Experiments and modeling of the deposition of nano-structured alumina–titania coatings by detonation waves

Deposition of nano-structured alumina–titania coatings by detonation waves has been studied experimentally. Experiments involved entrainment of conventional (Metco 130) and nano-structured/agglomerated alumina–titania powders with particle size distributions between 10 and 120 μm into the convective flow behind detonation waves. Stoichiometric mixtures of acetylene–oxygen was utilized in these experiments. Small quantities of powder (0.8–1.2 g) were placed in the detonation tube at fixed distances from the deposition location. The variation of the initial powder location along the detonation tube allowed different residence times of particles in the flow before deposition, thus allowing variation of the thermal and kinetic states of the particles prior to deposition. In this article, experimental results are presented in terms of the microstructure of the deposited coatings and coating micro-hardness. The coating characterization was performed as a function of the initial powder location. It was found that the cross-sections of the coatings contained significant regions with feature sizes of several hundred nanometers. Due to the low temperatures and high velocities attained by particles in the detonation process, coating structure is quite different than those obtained in plasma deposition. Results from modeling of particle motion and heat-up suggest that different size powder particles are accelerated and heated-up at different rates in the detonation process. Small particles are more rapidly accelerated and reach higher temperatures as compared to the larger sizes. This leads to upper layers of the coatings having the larger particles that may have very little or no melting versus lower layers of the coating consisting of small particles that undergo significant melting. The un-melted portions containing significant nano-structure from the feedstock material are believed to be mostly responsible for the retained nano-structure. Coating micro-hardness is found to increase with the increasing stand-off distance between the initial powder location and the deposition point. For each sample, higher hardness values were found in the lower layers of the coating consisting of melted regions. Metco 130 detonation coatings exhibited a stronger dependence of micro-hardness on the stand-off distance between the initial powder and the deposition location as compared to the nano-structured powder. Detonation coatings generated from nano-structured powder have consistently 30–60% higher micro-hardness values above those for typical plasma coatings utilizing the same material.