High velocity oxy-fuel thermal spraying was used to prepare free-standing CoNiCrAlY ) bond coat alloy samples approximately 0.5 mm thick. Creep tests were conducted at 750 C on these samples using a small punch (SP) creep test method. The samples were characterised before and after creep testing using scanning electron microscopy with electron backscatter diffraction (EBSD). EBSD revealed a two phase fcc -Ni and bcc B2 β-NiAl microstructure with grain sizes ~ 1-2 m for both phases, which did not change significantly following testing. The constant temperature SP test data were characterised by a minimum creep strain rate, , and a total time to failure, t f , at different applied stresses. The data are fitted to conventional power law equations with a stress exponent for creep close to 8 in the Norton power law and between 7 and 10 in the Monkman-Grant creep rupture law. Creep rupture was predominantly due to creep cavitation voids nucleating at both the γ -β interphase boundaries and the - grain boundaries leading to final failure by void linkage. However, rupture life was influenced by the quantity of oxide entrained in the coating during the spray deposition process.
Free-standing VPS and HVOF CoNiCrAlY coatings were produced. The as-sprayed HVOF coating retained the c/b microstructure of the feedstock powder, and the VPS coating consisted of a single (c) phase. A 3-h, 1100°C heat treatment in vacuum converted the single-phase VPS coating to a two-phase c/b microstructure and coarsened the c/b microstructure of the HVOF coating. Oxidation of freestanding as-sprayed and heat-treated coatings of each type was carried out in air at 1100°C for a duration of 100 h. Parabolic rate constant(s), K p , were determined for free-standing, as-sprayed VPS and HVOF coatings as well as for free-standing coatings that were heat treated prior to oxidation. The observed increase in K p following heat treatment is attributed to a sintering effect eliminating porosity from the coating during heat treatment. The lower K p values determined for both HVOF coatings compared to the VPS coatings is attributed to the presence of oxides in the HVOF coatings, which act as the barrier to diffusion. Oxidation of the as-sprayed coatings produced a dual-layer oxide consisting of an inner a-Al 2 O 3 layer and outer spinel layer. Oxidation of the heat-treated samples resulted in a singlelayer oxide, a-Al 2 O 3 . The formation of a thin a-Al 2 O 3 layer during heat treatment appeared to prevent nucleation and growth of spinel oxides during subsequent oxidation.
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AbstractIt is commonly observed that there is a performance gap between the corrosion resistance of thermally sprayed coatings and the equivalent bulk material. This is attributed to the significantly modified microstructure of the sprayed coatings. However, currently there is no detailed understanding of which aspects of microstructural modification are primarily responsible for this performance gap. In this work several deliberately microstructurally modified versions of the Ni-based superalloy Inconel 625 were produced. These were subjected to potentiodynamic electrochemical testing in 0.5M H 2 SO 4 to investigate the links between specific microstructural features and electrochemical behaviour. Samples were prepared by high velocity oxy-fuel (HVOF) thermal spraying, laser surface remelting using a high power diode laser and conventional powder sintering. Microstructural features were examined by optical and scanning electron microscopy and X-ray diffraction.Potentiodynamic testing was carried out on the following forms of Inconel 625: wrought sheet; HVOF sprayed coatings; sintered powder compacts; laser melted wrought sheet and HVOF sprayed coatings. Using the corrosion behaviour, i.e. passive current density, of the wrought sheet as a baseline, the performance of different forms of Inconel 625 were compared. It is found that a fine dendritic structure (with associated microsegregation) produced by laser remelting wrought sheet has no significant effect on corrosion performance.Up to 12% porosity in sintered powder samples increases the passive current density by a factor of only around 2. As observed previously, the passive current density of HVOF sprayed coatings is 20 -40 times greater. However, HVOF coatings subjected to laser surface remelting are found to have a passive current density close to that of wrought material. It is concluded that, whilst porosity in coatings produces some decrease in corrosion resistance, the main contributing factor is the galvanic corrosion of localised Cr-depleted regions which are associated with oxide inclusions within HVOF sprayed samples.
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