Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
s~
60SD-127
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RANDOMLY-ORIENTED POLYCRYSTALLINE SILICON CAR~IDE COATINGS
FOR AB~ASIVE GRAINS
Background of the Invention
The present invention relates to silicon carbide
coated abrasive grains, such as diamond and cubic
boron nitride, and more particularly to unique randomly-
oriented polycrystalline silicon carbide coatings
therefor.
Taylor (U.S. Patent No. 3,520,667 dated July l4,
1970) proposes to coat diamond abrasive particles
with an adherent surface coating of silicon carbide by
suspending the diamond particles in a gaseous atmosphere
of a volatile silicon compound which is thermally
decomposed to form a silicon carbide coating on the
diamond particles by reaction with the carbon present
in the volatile silicon compound or added gaseous
source of carbon. Kuratomi (U.S. Patent No.3,617,347
dated November 2~ 1971) uses an analogous vapor
deposition technique to coat diamond powder with a thin
film of silicon carbide formed from the gaseous
silicon compound and the diamond particle itself which
thin silicon carbide coating then is .recoated with
elemental si.licon. Silicon-based alloy coatings for
abrasive grains have been proposed by Otopkov (U.S~
Pa~ent No. 4,184,853 dated January 22, 1980) wherein
the coating is a three-component system which includes
silicon and two other metals selected from a rscited
group of metals, and by Ca~eney (U.S. Pa~n-t No.3,929,432
. ~J
~9~
60SD-127
dated December 30, 1975) wherein a titanium alloy
coats the diamond grainO U.S. Patent No. 4,174,971
to Schrewelius dated November 20, 1979, describes
the in situ formation of silicon carbide by a
pyrolysis process.
A prime use of these various coated abrasive
particles is in the formation of grinding elements
wherein the coated abrasive particles are bonded
together with resin, metal, vitreous material, or
the like. A prevalent problem in using such bonded
coated abrasive particles is that the particles tend
to pull out from the bonding matrix, thus resulting
in decreased grinding performance and a loss of
valuable abrasi~e particles. The coatings on the
abrasive particles provide a degree of increased
retention of the particles within the bonding matrix.
Still, improvements in this art are needed.
Broad Statement o'f the Ihvention
One aspect of the present invention is a composite
abrasive particle comprising a core abrasive crystal
selected from the group of diamond and cubic boron
nitride, and a silicon carbide coating adherently
bonded to substantially all of the exterior surfaces
of said core crystal. The improved silicon carbide
coating is characterized by silicon carbide crystals
having a random orientation substantially independent
of the structure of the core crystal, the outer surface
o~ the coating being conformationally irregular.
Another aspect of the present invention is a
composite a~rasive aggregate comprising core crystals
selected from diamond, subic boron nitride, and mixtures
thereof, disposed within a silicon carbide matrix which
matrix is ~dherently bonded to substantially all of the
exterior surfaces of each of the core crystals. The
silicon carbide matrix is characterized by silicon
60SD~127
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carbide crystals having a random orientation substantially
independent of the structure of the core crystals. The
matrix interconnecting said core crystals has an open
structure and the outer surface of the matrix is
conformationally irregular.
A method for making the co~posite abrasive particle
described ahove comprises~ providing core crystals coated
with non-diamond carbonaceous material; infiltrating
said crystals with fluid silicon under a partial
vacuum at a temperature of above 1400C, cooling and
recovering the resulting mass of core crystals bonded
together by a bonding medium of silicon carbide and
elemental silicon, the outer surfaces of said core
crystals having silicon carbide adherently bonded to
substantially all of the exterior surfaces thereof;
leaching substantially all of said silicon from said
bonded mass with a silicon leaching agent; and
subdividing (milling) the resulting leached mass and
recovering said composite abrasive particles. Fibrous
graphit~ can be provided in a uniform mixture with the
coated core crystals to promote silicon infiltration.
There are two distinctions between the process as
used to make the composite abrasive par-ticles described
above and the composite abrasive aggregate. First, in
the case of aggregates, the core crystals coated with
non-diamond carbonaceous material should be tightly
packed ~ince a high degree of contact between the
coated crystals is needed. This high packing density
may be used in the case of the single particles as
well, but is not required. The high crystal concentration
also dictates that a minium amount of fibrous graphite
be used.
Secondly, and more importantly, to make aggregates,
a relatively light or low energy milling (eg. less
milling time) is required with sieving done more frequently
to remove aggregates of the desired size (more than twice
~ 19 ~5~ 6OSD-127
a,
the size o~ the starting core crystals). To obtain just
coated diamond or CBN, a more vigorous milling can
be used with sieving done at a size only slightly
larger than the core crystal. For example, starting
with 140/170 mesh (106/90 micron) diamond, silicon
carbide coated diamond would be sieved out in the
100/120 mesh (150/125 micron) size while aggregates
would be obtained by not milling beyond about 40/60
mesh (425/~50 micron.s).
Another aspect of the present invention is an
improved abrasive element of coated abrasive particles
embedded in a bonding matrix. The improvement is that
said coated abrasive particles comprise a core abrasive
crystal selected from the group of diamond and cubic
horon nitride, and a silicon carbide coating adherently
bonded to substantially all of the exterior surfaces
o~ the core crystal, said silicon carbide coating
characterized by silicon carbide crystals having a
random orientation substantially independent of the
structure of said core crystal, the outer surface of
said coating being conformationally irregular.
Additionally, the coated abrasive particles in the
improved abrasive element can comprise composite
abrasive aggregates wherein the silicon carbide coated
core abrasive crystals are interconnected by a silicon
carbide matrix having an open structure. The preferred
bonding matrix can be selected from the group of resin,
metal, and vitrous material, and the preferred abrasive
element is a grinding element.
Ad~antages of the present invention include a
silicon carbide coating which is tightly bonded to
the core particle and which provides excellent
mechanical bonding in the bonding matrix. Also, the
unique silicon carbide coating of the present invention
provides a highly wear-resistent region, intermediate
between the core abrasi~e crystal and the bonding
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matrix, around each abrasive grain which reduces loss
of the coated crystal by chip erosion during use of the
abrasive element. FurthQr, the composite abrasive
aggregates of the present invention have an open
structure between silicon carbide coated grains which,
when filled with bonding matrix material during
abrasive fabrication, provides for a positive locking
of the aggregate in the bonding matrix. ~dditionally,
since no metal coating is contained on ~he composite
abrasive particles or aggregates of the present invention,
the silicon carbide coating can be added at the expense
of additional secondary abrasive in the abrasive element
without detrimental effects even at higher concentrations
of the coated abrasive particles in the abrasive element.
These and other advantages will beeome readily apparent
to those skilled in the art based upon the disclosure
herein contained.
Brief Description of the Drawings
Figs. lA, lB, and lC are photomicrographs at three
different magnifications (lOOX, 500X~ and 1500X,
respectively) of the randomly-oriented polycrystalline
silicon carbide coated diamond grains of the present
invention; and
Figs. 2A, 2B, and 2C are photomicrographs at
three different magnifications (lOOX, 500X, and 1500X,
respectively) of comparative silicon carbide coated
diamond grains.
Detailed Description of the Drawings
- The comparative silicon carbide coated diamond
particles shown in Figs. 2A-2C were made by the preferred
infiltration, leaching, and subdividing process of the
present invention, except that the diamond grains were
uncoated, i.e. not coated with non diamond carbonaceous
material so that the silicon carbide coating was formed
b~ a chemical reaction of diamond carbon with silicon.
The silicon carbide coated diamond particles in Figs.
60SD~127
lA-lC were made hy the pr~ferred method of the present
invention wherein the diamond grits were coated with
a non-diamond carbonaceous material for infiltration
thexeof with fluid silicon. Note the relatively
smooth outex surface of the comparative partioles and
the apparent related orientation of the silicon
carbide crystals of the coatin~. rrhis is to
be contrasted with the exterior surface of the no~l
particles o the present invention wherein an apparently
total random orientation of the silicon carbide
crystals can be seen. The silicon carbide crystals
on the novel particles also appear to be much smaller
in overall exterior dimensions (eg.about 0.1 - 10 microns)
than the silicon carbide crys~als of the comparative
particles.
Note further that the differen~ faces of the core
diamond crystal are distinctly de~ined for the com-
parative particles. The comperative particles also
display a relationship or ordered pattern between the
diamond surace w~ich supplied the carbon and the
formed silicon carbide crystals. Note the silicon
carhide crystal shape and orientation in Figure 2A on
the octahedral (6-sided~ surfaces compared to the
cubic (4 sided) surfaces. This illustrates the
dependance of the coating sur~ace upon the underlying
diamond surface which suppliea the carbon atoms for the
formation of silicon carbide. On the oth~r hand~
the exterior surface of the novel particles o~ the
- present invention exhibit a completely random
orientation of silicon carbide crystals independent
of the underlying diamond surface. The outer surface
of the novel particles also is conformationally
irregular and extremely rough in texture. The random
orientation of the silicon carbide cr~tals on the
novel coated particles of the present invention
apparently s~ems from ~he fac~ that the silicon carbide
s~o
60SD-127
-- 7
nucleation arises from the silicon carbide crystals
of the coating rather than from the diamond core
particle. Apparently, nucleation of the silicon carbide
crystals on the comparati~e samples during their for-
mation is based on the core diamond particle itself.Also, since nucleation of the silicon carbide in the
coating of the novel particles is based upon the silicon
carbide crystals in the coating itself, excellent
intercrystal silicon carbide crystal bonding is
displayed by the novel coated particles of the present
invention. ~ased upon the texture and arrangement
of the silicon carbide crystals of the coatings of
the two different particles shown in the photographs,
one would expect that the novel coated particles of the
present. invention could be bound in a matrix to a much
better degree than the comparative coated crystals. In
fact, this has been demonstrated to be true as the
example will amply demonstrate.
Detailed Description of the Invention
Referring to the pre~erred method for ma]cing the
composite abrasive particles and aggregates of the
present invention, the preferred core abrasive
crystals is selected from the group consisting of
diamond, cubic boron nitride, and mixtures thereof
when ma~ing the novel abrasive aggregates of the
present in~ention. It must be recognized, though,
that any abrasive particle (eg~ silicon carbide itself,
aluminum oxide, etc.) may serve as the core abrasive
- cxystal for coatiny with silicon carbide according
to the precepts of the pxesent inventionO In view of
the cr~stal arrangement and morphology of the silicon
carbide crystals in the coating as shown in the
drawing, it is apparent that the source of carbon for
foxming the silicon carbide coating should not come
from the core ahrasive crystal itself~ but rather an
external source of carbon should be used~ Thus, the
60SD-127
core abrasive crystals provided with a coating of non-
diamond carbonaceous material. The particle size range
for the preferred diamond and cubic boron nitride
abrasive crystals range from about 325/400 to 60/80
mesh (United States standard sieves series) (45/38 to
250/1~0 microns) with particles of about 100/120 mesh
and smaller being preferred. Coating of the core
crystals with non-diamond carbonaceous material and the
silicon infiltration s~ep of the present process
desirably are practiced according to -the general procedure
disclosed in U.S. Patent No. 4,220,455 to St. Pierre
dated September 2, 1980. Note that somewhat similar
processes also are disclosed in U.S.Patent Nos.4,148,894
to Hillig et al dated April 10~ 1979, 4,233,433 dated
~ecember 9, 1980; and 4,242,106 to Morelock dated
December 30, 1980.
The carbonaceous material can be elemental non-
diamond carbon, an organic material, or mixtures
thereof. The organic material decomposes completely at
an elevated temperature below 1400C, and ordinarily
decomposes completely at a temperature ranging from
about 50C to about 1200C, to produce elemental
non-diamond carbon and gaseous product of decomposition.
Representative of the organic materials useful
in the present process are polymers of aromatic
hyd ~carbons such as polyphenylene and polymethylphenylene
derivatives of polynuclear aromatic hydrocarbons
contained in coal tar such as dibenzanthracene and
chrysene. Additional examples of useful organic
materials are the epoxy resins such as the reaction
product of epichlorohydrin and Bisphenol-A. Still
additional examples of useful organic materials are
phenolic resins obtained by the condensation of phenol
or sub~tituted phenols with aldehydes such as formalde-
hyde~ acetaldehyde, and furfural. Specific examplesaxe the condensation products of phenol~formaldehyde,
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6OSD-127
resorcinol-formaldehyde, aniline-formaldehyde, and
cresolformaldehyde.
The non-diamond carbonaceous material coated core
crystals nex~ are infiltrated with fluid silicon under
partial vacuum at a temperature of above 1400 C,
preferably using the apparatus and general procedure
shown and described in U.S. Patent No. 4,220,455 to
St. Pierre dated September 2, 1980. While the coated
core crystals can be admixed with additional carbon
for infiltrating with silicon, such is not necessary
in forming ~he silicon carbide coated composite abrasive
particles of the present invention. As noted in U.S.
Patent No. 4,220,455, the coated core crystals and a
source of silicon are heated to an infiltration
temperature which is above 1400C. When the carbonaceous
material coating the core crystals is an organic
material, such organic material decomposes completely
at a temperature below 1400C producing elemental non-
diamond carbon and gaseous products of decomposition.
Infiltration is carried out at a temperature
above 1400C at which silicon becomes fluid and which
has no significant deleterious effect on the crystals.
For cubic boron crystals infiltration temperatures
significantly higher than about 1450C are not useful
~ince they are likely to cause conversion to hexagonal
boron nitride~ On the other hand, for diamond crystals,
temperatures higher than 1550C provide no significant
advantage. By a temperature at which silicon becomes
fluid it is meant herein a temperature at which the
silicon is readily 10wable. The fluid silicon is
highly moblle and highly reactiwe with elemental non-
d~amond carbon, i.e~ it has an affinity for elemental
non~diamond carbon, wetting it and reacting with it to
foxm silicon carbide. Specifically, when sili~on is at
its melting temperature, which has been given in the
art to range from about 1412C to about 1430C, it has
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60SD-127
-- 10 --
a high viscosity, bu-t as its temperature ls raised, it
becomes less viscous and at a temperature about ten
degress higher than its melting point, it becomes fluid.
The temperature at which the silicon is fluid is the
temperature at which it will infuse or infiltrate
through the capillary~size passages, interstices or
voids of the moldconfined mixture of crystals. With
increase in temperature, the flowability of the fluid
silicon increases resulting in a faster rate of reaction.
Sufficient silicon is infiltrated throughout the
mold-confined mass or mixture infusing or infiltrating
through the voids or pores of the mixture by capillary
action to react with the total amount of elemental
non diamond carbon (including fibrous graphite) present
in the confined mixture forming silicon carbide, and
also to fill any pores or voids which may remain after
formation of the silicon carbide. Any pores remaining
after formation of silicon carbide are filled during
infiltration with elemental silicon. Also during
infiltration, the silicon reacts with the elemental
non-diamond carbon coating on the surfaces of the
crystals forming a protective adherent coating of
silicon carbide on the diamond surfaces and causing
no loss or no significant loss of crystals and no
change or no significant change in the shape and
dimensions of -the crystals. The resulting infiltrated
mass is cooled in an atmosphere which has no significant
deleterious effect on said infiltrated mass~ preferably
it is furnace cooled in ~he partial vacuum to about
room temperature, and the resulting polycrystalline
body is recovered.
The period of time for full infiltration by the
silicon is determinable empirically and depends largerly
on the si2e of the shaped mixture, and frequently,
infiltration hy the fluid silicon through the mold-
confined shaped mixture is completed within about lS
60SD-127
11 -
minutes.
The cooled and recovered resulting mass of core
crystals bonded togther by a bonding medium of silicon
carbide and elemental silicon then is subjected to the
action of a silicon leaching agent for leaching
substantially all of the silicon from the resulting
bonded mass. The silicon leaching agent generally
is a suitable acid with a mixture of nitric and
hydrofluoric acids in a volume ratio of 9:1, respectively,
~eing preferred. The leaching operation desirably
5s practiced by immersion of the bonded mass in the
silicon leaching agent which may be held at room
temperature or more preferably at a temperature between
abou-t 100 and 120C. Note that sequential leaching
of the honded mass or on subdivided portions of the
bonded mass may be practiced as is necessary, desirable,
or convenient.
The leached bonded mass is quite friable and is
composed primarily of silicon carbide coated core
abrasive crystals and silicon carbide particles. This
leached mass is subdivided by crushing or other
conventional technique to provide the composite abrasive
particles or aggregates of the present invention in
admixture with silicon carbide particles. The sub-
divided mass, then is subjected to a separation step
by screening or other conventional classification
technique for separating and recovering the composite
abrasive particle~ or aggregates o~ the present
invention.
Several unexpected and beneficial results of this
process are achieved. The silicon carbide coating on
the core abrasive particles is in a thickness ranging
from about 0~1 to 10 microns with a thickness of about
0.5 to 1 micron being preferred. It will be appreciated
that due -to the highly rough texture and outer surface
conformational irregularity of the particles of the
60SD-127
- 12
present invention that precise measurements of the
coating thickness are difficulk and the figures
given here, while believed to be accurate, are not a
limitation on the present invention. Still, the
coating thickness of the silicon carbide on the core
abrasive crystal generally is much greater than can be
conveniently achieved by formation of silicon carbide
coatings deposited on the core abrasive crystals by
conventional vapor deposition techniques. Additionally~
the silicon carbide coating is very tightly bonded to
the core crystal. Thicker coatings are possible if
the vapor process is carried out for long periods of
time. Vapor deposition is inherently a slow process
since the layer must be built up atom by atom. Additionally,
for this reason vapor deposited coatings tend to be
smoother or follow the roughness of the substrate. In
the present invention, the coating is rougher because
the non-diamond carbonaceous coating is rough. Note
the difference in the Figures. Further, the random
orientation of the silicon carbide crystals is a noted
unique feature. Additionally, the silicon carbide
crystals in the coating generally are quite small in
particle size, especially when compared to silicon
carbide which is formed by reaction of silicon with the
~5 carbon content of diamond, for example.
With respect to the composite aggregates of the
present invention, the aggregates are composed of the
silicon carbide coated crystals which are interconnected
by a silicon carbide matrix in an open structure. This
3Q means that channels and passageways run through the
aggregates ~rom the interior thereof to the outside.
The open structure or porosity of -the silicon carbide
matrix binding the coated composite abrasive particles
is unique and well adapted for bonding of the aggreyates
~5 in the formation of abrasive elements, for example.
The composite abrasive particles and aggregates of
3~
6 0 SD- 1 2 7
13
the present invention desirably are used in the formation
of abrasive elements wherein the coated ahrasive
particles or aggregates are embedded in a bonding matrix
for formation of an abrasive element which preferably
is a grinding element. The novel composite abrasive
particles and aggregates of the present invention in
such use provide much better mechanical bonding of the
abrasive particles to the matrix as well as much better
bondlng of the silicon carbide coating of the core
abrasive crystals, especially when the composite abrasive
particles of the present invention are compared to
conventional metal coated abrasive particles. Further,
the silicon carbide coating provides an area of wear
resistance about the core abrasive particle which is
intermediate between the core abrasive crystal itself
and the bonding matrix. This further contributes to
the retention of -the composite abrasive particles and
aggregate~ in the bonding matrix during use o~ the
abrasive elements, since it reduces erosion of the
bondîng matrix around the diamond/CBN crys-tals by
workpiece chips generated in the grinding action.
A variety of conventional bonding matrices may
be used to bond -the composite abrasive particles and
aggregates of the present invention. Such conventional
bonding matrices include resin, metal, and vitreous
materials.
The resin most frequently used in resin-bonded
grinding elements or other abrasive elements is a
phenol-formaldehyde reaction product. However, other
resins or organic polymers may be used such as melamine
or urea-~ormaldehyde resins, epoxy resins~ polyesters,
po'yamides, and polyimides. A resinoid grinding, wheel
with the composite abrasive particles and aggregates of
the present invention can be formed, for e~ample, in the
manner as taught in U.S. Patent No. 3,6~5,706 to Bovenkerk
dated February 29, 1972~ Note that the silicon carbide
60SD-127
- 14 -
coating can be added to the abrasive emenet at the expense
of secondary abrasive particles customarily in resin
grinding elem2nts without detrimentally effecting the
performance of the grinding element. The improved
resinoid grinding elements of the present invention
having diamond as the core crystal are particularly
useful in the wet or dry grindlng of various workpieces
and especially in grinding non-ferrous materials such
as cemented tungsten carbide and cemented tungsten
carblde/steel combinations.
It is feasible to modify the composite abrasive
particles or aggregates of this invention by coating
techniques~ They could be coated with a metal (eg.
nickel or copper) over the silicon carbide coating by
any of the well known techniques such as electroless
coating (see U.S. Patents 3,556,839 to Roy dated
January 19, 1971 and 3,957,461 to Lindstrom dated May
18, 1976, Example 6 and Lowenhein, F.A., Electroplating,,
Chapters 17 and 19, McGraw Hill Book Co. (1978). Further
re~erences to various metals which are suitable for
coating the p~rticles of the pxesent invention can be
found in U.S. Patent 3,957,461 to Lindstrom dated May
18, 1276~see especially column 8).
The ~olume percent concentration of coated abrasive
particles, when used in a resin bond matrix, is normally
between a~out 5 and 35 percen~.
The aggregates as described herein also can be
infiltrated with metal (eg. silver or copper) by: (a)
mixing them with metal powder; and (b) furnace heating
the mixture ~preferably under non-Qxidizing conditions)
at su~ficient temperature to melt the metal allowing
it to infiltrate into the interstices of the aggregates,
the resulting infiltrated masses may be milled and
sieved to si~eO Two infiltration process known to the
art are described in U.S. Patents 2,216,728 Example I
and 4,024,675 to Naidich dated May 24, 1977.
~1 ~ ~ r~ ~ c- a
60SD-127
For further details as to suitable metal, resin, and
vitreous matrices for bond.ing the composi~e abrasive
particles and aggregates of the present invention,
reference is made to U.S~ Patents Nos. 2,137,329,
2,561,709 to Norling dated July 24, 1951, 3,203,775
Cantrell August 31, 1965 and 3,894,673 to Lowder et al
dated July 5, 1975 for metals; 3,385,684 to Voter
dated May 28, 1968; 3,664,819 Slovi et al dated May
23, 1972, 3,779,727 to Siqui et al dated December 18,1973
10 and 3,957,461 to Lindstrom et al dated May 18, 1976
for resins; and 2,216,728 (page 3) for vitreous bond.
Metal bonded grinding elements can be used in glass
edging, lens generating, grinding ceramics and other
non-metall.ic materials as well as other articles as is
well known in the art. A variety of ferrous and non-
ferrous metals and alloys can be suitably ground with
vitreous bonded grinding elements also.
The following example shows how the present
invention can be practiced, but should not be construed
as limitiny. In this application, all units are in the
metric system, all mesh sizes are in United States
standarded sieves series, and all proportions are by
volume, unless otherwise expressly indicated.
EXAMPLE
WET GRINDING TESTS
Abrasive Preparation
Two abrasives were silicon carb.ide coated:
140/170 mesh ~106/90 micron) resin bond diamond
obtained as RVG from General Electric Company
140,~170 mesh metal bond diamond obtained as MGB
~rom General Electric Company
Each was non-diamond carbon coated by pyrolyzing
methane at about 1200C. The coating weight was about
seve.n percent (7~) by weight for both abrasives.
Each coated abrasive was silicon infiltrated
5~
- 16 ~ 60SD-127
according to U.S. Patent 4,200,455 to St. Pierre dated
September 2, 1980 and U.S. Patent No. ~,3~1,271 to Hayden
dated April 26, 1983O Each carbon coated abrasive was
mixed with fibrous graphite (which had previously been
heat treated for 1-5 minutes at 1300 to 1550C. under
a vacuum of less than about 1 Torr) in the approximate
weight ratio 100:6 coated diamond to fibrous graphite.
This mixture was placed into a hexagonal boron nitride
coated graphite mold with a diamond concentration of
about 28 percent (23%) by volume. The mixtures then
were silicon infiltrated. The infiltrated bodies were
placed in a bath of nitric acid and hydrofluoric acid
in a volume ratio of ahout ~:1. This bath was heated
to about 120C. and held at this temperature until no
further reaction was evident, about 4 hours. The
resulting friable mass was rinsed in water, dried, and
lightly crushed in a mortar and pestle. Because of
the low diamond concentration,~only light crushing
was required to separate the silicon carbide coated
diamond particles. Particles larger than about 140
U.S. mesh were sieved out and acid treated as before
until no further reaction was evident, about 30 minutes.
These particles were rinsed in water and dried. The
100/140 mesh particles were sieved out and used for
grinding tests. The particles essentially consisted
of individual 140/170 mesh diamond grits uniformly
coated with silicon carbide.
The silicon carbide coating level was measured
by chemically removing the silicon carbide from a
representative sample in fused potassium hydroxide.
The result in percent by weight was as follows:
Type SiC Coating Level (%)
Resin Bond Diamond 36
Metal ~ond Diamond 23
Prepara-tion of Grinding Wheels
lAl type peripheral grinding wheels containing
q r-~
~_ ~ 9;;1~ J~
60SD~127
- 17 -
each t~pe of abrasi~e and 140/170 mesh RVG uncoated,
resin bond diamond for comparision were made. All
wheels contained diamond at 22.5 percent by volume
concentration and were 125 mm diameter, 9.5 ~m wide and
a 9.5 mm thick abrasive section. The wheels were
phenolic resin bonded with fine SiC secondary abrasive
in a 1:25 weight ratio to the resin. A minimum of two
wheels containing each abrasive type was ~ested.
Grinding Tests
Wet grinding tests were run on a Norton surface
grinder. A BioCool 500 (a rust inhibitor) and water
mixture was used as the coolant. The workpiece ma-terial
was Carboloy Grade 370 cemented carbide; a wear resistant
grade with a composition by weight of WC 72%, TiC 8%,
TaC 11.5%, and Co. 8.5%.
Prior to testing, each wheel was mounted on the
grinder and trued using a silicon carbide wheel on a
brake-controlled trueing device. The wheel was subjected
to a precondikioning grind until energy re~uirements
and rate of wheel wear had stabilized.
Testing was done at the following conditions:
Wheel Speed: 24.5 m/sec
Table Speed: 15.0 m/min
Downfeed: 0.038mm
Crossfeed: 1.27 mm
Workpiece Size: 145cm2
Grindihg Test ~esults
The grindiny test results are expressed in terms
of grinding ratio, GR, which is defined as the volume
o workpiece material removed divid~d by the volume
of wheel consumed. GR is a measure of wheel efficiency.
The grinding test results were as follows. The
G~ reported is the average of two wheels with a minimum
of 3 tests per wheel~
"
3~5~
60SD-127
- 18 -
Abrasive Type Grinding Ratio Rel. GR*
Uncoated resin bond diamond 50 loO
Resin bond diamond 120 2.4
Metal bond diamond 38 0.76
*Rel. GR is -the relative grindiny ratio, calculated
by dividing the grinding ratio for each abrasive type
by the GR for the uncoated abrasive.
As can be seen, the GR and SiC coated resin bond
diamond is unexpectedly large compared to uncoated
resin bond diamond. Metal ~ond diamond is not recommended
for grinding cemented carbide since it is less friable
and, thus, does not remain sharp. SiC coated metal
bond diamond also has thi~ disadvantage. This is
evidence that the coating process does not degrade the
metal bond crystal to where its grinding performance is
effected.
