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Title: CrAlN-based nanoscale multilayer coatings dedicated to wear and oxidation protection
Author: Reinhard, Christina
ISNI:       0000 0001 3513 4648
Awarding Body: Sheffield Hallam University
Current Institution: Sheffield Hallam University
Date of Award: 2007
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This study explores the wear and oxidation resistance of novel CrAlYN/CrN nanoscale multilayer coatings grown by a combined high power impulse magnetron sputtering (HIPIMS) / unbalanced magnetron (UBM) sputtering process. Firstly, it is found that a HIPIMS substrate etching pre-treatment prior to the coating deposition effectively removes contaminants from the substrate surface and provides the base for epitaxial growth. Subsequently, an improvement in the adhesion and the tribological performance of CrAlYN/CrN is observed compared to coatings pre-treated with a conventional cathodic arc pre-treatment. Secondly, the effect of the deposition conditions on the microstructure and the subsequently arising coating properties is investigated. A variation in the energy of ions impinging at the substrate during the coating growth permits a change in the microstructure from polycrystalline columnar to partially single crystalline. Depending on the microstructure, CrAlYN/CrN reveals a maximum high hardness of 51 GPa and extremely low wear rated of 10[-17] m3N[-1]m[-1]. Modifications of the microstructure from nano-layered columnar to near-monolithic columnar are discovered with a change in the chemical composition. These structural changes only have a mild effect on the mechanical and tribological performance. Finally, the oxidation performance of CrAlYN/CrN is extensively investigated. Annealing in air leads to simultaniously occurring processes with (i) the thermal degradation of the nanolayered CrAlYN/CrN into Cr[2]N and AIN and (ii) the surface oxidation with the formation of corundum-type oxides and at long exposure times (iii) diffusion of substrate elements through the coating. The observed phase transformation into Cr[2]N and AIN may be promoted by the diffusion of the substrate element Cr into the coating. The results suggest that the oxidation performance of CrAlYN/CrN can be improved if the coating is grown under high energetic bombardment. The coating is prevented from cracking and therefore protects the substrate material more effectively from oxidative attack. Furthermore, it is found that the oxidation resistance can also be improved when changing the chemical composition. Highly Al-containing coatings form Cr[2]O[3]-Al[2]O[3] oxides which protect the underlying more effectively than (Cr,Al)[2]O[3] oxides grown on low Al-containing CrAlYN/CrN.
Supervisor: Luo, Quanshun Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available