Polycrystalline Cubic Boron Nitride Grains Research Articles

Effects of hardness and grain size on wear resistance of polycrystalline cubic boron nitride

Exploring the factors affecting the wear resistance of superhard materials is interesting and has practical industrial applications. Characterizing the hardness and grain size of superhard materials is more convenient and simple than measuring their wear resistance. Therefore, we expect to obtain a quantitative relationship between the hardness and grain size of superhard materials and their wear resistance. In this work, a series of polycrystalline cubic boron nitride (PcBN) compacts with different grain sizes and hardnesses were sintered under high temperature and pressure. Furthermore, turning tools were manufactured using these PcBN samples. The edge wear of the turning tool during the turning of granite was examined. Based on aforementioned measurements, the relationship between hardness and wear resistance is examined with same grain size (1–2 μm) and the influence of PcBN grain size on wear resistance is investigated under the same hardness (44.0 ± 1.2 GPa). The results show that the wear resistance of PcBN exhibits a regression parabolic and linearly increasing trends with the increase in hardness and grain size, respectively. Further investigation reveals that the compactness of PcBN materials, the bonding strength between the crystal grains, and the depth of the embedded crystal grains are critical factors affecting the wear resistance. These works can serve as a practical guide for the preparation as well as research and development (R&D) of PcBN and are expected to be extended to the R&D and application of other superhard materials and hard ceramics.

Effect of wear behaviour of single mono- and poly-crystalline cBN grains on the grinding performance of Inconel 718

Polycrystalline cubic boron nitride (PcBN) grains were fabricated by combining the monocrystalline cBN (McBN) nanoparticles and inter-abrasive ceramic materials via high temperature and pressure techniques. Grinding performance of Inconel 718 with single McBN and PcBN grains, including grinding force, force ratio, ground surface quality was investigated. Characterization of the wear morphology evolution of worn grains and scratches of PcBN grains were discussed. In addition, the fracture behaviour of PcBN grains was evaluated as the varying of the undeformed chip thickness. Results show that PcBN grains have the smaller grinding force and force ratio, more stable grain wear rate in comparison to McBN grains. Additionally, the better wear-resistance and grinding performance owing to its multi-cutting edges structure in terms of the grain wear morphology evolution were achieved for PcBN grains regardless of the undeformed chip thickness.

Micro-fracture variation and grinding performance of PCBN superabrasive grains in high-speed grinding

In this work, a finite element analysis model of micro-fracture of polycrystalline cubic boron nitride (PCBN) superabrasive grains based on Voronoi diagrams was established to study the fracture behaviour during the single-grain high-speed grinding process of Inconel 718 nickel-based superalloys. The profile evolution of grain was predicted by iterating the profile of fractured grain and analysed by combination with the experimental results. The effects of internal stresses and microstructure of PCBN grain on grain fracture were investigated. The profile evolution and chip formation process of PCBN grain due to micro-fracture were analysed, and the effect of micro-fracture on the grinding performance of PCBN grain was studied. Results showed that the main fracture modes of PCBN grain were micro-fracture near cutting edges and crack propagation inside the grain. Tensile stress was the main internal affecting factor of the fracture behaviour and led to more consumed volume than compressive stress. The microstructure of PCBN grain hindered the stress transfer and made the grain susceptible to cracks and micro-fractures. The number of cutting edges involved in the cutting process varied due to micro-fractures. The balance between the difficulty in chip formation and energy loss could only be reached when the percentage of fractured regions ranged from 40% to 50%. Micro-fracture changed the profile and sharpness of PCBN grain, thereby forming a ‘passivation-sharpening’ cycle.

Micro-fracture mechanism of polycrystalline CBN grain during single grain scratching tests based on fractal dimension analysis

The wear phenomenon is an important issue that affects the grinding performance of polycrystalline cubic boron nitride (PCBN) grinding wheel. To explore the wear mechanism, single grain scratching tests were conducted on the nickel-based superalloy Inconel 718. Fractal theory was applied to evaluate the grain wear process, and the influences of undeformed chip thickness agmax and grinding wheel speed vs on grain wear were analysed. The variation in fractal dimension, grinding force and grinding force ratio were discussed. Results show that the micro-fracture is caused by the crack, and the adhered grinding chip leads to the attrition wear of PCBN grain. The average specific material removal volume of monocrystalline CBN grain is approximately 10.7% that of PCBN grain when macro-fracture occurs. The effect of agmax on grain macro-fracture is stronger than that of vs. Furthermore, the grinding parameters should be set as follows: vs of 80 m/s and agmax in the range of 0.1–0.67 μm. These settings help improve the grain micro-fracture phenomenon in high-speed grinding.

Understanding the self-sharpening characteristics of polycrystalline cubic boron nitride super-abrasive in high-speed grinding of Inconel 718

This study aims to understand the self-sharpening characteristics of the polycrystalline cubic boron nitride (PCBN) super-abrasive grains in high-speed grinding of Inconel 718. Comparative single grain grinding operations by separately using the brazed PCBN and conventional CBN abrasive grains are carried out. An in-depth analysis on the grain fracture morphologies and the workpiece scratch profiles during grinding are provided. The cutting mechanism and the material removal difference induced by the PCBN and conventional CBN grain fracture behaviour during grinding are revealed. The obtained results indicate that, the PCBN abrasive grain tends to intergranular fracture during grinding, which not only leads to the grain micro-fracture behaviour but also provides plenty of micro cutting-edges of microcrystalline CBN particles to participate in the grain cutting process. The surface roughness of the workpiece scratch grooves produced by the PCBN abrasive grain is 0.30 μm in average, which is 0.10 μm higher than that produced by the conventional CBN abrasive grain. This is mainly attributed to the additional cutting effects of the outcropped microcrystalline CBN particles on the PCBN grain fracture surface in the actual grinding process, and the additional grain cutting effects on the workpiece materials contribute to the self-sharpening characteristics of the PCBN. Finally, a mechanical analysis model of the single abrasive grain in grinding is also developed to provide a more comprehensive understanding on the self-sharpening mechanism of PCBN, and the established mechanical model can be correctly demonstrated by the calculated grinding force ratios in the single grain grinding operations.

Effect of grinding wheel speed on self-sharpening ability of PCBN grain during grinding of nickel-based superalloys with a constant undeformed chip thickness

Cubic boron nitride (CBN) superabrasive wheels are of great potential in grinding of difficult-to-machine metallic materials. The grain wear and its evolution directly determine the sharpness of the grinding wheels, and therefore greatly affect the grinding efficiency, surface integrity and the tool life. In order to effectively control the grain wear, the present work is aimed to elucidate the wear behavior difference of polycrystalline CBN (PCBN) abrasive grains and conventional CBN grains, and meanwhile analyze the influence of grinding wheel speed (ranged from 20 to 120 m/s) on the PCBN grain wear. The wear behavior was also quantitatively characterized based on the fractal theory. Results obtained show that the self-sharpening ability of PCBN grain is superior to CBN grain considering the fractal dimension and specific grinding force. The radial wear and specific grinding force with the grinding wheel speed of 120 m/s increase severely by 29.6% and 117.3% compared with that of grinding wheel speed of 80 m/s, which is in accord with the change of fractal dimension. When considering the wear evolution behavior of PCBN grains, a critical grinding wheel speed, e.g., 80 m/s, could be determined. If the grinding wheel speed is above 80 m/s, the grain macro-fracture wear happens rapidly; however, if it is below 80 m/s, the grain micro-fracture wear increases with increasing of the grinding wheel speed. According to the finite element analysis, the micro-fracture of the PCBN grain is mainly caused by the intergranular fracture of the grain, while the attritious wear is due to the wear of the grain cutting edge.

Numerical simulation analysis and experimental validation on wear/fracture mechanisms of polycrystalline cubic boron nitride superabrasives in high-speed grinding

In this study, a two-dimensional finite element model is proposed to investigate the wear/fracture mechanisms of polycrystalline cubic boron nitride (PCBN) superabrasives in high-speed grinding process. The special geometric microstructures of PCBN grains are constructed by using the classic Voronoi tessellation technique, and cohesive elements are embedded into the geometric model of PCBN grains as the potential crack propagation paths for simulating the wear/fracture behaviours of PCBN grains under grinding loads. The effects of uncut chip thickness per grain (agmax) on the stress distribution characteristics and wear/fracture behaviours of PCBN grains during grinding are discussed in detail. Results show that the wear behaviour of PCBN grains during grinding mainly occurs around the grain vertex region; however, the fracture behaviour, leading to the quick failure of PCBN grains, is prone to appear around the grain–filler bonding interface, which is usually on the opposite side of the in-feed direction. Moreover, to separate the PCBN grains from the macro-fracture during grinding, the uncut chip thickness per grain should be kept smaller than 1.0 µm to prevent the unfavourable fracture behaviour from appearing around the grain–filler bonding interface. Furthermore, the corresponding single-grain grinding trials are performed to validate the numerical simulation results by evaluating the wear/fracture morphologies of the PCBN superabrasives in the actual grinding operation.

Compressive Strength and Interface Microstructure of PCBN Grains Brazed with High-Frequency Induction Heating Method

In order to prepare monolayer brazed superabrasive wheels, the polycrystalline cubic boron nitride (PCBN) grains were brazed to AISI 1045 steel matrix with Ag–Cu–Ti filler alloy using the high-frequency induction heating technique. The compressive strengths of brazed grains were measured. Morphology, chemical composition and phase component of the brazing resultant around PCBN grain were also characterized. The results show that the maximum compressive strength of brazed grains is obtained in the case of brazing temperature of 965 °C, which does not decrease the original grain strength. Strong joining between Ag–Cu–Ti alloy and PCBN grains is dependent on the brazing resultants, such as TiB2, TiN and AlTi3, the formation mechanism of which is also discussed. Under the given experimental conditions, the optimum heating parameters were determined to be current magnitude of 24 A and scanning speed of 0.5 mm/s. Finally, the brazing-induced residual tensile stress, which has a great influence on the grain fracture behavior in grinding, was determined through finite element analysis.

Investigation on stress distribution and wear behavior of brazed polycrystalline cubic boron nitride superabrasive grains: Numerical simulation and experimental study

The polycrystalline cubic boron nitride (PCBN) superabrasive grains, which are comprised of microcrystalline CBN particles and AlN ceramic binder, have a distinguished advantage in self-sharpness during grinding by means of controllable fracture wear. The present work intends to clarify the mechanism of grain fracture based on the analysis of brazing-induced residual stress and the resultant stress in grinding; as such, the grain fracture wear could be predicted and controlled effectively. A finite element model based on Voronoi tessellation method has been first established for PCBN grains. The effects of embedding depth, volume fraction, and grinding loads on the stress distribution within PCBN grains are discussed. It is found that the distribution patterns of microcrystalline CBN particles generally have less influence on grain fracture. Large tensile stress is produced at the interfaces of microcrystalline CBN particles, AlN ceramic binder and Ag-Cu-Ti filler. Particularly, the largest tensile stress is generated near the grain vertex in the case of the embedding depth of 50%, volume fraction of 80%, and uncut chip thickness of 0.6μm. The simulation results are verified experimentally through characterizing the wear topography evolution of brazed PCBN grains in grinding.

Understanding the residual stress distribution in brazed polycrystalline CBN abrasive grains

This article investigates the brazing-induced residual stresses in the polycrystalline cubic boron nitride (PCBN) abrasive grains based on finite element simulation. The effects of embedding depth and gap thickness on the residual stress distribution in brazed PCBN grains are discussed. Results obtained show that, compared with the Ag-Cu-Ti alloy joining CBN grains and AISI 1045 steel substrate, the AlN binder materials in the polycrystalline CBN grains always have a greater effect on the brazing-induced residual stresses; meanwhile, large residual tensile stresses usually exist at the interface between the microcrystalline CBN particles and AlN binders. In comparison to the embedding depth, the gap thickness has little influence on the brazing-induced residual stresses. Furthermore, compared with the other embedding depth, in the case of the embedding depth of 40 %, the residual stresses in the brazed PCBN grain are generally the lowest in the grain bottom and the largest in the grain top surface. Finally, the finite element model applied in the current simulation work is validated through measurement experiments of the brazing-induced residual stresses in the monocrystalline CBN grain.

Surface fractal evolution of fracture behavior of polycrystalline cBN grains in high-speed grinding

High-speed grinding experiments on nickel-based superalloy Inconel718 were carried out with single grain of polycrystalline cubic boron nitride (PcBN). Surface fractal evolution of fracture behavior of PcBN grains was discussed. The fracture reason of PcBN grains was studied from the viewpoint of grinding load based on finite element method (FEM). Results obtained show that micro fracture behavior is the predominant wear pattern of the PcBN grain cutting edges. Because the tested grains have different wear patterns, the statistical relationship is established between the fractal dimension of the grain cutting edges and the attritious wear flat percentage. The dynamic grinding force is the determining factor for the micro fracture behavior of the grain cutting edges in the particular region, where the grain firstly contacts the workpiece in the initial stage of the grinding process.

Comparative Investigation on Brazing Behavior, Compressive Strength, and Wear Properties of Multicrystalline CBN Abrasive Grains

In order to fabricate the abrasive wheels with good grain self-sharpening capacity, two types of multicrystalline CBN grains, that is, polycrystalline CBN (PCBN) and binderless CBN (BCBN), were brazed using Cu-Sn-Ti alloy, respectively. Comparative investigation on the brazing interface, compressive strength, and wear properties of the different grains was carried out. Results obtained show that the PCBN grains have more intricate reaction, more complicated resultants, and thicker reaction layer than the BCBN counterparts under the identical brazing conditions. Though the average compressive strength of the PCBN grains is similar to that of BCBN ones, stronger self-sharpening action by virtue of the microfracture behavior takes place with BCBN grains during grinding. As a consequence, compared to the brazed PCBN wheels and the conventional monocrystalline CBN (MCBN) ones, longer service life is obtained for the brazed BCBN wheels.

Reconstruction and Analysis of Wear Topography of PcBN Abrasive Grain

This paper mainly deals with the reconstruction and fractal analysis of wear topography of brazed polycrystalline cubic boron nitride (PcBN) grain in grinding process. On the basis of fractal analysis, the uncertainty in shape and complexity in wear mechanism are analyzed by means of the construction of micro fracture of PcBN grain after producing the brazed abrasive tool with PcBN grains and carrying out grinding test. The main results are summarized as follows: (1) Actual behavior of self-sharpening phenomenon due to micro fracture in the grinding process can be grasped and reconstructed using SEM and 3D video microscope. The reconstruction model can clearly express the complicated wear features and characteristics of the grain. (2) The fractal dimension can relate the performance of PcBN grain to its micro fracture, and then the self-sharpening phenomenon due to micro fracture can be evaluated quantitatively on the basis of fractal analysis.

Fractal analysis of wear topography of brazed polycrystalline cBN abrasive grains during grinding nickel super alloy

In order to characterize quantificationally the wear topography of the brazed polycrystalline cBN (PcBN) grains during grinding, the reconstruction model of grain topography is established through the photographs grasped with three-dimensional (3D) optical video microscope. The relationship between 3D fractal dimension and the complicated topography change of the PcBN grains is investigated based on fractal theory. The results obtained show that it is reasonable to calculate 3D fractal dimension according to the protrusion height of the abrasive grain. The fractal dimension of the grain wear topography formed due to microfracture is higher than that formed due to large fracture and attritious wear. The fractal dimension range of the wear topography of the brazed PcBN grains is limited to 2.0325–2.0475 with a concentrated value of 2.04 under the present experimental condition.

Interface characteristics and fracture behavior of brazed polycrystalline CBN grains using Cu–Sn–Ti alloy

Polycrystalline CBN grains were brazed onto AISI 1045 steel matrix using Cu–Sn–Ti brazing alloy. Brazing temperature was 880°C, 900°C, 920°C respectively, and the holding time was 8min. Interface characteristics, elemental distribution and fracture behavior were analyzed in detail by means of scanning electron microscopy, energy dispersive spectroscopy and testing machine for grain compressive strength. The maximum value of the mean compressive strength of brazed PCBN grains reached 879.3MPa when the brazing temperature was 900°C. The resultants layer composed of TiN, TiB2, TiB and TiAl3 was formed at grain/alloy interface. Elemental distribution showed that Ti mainly concentrated within the interface zone. The intercrystalline fracture along the CBN/CBN particle boundary was the main fracture modes of brazed polycrystalline CBN grains.

Self-Sharpening Abrasive Composite Bulks with PCBN Grains

Abrasive composite bulks consisting of polycrystalline cubic boron nitride (PCBN) grains, Cu-Sn-Ti alloy and graphite particles were sintered at the heating temperature of 920 °C for the dwell time of 30 min. The bending strength of abrasive composite bulks was measured. The interfacial microstructure and the phases were characterized. In addition, the dressing experiment was carried out to detect the self-sharpening behavior of the composite bulks. Results obtained show that the abrasive composite bulks in this investigation give higher bending strength than that of the vitrified abrasive wheels. The compounds of TiN, TiB2were formed and the PCBN grains were embedded firmly. Strong fixing of the bulks to the PCBN grains led to the breakage of the PCBN grains when the abrasive composite bulks fractured. The intergranular fracture mode of the PCBN grains ensured the self-sharpening effect of the grains.

High-efficiency grinding of cold-work die steel with ultrafine-crystalline cBN abrasive grains

This paper presents the grinding characteristics of newly developed ultrafine-crystalline cBN (UcBN) abrasive grains in high-efficiency grinding of cold-work die steel. This new UcBN grain possesses an ultrafine crystal structure composed of sub-micron sized primary crystal grains. The fracture strength of the UcBN grain is about 3.7 times higher than that of conventional monocrystalline cBN grain. The grinding ratio in grinding with the UcBN grain is eight to 11 times higher than those in grinding with conventional typical monocrystalline and polycrystalline cBN grains.

Characterization and Performance of Monolayer Brazed Polycrystalline CBN Abrasive Tools

Brazing experiments of polycrystalline CBN abrasive grains and AISI 1045 steel matrix using 95(72Ag-28Cu)-5Ti (wt.%) filler alloy were carried out at the heating temperature of 900 °C for the dwell time of 8 min. The microstructure of the brazing interface among PCBN grain, Ag-Cu-Ti alloy and steel matrix, was characterized with optical microscope, scanning electron microscope and X-ray diffraction equipment. Grinding performance of the brazed polycrystalline CBN abrasive tools was evaluated experimentally by comparison with monocrystalline CBN counterparts. The results show that the reaction layer between polycrystalline CBN abrasive grain and Ag-Cu-Ti filler layer consists of the compounds of Ti-nitride, Ti-borides and Ti3AlN. The resultants have played an important role in terms of strong chemical joining at the grain-filler interface. The brazed abrasive tools with polycrystalline CBN grains have given higher material removal rate and longer service life than that with monocrystalline CBN ones.

Grain wear of brazed polycrystalline CBN abrasive tools during constant-force grinding Ti–6Al–4V alloy

Constant-force grinding experiments of titanium alloy Ti–6Al–4V were carried out with brazed polycrystalline cubic boron nitride (PCBN) abrasive wheel heads. Stock removal and tool wear of PCBN abrasive tools were evaluated by comparison with that of monocrystalline CBN counterparts. Grain wear of the brazed PCBN abrasive tools were examined using optical microscopy and scanning electron microscopy. Fracture mechanism of the PCBN abrasive grains was discussed. The results show that the brazed abrasive tools with PCBN grains give larger stock removal by 25% and the longer service life by 30% than the tools with CBN grains. PCBN abrasive grains have the capability to maintain sharp cutting edges by means of the fracture behavior of microcrystalline CBN particles. In case of strong fixing of the PCBN grains in the Ag–Cu–Ti filler layer, the crack of PCBN abrasive grains are mainly dominated in the intergranular mode dependent upon the joining effects of AlN binder material among the adjacent microcrystalline CBN particles.

5511379 Process for the preservation of products at low temperature in an insulated chamber, installation for practicing the process, insulated chamber and container for such a chamber

The polycrystalline cubic boron nitride (PCBN) superabrasive grains, which are comprised of microcrystalline CBN particles and AlN ceramic binder, have a distinguished advantage in self-sharpness during grinding by means of controllable fracture wear. The present work intends to clarify the mechanism of grain fracture based on the analysis of brazing-induced residual stress and the resultant stress in grinding; as such, the grain fracture wear could be predicted and controlled effectively. A finite element model based on Voronoi tessellation method has been first established for PCBN grains. The effects of embedding depth, volume fraction, and grinding loads on the stress distribution within PCBN grains are discussed. It is found that the distribution patterns of microcrystalline CBN particles generally have less influence on grain fracture. Large tensile stress is produced at the interfaces of microcrystalline CBN particles, AlN ceramic binder and Ag-Cu-Ti filler. Particularly, the largest tensile stress is generated near the grain vertex in the case of the embedding depth of 50%, volume fraction of 80%, and uncut chip thickness of 0.6 μm. The simulation results are verified experimentally through characterizing the wear topography evolution of brazed PCBN grains in grinding.