Shelling-resistant abrasive grain, a method of making the same, and abrasive products

Sintered ceramic abrasive grains having increased surface area and a method of making the same are provided. The abrasive grains are incorporated into improved "shelling-resistant" abrasive articles such as coated abrasives, grinding wheels and non-woven abrasives. The ceramic abrasive grains are made by providing first particles comprising alpha-alumina precursor, introducing fine particles which can be sintered directly to the surface of the first particles upon sintering of the same, and heating the mixture of particles under sintering conditions to cause autogenous bonding of the fine particles to the surface of each of the first particles.

TECHNICAL FIELD 
This invention relates to shelling-resistant ceramic abrasive grain, a 
method of making the same, and abrasive products which contain the same 
BACKGROUND ART 
Shelling is a term sometimes employed by those familiar with the abrasive 
industry, particularly the coated abrasive industry, which refers to the 
phenomenon whereby abrasive grains are released prematurely from a bond 
system which is typically intended to hold the abrasive grain throughout 
the useful life of the abrasive grains. The term "shelling" may have been 
adopted because someone thought the phenomenon of the abrasive grains 
being released from the bond system was similar to the releasing of 
kernels of corn from an ear of corn during a corn shelling operation. 
While there are some exceptions, e.g., where the abrasive product is used 
as a source of abrasive grain for a slurry, shelling of abrasive grain 
from abrasive products is unwanted because it has the effect of reducing 
the efficiency of the abrasive product because of the loss of the abrading 
surfaces which would have been provided by the missing abrasive grain, and 
for other reasons explained below. The decreased efficiency by shelling 
can be noted in all types of abrasive products such as bonded abrasive 
products, e.g., grinding wheels, and non-woven abrasive products, but is 
particularly notable in coated abrasive products where substantially all 
of the abrasive grains are held on a sheet by the bond system with one end 
of each grain typically being exposed or nearly exposed. The loss of 
abrasive grain by shelling from coated abrasive products can provide 
non-abrasive areas on the coated abrasive surface, reducing abrasive 
efficiency and possibly resulting in uneven surface finishing. 
The shelling problem can be particularly significant when the abrasive 
product is used for high stock removal applications. During such use, the 
abrasive product must be able to withstand high pressures and rotative 
speeds and still provide sufficient abrasive cut. For example, a coated 
abrasive disc can traverse 12,000 revolutions per minute (rpm) and be 
subjected to an interface pressure as high as 15 kg/cm.sup.2. While these 
severe conditions are preferred because they usually result in increased 
cut rates, they also severely tax the adhesive bond between the abrasive 
grain and the bond system. If this bond fails, abrasive grains are ejected 
(or shelled) from the coated abrasive product at an extremely high rate of 
speed which could result in serious injury to the operator, particularly 
when the abrasive grains are of a large grit size. This hazard is of grave 
concern to those in the abrasives industry and it sometimes results in 
products not being used to their full potential because of the possible 
safety hazard. 
Furthermore, many abrasive products, particularly coated abrasive products, 
utilize moisture-susceptible resinous bond systems, the most popular 
comprising phenolic resin. Bond systems based upon phenolic resins are 
known to increase the potential for an abrasive product to shell as the 
moisture level of the grinding conditions is increased. 
Many solutions to the shelling problem have been proposed, but they have 
been either impractical, expensive, of little utility, or not particularly 
suited for use with sintered ceramic abrasive grains. 
The earliest known reference specifically directed to solving the shelling 
problem is U.S. Pat. No. 1,528,543 (Hartmann) which discloses a process 
for treating the surfaces of crystalline mineral materials to scratch and 
roughen the grain surfaces and thus increase their bonding qualities. 
Various other references disclose the adhesion of smaller particles to the 
surface of larger particles to increase surface area by utilization of a 
bonding layer or flux to obtain particles having surface bonded smaller 
particulate material. For example, U.S. Pat. No. 3,269,815 (Koopman) 
discloses coating abrasive grain with finely divided particles of solid 
material such as carbides and nitrides by cementing these particles to the 
abrasive grain by a thin ceramic film such as glass. The thin ceramic film 
will soften at a temperature less than the softening or melting of the 
abrasive grain or the solid materials to thereby cement the finely divided 
particles to the abrasive grain to promote resin adherence and increase 
the bonded strength between the particles of abrasive grain when in a 
bonded abrasive article. Additionally, U.S. Pat. No. 4,472,173 (Bruning et 
al) discloses corundum abrasive grain which is coated with ground frit, a 
binder and fine grain highly abrasive material for the purpose of 
improving adhesion of the abrasive grain in its processing to resin-bonded 
abrasives. 
Additionally, certain references disclose forming compacts of smaller 
particles which would inherently have a greater surface roughness. For 
example, U.S. Pat. No. 4,252,544 (Takahashi) discloses alumina abrasive 
grain constructed of electro-fused or high temperature calcined alumina 
coarse crystal particles and alumina fine crystal particles which are 
located between the alumina coarse crystal particles. These abrasive 
grains are manufactured by forming alumina coarse powder of a particular 
type, forming alumina fine powder of a particular type, kneading the 
alumina coarse powder and alumina fine powder in the presence of water or, 
if necessary, primary binder, extruding the kneaded material by means of a 
mechanical extruder, drying the extruded material, cutting the extruded 
material to a predetermined length, and sintering the dried and cut pieces 
of the extruded material at a temperature higher than that of the 
calcining temperature of the fine powdered alumina and lower than 
1700.degree. C. 
There is no indication in any of the aforementioned references that the 
teachings could be utilized to produce alpha-alumina based ceramic 
abrasive grain having desirable physical properties without altering the 
same and without modifying the ceramic abrasive grains by the application 
of binder layers or flux. The type of ceramic abrasive grain desired to be 
improved according to the present invention is, for example, that 
disclosed in assignee's U.S. Pat. No. 4,744,802 (Schwabel). Other 
references which disclose the preparation of alumina-based ceramic 
abrasive grain of this type include U.S. Pat. Nos. 4,314,827 (Leitheiser 
et al), 4,518,397 (Leitheiser et al) and 4,574,003 (Gerk). 
None of the above mentioned references discloses ceramic particulate 
material having small separated masses of inorganic material autogenously 
bonded to the surface thereof to provide an improved abrasive grain which 
is shelling resistant, or abrasive products such as bonded abrasive 
products, non-woven abrasive products, and coated abrasive products, which 
contain the same. 
SUMMARY OF THE INVENTION 
According to the present invention, improved abrasive grains comprising 
ceramic particulate material having small separated protuberant masses of 
inorganic material autogenously bonded surface thereof are provided. The 
term "autogenously bonded" means that separated surface masses are bonded 
to the surface of each ceramic particle without any type of external 
bonding medium such as flux, vitreous bonding material, organic binder, 
glass, or the like. Bonding of the surface particulate materials is solely 
as a result of binder-free adhesion between the precursor of the material 
forming the abrasive grain and the precursor of the material forming the 
inorganic mass which ultimately forms a permanent bond therebetween on 
firing to sinter the precursor materials to form a ceramic material. 
It has been discovered, quite unexpectedly, that inorganic surface masses 
may be formed as protuberances on ceramic abrasive grains during the 
sintering operation without deliterious effect of the abrading properties 
of the resultant abrasive grain. The resultant abrasive grain has an 
expanded surface area provided by the attached protuberant masses which 
thereby provides a structure which is more shelling resistant than the 
conventional protuberant-free surface and has unexpectedly improved 
performance in some cases. 
More particularly, the invention provides a method of making ceramic 
abrasive grain each grain of which is characterized by having autogenously 
bonded to the surface thereof a multitude of small separated protuberant 
masses of inorganic material. The method comprises the steps of providing 
mass of first particles comprising alpha-alumina precursor material, each 
particle of which is sinterable to alpha-alumina-based abrasive grain, 
introducing into the mass second particles much finer than the first 
particles, the second particles being capable of autogenous bonding to the 
surface of the first particles upon sintering of the first particles to a 
hard protuberant mass of inorganic material, and heating the particles 
under sintering conditions to sinter the first particles and cause 
autogenous bonding of the second particles to the surface of each of the 
first particles. 
The process contemplates introducing the second particles by tumbling the 
first particles to cause the first particles to wear away on their 
surfaces to generate the second particles. This is done without 
substantially affecting the particle size of the first particles. The 
tumbling is continued until attachment of the generated second particles 
to the surface of each of the first particles results. The method also 
contemplates the introduction by addition of second particles to the first 
particles of second particles which are of a material which is different 
from the composition of the material forming the first particles. The 
second particles are then tumbled with the first particles to cause the 
second particles to attach to the surface of the first particles. 
The preferred method involves preparing the first particles by providing a 
mixture of an aqueous dispersion of alumina-hydrate, gelling the mixture, 
drying the gelled mixture to product a dried solid, and crushing the dried 
solid to produce the first particles. 
The method may include the step of calcining at least the first particles 
to substantially remove bound volatile materials prior to introducing the 
second particles into the mass of first particles. 
As previously stated, the abrasive grains of the invention comprise 
alpha-alumina based ceramic particles having autogenously bonded masses of 
inorganic material to provide a multiplicity of surface protuberances on 
each abrasive grain. The preferred abrasive grain according to the 
invention has surface protuberances which comprise a material selected 
from the group consisting of alumina, alpha alumina, alumina:zirconia, 
zirconia, silicon nitride, rare earth metal oxides, yttria, chromia, 
ceria, titanium carbide, titanium nitride, silicon alumina oxynitride, 
silicon aluminum oxycarbide, yttrium alumina-garnet, hexagonal rare earth 
aluminate, aluminum oxynitride, oxides of zinc, magnesium or nickel and 
mixtures thereof. 
The abrasive grain according to the present invention also contemplates the 
inclusion of at least one modifying additive in the alpha-alumina based 
ceramic. The modifying additive preferably is an oxide of one or more 
metals selected from the group consisting of magnesium, zinc, zirconium, 
hafnium, cobalt, nickel, yttrium, praseodymium, samarium, ytterbium, 
neodymium, lanthanum, gadolinium, cerium, dysprosium, and erbium. 
The invention also provides abrasive products made with the abrasive grain 
of the invention. The abrasive products may be in the form of coated 
abrasive products, bonded abrasive products, e.g., grinding wheels, honing 
stones, and the like, which may include a vitreous or non-vitreous binder, 
e.g., an organic binder, or non-woven abrasive products. 
The ceramic abrasive grains according to the invention have increased 
surface area which results in improved adhesion to the bond system in 
abrasive type products. By improving the adhesion, the abrasive products 
can be utilized at higher pressures, which results in an increase in the 
stock removal rates. Additionally, abrasive products containing the 
abrasive grain of the invention will abrade significantly more stock from 
a workpiece than a corresponding abrasive product made without surface 
modification of the abrasive grain as herein described.

DETAILED DESCRIPTION 
This invention pertains to alpha-alumina based ceramic abrasive grains 
which are prepared by drying and firing particulate precursor material 
below the melting temperature of the ceramic to achieve sintering of the 
dehydrated starting materials. During heating the precursor material will 
be transformed to a dehydrated metal oxide structure which will densify as 
heating continues. The most common alpha-alumina-based ceramic may be 
modified with oxides of metals such as magnesium, nickel, zinc, yttria, 
rare earth oxides, zirconia, hafnium, chromium or the like. The preferred 
method of making the ceramic abrasive grain is by the so called sol-gel 
process disclosed in the following U.S. Pat. Nos.: 4,314,827; 4,518,397; 
4,574,003; 4,623,364; and 4,744,802, each of which is incorporated herein 
by reference. 
To prepare the sol gel alumina-based ceramic abrasive grains, a dispersion 
comprising from about 2 to almost 60 weight percent alpha alumina oxide 
monohydrate (e.g., boehmite) is first formed. The boehmite can either be 
prepared from various techniques well known in the art or can be acquired 
commercially from a number of suppliers. Examples of commercially 
available materials include Disperal.RTM., produced by Condea Chemie, GMBH 
and Catapal.RTM., produced by Vista Chemical Company. These aluminum oxide 
monohydrates are in the alpha-form, are relatively pure (including 
relatively little, if any, hydrate phases other than monohydrate), and 
have a high surface area. 
The dispersion may contain a precursor of a modifying additive which can be 
added to enhance some desirable property of the finished product or 
increase the effectiveness of the sintering step. These additives are in 
the form of soluble salts, typically water soluble, and typically consist 
of a metal-containing compound and can be a precursor of the oxides of 
magnesium, zinc, cobalt, nickel, zirconium, hafnium, chromium, titanium, 
yttrium, rare earth oxides, and mixtures thereof. The exact proportions of 
these components that are present in the dispersion are not critical to 
this invention and thus can vary to convenience. 
A peptizing agent is usually added to the boehmite dispersion to produce a 
more stable hydrosol or colloidal dispersion. Monoprotic acids or acid 
compounds which may be used as the peptizing agent include acetic, 
hydrochloric, formic and nitric acid. Nitric acid is a preferred peptizing 
agent. Multiprotic acids are normally avoided since they rapidly gel the 
dispersion making it difficult to handle or mix in additional components. 
Some commercial sources of boehmite contain an acid titer (such as 
absorbed formic or nitric acid) to assist in forming a stable dispersion. 
The dispersion can be formed by any suitable means which may simply be the 
mixing of aluminum oxide monohydrate with water containing a peptizing 
agent or by forming an aluminum oxide monohydrate slurry to which the 
peptizing acid is added. Once the dispersion is formed, it preferably is 
then gelled. The gel can be formed by any conventional technique such as 
the addition of a dissolved or dispersed metal containing modifying 
additive, e.g., magnesium nitrate, the removal of water from the 
dispersion of some combination of such techniques. 
The dispersion may contain a nucleating agent to enhance the transformation 
to alpha alumina. Suitable nucleating agents include fine particles of 
alpha alumina, alpha ferric oxide or its precursor and any other material 
which will nucleate the transformation. The amount of nucleating agent is 
sufficient to effect nucleation. Nucleating such dispersions is disclosed 
in assignee's U.S. Pat. No. 4,744,802. 
Once the gel has formed it may be shaped by any convenient method such as 
pressing, molding, coating or extrusion and then carefully dried to 
produce a dried solid material. 
The gel can be extruded or simply spread out to any convenient shape and 
dried, typically at a temperature below the frothing temperature of the 
gel. Any of several dewatering methods, including solvent extraction, can 
be used to remove the free water of the gel to form the solid. 
After the gel is dried, the dried solid can be crushed or broken by any 
suitable means, such as a hammer or ball mill or a roll crusher, to form 
abrasive grain precursor particles hereafter referred to as precursor 
particles. Any method for comminuting the solid can be used and the term 
"crushing" is used to include all such methods. 
The ceramic abrasive grain of the invention with increased surface area may 
be prepared by tumbling the non-sintered particles. Non-sintered particles 
refer to particles that have been dried but not fully sintered, typically 
having a density which is less than 60% of theoretical density in such a 
condition. Calcined non-sintered particles are typically about 50% of 
theoretical density. The preferred method of making the non-sintered 
particles is disclosed in U.S. Pat. No. 4,744,802 assigned to the assignee 
of the present application. Tumbling of the non-sintered particles may be 
accomplished either prior to calcining or prior to final firing 
(sintering). The non-sintered particles are relatively soft and, when they 
tumble together in a mixer, the edges break off to produce fines which are 
redeposited upon the surface of the abrasive grains via mechanical 
impingement as tumbling is continued. In a first embodiment the 
non-sintered particles are tumbled in a mixer to generate fine particulate 
material. Tumbling may be accomplished by charging a mixer with a 
specified amount of non-sintered particles. At this point in the process, 
the non-sintered particle will be relatively soft. Tumbling results in 
contact between non-sintered particles which generates fine particles. 
Continued tumbling causes the fines to redeposit on the surfaces of the 
non-sintered particles via mechanical impingement. During final firing, 
the fine particles sinter along with the non-sintered particle to form 
abrasive grains. The redeposited fine particles form as separated 
protuberant masses on the surface of the abrasive grain, thereby 
increasing the surface area of the abrasive grain which results in 
improved adhesion to the bond system. 
In a second embodiment, the non-sintered particles are tumbled in a mixer 
with added fine particles which are of a different composition from that 
of the non-sintered particles yet which form inorganic protuberant masses 
on the surface of the abrasive grains. These fines, along with the fines 
of the non-sintered particles that are broken from the surface of the 
non-sintered particles during the tumbling, are redeposited onto the 
surface of the non-sintered particles during continued tumbling. Final 
sintering causes the fine particles, both the added and the generated, to 
sinter along with non-sintered particles to form abrasive grain having 
surface protuberances. 
In order to facilitate the tumbling, non-sintered particles are charged to 
a mixer in a specified weight ratio. In general, the mixer should be about 
10 to 75% full, preferably about 20 to 50% full by volume with 
non-sintered particles. The non-sintered particles are tumbled for a time 
sufficient to generate an adequate amount of fine particles without 
undesirably rounding the edges of the non-sintered particles and thereby 
on sintering producing an abrasive grain which has undesirably rounded 
edges which does not abrade as well as an abrasive with sharp edges. Some 
experimentation may be needed to predict the exact time necessary to 
tumble the non-sintered particles, since the time is a function of the 
energy generated. Non-sintered particles which have been prefired or 
calcined require more tumbling time, since the particles are harder than 
non-calcined particles. The tumbling times may easily be determined by 
those skilled in the art. In general, the more energy generated for a 
given time period, the less tumbling time required. This energy is the 
mechanical energy generated by the non-sintered particles interacting with 
one another but is also a function of the mixer design. It is also 
preferred to use a mixer with baffles or flanges to increase the generated 
energy and thus reduce the tumbling time. Although any type of mixer may 
be utilized, a closed system is preferred to avoid fine particle loss. Not 
all mixer designs will provide the necessary conditions to cause the fine 
particles to adhere to the non-sintered particles via mechanical 
impingement. For example, a vibratory mill does not provide the necessary 
interactions between the particles to cause the fines to break off and 
deposit themselves on the unsintered particles. The preferred mixer design 
is a ball mill without the abrasive media. Typical tumbling times are on 
the order of 5 to 180 minutes, preferably 20 to 50 minutes. 
In the first embodiment, the non-sintered particles alone are tumbled to 
produce fines of the same chemical composition as the non-sintered 
particles. In the second embodiment, the non-sintered particles are 
tumbled with added fines of a different composition, resulting in a blend 
of these with fines of the same chemical composition which are generated 
as the non-sintered particles are tumbled. The second embodiment has an 
unexpected benefit in that it permits the tailor-making of abrasive grain 
surfaces by selecting the appropriate fine particles to be tumbled with 
the non-sintered particles, it being well known in the art that the 
surface of the abrasive grain has a significant role during abrading 
applications. 
After crushing (and tumbling if done at this time), the dried gel particles 
are then calcined to remove essentially all volatiles and transform the 
various components of the grains into ceramics. The dried gel particles 
are generally heated (calcined) at a temperature between about 400.degree. 
C. and about 800.degree. C. and held within this temperature range until 
the free water and over 90 weight percent of any bound water is removed. 
Tumbling can occur either before or after the calcining step so long as it 
occurs before the final firing step. It is preferred that the tumbling be 
done before the calcining step for ease of handling. 
The calcined particles are then sintered by heating to a temperature of 
between about 1000.degree. C. and about 1650.degree. C. and holding within 
this temperature range until substantially all of the alpha alumina 
precursor material (e.g., alpha alumina monohydrate) is converted to alpha 
alumina. Of course, the length of time to which the ceramic must be 
exposed to the sintering temperature to achieve this level of conversion 
and will depend upon various factors but usually from about 5 to about 30 
minutes is sufficient. 
Other steps can be included in this process, such as rapidly heating the 
material from the calcining temperature to the sintering temperature, 
sizing granular material, centrifuging the dispersion to remove sludge 
waste, etc. Moreover, this process can be modified by combining two or 
more of the individually described steps, if desired. 
It is conceivable that a ceramic abrasive grain may go through several high 
temperature firing cycles. The tumbling must be done prior to the last or 
sintering firing cycle. The tumbling process should not be accomplished 
after sintering, since it would cause small parts of the abrasive grains 
to break off with no means to sinter them onto the abrasive grain surface. 
FIG. 1 shows the protuberant surface of an abrasive grain according to the 
invention while FIG. 2 by contrast shows the very smooth surface of a 
conventional ceramic abrasive grain. It should be noted that the increased 
surface area of the abrasive grain of the invention does not result 
because of an increase of porosity of the abrasive grain. The abrasive 
grain of the invention preferably has a density of at least 90% of 
theoretical. While porous abrasive grains tend to have a large surface 
area, their performance is substantially inferior because of decreased 
strength and integrity. 
The surface area ratio is one means by which the abrasive grain of the 
invention may be characterized. The abrasive grain of the invention has a 
surface area ratio of at least 1.5, preferably at least 2.0. Surface area 
ratio, a measure of the increase in surface area of the abrasive grain, is 
determined by dividing the surface area of the abrasive grain of the 
invention by the surface area of the standard untreated graded abrasive 
grains for a given grade. The surface area ratio is a more meaningful way 
to characterize the increase in surface area since every abrasive grain 
grade has a different particle size and would thus have a different 
surface area. The grading standard for abrasive grains may be found in 
American National Standard Institute (ANSI) Standard No B74.18, 1984; the 
coarse and control fractions were used for a given grade to calculate 
surface area ratios. Untreated graded standard abrasive grains may be 
purchased from Minnesota Mining and Manufacturing (3M) Company of St. 
Paul, Minn., under the trade designation Cubitron abrasive grain. The 
tumbled abrasive grains were made according to the teachings in U.S. Pat. 
No. 4,744,802, except that the abrasive grains were tumbled for 20 minutes 
in a ball mill containing no grinding or abrasive media prior to 
prefiring. The ball mill was approximately 1.82 meter in length with a 
1.82 meter inside diameter and was rotated at 16 rpm. Baffles (5.1 cm high 
and 5.1 cm thick) were spaced around the inside circumference of the ball 
mill at approximately 46 cm intervals. The remainder of the steps were the 
same as the teachings of U.S. Pat. No 4,744,802. 
Listed below are the surface areas of the untreated ANSI Standard graded 
abasive grain and the tumbled abrasive grain made according to the 
invention and the calculated surface are ratio. The surface area was 
measured on a Quantosorb, Model QS 13 surface area measuring device which 
was obtained from Quanthchrme Company, Syusett, N.Y. The samples were 
soaked in nitrogen gas for 20 minutes prior to testing. 
______________________________________ 
Surface Area Measurements 
ANSI Tumbled 
Grains Grains 
Grade (m.sup.2 /g) (m.sup.2 /g) 
Ratio 
______________________________________ 
36 0.045 0.26 5.8 
50 0.041 0.29 7.1 
80 0.19 0.40 2.1 
120 0.12 0.47 3.8 
______________________________________ 
The protuberant coating on the surface of the abrasive grain resulting from 
attachment thereto of the fines is discontinuous, leaving exposed portions 
of the original surface of abrasive grain with an irregular coating of the 
fine inorganic particles. This discontinuous coating has greater surface 
area than a continuous coating, leading to improved adhesion to the bond 
system. 
The materials from which the fine particles are chosen are sinterable to 
form a ceramic typically based on metal oxide, nitride, carbide, 
oxynitride, or oxycarbide. Examples of typical addition fines will sinter 
to the following: alumina, aluminum oxynitride, metal oxides of zinc, 
magnesium or nickel, alpha alumina, alumina zirconia, zirconia oxide, 
silicon nitride, rare earth oxides, yttrium oxide, cerium oxide, titanium 
carbide, titanium nitride, silicon alumina oxynitride, silicon aluminum 
oxycarbide, yttrium alumina- garnet, and hexagonal rare earth aluminates. 
The addition fines may be added as a precursor of the final sintered 
composition. The addition fines are preferably added directly to the mixer 
in a ratio from 1 part fines to 99 parts precursor particles to 30 parts 
fines to 70 parts precursor particles. The addition fines can be added to 
the mixer in a powder form. It has been verified via a scanning electron 
microscope and Energy Dispersion X-ray Analysis (EDAX) which determines 
elemental analysis that the addition fines are indeed attached to the 
surface of the abrasive grain. 
Some addition fines may also co-react with the alumina-based abrasive grain 
during sintering. For example, the oxides of cobalt, nickel, zinc and 
magnesium will typically form a spinel structure with the alumina. Yttria 
will typically react with alumina to form 3Y.sub.2 O.sub.3 -5Al.sub.2 
O.sub.3, a garnet crystal phase. Praseodymium, samarium, ytterbium, 
neodymium, lanthanum, gadolinium, cerium, dysprosium, erbium, and mixtures 
of two or more of these rare earth metals will typically react with 
alumina to form a garnet, beta alumina, perovskite, or hexagonal rare 
earth aluminate crystal structure. 
The average particle size of the fines should initially be between about 
0.05 and 50 micrometers. The average particle size of the non-sintered 
particle is between 50 micrometers to 1200 micrometers. In general, the 
large particle size fines do not work as well as the small particle fines 
on the small particle size non-sintered particles. For example, an 
abrasive grain having a particle size of 120 micrometers, typically would 
have fines on the order of 0.2 to 5 micrometers. Likewise, an abrasive 
grain having a particle size of 600 micrometers, preferably has fines on 
the order of 0.2 to 10 micrometers. The preferred addition fine average 
particle size is between 0.1 and 15 micrometers. If the particle size of 
the fines is too large, adhesion to the surface of the abrasive grain may 
be inadequate. If the particle size of the fines is too small, there would 
be little or no increase in surface area. 
The ceramic abrasive grains of the present invention are conveniently 
handled and incorporated into various abrasive products according to well 
known techniques to make, for example, coated abrasive products, bonded 
abrasive products, and lofty non-woven abrasive products. The methods of 
making such abrasive products are well known to those skilled in the art. 
A coated abrasive product includes a backing, for example, formed of paper, 
fiber, fabric (e.g., woven or non-woven fabric such as paper) which may be 
saturated with a filled binder material, a polymer film such as that 
formed of oriented heat set polypropylene or polyethylene terephthalate 
which may be first primed, if needed, with a priming material, or any 
other conventional backing material. The coated abrasive also includes a 
binder material, typically in layers including a make or maker coat, a 
size or sizing coat and possibly a supersize coat. Conventional binder 
materials include phenolic resins. 
Non-woven abrasive products typically include an open porous lofty polymer 
filament structure having the ceramic abrasive grains distributed 
throughout the structure and adherently bonded therein by an adhesive 
material. The method of making such non-woven abrasive products is well 
known. 
Bonded abrasive products typically consist of a shaped mass of abrasive 
grains held together by an organic or ceramic binder material. The shaped 
mass is preferably in the form of a grinding wheel. The preferred binder 
materials for the ceramic abrasive grains of the invention are organic 
binders. Ceramic or vitrified binders may be used if they are curable at 
temperatures and under conditions which will not adversely affect the 
ceramic abrasive grains of the present invention. 
A coupling agent is preferably present in the bond system, particularly 
when the bond system is part of a coated abrasive that is used in high 
humidity or wet grinding conditions. Typical examples of coupling agents 
and their useful quantities in abrasive products may be found in 
assignee's U.S. patent application Ser. No. 132,485, filed Dec. 14, 1987, 
incorporated herein by reference. 
EXAMPLES 
The following non-limiting examples will further illustrate the invention. 
Control Example and Examples 1-3 
The Control Example and Examples 1-3 compare the effect of tumbling on the 
performance of abrasive grain. 
Control Example 
The Control Example is abrasive grain made in a conventional way without 
tumbling with fines. 
The Control Example grain was made according to the following procedure. 
Into a continuous mixer were charged 12,000 ml of deionized water, 282.5 g 
of 11N analytical reagent grade nitric acid, 4780 g of alpha aluminum 
oxide monohydrate powder at 78% solids sold under the trade designation 
Disperal.RTM. and 761 grams of an alpha iron oxide precursor aqueous 
solution at 10% solids. The charge was dispersed until a homogeneous 
solution was obtained. The resulting dispersion and an aqueous solution of 
magnesium nitrate [Mg(NO.sub.3).sub.2.6H.sub.2 O] were metered though an 
in line mixer to form a gel in an amount to provide the following 
composition after the final firing: 93.5% alpha alumina, 4.5% MgO and 2% 
Fe.sub.2 O.sub.3. The resulting gel was then dried in a forced air above 
at 150.degree. C. to a friable solid. 
The resultant dried material was crushed using an impact hammer mill and a 
roll crusher. 
The dried, screened, crushed gel was calcined by being fed into the end of 
a kiln which was a 23 cm diameter 4.3 meter long stainless steel tube 
having a 2.9 meter hot zone, the tube being inclined at 2.4 degrees with 
respect to the horizontal, and rotating a 7 rpm, to provide residence time 
therein of about 15 minutes. The calciner had a hot zone feed end 
temperature of 350.degree. C. and exit end temperature of 800.degree. C. 
The fired product from the calciner was fed into a 1390.degree. C. kiln 
which was a 10.1 cm diameter 1.53 meter long silicon carbide tube included 
at 4.4 degrees with respect to the horizontal and having a 76 cm hot zone, 
rotating at 10 rpm, to provide a residence time therein of about 3.8 
minutes. The abrasive grain exited the kiln into room temperature air 
where it was collected in a metal container and allowed to cool to room 
temperature. The abrasive grain was a conventional grade 36 and had an 
average particle size of 700 micrometers. 
Abrasive Disc Preparation 
After the abrasive grain was made, it was utilized in a coated abrasive 
disc. The discs were prepared using conventional coated abrasive making 
procedures, conventional 0.76 mm vulcanized fiber backings and 
conventional calcium carbonate-filled phenolic make resin and conventional 
cryolite-filled phenolic size resin. The average make weight was 172 
grams/square meter, the average size grain weight was 696 grams/square 
meter. The make resin was precured for 90 minutes at 88.degree. C. and the 
size resin was precured for 90 minutes at 88.degree. C. followed by a 
final cure of 100.degree. C. for 10 hours. The coating was done using 
conventional techniques in a one-trip operation with curing in a forced 
air oven. The cured discs were first conventionally flexed to controllably 
break the hard bonding resins. The test results can be found in Table 1 
for the Shelling Test and Table 2 for the Cut Test. 
Shelling Test 
The Shelling Test is designed to measure the time it takes for the abrasive 
grain to shell from a coated abrasive disc. The test equipment included 
17.8 cm diameter test coated abrasive discs with a 2.22 cm mounting hole 
attached to a 16.5 cm diameter 1.57 mm thick hard phenolic backup pad 
which was in turn mounted on a 15.2 cm diameter steel flange. The test 
disc so supported was rotated counter-clockwise at 3550 rpm. The 1.8 mm 
peripheral edge of a 25 cm diameter 1010 carbon steel disc-shaped 
workpiece, deployed 18.5.degree. from a position normal to the abrasive 
disc and rotated counter- clockwise at 2 rpm, was placed into contact with 
the abrasive face of the abrasive disc under a load of 2.9 kg. The test 
endpoint was 20 minutes or when the disc begins to shell, i.e., a 
substantial portion of its abrasive grain flies off of the disc, whichever 
occurred first. At the end of the test the workpiece was weighed to 
determine the amount of metal cut (abraded) from the workpiece. The discs 
were humidified for one week prior to testing at the humidity level 
specified in Table I or II below. 
Cut Test 
For the Cut Test, the 17.8 diameter disc with a 2.22 cm center hole was 
attached to an aluminum back up pad have a beveled edge and used to grind 
the face of a 2.5 cm by 18 cm 1018 mild steel workpiece. The disc was 
driven at 5,500 rpm while the portion of the disc overlaying the beveled 
edge of the back up pad contacted the workpiece at 5.92 kg pressure 
generating a disc wear path of about 140 cm.sup.2. Each disc was used to 
grind a separate workpiece for one minute each for a total time of 12 
minutes each or for sufficient one minute time segments until no more than 
5 grams of metal were removed in any one minute grinding cut. 
EXAMPLE 1 
Example 1 was made and tested in the same manner as the Control Example 
except the abrasive grain was tumbled prior to calcining. Approximately 
1200 kilograms of the dried gel was charged to a ball mill containing no 
grinding or crushing media. The ball mill was approximately 1.82 meter in 
width and had a 1.82 meter inside diameter and rotated at 16 rpm. Baffles 
(5.1 cm high by 5.1 cm thick) were spaced at 46 cm intervals around the 
inside circumference of the ball mill. The non-sintered particles of dried 
gel were tumbled for 10 minutes. After tumbling, the remainder of the 
steps were the same as in the Control Example. 
EXAMPLE 2 
Example 2 was prepared and tested in the same manner as Example 1, except 
the tumbling time was 20 minutes. 
TABLE I 
______________________________________ 
(Shelling) 
Total Time to 
Cut Shell % Relative 
Example (grams) (minutes) 
Humidity 
______________________________________ 
Control 192 3.9 30 
1 320 7.1 30 
2 424 18 30 
Control 132 2.8 45 
1 280 6.5 45 
2 287 12.4 45 
Control 44 0.8 70 
1 98 1.4 70 
2 171 4.6 70 
______________________________________ 
TABLE II 
______________________________________ 
(Cut) 
Total Cut % Relative 
Example (grams) Humidity 
______________________________________ 
Control 1149 30 
1 1216 30 
2 1202 30 
______________________________________ 
From the above listed data, there was a dramatic improvement in performance 
on the Shelling Test, especially at high humidities, indicating improved 
adhesion between the bond system and the abrasive grains according to 
Examples 1 and 2 over that of the Control Example. The Cut Test results, 
where adhesion between the bond system and the abrasive grains is not as 
critical, showed a slight improvement in performance in Examples 1 and 2 
over the Control Example. 
EXAMPLES 3 THROUGH 5 
Examples 3 through 5 demonstrate the second embodiment of the invention in 
which added fine particles are tumbled with the precursor particles. The 
resulting abrasive grain had directly sintered to its surface both the 
added fine particles and fine particles originating from the precursor 
particles. 
EXAMPLE 3 
The abrasive grain and coated abrasive disc for Example 3 was made and 
tested in the same manner as the Control Example except fines containing 
oxides of alumina and zirconia were tumbled with the abrasive grain 
precursor particles prior to calcining. The abrasive fines were made 
according to Example 1 of U.S. Pat. No. 4,314,827. Approximately 5000 
grams of the precursor particles of dried gel and 500 grams of fines 
containing oxides of alumina and zirconia were charged to a mixer. The 
fines were in the range 0.25 to 3 micrometers in size. The mixer had a 
volume of 1.15 cubic meters and was obtained from the PattersonKeller Co. 
Inc., East Stroudsburg, as Model #P.K. 232213. The mixer did not have any 
baffles or flanges in its interior. The fines and the dried, screened, 
crushed gel precursor particles were tumbled for one hour. The remaining 
steps to produce the abrasive grain and the coated abrasive were the same 
as the Control Example. The test results can be found in Table III. 
EXAMPLE 4 
The abrasive grain and coated abrasive disc for Example 4 was made and 
tested in the same manner as Example 4 except fine inorganic particles 
were abrasive fines containing oxides of alumina, yttria and magnesia, not 
alumina zirconia. The abrasive fines were made according to U.S. Pat. No. 
4,770,671 and contained 90% alpha alumina, 8.5% yttrium oxide and 1.5% 
magnesium oxide. During the final firing, the magnesium in the fines 
co-reacted with the alpha alumina in the grain to form a spinel. Also at 
the same time the yttria co-reacted with the alpha alumina to form 
3Y.sub.2 O.sub.3 -5Al.sub.2 O.sub.3. 
EXAMPLE 5 
The abrasive grain and coated abrasive disc for Example 5 was made and 
tested in the same manner as the Control Example except silicon nitride 
fines were tumbled with the abrasive grain precursor particles prior to 
calcining. Also Example 5 was subjected to different calcining and firing 
conditions necessary for the silicon nitride fines. After tumbling, the 
resulting coated precursor particles were packed into a 5 cm diameter, 30 
cm long alumina tube which was placed into a 7.5 cm diameter stationary 
tube furnace where it was heated to a calcining temperature of 
1050.degree. C. in a nitrogen atmosphere after which a 95% nitrogen/5% 
hydrogen atmosphere was maintained. The temperature was increased over 
several hours to 1350.degree. C. where it was held for 30 minutes to 
sinter the abrasive grain and then cooled over several hours to room 
temperature. 
The abrasive grain was used to prepare abrasive discs which were tested as 
described above. Shelling Test results are shown in Table III. 
TABLE III 
______________________________________ 
(Shelling) 
Total Time to 
Cut Shell % Relative 
Example (grams) (minutes) 
Humidity 
______________________________________ 
Control 531 16 35 
3 647 20 35 
4 678 20 35 
5 562 20 35 
Control 208 5.2 70 
3 396 12 70 
4 378 12 70 
5 355 12 70 
______________________________________ 
It was very evident from the data shown in Table III that autogenously 
bonded added fines increased the adhesion between the abrasive grain and 
the bonding resin. 
EXAMPLES 6 AND 7 
Examples 6 and 7 compare abrasive grain tumbled before and after calcining. 
The test results can be found in Table IV. 
EXAMPLE 6 
Example 6 was made and tested in the same manner as Example 1 except that a 
different ball mill was utilized to tumble the abrasive grain. 
Approximately 182 grams of the dried gel was charged into a ball mill 
containing no grinding or crushing media. The ball mill had an inside 
diameter of approximately 1.82 meter and a width of 0.3 meter. Baffles 5.1 
cm high by 5.1 cm thick were spaced at approximately 46 cm intervals 
around the inside circumference of the ball mill. The ball mill rotated at 
16 rpm. The non-sintered particles were tumbled for 20 minutes. After 
tumbling, the remainder of the steps were as in Example 1. The 
non-sintered particles of Example 6 were not calcined prior to tumbling. 
EXAMPLE 7 
Example 7 was prepared and tested in the same manner as Example 6, except 
the non-sintered particles were calcined prior to tumbling. 
TABLE IV 
______________________________________ 
Total Time to Calcined 
Example Cut Shell Before % Relative 
Number (grams) (minutes) Tumbling 
Humidity 
______________________________________ 
Control 298 10 N/A 45 
6 415 14 no 45 
7 450 13 yes 45 
Control 90 1.9 N/A 70 
6 208 6.7 no 70 
7 149 3.4 yes 70 
______________________________________ 
It was concluded from the above data that tumbling either before or after 
calcining significantly improved the shelling resistance of the coated 
abrasive product. 
While this invention has been described in terms of specific embodiments, 
it should be understood that it is capable of further modifications. The 
claims herein are intended to cover those variations which one skilled in 
the art would recognize as the equivalent of what has been described here.