Cubic boron nitride (CBN) abrasive tool

Abstract: A monolayer CBN tool is fabricated by coating CBN grits with carbides of transitional metal directly by CVD to make the surface metallurgically ccmpatible to the readily and commercially available brazing alloys known for their strength and durability.

Description: In recent years grinding has gained its significance as a stock removal process for shaping and sizing both hard and soft material in contrast to what was realized in past as the metal finishing operation and a process to be worked on very hard material. To meet the ever increasing demand for improved productivity in the field of grinding various new techniques have been introduced and are being tried. These processes basically meant for high rate stock removal, with improved work piece quality and prolong wheel life. One of the most important requirement to be satisfied by the grinding wheels is the free cutting action. This necessitates availability of large chip clearance volume ahead of individual crystal which clearly indicates that the gap between the adjacent grits in the direction of cutting should be wide enough. At the same time the protrusion of the grit above the bond should be large enough. Along with these conditions another requirement to be fulfilled is that the bond between grits and matrix should be strong to retain the grit throughout its useful life. The bonding material should have desirable mechanical properties like strength, hardness, low adhesion, low solubility in the ground material, and resistance to yielding during actual grinding. The most cost effective method of preparation of such working surface of a wheel is the well known galvanic process. The process consists of interlocking the abrasive particles by galvanic deposition of metal like Ni or Co on a steel support (steel hub or steel mandrel). Till to date galvanic wheels with steel mandrel is the only type that can be used at a very high speed for safety reason. But it also suffers from several weaknesses. For effective retention of the grits in the bond, the grits are covered up to 60-80 % of their height thus reducing the space for chip disposal. In addition, formation of lump material in the form of nodules is a common feature in the surface of such tool which further reduces the space in between the adjacent grits. Thus the galvanically bonded tool often fails to offer free cutting action. It is understandable that effective bonding can be made even with less amount of binder if it is chemical or metallurgical in nature as obtained in liquid phase bonding or brazing. Such a process is disclosed in U.S. Patent No. 4.239.502. A suitable process is to first coat the CBN particles with a silver base alloy, coat the surface of the rim portion of metal wheel and then clamp the coated CBN particles to the coated rim while applying heat to bond. The main shortcoming of the process is the complicacy in practicing the process of grit coating and in obtaining separated grits. It is important that grits must be separated from one another before brazing and should not remain as a cluster of grits. Moreover, metal coated grits will generate large force and temperature as a result of metal-metal contact during grinding. Methods of applying and metallurgically bonding hard carbide particles on the surface of a metal substrate are disclosed in U.S. Patents No. 3.868.235, 3.378.361, 3.615.309, 3.248.189. The bonding materials are mainly Ni-Cr base alloys containing other elements like Fe, B, Si and in most cases can be obtained commercially. It has been found that inclusion of Mo and Co in these alloys improves the bond provided by the matrix (U.S. Patent 3.248.189) Cobalt-chromium-tungsten type "Stellite" alloys are also suitable (U.S. Patent 3.615.309). Diamond abrasive tool with a monolayer of abrasive grits and method of manufacture are disclosed in U.S. Patents No. 3.894.673 and 4.018.576. The method utilizes readily available, very hard and durable brazing alloys which are found to readily wet untreated diamond surface. Another Patent (No. PCT/US83/01946) teaches a method of making a monolayer abrading tool with tungsten carbide particles. A tape of powdered brazing material blended into a soft, flexible matrix is first secured to a steel substrate. Later the abrasive particles are imbedded to the tape. The tool is heated to a temperature at least equal to the liquidus temperature of the brazing material to set the abrasive particles and rapidly tool is cooled to solidify the brazing material to produce a metallic matrix. The shortcoming of such tools is that they cannot be used for grinding steel because of vigorous reaction between the work material and the abrasive particles causing rapid wear. In addition to that the brazing materials disclosed in the said patents are apparently limited to the bonding of hard carbide and diamond particles. Nothing is disclosed on joining of CBN by the processes disclosed or materials used. The present investigators have found that the Ni base alloys containing Cr which braze effectively uncoated diamond below 1200° C cannot provide strong bond for CBN. The grinding experiments showed that CBN tools fail because of premature grit dislodgement due to poor adhesion between the bond and grit surface. Situation does not change when the amount of Cr in the alloy is raised to 25 %. Thus it can be realized that Ni-Cr alloys cannot braze CBN grits because of the fact that Cr cannot effectively act as a wetting agent or react with CBN surface under the brazing conditions. The inability of Cr to react with CBN surface has also been established by the present inventors from the discovery of the fact that from gas phase reaction Cr cannot be deposited on CBN, whereas Cr can be deposited by the same CVD process on diamond through formation of chromium carbide at the interface even at 900° C easily. A recently published patent (U.S. Patent 4776862) discloses a method of fabrication of a monolayer diamond tool by precoating diamond particles with carbide forming metal e.g. Fe, Cr and Mo which form a carbide of the said elements when heat treated. The said carbide facilitates wetting of diamond surface by the braze alloy. Precoating is done by wetting the abrasive particles with mineral oil or organic binder and then applying fine carbide forming metal powders. The said coating process is cumbersome in the sense that it is very difficult to obtain discrete abrasive particles after precoating. This is of particular importance for fabricating tool with monolayer configuration. Uniform coating of the entire surface of the abrasive particle is also not very easy when finer grits are used. The fundamental limitation of the process is that elements like Fe, Cr, Mo cannot react with CBN so effectively as they can do with diamond producing wettable carbides. CBN is more chemically stable than diamond and instead of carbides, borides or nitrides would form if reaction takes place and which are definitely less wettable than the respective carbides. Therefore the above disclosed process is so limited in scope that it cannot be used to braze diamond. Thus there remains a need for a low cost practical method for brazing a monolayer of CBN grits to a steel substrate. Objectives 1. To provide a know-how which can be readily practiced on economically feasible level to manufacture CBN grinding wheels which can efficiently satisfy the requirements of modern grinding techniques like creep feed, fast feed grinding. 2. To provide a method of bonding a monolayer CBN crystals with readily and commercially available brazing filler metals which are best known for their strength, hardness, resistance to plastic deformation but which unfortunately cannot wet and bond, highly chemically stable material like CBN. 3. To provide a method for changing the surface chemistry of CBN particles without adversely affecting their grinding capability. 4. To change the surface chemistry of CBN crystals by depositing a thin film of material or materials which should have good adherence with CBN. On the other hand the film deposited should enhance the wettability and bondability of the "chemically inert" CBN by the readily available conventional filler metals. 5. To provide an economically feasible method of deposition of the said film on CBN crystals which can be employed for production of coated grits with a very high degree of reliability. Summary During the course of the invention it is discovered that a thin film of TiC which is mainly used in cutting tool and other tribological applications and which is deposited mainly on cemented carbide, ceramic and steel substrates by the well known CVD process can be deposited on CBN particles by the same well established CVD process with a view to changing the surface chemistry of CBN thus enabling it to bond with Ni base alloys. Similarly, carbide of elements like Zr or Hf can also be deposited by CVD. Similarly a secondary coating of carbide of elements group VI A like Cr, Mo, W or of elements groups IVA or VA can also be deposited by CVD in order to bring about a change in the surface chemistry and to make the surface metallurgically compatible to the brazing alloy. The advantage of using CVD for such coating can be well realised for its better throwing power, ability to coat grain like substrate with irregular shape with much better uniformity than any other process and over all economy. The coated particles can be obtained as discrete ones and can be readily used for bonding. Since the coated film is no metal but rather a carbide of a transition metal such a film of 1-2 µm (micron) thickness cannot adversely affect the grinding capability of CBN particles and such pregrinding operations like opening up of a wheel or removal of a coating from the surface is not necessary. It is interesting to discover that the alloys which just fail to wet and bond effectively uncoated CBN crystals readily cling to the surface of the carbide coated CBN grits when brazing is carried out below 1200° C just for 5-10 minutes under argon atmosphere. The fabrication process can be divided into two parts: A. Coating the CBN particles with hard metal carbides, preferably not above 1200° C to avoid any thermal degradation of CBN grits. B. Brazing the coated CBN particles with alloys preferably Ni-base alloys and preferably below 1200° C. One can have further understanding of the present invention from the following examples. Coating of CBN grits by CVD Example I CBN particles of sizes B251, B126 and B91 are placed in sieves having mesh sizes smaller than the respective grit sizes. The sieves are placed inside a vertical CVD reactor. Temperature of the reactor is kept at 1100° C. To deposit TiC, a mixture of H₂, TiCl₄ and CH₄ is passed over the CBN grits. Typical process parameters are: H₂ flow rate 10 l /min, CH₄ flow rate 250 ml/min. Concentration of TiCl₄ is 2 %. The system pressure is 133.10² Pa (100 Torr). The deposition is continued at 1100° C for 60 minutes. After completion of deposition the reactor is cooled down under argon atmosphere to 50° C . Discrete coated grits are obtained. Microscopic examination reveals that coating is both adherent as well as coherent. The process parameters used in this example are typically that of a standard CVD process and are merely illustrative and should not be considered as a limitation of the process. Many other available process parameters and conditions well standardized for CVD of TiC can be used for deposition on CBN grits with a view to change the surface chemistry along with a strong film-grit interface without deviating from the basic objective of the invention. Example II To obtain a further layer of chromium carbide over the TiC coating deposited on the CBN, the process is carried out in the following way. At the top of the reactor Cr granules are kept and HCl vapour is carried over these granules in a stream of argon to generate CrCl₂. CH₄ is carried by H₂ and its entry point into the reactor is kept below the level where Cr granules are placed. Chromium carbide is formed as a result of reaction between CrCl₂ and CH₄ and is deposited on the TiC film of the CBN grits. The exact type of chromium carbide depends on concentration of CrCl₂ and flow rate of CH₄. A typical deposition condition can be stated as follows. Reactor temperature : 1000° C Flow rate of HCl : 0.25 l/min Flow rate of argon: 5 l/min Flow rate of H₂: 1 l/min Flow rate of CH₄: 0.25 l/min Deposition time: 30 min Brazing of CBN grits to a steel substrate The suitable brazing materials are principally Ni-base and Co-base alloys which are commercially available for example from Wall Colmonoy Corporation of Detroit, Michigan or from Degussa AG, Hanau of West Germany. There can be other manufacturers of braze alloys. It should be understood that in the following examples of brazing , the alloys used are commercially available and the composition of the alloys are simply illustrative in nature and should not be considered as a restriction of the process. Many alterations in the brazing composition can be possible without departing from the basic objectives of the invention or the principle of the invented process. In principle all Ni base alloys with melting point of 1200° C can be used provided they are able to wet and bond the outermost carbide coating on CBN grits. The selection of a particular braze alloy may depend on the specific requirement of a tool such as mechanical strength of the bond, wear resistance, etc. Similarly, selection of brazing temperature and time may depend on its melting point, fluidity, reactivity with the film deposited on the CBN grits. It is apparent that brazing parameters should be chosen only to allow restricted reaction at the braze-film interface so that required wetting and bonding are achieved and no detrimental brittle phase is formed at the interface as a result vigorous reaction between the braze alloy and the film material under a brazing temperature-time combination. Example I The brazing material containing 76 % Ni, 14 % Cr and 10 % P (Nicrobraz 50/ Degussa 897) available in the form of a paste is applied on the steel substrate. The CBN grits coated with a coating of TiC are then applied on the braze layer uniformly in a single layer. The assembly is placed in a furnace chamber which is then evacuated to 13,33 Pa (10⁻¹ Torr) by a rotary pump. The pump is then stopped and the system is back filled with argon until the pressure becomes above atmospheric. This is followed by opening of the system to atmosphere. The flow rate of argon is 5 l/min and 10 % H² was added in the gas flow. The furnace is heated to 1040° C and kept at that temperature for 5 minutes. This is followed by cooling under the same atmosphere. The tool thus produced consists of a monolayer of CBN crystals firmly bonded to the steel substrate. Example II The brazing process carried out is the same as that of example I, except that TiC coated CBN grits of size B251 are applied on the braze layer in a regular pattern. After brazing, the coated grits are seen to be held not only strongly by the bonding matrix but with a better uniformity of bond level in the space between the grits is obtained than in example I. Example III The brazing process is carried out in the similar way wherein the brazing material is Cr free and contains 90 % Ni and 10 % P (Nicrobraze 1O/Degussa 876). After brazing the coated CBN grits are seen to be held strongly by the bonding matrix. Example IV The brazing process is carried out in the same way as that of example I but the brazing materials contains 25 % Cr, 65 % Ni and 10 % P (Nicrobraze 51). The brazing temperature was kept at 1050 ° C. After brazing the coated CBN grits are seen to be held tenaciously by the braze to the steel member. Example V Steps of the example I are carried out except that the outermost layer of the composite coating is chromium carbide and the brazing alloy consists of 82.4% Ni, 7 % Cr, 3 % Fe, 4.5 % Si, 3.1 % B (Nicrobraz LM / Degussa 1002). Brazing temperature is 1050 ° C. The brazing alloy readily wets the chromium carbide film and strongly bonds the CBN crystals. Example VI Coated CBN grits of example V are used with the brazing alloy containing 74 % Ni, 14 % Cr, 4.5 % Fe, 4.5 % Si, 3 % B (Nicrobraz 125/ Degussa 104). The steps of the example I are followed at a brazing temperature of 1080 ° C. Satisfactory wetting is achieved after brazing. Example VII Coated CBN grits of example V are used with a Cr free brazing alloy containing 92.5 % Ni, 4.5 % Si and 3 % B (Nicrobraz 130/Degussa 1041). The steps of the example I are followed at brazing temperature of 1080 ° C resulting in a satisfactory wetting and bonding of the coated CBN grits. Application I The product resulting from the practice of the invention represents a significant advancement in the construction of monolayer CBN tools. This advancement can be easily realized when the performance of the prototype tool is compared with that of the galvanic tool of similar type. Results of comparative grinding tests. In-feed Galvanic tool A Grit size 250/212 µm Galvanic tool B 250/212 µm Prototype tool 250/212 µm 10 µm Satifactory grinding Satisfactory grinding Satisfactory grinding 20 µm Force and power steadily increased followed by decrease and finally attained steady value. Force and power steadily increased with grinding followed by decrease and finally attained steady value. Force and power were very stable from the beginning. Magnitude of force and power were less than that developed by galvanic tool A and B. Normal force = 22 N 25 N 15 N Power = 150 W 174 W 105 W 30 µm Force and power increased very fast within 1 mm cut. Severe wheel loading indicating failure of the grinding wheel. Force and power increased very fast within 1 mm cut. Severe wheel loading indicating failure of the grinding wheel. Steady grinding Normal force = 20 N Power = 156 W 40 µm Wheel was not used Wheel was not used Steady grinding 60 mm was removed from the workpiece. Normal force = 34 N Power = 228 W Operation : Face grinding Workpiece material : 100 Cr 6 steel (hardness : 290 HV) Cutting speed : 30 m/s Table speed : 2 m/min Cutting fluid : 5 % soluble oil, Castrol Syntillo DC 282. Application II When grinding with galvanically bonded CBN wheels, a low cost method of improving the surface finish and accuracy of the workpiece is to shave the grit tips by a diamond tool. However, it is contended, "It is important that the grit tips are removed only in the µm range so that adequate chip clearance spaces are still available after the shaving process". In the view of the said quotation from the paper of K. Yegenoglu and M. Roth appeared in page 116 of Industrial Diamond Review, vol. 3/87, it is understandable that shaving process can only be used in a very restricted way in a galvanically bonded CBN wheel, so that necessary chip clearance space is not reduced. It might have been thought that such restricted shaving is a prerequisite for galvanically bonded CBN wheel for load free cutting. On the other hand the product obtained as a result of the present invention offers significant advantages in this respect. More specifically, it can be said that much grit shaving allowance is provided in the product in order to achieve better surface finish and accuracy of the workpiece but without the risk of reduction of the chip clearance space. The above mentioned advantage of the invented product can be demonstrated by the following experiment. Two CBN wheels were taken, one having galvanic bond and the second with brazed bond. The grit tips of the two wheels shaved with a diamond tool by 10 µm. Both the wheels were submitted to grinding of unhardened 100 Cr 6 steel under the following conditions. Operation : Face grinding Wheel speed: 30 m/s Table speed: 2 m/s In-feed : 20 µm Cutting fluid : 5 % soluble oil Castrol Syntillo DC282 Shaved brazed wheel showed improved surface finish of the workpiece and grinding force and power were very stable from the beginning of cut. Shaved galvanic wheel showed progressive rise of force and power with grinding passes and at the end of 1 mm cut severe wheel loading took place indicating failure of the grinding wheel.

Claims: A CBN abrasive tool for grinding, sawing, milling, honing of various grades of cast iron and steel, comprising a metal substrate and a single layer of CBN grits bonded to the said metal substrate by a braze alloy, wherein surface chemistry of the CBN grits is modified by depositing at least a film of carbide selected from transition elements of group IV A, V A or VI A strongly adhered to the CBN crystal surface. The CBN abrasive tool according to claim 1, wherein the braze alloy are conventional Ni-based alloys characterized by their inability to wet and bond untreated CBN grits effectively below 1200° C but are able to wet and bond the said CBN grits at temperature below 1200° C only because of surface modification The CBN abrasive tool according to claim 1, wherein the film is deposited by conventional CVD at temperature not above 1200° C. The CBN abrasive tool according to any of claims 1 to 3, wherein the film is TiC. The CBN abrasive tool according to claim 4, wherein said surface modified CBN grits are additionally coated with a second film of chromium carbide.

Inventors: CHATTOPADHYAY AJAY KUMAR, HINTERMANN HANS ERICH, CHATTOPADHYAY, AJAY KUMAR, HINTERMANN, HANS ERICH
Applicants: SUISSE ELECTRONIQUE MICROTECH, CENTRE SUISSE D'ELECTRONIQUE ET DE MICROTECHNIQUE S.A.
Application year: 1991
Main classification: B24D3
IPC codes: B24D3, C09C1, C09K3, C23C16