Self-bonded ceramic abrasive wheels

A grinding tool is described which comprises self-bonded particles of a ceramic abrasive and has a voids volume of from 5 to 65%.

BACKGROUND OF THE INVENTION 
This invention relates to grinding wheels made from a ceramic material and, 
particularly, to wheels which require no bonding medium to adhere and hold 
the ceramic materials in the structure. 
Conventional grinding wheels comprise abrasive grit particles held in a 
matrix that may be vitreous, metallic or resinous in nature. The function 
of the matrix material is to give the structure physical integrity and 
strength so that when contacted with a workpiece, the abrasive grits are 
held tightly enough to ensure that the workpiece is abraded before the 
grain is worn down or detached from the wheel. In addition, it is 
desirable that the wheel have "structure", that is to say, it should have 
a degree of porosity that is determined by the intended use. This porosity 
helps to dissipate the heat generated by the grinding action and thus 
reduce burning of the surface of the workpiece. It also makes easier the 
removal of swarf generated by the cutting action without clogging of the 
grinding area. 
Conventional grinding wheels are produced using two separate firing cycles. 
The first cycle fires the abrasive grits to their final crystalline form. 
The second is used to develop the bond which holds the grits together and 
to form the wheel. By controlling the amount of bond and abrasive 
material. It is possible to tailor the wheel to the intended use. Thus 
wheels for low wheel/work conformity, or high-pressure grinding such as 
external precision grinding or ball grinding, might use a wheel with a 
fairly low porosity; for example from 5 to 15% pore volume in the 
structure, while those where burning is a real problem might use a more 
open structure with a pore volume of 40 to about 65% or even higher. 
Grinding wheels do not, as a rule, have porosities greater than this 
because they need to have a certain structural strength to stand up to the 
strains of grinding operation. With somewhat higher porosities, say around 
80% and higher, structures typical of filters and catalyst support 
materials are obtained and these are quite different from those suitable 
for grinding wheels. Ceramic materials such as alpha alumina are very hard 
and abrasion resistant. Particles of this material can be induced to 
sinter together in the absence of a bond material to form a coherent 
structure. If the structure is only partially sintered such that it 
contains porosity levels suitable for a grinding tool, the mechanical 
integrity of the structure is inadequate to withstand the forces of 
grinding. In addition, the high sintering temperature required for an 
alpha alumina structure leads to growth of the alpha alumina crystals to 
such an extent that the grinding performance of a structure comprised of 
such crystals, which is related to the crystal size, is significantly 
reduced. If high pressure is used to limit the grain growth at the 
sintering temperature, the products obtained lack the level of porosity 
required for a grinding tool. 
An alternative approach is to use sol-gel technology to generate 
microcrystalline (generally sub-micron sized crystal structures), alpha 
alumina in situ in a mass having the form of the desired grinding tool 
such as a wheel. Coating a sol-gel derived grinding wheel, or for that 
matter, casting a sol, gel, slurry or slip, is unsatisfactory because of 
the significant amount of water which must be removed. The elimination of 
water is impeded by the thickness of the wheel through which the water 
must pass and the product may, in fact, be formed with voids or may even 
be crumbled by the internal forces generated upon drying. The problems are 
exacerbated if a sol-gel of an alumina precursor is used since this adds 
chemically bonded water that must be removed before conversion to the 
final fired alpha alumina form can be achieved. Thus, wheels of the 
thickness typically needed for grinding applications are difficult to 
obtain using this technique. Furthermore, the casting process inherently 
limits the range of porosity that can be attained and the products tend to 
have low porosity and to lack the internal strength to stand up to heavy 
grinding pressures. 
Nevertheless, there is great attraction in the development of a one-step 
approach utilizing a single firing step to make grinding wheels and there 
is little doubt that a casting process for producing grinding wheels 
without the above disadvantages would have great significance. Such a 
process would, however, need to be controllable in terms of the porosity 
of the product obtained, and yet not require the use of a matrix bonding 
medium. 
The present invention provides such a process and results in an abrasive 
wheel with a number of unique characteristics. The wheels produced by the 
process of the invention provide the benefits of controlled porosity in 
the context of a process that is essentially a single step and, 
consequently, very simple in operation. 
DESCRIPTION OF THE INVENTION 
The present invention provides a grinding tool having a self-bonded ceramic 
abrasive structure having from about 5 to about 65 vol. %, and preferably 
from about 30 to about 60 vol. %, and most preferably from about 40 to 
about 50% of voids. 
The tool is conventionally a wheel, but it could also be a hone or wheel 
segment or other convenient means of presenting an abrasive substance to a 
workpiece. Its characterizing feature is that it is in the form of a 
ceramic reticulated structure. The structure, therefore, does not require 
a bond to give the tool a form as is the case with tools formed of the 
more conventional abrasive particles, though a material may be impregnated 
or incorporated into the structure for other reasons to impart desirable 
characteristics. The structure is described as "reticulated" in the sense 
of being a continuous network of ceramic material that defines an open or 
closed cell porous body. 
The ceramic abrasive is self-bonded and by this is meant that the structure 
comprises essentially no added agent bonding distinct grains of the 
abrasive together. In fact, the tool comprises a porous structure in which 
the ceramic abrasive matrix is continuous. This matrix can further 
comprise reinforcing particles, fibers or filaments but, in general, these 
are not essential to give the structure its dimensional integrity. The 
porous structure has a generally uniform degree of porosity over the whole 
tool except where it is desired to provide a specific region with a higher 
density than another. The tool is not, however, characterized by random 
porosity variations. 
The nature of the ceramic abrasive is restricted by the ability of the 
individual particles to sinter together to form a hard, coherent structure 
capable of functioning as an abrasive tool. Suitable materials include 
alumina, modified aluminas, alumina/zirconia, silicon carbide, cubic boron 
nitride and the like. 
The preferred ceramic abrasive is alumina and the invention is described 
hereafter with particular reference to this material for the sake of 
simplicity. It is not, however, to be implied from this that there is any 
limitation to this material inherent in the invention. The preferred 
alumina used is a microcrystalline alumina by which is implied that the 
alumina is comprised essentially of sub-micron sized crystals of alpha 
alumina. Such products are commonly obtained by a seeded sol-gel process 
such as is described in U.S. Pat. Nos. 4,623,364 and 4,744,802. Other 
components may be present in the structure, such as fibrous or particulate 
ceramics or toughening additives such as zirconia and certain rare earth 
metal oxides. It is often desirable to increase the solids content by 
addition of up to about 55% of the solids weight of the mixture, of fine 
particles of alpha alumina or a precursor of alpha alumina such as gamma 
alumina. This would result in a composite gel rather than a sol or gel but 
would not otherwise change the nature of the process or the final product. 
It is also possible to use a slip or slurry comprising up to about 55% by 
weight of finely divided microcrystalline alpha alumina, or precursors 
thereof, in the process of the invention. 
The sol, gel, composite gel, slurry or slip should preferably have as high 
a solids content as possible since this reduces the amount of water that 
has to be lost during the drying and sintering stages. This can be done, 
for example, by forming a boehmite gel with a minor amount (from about 0.1 
to about 5% by weight for example) of sub-micron sized particles of alpha 
alumina to act as a seed material to reduce the temperature at which the 
conversion of the precursor to alpha alumina takes place and then adding 
an alpha alumina precursor that has been calcined at a temperature of from 
about 500 to about 900 degrees centigrade. The amount of the solid 
precursor that can be added in this way is limited by the ease with which 
the resulting high solids mix can be handled. 
The regular porosity characterizing the tools of the invention can be 
induced by combining the slip, slurry, sol or gel with a solid 
pore-forming material in the desired shape, removing the water and firing 
to a temperature that is high enough to cause (where a sol-gel of an alpha 
alumina precursor is used) phase transformation to alpha alumina and 
sintering of the alpha alumina. The pore forming material can be a 
particulate organic material, such as walnut shells or plastic beads of 
the appropriate dimensions, or it can, more preferably, be a reticulated, 
open-celled foam of an organic material. In either event the firing will 
also have the effect of burning the material out leaving pores in the 
finished product. The preferred particulate pore former is walnut shells 
since these have a combination of hardness, combustibility and dimensional 
stability that makes them ideal for this purpose. 
In a further embodiment of the above process the pore former may be a 
ceramic foam (which is understood to include honeycomb structures) and the 
interior walls are coated with same mixture described above which is then 
fired to convert it to alpha alumina. In such event, the ceramic foam used 
will have a pore volume in excess of about 70 volume % before treatment 
and the extent of treatment should be such as to leave a pore volume in 
the final product of from about 5 to about 65 volume %. 
The amount of solid pore former that may be used is determined by the 
desired porosity of the final product. Because of shrinkage during water 
loss, the percent voids is often much greater than the volume of pore 
former added. Thus, if a 40% voids volume product is required, then 20% by 
volume of a pore former may be all that is needed. This is not an exact 
relationship and the behavior of each particular system must be determined 
by practical tests to derive a basis for predicting the amount that will 
be necessary in each case to obtain a desired level of porosity. These 
pore formers are most useful with products of high porosity such that an 
open celled structure is produced through which the combustion products 
can readily escape. 
Another alternative means of generating the foam structure is by the use of 
physical or chemical blowing agents. These generate pores in the mix while 
it is in a plastic state. Chemical blowing agents are compounds that 
decompose at elevated temperatures to liberate a gas. Such agents, which 
can be added as part of the mix include chemicals, such as 
azodicarbonamide, ammonium hydrogen carbonate and benzene diazonium 
chloride. This gas expands at the elevated temperature to give the 
solidifying structure a porous nature. Alternatively, physical blowing 
agents can be used. These are gases that are injected into the mixture 
while it is at state of plasticity that the bubbles become trapped within 
the mixture and produce a foam structure. These kinds of blowing processes 
are difficult to control such that blowing occurs at a point at which the 
mixture is sufficiently reduced in water content to ensure that the pores, 
thus generated, do not immediately collapse when the gas escapes. It is, 
therefore, necessary to eliminate the water at comparatively low 
temperatures below those at which the pore formation occurs so that the 
mixture has sufficient dimensional stability to withstand the pressures 
generated by the gas. The physical properties of the mixture can 
advantageously be enhanced by the use of organic additives that confer 
green strength sufficient to allow expansion without disruption. It is 
also possible, where a sol-gel is used as the source of the alumina, to 
overcome the tendency of the foam bubbles to collapse by causing internal 
gelation using additives such as hexamethylene tetramine. This undergoes 
thermal decomposition to liberate ammonia which increases the pH of the 
mix and leads to gelation. Thus, bubble formation and solidification occur 
essentially simultaneously. A further means of obtaining the desired 
porosity is to extrude the material directly into a honeycomb structure 
from which a wheel can be obtained by trimming the extruded structure. 
The most preferred way of obtaining the porous structure of the invention 
is by infiltrating a sol, gel, slurry or slip of an alpha alumina 
precursor or of alpha alumina itself into the pores of an open-celled foam 
preform in the shape of the desired tool and thereafter heating to bring 
about solidification of the structure, conversion to alpha alumina and 
elimination of the material of the open celled preform. In such a process 
it is, of course, necessary to ensure complete penetration of the pores by 
the sol, gel or slip. The penetration can be aided by pressure, providing 
this is not so great as to cause collapse of the cells before they are 
filled. It is also advantageous to evacuate the air from the foam so as 
more readily to permit entry of the precursor without air blocks. 
The material from which the foam preform may be formed should have 
sufficient dimensional stability to withstand the conditions during the 
filling operation and yet be relatively easily removed during the 
sintering operation. In practice, it has been found that ceramic foams and 
thermoplastic resin foams, such as a polyurethane or polyolefin open 
celled foam, are suitable. 
It is also possible to use a preform in the form of a shaped matt of a 
bonded fibrous material. Such materials are readily available commercially 
and, providing they are made of a material that is readily and cleanly 
burned off at sintering temperatures, are quite suitable for use in the 
present invention. 
Finally, a ceramic reticulated structure with a porosity too great for 
practical use as an abrasive can be coated with an alumina based ceramic 
to reach the right level of porosity for abrasive use.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is now further described with reference to the 
following examples which are for the purpose of further describing 
preferred embodiments of the inventive concept and are not to be 
understood as implying any necessary limitation on the essential scope of 
the invention. 
EXAMPLE 1 
This example illustrates the production of a grinding wheel by infiltration 
of a foam with a sol-gel material, followed by drying and firing to form 
the abrasive wheel and to burn out the foam starting material. 
A foam polyurethane pad, in the shape of the final desired wheel and having 
a porosity of about 40%, was infiltrated using a composite gel comprising 
a seeded sol-gel of sub-micron sized boehmite seeded with sub-micron sized 
alpha alumina particles and prepared using the process described in U.S. 
Pat. No. 4,623,364 (which is incorporated by reference herein in its 
entirety) and a calcined gamma alumina sol-gel derived powder with a 
surface area of approximately 100 square meters per gram and a mean 
particle diameter (determined by use of a sedigraph technique) of 1.5 
micron. The sol-gel had a solids content of about 35 wt. % and the amount 
of calcined powder added wa 33 wt.% of the total weight. This mix was 
infiltrated into the foam polyurethane pad and the infiltrated pad was 
then dried at room temperature for two days and fired at 1300 degrees 
Centigrade for 60 minutes to yield the final product. The ramp rate to the 
firing temperature must be slow so as to prevent cracking. In fact, a ramp 
rate of about 25.degree. C. per hour is used. The firing burned out the 
polyurethane material leaving a microcrystalline product formed entirely 
of alpha alumina. The total volume porosity of the final product was 42%. 
The wheel was trued with a single point diamond using a 0.002 inch/rev. 
lead and a 0.0005 inch wide diameter of dress. The trued wheel was then 
used to grind 52100 steel (hardness Rc 60) using an internal grinding test 
procedure. The test conditions were as follows: 
Coolant -- 5% soln. of Trim VHPE 300; 
Wheel -- 1.5 inch diameter; speed 13700 rpm, 5384 ft/min. 
Workpiece Speed -- 165 rpm or 117 ft/min. 
Part diameter 2.7 in. 
Total Infeed -- 0.020 in. 
It was found that the wheel gave a G-ratio of 51.3 at a power draw of 10.1 
HP/inch. The surface finish was 41.7 microinch. 
EXAMPLE 2 
This example illustrates the use of pore inducing materials to obtain a 
product according to the invention. 
A boehmite seeded sol gel with a solids content (calculated on the basis of 
Al.sub.2 O.sub.3) of 24 wt. % similar to that described in Example 1, was 
mixed with 20 vol. % of walnut shells of average particle size 275 
micrometers. This material was thoroughly mixed and molded into the shape 
of a wheel which was then dried at room temperature for two days and then 
the temperature was raised at about 25.degree. C./hour, to a firing 
temperature of 1300.degree. C. and held there for 60 minutes to convert 
the boehmite to alpha alumina and to burn out the walnut shells. The final 
product had a porosity of 40% and had sufficient strength to be used as a 
grinding wheel.