Hidetoshi Umeda

Characteristics and manufacturing process of aluminum substrates for magnetic recording disks

1956年 IBMより世界最初のハードディスクRAMACが発 表された 。RAMACには直径24インチ(600 mm)のディス ク50枚が搭載され,大型冷蔵庫2台分の大きさの巨大なもの であったが,記録容量はわずか4.8 MBしかなかった。HDD の記録容量は図2に示すように年々飛躍的に上昇する一方, 基板の小型化が進み,基板の外径については75年には8イ ンチ,80年には5.25インチ,84年には3.5インチ,90年には 2.5インチが登場した。小径化の流れはさらに続き,98年に は1.8インチ,99年には1インチの基板が登場し,携帯音楽 プレーヤやデジタルカメラに採用されたが,これらの小径の HDDは,NANDフラッシュメモリを使用したSSDの低価格, 大容量化の進展とともに置き換えが進み,現在は,SSDとの 容量単価の差が大きい2.5インチと3.5インチのみとなってい る。 従来,大容量の3.5インチHDD用基板にはアルミニウム基 板が使用され,耐衝撃性の必要な2.5インチHDD用基板には ガラス基板が用いられてきたが,衝撃センサによるヘッドの 回避機構などの耐衝撃性の向上の効果もあり,アルミニウム 基板が2.5インチHDDにも使用されるようになっている。 HDDの出荷台数は1996年に1億台を超え,パソコンの普 及と用途の拡大に伴い,年々増加し2007年に5億台に到達し

Behavior of hydrogen in Al-4%Mg alloys during heat treatment

The effect of the annealing atmosphere, the annealing temperature etc. on the hydrogen behavior in several Al-4% Mg alloys during heat treatment, was investigated. The results have shown that the hydrogen content in the as-cast slab is uniform, while the hydrogen content in the slab tends to be higher near the surface than in the interior after annealing. Such a tendency becomes more marked when annealing time is prolonged and Si and Fe content is lowered. The condensation of hydrogen near the surface can be seen only when it is annealed in a wet atmosphere. When annealed in a dry atmosphere, the hydrogen content near the surface becomes lower than in the center of the specimen. The hydrogen in Al-Mg alloys tends to be released to outside intrinsically at temperatures around 400°C and above. It is revealed that oxide film formed on the surface prevents the hydrogen from being released to outside.

Effect of heat treatment condition and alloy composition on hydrogen content in Al-Mg alloys

The effect of heat treatment conditions such as atmosphere, temperature, annealing time and alloying elements on the hydrogen content in Al-Mg based alloys was investigated. The hydrogen content after annealing depends on the annealing atmospheres and alloying elements. When annealed in a wet atmosphere, the release of the hydrogen in the Al-Mg alloys to outside and the absorption of hydrogen from atmosphere into Al-Mg alloys are presumed to occur at the same time. The oxide layer on the surface is revealed to prevent the hydrogen from being released to outside. The spheroidal MgO particles can be seen on the surface of Al-Mg alloys after annealing at 550°C. The number of the MgO particles increases with increasing impurity elements such as Si and Fe, reducing the shielding effect against hydrogen permeration. Therefore, the condensation of hydrogen near the surface after annealing occurs more easily in an Al-Mg alloy of a high-purity than that of an ordinary purity.