Polycrystalline Cubic Boron Nitride Research Articles

Nc-AlTiN/a-Si3N4 and nc-AlCrN/a-Si3N4 nanocomposite coatings as protection layer for PCBN tools in hard machining

The cutting material polycrystalline cubic boron nitride (PCBN) is ideally suited for machining of difficult-to-cut materials. It exhibits properties such as high hardness and high chemical stability at elevated temperatures. However, due to the occurrence of tribo-oxidation and abrasion during machining of hardened steels abrupt tool failure is common. The deposition of coatings offers a possibility to protect the PCBN substrate. Initial research results indicate that nanocomposite coated cutting tools show a significantly improved tool life and process behavior in hard machining. This paper describes the results of machining tests with uncoated and nc-AlTiN/a-Si3N4 and nc-AlCrN/a-Si3N4 nanocomposite coated PCBN tools with regard to the tool life and wear form. Additionally, the wear mechanisms abrasion and tribo-oxidation were investigated in model wear experiments. Furthermore, FEM cutting simulations were used to investigate the thermal interaction between substrate and coating.

Experimental Investigation of Hard Turning Mechanisms by PCBN Tooling Embedded Micro Thin Film Thermocouples

Temperature-distribution measurements in cutting tools during the machining process are extremely difficult and remain an unresolved problem. In this paper, cutting temperature distributions were measured by thin film thermocouples (TFTCs) embedded into polycrystalline cubic boron nitride (PCBN) cutting inserts in the immediate vicinity of the tool-chip interface. The embedded TFTC array provides temperature measurements with a degree of spatial resolution (100 μm) and dynamic response (150 ns) that is not possible with currently employed methods due to the micro-scale junction size of the TFTCs. Using these measurements during hard turning, steady-state, dynamic, as well as chip morphology and formation process analyses were performed based on the cutting temperature and cutting force variations in the cutting zone. It has been shown that the temperature changes in the cutting zone depend on the shearing band location in the chip and the thermal transfer rate from the heat generation zone to the cutting tool. Furthermore, it became evident that the material flow stress and the shearing bands greatly affect not only the chip formation morphology but also the cutting temperature field distributions in the cutting zone of the cutting insert.

High speed ball nose end milling of hardened AISI A2 tool steel with PCBN and coated carbide tools

High speed machining (HSM) of tool steels in their hardened state is emerging as an attractive approach for the mold and die industry due to its potential for significant cost savings and productivity improvement. An experimental study was conducted to investigate the tool wear mechanism and surface integrity in high speed ball nose end milling of hardened AISI A2 tool steel using coated tungsten carbide and polycrystalline cubic boron nitride (PCBN) tools. It is found that coated carbide tools can only be used at low speed (120m/min) while high content PCBN tools are suitable for HSM range (470m/min). PCBN tools produce a damage free workpiece with better surface finish and less work hardening. Despite the higher tool cost, HSM with PCBN tools lead to reduction in both total cost and production time per part.

Using Pulse Laser Processing to Shape Cutting Edge of PcBN Tool for High-Precision Turning of Hardened Steel

Polycrystalline Cubic Boron Nitride (PcBN) cutting tools have excellent characteristics, such as a high degree of hardness and low chemical reactivity, so they have the potential to replace the high precision grinding of hardened steel with high precision cutting. However, there are some problems inherent in so-called sintered materials, including low formability and unsteady wear. In order to solve these problems, pulsed laser processing is applied to shape a cutting edge. In this study, first PcBN cutting tools processed by lasers are adopted to the high-precision turning of hardened steel with 58HRC. As the result, these PcBN cutting tools provide a steady cutting state and a smoother finished surface. Second, in order to investigate these advantages of PcBN cutting tools processed by laser, individual requirements of the cutting tool are investigated: edge sharpness, surface quality including micro-defects, surface hardness, and friction coefficient. The results indicate that tools processed by laser have sharper edges and smoother, harder, and lowerfiction surfaces, compared to ground tools. Above all, the increase in surface hardness improves cutting performance because of it provides higher wear resistance.

A thermo-mechanical model of dry orthogonal cutting and its experimental validation through embedded micro-scale thin film thermocouple arrays in PCBN tooling

Cutting temperature and its distribution in the cutting zone are a critical factor that significantly affects tool life and degrades part accuracy during metal removal operations. However, issues surrounding their modeling and experimental validation in the immediate cutting zone still remain an unresolved issue. A major impediment is the unavailability of adequate temperature measurement methods with sufficient temporal and spatial resolution to measure actual temperatures and validate predictive models. In this paper, a model for the dry orthogonal cutting process with thermo-mechanical coupling effects, i.e., interactions between the stress state, strain rates and the temperature softening of material in the plastic deformation zone, is proposed to predict cutting temperature distribution in the cutting zone. The feasibility and prediction accuracy of the model is verified by experimental measurements through Thin Film Thermocouple (TFTC) arrays embedded at the immediate vicinity of the cutting zone into Polycrystalline Cubic Boron Nitride (PCBN) tooling. The experimental verification is performed under hard turning conditions. It has been shown that the predictions of the proposed model are in very close agreement with the experimentally measured results including the cutting forces, chip thickness and cutting temperature distributions on the rake and flank faces in the cutting zone. Furthermore, the modeling results have also provided an essential understanding on the stress distributions at the tool/chip and work/tool interfaces as well as of the nature of the chip flow velocity along the rake face of the cutting tool.

Preparation of polycrystalline cubic boron nitride compact by high-pressure infiltration using cemented carbide

Polycrystalline cubic boron nitride (PcBN) compacts, using the infiltrating method in situ by cemented carbide (WC–Co) substrate, were sintered under high temperature and high pressure (HPHT, 5.2GPa, 1450°C for 6min). The microstructure morphology, phase composition and hardness of PcBN compacts were investigated by using scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). The experimental results show that the WC and Co from WC–Co substrate spread into cubic boron nitride (cBN) layer through melting permeability under HPHT. The binder phases of WC, MoCoB and Co3W3C realized the interface compound of PcBN compact, and the PcBN layer formed a dense concrete microstructure. Additionally the Vickers hardness of 29.3GPa and cutting test were performed when sintered by using cBN grain size of 10–14μm.

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.

Wear mechanisms of PVD-coated PCBN cutting tools

The wear characteristics of PVD coated (TiN, TiSiN, TiAlN, and AlCrN) polycrystalline cubic boron nitride (PCBN) cutting tools were compared to those of uncoated PCBN during single point turning of case hardened steel 16MnCr5. Post-cutting observations of the worn inserts were performed using high resolution field emission gun scanning electron microscope (FEG-SEM) in combination with focus ion beam (FIB) and analytical transmission electron microscopy (TEM). Substantial differences in the wear behaviour of the different coating materials could therefore be observed. In particular, a remarkable tendency for TiN to exhibit plastic deformation was revealed while TiSiN exhibited a more brittle behaviour evidenced by adhesive wear and microchipping. TiAlN and AlCrN on the other hand exhibited less workpiece adhesion. The wear mechanisms of the above thin coating layers are discussed and interpreted in light of thermal, mechanical, and frictional conditions occurring at the tool–chip contact.

Nanodiamond powder compaction via laser shockwaves: Experiments and finite element analysis

Abstract Laser shock sintering (LSS) is an innovative technique employing mechanical forces to densify powders. It can impart compressive stresses into material to improve the relative density and generate residual stresses to enhance the fatigue strength. In this work, LSS of nanodiamond powders embedded in polycrystalline cubic boron nitride (PCBN) substrate was experimentally investigated using a 1064 nm, 10 ns Q-switched Nd:YAG laser with a peak power density of 6.37 GW/cm2. Finite element simulation (FEM) was applied to predict the mechanical deformations induced by LSS. The dynamic analysis by ABAQUS/Explicit and static analysis by ABAQUS/Standard were performed in detail to predict the residual stress field and densification of nanodiamond powder compacts. The predicted densification of nanodiamond powder compacts is in agreement with the experimental data for single and multiple laser shocks.

Ultrahard nanotwinned cubic boron nitride

Cubic boron nitride (cBN) is a well known superhard material that has a wide range of industrial applications. Nanostructuring of cBN is an effective way to improve its hardness by virtue of the Hall-Petch effect--the tendency for hardness to increase with decreasing grain size. Polycrystalline cBN materials are often synthesized by using the martensitic transformation of a graphite-like BN precursor, in which high pressures and temperatures lead to puckering of the BN layers. Such approaches have led to synthetic polycrystalline cBN having grain sizes as small as ∼14 nm (refs 1, 2, 4, 5). Here we report the formation of cBN with a nanostructure dominated by fine twin domains of average thickness ∼3.8 nm. This nanotwinned cBN was synthesized from specially prepared BN precursor nanoparticles possessing onion-like nested structures with intrinsically puckered BN layers and numerous stacking faults. The resulting nanotwinned cBN bulk samples are optically transparent with a striking combination of physical properties: an extremely high Vickers hardness (exceeding 100 GPa, the optimal hardness of synthetic diamond), a high oxidization temperature (∼1,294 °C) and a large fracture toughness (>12 MPa m(1/2), well beyond the toughness of commercial cemented tungsten carbide, ∼10 MPa m(1/2)). We show that hardening of cBN is continuous with decreasing twin thickness down to the smallest sizes investigated, contrasting with the expected reverse Hall-Petch effect below a critical grain size or the twin thickness of ∼10-15 nm found in metals and alloys.

Cubic boron nitride competing with diamond as a superhard engineering material – an overview

Since the synthesis of the boron nitride in its cubic crystallographic structure, cBN, a material with exceptionally high hardness and special properties was developed for technical engineering applications. For practical use, cBN is, after diamond, the second known hardest material and is today being increasingly used as cutting and drilling tools in substitution for diamond-based tools owing to superior thermal stability and chemical inertness. In this study, the advances in the use of the cBN competing with diamond as a superhard industrial tooling for petroleum extraction, automobile manufacture and other engineering applications, was reviewed. The properties of both single crystals and polycrystalline cBN in the form of composites and thin films were assessed to characterize the possibilities of a next generation of superhard materials to replace diamond in tool and other technological devices.

Measurement of Transient Tool-Internal Temperature Fields During Hard Turning by Insert-Embedded Thin Film Sensors

This paper presents a novel approach for obtaining thermal data from the close vicinity (70–700 μm) of the tool-workpiece interface while machining hardened steel. Arrays of microthin film C-type thermocouples with a junction size of 5 μm × 5 μm were fabricated by standard microfabrication methods and have been successfully embedded into polycrystalline cubic boron nitride (PCBN) using a diffusion bonding technique. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were performed to examine material interactions at the bonding interface in order to determine optimal bonding parameters. Static and dynamic sensor performances have been characterized. The sensors exhibit excellent linearity up to 1300 °C, fast rise time of 150 ns, and sensitivity of ∼19 μV/ °C. The PCBN inserts instrumented with embedded thin film C-type thermocouples were successfully applied to measure internal tool temperatures as close as 70 μm to the cutting edge while machining hardened steel workpieces at industrially relevant cutting conditions. Correlations between temperature and cutting parameters have been established. The embedded microthin film sensor array provided unprecedented temporal and spatial resolution as well as high accuracy for microscale transient tool-internal temperature field measurements. Tool-internal temperature maps were generated from acquired data. In the frequency domain, obtained thermal data indicated the onset of regenerative machining chatter earlier and more effective than conventional force measurement by dynamometer.

Formation Mechanism of Grain Cutting Edges in Micro Dressing of Polycrystalline cBN Grinding Wheels

This paper presents a mechanism for the formation of grain cutting edges in the micro dressing of coarse-grain polycrystalline cubic boron nitrite (cBN) grinding wheels using a fine-grain diamond dresser. Many grain cutting edges having a flat surface consisting of ductile smooth surfaces as well as many brittle micro dents, are formed on the working surface of the coarse-grain (#80 mesh) cBN wheel by micro dressing using a fine-grain (#1200 mesh) diamond dresser. This result shows that the flat surfaces of cutting edges on the wheel surface are formed on the basis of a ductile removal process as well as a brittle micro-fracturing of cBN grains by the diamond cutting edges. Moreover, cylindrical grinding experiments with these wheel working surfaces were conducted to clarify the feasibilyty of creating a ground mirror-like surface. As a result, it was confirmed that high-quality mirror surfaces with roughness less than 0.03 m Ra can be efficiently formed using the working surface prepared by this micro dressing method.

Tool Life and Surface Integrity in High-speed Milling of Titanium Alloy TA15 with PCD/PCBN Tools

Titanium alloys are widely used in aeronautics that demand a good combination of high strength, good corrosion resistance and low mass. The mechanical properties lead to challenges in machining operations such as high process temperature as well as rapidly increasing tool wear. The conventional tool materials are not able to maintain their hardness and other mechanical properties at higher cutting temperatures encountered in high speed machining. In this work, the new material tools, which are polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) tools, are used in high-speed milling of Ti-6.5Al-2Zr-1Mo-1V (TA15) alloy. The performance and wear mechanism of the tools are investigated. Compared to PCBN tool, PCD tool has a much longer tool life, especially at higher cutting speeds. Analyses based on the SEM and EDX suggest that attrition, adhesion and diffusion are the main wear mechanisms of PCD and PCBN tools in high-speed milling of TA15. Oxidation wear is also observed at PCBN tool/workpiece interface. Roughness, defects, micro-hardness and microstructure of the machined surface are investigated. The recorded surface roughness values with PCD/PCBN tools are bellow 0.3 μm at initial and steady cutting stage. Micro-hardness analysis shows that the machined surface hardening depth with PCD and PCBN tools is small. There is no evidence of sub-surface defects with PCD and PCBN tools. It is concluded that for TA15 alloy, high-speed milling can be carried out with PCD/PCBN tools.

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.

Performance and wear mechanisms of whisker-reinforced alumina, coated and uncoated PCBN tools when high-speed turning aged Inconel 718

Inconel 718, an efficient superalloy for energy and aerospace applications, is currently machined with cemented carbide tools at low speed (vc ≈ 60 m/min) due to its unfavorable mechanical and thermal properties. The article presents results of a study of superalloy machinability with whisker-reinforced alumina, uncoated and coated polycrystalline cubic boron nitride (PCBN) tools. Turning of age-hardened Inconel 718 (45 HRC) was done under high-speed machining conditions (vc = 250…350 m/min). Aspects of tool life, tool wear, and generated surface quality were studied. Application of uncoated PCBN tools resulted in surface quality and force level superior to other tool materials. Considerable sideflow of workpiece material was found to affect surface quality, especially for coated PCBN and ceramic tools. It was found that protective function of the coating, which increases the tool life up to 20 %, is limited only to low cutting speed range. EDX and AFM analyses suggested dominance of chemical and abrasive wear mechanisms. EDX mapping of worn tools pointed absence of diffusional wear for PCBN tools and intensive degradation of whisker reinforcement in ceramic tools due to diffusion of Ni, Fe, and Cr.

In situ synthesis of cBN–Ti3AlC2 composites by high‐pressure and high‐temperature technology

In this paper, titanium aluminum carbide (Ti3AlC2) was studied as the binder of polycrystalline cubic boron nitride (PCBN) to overcome the weaknesses of traditional metal binders and ceramic binders. The composites of cubic boron nitride (cBN) and Ti3AlC2 were in situ synthesized from the mixtures of Ti, Al, TiC and cBN powders at high temperature (1050°C–1250°C) and under high pressure (4.5GPa) (HTHP). The Ti3AlC2 proportion in as-prepared composites increased as the processing temperature increased from 1050°C to 1150°C. At 1200°C, part of Ti3AlC2 decomposed to form new TiC. At 1250°C, cBN transformed to hBN and reacted with other materials. The composites processed at 1200°C had the optimal mechanical properties, including friction coefficient, hardness and mass loss of worn Si3N4 ball. From the microstructure of as-prepared composites, agglomeration of cBN particles existed; however, the cBN particles, in general, were evenly distributed in the matrix of Ti3AlC2. The cBN particles strongly bonded with the matrix with a clear interface.

Polycrystalline cubic boron nitride sintered compacts prepared from nanocrystalline TiN coated cBN powder

Polycrystalline cubic boron nitride (PcBN) samples were fabricated via high pressure high temperature (HPHT) sintering process using nanocrystalline TiN particle coated cBN powder. cBN powder was coated with TiN nanoparticles prior to sintering for the purpose of improving microstructural homogeneity. The effect of using coated cBN powders on the mechanical properties of the sintered compacts was investigated. Nano-sized TiN coated cBN powders resulted in a significant improvement in the homogeneity of the binder phase in bulk sintered compacts which would not have been obtainable by ordinary mechanical mixing technique. By using coated powders, TiN phase was found to be continuously distributed within the cBN network. Better distribution of TiN resulted in a complete reaction of TiN with cBN at the interface resulting in stronger bonding and higher interfacial strength. Compacts prepared using coated cBN powder exhibited improved mechanical properties and lower values of flank wear and surface roughness.

Friction Stir Spot Welding (FSSW) of Advanced High Strength Steel (AHSS)

Friction stir spot welding (FSSW) is applied to join advanced high strength steels (AHSS): galvannealed dual phase 780 MPa steel (DP780GA), transformation induced plasticity 780 MPa steel (TRIP780), and hot-stamped boron steel (HSBS). A low-cost Si3N4 ceramic tool was developed and used for making welds in this study instead of polycrystalline cubic boron nitride (PCBN) material used in earlier studies. FSSW has the advantages of solid-state, low-temperature process, and the ability of joining dissimilar grade of steels and thicknesses. Two different tool shoulder geometries, concave with smooth surface and convex with spiral pattern, were used in the study. Welds were made by a 2-step displacement control process with weld time of 4, 6, and 10 seconds. Static tensile lap-shear strength achieved 16.4 kN for DP780GA-HSBS and 13.2kN for TRIP780-HSBS, above the spot weld strength requirements by AWS. Nugget pull-out was the failure mode of the joint. The joining mechanism was illustrated from the cross-section micrographs. Microhardness measurement showed hardening in the upper sheet steel (DP780GA or TRIP780) in the weld, but softening of HSBS in the heat-affect zone (HAZ). The study demonstrated the feasibility of making high-strength AHSS spot welds with low-cost tools.