Title:
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Novel Nanostructured Coatings for Extreme Tribological Environments
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The degradation of material surfaces is a 'valve' that has limited technological
progress through the ages. Today's most challenging environments are those where
component surfaces are exposed to the simultaneous or sequential actions of corrosion and
mechanical wear. These phenomena can be mitigated by a better understanding of the
interactions between the multiple degradation mechanisms and the systematic development
of alternative material surfaces.
In this work, PVD coatings based on the Fe-Cr-Ni, Cr-Ti, Cr-B-(N) and W-(B)
material systems were developed using unbalanced magnetron sputtering and mainly
applied to AISI 316L, Ti-6AI-4V and Ortron 90 substrates respectively. For Fe-Cr-Ni and
Cr-Ti coatings, post deposition heat treatments were conducted to stimulate the formation
of the cr-FeCrNi and Laves Cr2Ti intermetallic phases respectively. In all cases, the
morphology, chemical composition, structure, mechanical properties, corrosion behaviour
and damage tolerance of the various coated materials were investigated. Special emphasis
was placed on the response of the coated materials and uncoated test-pieces to the
simultaneous action of corrosion and mechanical wear that resulted from sliding an Ah03
ball against the coated and uncoated materials in a 0.9 wt%NaCI solution. For one case
only, W-(B) coated AISI 52100, lubricated rolling/sliding contact durability was assessed.
For the corrosion-wear tests, the removal and regeneration of the passive layer (type I
corrosion-wear) dominated the material loss. A high coatings hardness for'Fe-Cr-Ni and CrTi
coatings was often beneficial to reduce the mechanical wear but the corrosion due to
wear frequently remained high. Partial post deposition oxidation of Fe-Cr-Ni coatings was
very effective in reducing the latter. Also of importance was the roughening of the Ah03
counterface due to the formation of tribo/transfer films or grain pull out. The material loss
for Cr-B-(N) coatings was dominated by a material transfer mechanism and Ah03 grain pull
out resulted in high mechanical wear of amorphous Cr-Ti coating. W-(B) coatings generally
had low material loss and their crystalline variants displayed a low p.. The latter became
inappropriate for conditions which promote high W dissolution. In lubricated rolling/sliding
tests W-(B) coated AISI 52100 performance was·dependent on coating crystal structure and
boron supersaturation. The crystalline coatings with low boron supersaturation displayed
the best rolling/sliding damage resistance.
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