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Abstract: In this work, a comparative evaluation has been carried out on the deposition efficiency of iron boride powder with a composition of Fe–17.9B–0.4C–1.6Si–0.3Al (wt.%) mixed with Ni powder (20 wt.%Ni) sprayed onto low carbon steel substrate by cold gas dynamic spray processes and the High Velocity Oxy-Fuel (HVOF) spraying method. The particle deformation and bonding mechanism were evaluated. Direct spraying of iron boride powders by low and high pressure cold gas spraying led to depositing a very thin single layer on the substrate, due to the intrinsic brittleness of the powder. Prior to spraying, reducing the particle size (−20μm) and increasing the internal energy of the powders by the ball milling process resulted in no appreciable improvement in deposition efficiency, implying that sprayed particles still had insufficient ductility to plastically deform and flow. Likewise, iron boride powders were spray dried and sintered with Ni particles (FeB-20wt.% Ni) to deposit layers in both low pressure and high pressure cold gas spray processes with various spray parameters. It was discovered that iron boride particles, surrounded by ductile Ni, experienced disintegration and cracking during deposition. The build up of iron boride layers was successfully obtained when spraying FeB20Ni powder by the HVOF deposition method. An abrasion wear test result of the HVOF coatings, 952 HV, was compared with HVOF sprayed WC-12Co coatings. [Copyright &y& Elsevier]

Abstract: Recently, a large local stress has been found, caused by the change of both the diffusion rate of oxygen through an existing oxide and the rate of chemical reaction at the oxide/oxidized material interface. Since high thermal stress occurs in the thermal barrier coating (TBC) system, the volume expansion of the newly grown oxide, and centrifugal force, the growth rate of the thermally grown oxide (TGO) may change depending on the temperature, the exposure time, and the stress. The aim of this study is to make clear the influence of stress on the growth rate of the TGO thickness under static oxidation. The results show that TGO thickening was affected by the increase of not only the exposure temperature but also the applied stress. The tensile stress in a longitudinal direction in a TBC system due to the applied load makes the oxidant transport and oxide flow more easy, i.e., the tensile stress promotes a volume expansion of newly formed oxide. The increase rate of the TGO thickness was approximately 34% when the applied stress increased from 0 to 205 MPa at 900 °C for 325 h, and approximately 25% when the stress increased from 0 to 150 MPa at 950 °C for 125 h. [Copyright &y& Elsevier]

Abstract: Brittle SiC coatings have been successfully applied on Ni–Cr-based superalloy substrates to improve their oxidation resistance at high temperatures. The coating has been formed through the plastic deformation of the substrate with particle segmentation. The overlaid SiC layer shows the ability to slow down the outward chromium diffusion and hence sustaining the elemental content of the substrate. The formation of MoSi2 and SiO2 layers beneath SiC layer plays an important role in the oxidation rate reduction at elevated temperature environment. Scratch adhesion test result shows an excellent adhesion of SiC coating, as the coating delamination did not happen up to a 3.7N load. Four-point bending tests also pointed out the strong adhesion of oxide scales, which is necessary for further oxidation prevention. [Copyright &y& Elsevier]