OS11(4)-17(OS11W0021) Fatigue Behavior of Thermally Sprayed Materials at High Temperature

Abstract

High temperature fatigue and thermal cycle damage behavior of SUS304 steel thermally sprayed Al_2O_3/NiCr atmospheric plasma was investigated. Surface deformation behavior was measured using an electronic speckle pattern interferometry (ESPI) system. Cracks and delaminations in the thermally sprayed coatings were observed after high temperature fatigue or thermal cycle testing with SEM. The strain distribution across specimen surface was obtained from ESPI measurements. The relationship between crack generation or delamination and strain distribution was discussed. Surface cracks occurred in the outer Al_2O_3 layer but stopped at the inner NiCr layer after one fatigue cycle when tensile stress was lager than 173MPa at 873K. Surface cracks propagated into the NiCr layer but stopped at the substrate, and local delamination occurred along the interface between the NiCr layer and substrate after N=1×(10)^5 cycles, when the fatigue test was carried out for σ_<max>=173 or 202MPa (R=0) at 873K. No cracks or delamination were observed after N=1×(10)^5 cycles for fatigue condition σ_<max>=115MPa (R=0) at 873K. Short cracks but no delamination were observed for fatigue condition σ_<max>=202MPa (R=0) at 573K. No influence of stress velocities (6.7 or 14Hz) was detected. No damage was observed after thermal cycle testing. The strain value obtained from the ESPI system was confirmed to be approximately the same as that from a strain gauge on an un-sprayed specimen at 293K. The thermal expansion or stress deformation at high temperature was easily measured with the ESPI system. The sprayed specimen surface strain was approximately the same as that of an un-sprayed specimen at 873K, showing that the deformation of the sprayed coating always follows substrate interface deformation at high temperature. Surface strain decreased with N=1×(10)^5 cycle fatigue showing evidence of the hardness effect due to fatigue. Strain values along cracks measured with the ESPI system were larger than those in other areas due to crack opening under the tensile load. In this paper, the larger strain zone is called "the strain concentration zone". Comparing crack location with strain concentration zones, a correspondence was found between the strain distribution obtained from the ESPI system and crack distribution in the thermally sprayed coating. Moreover, strain values decreased or disappeared entirely in the area where delamination in sprayed coating can be detected in situ by analyzing strain concentration or decrease with an ESPI system at high temperature.