Sintering of cubic boron nitride under high pressures and temperatures in the presence of boron carbide as the binding material

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1 Indian Journal of Engineering & Materials Sciences Vol. 12, August 2005, pp Sintering of cubic boron nitride under high pressures and temperatures in the presence of boron carbide as the binding material S K Singhal* & B P Singh Division of Engineering Materials, National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi , India Received 2 November 2004; accepted 30 May 2005 Sintering of cubic boron nitride (cbn) was carried out by subjecting fine powders of cbn to high pressures and temperatures (P = kb, T = C ) in the presence of boron carbide as the main binding material together with a small amount of aluminium nitride. The composites were characterized by X-ray diffraction, scanning electron microscopy, FTIR, Raman Spectroscopy and microhardness measurements. Best composites with a maximum microhardness of about 3700 kg/mm 2 were obtained at around 50 kb, 1450 C using 75 wt% of cbn in the mixture. No peaks due to hexagonal boron nitride were observed in the XRD patterns. IPC Code: C01B35/08, C01B31/36 Cubic boron nitride is a material next only to diamond in microhardness. The high hardness of cbn is derived from its fundamental structure and electronic charge distribution. It has a close similarity with diamond in these regards. Both the crystal structures consist of a tetrahedral arrangement of atoms with sp 3 hybridization and strong ionic-covalent bonding which result in short bond length and high bulk modulus. It is an excellent abrasive for grinding and cutting applications for ferrous materials. It is synthesized all over the world by subjecting hexagonal (hbn) and other modifications of boron nitride to high pressures and temperatures. This phase transformation is brought either directly or with the aid of suitable catalyst-solvent materials selected from the nitrides of alkali and alkaline earth metals. However, commercial applications of cbn crystals are restricted due to the limited size to which they can be grown and they do not have the required fracture toughness 1. Therefore, in order to enhance the utility of these small particles they are compacted into a conglomerate mass of sufficient size, toughness, hardness and chemical stability under high pressures and temperatures. Several methods have been reported for producing these sintered compacts of cbn. The methods involve either the direct conversion of hbn to cbn and simultaneously sintering at pressures > 77 kb and temperature in between C (refs 2-8) or by direct sintering *For correspondence ( sksinghal@mail.nplindia.ernet.in) of cbn powder at about 77 kb and temperatures higher than 2000 C (refs 9,10). The fine cbn particles can also be compacted into desired shape and size by the use of some binding materials at relatively lower pressures and temperatures than those required without using any additives and this process has generally been used by a number of firms involved in production of these sintered compacts of cbn although the exact nature of binding materials used by these firms are not disclosed. In order to improve the cutting performance of cbn tool tips, tips made using different compositions of cbn and the binding materials are produced by several manufacturers. Also the presence of small quantity of the binder phase is necessary as it takes care of the sudden shock produced at the cbn tool tip when it comes in contact with the workpiece at the machining conditions and thereby prevent further propogation of any crack that might have developed at the tip surface. The presence of binder phase therefore, increases the cbn tool life. Thus, in the production of cbn compacts the relative composition of cbn in the mixture, and nature of the binding material is very important. The binding materials should have the qualities such as high microhardness, high melting temperature, less chemical reactivity with cbn and catalytic activity for hbn to cbn conversion. Generally, these binding materials are selected from the carbides, carbonitrides, borides and silicides of Group IVa, Va and VIa of the periodic table or a mixture or solid solution compound with an addition of small amount

2 326 INDIAN J. ENG. MATER. SCI., AUGUST 2005 of Al and/or Si. The first high pressure compaction of cbn powder was reported by Wentorf and De Lai 11 and since then a number of papers have been published in this area using different binders. Fukunaga et al. 13 have used different compositions of Co-Al alloy as the binding material and carried out high pressure sintering at 55 kb and C for a period of 1 h. Akaishi et al. 15 reported the compaction of cbn powder by a new method called reaction sintering by subjecting partially graphitized cbn powder to high pressures and temperatures in the presence of magnesium boron nitride. In our earlier experiments we used a solid solution of TiC and TiN as the main binding material together with a small amount of Al 14. The sintering was performed at pressures ranging from 55 kb to 62 kb and temperature was varied from 1300 to 1600 C. The resultant cbn compacts produced in these experiments had a microhardness of about 3200 kg/mm 2. However, with such binders if the composites are made under extreme P-T conditions (particularly the temperature) the chances of back conversion from cbn to hbn may exist because of which it may not have the desired cutting tool properties. Therefore, in order to overcome this problem one of the components in the mixture should have catalytic action for converting hbn into cbn if at all it has been transformed into hbn during the high pressure sintering. Aluminium nitride has been shown to possess such properties 17. AlN is found to be a good catalyst for converting hbn into cbn. Therefore, in the present work we have chosen boron carbide (B 4 C) as the main binding material in the compaction of cbn together with a small percentage of AlN. B 4 C was chosen as the main binder because of its high microhardness (2800 kg/mm 2 ), strong bonding action towards cubic boron nitride and possesses all other required properties. Aluminium nitride was added to the reaction charge to take care of any hbn being formed under high P-T conditions by the reconversion from cbn powder. Experimental Procedure The high pressure sintering experiments were carried out on a belt type apparatus capable of generating pressures up to 80 kb and temperatures of the order of 2000 C. The equipment was calibrated with respect to pressure at room temperature by using standard pressure fixed points of phase transitions of Bi (I-II) 25.4 kb, Yb (fcc-bcc) at 39 kb and Ba (I -II) at 55 kb. Temperature calibration was done by inserting a Pt 13%Rh-Pt 20%Rh thermocouple at the centre of the reaction cell. No correction was made for the pressure effect on the e.m.f. of the thermocouple.the powdered mixture of cbn (2-4 μm, obtained from Warron Diamond Powder Co. Inc. USA) and the binding materials ( B 4 C < 1 μm, AlN 0.3 μm ) were mixed thoroughly using ethanol as the mixing agent. Ethanol was then dried out at about 50 C in a vacuum oven and the powder was degassed at 500 C in a vacuum of 10-2 torr for 2 h. It was then pre-compressed in a Zr foil to a disc of 6.0 mm diameter and 3-4 mm thickness, packed inside a NaCl sleeve and finally subjected to the desired high pressures and temperatures. Although the amount of cbn powder in the mixture could be varied most of the experiments in the present work were carried out by using cbn 75 wt.% and 95 wt.% respectively. High percentage of cbn powder in the mixture was used to enable the sintered body to have high wear resistance under aggressive conditions in cutting of ferrous alloys. As cbn is highly reactive towards aluminium or its compounds at high temperatures generated during machining operations thereby converting it into the softer form, i.e., hbn, a very small amount ( 2-5 wt.%) of AlN was added to the reaction charge. By sintering one means the bonding together of a mass of small particles into a larger, coherent piece. During this process the contact patches between particles grow and the total volume of the mass shrinks leading to a sufficiently dense and strong mass even though some voids may remain. Application of high pressure and temperature helps in densification and composite formation by particle rearrangement followed by considerable plastic deformation. For pure materials the sintering temperature is about 60-70% that of the melting. As the melting temperature of cbn is about 3500 C, a suitable sintering temperature would be about 2100 C. The required pressure for cbn stability then would be around 70 kb. However, because of the high hardness of the cbn particles, it is very difficult to achieve this pressure over the entire surfaces of all the consolidating particles. The pressure distribution in the compact at this stage is very uneven, being very high (>100 kb) where the cbn grains are in direct contact with each other, and in other places (most places) the local pressure is that of the voids. Therefore, at an average pressure of about 70 kb we have a network of local high pressure contact patches and a large area at a relatively low pressure. In the low pressure region the cbn is thermodynamically

3 SINGHAL & SINGH: SINTERING OF CUBIC BORON NITRIDE 327 unstable and the high temperature causes a significant amount of reconversion to hexagonal BN. Thus, at 70 kb the sintered cbn compact is a mixture of cbn and hbn and in order to suppress further cbn hbn transformation an increase in pressure (>100 kb) is necessary, which is very difficult to achieve on a practical commercial scale. Thus, the practical solution to this problem is to use some catalysts/binders as the sintering aids, which not only make it possible to sinter at lower pressures but also promote densification and bonding at the sintering temperature. In the present work the pressure was kept constant at 50 kb and the temperature was varied from 1400 to 1600 C for a duration of 30 min. These P-T conditions are well within the thermodynamically stable region of cbn. After the specimens subjected to the desired P-T conditions, the temperature was first decreased followed by the release of pressure in about 4 h. The zirconium foil was removed by grinding and the resultant cbn compacts were characterized by using different techniques such as X- ray diffractometry (XRD), scanning electron microscopy (SEM), FTIR, Raman spectroscopy and indentation technique. Phase identification was made after the high-pressure/high-temperature treatments using XRD. Results and Discussion Figure 1 shows a surface X-ray diffraction pattern of a cbn compact sintered at 50 kb and 1450 C for a duration of 30 min. The weight percentage of cbn used in this compact was 75% and that of binding Fig. 1 XRD pattern of a cbn compact sintered at 50 kb and 1450 C using 75:20:5 wt.% of cbn, B 4 C and AlN materials was about 20 wt.% for B 4 C and 5 wt.% for AlN.The main peak of cbn (111) appeared at 2 θ = 43.3 corresponding a d value of Å. The other phases as identified in the XRD were AlN, B 4 C. No peak corresponding to hbn was identified in the XRD pattern indicating that the back conversion from cbn to hbn did not occur at this P-T condition. In another cbn compact with a percentage composition of cbn, B 4 C and Al as 95:3:2 the general features were more or less the same as observed in earlier XRD pattern except the increase in peak height intensity of cbn as expected. In both the composites formation of α- Al 2 O 3 was observed through the reaction of AlN with oxygen impurity adsorb on the surfaces of fine cbn particles. However, as α-al 2 O 3 is also a very hard material (about 2000 kg/mm 2 ) and can be used as a binding material for cbn, its formation in the cbn compacts during the high pressure sintering does not adversely affect the cutting tool properties of the cbn compacts. No peak of hbn was observed in the XRD patterns indicating thereby that no back conversion of cbn to hbn took place under these conditions of high pressure sintering. The FTIR spectra of two samples of cbn composites reported in this work are shown in Figs 2a and 2b respectively. The FTIR spectra were recorded in a reflection mode. In both the spectra that a well defined peak was observed near 1070 cm 1 which is assigned to the TO mode of cbn corresponding to sp 3 bond. The cbn compacts synthesized in the present work were also characterized using Raman spectroscopy. Raman analysis gives information on phase composition, crystallinity and even stress. The laser power was kept below 50 mw for the Raman measurements to minimize the influence of temperature variation in the measurements. The Raman spectrum of a typical sample of a cbn composite is shown in Fig. 3a with a cbn content of 75 wt.%. It can be seen that both transverse (TO) and longitudinal optical (LO) cbn modes appear in this Raman spectrum. The peak appeared near 1050 cm -1 corresponds to TO mode of cbn and that appeared near1300 cm -1 corresponds to LO mode of cbn. Similar Raman spectrum was observed for other cbn compact sintered using 95 wt.% of cbn. In order to confirm these results the Raman analysis was also carried out for some of the commercially available cbn composites. Fig. 3b shows a Raman spectrum of one such sample of cbn composite. The general features of this spectrum were more or less similar to those observed in our cbn composites.

4 328 INDIAN J. ENG. MATER. SCI., AUGUST 2005 Fig. 2a FTIR spectra of a cbn compact sintered at 50 kb and 1450 C using 75:20:5 wt.% of cbn, B 4 C and AlN Fig. 3a Raman spectra of a cbn compact produced at 50 kb and 1450 C using 75:20:5 wt.% of cbn, B 4 C and AlN Fig. 2b FTIR spectra of a cbn compact sintered at 50 kb and 1450 C using 95:3:2 wt.% of cbn, B 4 C and AlN The cbn compacts synthesized as above were lapped and polished by fine diamond paste (1-5 μm) to obtain a mirror-like finish on the surface so that accurate microhardness measurements can be made. Knoop hardness measurements made under a load of 500 g showed a maximum value of microhardness of the cbn compacts of about 3700 and 3400 kg/mm 2 having a cbn content of 75 and 95 wt.% respectively. The lower microhardness in case of composites with higher cbn content may be explained by the fact that with high cbn content the number and size of micropores increases. In this case it is very difficult to fill all the spaces between the cbn particles because of the high rigidity of cbn particles and even under high pressures and temperatures they do not deform. The bulk density decreases apparently because the cbn grains come into contact with each other and Fig. 3b Raman spectra of a commercially available cbn compact number of packing voids remain among them. Thus, the microhardness in these composites may be relatively low (3400 kg/mm 2 ). In case of cbn composites with relatively low cbn content the spaces between cbn grains are filled by the binding materials during high pressure sintering because these binders have a lower rigidity than cbn, and more easily deform under high pressures to form a densely compacted powder body. Similar results have also been observed by Itoh et al. 18 in case of B 6 O-cBN composites by high pressure sintering. However, all such cbn composites with different cbn contents are very useful in the machining of hardened steel components and various ferrous materials under different machining conditions. It has been reported that the tools with large cbn content perform very well in continuous machining, whereas, the composite with lower cbn content performs best in interrupted

5 SINGHAL & SINGH: SINTERING OF CUBIC BORON NITRIDE 329 Fig. 4a SEM micrograph of a cbn compact produced at 50 kb and 1450 C using 75:20:5 wt.% of cbn, B 4 C and AlN Fig. 4b SEM micrograph of a cbn compact produced at 50 kb and 1450 C using 95:3:2 wt.% of cbn, B 4 C and AlN machining conditions 19,20. The microhardness of the cbn compacts produced in this study is slightly higher than microhardness observed for cbn composites obtained using a solid solution of TiC/TiN as the binding material which is around 3200 kg/mm 2. The SEM study of the composites showed strong bonding between cbn grains and the binding particles as shown in Figs 4a and 4b for different compositions of cbn. If the distribution of the binders is not uniform throughout the cbn compact some of the cbn grains orient themselves under high P-T condition to form cbn-cbn physical bonding. This will happen when there is a local deficiency of the binder phase. However, this will lead to an increase in brittleness locally. On the contrary, when the binders are segregated locally the cbn compact is expected to be soft. In case of uniform distribution of the binding phase within the cbn matrix optimum properties with respect to hardness and toughness results. These features have already been reported in our earlier work 17. The cbn grains are randomly oriented throughout the binding phase resulting in uniform hardness and abrasion resistance in all directions. Not much grain growth was observed in the sintered cbn discs. The size of the cbn compacts produced in this work was 6 mm diameter and 2-3 mm thickness. Large size of cbn compacts can be made using similar method. These composites are very useful in various cutting tool applications for ferrous materials. The cemented carbide tools do not resist plastic deformation especially at high temperatures developed at the point of contact between the tool and the workpiece and thus imposes restrictions on the economical use of these tools. Deformation of the tool may lead to rapid wear, often at the nose, or to fracture. However, sintered compacts of cbn can effectively machine hardened ferrous alloys and nickel based high temperature alloys that are almost impossible to machine otherwise because the cbn tools retain all useful properties up to 1300 C and more importantly it does not react with most of the metals and alloys. Conclusions The cbn composites produced in the present work were synthesized at moderate pressures and temperatures (50 kb, C) using B 4 C as the main binding material together with a small amount of AlN. The composites produced using 75 wt.% cbn had a Knoop microhardness of about 3700 kg/mm 2 which is much higher than the microhardness of cemented tungsten carbide (1800 kg/mm 2 ) and are stable up to about 1300 C where most of the other tools fail because of plastic deformation at high temperatures generated during the machining operation. Although cbn compacts produced in this study are of 6 mm diameter, the process can be upscale for producing large size cbn compacts required in machining of hardened ferrous alloys as well as cobalt and nickel based alloys. Acknowledgments The authors are thankful to Dr. R P Pant for recording the various XRD patterns and to Mr. K.N Sood for SEM study of the cbn composites reported in this work. Sincere thanks are also due to Dr. Nita Dilawar and Dr. Sushil Kumar for recording Raman and FTIR spectra of the cbn compacts.

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