Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MECHANOCHE~1ICAL POLISHIi~7G ABRASI~E
BACK(~ROUND OF THE INYENTIOI~l
1. Field oS the Invention
The invention is a mechanochemical polishing abrasive for use in the
S polishing of hard substrates such as ceramics, crystalline materials, glass and
similar materials which require highly polished surfaces.
The economics of polishing and machining ceramics can often be the
most costly part of the ceramic production process. Economic considerations
for polishing ceramics involve both the time and the consumable products
employed. I:or final polishing of ceramics, diamond abrasives are extensively
used, the diamond abrasives are expensive and the polishing process using
diamond abrasives is slow.
Conversely, the machining of ceramics can be very damaging. Unlike
the machining of metals which are ductile, ceramics are generally very
brittle. The brittle nature of ceramics makes them very sensitive to
subsurface fracturing. This subsurface damage adversely affects significant
physical properties of the ceramics. Such physical properties adversely
affected by machining include a reduction in the ceramics strength, changes
in the`ceramic magnetic properties, and even changes in the electronic
properties of the ceramics.
The machining of advanced ceramics has traditionally been
accomplished by hard abrasives such as diamond or silicon carbide. Though
this has produced surfaces that are acceptable under certain circumstances,
there still remains a certain degree of surface and subsurface damage
utilizing these compounds. The use of softer polishing abrasives, such as
colloidal silica, for machining advanced ceramics has been examined.
Colloidal silica has been shown to polish alumina, silica, and silicon.
In fact, colloidal silica has been used extensively to polish silicon
chips. However, the use of colloidal silica to polish advanced ceramics tends
to produce a polished advanced ceramic product with substantial phase relief.
The relief is believed to be caused by the chemical dissolution of selective
grains on the advanced ceramics by the colloidal silica.
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2. To Prior Art
The use of colloidal silica to polished silicon surfaccs, mctals, glass,
garnets and sapphires is disclosed in the article H. W. Gutsche and J. W.
Moody, "Polishing of Sapphire with Colloidal Silica", J. Electrical Chemicnl
Soc. 125, No. 1, Pages 136-138, (1978~. The article discloses that colloidal
silica has a chemical effect Otl the harder sapphire material allowing the
colloidal silica to polish the sapphire. The article is silent concerning
combining colloidal silica with other polishing substances to polish hardened
materials.
The use of CaCO3, BaC03 and MgO as mechanochemical polishing
abrasives is disclosed in a report by H. Bora and R.J. Stokes, Stlldy oJ
Mechanochemical Machining of ~eramics and the Effect on Thin Film Behavior,
United States Government Report N00014-80-C-0437-1 (April 30, 1981). The
report details the polishing of thin layers of MgO, Al2O3 and Si by various
abrasives including rock salt, Calcite, Fluorite and various other abrasives
including window glass. The three compounds mentioned above were
discovered to be capable of mechanochemically polishing one or more of the
above materials. None of the mechanochemical abrasives used were combined
with colloidal silica.
The mechanochemical polishing of sapphire, silicon, and quartz
crystals is disclosed by N. Yasunaga, U. Tarumi, A. Obara, "Mechanism and
Application of the Mechanochemical Polishing Method Using Soft Powder"
The Science of Ceramic Machining and Surface Finishing 11, NBS special
publication 562, U.S. Government Printing Office, Washin~ton, D.C., pages
171-183 (1979). The sapphire, silicon and quartz were polished with wet and
dry mechanochemical media. The mechanochemical media includcd BaCO3,
Fe3O4, CeCO2, SiOa CeO2, diamond and MnO2. The primary focus of the
article is a description of the formation of crystalline silica materials in themixed powder abrasive during the workpiece polishing. The crystalline
materials were produced by polishing the hard materials described above at
high temperatu}es and pressures using the mixed powder. The article does
not disclose the use of colloidal silica in any manner for mechanochemical
polishing.
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SUMMARY O~ THE IN~ENTI~N
It is an object of this invention to provide an inexpensive
mechanochemical polishing abrasive which is capable of polishing a hardencd
work p;ece without seleGtively abrading the surface particles of the hardened
work piece.
It is yet another object of this invention to provide a method for
polishing a hardened work piece utilizing an inexpensive mechanochemical
abrasive. This invention relates generally to a mechanochemical abrasive.
The mechanochemical abrasive comprises a slurry of colloidal silica
containing one or more mechanical abrasives.
In a variation of this embodiment, this invention is a mechanochemical
abrasive comprising a slurry of colloiclal silica and a mechanical abrasive
selected from one or more of the materials in the group comprising Fe203,
Fc304, MgO, BaCO3, CaC03, MnO2, CeO, SiOz, CeO2, Cr203, and Al20~
In a preferred embodiment, this invention is a mechanochemical
polishing abrasive comprising from about 13 to about 99.3 weight percent of
Q basic slurry of colloidal silica and from about 0.7 to about 2.0 weight
percent of a mechanical abrasive. The mechanical abrasive has a particle
size of from about 0.1 microns to about 10 microns. The mechanical abrasive
is selected from one or more of the materials in a group comprising Fe203,
Fe30b, MgO, BaCO3, CaC03, MnO2, CeO, SiO2, CeO2, Cr20~ and Al20~
In another embodiment, this invention is a method for polishing one
.~ or more workpieces with a mechanochemical polishing abrasive. The
mechanochemical polishing abrasive cornprises an aqueous slurry of colloidal
silica and a mechanical abrasive selected from one or more of the materials
in the group comprising Fe203, Fe304, MgO, BaCO3, CaC03, MnO2, CeO, SiO2,
CeO2, Cr203, and Al20~ The workpiece is polished by applying the
mechanochemical polishing abrasive to a workpiece and either directly or via
a polishing pad and contacting the polishing means with one or more
workpie~es for a period of time sufficient to polish a surface of the
workpiece.
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D~IPTION OF THE PR~FERRED E~qBODIME~lTS
The present invention relates to a mechanochemical polishing abrasive
and a method for polishing a workpiece utilizing the mechanochemical
polishing abrasive. The mechanochemical polishing abrasive of this invention
S comprises a colloidal silica material combined with a mechanical abrasive.
The colloidal silica material contained in the mechanochemical
polishing abrasive of this invention chern-ica!ly reacts with the various
surface cornponents whjch~ ~make~yp the hardened work~ieces that are polished
with the mechanochemic?l polishing abrasive. The precise chemical reaction
that occurs between the colloidal silica and the elements and molecules in the
surface of the workpiece is not totally understood. However, it is believed
that the colloidal silica reacts with the surface of the hardened workpiece to
produce a surface material on the workpiece that is softer than the
mechanical abrasive. As mentioned above, one drawback with the colloidal
silica is that it appears to attack preferred grains on the surface of the
hardened substrate so that a reliefed surface finish can result when the
workpiece surface is chemically altered by the colloidal silica.
Colloidal silica is typically supplied in an aqueous (water-containing)
slurry made up of up to 50% or more colloidal silica. One interesting feature
of the colloidal silica slurry is that the colloidal silica does not settle froma slurry even after a great period of time. The colloidal silica is generally
contained in an aqueous slurry, that is combined with water. However, for
purposes of the polishing abrasive of this invention, the colloidal silica need
not be, and in some cases, must not be in a slurry with water, but can be in
a slurry with some other liquid, such as an alcohol and organic solvent or the
like material. Water is not desired as a slurry of material in all cases
because water can adversely react with certain hardened workpieces to
produce a useless product.
The preferred colloidal silica will be an aqueous slurry of colloidal
silica. The weight percent content of the colloidal silica in the aqueous
slurry is not critical. However, it is preferred that the colloidal silica be
present in the aqueous slurry in an amount ranging from about 15 to about
50 weight percent or greater.
Other properties of the colloidal silica slurry such as pH, particle size
and the like are not absolutely critical to its usefulness in chemically reacting
with the surface of a hardened workpiece. However, it is preferred that the
pH of the colloidal silica slurry be greater than about seven. A colloidal
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- silica slurry having a pH greater than about seven is a basic slurry, and has
been found to react more efficiently with the surface of the materials which
can be polished with the mechanochemical polishing abrasive of this
invention.
The preferred aqueous slurry of colloidal silica can be any known
colloidal silica slurry. Generally, colloidal silica's are stabilized at a pH offrom about 8 to 14 and more commonly in a range from about 9 to 11. The
preferred colloidal silica has a pH ranging from about 9.8 to 10.2 and has an
average particle size of about 0.06 microns. The mechanochemical
polishing abrasive of this invention also includes a mechanical abrasive
The mechanical abrasive is generally a softer material than the material
making up the workpiece. However, the mechanical abrasive is generally
harder than the surface material of the workpiece resulting from the
chemical reaction between the colloidal silica components of the polishing
abrasive and the hardened workpiece. The purpose of the mechanical
abrasive is to abradc the softer reacted materials from the surface of the
hardened workpiece leaving behind a smooth workpiece surface. By abrading
the softer reaction product from the surface of the hardened workpiece the
mechanical abrasive continuously exposes a hardened surface to the colloidal
silica which chemically reacts with the exposed surface. In this way, thc
selective reac~ion of the colloidal silica with various selective grains on the
surface of the hardened workpieces can be reduced to a minimum producing
a highly polished, hardened substrate surface.
The mechanical abrasive useful in the mechanochemical abrasive of
this invention may be any material or combination of materials known in the
art to be useful as a mechanical abrasive material. The mechanical abrasive
will typically be softer, that is, not as hard as the material making up the
workpiece. The mechanical abrasive material may, for example, be selected
from one or more of the following compounds: Fe203, Fe304, MgO, BaCO3,
CaC03, MnOa CeO, SiOa CeOa Cr203, and Al2O} A preferred mechanical
abrasive is Fe2O3. It is also preferred that the mechanical abrasive have an
average particle size of from about 0.1 microns to about 10 microns and more
preferably from about 0.5 to 5.0 microns.
As previously mentioned, the mechanochemical polishing abrasive of
this invention comprises a slurry of colloidal silica and one or more
mechanical abrasives. The mechanochemical polishing abrasive of this
invention will typically contain from about 0.1 grams of a powdered
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mechanical abrasive with 100 ml of colloidal silica to enough rnechanical
abrasive combined with colloidal silica to turn the mechanochemical polishing
abrasive into a viscous slurry. The amount of mechanical abrasive necessary
to turn the mechanochemical abrasive into a viscous slurry will vary
depending upon the mechanical abrasive used and depending upon the silica
content of the colloidal silica.
Water or other diluent materials such as alcohols, solvents, etc. may be
added to the mechanical abrasive/colloidal silica mixture to reduce the
viscosity of the mechanochemical abrasive. A mechanochemical abrasive with
a low viscosi~y is easily applied to workpieces. It covers the workpieces
evenly and generally has better flow and abrasive properties.
Preferably, the mechanochemical abrasive of this invention will
comprise from about 0.07 to about 2.0 wt % of a powdered mechanical
abrasive from about 13 to about 99.2 wt % of colloidal silica and from about
0.7 to about 85 wt % ml water. Preferably, the water is deionized water.
The mechanochernical abrasive of this invention is useful for polishing
the surfaces of many different hardened workpieces. The mechanochemical
polishing abrasive of this invention may be used to polish the surfacc of any
material which is capable of chemically reacting with a colloidal silica slurry.Examples of such material include silicon (such as silicon wafers), sapphire,
metals, glass, alumina, silicon nitride (Si3N4), gallium arsenide (GaAs),
magnesium oxide (MgO), zirconia and other hardened ceramic and non-
ceramic materials.
The mechanochemical polishing abrasive of this invention can be used
to polish hardened workpieces by any polishing means known in the art for
polishing hardened workpieces. The polishing can be accomplished by any
polishing means including by hand using a pad and the mechanochemical
polishing abrasive, or by a machine using the liquid mechanochemical
t polishing abrasive of this invention. It is preferred that a pad be used in
conjunction with a machine to polish hardened workpieces with the polishing
abrasive of this invention. The polishing abrasive is applied to the pad or
the workpicces and the pad then frictionally contacts at least one surface of
the abrasive covered workpieces during the polishing step. The workpiece is
polished for a period of time sufficient to polish the surface of the
workpiece to a desired finish.
Polishing conditions, including pressure and temperature, may affect
workpiece polishing rate. However, the abrasive is effective in polishing
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workpieces over a wide range of pressures and temperatures. Specif ic
pressures and temperatures arc not required for the mechanochemical
polishing abrasiYe to be effective.
Certain preferred examples of the invention will be discussed bclow.
However, many other examples would also fall within ~he scope of present
invention.
EXAMPLES
Example 1:
In this example Alumina (Al2O3) was polished with various abrasives
including mechanical abrasives, chemical abrasives and the mechanochemical
abrasive of this invention. The abrasives were evaluated far their ability to
remove surface material from alumina workpieces over time.
The surface of the alumina workpicces were prepared by grinding
them with 30 micron diamond particles contained on a hard polymer
composite plate followed by grinding with 6 micron diamond particles on a
soft polymer composite plate. After the surface of the alumina was prepared,
a Knoop indent was made in the hardened workpiece with a 5 kg load,
indented at a loading rate of 70 microns/sec. with a loading time of 15
seconds. Material removal rates were made by measuring the reduction in the
diagonal length of the Knoop indent over time.
Both vibratory and semi-automated polishers were used. Vibratory
polishing was used to minimize the mechanical contribution. The experiments
were monitored for 24 hours. Semi-automated polishers were used to provide
added mechanical contribution.
The polishing was conducted on a TEXMETR polishing cloth sold by
Buehler.
A summary of the polishing results for alumina workpieces is found
in Table I below:
TABLE I
Polishing Pol~shing
Rate
Te~hnique Abrasive
microns/hr!
Vibratory Fe~03 0.002
Polishing colloidal silica 0.042
Fe203 + (colloidal
silica) 0. 125
(microns/n~in.)
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Mechanical - Fe2O3 nil
~otary colloidal silica 0.26
Polishing 1/4 microns diamond nil
3 microns diamond 0.21
Fe2O3 + (colloidal 0.35
silica)
The results indicate that, with either vibratory or mechanical rotary
polishing, the mechanochemical polishing abrasive of this invention
comprising colloidal silica slurry in combination with the mechanical
abrasive, Fe2O3, is able to polish alumina at a higher polishing rate than
either coiloidal silica or Fe2O3 alone. Additionally, the polishing rate for thecolloidal silica/Fe2O3 abrasive is far supe~ior to the cumulative polishing rateof the colloidai silica and the Fe2O3. (0.125 microns/hr versus 0.044
microns/hr for vibratory polishing and 0.35 microns/min. versus 0.26 microns
per min. for rotary polishing).
This example clearly shows that the mechanochemical polishing
abrasive of this invention unexpectedly has a superior polishing rate in
comparison to colloidal silica or mechanical abrasives alone or cumulatively.
ExamDle 11:
Using the same vibratory polishing method above, Knoop indented
samples of alumina, silicon nitride, and zirconia were polished using colloidal
silica, various mechanical abrasives and various mechanochemical abrasives
of this invention.
The mechanochemical polishing media consisted of a liter of solution
made up of 20 grarns of either Al2O3, CeO2, Cr2O3 or Fe3O2 along with, 490
ml of a aqueous colloidal silica. The aqueous colloidal silica had a pH of
about lû.0, an average particle size of about 0.05-0.07 microns, a specific
gravity of 1.390 and contained 50% solids. ~ ml of deionized water was
added to the mixturc to complete the formula.
Vibratory polishing was used to polish the various samples for a
period of 24 hours. The polishing rate for each abrasive in microns/hr was
then determined. The result of the vibratory polishing of the three samples
are found in Table II below:
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TA13LE II
Mechanochemlcal Polishing Rate (mi~rons/hr!
PolishinF Medill Alumina Silicon Nitride Zirconia
Al2O3 + colloidal silica 0.045 0.163 0.173
CezO3 ~ colloidal silica 0.027 0.300 0.1875
Ce203 0.006 0.135 --~----
- Fe203 + colloidal silica 0.125 0.300 0.448
Fe203 0.003 0.042 0.173
Cr203 + colloidal silica 0.148 0.188 0.233
Cr2O3 0.00 0.058 G.00
(CS) colloidal silica 0.044 -----
0.058
The colloidal silica used above is the same colloidal silica used in the
mechanochemical polishing abrasive. No water was added to the colloidal
silica before it was used for testing purposes.
A slurry was made of each of the mechanical abrasives used in the
testing by adding 5 ml of deionized water to about 20g of the powdered
mechanical abrasive.
Polishing pressures and temperatures were not critical to the results
but were maintained as uniform throughout the testing as possible. The
polishing temperature was held at about 25C while the polishing pressure
was kept at about 50 gm/cm2.
In most cases, the mechanochemical polishing abrasive of this
invention polished each material at a higher rate than the colloidal silica or
the mechanical abrasive alone or cumulatively. Only the Ce203 plus colloidal
silica, polishing alumina, showed a lower polishing rate than colloidal silica
alone.
