Magnetic disc file including a slider which exhibits reduced deformation during operation

The present invention provides a magnetic disc file which comprises a thin film magnetic disc for recording information, a motor for rotating the thin film magnetic disc, a magnetic head for writing and reading information, the head being provided with a flying slider, and a carriage for supporting the magnetic head and changing the position of the head in respect of the thin film magnetic disc, the flying height of the flying slider from the thin film magnetic disc being set in the range of 0.05 to 0.15 .mu.m, and the slider being not in contact with the thin film magnetic disc during the seeking operation on the thin film magnetic disc, the slider being a sintered body containing a metal oxide or oxides in an amount of 50 vol % or more based on the total of said body and containing 0.01 wt % to 2 wt % of fluorine based on the total of the body; a thin film magnetic head which comprises a flying slider of the same sintered body as mentioned above, and a head element formed on the surface of the slider; and a wafer for making thin film magnetic heads, which comprises the same sintered body as mentioned above.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetic disc file for use as an 
external store of a computer, a thin film magnetic head for use in said 
magnetic disc file, and a wafer for making said thin film magnetic head, 
more particularly to a magnetic disc file, thin film magnetic head and 
wafer for making said thin film magnetic head, affording a high recording 
density to the magnetic disc. 
2. Description of Related Art 
A magnetic disc file with a high speed of writing and reading information 
has been demanded as external store of a computer in the art. The magnetic 
disc file has been required to have a greater storing capacity with the 
amount of information to be computerized being increased. On the other 
hand, it is very important for sales that the size of the system is 
smaller. Therefore, it is required that the magnetic disc file should have 
a greater storing capacity, a smaller size and a higher speed of writing 
and reading information. 
The increase of a recording density on a magnetic disc is clearly a key 
point for affording a greater storing capacity and smaller size to the 
magnetic disc. 
It was already known that the reduction of the thickness of a recording 
medium layer is necessary to increase the recording density on the 
magnetic disc. That is, the magnetic head element is formed on the rear 
end of a flying slider, which flies with a small space maintained on the 
magnetic disc in writing or reading information, while a magnetic flux 
emitted for recording information at the end of the magnetic gap of the 
magnetic head element is spread in a recording medium in case the 
recording medium is thick. Therefore, when the recording density is to be 
increased, the recorded magnetic fluxes are closer to each other and have 
an adverse effect on each other and, as a result, a signal resolution is 
poor. Thus, the recording medium is required to be thinner for a high 
recording density. As a magnetic disc satisfying this requirement there 
has been a thin film magnetic disc such as a sputtering-type magnetic disc 
or plating-type magnetic disc. The thickness of magnetic layers of these 
discs can be controlled to be smaller, because the recording medium is 
formed by sputtering or plating. Since the recording mediums of these 
discs are continuous and dense, these discs have such an advantage that a 
relatively large recording magnetization can be obtained. However, as the 
recording medium is thinner, it is disadvantageous that the recording 
magnetization becomes smaller and the S/N is reduced. However, if the 
distance between the magnetic head element and the recording medium is 
smaller, this problem will be solved. Furthermore, the distance between 
the magnetic head element and the recording medium should be smaller for 
reducing the spread of the magnetic flux emitted at the end of the 
magnetic gap. Thus, in order to obtain a high recording density, it is 
essential that the thickness of the recording medium is not only reduced 
but also the distance between the magnetic head element and the recording 
medium is reduced, i.e., the flying height of the slider is lowered. 
The structure of the slider is shown in FIG. 3. FIG. 3 is a pictorial view 
of a tapered flat-type magnetic head comprising a slider 12 and a magnetic 
head element 13. The structure of the slider will be explained below. The 
surface of the slider facing the magnetic disc is divided by straight or 
curved contours in a plurality of rails. Between the rails there is formed 
a predetermined amount of steps (FIG. 3 shows an example of rails divided 
by straight contours). In this case, the surfaces of the divided rails, 
i.e., the flying faces 14 have a width determined depending upon the 
designed flying height of the slider. The lower the flying height, the 
smaller the width. Furthermore, the forepart of the flying face in respect 
of the relative movement with the magnetic disc is cut obliquely in a 
forward direction with a small angle of inclination. This is hereinafter 
referred to as a "tapered part". A relatively great lifting power for 
flying is exerted on the inner edge of the tapered part and the rear end 
of the flying face when the magnetic disc is rotated. Therefore, in the 
example shown in FIG. 3, a main lifting power for flying is exerted on the 
two inner edges of the tapered parts and the two rear ends of the flying 
faces. Thus, the slider is supported at the four corners thereof. This 
effectively inhibits the slider from rolling and pitching, and thus 
maintains the slider in a stabilized flying pose. Therefore, the flying 
face of the slider is normally divided into a plurality of parts, which 
are located approximately over the entire width of the slider. 
Furthermore, since a larger amount of the lifting power for flying is 
exerted on the inner edges of the tapered parts, the forepart of the 
slider is more strongly pushed up. That is, the rear end of the slider is 
closest to the magnetic disc. Furthermore, since the end of the magnetic 
gap of a magnetic head element is formed on the surface of the rear end of 
the slider in such a manner that the end of the gap is exposed on the same 
plane as that of the flying faces, the above-mentioned tapered parts of 
the flying slider allow the end of the magnetic gap to be closest to the 
magnetic disc. 
Now, the "flying height" of the flying slider is defined herein as a 
distance between the magnetic head element in a flying state and the 
magnetic disc, for convenience. An actual flying height is different from 
the set one and varies mainly depending upon parameters as mentioned 
below. Firstly, such difference in the flying height from the set value is 
based on a dimensional error in the width of the flying face of slider and 
an error in the force of a slider-supporting spring, secondly on 
vibrational fluctuation of the distance between the slider and the disc 
due to the warping of the disc surface, thirdly on temporal vibration of 
the slider when the slider rapidly moves from one position to another 
position (seeking movement) for writing and reading another information, 
and fourthly on many projections generated but yet retained after the 
projection-removing process by slightly sliding a so-called vanishing 
slider on the disc surface. The disc surface is not completely even and 
has many projections generated thereon. The too much removing with the 
vanish slider will injure the magnetic disc, so the removing is carried 
out in an amount varying depending upon the set flying height of the 
slider. However, all of the projections are not removed and, furthermore, 
some projections should preferably be retained in order to prevent the 
adsorption between the slider and the magnetic disc. The top of this 
projection is closer to the slider. The parameters above cause the flying 
height of the slider to be changed. In the present state of art that the 
flying height is as small as about 0.3 .mu.m, it is estimated that the 
first parameter above contributes to a changed amount of about .+-.10%, 
the second parameter to a changed amount of about .+-.10%, the third 
parameter to a changed amount of about .+-.10%, the fourth parameter to a 
changed amount of about -60% to -70%. The marks "+" and "-" mean the 
increase and decrease of the flying height, respectively. Therefore, the 
tolerance width of change in decrease of flying height is only 0 to 10%, 
almost about 5% in the worst case. 
Several hundred sliders as mentioned above are normally made from one piece 
of wafer at a time. The procedure are briefly shown in FIG. 4, which is a 
flow sheet of process steps for making a magnetic head. Firstly, (a) all 
magnetic head elements are formed at a time on one piece of wafer. Then, 
(b) this wafer is machined to be divided into individual sliders. The cut 
section of the slider is then subjected to the treatment such as 
additional machining, ion milling or etching to form a positive-pressure 
type flying slider (e) having a predetermined flying face. FIG. 4 also 
shows a negative-pressure type flying slider (d) of which two flying faces 
are connected at the foreparts thereof. 
Various metal oxides may be used for the slider and vary depending upon the 
objects. A typical example of the metal oxides is Al.sub.2 O.sub.3 which 
has a low density and a high Young's modulus. That is, a lightweight 
magnetic head is demanded in view of flying stability. A material having a 
high Young's modulus is used as a slider in order to reduce an amount of 
deformation during the processing. Furthermore, spinel type oxides or 
ZrO.sub.2 materials may be used. That is, at the stage of starting or 
stopping the rotation of the magnetic disc, a transitionally sliding 
mechanism (CSS mechanism) is generally employed to work on the magnetic 
head and magnetic disc. Therefore, the sliding with the magnetic head is 
mainly contributed to by the slider of the magnetic head. Thus, as a 
slider material is desired a material having a low hardness not to injure 
the magnetic disc by sliding. 
As mentioned above, various metal oxides may be used. The reasons for 
choosing the oxides are that such materials are inexpensive and sintering 
can relatively easily be carried out. Furthermore, the process steps of 
making a thin film magnetic head is complex, and the magnetic head is an 
article requiring a high precision and mass productivity. Particularly, 
when the wafer (substrate) undergoes damages such as chipping or 
dimensional deviation in finally machining a predetermined shape of 
slider, it is a problem that there is a large amount of loss, because the 
wafer was already made through the complex process steps. On the other 
hand, if a wafer is carefully machined so that such damages are to be 
reduced, it is impossible that a large amount of sliders are made within a 
given period of time. Therefore, a substrate which has a less amount of 
chippings and can be made with a high precision has been required. 
Many proposals have been made to improve the above-mentioned problems, 
particularly on Al.sub.2 O.sub.3 substrates. For example, it is disclosed 
in Japanese Patent KOKAI (Laid-Open) No. 61-158862. In some of the 
proposals, TiC and a small amount of oxides or metals are added to 
Al.sub.2 O.sub.3, in order to improve the machinability. 
Considering the future high-density magnetic recording disc file, an area 
recording density is required to be 100 Mb/in.sup.2 or more. In order to 
obtain such a recording density, the flying height of the slider is 
required to be set to 0.05 to 0.15 .mu.m. The flying height in this range 
is also effective to a negative-pressure type slider or perpendicular 
magnetic recording. Additional problems which have been found in prior art 
wafers or sliders are that there occurs such a phenomenon that the prior 
art sliders are deformed when machined and the edge of the rear end of the 
flying face of the slider goes down below the end of the magnetic head 
element. The thus deformed slider in a flying state is schematically shown 
in FIG. 5. Since the slider is deformed, it is seen that the edge of the 
rear end of the flying face of the slider is below the end of the magnetic 
head element The amount 19 of a slider 12 deformed is herein defined as 
being a distance from a base line connecting the two edges projected on 
the flying face 14 in the largest amounts to each other to the end of the 
magnetic head element 13 for writing and reading information. Such prior 
art sliders are not applicable to the case wherein the flying height is 
lower. In cutting the wafer, the cutting resistance of a grinder at the 
end thereof is not sufficiently reduced. Therefore, the cut section is 
curved by warping of the grinder, so that the slider is deformed. 
Furthermore, the cut section has a machining residual stress retained 
thereon due to the machining resistance. This stress can be reduced by 
polishing, but as shown in FIG. 3, there is a difference in the conditions 
of the machined slider such as the shape and surface finish thereof 
between the flying face side and the opposite side of the slider. 
Therefore, there is a difference in the amount of the machining residual 
stress between the two face sides. This causes the deformation of the 
slider. The amount of deformation is about 0.01 to 0.02 .mu.m in prior 
art. There is a tendency that the smaller the Young's modulus of the 
slider, the larger the amount. However, this was not a problem in prior 
art. If the flying height is required to be in a small range of 0.05 to 
0.15 .mu.m, however, this amount cannot be ignored. As already mentioned, 
the flying height of the slider, i.e., the distance between the magnetic 
head element and the magnetic disc surface may be changed. In the worst 
case, the actual flying height may be about 5% of the set value without 
considering the deformation of the slider at the rear end thereof. Thus, 
the deformed amount as shown above is at least 7% to 20% of the set flying 
height of 0.05 to 0.15 .mu.m, which is clearly beyond 5%. Therefore, when 
the rear end of the slider is deformed, the slider may dangerously impinge 
on the magnetic disc. However, since a film formed on the magnetic disc is 
very thin, such impinging must always be avoided so as not to injure the 
magnetic disc. 
Therefore, the deformation of the rear end of the slider as well as the 
above-mentioned change in the flying height of the slider must be taken 
into account from now. If the change in the flying height is taken into 
account, a tolerated deformation of the rear end of the slider is at most 
about 5% of the set flying height, i.e., about 0.003 to 0.008 .mu.m, which 
has not yet been obtained by prior art sliders. In order to solve this 
problem, the structure of the slider may be considered such that the 
flying face is close to the center of the slider to reduce an influence 
from the deformation of the slider. In this case, however, the slider is 
susceptible to rolling during flying and thus has a problem in respect of 
flying stability. 
SUMMARY OF THE INVENTION 
One of the present invention is to solve the above-mentioned problems of 
slider deformation and provide a magnetic disc file in which the slider 
can fly in a stable state without impinging on a magnetic disc during 
seeking operation, the flying height of the slider from the magnetic disc 
is in the range of 0.05 to 0.15 .mu.m, and the area recording density is 
100 Mb/in.sup.2 or more. 
Another object of the present invention is to provide a thin film magnetic 
head in which the flying height of the slider is in the range of 0.05 to 
0.15 .mu.m, and the deformation of the rear end of the slider is 5% or 
less of the set flying height. 
A further object of the present invention is to provide a wafer for making 
the thin film magnetic head in which the flying height of the slider is in 
the range of 0.05 to 0.15 .mu.m, and the deformation of the rear end of 
the slider is 5% or less of the set flying height.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
The objects of the present invention are achieved by using a sintered wafer 
(substrate) of a metal oxide material containing fluorine. A spinel type 
oxide material may contain an alkali element or alkaline earth element in 
place of the fluorine. Furthermore, a ZrO.sub.2 material may contain an 
alkali element or Ba. 
The proportion of the fluorine is not less than 0.01 wt % to not more than 
2 wt % based on the entire wafer. Furthermore, the proportion of the 
alkali element or alkaline earth element not less than 0.01 wt % to not 
more than 8 wt % based on the entire wafer. For the other purposes, 
additional carbides, nitrides or borides such as SiC, TiC and etc. may be 
incorporated in such an amount that the proportion of the oxide is not 
less than 50 vol %, into the wafer. Furthermore, the average grain size of 
the sintered wafer is preferably 5 .mu.m or smaller. 
The spinel type oxide is represented by the general formula of MR.sub.2 
O.sub.4, wherein M is one or more divalent metals selected from Mg, Ca, 
Sr, Ba, Ni, Co, Mn and etc., and R is one or more trivalent metals 
selected from Al, Cr, Fe and etc. Furthermore, since a sintered body of 
pure ZrO.sub.2 is easily broken due to phase-transformation, Y.sub.2l 
O.sub.3 is normally added to the ZrO.sub.2 to form a solid solution which 
is in a cubic form stable at room temperature The thus obtained stabilized 
zirconia is generally used. 
The present inventors have found that a metal oxide material containing 
fluorine added thereto is sintered to form a sintered body of the metal 
oxide containing grain boundaries which are easily broken. If grain 
boundaries are easily broken, the sintered body is preferentially broken 
at the boundaries, when a grinder cuts the sintered body. Thus, cuttings 
are easily formed and machining resistance is reduced. Therefore, 
machinability is good. Furthermore, the deformation of a grinder in 
machining or the residual stress of a slider when machined is considerably 
decreased Accordingly, the deformation of a slider contributed to by the 
deformation of a grinder and/or the residual stress of a slider is also 
remarkably decreased. Chipping occurring at the edges of a slider is less 
as the machining resistance is reduced. Furthermore, since cracks are easy 
to propagate along the grain boundaries, the chipping easily occurs at a 
unit of grain. Therefore, the size of chippings is smaller as crystal 
grain size is decreased. 
The amount of the fluorine contained is required to be 0.01 wt % or more of 
the entire body of wafer for obtaining such advantages as mentioned above. 
If it is too much, the wafer itself is brittle. Therefore, it is desirably 
2 wt % or less. 
The grain size of the sintered body of wafer is preferably 5 .mu.m or 
smaller for making smaller size chippings by allowing cracks to detour as 
mentioned above. 
In the case of the spinel type oxide material, use of an alkali element or 
alkaline earth element in place of the fluorine can afford the same 
advantages. Furthermore, in the case of ZrO.sub.2 materials, use of an 
alkali element or Ba can afford the same advantages. The amount of these 
elements contained is properly in the range of not less than 0.01 wt % to 
not more than 8 wt %. Carbides, nitrides or borides such as SiC or TiC may 
be added in such an amount that they do not exceed 50 vol % of the entire 
wafer. If they are contained in an amount exceeding 50 vol %, then the 
advantages of the oxides, for example, good sinterability is unpreferably 
lost. 
When a thin film magnetic head or the wafer for the thin film magnetic head 
is prepared by using the wafer as mentioned above, the machinability of 
the wafer is very good. 
According to the present invention, wafers of various metal oxides for 
magnetic heads can easily be prepared with a good precision. Therefore, a 
small size magnetic head with a high precision can be produced at an 
improved yield. 
As a result, impinging between the magnetic head and the magnetic disc can 
be avoided at a flying height of a slider is in the range of 0.05 to 0.15 
.mu.m. Therefore, writing and reading of information at an area recording 
density of 100 Mb/in.sup.2 or more can be stably carried out. 
The present invention will be illustrated below with reference to some 
examples. However, the present invention should not be limited to these 
examples. 
EXAMPLE 1 
FIGS. 1 and 2 are pictorial views showing the whole and plane, 
respectively, of an example of the magnetic disc files according to the 
present invention. The magnetic disc file of this example was comprised of 
a magnetic disc 1 for recording information, a DC motor 2 for rotating 
this disc, a magnetic head 3 for writing or reading the information, an 
actuator 4 and voice coil motor 5 for supporting the head and changing the 
position of the head in relation to the magnetic disc, and an air filter 6 
for maintaining the interior of the file clean. The actuator was comprised 
of a carriage 7, rail 8 and bearing 9. The voice coil motor was comprised 
of a voice coil 10 and magnet 11. In the example as shown in FIGS. 1 and 
2, 8 plates of the magnetic disc were mounted on the same rotary axis to 
increase a total storage capacity. 
FIG. 3 is a pictorial view of an example of the present invention, i.e., a 
tapered flat-type magnetic head which was comprised of a slider 12 and 
magnetic head elements 13 formed on the part of the surface of said 
slider. 
In this example, a track density was set to 2,000 T/in, a bit density to 50 
kb/in, and a recording wavelength to 1.4 .mu.m, in order to obtain an are 
recording density of 100 Mb/in.sup.2. For this, the magnetic head element 
was prepared to have a gap width of 10 .mu.m. Furthermore, the magnetic 
disc used had a magnetic layer of 0.06 .mu.m thick and which comprised 
Co-containing sputtered type magnetic layer having a coercivity of 1,600 
Oe. Furthermore, the designed flying height of the slider was set to 0.10 
.mu.m. For this, the width of the flying face of the slider was machined 
in 0.26 mm. 
Table 1 shows the slider material used and the deformation amount of the 
machined slider measured. 10 magnetic heads of each of the thus obtained 
magnetic heads were built in the magnetic disc file and allowed to fly by 
rotating the magnetic disc. The writing and reading properties were 
examined. A so-called over-writing property was in a good range of -23 to 
-27 dB. This was found to be usable as a store. 
Next, each magnetic head was reciprocated between the outside periphery and 
the inside periphery of the magnetic disc, i.e., the so-called seeking 
operation was repeated. After this repeating was made 10,000 times, the 
magnetic disc was removed out. Superficial scratches were investigated 
detailedly. The proportion of the magnetic discs having scratches thereon 
is shown in Table 1. There were found no scratches on the disc in the case 
that the amount of the slider deformation was 0.005 .mu.m which was 5% of 
the designed flying height of the slider, and there were scratches on the 
disc when the amount of deformation exceeded 5%. That is, it was found 
that, if the amount of deformation exceeded 5%, the magnetic head impinged 
on the magnetic disc. 
The similar results to the above were obtained by a negative-pressure type 
slider. 
TABLE 1 
__________________________________________________________________________ 
Amount of Slider 
Proportion of 
Deformation 
Magnetic Disc 
Slider Material 
(.mu.m) with Scratches 
Notes 
__________________________________________________________________________ 
MgAl.sub.2 O.sub.4 
0.020 4/10 Comp. Example 
Al.sub.2 O.sub.3 -- 
0.010 2/10 Comp. Example 
30 vol % TiC 
(ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
0.007 1/10 Comp. Example 
2 wt % BaO 
(ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
0.005 0/10 The invention 
5 vol % Al.sub.2 O.sub.3)-- 
5 vol % TiC-- 
2 wt % BaF.sub.2 
MgAl.sub.2 O.sub.4 -- 
0.003 0/10 The invention 
30 vol % SiC-- 
2 wt % BaF.sub.2 
Al.sub.2 O.sub.3 -- 
0.002 0/10 The invention 
30 vol % TiC-- 
2 wt % BaF.sub.2 
__________________________________________________________________________ 
EXAMPLE 2 
In the magnetic disc file of EXAMPLE 1, a track density was set to 3,000 
T/in, a bit density to 100 kb/in, and a recording wavelength to 0.65 
.mu.m, in order to obtain an area recording density of 300 Mb/in.sup.2. 
For this, the magnetic head element was prepared to have a gap width of 6 
.mu.m. Furthermore, the magnetic disc used had a magnetic layer of 0.05 
.mu.m thick and which comprised a Co-containing sputtered type magnetic 
layer having a coercivity of 1,650 Oe. Furthermore, the designed flying 
height of the slider was set to 0.05 .mu.m. For this, the width of the 
flying face of the slider was machined in 0.21 mm. The same slider 
materials as those same manner as in EXAMPLE 1. 
As a result, an overwriting property was in a good range of -23 to -27 dB 
in the area recording density of 300 Mb/in.sup.2 Scratches on the surface 
of the magnetic disc after seeking operation were searched and the result 
is shown in Table 2. From this table, it is found that there were no 
scratches on the disc when the amount of the slider deformation was below 
0.0025 .mu.m, i.e., below 5% of the designed flying height, and there were 
scratches if this amount exceeded 5%. That is, the magnetic disc impinged 
on the magnetic head when the deformation amount exceeded 5%. 
The similar results to the above were obtained by a negative-pressure type 
slider. 
TABLE 2 
__________________________________________________________________________ 
Amount of Slider 
Proportion of 
Deformation 
Magnetic Disc 
Slider Material 
(.mu.m) with Scratches 
Notes 
__________________________________________________________________________ 
Al.sub.2 O.sub.3 -- 
0.008 3/10 Comp. Example 
30 vol % TiC 
(ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
0.006 2/10 Comp. Example 
2 wt % BaF.sub.2 
(ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
0.004 1/10 Comp. Example 
5 vol % Al.sub.2 O.sub.3 -- 
5 vol % TiC-- 
2 wt % BaF.sub.2 
MgAl.sub.2 O.sub.4 -- 
0.002 0/10 The invention 
30 vol % SiC-- 
2 wt % BaF.sub.2 
Al.sub.2 O.sub.3 -- 
0.002 0/10 The invention 
30 vol % TiC-- 
2 wt % BaF.sub.2 
__________________________________________________________________________ 
EXAMPLE 3 
It was examined whether or not the magnetic disc and the slider impinged on 
each other, in the same manner as in EXAMPLES 1 and 2, in the case that 
the set flying height was 0.15 .mu.m. As a result, the similar results to 
those in EXAMPLES 1 and 2 were obtained in this example. These results are 
shown in FIG. 6 together with the results of EXAMPLES 1 and 2. This figure 
shows a relationship between the amount of the slider deformation and the 
rate at which said slider impinged on the magnetic disc in the case of a 
designed flying height in a range of 0.05 to 0.15 .mu.m. From this figure, 
it is found that the magnetic head did not impinge on the magnetic disc 
when the amount of deformation was below 5%. 
EXAMPLE 4 
An example of a magnetic head exhibiting a small amount of deformation is 
described below. In this example, the flying face of a slider had a width 
of 0.26 mm. 
Firstly, a method of producing an Al.sub.2 O.sub.3 slider material is 
described. 
Table 3 shows compositions of raw material mixtures, each of which was 
uniformly mixed with water as medium by a ball mill for 10 to 50 hours, 
and then greater particles were ground. Table 3 also shows a composition 
containing TiC. Since addition of TiC provides an effect that the grain 
size are made smaller, it is possible to add TiC in such an amount that 
the volume of Al.sub.2 O.sub.3 is less than 50%. In Nos. 12 and 13, CaO 
and BaO, respectively, were added in a carbonate form. 
The resultant slurry was dried under agitation and then charged into a 
metal mold and compacted. 
The resultant compact was hot press sintered in a furnace containing an 
inert gas atmosphere at the temperature as shown in Table 3 for 30 
minutes. The pressure of the hot pressing was 400 kgf/cm.sup.2. The 
surface of the resultant sintered body was mirror-like polished to a 
maximum surface roughness of 0.1 .mu.m to form a substrate for a thin film 
magnetic head. 
A part of the substrate was cut, etched and examined for the grain size 
thereof. The etched section was observed by a scanning electron 
microscope. The average grain size was determined by the intercept method. 
The results are shown in Table 3. For the compositions according to the 
present invention, it was about 1.5 .mu.m when TiC was added, and about 
3.0 .mu.m when TiC was not added. In both the cases, the grain size was 
lower than 5 .mu.m. The grain size may be controlled by changing the 
sintering temperature. The grain size of prior art materials is about 3 
.mu.m, which is approximately identical with that of the present 
invention. 
Then, the amount of fluorine retained in the substrate was analyzed. The 
results are shown in Table 3. In all the cases, it was within the range of 
0.37 to 0.85 wt %, which falls within the scope of the present invention. 
This broad range of the residual amount of the additive is considered to 
be contributed to by change in melting point, vapor pressure and atomic 
weight. Examination of the position of fluorine by an electron microscope 
revealed that the fluorine was partially segregated at the grain 
boundaries of Al.sub.2 O.sub.3. 
A thin film magnetic head as shown in FIG. 3 was made from the substrate. 
This magnetic head was constituted by a slider 12 made from the sintered 
body above and a magnetic head element 13. In the step of machining the 
slider to form slider channels, chipping should be noted. Therefore, the 
maximum size of chippings produced at the ridges of channels machined in 
the substrate with a diamond grinder of #1500 was examined. The feed rate 
of the grinder was 0.3 mm/sec. For comparison of machining resistance the 
consumed electrical power of the rotary wheel of the grinder during the 
machining was determined with relative values. The results are shown in 
Table 3. It is seen that the fluorine-containing materials according to 
the present invention had a smaller chipping size and lower machining 
resistance than those of the prior art materials, Al.sub.2 O.sub.3 -TiC 
(Nos. 11 to 13). Then, the amount of the slider deformation was also 
examined The results are shown in Table 3. It is seen that the material of 
the present invention having a lower machining resistance had a very small 
amount of deformation. That is, use of the fluorine-containing material 
according to the present invention can reduce the size of chippings 
produced when the material is machined in the thin film magnetic head. The 
machining can be carried out with precision. Furthermore, from the 
comparison of Nos. 7 and 9 with Nos. 12 and 13 it is seen that the 
effective element of BaF.sub.2 and CaF.sub.2 added to the Al.sub.2 O.sub.3 
was not Ba and Ca but fluorine. 
TABLE 3 
__________________________________________________________________________ 
Sintering 
Grain 
F Max. Size 
Machining 
Amount of Slider 
Temp. Size Content 
of Chip- 
Resistance 
Deformation 
No. Composition (.degree.C.) 
(.mu.m) 
(wt %) 
pings (.mu.m) 
(relative) 
(.mu.m) Notes 
__________________________________________________________________________ 
1 Al.sub.2 O.sub.3 --2 wt % MgF.sub.2 
1500 3.2 0.73 4.4 7.0 0.003 The Invention 
2 Al.sub.2 O.sub.3 --2 wt % CaF.sub.2 
1500 3.1 0.84 4.3 7.1 0.003 The Invention 
3 Al.sub.2 O.sub.3 --2 wt % SrF.sub.2 
1500 3.3 0.56 4.4 7.2 0.003 The Invention 
4 Al.sub.2 O.sub.3 --2 wt % BaF.sub.2 
1500 3.5 0.38 4.3 6.8 0.003 The Invention 
5 Al.sub.2 O.sub.3 --2 wt % LiF 
1500 2.9 0.37 4.0 7.2 0.003 The Invention 
6 Al.sub.2 O.sub.3 --30 vol % TiC-- 
1500 1.5 0.72 2.3 6.9 0.002 The Invention 
2 wt % MgF.sub.2 
7 Al.sub.2 O.sub.3 --30 vol % TiC-- 
1500 1.6 0.85 2.1 7.3 0.002 The Invention 
2 wt % CaF.sub.2 
8 Al.sub.2 O.sub.3 --30 vol % TiC-- 
1500 1.4 0.55 2.0 7.1 0.002 The Invention 
2 wt % SrF.sub.2 
9 Al.sub.2 O.sub.3 --30 vol % TiC-- 
1500 1.3 0.39 2.3 7.0 0.002 The Invention 
2 wt % BaF.sub.2 
10 Al.sub.2 O.sub.3 --30 vol % TiC-- 
1500 1.5 0.38 2.1 7.2 0.002 The Invention 
2 wt % LiF 
11 Al.sub.2 O.sub.3 --30 vol % TiC 
1600 2.9 8.8 15 0.010 Comp. Ex. 
12 Al.sub.2 O.sub.3 --2 wt % CaO 
1600 3.1 7.5 13 0.010 Comp. Ex. 
13 Al.sub.2 O.sub.3 --2 wt % BaO 
1600 3.1 7.6 13 0.010 Comp. 
__________________________________________________________________________ 
Ex. 
EXAMPLE 5 
An example of a spinel type oxide slider material is described below. 
Raw materials having the compositions as shown in Table 4 were sintered in 
the same manner as in EXAMPLE 4 to form substrates which were then 
examined. 
Table 4 shows compositions containing SiC. Since addition of SiC provides 
an effect that the grain size are made smaller, it is possible to add SiC 
in such an amount that the volume of the spinel is less than 50 vol % 
Li.sub.2 O, CaO and BaO were added in a carbonate form in Nos. 19, 25, 28, 
31, 34, 37, 40 and 43 to 45. The results are shown in Table 4. The grain 
size of the fluorine-containing materials according to the present 
invention was about 1.5 .mu.m when SiC was added and about 3 .mu.m when 
SiC was not added. In both the cases, the size was lower than 5 .mu.m. The 
grain size can be controlled by changing the sintering temperature. The 
grain size of prior art materials (Nos. 46 and 47) is about 3 .mu.m, which 
is approximately identical with that of the present invention. 
Furthermore, ten substrates were analyzed for the added alkali elements or 
alkaline earth elements and fluorine retained therein. However, the 
analysis of the fluorine was converted to the amount of the alkaline earth 
element in the case that the alkaline earth element added was the same as 
that of the spinel (Nos. 14, 20, 27 and 30), because they could not be 
separated from each other. The results are shown in Table 4. In all the 
cases, the amounts of the alkali elements and alkaline earth elements were 
0.14 to 1.4 wt %, and the amount of fluorine was 0.3 to 0.8 wt %, which 
all fall within the scope of the present invention. This broad range of 
the residual amount of the additive is, considered to be contributed to by 
change in melting point, vapor pressure and atomic weight. Examination of 
the position of the above element by an electron microscope revealed that 
those were partially segregated at the grain boundaries of the spinel 
crystal grains. 
Thin film magnetic heads as shown in FIG. 3 were made from the substrates. 
In the step of machining the slider to form slider channels, chipping 
should be noted. Therefore, the maximum size of chippings produced at the 
ridges of channels machined in the substrate with a diamond grinder of 
#1500 was examined. The feed rate of the grinder was 0.2 mm/sec. Machining 
resistance was compared by relative values. The results are shown in Table 
4. It is seen that the amount of chippings of the fluorine-containing 
material according to the present invention is extremely reduced as 
compared with that of a prior art material, MgAl.sub.2 O.sub.4 (No. 46), 
and the machining resistance of said fluorine-containing material is lower 
than that of Al.sub.2 O.sub.3 -TiC (No. 47). Then, the amount of the 
slider deformation was determined. The results are also shown in Table 4. 
It is seen that the flurine-containing material has a smaller amount of 
deformation as the machining resistance is lower. Furthermore, in the case 
of the spinel type oxide, it is seen that incorporation of an alkali 
element or alkaline earth element such as Li, Ba and Ca in place of 
fluorine affords the same advantages as afforded by incorporation of 
fluorine (Nos. 19, 25, 28, 31, 37, 40 and 43 to 45). 
TABLE 4 
__________________________________________________________________________ 
Amount Amount Amount 
Sin- of Residual 
of Max. of Slider 
tering 
Grain 
Alkali and 
Residual 
Size of 
Machining 
Deforma- 
Temp. 
Size 
Alkaline 
F Chippings 
Resistance 
tion 
No. Composition (.degree.C.) 
(.mu.m) 
Earth (wt %) 
(wt %) 
(.mu.m) 
(relative) 
(.mu.m) 
Notes 
__________________________________________________________________________ 
14 MgAl.sub.2 O.sub.4 --2 wt % MgF.sub.2 
1450 
3.3 0.5 0.8 4.2 7.2 0.005 
The Invention 
15 MgAl.sub.2 O.sub.4 --2 wt % CaF.sub.2 
1450 
3.0 0.9 0.8 4.0 7.0 0.005 
The Invention 
16 MgAl.sub.2 O.sub.4 --2 wt % SrF.sub.2 
1450 
3.5 1.3 0.5 4.3 7.8 0.005 
The Invention 
17 MgAl.sub.2 O.sub.4 --2 wt % BaF.sub.2 
1450 
3.2 1.2 0.3 4.1 7.1 0.005 
The Invention 
18 MgAl.sub.2 O.sub.4 --2 wt % LiF 
1450 
2.8 0.14 0.7 3.9 7.1 0.005 
The Invention 
19 MgAl.sub.2 O.sub.4 --2 wt % Li.sub.2 O 
1450 
3.2 0.9 4.1 7.0 0.005 
The Invention 
20 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.6 0.5 0.8 2.2 7.1 0.003 
The Invention 
2 wt % MgF.sub.2 
21 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.4 0.9 0.8 2.1 6.8 0.003 
The Invention 
2 wt % MgF.sub.2 
22 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.3 1.3 0.6 1.9 7.1 0.003 
The Invention 
2 wt % SrF.sub.2 
23 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.5 1.2 0.3 2.2 7.2 0.003 
The Invention 
2 wt % BaF.sub.2 
24 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.6 0.14 0.7 2.0 6.9 0.003 
The Invention 
2 wt % LiF 
25 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.5 0.9 1.9 7.0 0.003 
The Invention 
2 wt % Li.sub.2 O 
26 BaAl.sub.2 O.sub.4 --2 wt % CaF.sub.2 
1450 
3.5 0.9 0.8 4.3 7.1 0.005 
The Invention 
27 BaAl.sub.2 O.sub.4 --2 wt % BaF.sub.2 
1450 
3.2 1.2 0.3 4.1 6.8 0.005 
The Invention 
28 BaAl.sub.2 O.sub.4 --2 wt % Li.sub.2 O 
1450 
3.3 0.9 4.2 7.0 0.005 
The Invention 
29 BaAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.5 0.9 0.8 2.1 7.2 0.003 
The Invention 
2 wt % CaF.sub.2 
30 BaAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.4 1.2 0.3 2.0 6.9 0.003 
The Invention 
2 wt % BaF.sub.2 
31 BaAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.6 0.9 2.1 7.1 0.003 
The Invention 
2 wt % Li.sub.2 O 
32 MnAl.sub.2 O.sub.4 --2 wt % CaF.sub.2 
1450 
3.2 0.9 0.8 4.2 7.2 0.005 
The Invention 
33 MnAl.sub.2 O.sub.4 --2 wt % BaF.sub.2 
1450 
3.0 1.2 0.3 4.2 6.8 0.005 
The Invention 
34 MnAl.sub.2 O.sub.4 --2 wt % Li.sub.2 O 
1450 
3.4 0.9 4.3 6.9 0.005 
The Invention 
35 MnAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.5 0.9 0.8 2.0 7.0 0.003 
The Invention 
2 wt % CaF.sub.2 
36 MnAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.4 1.2 0.3 2.2 7.1 0.003 
The Invention 
2 wt % BaF.sub.2 
37 MnAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.5 0.9 1.9 6.8 0.003 
The Invention 
2 wt % Li.sub.2 O 
38 MgCr.sub.2 O.sub.4 --2 wt % CaF.sub.2 
1450 
3.2 0.9 0.8 4.2 7.1 0.005 
The Invention 
39 MgCr.sub.2 O.sub.4 --2 wt % BaF.sub.2 
1450 
3.0 1.2 0.3 4.1 6.9 0.005 
The Invention 
40 MgCr.sub.2 O.sub.4 --2 wt % Li.sub.2 O 
1450 
2.9 0.9 4.0 7.1 0.005 
The Invention 
41 MgCr.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.4 0.9 0.8 2.0 7.0 0.003 
The Invention 
2 wt % CaF.sub.2 
42 MgCr.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.5 1.2 0.3 1.9 6.9 0.003 
The Invention 
2 wt % BaF.sub.2 
43 MgCr.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.6 0.9 2.2 7.2 0.003 
The Invention 
2 wt % Li.sub.2 O 
44 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.5 1.3 2.2 6.9 0.003 
The Invention 
2 wt % CaO 
45 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 
1.5 1.4 2.2 7.2 0.003 
The Invention 
2 wt % BaO 
46 MgAl.sub.2 O.sub.4 
1450 
3.2 32.0 149.2 0.020 
Comp. Ex. 
47 Al.sub.2 O.sub.3 --30 vol % TiC 
1600 
2.9 5.5 12.3 0.010 
Comp. 
__________________________________________________________________________ 
Ex. 
EXAMPLE 6 
A slider material comprising ZrO.sub.2 is described below. 
Raw material powders having the compositions as shown in Table 5 were 
sintered in the same manner as in EXAMPLE 5 and then examined. As already 
mentioned, since a stabilizer is needed for stabilizing the crystal phase 
of ZrO.sub.2, Y.sub.2l O.sub.3 was selected in this example and added in 
an amount of 9 mol % based on ZrO.sub.2. Generally various additives such 
as TiC, SiC and Al.sub.2 O.sub.3 may be added in such an amount that 
ZrO.sub.2 or a total of metal oxides is not below 50 vol % of the sintered 
body. In Nos. 48 and 54, Li.sub.2 O and BaO, respectively, were added in a 
carbonate form. 
The results of the examination are shown in Table 5. Various grain sizes in 
the range of 3.6 to 8.5 were obtained. It is seen from Table 5 that the 
grain size can be reduced to 5 .mu.m or less by adding TiC or SiC. In No. 
57, the grain size was increased to 8.5 .mu.m, in spite of the addition of 
TiC. Electron microscopic analysis revealed that BaF.sub.2 was reacted 
with Al.sub.2 O.sub.3 to form a complex oxide of Ba and Al, which formed a 
liquid phase during the sintering. The liquid phase promoted the grain 
growth to increase the grain size. The residual amount of fluorine was 
0.08 to 0.8 wt %, and the residual amount of Ba and the alkali element was 
0.14 to 1.4 wt %, which amounts fall within the scope of the present 
invention. 
The size of chippings by machining is much lower for the 
fluorine-containing materials than for comparative materials of ZrO.sub.2 
and ZrO.sub.2 plus TiC (Nos. 59 and 60). The same things are applicable to 
the machining resistance and the amount of the slider deformation. 
Furthermore, the ZrO.sub.2 -containing materials may contain an alkali 
element or Ba in place of fluorine, which afforded the same advantages 
(Nos. 48 and 54). 
TABLE 5 
__________________________________________________________________________ 
Amount Amount Amount 
Sin- of Residual 
of Max. of Slider 
tering 
Grain 
Alkali and 
Residual 
Size of 
Machining 
Deforma- 
Temp. 
Size 
Alkaline 
F Chippings 
Resistance 
tion 
No. Composition (.degree.C.) 
(.mu.m) 
Earth (wt %) 
(wt %) 
(.mu.m) 
(relative) 
(.mu.m) 
Notes 
__________________________________________________________________________ 
48 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
6.2 0.9 5.4 8.5 0.007 
The Invention 
2 wt % Li.sub.2 O 
49 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
6.5 0.14 0.7 5.1 8.2 0.007 
The Invention 
2 wt % LiF 
50 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
6.1 0.5 0.8 5.3 8.4 0.007 
The Invention 
2 wt % MgF.sub.2 
51 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
6.3 0.9 0.8 5.2 8.1 0.007 
The Invention 
2 wt % CaF.sub.2 
52 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
6.2 1.3 0.5 5.3 8.3 0.007 
The Invention 
2 wt % SrF.sub.2 
53 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
6.7 1.2 0.3 5.5 8.2 0.007 
The Invention 
2 wt % BaF.sub.2 
54 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
6.6 1.4 5.3 8.5 0.007 
The Invention 
2 wt % BaO 
55 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
4.2 1.2 0.3 4.3 7.5 0.006 
The Invention 
5 vol % TiC-- 
2 wt % BaF.sub.2 
56 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
2.2 1.2 0.3 2.6 6.9 0.005 
The Invention 
5 vol % TiC-- 
5 vol % SiC-- 
2 wt % BaF.sub.2 
57 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
8.5 1.2 0.3 6.8 9.5 0.005 
The Invention 
5 vol % TiC-- 
5 vol % Al.sub.2 O.sub.3 -- 
2 wt % BaF.sub.2 
58 (ZrO.sub. 2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
3.6 0.3 0.08 
3.5 7.2 0.005 
The Invention 
5 vol % TiC-- 
5 vol % Al.sub.2 O.sub.3 -- 
0.5 wt % BaF.sub.2 
59 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3) 
1400 
6.6 42 180 0.024 
Comp. Ex. 
60 (ZrO.sub.2 --9 mol % Y.sub.2 O.sub.3)-- 
1400 
4.4 38 160 0.023 
Comp. Ex. 
5 vol % TiC 
__________________________________________________________________________ 
EXAMPLE 7 
In order to examine the influence of an amount of fluorine, raw materials 
of the composition as shown in Table 6 were sintered, machined and 
examined. The results are shown in Table 6. From this table, it is seen 
that the larger the amount of BaF.sub.2 added, the larger the residual 
amount of fluorine. However, it is seen that the amount of fluorine had no 
influence on the grain size. As a result, the size of chippings, machining 
resistance and deformation amount of the slider materials containing less 
than 0.01 wt % of residual fluorine were large. Thus, such slider 
materials were not proper (Nos. 67 and 71). In the case of the amount of 
residual fluorine being greater than 2 wt %, on the other hand, the 
machining resistance and the amount of deformation were slightly reduced, 
but the size of chippings became greater (Nos. 70 and 74). When the 
residual amount of fluorine was more than 2 wt %, the sintered body became 
brittle. The slidability of the head with the disc were almost unchanged. 
From the foregoing it is clear that the residual amount of fluorine should 
be within the range of not less than 0.01 wt % but not more than 2 wt % in 
respect of the machinability. The same things are applicable to the oxides 
other than ZrO.sub.2. 
TABLE 6 
__________________________________________________________________________ 
Sintering 
Grain 
F Max. Size 
Machining 
Amount of Slider 
Temp. Size Content 
of Chip- 
Resistance 
Deformation 
No. Composition (.degree.C.) 
(.mu.m) 
(wt %) 
pings (.mu.m) 
(relative) 
(.mu.m) Notes 
__________________________________________________________________________ 
67 Al.sub.2 O.sub.3 --0.03 wt % BaF.sub.2 
1500 3.4 0.006 
13 35 0.013 Comp. Ex. 
68 Al.sub.2 O.sub.3 --0.05 wt % BaF.sub.2 
1500 3.5 0.01 4.6 7.0 0.003 The Invention 
69 Al.sub.2 O.sub.3 --10 wt % BaF.sub.2 
1500 3.5 2.0 4.6 7.0 0.003 The Invention 
70 Al.sub.2 O.sub.3 --13 wt % BaF.sub.2 
1500 3.4 2.5 8.9 5.1 0.003 Comp. Ex. 
71 Al.sub.2 O.sub.3 --30 vol % TiC 
1500 1.4 0.006 
7.6 13 0.010 Comp. Ex. 
0.03 wt % BaF.sub.2 
72 Al.sub.2 O.sub.3 --0.03 vol % TiC 
1500 1.3 0.01 2.2 7.1 0.002 The Invention 
0.05 wt % BaF.sub.2 
73 Al.sub.2 O.sub.3 --0.03 vol % TiC 
1500 1.3 2.0 2.1 6.9 0.002 The Invention 
10 wt % BaF.sub.2 
74 Al.sub.2 O.sub.3 --0.03 vol % TiC 
1500 1.2 2.5 9.2 5.2 0.002 Comp. Ex. 
13 wt % BaF.sub.2 
__________________________________________________________________________ 
EXAMPLE 8 
Raw materials having the compositions as shown in Table 7 were sintered, 
machined and examined for investigating the influence of an alkali element 
and alkaline earth element on the slider. Li.sub.2 O and BaO were added in 
a carbonate form. The results are shown in Table 7. From this table, it is 
seen that the residual amounts of Li and Ba are increased as the amounts 
of Li.sub.2 O and BaO added are increased. However, the amounts of Li and 
Ba had no influence on the grain size. As a result, the size of chippings, 
machining resistance and amount of the slider deformation became greater 
in the case of the residual amounts of Li and Ba being less than 0.01 wt 
%. Thus, no good materials were obtained (Nos. 75 and 79). On the other 
hand, the machining resistance and amount of the slider deformation were 
slightly reduced but the size of chippings was greater in the case of the 
amounts of Li and Ba being more than 8 wt % (Nos. 78 and 82). Thus, it is 
seen that if the residual amounts of Li and Ba are 8 wt %, the sintered 
body is brittle. The slidability of the slider with the disc was almost 
the same in all the cases above. 
From the foregoing it is clear that the residual amounts of Li and Ba 
should be within the range of not less than 0.01 wt % and not more than 8 
wt % in respect to the machinability. The same things are applicable not 
only to the case of Li and Ba but also to the case of Mg, Ca, Sr and etc. 
and furthermore to the case of Li and Ba added to ZrO.sub.2. 
TABLE 7 
__________________________________________________________________________ 
Amount 
of Residual Amount of 
Sintering 
Grain 
Alkali and 
Max. Size of 
Machining 
Slider 
Temp. 
Size Alkaline 
Chippings 
Resistance 
Deformation 
No. Composition (.degree.C.) 
(.mu.m) 
Earth (wt %) 
(.mu.m) 
(relative) 
(.mu.m) 
Notes 
__________________________________________________________________________ 
75 MgAl.sub.2 O.sub.4 --0.013 wt % Li.sub.2 O 
1450 3.1 0.006 
13 49 0.016 Comp. Ex. 
76 MgAl.sub.2 O.sub.4 --0.02 wt % Li.sub.2 O 
1450 3.0 0.01 4.2 7.2 0.005 The Invention 
77 MgAl.sub.2 O.sub.4 --17 wt % Li.sub.2 O 
1450 3.2 8 4.3 7.0 0.005 The Invention 
78 MgAl.sub.2 O.sub.4 --19 wt % Li.sub.2 O 
1450 3.1 9 11 5.1 0.004 Comp. Ex. 
79 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 1.4 0.006 
12 6.2 0.003 Comp. Ex. 
0.007 wt % BaO 
80 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 1.6 0.01 2.2 7.3 0.003 The Invention 
0.01 wt % BaO 
81 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 1.5 8 2.3 7.2 0.003 The Invention 
9 wt % BaO 
82 MgAl.sub.2 O.sub.4 --30 vol % SiC-- 
1450 1.5 9 11 5.5 0.002 Comp. Ex. 
10 wt % BaO 
__________________________________________________________________________