Mechanical and barrier properties of MOCVD processed alumina coatings on Ti6Al4V titanium alloy

This study focuses on the implementation of different aluminum oxide coatings processed by metal-organic chemical vapor deposition from aluminum tri-isopropoxide on commercial Ti6Al4V titanium alloy to improve its high temperature corrosion resistance. Films grown at 350 degrees C and at 480 degrees C are amorphous and correspond to formulas AlOOH, and Al(2)O(3), respectively. Those deposited at 700 degrees C are composed of gamma-Al(2)O(3) nanocrystals dispersed in a matrix of amorphous alumina. Their mechanical properties and adhesion to the substrates were investigated by indentation, scratch and micro tensile tests. Hardness and rigidity of the films increase with increasing deposition temperature. The hardness of the coatings prepared at 350 degrees C and 480 degrees C is 5.8 +/- 0.7 GPa and 10.8 +/- 0.8 GPa respectively. Their Young's modulus is 92 +/- 8 GPa (350 degrees C) and 155 +/- 6 GPa (480 degrees C). Scratch tests cause adhesive failures of the films grown at 350 degrees C and 480 degrees C whereas cohesive failure is observed for the nanocrystalline one, grown at 700 degrees C. Micro tensile tests show a more progressive cracking of the latter films than on the amorphous ones. The films allow maintaining good mechanical properties after corrosion with NaCl deposit during 100 h at 450 degrees C. After corrosion test only the film deposited at 700 degrees C yields an elongation at break comparable to that of the as processed samples without corrosion. The as established processing-structure-properties relation paves the way to engineer MOCVD aluminum oxide complex coatings which meet the specifications of the high temperature corrosion protection of titanium alloys with regard to the targeted applications. (C) 2011 Elsevier B.V. All rights reserved.