Abstract:
The present invention relates to non-magnetic ceramic substrate material for magnetic head having similar thermal expansivity to the metal magnetic thin film, abrasion resistance during contact traveling of the magnetic tape and good processability and thus increasing lifetime by preventing exfoliation and crack of the magnetic thin film. The material comprises 10-35 mol % of CaO, 10-40 mol % of TiO 2 , 10-60 mol % of NiO and 10-50 mol % of CoO.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to non-magnetic material for magnetic head, and particularly to non-magnetic ceramic substrate material having increased lifetime by preventing exfoliation and crack of the magnetic thin film through using material for non-magnetic ceramic substrate having similar thermal expansivity to the metal magnetic thin film, low abrasion during contact travel of magnetic tape and good processability. 
     Recently, VTR head comes to have high performance as the function of the VTR comes to have high recording density. 
     Particularly, for the D-VTR (digital VTR) in which the velocity of the tape is faster than the conventional VTR, integrated head having low perturbation noise is used. 
     As for the conventional material for non-magnetic substrate material for such a head, barium titanate (BaTiO 3 ), calcium titanate (CaTiO 3 ), etc. are used. 
     However, since such material has thermal expansivity of 90-110×10 -7  /° C. which is lower than that of sendust or Fe based metal thin film material (120-150×10 -7  /° C.) which was used as the metal magnetic thin film, exfoliation or crack of the metal magnetic thin film is produced. And these deteriorate electromagnetic characteristic of the magnetic head and lower processing yield. 
     To solve the above-mentioned problems, recently, CoO-NiO based material (Japanese Laid open Patent hei 3-80412) and MnO-NiO based material (Japanese Laid open Patent hei 4-95204) were applied to form non-magnetic substrate material which have thermal expansivity of 120-150×10 -7  /° C. 
     In the Japanese Laid open Patent hei 3-80412, Co x  Ni 1-x  O 2  (0≦x≦0.2), non-magnetic substrate material having rock salt-type structure is disclosed. The material has thermal expansivity of about 150×10 -7  /° C. and hardness of about 500-700 by Vikers. 
     In this Patent, material having no chipping and good processability could be obtained by using CoO-NiO based hot press and by manufacturing particle size thereof to 5 μm or less. 
     In the Japanese Laid open Patent hei 4-95204, MnNiO 2  having rock salt-type crystal structure is used as the non-magnetic substrate material. In this material 20-40 mol % of MnO, 50-80 mol % of NiO are contained as main components and one or more Y 2  O 3 , CaO, Al 2  O 3 , ZrO 2 , etc. are contained to improve sintering characteristic and to adjust particle diameter. The thermal expansivity at this time is about 130-140. 
     In this patent, non-magnetic substrate consisting of 77-98 mol % of MnO as a main component and 2-23 mol % of TiO 2  is disclosed. At this time, if the amount of MnO is less than 77%, the expansivity is less than 100×10 -7  and so the thermal expansivity of the metal magnetic thin film and non-magnetic substrate material could not coincide and if the amount of MnO exceeds 98%, pores having 5 μm or over come to be produced and sintering characteristic is deteriorated. 
     To improve this problem, 2-23 mol % of TiO 2  is added to MnO to eliminate the pores and to improve sintering characteristic. 
     However, since the above-mentioned material such as barium titanate, calcium titanate, etc. have different thermal expansivity with the metal magnetic film, exfoliation or crack of the metal magnetic thin film is produced and this deteriorates electromagnetic characteristic and processing yield of magnetic head. And though CoO-NiO based material or MnO-NiO based material has high thermal expansivity, since their abrasion resistance are low, lifetime of the head decreases with the increase of the abrasion amount when travel velocity of tape is increased. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the problem by employing non-magnetic ceramic substrate material having different chemical composition from the conventional one and so by approaching the thermal expansivity to the metal magnetic thin film and imparting abrasion resistance. 
     To accomplish the above object, there is provided in the present invention non-magnetic ceramic substrate material for magnetic head comprising 10-35 mol % of CaO, 10-40 mol % of TiO 2 , 10-60 mol % of NiO and 10-50 mol % of CoO. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing abrasion variation of the magnetic head according to the travel time of the magnetic tape. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in detail below. 
     The composition of the present invention comprises 10-35 mol % of CaO, 10-40 mol % of TIC 2 , 10-60 mol % of NiO and 10-50 mol % of CoO. And one or more compounds among SIC 2 , Cr 2  O 3 , V 2  O 3  and ZrO 2  could be added at need. 
     The mixing amount of NiO and CoO is preferably 50-90 mol % in this composition. 
     If the amount is less than 50 mol %, the thermal expansivity becomes less than 120×10 -7  /° C. and if the amount exceeds 90 mol %, the sintering characteristic is deteriorated and large pores are possibly formed. 
     Thermal expansivity of TiO 2  is about 100×10 -7  /° C. If the amount of TiO 2  is less than 10 mol %, abrasion resistance is lowered, while the amount exceeds 40 mol %, liquid phase is formed resulting in abnormal crystal growth. This deteriorates processability and lowers the thermal expansivity to induce exfoliation of the metal magnetic thin film. 
     CaO reacts with TiO 2  to produce CaTiO 3  and of which thermal expansivity is about 110-120×10 -7  /° C. This is small when is compared with that of the metal magnetic thin film. However, if NiO and CoO having high thermal expansivity is mixed to the composition, the thermal expansivity of CaTiO 3  could be increased while maintaining its abrasion resistance. 
     At this time, if the amount of CaO is more than 35 mol %, CaO produces hydrate inducing deterioration of sintering characteristic and producing lots of pores. 
     Each composition is weighed in the range of mol % and wet mixed by means of alumina ball mill in pure water. 
     After drying, calcining at 900°-1100° C. for 2-3 hours, the mixture is pulverized by wet ball milling to the size of 3 μm. 
     Then 5-10% of 10% PVA was added, mixed and spray dried to the particle size of 100 μm. 
     After molding under 1-3 ton/cm 2 , the particles are cold isostatically pressed under 2-5 ton/cm 2 . 
     Then is sintered at 1250°-1450° C. for 2-5 hours under atmosphere and atmospheric pressure, and then hot pressed at 110°-1350° C. under 100-400 kg/cm 2  for 1-4 hours. 
     The thus obtained non-magnetic substrate has relative density of 99.5% or over and thermal expansivity of 130-150×10 -7  /° C. which is similar to that of the metal magnetic thin film. 
     The particle size is about 2-4 μm and fine hardness is 800-950 Hv which is larger than that of the conventional material and so could improve abrasion resistance of the head. 
     And one or more compounds among ZrO 2 , Al 2  O 3 , Cr 2  O 3 , SiO 2  and V 2  O 3  could be added to the basic composition of CaO-TiO 2  -NiO-CoO. 
     At this time, ZrO 2 , Al 2  O 3 , Cr 2  O 3 , etc. have the characteristic of restraining crystal growth and so have improved hardness. However, if the added amount is less than 0.1 wt %, the effect could not be obtained and if the amount exceeds 5 wt %, the sintering characteristic is rather deteriorated. 
     And during sintering, liquid phase of SiO 2 , V 2  O 3 , etc. could promote the densification of the material and prevent the production of the pores to harden. However, if the amount is less than 0.1 wt %, the effect could not be obtained and if the amount exceeds 5 wt %, the hardness and the thermal expansivity are decreased. 
     The present invention will be described in detail below. 
     Chemical components as shown in Table 1 are weighed and wet ball milled for 36 hours. Then calcined at 1000° C. for 2 hours. 
     After milling for 48 hours using alumina ball mill, the material is spray dried. 
     Molding under 1 ton/cm 2  to the size of 40×50×15 mm, cold isostatic pressing under about 2000 atm, sintering at 1350° C. for 2 hours and hot pressing at 1300° C. for 2 hours under 300 kg/cm 2  were sequentially carried out. 
     Physical properties of the thus obtained materials are illustrated in Table 1. 
     The thermal expansivity was 120-150×10 -7  /° C., fine hardness was 800-950 and particle diameter was 2-4 μm. 
     Physical properties of the materials obtained by adding ZrO 2 , Al 2  O 3 , SiO 2 , Cr 2  O 3 , V 2  O 3 , etc to CaO-TiO 2  -(NiO-CoO) base composition are illustrated in Table 2. 
     The attached drawing illustrates the variation of abrasion amount according to the travel time of the integrated head made of CaO-TiO 2  -(NiO-CoO) material of the present invention and the conventional MnO-NiO material. It is shown that the abrasion amount of CaO-TiO 2  -(NiO-CoO) material is far less than that of MnO-NiO material. 
     When accepting that about 15 μm abrasion corresponds to the lifetime limit, the lifetime of the head made of MnO-NiO material is 200 hours, while the lifetime of the head made of CaO-TiO 2  -(NiO-CoO) material is 1000 hours or over. 
     Accordingly, in case of applying the material of the present invention as the substrate material of the integrated head of Digital VCR in which relative velocity of the tape is very fast, even better abrasion resistance could be obtained when is compared with the conventional material. 
     
                                           TABLE 1__________________________________________________________________________samplecomposition (mol %)             S.D.  T.E.(×                         F.H.  P.D.No.  CaO   TiO.sub.2      NiO CoO             (g/cm.sup.2)                   10.sup.-7 /°C.)                         (kg/mm.sup.2)                               (μm)__________________________________________________________________________1    25 25 30  20 5.18  135.4 872   22    20 30 30  20 5.19  127.2 877   2.53    15 35 30  20 5.21  120.1 882   34    10 40 30  20 5.22  115.4 915   3.25    20 20 30  30 5.48  140.2 848   36    15 25 30  30 5.49  132.4 850   3.37    10 30 30  30 5.50  125.3 890   3.58    15 15 40  30 5.82  145.2 854   3.39    10 20 40  30 5.84  135.2 866   3.610    5 25 40  30 5.85  126.1 877   3.611   10 10 40  40 6.15  149.2 845   3.512    5 15 40  40 6.16  139.0 850   3.7__________________________________________________________________________ *In the Table, S.D. represents sintering density, T.E. represents thermal expansivity, F.H. represents fine hardness and P.D. represents particle diameter. 
    
     
                                           TABLE 2__________________________________________________________________________                      T.E. (                           F.H.samplecomposition (mol %)             A.A.                 S.D. × 10.sup.-7                           (kg/                               P.D.No.  CaO   TiO.sub.2       NiO          CoO             (wt %)                 (g/cm.sup.2)                      /°C.)                           mm.sup.2)                               (μm)__________________________________________________________________________1    20 20  30 30 ZrO.sub.2                 5.49 140.3                           863 2.1             1.02    20 20  30 30 Al.sub.2 O.sub.3                 5.50 141.0                           868 1.9             1.03    20 20  30 30 Cr.sub.2 O.sub.3                 5.48 140.7                           870 2.0             1.04    20 20  30 30 SiO.sub.2                 5.49 140.5                           875 2.5             1.05    20 20  30 30 V.sub.2 O.sub.5                 5.49 141.0                           877 2.6             1.06    15 15  40 30 ZrO.sub.2                 5.50 145.3                           869 2.5             2.07    15 15  40 30 Al.sub.2 O.sub.3                 5.51 145.2                           864 2.6             2.08    15 15  40 30 Cr.sub.2 O.sub.3                 5.50 145.1                           865 2.5             2.09    15 15  40 30 SiO.sub.2                 5.51 145.2                           879 2.8             2.010   15 15  40 30 V.sub.2 O.sub.3                 5.51 145.1                           882 2.8             2.0__________________________________________________________________________ *In the Table, A.A. represents additional agent, S.D. represents sinterin density, T.E. represents thermal expansivity, F.H. represents fine hardness and P.D. represents particle diameter.