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Title: Novel Nanostructured Coatings for Extreme Tribological Environments
Author: Mallia, Bertram
ISNI:       0000 0001 3617 6073
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2008
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available