Qianzhi Wang

Tribological and electrochemical properties of Cr–Si–C–N coatings in artificial seawater

Cr–Si–C–N coatings were deposited on Si(100) wafer and stainless steel 316L by using unbalanced magnetron sputtering via adjusting trimethylsilane (Si(CH3)3H, or TMS) flow, and their microstructure and hardness were analyzed by using EDS, SEM, XRD, XPS and nano-indenter. The electrochemical behavior of coatings in seawater were measured by using a CHI660D electrochemical workstation, and the tribological properties of coatings sliding against SiC balls in artificial seawater were investigated by using ball-on-disk tribometers. The results indicated that there were nanocrystallites of Cr(C, N) crystal and amorphous phases such as a-Si3N4 and a-C(a-CNx) in the CrSiCN coatings. With an increase in the TMS flow, the hardness of CrSiCN coatings first increased to the maximum value of 21.3 GPa, and then decreased to 13 GPa. As compared with the CrCN coating, the CrSiCN coatings exhibited a lower friction coefficient. The pitting corrosion only appeared on the wear tracks, and therefore the mechanical wear and wear-accelerated corrosion were the main material deterioration mechanisms in seawater. The impedance modulus and phase value became high in the low frequency range with an increase in the TMS flows. Because the amorphous phase of silicide dispersed in the grains could effectively improve the density of the coatings, thus the coatings deposited at higher TMS flows exhibited better corrosion resistance.

Influence of CrB2 target current on the microstructure, mechanical and tribological properties of Cr–B–C–N coatings in water

Cr–B–C–N coatings with different boron contents (24.6–27.2 at%) were deposited on Si(100) wafers and 316L stainless steels by using closed-field unbalanced magnetron sputtering via adjusting the CrB2 target current. The microstructures, mechanical and tribological properties of Cr–B–C–N coatings in water were characterized by using XRD, XPS, SEM, nano-indentation and ball-on-disk tribometers. The results showed that the soft amorphous BN phases were formed when boron elements were introduced into CrCN coatings. With an increase in the CrB2 target current, the B content increased from 24.9 to 27.2 at%, which caused the a-BN content in CrBCN coatings to increase gradually, and then the hardness of CrBCN coatings decreased from 18.6 ± 0.86 GPa to 16.3 ± 0.28 GPa. Due to the formation of H3BO3, the friction coefficient and wear rates of CrBCN/SiC tribopairs were lower than those of CrCN/SiC tribopairs. When the CrBCN coatings (25.1 at% B) were deposited at the CrB2 target current of 3 A, the lowest values of friction coefficient (0.16) and wear rate (2.1 × 10−7 mm3 N−1 m−1) were obtained simultaneously.

Thermodynamic predicting and atomistic modeling the favored compositions for Mg–Ni–Y metallic glasses

For the Mg–Ni–Y system, a typical Mg-based bulk metallic glass forming system, glass formation compositions are first predicted by thermodynamic calculations based on the extended Miedema’s model, suggesting that metallic glasses in the system are favored over a large composition range. Assisted by ab initio calculations, a realistic Mg–Ni–Y n-body potential is then constructed under a proposed modified tight-binding scheme. Based on the potential, an atomistic modeling scheme is further formulated for designing the favored, and even pinpointing the optimized, compositions at the atomic level. A hexagonal glass formation region is located for the Mg–Ni–Y system, reflecting the possible compositions energetically favoring metallic glass formation. An optimized stoichiometry sub-region is further pinpointed, within which the driving force for glass formation is prominently larger than that outside. The present study evaluates glass formation in the Mg–Ni–Y system from two different perspectives, and the results have implications for the entire family of Mg-based systems. The prediction schemes could be of great help for guiding the composition design of ternary glass formers.

Progress in Preparation and Modification of LiNi0.6Mn0.2Co0.2O2 Cathode Material for High Energy Density Li-Ion Batteries

Due to the advantages of high specific capacity, various temperatures, and low cost, layered LiNi0.6Co0.2Mn0.2O2 has become one of the potential cathode materials for lithium-ion battery. However, its application was limited by the high cation mixing degree and poor electric conductivity. In this paper, the influences of synthesis methods and modification such surface coating and doping materials on the electrochemical properties such as capacity, cycle stability, rate capability, and impedance of LiNi0.6Co0.2Mn0.2O2 cathode materials are reviewed and discussed. The confronting issues of LiNi0.6Co0.2Mn0.2O2 cathode materials have been pointed out, and the future development of its application is also prospected.

Atomistic study of chemical effect on local structure in Mg-based metallic glasses

By applying a recently constructed interatomic potential, molecular dynamics (MD) simulations were performed to investigate the structural origin of chemical effects in Mg–Cu–Ni ternary metallic glasses. The detailed evolution of local atomic structure in a series of Mgx(Cu42.5Ni57.5)100−x (x = 40–80) metallic glasses was tracked and comprehensively characterized by the pair correlation function, Voronoi tessellation, Honeycutt–Andersen bond pair and local chemistry analyses. Remarkable topological short-range orders (SROs) were found in Mg–Cu–Ni metallic glasses, with the characteristic motifs being icosahedra. The degree of icosahedral ordering varies distinctly with the alloy composition and is intimately correlated with the phase stability of metallic glasses. In contrast to the long-term understanding that five-fold bond pairs are a direct indication of icosahedral ordering, it was revealed that bond pairs are actually insensitive to the composition, and whether icosahedral ordering is preferred or not, fragmented pentagonal bipyramids are always populated. Furthermore, it was indicated that multiple chemical interactions among constituent atoms do result in chemical SROs in Mg–Cu–Ni metallic glasses, which are characterized by enriched Mg atoms in neighboring shells than expected from the nominal composition. The atomic-scale topological or chemical heterogeneity helps tune the local environments in Mg–Cu–Ni metallic glasses to achieve efficient packing and energy minimization for various compositions.