cBN Materials Research Articles

Study of thermodynamic properties and elastic constants of cubic boron nitride by statistical moment method

The statistical moment method has been applied to investigate temperature effects on the thermodynamic and elastic properties of the cubic boron nitride (cBN). The analytical expressions of thermal-induced atomic displacement, the mean-square displacement, elastic constants (Young’s modulus, bulk modulus, shear modulus, and longitudinal-wave elastic constant), and the bulk and longitudinal sound velocities of cBN compound have been derived. Our theoretical calculations are compared with previous experiments and computations showing reasonable agreement. We have shown that while the mechanical strength and Debye temperature of the cBN material are slightly reduced as temperature rises, the mean-square displacement function increases rapidly. The noticeable difference in mean-square displacements of B isotopes is observed at low temperature, and the disparity diminishes gradually at high temperatures. The calculations of the present work can be used as an appropriate reference for the future experiments.

Cutting performance of the nanotwinned cBN tool in nano-cutting of Ni-Cr-Fe alloy

As nickel-based superalloys are broadly used and difficult to machine material, it is challenging to achieve high machining efficiency while improving surface quality and enhancing tool life. Aiming at the cutting of Inconel-718 type nickel-based superalloys, a new type of tool in which the cBN tool incorporated naontwinned structures is proposed. The mechanical properties of cBN tool materials with the introduction of nanotwinned structures are studied by uniaxial tensile simulations of nanotwinned cBN (nt-cBN) and single-crystal cBN (sc-cBN) nanosheets. Compared with the sc-cBN materials, the introduction of nanotwinned structures improved the properties of strength, toughness, and stiffness of cBN materials. The tool performance of the nt-cBN tool and the sc-cBN tool have been studied when cutting a simplified nickel-based superalloys (Inconel-718) model using the molecular dynamics method. Tool wear mechanisms of nt-cBN tools are investigated by calculating and visualizing tool temperature, Von Mises stress, and shear stress distributions. The results show that the wear resistance performance of the nt-cBN tool has improved during high-speed cutting, with less plastic deformation of the tool, less diffusion wear, less adhesion wear, and especially the wear of the flank surface is significantly reduced. As a new type of tool for machining difficult-to-machine materials, nt-cBN tools have promising research prospects.

Mechanical response of nanoindentation and material strengthening mechanism of nt-cBN superhard materials based on molecular dynamics

In this paper, the mechanical response of nanoindentation process of nano-twinned cubic Boron Nitride (nt-cBN) materials is investigated by molecular dynamics method. The introduction of nano-twinned structure in cBN materials can lead to superior mechanical properties than those of single-crystal cBN (sc-cBN) materials. The mechanism of material strengthening is derived by studying the dislocation propagation during the indentation process. The limitation of dislocation propagation by nano-twinned layers leads to the accumulation of dislocations in the interlayer to form immovable dislocation multi-junctions, which leads to the increase of intracrystalline back stress and raises the yield stress of the material. In the high-pressure environment, dislocations can react with twin boundary layers and split into multiple dislocations, increasing the dislocation density and the number of dislocation multi-junctions in the material. The reaction between dislocations and twin-crystal plays an important role in twin-crystal strengthening.

Sintering of binderless cubic boron nitride and its modification by β-Si3N4 additive for hard machining applications

This study present the results of HP-HT sintering, microstructure, properties, and performance of binderless cBN tool material. Within the investigated sintering temperature range of 1900–2600 °C the optimum was found to be 2200–2300 °C. Lower temperature results in incomplete diffusion bonding between cBN grains, while higher temperature results in high degree of recrystallization of initial structure, grain growth, and even formation of hexagonal boron nitride in triple joints. Introduction of stress-inducing β-Si3N4 minor inclusions resulted in high overall mechanical and thermal properties: HK = 41 GPa; KIC = 12.6 MPa·m1/2; λ = 180 W/(m·K). Machining experiments in roughing of hardened tool steels show that binderless cBN material provides high performance in terms of resistance to tool cratering, chipping, and tool fracture.

IM (Indentation Method) Based Impact Fracture Characteristics of cBN Crystal and Their Influence on High Speed Wheel Performance

Cubic Boron Nitride (cBN) is considered a superabrasive due to its excellent material behavior, and is commonly used in the manufacturing of high end grinding tools, in particular for machining hard-to-machine materials, in order to achieve high productivity in combination with high precision. During the actual grinding process, the performance of grinding tools is significantly affected by the fracture response of employed cBN crystals to the external impact load related to the grinding speed. Therefore, two types of cBN crystals are selected in this paper and their fracture resisting behaviors are measured under various impact loading rates through the Indentation Method (IM). In order to establish the fracture behavior of cBN crystals and the overall grinding performance of cBN wheels, high speed surface grinding experiments are conducted and grinding performances are evaluated regarding the evolution of grinding force and power consumption, wheel wear and grinding efficiency, as well as the chemical stability of cBN materials on affinity. The experimental results show a good agreement of cBN crystal’s impact fracture properties on the cBN grinding wheel performance, which helps to understand cBN crystal’s high speed grinding characteristics.

An Experimental Investigation on Response of cBN Super Abrasive’s Grinding Performance and Failure Characteristics to the Grinding Speed

Cubic Boron Nitride (cBN), a kind of super abrasive material with excellent properties, is widely applied into the manufacturing of high end grinding tools especially for high speed machining fields. During the actual grinding process, abrasive failure phenomena take place, which affects the grinding tool performance, and then the final machining quality. Considering the high speed applicability of cBN grinding tools, it is of significance to investigate the failure mechanism and grinding performance of cBN abrasives at different grinding speeds, in particular under high speed condition. Therefore, this paper carries out an investigation of cBN abrasive failure behavior taking grinding speed sensitivity into consideration. Given that the random and diversity of multi-abrasives distributed upon grinding tools contributes to the complexity of the study, single cBN abrasive cutting experiments are conducted under four grinding speed levels of 40, 60, 80 and 100 m/s, respectively. When all experiments are completed, abrasive failure patterns can be identified through observation on the morphology of ground cBN abrasives bonded on cutting inserts using Scanning Electron Microscope (SEM). Based on the analysis of experimental results, it is shown that for the cBN abrasives studied in this paper, abrasive breakage occurs at low grinding speed, but as speed increases, cBN abrasives tend to get worn. The change of grinding forces, as well as roughness Ry values, can show the detailed failure behavior of tested cBN abrasives with time dependent features. In addition, G ratio rises with the increase of grinding speed, which demonstrates the high productivity potential of cBN high speed grinding. Finally, an in-depth analysis concerning on cBN material’s crystal characteristics and energy threshold (binding energy) is discussed, which provides an intrinsic explanation on the influence of grinding speed upon cBN abrasive failure mechanism.

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.

Oxidation resistance of B 6O-materials with different additives

The oxidation resistance of B 6O materials with and without Y 2O 3/Al 2O 3 additives and a typical cBN material were analysed in dry and wet air using TG measurements for up to 10 h and static experiments in a muffle furnace for up to 50 h. The results showed that the oxidation of the materials is sensitive to the composition of the atmosphere. In wet air with 17 vol.% H 2O a weight loss was measured for all materials. In dry air a parabolic increase of the mass and a formation of a glassy surface layer were observed for the B 6O materials whereas the cBN-material is characterized by the formation of a mostly crystalline layer. The oxidation resistance is the lowest for the pure B 6O and the highest for the investigated cBN material.

Preparation and characterization of SiO2–Al2O3–Na2O glass coated cBN abrasive particles via sol–gel route

SiO2–Al2O3–Na2O glass coated cubic boron nitride (cBN) abrasive particles were prepared by sol–gel technique. The results indicated that SiO2–Al2O3–Na2O glass was excellent material for oxidation protection of cBN abrasive grains because coefficient of thermal expansion of this glass closely matched that of cBN materials. The single particle compressive strength and impact toughness of this glass coated cBN abrasive particles were significantly increased. For the application of glass coated cBN abrasives to vitrified grinding wheels, it was evident that the glass coating provided high bonding strength between cBN abrasive grains and vitrified bond system.

An Investigation of Surface Grinding Characteristics for Titanium Alloy with CBN Wheel

This study performs an experiment to investigate the effect of process variables such as rotational cutting speeds of the wheel, feed rate of the work-table and grinding depth of cut on surface roughness and the fluctuations of grinding forces for Ti-6Al-4V titanium alloy. STP-1623 ADC surface grinding machine, grinding wheel with CBN material sintering and Ti-6Al-4V titanium alloy workpiece are used in the experiment. The roughness of the grinding surface was measured by the roughness measuring instruments and the fluctuations of grinding forces were measured through dynamometer after each surface layer ground from the workpiece in the experiment. The grinding performance can be ascertained from the signal fluctuations phenomena of the grinding forces both along normal and tangential directions, which may also be utilized as an index for the quality of surface finish judgment. The results show that excellent surface quality being always consistent with the stable grinding force fluctuations and can be obtained under the conditions of slow feed rate of the work-table, high revolutions of the wheel and shallow depth of cut.

Science and technology in the recent development of boron nitride materials

In this paper, we review recent developments relating to cubic boron nitride(cBN) abrasive grains and sintered cutting tools. The demand for high-speedmachining and the ecological benefits of using ferrous materials have led todevelopments in the area of heavy-duty dry cutting and grinding processes inrecent years. Optimization of the process of manufacturing cBN materials is animportant issue, both fundamentally and as regards applications. We reviewrecent developments in cBN applications and discuss the challenges arising fromnew processes encountered in basic cBN study at high pressure and hightemperature.