HOT-ISOSTATIC PRESSING OF Mn-Zn FERRITES FOR MAGNETIC RECORDING HEADS

(1)

HAL Id: jpa-00217031

https://hal.archives-ouvertes.fr/jpa-00217031

Submitted on 1 Jan 1977

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

HOT-ISOSTATIC PRESSING OF Mn-Zn FERRITES

FOR MAGNETIC RECORDING HEADS

C. Büthker, Th. Berben

To cite this version:

(2)

JOURNAL DE PHYSIQUE _{Colloque C l , supplkment au no}_{4, Tome 38, Avril 1977, page Cl-341}

HOT-ISOSTATIC PRESSING OF Mn-Zn FERRITES

FOR MAGNETIC RECORDING HEADS

C. BUTHKER and Th. BERBEN

N. V. Philips Ceramic Laboratory-Building BE2 Eindhoven, Netherlands

Resumb. - On produit des ferrites magnbtiques denses par frittage normal et pressage isosta- tique B chaud. Les matkriaux de depart sont des ferrites frittks normalement avec des pores fermb. La dimension des grains du materiau fini presd B chaud peut &re ajustQ dans une large gamme de valeurs, de 15 B l000 pm.

Abstract. - Dense magnetic ferrites are produced by normal sintering and isostatic hot pressing. The starting materials are normally sintered ferrites with closed porosity. The grain size in the final hot-pressed material can be adjusted in a broad range between 15 pm and 1 000 pm.

1. Introduction.

-

Hot-pressed Mn-Zn ferrites are widely used for magnetic heads, especially in video recording. Various methods have been published for hot pressing of Mn-Zn ferrites. Most commonly used is the uniaxial hot-pressing method. Other possibilities are the continuous and hot-isostatic pressing methods. The latter method was used by Hardtl [l] for densification of several ceramic materials. It soon appeared to be a very useful method for the densification of Mn-Zn ferrites. Characteristic for hot-isostatic pressing is that a sintered material of a closed porosity is fired for a second time at a gas pressure of 100 to 200 atm. No metal encapsulants are needed.

FIG. l . - Porosity as a function of hot-pressing time for different isostatic pressures at a hot-pressing temperature of

1 280 OC. The starting materials were sintered Mn-Zn ferrites with a type 1 microstructure (ref. [3]).

2. Densification of Mn-Zn ferrites. - The main requirements made on a material for video application are low porosity (< 0.1

X),

small size residual pores

(< 1 pm) and absence of pores at the grain boundaries. It is very difficult to obtain such a material by normal powder preparation and sintering since the microstructure then obtained is one where large pores are

FIG. 2. - Microphotograph of a polished and etched sample of a ferrite hot-pressed at l 180 O C and 150 kg/cmz. During

hot-pressing no grain growth has occurred.

(3)

FIG. 3 .

-

The same as figure 2, but now the sintered material used has a larger average grain size.

situated a t the grain boundaries and the grains are almost pore-free. This sintering behaviour has been explained by Reijnen [2]. Essential in his model is

the presence of cation vacancies during the sintering

process. These cation vacancies give rise to pore migration. By keeping the cation vacancy concentration at a low value during sintering Stassen et al. [3] managed to obtain a low porosity (0.5

%)

without hot pressing.

Although the progress in this field is encourageing, hot pressing is at this moment still necessary to obtain an optimal magnetic head material.

Stassen et al. [3] applied the hot-isostatic pressing

method on Mn-Zn ferrites. Two types of sintered starting-materials with a closed porosity were used in their experiments. In the first type (type I) the pores were at the grain boundaries and, in the other type (type 11) the pores were inside the grains.

Samples of the first type were hot pressed at 1 280 OC

under different conditions of time and pressure. Results are shown in figure 1 where the porosity of the hot-pressed samples is plotted as a function of the hot-pressing time for different pressures. It is clear from this figure that the porosity gradually decreases during hot pressing. It turns out [3] that pores a t grain boundaries have disappeared and small pores are

present inside the grains due to grain growth. The residual porosity is about 0.1

%.

If the pores of the sintered material are mainly situated inside large grains (type IT), a lower porosity cannot be obtained, no matter which hot preysing conditions are used. The pores inside the grains do not disappear unless they are very close to the grain boun- dary.

In a typical experiment a type IT material was hot- pressed at 1 320 OC and a pressure of 135 kg/cm2 for 15 hours, the porosity then decreased from 1.4 to 1.2 "/,.

From the two experiments it may be concluded that sintered Mn-Zn ferrites which have a normal type I microstructure are very suitable for hot isostatic pressing.

3. Grain size aspects.

-

Apart from the residual porosity, the final grain size can be of interest for the use of the material. Many process parameters have an influence on the final grain size. We have found two methods to adjust the desired grain size. The first one is to adjust the wanted grain size in the first sintering and to densify the material during hot pressing at relatively low temperatures so that further grain growth is avoided. Examples are shown in the figures 2 and 3. The materials were sintered under

FIG. 4.

-

Microphotograph of a polished and etched sample

(4)

HOT-ISOSTATIC PRESSING OF Mn-Zn FERRITES FOR MAGNETIC RECORDING HEADS Cl-343 different conditions resulting in a different grain

size. They were both hot pressed at 1 180OC and a pressure of 150 kg/cm2. Under these hot-pressing conditions further grain growth does not take place. Residual pores are mainly present at grain boundaries. The smallest attainable grain size is determined by the minimum grain size which can be realized in combi- nation with a closed porosity by normal sintering. In our experiments the minimum grain size was 15 pm (see Fig. 2). It must be possible to obtain even smaller grain sizes by using more reactive powders. The largest grain size obtained with method 1 is 50 pm (Fig. 3).

The second method is hot-pressing at high temperatures. Figure 4 shows an example of a material hot- pressed at 1 320 OC and 150 kg/cm2. The average grain size before hot-pressing was 15 pm. During hot- pressing the grains have grown to a very large size ; now the residual pores are predominantly present

inside the grains. By using more drastic hot-pressing conditions there is hardly a limit to the maximum grain size. However, an exact adjustment of the grain size and obtaining at the same time a uniform grain- size distribution appears to be much more difficult, because grain growth occurs very discontinuously. 4. Conclusions. - The hot-isostatic pressing method is suitable for the densification of Mn-Zn ferrites for recording head applications. Essential for the method is the use of a presintered material which has closed pores located at the grain boundaries.

When small grains are desired, the grain size can be adjusted in the pre-sintering stage provided the sub- sequent hot-pressing process does not lead to a further grain growth.

However, when large grains are desired, the grain size can be adjusted by choosing hot pressing conditions which produce the desired grain size.

References

[ l ] ~ R D T L , H . K . , Philips Tech. Rev. 35 (1975) 65.

[2] REIJNEN, P. J. L., Sci. Ceram 4 . (1968) 169.