JP4110339B2 - Cubic boron nitride sintered body - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は立方晶窒化ホウ素焼結体に関するもので、特に鉄系材料の切削工具として用いた場合に、耐摩耗性、耐欠損性に優れた立方晶窒化ホウ素焼結体に関する。
【０００２】
【従来の技術】
立方晶窒化ホウ素（以下、ｃＢＮという）は、ダイヤモンドに次ぐ硬度を有し、熱的化学的安定性の高い物質であり、従来より鉄系材料の切削工具として用いられている。現在、切削工具として一般に用いられているｃＢＮ焼結体は、ｃＢＮの粉末を、ＴｉＮ、ＴｉＣ、Ｃｏなどの結合材を用いて超高圧下で焼結されたもので、焼結体には１０〜６０体積％程度の結合材が含まれる。
通常、上記ｃＢＮ焼結体の原料に用いられるｃＢＮ粉末は、六方晶窒化ホウ素（以下、ｈＢＮという）を、アルカリ金属やアルカリ土類金属の窒化物やホウ窒化物を触媒として、高温高圧下で変換して合成されたｃＢＮの単結晶である。
【０００３】
従来より知られている直接変換によるｃＢＮ多結晶体は、例えば特開昭４７−３４０９９号、特開平３−１５９９６４、特公昭６３−３９４号、特開平８−４７８０１号各公報に示されているように、結晶性のよいｈＢＮ（六方晶窒化ホウ素）やｐＢＮ（熱分解窒化ホウ素）を用いていたので、十分なｈＢＮ→ｃＢＮ変換を行うのに２１００℃以上の温度が必要で、その結果、多結晶体を構成するｃＢＮ粒子の粒径が３〜５μｍと大きくなり、粒子間の結合力も弱く、高温での強度は低い。すなわち、従来の方法では、高温下で高い強度を有するｃＢＮ多結晶体は得られない。
【０００４】
【発明が解決しようとする課題】
ｃＢＮはへき開しやすく、また、触媒をインクルージョンとして含むため、強度があまり高くなく、特に高温下で強度が大きく低下してまう。このため、このようなｃＢＮ結晶を原料として作製された従来のｃＢＮ焼結体を切削工具として用いた場合、ｃＢＮ粒子の破壊、へき開によるマイクロチッピングなどにより工具刃先が摩耗したり、欠損（チッピング）しやすいという問題がある。 ｃＢＮ焼結体の切削性能、寿命の向上のためには、原料のｃＢＮ粉末をより強靱なものとする必要がある。一部で研削用砥粒として用いられている多結晶体砥粒は、単結晶の砥粒より強度的にやや改善される。しかし、従来の多結晶砥粒は、構成する一次粒子の粒径が数μｍから数十μｍと粗く不揃いで、また、粒子結合が不十分であり、強度的に十分とはいえない。
本発明は、上記の問題を解決するために開発されたもので、高強度で、耐熱性に優れたｃＢＮ単相の多結晶体を作製し、それを粉砕して、ｃＢＮ焼結体の原料とし、耐摩耗性、耐欠損性に優れたｃＢＮ焼結体を提供することを目的とする。
【０００５】
【課題を解決するための手段】
すなわち、直接変換によるｃＢＮ多結晶体作製において、出発物質に高純度化が可能な微粒もしくは低結晶性の常圧型ＢＮを用い、粒成長の起こらない温度範囲で変換焼結して、粒子同士の結合力、粒径及び未変換ｈＢＮ（圧縮型ｈＢＮとして焼結体内に残留）の残留量を制御した。その結果、従来にない高強度で耐熱性に優れた焼結体が得られることを見い出した。そして、この焼結体を粉砕したｃＢＮ多結晶体粉末を原料として、結合材を用いて、ｃＢＮ焼結体を作製し、切削工具として性能を評価したところ、耐摩耗性、耐欠損性とも従来のｃＢＮ焼結体に比べ、格段に優れた切削性能を有することがわかり、本発明に至った。本発明のｃＢＮ焼結体と従来のｃＢＮ焼結体のそれぞれの概念図を図１に示す。
【０００６】
すなわち、本発明は、（１）立方晶窒化ホウ素と、分散した結合材とからなる立方晶窒化ホウ素焼結体であって、該焼結体を構成する立方晶窒化ホウ素が、平均結晶粒径１μｍ以下、好ましくは０．５μｍ以下の微細な立方晶窒化ホウ素の結晶からなる多結晶体であり、
（２）前記立方晶窒化ホウ素の結晶からなる多結晶体は、圧縮型六方晶窒化ホウ素を０．０１〜０．５体積％、好ましくは０．０１〜０．３体積％含むことを特徴とする立方晶窒化ホウ素焼結体、
【０００７】
（３）前記立方晶窒化ホウ素の結晶からなる多結晶体は、ホウ素と酸素を含む化合物を、炭素と窒素の存在下で還元窒化することにより合成された低圧相窒化ホウ素を出発物質として、高温高圧下で立方晶窒素ホウ素に直接変換させると同時に焼結させることで作製した立方晶窒素ホウ素単相の焼結体を粉砕することにより得られる多結晶体であることを特徴とする上記（１）又は（２）に記載の立方晶窒化ホウ素焼結体、
（４）前記立方晶窒化ホウ素への直接変換は、ホウ素と酸素とを含む化合物の沸点以上の温度で、前記低圧相窒素ホウ素を非酸化性雰囲気で加熱したのち行われることを特徴とする上記（３）に記載の立方晶窒化ホウ素焼結体、
【０００８】
（５）前記立方晶窒化ホウ素の結晶からなる多結晶体が結合し、その連続した多結晶体の粒界に結合相が分散していることを特徴とする上記（１）〜（４）のいずれかに記載の立方晶窒化ホウ素焼結体、
（６）前記立方晶窒化ホウ素の結晶からなる多結晶体の含有量が、８０〜９５体積％、好ましくは８５〜９５体積％である上記（１）〜（５）のいずれかに記載の立方晶窒化ホウ素焼結体、
【０００９】
（７）前記結合相がＷ又は／及びＣｏを主成分とする化合物からなることを特徴とする上記（１）〜（６）のいずれかに記載の立方晶窒化ホウ素焼結体、
（８）前記結合相がＡｌを、好ましくは１８〜３０重量％含むことを特徴とする上記（１）〜（７）のいずれかに記載の立方晶窒化ホウ素焼結体、
（９）前記結合相がＡｌを主成分とする化合物を含むことを特徴とする上記（１）〜（６）のいずれかに記載の立方晶窒化ホウ素焼結体、
【００１０】
（１０）ホウ素と酸素を含む化合物を炭素と窒素の存在下で還元窒化して低圧相窒化ホウ素を合成し、得られた低圧相窒化ホウ素を出発物質として高温高圧下で立方晶窒化ホウ素に直接変換させると同時に焼結し、得られた立方晶窒化ホウ素単相の焼結体を粉砕し、これを結合材と混合して超高圧・高温条件下で焼結することを特徴とする立方晶窒化ホウ素焼結体の製造方法、
（１１）前記立方晶窒化ホウ素への直接変換は、ホウ素と酸素とを含む化合物の沸点以上の温度で前記低圧相窒化ホウ素を非酸化性雰囲気で加熱した後に行うことを特徴とする上記（１０）に記載の立方晶窒化ホウ素の結晶からなる多結晶体の製造方法を提供するものである。
上記（４）及び（１１）における非酸化性雰囲気としては通常、窒素、アルゴン、真空等を用いるのが好ましい。また、上記（９）におけるＡｌを主成分とする化合物としてはＡｌＮ、ＡｌＢ2 、Ａｌ2 Ｏ3 、ＡｌＳｉ、ＮｉＡｌ等が挙げられる。
【００１１】
【発明の実施の形態】
本発明のｃＢＮ焼結体の原料となるｃＢＮ多結晶体粉末は、ｈＢＮ→ｃＢＮ直接変換法で、粒子同士の結合力、粒径及び未変換ｈＢＮ（圧縮型ｈＢＮとして焼結体内に残留）の残留量を制御することにより作製された高強度、耐熱性ｃＢＮ多結晶体を粉砕することにより得られる。
この高強度、耐熱性ｃＢＮ多結晶体の作製は具体的には、出発原料に高純度で微粒もしくは低結晶性の常圧相ＢＮを用い、粒成長の起こらない温度範囲でｃＢＮに直接変換焼結することにより行う。
【００１２】
ここで出発原料に用いる高純度で微粒もしくは低結晶性の常圧型ＢＮは、酸化ホウ素やホウ酸を、炭素や有機物で還元し、窒化させて作製されたものが好ましい。通常、常圧型ＢＮの合成方法として、酸化ホウ素やホウ酸をアンモニアと反応させる方法が一般に工業的に行われている。しかし、このようにして得られたＢＮは、高温で熱処理するとｈＢＮへ結晶化、もしくは粒成長する。このため、この方法により微細で低結晶性の常圧型ＢＮを合成しても、不純物の酸化ホウ素を除去するための高温精製処理（窒素ガス中２０５０℃以上、真空中１６５０℃以上など）を行うと、ｈＢＮに結晶化、粒成長してしまう。これに対し、酸化ホウ素やホウ酸を炭素と窒素の存在下で還元窒化させた常圧型ＢＮは、高温で熱処理しても結晶化しない特徴があり、したがって、この方法で微粒で低結晶性の常圧型ＢＮを合成し、窒素ガス中２０５０℃以上又は真空中１６５０℃以上などの高純度精製処理を行うことで、酸化ホウ素や吸着ガスのない直接変換焼結に非常に適した常圧型ＢＮが得られる。上記の還元窒化はメラミンのような窒素と炭素を含む化合物を用いて行うこともできる。
【００１３】
このｃＢＮ多結晶体の合成（焼結）条件は、圧力６〜７ＧＰａ、温度１５５０℃〜２１００℃が好ましい。特に焼結温度が重要で、低いとｃＢＮへの変換が十分でなく、高すぎるとｃＢＮの粒成長が進行し、ｃＢＮ同士の結合力が小さくなる。ｃＢＮの粒成長の起こらない焼結温度は、出発原料の結晶性、粒径により変化する。
【００１４】
上記の適切な焼結温度範囲で焼結したｃＢＮ多結晶体は、平均粒径１μｍ以下、好ましくは０．５μｍ以下のｃＢＮからなる緻密な組織を有し、曲げ強度が高い。この多結晶体の破面を見ると、粒内破壊が支配的で、粒子同士の結合力が強いことを示している。１０００℃の高温でも強度が低下せず、むしろ室温より向上する傾向がある。高温下で、粒子内の転位の移動による塑性変形が起こり、それにより亀裂先端での応力集中が緩和され、破壊強度が向上すると考えられる。
【００１５】
一方、これにより高い温度で焼結した多結晶体は、平均粒径が１μｍを越え、破面を見ると主に粒界で破壊し、粒間結合が弱いことを示した。高温下ではさらに強度が低下、１０００℃では室温の約半分程度の強度となる。高温下では弱い粒界が更に弱化し、粒界で不均一な変形がおこるため、高温での強度が低下すると考えられる。
ここで、ｃＢＮ粒径のコントロールは直接変換焼結時の温度で行う。すなわち、１μｍ以下、特に０．５μｍ以下の微粒状態をコントロールするために、出発原料として微粒で低結晶性の常圧型のＢＮを用いそして低温域で直接変換焼結する必要がある。通常のｈＢＮやｐＢＮでは２１００℃以上にしなければｃＢＮに変換しないので１μｍ以下にコントロールできない。
【００１６】
また、このｃＢＮ多結晶体は０．０１〜０．５体積％の圧縮型ｈＢＮを含むのが特徴である。この程度の圧縮型ｈＢＮは多結晶体の強度に影響を及ぼさない。むしろ亀裂の進展を阻止し、靱性を向上させる効果がある。圧縮型ｈＢＮが０．０１体積より少ない多結晶体は靱性が低下し、０．５体積％を越えると、圧縮型ｈＢＮでの応力集中が大きくなり、強度が低下する。
こうして得られたｃＢＮ多結晶体は、微粒で、ｃＢＮ粒子同士が強固に結合して緻密な組織を有するため、高強度で、高温下でもその強度が低下することがない。１０００℃を越える温度では強度が向上するという従来のｃＢＮ焼結体にみられない特徴を有す。１０００℃以上の強度は、従来のｃＢＮ焼結体の強度の２倍以上となる。
この多結晶体を粉砕することで、従来のｃＢＮ結晶粒にない高強度で耐熱性の高いｃＢＮ多結晶粒が得られる。このｃＢＮ多結晶粒を切削工具用ｃＢＮ焼結体の原料とすることで、本発明の耐摩耗性、耐欠損性に優れたｃＢＮ焼結体が得られる。
本発明の焼結体組織はｃＢＮ多結晶体粉末の粒度、結合材の粒度及びそれ等の混合比率に応じて図１（ａ）に示されるように多結晶体の粒界に結合材が分散した相を形成している。このような焼結体を切削工具として用いると耐摩耗性、耐欠損性に優れた切削性能が得られる。
【００１７】
【実施例】
以下、本発明の具体的な実施例をあげ、本発明の効果を一層明らかにする。
（実施例１）
窒素雰囲気中で、酸化ホウ素（Ｂ2 Ｏ3 ）とメラミン（Ｃ3 Ｎ6 Ｈ6 ）を反応させて微細なｈＢＮの粉末を合成し、さらに、窒素雰囲気中、２１００℃で２時間処理した。得られたｈＢＮ粉末は、平均粒度０．１μｍで酸素含有量は０．１重量％であった。このｈＢＮ粉末を６ｔｏｎ／ｃｍ2 で型押し成形、この成形体を再度、高周波炉で、Ｎ2 ガス中、２１００℃で２時間処理した。
次にこの高純度化処理した試料をＭｏカプセルに入れ、ベルト型高圧発生装置で圧力６．５ＧＰａで１７００℃〜２１００℃の温度条件で１５分処理し、ｃＢＮに変換焼結した。
得られたｃＢＮ焼結体は表１に示す結晶粒子の大きさと圧縮型ｈＢＮを含む緻密な焼結体で、粒子同士が強固に結合した構造を有している。これらの焼結体を振動ミルで粉砕し、分級して、粒径２〜４μｍのｃＢＮ多結晶体粉末を得た。
次にＷ粉末を３２重量％、Ｃｏ粉末を５０重量％、Ａｌ粉末を１８重量％を混合し、これを超硬合金製ポット及びボールを用いて平均粒度１μｍ以下の結合材粉末を作製した。
【００１８】
これらのｃＢＮ多結晶体粉末と結合材粉末を体積比で９０対１０となるように混合し、混合粉末を作製した。次に、Ｍｏ製の容器にＷＣ−１０重量％Ｃｏ組成の超硬合金からなる円盤を挿入した後、これらの混合粉末を充填し、真空炉にて１０-4Ｔｏｒｒ、１０００℃で１０分間加熱して脱気した。次にこの容器を超高圧・高温装置に入れ、圧力５３ｋｂ、温度１４００℃で３０分間保持して焼結体を得た。
次に、これらの焼結体の組織を走査型電子顕微鏡で観察したところ、ｃＢＮ多結晶粒子が相互に結合し、その粒界に結合材が分散していることが認められた。
【００１９】
上記各焼結体を切削加工用チップに加工した。これらの切削加工用チップを用いて、直径が１２０ｍｍ長さ３００ｍｍであり、外周面に軸方向に垂直な６本の溝が等間隔に形成された、ＨＢ２００のＦＣ３００種からなる丸棒を切削した。切削条件は、切削速度１０００ｍ／ｍｉｎ、切り込み０．３ｍｍ、送り０．１ｍｍ／ｒｅｖ．、湿式である。溝で分割された外周面を１パスとし、刃先が欠損するまでの切削時間を表１に示す。
【００２０】
【表１】
（実施例２）
実施例１の焼結体で粒径０．５μｍ以下の結晶からなる焼結体を振動ミルで粉砕、分級して、粒径０．５〜２μｍのｃＢＮ多結晶体粉末を得た。
次にＷ、Ｃｏを主成分とした粉末に、Ａｌ粉末、ＷＣ粉末等を混合し、これを超硬合金製ポット及ボールを用いて、表２に示される組成を有する結合材粉末を作製した。
これらのｃＢＮ多結晶粉末と結合材粉末を表２に示す体積比で混合し、混合粉末を作製した。Ｍｏ製の容器にＷＣ−１０重量％Ｃｏ組成の超硬合金からなる円盤を挿入した後、これらの混合粉末を充填し、真空炉にて１０-4Ｔｏｒｒ、１０００℃で１０分間加熱して脱気した。次にこの容器を超高圧・高温装置に入れ、圧力５３ｋｂ、温度１４００℃で３０分間保持して焼結体を得た。
上記各焼結体を切削加工用チップに加工した。これらの切削加工用チップを用いて、直径が１２０ｍｍ、長さ２００ｍｍであり、ＨＲＣ５８の耐熱合金インコネル７１８の丸棒を切削した。切削条件は、切削速度１５０ｍ／ｍｉｎ、切り込み０．１５ｍｍ、送り０．０８ｍｍ／ｒｅｖ．、湿式である。工具が欠損又は逃げ面摩耗が０．２ｍｍを超えるまでの切削可能な時間を表２に示す。
【００２２】
【表２】
（実施例３）
実施例１の焼結体で粒径０．５μｍ以下の結晶からなる焼結体を振動ミルで粉砕、分級して、粒径２〜６μｍのｃＢＮ多結晶体粉末を得た。
次にＡｌを主成分とする表３に示される組成を有する結合材粉末を作製した。Ｔａ製の容器に上記の多結晶ｃＢＮ粉末を充填した後、表３の組成比になる量の結合材粉末をｃＢＮ粉末の上に積層して充填した後、この容器を超高圧・高温装置に入れ、圧力５３ｋｂ、温度１４００℃で３０分間保持して焼結体を得た。上記各焼結体を切削加工用チップに加工した。これらの切削加工用チップを用いて、直径が１００ｍｍ、長さ３００ｍｍであり、ＨＲＣ６２のＳＫＨ５１種の丸棒を切削した。切削条件は、切削速度６０ｍ／ｍｉｎ、切り込み０．３ｍｍ、送り０．１２ｍｍ／ｒｅｖ．、湿式である。工具が欠損するまでの時間を表３に示す。
【００２４】
【表３】
【発明の効果】
本発明によると多結晶体を粉砕することで、従来のｃＢＮ結晶粒にない高強度で耐熱性の高いｃＢＮ多結晶粒が得られ、このｃＢＮ結晶粒を切削工具用ｃＢＮ焼結体の原料とすることにより、本発明の耐摩耗性、耐欠損性に優れたｃＢＮ焼結体が得られる。本焼結体は、特に鉄系材料の切削工具として用いる場合に、耐摩耗性、耐欠損性について優れた効果を発揮する。
【図面の簡単な説明】
【図１】図１（ａ）、（ｂ）は、それぞれ本発明のｃＢＮ焼結体と従来のｃＢＮ焼結体の組織を示す模式図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a cubic boron nitride sintered body, and more particularly to a cubic boron nitride sintered body excellent in wear resistance and fracture resistance when used as a cutting tool for an iron-based material.
[0002]
[Prior art]
Cubic boron nitride (hereinafter referred to as cBN) has a hardness second to diamond and has high thermal and chemical stability, and has been conventionally used as a cutting tool for iron-based materials. Currently, a cBN sintered body generally used as a cutting tool is obtained by sintering cBN powder under a super-high pressure using a binder such as TiN, TiC, or Co. About ~ 60% by volume of binder is included.
Usually, the cBN powder used as a raw material for the cBN sintered body is hexagonal boron nitride (hereinafter referred to as hBN), a high-temperature and high-pressure catalyst using an alkali metal or alkaline earth metal nitride or boronitride as a catalyst. It is a single crystal of cBN synthesized by conversion.
[0003]
Conventionally known cBN polycrystals by direct conversion are disclosed in, for example, JP-A-47-34099, JP-A-3-159964, JP-B-63-394, and JP-A-8-47801. As described above, since hBN (hexagonal boron nitride) and pBN (pyrolytic boron nitride) having good crystallinity were used, a temperature of 2100 ° C. or higher is necessary to perform sufficient hBN → cBN conversion. The particle size of the cBN particles constituting the polycrystal is as large as 3 to 5 μm, the bonding force between the particles is weak, and the strength at high temperature is low. That is, the conventional method cannot obtain a cBN polycrystal having high strength at high temperatures.
[0004]
[Problems to be solved by the invention]
Since cBN is easy to cleave and contains a catalyst as an inclusion, the strength is not so high, and the strength is greatly reduced particularly at high temperatures. For this reason, when a conventional cBN sintered body produced using such a cBN crystal as a raw material is used as a cutting tool, the cutting edge of the tool is worn due to cBN particle destruction, cleaving, or chipping. There is a problem that it is easy to do. In order to improve the cutting performance and life of the cBN sintered body, it is necessary to make the cBN powder of the raw material tougher. Polycrystalline abrasive grains, which are partially used as abrasive grains for grinding, are slightly improved in strength than single crystal abrasive grains. However, the conventional polycrystalline abrasive grains are not sufficiently strong in strength because the primary particles constituting them are roughly irregular in particle size of several μm to several tens of μm, and the particle bonding is insufficient.
The present invention was developed in order to solve the above-described problems. A cBN single-phase polycrystal having high strength and excellent heat resistance was produced, and pulverized to obtain a raw material for a cBN sintered body. An object of the present invention is to provide a cBN sintered body having excellent wear resistance and fracture resistance.
[0005]
[Means for Solving the Problems]
That is, in the preparation of cBN polycrystals by direct conversion, the starting material is made of fine particles that can be highly purified or low-pressure normal pressure BN, and converted and sintered in a temperature range in which no grain growth occurs. The binding strength, particle size, and residual amount of unconverted hBN (residual in the sintered body as compressed hBN) were controlled. As a result, it has been found that a sintered body having a high strength and excellent heat resistance, which has never been obtained, can be obtained. The cBN polycrystalline body powder obtained by pulverizing the sintered body was used as a raw material to produce a cBN sintered body using a binder, and the performance was evaluated as a cutting tool. Conventionally, both wear resistance and fracture resistance were obtained. Compared to the cBN sintered body, it was found that the cutting performance was remarkably superior, and the present invention was achieved. The conceptual diagram of each of the cBN sintered body of the present invention and the conventional cBN sintered body is shown in FIG.
[0006]
That is, the present invention provides (1) a cubic boron nitride sintered body comprising cubic boron nitride and a dispersed binder, and the cubic boron nitride constituting the sintered body has an average crystal grain size. A polycrystalline body composed of fine cubic boron nitride crystals of 1 μm or less, preferably 0.5 μm or less ,
(2) The polycrystalline body composed of cubic boron nitride crystals contains 0.01 to 0.5% by volume, preferably 0.01 to 0.3% by volume, of compressed hexagonal boron nitride. stand-cubic boron nitride sintered body which,
[0007]
(3) The polycrystalline body composed of the cubic boron nitride crystal has a low temperature phase boron nitride synthesized by reducing and nitriding a compound containing boron and oxygen in the presence of carbon and nitrogen as a starting material. The above-mentioned (1) characterized in that it is a polycrystal obtained by pulverizing a cubic nitrogen-boron single-phase sintered body produced by direct conversion to cubic nitrogen-boron at the same time under high pressure and sintering. ) Or the cubic boron nitride sintered body according to (2),
(4) The direct conversion to cubic boron nitride is performed after heating the low-pressure phase nitrogen boron in a non-oxidizing atmosphere at a temperature equal to or higher than the boiling point of the compound containing boron and oxygen. The cubic boron nitride sintered body according to (3),
[0008]
(5) The above-mentioned (1) to (4), wherein the polycrystals composed of the cubic boron nitride crystals are bonded, and the binder phase is dispersed at the grain boundaries of the continuous polycrystals. Cubic boron nitride sintered body according to any one of the above,
(6) The cubic according to any one of the above (1) to (5), wherein the content of the polycrystalline body composed of cubic boron nitride crystals is 80 to 95% by volume, preferably 85 to 95% by volume. Sintered boron nitride sintered body,
[0009]
(7) The cubic boron nitride sintered body according to any one of (1) to (6) above, wherein the binder phase is composed of a compound containing W or / and Co as a main component.
(8) The cubic boron nitride sintered body according to any one of (1) to (7), wherein the binder phase contains Al, preferably 18 to 30 % by weight,
(9) The cubic boron nitride sintered body according to any one of (1) to (6), wherein the binder phase includes a compound containing Al as a main component,
[0010]
(10) A low pressure phase boron nitride is synthesized by reducing and nitriding a compound containing boron and oxygen in the presence of carbon and nitrogen, and the obtained low pressure phase boron nitride is directly used as a starting material for cubic boron nitride under high temperature and high pressure. Cubic crystals characterized by being sintered at the same time as being converted, pulverizing the obtained cubic boron nitride single-phase sintered body, mixing this with a binder, and sintering under ultrahigh pressure and high temperature conditions Manufacturing method of boron nitride sintered body,
(11) The direct conversion to cubic boron nitride is performed after the low-pressure phase boron nitride is heated in a non-oxidizing atmosphere at a temperature equal to or higher than the boiling point of the compound containing boron and oxygen (10). And 3) a method for producing a polycrystalline body comprising cubic boron nitride crystals.
As the non-oxidizing atmosphere in the above (4) and (11), it is usually preferable to use nitrogen, argon, vacuum or the like. Examples of the compound mainly composed of Al in the above (9) include AlN, AlB 2 , Al 2 O 3 , AlSi, NiAl, and the like.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The cBN polycrystalline powder used as a raw material for the cBN sintered body of the present invention is obtained by the hBN → cBN direct conversion method. It can be obtained by pulverizing a high-strength, heat-resistant cBN polycrystal produced by controlling the residual amount.
Specifically, this high-strength, heat-resistant cBN polycrystal is prepared by using high-purity, fine-grained or low-crystalline normal pressure phase BN as a starting material, and direct conversion firing to cBN in a temperature range where grain growth does not occur. This is done by tying.
[0012]
Here, the high-purity fine-grained or low-crystalline atmospheric BN used as a starting material is preferably prepared by reducing and nitriding boron oxide or boric acid with carbon or an organic substance. Usually, as a method for synthesizing normal pressure BN, a method of reacting boron oxide or boric acid with ammonia is generally carried out industrially. However, the BN thus obtained crystallizes or grows into hBN when heat-treated at a high temperature. For this reason, even if fine and low crystalline atmospheric BN is synthesized by this method, a high-temperature purification treatment (2050 ° C. or higher in nitrogen gas, 1650 ° C. or higher in vacuum, etc.) is performed to remove boron oxide as an impurity. Then, crystallization and grain growth occur in hBN. On the other hand, atmospheric pressure type BN obtained by reducing and nitriding boron oxide or boric acid in the presence of carbon and nitrogen has a characteristic that it does not crystallize even when heat-treated at a high temperature. By synthesizing normal pressure type BN and performing high-purity purification treatment such as 2050 ° C. or higher in nitrogen gas or 1650 ° C. or higher in vacuum, normal pressure type BN very suitable for direct conversion sintering without boron oxide or adsorption gas is obtained. can get. The above reductive nitriding can also be performed using a compound containing nitrogen and carbon such as melamine.
[0013]
The synthesis (sintering) conditions of this cBN polycrystal are preferably a pressure of 6 to 7 GPa and a temperature of 1550 ° C. to 2100 ° C. In particular, the sintering temperature is important. If the sintering temperature is low, the conversion to cBN is not sufficient, and if it is too high, the grain growth of cBN proceeds and the bonding strength between the cBNs decreases. The sintering temperature at which no grain growth of cBN occurs varies depending on the crystallinity and grain size of the starting material.
[0014]
The cBN polycrystal sintered in the above suitable sintering temperature range has a dense structure composed of cBN having an average particle size of 1 μm or less, preferably 0.5 μm or less, and has high bending strength. Looking at the fracture surface of this polycrystal, it is shown that intragranular fracture is dominant and the bonding force between the particles is strong. Even at a high temperature of 1000 ° C., the strength does not decrease, but rather tends to improve from room temperature. It is considered that plastic deformation occurs due to the movement of dislocations within the particles at high temperatures, thereby relaxing the stress concentration at the crack tip and improving the fracture strength.
[0015]
On the other hand, the polycrystalline body sintered at a high temperature thereby exceeded the average grain size of more than 1 μm, and when the fracture surface was observed, it was mainly broken at the grain boundary, indicating that the intergranular bond was weak. At a high temperature, the strength further decreases, and at 1000 ° C., the strength is about half that of room temperature. It is considered that the strength at high temperatures decreases because weak grain boundaries are further weakened at high temperatures and uneven deformation occurs at the grain boundaries.
Here, the control of the cBN particle size is performed at the temperature during direct conversion sintering. That is, in order to control the fine particle state of 1 μm or less, particularly 0.5 μm or less, it is necessary to use a fine-grained, low-crystalline normal pressure type BN as a starting material and perform direct conversion sintering in a low temperature range. Ordinary hBN and pBN cannot be controlled to 1 μm or less because they are not converted to cBN unless the temperature is 2100 ° C. or higher.
[0016]
The cBN polycrystal is characterized by containing 0.01 to 0.5% by volume of compressed hBN. This level of compressed hBN does not affect the strength of the polycrystal. Rather, it has the effect of preventing the growth of cracks and improving toughness. A polycrystal having a compression type hBN of less than 0.01 volume has low toughness, and if it exceeds 0.5 volume%, the stress concentration in the compression type hBN increases and the strength decreases.
The cBN polycrystal obtained in this way is fine and has a dense structure in which the cBN particles are firmly bonded to each other, so that the strength is high and the strength does not decrease even at high temperatures. It has a characteristic not seen in the conventional cBN sintered body that the strength is improved at a temperature exceeding 1000 ° C. The strength of 1000 ° C. or higher is twice or more the strength of the conventional cBN sintered body.
By crushing this polycrystal, cBN polycrystal grains having high strength and high heat resistance not found in conventional cBN crystal grains can be obtained. By using this cBN polycrystalline grain as a raw material for a cBN sintered body for a cutting tool, the cBN sintered body having excellent wear resistance and fracture resistance according to the present invention can be obtained.
In the sintered body structure of the present invention, as shown in FIG. 1 (a), the binder is dispersed at the grain boundaries of the polycrystal according to the particle size of the cBN polycrystal powder, the particle size of the binder and the mixing ratio thereof. Phase is formed. When such a sintered body is used as a cutting tool, cutting performance excellent in wear resistance and fracture resistance can be obtained.
[0017]
【Example】
Hereinafter, specific examples of the present invention will be given to further clarify the effects of the present invention.
(Example 1)
Boron oxide (B 2 O 3 ) and melamine (C 3 N 6 H 6 ) were reacted in a nitrogen atmosphere to synthesize fine hBN powder, and further treated at 2100 ° C. for 2 hours in a nitrogen atmosphere. The obtained hBN powder had an average particle size of 0.1 μm and an oxygen content of 0.1% by weight. This hBN powder was stamped and molded at 6 ton / cm 2 , and this molded body was again treated at 2100 ° C. for 2 hours in N 2 gas in a high frequency furnace.
Next, this highly purified sample was put in a Mo capsule, treated with a belt type high pressure generator at a pressure of 6.5 GPa under a temperature condition of 1700 ° C. to 2100 ° C. for 15 minutes, and converted to cBN and sintered.
The obtained cBN sintered body is a dense sintered body containing the size of crystal grains and compression type hBN shown in Table 1, and has a structure in which the particles are firmly bonded. These sintered bodies were pulverized with a vibration mill and classified to obtain cBN polycrystalline powder having a particle size of 2 to 4 μm.
Next, 32 wt% of W powder, 50 wt% of Co powder, and 18 wt% of Al powder were mixed, and a binder powder having an average particle size of 1 μm or less was prepared using a cemented carbide pot and balls.
[0018]
These cBN polycrystal powder and binder powder were mixed so that the volume ratio was 90:10 to produce a mixed powder. Next, after inserting a disc made of a cemented carbide of WC-10 wt% Co into a Mo container, these mixed powders were filled and heated at 10 −4 Torr and 1000 ° C. for 10 minutes in a vacuum furnace. Then degassed. Next, this container was put into an ultrahigh pressure / high temperature apparatus and held at a pressure of 53 kb and a temperature of 1400 ° C. for 30 minutes to obtain a sintered body.
Next, when the structure of these sintered bodies was observed with a scanning electron microscope, it was found that the cBN polycrystalline particles were bonded to each other and the binder was dispersed at the grain boundaries.
[0019]
Each of the sintered bodies was processed into cutting chips. Using these cutting tips, a round bar made of FC300 of HB200 having a diameter of 120 mm and a length of 300 mm, and six grooves perpendicular to the axial direction formed on the outer peripheral surface at equal intervals was cut. . Cutting conditions were a cutting speed of 1000 m / min, a cutting depth of 0.3 mm, and a feed of 0.1 mm / rev. It is wet. Table 1 shows the cutting time until the outer peripheral surface divided by the groove is one pass and the cutting edge is lost.
[0020]
[Table 1]
(Example 2)
The sintered body of the sintered body of Example 1 made of crystals having a particle size of 0.5 μm or less was pulverized and classified by a vibration mill to obtain a cBN polycrystalline powder having a particle size of 0.5 to 2 μm.
Next, Al powder, WC powder, and the like were mixed with the powder mainly composed of W and Co, and a binder powder having the composition shown in Table 2 was prepared using a cemented carbide pot and ball. .
These cBN polycrystalline powder and binder powder were mixed at a volume ratio shown in Table 2 to prepare a mixed powder. After inserting a disc made of a cemented carbide of WC-10 wt% Co into a container made of Mo, these mixed powders were filled and removed by heating at 10 −4 Torr and 1000 ° C. for 10 minutes in a vacuum furnace. I worried. Next, this container was put into an ultrahigh pressure / high temperature apparatus and held at a pressure of 53 kb and a temperature of 1400 ° C. for 30 minutes to obtain a sintered body.
Each of the sintered bodies was processed into cutting chips. Using these cutting tips, a round bar of HRC58 heat-resistant alloy Inconel 718 having a diameter of 120 mm and a length of 200 mm was cut. Cutting conditions were a cutting speed of 150 m / min, a cutting depth of 0.15 mm, and a feed of 0.08 mm / rev. It is wet. Table 2 shows the possible cutting time until the tool is broken or the flank wear exceeds 0.2 mm.
[0022]
[Table 2]
(Example 3)
The sintered body comprising the crystals having a particle size of 0.5 μm or less was pulverized and classified with the sintered body of Example 1 to obtain cBN polycrystalline powder having a particle size of 2 to 6 μm.
Next, a binder powder having a composition shown in Table 3 containing Al as a main component was produced. After filling the above-mentioned polycrystalline cBN powder in a Ta-made container, the binder powder in an amount corresponding to the composition ratio shown in Table 3 is stacked on top of the cBN powder, and this container is then placed in an ultrahigh pressure / high temperature apparatus. And a pressure of 53 kb and a temperature of 1400 ° C. for 30 minutes to obtain a sintered body. Each of the sintered bodies was processed into cutting chips. Using these cutting tips, a SKH51 type round bar of HRC62 having a diameter of 100 mm and a length of 300 mm was cut. Cutting conditions were a cutting speed of 60 m / min, a cutting depth of 0.3 mm, and a feed of 0.12 mm / rev. It is wet. Table 3 shows the time until the tool is lost.
[0024]
[Table 3]
【The invention's effect】
According to the present invention, by crushing the polycrystalline body, cBN polycrystalline grains having high strength and high heat resistance, which are not found in conventional cBN crystalline grains, are obtained, and the cBN crystalline grains are used as a raw material for cBN sintered bodies for cutting tools. By doing so, the cBN sintered body excellent in wear resistance and fracture resistance of the present invention is obtained. The sintered body exhibits excellent effects on wear resistance and fracture resistance, particularly when used as a cutting tool for iron-based materials.
[Brief description of the drawings]
FIGS. 1A and 1B are schematic views showing the structures of a cBN sintered body of the present invention and a conventional cBN sintered body, respectively.
Claims (8)
この多結晶体は、平均結晶粒径１μｍ以下の微細な立方晶窒化ホウ素の結晶からなると共に、圧縮型六方晶窒化ホウ素を０．０１〜０．５体積％含むことを特徴とする立方晶窒化ホウ素焼結体。And cubic boron nitride, a cubic boron nitride sintered body consisting of a dispersed binder, cubic boron nitride constituting the sintered body is a polycrystalline body comprising a crystal of standing-cubic boron nitride Yes,
This polycrystal is composed of fine cubic boron nitride crystals having an average crystal grain size of 1 μm or less and contains 0.01 to 0.5% by volume of compressed hexagonal boron nitride. Boron sintered body.
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| JP5002886B2 (en) * | 2004-07-09 | 2012-08-15 | 住友電気工業株式会社 | Method for producing cubic boron nitride polycrystal |
| CN107207365A (en) * | 2014-12-31 | 2017-09-26 | 戴蒙得创新股份有限公司 | Polycrystal cubic boron nitride (PCBN) comprising micro-crystal cubic boron nitride (CBN) and preparation method thereof |
| EP3808715A4 (en) * | 2018-06-18 | 2022-03-09 | Sumitomo Electric Hardmetal Corp. | Polycrystalline cubic boron nitride and production method therefor |
| JP7269967B2 (en) * | 2018-06-28 | 2023-05-09 | ダイヤモンド イノヴェーションズ インコーポレイテッド | PCBN sintered compact |
| WO2024005058A1 (en) * | 2022-06-28 | 2024-01-04 | 京セラ株式会社 | Insert and cutting tool |
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