炭素は焼結過程において添加した酸化物助剤と反応してＣＯガスとなって揮散するため、得られる焼結体中に多数の粗大な気孔が残存することになる。したがって、本発明の窒化珪素粉末中に存在する、２５μｍ以上の粗大粒子並びに凝集粒子及び／又は融着粒子の炭素含有率は５．０ｗｔ％以下、好ましくは、２．５ｗｔ％以下であることが好ましい。２５μｍ以上の粗大粒子並びに凝集粒子及び／又は融着粒子の炭素含有率が５．０ｗｔ％以下であれば、焼結体中の粗大気孔は著しく減少し、焼結体の機械的特性や耐熱性、耐食性にほとんど影響を及ぼさなくなる。
【００１３】
また、１０μｍ以上の粗大粒子並びに凝集粒子及び／又は融着粒子の炭素含有率が４．０ｗｔ％以下、好ましくは、２．０ｗｔ％以下、さらには、５μｍ以上の粗大粒子並びに凝集粒子及び／又は融着粒子の炭素含有率が３．２ｗｔ％以下、好ましくは、１．６ｗｔ％以下であることが好ましい。
窒化珪素粉末中の炭素含有異物は、焼結後に粗大気孔として焼結体中に残存し、これが破壊の起点となる。
窒化珪素粉末中に存在する２５μｍ以上の炭素含有異物量が多くなると、得られる焼結体中に特に低強度な欠陥部が生成し、強度試験におけるバラツキが増大する。強度測定データをワイブル統計により解析した場合には、ワイブル係数が低下すると共に、低強度側に異常データ点を生じ、解析自体の信頼性が無くなる。もちろん、材料としての信頼性も欠如する。
また、１０〜２５μｍおよび／または５〜１０μｍの炭素含有異物量が多くなると、同様に焼結体中に低強度な欠陥部が多数生成する。この場合には欠陥部の寸法はやや小さいものの、欠陥の数量が多いため、平均強度も低下し、バラツキも増大する。ワイブルプロット上では特に低強度な異常データ点は生じないが、ワイブル係数は低下し、材料としての信頼性は低下する。
【００１４】
本発明における粗大粒子量の測定は、以下に示す粉末中の微量の粗大粒子の抽出方法及び抽出された粗大粒子の重量測定法により行った。
まず、５００ｍｌのテフロンビーカーに粉末試料２０ｇを採集し、これにヘキサメタリン酸ナトリウム（ＮａＨＭＰ）０．０１ｗｔ％を添加した超純水（イオン交換後、蒸留し、さらに孔径０．１μｍのフィルターを通過させたもの）２００ｍｌを加えて粉末を懸濁させ、テフロン製スプーンで攪拌しながら、超音波を１０分間照射して、分散させる。
次いで、分散スラリーを吊り下げ型の電磁振動ふるい器に取り付けた所定の目開き（５μｍ、１０μｍ、２５μｍ）のナイロン製篩を通過させる。ナイロン製篩は、着脱式の型枠に固定し、超音波洗浄器の水面から約５mm上の部分に設置し、超音波洗浄を併用して、目詰まりなく篩操作ができるようにする（水の表面張力により、超音波洗浄器の水面は、ナイロン製篩の篩面に吸い付く）。
【００１５】
次に、篩面をＮａＨＭＰ０．０１ｗｔ％を添加した超純水５００ｍｌで１０〜３０回洗浄した後、篩を取り出し、型枠をはずして、ナイロン製篩のみを取り出す。これを折ってφ６０×３０mmの秤量瓶に入れる。
秤量瓶（ナイロン製篩と抽出粗大粒子の入ったもの）を乾燥器で１２０℃、３０分間乾燥した後、デシケーター中で２時間放冷する。
温度一定の恒温室内に設置された精密天秤の天秤室内に、秤量瓶を１５分間放置し、温度を一定にした後に、秤量瓶の重量を測定する。重量は５回測定し、その平均値を採用する。
また、ナイロン製篩上の残さを回収した後、超純水中で超音波洗浄して、付着物を完全に取り除き、これを秤量瓶に入れて、同様の操作で重量を測定し、前記の測定値から差し引く。
これにより、安定した精密重量測定を実施できるようになった。
【００１６】
次に、本発明の窒化珪素粉末を製造する方法について説明する。
本発明の窒化珪素粉末は、金属珪素粉末の直接窒化法、シリカ粉末の還元窒化法、イミド分解法等により、粗大粒子および炭素含有異物を低減する条件で製造された粉末を、解砕処理した後、さらに分級装置で分級することにより製造される。窒化珪素粉末中の粗大粒子量および炭素含有異物を容易に低減できるという点では、イミド分解法が最も適している。
【００１７】
イミド分解法では、例えば、イミド中に残存するトルエンの量を０．４ｗｔ％以下に、イミドの比表面積を５００〜９００ｍ2 ／ｇに、軽装密度を０．０３５〜０．０７５ｇ／ｃｍ3 に調整し、１４００〜１７００℃の温度条件下で結晶化させることにより結晶質窒化ケイ素粉末を製造する。
次いで、前記の焼成により得られた結晶質窒化ケイ素粉末を、酸素を４〜３０％含有し、残部が不活性ガスからなる雰囲気中でミル処理する。雰囲気ガスとしては、酸素を４〜３０％含有し、残部が窒素、ヘリウム、アルゴン等の不活性ガスからなる雰囲気であればよく、例えば、空気雰囲気が好ましく用いられる。ミル処理方法としては、特に制限はなく、通常用いられるミル処理装置、例えば、振動ミル、アトライター等が用いられる。このミル処理により焼成時に起こった粒子間の融着や凝集を壊すことができる。
【００１８】
金属珪素粉末の直接窒化法では、例えば、α相分率７０％以上及び比表面積１０ｍ2 ／ｇ以上の窒化ケイ素粉末を比表面積１０ｍ2 ／ｇ以上及び酸素含有量２．０重量％以下の金属珪素粉末に５〜２０重量％添加混合し、混合物を、水素ガスと窒素ガスとの混合雰囲気下あるいはアンモニアガスと窒素ガスとの混合雰囲気下、昇温速度５〜２５℃／ｈで１４００〜１６００℃まで昇温することにより、窒化珪素のインゴットを得る。得られたインゴットを１５００〜１７００℃で熱処理した後、常法により、粗砕、中砕し、さらに、ボールミル、振動ミル、ジェットミル、アトライターミル、パールミル等で湿式または乾式粉砕して粒度を調整する。粉砕の際に混入した不純物を無機酸（例えば、フッ化水素酸と硫酸との混合物）で処理して、溶解除去した後、乾燥処理して窒化珪素粉末を製造する。アトライター粉砕を例にとれば、粉砕時間を０．８時間以上にして十分に粉砕し、酸処理後のろ過ケーキの乾燥を１００℃以下の温度で行うことにより、粗大粒子量の少ない窒化珪素粉末を製造することができる。
【００１９】
シリカ還元法では、例えば、比表面積１０ｍ2／ｇ以上の窒化珪素粉末、比表面積１００ｍ2／ｇ以上のシリカ粉末及び比表面積５０ｍ2／ｇ以上のカーボンブラックとを一定の割合で混合したものを原料とし、これを窒素ガス気流中で加熱することにより、窒化珪素粉末を製造する。
また、シリカ１重量部に対して、カーボン２重量部以上、窒化珪素０．１重量部以上を添加して、湿式で均一混合した後、造粒したものを原料として使用し、焼成温度を１４５０℃以下に設定すると共に、生成粉末の大気中加熱による脱炭処理温度を６８０℃以下に設定して、４時間以上かけてゆっくり脱炭することにより、粗大粒子量の少ない窒化珪素粉末を製造することができる。この方法により製造された窒化珪素粉末は、ナイロン製の振動ミルにより軽く解砕処理して、所望の特性の粉末を得る。
【００２０】
次に、上記の粗大粒子及び炭素含有異物を低減する条件で製造された粉末を、分級装置で分級する。
風力分級機は、強制渦中に浮遊した粒子を、遠心力と流体抗力の差によって微粒子と粗粒子に分離する装置である。
本発明で使用した風力分級機では、導入空気はローターとローターリングの隙間から高速の気流となって流入し、分散ゾーンに入る。一方、原料投入口より供給された粉体は、空気の流れに乗った状態で、高速気流による剪断力を受けると共に、分散羽根と分散円板の作用により均一に分散されながら分級室に送り込まれる。ここで、個々の粒子は回転流により外方向に働く遠心力と、半径方向に流れる空気流による内方向に向かう空気抗力とを受ける。風力分級において分離される粒径は、粒子に働く遠心力と空気抗力との釣り合いによって決まる。即ち、粗粒子はより大きな遠心力を受け、微粒子は空気抗力の影響を大きく受けて、それぞれ異なる軌跡を描いて移動する。分級される粗粒子は遠心力によって分級ローターの外側に飛ばされ、ローターリング内壁に到達した後、ローターリングの粗粉排出口を経て、エアーシール状態で取り出される。微粒子は空気流と共に分級ローターの内部に送り込まれ、分散羽根によって整流された空気流に乗って、分級ローター、バランスローター、ケージングを通り、ローター中心部の細粉排出口を経て、サイクロンあるいはバグフィルターなどによって捕集される。
【００２１】
分級点の調整は、分級ローターの回転速度と通過空気流量の変更によって行われる。通過空気流量は可変範囲が限定されるので、通常は、分級ローターの回転数で調整を行うことが多いが、広範囲の分級点を設定する上では、両者を独立に制御することが望ましい。
ローター回転数が高いほど、通過空気流量が少ないほど、分級点が微粒子側にシフトするので、両者の条件設定により、０．５〜５０μｍの範囲での分級が可能となる。例えば、ローター回転数を２４００ｒｐｍ（分散羽根外周での周速度３５ｍ／ｓに相当）に、通過空気流量を５．５ｍ3 ／ｍｉｎ（分散羽根入口空気流速２ｍ／ｓに相当）に設定することにより、粒径８μｍ以上の粗粒子を効率的に分離、除去することができる。
粉流体の分級精度を高める上で重要なことは、供給された粉体を単一粒子に近い状態まで分散させると共に、分級室内の空気の流れを均一に保つことである。その為には、分級ローターと分散羽根とを一体で回転させて気流の乱れを抑制することや、外部ブロアーの吸引によって起こる半径方向の不均一流や局部的渦流などを抑制することに注力する必要がある。
【００２２】
本発明で使用する窒化珪素粉末中には、低炭素含有量の粗大粒子と高炭素含有量の炭素含有異物とが存在し、これらは分級装置内部での分散のされ具合が異なる。この為、分級ローターの回転数を上げて分級点を微粒子側にシフトさせると、低炭素含有量の粗大粒子が優先的に分離、除去され、逆に、通過空気流量を少なくして分級点を微粒子側にシフトさせると、高炭素含有量の炭素含有異物が優先的に分離、除去されるという現象が発生した。したがって、分級ローターの回転速度と通過空気流量を組み合わせた条件設定が重要であり、特定の条件範囲でのみ、高生産性ならびに高粉末回収率で、低炭素含有量の粗大粒子と高炭素含有量の炭素含有異物との両方を効率的に分離、除去することが可能となった。
【００２３】
本発明の窒化ケイ素粉末は、従来の窒化ケイ素粉末の場合と同様な方法、例えば、酸化アルミニウム、酸化イットリウム、酸化マグネシウム等の焼結助剤と混合し、混合物を所定の形状に成形した後、焼結することにより、窒化ケイ素セラミックス（焼結体）を製造することができる。上記成形圧力は、０．５〜１０ton ／cm2程度とすれば良く、また上記焼結条件は、焼結温度１５００〜２０００℃、雰囲気圧力０．５〜１００気圧、焼結時間１〜１０時間程度とすれば良い。
【００２４】
本発明の窒化ケイ素粉末を用いて製造された、窒化ケイ素セラミックス（焼結体）は、特に、高強度、高ワイブル係数で信頼性の高いことから、本発明の窒化ケイ素粉末は、高度の信頼性と製品寿命が要求されるターボローター、エンジンバルブ、ディーゼルエンジン副燃焼室等の熱機関用部品や機械部品として用いられる窒化ケイ素セラミックスの製造用原料として、特に好適なものである。
【００２５】
【実施例】
以下に本発明の実施例を比較例と共に挙げ、本発明を更に詳しく説明する。
実施例１〜９及び比較例１〜９
下記の製造方法（イミド分解法）及び下記〔表１〕に示す製造条件により、窒化ケイ素粉末をそれぞれ製造した。得られた窒化ケイ素粉末の粉末特性を、下記〔表２〕に示す。
【００２６】
〔窒化ケイ素粉末の製造方法〕
液体アンモニアと予め調製した四塩化ケイ素２０〜３５重量％、残部トルエンよりなる溶液とを反応槽に供給して得られた反応生成物を、液体アンモニアで何回もバッチ洗浄して、精製シリコンジイミドを得た。
反応の際の四塩化ケイ素と液体アンモニアとの比率（体積基準）を１／５０〜２／５０の範囲で変化させることにより、比表面積５００〜８５０ｍ2／ｇのシリコンジイミドを合成した。
また、生成シリコンジイミドを乾燥する際の乾燥時間と撹拌回転数を変えることにより、シリコンジイミドの軽装密度を０.０３５〜０.０７５ｇ／ｃｍ3の範囲で変化させた。
【００２７】
生成したシリコンジイミドを、下記〔表１〕に記載した酸素濃度を有する窒素ガスを流通させながら１０００℃で加熱分解させて、非晶質窒化ケイ素粉末を得た。次いで、得られた非晶質窒化ケイ素粉末を振動ミルにて摩砕処理した後、表面を炭化珪素で被覆した黒鉛製容器に充填して、電気炉にて、窒素雰囲気下、〔表１〕に記載の条件（昇温速度、最高温度及び同温度での保持時間、炉内ＣＯ濃度）で加熱、焼成して、灰白色の窒化ケイ素粉末を得た。
尚、炉内のＣＯ濃度は、流通させる窒素ガスの純度（酸素濃度、露点）と流量により調整した。
この結晶質窒化珪素粉末を振動ミルに投入し、酸素含有量１０％、残部が不活性ガスよりなる雰囲気下、振幅８ｍｍで所定の時間、ミル処理を行った後、風力分級機に供給して、表１に記載した所定の条件下で分級処理を行い、粗大粒子を除去した。ローターとリングの隙間部に入る空気の平均流速および分散羽根入口の平均空気流速は共に、通過空気流量に比例し、それぞれ３３ｍ／ｓ〜１００ｍ／ｓおよび０．８ｍ／ｓ〜２．５ｍ／ｓであった。
得られた窒化珪素粉末の粉体特性を、表２に示す。
【００２８】
なお、粗大粒子量は、前述の測定方法により行った。
比表面積は、島津−マイクロメリテックス製フローソーブ２３００形を使用して、ＢＥＴ一点法により測定した。
酸素含有量は、ＬＥＣＯ社製ＴＣ−１３６型酸素・窒素同時分析装置を使用して不活性ガス融解−赤外線吸収法により測定した。
炭素含有量は、ＬＥＣＯ社製ＷＲ−１２型炭素分析装置を使用して、燃焼−熱伝導度法により測定した。
〔Ｘ線回折測定〕
ターゲットが銅の管球とグラファイトモノクロメーターを使用し、定時ステップ走査法により、得られた窒化ケイ素粉末の粉末Ｘ線回折パターンを測定した。回折角（２θ）１５〜８０゜の範囲を０．０２゜刻みでステップスキャンし、リートベルト解析によりα分率とβ分率を求めた。
【００２９】
【表１】
【表２】
実施例１０〜１３及び比較例１０〜１５
比表面積１０ｍ２／ｇ以上、酸素含有量２．０重量％以下の金属珪素粉末を使用して、前述の直接窒化法により、表３に示す製造条件で窒化珪素粉末を製造した。得られた窒化珪素粉末を風力分級機を使用して分級し、粗大粒子を分離、除去した。
得られた窒化珪素粉末の特性を表４に示す。
【００３２】
【表３】
【表４】
使用試験例
実施例１〜１３及び比較例１〜１５で得られた窒化珪素粉末を原料に用いて、下記の製造方法によりそれぞれの焼結体を作製した。
〔焼結体の製造方法〕
窒化珪素粉末９３重量部に焼結助剤としてＹ2Ｏ3５重量部とＡｌ2Ｏ3２重量部を添加し、エタノールを加えて、ボールミルにて４８時間湿式混合した後、乾燥した。
乾燥、粒度調整を行った顆粒を、３００ｋｇ／ｃｍ2の成形圧で７５×４５×６ｍｍの形状に金型成形した後、これを２ｔｏｎ／ｃｍ2の圧力でラバープレス成形して、グリーン成形体を作製した。この成形体を窒化珪素製ルツボに充填し、電気炉にて、１気圧の窒素ガス雰囲気中、昇温速度１００℃／ｈで昇温し、１７６０℃で４時間保持して、窒化ケイ素質焼結体を得た。
【００３５】
得られた焼結体の嵩密度はアルキメデス法で測定した。焼結体よりＪＩＳ Ｒ１６０１に準拠した３×４×４０ｍｍ相当の抗折試験片を切り出し、ＪＩＳ Ｒ １６０１に準拠して、外スパン３０ｍｍ、内スパン１０ｍｍ、クロスヘッドスピード０.５ｍｍ／ｍｉｎの条件で四点曲げ試験を行った。室温における曲げ強度は４０本の平均値である。高温での曲げ試験は、窒素雰囲気中で試験片を１３００℃に１０分間保持した後、８本以上の試験片について強度測定を行い、平均値を算出した。また、破壊靭性値はＪＩＳ Ｒ １６０７規定のＳＥＰＢ法で測定した。
到達密度、曲げ強度（室温強度、室温強度のワイブル係数及び高温強度）、及び破壊靭性値の測定結果を下記〔表５〕に示す。
本発明の窒化珪素粉末は、高強度で、特性のバラツキの少ない高信頼性の窒化珪素セラミックスを再現性良く安定して製造できることがわかる。
【００３６】
【表５】
【発明の効果】
本発明の窒化ケイ素粉末は、高強度で、特性のバラツキの少ない高信頼性の窒化ケイ素セラミックスを再現性良く安定して製造できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an easily sinterable silicon nitride powder from which a high-strength, highly reliable silicon nitride sintered body can be stably obtained.
Silicon nitride, a structural ceramic, is a material with excellent mechanical properties such as strength, toughness, thermal shock resistance, and various properties such as heat resistance and corrosion resistance. Cutting chips, engine parts, gas turbine members, casting nozzles It is used for bearing members and the like.
[0002]
[Prior art and its problems]
Known methods for producing silicon nitride powder, which is a raw material for producing a silicon nitride sintered body, are (1) direct nitridation of metal silicon, (2) silica reduction nitridation, and (3) thermal decomposition of silicon diimide. ing. Silicon nitride powder produced by these methods has recently been improved to high purity, ultrafine powder, high α phase content, etc. in order to improve the mechanical properties and heat resistance of the sintered body. In terms of characteristics such as the amount of impurities, oxygen content, particle size, and specific surface area, powders of the same degree have been produced.
[0003]
In general, characteristics such as mechanical properties and heat resistance of a silicon nitride sintered body are greatly influenced by coarse primary particles, aggregated particles, and / or fused particles present in a raw material silicon nitride powder. It is known to receive. The powder produced by the method (1) contains a large amount of coarse particles of coarse particles produced during the direct nitriding reaction, and the powder produced by the method (2) comprises coarse particles obtained by fusing the raw silica powder. The powder produced by the method (3) contains many coarse particles due to aggregation of fine particles.
[0004]
Coarse primary particles, coarse particles such as aggregated particles and / or fused particles in the silicon nitride powder are hard and must be crushed or pulverized by dry or wet ball mill mixed pulverization during raw material preparation in the sintered body production process. Is difficult. The silicon nitride sintered body is obtained by sintering a compact of a mixed powder to which a sintering aid is added. The silicon nitride powder and the sintering aid are usually mixed by wet ball mill mixing. Accordingly, the coarse particles in the silicon nitride powder remain as they are in the mixed powder with the sintering aid. The remaining coarse particles are usually removed from the sintering aid mixed slurry using a sieve having an opening diameter of about 50 μm. When the opening diameter of the sieve is smaller than 50 μm, clogging of the sieve increases rapidly, so that it is about 25 μm at the smallest. Usually, a sieve of about 45 μm is used in consideration of workability.
In other words, in order to provide a silicon nitride powder that is a raw material for producing a silicon nitride-based sintered body, an easily sinterable silicon nitride powder that can stably obtain a sintered body having high strength and little variation in characteristics. It is necessary to control the amount of coarse particles such as coarse primary particles of 50 μm or less, aggregated particles and / or fused particles in the silicon nitride powder to a certain value or less.
[0005]
The content of coarse particles such as coarse primary particles, agglomerated particles and / or fused particles in the silicon nitride powder is qualitatively determined by the scanning electron micrograph and transmission electron micrograph. Or a particle size distribution measurement by a particle size distribution measuring device such as a centrifugal sedimentation light transmission method, an X-ray transmission method, a light scattering method, a laser diffraction method, a Coulter counter method, or the like.
However, the measurement of coarse particles by scanning or transmission electron micrographs is a measurement of a very small portion and lacks reproducibility, and it is difficult to determine the particle size distribution of the aggregated particles, so that quantitative measurement cannot be performed. Moreover, the measurement of coarse particles by each particle size distribution measuring device is mainly the measurement of secondary particles, and since the measurement limit is in units of%, a small amount of coarse particles of 0.1% or less can be obtained with good reproducibility. It was difficult to measure.
For this reason, there is a problem that it is not possible to study the production of easily sinterable silicon nitride powder that can stably obtain a sintered body having high strength and little variation in characteristics by reducing the amount of these coarse particles. .
[0006]
OBJECT OF THE INVENTION
In order to solve the above-mentioned problems, the present invention examines a method for measuring the amount of coarse particles in silicon nitride powder, and based on this result, a sintered body having high strength and little variation in characteristics can be stably obtained. It is an object to provide a functional silicon nitride powder.
[0007]
[Means for solving problems]
The present inventors use a high-precision sieving method as a method for measuring the amount of coarse primary particles, aggregated particles, and / or fused particles of 50 μm or less, which has conventionally been difficult to quantitatively measure. Thus, it was possible to obtain a silicon nitride powder capable of stably producing a sintered body having high strength and little variation in characteristics.
[0008]
That is, the present invention is a silicon nitride powder having a specific surface area of 4 to 25 m 2 / g and an oxygen content of 0.7 to 2.0 wt%, and is present in the silicon nitride powder and having a particle size of 25 μm or more. The nitriding is characterized in that the weight fraction of coarse particles and aggregated particles and / or fused particles is 1000 ppm or less , and the weight fraction of coarse particles of 10 μm or larger and aggregated particles and / or fused particles is 2000 ppm or less. It relates to silicon powder.
[0009]
The silicon nitride powder of the present invention has a specific surface area of 4 to 25 m 2 / g. In the silicon nitride powder having a specific surface area of less than 4 m 2 / g, the proportion of coarse particles grown is increased, and in the silicon nitride powder having a specific surface area of more than 25 m 2 / g, the coarse particles are formed by agglomeration and fusion of fine particles. Particles increase. The specific surface area is an important factor governing the sinterability and sintered body characteristics. When the specific surface area is less than 4 m 2 / g, densification becomes difficult and a high-density sintered body is obtained. I can't. When the specific surface area exceeds 25 m 2 / g, variation in strength of the obtained silicon nitride-based sintered body increases and reliability decreases.
Furthermore, the silicon nitride powder of the present invention has an oxygen content of 0.7 to 2.0 wt%.
Similar to the specific surface area, the oxygen content of the silicon nitride powder is an important factor governing the sinterability and sintered body characteristics, and if the oxygen content is less than 0.7 wt%, densification becomes difficult. A high-density sintered body cannot be obtained. If the oxygen content exceeds 2.0 wt%, the high temperature strength and fracture toughness of the resulting silicon nitride sintered body will be reduced.
[0010]
The silicon nitride powder of the present invention desirably has an α fraction of 85% or more. A high β fraction silicon nitride powder with an α fraction of less than 85% is produced when the crystallization temperature reaches a high temperature of 1550 ° C. or higher, and under this high temperature, the silicon nitride particles grow, fuse and agglomerate. And coarse particles increase.
[0011]
Furthermore, the silicon nitride powder of the present invention has a weight fraction of coarse primary particles of 25 μm or more and aggregated particles and / or fused particles present in the silicon nitride powder of 1000 ppm or less, preferably 800 ppm or less.
The weight fraction of coarse particles of 10 μm or more and aggregated particles and / or fused particles is 2000 ppm or less, preferably 1500 ppm or less, and further, the weight fraction of coarse particles of 5 μm or more and aggregated particles and / or fused particles. The rate is 3000 ppm or less, preferably 2100 ppm or less.
When the weight fraction of coarse particles of 25 μm or more in the silicon nitride powder is more than 1000 ppm, the dissolution rate when the silicon nitride particles are dissolved in the grain boundary phase composed of a sintering aid or the like in the sintering process of silicon nitride. Shifts and delays, preventing the progress of densification. In addition, grain growth with coarse particles as the core progresses during densification, resulting in a sintered body structure with an abnormal grain growth part, creating microcracks around the abnormal grain growth part, and increasing the strength of the sintered body. Reduce.
When the weight fraction of coarse particles of 25 μm or more in the silicon nitride powder is 1000 ppm or less, the dissolution rate of the silicon nitride particles in the grain boundary phase is uniform and fast, the sintered body structure becomes uniform, and the strength and reliability are high. A silicon nitride-based sintered body.
[0012]
On the other hand, silicon nitride powder is manufactured by firing at 1400 to 1700 ° C. in an electric furnace in any manufacturing method. As a furnace material (a heater, a heat insulating material, a core tube, a hearth plate, etc.) of an electric furnace, since a carbonaceous substance is normally used, carbon is inevitably mixed in the obtained product powder. Since the mixed carbonaceous material grows into coarse aggregates, the amount of carbon contained in coarse particles of 25 μm or more, and aggregated particles and / or fused particles increases to several wt%. When the carbon content becomes high in this way, carbon reacts with the oxide aid added in the sintering process and volatilizes as CO gas, so that a large number of coarse pores are formed in the obtained sintered body. Will remain. Therefore, the carbon content of the coarse particles of 25 μm or more and the agglomerated particles and / or fused particles present in the silicon nitride powder of the present invention is 5.0 wt% or less, preferably 2.5 wt% or less. preferable. If the carbon content of coarse particles of 25 μm or more and agglomerated particles and / or fused particles is 5.0 wt% or less, the coarse atmospheric pores in the sintered body are significantly reduced, and the mechanical properties and heat resistance of the sintered body are reduced. Has little effect on corrosion resistance.
[0013]
The coarse particles of 10 μm or more and the aggregated particles and / or fused particles have a carbon content of 4.0 wt% or less, preferably 2.0 wt% or less, and further, coarse particles of 5 μm or more and aggregated particles and / or It is preferable that the carbon content of the fused particles is 3.2 wt% or less, preferably 1.6 wt% or less.
The carbon-containing foreign material in the silicon nitride powder remains in the sintered body as coarse air holes after sintering, and this becomes a starting point of destruction.
When the amount of carbon-containing foreign matter of 25 μm or more present in the silicon nitride powder is increased, defective portions having particularly low strength are generated in the obtained sintered body, and variation in the strength test is increased. When the intensity measurement data is analyzed by Weibull statistics, the Weibull coefficient is lowered and an abnormal data point is generated on the low intensity side, and the reliability of the analysis itself is lost. Of course, it lacks reliability as a material.
Moreover, when the amount of carbon-containing foreign matter of 10 to 25 μm and / or 5 to 10 μm increases, many low-strength defect portions are similarly generated in the sintered body. In this case, although the size of the defect portion is slightly small, the number of defects is large, so that the average strength is lowered and the variation is also increased. On the Weibull plot, an abnormal data point having a particularly low intensity does not occur, but the Weibull coefficient is lowered and the reliability as a material is lowered.
[0014]
The measurement of the amount of coarse particles in the present invention was performed by the following method for extracting a small amount of coarse particles in the powder and measuring the weight of the extracted coarse particles.
First, 20 g of a powder sample was collected in a 500 ml Teflon beaker, and ultrapure water to which sodium hexametaphosphate (NaHMP) 0.01 wt% was added (distilled after ion exchange and further passed through a filter having a pore diameter of 0.1 μm. 2) Add 200 ml to suspend the powder, and disperse by irradiating with ultrasonic waves for 10 minutes while stirring with a Teflon spoon.
Next, the dispersed slurry is passed through a nylon sieve having predetermined openings (5 μm, 10 μm, 25 μm) attached to a hanging type electromagnetic vibration sieve. Nylon sieve is fixed to a removable formwork and placed on the surface of the ultrasonic cleaner about 5mm above the water surface. Ultrasonic cleaning is also used to enable the sieve operation without clogging (water Due to the surface tension of the ultrasonic cleaner, the water surface of the ultrasonic cleaner sticks to the screen surface of the nylon sieve).
[0015]
Next, the screen surface is washed 10 to 30 times with 500 ml of ultrapure water to which 0.01 wt% of NaHMP is added, and then the screen is taken out, the mold is removed, and only the nylon screen is taken out. This is folded and placed in a weighing bottle of φ60 × 30 mm.
A weighing bottle (containing a nylon sieve and extracted coarse particles) is dried at 120 ° C. for 30 minutes in a dryer and then allowed to cool in a desiccator for 2 hours.
The weighing bottle is allowed to stand for 15 minutes in a balance room of a precision balance installed in a constant temperature room at a constant temperature. After the temperature is kept constant, the weight of the weighing bottle is measured. The weight is measured 5 times and the average value is adopted.
Also, after collecting the residue on the nylon sieve, ultrasonically washed in ultrapure water to completely remove the deposits, put this in a weighing bottle, measure the weight in the same manner, Subtract from the measured value.
As a result, stable precise weight measurement can be performed.
[0016]
Next, a method for producing the silicon nitride powder of the present invention will be described.
The silicon nitride powder of the present invention is obtained by crushing a powder produced under conditions that reduce coarse particles and carbon-containing foreign matters by a direct nitriding method of metal silicon powder, a reductive nitriding method of silica powder, an imide decomposition method, or the like. Thereafter, it is produced by further classifying with a classifier. The imide decomposition method is most suitable in terms of easily reducing the amount of coarse particles and carbon-containing foreign matter in the silicon nitride powder.
[0017]
In the imide decomposition method, for example, the amount of toluene remaining in the imide is 0.4 wt% or less, the specific surface area of the imide is 500 to 900 m 2 / g, and the light weight density is 0.035 to 0.075 g / cm 3 . Crystalline silicon nitride powder is manufactured by adjusting and crystallizing under the temperature condition of 1400-1700 ° C.
Next, the crystalline silicon nitride powder obtained by the firing is milled in an atmosphere containing 4 to 30% oxygen and the balance being an inert gas. The atmosphere gas may be any atmosphere that contains 4 to 30% oxygen and the balance is made of an inert gas such as nitrogen, helium, or argon. For example, an air atmosphere is preferably used. There is no restriction | limiting in particular as a mill processing method, For example, a mill processing apparatus normally used, for example, a vibration mill, an attritor, etc. are used. This mill treatment can break the fusion and aggregation between particles that occurred during firing.
[0018]
In the direct nitriding method of the metal silicon powder, for example, a silicon nitride powder having an α phase fraction of 70% or more and a specific surface area of 10 m 2 / g or more is a metal having a specific surface area of 10 m 2 / g or more and an oxygen content of 2.0% by weight or less. 5 to 20% by weight of silicon powder is added and mixed, and the mixture is 1400 to 1600 at a heating rate of 5 to 25 ° C./h in a mixed atmosphere of hydrogen gas and nitrogen gas or in a mixed atmosphere of ammonia gas and nitrogen gas. By raising the temperature to 0 ° C., an ingot of silicon nitride is obtained. After heat-treating the obtained ingot at 1500 to 1700 ° C., it is roughly crushed and crushed by a conventional method, and further wet or dry pulverized with a ball mill, vibration mill, jet mill, attritor mill, pearl mill, etc. adjust. Impurities mixed during pulverization are treated with an inorganic acid (for example, a mixture of hydrofluoric acid and sulfuric acid), dissolved and removed, and then dried to produce silicon nitride powder. Taking attritor pulverization as an example, silicon nitride with a small amount of coarse particles is obtained by sufficiently pulverizing with a pulverization time of 0.8 hours or more and drying the filter cake after acid treatment at a temperature of 100 ° C. or less. A powder can be produced.
[0019]
In the silica reduction method, for example, a silicon nitride powder having a specific surface area of 10 m 2 / g or more, a silica powder having a specific surface area of 100 m 2 / g or more, and a carbon black having a specific surface area of 50 m 2 / g or more mixed at a constant rate. Silicon nitride powder is produced by using this as a raw material and heating it in a nitrogen gas stream.
Further, 2 parts by weight or more of carbon and 0.1 part by weight or more of silicon nitride are added to 1 part by weight of silica, and after wet mixing uniformly, the granulated material is used as a raw material, and the firing temperature is 1450. The temperature of decarburization by heating the generated powder in the atmosphere is set to 680 ° C. or lower while being slowly decarburized over 4 hours to produce silicon nitride powder with a small amount of coarse particles be able to. The silicon nitride powder produced by this method is lightly crushed by a nylon vibration mill to obtain a powder having desired characteristics.
[0020]
Next, the powder produced under the conditions for reducing the coarse particles and the carbon-containing foreign matter is classified with a classifier.
The wind classifier is a device that separates particles suspended in a forced vortex into fine particles and coarse particles by the difference between centrifugal force and fluid drag.
In the wind classifier used in the present invention, the introduced air flows as a high-speed air stream from the gap between the rotor and the rotor ring and enters the dispersion zone. On the other hand, the powder supplied from the raw material inlet is subjected to a shearing force due to a high-speed air flow while riding on the air flow, and is sent to the classification chamber while being uniformly dispersed by the action of the dispersion blades and the dispersion disk. . Here, each particle receives a centrifugal force acting outward by a rotating flow and an air drag force directed inward by a radially flowing air flow. The particle size separated in the air classification is determined by the balance between centrifugal force acting on the particles and air drag. That is, coarse particles are subjected to a greater centrifugal force, and fine particles are greatly affected by air drag, and move along different trajectories. The coarse particles to be classified are blown to the outside of the classification rotor by centrifugal force, reach the inner wall of the rotor ring, and then taken out in an air-sealed state through the coarse powder discharge port of the rotor ring. The fine particles are fed into the classification rotor together with the air flow, ride on the air flow rectified by the dispersion vanes, pass through the classification rotor, balance rotor, and caging, pass through the fine powder discharge port in the center of the rotor, and then the cyclone or bag filter It is collected by.
[0021]
Adjustment of the classification point is performed by changing the rotation speed of the classification rotor and the flow rate of the passing air. Since the variable range of the flow rate of the passing air is limited, the adjustment is usually performed by the number of rotations of the classification rotor, but it is desirable to control both independently in setting a wide range of classification points.
Since the classification point shifts to the fine particle side as the rotor rotation speed is higher and the passing air flow rate is smaller, classification in the range of 0.5 to 50 μm is possible by setting both conditions. For example, by setting the rotor rotational speed to 2400 rpm (corresponding to a peripheral speed of 35 m / s on the outer periphery of the dispersion blade) and the passing air flow rate to 5.5 m 3 / min (corresponding to the dispersion blade inlet air flow velocity of 2 m / s). Coarse particles having a particle size of 8 μm or more can be efficiently separated and removed.
In order to improve the classification accuracy of the powder fluid, it is important to disperse the supplied powder to a state close to a single particle and to keep the air flow in the classification chamber uniform. To that end, the classifying rotor and dispersing blades are rotated together to reduce the turbulence of the air flow, and to concentrate on the radial non-uniform flow and local vortex flow caused by the suction of the external blower. There is a need.
[0022]
In the silicon nitride powder used in the present invention, coarse particles having a low carbon content and carbon-containing foreign matters having a high carbon content exist, and these are differently dispersed in the classifier. For this reason, when the rotation speed of the classification rotor is increased and the classification point is shifted to the fine particle side, coarse particles having a low carbon content are preferentially separated and removed, and conversely, the classification air point is reduced by reducing the flow rate of the passing air. When shifted to the fine particle side, a phenomenon that a carbon-containing foreign substance having a high carbon content was preferentially separated and removed occurred. Therefore, it is important to set conditions that combine the rotational speed of the classification rotor and the flow rate of the passing air. Only in specific ranges, high productivity and high powder recovery rate, low carbon content coarse particles and high carbon content. It was possible to efficiently separate and remove both the carbon-containing foreign matter.
[0023]
The silicon nitride powder of the present invention is mixed with a sintering aid such as aluminum oxide, yttrium oxide, and magnesium oxide in the same manner as in the case of conventional silicon nitride powder, and after the mixture is formed into a predetermined shape, By sintering, silicon nitride ceramics (sintered body) can be produced. The molding pressure may be about 0.5 to 10 ton / cm 2, and the sintering conditions are a sintering temperature of 1500 to 2000 ° C., an atmospheric pressure of 0.5 to 100 atm, and a sintering time of 1 to 10 hours. It should be a degree.
[0024]
Since the silicon nitride ceramics (sintered body) manufactured using the silicon nitride powder of the present invention has high strength, high Weibull coefficient and high reliability, the silicon nitride powder of the present invention is highly reliable. It is particularly suitable as a raw material for producing silicon nitride ceramics used as heat engine parts and machine parts such as turbo rotors, engine valves, diesel engine auxiliary combustion chambers, etc., which are required to have high performance and product life.
[0025]
【Example】
Examples of the present invention are given below together with comparative examples to explain the present invention in more detail.
Examples 1 to 9 and Comparative Examples 1 to 9
Silicon nitride powders were respectively produced according to the following production method (imide decomposition method) and production conditions shown in [Table 1] below. The powder characteristics of the obtained silicon nitride powder are shown in [Table 2] below.
[0026]
[Method for producing silicon nitride powder]
Purified silicon diimide is obtained by batch-washing the reaction product obtained by supplying liquid ammonia with a solution of 20 to 35% by weight of silicon tetrachloride prepared in advance and the remaining toluene into the reaction vessel, and batch-washing with liquid ammonia several times. Got.
Silicon diimide having a specific surface area of 500 to 850 m 2 / g was synthesized by changing the ratio (volume basis) of silicon tetrachloride and liquid ammonia during the reaction in the range of 1/50 to 2/50.
Moreover, the light weight density of silicon diimide was changed in the range of 0.035-0.075 g / cm < 3 > by changing the drying time and stirring rotation speed at the time of drying produced | generated silicon diimide.
[0027]
The produced silicon diimide was thermally decomposed at 1000 ° C. while flowing nitrogen gas having the oxygen concentration described in the following [Table 1] to obtain amorphous silicon nitride powder. Next, the obtained amorphous silicon nitride powder was ground with a vibration mill, and then filled into a graphite container whose surface was coated with silicon carbide. In an electric furnace, in a nitrogen atmosphere, [Table 1] Were heated and fired under the conditions described in (temperature increase rate, maximum temperature and holding time at the same temperature, CO concentration in the furnace) to obtain a grayish white silicon nitride powder.
The CO concentration in the furnace was adjusted by the purity (oxygen concentration, dew point) and flow rate of the nitrogen gas to be circulated.
This crystalline silicon nitride powder is put into a vibration mill, and after milling for a predetermined time at an amplitude of 8 mm in an atmosphere containing an oxygen content of 10% and the balance being an inert gas, the powder is supplied to an air classifier. Then, classification was performed under the predetermined conditions described in Table 1 to remove coarse particles. The average flow velocity of the air entering the gap between the rotor and the ring and the average flow velocity of the dispersion blade inlet are both proportional to the passing air flow rate and are 33 m / s to 100 m / s and 0.8 m / s to 2.5 m / s, respectively. Met.
Table 2 shows the powder characteristics of the obtained silicon nitride powder.
[0028]
The amount of coarse particles was measured by the measurement method described above.
The specific surface area was measured by a BET one-point method using a flowsorb type 2300 manufactured by Shimadzu-Micromeritex.
The oxygen content was measured by an inert gas melting-infrared absorption method using a TC-136 type oxygen / nitrogen simultaneous analyzer manufactured by LECO.
The carbon content was measured by a combustion-thermal conductivity method using a WR-12 type carbon analyzer manufactured by LECO.
[X-ray diffraction measurement]
The powder X-ray diffraction pattern of the obtained silicon nitride powder was measured by a regular step scanning method using a copper tube and a graphite monochromator. A step angle scan of the diffraction angle (2θ) in the range of 15 to 80 ° in steps of 0.02 ° was performed, and α and β fractions were obtained by Rietveld analysis.
[0029]
[Table 1]
[Table 2]
Examples 10 to 13 and Comparative Examples 10 to 15
Using a metal silicon powder having a specific surface area of 10 m 2 / g or more and an oxygen content of 2.0 wt% or less, a silicon nitride powder was produced under the production conditions shown in Table 3 by the direct nitridation method described above. The obtained silicon nitride powder was classified using an air classifier to separate and remove coarse particles.
Table 4 shows the characteristics of the obtained silicon nitride powder.
[0032]
[Table 3]
[Table 4]
Usage Test Examples Using the silicon nitride powders obtained in Examples 1 to 13 and Comparative Examples 1 to 15 as raw materials, respective sintered bodies were produced by the following production methods.
[Method for producing sintered body]
To 93 parts by weight of silicon nitride powder, 5 parts by weight of Y 2 O 3 and 2 parts by weight of Al 2 O 3 were added as sintering aids, ethanol was added, wet mixed in a ball mill for 48 hours, and then dried.
The granules after drying and particle size adjustment are molded into a shape of 75 x 45 x 6 mm at a molding pressure of 300 kg / cm 2 and then rubber press-molded at a pressure of 2 ton / cm 2 to produce a green molded body did. This formed body is filled in a silicon nitride crucible, heated in an electric furnace at a rate of temperature increase of 100 ° C./h in a nitrogen gas atmosphere of 1 atm, and maintained at 1760 ° C. for 4 hours. A ligature was obtained.
[0035]
The bulk density of the obtained sintered body was measured by the Archimedes method. A bending test piece equivalent to 3 × 4 × 40 mm conforming to JIS R1601 was cut out from the sintered body, and in accordance with JIS R1601, the outer span was 30 mm, the inner span was 10 mm, and the crosshead speed was 0.5 mm / min. A four-point bending test was performed. The bending strength at room temperature is an average value of 40 pieces. In the bending test at a high temperature, after holding the test piece at 1300 ° C. for 10 minutes in a nitrogen atmosphere, the strength of eight or more test pieces was measured and the average value was calculated. The fracture toughness value was measured by the SEPB method defined in JIS R 1607.
The measurement results of the ultimate density, bending strength (room temperature strength, room temperature strength Weibull coefficient and high temperature strength), and fracture toughness value are shown in Table 5 below.
It can be seen that the silicon nitride powder of the present invention can stably produce a highly reliable silicon nitride ceramic with high strength and little variation in characteristics.
[0036]
[Table 5]
【The invention's effect】
The silicon nitride powder of the present invention can stably produce a highly reliable silicon nitride ceramic with high strength and little variation in characteristics.
Claims (4)
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| KR102643831B1 (en) | 2018-02-28 | 2024-03-07 | 가부시끼가이샤 도꾸야마 | Method for producing silicon nitride powder |
| WO2021210507A1 (en) | 2020-04-13 | 2021-10-21 | 株式会社トクヤマ | Silicon nitride powder for sintering |
| WO2025206127A1 (en) * | 2024-03-28 | 2025-10-02 | Ｕｂｅ株式会社 | Crystalline silicon nitride powder, silicon nitride-based sintered body, and method for producing silicon nitride-based sintered body |
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