Preparation and Application Research of Polycrystalline Cubic Boron Nitride

Cubic boron nitride (cBN) was first synthesized by Wentorf in 1957 under high temperature and high-pressure conditions. It is an artificial crystal with hardness second only to diamond, but its thermal stability and chemical stability are superior to diamond. It is widely used for processing ferrous metals, high-temperature-resistant alloys, titanium alloys, and other materials that are difficult to process with diamond tools. Single crystal cubic boron nitride has a small size and anisotropy, with cleavage planes, limiting its application in key industrial fields such as mechanical and aerospace technologies. Therefore, scientists at home and abroad have paid great attention to developing polycrystalline cubic boron nitride.

1. Diversity of Preparation Methods

Polycrystalline cubic boron nitride (PcBN) is a cubic boron nitride bulk material with the advantages of large size, isotropy, and no cleavage plane. Currently, its primary manufacturing method is high-temperature and high-pressure sintering, in which boron nitride powder is used as the raw material, either with or without the addition of binders, and sintered together at high temperatures and pressures to form a composite, either a non-pure phase sintered body or a pure phase sintered body.

In non-pure phase sintered bodies, binders are added primarily to reduce the sintering conditions. Binders are mainly divided into three categories:

1. Metal or alloy binders, which have good mechanical wear resistance but can soften under high temperature and pressure, affecting wear resistance.

2. Ceramic binders, which exhibit good high-temperature hardness but have poor impact resistance and thermal conductivity, making them unsuitable for interrupted cutting.

3. Metal-ceramic binders, which can partially overcome the shortcomings of the above two types.

In general, non-pure phase sintered PcBN is moving toward higher wear resistance, toughness, and excellent performance. However, this method reduces hardness, falling short compared to PcBN obtained through pure-phase sintering. Pure-phase sintering mainly involves using different initial materials such as hexagonal boron nitride (hBN), pyrolytic boron nitride (pBN), onion-like boron nitride (oBN), and cBN, sintered at high temperatures and pressures without the use of binders.

Currently, there are two main types of pure-phase nano-cubic boron nitride:

- The first involves using hBN as the initial powder, converting it to polycrystalline cubic boron nitride under high temperature and high pressure through phase transformation.

- The second uses cBN as the initial powder, which is sintered at high temperature and pressure to form polycrystalline cubic boron nitride. SOLOZHENKO and others used low-crystallinity pBN as the initial material to prepare polycrystalline cubic boron nitride under conditions of 20GPa and 1500°C, achieving a Vickers hardness of 85GPa.

ICHIDA used pyrolytic boron nitride as the starting material and synthesized super-hard polycrystalline cubic boron nitride with grain sizes less than 100nm, achieving a Knoop hardness of up to 55.2GPa by heating it for 1-6 minutes under 25GPa and 1950°C.

TIAN used onion-like hexagonal boron nitride as the initial material and prepared nano-polycrystalline cubic boron nitride under 12-25GPa and 1600-2200°C, achieving a Vickers hardness of over 100GPa, thermal stability up to 1294°C, and a fracture toughness greater than 12MPa·m^1/2.

TAKASHI used high-purity hexagonal boron nitride as the raw material and adopted a high-pressure direct phase transformation process to obtain high-purity cBN sintered bodies and highly oriented wurtzite boron nitride (wBN) crystals. The cBN sintered body achieved a hardness of 61GPa, and the hardness of wBN was about 80%-90% of that of the cBN sintered body.

Mechanism analysis shows that during the phase transformation from hBN to cBN, a diffusionless phase transformation from hBN to wBN occurs. As the synthesis conditions increase, a stable phase transformation of wBN appears.

During the synthesis of polycrystalline cubic boron nitride using hBN as the initial material under high temperature and high pressure, phase transformations are accompanied by certain volume shrinkage, resulting in poor molding properties of the synthesized samples. To address this issue, researchers have studied the synthesis of polycrystalline cubic boron nitride bulk materials using cBN micropowder as the initial material.

Yongkai Wang, Xiangfa Zhang, and others synthesized polycrystalline cubic boron nitride using the direct conversion method at 15GPa and 1500-2100°C, with hBN as the initial material. They characterized the microstructure and mechanical properties of the polycrystalline cubic boron nitride bulk material using X-ray diffraction, scanning electron microscopy, and a Vickers hardness tester.

Their research concluded that under suitable temperature and pressure conditions, they successfully prepared pure-phase, semi-transparent, nano-polycrystalline cubic boron nitride bulk materials. The sample grains were composed of nanocrystalline and flake-like grains, with particle sizes ranging from 70-130nm, and flake-like grains measuring approximately 2μm. At the same pressure, the grain size of the polycrystalline cubic boron nitride bulk material increased with the synthesis temperature, and the hardness decreased as the synthesis temperature increased. The maximum hardness reached 64.43GPa, with a fracture toughness of 10.47MPa·m^1/2.

Nano-polycrystalline cubic boron nitride has high hardness and isotropy, meeting the demands for efficient and green processing, and enabling self-controlled development of a new generation of tool materials for the high-precision mechanical industry in the steel industry. Developing high-quality, large-sized bulk nano-polycrystalline cubic boron nitride materials through direct conversion will become a research hotspot.

2. Application Technical Fields

Due to cBN's hardness and thermal conductivity being only second to diamond, its excellent thermal stability, and the fact that it does not oxidize when heated to 1000°C in the atmosphere, as well as its extremely stable chemical inertness towards ferrous metals, it has garnered significant attention as a cutting tool material. cBN superhard cutting tool materials, owing to their high hardness, high thermal stability, and good chemical inertness, greatly reduce the tendency for single-crystal cleavage. As the cutting tool wears, new cutting edges are continuously exposed, making it one of the widely used cutting tool materials for high-speed cutting, hard machining, dry cutting, and green processing. 2.1 Green Low-Carbon Economy and PcBN Tools High production efficiency and high quality are the two primary goals pursued by advanced manufacturing technologies. High-speed cutting, which represents the mainstream direction of modern machining, has rapidly developed in line with the trends of high efficiency, high precision, flexibility, and greenness in 21st-century machining.

Characteristics of high-speed cutting:

1. Cutting speeds are increased, leading to an increased material removal rate per unit time. The cutting zone temperature is higher, which increases toughness, and the feed rate can also be correspondingly increased, thereby exponentially improving cutting efficiency, reducing energy consumption, and lowering costs.

2. When the cutting speed exceeds a certain critical value, more than 95% of the cutting heat is swiftly carried away by the chips, while the workpiece remains essentially cool.

3. In high-speed cutting, the vibration and the excitation frequency of the machine tool are very high, far exceeding the natural frequency range of the machine tool-tool-workpiece system, resulting in stable cutting, minimal vibration, and higher processing quality, thereby reducing machining processes. Requirements for tools in high-speed cutting:

1. High reliability;

2. High thermal, impact, and high-temperature mechanical properties;

3. The ability to adapt to the needs of difficult-to-machine materials and new processing methods.

The low-carbon economy and green manufacturing technology and applications are the future development directions of China’s machine tool industry. In accordance with the concepts of green manufacturing, energy conservation, resource savings, minimal pollution, and environmental protection, the most ideal and effective processing method to eliminate the negative effects of cutting fluids is dry cutting. Compared to wet cutting, dry cutting can significantly improve production efficiency. The mechanism is that at high cutting speeds, the heat generated is concentrated at the front of the tool, raising the temperature of the material near the cutting area to a red-hot state, reducing the yield strength, and thus improving cutting efficiency. The prerequisite for adopting dry cutting processes is that under relatively high cutting temperatures, the strength of the material being cut significantly decreases, making it easier to cut; at the same time, the tool material must have good red hardness, wear resistance, and adhesion under the same conditions. PcBN tools are very suitable for high-speed cutting of hard materials, maintaining high hardness even at cutting temperatures reaching 1000°C, allowing for long-duration processing of high-precision parts (with minimal dimensional variability). This greatly reduces the frequency of tool changes and the downtime required for tool wear compensation, making them suitable for CNC machine tools and highly automated processing equipment. In many instances of metal cutting, processes such as turning, boring, and milling can replace grinding, allowing machined parts to achieve high precision and good surface quality while significantly increasing production efficiency. For example, in the processing of surface sprayed (coated) materials, using tools made of other materials leads to extremely low tool life and makes grinding methods unfeasible, whereas PcBN is the only suitable tool material. Machining is advancing towards high-speed, composite, intelligent, and environmentally friendly directions. The development trend of green cutting technology indicates that high-speed cutting, high-stability processing, and hard machining align best with the characteristics of green processing. The organic combination of dry cutting and high-speed cutting will represent an ideal, efficient, low-consumption, high-quality processing method with minimal environmental pollution and comprehensive benefits, becoming the mainstream of future machining. As an important means of modern cutting processing, PcBN superhard tools possess unparalleled advantages in terms of processing precision, cutting efficiency, and tool life. They are widely used in advanced machining processes such as high-speed cutting, high-stability processing, and hard machining, and will become an important component of green cutting.

2.2 PcBN: The Preferred Tool Material for Difficult-to-Machine Materials

2.2.1 High-Speed and Ultra-High-Speed Cutting

High-speed cutting can improve cutting efficiency, reduce processing time, and lower production costs. The BN7000 produced by Sumitomo in Japan can achieve maximum speeds of up to 2000 m/min for machining gray cast iron, while Mitsubishi's MBC010 can reach speeds of 400 m/min for machining hardened steel, which is a level of speed unattainable by other tools such as carbide and ceramic tools.

2.2.2 Hard Cutting

PcBN is typically used to process materials with a hardness (HRC) greater than 50, achieving hard cutting as a substitute for grinding to complete the final machining of materials. This "turning instead of grinding" method allows for the machining of workpieces with various geometric shapes, resulting in high cutting efficiency and shorter processing times, thus reducing production costs. The cutting heat generated during the process is relatively low, minimizing the risk of burning and micro-cracking on the machined surface, and helps maintain the integrity of the surface properties of the workpiece. Hard cutting does not require the use of cooling fluids, thereby avoiding the environmental pollution caused by waste fluids generated during processing.

2.2.3 Dry Cutting

In wet cutting processes, various issues arise from the use of cutting fluids, such as the increased production costs due to the use, transportation, recycling, and filtration of cutting fluids; the health hazards posed by the mist generated from cutting heat; and the environmental pollution from leaks and spills, which can lead to safety and quality incidents. Dry cutting technology has been developed in response to the increasing global environmental requirements and sustainable development strategies. It is significant for saving resources, protecting the environment, and reducing costs. In recent years, dry cutting methods have become a focal research topic in the machinery manufacturing industry.

2.2.4 Automated Processing

PcBN has high hardness and wear resistance, enabling it to produce high-precision parts (with minimal dimensional variability) for extended periods under high-speed cutting conditions, significantly reducing the frequency of tool changes and the time spent on tool wear compensation. Therefore, it is very suitable for CNC machine tools and highly automated equipment, allowing the efficient performance of the equipment to be fully utilized.

2.2.5 Machining Difficult-to-Machine Materials

For difficult-to-machine materials like superalloys, stainless steel, and titanium alloys, other tool materials exhibit extremely low tool life. However, due to the excellent properties of PcBN, it demonstrates outstanding advantages. For instance, when machining high-temperature alloy wear-resistant cast iron used in oil power station equipment, PcBN tools achieve more than four times the cutting efficiency compared to carbide tools, with the cost per piece dropping to one-fifth of the original. Therefore, PcBN material has become the preferred tool material for machining these difficult-to-machine materials.

2.3 PcBN Dry Cutting of Hardened Steel

To reduce power consumption and improve production efficiency, more manufacturers in the machinery manufacturing industry are utilizing PcBN tools to implement processes that substitute turning for grinding and milling for grinding, especially in the machining of hardened steel. During the machining of hardened steel, the cutting edge of the tool must withstand significant cutting pressures, leading to micro-chipping and failure of the tool, which results in unstable tool life. Characteristics of machining hardened steel include:

1. High hardness and strength, with hardness reaching over HRC 50 and strength up to 2600 MPa.

2. When turning hardened steel, the cutting edge must endure substantial impact forces, making it prone to cutting vibrations during the machining process.

3. Hardened steel has a low thermal conductivity, making it difficult for cutting heat to be dissipated by the chips, resulting in heat concentration at the cutting tip of the tool.

4. The long chips produced from cutting hardened steel can easily wrap around and scratch the surface of the workpiece; thus, the tool must feature chip-breaking grooves. These characteristics determine that hardened steel is classified as a difficult-to-machine material. Currently, the most advanced method is to use PcBN tools for dry machining. Welded PcBN tools are prone to losing their tips due to the high cutting temperatures and cutting forces, leading to layer chipping and micro-chipping that can cause failure. Solid PcBN tools can also experience micro-chipping and wear; worn tools exert greater cutting forces during machining, exacerbating wear and creating a vicious cycle. Manufacturing uniformly wear-resistant PcBN tools with minimal wear is key to machining hardened steel. The grain size of cBN is crucial, employing a mixture of fine particles in the range of 1-5 µm, which is one of the critical factors for producing uniformly wear-resistant PcBN tools.

2.4 Cutting Performance of PcBN Tools in Turning Hard Ni-Based Superalloys

There has been relatively little systematic research on the factors affecting the cutting performance of PcBN tools when machining Ni-based superalloys. To promote their application in this field, Li Tingke and others conducted targeted studies on the impact of tool geometric parameters, PcBN materials, cutting quantities, and cutting processes on cutting performance.

2.4.1 Influence of Tool Geometric Parameters on Wear

Using a single-factor approach, the influence of the tool tip radius (0.4 mm, 0.6 mm, 0.8 mm, 1.0 mm) and negative chamfer parameters (--20 degrees x 0.1 mm, --20 degrees x 0.2 mm, --28 degrees x 0.1 mm) on tool wear (measured after 20 seconds of cutting by assessing wear on the tool face) was studied. The results are shown in Figures 1 and 2.

Figure 1: Effect of Tool Tip Radius on Wear

As shown in Figure 1, appropriately increasing the tool tip radius is beneficial for heat dissipation, which can reduce tool wear. When the radius exceeds 0.8 mm, the wear rate tends to stabilize, suggesting that a tool tip radius of 0.8-1.0 mm is advisable.

Figure 2: Effect of Negative Chamfer Parameters on Wear

From Figure 2, it can be concluded that a smaller chamfer width of 0.1 mm and a larger chamfer angle of 28 degrees should be selected during cutting. The main role of the chamfer is to enhance the cutting edge and reduce tool wear. If the chamfer width is too large, the chips may flow out along the chamfer, causing the negative chamfer to act like a negative rake angle on the front face of the tool, leading to increased cutting forces and difficulties in cutting. Appropriately increasing the chamfer angle can improve the heat dissipation conditions.

2.4.2 Effect of PcBN Material and Cutting Parameters on Wear

Using three different grades of PcBN tools, wear on the back face of the tools was measured under different cutting speeds after 20 seconds of cutting, with the results shown in Figure 3.

Figure 3: Comparison of Wear at Different Cutting Speeds

From Figure 3, it is visually evident that under the same conditions, the wear amount for DBW83 is the smallest, followed by BZN6000, while BIN100 exhibits the highest wear. The experiment found that both BZN6000 and BIN100 experienced micro-chipping and grooving at cutting speeds greater than 56 m/min, while DBW85 showed uniform wear.

2.4.3 Effect of Cutting Process on Wear

In the machining of nickel-based superalloys, due to the high cutting temperatures, the experiment employed both wet and dry cutting methods. The cutting performance of PcBN tools was studied (measuring wear on the back face after 20 seconds of cutting), and the results are shown in Figure 4.

Figure 4: Comparison of Wear in Dry Cutting Tools

From Figure 4, it can be observed that PcBN tools exhibit less wear in dry cutting compared to wet cutting, with a reduction of 40%-50%. This is attributed to the fact that cutting fluids can carry away some of the heat, thereby lowering the cutting temperature. Figure 4 also indicates that, whether in dry or wet conditions, the wear of DBW85 is consistently lower than that of BZN6000.