M Hosseini Najafabadi - MSc, Steel Institute, Isfahan University of Technology, Isfahan, Iran
F Ashrafizadeh - Professor,
M Monirvaghefi - Associate Professor, Department of Materials Engineering, Isfahan University of Technology
H Saleki Najafabadi - MSc student, Department of Physics, University of Guilan, Rasht, Iran

Conventional coatings of nickel phosphorus (Ni-P) and composite coating with carbon nano-tubes (Ni- P-CNT) were deposited on commercially pure aluminium and blackening process was carried out to improve the optical properties of the surface. The effect of CNT on structure, hardness, absorption and emission coefficient, and durability of the coatings was studied. In order to examine surface topography of the coatings, atomic force microscopy was employed and chemical composition of the coatings was identified by energy dispersive spectroscopy. The optical properties were determined by means of spectrophotometer and infrared spectroscopy. The results indicated that addition of CNTs increased microhardness to about 850 HV0.1.and affected optical properties. Black Ni-P-CNT composite coatings had an absorption coefficient 2% higher than the black Ni-P coatings, approaching 99%. However, emission coefficient was decreased from 32% to 18% in Ni-P-CNT. Durability test indicated that addition of CNTs increased the lifetime of the composite coatings in weather condition. It is concluded that such composite coatings have comparable properties with more expensive PVD coatings and can be used for solar absorber applications.

Composite coating; Ni-P-CNT; Electroless; Optical properties; Solar absorber