Naizhou Du

The corrosion resistant of Laser Powder Bed Fusion (LPBF) dual-phase NiTi alloys remains controversial, and the underlying mechanisms are not fully understood. In this study, a tailored LPBF combined with annealing strategy was employed to introduce martensite-rich (~46.5% martensite) dual-phase NiTi alloys (HT500) with homogeneous micro-strain. Despite rapid initial anodic dissolution, the HT500 sample exhibits pronounced repassivation, resulting in an extended passive region of approximately 1600 mV and only a limited number of shallow pits. This passive region is substantially wider than those reported for predominantly austenitic LPBF NiTi alloys (typically ~300 mV) and commercial wrought NiTi alloys (typically ~700 mV). It can be attributed to the synergistic effects of the evolution of Ti4Ni2Ox nanoprecipitates and the homogeneous micro-strain distribution. The net-like and ellipsoidal Ti4Ni2Ox nanoprecipitates initially dominate the local micro-galvanic coupling due to high micro-galvanic driving force (360 mV), resulting in a rapid increase in anodic current density. Subsequently, their partial detachment during polarization progressively weakens the micro-galvanic driving force (160 mV) and suppresses localized corrosion susceptibility. Meanwhile, continuous regeneration of the passive film promotes current stabilization, while the homogeneous micro-strain distribution suppresses localized corrosion and limits pit propagation. These findings provide new insight into the superior pitting resistance of martensite-rich dual-phase NiTi alloys over predominantly austenitic counterparts, emphasizing the importance of nanoprecipitates and micro-strain beyond phase constitution alone.

Plasma nitriding is a pivotal surface modification technique for enhancing the tribological and mechanical performance of titanium alloys, yet the cross-scale transition from initial nitrogen adsorption to deep infiltration remains inadequately understood. In this study, a multiscale computational framework, integrating first-principles calculations based on density functional theory (DFT) and molecular dynamics (MD) simulations, was employed to systematically investigate the nitriding mechanisms of α-Ti. DFT calculations reveal that nitrogen atoms preferentially occupy octahedral sites (O-sites) due to strong hybridization between N-2p and Ti-3d orbitals, creating a “thermodynamic trap” that governs the initial adsorption. Transitioning to the dynamic regime, MD simulations utilizing a stochastic sampling algorithm demonstrate that ion implantation behavior is highly site-dependent and significantly influenced by thermal lattice vibrations. Increasing the incident energy from 1 to 200 eV was found to overcome the surface energy barrier and the “site-blocking effect” induced by pre-implanted nitrogen atoms. Furthermore, continuous flux simulations illustrate a structural stratification process, where high-nitrogen-potential environments lead to the formation of a gradient interstitial solid solution. Aberration-corrected STEM observations further reveal locally ordered lattice fringes and local lattice distortion in the nitrided diffusion layer, providing experimental evidence consistent with the simulated nitrogen-associated interstitial solid-solution structure. This evolution is accompanied by the accumulation of significant compressive residual stresses and lattice expansion, which provide an atomistic basis for surface hardening. These findings bridge the gap between static thermodynamic preferences and dynamic kinetic evolution, providing a theoretical foundation for optimizing plasma nitriding parameters in industrial applications.

Constructing the periodically modulated multilayer architectures is a prevalent strategy to resolve the intrinsic trade-off between hardness and stress in diamond-like carbon (DLC) films. However, the specific dynamic contributions of distinct modulated layers to structural evolution under complex loading remain elusive. Here, the structure-property relationships of modulated DLC films were investigated at the atomic scale, spanning from energetic deposition to dynamic tribology. A counterintuitive tribological reversal governed by pressure was identified, where increasing modulation period caused a degradation in frictional performance under moderate pressure but an enhancement under high pressure. The modulation period acted as a kinetic regulator governing competitive evolution pathways. In particular, under highpressure conditions, frequent interfaces facilitated interfacial densification via energetic subplantation, transforming soft layers into a hard tribofilm that effectively supported loads through stress segmentation. Conversely, under lower pressures, shear stress was dominant, leading to the destabilization of thin layers and a consequent loss of load-bearing capacity. These findings establish a unified theoretical framework linking performance to a critical transition from shear-dominated softening to adaptive pressure-driven hardening, and thus provide essential criteria for tailoring coating architectures to specific operational contact pressures.

The atomic-scale friction behavior of amorphous carbon (a-C) coating against BCC-Fe substrate under high pressure and lubricant conditions was systematically investigated using reactive molecular dynamics simulations, especially its dependence on the hydrogenated degree of a-C surface. Results indicate that hydrogenation can significantly reduce the friction coefficient of a-C/Fe system, which achieves a 51% reduction for the system with 24.9 at.% H content compared to the hydrogen-free case. However, as the surface H content is 18.8 at.%, the system exhibits the most stable friction profile and interfacial structure, originating from both the effective passivation of dangling bonds and the optimization of lubricant distribution. In addition, note that threshold exists in the hydrogenated degree of a-C surface, beyond which the structural stability of the friction pair is markedly enhanced. This study elucidates the competing roles of chemical passivation and physical interactions at the interface, providing atomic-scale insights for the design of high-performance carbon-based coatings.

The combination of fluid lubricants and textured amorphous carbon (a-C) can provide an ultralow friction state, which can improve the reliability and service life of dynamic machinery. However, the coupling effects of the contact pressure and oil content on the friction-reducing efficiency is still lack of study, and the corresponding friction mechanism is also not fully understood, which cannot be achieved by experiment due to the limitation of in situ characterization. In this study, using the reactive molecular dynamics simulation, the insight into the evolution of interfacial structures induced by both contact pressures and oil contents on a-C surface was systematically investigated to explore the fundamental mechanism. In particular, the friction difference between textured and untextured a-C films was evaluated comparatively. Results indicate that the tribological performance strongly depends on the interfacial lubrication state, which is jointly determined by the oil content and contact pressure; the best operating condition to achieve ultralow friction coefficient (0.002) is obtained, and the evolution of friction coefficient with oil content and contact pressure is highly dominated by the lubricant mobility, cross-linking between mating a-C surfaces, or competition/synergy of the H stress state from the lubricant with interfacial passivation. Furthermore, the difference in friction reduction between textured and untextured systems is unveiled; with the increase of contact pressure, the role of texturing a-C surface in antifriction changes from positive to negative effect, which is related to the transformation of interfacial hybridized structure and anomalous flow of lubricant. These results can significantly enhance the understanding of composite lubrication systems through computation and also provide a roadmap for the R&D of the advanced lubrication system according to the working conditions.