US2897076A - Abrasive article - Google Patents

Description

United States Patent A'R'FICLE Arthur M. Suga, Philadelphia, Pa., assignor to Simonds Abrasive Company, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Application July 31, 1956 Serial N0. 601,127

11 Claims. (Cl. 51-307) Abrasive bodies, that is dense, rigid structures, having abrading properties, such as grinding wheels and the like, are well known. In such bodies abrasive grain is bound together to form a dense, rigid structure generally by means of a ceramic or resinoid bond. In those abrasive articles having a ceramic bond, abrasive grain comprises the discontinuous phase bound in a continuous phase of the ceramic bond. The bond material acts as a tool post to hold abrasive grain in the wheel. When the amount of bond is increased, the size'of the posts connecting each abrasive grain to adjoining grains is increased accordingly. The amount of bonding material determines the hardness or softness of the grinding wheel, since the abrasive grain remains the same in both strength and grade. One measure of the hardness of abrasive articles, such as grinding wheels, maybe expressed in terms of grinding ratio, i.e., the relationship between material removed to wheel wear. This measure of hardness is indicated by the expression:

wherein M is the amount of material removed per unit time, :and W is wheel wear measured in loss of weight of thewheel per unit of time. r

The varying of the hardness or' the softness of an abrasive body by increasing or decreasing the amount of ceramic bonding agent present has distinct disadvantages. As the quantity of vitreous bond is decreased to provide a softer grinding wheel, the mechanical strength of the grinding wheel is decreased since, as previously mentioned, the size of the posts connecting abrasive grain to the adjoining grain is also decreased. Thus, a soft wheel, that is one having a low grinding ratio, also may be mechanically weak and will be subject to fracture more easily than a hard wheel. The bond should also hold the abrasive grain together in the wheel in such a way that each particle performs its cutting work efliciently. For example, abrasive particles should be retained inthe Wheel until they become'dull, and as they become dull, bonding material should break away, releasing the dull particles and exposing new sharp abrasive points. Unfortunately, an abrasive body containing a proper amount of ceramic bond to provide the abrasive article with the desired hardness or softness, may not hold the abrasive grainfor such a period of time as.'to permit each particle temperature below about 1750 F.

2,897,076: I Patented July 28, 1959 2 remain in the wheel after they have become dull, thereby preventing exposure of new sharp abrasive points.

Further, it is important that the abrasive grain be firmly bound by the particular ceramic bond employed, otherwise the abrasive particles may be dislodged from the abrasive article before their useful abrasive life has been utilized. In order to effect a' tenacious bond be tween the abrasive grain and ceramic bond, it is essential that the bond substantially completely wet the surface of the abrasive grain. During the maturing of many ceramic bonds, the surface of the abrasive grain may-be destroyed as by oxidation, which is particularly true, in the case of a carbide type abrasive, such as silicon carbon, resulting in an unclean surface which is not easilywetted by the ceramic bond.

It is the principal object of this invention to provide abrasive bodies of various degrees of hardness having substantially uniformly high mechanical strength.

Another object of this'invention is to provide a grind.- ing wheel having a ceramic bond formed from a glassy frit and a fluorine-containing mica, which bond tightly binds the abrasive grain so that each abrasive particle is not released from the bond until its useful grinding life has been expired. '5

A further object of this invention is the provision of grinding wheels having a ceramic. bond formed from; a prefired glassy frit and a fluorine-containing mica in which the hardness of the wheels may be varied without varyiing the quantity of bond, thereby maintaining a' high degree of mechanical strength in the grinding wheels.

A still further object of this invention is the'provision of abrasive bodies having a ceramic bond which do. not

suffer from the disadvantages of the prior discussed abrasive articles having a ceramic bond. Further objects will become apparentfrom a consideration of the following specification and claims. r l The abrasive body of the present invention comprises abrasive grain bonded in a ceramic bond formed of a fused, glassy frit'and a fluorine-containing mica, the glassy frit having a fusing temperature below the sintering temperature of the mica. The preferred prefired, glassy frit of the instant invention is a borosilicate frit containing from about 5% to about 40%, by weight, of boron trioxide.- The micas employed are fluorine-containing silicates which can be split into very thin transparent sheets. These micas may be represented by the general formula:

RR'R"R'XSi O F wherein R, R, R", R"" and X represent a large number of cations to provide an extensive series of fluorinecontaining compounds. A preferred mica has a sintering The present invention is based upon the discovery that abrasive articlespossessing a number of very desirable, yet unexpected, properties may be obtained by the provision of a ceramic bond formed from a'prefired, glassy frit and a fluorine-containing mica, the glassy frit having a fusing temperature below'the sintering temperature of the mica. As discussed previously, in varying the hardness of an abrasive body having a ceramic bond, such as a grinding wheel, it has been a practice to vary the quantity of bond. For example, in: order to form a soft grinding wheel, it has been the practice to. reduce the quantity of bond, which of course results in a reduction in the mechanical strength of the grinding wheel.

, According to the present invention,.it has been found to per-form eflicient cutting. This is. so since the'size-- that when a fluorine-containing mica is :employed" with a prefired, glassy frit which fuses belowthe sintering point of themica to-form a ceramic bond. for a grinding wheel, theproportion of mica, and glassy frit present may be varied over a 'wide range to provide grinding'wheels of varying degrees, of' hardness. "Yet, the linechanical strength of such grinding wheels is maintained at a substantially uniformly high level, for the total quantity of bond employed need not be varied and may be present in an amount which insures high mechanical strength; In addition to' this property of the abrasive articles to maintain high mechanical strength through a wide range of grinding ratios, the abrasive articles having the bond of this invention, atcertain particular bond compositions, possess unexpectedly high grinding ratios, which ratios are not obtained when a bond consisting essentially of mica or of the fused glassy frit alone is employed.

The desirability of having a bond hold the abrasive grain at the grinding surface in such a way that each particle performs its cutting work eflicient-ly, i.e., the abr'asiveparticles are retained in the bond until their useful abrasive "life is exhausted, at which time they are permitted to break away from the bond to expose new sharp abrasive points has been discussed previously. It was found that the ceramic bond of the instant inyention tenaciously holds on to the abrasive grain to permit itto exert its fullest grinding action before being released from the abrasive article. It appears that the fluorine in the micas employed in the abrasive articles of this invention cleans the surfaces of the abrasive grain so that the ceramic bond may adhere more tenaciously -to the grains which are ordinarily diflicult to wet with bonding agent. The amount of fluorine needed to clean the surfaces of the abrasive particles is extremely small;

however, the time of release of free fluorine by the mica for this purpose appears to be of great importance. It is believed that at the sintering temperature of the mica, fluorine is released to clean the surfaces of the abrasive grain, at which time the abrasive grain is immediately wet by the mica and fused frit. If the fluorine is made available at a lower temperature, other volatile constituents, which volatilize subsequent to fluorine, may again contaminate the particle surfaces which have been cleaned by fluorine, thereby destroying the cleaning efliect of the fluorine and presenting a surface which is no longer easily wetableyby the ceramic bond. On the other hand, if fluorine is made available only at temperatures above the mica sintering temperature, fluorine for cleaning abrasive particle surfaces will be released at temperatures which may cause decomposition of the mica. If attempts .;are made to add fluorine in chemically combined form by means of other compounds, as for example cryolite, obtaining the desired result is extremely difiicult because .of the great difficulty in determining the amount of fluorine compound to be employed and the manner of its addition. a

A further valuable property of the abrasive articles 'of this invention, is that the articles can be machined with ordinary cutting tools, particularly where the bond lcontains high proportions of fluorine-containing mica. Machinability of abrasive articles is extremely desirable since the machining thereof is often necessary to provide abrasive articles, as for example grinding wheels, with proper shape.

Still another advantage of the abrasive articles of this invention is that they can be produced by the hot press method, which method provides articles of greater "density. Also, by means of the hot press method, the .chance of fracturing abrasive articles during their manufacture is greatly reduced. Such fracture during manufacture is a serious problem, particularly where the abrasive grain is a costly material, such as diamonds, which, because of their great value, the economics of manufacturing abrasive articles with this abrasive grain requires its recovery from imperfectly manufactured articles. Thus, considerable advantage is to be gained by 1the instant invention in the preparation of diamondcontaining abrasive articles, such as diamond wheels.

Theabrasive grain, from which the abrasive body is prepared, may be selected from a wide variety of materials well known in theart. The particular abrasive viously, theiabrasive articles of this invention may be formed by the hot-press method, by which method the chances of fracture of the abrasive article during the manufacture are greatly reduced.- Obviously, a minimum of manufacturing difficulties are .desirable where the abrasive grain employed is a costly material such as diamonds, and wastein the form of imperfectly manufactured articles represents a prohibitive expense. The size of the abrasive grain may vary widely depending upon the properties desired in the final product. The selection of a particular material for use as an abrasive grain, the particle size, distribution thereof, are considerations well known in the art and will present no problem to those skilled in the 'art. The proportion of abrasive grain in the product varies somewhat as is known to those skilled in the art.

With most types of abrasive grain, the grain may make up as low as about 40% of the final abrasive product, but generally, it will range between about 60% and about 96%, by weight, of the product. Preferably, the abrasive grain makes up between about 75% and about of the product. Where the abrasive grain comprises diamonds, because of their great cost, the abrasive grain preferably makes up between about 10% and about 25%, by weight, of the final product. The remainder of the product comprises ceramic bond, and in certain instances, filler material hereinafter described.

The bond of the abrasive body will, as stated, comprise a ceramic material formed of a fused, glassy frit, i.e. a glass, and a fluorine-containing mica in separate phases. The glassy frit should have a fusing temperature which is below the sintering temperature of the fluorinecontaining mica. This is necessary to insure that the fluorine of the mica will be released to clean the surfaces of the abrasive grain at the proper time. If the glassy frit fuses at a temperature greater than the temperature at which the mica sinters, the mica will melt, decompose and lose certain valuable physical properties which con tribute to the advantageous properties of the ultimate abrasive products. Further, since the glass has a lower fusion point in thersintering temperature of the mica, the sintering temperature of the mica is reduced so that the abrasive articles of this invention may advantageously be formed by hot pressing methods employing moderate temperatures which temperatures do not adversely alter the desirable abrasive properties of the abrasive grain, as for example by causing diamonds to undergo graphitization.

The glass portion of the bond, in the form of a fused frit, comprises between about 45% to about 75%, by weight,.of silica, SiO and from about 5% to about 40%, by weight, of boron trioxide, B 0 The frit may also contain alumina, A1 0 in amounts up to about 15%, and alkali and alkaline earth metal oxides in amounts up to about 10%. The boron trioxide acts as a fluxing constituent, and together with alkaline earth metal oxides, such as magnesia and lime, with or without the addition, of minor amounts of alkali metal oxide fluxes, reduces the temperature at which the frit softens so that its fusing point is below the sintering temperature of the fluorine-containing mica. The aluminasingredient is a more or'less neutral material and has valuable properties, which include the quality ofreducing the reactivity of the bond toward abrasive grain, such as silica carbide. Since the alumina increasesthe viscosity of the fused glassy frit, amounts of alumina in excessof about 15%-should not be employed, since temperaturesgreater than the sinteringtemperature. of the mica may be necessary in orderto provide the glassy frit with the desired fluidity to wet the surfaces of the abrasive grain. A preferred frit comprises from about 40% to about 60%, by weight, of silica, from about 20% to about 35% of boron trioxide,'from about 10% to about of alumina, and from about 5% to about 15% of alkali and alkaline earth metal oxides and mixtures thereof. 7 i

The bond will generally include other ingredients which may be termed impurities, For example, titania, which is probably not detrimental up to several percent may be present in some of the ceramic bonds made in accordance with this invention. However, it may be considered, a relatively inert-ingredient. Another impurity which is usually present, simply because it is practically impossible to eliminate 'all of it, is iron oxide,

Fe O which is more or less of a flux. If all of the iron oxide could be eliminated, the invention could well be carried out without its presence, but on the other hand it is believed that even as much as about 2% might, in some cases, be present Without producing detrimental eifects. v

The composition, inthe form of a frit, having the above-described chemical composition, may be prepared in accordance with conventional practice; As is well known, in preparing the frit, compounds ultimately providingthe desired oxide analysis, in the present case providing the above-described oxide analysis, arefmixed toe gether in fine particle size andmelted, and the molten mass quickly cooled, by Water-quenching and dried. With respect to the-compoundsemployed, such compounds are well known in the art, and the provision of the above-described chemicalanalysis will presentno problem to those-.skilledinthe ceramic art. Thus, the silica may be provided, for example, by quartz, sand or the like; boron trioxide, B 0 may be provided for example, 'by' borates, such as sodium borate, for instance, borax, boric acid, and the like; the alumina, A1 0 may be provided, for example, by aluminum meta-- or orthophosphate, aluminum hydroxide, or the free alumina itself may be used; alkaline earth metal oxides, for example, calcium oxide, CaO, may be provided in the form of unslaked lime of the like; alkali metal oxides, such as sodium oxide, Na O, may be provided, for'example,

by sodium carbonate, soda ash, sodium carbonate monohydrate, and phosphates, for instance, tetrasodium pyrophosphate, sodium tripolyphosphate, sodium hexametaphosphate, andborates, for instance, borax and the like.

Materials, in fineparticle size, providing the desired chemical analysis upon melting thereof, are mixed together in accordance. with the common practiceand heated to an elevated temperature to provide a molten, pourable mass. The molten mass is then quickly chilled, such as, by pouring it into a water bath, and such quick chilling causes the glass-like mass to fracture in to small pieces. These small pieces are recovered and dried to provide the frit having .the chemical analysis described above, Thevresulting frit may be too coarse for direct use in the ceramic bond, and may, therefore,- be ground to the desired size in accordancewith, well known practice. The particle size-of the glassy frit should be less than about 100 mesh (US. Bureau of Standards) and preferably finer than about 200 mesh.

As previously stated, the micas employed in.the abrasive article of the instant invention are fluorinecontainingpmicas, which preferably have a sintering temperature-belowabout1750 F. Thesefiuorine-contaim ing micas are, Well known. and are. describedin detail in Fiat FinalReports #746, #747 and #748 (1946) of the UnitediStates Oflicefof Military Government for Germany. These-.micasmay. berepresented by the general formula: I

6 whereinR is an element selected from the group consist: ing of sodium and potassium; R is an element selected from the group consisting of magnesium, manganous manganese, ferrous iron, ferric iron, cobalt, nickel and aluminum; R" and R' are elements selected from the group consisting of magnesium, manganousmanganese, cobalt, ferrous iron, nickel and lithium; and taken to gether aluminum and ferric iron, and X is an element selected from the group consisting of aluminum, boron and silicon, the sum of the valencies of R, R and R being 5 when X is a tetravalent element and 6. when X'is' a trivalent element. The following fluorine-containing micas are typical of those which may be employed in the ceramic bond of the abrasive articlesof this invention The fluorine-containing micas suitable for the purposes of this invention may be. prepared either by reaction in the. sol-idstate, or by crystallization from 'melts.' For example, the micas may be prepared :by melting materials ultimately providing the desired mica in proportions in: dicated by the particular mica'formula, under conditions minimizing volatilization of fluorides, and then crystallizing the mica from the melt by slow cooling.- .The fluorine-containing micas of this type crystallize'readily from a melt of such composition. The micas can also be formed by a solid state reaction For example, a mica having the formula KMg MnAlSi O F may be prepared by sinteringtogether materials such. as K SiF MgO, MnCO Al O .2SiO anclv SiO and sintering the mixture in. a crucible atabout 2000 F. in a reducing atmosphere. Thefluorine-containing mica, prepared for example by the method described above, may then be ground by conventional methods to a particle size suitable for forming the ceramic bond of the abrasive article of this invention which should be generally less .than about 100 mesh and preferably finer than about 200 mesh., V f

As mentioned heretofore, the abrasivearticles'of this invention comprise abrasive grain, ceramic. bond, and in certain instances also a filler material. Thus, in the abrasive articles of thisinvention, the ceramic bond, may make up all vof that portion of the productwhieh not abrasive grain, i-.e., with most abrasive grain up to about 60%.of the final product, and when the abrasive grain comprises diamonds, up ,to aboutv 90% of the product. Generally, with most types of abrasive grain, the ceramic bond will comprise between about 4% and about 40%, and preferably between 10% and 25%, by weight, of the-final product. If the abrasive grain comprises diamonds, the ceramic bond preferably comprises between about and by weight, of the final product In additionto. ceramic bond, the abrasive articles of this invention may have incorporated therein a filler material. Filler materials are substantially insoluble, inorganic materials which are capable of existing in discrete, finely-divided formjand are not fused at the bond maturing temperature. Examples, of such materials are alumina and carbides such as silicon carbideyboroncarbideftungsten" carbide, tantalum carbideyandgthe' like.

These filler materials, which are themselves abrasives, all

of the bond with respect to the abrasive grain. The preferredfiller is acarbide filler, such as finely-divided silicon carbide.

; .Advantageously, the abrasive article of this invention may be formed bythe hot vpress method, which method produces an abrasive article of greater density, and by which method the likelihood of fracture of thernolded abrasive article is substantially reduced. The hot pressing techniques employed in the manufacture of the abrasive article of the instant invention are those well known in the art, and comprise broadly forming a thorough mixture of abrasive grain, ground mica, ground prefired glassy frit and filler, if any, placing the mixture in a graphite mold and subjecting the mixture in the mold to heat and pressure. Pressures up to about 1500 p.s.i. are generally employed with temperatures sufiiciently high to cause the mica to sinter. However, hot press temperatures should not greatly exceed the sintering temperature of the mica, otherwise, the mica may be decomposed. Temperatures employed generally range between about 1200 and 1750 F.

In the abrasive articles of the present invention, the abrasive grain comprises the discontinuous phase and the ceramic bond comprises the continuous phase. It is believed that the fluorine'containing mica and fused, prefired glassy frit do not unite to form a single ceramic material, but rather retain their separate identity. For example, if the ceramic bond includes a high percentage of glass, as for example 80% glass, the fused glassy frit will comprise the continuous phase, and the mica will be distributed throughout the fused glassy frit in the form of discrete particles. On the other hand, if the proportion of mica present in the ceramic bond is substantially greater than that of the fused frit, the mica, upon sintering, will become the continuous phase with discrete particles of fused, glassy frit dispersed throughout the mica. Regardless of the actual physical form of the ceramic bond, the ceramic bond of this invention produces advantageous results. I

In the abrasive articles of this invention, the proportions of mica and glassy frit in the ceramic bond may be varied to. provide the abrasive articles with various degrees of hardness. Because of certain high grinding ratios which are obtainable with ceramic bonds of certain compositions, it is often desirable that such bonds be employed where a very hard abrasive body is desired.

Generally, bond compositions providing abrasive bodies having very high grinding ratios will contain from about 45% to about 65%, by weight, and preferably between about 50% and about 60%, of fluorine-containing mica.

Preparation of the product of this invention will be .more clearly understood from a consideration of the following specific examples which are given for the purpose of illustration and are not intended to limit the scope of this invention in any way.

Example I Percent by weight Prefired, glassy frit, less than 200 mesh 25 vFluorinecontaining mica, less than 200 mesh 30 Diamonds, 100 to 140 frit -l 2s Silicon carbide filler, less than 200 mesh.. 20

employed has the general 8 formula KMg LiSi Q F and the prefired glassy frit has" the following composition: a p 7 Percent by weight Silica, Si0 I I V V 44.48 Boric oxide, B 0 28.88 Alumina, A1 0 13.85 Magnesia, MgO 10.27 Lime, CaO 2.05 Titania, TiO- v V V p 0.80 Ferric oxide, Fe Q 0.25

The above-described dry mixture of materials is placed in a graphite mold of proper size, the mold is placed in a furnace and heated to the sintering temperature of the mixture which is about 1755 F. while a pressure of about 1500 p'.s.i.g. is applied to the mold until maximum density of the wheel is obtained. The mold is removed from the furnace andthe wheel is stripped from the mold after it has cooled about 100? F. below the sintering temperature of the mixture; Temperatures throughout the molding process are obtained by means of a thermocouple located in the mold shell. 7 a

The wheel is mounted of a steel mandrel and the wheel is driven at high speed by compressed air. The wheel is employed to grind a sample of tungsten carbide. A

Example I is repeated with the exception that a boron mica having the general formula KMg Bsi O F is substituted for the lithium mica. The sintering temperature of the mixture is 1665 F. A grinding ratio of 258:1 is

obtained.

Example 111 Example I isv repeated, however, the mica is omitted, addition of glassy frit being substituted for the mica. A grinding ratio of 50:1 is obtained.

I claim:

1. An abrasive body comprising abrasive grain bonded in a ceramicbond consisting essentially of a fluorinecontaining mica and a borosilicate glass consisting essentially of from about 40% to about 60%, by weight, of silica, from about 20% to about 35% of boron trioxide, from about 10% to about 15% of alumina, and from about 5% to about 15% of a material selected from the groupconsisting of alkali and alkaline earth metal oxides, and mixtures thereof, said mica and said glass comprising separate phases of said bond, and said mica phase comprising from about 45% to about 65%, by weight, of said bond. 2. An abrasive body comprising abrasive grain bonded in a ceramic bond consisting essentially of a fluorine-containing mica of the general formula:

RRRRXSi O F wherein R is an element selected from the group consisting of sodium and potassium; R is an element selected from the group consisting of magnesium, manganous manganese, ferrous iron, ferric iron, cobalt, nickel and aluminum; R" and R are elements selected from the group consisting of magnesium, ferrous iron, manganous manganese, cobalt, nickel and lithium, and taken together aluminum and ferric iron, and X is an element selected from the group consisting of boron, aluminum and silicon, the sum of the valencies of R, R" and R'" being 5 when X is a tetravalent element and 6 when X is a trivalent element;'and a borosilicate glass consisting essentially of from about 40% toabout 60%, by Weight, of silica, from about 20% to about 35% of boron trioxide, from about 10% to about 15% of alumina, and from about 5% to about 15% of .a material selected from the group consisting of alkaliand alkaline earth metal oxides,

9 and mixtures thereof, said mica and said glass comprising separate phases of said bond, and said mica comprising from about 45% to about 65%, by weight, of said bond. 3. An abrasive body comprising abrasive grain bonded in a ceramic bond consisting essentially of a fluorine-containing mica of the general formula:

KM oFz and a borosilicate glass consisting essentially of about 45% silica, about 14% alumina, about 29% boron trioxide, about 2% calcium oxide and about 10% magnesium oxide, said mica and said glass comprising separate phases of said bond, and said mica comprising from about 45 to about 65%, by Weight, of said bond.

4. The product of claim 1 in which the said fluorinecontaining mica comprises from about 50% to about 60%, by weight, of said ceramic bond.

5. The product of claim 1 in which said mica has a sintering temperature below about 175 F.

6. The product of claim 1 in which said abrasive grain comprises diamonds.

7. The product of claim 2 in which the said fluorinecontaining mica comprises from about 50% to about 60%, by weight, of said ceramic bond.

8. The product of claim 2 in which said mica has a sintering temperature below about 1750 F..

9. The product of claim 2 in which said abrasive grain comprises diamonds.

10. The product of claim 3 in which the said fluorinecontaining mica comprises from about 50% to about 60%, by weight, of said ceramic bond.

11. The product of claim 3 in which said prefired glassy frit comprises about by weight, of said body, said mica comprises about said abrasive grain is diamonds and comprises about 25 and the remainder of said body essentially comprises a finely-divided silicon carbide filler.

References Cited in the file of this patent UNITED STATES PATENTS Harvey Apr. 13, 1943 2,730,439 Houchins Jan. 10, 1956

Claims (1)

1. AN ABRASIVE BODY COMPRISING ABRASIVE GRAIN BONDED IN A CERAMIC BOND CONSISTING ESSENTIALLY OF A FLUORINECONTAINING MICA AND A BOROSILICATE GLASS CONSISTING ESSENTIALLY OF FROM ABOUT 40% TO ABOUT 60%, BY WEIGHT, OF SILICA, FROM ABOUT 20% TO ABOUT 35% OF BORON TRIOXIDE, FROM ABOUT 10% TO ABOUT 15% OF ALUMINA, AND FROM ABOUT 5% TO ABOUT 15% OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH METAL OXIDES, AND MIXTURES THEREOF, SAID MICA AND SAID GLASS COMPRISING SEPARATE PHASES OF SAID BOND, AND SAID MICA PHASE COMPRISING FROM ABOUT 45% TO ABOUT 65%, BY WEIGHT, OF SAID BOND.