In general, the substrate members (or substrates) for magnetic recording disks are required to possess the following properties that:
1. They have an improved surface roughness after polishing so as to achieve stable floating of magnetic heads and stable recording properties in association with a head floating height of as low as 0.3 micron or less.
2. They are free from any projections or pinhole-like indentations which are essentially attributable to a deficiency of the magnetic thin film formed on the surface of the substrate.
3. They have mechanical strength sufficient to withstand machining, polishing or high-speed rotation during use.
4. They have corrosion, weather and heat resistance.
Hitherto, the substrates for magnetic disks have made use of aluminum alloys. However, the Al alloy materials provide insufficient substrate materials for high-density magnetic recording disks due to an unfavorable surface state of the substrate which includes protrusions, indentations and a wave-like configuration. Namely, in the Al alloy materials, the crystal anisotropy, material deficiency and non-metallic inclusions thereof remain on the surface of the substrates and give rise to protrusions or indentations upon machining and/or polishing whereupon certain hard spots fall away from the substrates to leave behind indentations, so that polishing only results in a surface roughness of at most about 200 angstroms.
The machined quality of magnetic disk substrates directly affects the run-outs and acceleration component caused by the magnetic recording disks, the signal error of magnetic recording media, and the like.
As the Al alloys are metallic materials, they have a Vickers hardness on the order of Hv 100 (that of ceramics is more than 600) and a bending strength on the order of 1000 kg/cm.sup.2 (that of ceramics is more than 4000 kg/cm.sup.2). For that reason, more stringent limitations are now imposed upon the dimensional accuracy stipulated in respect of scratch, flaw, smoothness and wave configuration as the recording density increases, and involve more difficulty in machining. Machining using abrasive grains also causes incidental packing of abrasive particles, which entails another problem. In the case of the Al alloy substrates, a great deal of care should also be taken with regard to cleanliness, rust prevention, contamination, etc. in the production steps inclusive of lathe turning and polishing and in the storage period in order to secure the surface corrosion and weather resistance thereof and the prevention of the surface contamination thereof.
For the purpose of improving the Al alloy substrates, it has been known to form a film with a high hardness on the surface thereof. As an example, it has been proposed to form an alumite layer on the surface of Al alloys, thereby increasing hardness, in order to improve abrasive machinability. However, traces of impurities (Fe, Mn, Si) contained in the Al alloys precipitate as intermetallic compounds during the formation of alumite, which are responsible for the occurrence of the indentation deficiency after the alumite treatment. It is extremely difficult to further improve the purity of mother alloys in view of the production process. In addition, the Al alloys cause a handling problem in view of corrosion resistance and cleanliness. Furthermore, the production of thin film magnetic recording media by plating or sputtering poses problems in connection with the occurrence of chemical reactions and diffusion between the Al alloys and the magnetic films. In some cases, it is required to apply a heat treatment to the magnetic films depending upon the type of steps applied. However, such a heat treatment will readily cause deformation and lowering of the dimensional accuracy of the Al alloy substrates, simultaneously causing increases in the surface vibration and acceleration. Thus, it is difficult to apply that treatment.
Although there is available a method for forming oxides such as SiO.sub.2, Al.sub.2 O.sub.3 and the like on the Al substrates by sputtering, this method is disadvantageous in that the adhesion of the Al substrate to the sputtered oxides is weak.
Ceramic materials have become widely used in various fields due to their superiority over Al alloy base disk substrates with respect to heat resistance, wear resistance, weather resistance, and insulation and mechanical strength. In the substrates for magnetic disks having recording media treated on the substrate surface, however, there is a strong demand for the surface thereof to be free of any holes and strain in association with the thinning and high-densification of the recording media.
Generally, the methods for producing ceramic substrates embrace single crystallization; forming with a mold, rubber press, doctor blade, etc. followed by sintering; and hot pressing (HP process) as well as hot isostatic pressing (HIP process) for obtaining a high density. However, the single crystallization is not only high in the production cost, but also encounters difficulty in the production of any substrates having an increased diameter. On the other hand, although it is possible to produce highly densified substrates by the HIP or HP processes, there arise certain reliability problems such as the occurrence of drop-outs, head crush, etc. due to minute surface deficiencies (micropores of 5 microns or less) of the resulting substrates, where they are used for magnetic recording disks.
In general, the mechanochemical polishing method, which is applicable to the disk substrates, etc. as the surface polishing method, has been known to finish the surface of Si substrates, GGG crystals, ferrite, etc. without incurring deterioration of the surface physical properties thereof. However, where the mechanochemical polishing method is applied and ceramic materials in which micropores exist, such pores are exposed to open on the surface, resulting in insufficient substrates for magnetic disks. Where the mechanochemical polishing method is applied to the alumina base ceramic materials, on the other hand, it is likely that exposure of micropores takes place simultaneously with the occurrence of a stepwise surface difference between crystals due to the difference in the rate of chemical erosion on the different surfaces of component grains or crystal grains.