Low-Temperature Transformation of Amorphous Sol–Gel TiO2 Powder to Anatase During Cold Spray Deposition

Abstract

Nanocrystalline TiO 2 coatings were successfully deposited on metallic substrate surfaces through lowpressure cold spray. Sol-gel amorphous TiO 2 dry gel was for the first time used as a feedstock powder. Agglomerated morphology of the amorphous powder with high-density packing of particles and reduced pores number was an important factor for successful low-pressure cold spraying. However, in the present study one more factor contributing to anchoring of TiO 2 particles by cold gas spraying onto a substrate was indicated. This factor enabling effective powder deposition and coating production is based on an exotic for conventional ceramic materials' plastic deformation of amorphous powder. Furthermore, the detailed analysis of SEM, Raman and XRD measurements clearly demonstrated that amorphous titanium dioxide is transformed into crystalline anatase form during the process of low-pressure cold spraying.

Prospects of Low-Pressure Cold Spray for Superhydrophobic Coatings

“…As the method developed, higher-strength metals and alloys were also deposited basing on the crystal grain refinement. Then, polymer coatings were formed due to application of lower velocities (about 100 m·s −1 ) and finally, even ceramic materials were employed [11][12][13][14][15] which was possible due to particle fragmentation and particle interlocking. However, attempts to deposit conventional (hard and brittle) ceramics are still not common which is mainly caused by the lack of understanding of the bonding mechanism nature.…”

“…In case of high-temperature pre-treatment of feedstock powder typically applied for ceramics, such as sintering, the binding of separate particles chemically and physically is observed. However, it is chemical diffusion which plays the predominant role which finally leads to solid-state neck formation characterized by high binding energy [2,13]. For soft non-sintered agglomerates, the binding energy of individual particles is much lower because the particles are joined by purely physical interactions [39], for instance through a suitable liquid binder.…”

“…Therefore, it is believed that only so-called soft agglomerates are preferred to be sprayed using the LPCS method [40,41]. Soft agglomerates network can be achieved either by adding a suitable binder to the powder [30,32,35] or by using a fresh sol-gel powder [13]. It has been shown very recently that organic liquid substances remained after the sol-gel synthesis act as binders responsible and sufficient for agglomeration [13].…”

“…Soft agglomerates network can be achieved either by adding a suitable binder to the powder [30,32,35] or by using a fresh sol-gel powder [13]. It has been shown very recently that organic liquid substances remained after the sol-gel synthesis act as binders responsible and sufficient for agglomeration [13]. However, the properties of as-prepared coatings are considered rather low post-treatment including heat or mechanical treatment are suggested [11,15,42,43].…”

“…The possibility of a potential repair may be also advantageous [42][43][44][45]. However, it is worth emphasizing that in the case of superhydrophobic coating analyzed in this work, mechanical strength does not dominate over its functionality.Driven by the concept to ensure high functionality and considering that only soft agglomerates are preferred to be deposited using the LPCS method [13], the use of the sol-gel method to optimize feedstock powder seems to be an obvious solution. First of all, the functionality of feedstock powder may be easily achieved on the sol-gel route, because carefully planned synthesis may provide the set of new, programmable features.…”