Due to its high hot hardness and thermal stability, cubic boron nitride (cBN) satisfies modern requirements for machining efficiency and environmental protection. Consequently, PCBN tool materials suitable for dry cutting have become a major research hotspot. The performance of products varies across different binder systems. Research has demonstrated that among the numerous factors affecting the performance of PCBN materials, bonding is one of the critical issues. This section provides a detailed analysis of these specific aspects.
Self-propagating high-temperature synthesis (SHS), also known as combustion synthesis, offers advantages such as simpler equipment, shorter reaction times, and lower energy consumption compared to conventional processes.
The ternary compound Ti₃AlC₂ is an important ceramic material that combines the excellent properties of both metals and ceramics, including low density, high melting point, good electrical and thermal conductivity, oxidation resistance, and thermal shock resistance, alongside excellent machinability. High-precision ceramic components can be fabricated through machining. Compared to traditional oxide ceramic binders, Ti₃AlC₂ contains the element Ti, which reacts with the boron and nitrogen on the cBN surface to form a transition layer. This significantly enhances the bonding force between the Ti₃AlC₂ matrix and the cBN grains.
Using the SHS method, this experiment utilized Ti, Al, TiC, graphite powder, and cBN as raw materials to prepare a titanium-aluminum-carbon-based binder cBN material, investigating the effects of the TiC source and Al content on the fabrication of the composite.
For the compacts in the Ti–TiC–Al system containing 25% cBN after SHS, XRD phase analysis indicated that the primary phases in the samples included TiC, Ti₃Al, TiAl, TiN, TiB₂, and unreacted cBN. When the Al content was low, the diffraction peak intensity of the ternary phase Ti₃AlC₂ was weak, indicating a small yield. As the Al content increased, the diffraction peak intensity of Ti₃AlC₂ strengthened significantly, showing that its synthesized volume grew accordingly. Furthermore, phases such as TiB₂, TiN, and AlN were detected in the XRD results, confirming that cBN chemically reacted with Ti and Al during sintering to form corresponding borides and nitrides.
For the Ti–Al–C system also containing 25% cBN subjected to SHS, XRD analysis showed that characteristic diffraction peaks of Ti₃AlC₂ were clearly detected in all specimens. Compared to the Ti–TiC–Al system, the Ti₃AlC₂ diffraction peaks in this system were more distinct, and their intensity continuously strengthened with increasing Al content. When the atomic ratio of Al reached 1.27, the relative content of Ti₃AlC₂ reached its maximum.
Analysis suggests that low-melting-point Al forms a liquid phase in the early stage of sintering, promoting the diffusion of Ti and C elements and increasing the contact area between particles, thereby accelerating the synthesis reaction of Ti₃AlC₂. Meanwhile, the exothermic reaction between Ti and Al releases a substantial amount of heat, and the resulting Ti–Al intermediate compounds further provide favorable conditions for the generation and crystal growth of Ti₃AlC₂.
Following the introduction of cBN into both systems, the B and N elements within its components interdiffuse with the surrounding Ti and Al at high temperatures. This in-situ forms a transition layer of borides and nitrides (such as TiB₂, TiN, and AlN) between the cBN particles and the matrix, which helps enhance interface bonding. However, when a larger grain size of cBN is used, the particles partially hinder the diffusion of other raw materials, leading to a reduced reaction rate and a slowdown in the overall sintering kinetics.
Based on thermodynamics and the non-stoichiometric defect structure theory of TiN, this study aims to explore the reaction mechanism between a TiN₀.₃/AlN composite material and Al. By tailoring the interfacial structural characteristics, the study seeks to improve the bonding strength and toughness of the TiN₀.₃/AlN composite, making it better suited as a binder for PCBN cutting tools used in dry cutting.
The interface structure revealed that the reaction zone includes a layered structure near the Al, where AlN is distributed within the TiN matrix phase. From AlN to TiN₀.₃, the Al composition in the matrix phase exhibits a gradient variation, which can be represented as: Al / TiN + TiN + Al / TiN₀.₃.
The elemental surface distribution at the AlN diffusion interface confirmed the composition of each reaction region.
Microscopic observation of the TiN₀.₃/AlN composite binders sintered under different high-temperature and high-pressure (HTHP) conditions revealed that: at lower sintering temperatures, the material exhibited a relatively uniform but insufficiently reacted microstructure. As the sintering temperature increased, the interfacial reaction intensified significantly, and the originally added AlN phase gradually decomposed, being replaced by newly formed AlN particles—these new AlN particles tended to arrange themselves in a ring pattern, encapsulating the TiN phase at the center to form an ordered core-shell structure. However, when the sintering temperature was raised further, this ordered structure was destroyed due to over-reaction or grain coarsening, resulting in a decline in interface clarity and a tendency toward an inhomogeneous microstructure.
The bonding phases of PCBN compacts include metals, ceramics, and cermets, such as Al, AlN, TiN, and Ti(C,N). Because Al has a low melting point, it can react with cBN after melting to form the ceramic phase AlN. Since AlN possesses high hardness, high thermal conductivity, and a thermal expansion coefficient close to that of cBN, it helps in fabricating high-performance PCBN compacts. Therefore, adding Al powder has become a focal point in manufacturing PCBN compacts.
Xu Hongliang et al. prepared composite compacts using cBN, Al powder, and cemented carbide as raw materials via HTHP sintering, analyzing their XRD, SEM, and mechanical properties. They found that:
1.Two crystalline phases appeared in the XRD patterns of the compacts, indicating that under HTHP conditions, molten Al reacts with cBN as follows: Al + BN → AlN + B, thereby forming AlN. The diffraction peak intensity of AlN strengthened with the increase in Al addition, showing a corresponding increase in Al content.
2.The cBN grains (appearing black) in the composite layer were uniformly distributed, indicating that the wet-mixing process achieved homogeneous blending.
3.Mechanical properties were closely related to the amount of Al added. As the Al content increased, the hardness first rose and then fell, indicating the existence of an optimal value.
To obtain high-strength, highly wear-resistant PCBN sintered bodies, the content of metallic and ceramic phases must be strictly controlled. Whether the formulation ratio is accurate and whether the chosen binders are uniformly mixed are critical factors in producing high-quality PCBN.
SEM microstructures of PCBN compacts prepared with different TiC/Al binder ratios showed that the cBN particles were uniformly distributed. The dark-gray, white, and light-gray regions each formed a networked, dense structure, with no local agglomeration of either the binder phase or the cBN grains.
The different colored regions corresponded to distinct phases: the white regions primarily contained Ti and C elements, dominated by TiC; the light-gray regions were dominated by the Al element, likely representing AlN or AlB₂; and the dark-gray regions represented cBN.
The most commonly used substrate material is cemented carbide, which is sintered from a mixture of WC/TiC/TaC particles and metallic binders (Fe, Co, Ni, etc.). The WC–Co system, using Co as the binder, is the most prevalent substrate material. Its wear resistance increases as the grain size decreases and the Co content drops, while its impact resistance enhances with larger grain sizes and higher Co content. The required performance of the cemented carbide substrate can be tailored by adjusting the WC grain size and Co content.
Currently, there are three primary types of PCBN binders used globally:
1.Metallic and Metal Alloy Binders: Metallic binders must satisfy the cutting performance needs of PCBN tools. The primary elements should be chosen from Fe, Ni, or Co, accounting for 40%–50% of the binder mass. Alloying elements should be selected from Cr, Mo, W, Ta, Y, Nb, Ti, Zr, V, Hf, or Al, accounting for 5%–60%. Trace elements can be selected from C, Mg, S, Si, Cu, P, B, N, or Sn.
2.Ceramic Binders: For instance, Element Six utilizes various ceramic-based binders, including carbides, nitrides, carbonitrides, and sintered carbonitrides, to synthesize PCBN.
3.Cermet (Hybrid) Binders: PCBN synthesized with pure metallic binders exhibits good toughness but tends to soften at high temperatures, resulting in poor hardness and wear resistance. Conversely, pure ceramic binders resolve high-temperature softening but suffer from low toughness, making them prone to chipping and reducing tool life. Hybrid cermet binders combine the strengths of both, compensating for their respective defects.
Research indicates that multimodal (mixed) grain sizes outperform single (monodisperse) grain sizes, and a wide grain size distribution yields better blending effects than a narrow one. This is because the cBN grain size directly affects the toughness of PCBN: larger grain sizes enhance mechanical wear resistance but lower fracturing resistance, making the tool cutting edge less sharp. Mixed grain sizes effectively optimize grain packing and increase bulk density, leading to more complete crystallization.
It is recommended to adopt a coarse-and-fine mixed grain method: coarse grains ranging from 10–300 μm and fine grains from 0–10 μm, with a coarse-to-fine ratio of approximately 3:1. Multiple grain sizes can also be blended. The grain size of the binder should ideally be controlled within 0.001–0.15 mm.
Thick PCBN layers are not only difficult to synthesize and expensive, but they are also prone to defects like cracks and internal residual stresses, leading to edge chipping during service. Double-layer or multi-layer structures can be implemented to achieve thicker PCBN dimensions. The interface between the substrate and the cBN layer can be a smooth plane or a textured surface with corrugated grooves.
The wear behavior of PCBN abrasive grains caused by fracturing during grinding is a key factor affecting machining quality.
Researchers observed the grinding process of PCBN grains under specified conditions using 3D scanning electron microscopy (3D-SEM) and conventional SEM. They applied image reconstruction methods based on fractal theory to analyze the post-grinding wear morphology of the abrasive grains.
The fractal dimension is a quantitative index used to characterize the complexity of geometric shapes. Generally, a larger fractal dimension indicates a more complex shape with richer details.
When attempting to evaluate the wear morphology changes of PCBN grains during grinding using the fractal dimension:
When the grains undergo macro-wear or large-scale fracturing, a flat plateau forms at the top, yielding a regular profile with fewer details and a smaller fractal dimension.
When the grains undergo micro-fracturing, the boundary profile becomes spatially complex and highly irregular, resulting in a larger fractal dimension.
Using various domestic and international grades of PCBN as samples, hardness and microstructural analyses were conducted via digital microhardness testers and SEM.
The results demonstrated that the microhardness of different PCBN grades varies significantly, primarily influenced by the cBN content, binder type, and grain size distribution. Samples with high cBN content (e.g., DBW85, BN700) exhibited significantly higher hardness than low-content grades (e.g., DBC50, BN250). Concurrently, a mixed grain structure helps increase densification, thereby elevating hardness values.
PCBN compacts are typically fabricated by directly sintering a mixture of cBN and binders onto a cemented carbide substrate under HTHP. Although the initial interface is flattened to approximate a plane, electron microscopy reveals distinct interdiffusion and material penetration between the cBN cutting layer and the cemented carbide substrate, showing a non-ideal planar morphology.
This penetration behavior is jointly influenced by synthesis temperature, pressure, holding time, contact area, and material composition, and it serves as the decisive factor for bonding strength. Because of the significant mismatch in thermal expansion coefficients and elastic moduli between the cBN layer and the cemented carbide, stress concentrations during service can easily cause cracking or delamination. Failures are mostly concentrated within the cBN layer, roughly 0.1 mm away from the interface.
Therefore, the key to improving bonding strength lies in promoting effective interpenetration between the two phases. While three types of cemented carbides can serve as substrates, grades featuring a high elastic modulus and strong chemical affinity with cBN are preferred.
Microstructural analysis was conducted after samples were finely polished on a polishing machine using W10 and W5 diamond abrasives. The results showed that the PCBN material possessed a uniform microstructure, with cBN particles densely distributed within the binder, showing no obvious voids or agglomeration, indicating an excellent fabrication process.
Additionally, the introduced Si₃N₄ whiskers exhibited a typical lath-like morphology. This structure helps impede crack propagation, effectively enhancing the overall strength and fracture toughness of the PCBN composite.
When the synthesis time and pressure are held constant, the hardness of the compact first increases with rising temperature and then tends to stabilize. Excessively high temperatures, however, can trigger delamination or the precipitation of metallic veins.
When the synthesis temperature and time are constant, wear resistance increases with rising pressure but levels off after reaching a certain value. Appropriately increasing the pressure drives thorough densification of the PCBN, reducing porosity and bringing the cBN particles into a tighter arrangement.
According to powder compaction theory, higher pressure leads to more pronounced pore shrinkage and a denser material. Increased densification not only bolsters wear resistance but also improves electrical conductivity due to a more continuous conductive network. This is highly beneficial for electrical discharge machining (EDM) wire-cutting operations.
PCBN suffers from relatively poor toughness and is prone to edge chipping during cutting, which limits its broader application.
Ti(C,N) combines the advantages of both TiC and TiN: high melting point, high hardness, excellent thermal conductivity, electrical conductivity, and chemical stability. In Ti(C,N)/Al₂O₃ composite ceramics, the two-phase particles interweave with each other, inhibiting grain growth and providing a toughening and reinforcing effect that enhances mechanical properties. In cermet tools, Ti(C,N) significantly improves flexural strength and fracture toughness, maintaining superb red hardness and a low friction coefficient during high-speed cutting, which yields an excellent surface finish.
Experiments revealed that Co can significantly lower the sintering energy consumption of high-cBN-content compacts and substantially reduce electrical resistance, making them suitable for EDM wire-cutting and improving machinability.
Since composition and microstructure dictate performance, studying the internal structure of PCBN is vital.
Observations showed that the detected Co content was higher than the added amount, and the element W—which was not included in the raw formulation—was also detected. This indicates that W and Co migrated and penetrated from the cemented carbide substrate into the pores of the cBN layer. The penetration volume of W and Co in high-cBN compacts was greater than in low-cBN samples, which may be tied to the synthesis parameters.
As a metallic binder, Co promotes PCBN sintering and forms solid solutions with cBN and other binders, thereby enhancing the sintering strength. Within a certain range, the flexural strength of PCBN increases with higher Co content. Consequently, high-cBN-content compacts exhibit higher strength than low-content ones due to greater Co penetration.
Based on their structure, PCBN materials can be divided into composite compacts with a cemented carbide substrate and solid (indexable) PCBN inserts without a substrate. Solid inserts can be directly used as cutting inserts after chamfering and edge grinding.
Solid inserts eliminate the risk of tip detachment caused by brazing failures in composite compacts. They offer multiple cutting edges, reducing the cost per edge, and exhibit superior mechanical performance and thermal conductivity. At the end of their service life, they can be reground or downgraded for reuse, offering significant practical value.
In application, the appropriate cBN grain size, content, and binder type must be selected based on the target workpiece:
Finer cBN grain sizes yield higher wear resistance; fine grains increase the grain boundary area, enhancing bonding strength and resistance to crack propagation.
Higher cBN content results in higher hardness and wear resistance.
Different binders cater to distinct application scenarios.
The principles for selecting cBN grain size are detailed in Section 1.5. Additionally, during assembly, the height of the "gas storage chamber" must be moderate. If it is too small, gases cannot escape efficiently, leading to pinholes and gas pockets on the PCBN surface. If it is too large, molten salts can easily infiltrate into the Mo cup during HTHP sintering, corroding the cemented carbide substrate and the cBN layer, which degrades performance. A moderate height ensures effective outgassing while preventing molten salt infiltration, guaranteeing a defect-free product appearance.
With the rapid development of the machinery manufacturing industry, difficult-to-machine materials are being widely adopted, posing severe challenges to cutting tools. Developing superhard cutting tools with high wear resistance, exceptional thermal stability, and strong impact resistance has become an inevitable trend. High-efficiency machining, hard machining, dry machining, ultra-precision machining, and the machining of advanced difficult-to-cut materials define the future direction of the tooling industry.
Owing to their unique advantages in processing high-hardness ferrous metals, cBN tools have become essential instruments for achieving precision and ultra-precision machining.
The hardness and thermal conductivity of cBN are second only to diamond, and its thermal stability is outstanding—it does not oxidize when heated up to 1000°C in the atmosphere and remains chemically inert toward ferrous metals. As a tool material, cBN combines high hardness, high stability, and chemical inertness. Its single-crystal cleavage tendency is drastically reduced, and continuous micro-wear during cutting exposes fresh cutting edges. It is widely used in high-speed cutting, hard cutting, dry cutting, and green manufacturing.
This section provides a thorough analysis of these specific application issues.
Green manufacturing emphasizes energy conservation, material savings, low pollution, and environmental protection. The most effective way to eliminate the adverse environmental impacts of cutting fluids is dry cutting. Compared to wet cutting, dry cutting can significantly boost production efficiency.
The premise of dry cutting is that the strength of the workpiece material drops significantly under high cutting temperatures. In many scenarios, it enables "turning instead of grinding" and "milling instead of grinding," achieving high precision and excellent surface quality.
High-Speed Cutting: Boosts efficiency, shortens processing time, and lowers costs.
Enhanced Manufacturing Flexibility: By altering the cutting edge geometry and feed paths, complex workpieces can be machined. The cutting efficiency is high, cycle times are short, and costs are low. Less cutting heat enters the workpiece, preventing surface burns or micro-cracks, which helps preserve the surface integrity of critical components. Hard cutting requires no coolant, avoiding environmental pollution.
To save energy and increase efficiency, an increasing number of manufacturers are using PCBN tools to achieve "turning instead of grinding" for hardened steel. However, during the machining of hardened steel, the tool cutting edge withstands high pressures, making it prone to micro-chipping and causing unstable tool life.
The current state-of-the-art method relies on dry machining with PCBN tools.
Studies show that the densification and durability of PCBN materials do not always increase symmetrically with higher grinding ratios and hardness. Therefore, it is inappropriate to use the grinding ratio and hardness as the sole metrics for evaluating PCBN cutting performance.
Hardened steel is widely used in machinery manufacturing. Due to its high hardness, high strength, poor thermal conductivity, high cutting temperatures, and tendency to cause edge chipping, it represents a classic difficult-to-machine material. It constitutes a massive share of machined materials and holds immense market potential. Low-cBN-content PCBN tools are typically selected for this type of machining.
To verify the impact resistance of PCBN, interrupted turning experiments were conducted on three types of tools: completing 3 passes (enduring over 40,000 interrupted impacts) was deemed a benchmark for good impact toughness.
Generally, the strength of PCBN decreases as the cBN content lowers. As the cutting distance extends, flank wear progressively increases, though the wear rate varies slightly across different stages.
Appropriately increasing the tool nose radius is beneficial for heat dissipation and minimizes tool wear. When the radius exceeds 0.8 mm, the wear rate tends to flatten out. A tool nose radius of R = 0.8–1.0 mm is recommended.
In continuous cutting (v = 80 m/min), both A and B inserts achieved a cutting distance of over 5500 m. Under lightly interrupted conditions, this dropped to about 1000 m, and under heavily interrupted conditions, it plummeted to just 250–450 m. This indicates that the interruption mode drastically impacts tool life.
In continuous cutting, the cutting force remains constant. In interrupted cutting, the cycle of engaging and disengaging the workpiece causes drastic, sudden fluctuations in both the magnitude and direction of cutting forces, making the tool highly vulnerable to catastrophic failure.
Under interrupted conditions, tool life shortens as the cutting speed increases:
At v = 120 m/min → cutting distance is approx. 1000 m;
At v = 150 m/min → 500–750 m;
At v = 180 m/min → 250–450 m.
The impact intensity of heavy interruption is roughly 4 times that of light interruption, and tool life drops precipitously as the interruption intensity escalates.
Rough milling the six main faces of an engine cylinder block is traditionally the least efficient and most expensive process, usually relying on coated cemented carbide face milling cutters.
To boost efficiency and cut costs, an engine plant in Southwest China adopted solid PCBN inserts independently developed by Zhengzhou Halnn Superhard materials Co., Ltd. They performed high-feed dry rough milling on the top face of alloy cast iron cylinder blocks, achieving highly satisfactory results.
The tool life of different PCBN grades varies markedly, dictated fundamentally by their properties: high-cBN-content tools (e.g., DBW85, BN700) possess high hardness and strong wear resistance. Co and Al binders improve toughness and thermal conductivity, keeping cutting forces small and temperatures low during high-speed cutting. This minimizes mechanical and adhesive wear, leading to an extended tool life.
1.When four types of tools were used for the interrupted cutting of Austempered Ductile Iron (ADI), the radial force (F_p) was the largest, followed by the principal cutting force (F_c), while the axial force (F_f) was the smallest. Since the three-directional forces exhibited similar trends with velocity variations, the radial force (F_p) was chosen as the primary subject of study.
2.Different PCBN tool compositions exhibited distinct cutting force characteristics:
Low-cBN-content tools (DBC50, BN250): Cutting force increased as speed increased, with DBC50 > BN250;
High-cBN-content tools (DBW85, BN700): Cutting force varied complexly with speed:
When v < 100 m/min, cutting force increased with speed.
As v continued to increase, the cutting force decreased.
When v > 150 m/min, the cutting force rose once again.
This indicates that the PCBN composition exerts a substantial influence on tool performance.
1.The difference between the peak impact force and the quasi-static force (defined as the "impact amplitude") truly reflects the magnitude of the shock at the moment of entry and exit. A larger difference increases the likelihood of edge chipping.
The experiment found that the impact force first increased and then decreased with speed. The reasons are as follows:
2.At lower speeds, ADI exhibits high hardness; increasing the rotational speed increases the total impulse, causing the impact force to rise.
At higher speeds, due to the poor thermal conductivity of ADI, the temperature in the tool-chip contact zone rises sharply, approaching the material's softening point. As the material softens, elastic pressure drops and the friction coefficient changes, causing the impact force to decrease instead.
3.Tool life discrepancies remain dictated by performance:
DBW85 and BN700: High hardness, high wear resistance, with Co/Al binders improving toughness and thermal conductivity. They yield low wear and long life under high speeds.
DBC50 and BN250: Utilizing TiC/TiN as binders, they resist high temperatures but offer poor impact resistance. They are prone to chipping during high-speed interrupted cutting, leading to rapid wear and shorter tool life.
Flank wear land width (VB) is universally used as the tool life criterion.
1.Effect of cBN Content on Tool Life: In the 0–1500 m cutting distance phase under either dry or wet cutting, the wear volumes of PCBN50, 60, and 70 tools did not differ significantly. However, the overarching trend indicated that under specified cutting conditions, lower cBN content yielded slightly less flank wear.
2.Effect of Cutting Mode on Tool Life: Taking PCBN50 as an example, dry cutting up to 3000 m resulted in VB = 0.29 mm, whereas wet cutting resulted in VB = 0.20 mm. Because cutting fluid provides lubrication and flushing actions, it reduces wear.
Microstructural analysis of the surface layer of dry-cut workpieces indicated that the cross-section can be divided into four distinct characteristic zones:
1.White layer;
2.Black layer (sub-surface layer) beneath the white layer;
3.A transition zone approximately 10 μm thick beneath the black layer;
4.The base matrix microstructure.
As tool wear intensifies, the thicknesses of both the white and black layers increase. In contrast, wet cutting—due to its cooling effect—maintains a lower cutting temperature; only the topmost surface layer is lightly affected, yielding a thin, blurred white layer with minimal contrast against the base matrix. This proves that coolant effectively suppresses white layer formation and reduces surface layer defects.
Cylinder liners are frequently centrifugally cast from boron cast iron, which requires a Type-A graphite distribution to achieve high wear resistance and mechanical properties, classifying it as a difficult-to-machine material. Machining graphite cast iron containing P and Cu presents severe challenges to tool life, surface roughness, and cutting speeds.
A negative rake angle has a pronounced effect on cutting temperatures and flank wear. Employing edge preparation (honing/chamfering) or specialized tool geometries can lower cutting temperatures and extend tool life.
Titanium alloys offer high specific strength, oxidation resistance, excellent heat resistance, and thermal stability. They are widely utilized in aerospace (with its structural weight share in aircraft rising from 5% to over 14%), spacecraft, marine vessels, automobiles, chemical processing, and medical sectors. However, their poor machinability limits wider adoption.
Their poor machinability is mainly manifested in:
1.High cutting temperatures;
2.Large cutting forces per unit area;
3.High chemical reactivity;
4.Low elastic modulus.
Dry cutting has evolved in tandem with advancements in high-temperature tool materials. It refers to a process that relies on specific tools and parameters to achieve ideal machining results without using any cutting fluids. Its underlying mechanism is that high-speed cutting generates concentrated heat that localizes in the cutting zone, softening the workpiece material (lowering its yield strength), thereby boosting machining efficiency.
PCBN possesses superb high-temperature hardness and thermal stability, enabling the use of aggressive cutting speeds. The resulting cutting heat softens the workpiece to facilitate cutting while ensuring acceptable tool life.
Cutting temperature is primarily governed by cutting speed, followed by feed rate (whose influence intensifies as feed increases); the depth of cut also exerts a substantial impact, which exacerbates as workpiece hardness increases.
PCBN tools can achieve excellent surface roughness when processing various materials, far outperforming conventional cemented carbide tools.
Tool wear directly affects machining efficiency, quality, and cost. Wear during PCBN dry cutting is the cumulative result of multiple concurrent mechanisms rather than being dominated by a single isolated mode.
Authors: Wang Guangzu, Wang Yun, Qin Yu