The research team, led by Guangpei Lin of Wuhan University of Science and Technology together with colleagues at the Guangdong Academy of Sciences, Jinan University and City University of Hong Kong, used laser direct energy deposition, or LDED, to fabricate composite coatings in which varying amounts of diamond powder were blended into FeCoCrNi high-entropy alloy feedstock. LDED is an additive manufacturing technique in which a focused laser beam melts powder as it is fed through a nozzle, building up dense metallic layers layer by layer. Because FeCoCrNi is a canonical high-entropy alloy, containing near-equal atomic fractions of iron, cobalt, chromium and nickel, it offers an unusually robust and ductile matrix in which to embed brittle reinforcement particles.

Embedding diamond in metal is notoriously difficult. Diamond is the hardest known bulk material, giving it enormous potential as a wear-resistant reinforcement, but it is also thermodynamically unstable at the temperatures reached in a laser melt pool. At high temperature and in contact with certain molten metals, diamond can graphitize, converting from its prized cubic crystal structure into soft graphite, which destroys its load-bearing capacity. Worse, many metals do not wet diamond well, meaning the molten alloy fails to bond to the particle surface and leaves gaps that act as ready-made crack starters under mechanical load.

The new study reveals that the FeCoCrNi system overcomes both obstacles through a fortunate combination of physics and chemistry. During deposition, the diamond particles, being far less dense than the surrounding molten alloy, experience buoyancy forces. At the same time, steep temperature gradients across the melt pool drive Marangoni convection, a circulating flow generated by surface tension differences that stirs the liquid metal. Acting together, these forces preferentially transport the diamond particles upward, enriching them at the coating surface. This is a significant advantage: the very region of the coating that experiences the most severe sliding contact and abrasion in service is precisely the region that ends up with the highest concentration of the superhard phase.

Wettability, the ability of the melt to spread over and adhere to the diamond surface, proved equally important. The researchers found that the compositional compatibility between cobalt-rich coating material and the FeCoCrNi substrate improved wetting of the diamond by the melt, allowing the liquid alloy to embrace the particles closely rather than leaving deleterious voids around them. Good wetting is a prerequisite for strong interfaces in any metal-matrix composite, because load applied to the coating must transfer efficiently from the ductile alloy into the stiff, hard particles for those particles to shield the surface from wear.

The decisive chemical trick, however, involves chromium. As the melt pool solidifies, chromium atoms from the alloy react in situ with carbon atoms at the diamond surface, forming a thin chromium carbide layer at the particle-matrix interface. This carbide layer performs two critical functions simultaneously. First, it acts as a diffusion barrier and chemical buffer that suppresses the graphitization damage that would otherwise degrade the diamond during the thermal cycle. Second, it creates a strong, adherent bridge between particle and matrix, enhancing the interfacial bond strength so that the diamond can fully exploit its load-bearing reinforcement role. The diamonds also promote the formation of additional carbides in their vicinity, further raising the hardness of the surrounding matrix.

The team systematically varied the diamond content and found a clear optimum. At 20 percent diamond, the coating exhibited the best combination of microstructure, interfacial bonding and tribological performance. In wear testing, this optimal coating reduced wear loss by 16.2 percent and wear volume by 40.5 percent relative to a pure FeCoCrNi coating produced under the same conditions. Detailed examination of worn surfaces showed that the diamond-rich surface layer shields the underlying material, shifting the dominant wear mechanism to relatively benign abrasive wear. Under impact loads and compressive stresses, some diamond particles do flake out of the surface, leaving small pits, but the overall damage remains far milder than in the unreinforced alloy.

Just as instructive is what happens when the diamond content departs from the optimum. Excessive diamond loading disrupts the continuity of the metallic matrix and weakens interfacial bonding, so particles detach early during sliding. Once freed, these detached particles roll between the coating and the counterface as third-body abrasives, gouging the surface and accelerating material removal, a self-defeating outcome that the moderate, 20 percent formulation avoids. Moderate loading keeps the matrix continuous, maintains stable bonding and prevents the early particle detachment that would seed third-body abrasion. The result is a coating in which each diamond particle remains anchored, load-bearing and protective throughout its service life.

The implications extend across industries in which surface wear dictates component lifetimes: mining tools, drilling and cutting equipment, forming dies, pumps and aerospace actuators all depend on hard coatings, and laser direct energy deposition is already attractive for repairing and resurfacing expensive parts in place. By showing that a high-content diamond reinforcement can survive additive manufacturing and deliver measurable tribological gains in a ductile high-entropy alloy, the study offers a practical recipe for next-generation protective coatings. The work also contributes fundamental insight into how buoyancy and Marangoni convection can be harnessed, rather than merely tolerated, to position reinforcement particles where they are most useful, and how a single reactive alloying element, chromium, can be recruited to protect a fragile superhard phase from thermal destruction during processing.

The research was supported by the National Natural Science Foundation of China, the Guangdong Provincial Key R&D Program, the Advanced Materials National Science and Technology Major Project, the Guangdong Basic and Applied Basic Research Foundation, the Guangdong-Hong Kong Joint Laboratory of Modern Surface Engineering Technology and several provincial and municipal programs. Correspondence for the study is handled by Zhaobing Cai of Wuhan University of Science and Technology and Bingwen Lu of the Guangdong Academy of Sciences. As additive manufacturing continues to mature from prototyping into production of demanding engineering components, strategies that unite process physics with interface chemistry, as demonstrated here, are likely to define the next wave of wear-resistant surface engineering.

Subject of Research: Diamond-reinforced FeCoCrNi high-entropy alloy coatings fabricated by laser direct energy deposition

Article Title: High-content superhard diamond enhances hardness and wear resistance in LDED FeCoCrNi high-entropy alloy

Article References: Lin, G., Cai, Z., Gu, L., Dong, Z., Feng, L., Huang, X., Dai, S., Zhang, P., Yan, X., & Lu, B. (2026). High-content superhard diamond enhances hardness and wear resistance in LDED FeCoCrNi high-entropy alloy. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13720-w

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13720-w

Keywords: high-entropy alloy, diamond composite, laser direct energy deposition, wear resistance, hardness, chromium carbide, Marangoni convection, wettability, abrasive wear, FeCoCrNi, coatings, tribology

The distinction between hardness and toughness is central to the advance. Hardness describes a material’s resistance to permanent deformation, such as indentation, scratching or wear. Fracture toughness, by contrast, measures how effectively a material resists the growth of an existing crack. These properties are often difficult to maximize simultaneously because structures that block deformation can also leave a material unable to absorb the energy released at a crack tip. In diamond, the rigid three-dimensional network of carbon atoms joined predominantly by sp³ bonds produces extraordinary stiffness and hardness. But that same rigidity offers limited opportunity for the material to dissipate mechanical energy when a crack begins to propagate. Conventional approaches to improving diamond toughness have therefore focused on intrinsic modifications, including changing the stacking sequence of atomic planes, introducing faults, creating nanotwins or adding amorphous regions. The new work takes an extrinsic approach: rather than relying exclusively on defects or redesigned diamond grains, it inserts a separate, continuous toughening network into the spaces between them.

The reinforcing phase consists of multi-walled carbon nanotubes, or MWCNTs. These are cylindrical carbon structures made from multiple concentric graphene-like shells. Within each shell, carbon atoms are connected through sp² bonding, the bonding arrangement associated with graphite and graphene. The nanotubes are therefore chemically and structurally different from the diamond grains, whose carbon atoms are connected through sp³ bonds. In the reported composite, highly dispersed MWCNTs occupy the gaps between diamond grains and form a continuous network extending through the material in three dimensions. This geometry is important: isolated nanotubes or randomly distributed carbon inclusions would not necessarily provide a reliable path for transferring stress or stopping cracks. A connected network can interact with cracks wherever they travel, while the surrounding diamond framework maintains the load-bearing structure responsible for high hardness. The result is a heterogeneous material in which the diamond and nanotube phases are not simply mixed, but integrated through a vast population of engineered interfaces.

The researchers attribute the unusual performance to atomic continuity across those interfaces. At the boundaries between the MWCNT network and diamond, carbon atoms form mixed sp²–sp³-hybridized bonding interactions. These bonds create a more coherent transition between the relatively flexible nanotube phase and the rigid diamond phase than would be possible with a weak or abrupt boundary. When a crack reaches such an interface, its energy can be redirected into several processes rather than being concentrated entirely in the diamond lattice. The interface may promote crack deflection, alter the crack path, and distribute stresses over a larger volume. The nanotube network can also deform, stretch or otherwise absorb mechanical energy in ways that a continuous diamond crystal cannot. Together, these effects reduce the driving force at the crack tip. In fracture mechanics terms, the composite increases the amount of energy required for a crack to advance, raising the measured critical stress-intensity factor.

The architecture also addresses a problem that has limited earlier attempts to toughen diamond with carbon additives. Introducing a softer or less rigid phase can improve resistance to fracture, but it may reduce hardness if the reinforcing material interrupts the diamond skeleton. The reported composite avoids that outcome by preserving a three-dimensional diamond framework with robust carbon–carbon, or D–D, bonding. The diamond grains remain connected into a mechanically continuous structure rather than being separated by thick layers of nanotubes. MWCNTs are concentrated in the intergranular gaps, where they can perform their toughening role without displacing enough diamond to compromise the dominant load-bearing network. This balance appears to be responsible for the combination of high hardness and high toughness. The measured hardness of about 91.6 GPa is lower than the idealized hardness often associated with perfect diamond, but it remains in the range expected for an exceptionally hard engineering material while delivering a substantial gain in damage tolerance.

To evaluate resistance to cracking, the researchers used a single-edge notched beam approach. In this type of fracture test, a carefully prepared notch acts as a controlled starting point for a crack. The specimen is loaded in bending, and the force required to extend the crack is used to calculate fracture toughness. The method is particularly useful for comparing materials that may be extremely hard but fail suddenly once a crack begins. The reported average value of 31.9 MPa m¹ᐟ² and the maximum of 36.4 MPa m¹ᐟ² indicate that the composite can withstand considerably greater crack-driving forces than ordinary single-crystal diamond under the stated testing conditions. The researchers describe the improvement as approximately fivefold relative to single-crystal diamond and report that the composite’s toughness surpasses that of tungsten alloys. Such comparisons depend on specimen geometry, notch quality, loading direction and measurement protocol, but the magnitude of the reported increase highlights the potential significance of the interface design.

The work illustrates why interfaces are becoming a major focus in materials science. In many advanced solids, the boundary between two phases is not merely a structural imperfection; it can be designed as an active mechanical component. At the MWCNT–diamond boundary, the transition between sp² and sp³ bonding creates a region with a different balance of stiffness, strength and deformability from either material alone. Under stress, this interfacial zone can mediate the transfer of force from the diamond framework to the nanotubes. Efficient load transfer prevents the nanotubes from pulling out without resistance, while the nanotubes’ ability to accommodate strain helps prevent the diamond grains from carrying the entire burden of crack-tip stresses. The three-dimensional continuity of the network is equally important because it avoids the localized behavior of isolated reinforcements. A crack encountering one nanotube can still interact with connected nanotubes elsewhere, creating a distributed energy-dissipation system throughout the composite.

The findings could be relevant to technologies that require both extreme wear resistance and improved tolerance of impact or defect-driven failure. Diamond-based materials are already used in cutting, drilling, grinding, polishing and other applications in which hardness and thermal or chemical stability are valuable. In those settings, brittleness can shorten service life because small flaws generated during manufacturing or operation may grow under repeated loading. A tougher diamond composite could, in principle, maintain a sharp or wear-resistant working surface while reducing the likelihood of sudden chipping or fracture. Potential uses would depend on whether the material can be produced in sufficiently large, uniform forms and whether its properties remain stable under the temperatures, pressures and chemical environments encountered in service. The current study establishes a material concept and reports strong mechanical performance, but it does not by itself demonstrate industrial deployment or resolve every manufacturing challenge associated with nanotube dispersion and interface control.

The broader message is that the hardest materials need not be designed as flawless, single-phase crystals. By combining a rigid diamond scaffold with a continuous nanoscale network, the researchers have created a structure in which strength, hardness and fracture resistance arise from different but cooperating features. The diamond framework supplies the resistance to indentation, while the MWCNT network and its atomic-scale interfaces provide routes for dissipating energy when damage begins. That division of labor is the key to the composite’s reported performance. Rather than weakening diamond to make it tougher, the approach surrounds its most vulnerable regions with a carbon network capable of interacting with cracks and redistributing stress. If the architecture can be reliably scaled and its behavior validated under practical loading conditions, it could offer a new path toward damage-tolerant superhard materials. For now, the result demonstrates a striking principle of materials design: sometimes the best way to toughen an extraordinary crystal is to engineer what lies between its grains.

Subject of Research: Multi-walled carbon nanotube network-toughened diamond composite and atomic interface engineering

Article Title: Multi-walled carbon nanotube network-toughened diamond composite via atomic interface continuity

Article References: Zhang, J., Qiu, K., Xu, T. et al. “Multi-walled carbon nanotube network-toughened diamond composite via atomic interface continuity.” Nature Synthesis (2026). https://doi.org/10.1038/s44160-026-01113-5

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s44160-026-01113-5

Keywords: diamond composite, carbon nanotubes, fracture toughness, superhard materials, sp²–sp³ bonding, atomic interfaces, crack resistance, materials engineering