JP3561476B2 - Boron carbide-chromium diboride sintered body and method for producing the same - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、炭化硼素−二硼化クロム焼結体及びその製造方法に関するものであり、更に詳しくは、炭化硼素粒子が導電性の良好な二硼化クロム相の３次元ネットワーク構造により結合された微細な組織を有し、高い曲げ強度を有し、かつ高い導電率を有する炭化硼素（Ｂ４ Ｃ）−二硼化クロム（ＣｒＢ２ ）焼結体、更に、高い曲げ強度と高い導電率を有すると共に、高い破壊靭性値を兼ね備えた炭化硼素−二硼化クロム焼結体、及びそれらの製造方法に関するものである。
【０００２】
【従来の技術】
炭化硼素焼結体は、軽量で高い硬度を有し、耐摩耗性や耐腐食性に優れた材料であり、現状では、サンドブラストノズル、線引きダイス、押し出しダイス等に使用されている。しかしながら、炭化硼素焼結体は、低強度、低靭性であるという欠点を有している。例えば、文献〔Ｋ．Ａ．Ｓｃｈｗｅｔｚ，Ｊ．ＳｏｌｉｄＳｔａｔｅ Ｃｈｅｍｉｓｔｒｙ，１３３，１７７−８１（１９９７）〕では、様々な焼結条件にて炭化硼素焼結体をＨＩＰ処理にて作製しているが、６００ＭＰａ以上の曲げ強度を持つ炭化硼素焼結体は得られていない。また、曲げ強度が高いほど破壊靭性値は低下する傾向があり、最も高い強度が得られた炭化硼素焼結体では、破壊靭性値は３ＭＰａ・ ｍ１／２ 以下の低い値となる。更に、炭化硼素焼結体は、難焼結性であることから、ホットプレス法やＨＩＰ処理により作製されるが、その硬度が極めて高いことから、通常の研削・研磨法では加工し難く、また、炭化硼素焼結体の導電率が１０〜３００Ｓ／ｍのレベルと低いために、放電加工が困難である、という問題があった。
上記のように、炭化硼素焼結体は、低強度、低靭性、難加工性であることから、現状では、極めて限定された用途にのみ使用されている。
【０００３】
【発明が解決しようとする課題】
このような状況の中で、本発明者らは、上記従来技術に鑑みて、高強度、高靱性で、かつ高い導電率をもつ新しい炭化硼素−二硼化クロム焼結体の研究／開発を鋭意進める過程で、特定の物性を有する炭化硼素粉末に二硼化クロムを５〜２５ｍｏｌ％添加し、２０ＭＰａ以下の低い加圧条件下で二硼化クロムの液相が発生する液相焼結を行い、導電性の高い二硼化クロム相が３次元的にネットワーク構造を形成した微細な組織を有する焼結体を作製することで、優れた特性をもつ新しい炭化硼素−二硼化クロム焼結体が得られることを見出し、本発明を完成するに至った。
即ち、本発明は、６００ＭＰａ以上の四点曲げ強度を有し、５×１０２ Ｓ／ｍ以上の導電率を持つ炭化硼素−二硼化クロム焼結体、及び、６００ＭＰａ以上の四点曲げ強度と５×１０２ Ｓ／ｍ以上の導電率を維持すると共に、３． ０ＭＰａ・ ｍ１／２ 以上の破壊靭性値を兼ね備えた炭化硼素−二硼化クロム焼結体、及びそれらを２０ＭＰａ以下の低い加圧条件下で二硼化クロムの液相が発生する液相焼結を行うことによって製造する方法を提供することを目的とするものである。
【０００４】
【課題を解決するための手段】
上記課題を解決するための本発明は、以下の技術的手段から構成される。
（１）２０ＭＰａ以下の低い加圧条件下で二硼化クロムの液相が発生する液相焼結を行うことによって作製される、炭化硼素（Ｂ４ Ｃ）に二硼化クロム（ＣｒＢ２ ）を５〜２５ｍｏｌ％含有する炭化硼素−二硼化クロム焼結体であって、炭化硼素の最大粒子径が３μｍより大きく２０μｍ以下であり、６００ＭＰａ以上の四点曲げ強度、及び５×１０２ Ｓ／ｍ以上の導電率を有することを特徴とする炭化硼素−二硼化クロム焼結体。
（２）３．０ＭＰａ・ｍ１／２ 以上の破壊靱性値を有することを特徴とする前記（１）記載の炭化硼素−二硼化クロム焼結体。
（３）２０ＭＰａ以下の低い加圧条件下で二硼化クロムの液相が発生する液相焼結を行うことにより炭化硼素−二硼化クロム焼結体を製造する方法であって、平均粒径（Ｄ５０）が１μｍ以下で、最大粒子径が８μｍ以下であり、比表面積が１０ｍ２ ／ｇ以上である炭化硼素粉末に、二硼化クロム粉末を５〜２５ｍｏｌ％添加し、炭化硼素−二硼化クロム焼結体中の炭化硼素の最大粒子が３μｍより大きく２０μｍ以下となるように、１９５０〜２１００℃の温度範囲で２０ＭＰａ以下の圧力下にて加圧焼結することを特徴とする炭化硼素−二硼化クロム焼結体の製造方法。
（４）前記炭化硼素粉末中のアルミニウム含有量が１５００ｐｐｍ以下であることを特徴とする前記（３）記載の炭化硼素−二硼化クロム焼結体の製造方法。
【０００５】
【発明の実施の形態】
次に、本発明について更に詳細に説明する。
炭化硼素は、難焼結性であることから、粒成長が起こり易い極めて高い温度で焼結されるが、従来の技術では、得られた焼結体組織は、大きな粒子を含むこととなり、曲げ強度の劣化を招く。
本発明では、特定の物性を有する炭化硼素粉末を用いて、ＣｒＢ２ を主成分とする液相が発生する温度領域にて、特定の条件下でホットプレス法等の加圧焼結を行うこと、即ち、２０ＭＰａ以下の低い加圧条件下で二硼化クロムの液相が発生する液相焼結を行うことで、炭化硼素の最大粒子サイズが３μｍより大きく２０μｍ以下であり、導電性の高い二硼化クロム相が３次元的にネットワーク構造を形成した微細な組織を得ることによって、６００ＭＰａ以上の高い四点曲げ強度及び５×１０２ Ｓ／ｍ以上の導電率を有する炭化硼素−二硼化クロム焼結体を作製する。
【０００６】
本発明では、炭化硼素粉末として、レーザー回折散乱分析計（マイクロトラック） にて測定した平均粒径（Ｄ５０） が１μｍ以下０．１μｍ以上、最大粒子径が８μｍ以下１μｍ以上のものが用いられる。これらは、好ましくは、平均粒径（Ｄ５０）が０．６μｍ以下〜０．３μｍ以上、最大粒径が６μｍ以下〜１．５μｍ以上である。平均粒径（Ｄ５０）が１μｍより大きいと焼結性が劣り、１９５０〜２１００℃の温度範囲で緻密な焼結体が得らず、また、緻密化するためには、粒成長が起こり易いより高い焼結温度にする必要があり、曲げ強度の劣化を招く。比表面積（ＢＥＴ） については、焼結性が良好な１０ｍ２ ／ｇ以上３０ｍ２ ／ｇ以下の炭化硼素粉末が用いられ、好ましくは１５ｍ２ ／ｇ以上のものが用いられる。
【０００７】
また、炭化硼素粉末中のアルミニウム不純物は、焼結中の炭化硼素の急激な粒成長を助長するので、１５００ｐｐｍ以下でなければならず、好ましくは１０００ｐｐｍ以下であることが望ましい。
前記の物性を有する炭化硼素粉末は、ふるい分け、沈降分離、粉砕等の手段によって調製して得られるが、前記の物性を有する市販品を入手して使用してもよい。
【０００８】
上記の物性を有する炭化硼素粉末に、二硼化クロム粉末を５〜２５ｍｏｌ％添加する。二硼化クロムの添加量が５ｍｏｌ％未満であると、十分な量の二硼化クロム系の液相が生成しないことから、緻密な焼結体が得られず、破壊靭性値の改善効果も十分でない。また、２５ｍｏｌ％より多い場合には、焼結体の密度が３． ０ｇ／ｃｍ３ よりも高くなり、炭化硼素系焼結体の軽量性の特徴が損なわれ、硬度も低下する。
次に、これらを混合して炭化硼素−二硼化クロムの混合粉を調製し、これを２０ＭＰａ以下１ＭＰａ以上の圧力で、真空中あるいはＡｒ等の不活性ガス雰囲気中、１９５０〜２１００℃の焼結温度範囲で、二硼化クロム系の液相を発生させた状態にて液相焼結を行う。
本発明では、２０ＭＰａ以下の低い加圧条件下で二硼化クロムの液相が発生する液相焼結を行うことが重要であり、それにより、通常の固相焼結では得られない優れた特性を有する炭化硼素−二硼化クロム焼結体を作製することができる。
【０００９】
二硼化クロム粉は、焼結中に一部の炭化硼素と反応して溶融し、二硼化クロム系の液相を発生し、炭化硼素粒子間に浸透することから、炭化硼素粉末に比較して、粒径の大きな原料粉でも使用でき、平均粒径（Ｄ５０）が８μｍ以下の二硼化クロム粉を用いることができるが、好ましくは平均粒径（Ｄ５０）が４μｍ以下のものを使用するとよい。
【００１０】
焼結温度が１９５０℃より低い場合には、十分に緻密な炭化硼素焼結体が作製できず、また、二硼化クロム系の液相が発生しないことから、二硼化クロム相が３次元ネットワークを形成できず、導電率が５×１０２ Ｓ／ｍ未満となるので好ましくない。
また、２１００℃より高い焼結温度では、粒成長により微細な焼結体組織が得られず、曲げ強度の低下を招くので好ましくない。焼結時の圧力が２０ＭＰａより大きい場合は、炭化硼素粒子同士の固相焼結が促進されることにより、二硼化クロム相の３次元ネットワーク形成が阻害されて、導電率が低くなる。また、二硼化クロム系の液相量が多い場合には、焼結時の圧力が２０ＭＰａより大きいと、焼結中に液相が流出する。
【００１１】
高い密度を有する炭化硼素−二硼化クロム焼結体を得るために、１９５０〜２１００℃の温度範囲で２０ＭＰａ以下１ＭＰａ以上の圧力下にて、二硼化クロム系の液相を発生させて加圧焼結を行った後、引き続き、二硼化クロム系の液相が発生しない１９００℃以上１９５０℃未満の温度領域まで下げて、より高い３０ＭＰａ以上の圧力にて焼結する二段階の焼結を行うことも適宜可能である。
【００１２】
本発明の炭化硼素−二硼化クロム焼結体は、導電性の良好な二硼化クロム相が３次元ネットワークを形成しており、最大粒子サイズが３μｍより大きく２０μｍ以下の炭化硼素粒子が二硼化クロム相によって結合された組織を有している。二硼化クロムの熱膨張率が炭化硼素より大きいため、破壊の進行時に炭化硼素粒子と二硼化クロム相との界面近傍にて亀裂の伝播の迂回やマイクロクロックが発生することによって破壊靭性値が改善される。また、最大粒子サイズが３μｍより大きく２０μｍ以下と微細であること、二硼化クロム系の液相による溶解・析出機構によって炭化硼素粒子の突起部分が消失して応力集中が緩和されること、炭化硼素粒子が二硼化クロム相によって結合されるため、加工時に炭化硼素粒子の脱落が抑制されること、及び破壊靭性値が改善されること等により、強度が改善され、６００ＭＰａ以上の高い曲げ強度が得られる。
【００１３】
【実施例】
以下に、本発明の内容を、実施例及び比較例により具体的に説明する。
実施例１〜２
（１）炭化硼素−二硼化クロム焼結体の作製
表１に示す物性を有する炭化硼素粉末Ｉ及びＩＩに、平均粒径（Ｄ５０）：３．５μｍの二硼化クロム粉をそれぞれ２０ｍｏｌ％配合し、メタノール溶媒を用いて、遊星ボールミルにて回転数：１８５ｒｐｍ、３０分の混合を行った後、エバポレーターで乾燥させ、更に、１５０℃、２４時間の乾燥を行った後に、開き目２５０μｍのふるいに通して、二種類の炭化硼素−二硼化クロム混合粉を調製した。黒鉛製ダイス中に炭化硼素−二硼化クロム混合粉を充填し、７．５ＭＰａで成形した後、焼成炉に取り付けた。５ＭＰａに加圧した状態にて、拡散ポンプを用いて２． ０×１０−１〜２． ０×１０−２Ｐａの圧力に真空引きをしながら、４０℃／ｍｉｎの昇温速度にて加熱を行った。１０００℃に到達した時に真空引きを終了してＡｒガスを導入し、Ａｒ流量：２リットル／ｍｉｎ、ガス圧力：０． １０３ＭＰａの雰囲気中で１５００℃まで加熱した。１５００℃から２０５０℃までは１０℃／ｍｉｎ昇温速度にて加熱した。２０５０℃に到達した後、１時間保持し、一段階めのホットプレス焼結を行った。保持後、１５℃／ｍｉｎの速度で１９００℃まで温度を下げ、圧力を５０ＭＰａに上げて、この温度で４０分の保持を行って、二段階めの焼結を行って、炭化硼素−二硼化クロム焼結体を作製した。
【００１４】
（２）測定方法
炭化硼素焼結体の四点曲げ強度、及び破壊靭性値をそれぞれＪＩＳ Ｒ１６０１、ＪＩＳ Ｒ１６０７に基づいて測定した。テストピースの表面は、平面研削盤 ４００番にて仕上げた。また、アルキメデス法にてテストピースの密度を測定し、相対密度を計算した。テストピースの表面をラッピングし、エッチング処理を行った後にＳＥＭ観察を行い、炭化硼素の最大粒子サイズをもとめた。更に、導電率の測定を行った。
【００１５】
（３）結果
評価の結果を表２に示す。何れの実施例も６００ＭＰａ以上の四点曲げ強度、３．０ＭＰａ・ｍ１／２ 以上の破壊靱性値、及び５×１０２ Ｓ／ｍ以上の導電率を有している。
【００１６】
比較例１〜２
表１に示す物性を有する炭化硼素粉末ＩＶ及びＶを用いて、実施例１〜２と同様の手順にて、炭化硼素−二硼化クロム焼結体を作製し、評価を行った。
表２に評価結果を示す。比較例１では、比表面積（ＢＥＴ） が１０ｍ２ ／ｇ以下の炭化硼素粉末を用いたが、緻密な焼結体ができずに、曲げ強度及び破壊靭性値は低い値となった。また、比較例２では、アルミニウム含有量が多い炭化硼素粉を用いたが、焼結中に炭化硼素粒子の急激な成長が起こり、最大粒子径が大きくなり、曲げ強度が低下した。
【００１７】
比較例３
表１に示す物性を有する炭化硼素粉末Ｉを用い、二硼化クロム粉の配合を行わないこと以外は、実施例１〜３と同様の手順にて炭化硼素焼結体を作製し、評価を行った。
表２に評価結果を示す。緻密な焼結体ができずに、曲げ強度及び破壊靭性値は低い値となった。
【００１８】
実施例３〜５
実施例１〜２と同じ二硼化クロム粉を実施例３では炭化硼素粉末ＩＩＩ に２０ｍｏｌ％、実施例４では炭化硼素粉末Ｉに２０ｍｏｌ％、実施例５では炭化硼素粉末Ｉに２２．５ｍｏｌ％配合し、メタノール溶媒を用いて、遊星ボールミルにて回転数：２７０ｒｐｍ、１時間の混合を行った後、実施例１〜２と同様な手順にて、炭化硼素−二硼化クロム混合粉を調製した。二段階めの焼結を行わなかったこと以外は、同様の方法にて焼結を行って炭化硼素−二硼化クロム焼結体を作製し、評価を行った。
評価の結果を表２に示す。何れの実施例も６００ＭＰａ以上の四点曲げ強度、３．０ＭＰａ・ｍ１／２ 以上の破壊靱性値、及び５×１０２ Ｓ／ｍ以上の導電率を有している。
【００１９】
比較例４
原料粉ＩＩを用い、５０ＭＰａの圧力下にて１９２５℃で焼結を行ったこと以外は、実施例３と同様の手順にて、炭化硼素−二硼化クロム焼結体を作製し、評価を行った。
評価の結果を表２に示す。焼結温度が低く、二硼化クロム系の液相が発生しないため、緻密な焼結体が得られず、曲げ強度は低い値となった。また、二硼化クロム相の３次元ネットワークが形成されないため、導電率も低い値となった。
【００２０】
比較例５
原料粉ＩＩを用い、二硼化クロム粉の配合を行わずに５０ＭＰａの圧力下にて焼結を行ったこと以外は、実施例３と同様の手順にて、炭化硼素−二硼化クロム焼結体を作製し、評価を行った。
評価の結果を表２に示す。二硼化クロム系の液相が発生しないため、緻密な焼結体が得られず、曲げ強度は低い値となった。また、二硼化クロム相の３次元ネットワークが形成されないため、導電率も低い値となった。
【００２１】
【表１】
【表２】
【発明の効果】
以上詳述した通り、本発明は、二硼化クロム系の液相を発生させた状態で液相焼結を行って作製された、炭化硼素−二硼化クロム焼結体とその製造方法に係るものであり、本発明により、６００ＭＰａ以上の四点曲げ強度、５×１０２ Ｓ／ｍ以上の導電率、３．０ＭＰａ・ｍ１／２ 以上の破壊靱性値を有する炭化硼素−二硼化クロム焼結体を製造し、提供することができる。
また、本発明により、二硼化クロム系の液相を発生させた状態で液相焼結を行うことで、炭化硼素粒子が導電性の良好な二硼化クロム相の３次元ネットワーク構造により結合された微細な組織を有する炭化硼素−二硼化クロム焼結体を作製することができる。
本発明の炭化硼素−二硼化クロム焼結体は、強度・靭性が高く、機械的特性が優れ、放電加工が可能で、加工性も良好であるので、摺動部品、切削工具や新しい耐摩耗性部品等、いろいろな用途で用いられることができ、産業上有用である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a boron carbide-chromium diboride sintered body and a method for producing the same, and more particularly, to a method in which boron carbide particles are bonded by a three-dimensional network structure of a chromium diboride phase having good conductivity. It has a fine structure, having a high bending strength, and boron carbide having a high conductivity (B 4 C) - diboride chromium (CrB 2) sintered body, further, a high bending strength and high electrical conductivity The present invention relates to a boron carbide-chromium diboride sintered body having a high fracture toughness value and a method for producing the same.
[0002]
[Prior art]
BACKGROUND ART A boron carbide sintered body is a material that is lightweight, has high hardness, and is excellent in wear resistance and corrosion resistance, and is currently used for a sandblast nozzle, a drawing die, an extrusion die, and the like. However, the boron carbide sintered body has disadvantages of low strength and low toughness. For example, the literature [K. A. Schwetz, J .; In Solid State Chemistry, 133, 177-81 (1997)], boron carbide sintered bodies are manufactured by HIP processing under various sintering conditions. However, boron carbide sintered bodies having a bending strength of 600 MPa or more are used. Not obtained. Also, the higher the bending strength, the lower the fracture toughness value tends to be. In the boron carbide sintered body having the highest strength, the fracture toughness value is a low value of 3 MPa · m 1/2 or less. Further, since the boron carbide sintered body is difficult to be sintered, it is manufactured by a hot press method or a HIP process. However, since its hardness is extremely high, it is difficult to process by a normal grinding and polishing method. Since the electrical conductivity of the boron carbide sintered body is as low as 10 to 300 S / m, there is a problem that electric discharge machining is difficult.
As described above, since the boron carbide sintered body has low strength, low toughness, and difficulty in processing, it is currently used only for extremely limited applications.
[0003]
[Problems to be solved by the invention]
Under these circumstances, the present inventors have studied and developed a new boron carbide-chromium diboride sintered body having high strength, high toughness, and high electrical conductivity in view of the above prior art. In the process of intensive progress, 5 to 25 mol% of chromium diboride is added to boron carbide powder having specific physical properties, and liquid phase sintering in which a liquid phase of chromium diboride is generated under a low pressure condition of 20 MPa or less. A new boron carbide-chromium diboride sintering with excellent properties is produced by producing a sintered body having a fine structure in which a highly conductive chromium diboride phase forms a three-dimensional network structure. They found that a body could be obtained and completed the present invention.
That is, the present invention provides a four-point bending strength of 600 MPa or more, a boron carbide-chromium diboride sintered body having a conductivity of 5 × 10 2 S / m or more, and a four-point bending strength of 600 MPa or more. And maintain a conductivity of 5 × 10 2 S / m or more, and 3. A boron carbide-chromium diboride sintered body having a fracture toughness value of 0 MPa · m 1/2 or more, and a liquid phase firing method for producing a chromium diboride liquid phase under a low pressure condition of 20 MPa or less. It is an object of the present invention to provide a method of manufacturing by performing knotting.
[0004]
[Means for Solving the Problems]
The present invention for solving the above-mentioned problems includes the following technical means.
(1) Boron carbide (B 4 C) produced by performing liquid phase sintering in which a liquid phase of chromium diboride is generated under a low pressure condition of 20 MPa or less, chromium diboride (CrB 2 ) , A boron carbide-chromium diboride sintered body containing 5 to 25 mol% of, the maximum particle diameter of boron carbide is more than 3 μm and 20 μm or less, a four-point bending strength of 600 MPa or more, and 5 × 10 2 S / Boron carbide-chromium diboride sintered body having a conductivity of at least / m.
(2) The boron carbide-chromium diboride sintered body according to the above (1), which has a fracture toughness value of 3.0 MPa · m 1/2 or more.
(3) A method for producing a boron carbide-chromium diboride sintered body by performing liquid phase sintering in which a liquid phase of chromium diboride is generated under low pressure conditions of 20 MPa or less, wherein A chromium diboride powder is added to a boron carbide powder having a diameter (D50) of 1 μm or less, a maximum particle diameter of 8 μm or less, and a specific surface area of 10 m 2 / g or more. Pressure sintering under a pressure of 20 MPa or less in a temperature range of 1950 to 2100 ° C. so that the maximum particle of boron carbide in the chromium boride sintered body is more than 3 μm and 20 μm or less. A method for producing a boron-chromium diboride sintered body.
(4) The method for producing a boron carbide-chromium diboride sintered body according to (3), wherein the aluminum content in the boron carbide powder is 1500 ppm or less.
[0005]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, the present invention will be described in more detail.
Since boron carbide is difficult to sinter, it is sintered at an extremely high temperature at which grain growth is likely to occur. However, according to the conventional technology, the obtained sintered body structure contains large particles, and is bent. This leads to deterioration in strength.
In the present invention, pressure sintering such as a hot press method is performed under specific conditions using a boron carbide powder having specific physical properties in a temperature range where a liquid phase mainly composed of CrB 2 is generated. That is, by performing liquid phase sintering in which a liquid phase of chromium diboride is generated under a low pressure condition of 20 MPa or less, the maximum particle size of boron carbide is larger than 3 μm and 20 μm or less, and the conductivity is high. By obtaining a fine structure in which the chromium diboride phase forms a three-dimensional network structure, boron carbide-diboron having a high four-point bending strength of 600 MPa or more and a conductivity of 5 × 10 2 S / m or more is obtained. A chromium oxide sintered body is produced.
[0006]
In the present invention, as the boron carbide powder, those having an average particle diameter (D50) of 1 μm or less and 0.1 μm or more and a maximum particle diameter of 8 μm or less and 1 μm or more measured by a laser diffraction scattering analyzer (Microtrac) are used. These preferably have an average particle size (D50) of 0.6 μm or less to 0.3 μm or more, and a maximum particle size of 6 μm or less to 1.5 μm or more. If the average particle size (D50) is larger than 1 μm, the sinterability is inferior, and a dense sintered body cannot be obtained in a temperature range of 1950 to 2100 ° C. A high sintering temperature is required, leading to a deterioration in bending strength. As for the specific surface area (BET), a boron carbide powder having a good sinterability of 10 m 2 / g or more and 30 m 2 / g or less is used, and preferably a powder of 15 m 2 / g or more is used.
[0007]
Further, the aluminum impurity in the boron carbide powder promotes rapid grain growth of boron carbide during sintering, so that it must be 1500 ppm or less, preferably 1000 ppm or less.
The boron carbide powder having the above-mentioned physical properties is obtained by being prepared by means such as sieving, sedimentation, and pulverization, but a commercially available product having the above-mentioned physical properties may be obtained and used.
[0008]
Chromium diboride powder is added to the boron carbide powder having the above-mentioned properties in an amount of 5 to 25 mol%. If the addition amount of chromium diboride is less than 5 mol%, a sufficient amount of chromium diboride-based liquid phase is not generated, so that a dense sintered body cannot be obtained and the effect of improving the fracture toughness value is also reduced. not enough. When the content is more than 25 mol%, the density of the sintered body is 3. It becomes higher than 0 g / cm 3 , and the light weight characteristics of the boron carbide based sintered body are impaired, and the hardness is also reduced.
Next, these are mixed to prepare a mixed powder of boron carbide and chromium diboride, which is sintered at 1950-2100 ° C. in a vacuum or an inert gas atmosphere such as Ar at a pressure of 20 MPa or less and 1 MPa or more. The liquid phase sintering is performed in a state in which a chromium diboride-based liquid phase is generated within the setting temperature range.
In the present invention, it is important to carry out liquid phase sintering in which a liquid phase of chromium diboride is generated under a low pressure condition of 20 MPa or less, whereby an excellent solid phase sintering that cannot be obtained by ordinary solid phase sintering is obtained. A boron carbide-chromium diboride sintered body having properties can be produced.
[0009]
Chromium diboride powder reacts with some boron carbide during sintering and melts, generating a chromium diboride-based liquid phase and penetrating between boron carbide particles. A raw material powder having a large particle size can be used, and a chromium diboride powder having an average particle size (D50) of 8 μm or less can be used. Preferably, a powder having an average particle size (D50) of 4 μm or less is used. Good to do.
[0010]
If the sintering temperature is lower than 1950 ° C., a sufficiently dense boron carbide sintered body cannot be produced, and a chromium diboride-based liquid phase is not generated. A network cannot be formed and the conductivity is less than 5 × 10 2 S / m, which is not preferable.
On the other hand, if the sintering temperature is higher than 2100 ° C., a fine sintered body structure cannot be obtained due to grain growth, resulting in a decrease in bending strength. When the pressure at the time of sintering is higher than 20 MPa, solid phase sintering of boron carbide particles is promoted, so that the formation of a three-dimensional network of the chromium diboride phase is inhibited, and the electrical conductivity is lowered. When the amount of the chromium diboride-based liquid phase is large, if the pressure during sintering is higher than 20 MPa, the liquid phase flows out during sintering.
[0011]
In order to obtain a boron carbide-chromium diboride sintered body having a high density, a chromium diboride-based liquid phase is generated at a temperature range of 1950 to 2100 ° C. under a pressure of 20 MPa or less and 1 MPa or more. After the pressure sintering, the two-stage sintering is performed by lowering the temperature to a temperature range of 1900 ° C. or more and less than 1950 ° C. where no chromium diboride-based liquid phase is generated and sintering at a higher pressure of 30 MPa or more. Can be appropriately performed.
[0012]
In the boron carbide-chromium diboride sintered body of the present invention, a chromium diboride phase having good conductivity forms a three-dimensional network, and boron carbide particles having a maximum particle size of more than 3 μm and 20 μm or less are formed. It has a tissue connected by a chromium boride phase. Since the thermal expansion coefficient of chromium diboride is larger than that of boron carbide, the fracture toughness value is generated by the detour of crack propagation and microclock generation near the interface between the boron carbide particles and the chromium diboride phase during the progress of fracture. Is improved. In addition, the maximum particle size is as fine as 3 μm or more and 20 μm or less, the protrusions of the boron carbide particles disappear due to the dissolution / precipitation mechanism of the chromium diboride-based liquid phase, and the stress concentration is reduced. Since the boron particles are bonded by the chromium diboride phase, the strength is improved by suppressing the falling of the boron carbide particles during processing, and the fracture toughness value is improved, and the high bending strength of 600 MPa or more is obtained. Is obtained.
[0013]
【Example】
Hereinafter, the content of the present invention will be specifically described with reference to Examples and Comparative Examples.
Examples 1-2
(1) Preparation of Boron Carbide-Chromium Diboride Sintered Body 20 mol% of chromium diboride powder having an average particle diameter (D50): 3.5 μm was added to each of boron carbide powders I and II having the physical properties shown in Table 1. After mixing, using a methanol solvent, mixing at a rotation speed of 185 rpm for 30 minutes in a planetary ball mill, drying with an evaporator, and further drying at 150 ° C. for 24 hours, the opening of 250 μm was obtained. The mixture was passed through a sieve to prepare two types of boron carbide-chromium diboride mixed powder. A graphite die was filled with a mixed powder of boron carbide and chromium diboride, molded at 7.5 MPa, and then attached to a firing furnace. 1. While pressurized to 5 MPa, use a diffusion pump. 0 × 10 −1 to 2. Heating was carried out at a heating rate of 40 ° C./min while evacuating to a pressure of 0 × 10 −2 Pa. When the temperature reached 1000 ° C., the evacuation was terminated and Ar gas was introduced, and the Ar flow rate was 2 liters / min and the gas pressure was 0.1 mL. Heated to 1500 ° C. in an atmosphere of 103 MPa. Heating was performed at a rate of 10 ° C./min from 1500 ° C. to 2050 ° C. After the temperature reached 2050 ° C., the temperature was maintained for one hour, and the first-stage hot press sintering was performed. After the holding, the temperature was lowered to 1900 ° C. at a rate of 15 ° C./min, the pressure was increased to 50 MPa, the holding was performed at this temperature for 40 minutes, and the second stage of sintering was performed. A chromium oxide sintered body was produced.
[0014]
(2) Measurement Method The four-point bending strength and the fracture toughness value of the boron carbide sintered body were measured based on JIS R1601 and JIS R1607, respectively. The surface of the test piece was finished with a surface grinder No. 400. Further, the density of the test piece was measured by the Archimedes method, and the relative density was calculated. After lapping the surface of the test piece and performing an etching treatment, SEM observation was performed to determine the maximum particle size of boron carbide. Further, the conductivity was measured.
[0015]
(3) Results The results of the evaluation are shown in Table 2. Each example has a four-point bending strength of 600 MPa or more, a fracture toughness value of 3.0 MPa · m 1/2 or more, and a conductivity of 5 × 10 2 S / m or more.
[0016]
Comparative Examples 1-2
Using boron carbide powders IV and V having the physical properties shown in Table 1, a boron carbide-chromium diboride sintered body was prepared in the same procedure as in Examples 1 and 2, and evaluated.
Table 2 shows the evaluation results. In Comparative Example 1, although a boron carbide powder having a specific surface area (BET) of 10 m 2 / g or less was used, a dense sintered body was not formed, and the flexural strength and fracture toughness were low. Further, in Comparative Example 2, although boron carbide powder having a high aluminum content was used, rapid growth of boron carbide particles occurred during sintering, the maximum particle diameter was increased, and the bending strength was reduced.
[0017]
Comparative Example 3
Using boron carbide powder I having the physical properties shown in Table 1, except that the chromium diboride powder was not blended, a boron carbide sintered body was prepared in the same procedure as in Examples 1 to 3, and evaluated. went.
Table 2 shows the evaluation results. Since a dense sintered body could not be formed, the flexural strength and fracture toughness were low.
[0018]
Examples 3 to 5
The same chromium diboride powder as in Examples 1 and 2 was used in Example 3 at 20 mol% in boron carbide powder III, in Example 4 at 20 mol% in boron carbide powder I, and in Example 5 at 22.5 mol% in boron carbide powder I. After mixing and mixing with a planetary ball mill using a methanol solvent at a rotation speed of 270 rpm for 1 hour, a boron carbide-chromium diboride mixed powder is prepared in the same procedure as in Examples 1-2. did. Sintering was performed in the same manner except that the second-stage sintering was not performed to produce a boron carbide-chromium diboride sintered body, which was evaluated.
Table 2 shows the results of the evaluation. Each example has a four-point bending strength of 600 MPa or more, a fracture toughness value of 3.0 MPa · m 1/2 or more, and a conductivity of 5 × 10 2 S / m or more.
[0019]
Comparative Example 4
A boron carbide-chromium diboride sintered body was prepared and evaluated in the same procedure as in Example 3 except that sintering was performed at 1925 ° C. under a pressure of 50 MPa using the raw material powder II. went.
Table 2 shows the results of the evaluation. Since the sintering temperature was low and a chromium diboride-based liquid phase was not generated, a dense sintered body could not be obtained, and the bending strength was low. Further, since the three-dimensional network of the chromium diboride phase was not formed, the conductivity also became a low value.
[0020]
Comparative Example 5
The same procedure as in Example 3 was carried out except that the raw material powder II was used and sintering was performed under a pressure of 50 MPa without blending the chromium diboride powder. The aggregate was produced and evaluated.
Table 2 shows the results of the evaluation. Since no chromium diboride-based liquid phase was generated, a dense sintered body could not be obtained, and the bending strength was low. Further, since the three-dimensional network of the chromium diboride phase was not formed, the conductivity also became a low value.
[0021]
[Table 1]
[Table 2]
【The invention's effect】
As described in detail above, the present invention relates to a boron carbide-chromium diboride sintered body produced by performing liquid phase sintering in a state where a chromium diboride-based liquid phase is generated, and a method for producing the same. According to the present invention, the present invention provides a four-point bending strength of 600 MPa or more, a conductivity of 5 × 10 2 S / m or more, and a boron carbide-diboride having a fracture toughness of 3.0 MPa · m 1/2 or more. A chromium sintered body can be manufactured and provided.
Further, according to the present invention, by performing liquid phase sintering in a state where a chromium diboride-based liquid phase is generated, the boron carbide particles are bonded by a three-dimensional network structure of the chromium diboride phase having good conductivity. Thus, it is possible to produce a boron carbide-chromium diboride sintered body having the obtained fine structure.
The boron carbide-chromium diboride sintered body of the present invention has high strength and toughness, has excellent mechanical properties, can be subjected to electric discharge machining, and has good workability. It can be used in various applications such as wear parts, and is industrially useful.
Claims (4)
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Free format text: JAPANESE INTERMEDIATE CODE: R350 |
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| EXPY | Cancellation because of completion of term |