The nano-diamond powder used in this work was purchased from Henan Zhenzuan Abrasive Tools Co., Ltd. (Zhengzhou, Henan, China). The information on particles was shown in Tables 1,2.

Considering the comprehensive balance of cost and quality, this study adopts nano-diamond particle material with an average particle size of 30 nm for thermal diffusion experiments. Its surface morphology was observed using a scanning electron microscope (SEM) ZEISS Sigma300 SEM (Oberkochen, Germany) as shown in Fig. 1.

Fig. 1.

Microscopic morphology diagram of nano-diamond. The red circle shows micron-scale hard agglomerates and detonation-derived unrefined particles in nanodiamond powder. This inherent high-surface-energy effect explains why the weight-based CV% in Table 2 is markedly higher than the number-based value. Scale bar = 10 μm

12Cr2Ni4A samples with dimensions of 20 mm $\times$ 10 mm $\times$ 10 mm were used. The sample surfaces were finely ground by using 600-grit SiC sandpaper (Shanghai Grinding Wheel Factory Co., Ltd., Shanghai, China), aiming to control the surface roughness within the range of 0.3–0.5 µm. Subsequently, ultrasonic cleaning was performed on the samples to remove ethanol residues. After confirming that the treated sample surfaces were flat and clean, nano-diamond powder with an average particle size of 30 nm was uniformly coated onto the surface of the prepared 12Cr2Ni4A steel samples using a clean spatula. By precisely controlling the powder amount and spreading technique, a continuous and uniform pre-deposited powder layer was formed on the surface to be treated. This method ensured maximum intimate contact between the nano-diamond and the metal substrate before the initiation of thermal diffusion, laying the foundation for the subsequent high-temperature diffusion process.

Thermal diffusion was conducted in a tubular furnace (Shenyang Kejing Auto-instrument Co., Ltd., Shenyang, Liaoning, China). During the thermal diffusion process, the system pressure was strictly controlled within 0–10 MPa. Under an argon atmosphere, the temperature was increased at a constant rate of 10 ℃/min to a preset temperature range (950 ℃ to 990 ℃, with 10 ℃ intervals), and held at this temperature for 4 hours.

After thermal diffusion, the samples were subjected to a 90-minute holding treatment at 850 ℃, followed by rapid cooling using special quenching oil. Finally, the samples were tempered for 2 hours and then air-cooled (Fig. 2).

Fig. 2.

Schematic diagram of the fabrication process.

The material hardness performance test was carried out using an HV-1000A Vickers optical microhardness tester (Zhengzhou Huayin Instrument Co., Ltd., Zhengzhou, Henan, China), as shown in Fig. 3a. A square pyramid diamond indenter was employed: A standard load of 9.8N was applied to the 12Cr2Ni4A steel sample surface, and the load was maintained for 10 seconds. The hardness value was calculated based on the average pressure per unit surface area of the square pyramid indentation. A ten-point measurement method was applied for multi-point testing, and the arithmetic mean of hardness values was calculated to yield reliable test data.

Fig. 3.

Experimental instruments. (a) Huayin HV-1000A Vickers Optical Microhardness Tester. (b) DX-2700A X-ray Diffractometer. (c) Keyence VHX-5000 Super Depth of Field 3D Digital Microscope.

To determine the phase composition of 12Cr2Ni4A steel post-thermal diffusion, an X-ray diffraction (XRD) analysis was performed on the cross-section of the thermally diffused sample by using a DX-2700A X-ray diffractometer (Dandong Haoyuan Instrument Co., Ltd., Dandong, Liaoning, China), as shown in Fig. 3b.

A VHX-5000 super-depth-of-field 3D digital microscope, as shown in Fig. 3c (VXH-500, Keyence (Japan) Co., Ltd., Osaka, Japan), was employed for observing and analyzing the surface morphology of the 12Cr2Ni3A steel disk sample post-pin-on-disk test. The influence of different thermal diffusion treatment temperatures on the friction and wear behavior of 12Cr2Ni4A steel pin samples was investigated.

A pin-on-disk rotational friction and wear test was conducted by using an MMW-1A vertical universal friction and wear tester (Jinan Jingcheng Testing Technology Co., Ltd., Jinan, Shandong, China). The pin sample was taken as the test specimen, while the disk sample was 12Cr2Ni3A steel (55 mm in diameter). Prior to the experiment, the pin sample was weighed using a PR series balance (OHAUS Instruments (Shanghai) Co., Ltd., Shanghai, China). The test was conducted at room temperature with Mobil 600 XP 100 lubricant (ExxonMobil (Tianjin) Co., Ltd., Tianjin, China), 50 N normal load, 300 r/min rotational speed, and 60 min experimental time. The pin sample was cleaned and reweighed to calculate wear loss as illustrated in the schematic diagram of the friction and wear test in Fig. 4.

Fig. 4.

Schematic diagram of friction and wear test.

A molecular dynamics simulation method was employed to simulate the thermal diffusion process of nano-diamonds in 12Cr2Ni4A alloy. The microstructural evolution patterns of nano-diamonds and 12Cr2Ni4A were analyzed. The thermal diffusion mechanism of nano-diamonds was revealed. The main elemental composition of 12Cr2Ni4A alloy was Fe, Cr, and Ni atoms. The FeCrNi alloy model structure was used to represent the 12Cr2Ni4A alloy in the molecular dynamics thermal diffusion simulation, with the model structure shown in Fig. 5. The dimensions of the simulation box along the X[1 0 0], Y[0 1 0], and Z[0 0 1] directions were 115 Å, 45 Å, and 75 Å, respectively, with slight adjustments made to satisfy periodic boundary conditions. The FeCrNi alloy model measured 115 Å, 45 Å, and 57 Å along the X, Y, and Z directions, adopting a face-centered cubic (FCC) structure (a = 3.56). The nano-diamond model had a thickness of 12 Å, and the total number of atoms in the model was 56,362. The bottom section (0–15 Å) of the alloy model was designated as a rigid body to maintain substrate stability, while the remaining section (15–75 Å) followed Newton’s second law to simulate the thermal diffusion motion of nano-diamonds. To avoid non-physical interactions between the nano-diamonds and the alloy model in the initial stage, the initial position of the nano-diamonds was set 0.5 nm above the surface of the FeCrNi alloy, ensuring the accuracy and reliability of the simulation results.

Fig. 5.

Thermal diffusion model of nano-diamond particles and FeCrNi alloy.

Two types of potential functions were used to describe the interactions between different atoms, the Embedded Atom Method (EAM) and the Lennard-Jones (L-J) potential function. The EAM potential [22, 23, 24] is used to characterize the interactions between Fe, Cr, and Ni atoms, with its formula shown in Eqn. 1.

(1)$$E_{ij}=F_a\left[{\sum }_{i\ne j}{\rho }_{\beta }\left(r_{ij}\right)\right]+\frac{1}{2}{\sum }_{i\ne j}{\phi }_{\alpha \beta }\left(r_{ij}\right)$$

Where $\alpha$ and $\beta$ denote the element types of atoms i and j, rij is the interatomic distance, $\phi$$\alpha$$\beta$ represents the two-body potential (also referred to as the pair potential), F$\alpha$ is the many-body potential, and $\rho$$\beta$ is the electron density contributed by atom j to atom i.

The Lennard-Jones (L-J) potential [25, 26] is used to describe the non-bonded interactions between nano-diamonds and Fe, Cr, Ni atoms [27, 28], respectively. LAMMPS (LAMMPS 64-bit 19Nov2024-MSMPI) (Large-scale Atomic/Molecular Massively Parallel Simulator) includes the L-J potential and its various derived versions. The version adopted in this work is the “pair lj/cut” potential, which truncates the L-J potential at a distance rc. Its formula is shown in Eqn. 2.

(2)

Where $\epsilon$ij is the minimum energy, $\sigma$ij is the distance at minimum energy (also known as the zero-crossing distance), and rc is the cutoff radius. The parameters of the L-J potential function used in this work are shown in Table 3.

This study investigated the thermal diffusion behavior of the nano-diamond/FeCrNi alloy system. For simulation parameter configuration—aimed at balancing calculation reliability and computational efficiency—the system was configured under the number-volume-energy (NVE) microcanonical ensemble with a 0.001 ps time step. Atomic initial velocities were assigned according to the Maxwell-Boltzmann distribution to accurately capture the system’s thermodynamic properties. To maintain isothermal conditions during simulation, the Rescale temperature control algorithm was adopted. The simulation temperature protocol was strictly aligned with thermal diffusion experimental conditions: starting from an initial temperature of 293K, the system was heated to 1243K at a constant rate of 10 K/s, followed by isothermal simulation at this target temperature. To acquire detailed kinetic data, molecular dynamics trajectory information was recorded every 0.1 picoseconds.

The wear mass loss was shown in Fig. 9. The wear resistance of the 12Cr2Ni4A steel pin specimens was significantly improved. Under different treatment conditions, the wear loss of the specimens exhibited a declining trend. The specimen treated with nano-diamond at 970 °C demonstrated the most excellent anti-wear performance, with a wear loss of 0.086 g. The wear mass loss reduced 59.81% compared to the untreated specimen. The specimen subjected to thermal diffusion treatment at 990 °C reduced 12.62% compared to the untreated specimen. The wear losses of specimens treated at 960 °C and 980 °C decreased to 0.136 g and 0.107 g, reduced 35.98% and 50.00%, respectively.

Fig. 9.

Wear mass loss of experimental pin after thermal diffusion friction.

The friction coefficient curve was shown in Fig. 10. Specimens subjected to nano-diamond thermal diffusion treatment manifest pronounced regular variations in friction characteristics. During the running-in stage, the friction coefficient exhibits notable fluctuations. As the test progresses, it advances into a stable stage, with the friction coefficient stabilizing within a defined range. In the later stage of the experiment, the friction coefficient demonstrates a gradual upward trend.

Fig. 10.

Friction coefficient curves of tribology test.

The average friction coefficient was demonstrated in Fig. 11. The average friction coefficient of the untreated specimen was 0.6285. The wear resistance of 12Cr2Ni4A gear steel pin specimens was significantly enhanced with nano-diamond thermal diffusion treatment.

Fig. 11.

Average friction coefficient of nano-diamond samples after thermal diffusion friction and wear experiment.

The 970 ℃ treated specimen exhibited the most pronounced reduction, with its average friction coefficient decreasing to 0.3686 (41.35% reduction compared to the untreated specimen). The friction coefficients of specimens treated at 960 ℃, 980 ℃ and 990 ℃ declined to 0.4974, 0.4408 and 0.5228, corresponding to reductions of 20.86%, 29.86% and 16.83% respectively.

The wear scar areas of 12Cr2Ni3A steel disk specimens in each group were characterized using a super-depth-of-field microscope, as illustrated in Fig. 12.

Fig. 12.

Wear trace morphology of experimental disc. (a) 950 ℃, (b) 960 ℃, (c) 970 ℃, (d) 980 ℃, (e) 990 ℃.

The untreated specimen exhibited a surface groove depth of 0.2 µm, while the specimen treated at 970 ℃ had a groove depth of 8.27 µm in the wear area. The groove depth exhibited a trend of initial increase followed by subsequent decrease as the thermal diffusion treatment temperature rose. The wear scar areas of all nano-diamond treated specimens display grooves of varying depths.

The non-linear relationship between thermal diffusion temperature and surface hardness of 12Cr2Ni4A steel, is essentially the result of the competition between thermal activation-induced strengthening and high-temperature-induced microstructure deterioration.

At 950–970 ℃, the rising temperature provides sufficient activation energy for carbon atoms from nano-diamonds. On one hand, active carbon atoms diffuse into the interstitial sites of Fe-Cr-Ni matrix lattice, causing lattice distortion and realizing interstitial solid solution strengthening, which improves the deformation resistance of the matrix. On the other hand, diffused carbon atoms combine with Fe, Cr, Ni elements to form nano-scale dispersed Fe-C/Cr-C/Ni-C carbides, which act as strong obstacles to dislocation movement during plastic deformation, bringing a significant dispersion strengthening effect. The peak carbide diffraction intensity at 970 ℃ corresponds exactly to the maximum hardness value, which directly proves that carbide formation is the core source of hardness improvement.

When the temperature exceeds 970 ℃, the strengthening effect weakens sharply. This is mainly attributed to two factors: first, excessively high temperature leads to austenite grain coarsening of 12Cr2Ni4A steel, which reduces the grain boundary strengthening effect after cooling; second, Ostwald ripening of carbide phases occurs under high temperature, the fine dispersed carbides merge and grow up, which greatly weakens the dispersion strengthening effect. The combined effect of the two leads to the only 26.09% hardness improvement at 990 ℃.

The significant improvement of wear resistance and friction reduction performance of the modified 12Cr2Ni4A steel, is the synergistic result of hardness enhancement, self-lubrication effect and tribochemical film formation.

In line with Archard’s wear law, the wear rate of material is inversely proportional to its hardness under the same wear conditions. The 110.68% hardness increase of the 970 ℃ treated sample directly enhances the material’s resistance to plastic deformation and micro-cutting during friction, which is the core reason for the 59.81% wear mass loss reduction and 41.35% friction coefficient reduction. The change of wear scar groove depth also reflects the transition of wear mechanism: the main wear mechanism shifts from severe adhesive wear for the untreated sample to mild abrasive wear for the modified sample, which further confirms the improvement of wear resistance.

In addition, the dual existence form of carbon elements in the modified layer also plays a key role in friction reduction. During friction, intact nano-diamond particles act as nano-scale ball bearings between friction pairs, reducing the shear stress of the contact surface; meanwhile, under the action of friction heat and shear force, free carbon atoms form a graphitized carbon-based lubricating film on the friction surface, which effectively isolates the direct contact between metal matrixes, reduces the fluctuation of friction coefficient in the running-in stage, and prolongs the stable friction stage. For the 990 ℃ treated sample, the limited hardness improvement and coarse grain structure lead to the deterioration of surface load-bearing capacity, the lubricating film is easy to break during friction, so the anti-wear and friction reduction performance is significantly weakened.

The molecular dynamics simulation results provide direct atomic-scale evidence for macroscopic experimental phenomena, and reveal the thermal diffusion kinetic mechanism of nano-diamond in 12Cr2Ni4A steel matrix.

Under thermal activation, carbon atoms from nano-diamonds diffuse into the Fe-Cr-Ni matrix through interstitial sites, which is consistent with the classical interstitial diffusion mechanism. The diffusion zone thickness increases significantly with prolonged thermal action time, which directly explains the enhanced strengthening effect with rising temperature at 950–970 ℃: higher temperature and longer holding time promote more sufficient carbon diffusion, thus bringing a more significant strengthening effect.

Meanwhile, the simulation reveals the dual existence form of nano-diamond during diffusion: intact particles in the near-interface region for dispersion strengthening, and decomposed free carbon atoms in the deep matrix for solid solution and carbide precipitation strengthening. The Fe-C/Cr-C/Ni-C bonding evolution observed in the simulation is exactly the microscopic origin of the carbide diffraction peaks detected by XRD, which builds a complete bridge between atomic-scale diffusion behavior and macroscopic mechanical properties of the material.

Compared with conventional surface modification technologies for 12Cr2Ni4A steel, the nano-diamond thermal diffusion technology proposed in this study has significant core advantages. Existing technologies such as vacuum carburization combined with Ti+N ion implantation can only improve nano-hardness by ~35% with a strengthened layer thickness of only ~200 nm [5], and rare earth-assisted carburization usually achieves a hardness improvement of 40–50% [6]. In contrast, the proposed technology achieves a 110.68% hardness increase and 59.81% wear mass loss reduction, and can form a micron-scale thick strengthened layer, which can meet the long-term service requirements of heavy-duty gear components under harsh working conditions. Meanwhile, this process has the characteristics of simple operation, low equipment requirement and strong applicability, with broad industrial application prospects.

The molecular dynamics model employed in this study involves simplifications of the actual microstructure of 12Cr2Ni4A steel. The multicomponent alloy is reduced to an idealized Fe-Cr-Ni crystal model, without considering complex defects such as grain boundaries. The LennardJones potential is used to describe the non-bonded interactions between metal and carbon atoms, which is suitable for capturing atomic diffusion trends but cannot accurately describe the formation and breaking of chemical bonds. These simplifications are intended to focus on revealing the core qualitative mechanisms of carbonatom diffusion and interfacial interactions. The consistency between the model’s predictions and the macroscopic performance improvements validates its effectiveness within the research framework.

This study confirms the strengthening effect of nano-diamond thermal diffusion on 12Cr2Ni4A steel, yet certain limitations remain, such as the lack of surface engineering treatment on the nano-diamonds to optimize their interfacial compatibility. Future research could expand into the validation of diverse high-end application scenarios, for instance, evaluating the performance of this modified material under extreme operating conditions in fields such as aerospace and biomedical applications.

The tribological conclusions of this study are derived from a single, standardized set of operating conditions, which, to some extent, limits the universality of the findings. All experiments were conducted with Mobil 600 XP 100 oil, at room temperature, 50 N load and 300 r/min rotational speed. The performance of the modified layer under more complex conditions was not investigated. Future research can systematically test the tribological performance of the modified layer under the coupled influence of multiple factors, including lubrication, load, and speed.