JP2004091243A - Method for manufacturing sintered compact of silicon nitride, and sintered compact of silicon nitride - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a sintered compact of silicon nitride that does not necessititate expensive equipment and complex control and is capable of efficeintly nitriding silicon even if the ratio of silicon in a raw material powder is high and has good dimentional precision, and further that has high density(dense) and high strength and is excellent in a void property, and to provide a sintered compact of silicon nitride. <P>SOLUTION: The sintered compact of silicon nitride is obtained by grinding a mixed powder of the silicon nitride powder of which the particle diameter is 5-40μm when the cumulative particle size distribution is 90% and the silicon powder of which the weight ratio to the silicon nitride powder is 1 or more as main components and 1-20wt.% of the group 3a element in the periodical table calculated in term of the oxide, 1-10wt.% of alminum calculated in term of the oxide and 1-10 wt% of excess oxygen calculated in terms of silicon oxide, so that the BET specific surface area becomes 6-14 m<SP>2</SP>/g, molding the resultant to a desired shape, heat-treating the molding in a nitrogen-containing atmosphere at 1,000-1,500°C to nitride the silicon powder as much as 90% or more of the nitriding ratio, and further firing the nitride in a non-oxidating atmosphere contaning nitogen. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、窒化珪素質焼結体の製造方法および窒化珪素質焼結体に関する。
【０００２】
【従来の技術】
窒化珪素質焼結体は、機械的特性、耐熱性、耐食性等、特に比強度および比剛性が優れていることから、機械産業部品分野において金属部材に変わる機械部材として応用が進められている。また窒化珪素質焼結体は、耐熱性、耐熱衝撃性および耐酸化性に優れることからエンジニアリングセラミックス、特にターボローター等の熱機関用においても応用が進められている。
【０００３】
この窒化珪素質焼結体は、一般には窒化珪素に対してＹ_２Ｏ_３、Ａｌ_２Ｏ_３、ＭｇＯなどの焼結助剤を添加した原料を、所望の形状に成形した後、常圧または窒素加圧雰囲気中で焼成して緻密化することにより得られる。前記焼結助剤は用途に応じて選定される。このような窒化珪素質焼結体を作製する過程において、焼成時には液相焼結に伴う必然的な焼成収縮が生じる。このため、複雑な形状の製品に対しては、寸法精度を高めるために精密な寸法設計、成型方法の改善、出発原料の改良等が行われている。
【０００４】
しかし、上記のような設計、改善、改良等を実施しても、高い精度で正確に製品の寸法を制御するのは非常に難しい。このため、所望の製品形状となるように焼成後に別途研磨加工を施す必要があるため、トータルコストが増大するという問題がある。そこで、出発原料に珪素を添加し、焼成前に窒素雰囲気中で珪素を窒化処理して成形体の密度を高めた後、焼成することによって焼成時の収縮を抑えて緻密な焼結体を得る、いわゆる反応焼結法が用いられている。
【０００５】
【発明が解決しようとする課題】
しかしながら、従来の反応焼結法では、出発原料に添加する珪素の比率が高くなるほど窒化が困難となるので、窒化工程を含まない一般焼結法と比較しても焼結性が劣って緻密な焼結体が得にくくなるという問題がある。特に、焼結体中のボイドにおいては、全体的なボイド量に加え、平均ボイド径、さらには、機械的特性に致命的な最大ボイド径が大きくなるという問題があり、最終的に得られる焼結体の高緻密化が難しく、機械的特性も一般焼結法に比べて劣るという問題があった。
【０００６】
そこで、出発原料に添加する珪素の比率が高い場合には、加圧窒素雰囲気で窒化する方法（特開平１−５２６７８号公報）、水素ガスを含む雰囲気下で窒化する方法（特開平７−１３８０７４号公報）などが提案されている。これらの方法では、珪素の比率が高い場合でも、ある程度窒化が可能となり窒化率を向上させることができるが、窒化のための装置が高価で、またその窒化制御が複雑になるなどの問題があった。
【０００７】
したがって、本発明の目的は、高価な装置や複雑な制御を必要とせず、原料粉末中の珪素の比率が高い場合でも珪素を効率的に窒化させることができ、寸法精度が良好で、しかも高密度（緻密）で高い強度を有し、ボイド特性に優れた窒化珪素質焼結体の製造方法および窒化珪素質焼結体を提供することである。
【０００８】
【課題を解決するための手段】
本発明者らは、反応焼結法により焼結体を作製するに際して、特に高密度で高強度な特性に加え、焼結体中に残存するボイドを制御する方法について検討を重ねた結果、粒径の比較的大きな窒化珪素粉末と、珪素粉末とを所定の割合で混合し、さらに希土類酸化物の少なくとも１種、アルミニウム酸化物および酸化珪素分を所定の割合で含む混合粉末を、所定の比表面積になるように粉砕した後、所定の窒化条件で窒化し、焼成することによって、たとえ珪素粉末を高い比率で含有している場合であっても、珪素を効率的に窒化させることができ、寸法精度が良好で、しかも高密度で高い強度を有し、ボイド率およびボイド径が小さい窒化珪素質焼結体を得ることができるという新たな事実を見出し、本発明を完成するに至った。
【０００９】
すなわち、本発明の窒化珪素質焼結体の製造方法は、累積粒度分布９０％における粒径が５〜４０μｍである窒化珪素粉末およびこの窒化珪素粉末に対する重量比率が１以上である珪素粉末を主成分とし、周期律表第３ａ族元素の１種類以上を酸化物換算で１〜２０重量％、アルミニウムを酸化物換算で１〜１０重量％、過剰酸素を酸化珪素換算で１〜１０重量％含む混合粉末を、ＢＥＴ比表面積が６〜１４ｍ^２／ｇになるように粉砕し、ついで成形して成形体を得た後、この成形体を１０００〜１５００℃の窒素含有雰囲気中で熱処理して窒化率が９０％以上である窒化体を得、該窒化体中の窒化珪素結晶は、短軸粒子径（平均短軸粒子径、以下同じ。）０．３μｍ未満の窒化珪素結晶を面積比率で５０〜９５％含み、残部が短軸粒子径０．３μｍ以上の窒化珪素結晶であり、これらの窒化珪素結晶のα化率が５０％以上であり、さらに、この窒化体を窒素を含む非酸化性雰囲気中で焼成することを特徴とする。
【００１０】
上記のようにして得られる窒化珪素質焼結体は、原料粉末に珪素粉末を主成分として使用することにより、焼成後の寸法精度を向上させることができるので、研磨加工を施す必要がなくなるだけでなく、珪素粉末は安価であるので、原料コストを低減することができる。これにより、最終製品である窒化珪素質焼結体のトータルコストを低減することができる。ここで、過剰酸素とは、焼結体中に含まれる全酸素量から周期律表第３ａ族元素酸化物およびアルミニウム酸化物として化学量論量で混入した酸素を除いた残りの酸素量であり、実際には窒化珪素原料中の不純物酸素から構成されるものである。
【００１１】
ここで、短軸粒子径とは、窒化珪素結晶粒子の寸法を長軸・単軸の２軸方向で測定した時の、短軸寸法を意味する。また、α化率とは、窒化珪素結晶中のα型窒化珪素相の含有比率（α−Ｓｉ_３Ｎ_４／α−Ｓｉ_３Ｎ_４＋β−Ｓｉ_３Ｎ_４）を意味する。
【００１２】
また、本発明の窒化珪素質焼結体の製造方法では、前記成形体の理論密度に対する相対密度比率が４０〜６５％であるのが好ましい。ここで、成形体の理論密度に対する相対密度比率とは、粉末の混合則で計算して得られる成形体の理論密度に対する、重量／体積で算出した成形体密度の比率である。
【００１３】
本発明の窒化珪素質焼結体は、ボイド率が２．０％以下、最大ボイド径が３０μｍ以下、焼結体の理論密度に対する相対密度比率が９５％以上、室温における４点曲げ抗折強度が９００ＭＰａ以上であることを特徴とし、上記の製造方法により得られるものである。ここで、ボイド率とは、焼結体表面を鏡面研磨した後、金属顕微鏡によりその表面を観察し、画像解析により測定面積中のボイド総面積を面積比率にて算出した値である。また、焼結体の理論密度に対する相対密度比率とは、混合則で計算して得られる焼結体の理論密度に対する、アルキメデス法により測定した焼結体密度の比率である。
【００１４】
【発明の実施の形態】
以下、本発明の窒化珪素質焼結体の製造方法およびこの方法により得られる窒化珪素質焼結体について詳述する。
【００１５】
＜窒化珪素質焼結体の原料粉末（混合粉末）＞
本発明では、主成分として窒化珪素粉末および珪素粉末を含み、焼結助剤として少なくとも周期律表第３ａ族元素化合物、アルミニウム酸化物および酸化珪素を含む混合粉末を使用する。ここでいう酸化珪素とは、珪素粉末や窒化珪素粉末などに不可避的に含まれる過剰酸素を酸化珪素に換算したものである。具体的には、焼結体の全酸素量から他の助剤成分である周期率表第３ａ族元素化合物、アルミニウム酸化物に含まれる酸素を引いた残部の酸素量を酸化珪素に換算したものである。
【００１６】
前記窒化珪素粉末としては、累積粒度分布９０％における粒径が５〜４０μｍ、好ましくは１０〜２５μｍであるものを用いる。この粒径が５μｍ未満となると、後述する窒化工程で得られる窒化体中において、珪素粉末由来の窒化珪素結晶（珪素粉末が窒化して生じる窒化珪素結晶）の結晶径と、窒化珪素粉末由来の窒化珪素結晶の結晶径との差が小さくなるため、これらの結晶は全体的に微細で均一な組織となり、焼成時に結晶が均一に粒成長するので、得られる焼結体はアスペクト比の低い結晶組織を有するものとなる。このようなアスペクト比の小さな結晶組織ではボイドを焼結体外へ拡散させるのが困難となるため、良好なボイド特性が得られず、所望の機械的特性も得られないおそれがある。
【００１７】
一方、粒径が４０μｍを超えると、後述する粉砕工程において、混合粉末を目標とする比表面積まで粉砕するのに長時間を要し、非効率的である。また、珪素粉末との粒径差が大きくなりすぎるため、最終的に得られる焼結体は粗大な結晶を含みやすくなり、これが焼結体の強度を低下させる要因（焼結体の破壊源）となるおそれがある。このような窒化珪素粉末は、混合粉末総量に対して、通常、０．５〜４５重量％程度、好ましくは５〜４０重量％程度混合するのがよい。
【００１８】
前記珪素粉末としては、平均粒径１０μｍ以下、好ましくは５μｍ以下の微細な粉末であるのがよい。これにより、窒化工程において珪素の窒化が容易となり窒化率が向上するとともに、焼結体中において珪素粉末由来の窒化珪素結晶を微細なものとすることができる。
【００１９】
また、珪素粉末は、前記窒化珪素粉末に対する重量比率が１以上、好ましくは２以上となるように混合するのがよく、具体的には、混合粉末総量に対して、通常、０．５〜９０重量％程度、好ましくは４０〜８５重量％程度混合するのがよい。重量比率が１以上であることによって、窒化工程で得られる窒化体中の窒化珪素結晶組織は、珪素粉末由来の窒化珪素結晶と、窒化珪素粉末由来の窒化珪素結晶の形状、特に短軸粒子径に差異が生じることで、窒化体全体として短軸粒子径にバラツキを持たせることができる。
【００２０】
一方、重量比率が１未満となると、窒化珪素粉末由来の窒化珪素結晶の比率が高くなる。これにより、焼成前に珪素を窒化することで密度を高め、焼成後の焼結体の寸法変化を抑制する効果が低減し、機械的特性が低下するおそれがある。また、珪素粉末は比較的安価であるため、珪素粉末の混合比率が低下するとコストが上昇するという欠点もある。
【００２１】
焼結助剤である周期律表第３ａ族元素化合物は、第３ａ族元素であるＹやランタノイド元素などの酸化物を用いることができ、特に、耐熱性向上の点からＹ，Ｙｂ，Ｅｒ，Ｌｕの酸化物が好ましい。このような周期律表第３ａ族元素化合物は単独で、あるいは２種以上を同時に用いてもよい。この周期律表第３ａ族元素化合物の含有量は１〜２０重量％、好ましくは３〜１２重量％であるのがよい。含有量が２０重量％を超えると、窒化工程においてβ型窒化珪素が生成しやすくなり、アスペクト比の大きな微細な柱状結晶が得にくくなるので、所望の機械的特性が得られないおそれがある。一方、含有量が１重量％未満となると、焼成工程において十分に焼成が進行せず、緻密な焼結体が得られないおそれがある。
【００２２】
アルミニウム酸化物の含有量は１〜１０重量％、好ましくは４〜１０重量％であるのがよい。また、酸化珪素の含有量は１〜１０重量％、好ましくは２〜７重量％であるのがよい。アルミニウム酸化物および酸化珪素の含有量が１０重量％を超えると、焼結体の強度が低下し、大きなボイドが生成しやすくなるおそれがある。一方、これらの含有量が１重量％未満となると、窒化工程において窒化が十分に生じないため、十分な焼結が進行せず、緻密な焼結体が得られないおそれがある。
【００２３】
＜粉砕工程＞
次に、粉砕工程について説明する。上記で説明した混合粉末は、窒化を効率的に行い、かつ緻密な焼結体を得るために、ＢＥＴ比表面積が６〜１４ｍ^２／ｇ、好ましくは８〜１２ｍ^２／ｇになるように粉砕され微細化される。ＢＥＴ比表面積が６ｍ^２／ｇ未満となると、窒化工程において窒化が十分に生じず、珪素粉末から微細な窒化珪素結晶が得にくくなるとともに、凝集結晶が多く発生するおそれがある。これにより、緻密な焼結体が得られなくなり、所望の機械的特性が得られないおそれがある。一方、ＢＥＴ比表面積が１４ｍ^２／ｇを超えると、珪素粉末の粉砕によって現れる新しい表面に酸素が吸着し、窒化前の成形体中の酸化珪素の重量が増大するため、成形体中の組成が所定の範囲から変動し、焼成後に得られる焼結体はボイドの多い多孔質なものとなりやすい。混合粉末の粉砕に用いる粉砕機は特に限定されず、例えば振動ミル、回転ミルなどを用いることができる。
【００２４】
＜成形工程＞
上記粉砕工程で得られた粉末を、例えばプレス成型、鋳込み成型、押し出し成型、射出成型、排泥成型、冷間静水圧成型などの公知の手段により所望の形状に成形し、成形体を得る。この成形体は、その理論密度に対する相対密度比率が４０〜６５％、好ましくは４５〜６２％となるように成形されるのがよい。成形体の密度を上記範囲に制御するには、成形時において粉末に付加される圧力を調節すればよい。相対密度比率が４０％未満となると、次工程において窒化反応が急激に進行し、β型窒化珪素の生成が促進され、焼成により得られる焼結体の窒化珪素結晶が微細な柱状結晶とならないおそれがある。一方、相対密度比率が６５％を超えると、窒化反応が十分に進行せず、窒化体中に珪素粉末が残留するおそれがある。
【００２５】
＜窒化工程＞
上記成形工程で得られた成形体を、窒素を含有する雰囲気中において１０００〜１５００℃、好ましくは１１００〜１４００℃で熱処理することにより窒化体を得る。このとき、上記の温度範囲内で温度を段階的に上昇させて珪素を徐々に窒化させるのがよい。これにより、窒化率がより向上する。また、１〜５０ａｔｍの窒素加圧雰囲気中で窒化させたり、水素などのガスを混合することによっても窒化を促進させることができるが、これらの方法を用いるとコストが増大するおそれがある。
【００２６】
成形体を上記のような窒化条件で窒化することによって、９０％以上の高い窒化率で窒化された窒化体を得ることができる。この窒化体中の窒化珪素結晶は、主として珪素粉末由来で、短軸粒子径が０．３μｍ未満、好ましくは０．２μｍ以下の窒化珪素結晶を面積比率で５０〜９５％、好ましくは７０〜９０％含み、残部が主として窒化珪素粉末由来で、短軸粒子径０．３μｍ以上の窒化珪素結晶であるのがよい。また、これらの窒化珪素結晶のα化率は５０％以上、好ましくは６５〜９２％であるのがよい。
【００２７】
上記面積比率が５０％未満となると、窒化珪素結晶の粒成長の核となるβ型窒化珪素のうち、微細なものの存在比率が低くなるので、焼結体が緻密なものとならず、高い強度が得られないおそれがある。一方、面積比率が９５％を超えると、粒成長の核となるβ型窒化珪素のうち、微細なものの存在比率が多くなりすぎるので、結晶の粒成長の不均一さが生じにくくなり、結晶が均一に成長し、得られる焼結体は微細な結晶組織となりやすいが、全体的にアスペクト比の低い、同形状の結晶が多く存在してしまうため、所望のボイド特性が得られなくなるおそれがある。また、α化率が上記範囲にあることで、緻密で、高い強度を有し、ボイド特性に優れた窒化珪素質焼結体を得ることができる。
【００２８】
＜焼成工程＞
最後に、上記窒化工程で得られた窒化体を、窒素を含む非酸化性雰囲気中において、公知の焼成方法、例えばホットプレス焼成、常圧焼成、窒素加圧焼成、さらには、これらの焼成後に熱間静水圧焼成（ＨＩＰ）処理、ガラスシールＨＩＰ焼成などで焼成することにより窒化珪素質焼結体を得ることができる。焼成時の温度は、１６００〜２０００℃、好ましくは１６５０〜１９００℃であるのがよい。焼成温度を高温にしすぎると、窒化珪素結晶が過度に粒成長し、強度が低下するおそれがある。焼成時間は、通常、５〜１５時間程度とされる。こうして得られた窒化珪素質焼結体は、理論密度に対する相対密度比率が９５％以上で、ボイド率が２．０％以下で、最大ボイド径が３０μｍ以下で、室温における４点曲げ抗折強度が９００ＭＰａ以上である。
【００２９】
本発明では、窒化体において、珪素粉末由来の窒化珪素結晶の短軸粒子径と、窒化珪素粉末由来の窒化珪素結晶の短軸粒子径に差異を生じさせることで、焼成時の粒成長を不均一なものとし、アスペクト比の高い窒化珪素結晶を析出させ、機械的特性を向上させるとともに、ボイド特性を向上させている。
【００３０】
焼成時の粒成長は、主として珪素粉末由来の短軸粒子径０．３μｍ未満の窒化珪素結晶中に含まれるβ型窒化珪素結晶と、主として窒化珪素粉末由来の短軸粒子径０．３μｍ以上の窒化珪素結晶中に含まれるβ型窒化珪素結晶とが核となり進行する。このとき、窒化珪素粉末と珪素粉末の比率を前記のように特定することで、粒成長の核は珪素粉末由来の窒化珪素結晶の方に多く含まれるため、微細なβ型窒化珪素を核とした粒成長が主として生じ、微量であるが粗大なβ型窒化珪素を核とした粒成長が同時に生じることとなる。このようにして、粒成長に不均一さが生じ、窒化珪素結晶のアスペクト比が高くなり、緻密で高い強度を有し、ボイド特性にも優れた窒化珪素質焼結体を得ることができる。
【００３１】
【実施例】
以下、実施例を挙げて本発明を詳細に説明するが、本発明は以下の実施例のみに限定されるものではない。
【００３２】
実施例
原料粉末として、平均粒径４．２μｍ、酸素量１．４重量％の珪素粉末と、α化率９０％、酸素量１．５重量％、累積粒度分布９０％における粒径が表１に示す値である窒化珪素粉末と、表１に示す周期律表第３ａ族酸化物と、アルミニウム酸化物とを用い、これらが表１に示す組成になるように調合し、ついで表１に示すＢＥＴ比表面積まで振動ミルにて粉砕した後、冷間静水圧プレス機にて荷重を加えて成形体を作製した。ついで、得られた成形体を常圧で、表２に示す窒化温度パターンにて窒化を行って窒化体を得た。この窒化体を１７５０℃、窒素圧１ａｔｍで１０時間焼成することにより窒化珪素質焼結体を作製した。
【表１】

【表２】

【００３３】
表１，２に示す各物性値は以下に示す方法で測定した。
１．ＢＥＴ比表面積
粉砕した混合粉末を２００℃で１０分以上加熱した後、流動式窒素吸着法にて算出した。
２．成形体の理論密度に対する相対密度比率
得られた成形体について重量／体積により密度を測定し、成形体の理論密度に対する成形体の実際の密度の比率を算出した。ただし、成形体の相対密度の算出にあたり、珪素（Ｓｉ）の理論密度は２．４（ｇ／ｃｍ^３）、窒化珪素（Ｓｉ_３Ｎ_４）の理論密度は３．１８（ｇ／ｃｍ^３）、アルミナ（Ａｌ_２Ｏ_３）の理論密度は３．９８（ｇ／ｃｍ^３）、酸化イットリウム（Ｙ_２Ｏ_３）の理論密度は５．０（ｇ／ｃｍ^３）、酸化エルビウム（Ｅｒ_２Ｏ_３）の理論密度は８．６（ｇ／ｃｍ^３）、酸化イッテルビウム（Ｙｂ_２Ｏ_３）の理論密度は９．２（ｇ／ｃｍ^３）、酸化ルテチウム（Ｌｕ_２Ｏ_３）の理論密度は９．４（ｇ／ｃｍ^３）、酸化珪素（ＳｉＯ_２）の理論密度は２．６５（ｇ／ｃｍ^３）とした。
３．窒化率
成形体と窒化体との重量を比較して、その重量増加量から混合した珪素が窒化された割合を算出した。
４．窒化珪素結晶の面積比率
まず、窒化体の表面を鏡面仕上げし、強酸にてエッチング処理した後、走査型電子顕微鏡にて１５０００倍で写真撮影を行った。この写真（４８μｍ^２）中から３μｍ×３μｍの格子に分割し、これらの中から３ヶ所を選定し、さらにこれらを０．３μｍ×０．３μｍの格子に分割して観察した。０．３μｍ×０．３μｍの格子上で確認される窒化珪素結晶の短軸粒子径において、一辺０．３μｍの格子より短い結晶を含む格子数と、格子４つ分の一辺０．６μｍの格子数を、全体の格子数１００で割ることによって、短軸粒子径０．３μｍ未満の窒化珪素結晶の面積比率を算出した。
５．α化率
窒化後の成形体の一部を粉砕しＸ線回折測定により、α型窒化珪素量を求めた。α型窒化珪素量の算出には、α−Ｓｉ_３Ｎ_４の（１０２）、（２１０）のピーク強度をＨ（１０２）、Ｈ（２１０）、β−Ｓｉ_３Ｎ_４の（１０１）、（２１０）のピーク強度をｈ（１０１）、ｈ（２１０）とした時、Σ［Ｈ（１０２）＋　Ｈ（２１０）］／Σ［Ｈ（１０２）＋　Ｈ（２１０）＋　ｈ（１０１）＋　ｈ（２１０）］の式を用いて比率を求めた。
６．焼結体の理論密度に対する相対密度比率
得られた焼結体についてアルキメデス法により比重を測定し、焼結体の理論密度に対する焼結体の実際の密度の比率を算出した。その焼結体の相対密度の算出は、成形体の相対密度の算出方法と同様である。
７．抗折強度
上記で得られた焼結体を３ｍｍ×４ｍｍ×４０ｍｍのテストピース形状に切断・研磨し、ＪＩＳ　Ｒ１６０１に基づき室温にて４点曲げ抗折強度試験を実施した。
８．アスペクト比
焼結体の表面を鏡面仕上げし、強酸にてエッチング処理した後、金属顕微鏡（ニレコ社製ＬＵＺＥＸ−ＦＳ）にて窒化珪素結晶を観察し、画像解析装置にて窒化珪素結晶のアスペクト比を測定した。なお、金属顕微鏡による観察では、測定倍率を２０００倍、測定面積を１００μｍ^２、測定ポイントを１０ケ所とした。
９．ボイド特性
焼結体の表面を鏡面仕上げし、金属顕微鏡（ニレコ社製ＬＵＺＥＸ−ＦＳ）にてボイドの発生状況を観察し、画像解析装置にてボイド率、最大ボイド径を測定した。なお、金属顕微鏡による観察では、測定倍率を１００倍、測定面積を９．０×１０^４μｍ^２、測定ポイントを１０ケ所とした。
１０．寸法変化率
上記で作製した成形体および焼結体の寸法を測定し、成形体の寸法に対する比率（１−焼結体寸法／成形体寸法）を寸法変化率（％）とした。
【００３４】
表２から、窒化珪素粉末に対する珪素粉末の重量比率が１未満になると、焼結体の寸法変化率が大きくなっていることがわかる（Ｎｏ．３６〜４０）。
【００３５】
周期律表第３ａ族元素の酸化物の含有量が１重量％未満になると、焼結性およびアスペクト比の低下が見られ（Ｎｏ．４１）、２０重量％を超えると、粗大な窒化珪素結晶が多くなり、抗折強度が低下する傾向にある（Ｎｏ．４２）。
【００３６】
アルミニウム酸化物の含有量が１重量％未満になると、焼結性の低下が見られ（Ｎｏ．４３）、１０重量％を超えると、焼結性は向上するが、焼成温度が最適ではないため窒化珪素結晶が粗大化して機械的特性が低下した（Ｎｏ．４４）。これは、珪素酸化物の含有量においても同様の結果が得られ、特に珪素酸化物の含有量が１０重量％を超えると、焼結体中にポーラスな組織が増大し、抗折強度が著しく低下した（Ｎｏ．４５，４６）。
【００３７】
窒化珪素粉末の累積粒度分布９０％における粒径が５μｍ未満になると、窒化後の窒化珪素結晶組織は微細であるが、焼結体のアスペクト比が低くなり、高い機械的特性が得られなかった（Ｎｏ．４７，４９〜５４）。また、粒径が４０μｍを超えると、アスペクト比の高い窒化珪素結晶が得られているが、全体的に粗大な結晶が多くなり、機械的特性の劣る結果となった。窒化珪素結晶の破壊面を観察すると、微細な柱状結晶群の中に異常成長した柱状結晶が混ざった組織になっており、破壊源になっている様子が確認された（Ｎｏ．４８）。
【００３８】
混合粉末の粉砕後のＢＥＴ比表面積が６ｍ^２／ｇ未満となると、焼結性の面で緻密体が得にくくなり、また結晶においては、全体的にアスペクト比の低い粗大な結晶が多く存在する組織になっていることが確認された（Ｎｏ．４９，５０）。また１４ｍ^２／ｇを超えると、焼結体中のボイド率が大きくなる傾向があり、最大ボイド径も大きくなることが確認された。これに伴い、機械的特性も劣る結果となった（Ｎｏ．５１，５２）。特に比表面積が１５ｍ^２／ｇを超えると、焼結体の部分的な箇所に、粗大な空孔を多く確認することができ、緻密な焼結体を得ることができなかった（Ｎｏ．５２）。
【００３９】
窒化体中において、短軸粒子径が０．３μｍ未満の窒化珪素結晶が面積比率で５０％未満となると、緻密な焼結体を得ることはできるが、面積比率が５０〜９５％の範囲にある試料に比べて機械的特性の点で劣る結果となった（Ｎｏ．３２，３３，３８〜４０，４２，４４，４６，５３）。面積比率が９５％を超えると、全体的に微細で緻密な窒化珪素結晶組織を得ることができるが、結晶のアスペクト比率が低くなり、また、焼結体中のボイド率が大きくなる傾向にあった（Ｎｏ．３０，３１）。また、窒化後の窒化珪素結晶のα化率が５０％未満となると、焼結体の機械的強度が劣る結果となった（Ｎｏ．３４，３５，４２，５３）。
【００４０】
成形体の理論密度に対する相対密度比率が４０％未満になると、その成形体は高い窒化率を示し焼結性の点で優れた結果が得られたが、焼成時の収縮が非常に大きく、寸法変化率が２０％以上となり、かつ結晶のアスペクト比も小さく、抗折強度が低い値となった（Ｎｏ．５３）。一方、相対密度比率が６５％を超えると、混合した珪素粉末が完全に窒化されず、緻密な焼結体が得られなかった（Ｎｏ．５４）。
【００４１】
窒化温度が低い、または窒化に費やした合計時間が短くなると、十分に窒化されず、得られる焼結体は緻密なものとはならなかった（Ｎｏ．５５〜５８）。また、窒化温度が高い、または窒化に費やした合計時間が長くなると、珪素粉末から得られる窒化珪素は、β型窒化珪素結晶として析出する量が多くなり、抗折強度が劣る結果となった（Ｎｏ．３４，３５，５９）。
【００４２】
一方、試験Ｎｏ．１〜２９では、原料粉末中の珪素粉末を高い比率で混合した場合であっても、珪素粉末が効率的に窒化し、焼結体の相対密度比率が９９％以上の緻密体が得られ、かつ抗折強度が１０００ＭＰａ前後と高い抗折強度を有した窒化珪素質焼結体が得られた。また、所定の窒化珪素粉末を混合し、粉砕条件を制御することで得られる成形体を、窒素含有雰囲気中で窒化し、窒化後の窒化珪素結晶の結晶径分布を制御することで、高い機械的特性を有しながらも、ボイド特性に優れた焼結体を得ることができることが分かる。さらに、珪素粉末を高い比率で混合することで、寸法変化率を低く抑え、簡易でかつ安価な窒化珪素質焼結体を製造することができた。
【００４３】
【発明の効果】
本発明によれば、珪素粉末を高い比率で混合した場合であっても、寸法変化が小さく、緻密でかつ抗折強度に優れ、ボイド特性にも優れた窒化珪素質焼結体を得ることができるという効果がある。これにより得られる窒化珪素質焼結体は、高い寸法精度、比強度・比剛性を要求される機械部品分野および自動車用部品分野の構造材料に適用することができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a silicon nitride based sintered body and a silicon nitride based sintered body.
[0002]
[Prior art]
BACKGROUND ART Silicon nitride-based sintered bodies have excellent mechanical properties, heat resistance, corrosion resistance, etc., and particularly excellent in specific strength and specific rigidity. Therefore, silicon nitride-based sintered bodies are being applied as mechanical members in place of metal members in the field of mechanical industrial components. Further, since silicon nitride sintered bodies are excellent in heat resistance, thermal shock resistance and oxidation resistance, they are also being applied to engineering ceramics, especially for heat engines such as turbo rotors.
[0003]
This silicon nitride-based sintered body is generally made of Y with respect to silicon nitride. ₂ O ₃ , Al ₂ O ₃ And a raw material to which a sintering aid such as MgO has been added is formed into a desired shape, and then fired in a normal pressure or nitrogen pressurized atmosphere to densify. The sintering aid is selected according to the application. In the process of producing such a silicon nitride-based sintered body, during firing, inevitable firing shrinkage accompanying liquid phase sintering occurs. For this reason, for products having a complicated shape, precise dimensional design, improvement of a molding method, improvement of a starting material, and the like are performed in order to enhance dimensional accuracy.
[0004]
However, even if the above-mentioned design, improvement, improvement, and the like are performed, it is very difficult to accurately and precisely control the dimensions of the product. For this reason, it is necessary to separately perform polishing after baking so as to obtain a desired product shape, so that there is a problem that the total cost increases. Therefore, silicon is added to the starting material, and before the sintering, the silicon is nitrided in a nitrogen atmosphere to increase the density of the molded body, and then sintered to suppress shrinkage during sintering and obtain a dense sintered body. That is, a so-called reaction sintering method is used.
[0005]
[Problems to be solved by the invention]
However, in the conventional reaction sintering method, the higher the ratio of silicon added to the starting material, the more difficult the nitridation becomes. There is a problem that it is difficult to obtain a sintered body. In particular, in the case of voids in the sintered body, there is a problem that, in addition to the total void amount, the average void diameter, and further, the maximum void diameter that is fatal to mechanical properties, increase. There is a problem that it is difficult to achieve a high density of the sintered body and the mechanical properties are inferior to those of a general sintering method.
[0006]
Therefore, when the ratio of silicon added to the starting material is high, a method of nitriding in a pressurized nitrogen atmosphere (JP-A-1-52678) and a method of nitriding in an atmosphere containing hydrogen gas (JP-A-7-138074) Publication). In these methods, even when the ratio of silicon is high, nitriding is possible to some extent and the nitriding ratio can be improved. However, there are problems such as an expensive apparatus for nitriding and complicated control of the nitriding. Was.
[0007]
Therefore, an object of the present invention is to eliminate the need for expensive equipment and complicated control, to efficiently nitride silicon even when the ratio of silicon in the raw material powder is high, to obtain good dimensional accuracy, and to achieve high dimensional accuracy. An object of the present invention is to provide a method for producing a silicon nitride-based sintered body having high density and high strength and having excellent void characteristics, and a silicon nitride-based sintered body.
[0008]
[Means for Solving the Problems]
The present inventors have repeatedly studied a method of controlling voids remaining in a sintered body in addition to high-density and high-strength characteristics when producing a sintered body by a reaction sintering method. A silicon nitride powder having a relatively large diameter and a silicon powder are mixed at a predetermined ratio, and a mixed powder containing at least one rare earth oxide, aluminum oxide and silicon oxide at a predetermined ratio is mixed at a predetermined ratio. After pulverizing to a surface area, nitriding under predetermined nitriding conditions and firing, even if the silicon powder is contained in a high ratio, silicon can be efficiently nitrided, The present inventors have found a new fact that a silicon nitride sintered body having good dimensional accuracy, high density and high strength, and having a small void ratio and a small void diameter can be obtained, and completed the present invention.
[0009]
That is, the method for producing a silicon nitride-based sintered body of the present invention mainly includes a silicon nitride powder having a particle size of 5 to 40 μm in a cumulative particle size distribution of 90% and a silicon powder having a weight ratio to the silicon nitride powder of 1 or more. As a component, it contains 1 to 20% by weight of an oxide equivalent of 1 or more of Group 3a elements of the periodic table, 1 to 10% by weight of aluminum in terms of oxide, and 1 to 10% by weight of excess oxygen in terms of silicon oxide. The mixed powder has a BET specific surface area of 6 to 14 m. ² / G, and then molded to obtain a molded body. The molded body is heat-treated in a nitrogen-containing atmosphere at 1000 to 1500 ° C to obtain a nitride having a nitriding ratio of 90% or more. The silicon nitride crystal in the nitride contains 50 to 95% by area ratio of silicon nitride crystal having a minor axis particle diameter (average minor axis particle diameter, the same applies hereinafter) of less than 0.3 μm, and the remainder has a minor axis particle diameter of 0%. 0.3 .mu.m or more, characterized in that these silicon nitride crystals have an alpha conversion of 50% or more, and that the nitride is fired in a non-oxidizing atmosphere containing nitrogen.
[0010]
The silicon nitride-based sintered body obtained as described above can improve dimensional accuracy after firing by using silicon powder as a main component as a raw material powder, so that it is not necessary to perform polishing. In addition, since silicon powder is inexpensive, raw material costs can be reduced. Thereby, the total cost of the silicon nitride based sintered body as the final product can be reduced. Here, the excess oxygen is an amount of oxygen remaining after removing a stoichiometric amount of oxygen mixed as a Group 3a element oxide and an aluminum oxide from the total amount of oxygen contained in the sintered body. In fact, it is composed of impurity oxygen in the silicon nitride raw material.
[0011]
Here, the minor axis particle diameter means a minor axis dimension when the dimensions of the silicon nitride crystal particles are measured in two axial directions of a major axis and a uniaxial. Further, the α-rate is defined as the content ratio of α-type silicon nitride phase in silicon nitride crystal (α-Si ₃ N ₄ / Α-Si ₃ N ₄ + Β-Si ₃ N ₄ ).
[0012]
In the method for producing a silicon nitride-based sintered body of the present invention, it is preferable that a relative density ratio of the compact to a theoretical density is 40 to 65%. Here, the relative density ratio to the theoretical density of the compact is the ratio of the density of the compact calculated by weight / volume to the theoretical density of the compact obtained by the mixing rule of the powder.
[0013]
The silicon nitride sintered body of the present invention has a void fraction of 2.0% or less, a maximum void diameter of 30 μm or less, a relative density ratio to a theoretical density of the sintered body of 95% or more, and a four-point bending strength at room temperature. Is not less than 900 MPa, and is obtained by the above-mentioned manufacturing method. Here, the void ratio is a value obtained by mirror-polishing the surface of a sintered body, observing the surface with a metallographic microscope, and calculating the total area of voids in the measured area by image analysis based on the area ratio. Further, the relative density ratio to the theoretical density of the sintered body is a ratio of the sintered body density measured by the Archimedes method to the theoretical density of the sintered body calculated by the mixing rule.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the method for producing a silicon nitride based sintered body of the present invention and the silicon nitride based sintered body obtained by this method will be described in detail.
[0015]
<Raw material powder (mixed powder) of silicon nitride sintered body>
In the present invention, a mixed powder containing silicon nitride powder and silicon powder as main components and at least a Group 3a element compound of the periodic table, aluminum oxide and silicon oxide as sintering aids is used. Here, silicon oxide is obtained by converting excess oxygen inevitably contained in silicon powder, silicon nitride powder, or the like into silicon oxide. Specifically, the remaining oxygen content obtained by subtracting the oxygen contained in the periodic table group 3a element compound and aluminum oxide, which are other auxiliary components, from the total oxygen content of the sintered body is converted into silicon oxide. It is.
[0016]
As the silicon nitride powder, one having a particle size of 5 to 40 μm, preferably 10 to 25 μm at a cumulative particle size distribution of 90% is used. When the particle size is less than 5 μm, in the nitride obtained in the nitriding step described later, the crystal diameter of silicon nitride crystal derived from silicon powder (silicon nitride crystal generated by nitriding silicon powder) and the diameter of silicon nitride powder Since the difference from the crystal diameter of the silicon nitride crystal is small, these crystals have a fine and uniform structure as a whole, and the crystals grow uniformly during firing, so that the obtained sintered body has a low aspect ratio. You have an organization. In such a crystal structure having a small aspect ratio, it is difficult to diffuse the voids out of the sintered body, so that good void characteristics may not be obtained and desired mechanical characteristics may not be obtained.
[0017]
On the other hand, when the particle size exceeds 40 μm, it takes a long time to pulverize the mixed powder to a target specific surface area in a pulverization step described later, which is inefficient. In addition, since the difference in particle size from the silicon powder becomes too large, the finally obtained sintered body tends to contain coarse crystals, which causes a reduction in the strength of the sintered body (a source of fracture of the sintered body). May be caused. Such a silicon nitride powder is usually mixed in an amount of about 0.5 to 45% by weight, preferably about 5 to 40% by weight, based on the total amount of the mixed powder.
[0018]
The silicon powder is a fine powder having an average particle diameter of 10 μm or less, preferably 5 μm or less. This facilitates the nitridation of silicon in the nitriding step, improves the nitriding rate, and makes the silicon nitride crystal derived from silicon powder fine in the sintered body.
[0019]
Further, the silicon powder is preferably mixed so that the weight ratio with respect to the silicon nitride powder is 1 or more, preferably 2 or more. %, Preferably about 40 to 85% by weight. When the weight ratio is 1 or more, the silicon nitride crystal structure in the nitride obtained in the nitriding step has a shape of silicon nitride crystal derived from silicon powder and a shape of silicon nitride crystal derived from silicon nitride powder, particularly a short axis particle diameter. Causes a variation in the minor axis particle diameter of the nitride as a whole.
[0020]
On the other hand, when the weight ratio is less than 1, the ratio of silicon nitride crystals derived from silicon nitride powder increases. Thereby, the density is increased by nitriding the silicon before firing, the effect of suppressing the dimensional change of the sintered body after firing is reduced, and the mechanical properties may be reduced. In addition, since silicon powder is relatively inexpensive, there is a disadvantage that the cost increases when the mixing ratio of the silicon powder decreases.
[0021]
As the sintering aid, an oxide of a Group 3a element such as Y or a lanthanoid element can be used as the compound of the Group 3a element of the periodic table. Particularly, from the viewpoint of improving heat resistance, Y, Yb, Er, Lu oxides are preferred. Such group 3a element compounds of the periodic table may be used alone or in combination of two or more. The content of the Group 3a element compound of the periodic table is preferably 1 to 20% by weight, and more preferably 3 to 12% by weight. If the content exceeds 20% by weight, β-type silicon nitride is likely to be generated in the nitriding step, and it becomes difficult to obtain fine columnar crystals having a large aspect ratio, so that desired mechanical properties may not be obtained. On the other hand, if the content is less than 1% by weight, firing does not proceed sufficiently in the firing step, and a dense sintered body may not be obtained.
[0022]
The content of the aluminum oxide is 1 to 10% by weight, preferably 4 to 10% by weight. The content of silicon oxide is 1 to 10% by weight, preferably 2 to 7% by weight. When the content of the aluminum oxide and the silicon oxide exceeds 10% by weight, the strength of the sintered body is reduced, and large voids may be easily generated. On the other hand, if the content is less than 1% by weight, nitriding does not sufficiently occur in the nitriding step, so that sufficient sintering does not proceed and a dense sintered body may not be obtained.
[0023]
<Pulverizing process>
Next, the pulverizing step will be described. The mixed powder described above has a BET specific surface area of 6 to 14 m in order to efficiently perform nitriding and obtain a dense sintered body. ² / G, preferably 8 to 12 m ² / G and finely divided. BET specific surface area is 6m ² If it is less than / g, sufficient nitridation does not occur in the nitriding step, making it difficult to obtain fine silicon nitride crystals from silicon powder, and there is a possibility that a large number of aggregated crystals will be generated. As a result, a dense sintered body cannot be obtained, and desired mechanical properties may not be obtained. On the other hand, the BET specific surface area is 14m ² / G, oxygen is adsorbed on a new surface generated by pulverization of silicon powder, and the weight of silicon oxide in the compact before nitriding increases, so that the composition in the compact varies from a predetermined range, and The sintered body obtained later tends to be porous with many voids. The pulverizer used for pulverizing the mixed powder is not particularly limited, and for example, a vibration mill, a rotary mill, or the like can be used.
[0024]
<Molding process>
The powder obtained in the above-mentioned pulverization step is formed into a desired shape by a known means such as press molding, casting, extrusion molding, injection molding, sludge molding, and cold isostatic pressing to obtain a molded body. This molded body is preferably molded so that the relative density ratio to the theoretical density is 40 to 65%, preferably 45 to 62%. In order to control the density of the compact within the above range, the pressure applied to the powder during molding may be adjusted. When the relative density ratio is less than 40%, the nitridation reaction proceeds rapidly in the next step, the formation of β-type silicon nitride is promoted, and the silicon nitride crystal of the sintered body obtained by firing may not be a fine columnar crystal. There is. On the other hand, when the relative density ratio exceeds 65%, the nitriding reaction does not sufficiently proceed, and silicon powder may remain in the nitride.
[0025]
<Nitriding process>
A nitride is obtained by subjecting the molded body obtained in the above molding step to a heat treatment at 1000 to 1500 ° C., preferably 1100 to 1400 ° C. in an atmosphere containing nitrogen. At this time, it is preferable to gradually raise the temperature within the above temperature range to gradually nitride silicon. Thereby, the nitriding ratio is further improved. In addition, nitriding can be promoted by nitriding in a nitrogen pressurized atmosphere of 1 to 50 atm or by mixing a gas such as hydrogen, but using these methods may increase the cost.
[0026]
By nitriding the compact under the above-described nitriding conditions, a nitride nitrided at a high nitriding rate of 90% or more can be obtained. The silicon nitride crystal in the nitride is mainly derived from silicon powder, and has a minor axis particle diameter of less than 0.3 μm, preferably 0.2 μm or less, in an area ratio of 50 to 95%, preferably 70 to 90%. %, And the remainder is mainly silicon nitride powder, and is preferably a silicon nitride crystal having a short-axis particle size of 0.3 μm or more. Further, the α-rate of these silicon nitride crystals is preferably 50% or more, and more preferably 65 to 92%.
[0027]
When the area ratio is less than 50%, the proportion of fine β-type silicon nitride, which is a nucleus for the growth of silicon nitride crystal grains, becomes low, so that the sintered body does not become dense and has high strength. May not be obtained. On the other hand, if the area ratio exceeds 95%, the proportion of fine particles in the β-type silicon nitride, which is a nucleus for grain growth, becomes too large, so that it is difficult for the grain growth of the crystal to become nonuniform, and the crystal is grown. Although the sintered body obtained by uniform growth tends to have a fine crystal structure, since there are many crystals of the same shape with a low aspect ratio as a whole, a desired void characteristic may not be obtained. . Further, when the α-formation ratio is within the above range, a dense, high-strength, silicon nitride-based sintered body having excellent void characteristics can be obtained.
[0028]
<Firing step>
Finally, the nitride obtained in the nitriding step is subjected to a known firing method in a non-oxidizing atmosphere containing nitrogen, for example, hot press firing, normal pressure firing, nitrogen pressure firing, and further, after firing these. A silicon nitride sintered body can be obtained by firing by hot isostatic pressing (HIP) treatment, glass seal HIP firing, or the like. The temperature at the time of firing is 1600 to 2000 ° C, preferably 1650 to 1900 ° C. If the firing temperature is too high, the silicon nitride crystals may grow excessively, and the strength may be reduced. The sintering time is usually about 5 to 15 hours. The silicon nitride sintered body thus obtained has a relative density ratio to the theoretical density of 95% or more, a void fraction of 2.0% or less, a maximum void diameter of 30 μm or less, and a four-point bending strength at room temperature. Is 900 MPa or more.
[0029]
In the present invention, in the nitride, the difference between the short-axis particle diameter of the silicon nitride crystal derived from the silicon powder and the short-axis particle diameter of the silicon nitride crystal derived from the silicon nitride powder prevents the growth of particles during firing. A silicon nitride crystal having a high aspect ratio is deposited to be uniform to improve mechanical properties and void properties.
[0030]
The grain growth at the time of firing mainly includes β-type silicon nitride crystals contained in silicon nitride crystals having a minor axis particle diameter of less than 0.3 μm derived from silicon powder and those having a minor axis particle diameter of at least 0.3 μm derived from silicon nitride powders. The β-type silicon nitride crystal contained in the silicon nitride crystal advances as a nucleus. At this time, by specifying the ratio between the silicon nitride powder and the silicon powder as described above, the nucleus of the grain growth is more contained in the silicon nitride crystal derived from the silicon powder, so that fine β-type silicon nitride is used as the nucleus. Grain growth mainly occurs, and grain growth with a small but coarse β-type silicon nitride as a nucleus occurs simultaneously. In this way, the grain growth becomes non-uniform, the aspect ratio of the silicon nitride crystal becomes high, and a dense and high-strength silicon nitride-based sintered body having excellent void characteristics can be obtained.
[0031]
【Example】
Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to the following Examples.
[0032]
Example
Table 1 shows, as raw material powder, a silicon powder having an average particle size of 4.2 μm and an oxygen content of 1.4% by weight, and a particle size at an α conversion of 90%, an oxygen content of 1.5% by weight and a cumulative particle size distribution of 90%. Of silicon nitride powder, oxides of Group 3a of the Periodic Table shown in Table 1, and aluminum oxide, which were blended so as to have a composition shown in Table 1, and a BET ratio shown in Table 1. After pulverizing to a surface area with a vibration mill, a load was applied with a cold isostatic press to produce a molded body. Next, the obtained molded body was nitrided at normal pressure according to a nitriding temperature pattern shown in Table 2 to obtain a nitrided body. This nitride was fired at 1750 ° C. under a nitrogen pressure of 1 atm for 10 hours to produce a silicon nitride sintered body.
[Table 1]

[Table 2]

[0033]
Each physical property value shown in Tables 1 and 2 was measured by the following method.
1. BET specific surface area
After the pulverized mixed powder was heated at 200 ° C. for 10 minutes or more, it was calculated by a fluid nitrogen adsorption method.
2. Relative density ratio to theoretical density of compact
The density of the obtained molded article was measured by weight / volume, and the ratio of the actual density of the molded article to the theoretical density of the molded article was calculated. However, in calculating the relative density of the compact, the theoretical density of silicon (Si) is 2.4 (g / cm). ³ ), Silicon nitride (Si ₃ N ₄ ) Has a theoretical density of 3.18 (g / cm ³ ), Alumina (Al ₂ O ₃ ) Has a theoretical density of 3.98 g / cm ³ ), Yttrium oxide (Y ₂ O ₃ ) Is 5.0 (g / cm) ³ ), Erbium oxide (Er) ₂ O ₃ ) Has a theoretical density of 8.6 (g / cm ³ ), Ytterbium oxide (Yb ₂ O ₃ ) Has a theoretical density of 9.2 (g / cm ³ ), Lutetium oxide (Lu) ₂ O ₃ ) Has a theoretical density of 9.4 (g / cm ³ ), Silicon oxide (SiO ₂ ) Has a theoretical density of 2.65 (g / cm ³ ).
3. Nitriding rate
The weight of the molded body was compared with that of the nitride, and the proportion of the mixed silicon nitrided was calculated from the weight increase.
4. Silicon nitride crystal area ratio
First, the surface of the nitride was mirror-finished, etched with a strong acid, and photographed at a magnification of 15,000 with a scanning electron microscope. This photo (48μm ² ) Was divided into 3 μm × 3 μm grids, three places were selected from these, and these were further divided into 0.3 μm × 0.3 μm grids and observed. In the short-axis particle diameter of the silicon nitride crystal observed on the 0.3 μm × 0.3 μm lattice, the number of lattices including a crystal shorter than the 0.3 μm-side lattice and the lattice of 0.6 μm on one side for four lattices By dividing the number by the total number of lattices of 100, the area ratio of silicon nitride crystals having a minor axis particle diameter of less than 0.3 μm was calculated.
5. α rate
A part of the compact after nitriding was pulverized, and the amount of α-type silicon nitride was determined by X-ray diffraction measurement. To calculate the amount of α-type silicon nitride, α-Si ₃ N ₄ The peak intensities of (102) and (210) of H (102), H (210) and β-Si ₃ N ₄ Assuming that the peak intensities of (101) and (210) are h (101) and h (210), Σ [H (102) + H (210)] / Σ [H (102) + H (210) + h (101) + h (210)].
6. Relative density ratio to theoretical density of sintered body
The specific gravity of the obtained sintered body was measured by the Archimedes method, and the ratio of the actual density of the sintered body to the theoretical density of the sintered body was calculated. The calculation of the relative density of the sintered body is the same as the method of calculating the relative density of the compact.
7. Flexural strength
The sintered body obtained above was cut and polished into a test piece of 3 mm × 4 mm × 40 mm, and a four-point bending strength test was performed at room temperature based on JIS R1601.
8. aspect ratio
After the surface of the sintered body is mirror-finished and etched with a strong acid, the silicon nitride crystal is observed with a metallographic microscope (Luzex-FS manufactured by NIRECO) and the aspect ratio of the silicon nitride crystal is measured with an image analyzer. did. In the observation with a metallographic microscope, the measurement magnification was 2000 times and the measurement area was 100 μm. ² And 10 measurement points.
9. Void properties
The surface of the sintered body was mirror-finished, and the occurrence of voids was observed with a metallographic microscope (LUZEX-FS manufactured by Nireco), and the void ratio and the maximum void diameter were measured with an image analyzer. In the observation with a metal microscope, the measurement magnification was 100 times and the measurement area was 9.0 × 10 ⁴ μm ² And 10 measurement points.
10. Dimensional change rate
The dimensions of the molded body and the sintered body produced as described above were measured, and the ratio to the dimension of the molded body (1−dimension of the sintered body / dimension of the molded body) was defined as a dimensional change rate (%).
[0034]
Table 2 shows that when the weight ratio of the silicon powder to the silicon nitride powder is less than 1, the dimensional change rate of the sintered body increases (Nos. 36 to 40).
[0035]
When the content of the oxide of the Group 3a element of the periodic table is less than 1% by weight, the sinterability and the aspect ratio are reduced (No. 41). When the content exceeds 20% by weight, coarse silicon nitride crystals are used. And the bending strength tends to decrease (No. 42).
[0036]
When the content of aluminum oxide is less than 1% by weight, the sinterability is reduced (No. 43). When the content exceeds 10% by weight, the sinterability is improved, but the firing temperature is not optimal. The silicon nitride crystal was coarsened and the mechanical properties were reduced (No. 44). The same result can be obtained with respect to the content of silicon oxide. Particularly, when the content of silicon oxide exceeds 10% by weight, the porous structure increases in the sintered body, and the transverse rupture strength is remarkably increased. (No. 45, 46).
[0037]
If the particle size at 90% of the cumulative particle size distribution of the silicon nitride powder is less than 5 μm, the silicon nitride crystal structure after nitriding is fine, but the aspect ratio of the sintered body is low, and high mechanical properties cannot be obtained. (No. 47, 49-54). On the other hand, when the particle size exceeds 40 μm, a silicon nitride crystal having a high aspect ratio is obtained, but the number of coarse crystals increases as a whole, resulting in inferior mechanical properties. When the fracture surface of the silicon nitride crystal was observed, it was confirmed that the structure had a structure in which columnar crystals abnormally grown in a group of fine columnar crystals were mixed, and appeared to be a fracture source (No. 48).
[0038]
BET specific surface area after grinding of mixed powder is 6m ² / G, it is difficult to obtain a dense body in terms of sinterability, and it has been confirmed that the crystal has a structure in which many coarse crystals having a low aspect ratio are present as a whole (No. .49, 50). Also 14m ² / G, the void fraction in the sintered body tends to increase, and it has been confirmed that the maximum void diameter also increases. Along with this, the mechanical properties were also inferior (No. 51, 52). Especially the specific surface area is 15m ² / G, a large number of coarse pores could be confirmed in a part of the sintered body, and a dense sintered body could not be obtained (No. 52).
[0039]
In the nitride, if the silicon nitride crystal having a minor axis particle diameter of less than 0.3 μm is less than 50% in area ratio, a dense sintered body can be obtained, but the area ratio is in the range of 50 to 95%. The results were inferior in mechanical properties as compared to a certain sample (Nos. 32, 33, 38 to 40, 42, 44, 46, 53). When the area ratio exceeds 95%, a fine and dense silicon nitride crystal structure can be obtained as a whole, but the aspect ratio of the crystal tends to decrease and the void ratio in the sintered body tends to increase. (Nos. 30, 31). Further, when the rate of α-formation of the silicon nitride crystal after nitriding was less than 50%, the mechanical strength of the sintered body was inferior (Nos. 34, 35, 42, 53).
[0040]
When the relative density ratio with respect to the theoretical density of the molded body was less than 40%, the molded body exhibited a high nitridation rate and excellent results in terms of sinterability. The rate of change was 20% or more, the aspect ratio of the crystal was small, and the transverse rupture strength was low (No. 53). On the other hand, when the relative density ratio exceeded 65%, the mixed silicon powder was not completely nitrided, and a dense sintered body was not obtained (No. 54).
[0041]
When the nitriding temperature was low or the total time spent for nitriding was short, the nitriding was not sufficiently performed, and the resulting sintered body was not dense (Nos. 55 to 58). Also, when the nitriding temperature is high or the total time spent for nitriding becomes long, the amount of silicon nitride obtained from silicon powder precipitated as β-type silicon nitride crystal increased, resulting in inferior bending strength ( No. 34, 35, 59).
[0042]
On the other hand, Test No. In the case of 1 to 29, even when the silicon powder in the raw material powder is mixed at a high ratio, the silicon powder is efficiently nitrided, and a dense body having a relative density ratio of the sintered body of 99% or more is obtained, In addition, a silicon nitride sintered body having a high bending strength of about 1000 MPa was obtained. In addition, a compact obtained by mixing a predetermined silicon nitride powder and controlling the pulverization conditions is nitrided in a nitrogen-containing atmosphere, and by controlling the crystal diameter distribution of the silicon nitride crystal after nitriding, a high mechanical property is obtained. It can be seen that a sintered body having excellent void characteristics can be obtained while having the characteristic characteristics. Furthermore, by mixing the silicon powder at a high ratio, the dimensional change rate was suppressed low, and a simple and inexpensive silicon nitride sintered body could be manufactured.
[0043]
【The invention's effect】
According to the present invention, even when silicon powder is mixed at a high ratio, it is possible to obtain a silicon nitride-based sintered body that has small dimensional change, is dense, has excellent bending strength, and has excellent void characteristics. There is an effect that can be. The silicon nitride-based sintered body thus obtained can be applied to structural materials in the field of mechanical parts and the field of automobile parts, which require high dimensional accuracy, specific strength and specific rigidity.

Claims (3)

累積粒度分布９０％における粒径が５〜４０μｍである窒化珪素粉末およびこの窒化珪素粉末に対する重量比率が１以上である珪素粉末を主成分とし、周期律表第３ａ族元素の１種類以上を酸化物換算で１〜２０重量％、アルミニウムを酸化物換算で１〜１０重量％、過剰酸素を酸化珪素換算で１〜１０重量％含む混合粉末を、ＢＥＴ比表面積が６〜１４ｍ^２／ｇになるように粉砕し、ついで成形して成形体を得た後、この成形体を１０００〜１５００℃の窒素含有雰囲気中で熱処理して窒化率が９０％以上である窒化体を得、該窒化体中の窒化珪素結晶は、短軸粒子径０．３μｍ未満の窒化珪素結晶を面積比率で５０〜９５％含み、残部が短軸粒子径０．３μｍ以上の窒化珪素結晶であり、これらの窒化珪素結晶のα化率が５０％以上であり、さらに、この窒化体を窒素を含む非酸化性雰囲気中で焼成することを特徴とする窒化珪素質焼結体の製造方法。A silicon nitride powder having a particle size of 5 to 40 μm at a cumulative particle size distribution of 90% and a silicon powder having a weight ratio to the silicon nitride powder of 1 or more as a main component, and oxidizing at least one element of Group 3a element of the periodic table. BET specific surface area of a mixed powder containing 1 to 20% by weight in terms of a substance, 1 to 10% by weight in terms of an oxide in terms of aluminum, and 1 to 10% in terms of silicon oxide in terms of silicon oxide has a BET specific surface area of 6 to 14 m ² / g. And then molded to obtain a molded body, and then heat-treating the molded body in a nitrogen-containing atmosphere at 1000 to 1500 ° C. to obtain a nitride having a nitriding ratio of 90% or more. Is a silicon nitride crystal having an area ratio of 50 to 95% of a silicon nitride crystal having a minor axis particle diameter of less than 0.3 μm, and the remaining silicon nitride crystal having a minor axis particle diameter of 0.3 μm or more. Has a pregelatinization rate of 50% or more And a method for producing a silicon nitride sintered body, characterized by firing the nitride in a non-oxidizing atmosphere containing nitrogen. 前記成形体の理論密度に対する相対密度比率が４０〜６５％である請求項１記載の窒化珪素質焼結体の製造方法。The method for producing a silicon nitride-based sintered body according to claim 1, wherein a relative density ratio of the compact to a theoretical density is 40 to 65%. ボイド率が２．０％以下、最大ボイド径が３０μｍ以下、焼結体の理論密度に対する相対密度比率が９５％以上、室温における４点曲げ抗折強度が９００ＭＰａ以上であることを特徴とする、請求項１または２記載の方法により得られる窒化珪素質焼結体。A void ratio of 2.0% or less, a maximum void diameter of 30 μm or less, a relative density ratio to a theoretical density of the sintered body of 95% or more, and a four-point bending strength at room temperature of 900 MPa or more, A silicon nitride based sintered body obtained by the method according to claim 1.