JP3965246B2 - Silicon nitride sintered body, method for producing the same, and insulator material for ceramic heater - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、窒化珪素質焼結体及びその製造方法並びにセラミックヒータ用絶縁体材料に関する。更に詳しくは、本発明は、熱膨張係数が大きく、且つ優れた耐熱衝撃性を有する窒素珪素質焼結体及びその製造方法並びにセラミックヒータ用絶縁体材料に関する。本発明の窒化珪素質焼結体は、セラミックヒータ用の絶縁体材料、或いは窒化珪素よりも熱膨張率の大きい金属と組み合わせて何らかの構造部材等を構成するための窒化珪素質焼結体として有用である。
【０００２】
【従来の技術】
窒化珪素焼結体は、機械的特性、耐熱性及び耐食性等に優れているため、抵抗発熱体を埋設したセラミックヒータの絶縁体材料等として使用されている。この絶縁体材料に必要とされる特性は、高温における強度が大きいこと及び耐熱衝撃性が高いこと等である。また、抵抗発熱体として一般に用いられているタングステン、炭化タングステン及び珪化モリブデン等は、窒化珪素よりも熱膨張係数が大きく、加熱時或いは発熱時に絶縁体に亀裂を生ずる等の問題がある。そのため、絶縁体材料の熱膨張係数を抵抗発熱体のそれと同程度にまで大きくして、亀裂の発生等を抑える必要がある。
【０００３】
絶縁体材料の熱膨張係数を大きくするために、窒化珪素より熱膨張係数が大きい金属の炭化物、窒化物及び珪化物等の粒子をマトリックスに分散させる技術が知られている（特開昭６４−６１３５６号公報等）。更に、窒化珪素と珪化モリブデンによって抵抗発熱体と絶縁体との双方を作製し、それらの粒径を制御することによって、抵抗発熱体には導電性を、絶縁体には絶縁性を付与する技術が知られている（特公平５−４６６７４号公報等）。また、これらの従来の技術では、焼結性の低下を抑え、強度の大きい焼結体を得るため、マトリックスに分散させる熱膨張係数の大きい化合物は、粒径が１μｍ以下の微細な粒子の状態で用いられており、体積比で数％〜３０％程度が配合されている。
【０００４】
更に、窒化珪素焼結体はヤング率が低く、熱伝導性が小さく、耐熱衝撃性に優れているが、熱膨張係数の大きい化合物は、通常、窒化珪素よりヤング率が高いため、これらを配合することにより焼結体の耐熱衝撃性が低下する。この耐熱衝撃性の低下を抑えるため、従来より、特に、高温における焼結体の強度を大きくすることを目的として、窒化珪素原料或いは焼結助剤の組成及び粒径等の検討がなされている。しかし、優れた耐熱衝撃性を維持しつつ、熱膨張係数の大きい窒化珪素焼結体を得ることは未だ容易ではない。
【０００５】
【発明が解決しようとする課題】
本発明は、上記の問題を解決するものであり、焼結体の熱膨張係数を大きくする化合物として炭化珪素を使用し、特に、この炭化珪素の平均粒径を特定することにより、熱膨張係数が大きく、且つ優れた耐熱衝撃性がそのまま維持される窒化珪素質焼結体を提供することを目的とする。また、本発明は、特定の平均粒径を有する炭化珪素粉末等の原料粉末を、特定の温度及び圧力において、特定の時間焼成することにより、上記の優れた性能の窒化珪素質焼結体を製造する方法を提供することを目的とする。
【０００６】
【課題を解決するための手段】
熱膨張係数を大きくする化合物として、従来、焼結性及び強度の低下を抑えるため、微細な粒子からなるものを多量に配合し、分散させていた。しかし、この微粒子は焼結体の粒界に連続して存在することが多く、加熱或いは発熱の後、急冷させた場合に、焼結体の粒界において大きな収縮を生じ、これが起点となってマクロクラックが発生する傾向にあった。一方、本発明では、熱膨張係数を大きくする化合物として、従来に比べて粒径の大きい炭化珪素を使用している。このように粒径の大きい炭化珪素は、粒界において点在し、不連続であるため、加熱、発熱後の急冷による収縮は局所的であり、これがクラック発生の起点になることが抑えられる。尚、熱膨張係数を大きくする化合物として配合する炭化珪素は、高温で安定であり、その粒径等が焼成によってほとんど変化しないため、この目的において好適である。
【０００７】
第１発明は、窒化珪素粉末、希土類元素化合物粉末、クロム化合物粉末及び平均粒径３〜１０μｍの炭化珪素粉末を混合し、１７００〜１８００℃の温度、２０ＭＰａ以上の圧力で、１５〜６０分間加熱し、加圧する窒化珪素質焼結体の製造方法において、上記希土類元素化合物及び上記クロム化合物の組み合わせは、Ｅｒ ２ Ｏ ３ とＣｒＳｉ ２ との組み合わせ、Ｙ ２ Ｏ ３ とＣｒ ２ Ｏ ３ との組み合わせ、又はＹｂ ２ Ｏ ３ とＣｒ ３ Ｃ ２ との組み合わせであり、上記希土類元素化合物粉末の配合量は、上記窒化珪素粉末、上記希土類元素化合物粉末及び上記クロム化合物粉末の合計量を１００重量部とした場合に、８〜９重量部であり、上記クロム化合物粉末の配合量は、上記窒化珪素粉末、上記希土類元素化合物粉末及び上記クロム化合物粉末の合計量を１００重量部とした場合に、１〜４重量部であり、上記炭化珪素粉末の配合量は、上記窒化珪素粉末、上記希土類元素化合物粉末及び上記クロム化合物粉末の合計１００重量部に対して、５〜８重量部であることを特徴とする。
【０００８】
また、焼結体の「熱膨張係数」を「３．５×１０−６／Ｋ以上」とすることにより、セラミックヒータの絶縁体材料として用いた場合に、タングステン等の抵抗発熱体との熱膨張の差による亀裂等の発生が抑えられる。抵抗発熱体の熱膨張係数が、通常、（４．０〜４．５）×１０−６／Ｋ程度であることを考慮すると、この熱膨張係数は、特に（３．５〜４．０）×１０−６／Ｋ、更には（３．８〜４．０）×１０−６／Ｋとすることが好ましい。この範囲の熱膨張係数とすることによって、亀裂等の発生をより確実に抑えることができる。この熱膨張係数は、熱膨張の大きい希土類元素化合物、クロム化合物及び炭化珪素の配合量等によって容易に調整することができる。
【０００９】
第１発明において、上記希土類元素化合物及び上記クロム化合物の組み合わせは、上述のように、Ｅｒ２Ｏ３とＣｒＳｉ２との組み合わせ、Ｙ２Ｏ３とＣｒ２Ｏ３との組み合わせ、又はＹｂ２Ｏ３とＣｒ３Ｃ ２ との組み合わせである。
【００１０】
上記「炭化珪素」は、その結晶構造がα型であっても、β型であってもよいが、安価であるためα型のものがより好ましい。炭化珪素の平均粒径が３μｍ未満では、この炭化珪素が、窒化珪素粒子の粒界に連続的に存在し易く、これがクラック発生の起点となり、耐熱衝撃性が低下する。更に、焼成過程における窒化珪素粒子の針状化が抑制され、破壊靭性も低下する。一方、炭化珪素の平均粒径が１０μｍを越える場合は、耐熱衝撃性が低下するとともに、焼結性が低下し、緻密化が損なわれる。この炭化珪素の平均粒径は画像処理装置によって測定することができる。
【００１１】
炭化珪素を除いた窒化珪素質焼結体を１００重量部とした場合に、炭化珪素の含有量は、５〜２０重量部とすることが好ましい。この炭化珪素の含有量が５重量部未満では、焼結体の熱膨張係数を十分に大きくすることができず好ましくない。一方、炭化珪素の含有量が２０重量部を越える場合は、炭化珪素の粒子をクラックの起点にならない程度に粒界に点在させることは容易ではなく、その平均粒径を１０μｍを越えて大きくする必要があり、その場合は焼結性の低下を招くため好ましくない。また、炭化珪素の含有量を１０重量部未満とした場合は、より破壊靭性に優れた焼結体が得られ、１０重量部を越える場合はより熱膨張係数の大きい焼結体を得ることができる。このように、炭化珪素の含有量によって焼結体の熱膨張係数と破壊靭性とを所望のレベルに調整することもできる。
【００１２】
第２発明は、第１発明記載の窒化珪素質焼結体の製造方法により得られることを特徴とする窒化珪素質焼結体である。
第３発明は、第１発明記載の窒化珪素質焼結体の製造方法により得られる窒化珪素質焼結体からなることを特徴とするセラミックヒータ用絶縁体材料である。
【００１３】
焼成温度が１７００℃未満では、焼結性が低下し、十分に緻密化することができない。この温度が１８００℃を越える場合は、焼成過程においてＣｒ化合物が凝集し、強度が小さくなり、耐熱衝撃性が低下する。また、圧力が２０ＭＰａ未満では、焼結性が低下し、緻密な焼結体を得ることができず、耐熱衝撃性も低下する。この圧力を５０ＭＰａ及び通常は２０〜４０ＭＰａにすれば十分に緻密化することができる。更に、焼成時間が１５分未満では、十分に緻密化することができず、６０分を越えて長時間である場合は、Ｃｒ化合物が凝集し、強度が小さくなり、耐熱衝撃性が低下する。
【００１４】
【発明の実施の形態】
以下、実施例によって本発明をより詳しく説明する。
［１］原料粉末組成と物性との相関
実験例１〜１４
平均粒径０．７μｍ、α率９７％のＳｉ３Ｎ４粉末と、平均粒径１．０〜３．０μｍのＹ２Ｏ３、Ｅｒ２Ｏ３又はＹｂ２Ｏ３粉末と、平均粒径１．０〜３．０μｍのＣｒ２Ｏ３、Ｃｒ２Ｎ、Ｃｒ３Ｃ２又はＣｒＳｉ２粉末と、平均粒径０．１〜１６．０μｍで結晶構造がα型又はβ型の炭化珪素粉末とを、表１に示す量比で混合した。但し、炭化珪素粉末の量比は、窒化珪素粉末、希土類元素化合物粉末及びＣｒ化合物粉末の合計量を１００重量部とした場合の数値である。
【００１５】
この混合粉末を、窒化珪素製のボールミルに投入し、エタノールを加えて１６時間湿式混合した。その後、湯煎乾燥し、得られた粉末を温度１８００℃、圧力２５ＭＰａで、３０分間ホットプレスして焼成し、３５×３５×５ｍｍの形状の焼結体を得た。
【００１６】
【表１】
上記のようにして得られた焼結体の相対密度、熱膨張係数、熱衝撃抵抗温度及び破壊靭性値（ＫＩＣ）を測定した。結果を表２に示す。尚、これらの物性値の測定方法は以下のとおりである。また、必要に応じて上記の焼結体を切削加工して試験片とした。
（１）相対密度；アルキメデス法により密度を測定し、混合則により算出した理論密度によって除して算出した。
（２）熱衝撃抵抗温度（耐熱衝撃性）；φ３．５×３０ｍｍの試験片を水中急冷法によって処理し、蛍光探傷試験によりクラックが発生する温度を測定した。尚、表２において「クラック発生せず」とは、用いた装置の測定可能な最高温度である９００℃においてもクラックが発生せず、９００℃を越える高い熱衝撃抵抗温度を有する耐熱衝撃性に優れた焼結体であることを意味する。
【００１８】
（３）熱膨張係数；窒素雰囲気下、１５（長さ）×４（幅）×３（厚さ）ｍｍの試験片を３０℃から１３５０℃にまで昇温させ、前記の式によって求められる熱膨張係数を算出した。
（４）破壊靭性値；ＪＩＳ Ｒ １６０７に従って測定した。
【００１９】
【表２】
表２の結果によれば、実験例１〜９では、相対密度は９８％以上であり、十分に緻密化されている。また、優れた耐熱衝撃性と大きな熱膨張係数とを併せ有する焼結体が得られていることが分かる。一方、炭化珪素の平均粒径が第１発明の下限値未満である実験例１０及び下限値を大きく下回る実験例１４では、耐熱衝撃性が大きく低下し、実験例１４では、破壊靭性も相当に劣っている。また、この平均粒径が第１発明の上限値を越える実験例１１でも、耐熱衝撃性が低下するとともに緻密化も十分ではないことが分かる。更に、炭化珪素の含有量が第１発明の下限値未満である実験例１２では、熱膨張係数が十分に向上せず、この含有量が第１発明の上限値を越える実験例１３では、緻密化が十分ではなく、耐熱衝撃性も低下する傾向にある。
【００２１】
［２］炭化珪素の平均粒径と焼結体の微細構造との相関
実験例５（炭化珪素の平均粒径；４．７μｍ）と、従来のように炭化珪素の微粒子を含有する実験例１４（炭化珪素の平均粒径；０．１μｍ）の焼結体について、組織の微細構造を走査型電子顕微鏡によって観察し、写真撮影をした。図１は実験例５の焼結体の写真であり、図２は実験例１４の焼結体の写真である。
【００２２】
図１では、従来に比べて粒径の大きい炭化珪素が含有されているため、この炭化珪素の粒子が窒化珪素粒子の粒界に点在し、また、窒化珪素粒子は針状の粗大粒子となっていることが分かる。このような構造であるため、加熱、発熱後に急冷された場合に、炭化珪素粒子の収縮が局所的となり、マクロクラックの発生が抑えられ、耐熱衝撃性が向上する。一方、図２では、炭化珪素粒子が微細であるため、この炭化珪素粒子が窒化珪素粒子の粒界に連続して存在しており、窒化珪素粒子の針状化が抑制されている。このような構造であるため、加熱、発熱後に急冷された場合に、炭化珪素粒子により構成される部分の収縮が大きくなり、これが破壊の起点となってマクロクラックが発生し、耐熱衝撃性が低下する。
【００２３】
［３］焼成の温度、圧力及び時間と、得られる焼結体の物性との相関
実験例１５〜２０
平均粒径０．７μｍ、α率９７％のＳｉ３Ｎ４粉末、平均粒径１．２μｍのＹｂ２Ｏ３粉末、及び平均粒径１．５μｍのＣｒ２Ｏ３粉末を使用し、これらの粉末の全量を１００重量部とした場合に、Ｓｉ３Ｎ４が８５重量部、Ｙｂ２Ｏ３が１４重量部、Ｃｒ２Ｏ３が１重量部となるように配合し、これにさらに平均粒径４．７μｍのＳｉＣ粉末を１０重量部配合した。
【００２４】
この混合粉末を、窒化珪素製のボールミルに投入し、エタノールを加えて１６時間湿式混合した。その後、湯煎乾燥し、得られた混合粉末を内容積３５×３５×５ｍｍのカーボン型に収容し、表３の条件によってホットプレスし、焼成した。
【００２５】
【表３】
得られた焼結体の相対密度、曲げ強さ、熱膨張係数及び熱衝撃抵抗温度を測定した。結果を表４に示す。相対密度、熱膨張係数及び熱衝撃抵抗温度の測定方法は前記のとおりである。また、曲げ強さは以下のようにして測定した。
（５）曲げ強さ（室温において測定した３点曲げ強さ）；焼結体を切削して試験片を作製し、ＪｌＳ Ｒ １６０１に従って３点曲げ強度を測定した。尚、試験片の上下面の粗さをＪｌＳ Ｂ ０６０１に規定された０．８Ｓ以下としたものを使用して測定した。
【００２７】
【表４】
表４の結果によれば、実験例１５〜１７では、相対密度は９９％以上であり、十分に緻密化されている。また、優れた耐熱衝撃性と大きい熱膨張係数とを併せ有する焼結体が得られていることが分かる。一方、時間が上限値を大きく越えている実験例１８では、緻密化は十分であるものの、耐熱衝撃性及び曲げ強さはより低下していることが分かる。更に、温度が下限値未満である実験例１９では、緻密性が低下し、耐熱衝撃性及び曲げ強さも大きく低下する。また、温度が上限値を越えている実験例２０では、緻密化は十分であるものの、耐熱衝撃性及び曲げ強さはさらに大きく低下していることが分かる。
【００２９】
尚、本発明においては、上記の具体的な実施例に示すものに限られず、目的、用途に応じて本発明の範囲内で種々変更した実施例とすることができる。即ち、希土類元素化合物、Ｃr化合物の他にも、焼結体の熱膨張係数を大きくする作用を有する化合物を、炭化珪素と組み合わせて、焼結体の緻密化等が損なわれない範囲の量比で配合し、含有させることができる。
【００３０】
【発明の効果】
第１発明によれば、熱膨張係数を大きくするための化合物として炭化珪素等を使用し、この炭化珪素の平均粒径を特定することにより、十分に緻密化され、特に、熱膨張係数が大きく、且つ優れた耐熱衝撃性をも有する窒化珪素質焼結体を得ることができる。この窒化珪素質焼結体は、抵抗発熱体を埋設したセラミックヒータ等の絶縁体材料として有用であり、抵抗発熱体との熱膨張係数の差による亀裂の発生等が十分に抑えられる。
【００３１】
また、第１発明によれば、平均粒径が比較的大きい炭化珪素粉末を用い、焼成の温度、圧力及び時間を特定することにより、優れた特性を有する窒化珪素質焼結体を容易に製造することができる。
【図面の簡単な説明】
【図１】 実験例５の窒化珪素質焼結体の表面の走査型電子顕微鏡写真である。
【図２】 実験例１４の窒化珪素質焼結体の表面の走査型電子顕微鏡写真である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a silicon nitride sintered body, a method for manufacturing the same, and an insulator material for a ceramic heater. More specifically, the present invention relates to a silicon nitride sintered body having a large thermal expansion coefficient and excellent thermal shock resistance, a method for producing the same, and an insulator material for a ceramic heater. The silicon nitride-based sintered body of the present invention is useful as an insulating material for ceramic heaters or a silicon nitride-based sintered body for constituting any structural member in combination with a metal having a higher thermal expansion coefficient than silicon nitride. It is.
[0002]
[Prior art]
Since silicon nitride sintered bodies are excellent in mechanical properties, heat resistance, corrosion resistance, and the like, they are used as insulator materials for ceramic heaters in which resistance heating elements are embedded. The characteristics required for this insulator material are high strength at high temperatures and high thermal shock resistance. In addition, tungsten, tungsten carbide, molybdenum silicide, and the like, which are generally used as resistance heating elements, have a larger coefficient of thermal expansion than silicon nitride, and have problems such as causing cracks in the insulator during heating or heat generation. Therefore, it is necessary to increase the thermal expansion coefficient of the insulator material to the same level as that of the resistance heating element to suppress the occurrence of cracks.
[0003]
In order to increase the thermal expansion coefficient of an insulator material, a technique is known in which particles of metal carbide, nitride, silicide, etc. having a larger thermal expansion coefficient than silicon nitride are dispersed in a matrix (Japanese Patent Laid-Open No. Sho 64-64). 61356 publication etc.). Furthermore, both resistance heating elements and insulators are made of silicon nitride and molybdenum silicide, and the particle size is controlled to provide conductivity to resistance heating elements and insulation to insulators. Is known (Japanese Patent Publication No. 5-46674, etc.). Further, in these conventional techniques, in order to suppress a decrease in sinterability and obtain a sintered body having a high strength, a compound having a large thermal expansion coefficient dispersed in a matrix is in a state of fine particles having a particle size of 1 μm or less. And about several to 30% is blended by volume ratio.
[0004]
Furthermore, the sintered silicon nitride has a low Young's modulus, low thermal conductivity, and excellent thermal shock resistance. However, compounds with a large thermal expansion coefficient usually have a higher Young's modulus than silicon nitride, so these are blended together. By doing so, the thermal shock resistance of the sintered body is lowered. In order to suppress this decrease in thermal shock resistance, studies have been made on the composition, particle size, etc. of silicon nitride raw materials or sintering aids for the purpose of increasing the strength of sintered bodies at high temperatures. . However, it is not yet easy to obtain a silicon nitride sintered body having a large thermal expansion coefficient while maintaining excellent thermal shock resistance.
[0005]
[Problems to be solved by the invention]
The present invention solves the above problem, and uses silicon carbide as a compound that increases the thermal expansion coefficient of the sintered body. In particular, by specifying the average particle diameter of this silicon carbide, the thermal expansion coefficient An object of the present invention is to provide a silicon nitride sintered body that is large and that maintains excellent thermal shock resistance as it is. In addition, the present invention provides a silicon nitride sintered body having excellent performance described above by firing a raw material powder such as silicon carbide powder having a specific average particle diameter at a specific temperature and pressure for a specific time. The object is to provide a method of manufacturing.
[0006]
[Means for Solving the Problems]
As a compound that increases the coefficient of thermal expansion, conventionally, a large amount of fine particles have been blended and dispersed in order to suppress a decrease in sinterability and strength. However, these fine particles are often present continuously at the grain boundaries of the sintered body, and when heated or heated and then rapidly cooled, they cause large shrinkage at the grain boundaries of the sintered body, which is the starting point. There was a tendency for macro cracks to occur. On the other hand, in the present invention, silicon carbide having a particle size larger than that of the conventional one is used as the compound for increasing the thermal expansion coefficient. Since silicon carbide having such a large particle size is scattered at the grain boundaries and is discontinuous, shrinkage due to rapid cooling after heating and heat generation is local, and it is suppressed that this becomes a starting point of crack generation. Silicon carbide blended as a compound that increases the coefficient of thermal expansion is suitable for this purpose because it is stable at high temperatures and its particle size and the like hardly change by firing.
[0007]
The first invention is a mixture of silicon nitride powder, rare earth element compound powder, chromium compound powder and silicon carbide powder having an average particle size of 3 to 10 μm, and is heated for 15 to 60 minutes at a temperature of 1700 to 1800 ° C. and a pressure of 20 MPa or more. the combination of and, in the manufacturing method of pressurizing the silicon nitride sintered body, a combination of the rare earth element compound and the chromium compound, combination of Er 2 O 3 and CrSi 2, and Y 2 O 3 and Cr 2 O 3 Or a combination of Yb 2 O 3 and Cr 3 C 2, and the amount of the rare earth element compound powder is 100 parts by weight of the total amount of the silicon nitride powder, the rare earth element compound powder, and the chromium compound powder. 8 to 9 parts by weight, and the compounding amount of the chromium compound powder is the silicon nitride powder, the rare earth element compound powder, and the chromization. When the total amount of the product powder is 100 parts by weight, it is 1 to 4 parts by weight, and the amount of the silicon carbide powder is 100 weights in total of the silicon nitride powder, the rare earth element compound powder, and the chromium compound powder. 5 to 8 parts by weight with respect to parts.
[0008]
In addition, by setting the “thermal expansion coefficient” of the sintered body to “3.5 × 10 −6 / K or more”, when it is used as an insulator material for a ceramic heater, Generation of cracks and the like due to the difference in expansion is suppressed. Considering that the thermal expansion coefficient of the resistance heating element is normally about (4.0 to 4.5) × 10 −6 / K, this thermal expansion coefficient is particularly (3.5 to 4.0). × 10 −6 / K, more preferably (3.8 to 4.0) × 10 −6 / K. By setting the thermal expansion coefficient within this range, the occurrence of cracks and the like can be suppressed more reliably. This thermal expansion coefficient can be easily adjusted by the blending amount of the rare earth element compound, the chromium compound and the silicon carbide having a large thermal expansion.
[0009]
In the first invention, the combination of the rare earth element compound and the chromium compound is, as described above, a combination of Er 2 O 3 and CrSi 2 , a combination of Y 2 O 3 and Cr 2 O 3 , or Yb 2 O. 3 and Cr 3 C 2 .
[0010]
The “silicon carbide” may have an α-type or β-type crystal structure, but an α-type is more preferable because it is inexpensive. When the average particle size of silicon carbide is less than 3 μm, this silicon carbide tends to be continuously present at the grain boundaries of the silicon nitride particles, which becomes the starting point of crack generation, and the thermal shock resistance decreases. Furthermore, acicularization of the silicon nitride particles during the firing process is suppressed, and the fracture toughness is also reduced. On the other hand, when the average particle diameter of silicon carbide exceeds 10 μm, the thermal shock resistance is lowered, the sinterability is lowered, and the densification is impaired. The average particle diameter of the silicon carbide can be measured by an image processing apparatus.
[0011]
When the silicon nitride sintered body excluding silicon carbide is 100 parts by weight, the silicon carbide content is preferably 5 to 20 parts by weight. If the silicon carbide content is less than 5 parts by weight, the thermal expansion coefficient of the sintered body cannot be sufficiently increased, which is not preferable. On the other hand, when the content of silicon carbide exceeds 20 parts by weight, it is not easy to make the silicon carbide particles interspersed at the grain boundaries to the extent that they do not become the starting point of cracks, and the average particle size exceeds 10 μm. In such a case, the sinterability is deteriorated, which is not preferable. Moreover, when the content of silicon carbide is less than 10 parts by weight, a sintered body with more excellent fracture toughness can be obtained, and when it exceeds 10 parts by weight, a sintered body with a larger thermal expansion coefficient can be obtained. it can. Thus, the thermal expansion coefficient and fracture toughness of the sintered body can be adjusted to a desired level depending on the silicon carbide content.
[0012]
The second invention is a silicon nitride sintered body obtained by the method for producing a silicon nitride sintered body according to the first invention.
A third invention is an insulator material for a ceramic heater, characterized by comprising a silicon nitride sintered body obtained by the method for producing a silicon nitride sintered body according to the first invention.
[0013]
When the firing temperature is less than 1700 ° C., the sinterability is lowered, and it cannot be sufficiently densified. When this temperature exceeds 1800 ° C., Cr compounds are aggregated in the firing process, the strength is reduced, and the thermal shock resistance is lowered. On the other hand, when the pressure is less than 20 MPa, the sinterability is lowered, a dense sintered body cannot be obtained, and the thermal shock resistance is also lowered. If this pressure is 50 MPa and usually 20 to 40 MPa, it can be sufficiently densified. Furthermore, if the firing time is less than 15 minutes, it cannot be sufficiently densified, and if it exceeds 60 minutes for a long time, the Cr compound aggregates, the strength decreases, and the thermal shock resistance decreases.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to examples.
[1] Correlation between raw material powder composition and physical properties Experimental Examples 1-14
Si 3 N 4 powder having an average particle size of 0.7 μm and an α rate of 97%, Y 2 O 3 , Er 2 O 3 or Yb 2 O 3 powder having an average particle size of 1.0 to 3.0 μm, and an average particle size 1.0 to 3.0 μm Cr 2 O 3 , Cr 2 N, Cr 3 C 2 or CrSi 2 powder and silicon carbide powder having an average particle size of 0.1 to 16.0 μm and an α-type or β-type crystal structure Were mixed in the quantitative ratio shown in Table 1. However, the amount ratio of the silicon carbide powder is a numerical value when the total amount of the silicon nitride powder, the rare earth element compound powder and the Cr compound powder is 100 parts by weight.
[0015]
This mixed powder was put into a ball mill made of silicon nitride, ethanol was added and wet mixed for 16 hours. Thereafter, the bath was dried in hot water, and the obtained powder was hot-pressed and fired at a temperature of 1800 ° C. and a pressure of 25 MPa for 30 minutes to obtain a sintered body having a shape of 35 × 35 × 5 mm.
[0016]
[Table 1]
The relative density, thermal expansion coefficient, thermal shock resistance temperature, and fracture toughness value (K IC ) of the sintered body obtained as described above were measured. The results are shown in Table 2. In addition, the measuring method of these physical property values is as follows. Moreover, said sintered compact was cut as needed and it was set as the test piece.
(1) Relative density: Calculated by measuring the density by Archimedes method and dividing by the theoretical density calculated by the mixing rule.
(2) Thermal shock resistance temperature (thermal shock resistance): A test piece of φ3.5 × 30 mm was treated by an underwater quenching method, and a temperature at which a crack was generated was measured by a fluorescent flaw detection test. In Table 2, “no cracks” means that no cracks occur even at 900 ° C., which is the highest measurable temperature of the apparatus used, and the thermal shock resistance has a high thermal shock resistance temperature exceeding 900 ° C. It means that it is an excellent sintered body.
[0018]
(3) Coefficient of thermal expansion: heat obtained from the above equation by heating a test piece of 15 (length) × 4 (width) × 3 (thickness) mm from 30 ° C. to 1350 ° C. in a nitrogen atmosphere. The expansion coefficient was calculated.
(4) Fracture toughness value: measured according to JIS R 1607.
[0019]
[Table 2]
According to the results of Table 2, in Experimental Examples 1 to 9, the relative density is 98% or more, which is sufficiently densified. Moreover, it turns out that the sintered compact which has the outstanding thermal shock resistance and a big thermal expansion coefficient is obtained. On the other hand, in Experimental Example 10 in which the average particle diameter of silicon carbide is less than the lower limit of the first invention and Experimental Example 14 in which the average particle diameter is much lower than the lower limit, the thermal shock resistance is greatly reduced, and in Experimental Example 14, the fracture toughness is considerably high. Inferior. It can also be seen that even in Experimental Example 11 where the average particle size exceeds the upper limit of the first invention, the thermal shock resistance is lowered and the densification is not sufficient. Furthermore, in Experimental Example 12 in which the content of silicon carbide is less than the lower limit of the first invention, the thermal expansion coefficient does not sufficiently improve, and in Experimental Example 13 in which the content exceeds the upper limit of the first invention, However, the thermal shock resistance tends to decrease.
[0021]
[2] Correlation between average particle size of silicon carbide and microstructure of sintered body Experimental example 5 (average particle size of silicon carbide; 4.7 μm) and experimental example 14 containing silicon carbide fine particles as in the past Regarding the sintered body (average particle diameter of silicon carbide; 0.1 μm), the microstructure of the structure was observed with a scanning electron microscope and photographed. FIG. 1 is a photograph of the sintered body of Experimental Example 5, and FIG. 2 is a photograph of the sintered body of Experimental Example 14.
[0022]
In FIG. 1, since silicon carbide having a larger particle size than conventional ones is contained, the silicon carbide particles are scattered at the grain boundaries of the silicon nitride particles, and the silicon nitride particles are needle-like coarse particles. You can see that Due to such a structure, when quenched after heating and heat generation, the silicon carbide particles shrink locally, the occurrence of macro cracks is suppressed, and the thermal shock resistance is improved. On the other hand, in FIG. 2, since the silicon carbide particles are fine, the silicon carbide particles are continuously present at the grain boundaries of the silicon nitride particles, and the needle-like formation of the silicon nitride particles is suppressed. Because of this structure, when it is rapidly cooled after heating and heat generation, the shrinkage of the part composed of silicon carbide particles becomes large, which causes a macro crack as a starting point of destruction, and the thermal shock resistance decreases. To do.
[0023]
[3] baking temperature, the pressure and time, correlation experimental example of the physical properties of the resulting sintered body 15 to 2 0
Si 3 N 4 powder having an average particle size of 0.7 μm, α rate of 97%, Yb 2 O 3 powder having an average particle size of 1.2 μm, and Cr 2 O 3 powder having an average particle size of 1.5 μm were used. When the total amount of powder is 100 parts by weight, it is blended so that Si 3 N 4 is 85 parts by weight, Yb 2 O 3 is 14 parts by weight, and Cr 2 O 3 is 1 part by weight. 10 parts by weight of SiC powder having a diameter of 4.7 μm was blended.
[0024]
This mixed powder was put into a ball mill made of silicon nitride, ethanol was added and wet mixed for 16 hours. Thereafter, the mixture was dried in hot water, and the obtained mixed powder was placed in a carbon mold having an internal volume of 35 × 35 × 5 mm, hot-pressed and fired under the conditions shown in Table 3.
[0025]
[Table 3]
The relative density, bending strength, thermal expansion coefficient, and thermal shock resistance temperature of the obtained sintered body were measured. The results are shown in Table 4. The methods for measuring the relative density, thermal expansion coefficient, and thermal shock resistance temperature are as described above. The bending strength was measured as follows.
(5) Bending strength (3-point bending strength measured at room temperature): The sintered body was cut to prepare a test piece, and the 3-point bending strength was measured according to JlS R 1601. In addition, it measured using what made the roughness of the upper and lower surfaces of a test piece 0.8S or less prescribed | regulated to JlS B 0601.
[0027]
[Table 4]
According to the results of Table 4, in Experimental Examples 15 to 17, the relative density is 99% or more, which is sufficiently densified. It can also be seen that a sintered body having both excellent thermal shock resistance and a large thermal expansion coefficient is obtained. On the other hand, in Experimental Example 18 in which the time greatly exceeds the upper limit value, it can be seen that although the densification is sufficient, the thermal shock resistance and the bending strength are further reduced. Further, in Experimental Example 19 in which the temperature is lower than the lower limit value, the denseness is lowered, and the thermal shock resistance and the bending strength are greatly reduced. In Experimental Example 20 in which the temperature exceeds the upper limit value, it can be seen that although the densification is sufficient, the thermal shock resistance and the bending strength are further greatly reduced.
[0029]
In addition, in this invention, it can restrict to what is shown to said specific Example, It can be set as the Example variously changed within the range of this invention according to the objective and the use. That is, in addition to the rare earth element compound and the Cr compound, a compound having an effect of increasing the thermal expansion coefficient of the sintered body is combined with silicon carbide so that the densification of the sintered body is not impaired. It can mix | blend and contain.
[0030]
【The invention's effect】
According to the first invention, silicon carbide or the like is used as a compound for increasing the thermal expansion coefficient, and by specifying the average particle diameter of the silicon carbide, the silicon carbide is sufficiently densified, and particularly has a large thermal expansion coefficient. In addition, a silicon nitride sintered body having excellent thermal shock resistance can be obtained. This silicon nitride-based sintered body is useful as an insulating material such as a ceramic heater in which a resistance heating element is embedded, and generation of cracks due to a difference in thermal expansion coefficient from the resistance heating element is sufficiently suppressed.
[0031]
In addition, according to the first invention, by using silicon carbide powder having a relatively large average particle diameter and specifying the firing temperature, pressure and time, a silicon nitride-based sintered body having excellent characteristics can be easily produced. can do.
[Brief description of the drawings]
1 is a scanning electron micrograph of the surface of a silicon nitride sintered body of Experimental Example 5. FIG.
2 is a scanning electron micrograph of the surface of a silicon nitride-based sintered body of Experimental Example 14. FIG.
Claims (3)
上記希土類元素化合物及び上記クロム化合物の組み合わせは、Ｅｒ２Ｏ３とＣｒＳｉ２との組み合わせ、Ｙ２Ｏ３とＣｒ２Ｏ３との組み合わせ、又はＹｂ２Ｏ３とＣｒ３Ｃ ２ との組み合わせであり、
上記希土類元素化合物粉末の配合量は、上記窒化珪素粉末、上記希土類元素化合物粉末及び上記クロム化合物粉末の合計量を１００重量部とした場合に、８〜９重量部であり、
上記クロム化合物粉末の配合量は、上記窒化珪素粉末、上記希土類元素化合物粉末及び上記クロム化合物粉末の合計量を１００重量部とした場合に、１〜４重量部であり、
上記炭化珪素粉末の配合量は、上記窒化珪素粉末、上記希土類元素化合物粉末及び上記クロム化合物粉末の合計１００重量部に対して、５〜８重量部であることを特徴とする窒化珪素質焼結体の製造方法。Nitriding in which silicon nitride powder, rare earth element compound powder, chromium compound powder and silicon carbide powder having an average particle size of 3 to 10 μm are mixed, heated at a temperature of 1700 to 1800 ° C. and a pressure of 20 MPa or more for 15 to 60 minutes, and pressurized. In the method for producing a silicon-based sintered body,
A combination of the rare earth element compound and the chromium compound is a combination of Er 2 O 3 and CrSi 2 , a combination of Y 2 O 3 and Cr 2 O 3 , or a combination of Yb 2 O 3 and Cr 3 C 2. Yes,
The amount of the rare earth element compound powder is 8 to 9 parts by weight when the total amount of the silicon nitride powder, the rare earth element compound powder and the chromium compound powder is 100 parts by weight,
The blending amount of the chromium compound powder is 1 to 4 parts by weight when the total amount of the silicon nitride powder, the rare earth element compound powder and the chromium compound powder is 100 parts by weight,
The silicon nitride powder is mixed in an amount of 5 to 8 parts by weight with respect to 100 parts by weight as a total of the silicon nitride powder, the rare earth element compound powder and the chromium compound powder. Body manufacturing method.
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