Fretting of solvent cast polyimide coatings was investigated in a ball-on-flat
geometry as a function of relative humidity. Polyimides were synthesized from
benzophenone tetracarboxylic dianhydride (BTDA) and bisanline (Bis P), 6-fluoro bis
dian hydride (6FDA) and Bis P, and pyromellitic dianhydride (PMDA) and bis A phenyl
phosphine oxide (BAPPO). Coating life - the time for the steel ball to wear through
the coating - shortened with increasing humidity. Iron oxides or other reaction products
from 52100 ball generated at high humidity acted as abrasives and accelerated the wear
of the coatings. Variation of coating life among the three polyimides was attributed to
the residual stress, which developed upon cooling from the annealing temperature due
to the mismatch of thermal expansion coefficients between the polymer and the metal
substrate. Calculated from elasticity theory, the normal stress dropped shortly after the
start of the test, remained relatively constant, and increased toward the end. This
variation correlated with the wear rate and accounted for the non-linear increase of
coating life with coating thickness. Sub-surface shear stress and surface tensile stress
predicted well, respectively, the debonded shape and the inter-crack spacing of Hertzian
cracks.
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