Magnetomechanical damping in plasma sprayed iron–chromium based coatings

Abstract

The damping capacity (Q−1) of thermally sprayed Fe–Cr–X coatings has been investigated in the range of frequencies between f=10 Hz and 10 kHz, and deformations between ε=10−4 and 10−3, using a cantilever method. The magnetomechanical hysteresis loss was determined based on the modal analysis technique of flat beam and was found to be very sensitive to internal stress of the samples. Heat treatments usually enhanced the loss capability, but only high-temperature annealing restored the whole damping capacity of the plasma sprayed coatings to be comparable to that of cast alloys of the same chemical composition. The variation of Q−1 versus vibration amplitude first increased rapidly, passed through a maximum, and then declined relatively slowly to its initial values. The position of the maximum damping (Q−1Max) was shifted towards lower strains with annealing time and temperature. As expected in ferromagnetic materials and in contrast to strain amplitude effect, the vibration frequency did not significantly influence damping behavior. The structure of magnetic domains were observed using the magneto-optical Kerr effect and their modification following heat treatments was associated with different values of the damping capacity. Accordingly, the beneficial effect of annealing on damping capacity was found to arise on the one hand from improved mobility of unpinned domain walls and on the other hand from growth of 90° domains leading to equiaxed domain configurations. The irreversible movement of domain walls upon application of an external stress in general occurred suddenly and abruptly between two pinned positions.