The effect of pre-heat and post-heat treatment on the hardness and blackening process of electroless NiP deposits on titanium alloy

The black coatings were fabricated via electroless NiP deposits on Ti-6Al-4V alloy substrate with heat-treatment and blackening process. Two types of coatings were obtained: coatings with heat-treatment after blackening and coatings with blackening after haet-treatment. After parameter optimization, the temperature and pH of the solution were adjusted to 85 degrees C-95 degrees C and 4.3-4.7, respectively. The substrate was coated after the addition of modifying components to the solution. After refluxing the samples, the blackening process was done for 40 s in nitric acid (9molar). An SEM-EDS analyze used to assess the morphology and chemical composition of the coating. Moreover, adhesion strength and optical absorptance of the coating were measured by Pull Of test and Shimadzu UV-3100 analysis, respectively. The optical absorptance was measured by 0.89 and 0.95, respectively, for Oxalic acid and Nitric/Flouridric acid pretreated samples. Results showed that heat-treatment after blackening causes coating with higher micro-hardness and optical absorption. The blackened electroless NiP coating on Ti substrate with the heat-treatment of 4 h after the blackening process provided high solar absorption of 0.99, which is exceptionally suitable as a solar absorber coating for space and allied applications.