Ultra-Hard Cutting Tool Materials and Their Selection Methods

With the advancement of modern science and technology, a growing number of high-hardness engineering materials are being adopted. However, traditional turning techniques often fall short—or simply cannot handle—the machining of certain ultra-hard materials. Coated carbides, ceramics, PCBN, and other superhard tool materials, thanks to their exceptional properties such as high-temperature hardness, wear resistance, and thermochemical stability, provide the most fundamental prerequisites for cutting and machining these challenging materials, leading to significant efficiency gains in production.

Ultra-Hard Cutting Tools and Their Selection

The materials used in superhard cutting tools, along with their tool structures and geometric parameters, are fundamental elements for achieving hard turning. Therefore, selecting the right superhard tool material and designing an optimal tool structure and set of geometric parameters are critical to consistently enabling stable hard-turning operations.

1. Ultra-Hard Cutting Tool Materials and Their Selection

Coated Cemented Carbide

On tough, high-performance cemented carbide tools, a single or multiple layers of wear-resistant coatings such as TiN, TiCN, TiAlN, and Al3O2 are applied, with coating thicknesses ranging from 2 to 18 μm. These coatings typically exhibit significantly lower thermal conductivity compared to both the tool substrate and the workpiece material, thereby reducing the thermal impact on the tool substrate. At the same time, they effectively enhance friction and adhesion characteristics during the cutting process, helping to minimize the generation of cutting heat.

Coatings can be classified into two types based on their formation methods: Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). PVD coatings (2–6 μm thick) primarily include materials such as TiN, TiCN, and TiAlN, with their compositions continually expanding—recently, for instance, TiZrN has been added. The maximum compressive stresses of TiN and TiC coatings reach 3580 MPa and 3775 MPa, respectively. Unfortunately, reliable elastic modulus data for TiAlN coatings are currently unavailable, making it difficult to determine an accurate compressive stress value; however, high-speed cutting tests have shown that TiAlN delivers the best overall performance. Figure 1 illustrates how the hardness of these three coatings changes with temperature, revealing that hardness peaks at room temperature. Above a certain temperature [Y], TiAlN coating hardness surpasses both TiCN and TiN coatings. Figure 2 compares tool life under two different cutting speeds—v1 = 193.5 m/min and v2 = 380 m/min—when machining the nickel-based superalloy Inconel 178. The results clearly demonstrate that tools equipped with TiCN and TiAlN coatings exhibit significantly superior cutting performance compared to those coated with TiN.

Although PVD coatings have demonstrated numerous advantages, certain coatings—such as Al2O3 and diamond—tend to rely on CVD coating technology instead. Al2O3 is a highly heat-resistant and oxidation-proof coating that effectively insulates the tool body from the intense heat generated during cutting. Moreover, CVD coating techniques allow for the seamless integration of multiple coating benefits, enabling optimal cutting performance and meeting the demands of machining processes. For instance, TiN boasts low friction properties, reducing wear on the coating structure; TiCN helps minimize flank wear; TiC offers exceptional hardness; and Al2O3 provides outstanding thermal insulation—all contributing to superior cutting efficiency.

Coated carbide tools have significantly improved in terms of strength, hardness, and wear resistance compared to uncoated carbide tools. When turning workpieces with a hardness ranging from HRC45 to 55, cost-effective coated carbide inserts can enable high-speed machining. In recent years, several manufacturers have enhanced tool performance dramatically by adopting advanced coating materials and other innovative techniques. For instance, companies in the U.S. and Japan are leveraging Swiss AlTiN coating materials along with cutting-edge proprietary coating technologies to produce inserts boasting exceptional hardness levels—up to HV4500–4900. These cutting-edge tools can efficiently machine mold steels with a hardness of HRC47–58 at speeds as high as 498.56 m/min. Remarkably, even under extreme cutting temperatures reaching 1500–1600°C, these inserts maintain their hardness without oxidation, delivering a tool life that’s four times longer than conventional coated blades—yet at just 30% of the cost. Additionally, they exhibit excellent adhesion properties.

Ceramic materials

As ceramic tool materials continue to evolve through advancements in their compositional structures and pressing techniques—particularly with the progress in nanotechnology—the toughening of ceramics has become a reality. In the near future, ceramics may spark the third revolution in cutting technology, following high-speed steel and cemented carbides. Ceramic tools boast exceptional properties, including ultra-high hardness (HRA 91–95), impressive strength (flexural strength ranging from 750 to 1000 MPa), excellent wear resistance, outstanding chemical stability, superior anti-stick performance, low friction coefficients, and cost-effectiveness. Moreover, these ceramic tools maintain remarkable thermal stability, retaining a hardness of HRA 80 even at temperatures as high as 1200°C.

Under normal cutting conditions, ceramic tools exhibit exceptionally high durability, allowing cutting speeds to be 2 to 5 times faster than those achievable with cemented carbide. They are particularly well-suited for machining high-hardness materials, precision finishing, and high-speed operations—capable of cutting a wide range of hardened steels and hardened cast irons with hardness levels up to HRC65. Commonly used types include alumina-based ceramics, silicon nitride-based ceramics, metal ceramics, and whisker-reinforced ceramics.

Alumina-based ceramic tools exhibit higher red hardness compared to cemented carbides, meaning the cutting edges typically remain free from plastic deformation even under high-speed cutting conditions. However, they suffer from very low strength and toughness. To enhance their toughness and improve impact resistance, ZrO₂ or a mixture of TiC and TiN is often added. Alternatively, pure metals or silicon carbide whiskers can be incorporated into the material. On the other hand, silicon nitride-based ceramics not only boast exceptional red hardness but also demonstrate superior toughness. Compared to alumina-based ceramics, silicon nitride tools have a notable drawback: they tend to promote high-temperature diffusion when machining steel, which accelerates tool wear. As a result, silicon nitride ceramics are primarily suited for intermittent turning of gray cast iron and milling operations on gray cast iron materials.

Cemented carbide is a material whose matrix is based on carbides, with TiC serving as the primary hard phase (0.5–2 μm in size). These materials are bonded together using Co or Ti binders, making them similar to cemented carbides in terms of tool performance—but they exhibit lower affinity, superior friction properties, and enhanced wear resistance. While cemented carbide can withstand higher cutting temperatures, metal ceramics lack the impact resistance, toughness required for heavy-duty cutting, and strength at low speeds with large feeds that are characteristic of conventional cemented carbides. In recent years, thanks to extensive research, continuous improvements, and the adoption of innovative manufacturing techniques, both the flexural strength and toughness of metal ceramics have significantly improved. For instance, Mitsubishi Metal Corporation in Japan has developed the new metal ceramic grade NX2525, while Sandvik of Sweden has introduced its advanced CT series of metal ceramic inserts—as well as a coated metal ceramic blade lineup—featuring grain structures with diameters as fine as less than 1 μm. These cutting-edge materials now boast flexural strengths and wear resistances far surpassing those of standard metal ceramics, dramatically expanding their range of applications.

Cubic Boron Nitride (CBN)

The hardness and wear resistance of CBN

CBN-high-content composite polycrystalline cubic boron nitride (PCBN) tools exhibit high hardness, excellent wear resistance, superior compressive strength, and strong impact toughness. However, their drawbacks include poor thermal stability and low chemical inertness, making them suitable primarily for machining heat-resistant alloys, cast iron, and iron-based sintered metals. On the other hand, PCBN tools with lower CBN particle content—where ceramics are used as the binder—offer reduced hardness but compensate for the weaker thermal stability and chemical inertness of the former material, making them ideal for cutting quenched steel.

CBN-high-content composite polycrystalline cubic boron nitride (PCBN) tools exhibit high hardness, excellent wear resistance, superior compressive strength, and strong impact toughness. However, their drawbacks include poor thermal stability and low chemical inertness, making them suitable primarily for machining heat-resistant alloys, cast iron, and iron-based sintered metals. On the other hand, PCBN tools with lower CBN particle content—where ceramics are used as the binder—offer reduced hardness but compensate for the weaker thermal stability and chemical inertness of the former material, making them ideal for cutting淬硬钢.

When machining gray cast iron and quenched hardened steel, ceramic tools or CBN tools can be selected. To make the best choice, a cost-benefit analysis and an evaluation of machining quality should be conducted. Figure 3 illustrates the flank wear patterns on Al2O3, Si3N4, and CBN tools after machining gray cast iron. As shown in the figure, PCBN tool material outperforms both Al2O3 and Si3N4 in terms of cutting performance. However, when dry-machining hardened steel, the cost of Al2O3 ceramics remains lower than that of PCBN materials. Ceramic tools exhibit excellent thermochemical stability, though they lag behind PCBN tools in terms of toughness and hardness. Therefore, ceramic tools are ideally suited for cutting materials with hardness below HRC 60 and when using small feed rates. On the other hand, PCBN tools are more appropriate for machining workpieces with hardness exceeding HRC 60, particularly in automated and high-precision applications. Additionally, under identical flank wear conditions, workpieces machined with PCBN tools tend to maintain more stable residual stresses on their surfaces compared to those cut with ceramic tools, as depicted in Figure 4.

When dry-machining quenched steel with PCBN tools, the following principles should also be followed: Within the limits of machine tool rigidity, select the largest possible cutting depth whenever feasible. This approach generates enough heat in the cutting zone to locally soften the metal ahead of the cutting edge, effectively reducing wear on the PCBN tool. Additionally, at smaller cutting depths, it’s important to consider that PCBN tools have poor thermal conductivity, causing heat generated in the cutting zone to accumulate rather than dissipate quickly. As a result, the shear zone can experience significant localized softening of the metal, further minimizing wear on the cutting edge.

2. Blade Structure and Geometric Parameters of Superhard Cutting Tools

Properly determining the blade shape and its geometric parameters is crucial for maximizing the cutting performance of the tool. In terms of tool strength, the sharpness of various blade shapes—from highest to lowest—ranks as follows: circular, 100° diamond, square, 80° diamond, triangular, 55° diamond, and 35° diamond. Once the blade material has been selected, it’s essential to choose the blade shape that offers the highest possible strength. For hard turning applications, blades with the largest possible tip radius are also recommended; use circular blades or those with a large tip radius for roughing operations, while maintaining a tip radius of approximately 0.8 μm during finishing processes.

Hardened steel chips appear as reddish, brittle, and crumbly ribbon-like formations—highly fragile, prone to breaking easily, and non-adhesive. The surface finish achieved during hardened steel cutting is typically excellent, with built-up edges rarely forming. However, the cutting forces involved tend to be significant, particularly the radial cutting force, which often exceeds the main cutting force. Therefore, it’s advisable to use tools with a negative rake angle (go ≥ -5°) and a relatively large relief angle (ao = 10°–15°). The principal cutting edge angle should be selected based on the rigidity of the machine tool, usually ranging from 45° to 60°, to minimize chatter between the workpiece and the tool.