Heat Resistance,High-Temperature Tribological Characteristics,and Electrochemical Behavior of Arc-PVD Nanostructural Multilayer Ti–Al–Si–N Coatings

The present paper has aimed at studying heat resistance, electrochemical behavior, and tribological characteristics at high temperatures of superhard (~48 ± 2 GPa), multilayered with a modulation period of 17–18 nm, and nanostructured (nc)AlN-(am)Si₃N₄/(nc)TiN coatings obtained with an ion-plasma vacuum arc. The heat resistance of the coatings studied in the temperature range of up to 800°C inclusive was mainly determined by the oxidation of their surface layers without the substrate intrusion. Having a high coefficient of friction from 0.6 at 20°C to 0.8–0.85 at elevated temperatures, the coatings are characterized by virtually no wear, which was confirmed by profilometry measurements of friction zones. The obtained results concerning electrochemical behavior indicate that the Ti–Al–Si–N coatings are highly efficient in the protection of a cutting tool from corrosion in both acidic and alkaline media.     

Thermal Properties of Mg–Al/AlN Composites Fabricated by Powder Metallurgy

Magnesium matrix composites reinforced with AlN particles were fabricated by the powder metallurgy technique.Different mixing methods were used in this study to control the distribution of Al N particles.The microstructure,thermal diffusivity and thermal expansion of the Mg–Al/Al N composites using different mixing methods were investigated.The results showed that the intergranular and intragranular distributions of Al N particles were obtained,respectively,by controlling the mixing methods.The composite with intragranular particles exhibited lower thermal diffusivity because of the existences of more interfaces,defects and grain boundaries,which acted as scattering centers and reduced the mean free path of electrons and phonons.The existence of Al N particles lowered the coefficient of thermal expansion(CTE)and enhanced the dimensional stability of the composites.And the use of the improved mixing method further reduced the CTE of Mg–Al/Al N composites.     

Thermal and thermo-mechanical properties of Ti–Al–N and Cr–Al–N coatings

Metastable Ti–Al–N and Cr–Al–N coatings have been proven to be an effective wear protection due to their outstanding mechanical and thermal properties. Here, a comparative investigation of mechanical and thermal properties, for Ti–Al–N and Cr–Al–N coatings deposited by cathodic arc evaporation with the compositions (c-Ti_0.52Al_0.48N, c/w-Ti_0.34Al_0.66N and c-Cr_0.32Al_0.68N) widely used in industry, has been performed in detail. The hardness of Ti_0.52Al_0.48N and Ti_0.34Al_0.66N coatings during thermal annealing, after initially increasing to the maximum value of ~ 34.1 and 38.7 GPa with T_a up to 900 °C due to the precipitation of cubic Al-rich and Ti-rich domains, decreases with further elevated T_a, as the formation of w-AlN and coarsening of precipitated phases. A transformation to Cr₂N and finally Cr via N-loss in addition to w-AlN formation during annealing of the Cr_0.32Al_0.68N coating occurs, and thus results in a continuous decrease in hardness. Among our coatings, the mixed cubic-wurtzite Ti_0.34Al_0.66N coating exhibits the highest thermal hardness, but the worst oxidation resistance. The Cr_0.32Al_0.68N coating shows the best oxidation resistance due to the formation of dense protective α-Al₂O₃-rich and Cr₂O₃-rich layers, with only ~ 1.4 μm oxide scale thickness, after thermal exposure for 10 h at 1050 °C in ambient air, whereas Ti–Al–N coatings are already completely oxidized at 950 °C.     

Erosion and Abrasion Resistance,Mechanical Properties,and Structure of the TiN,Ti–Cr–Al–N and Cr–Al–Ti–N Coatings Deposited by CFUBMS

Protection of Metals and Physical Chemistry of Surfaces - The results of structural studies and mechanical, abrasion, and erosion tests of single-layer and multi-layer TiN,...     

Corrosion Behavior of Fe–Al Coatings Fabricated by Pack Aluminizing Method

In this study,the two kinds of Fe–Al coatings were fabricated by pack aluminizing on low-carbon steel at different temperatures.The corrosion behavior of the Fe–Al coatings in artificial seawater was investigated by the electrochemical and weight loss techniques.Results show that the thickness of coating layer increases with increasing aluminizing temperature.The coatings exhibit high micro-hardness and good metallurgical bonding with the substrate.In comparison with the steel substrate,the corrosion current density Icorrof the Fe–Al coatings is always lower than that of substrate,about 1/38 or 1/33 after 2 h immersion,and 1/3 or 1/6 for 720 h immersion.As can be seen from the weight loss curve,the Fe–Al coatings show less loss than that of the substrate within 30-day immersion.The corrosion products formed on the surface of the coatings include oxides of Al,Mg,Fe and Ca,and pitting defect has also been found.The Fe–Al coating with higher content of Fe_2Al_5 has better corrosion resistance.     

Microstructural and Mechanical Properties of TiCX–Ni3(Al,Ti)/Ni Functionally Graded Composites Fabricated from Ti3AlC2 and Ni Powders

Metals and Materials International - In this paper, a novel type of functionally graded material (FGM) was successfully fabricated from pure Ni and Ti3AlC2 powder mixtures by hot-press sintering...     

Microstructure and Thermal Conductivity of Al–Graphene Composites Fabricated by Powder Metallurgy and Hot Rolling Techniques

The objective of this research is to improve the thermal conductivity and mechanical properties of Al/GNPs(graphene nanoplatelets) nanocomposites produced by classical powder metallurgy and hot rolling techniques. The microstructural evaluation confirmed the uniform dispersion of GNPs at low content and agglomeration at higher contents of GNPs. The structure of graphene was studied before and after the mixing and the Raman spectrum proofs that the wet mixing has a great potential to be used as a dispersion method. There was no significant peak corresponding to the Al_4C_3 formation in both the differential scanning calorimetry curves and X-ray diffraction patterns. The microstructural observation in both fabrication techniques showed grain refinement as a function of the GNPs content. Moreover, the introduction of the GNPs not only improved the Vickers hardness of the composites but also decreased their density. The thermal conductivity investigations showed that in both the press-sintered and hot-rolled samples, although the thermal conductivity of composites was improved at low GNPs contents, it was negatively affected at high GNPs contents.     

Microstructure and Properties of Electrodeposited nc-TiO2/Ni–Fe and Ni–Fe Coatings

Metals and Materials International - In this study, nc-TiO2/Ni–Fe composite coatings, and Ni–Fe alloys as equivalents to their matrices, were obtained from citrate-sulphate baths in the...     

Electrochemical Deposition and Properties of Nickel—Chromium–Graphene Oxide Composite Coatings

Protection of Metals and Physical Chemistry of Surfaces - Composite electrochemical coatings (CECs) based on nickel–chromium alloy modified with multilayer graphene oxide (GO) have been...     

Interdiffusion Behavior of Ni–Cr–Al–Y Coatings Deposited by Arc-Ion Plating

Ni–Cr–Al–Y coatings were deposited on the Ni3Al–base superalloys IC-6 and K17 by arc-ion plating. The results indicated that a small amount of substrate atoms, such as Co, Ti, Mo, etc., existed in the Ni–Cr–Al–Y coatings near the substrates, probably due to sputter and antisputter. For the alloy K17, the impeding effect of Al was not obvious, because the temperatures of the substrate and the coating were high (420–480°C) during the deposition process and interdiffusion was accelerated. However, for alloy IC-6 which contains Al, as well as a high concentration of Mo, the diffusion of Cr was impeded. Vacuum heat treatment at 1050°C drastically increased diffusivities and the presence of Al and Mo was not enough to prevent some Cr diffusion. Thus, the coating became more uniform and close to the desired composition.     

Pseudo-Capacitors: SPPS Deposition and Electrochemical Analysis of α-MoO3 and Mo2N Coatings

Solution precursor plasma spraying (SPPS) is a novel thermal spray process in which a solution precursor is injected into the high-temperature zone of a DC-arc plasma jet to allow solvent evaporation from the precursor droplets, solute precipitation, and precipitate pyrolysis prior to substrate impact. This investigation explored the potential of SPPS to fabricate α-MoO₃ coatings with fine grain sizes, high porosity levels, and high surface area: characteristics needed for application as pseudo-capacitor electrodes. Since molybdenum nitride has shown a larger electrochemical stability window and higher specific area capacitance, the α-MoO₃ deposits were subsequently converted into molybdenum nitride. A multistep heat-treatment procedure resulted in a topotactic phase-transformation mechanism, which retained the high surface area lath-shaped features of the original α-MoO₃. The electrochemical behaviors of molybdenum oxide and molybdenum nitride deposits formed under different deposition conditions were studied using cyclic voltammetry to assess the influence of the resulting microstructure on the charge storage behavior and potential for use in pseudo-capacitors.     

Effects of Si Addition and Long-Term Thermal Exposure on the Tensile Properties of a Ni–Mo–Cr Superalloy

The effect of long-term thermal exposure on the grain boundary carbides and the tensile behavior of two kinds of Ni–Mo–Cr superalloys with different silicon contents(0 and 0.46 wt%) was investigated. Experimental results showed granular M2C carbides formed at the grain boundaries after exposure for 100 h for the non-silicon alloy. Furthermore, these fine granular M2C carbides will transform into plate-like M6C carbides as exposure time increases. For the Si-containing alloys,only the granular M6C carbides formed at the grain boundaries during the whole exposure time. The coarsening of the grain boundary carbides occurred in both alloys with increasing exposure time. In addition, the coarsening kinetics of the grain boundary carbides for the non-silicon alloy is faster than that of the standard alloy. The tensile properties of both alloys are improved after exposure for 100 h due to the formation of nano-sized grain boundary carbides. The grain boundary carbides are coarsened more seriously for non-silicon alloys than that of Si-containing alloys, resulting in a more significant decrease in the tensile strength and elongation for the former case. Silicon additions can effectively inhibit the severe coarsening of the grain boundary carbides and thus avoid the obvious deterioration of the tensile properties after a long-term thermal exposure.     

Electrochemical and Spectroscopic Study on the Corrosion of Ti–5Al and Ti–5Al–5Cu in Chloride Solutions

Metals and Materials International - In this study, manufacturing of Ti–5Al and Ti–5Al–5Cu alloys were accomplished employing mechanical alloying technique. The corrosion...     

Stability of B2 ordered phase in the Ti-rich portion of Ti–Al–Cr and Ti–Al–Fe ternary systems

The stability region of the B2 phase at 1000°C in the Ti-rich part of the Ti–Al–Cr and Ti–Al–Fe ternary systems are investigated by energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) using two-phase alloys and diffusion couples. It is established that the critical boundaries of the A2/B2 continuous ordering transition are functions of both the Al and Fe or Cr contents, and the phase equilibria between the α₂ and the β and between the β and FeTi (B2) phases are strongly affected by the A2/B2 order–disorder transition. By extrapolating these ternary data to the Ti–Al binary and using the Bragg–Williams–Gorsky approximation a metastable A2/B2 ordering boundary is postulated to exist at 1000°C in the vicinity of 23.5 at%Al in the Ti–Al binary system.     

Structural Phase State and Thermal Cyclic Stability of the Thermal Barrier Zr–Si–O Coatings Deposited on a Copper Substrate by the Microplasma Method

Protection of Metals and Physical Chemistry of Surfaces - In this work, preparation of thermal barrier coatings based on zirconium oxide is shown. The phased treatment of the copper substrate is...     

Environmental Resistance of Mo–Si–B Alloys and Coatings

For high temperature application beyond the range of Ni-base superalloys, multiphase Mo–Si–B alloys with compositions, that yield the ternary intermetallic Mo₅SiB₂ (T₂) phase as a key microstructure constituent together with the Mo and Mo₃Si phases, offer an attractive balance of high melting temperature, oxidation resistance and mechanical properties. The investigation of reaction kinetics involving the T₂ phase enables the analysis of oxidation in terms of diffusion pathways and the design of effective coatings. From this basis kinetic biasing is used together with pack cementation to develop multilayered coatings and in situ diffusion barriers with self-healing characteristics for enhanced oxidation resistance. While a combustion environment contains water vapor that can accelerate attack of silica based coatings, the current pack cementation coatings provide oxidation resistance in water vapor up to at least 1500 °C. An exposure to hot ionized gas species generated in an arc jet confirms the robust coating performance in extreme environments.     

Thermal conductivity mapping of the Ni–Al system and the beta-NiAl phase in the Ni–Al–Cr system

Micron-scale-resolution thermal conductivity mapping on graded compositions created in diffusion-multiple samples can be used to rapidly establish composition-phase-property relationships and to reveal the effects of solid-solutioning, order-disorder transition, compositional point defect, and site preference on thermal conductivity.

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Characterizations on Mechanical Properties and In Vitro Bioactivity of Biomedical Ti–Nb–Zr–CPP Composites Fabricated by Spark Plasma Sintering

To alleviate the bio-inert of Ti alloys as hard tissue implants, Ti–35Nb–7Zr–xCPP(calcium pyrophosphate,x = 5, 10, 15, 20 wt%) composites were prepared by mechanical alloying(MA) and following spark plasma sintering(SPS). Mechanical behaviours and in vitro bioactivity of these composites were investigated systematically. Results showed that the composites consisted of β-Ti matrix, α-Ti, and metal–ceramic phases such as CaO, CaTiO_3, CaZrO_3, and Ti_xP_y. With increasing CPP content, the composites had higher strength(over 1500 MPa) and higher elastic modulus, but suffered almost zero plastic deformation together with lower relative density. When the CPP contents were 5 and 10 wt%,the compressive elastic moduli were 44 and 48 GPa, respectively, which were close to those of natural bones. However, the compressive elastic modulus of the composites increased significantly when CPP contents exceed 10 wt%, thus deteriorating the mechanical compatibility of the composites owing to more α-Ti and metal–ceramic phases. Besides, the surface of Ti–35Nb–7Zr–10CPP composite was deposited as a homogeneous apatite layer during soaking in simulated body fluid(SBF). It indicates a good bioactivity between the implant materials and living bones.     

Structural Stability,Electronic Structure and Mechanical Properties of Li–N–H System

Ab initio calculations are performed to investigate the ground state properties, structural phase transition,electronic structure and mechanical properties of lithium nitride(Li3N), lithium imide(Li2NH) and lithium amide(LiNH2).The computed ground state properties like equilibrium lattice constant, cell volume, valence electron density, cohesive energy, bulk modulus and its derivatives are in good agreement with available experimental data. The structural phase transitions from a-P6/mmm to b-P63/mmc phase at a pressure of 17.5 GPa in Li3N and cubic(Fm3m) to hexagonal(P63/mmc) phase at a pressure of 102 GPa in lithium imide(Li2NH) are observed. A new high pressure hexagonal(P63/mmc)phase is predicted for Li2NH. Electronic structure reveals that Li3N and LiNH2 are semiconductors, whereas Li2 NH is an insulator. The calculated elastic constants indicate that these materials are mechanically stable at ambient condition.     

Electrochemical Properties and Microstructure of Al/Pb–Ag and Al/Pb–Ag–Co Anodes for Zinc Electrowinning

The Al/Pb–0.8%Ag and Al/Pb–0.75%Ag–0.03%Co(in mass fraction) anodes used in zinc electrowinning are prepared through the electrodeposition of lead methanesulfonate electrolyte onto an aluminum matrix.The results of anode polarization curves,Tafel curves,and EIS characterizations indicated that the Al/Pb–0.75%Ag–0.03%Co anode has higher electrocatalytic activity and corrosion resistance than the Al/Pb–0.8%Ag anode.SEM observations on the fruit surfaces demonstrated the crystals on the Al/Pb–0.8%Ag anode are larger than on the Al/Pb–0.75%Ag–0.03%Co anode.After 24 h of anodic polarization,SEM observations and XRD analysis showed that the MnO2–PbO2layer on the Al/Pb–0.75%Ag–0.03%Co anode surface is characterized by dendritic crystals,and the PbSO4–PbO2layer under the MnO2–PbO2layer is characterized by uniform and chaotic orientation tetragonal symmetry crystallites of PbSO4.However,the MnO2–PbO2layer on the Al/Pb–0.8%Ag anode surface is characterized by granular crystals,and the PbSO4–PbO2layer under the MnO2–PbO2layer is characterized by well-organized orientation crystallites of PbSO4,which are concentrated in certain zones.