Key Engineering Materials Vols. 373-374

Abstract: Thermal barrier coatings (TBCs) were prepared by the recently-developed detonation gun spray process. The oxide scale formation and micro-damage evolution of these coatings during oxidation in air at 1100 °C were investigated. It was found that duplex oxide scales, the upper oxide mixture scale and α-Al2O3 subscale, form at the interface between bond coat (BC) and top coat (TC) during the oxidation. Microcracks usually nucleate within the porous oxide mixture layer. With the increase of oxidation time, some microcracks coalesce to form a long crack. Residual stress in the thermally grown oxides (TGO) was measured using photo-stimulated luminescence spectroscopy. It was found that compressive residual stress exhibits a fast increase at the beginning of oxidation up to maximum value for about 10h. Then, the compressive stress begins to decline up to 100h. Local stress distribution showed that the micro-damage in the TGO causes a remarkable decrease in the magnitude of compressive residual stress.

Abstract: The thrust surface of W615 engine is remanufactured by plasma transferred arc (PTA) powder surfacing technology. The Ni15 Nickle-based alloy powder is selected as deposited metal. By optimizing the welding procedure, the coating is free of crack and pore. The microstructure characteristics of deposition layer are investigated by metallographic microscope, and the microhardness of the overlay is measured by vickers hardness tester. It is found that the heat affected zone is narrow, the rate of dilution is low and the microhardness of deposition layer is about HV130 which is close to that of base metal. Furthermore, the hardness of welding surface can meet the needs of the engine.

Abstract: Cr3C2-NiCr compound coating was acquired on 20 steel substrate by plasma spraying process. The microstructure of the coating is dense with low porosity. Carbide lamellartiy inlays into the nickel-base solid solution in the coating. Main phases of the coating are Cr3C2, Cr23C6, Cr7C3 and nickel-base solid solution, etc. Average microhardness of the coating is about HV0.1756.9. Microhardness of some ceramic phases in the coating is as high as HV0.11113, which helps to the improving of wear resistance of the coating. The wear-resisting property of the coating is good under grain-abrasion condition and excellent under lubricated wear condition.

Abstract: Al2O3 coatings were fabricated by multi-functional micro-plasma spray through axial powder feed. The flight particle velocity and temperature were measured by SprayWatch-2i. With the rising of Ar flow, the velocity of Al2O3 particles increases, but the temperature of particles decreases gradually. Al2O3 coatings were analyzed by SEM and XRD. The microstructure of coatings is density, low porosity and uniform morphology. X-ray diffraction results show that α-Al2O3 is the main phase in the original Al2O3 powders, but Al2O3 coatings consist of γ-Al2O3 and a small amount of α-Al2O3. The microhardness and bonding strength of Al2O3 coatings were also measured, which is HV0.11183~1387 and 28.6MPa, respectively.

Abstract: A protective zinc-aluminum spray coating and a post heat treatment was carried out onto AZ91D magnesium alloy. The effect of post heat treatment on the interfacial characteristics, corrosion behaviors, micro-hardness, and thermal shock resistance of the zinc-aluminum coating were evaluated in this paper. It was found that a dense inter-coating formed at the Zn/Mg substrate interface followed by the post heat treatment. The results of EDS analysis indicated that the diffusion took place among Mg, Al and Zn atoms. The Zn-Al deposited coating with heat treatment had much more corrosion resistant and higher resistant to thermal shock. It was harder than as-received AZ91D Mg alloy.

Abstract: Nonastructured Al2O3-13wt%TiO2 (AT13) coatings were deposited by multi-function micro-plasma spray and Metco 9M plasma spray, respectively. Constituent phases and the microstructure of the powder particles and coatings prepared were examined with the aid of scanning electronic microscope (SEM) and X-ray diffraction (XRD). Mechanical properties including hardness and bonding strength were also evaluated by microhardness tester and electron tensile tester. Multi-function micro plasma sprayed nanostructured AT13 Coating is fully-melted, dense and uniform. However, AT13 Coating deposited by Metco 9M plasma spray is partial-melted. The microhardness of multi-functional micro plasma sprayed AT13 Coating is HV975.7~1441.7, much higher than that of Metco 9M plasma sprayed AT13 Coating (HV655.3~946.6). The bonding strength results present the same, increased from 19.8 MPa to 42.7 MPa.

Abstract: Cored wires and high velocity arc spraying (HVAS) technologies have been applied to produce Zn-Al-Mg-RE coating on A3 steels. The durability of coating in terms of corrosion resistance is the focus of this paper. The detailed degradation mechanism has been investigated using electrochemical-monitoring techniques supported by precise pre-test and post-test microscopical examination using Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). Zn-Al-Mg-RE coating shows a typical aspect of layered heterogeneous structure, the existing of ZnAl2O4 and MgAl2O4 implies the good anti-corrosion feature. The polarization curves showed that Zn-Al-Mg-RE coating exhibits passive behavior at 0.5h and 16h immersion, but at 600h, it begins to dissolute. In contrast, the anti-corrosion performance isn’t cut down. The corrosion potential moves to the noble direction step by step, along with the current density drops gradually. After corrosion, it has been observed that the coating surface is jointed by flaky lamellar products and dense without obvious defects and pores. It is postulated that with the addition of RE, the porosity of the coating is reduced, meanwhile, the stability of the products layer can reinforce the self-sealing effect of the coating distinctively.

Abstract: YSZ molten droplets created by plasma spraying were deposited on the YSZ substrate preheated to different temperature from 75oC to 1000oC. The results showed that there is little change in crack spacing when substrate temperature is less than about 740oC, and a significant increase in crack spacing from 3.54 μm at 740oC to 10.91 μm at 1000oC was observed. A simple qualitative model was proposed on the basis of the origin of thermal stresses to explain the influence of substrate initial temperature on crack density characterized by crack spacing. The analysis indicates that the results can be ascribed to the changes in tensile stress and splat/substrate adhesion resulting from changes of substrate initial temperature.

Abstract: Nanostructured FeAl/WC intermetallic composite coatings were prepared by cold spaying of the ball-milled powders. The effect of annealing on the coating structure and microhardness was examined. It was found that the nanocrystalline structure of the milled feedstock was retained in the cold sprayed coatings. The FeAl intermetallic phases were formed from the milled Fe(Al) solid solution during the post-spraying annealing at 550oC. The microhardness of the as-sprayed coating was about 680HV0.1 and it decreased a little with increasing the annealing time at 550oC.

Abstract: The wear-resisting property and hardness of arc-sprayed coatings is affected by spraying parameters, such as compressed air pressure, spraying current, spraying distance and wire feeding speed. In this paper, the four factors orthogonal experiment method was applied to optimize those four parameters. T12 high-carbon steel wire and SCDP-3 spray gun were used in the experiment. SKODA-Savin plunge wear test machine and the HB3000 Brinell hardness tester were used to measure the wear resistance and hardness of the coating. Based on the analyses of experimental data, the relationship curves of four factors versus the wear resistance and hardness were set up separately. The variance analysis of data shows that the gas pressure and spraying distance have the most significant effect on the wear resistance, wire feeding speed has the effect on the wear resistance, and the spraying current has less effect on the wear resistance. Four factors significantly affect the coating hardness, however the compressed air pressure has the greatest effect on the coating hardness. By the optimization analysis, the optimum parameters are obtained.