JP4567853B2 - Sintered silicon nitride - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、適度な電気抵抗値を有する窒化珪素焼結体に関する。
【０００２】
【従来の技術】
近年、ハードディスクドライブ（ＨＤＤ）等の磁気記録装置、光ディスク装置またはＤＶＤ、モバイル製品、各種ゲーム機器などの発達は目覚しいものがある。これらは通常、スピンドルモータ等の回転駆動装置により回転軸を高速回転させることにより各種ディスクドライブを機能させている。
従来、このような回転軸を支えるベアリング（軸受）部材、特にベアリングボールには軸受鋼等の金属が用いられていた。しかしながら、軸受鋼等の金属は耐摩耗性が十分ではないことから、例えば前記電子機器等のように5,000rpm以上の高速回転が要求される分野においては寿命のバラツキが大きく信頼性のある回転駆動を提供できずにいた。
このような不具合を解決するために近年はベアリングボールに窒化珪素を用いることが試みられるようになっていた。窒化珪素はセラミックスの中でも摺動特性に優れることから耐摩耗性は十分であり、高速回転を行ったとしても信頼性のある回転駆動を提供することができていることが確認されている。
【０００３】
【発明が解決しようとする課題】
しかしながら、窒化珪素製ベアリングボールは電気的に絶縁物であることから高速回転を行った際に発生する静電気を軸受鋼等の金属部材により作製された回転軸部、ボール受け部（いわゆるベアリングボール以外のベアリング部材の構成要素）に上手く静電気が発散されないと言った問題が発生してしまうことが分かった。
このように静電気が上手く発散されず必要以上に帯電してしまうと電子機器、例えばハードディスクドライブ等のように磁気的信号を用いる記録媒体に悪影響を与えてしまい、その結果ハードディスク等の電子機器そのものを破壊してしまうと言った現象が起きていた。
さらに、ハードディスクドライブの小型化、高容量化に伴い回転数も8,000rpm、さらには10,000rpm以上とさらなる高速回転が要求されている。このような高速回転が行われるとベアリングボールは摺動により加熱される。このとき従来の窒化珪素製ベアリングボールでは熱伝導率が20W/m・k程度と低く摩擦熱を上手く発散できずにいた。この放熱性の観点は高速回転になればなるほど問題となり、特に高速回転を長時間行うこと対しての対応は十分ではなかった。
【０００４】
一方、従来から電気抵抗値が10-3Ω・cm程度を示す低電気抵抗の窒化珪素焼結体は存在している。このような窒化珪素焼結体は主に切削工具などに使われているが、低電気抵抗を実現するために炭化物などの導電性付与粒子を多量に添加させねばならない。多量の導電性付与粒子を添加した窒化珪素焼結体は確かに電気抵抗値は下がるものの、多量に添加された導電性付与粒子同士が凝集し易く、凝集粒子が窒化珪素焼結体中に多く分散され易くなってしまう。
例えば、ベアリングボールのように常に全体から圧縮荷重を受けるような用途においては、このような凝集粒子が多数あるとそこから亀裂が入り易く摺動特性が劣化してしまう。従って、ベアリングボールのように全体から圧縮荷重を受けながら使用されるものにおいては凝集粒子があまり多くない方が好ましい。
本発明は上記したような問題を解決するためになされたものであって、所定の電気抵抗値を有し、導電性付与粒子の分散状態を制御した導電性を有する窒化珪素焼結体を提供することを目的とする。
さらにこのような導電性を有する窒化珪素焼結体を、ハードディスク等の電子機器用摺動部材、例えばベアリングボールに適用することにより必要以上に静電気が帯電することを防止することができる。また、熱伝導率が40W/m・k以上であるため摺動の際の熱を効率よく発散できることから電子機器用摺動部材に適している。従って、本発明においては導電性を有する窒化珪素焼結体を用いた摺動部材並びにベアリングボールを提供することも目的とする。
【０００５】
【課題を解決するための手段】
本発明では上記目的を為し得るために、窒化珪素焼結体中に存在する導電性付与粒子の分散状態を特定している。具体的には、窒化珪素焼結体中に導電性付与粒子として炭化物粒子と窒化物粒子を分散含有し、分散した導電性付与粒子同士の粒子間距離が１μｍ未満である導電性付与粒子の凝集部が単位面積当たりの面積率で30%以下であり、電気抵抗値が107〜102Ω・cmである窒化珪素焼結体である。
また、該凝集部同士の距離が２〜１０μｍであることが好ましい。また、導電性付与粒子の凝集部を形成していない導電性付与粒子同士の粒子関距離は１〜１０μｍであることが好ましい。
該炭化物粒子は4a族，5a族，6a族，7a族元素、珪素、硼素の炭化物の少なくとも１種以上からなることが好ましく、該窒化物粒子は4a族の窒化物の少なくとも１種であることが好ましい。さらに、導電性付与粒子の凝集部の最大径が５μｍ以下であることが好ましく、また、熱伝導率が40W/m・k以上であることが好ましい。
このような窒化珪素焼結体を、摺動部材、例えばベアリングボールに適用すると特に効果的である。特に電子機器用の摺動部材、例えばハードディスクドライブ等の電子機器の回転駆動に適用するベアリングボールの場合、回転駆動に伴い発生する静電気を必要以上に帯電することを防止できると共に、熱伝導率が高いことから放熱性も優れている。
【０００６】
【発明の実施の形態】
以下、本発明の実施の形態について説明する。
本発明の窒化珪素焼結体は、導電性付与粒子として炭化物粒子と窒化物粒子を含有し、その導電性付与粒子の分散状態に関し、導電性付与粒子同士の粒子間距離が１μｍ未満である導電性付与粒子の凝集部が単位面積当たり面積率で30%以下としている。
【０００７】
この導電性付与粒子の凝集部の面積率の求め方は、まず、焼結体表面もしくは断面を鏡面（表面粗さRaで0.01μｍ以下）に加工し、その表面における単位面積30μｍ×30μｍ（もしくはそれ以上の面積）をＥＰＭＡによるカラーマップを撮り、そのカラーマップに対して単位面積内に存在する導電性付与粒子の凝集部のすべての面積をカウントする方法が有効である。本発明では、このような方法を単位面積当たり任意の４ヶ所以上行いその平均値により求めることが好ましい。なお、炭化物粒子と窒化物粒子を同時にＥＰＭＡで分析できないときは、それぞれを分析したものを合成して面積率を求める方法で対応可能である。
カラーマップについては倍率2000倍（50μｍを10cmで表示）以上が好ましく、この程度もしくはそれ以上の倍率において窒化珪素焼結体の表面（もしくは断面）の鏡面部を観察した場合、単位面積中の導電性付与粒子の凝集部の面積を求める上での判断のバラツキが小さくなる。また、その窒化珪素焼結体中の凝集した導電性付与粒子の面積を求める上で単位面積30μｍ×30μｍあれば凝集した導電性付与粒子の面積の測定誤差が少ないことから本発明では単位面積30μｍ×30μｍを適用した。
【０００８】
また、窒化珪素焼結体中の単位面積30μｍ×30μｍ中の導電性付与粒子の面積の測定場所については後述する均一混合を用いているのであれば導電性付与粒子が均一混合されていることから簡易的に表面１ヶ所のみの測定であっても問題はないが、通常、焼結体の表面または断面の少なくとも計４ヶ所について任意の場所を、各測定個所のカラーマップに対して30μｍ×30μｍに相当する面積中の凝集部の面積率を測定し、その平均値で示すことが好ましい。
なお、ＥＰＭＡカラーマップにて判断するときベアリングボールのように球面状をカラーマップにとるとカラーマップの端部が湾曲して写るため正確に表面の凝集した導電性付与粒子の存在状態を示さないことが考えられるが単位面積30μｍ×30μｍのように微小な範囲を撮影する上ではこの問題は考慮しなくても実質的に問題はない。このような観点からも単位面積は30μｍ×30μｍ程度が好ましい。
【０００９】
本発明においては、導電性付与粒子同士の粒子間距離が１μｍ未満である導電性付与粒子の凝集部が単位面積当たり面積率で30%以下であることが好ましく、さらに好ましくは2〜10%である。
摺動特性のみを考慮すると導電性付与粒子の凝集部は存在しない（面積率で０％）ことが好ましいが、凝集部が全く存在しない形態では電気抵抗値のバラツキが大きいことが判明した。
【００１０】
窒化珪素焼結体を構成する窒化珪素結晶粒子は前述のように絶縁体であることから通常、電気抵抗値は1010Ω・cm以上である。そのため本発明では電気抵抗値を所定の値にするために導電性付与粒子を添加している。電気抵抗値を下げることのみに着目すれば導電性付与粒子を添加するだけで十分であるが、例えば電子機器用のベアリングボールに適用した場合、個々のベアリングボールの電気抵抗値にバラツキが存在すると静電気の帯電防止効果にバラツキが生じてしまう。静電気は基本的に電気抵抗値の高いところ（絶縁性の高いところ）に帯電することから、静電気の帯電防止効果にバラツキが生じてしまうとその中で最も電気抵抗値の高いところに静電気が集中してしまい電子機器に静電気による不具合を生じてしまうこともある。このような現象は回転数が5,000rpm程度ではさほど問題とはならないが、回転数が9,000rpm以上の高速回転になると少しずつ確認され始めている。特に、静電気の帯電による電子機器への不具合は瞬間的な帯電量によっても影響されるためベアリングボールのように複数のボールを組合せて使用する摺動部材においては個々のベアリングボールの電気抵抗値のバラツキを無くすことは重要なことである。
【００１１】
従って、本発明では導電性付与粒子の凝集部をあえて具備させることにより個々の窒化珪素焼結体の電気抵抗値のバラツキを改善したものである。導電性付与粒子の凝集部の面積率は単位面積当たり30%以下、好ましくは2〜10%であり、このような形態にすることにより電気抵抗値のバラツキを±15%／100個に抑えることができる。
【００１２】
導電性付与粒子同士の粒子間距離が１μｍ未満である導電性付与粒子の凝集部の面積率が２％未満である場合は、凝集部の割合が少ないため電気抵抗値のバラツキが大きくなる。例えば、凝集部が少ない場合では窒化珪素焼結体の電気抵抗値があまり下がらず、目的の電気抵抗値より大きくなってしまうことが多い。前述のように窒化珪素焼結体は絶縁体であるから電気抵抗値を所定の値にするためには導電性付与粒子の存在は必要である。しかしながら、導電性付与粒子同士の距離があまり離れていると電気抵抗値を下げる効果が十分ではなくなる。そのため、一部の導電性付与粒子を凝集させることにより電気抵抗値を所定の値の範囲内に安定させる（バラツキをなくす）ことが可能となる。
一方、導電性付与粒子同士の粒子間距離が１μｍ未満である導電性付与粒子の凝集部の面積率が３０％を超える場合は、確かに電気抵抗値のバラツキは小さくなるものの、窒化珪素焼結体中の凝集部が多いことから凝集部が破壊起点となってしまい強度の低下を招く。また、ベアリングボールなどの摺動部材においては転がり寿命を低下させてしまう。
【００１３】
また、凝集部が所定の面積率あれば電気抵抗値のバラツキを抑えることが可能であるが、凝集部同士の距離があまり近くなると見掛け上凝集部が大きくなりすぎ、見掛け上大きな凝集部が破壊起点となってしまい摺動特性を劣化させてしまう。そのため凝集部同士の距離は2〜10μｍであることが好ましい。凝集部同士の距離が２μｍ未満では、見掛け上、凝集部が必要以上に大きくなってしまい易く後述する凝集部の最大径が５μｍを超え易い。一方、凝集部同士の距離が１０μｍを超えてしまうと凝集部が存在する効果が薄くなり電気抵抗値のバラツキが大きくなり易い。
なお、凝集部を形成する導電性付与粒子は炭化物粒子のみ、窒化物粒子のみであってもよいし、炭化物粒子と窒化物粒子の両方が混在した状態であっても良い。
【００１４】
また、凝集していない導電性付与粒子についてもあまり導電性付与粒子同士の粒子間距離が離れてしまうと導電性付与効果が小さくなってしまうことから、粒子間距離は１〜１０μｍの範囲内であることが好ましい。特に、凝集していない導電性付与粒子の粒子間距離は３〜１０μｍの範囲であることが好ましく、このような範囲であれば窒化珪素焼結体の本来持つ摺動特性を劣化させないで済むと共に、前述の凝集部の面積率と組合せると電気抵抗値のバラツキを±10%／100個とさらに向上させることが可能となる。つまり、本発明では窒化珪素焼結体中の導電性付与粒子同士の粒子間距離に所定の形態を具備させることにより電気抵抗値のバラツキを抑制したものである。
【００１５】
以上のように、導電性付与粒子同士の粒子間距離が１μｍ未満である導電性付与粒子の凝集部が単位面積当たりの面積率で30％以下が良く、好ましくは２〜１０％である。このような導電性付与粒子の分散状態を具備する窒化珪素焼結体は電気抵抗値を107〜102Ω・cmとなると共に、電気抵抗値のバラツキを±15%／100個に抑えることができる。また、凝集していない導電性付与粒子同士の粒子間距離を制御することによりさらに電気抵抗値のバラツキを抑えることが可能となる。
さらに、本発明の窒化珪素焼結体は窒化珪素焼結体の持つ耐摩耗性や強度の良さをいかせることから摺動部材に適しており、特に電子機器用摺動部材、例えば電子機器用ベアリングボールに用いることにより回転駆動に伴う静電気を効率よく発散でき必要以上に帯電することを抑制することができると共に、優れた摺動特性を示すことが可能となる。
【００１６】
また、導電性付与粒子の粒子間距離が１μｍ未満である導電性付与粒子の凝集部が所定の面積率を具備していたとしても導電性付与粒子の凝集部があまり大きいと窒化珪素焼結体の強度並びに耐摩耗性（摺動特性）を低下させてしまう。そのため導電性付与粒子同士の凝集部の最大径は５μｍ以下、さらには３μｍ以下であることが好ましい。なお、本発明において導電性付与粒子同士の凝集部とは、導電性付与粒子が直接接触しているもの（粒子間距離０μｍ）および導電性付与粒子同士の粒子間距離が１μｍ未満のものを示すものとする。
【００１７】
次に、導電性付与粒子の材質について説明する。導電性付与粒子の材質は窒化珪素焼結体の電気抵抗値を下げることができる炭化物および窒化物であれば特に限定されるものではないが、好ましくは炭化物粒子は4a族，5a族，6a族，7a族元素、珪素、硼素の炭化物の少なくとも１種以上からなる化合物であり、さらに好ましくはタンタル、チタン、ニオブ、タングステン、珪素、硼素の炭化物の少なくとも１種以上である。また、窒化物粒子は4a族元素の窒化物の少なくとも１種である。
【００１８】
本発明の窒化珪素焼結体は、例えばベアリングボールなどの摺動部材に使用されるため含有する導電性付与粒子も当然ながら窒化珪素焼結体と共に摺動される。
このため、導電性付与粒子にもある程度の摺動特性は要求されることから前述の炭化物が好適である。該炭化物は摺動特性が優れているだけでなく、熱伝導性にも優れていることから窒化珪素焼結体の熱伝導率を40W/m・k以上にし易い。
【００１９】
また、窒化物粒子は4a族元素の窒化物が好ましく、特に好ましくは窒化チタンである。4a族元素の窒化物は導電性付与効果のみではなく焼結助剤としての効果も得られることから好ましく、特に窒化チタンはその効果が顕著であることから好ましい。さらに4a族元素の窒化物を分散含有させるときに、4a族元素の酸化物を含有させ、焼結時に窒化物へと析出させるとより焼結性を向上させることができる。
【００２０】
炭化物粒子および窒化物粒子の含有量は所定量の凝集部を含有しているのであれば特に限定されるものではないが、炭化物粒子は10〜35wt%、窒化物粒子は0.1〜5wt%である。前述のように炭化物粒子は摺動特性に優れていることから10〜35%含有させても窒化珪素焼結体の強度や摺動特性を必要以上に低下させることはないが、窒化物粒子自体は比較的強度が弱く脆性材料であることから5wt%を超えて含有させると窒化珪素焼結体の強度および摺動特性を低下させてしまう。
【００２１】
窒化珪素焼結体中に存在する導電性付与粒子の最大径2μｍ以下、好ましくは0.3〜1.2μｍである。本発明の導電性付与粒子の最大径とは個々の導電性付与粒子のサイズであり、窒化珪素焼結体の表面鏡面部のＥＰＭＡにおけるカラーマップを見たときの導電性付与粒子粒子の最も長い対角線を最大径とする。また、導電性付与粒子同士の凝集部の最大径についても同様の方法にて測定するものとする。
なお、導電性付与粒子同士の粒子間距離が１μｍ未満の導電性付与粒子の凝集部の面積を求める上では、凝集部中の多数の粒子を一つ一つをカウントするものとする。また、凝集部が実質的に円形である場合には、面積＝π×（最大径／２）2により求めることも可能であるが、現在では画像処理により求める方法が効果的である。
【００２２】
また、炭化物粒子の平均粒径と窒化物粒子の平均粒径を比較した場合、炭化物粒子の平均粒径≦窒化物粒子の平均粒径であることが好ましい。具体的には、炭化物粒子の平均粒径は0.3〜1μｍ、窒化物粒子の平均粒径は1〜2μｍであることが好ましい。炭化物粒子は窒化物粒子と比べて多く含有させることから凝集部を形成しやすいため、粒径を窒化物粒子より小さくすることにより凝集部の最大径を5μｍ以下にし易くなり窒化珪素焼結体の強度や摺動特性を向上させることができる。
【００２３】
このような形態を示す窒化珪素焼結体の電気抵抗値が107〜102Ω・ｃｍである。
本発明の窒化珪素焼結体は特に用途が限定されるものではないが、ハードディスクドライブなどの電子機器を回転駆動させるためのモータ機器に具備される摺動部材、例えばベアリングボールに用いることが最適である。
このとき電気抵抗値が107Ω・cmを超えるようであるとベアリングボールの摺動時に発生する静電気の帯電を効率よく防ぐことが難しく、逆に102Ω・cm未満であると静電気の帯電を防ぐことは可能であるものの窒化珪素焼結体中に導電性付与粒子が大量に添加されている状態となり易くなるため窒化珪素焼結体が本来持つ耐摩耗性や強度の良さを十分いかせなくなるのであまり好ましくはない。
【００２４】
また、本発明の窒化珪素焼結体は導電性付与粒子を添加していることから熱伝導率40W/m・k以上と向上させることができる。本発明の窒化珪素焼結体は、主として電子機器用摺動部材に用いるものである。電子機器は、例えば半導体装置用基板を見て分かる通り、熱の問題は非情に重要である。このため、電子機器用の摺動部材であっても放熱性に優れていることは重要である。特に、ハードディスク等の電子機器の回転駆動に用いるベアリングボールを熱伝導率が40W/m・k以上と放熱性に優れた本発明の窒化珪素焼結体で形成すると、前述の静電気の帯電を防止するだけでなく、回転駆動に伴う摩擦熱をも効率よく発散できるようになり、静電気の帯電防止および放熱性の両方の効果を得ることができる。
【００２５】
ベアリング部材の場合、回転軸およびボール受け部は軸受鋼等の金属部材で形成されていることが多く、摺動時の熱による変形等の問題は起き易い。特に電子機器においては回転速度が8,000rpm以上、さらには10,000rpm以上と高速回転化していく傾向にあり、従来より放熱性の問題は起き易くなっている。従って、熱伝導率の高い本発明の窒化珪素焼結体を用いたベアリングボールは電子機器に適しており、特に回転軸およびボール受け部が軸受鋼等の金属部材からなるベアリング部材に最適であると言える。
【００２６】
さらに、ベアリングボールの直径が3ｍｍ以下、さらには2ｍｍ以下であることが好ましい。本発明の窒化珪素焼結体は、熱伝導率が40W/m・k以上と高いが、回転軸等を構成する金属部材と比較すると熱伝導率という観点は劣ってしまう。そのため、放熱性という観点では窒化珪素製ベアリングボールは熱抵抗体となってしまうことから、直径が3ｍｍ以下、さらには2ｍｍ以下と小さくすることによりベアリング部材としての熱抵抗を下げることができる。
【００２７】
なお、ここまでは主に導電性付与粒子について説明してきたが本発明においては他の成分、例えば焼結助剤を添加してよいことは言うまでもない。焼結助剤としては一般的に使用されているものでよく、酸化イットリウム等の希土類化合物、酸化マグネシウム等の金属酸化物が好適である。また、酸化アルミニウムや窒化アルミニウム等のアルミニウム化合物などを併用してもよい。添加量としては特に限定されるものではないが3〜20wt%が好ましい。
【００２８】
次に製造方法について説明する。製造方法は導電性付与粒子の分散状態に関し、導電性付与粒子の粒子間距離が１μｍ未満である導電性付与粒子の凝集部が面積率で３０％以下である窒化珪素焼結体を得られるのであれば特に限定されるものではないが、例えば次のような方法がある。
【００２９】
まず、窒化珪素粉末、焼結助剤、導電性付与粒子粉末を所定量均一混合した後、造粒、成形、脱脂、焼結する方法である。
特に、導電性付与粒子粉末が必要以上に凝集することを防ぐことが重要である。
導電性付与粒子の凝集が必要以上に起きると導電性付与粒子の凝集部が面積率で３０％以上の部分ができ易くなると共に導電性付与粒子同士の凝集部の最大径が5μｍを超え易くなってしまう。
【００３０】
そのため例えば、凝集部を形成するための導電性付与粒子を予め造粒し、凝集部の最大径が5μｍ以下になるよう造粒粉を形成し、所定の面積率を満たすように添加した後、凝集させない導電性付与粒子を別途添加混合する方法がある。
また、添加混合する際に、更なる凝集部を形成させないようにするために例えば次のような方法が有効である。まず、１ロット分の原料粉末を混合するにあたり、各原料粉末をそれぞれ２分割以上、好ましくは３〜５分割して比較的少量ずつ混合したものを最終的に１つに混ぜ合わせる方法である。
１ロット分で導電性付与粒子粉末の凝集部が必要以上に存在しない混合粉末が得られれば特に問題ではないが、このような場合において凝集部の少ない均一混合を行おうとすると混合時間が必要以上に長くなってしまうことが多く、必ずしも製造性が良いとは言えない。また、１度に大量に各原料粉末を混ぜ合わせると最終的な窒化珪素焼結体となったときに導電性付与粒子が凝集し凝集部が面積率で３０％以上の場所ができ易くなると共に凝集部の最大径が5μｍを超え易い。
【００３１】
別の方法では、まず窒化珪素粉末および焼結助剤を混合する。その混合粉の中に導電性付与粒子粉末を添加する際、添加する凝集部を形成させないための導電性付与粒子粉末を数回に分けて添加する方法が有効である。例えば、導電性付与粒子粉末の添加量を２分割以上、好ましくは３〜５分割し、１回目の添加を行い所定時間経過した後（30分以上が好ましい）２回目以降を順に添加する方法である。導電性付与粒子粉末を少量ずつ添加混合することにより導電性付与粒子粉末同士の更なる凝集を防ぐことが可能となり導電性付与粒子が面積率で３０％以下かつ導電性付与粒子の凝集部の最大径が5μｍ以下である窒化珪素焼結体を得易い。
【００３２】
このような方法によって原料粉末を均一混合すれば導電性付与粒子粉末同士が必要以上に凝集することを抑えることができるので、仮に凝集部が存在したとしても窒化珪素焼結体中での導電性付与粒子の凝集部の最大径を5μｍ以下、好ましくは3μｍ以下にすることが可能となる。特に、直径3ｍｍ以下、さらには2ｍｍ以下の小型ベアリングボールを作製する場合は、導電性付与粒子の凝集部を必要以上に形成させないことが重要である。ベアリングボールが小型になればなるほど凝集部の影響を受け易いためである。
【００３３】
各原料粉末の大きさは特に限定されるものではないが、窒化珪素粉末の平均粒径は0.2〜3μｍ、焼結助剤は平均粒径2μｍ以下が好ましい。
また、導電性付与粒子粉末のサイズは平均粒径3μｍ以下、好ましくは0.1〜1.2μｍである。導電性付与粒子が0.1μｍ未満であるとベアリングボールに適用した場合、表面加工時または摺動時に表面から脱粒し易くなる。一方、3μｍを超えるとわずかな凝集だけで最大径が5μｍを超えてしまうので好ましくない。さらには前述の最大径を制御し易いように平均粒径のバラツキが少ない例えば標準偏差1.5μｍ以下の粉末を用いることが好ましい。
【００３４】
さらにベアリングボールとしての摺動特性を損なわないためには前記サイズを満たしていたとしても導電性付与粒子粉末としてウイスカーや繊維を用いることは好ましくはなく、粒子状粉末を用いることが望ましい。ウイスカーや繊維は、その形状から表面にトゲのような凸部を有しておりベアリングボールの表面にこのようなものが存在していた場合耐摩耗性を劣化させてしまう。
【００３５】
成形方法については、窒化珪素製焼結体またはベアリングボールを製造するための方法が適用可能である。従って、通常の成形方法や静水圧成形（ＣＩＰ）などが適用可能であり、ベアリングボールを製造する際は静水圧成形が好適である。
【００３６】
焼結方法についても窒化珪素製焼結体並びにベアリングボールを製造するための方法が適用可能である。従って、常圧焼結、加圧焼結、熱間静水圧プレス（ＨＩＰ）焼結が適用可能であり、ベアリングボールを製造する際は常圧焼結または加圧焼結を行った後にＨＩＰ焼結を行うことが好ましい。
以上のような工程を経た後、ベアリングボールとして使用する場合はJIS規格で定められた表面粗さを得るための表面研磨加工を施す。
【００３７】
【実施例】
（実施例１〜４、比較例１〜２、参考例１）
導電性付与粒子粉末として平均粒径0.7μｍ（標準偏差1.3μｍ以下）の炭化珪素粉末を20wt%、平均粒径0.9μｍ（標準偏差1.5μｍ以下）の酸化チタン粉末を1wt%、焼結助剤として平均粒径0.8μｍの酸化イットリウム粉末を5wt%、平均粒径0.9μｍの酸化アルミニウム粉末を4wt%、残部平均粒径0.7μｍの窒化珪素粉末を用意した。各原料粉を混合する前に導電性付与粒子同士の凝集部を形成するための炭化珪素粉末を予め凝集部の最大径が2μｍ以下になるよう造粒し、各原料粉末をそれぞれ3分割して混合して3つの混合粉末を得た後に、この3つの混合粉末を合せて混合して混合原料粉末を製造することにより導電性付与粒子粉末の凝集部が所定量含有した混合原料粉末を用意した。
この混合原料粉末をCIP法により成形し、不活性雰囲気中1600〜1900℃常圧焼結、続いて1600〜1900℃の温度でHIP焼結を行い表１に示した窒化珪素焼結体を作製した。
なお、各実施例はサイズ3×4×40mmの四角柱状の試料とし、さらにJIS規格で認定されたベアリングボールのグレード３に相当する表面研磨加工を施したものとする。また、凝集部を形成していない導電性付与粒子同士の粒子間距離はいずれも３〜１０μｍの範囲内であり、凝集部同士の距離は２〜１０μｍの範囲内であった。
【００３８】
このような各実施例に対し、電気抵抗値、電気抵抗値のバラツキ、３点曲げ強度（室温）、熱伝導率を測定した結果を併せて表１に示した。電気抵抗値は各試料の上下をラップ加工し同一平面上に２ヶ所電極を設置し、室温にてその間の抵抗を絶縁抵抗計で測定した。熱伝導率は試料を3×3×10mmに追加加工したものを用いレーザーフレッシュ法により測定した。各測定においては各実施例にかかる試料を100個用意し、その平均値にて示した。また、電気抵抗値のバラツキについては平均値に対して最も差の大きかった電気抵抗値を平均値に対する差としてパーセント（％）で表示した。
なお、各測定値において、本実施例では便宜的に試料形状を四角柱状としたが、例えば真球状のベアリングボールについて各特性を測定する場合でも同様にラップ加工を施すことにより対応可能である。
また、各窒化珪素焼結体中の導電性付与粒子の凝集部の面積率の測定は、各試料を表面粗さRaが0.01μｍ以下まで研磨加工を施し研磨面の表面の任意の４ヶ所（単位面積30μｍ×30μｍに相当する任意の面積）を選び、各測定個所のカラーマップ（倍率2000倍）を使用した。
【００３９】
比較のために導電性付与粒子を一度に過量添加することにより凝集部の面積率を本発明の範囲外としたものを比較例１として用意した。また、導電性付与粒子を添加しないこと以外は実施例と同様の窒化珪素焼結体を比較例２とした。参考例１として導電性付与粒子の凝集部が少ないものを用意した。
【００４０】
【表１】
表１から分かる通り、本発明の窒化珪素焼結体は電気抵抗値が107〜102Ω・cmの範囲において３点曲げ強度は1000MPa以上、熱伝導率は40W/m・k以上であることが分かった。
それに対して、比較例１は導電性付与粒子の凝集部の割合が多いため電気抵抗値のバラツキは小さいものの強度は低下してしまった。一方、導電性付与粒子を添加しない比較例２は電気抵抗値が1010Ω・cm以上であり、熱伝導性も悪かった。
また、参考例１で示した通り、導電性付与粒子の凝集部の面積率が０．３％のものは電気抵抗値のバラツキが大きくなってしまった。
なお、実施例１〜４の窒化珪素焼結体中の導電性付与粒子の凝集部の最大径はいずれも3μｍ以下であった。それに対し、一度に過量に添加した比較例１は凝集部が10μｍ以上となっている個所が複数発見されており、強度低下の原因となったと考えられる。
このような電気抵抗値等の特性を持つ窒化珪素焼結体は後述するハードディスクドライブ等の電子機器用ベアリングボールに用いると静電気による不具合を無くすことが可能となる。
【００４２】
（実施例５〜８、比較例４〜６、参考例２）
次に、実施例１と同様の製造工程により電気抵抗値および導電性付与粒子の凝集部の割合を変えた窒化珪素焼結体からなる直径2mmのベアリングボールを作製した。各ベアリングボールは表面研磨をグレード３のものとした。
各ベアリングボールをハードディスクドライブを回転駆動させるためのスピンドルモータのベアリング部材に１０個一組にして組込んだ。なお、その他のベアリング部材として、軸受鋼SUJ2製の回転軸部並びにボール受け部を用いた。
該モータを回転速度8,000rpmと11,000rpmで200時間連続稼動させたときの静電気による不具合の有無を調べた。静電気による不具合とは、200時間の連続稼動後にハードディスクドライブが通常通り可動するか否かにより判定した。なお、各静電気による不具合の有無はハードディスクドライブを各100台用意し測定を行った。
比較のために導電性付与粒子の凝集部の面積比が本発明の範囲外のものを比較例４、導電性付与粒子を含有させないものを比較例５、電気抵抗値を小さくしたものを比較例６、導電性付与粒子の凝集部割合が本発明の好ましい範囲を外れたものを参考例２として同様の測定を行った。その結果を表２に示す。
【００４３】
【表２】
表２から分かる通り、本実施例にかかるベアリングボールを用いたものは静電気による不具合がないことが分かった。それに対し、比較例５は電気抵抗値が本発明より非情に高いことから静電気による不具合を発祥してしまった（１００台中１〜３台）。
また、比較例４および比較例６は静電気による不具合は発生しなかったが、ベアリングボールの強度が不十分であることから200時間後のベアリングボールには若干の破損が確認され、あまり長時間の稼動には向かないことが確認された。
これは凝集部の割合が多すぎたことおよび凝集部が多いために凝集部の最大径が5μｍを超えてしまったためであると考えられる。
また、参考例２のものは8,000rpm程度の回転速度では静電気による不具合は確認されなかったが、11,000rpmではハードディスクドライブが完全に停止していないものの若干の不具合を示すもの（１００台中１台）が確認されたので「ややあり」と表記した。これは、電気抵抗値のバラツキが大きいため電気抵抗値の最も大きなベアリングボールに静電気が瞬間的に集中してしまったためであると考えられる。
【００４５】
（実施例９〜１３、比較例７〜９）
次に、実施例５〜８および比較例４〜６のベアリングボールを用いベアリングボールの転がり寿命の測定を行った。なお、本実施例にかかるベアリングボールは導電性付与粒子の凝集部の最大径はいずれも5μｍ以下であった。また、比較例４のベアリングボールを用いた比較例７の導電性付与粒子の凝集部の最大径は9μｍであり、比較例６のベアリングボールを用いた比較例９の導電性付与粒子の凝集部の最大径は23μｍであった。
転がり寿命の測定に関しては、スラスト型軸受試験機を用い、相手材としてSUJ2鋼製の平板上を回転させる方法で荷重は一球あたり最大接触応力5.9GPa、回転数1200rpm、タービン油の油浴潤滑条件下で最高400時間まで行いベアリングボールの表面が剥離するまでの時間を測定した。その結果を表３に示す。
【００４６】
【表３】
表３から分かる通り、本実施例にかかるベアリングボールにおいて導電性付与粒子の凝集部の面積率が本発明の範囲内のものは導電性付与粒子のを添加していない比較例８と同等の優れた転がり寿命を示すことが分かった。
それに対し、比較例７および比較例９のように導電性付与粒子の凝集部の面積率が30％を超えて50%程度になると摺動特性は劣化することが分かった。これは、結果として窒化珪素マトリックス中に導電性付与粒子が多くなりすぎてしまい窒化珪素焼結体の持つ摺動特性の良さをいかせなくなってしまっためであると言える。また、導電性付与粒子の凝集部の最大径が5μｍを超えているため凝集部が破壊起点となってしまったものと考える。
【００４８】
（実施例１３〜１４、参考例４）
導電性付与粒子の凝集部の最大径の影響を調べるため、該凝集部の最大径を変えた以外は実施例７と同様のベアリングボールを用意した。各ベアリングボールに対し、実施例１１と同様の転がり寿命試験を行った。また、併せて圧砕強度、３点曲げ強度（室温）の測定も行った。
圧砕強度の測定は、旧JIS規格B1501に準じた測定法により、インストロン型試験機で圧縮加重をかけ、破壊時の荷重を測定することにより対応した。その結果を表４に示す。
【００４９】
【表４】
表４から分かる通り、導電性付与粒子の凝集部の最大径が５μｍ以下のものは転がり寿命に優れ、かつ圧砕強度も220MPa以上と優れた特性を示すことが分かった。
それに対し、本発明の好ましい範囲を外れている参考例４のものは導電性付与粒子の凝集部の面積率が本発明の範囲内であるにも関わらず各特性が劣化することが分かった。これは導電性付与粒子の凝集部の最大径が大きすぎるためこの凝集部が破壊起点になってしまったためであると考えられる。
言い換えると、導電性付与粒子同士の粒子間距離が１μｍ未満である導電性付与粒子の凝集部の面積が本発明の範囲内であっても導電性付与粒子同士の凝集部の最大径が5μｍを超えるようなものは、ベアリングボールに適したものとは言えないと言える。
【００５１】
（実施例１５〜１６）
導電性付与粒子粉末として平均粒径0.8μｍ以下（標準偏差1.5μｍ以下）の炭化珪素粉末、平均粒径1.0μｍ（標準偏差1.5μｍ以下）の酸化チタン粉末、焼結助剤として平均粒径1.5μｍ以下の酸化イットリウム粉末を5wt%、平均粒径0.8μｍ以下の酸化アルミニウム粉末を3wt%、残部を平均粒径0.5μｍの窒化珪素粉末を用意した。
まず、導電性付与粒子の中で凝集部を形成させる分量について最大径3μｍ以下になるように造粒を行い凝集造粒粉を用意する。
次に、実施例１５として窒化珪素粉末と焼結助剤粉末を混合し、所定量の炭化珪素粉末を３回に分割して１時間間隔を空けて添加混合し、最後に凝集造粒粉を所定量添加混合することにより混合原料粉末を作製した。
【００５２】
実施例１６として、各原料粉末を３分割し、それぞれ混合した後、全体を混ぜ合わせ、最後に凝集造粒粉を添加混合した混合原料粉末を用意した。参考例５として、一度に全ての原料粉末を混合した混合原料粉末を用意した。
この各混合原料粉末をＣＩＰ法により成形し、不活性雰囲気中1740℃常圧焼結、続いて1000気圧1700℃でＨＩＰ焼結を行い直径２ｍｍの窒化珪素製ベアリングボールおよび3×4×40ｍｍの四角柱状の試料を作製した。
このような各試料を100個ずつ作製し、導電性付与粒子の凝集部の面積率および凝集部の最大径を測定した。凝集部の最大径は任意の30μｍ×30μｍを４ヶ所測定し、その中にあった最も大きな凝集部の最大径を示した。その結果を表５に示す。
【００５３】
【表５】
表５から分かる通り、実施例１５または実施例１６の添加混合方法によれば本発明の好ましい形態を具備する窒化珪素焼結体を作製できることが分かった。
それに対し、参考例５では導電性付与粒子の凝集部が10〜20μｍと大きな凝集部ができてしまった。このような窒化珪素焼結体では、強度が低下すると共に転がり寿命も低下してしまうことは前述の実施例の通りである。
【００５５】
（実施例１７〜２６）
次に、導電性付与粒子を表６にある材質に変える以外は実施例２と同一の窒化珪素焼結体を作製した。作製した各窒化珪素焼結体に対し、実施例２と同様の測定を行った。
【００５６】
【表６】
表６から分かる通り、導電性付与粒子の材質を変えたとしても電気抵抗値、３点曲げ強度、熱伝導率はいずれもすぐれた特性を示すことが分かった。
【００５８】
（実施例２７〜４２）
実施例１７〜２６の窒化珪素焼結体を用いた以外は実施例１０と同じベアリングボールを作製し、実施例１３と同様の方法により圧砕強度および転がり寿命特性を測定した。
測定した結果、いずれのべアリングボールも圧砕強度は210MPa以上、転がり寿命は400時間以上と優れた特性を示すことが分かった。
以上のことから本発明の窒化珪素および摺動部材においては導電性付与粒子の材質を変えたとしても優れた特性を示すと言える。
【００５９】
【発明の効果】
以上のように本発明の窒化珪素焼結体は、導電性付与粒子として炭化物粒子および窒化物粒子を含み、該導電性付与粒子同士の粒子間距離が１μｍ未満である導電性付与粒子の凝集部の面積を特定することにより、所定の電気抵抗値を有するためハードディスクドライブ等の電子機器の摺動部材、例えば回転駆動させるためのモータに搭載するベアリング部材のベアリングボールに用いた場合、回転駆動に伴う静電気の帯電を防止することが可能となる。
また、導電性付与粒子として炭化物等を用いることにより焼結体自体の熱伝導率を向上させることができるため回転駆動に伴う摩擦熱を効率よく放熱することも可能となる。さらに電気抵抗値のバラツキを抑えていることから、回転速度が8000rpm以上、さらには10000rpm以上と高速回転を行ったとしても静電気による不具合の発生を効率的に抑制することができる。
さらに、導電性付与粒子同士の凝集を防ぐことにより摺動特性等を向上させることができる。
このような形態にすれば窒化珪素焼結体からなるベアリングボールは窒化珪素が持つ摺動特性のよさを必要以上に低減させずに済み、ハードディスクドライブなどの電子機器に用いた場合、優れた摺動特性を示す。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a silicon nitride sintered body having an appropriate electric resistance value.About.
[0002]
[Prior art]
In recent years, there have been remarkable developments in magnetic recording devices such as hard disk drives (HDD), optical disk devices or DVDs, mobile products, and various game machines. These usually function various disk drives by rotating a rotating shaft at a high speed by a rotary drive device such as a spindle motor.
Conventionally, metals such as bearing steel have been used for bearing members that support such rotating shafts, particularly for bearing balls. However, since metals such as bearing steel are not sufficiently wear-resistant, for example, in the fields where high-speed rotation of 5,000 rpm or more is required, such as the above-mentioned electronic devices, the rotational drive has a large variation in life and is reliable. Could not provide.
In order to solve such problems, attempts have been made in recent years to use silicon nitride for bearing balls. Since silicon nitride is excellent in sliding characteristics among ceramics, it has been confirmed that it has sufficient wear resistance and can provide a reliable rotational drive even if it is rotated at high speed.
[0003]
[Problems to be solved by the invention]
However, since a silicon nitride bearing ball is an electrically insulating material, static electricity generated during high-speed rotation is generated by a rotating shaft portion and ball receiving portion (other than a so-called bearing ball) made of a metal member such as bearing steel. It has been found that the problem that static electricity is not dissipated well in the component of the bearing member of the above-mentioned bearing occurs.
Thus, if static electricity is not dissipated well and it is charged more than necessary, electronic devices such as hard disk drives will adversely affect recording media that use magnetic signals. There was a phenomenon that said it would be destroyed.
Furthermore, along with the miniaturization and increase in capacity of hard disk drives, the rotation speed is also 8,000 rpm, and even higher speeds of 10,000 rpm or more are required. When such high speed rotation is performed, the bearing ball is heated by sliding. At this time, the conventional silicon nitride bearing ball has a low thermal conductivity of about 20 W / m · k and could not dissipate frictional heat well. This viewpoint of heat dissipation becomes more problematic as the rotation speed becomes higher, and the response to the high speed rotation for a long time is not sufficient.
[0004]
On the other hand, the electrical resistance value has been 10-3There is a silicon nitride sintered body having a low electrical resistance that exhibits about Ω · cm. Such a silicon nitride sintered body is mainly used for cutting tools and the like, but in order to realize low electric resistance, a large amount of conductivity imparting particles such as carbides must be added. Although the silicon nitride sintered body to which a large amount of conductivity-imparting particles have been added certainly reduces the electrical resistance value, the conductivity-imparting particles to which a large amount has been added tend to aggregate together, and there are many aggregated particles in the silicon nitride sintered body. It becomes easy to be dispersed.
For example, in applications such as bearing balls that always receive a compressive load from the whole, if there are many such agglomerated particles, cracks are liable to occur from them and the sliding characteristics deteriorate. Therefore, it is preferable that the aggregated particles are not so many in a bearing ball that is used while receiving a compressive load from the whole.
The present invention has been made in order to solve the above-described problems, and provides a silicon nitride sintered body having a predetermined electric resistance value and having conductivity in which the dispersion state of the conductivity imparting particles is controlled. The purpose is to do.
Further, by applying such a conductive silicon nitride sintered body to a sliding member for an electronic device such as a hard disk, for example, a bearing ball, it is possible to prevent static electricity from being charged more than necessary. Further, since the heat conductivity is 40 W / m · k or more, the heat at the time of sliding can be efficiently dissipated, so that it is suitable for a sliding member for electronic equipment. Accordingly, an object of the present invention is to provide a sliding member and a bearing ball using a conductive silicon nitride sintered body.
[0005]
[Means for Solving the Problems]
In the present invention, in order to achieve the above object, the dispersion state of the conductivity-imparting particles present in the silicon nitride sintered body is specified. Specifically, carbide particles and nitride particles are dispersed and contained as conductivity-imparting particles in the silicon nitride sintered body, and the agglomeration of conductivity-imparting particles in which the distance between the dispersed conductivity-imparting particles is less than 1 μm. The area ratio is 30% or less per unit area, and the electric resistance value is 107~Ten2This is a silicon nitride sintered body of Ω · cm.
Moreover, it is preferable that the distance of this aggregation part is 2-10 micrometers. Moreover, it is preferable that the particle | interval distance of the electroconductivity provision particle | grains which are not forming the aggregation part of electroconductivity provision particle | grains is 1-10 micrometers.
Preferably, the carbide particles are composed of at least one kind of carbides of Group 4a, Group 5a, Group 6a, Group 7a, silicon, boron, and the nitride particles are at least one kind of nitride of Group 4a. Is preferred. Furthermore, the maximum diameter of the aggregated portion of the conductivity imparting particles is preferably 5 μm or less, and the thermal conductivity is preferably 40 W / m · k or more.
Such a silicon nitride sintered body is particularly effective when applied to a sliding member such as a bearing ball. In particular, in the case of a bearing ball that is applied to the rotation drive of electronic devices such as a sliding member for an electronic device, such as a hard disk drive, it is possible to prevent the static electricity generated by the rotation drive from being charged more than necessary and the thermal conductivity is reduced. Because of its high temperature, it has excellent heat dissipation.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
The silicon nitride sintered body of the present invention contains carbide particles and nitride particles as the conductivity-imparting particles, and the conductivity between the conductivity-imparting particles is less than 1 μm with respect to the dispersion state of the conductivity-imparting particles. The aggregation part of the property-imparting particles is 30% or less in terms of the area ratio per unit area.
[0007]
The method for determining the area ratio of the agglomerated portion of the conductivity-imparting particles is as follows. First, the sintered body surface or cross section is processed into a mirror surface (surface roughness Ra is 0.01 μm or less), and the unit area 30 μm × 30 μm (or the surface) It is effective to take a color map by EPMA over an area larger than that and count the total area of the aggregated portions of the conductivity-imparting particles existing within the unit area with respect to the color map. In the present invention, it is preferable to obtain such an average value by performing four or more arbitrary methods per unit area. When carbide particles and nitride particles cannot be analyzed by EPMA at the same time, it can be dealt with by a method of determining the area ratio by synthesizing those analyzed.
For the color map, a magnification of 2000 times (50 μm is displayed in 10 cm) or more is preferable. When the mirror surface portion of the surface (or cross section) of the silicon nitride sintered body is observed at this or higher magnification, the conductivity in the unit area Variation in determination in determining the area of the aggregation portion of the property-imparting particles is reduced. Further, in determining the area of aggregated conductivity-imparting particles in the silicon nitride sintered body, if the unit area is 30 μm × 30 μm, the measurement error of the area of the aggregated conductivity-imparting particles is small, so in the present invention, the unit area is 30 μm. × 30 μm was applied.
[0008]
In addition, as for the measurement place of the area of the conductivity imparting particles in the unit area 30 μm × 30 μm in the silicon nitride sintered body, if the uniform mixing described later is used, the conductivity imparting particles are uniformly mixed. There is no problem even if it is simple to measure only one surface, but usually at least 4 locations on the surface or cross section of the sintered body are placed at 30μm × 30μm for each measurement location color map. It is preferable to measure the area ratio of the agglomerated portion in the area corresponding to, and to show the average value.
In addition, when judging from the EPMA color map, if the spherical shape is taken as a color map like a bearing ball, the end of the color map is curved so that it does not accurately indicate the presence state of the conductivity-imparting particles aggregated on the surface. However, in photographing a minute range such as a unit area of 30 μm × 30 μm, there is substantially no problem even if this problem is not taken into consideration. From such a viewpoint, the unit area is preferably about 30 μm × 30 μm.
[0009]
In the present invention, the agglomerated part of the conductivity-imparting particles in which the interparticle distance between the conductivity-imparting particles is less than 1 μm is preferably 30% or less in terms of area ratio per unit area, more preferably 2 to 10%. is there.
Considering only the sliding characteristics, it is preferable that no agglomerated portion of the conductivity-imparting particles is present (0% in terms of area ratio), but it has been found that the variation in electric resistance value is large in the form where no agglomerated portion is present.
[0010]
Since the silicon nitride crystal particles constituting the silicon nitride sintered body are insulators as described above, the electrical resistance value is usually 10TenΩ · cm or more. Therefore, in the present invention, conductivity imparting particles are added to make the electric resistance value a predetermined value. If only focusing on lowering the electrical resistance value, it is sufficient to add the conductivity-imparting particles, but for example, when applied to a bearing ball for electronic equipment, there is a variation in the electrical resistance value of each bearing ball. Variations in the antistatic effect of static electricity will occur. Static electricity is basically charged at places with high electrical resistance (where insulation is high), so if there is a variation in the static electricity prevention effect, static electricity is concentrated at the highest electrical resistance. As a result, the electronic device may be damaged due to static electricity. Such a phenomenon is not so much a problem when the rotational speed is about 5,000 rpm, but is gradually being confirmed when the rotational speed becomes high speed of 9,000 rpm or more. In particular, malfunctions in electronic equipment due to electrostatic charging are also affected by the instantaneous charge amount. Therefore, for sliding members that use a combination of multiple balls, such as bearing balls, the electrical resistance of each bearing ball It is important to eliminate variations.
[0011]
Therefore, in the present invention, the dispersion of the electric resistance values of the individual silicon nitride sintered bodies is improved by intentionally providing the agglomerated portion of the conductivity imparting particles. The area ratio of the agglomerated portion of the conductivity-imparting particles is 30% or less per unit area, preferably 2 to 10%. By adopting such a form, the variation in electric resistance value is suppressed to ± 15% / 100. Can do.
[0012]
When the area ratio of the agglomerated part of the conductivity-imparting particles in which the interparticle distance between the conductivity-imparting particles is less than 1 μm is less than 2%, the variation in the electric resistance value increases because the ratio of the agglomerated part is small. For example, when there are few agglomerated parts, the electrical resistance value of the silicon nitride sintered body does not decrease so much and often becomes larger than the target electrical resistance value. As described above, since the silicon nitride sintered body is an insulator, the presence of the conductivity-imparting particles is necessary to make the electric resistance value a predetermined value. However, if the distance between the conductivity imparting particles is too large, the effect of lowering the electric resistance value is not sufficient. Therefore, it is possible to stabilize the electrical resistance value within a predetermined value range (aggregate variation) by aggregating some of the conductivity imparting particles.
On the other hand, when the area ratio of the agglomerated portion of the conductivity-imparting particles having an interparticle distance of less than 1 μm between the conductivity-imparting particles exceeds 30%, the variation in the electric resistance value is certainly reduced, but the silicon nitride sintered Since there are many agglomerated parts in the body, the agglomerated part serves as a starting point for destruction and causes a decrease in strength. In addition, the rolling life of a sliding member such as a bearing ball is reduced.
[0013]
In addition, if the agglomerated part has a predetermined area ratio, it is possible to suppress variation in the electrical resistance value, but if the distance between the agglomerated parts is too close, the agglomerated part appears to be too large and the apparently large agglomerated part is destroyed. As a starting point, the sliding characteristics are deteriorated. Therefore, the distance between the agglomerated parts is preferably 2 to 10 μm. If the distance between the agglomerated parts is less than 2 μm, the agglomerated parts are likely to be larger than necessary, and the maximum diameter of the agglomerated parts described later tends to exceed 5 μm. On the other hand, if the distance between the agglomerated parts exceeds 10 μm, the effect of the existence of the agglomerated parts becomes thin, and the variation in the electric resistance value tends to increase.
The conductivity-imparting particles that form the agglomerated part may be only carbide particles or only nitride particles, or may be a state in which both carbide particles and nitride particles are mixed.
[0014]
Moreover, since the conductivity imparting effect is reduced if the interparticle distance between the conductivity imparting particles is too much for the non-aggregated conductivity imparting particles, the interparticle distance is within a range of 1 to 10 μm. Preferably there is. In particular, the interparticle distance of the non-aggregated conductivity imparting particles is preferably in the range of 3 to 10 μm, and in such a range, the inherent sliding characteristics of the silicon nitride sintered body can be prevented from being deteriorated. In combination with the above-described area ratio of the agglomerated portion, the variation in electric resistance value can be further improved to ± 10% / 100. That is, in the present invention, variation in electric resistance value is suppressed by providing a predetermined form in the inter-particle distance between the conductivity-imparting particles in the silicon nitride sintered body.
[0015]
As described above, the agglomerated portion of the conductivity-imparting particles whose interparticle distance between the conductivity-imparting particles is less than 1 μm is preferably 30% or less per unit area, and preferably 2 to 10%. A silicon nitride sintered body having such a dispersed state of conductivity imparting particles has an electrical resistance value of 107~Ten2In addition to Ω · cm, variation in electrical resistance can be suppressed to ± 15% / 100. In addition, it is possible to further suppress variation in the electric resistance value by controlling the interparticle distance between the conductivity imparting particles that are not aggregated.
Furthermore, the silicon nitride sintered body of the present invention is suitable for a sliding member because it takes advantage of the wear resistance and strength of the silicon nitride sintered body, and in particular, a sliding member for electronic equipment, for example, a bearing for electronic equipment. By using it for a ball, it is possible to efficiently dissipate static electricity associated with rotational driving and to suppress charging more than necessary, and to exhibit excellent sliding characteristics.
[0016]
Further, even if the agglomerated portion of the conductivity-imparting particles having a distance between the particles of the conductivity-imparting particles of less than 1 μm has a predetermined area ratio, if the agglomerated portion of the conductivity-imparting particles is too large, the silicon nitride sintered body The strength and wear resistance (sliding characteristics) of the steel will be reduced. Therefore, the maximum diameter of the agglomerated part between the conductivity-imparting particles is preferably 5 μm or less, more preferably 3 μm or less. In the present invention, the agglomerated part between the conductivity-imparting particles refers to those in which the conductivity-imparting particles are in direct contact (interparticle distance 0 μm) and those where the interparticle distance between the conductivity-imparting particles is less than 1 μm. Shall.
[0017]
Next, the material of the conductivity imparting particles will be described. The material of the conductivity imparting particles is not particularly limited as long as it is a carbide and nitride that can lower the electric resistance value of the silicon nitride sintered body, but preferably the carbide particles are Group 4a, Group 5a, Group 6a , 7a group element, silicon, and boron carbide, more preferably at least one tantalum, titanium, niobium, tungsten, silicon, boron carbide. The nitride particles are at least one kind of nitride of group 4a element.
[0018]
Since the silicon nitride sintered body of the present invention is used for a sliding member such as a bearing ball, for example, the conductivity imparting particles contained are naturally slid together with the silicon nitride sintered body.
For this reason, since the conductivity imparting particles are required to have a certain degree of sliding characteristics, the above-mentioned carbides are preferable. Since the carbide is not only excellent in sliding characteristics but also excellent in thermal conductivity, the thermal conductivity of the silicon nitride sintered body is easily set to 40 W / m · k or more.
[0019]
The nitride particles are preferably nitrides of Group 4a elements, and particularly preferably titanium nitride. The nitride of the 4a group element is preferable because not only the conductivity imparting effect but also the effect as a sintering aid is obtained, and titanium nitride is particularly preferable because the effect is remarkable. Further, when the nitride of the 4a group element is dispersedly contained, the sinterability can be further improved by adding an oxide of the 4a group element and precipitating it into the nitride at the time of sintering.
[0020]
The content of the carbide particles and the nitride particles is not particularly limited as long as it contains a predetermined amount of agglomerated part, but the carbide particles are 10 to 35 wt%, and the nitride particles are 0.1 to 5 wt%. . As described above, the carbide particles have excellent sliding properties, so even if contained 10 to 35%, the strength and sliding properties of the silicon nitride sintered body will not be reduced more than necessary, but the nitride particles themselves Is a brittle material with relatively low strength, and if it exceeds 5 wt%, the strength and sliding properties of the silicon nitride sintered body are lowered.
[0021]
The maximum diameter of the conductivity-imparting particles present in the silicon nitride sintered body is 2 μm or less, preferably 0.3 to 1.2 μm. The maximum diameter of the conductivity-imparting particles of the present invention is the size of each conductivity-imparting particle, and is the longest of the conductivity-imparting particle particles when the color map in the EPMA of the surface mirror surface portion of the silicon nitride sintered body is viewed. The diagonal line is the maximum diameter. Further, the maximum diameter of the agglomerated portion between the conductivity-imparting particles is also measured by the same method.
In addition, when calculating | requiring the area of the aggregation part of the electroconductivity provision particle | grains whose particle | grain distance between electroconductivity provision particles is less than 1 micrometer, many particles in an aggregation part shall be counted one by one. Further, when the aggregated portion is substantially circular, area = π × (maximum diameter / 2)2However, at present, a method using image processing is effective.
[0022]
Further, when the average particle size of the carbide particles and the average particle size of the nitride particles are compared, it is preferable that the average particle size of the carbide particles ≦ the average particle size of the nitride particles. Specifically, the average particle size of the carbide particles is preferably 0.3 to 1 μm, and the average particle size of the nitride particles is preferably 1 to 2 μm. Since the carbide particles are contained in a larger amount than the nitride particles, it is easy to form an agglomerated part. Therefore, by making the particle size smaller than that of the nitride particles, the maximum diameter of the agglomerated part is easily reduced to 5 μm or less. Strength and sliding characteristics can be improved.
[0023]
The electrical resistance value of the silicon nitride sintered body showing such a form is 107~Ten2Ω · cm.
The silicon nitride sintered body of the present invention is not particularly limited in use, but is optimally used for a sliding member provided in a motor device for rotating an electronic device such as a hard disk drive, for example, a bearing ball. It is.
At this time, the electric resistance value is 107If it exceeds Ω · cm, it is difficult to efficiently prevent static electricity generated when the bearing ball slides.2If it is less than Ω · cm, it is possible to prevent electrostatic charging, but the silicon nitride sintered body tends to be in a state where a large amount of conductivity imparting particles are added to the silicon nitride sintered body. This is not preferable because the wearability and strength are not sufficient.
[0024]
In addition, since the silicon nitride sintered body of the present invention includes conductivity-imparting particles, the thermal conductivity can be improved to 40 W / m · k or more. The silicon nitride sintered body of the present invention is mainly used for a sliding member for electronic equipment. As for electronic equipment, for example, as can be seen from the substrate for a semiconductor device, the problem of heat is extremely important. For this reason, even if it is a sliding member for electronic devices, it is important that it is excellent in heat dissipation. In particular, if the ball bearing ball used for the rotation drive of electronic devices such as hard disks is formed of the silicon nitride sintered body of the present invention having a thermal conductivity of 40 W / m · k or more and excellent heat dissipation, the above-described electrostatic charging is prevented. In addition, it is possible to efficiently dissipate the frictional heat associated with the rotational drive, and it is possible to obtain both the effects of preventing static electricity and radiating heat.
[0025]
In the case of a bearing member, the rotating shaft and the ball receiving portion are often formed of a metal member such as bearing steel, and problems such as deformation due to heat during sliding are likely to occur. In particular, in electronic equipment, the rotational speed tends to increase at a high speed of 8,000 rpm or higher, and further 10,000 rpm or higher, and heat dissipation problems are more likely to occur than in the past. Therefore, the bearing ball using the silicon nitride sintered body of the present invention having a high thermal conductivity is suitable for an electronic device, and particularly suitable for a bearing member in which the rotating shaft and the ball receiving portion are made of a metal member such as bearing steel. It can be said.
[0026]
Furthermore, the diameter of the bearing ball is preferably 3 mm or less, and more preferably 2 mm or less. The silicon nitride sintered body of the present invention has a high thermal conductivity of 40 W / m · k or more, but the viewpoint of thermal conductivity is inferior compared with a metal member constituting a rotating shaft or the like. Therefore, since the silicon nitride bearing ball becomes a thermal resistor in terms of heat dissipation, the thermal resistance as a bearing member can be lowered by reducing the diameter to 3 mm or less, and further to 2 mm or less.
[0027]
Heretofore, the conductivity imparting particles have been mainly described, but it goes without saying that other components such as a sintering aid may be added in the present invention. As the sintering aid, those commonly used may be used, and rare earth compounds such as yttrium oxide and metal oxides such as magnesium oxide are suitable. Moreover, you may use together aluminum compounds, such as aluminum oxide and aluminum nitride. The addition amount is not particularly limited, but is preferably 3 to 20 wt%.
[0028]
Next, a manufacturing method will be described. Since the manufacturing method relates to the dispersion state of the conductivity-imparting particles, a silicon nitride sintered body in which the aggregated portion of the conductivity-imparting particles having an interparticle distance of less than 1 μm is 30% or less in area ratio can be obtained. Although there is no particular limitation as long as it is present, for example, there are the following methods.
[0029]
First, after a predetermined amount of silicon nitride powder, sintering aid, and conductivity-imparting particle powder are uniformly mixed, granulation, molding, degreasing, and sintering are performed.
In particular, it is important to prevent the conductivity imparting particle powder from aggregating more than necessary.
When the aggregation of the conductivity-imparting particles occurs more than necessary, the aggregated portion of the conductivity-imparting particles tends to be 30% or more in area ratio and the maximum diameter of the aggregation portion of the conductivity-imparting particles easily exceeds 5 μm. End up.
[0030]
Therefore, for example, the conductivity-imparting particles for forming the agglomerated part are pre-granulated, granulated powder is formed so that the maximum diameter of the agglomerated part is 5 μm or less, and added to satisfy a predetermined area ratio, There is a method in which conductivity imparting particles that are not aggregated are separately added and mixed.
In addition, for example, the following method is effective in order to prevent the formation of further agglomerated parts during addition and mixing. First, when mixing the raw material powder for one lot, each raw material powder is divided into two or more, preferably 3 to 5, and mixed in a relatively small amount and finally mixed into one.
There is no particular problem if a mixed powder in which the agglomerated part of the conductivity imparted particle powder does not exist more than necessary in one lot is obtained, but in such a case, if uniform mixing with few agglomerated parts is attempted, the mixing time is more than necessary. Therefore, it is not necessarily good for manufacturability. In addition, when the raw material powders are mixed in large quantities at once, the conductivity imparting particles are aggregated when the final silicon nitride sintered body is formed, and the aggregated portion is easy to be formed at an area ratio of 30% or more. The maximum diameter of the agglomerated part tends to exceed 5 μm.
[0031]
In another method, silicon nitride powder and sintering aid are first mixed. When adding the conductivity-imparting particle powder to the mixed powder, it is effective to add the conductivity-imparting particle powder in several times so as not to form the agglomerated portion to be added. For example, the addition amount of the conductivity-imparting particle powder is divided into two or more, preferably 3 to 5, and after the first addition and a predetermined time has elapsed (30 minutes or more are preferred), the second and subsequent times are added in order. is there. By adding and mixing the conductivity-imparting particle powder little by little, it becomes possible to prevent further aggregation of the conductivity-imparting particle powders, and the conductivity-imparting particles are 30% or less in area ratio and the maximum aggregated portion of the conductivity-imparting particles. It is easy to obtain a silicon nitride sintered body having a diameter of 5 μm or less.
[0032]
If the raw material powder is uniformly mixed by such a method, it is possible to prevent the conductivity-imparting particle powders from aggregating more than necessary, so even if there is an agglomerated part, the conductivity in the silicon nitride sintered body The maximum diameter of the agglomerated part of the imparted particles can be 5 μm or less, preferably 3 μm or less. In particular, when producing a small bearing ball having a diameter of 3 mm or less, and further 2 mm or less, it is important not to form an agglomerated portion of the conductivity imparting particles more than necessary. This is because the smaller the bearing ball, the more easily affected by the agglomerated portion.
[0033]
The size of each raw material powder is not particularly limited, but the silicon nitride powder preferably has an average particle size of 0.2 to 3 μm, and the sintering aid preferably has an average particle size of 2 μm or less.
The size of the conductivity-imparting particle powder is an average particle size of 3 μm or less, preferably 0.1 to 1.2 μm. When the conductivity-imparting particles are less than 0.1 μm, when applied to a bearing ball, the particles are easily separated from the surface during surface processing or sliding. On the other hand, if it exceeds 3 μm, it is not preferable because the maximum diameter exceeds 5 μm with only slight aggregation. Furthermore, it is preferable to use a powder having a small average particle size variation, for example, a standard deviation of 1.5 μm or less so that the above-mentioned maximum diameter can be easily controlled.
[0034]
Further, in order not to impair the sliding characteristics as a bearing ball, it is not preferable to use whiskers or fibers as the conductivity-imparting particle powder even if the size is satisfied, and it is desirable to use particulate powder. Whisker and fiber have protrusions such as thorns on the surface due to their shape, and when such a material is present on the surface of the bearing ball, the wear resistance is deteriorated.
[0035]
As a forming method, a method for manufacturing a silicon nitride sintered body or a bearing ball can be applied. Therefore, a normal molding method, isostatic pressing (CIP), etc. can be applied, and isostatic pressing is suitable when producing bearing balls.
[0036]
As for the sintering method, a method for producing a silicon nitride sintered body and a bearing ball can be applied. Therefore, normal pressure sintering, pressure sintering, and hot isostatic pressing (HIP) sintering can be applied. When producing bearing balls, HIP sintering is performed after atmospheric pressure sintering or pressure sintering. It is preferable to carry out ligation.
After passing through the above steps, when used as a bearing ball, surface polishing is performed to obtain the surface roughness defined by the JIS standard.
[0037]
【Example】
(Examples 1-4, Comparative Examples 1-2, Reference Example 1)
20 wt% silicon carbide powder with an average particle size of 0.7 μm (standard deviation 1.3 μm or less), 1 wt% of titanium oxide powder with an average particle size of 0.9 μm (standard deviation 1.5 μm or less), a sintering aid As an example, 5 wt% of yttrium oxide powder having an average particle diameter of 0.8 μm, 4 wt% of aluminum oxide powder having an average particle diameter of 0.9 μm, and a silicon nitride powder having a balance average particle diameter of 0.7 μm were prepared. Before mixing each raw material powder, granulate the silicon carbide powder for forming the agglomerated part between the conductivity imparting particles so that the maximum diameter of the agglomerated part is 2 μm or less, and divide each raw material powder into 3 parts respectively After mixing to obtain three mixed powders, these three mixed powders were mixed and mixed to produce a mixed raw material powder, thereby preparing a mixed raw material powder containing a predetermined amount of the agglomerated portion of the conductivity-imparting particle powder. .
This mixed raw material powder is molded by CIP method and sintered at 1600-1900 ° C under normal pressure in an inert atmosphere, followed by HIP sintering at a temperature of 1600-1900 ° C to produce the silicon nitride sintered body shown in Table 1. did.
In each example, a square columnar sample having a size of 3 × 4 × 40 mm is used, and a surface polishing process corresponding to bearing ball grade 3 certified by JIS standards is performed. In addition, the interparticle distance between the conductivity-imparting particles not forming the agglomerated part was in the range of 3 to 10 μm, and the distance between the agglomerated parts was in the range of 2 to 10 μm.
[0038]
Table 1 shows the results of measuring the electrical resistance value, the variation in the electrical resistance value, the three-point bending strength (room temperature), and the thermal conductivity for each of these examples. The electrical resistance value was obtained by lapping the top and bottom of each sample, placing two electrodes on the same plane, and measuring the resistance between them at room temperature with an insulation resistance meter. The thermal conductivity was measured by a laser fresh method using a sample additionally processed to 3 × 3 × 10 mm. In each measurement, 100 samples according to each example were prepared, and the average value was shown. Further, regarding the variation in the electric resistance value, the electric resistance value having the largest difference with respect to the average value was displayed as a percentage (%) as a difference with respect to the average value.
In each measurement value, the sample shape is a quadrangular prism for the sake of convenience in this embodiment. However, for example, even when measuring each characteristic of a true spherical bearing ball, it can be handled by performing lapping similarly.
In addition, the area ratio of the agglomerated portion of the conductivity-imparting particles in each silicon nitride sintered body was measured by polishing each sample to a surface roughness Ra of 0.01 μm or less, and at any four locations on the surface of the polished surface ( An arbitrary area corresponding to a unit area of 30 μm × 30 μm) was selected, and a color map (magnification 2000 times) of each measurement location was used.
[0039]
For comparison, a comparative example 1 was prepared by adding an excessive amount of conductivity-imparting particles at one time so that the area ratio of the aggregated portion was outside the range of the present invention. Further, a silicon nitride sintered body similar to the example was used as Comparative Example 2 except that the conductivity imparting particles were not added. As Reference Example 1, one having less agglomerated portions of the conductivity imparting particles was prepared.
[0040]
[Table 1]
As can be seen from Table 1, the silicon nitride sintered body of the present invention has an electric resistance value of 107~Ten2In the range of Ω · cm, the three-point bending strength was 1000 MPa or more, and the thermal conductivity was 40 W / m · k or more.
On the other hand, in Comparative Example 1, since the ratio of the agglomerated portion of the conductivity imparting particles is large, the electrical resistance value variation is small, but the strength is lowered. On the other hand, Comparative Example 2 in which no conductivity imparting particles are added has an electric resistance value of 10TenIt was Ω · cm or more and the thermal conductivity was poor.
Further, as shown in Reference Example 1, when the area ratio of the agglomerated portion of the conductivity imparting particles was 0.3%, the variation in electric resistance value was large.
In addition, all the maximum diameters of the aggregation part of the electroconductivity provision particle | grains in the silicon nitride sintered compact of Examples 1-4 were 3 micrometers or less. On the other hand, in Comparative Example 1 added in an excessive amount at a time, a plurality of locations where the agglomerated part was 10 μm or more were found, and it is considered that the strength was reduced.
When such a silicon nitride sintered body having characteristics such as an electric resistance value is used for a bearing ball for an electronic device such as a hard disk drive, which will be described later, it is possible to eliminate problems caused by static electricity.
[0042]
(Examples 5-8, Comparative Examples 4-6, Reference Example 2)
Next, a bearing ball having a diameter of 2 mm made of a silicon nitride sintered body in which the electrical resistance value and the ratio of the aggregated portions of the conductivity imparting particles were changed by the same production process as in Example 1 was produced. Each bearing ball had a surface polished grade 3.
Each bearing ball was assembled into a set of 10 bearing members of a spindle motor for rotating the hard disk drive. As other bearing members, a rotating shaft portion and a ball receiving portion made of bearing steel SUJ2 were used.
The motor was examined for the presence of defects caused by static electricity when it was continuously operated at 8,000 rpm and 11,000 rpm for 200 hours. The failure due to static electricity was judged by whether or not the hard disk drive could move normally after 200 hours of continuous operation. In addition, 100 hard disk drives were prepared and measured for the presence or absence of defects due to static electricity.
For comparison, Comparative Example 4 has an area ratio of agglomerated portions of conductivity imparting particles outside the range of the present invention, Comparative Example 5 does not contain conductivity imparting particles, and Comparative Example has a smaller electrical resistance value. 6. The same measurement was performed as Reference Example 2 in which the proportion of the aggregated portion of the conductivity imparting particles was outside the preferred range of the present invention. The results are shown in Table 2.
[0043]
[Table 2]
As can be seen from Table 2, it was found that the bearing ball according to this example was free from static electricity. On the other hand, since the electrical resistance value was higher than the present invention in Comparative Example 5, problems due to static electricity were born (1 to 3 units out of 100 units).
Further, Comparative Example 4 and Comparative Example 6 did not cause a problem due to static electricity, but the bearing ball was insufficient in strength after 200 hours due to insufficient strength of the bearing ball. It was confirmed that it was not suitable for operation.
This is considered to be because the maximum diameter of the agglomerated part exceeded 5 μm because the ratio of the agglomerated part was too large and there were many agglomerated parts.
In the case of Reference Example 2, no trouble due to static electricity was confirmed at a rotation speed of about 8,000 rpm, but the hard disk drive did not stop completely at 11,000 rpm, but showed some trouble (1 out of 100) Because it was confirmed, it was written as “somewhat there”. This is considered to be because static electricity was instantaneously concentrated on the bearing ball having the largest electrical resistance value due to the large variation in the electrical resistance value.
[0045]
(Examples 9-13, Comparative Examples 7-9)
Next, the rolling life of the bearing balls was measured using the bearing balls of Examples 5 to 8 and Comparative Examples 4 to 6. In the bearing balls according to this example, the maximum diameter of the agglomerated portion of the conductivity imparting particles was 5 μm or less. The maximum diameter of the agglomerated part of the conductivity-imparting particles of Comparative Example 7 using the bearing ball of Comparative Example 4 is 9 μm, and the agglomerated part of the conductivity-imparting particles of Comparative Example 9 using the bearing ball of Comparative Example 6 The maximum diameter was 23 μm.
For the measurement of rolling life, a thrust type bearing tester is used, and the load is a maximum contact stress of 5.9 GPa per ball, rotating at 1200 rpm, turbine oil bath lubrication by rotating on a SUJ2 steel flat plate as the counterpart material Under the conditions, it was performed up to 400 hours, and the time until the bearing ball surface peeled was measured. The results are shown in Table 3.
[0046]
[Table 3]
As can be seen from Table 3, in the bearing ball according to this example, the area ratio of the agglomerated portion of the conductivity-imparting particles is within the range of the present invention, which is equivalent to Comparative Example 8 in which the conductivity-imparting particles are not added. It was found to show a rolling life.
On the other hand, it was found that the sliding characteristics deteriorate when the area ratio of the agglomerated portion of the conductivity imparting particles exceeds 30% and reaches about 50% as in Comparative Example 7 and Comparative Example 9. As a result, it can be said that the conductivity imparting particles increase in the silicon nitride matrix, and the good sliding characteristics of the silicon nitride sintered body cannot be used. Moreover, since the maximum diameter of the agglomerated part of the conductivity imparting particles exceeds 5 μm, it is considered that the agglomerated part has become a starting point of fracture.
[0048]
(Examples 13 to 14, Reference Example 4)
In order to examine the influence of the maximum diameter of the aggregated portion of the conductivity imparting particles, a bearing ball similar to that in Example 7 was prepared except that the maximum diameter of the aggregated portion was changed. Each bearing ball was subjected to the same rolling life test as in Example 11. In addition, crushing strength and three-point bending strength (room temperature) were also measured.
The crushing strength was measured by applying a compression load with an Instron type tester and measuring the load at the time of breakage by a measurement method according to the old JIS standard B1501. The results are shown in Table 4.
[0049]
[Table 4]
As can be seen from Table 4, when the maximum diameter of the agglomerated part of the conductivity imparting particles is 5 μm or less, the rolling life is excellent, and the crushing strength is 220 MPa or more.
On the other hand, it was found that the characteristics of Reference Example 4 outside the preferred range of the present invention are deteriorated even though the area ratio of the agglomerated portion of the conductivity imparting particles is within the range of the present invention. This is presumably because the aggregated portion of the conductivity-imparting particles is too large, and this aggregated portion has become the starting point of fracture.
In other words, even if the area of the agglomerated portion of the conductivity-imparting particles whose interparticle distance between the conductivity-imparting particles is less than 1 μm is within the range of the present invention, the maximum diameter of the agglomerated portion between the conductivity-imparting particles is 5 μm. It can be said that anything exceeding this is not suitable for a bearing ball.
[0051]
(Examples 15 to 16)
Silicon carbide powder having an average particle size of 0.8 μm or less (standard deviation of 1.5 μm or less) as a conductivity imparting particle powder, titanium oxide powder having an average particle size of 1.0 μm (standard deviation of 1.5 μm or less), and an average particle size of 1.5 as a sintering aid An yttrium oxide powder having a particle size of 5 μ% or less, an aluminum oxide powder having an average particle size of 0.8 μm or less of 3 wt%, and the balance of silicon nitride powder having an average particle size of 0.5 μm were prepared.
First, granulation is performed so that the maximum diameter is 3 μm or less with respect to the amount for forming the agglomerated part in the conductivity-imparting particles to prepare agglomerated granulated powder.
Next, as Example 15, silicon nitride powder and sintering aid powder were mixed, a predetermined amount of silicon carbide powder was divided into three times, added and mixed at intervals of 1 hour, and finally the agglomerated granulated powder was mixed. A mixed raw material powder was prepared by adding and mixing a predetermined amount.
[0052]
As Example 16, each raw material powder was divided into three parts and mixed, and then the whole was mixed, and finally mixed raw material powder was prepared by adding and mixing agglomerated granulated powder. As Reference Example 5, a mixed raw material powder was prepared by mixing all the raw material powders at once.
Each of these mixed raw material powders is molded by the CIP method and sintered at 1740 ° C. under normal pressure in an inert atmosphere, followed by HIP sintering at 1000 atm. 1700 ° C., and a silicon nitride bearing ball having a diameter of 2 mm and a 3 × 4 × 40 mm A square columnar sample was prepared.
One hundred such samples were prepared, and the area ratio of the agglomerated part of the conductivity imparting particles and the maximum diameter of the agglomerated part were measured. The maximum diameter of the agglomerated part was measured at four arbitrary 30 μm × 30 μm points, and the maximum diameter of the largest agglomerated part was shown. The results are shown in Table 5.
[0053]
[Table 5]
As can be seen from Table 5, according to the additive mixing method of Example 15 or Example 16, it was found that a silicon nitride sintered body having a preferred embodiment of the present invention can be produced.
On the other hand, in Reference Example 5, the agglomerated part of the conductivity imparting particles was as large as 10 to 20 μm. In such a silicon nitride sintered body, the strength is lowered and the rolling life is also lowered as in the above-described embodiment.
[0055]
(Examples 17 to 26)
Next, the same silicon nitride sintered body as in Example 2 was produced except that the conductivity imparting particles were changed to the materials shown in Table 6. The same measurement as in Example 2 was performed on each of the produced silicon nitride sintered bodies.
[0056]
[Table 6]
As can be seen from Table 6, even when the material of the conductivity-imparting particles was changed, the electrical resistance value, the three-point bending strength, and the thermal conductivity all showed excellent characteristics.
[0058]
(Examples 27 to 42)
Except for using the silicon nitride sintered bodies of Examples 17 to 26, the same bearing balls as in Example 10 were produced, and the crushing strength and rolling life characteristics were measured in the same manner as in Example 13.
As a result of the measurement, it was found that all the bearing balls showed excellent characteristics such as a crushing strength of 210 MPa or more and a rolling life of 400 hours or more.
From the above, it can be said that the silicon nitride and the sliding member of the present invention show excellent characteristics even when the material of the conductivity imparting particles is changed.
[0059]
【The invention's effect】
As described above, the silicon nitride sintered body of the present invention includes carbide particles and nitride particles as the conductivity-imparting particles, and the agglomerated portion of the conductivity-imparting particles whose interparticle distance between the conductivity-imparting particles is less than 1 μm. By specifying the area, the sliding member of an electronic device such as a hard disk drive because it has a predetermined electric resistance value, for example, when used for a bearing ball of a bearing member mounted on a motor for rotational driving, is used for rotational driving. It is possible to prevent the accompanying electrostatic charge.
Moreover, since the thermal conductivity of the sintered body itself can be improved by using carbide or the like as the conductivity-imparting particles, it is also possible to efficiently dissipate the frictional heat accompanying the rotational drive. Further, since the variation in the electric resistance value is suppressed, even when the rotation speed is 8000 rpm or more, and further 10000 rpm or more, the occurrence of malfunction due to static electricity can be efficiently suppressed.
Furthermore, sliding characteristics and the like can be improved by preventing aggregation of the conductivity-imparting particles.
With this configuration, the bearing ball made of a silicon nitride sintered body does not unnecessarily reduce the good sliding characteristics of silicon nitride, and when used in an electronic device such as a hard disk drive, it has excellent sliding properties. Dynamic characteristics are shown.
Claims (4)
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| Application Number | Priority Date | Filing Date | Title |
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| JP2000249327A JP4567853B2 (en) | 2000-08-21 | 2000-08-21 | Sintered silicon nitride |
| US09/927,930 US6642165B2 (en) | 2000-08-21 | 2001-08-13 | Wear resistant member for electronic equipment, and bearing and spindle motor therewith |
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| JP2000249327A JP4567853B2 (en) | 2000-08-21 | 2000-08-21 | Sintered silicon nitride |
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| JP4820506B2 (en) * | 2001-08-27 | 2011-11-24 | 株式会社東芝 | Wear-resistant member for electronic equipment and bearing for electronic equipment using the same |
| JP4874475B2 (en) * | 2001-08-28 | 2012-02-15 | 株式会社東芝 | Abrasion resistant member for electronic equipment, method for producing the same, and bearing for electronic equipment using the same |
| JP4497787B2 (en) | 2002-04-04 | 2010-07-07 | 株式会社東芝 | Rolling ball |
| JP5295983B2 (en) * | 2010-01-12 | 2013-09-18 | 株式会社東芝 | Method for producing wear-resistant member made of silicon nitride |
| JP7353820B2 (en) * | 2019-06-21 | 2023-10-02 | 株式会社東芝 | Silicon nitride sintered body and wear-resistant parts using the same |
Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH05213674A (en) * | 1992-01-31 | 1993-08-24 | Asahi Glass Co Ltd | Ceramic material |
| JPH08152020A (en) * | 1994-11-30 | 1996-06-11 | Kyocera Corp | Dynamic pressure bearing made of ceramic |
| JPH1095674A (en) * | 1996-09-20 | 1998-04-14 | Nissan Motor Co Ltd | Silicon nitride/titanium nitride particle group sintered compact and its production |
| JP2000143351A (en) * | 1998-10-30 | 2000-05-23 | Kyocera Corp | High-toughness silicon nitride-based sintered compact |
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| JP2616931B2 (en) * | 1987-09-01 | 1997-06-04 | 株式会社デンソー | Glow plug heater support |
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Patent Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH05213674A (en) * | 1992-01-31 | 1993-08-24 | Asahi Glass Co Ltd | Ceramic material |
| JPH08152020A (en) * | 1994-11-30 | 1996-06-11 | Kyocera Corp | Dynamic pressure bearing made of ceramic |
| JPH1095674A (en) * | 1996-09-20 | 1998-04-14 | Nissan Motor Co Ltd | Silicon nitride/titanium nitride particle group sintered compact and its production |
| JP2000143351A (en) * | 1998-10-30 | 2000-05-23 | Kyocera Corp | High-toughness silicon nitride-based sintered compact |
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