查看更多>>摘要：SiC-ZrC-based composite coatings on carbon/carbon composites via ZrC polymer precursor pyrolysis (PPP) plus gaseous reactive infiltration (GRI) of Si or ZrSi2, separately denoted as GSIC and GZSIC, were comparatively studied to explore the feasibility and effectiveness of GRI of ZrSi2. This composite method by applying ZrSi2 or Si was proven to similarly produce dense coatings with enhanced coating/substrate interfacial bonding strength for the formed zig-zag transition layer. Furthermore, the GRI of ZrSi2 instead of Si further increased the UHTC (ZrC) content in the as-deposited coatings resulted from the reaction of infiltrated ZrSi2 and pre-introduced graphite, enhancing the ablation resistance of the coatings for carbon/carbon composites due to the formed stable Zr-rich Zr-Si-O multiphase oxide. The average mass and linear ablation rates were-0.12 mg/s and 1.56 mu m/s for GSIC and-0.3 mg/s and-0.53 mu m/s for GZSIC specimens, respectively. The average linear ablation rate of the GZSIC specimen was decreased by similar to 134% when compared to the GSIC specimen. These results further indicate that the PPP + GRI method with metal silicide MeSi2 (Me = Zr, Ti, Hf, Cr, etc.) will contribute to the optimal fabrication of dense composite coatings with controllable microstructure, adequate evenly-distributed ultra-high temperature ceramic (UHTC) phases, enhanced interfacial bonding strength and wide circumstance suitability for potential applications. The related work is ongoing in our laboratory.
查看更多>>摘要：TiAlCrY has been used as a bonding layer for the thermal barrier coating (TBC) system on TiAl alloys, due to its excellent high temperature oxidation resistance. However, it was found that vertical cracks formed in the TiAlCrY layer during the thermal cycling tests, which would fasten the failure of TBC system. In this work, a TiAlCrNb buffer layer was introduced between TiAlCrY layer and the substrate to alleviate the nucleation and propagation of cracks in the TiAlCrY layer from the perspective of alleviating the stress concentration, aiming to improve the thermal cycling lifetime of the YSZ/TiAlCrY system. The results show that the thermal cycling lifetime of the newly designed YSZ/TiAlCrY/TiAlCrNb system was over 245 cycles by means of water-quenching, which was almost twice of the system without the buffer layer and 3-4 times of the traditional YSZ/MCrAlY system on superalloys. The influence of the TiAlCrNb buffer layer was analyzed based on the combined analysis of microstructure, residual stresses and mechanical properties. This work implies that a proper buffer layer could effectively improve the thermal cycling lifetime of TBCs on TiAl alloys.
查看更多>>摘要：Refractory multi-principal element alloys (RMPEAs) have become one of the most promising candidates for hightemperature structural and hard protective coating materials due to their high strength and thermal stability. In present work, Mo-Ta-W RMPEA thin hard films were successfully fabricated via direct current (DC) multi-target magnetron co-sputtering technology. The phase structure, microstructure, surface roughness and mechanical properties of the Mo-Ta-W RMPEA thin films were investigated. The as-deposited Mo-Ta-W RMPEA thin films with a wide range of compositions presented a single body-centered cubic (BCC) solid solution structure. Nanocrystalline with average grain size of 10- 15 nm (estimated according to the Scherrer equation) was formed in these films. The surface morphology of the films exhibited the flake-like shape with small roughness value of Ra = 4- 8 nm, and cross-sectional structure revealed columnar dendrite. The Mo-Ta-W RMPEA thin films displayed superior mechanical properties with the hardness of ~20 GPa and the elastic modulus of beyond 270 GPa. The Mo-Ta-W alloy thin films had the high H/E and H3/E2 values, indicating that the alloy thin films should have excellent wear resistance. Hence, these Mo-Ta-W RMPEA thin films can be considered as candidates for hard protective coating applications.
查看更多>>摘要：The Si-MoSi2 ceramic coatings were prepared on TZM alloy through a hot dip silicon-plating (HDS) process. The hot dip experiments results showed that Si-MoSi2 ceramic coatings have a very dense surface morphology and low roughness (0.258 +/- 0.009 to 0.347 +/- 0.019 mu m). The Si-MoSi2 ceramic coatings presents a typical layered structure with the outermost silicon-rich MoSi2 layer, the intermediate layer is the pure MoSi2 layer, and the Mo5Si3/Mo5Si3C diffusion layer between the MoSi2 layer and the TZM substrate. The oxidation tests showed that MoSi2 ceramic coating maintained a complete appearance after high temperature oxidation at 1500 ? for 4 h. The self-healing SiO2 protective film effectively inhibits the diffusion of oxygen and reduces the consumption rate of MoSi2 layer. HDS Si-MoSi2 ceramic coating presents a very excellent oxidation resistance at high temperature, which is mainly attributed to the uniform and dense coating structure and high surface silicon concentration.
Valenti, Laura E.Bonnet, Laura, VGaliano, Mauricio R.Giacomelli, Carla E....
15页
查看更多>>摘要：The wide use of Titanium and its alloys as biomaterial is due to their excellent mechanical properties and chemical stability, good corrosion resistance and biocompatibility. Their main limitations (poor biological activity leading to deficient osseointegration and high surface potential for bacterial colonization) can be overcome with a simple two-step strategy to modify Ti6Al4V substrates with a hybrid coating that includes: (1) the electrodeposition of a calcium phosphate phase to provide them with bioactive functionality, and (2) the incorporation of Ag-NPs to the calcium phosphate coating to add antibacterial functionality. The substrates were characterized by Raman spectroscopy, X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Further, they were evaluated for antibacterial properties and cytocompatibility. The bioactive coating was composed by octacalcium phosphate and hydroxyapatite with granular morphology that originates pores and increases its total exposed area. Hybrid substrates showed homogeneously distribution of silver throughout the surface, excellent antibacterial ability against adherent and planktonic Staphylococcus aureus and a very good behavior for supporting cell adhesion and proliferation, resulting in proper environments for the integration with bone tissue without the release of toxic species that can affect the cell viability.
Bhowmick, SukantaShirzadian, ShayanAlpas, Ahmet T.
12页
查看更多>>摘要：Diamond-like carbon (DLC) coatings show a low coefficient of friction (COF) and high wear resistance against aluminum at elevated temperatures, yet they exhibit a high running-in COF (mu(R)) prior to reaching a low and stable steady-state COF (mu(s)). This study shows that incorporating titanium (Ti) atoms into the DLC structure would reduce the mu(R). During pin-on-disk tests conducted on Ti incorporating hydrogenated-DLC (Ti-H-DLC) with 6.2 at.% Ti subjected to dry sliding against 319 Al (Al-6.5% Si), mu(s) values decreased from 0.27 at 25 ?C to 0.11 at 200 ?C. The specific wear rate of Ti-H-DLC decreased from 2.44 x 10(-5) mm(3)/Nm at 25 ?C to 0.71 x 10(-5) m(3)/Nm at 200 ?C. A typical DLC with 40 at.% H (H-DLC) tested at 200 ?C showed a low mu(s) of 0.08 and a wear rate of 1.11 x 10(-5) mm(3)/Nm. However, at 200 ?C, Ti-H-DLC showed a lower mu(R) of 0.16 compared to mu(R) = 0.78 of H-DLC, and the duration of the running-in period for Ti-H-DLC, t(R) = 3 revolutions, was shorter than H-DLC with t(R) of 200 revolutions. Comparisons made with other DLCs, including, NH-DLC, W-DLC, ta-C, and Si-O-H-DLC, in addition to H-DLC, all tested using the same method, revealed that, in the temperature range of 100-250 ?C, Ti-H-DLC showed a better running-in behavior making Ti-H-DLC a suitable tool coating for manufacturing processes where high-temperature running-in sliding friction is important, including warm forming and (single-point) turning of aluminum alloys.
查看更多>>摘要：In this research, multilayer DLC-Si films were effectively formed on the 2024 Al alloy utilizing cage-like hollow cathode discharge plasma-enhanced chemical vapour deposition (PECVD). A scanning electron microscopy (SEM), atomic force microscope (AFM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and Nano Indenter G200 device were utilized to characterize the microstructural features of the coatings. Various electrochemical methods were used to investigate the anticorrosion performance of DLC films in 3.5 wt% electrolytic solution. The findings revealed that an increment in the bias voltage increases the condensation of the DLC layer and increments the thickness of the coatings. Furthermore, the outcomes revealed that the anodic current density and passive current decrements with an increment of bias voltage. This behavior is attributable to a good barrier impact of the amorphous DLC film. The multilayer film introduces new interfaces, resulting in the deterrence of Cl- ions penetrations. Further, the values of activation energy (delta E), enthalpy (delta H), and entropy of activations (delta S) for the samples in 3.5 wt% NaCl were determined and presented.
Tolentino-Hernandez, R. V.Garcia-Pastor, F. A.Baez-Medina, H.Jimenez-Melero, E....
10页
查看更多>>摘要：In this work graphene oxide (GO) coatings were obtained on Nb substrates by electrophoretic deposition (EPD), using a ramp of stepped potential and variable deposition time, to evaluate the feasibility as a protective coating against the exposition to a 20 W Yb laser (1070 nm). Laser irradiation constitutes a first approximation to the potential damage by local high heat loads that could occur in a Tokamak-type fusion reactor. GO coatings can act as the first barrier against exposure to the fusion reactor plasma, but the coating may experience local high heat loads and simultaneous exposure to energetic particles. The GO/Nb coatings were irradiated with different laser power-frequency settings. With X-ray photoelectron spectroscopy, the bonding evolution of the coatings was followed for each deposition time. Scanning electron microscopy was used to evaluate the coating morphology before and after laser irradiation, whereas Raman spectroscopy was used to evaluate the structural evolution of GO coatings, crystallinity, and disorder in the graphene oxide. An increase of the crystallite size due to sp2 restoration was observed for the films prepared up to 10 s/V. Atomic force microscopy was used to study the film morphology and to estimate the film thickness by comparing the z-offset between the substrate and coating topography images. Film thickness reduces from ca. 280 nm to 100 nm with the increase in the time at each voltage step. Coating tolerance against laser-induced damage was observed up to 34% of the full laser power, where the coating was damaged and local metal melting was observed. The coating damage occurs between 4.9 x 108 MW/m2 and 2.5 x 109 MW/m2 and higher power levels triggered the occurrence of melting in the metallic substrate. The reduction in laser damage is attributed to the enhanced thermal dissipation by the sp2 dominion as the crystallite size increases. GO coatings prepared by electrophoresis are shown to be promising to protect nuclear components against damage for high heat loads.
查看更多>>摘要：Plasma electrolytic oxidation (PEO) can be used to fabricate black ceramic films on aluminum alloy by adding transition-metal anions into the electrolyte. Although the coloration was attributed to transition-metal oxides, a thorough explanation of the relationship between oxides and black color was required. In this study, black PEO films were created by incorporating ammonium metavanadate or sodium tungstate into the electrolyte. The research focused on alumina band-gap modification to explain the mechanism of black coloring. The absorption spectrum and X-ray photoelectron spectroscopy (XPS) valence band characterization revealed that the bandgap of alumina after doping was significantly narrowed. The density of state of alumina, including before doping, after doping, and introducing point defect was simulated by first-principles density functional theory calculation. It was discovered that doping transition metals into alumina decrease the conduction band while defects in alumina cause an upper shift of the valence band. The narrowing of the bandgap resulted in the absorption of all visible light and the appearance of black.
查看更多>>摘要：Mechanical energy-induced deposition on NiTi shape memory alloys (SMAs) is difficult to accomplish due to the pseudoelasticity of NiTi SMAs. In this study, a high-frequency ball-bombardment method called ultrasonic mechanical coating and armouring (UMCA) is introduced to synthesize a hydroxyapatite (HA)-containing coating on a NiTi SMA. The effects of treatment duration, Ti addition, and ball-to-powder ratio on the HA content of the coating are investigated. Results show that the HA content of the coating largely increased during the initial 10 s and decreased with further ball-bombardment. The addition of Ti powder into the chamber assisted HA anchoring on the NiTi SMA surface. Using a ball-to-powder ratio of 1:1 induced the highest HA content on the NiTi SMA as compared to the 1:2 and 2:1 ratios. Coating consisting of 10-50 nm nanoparticles of HA and Ti formed under localized compressive forces were induced through shot collisions. The bombardment also created a local amorphous structure of the NiTi SMA near the coating/NiTi SMA interface. Laser annealing induced oxidation of the coating, which improved the coating adhesion and corrosion resistance of the NiTi SMA in Hank's balanced salt solution.
NSTL
Elsevier