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Production of Cr-Al2O3 Nano-Composite Coatings by Electrodeposition

Salehi Doolabi, Mohsen | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 41284 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Sadrnezhad, Khatiboleslam
  7. Abstract:
  8. Electroplating is an appropriate method for co-deposition of metallic, non-metallic and polymeric fine particles onto a metal matrix for production of functional coatings. In this study, chromium and Cr–Al2O3 composite coatings are electrodeposited from trivalent chromium bath by using DC and square pulse current. Dependence of the amount of Al2O3 nano-particles trapped into the coatings on the Al2O3 bath concentration, average current density, particle size, agitation rate and type of the current is investigated. These parameters are optimized for the highest co-deposition amount of the Al2O3 onto chromium layer. Optimum values are 20g/l Al2O3, 200 rpm stirring rate and 25A/dm2 current density. It is demonstrated that the composite electrodeposits prepared by pulse plating procedure incorporates higher percentages than those obtained by direct plating conditions, especially at low duty cycles. SEM images clearly show that the Al2O3 distribution in the nano-composite coatings at low Al2O3 bath concentrations is uniform across the coating thickness; while at high concentrations, Al2O3 nano-particles on the surface are agglomerated. Effect of Al3+ on dispersibility of the Al2O3 particles in electrolyte is investigated. It is deduced that Al3+ ions promote the Al2O3 particles to uniformly disperse into the chromium coating. The deposition efficiency with and without Al2O3 nano-particles in bath is also studied. It is found out that the cathodic efficiency increases with increasing of the Al2O3 bath concentration. Hardness of the electrodeposited chromium composite coatings are strongly influenced by the amount of Al2O3 particles incorporated into the chromium layer and the maximum hardness is obtainable at 3g/l Al2O3 bath concentration. The influence of the duty cycle on thickness, current efficiency and hardness of chromium and Cr–Al2O3 composite coatings are also investigated. The results show that the thickness, the current efficiency and the layer hardness all maximize at 40% duty cycle. Corrosion behavior of chromium and nano-composite coatings in 3.5% NaCl solution is evaluated at room temperature. It is concluded that the corrosion resistance of the nano-composite coating increases with bath content of the Al2O3 nano-particles. The results indicate that the corrosion rate of the chromium coatings produced by the pulse current is lower than that produced by the direct current
  9. Keywords:
  10. Pulse Current ; Nano-Composite Coating ; Alumina Particles ; Nano-Composite Coating ; Trivalent Chromium Bath ; Alumina Particles

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