Papers by Author: F.H. Yuan

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: A cast nickel based superalloy M963 was coated by high-velocity oxy-fuel (HVOF) spraying process. The effect of HVOF MCrAlY coating on thermo-mechanical fatigue (TMF) and isothermal fatigue (IF) in M963 was studied to understand fatigue life and failure mechanisms in coated and uncoated M963 alloy. Two types of TMF tests, i.e. in-phase (IP) and out-of-phase (OP), were performed in temperature range of 450~900°C, and IF tests were conducted at 900°C. It was found that the coating had a detrimental effect on fatigue life under OP TMF, while a beneficial effect of the coating existed under IP TMF and IF. Crack initiation time in the coated specimen was shorter than that in the uncoated specimen and the former’s crack density was higher than the latter’s one under OP TMF. The relationship of deformation and fracture response with fatigue life was discussed based on microscopic analysis.

Abstract: NiCrAlY coatings were prepared by the detonation spraying (DS) and high velocity oxygen fuel (HVOF) spraying processes, respectively. The DS NiCrAlY coating has higher oxidation rate than the HVOF NiCrAlY coating at 1100 oC due to its rougher surface and more mixed oxides. The residual compressive stress in thermally grown oxides (TGO) on the DS NiCrAlY coating is higher than that on the HVOF coating because the θ-Al2O3 to α-Al2O3 transformation causes the tensile stress in the TGO on the HVOF coating.