Abstract:
The erosion resistance of 50
m metal-ceramic multilayer coatings has been
investigated under impact conditions comparable to those in a gas turbine
compressor cascade. lt was possible to improve upon the erosion resistance of
Ti-6Al-4V
by a significant margin.
The influence of
layer mechanical properties, layer thickness, ceramic content and
coating process on erosion resistance has been studied over a range of impact
conditions. The most suitable
coating formulation for maximum erosion resistance
changed with particle impact energy. Under low energy impact conditions (<55
joules) coatings with a high ceramic content demonstrated the highest erosion
resistance. As
particle impact energy increased, coatings with a high ceramic
content
perfonned poorly, and those containing a high volume fraction (50%)
ductile metal
layer, with thin metal and ceramic layers become more successful.
Three
principal damage types were observed: lateral fracture, tensile fracture and
plastic definition. The most severe coating losses resulted from spallation due to
lateral fracture.
Coatings containing a high proportion of ductile metal with thin
metal and ceramic
layers were successful because such coatings had a high
resistance to lateral fracture. Erosion resistance was
greatest when the metal layer
had a
high yield strength and elastic modulus; such a combination of properties
also resisted
plastic definition.
Scratch
testing was investigated as a simple alterative technique for assessing
coating erosion resistance. Repeated pass scratch testing generated similar damage
modes to those of
particle impact, but there was poor correlation between coating
erosion rate and the threshold load for scratch
damage.
