Hardened alumina material

The disclosure refers to the art of remarkably increasing the hardness of alumina materials, such as alumina abrasives, alumina ceramics or aluminum oxide single crystal (sapphire), containing aluminum oxide (Al.sub.2 O.sub.3) as the main component, or the hardness of products using the same. The alumina material is partially or wholly hardened to have a micro-Vickers hardness of no less than 2600 kgf/mm2 (under a test load of 300.times.9.807 mN) by inclusion of an oxide or fluoride additive which is solid-soluble in aluminum oxide. Preferable examples of the additive include TiO.sub.2 and/or Y.sub.2 O.sub.3.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to the art of remarkably increasing the 
hardness of alumina materials, such as alumina abrasives, alumina 
ceramics, aluminum oxide single crystal (sapphire) or the like, which 
contain aluminum oxide (Al.sub.2 O.sub.3) as the main component as well as 
the hardness of products prepared by using such a material. 
2. Description of the Related Art: 
An example of alumina abrasive having high-purity aluminum oxide (Al.sub.2 
O.sub.3) is the WA abrasive. As shown in FIG. 1, this abrasive has a 
Vickers hardness of about 2000 kgf/mm2 (see WA# 24 in FIG. 1). FIG. 1 is a 
graph showing the results obtained by hardness investigation of various 
abrasive particles. It should be noted that even silicon carbide abrasive 
particles exhibit a Vickers hardness of only about 3000 kgf/mm2 , as 
indicated by GC#30 (available from A company), GC#30 (available from B 
company), C#24 (available from A company) and C#24 (available from B 
company) in FIG. 1. 
Therefore, if the hardness of the WA abrasive can be increased, it is 
applicable to a grinding process which requires the use of harder 
abrasives, such as C, GC or CBN, containing silicon carbide. 
An alumina ceramic cutting tool (so-called white ceramic tool) made by 
compacting and sintering fine particles of high purity aluminum oxide 
(Al.sub.2 O.sub.3) has a Vickers hardness of only about 1800 kgf/mm2 at 
best (see the non-treated tool (A) in FIG. 5). In this case, again, if the 
hardness of such a ceramic tool can be increased, the tool performance can 
be improved by restraining tool wear progression. 
Various kinds of ceramic products made by sintering aluminum oxide material 
(Al.sub.2 O.sub.3) have found use not only in the industrial fields but 
also in various other fields. In the industrial field, the alumina ceramic 
material is used as a bearing material for example. In general, alumina 
bearing balls conventionally used for ball bearings have a Vickers 
hardness of about 1800 kgf/mm2 . In this case, again, if the hardness of 
the ceramic products can be increased, their performance can be improved 
with respect to abrasion resistance and durability for example, thereby 
enabling expansion of the applicable fields. 
Further, a single crystal of aluminum oxide (Al.sub.2 O.sub.3) made by 
Verneul's method for example is used as an artificial jewel such as 
sapphire, ruby, alexandrite, topaz, aquamarine and spinel when containing 
additives or impurities. Besides, the single crystal is also used as a 
laser element, or as an abrasion resistant material for bearings in 
precision machines and wristwatchs or for record player needles. Because a 
single crystal artificial sapphire containing a small amount of impurities 
is colorless and transparent (a colorless single crystal of aluminum oxide 
(Al.sub.2 O.sub.3) being also called sapphire), the single crystal is used 
as a transparent cover plate for a wristwatch for example,. 
In the graph of FIG. 13 showing the results of hardness investigation for 
sapphire surfaces, the single crystal of aluminum oxide described above 
exhibits a Vickers hardness of slightly less than 2000 kgf/mm2 with cracks 
formed at the periphery of an indentation under a test load of 1000 gf, 
whereas it exhibits a Vickers hardness of slightly less than 2500 kgf/mm2 
with no cracks formed at the periphery of a crater under a test load of 50 
gf, as indicated by "Non-Treated Surface". Therefore, if the hardness of 
the single crystal of aluminum oxide can be increased, it is possible to 
improve the abrasion resistance, damage resistance and durability of the 
product made of such a single crystal. Further, if a cutting tool is 
produced by grinding hardened Z sapphire, it becomes possible to use the 
cutting tool for mirror grinding of ferrous metal which is impossible by a 
single-crystal diamond cutting tool unless a special measure such as 
ultrasonic machining is taken. 
Therefore, an object of the present invention is to provide an alumina 
material whose surface or interior is remarkably hardened. 
Another object of the present invention is to provide an alumina ceramic 
material which is remarkably hardened at least at its surface. 
Still another object of the present invention is to provide a single 
crystal of aluminum oxide which is remarkably hardened at least at its 
surface. 
Still another object of the present invention is to provide a method of 
remarkably hardening at least the surface of alumina material containing 
aluminum oxide as the main component. 
SUMMARY OF THE INVENTION 
Under the circumstances described above, the present invention has been 
completed as a result of various considerations to remarkably increase the 
hardness of the alumina materials and products described above. The 
primary concept of the present invention resides in partially or wholly 
hardening an alumina material containing aluminum oxide as the main 
component to have a micro-Vickers hardness of no less than 2600 kgf/mm2 
(under a test load of 300.times.9.807 mN) by inclusion of an oxide or 
fluoride additive which is solid-soluble in aluminum oxide. 
The alumina material described above refers to any form of alumina such as 
abrasive (abrasive particles), sintered ceramic, single crystal of 
alumina. 
The following methods may be applied for adding the additive to the alumina 
material described above. 
1. High-purity aluminum oxide (Al.sub.2 O.sub.3) is mixed with a suitable 
form of additive or a salt thereof, or with a solution of the additive or 
its salt which is later thermally decomposed, thereby providing material 
powder. Then, the material powder is subjected to thermal treatment for 
causing solid-solution of the additive in the aluminum oxide. 
2. Given high-purity aluminum oxide powder is mixed with additive powder to 
provide material powder which is then subjected to thermal treatment for 
causing solid-solution of the additive in the aluminum oxide. 
3. A given alumina-based abrasive material, alumina-based ceramic cutting 
tool, alumina-based ceramic material, alumina-based ceramic product, or 
alumina single crystal is brought into embedment in or or into contact 
with additive powder which is thermally treated to undergo inward 
diffusion by solid-solution. 
An oxide or fluoride which is solid-soluble in aluminum oxide (Al.sub.2 
O.sub.3) has been found suitable as the additive. Examples include 
Fe.sub.2 O.sub.3, SiO.sub.2, K.sub.2 O, MnO, Na.sub.2 O, Li.sub.2 O, 
Ga.sub.2 O.sub.3, Gd.sub.2 O.sub.3, N.sub.2 O.sub.5, SO.sub.3, Ho.sub.2 
O.sub.3, In.sub.2 O.sub.3, La.sub.2 O.sub.3, Li.sub.2 O, TiO.sub.2, 
Lu.sub.2 O.sub.3, ZrO.sub.2, Y.sub.2 O.sub.3, Nd.sub.2 O.sub.3, P.sub.2 
O.sub.5, SrO, PbO, Sc.sub.2 O.sub.3, RF.sub.2, ZnO, Sm.sub.2 O.sub.3, 
SnO.sub.2, Ta.sub.2 O.sub.5, Tm.sub.2 O.sub.5, UO.sub.2, V.sub.2 O.sub.5, 
Yb.sub.2 O.sub.3, NaF, RbF, AlF.sub.3, CaF.sub.2, NaF, LiF, MgF.sub.2, 
B.sub.2 O.sub.3, BaO, BeO, ThO.sub.2, Bi.sub.2 O.sub.3, CO.sub.2, CaO, 
Cr.sub.2 O.sub.3, K.sub.2 O, CeO.sub.2, Dy.sub.2 O.sub.3, Er.sub.2 
O.sub.3, Eu.sub.2 O.sub.3. One or more of these additives may form 
solid-solution with aluminum oxide. 
Experiments have shown that the proportion of the additive relative to the 
aluminum oxide in solid-solution may be no more than 3% by weight, 
preferably no more than 2.5% by weight, for providing a hardness increase. 
In this case, the alumina material undergoes a remarkable hardness 
increase to have a micro-Vickers hardness of at least 2600 kgf/mm2 (under 
a test load of 300.times.9.807 mN) and 4000 kgf/mm2 at maximum. 
Other aspects and advantages of the present invention will become apparent 
from the detailed description of the preferred embodiments given with 
reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following examples, the method 3 described above was adopted as a 
method for adding additives to alumina materials. Further, the description 
will be successively given to each case of adding TiO.sub.2 or the like 
to: high purity WA abrasive particles; alumina ceramic cutting tools 
(so-called white ceramic tool) made by compressing and sintering fine 
particles of high purity aluminum oxide (Al.sub.2 O.sub.3); general-use 
alumina ceramic materials obtained by adding a sintering agent to fine 
particles of aluminum oxide (Al.sub.2 O.sub.3) for shaping thereof in a 
suitable manner and for subsequent sintering thereof; and single crystals 
of aluminum oxide (sapphire). For heat treatment, the electric heating 
furnace shown in FIG. 2 was used. 
EXAMPLE 1 
&lt;high purity WA abrasive particles&gt; 
Table 1 shows the constituents and features of various kinds of abrasive 
particles. Of these abrasive particles, particulate WA abrasive having an 
aluminum oxide purity of no less than 99. 5% was used in this example. 
TABLE 1 
______________________________________ 
Features of Various Abrasive Particles 
Symbol of 
Abrasive Partic- 
les Constituents and Features 
______________________________________ 
WA white alumina abrasive particles 
containing no less than 99.5% of Al.sub.2 O.sub.3 and 
no Ti.sub.2 O.sub.3 
A alumina abrasive particles containing 
about 95% of Al.sub.2 O.sub.3 and a little Ti.sub.2 O.sub.3 
PA light purple alumina abrasive particles 
C black abrasive particles made by 
pulverizing ingots containing an ordinary 
purity of silicon carbide 
GC green abrasive particles made by 
pulverizing ingots containing a high 
purity of silicon carbide 
______________________________________ 
From "Dictionary of Technical Terms for Grinding & Polishing" (1972) 
edited by Particulate Abrasive Processing Society 
FIG. 3 is a graph illustrating a hardness increase of WA abrasive particles 
as a result of TiO.sub.2 -addition. For obtaining the results shown in 
this figure, the particulate WA abrasive was separately mixed with each of 
anatase, rutile and amorphous powder of TiO.sub.2 and heated in the 
furnace of FIG. 2 at a temperature of 1573 K for 50 hours before taking 
out. Then, only the WA abrasive particles was picked up and embedded in 
resin, and the Vickers hardness of the particulate WA abrasive was 
measured by cross-sectional polishing thereof. For comparison, the same 
graph also shows the hardness of non-treated WA#24 abrasive particles, 
WA#4 abrasive particles subjected only to heat treatment at 1573 K for 20 
hours, A#24 abrasive particles treated by TiO.sub.2 -addition at 1573 K 
for 20 hours, and non-treated A#24 abrasive particles in addition to the 
hardness of the WA#24 abrasive particles treated by TiO.sub.2 -addition in 
the above-described manner. 
The measurements of the Vickers hardness were performed according to JIS 
R1610-1991 "Vickers Hardness Testing Method for Fine Ceramics". The 
measurements were performed under different test loads. However, when the 
test load was greater than 300 gf, cracks were often formed at corners of 
the Vickers indentation. Occurance of cracks is indicated by blackening of 
the points in the graph. 
As shown in FIG. 3, "non-treated WA#24 abrasive particles" exhibited a 
Vickers hardness of about 2000 kgf/mm2, whereas "WA#24 abrasive particles 
treated by TiO.sub.2 -addition" exhibited an increased Vickers hardness of 
3000-4000 kgf/mm2. In FIG. 3, "WA#24, Heat Treatment only" indicates that 
the WA abrasive particles were subjected only to heat treatment without 
embedment in TiO.sub.2 powder, wherein the hardness of the abrasive 
particles did not increase to remain nearly as hard as "non-treated 
WA#24". Thus, it can be recongnized that the hardness of the abrasive 
particles does not increase by heat treatment alone. 
Further, in FIG. 3, "A#24, TiO.sub.2 -Addition Treatment" indicates that 
the A abrasive particles were embedded in TiO.sub.2 powder for heat 
treatment at 1573 K for 20 hours, which case provided substantially the 
same value as that of "non-treated A#24". Thus, it is confirmed that 
addition of TiO.sub.2 to the A#24 abrasive particles does not provide a 
hardness increase. 
Table 2, taken from JIS R6123-1987 "Explanation of Chemical Analysis Method 
for Alumina Abrasives", shows averages and ranges of impurities in 
abrasive particles as a result of experiments conducted by eight 
companies. This table shows that the amount of TiO.sub.2 contained in WA 
abrasive particles is too small to be detected. TiO.sub.2 contained in A 
abrasive particles is about 3%. Since the particulate A abrasive indicated 
by "A#24, TiO.sub.2 -addition treatment" contains no less than about 3% of 
TiO.sub.2, addition of TiO.sub.2 beyond about 3% will not provide a 
hardness increase. Therefore, the amount of TiO.sub.2 to be added to the 
WA abrasive particles should be no more than 3% for hardness increase. 
TABLE 2 
______________________________________ 
Averages and Ranges of Experimental Results on 
Alumina Abrasives Conducted by Eight Companies 
Abrasive 
Impurity Constituents (wt %) 
Particles 
SiO.sub.2 
Fe.sub.2 O.sub.3 
TiO.sub.2 
CaO MgO ZrO.sub.2 
Na.sub.2 O 
______________________________________ 
WA x 0.018 0.019 -- -- -- -- -- 
R 0.006 0.003 -- -- -- -- -- 
A x 0.807 0.105 2.984 0.087 
0.157 0.192 
0.389 
R 0.051 0.013 0.043 0.012 
0.009 0.013 
0.017 
PA x 0.022 0.023 -- -- -- -- 0.233 
R 0.001 0.004 -- -- -- -- 0.020 
______________________________________ 
x: Averages of Eight Companies From JIS R61231987 p28 
R: Range from the Averages "Explanation of Chemical Analysis Method for 
Alumina Abrasives 
FIG. 4 shows the results obtained by investigating the hardness increase of 
WA particles under different conditions of TiO.sub.2 -addition treatment. 
It has been found that the hardness increase becomes less as the 
high-temperature treatment time becomes longer, which means a longer 
treatment time at a high temperature results in an increase of TiO.sub.2 
to be added to the WA particles. Therefore, it is appreciated that there 
is a proper amount of TiO.sub.2 to be added to the WA particles for 
realizing a desired hardness thereof. The proper amount is estimated to be 
no more than about 3%, as already discussed by referring to Table 2. 
EXAMPLE 2 
&lt;White Ceramic Tool&gt; 
FIG. 5 shows an example wherein the hardness of an alumina ceramic cutting 
tool (so-called white ceramic tool) became larger by addition of 
TiO.sub.2. The tool was made by compacting and sintering fine particles of 
high purity aluminum oxide (Al.sub.2 O.sub.3). FIG. 5 shows the 
measurement results of the Vickers hardness at a surface of the white 
ceramic tool together with the results obtained for a non-treated tool. 
For measurement, the white ceramic tool was embedded in TiO.sub.2 powder 
for heating at 1573 K for 20 hours in the furnace of FIG. 2 and then taken 
out for polishing at the cross section shown in FIG. 5. The measurement of 
the Vickers hardness was performed according to the above-described JIS 
method under a constant test load of 500 gf. It is appreciated from the 
same figure that the hardness of the tool treated by the TiO.sub.2 
-addition increased to a maximum of about 3000 kgf/mm2 at the cross 
section which is about 1 mm deep from the surface, whereas the non-treated 
tool exhibited an uniform Vickers hardness of 1800 kgf/mm2 at any cross 
section. 
FIGS. 6 and 7 show depthwise hardness distribution at a cross section of 
the tool to illustrate the influences of the TiO.sub.2 -addition treatment 
conditions on the hardness increase. It is appreciated from FIGS. 6 and 7 
that the maximum hardness position shifts inward by adopting the treatment 
conditions which necessitate an increase of TiO.sub.2 amount to be added 
to the tool surface. It is thus inferred that, in the case of the white 
ceramic tool as well, there is a proper amount of added TiO.sub.2, which 
is estimated to be no more than about 3%, for realizing the maximum 
hardness of the sintered aluminum oxide cutting tool, like the abrasive 
described above. 
FIG. 8 shows the results obtained by investigation as to wear progression 
with respect to a treated alumina ceramic cutting tool (so-called white 
ceramic tool) in comparison with a non-treated tool. The treated cutting 
tool was made to have an increased hardness by TiO.sub.2 -addition to 
sintered high-purity aluminum oxide (Al.sub.2 O.sub.3), and used to cut 
structural carbon steel S55C at four different cutting speeds. The 
TiO.sub.2 -added tool was scarcely different in flank wear from the 
non-treated tool but provides less progression in crater wear. Thus, it is 
recognized that TiO.sub.2 -addition to this kind of ceramic tool restrains 
tool wear progression. 
EXAMPLE 3 
&lt;Aluminum Ceramic Material&gt; 
FIG. 9 shows how the hardness of a general-use alumina ceramic material 
increases by TiO.sub.2 -addition treatment. The alumina ceramic material 
was obtained by adding a sintering agent to fine particles of aluminum 
oxide (Al.sub.2 O.sub.3) followed by suitably molding and sintering the 
aluminum oxide particles. The TiO.sub.2 -addition treatment was performed 
in the same manner as is the case with the above-described white ceramic 
tool except for the heating temperature and heating time. In contrast to a 
non-treated material, the maximum Vickers hardness near the surface of the 
TiO.sub.2 -added ceramic material becomes as great as 2800 HV. In this 
case again, the hardness peak shifts into the material from the surface as 
the heating time becomes longer. This is because TiO.sub.2 will permeate 
into the material from its surface as the treatment time becomes longer, 
thereby causing the maximum hardness concentration portion (about no more 
than 3%) to gradually shift inward from the surface. 
FIG. 10 shows the results obtained with a different TiO.sub.2 -addition 
treatment than that for FIG. 9, together with the results obtained by heat 
treatment alone. Specifically shown is comparison between a case where 
where TiO.sub.2 -addition treatment was performed at 1273 K for 5 hours 
three times and another case where TiO.sub.2 -addition treatment was 
performed at 1273 K for 5 hours three times followed by heat treatment 
alone at 1273 K for 5 hours. As seen in the same figure, if the TiO.sub.2 
-addition treatment is followed by the heat treatment alone, the hardness 
distribution becomes gentle. This is presumably because the heating causes 
TiO.sub.2 to permeate inward to make the TiO.sub.2 -concentration 
distribution gentle. Thus, by performing such a two-stage treatment, it is 
possible to increase the thickness of the hardened surface layer. 
FIG. 11 shows the hardness increase obtained by addition of Y.sub.2 O.sub.3 
to general-use alumina ceramic materials similar to the one described 
above. As seen from the same figure, the Y.sub.2 O.sub.3 -addition 
treatment provides a maximum Vickers hardness of as great as 2800 kgf/mm2 
near the surface of of the ceramic material, as opposed to a non-treated 
case. In this case again, the hardness peak shifts inward from the surface 
as the total treatment time becomes longer. This is also because Y.sub.2 
O.sub.3 will permeate into the material from its surface as the treatment 
time becomes longer, thereby causing the maximum hardness concentration 
portion (about no more than 3%) to gradually shift inward from the 
surface. 
FIG. 12 shows hardness distribution with respect to similar general-use 
alumina ceramic materials which were respectively treated as follows: 
1TiO.sub.2 -addition treatment at 1273 K for 5 hours; 2Y.sub.2 O.sub.3 
-addition treatment at 1273 K for 5 hours, and 3TiO.sub.2 -addition 
treatment at 1273 K for 5 hours followed by Y.sub.2 O.sub.3 -addition 
treatment at 1273 K for 5 hours. As can be seen from the same figure, a 
portion near the surface of the ceramic material can be properly hardened 
by adding different additives in combination as long as the additives are 
solid-soluble in the ceramic material for hardening thereof. 
EXAMPLE 4 
&lt;Single Crystal of Aluminum Oxide (sapphire)&gt; 
FIG. 13 shows the surface hardness of a single crystal (sapphire) of 
aluminum oxide (Al.sub.2 O.sub.3) which was treated by TiO.sub.2 
-addition, in comparison with non-treated crystals. As seen from the same 
figure, the TiO.sub.2 -addition treatment provided a Vickers hardness 
increase to 3000-4000 kgf/mm2. The single crystal of aluminum oxide thus 
treated by TiO.sub.2 -addition was polished into a cutting tool which 
provided a remarkably lower tool wear than a non-treated cutting tool when 
tested for cutting a ferrous metal. 
FIG. 14 shows the measurement results with respect to the crystal surface 
hardness under various conditions of TiO.sub.2 -addition treatment. Within 
the range of the tests, the surface hardness increased as the treatment 
temperature became higher and the treatment time became longer. 
FIG. 15 shows the results obtained by investigating the hardness 
distribution inside the crystal. In this investigation, two crystal facets 
were subjected to addition treatment under two different treatment 
conditions. There was substantially no difference between the two facets, 
and a higher hardness was provided closer to the surface as long as the 
tests revealed. 
Of course, the scope of the present invention covers not only any alumina 
materials which consists mainly of alumina oxide but also any products 
such as tools made of such a material, regardless of forms. Further, 
surface hardening of the above-described materials or the like may be 
facilitated by adding TiO.sub.2 or other alumina-soluble additive to a 
given solid alumina material according to the method "3" above. 
Alternatively, the addition treatment may be performed by method "1" or 
"2" above during a tool-forming process. In any case, it is necessary to 
perform a heat treatment for a predetermined time at a predetermined 
temperature to properly dissolve the additive in alumina. As evident from 
the above examples, the former method hardens white ceramic tools, 
sapphire or the like only in the vicinity of the surface. On the other 
hand, the latter method provides uniform hardening inside the material or 
product. In the latter case, it is important to set the concentration of 
the additive at 3% or below for realizing a proper hardness increase. 
Measurements of micro-Vickers hardness vary slightly depending on the test 
load. Normally, for an identical specimen, a hardness value measured under 
a smaller test load is larger than that measured under a greater test 
load. Therefore, a specimen which provides a measurement of 2600 kgf/mm2 
under a test load of 500.times.9.807 mN for example will give a 
measurement of 2600 kgf/mm2 or more under a test load of 300.times.9.807 
mN, presumably over 3000 kgf/mm2 in consideration of the tendency shown in 
FIG. 14. 
As described above, according to the present invention, it is possible to 
increase the hardness of an alumina material or product to as high as 
2600-4000 kgf/mm2 from a conventionally highest value of 2000 kgf/mm2 . As 
a result, the hardened alumina material of the present invention, when 
applied to make an abrasive, a cutting tool or a ceramic product, provides 
a remarkably prolonged service life or a quality enhancement of the 
alumina ceramic product. Thus, the industrial significance of the present 
invention is exceptionally great.