Fluorinated Diamond Bonded in Fluorocarbon Resin

The invention described herein relates to a method of bonding diamonds in a support matrix and articles of manufacture formed from the resulting matrices. More particularly, it relates to a method of first fluorinating the diamonds and then bonding said fluorinated diamonds in a matrix of a desired material such as a fluorocarbon resin.

Diamond is the most versatile abrasive material known. Diamond dust or grit of varying sizes is used extensively in industry for grinding, sawing, drilling, honing, dressing, lapping (preparing very smooth surfaces), and polishing. While certain of these operations, such as lapping and polishing, may readily be performed with the diamond abrasive in a loose powder form, the others typically require the diamond grit to be bonded in a matrix which in turn is formed into a tool in which the grit has geometrically undefined, i.e., random, cutting edges. The matrices are typically formed of resins or metal claddings.

The term "bonded" is somewhat of a misnomer when applied to diamonds. The surface of a diamond is smooth and dense and does not readily form adhesive bonds. This is generally true of all of the hard, refractory solids. As a consequence, there is a tendency in present resin bonded matrices for the diamond grit to pull free and also at heavy grit loadings for the matrices to crack and separate at high stresses. These problems are avoided by metal laminated matrices which, however, are quite expensive compared to resin matrices.

Accordingly, it is an object of the present invention to provide a method of bonding diamonds in a support matrix.

Another object of the invention is to provide articles of manufacture produced from diamonds bonded in a support matrix.

A further object of the invention is to provide resin bonded diamond grinding wheels which are less susceptible to cracking and separating under heavy stress loads.

Still another object of the invention is to provide a method of bonding diamonds in a support matrix by standard hot pressing techniques.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

In its broad aspect, the invention encompasses a method of bonding diamond to a desired material which comprises first fluorinating the surface of the diamond and then bonding the desired material to the fluorinated surface of the diamond. In a preferred embodiment the diamond is in the form of diamond grit which is distributed through the matrix of a fluorocarbon resin such as a copolymer of perfluoroethylene-propylene. The fluorinated diamond grit is slurry mixed with the copolymer in a desired ratio, and the mixture is then dried to remove all liquids. By hot pressing the dried mixture at an appropriate temperature and pressure, a continuous matrix of the copolymer is readily formed to which the fluorinated diamond grit is bonded.

The invention also encompasses articles of manufacture made in accordance with the method of the invention. The articles thus made are advantageous over those of the prior art resin bonded diamond articles in that the bond is better and the articles are capable of undergoing quite high stress loadings without separating or cracking. These advantages are particularly significant in the manufacture of diamond grinding wheels. Finally, the invention permits well-known fluorocarbon resins of the type sold under the tradename of TEFLON to be used as the binding matrix.

It is known that diamond surfaces which have been cleaned by low energy (600 V) Ar.sup.+ ion bombardment can readily be saturated with chemisorbed fluorine by exposing them to F atoms generated by microwave-discharged SF.sub.6 vapor at low pressure (10.sup.-3 torr). A monolayer of covalently bonded fluoride forms within a few minutes. The fluorinated surfaces contain .tbd.CF, .dbd.CF.sub.2, and --CF.sub.3 groups. After 20 minutes exposure under the conditions noted, these groups were present in the following respective percentages: 61, 30, and 9%.

A fluorinated diamond surface formed in this manner has certain of the characteristics of the fluorocarbon resins sold under the trademark TEFLON by E. I. du Pont de Nemours and Co. Because of this fact fluorinated diamonds are more readily bonded to a polymeric material than are nonfluorinated diamonds. This is especially true if the polymeric material is itself fluorinated. Thus, fluorocarbon resins such as TEFLON are particularly well suited for use in the practice of the present invention.

The preferred fluorocarbon resin is TEFLON FEP which is a copolymer of tetrafluoroethylene (CF.sub.2 .dbd.CF.sub.2) and hexafluoropropylene (CF.sub.2 .dbd.CF--CF.sub.3) having the general chain formula ##STR1## It will be apparent that the ratio of tetrafluoroethylene to hexafluoropropylene may be varied in the copolymer. The preferred ratio is that in the resin sold as TEFLON 120 FEP.

In the preferred embodiment, industrial diamond dust or grit of a desired grit size is treated in a mild (0.5-1 torr) argon or helium microwave discharge (.about.75 W, 1350 MHz) for about 1 hour. The grit is stirred at 15 minute intervals to make certain that the various surfaces are exposed to the Ar or He plasma. The diamond grit thus treated is then fluorinated using a 4:1 ratio of argon or helium to SF.sub.6 under the same microwave discharge conditions for 0.5 to 1 hour with occasional stirring. Although SF.sub.6 is the preferred source of fluorine atoms, any fluorinated compound which produces fluorine atoms in the microwave discharge, as, for example, CF.sub.4 or BF.sub.3, is suitable for use, provided only that any other discharge product does not coat out on the diamond grit and thereby adversely affect the fluorination.

An aqueous dispersion or emulsion of TEFLON 120 FEP is mixed with the fluorinated diamond grit to form a paste. The amount of TEFLON 120 FEP is not critical, provided only that it is present in an amount sufficient to act as an effective binder for the diamond grit when hot pressed. Amounts ranging from less than 10 vol.% to 30 vol.% have been used. An amount of toluene sufficient to "break" the emulsion is then stirred into the paste. Typically, a few milliliters is sufficient for this purpose. The mixture is then slurried with an excess of acetone until foaming stops. The grit is allowed to settle and the acetone is decanted.

Finally, the wet mixture remaining is dried under vacuum at 250.degree. C. This serves to volatilize and remove the wetting agents used in forming the TEFLON 120 FEP emulsion as well as the water in which the emulsion is formed. The very small beads of TEFLON 120 FEP which were dispersed in the emulsion adhere to the fluorinated diamond grit by electrostatic attraction. As a consequence, in the resultant dried powder, each bit of diamond grit is surrounded by such beads. It is this dried powder which can be readily hot pressed to form a matrix in a desired shape of fluorocarbon resin in which the fluorinated diamond grit is substantially uniformly dispersed and bonded.

Thus, for example, in the manufacture of a grinding wheel in accordance with the invention, a soft aluminum cylinder is centered in a hardened steel die of a desired diameter, and the dried powder is poured around the aluminum cylinder. The powder and cylinder are pressed at 20 kpsia and 325.degree. C. Under these conditions the TEFLON 120 FEP melts to form a continuous matrix in which the fluorinated diamond grit is heat bonded. At the same time, the aluminum cylinder is deformed into a plate or hub having the matrix strongly bonded to its outer diameter. The aluminum plate or hub is then drilled out at its center to form a grinding wheel which may readily be mounted to any appropriate grinder.

The temperature and pressure given in the foregoing example are not critical. Thus, any temperature in the range of 288.degree.-399.degree. C. may be used. The only limitation is that the temperature be sufficient to heat bond the beads of TEFLON 120 FEP to each other and to the fluorinated diamond grit but not sufficiently high to degrade the resulting matrix. Likewise, the pressure is only required to be sufficient to adequately compact the powder and deform the central cylinder sufficiently to form the desired wheel.

The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use of the invention as set forth herein. This description sets forth the best mode presently contemplated for the practice of the invention. Finally, it is intended that the scope of the invention be defined by the claims appended hereto.