Self-propagating high-temperature synthesis (SHS) is a good way to prepare ZrB2-ZrC/metal cermet composites. In this work, ZrB2-ZrC/Ni cermet composites with various Ni contents were successfully ...fabricated by SHS using the Ni-Zr-B4C system. The effects of Ni content and particle size of the B4C powder on the SHS reaction were investigated. The results indicated that with an increase in Ni content, the adiabatic temperature, maximum combustion temperature, ignition delay time, and ceramic particle size in the product all showed a gradually decreasing trend. The SHS products and the ignition of the SHS reactions were significantly dependent on the B4C particle size. The formation mechanism of ZrB2 and ZrC during SHS from the Ni-Zr-B4C system was proposed based on the combustion wave quenching experiment.
The reaction mechanism of ZrB2-ZrC formation in a 30% Ni-Zr-B4C system under argon was revealed by using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron ...microscopy (SEM). The results indicated that the reaction mechanism in the Ni-Zr-B4C system was complex. Initially, NixZry and NixBy intermetallics were formed via solid-state diffusion reactions between Ni, B4C and Zr. Then, the eutectic reaction between Ni2B and Ni4B3 lead to the formation of Ni-B liquid. The free C atoms dissolved into the Ni-B liquid to form a Ni-B-C ternary liquid, and then part of the Zr powder dissolved into the surrounding Ni-B-C ternary liquid to form Ni-Zr-B-C quaternary liquid. Finally, ZrB2 and ZrC formed and precipitated out of the saturated liquid. The eutectic liquid plays an important role during the formation of ZrB2-ZrC.
•MAO/Ni–P coat is designed to improve the corrosion resistance of Mg alloy.•HF activation is necessary for the deposition of Ni–P coat on MAO layer.•MAO/Ni–P coat has dense top coat and stable ...bonding interface.•MAO/Ni–P coat shows more prominent corrosion resistance than the monolithic coat.
Microarc oxidation (MAO) coat was designed as an intermediate layer for the electroless plated Ni–P top coat, providing inert surface and necessary hardness for Mg alloy substrate. The composite coat was successfully prepared to improve the corrosion resistance of Mg alloy. The preparation and the characterization of the composite coat were investigated. The results show that the pre-treatment of MAO before electroless plating plays an important role in the deposition of compact composite coat. The activation (by HF solution) makes the MAO coat dense with uniform cracks which supply excellent bonding interface for Ni–P coat. Compared with monolithic MAO or Ni–P coat, the composite coat has excellent corrosion resistance and stable bonding interface. There is main pit corrosion at substrate after the corrosive medium penetrating through the whole coat. With the inert MAO interlayer, the electrochemical corrosion between the Ni–P and substrate is effectively inhibited.
A new functionally graded thermal barrier coating (FG-TBC) based on LaMgAl11O19 (LaMA)/YSZ has been designed and prepared via air plasma spraying. The microstructure and phase stability are ...investigated by X-ray diffraction, SEM and high-temperature DSC analysis. Results indicate that all the LaMA and LaMA-containing intermediate composite coatings suffer irreversible phase transformations induced by the recrystallization of amorphous LaMA coating and gamma to alpha -Al2O3 transition during heating, and LaMA and YSZ coatings are chemically compatible at elevated temperature. The appearances of LaMA platelet-like grains during the recrystallization process, impart the intermediate LaMA-containing composite coating layers with improved sintering resistance, reduced densities and hardness. The wear behavior and the influences of heat treatments on the microhardness of the coatings are also investigated. Coefficient of thermal expansion changes gradually from the YSZ bottom layer to the LaMA top layer, which would result in a reduced residual stress level compared to the LaMA/YSZ double ceramic layered TBC system. Such a FG-TBC displays an excellent burner-rig thermal cycling lifetime of ~11,749cycles at the surface testing temperature about 1372 degree C, showing the great potential for advanced TBCs applications in the future.
The reaction sequence and phase formation mechanism in self-propagating high temperature synthesis (SHS) processing of the Al-Ti-B4C compacts were explored through a delicate microstructure and phase ...analysis on the combustion-wave quenched samples in combination with differential thermal analysis. The reaction sequence could be described as Al +Ti + B4C TiA13 + Ti + B4C - > - TiAl' + B4C -+/- TiAl' + B13C2 + TiC, + TiaAlbCc -4- TiAln + + TiB2 + TiaAlbCc Al + TiC, + TiB2. The phase formation mechanism could be ascribed to the displacive reaction between B4C and the Ti-Al melt and is essentially dependent on the dissociation and diffusion rates of carbon and boron from the B4C crystal. The reasons for the prior formation of TiCX over TiB2 and the separated residence of the TiCX and TiB2 grains in the microstructure were addressed.
New LaMgAl11O19 (LaMA)/YSZ double ceramic top coat thermal barrier coatings (TBCs) with the potential application in advanced gas-turbines and diesel engines to realize improved efficiency and ...durability were prepared by plasma spraying, and their thermal cycling failure were investigated. The microstructure evolutions as well as the crystal chemistry characteristics of LaMA coating which seemed to have strong influences on the thermal cycling failure of LaMA and the new double ceramic top coat TBCs based on LaMA/YSZ system were studied. For double ceramic top coat TBC system, interface modification of LaMA/YSZ by preparing thin composite coatings seemed to be more preferred due to the formations of multiple cracks during thermal cycling making the TBC to be more strain tolerant and as well as resulting in an improved thermal cycling property. The effects of the TGO stresses on the failure behavior of the TBCs were discussed through fluorescence piezo-spectroscopy analysis. a-ordm The failure of LaMgAl11O19/YSZ double layered TBCs was reported for the first time. a-ordm The nanocrystallization phenomenon of the LaMA coating was put forward. a-ordm Single layered TBC may exhibit a life close to the corresponding double layered TBCs.
TiC-TiB2 particulate locally reinforced steel matrix composites were fabricated by a novel TE-casting route from an Al-Ti-B4C system with various B4C particle sizes. The formation mechanism of TiC ...and TiB2 in the locally reinforced regions was investigated. The results showed that TiC and TiB2 are formed and precipitated from Al-Ti-B-C melt resulting from the dissociation of B4C into Al-Ti melt when the concentrations of B and C atoms in the Al-Ti-B-C melt become saturated. However, in the case of coarse B4C powders ( greater than or equal to 40 mu m) used, the primary reaction in the Al-Ti-B-C melt is quite limited due to the poor dissociation of B4C. The poured steel melt infiltrates into the primary reaction product and thus leads to the formation of Al-Fe-Ti-Ba "C melt, thanks to the favorable reaction of molten Fe with remnant B4C, and then TiC and TiB2 are further formed and precipitated from the saturated Al-Fe-Ti-Ba "C melt. The relationship between the mechanisms of thermal explosion (TE) synthesis of TiC and TiB2 in the electric resistance furnace and during casting was proposed.
NiCoCrAlY/8wt.%Y2O3–ZrO2 coating was plasma sprayed on aluminum alloy to evaluate the effect of ceramic thickness on residual stress and bonding strength. A new stress calculation method based on ...Stoney equation and substrate-removal technique was proposed. Stress in both bond coat and ceramic was studied. With the increase of ceramic thickness, the residual stress in both layers was firstly compressive then turned tensile. The large thermal expansion coefficient of the substrate played an important role in residual stress formation when the ceramic was thin. However, the intrinsic deposition stress took a dominant position when the ceramic coating turned thicker. The bonding strength decreased and the location of the fractured surface moved toward the ceramic surface. The moving of the surface was mainly resulted from the variation of stress gradient and the weakness of high porosity zone near the bond coat–ceramic interface.
► Duplex thermal barrier coating was deposited on an unconventional substrate. ► The residual stress was evaluated by combined use of two experimental methods. ► The influence of different ceramic thickness was investigated. ► The relationship between residual stress and bonding strength was clarified.
The α-Ni(OH)
2
coated with manganese (Mn) materials was synthesized by the chemical precipitation method, and the electrochemical properties were investigated. The results showed that the ingredient ...of the coating is a mixture of MnO
2
and Mn
2
O
3
, and the coating can inhibit the intercalated anions escape and stabilize the α-Ni(OH)
2
phase structure in KOH alkaline solution. The coated electrodes have higher discharge capacities and higher capacity retention rates than the uncoated α-Ni(OH)
2
electrode, the discharge capacity of Mn-coated electrodes is over 350 mAh g
−1
, and the capacity retention rate exceeds 92%, separately. The reason may be that the Mn coatings can block or decelerate water molecule and the anion to escape from the interlayers, therefore prevent α phase structural collapse and maintain high discharge capacity during a long lifespan cycle. Therefore, the Mn coating α-Ni(OH)
2
is a promising candidate for high-power Ni-MH batteries.
PDF
