Key Engineering Materials Vols. 368-372

Abstract: Ti(C,N)-based cermets were coated with a TiN/TiAlN coating using ion plating technology. The sliding wear test was performed for the coated cermets and the microstructure, composition and surface roughness of the coated cermets under different velocities and loads were characterized. The results showed that the friction coefficients of the coated cermets were lower than that of the neat cermets. Under the same load, the adhesion phenomenon of the counterpart materials on the specimens was improved and the mean friction coefficient increased with increasing sliding speed. Under the same sliding velocity, the average friction coefficient of the coated cermets was lower under higher load. The wear mechanisms were mainly adhesive wear and abrasive wear.

Abstract: TiN/NbN multilayers on steel substrates (SKD11) are produced using DC magnetron sputtering process. The multilayer obtained are characterized in composition by means of X-ray diffraction techniques, Microhardness and adhesion to the substrate were studied by atomic force microscopy and scratch tests. The morphological analysis and coating structure are studied using scanning electron microscopy, and atomic force microscopy. The film thickness is measured by a stylus profiler (XP-2 stylus profiler). Wear tests were performed on pin-on-disk configuration and dry sliding conditions, at 5N load by using hardened steel ball. On mechanical properties, higher Young’s modulus and hardness values follow to increase the TiN/NbN layers number. The multilayer films of 64 layers at 500°C 1h annealing indicated that the microhardness and Young’s modulus had the highest values.

Abstract: Ti(CN) -based ceramics coating was deposited on TC4 titanium alloy substrate using the home- made reactive electric spark deposition system, which is under nitrogen-sealed atmosphere and using the graphite electrode. It was showed that: (1) the single-pulse deposition zone presents irregular spattering shape and the phase of coatings was made up of TiC0.51N0.12 spherocrystal and dendritic crystal, Ti0.80V0.20 and C, (2) metallurgical bond between coating and substrate is realized, (3) the Ti(CN) ceramic particle, whose average grain size is 626nm, distributes dispersively among the coating and is in-situ synthesized by the reaction among titanium from the substrate, carbon from the graphite electrode and nitrogen from the shielding nitrogen gas.

Abstract: The applications of titanium and titanium alloys under severe wear conditions are highly restricted due to their low hardness and poor tribological properties. To improve the hardness and wear resistance, laser cladding of commercial purity titanium was carried out with a 5 kW continuous wave CO2 laser. WC powders and Ti powders were mixed for cladding in different proportions. The phases of clad layer were characterized by scanning electron microscopy, X-ray diffractometer and energydispersive X-ray spectroscopy. The mechanism of the reaction between melted WC and Ti is discussed.

Abstract: Three compositions, (Ni75Si25)-5Cr, (Ni75Si25)-10Cr and (Ni75Si25)-15Cr, have been cladded onto Ni-based suaperalloy substrate by pre-placing laser cladding process with a 5 kW continuous wave CO2 laser. Ni75Si25 was also cladded with the same method for comparison. The process parameters have been optimized to obtain defect free claddings. The microstructure and the hardness of the cladded layers were characterized by optical microscopy, scanning electron microscopy, X-ray diffraction and microhardness measurement. The corrosion resistance of the cladded layers was measured in a sulfuric acid solution.

Abstract: Nanostructured surface layer was produced on the Ti-6Al-4V alloy by surface mechanical attrition treatment (SMAT) technique, and then the ceramic composite coatings of surface nanocrystallization/ micro-arc oxidation (SNC/MAO) were prepared on the nanocrystallized surface of Ti-6Al-4V alloy by pulsed single-polar micro-arc oxidation in NaAlO2 solution. The phase composition, morphology of surface and section and element content of the nanostructured surface layer and the ceramic coatings were investigated by XRD, TEM, SEM and EDS, respectively. The results showed that after the SMAT technique treatment for a short period of time, the surface layer was refined into ultrafine grains. The thickness of the coatings on the substrate treated by SMAT technique is about 10μm by micro-arc oxidation, which is thicker than the ones on the substrate untreated by SMAT technique, and the ceramic coatings are mainly composed of Al2TiO5. Besides, a bright interface layer comes out between the substrate and the coating when the substrate treated by SMAT technique. The influences of the SMAT technique on the surface morphology of the ceramic coatings also changed. The coatings using the SMAT technique were more compact and less porous than the ones on the substrate untreated by SMAT technique. The content of elements in the nanostructured surface layer on Ti-6Al-4V alloy had changed after the SMAT technique: the content of Ti increased, the content of V decreased, and the content of Al changed a little. The contents of Al and P in the coating increased while the content of Ti decreased.

Abstract: Thermal conductivity of zirconia and yttrium stabilized zirconia (YSZ) is calculated with perturbation molecular dynamics method (PMD). The results showed that thermal conductivity of YSZ is lower than that of pure zirconia and PMD is a very effective way in thermal conductivity calculation for ceramics. In higher temperature region, the calculated values show a different tendency with the experiment ones, which is because that photon conduction is not considered in PMD method. By taking photon effects into account, the calculated thermal conductivity curve is found fairly well coherent with measurements. In addition, the thermal expansion of zirconia is also presented by calculating the volumes at different temperatures. The results and methods in this paper have been proved to be very helpful in further design of new ceramic materials in the field of thermal barrier coatings (TBCs).

Abstract: Based on La2Zr2O7 ceramic for thermal barrier coatings, LaSmZr2O7 ceramic doped with samarium ion was successfully prepared using solid state reaction method. The pellets were sintered at 1600°C for 10 hours in air. The phase structure, thermal conductivity and thermal expansion coefficient of LaSmZr2O7 ceramic and La2Zr2O7 ceramic were measured by XRD, laser-flash device, high-temperature dilatometry, respectively. The results show that the crystal structure of LaSmZr2O7 ceramic is not affected by the doped samarium ion in the zirconium lattice. The thermophysical results show that the thermal conductivity of the LaSmZr2O7 ceramic is lower than that of La2Zr2O7 ceramic, while the thermal expansion coefficient is higher than that of La2Zr2O7 ceramic. These results indicate that LaSmZr2O7 ceramic or Ln2Zr2O7 ceramics doped with other rare earth ions could be candidate materials for future thermal barrier coatings.

Abstract: Pyrochlore structure oxide (La0.5Sm0.5)2(Zr0.9Ce0.1)2O7 was prepared by mixing the appropriate amounts of ZrO2, La2O3, Sm2O3 and CeO2, followed by reacting at 1600°C for 10 h. It was shown that the thermal conductivity was much lower than that of 8YSZ, which was mainly ascribed to the oxygen vacancies and lattice strain in lattice of (La0.5Sm0.5)2(Zr0.9Ce0.1)2O7.

Abstract: A complex rare-earth zirconate (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.3)2O7 powder for thermal barrier coatings (TBCs) was synthesized by coprecipitation method. The phase composition, microstructure and the thermophysical properties were investigated. XRD results revealed that single phase (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.3)2O7 with pyrochlore structure was prepared and the SEM result showed that the microstructure of the product was dense and no other phases existed among the particles. With the temperature increasing, the thermal expansion coefficient (CTE) of the ceramic increased, while the thermal conductivity decreased. The results indicated that CTE of the ceramic was slightly higher than that of La2Zr2O7 and the thermal conductivity of the ceramic was lower than that of La2Zr2O7. These results imply that (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.3)2O7 can be explored as the candidate material for the ceramic layer in TBCs system.