Quansheng Wang

Microstructural Characteristics and Oxidation Behavior of Low-Pressure Cold-Sprayed CoNiCrAlY Coatings

CoNiCrAlY coatings were deposited by low-pressure cold spraying and subsequently heat-treated at 1050 °C for 4 h in a vacuum environment. The microstructural characteristics and oxidation behavior of CoNiCrAlY coatings were investigated. The as-sprayed coating exhibited low porosity and oxygen content. The high plastic deformation of the sprayed particles led to significant refinement of γ-matrix and dissolution of β-(Ni,Co)Al phase in the as-sprayed coating. After heat treatment, the single phase (γ) in the as-sprayed coating was converted into a γ/β microstructure, and a continuous single α-Al2O3 scale was formed on the coating surface. Vacuum heat treatment can postpone the formation of spinel oxides within 100 h. After being oxidized at 1050 °C for 400 h, the heat-treated coating exhibited better oxidation resistance than the as-sprayed coating. The reduced growth rate of the oxide scale and the suppression of the formation of spinel oxides can be attributed to the vacuum heat treatment, as well as the intrinsic microstructure of the cold-sprayed coating. Finally, the effects of the microstructural changes induced during the cold spraying process on the growth of the thermally grown oxide and the oxidation mechanisms of the CoNiCrAlY coatings were discussed.

Oxidation of Ni-based single crystal after grit-blasting during exposure at high temperature

Abstract The effect of grit-blasting on the oxidation of Ni-based single crystal at 1150 and 1250 °C was studied. The oxide scales formed on the samples with or without grit-blasting were characterized by SEM, XRD, EDS, and EPMA. The results indicate that grit-blasting introduces a plenty of oxide nodules with lamellar structure into the oxide scale of samples after oxidation. The oxide nodule is composed of external and internal part. The external part is multi-layered with outer Cr2O3 layer, inner Al2O3 layer and transitional layer. The internal part includes several alternative alloy layers lack of Al and Al2O3 layer. Hf and Ta segregate at the oxide/alloy and oxide/oxide interface in two parts. Two recrystallized grains are formed under the blasted alloy surface after grit-blasting and heat exposure. The formation of oxide nodules accelerates the development of equiaxed recrystallization. Grit-blasting introduces abundant paths of oxygen diffusion and residual stress into the alloy, promoting the formation of oxide nodules and recrystallization.

Anti-ablation properties of MoSi2–W multi-layer coating system deposited by atmospheric plasma spray

Abstract Carbon fibre reinforced carbon (C/C) composites have become an important material for hyper thermal conditions and have been widely used in aerospace and aviation industry. However, the composites will be ablated above 723 K in an oxygen-containing atmosphere. The MoSi2–W multi-layer anti-ablation coating system was deposited by atmospheric plasma spray. The supersonic flame caused by kerosene and oxygen was selected to examine the property of the coatings. Samples of three groups were ablated by the flame at 1600°C. The speed of gas flow was 850 m s–1. The MoSi2–W multi-layer anti-ablation coating system can protect the graphite matrix effectively. The products of ablation include SiO2, Mo5Si3 and MoO3. With the prolongation of time, the aggregates sinter and grow up. The network structure of the coatings is caused by the evaporation of MoO3 and loss of SiO2.

Preparation of Mo(Si,Al)2 feedstock used for air plasma spraying

Abstract In order to prepare high quality Mo(Si,Al) 2 feedstock characterized with C40 phase, higher Al doping amount and excellent flowability, Mo(Si 1– x ,Al x ) 2 with different Al contents ( x =0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized by self-propagating high-temperature synthesis first and Mo(Si 0.6 ,Al 0.4 ) 2 was confirmed as the suitable material through X-ray diffraction analysis. A series of tests with different parameters of induction plasma spheroidization were applied to improving the flowability of feedstock. Mo(Si,Al) 2 feedstock with excellent flowability (26.2 s/50 g) was prepared through adding hydrogen into sheath gas and decreasing the powder feeding rate. The composition segregation occurred in the spheroidized powder after Al consumption and oxidation. The inhomogeneous structure of the same particle was caused by the asymmetric heating and cooling when particle passed through the plasma jet.