Cold sprayed Ti-6Al-4V coating adhesion and cohesion strength enhancement by surface preparation optimization

The effect of substrate surface preparation on the adhesion and cohesion strength of Ti-6Al-4V cold sprayed coatings deposited on Ti-6Al-4V substrates is investigated. Substrates were prepared by conventional methods including polishing and grit blasting and by laser treatments including in-situ pulsed laser ablation and continuous laser pre-heating. Cohesion strength was evaluated by the scratch test method while adhesion strength was evaluated by the standard ASTM C633 pull test method and the laser shock (LASAT) technique. Contrary to what is commonly observed for thermal spray coatings, the highest adhesion strength is obtained with mirror finish substrate for Ti-6Al-4V cold sprayed coatings. The condition of high surface roughness obtained by grit blasting results in only half the adhesion strength of the mirror finish one.  Between those roughness boundary cases, the adhesion strength passes through a minimum where high density of voids are observed at the coating/substrate interface. These voids result from the low deformability of the cold sprayed Ti-6Al-4V particles that fail to fill the asperities created by the roughening procedure. As the asperities size increases, the particles better fill the cavities and higher mechanical anchors develop. Although the mirror surface finish was obtained by polishing, similar surface finish and same adhesion strength can be achieved using pulsed laser ablation and laser pre-heating.  This makes possible the application of this method for spray forming of large industrial parts or for one-step repair.