Structural and mechanical properties of cBN composites by regulating particle size distribution and holding time
Introduction
Cubic boron nitride (cBN) is favored in the processing of automobiles, energy and electronics due to its high hardness and thermodynamic stability, especially its chemical inertness with ferrous elements [[1], [2], [3], [4]]. For a long time, researchers interested in cBN composites are mainly focused on optimizing the sintering additives, sintering parameters and particle size [[5], [6], [7]]. The properties of cBN composites are directly related to the cBN grain size. The larger the grain size, the better the wear resistance of cBN tools, and the weaker the impact resistance, the worse the cutting edge retention of cBN tools. A small grain size shows the opposite result [8,9]. The performance of composite has a greater relationship with the powder's bulk density. So far, there are few studies on the packing density in the preparation of cBN composites. When the packing density is low, the crack propagation path is relatively straight with less deflection. The crack propagation path with high packing density is tortuous and has obvious crack deflection and micro-crack generation [10]. Therefore, an increase in packing density can effectively induce crack deflection and improve the composite performance [[11], [12], [13]]. The properties of cBN composite with two or more grain sizes are better than that of tools prepared with single particle size, and the performance of cBN cutting tools with a wide grain size distribution range in the mixed particle size is better than that of tools with a narrow grain size distribution range [14]. To achieve the closest packing configuration, the size and proportion of cBN grains need to meet certain requirements. Several common types of dense packing powders are the Horsfield model [15], the Anderasen equation and the Dinger-Funk equation [16]. In the actual powders, the particle size is continuously distributed, that is, grains with various sizes may exist. The Anderasen equation only specifies the maximum value, while the Dinger-Funk equation can express the best ratio of grains size in a limited area.
Holding time is an important sintering parameter in high-temperature and high–pressure technology and directly affects the properties of the obtained specimens. cBN composite with a short holding time presents a poor bonding state and the limited homogenization of grains. While too long holding time would cause particle abnormal growth and decreased properties, which increase energy consumption and do not conducive to environmental protection [[17], [18], [19]].
Considering the particle size gradation and sintering parameter effect, this article selected four cBN particle size gradations assuming cBN grains were spherical and applying Gaussian fitting based on the Dinger-Funk equation. The main goals of this work are to study the structural and mechanical properties of cBN composites via choosing the appropriate particle size distribution and further optimizing the holding time. Therefore, composition, mechanical property and microstructures of as-prepared specimens sintered under HPHT were discussed.
Section snippets
Sample preparation
Firstly, cBN compact was prepared using cBN powders and TiN/Al/Co as the starting material with a mass ratio of 77:23. Then, the mixtures were treated in a ball milled for 5 h with a speed of 400 rpm. The milling mentioned above was performed in a planetary ball (ball:ethanol: powder = 1.5:2:1). After the milled process, the obtained powders were pressed into a graphite mold and continuously dried at 380 °C in H2 for 24 h. Next, powders were subjected to high pressure of 5.5 GPa in a six-anvil
Determination of cBN particle size gradation
Assuming that cBN grains were spherical, the densest packing cBN powders were obtained by adjusting the R-require value and keeping the curves smooth based on Gaussian fitting. Take an appropriate interval to fit it according to the Dinger-Funk equation (Eq. (1)), the specific method was as follows:
- (1)
Since the objective function (Eq. (1)) was the cumulative particle size distribution, the probability density distribution could be obtained by deriving this function. The ideal particle size
Conclusions
This paper proposed a high-temperature and high-pressure technology for preparing cBN composites and carried out a further in-depth study on the relationship of the properties of cBN composites and cBN particle size distribution and holding time.
- (1)
According to the dense packing principle, four particle size gradations of cBN were selected based on Gaussian Fitting. In particle size gradation of G1, the synergistic stacking effect of large, middle, and small-size cBN grains reduced the appearance
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgment
In this work, we acknowledge the contribution of Professor Xiangfa Zhang and Xing Wei from Zhengzhou Zhongnan Jete Superabrassive.
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