Fretting of Amorphous Alloys
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FRETTING OF AMORPHOUS ALLOYS
REINHARD BECKER , GERD SEPOLD Bremer Institut fOr angewandte Strahltechnik Ermlandstr.59, 2820 Bremen-71, Germany
ABSTRACT Amorphous surface layers produced by bonding metglas ribbons onto steel specimens were tested for their resistance to fretting. Friction and weight loss were determined and the worn surfaces were examined optically. There was no general tendency towards a better wear resistance of the tested amorphous metals as compared to crystallized metals or to a simple structural steel. The results will be discussed.
INTRODUCTION Amorphous alloys are expected to be wear resistant because of their high hardness and fatigue strength /l/. In this work amorphous metals were tested for their resistance to fretting, which is defined as a wear process occurring between two surfaces having oscillatory relative motion of small amplitude /2/. EXPERIMENTS Amorphous surface layers were produced by adhesive bonding metglas ribbons onto steel specimens. As amorphous ribbons Co Fe Mo Si B /3/ and Fe Si B /4/ were chosen. Part of them were crystallizeA by a heai reatment (1M0Y,5 8h). CoFeMoSiB proved very ductile whereas FeSiB was somewhat brittle. As to the crystallized metals CoFeMoSiB was very brittle, FeSiB showed no great change. In addition to the bonded ribbons, amorphous and crystallized foils were tested as interfacial elements between steel surfaces. Fretting tests were carried out on a testing equipment described by /5/. Weight loss and friction were determined and the worn surfaces were examined optically. Conditions:
no. of cycles normal pressure amplitude frequency atmosphere temperature humidity
500.000 2 5 / 9 / 20 MN/mi 50 Pm 1.800 cpm air 293 OK 60 rel.%
RESULTS VISUAL EXAMINATION Visual examination of the worn surfaces revealed scars and corrosion products, which are usual for fretting. There was a remarkable presence of pittings and cracks as a sign of fatigue damage. The worn surfaces especially of amorphous CoFeMoSiB showed crude grooves and resembled those surfaces having suffered adhesive wear (fig.1). After short fretting tests of 1.000 cycles often macroscopic welding was observed between the amorphous CoFeMoSiB and FeSiB surfaces.
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Fig.
1.
worn amorphous CoFeMoSiB-surface
100 x
The worn amorphous surfaces seemed to have suffered structural changes as they showed a distinct brittleness. This could be caused by structural relaxation or even crystallization of the surfaces under fretting. However, up to now no crystallized regions were found by optical and electronmicroscopic investigations. A distinct difference of the damage between the "dull" and the "bright" side of the worn metglas ribbons was observed. There was a higher damage of the bright side which is the side of slightly lower cooling rates during melt spinning. It was obvious that fretting was influenced by the slightly different amorphous structure of the two sides. Frictional behaviour Friction under fretting is goverened by adhesion in the beginning and later by corrosion products bet
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