Formation of Cold-Sprayed Ceramic Titanium Dioxide Layers on Metal Surfaces
Titanium dioxide (TiO2) coatings have potential applications in biomedical implants or as photo-catalytic functional systems. Cold spraying is a well-established method for metal on metal coatings. In cold spraying, the required heat for bonding is provided by plastic deformation of the impacting ductile particles. In contrast, few authors have investigated the impact phenomena and layer formation process for spraying brittle ceramic materials on ductile metal surfaces. In this study, the formation of TiO2 coatings on aluminum, copper, titanium, and steel substrates was investigated by SEM, TEM, XRD, and Raman spectroscopy. The results show that the deposition efficiency depends on spray temperature, powder properties, and in particular on substrate ductility, even for impact of ceramic particles during a second pass over already coated areas. Ceramic particles bond to metallic substrates showing evidence of shear instabilities. High-resolution TEM images revealed no crystal growth or phase transitions at the ceramic/metal interfaces.
Keywordsbonding mechanism cold spraying photocatalytic activity titanium dioxide
Since its invention in 1986 by Anatoli Papyrin and his team (Ref 1), cold gas spraying has developed from a niche laboratory method of material deposition to a widely accepted spray technique with manifold applications. For metallic materials, bonding mechanisms are fairly well understood (Ref 2-4). Cold gas spraying is based on an acceleration of particles in a gas stream to high velocities of up to 1200 m/s. As opposed to conventional thermal spray techniques, process gas temperatures are low enough and exposure time to the hot gas stream is short enough to avoid melting of the particles. Bonding of cold-sprayed particles occurs when the particles impact with high velocities on the substrate. The resulting shear stresses lead to plastic deformation. Kinetic energy is converted to thermal energy, which cannot dissipate within the short time frame of the impact (quasi-adiabatic conditions). Metallic materials soften at the boundaries, which leads to more plastic deformation and heat generation, resulting in shear instabilities. Basic principles can also be transferred to cold spraying of metals on ceramic substrates and the deposition of metal ceramic composite layers (Ref 5-7). For these examples, it can be assumed that bonding occurs if one ductile component attains shear instabilities by plastic deformation. So far only few authors have investigated the kinetic impact of pure ceramic particles on metal surfaces. This is not surprising. The above-mentioned process based on plastic deformation does not work with brittle reaction partners. A brittle ceramic would not go through plastic deformation, it would rather break. Nevertheless, it is possible to spray ceramic particles onto metallic substrates. Li et al. (Ref 8) and others (Ref 9) have shown that it is possible to spray TiO2 with high photocatalytic activities using cold gas spraying, but coating thicknesses were limited to 10 μm using agglomerated nanopowders. Other research has reported the possible build-up of thick ceramic/titania coatings with aerosol spraying (Ref 10), vacuum cold spraying (Ref 11) and recently also high pressure cold spraying (Ref 12). However, the bonding mechanism is still unknown. Mechanical compaction and entanglement by the later impacting particles (Ref 11) and chemical formation (Ref 10, 12) have been proposed.
In this study, we report on requirements for bonding ceramic particles on metallic substrates in cold spraying, with titanium dioxide (TiO2) as a model system. In contrast to former studies (Ref 10-12), dense TiO2 anatase particles with nanosized crystallites were used to investigate the coatings build-up. TiO2 is of special interest because of its photocatalytical properties which have been electrochemically explored by Fujishima and Honda (Ref 13). This article focuses on the coating creation and crystallographic structures of the cold-sprayed coatings. Data on the photocatalytic activity of the cold-sprayed coatings in comparison with other thermal spray technologies has been reported elsewhere (Ref 14).
Process parameters used in this study for cold spraying of TiO2 layers
Gas pressure, bar
Spray distance, mm
Particle velocities were measured by a Tecnar ColdSpray Meter. Powder morphologies and layer cross sections were investigated by optical microscopy (OM) and scanning electron microscopy (SEM) in as-polished conditions. Specimen for TEM were prepared with a focused ion beam (FIB) FEI Nova 600 Nano Lab with electron optics operated at 5 kV, sample preparation done with 30 kV Ga-ions, and last finish with 5 kV Ga-ions. Samples were prepared from cross sections of the coatings. SEM was performed with a PHILIPS CM 200 UT-FEG operated at 200 kV, and a PHILIPS CM 30 operated at 300 kV. Micro-Raman spectroscopy was used to detect possible phase transitions in the sprayed coatings. The mass gain was measured by weighing the substrate before and after the coating process. The deposition efficiency was determined as the ratio between the deposition rate and the powder feed rate.
Cold Spraying and Particle Velocities
Measured particle velocities for the selected parameter sets
Average particle velocity, m/s
200 °C, 30 bar
590 ± 49
400 °C, 30 bar
661 ± 47
600 °C, 40 bar
713 ± 60
800 °C, 40 bar
833 ± 66
Coating Microstructures and Impact Morphologies
Crystallographic Coating Structures
The plastic deformation, that is needed for the shear instability to arise, needs to derive from the ductile substrate. This explains why there seems to be a saturation limit for the deposition of the ceramic on the metal substrate, as seen in Fig. 2 and 4: once the surface is covered with TiO2, there is no further ductile component in direct contact to impacting particles. Thus, no shear instability can evolve to bond further impacting particles. The chemical bonding that has been observed by other authors (Ref 10-12) does not seem to have occurred for the rather solid particles in this study. Mechanical entanglement does not seem to be sufficient as well, since a thicker ceramic coating did not build up, no matter what spray conditions were used. However, chemical bonding cannot be ruled out completely since it would depend largely on the nanostructure and respective powder properties. It was proposed that the build-up of thicker coatings is due to breaking up crystallites and thus creating dangling bonds (Ref 16). If the impact energy in our case is dissipated by merely splitting the particles along the grain boundaries, there might not be enough chemical energy to result in a coating build-up.
Although the impacting particles seem not to bond to deposited ceramic layers, an increasing coating thickness could be observed for the aluminum substrate (Fig. 2), but not for the harder substrate materials as stainless steel and titanium (Fig. 4). The reason for this could be the different impact characteristics: at a similar impact momentum, jetting of substrate material is more pronounced for softer than for harder metals. Thus, by drawing out more substrate material softer substrates can locally supply fresh metal surfaces even after a couple of passes. Figure 7(b) shows an example of a coating, where parts of the aluminum substrate are embedded in-between the TiO2 particles. Sequences of a possible model process are probably as follows: (i) the ceramic particles impact and deform the substrate material, (ii) the shear instability leads to jetting which forms fresh metallic surfaces, in some case even on top of neighboring, already impacted ceramic particles in the close vicinity. (iii) These new metallic surfaces show plastic deformation under following impacts and are able to bond further incoming particles with shear instabilities. This process would be impossible for harder substrates, where the impacting particles do not reach the momentum necessary to deform the substrate significantly, as for stainless steel as shown in Fig. 8(b). If the growth in layer thickness on the AlMg3 substrate were due to bonding between ceramic particles, we should see an increase in layer thickness also for the titanium and steel substrates with more and more spray gun passes. This is, however, not the case. Burlacov et al. (Ref 17) reported similar effects of jetted substrates acting as a binder by cold spraying TiO2 on thermoplastic polysulfone material. The authors describe that softened polymer was acting as an additional binding agent to allow to form subsequent layers of TiO2, similar to the embedded aluminum that can be seen in Fig. 7(b) in the present study. In another study by Yang et al. (Ref 18), a similar upper limit for the deposition of TiO2 with cold spraying on metals has formed, above which no further increase in layer thickness is possible, even though using an agglomerated, nanoporous powder.
There is further evidence that the bonding of TiO2, even for the hard metal substrates, needs to arise from substrate deformation: the TEM images as well as the Raman spectroscopy results (Fig. 10-12) showed clearly that the TiO2 crystals remained unchanged during the process. Even at the very edge of the boundary layer between substrate and particle, where the highest temperatures are to be expected, no thermal influence on the TiO2, like, e.g., crystal growth, was visible (Fig. 11). Therefore, the particle impact temperature, which is an important factor for metal-metal coatings, does not seem to play a role in this case, except for heating the substrate to make it more ductile. For all types of metallic substrates, the impact velocity is the decisive factor for enhancing jetting and embedding more TiO2 particles explaining the deposition efficiency increase in Fig. 3.
An interesting aspect is the negative correlation between substrate hardness and deposition efficiency during the first pass of the spray gun. Schmidt et al. (Ref 3) have shown that the critical velocity is the decisive value to determine whether a significant deposition occurs in the cold spray process for metal-metal coatings. The critical velocity is rising with HV as well, just as in our case, where less material was deposited at harder substrates. In the present case, the spray particles, although showing a similar or lower hardness as compared to the substrates, do not build up thick coatings because of their brittleness.
The results show that bonding in cold gas spraying of ceramics like TiO2 can be realized by ductile substrates that allow shear instabilities to happen. TiO2 particles interact as solid spheres with the substrate bonding in a ring-like zone. Due to fracture under the elastic rebound forces, the brittle spray particles break and only small remnants remain in the bonding zones. In the present case, the ceramic spray particles did not contribute to bonding and therefore could not build up layers on their own, i.e., with impacts that hit another ceramic particle rather than the substrate surface. Only if substrate material is brought to the surface and is available to bind other particles, a second layer or parts of it are likely to be attached to the coating on impact, as it was the case for AlMg3 in this study. Causing more severe deformation of the substrate, higher particle velocities increase the deposition efficiency and also the saturation limit for the deposited TiO2.
The authors would like to thank the laboratory staff, in alphabetical order Thomas Breckwoldt, Dieter Müller, Norbert Németh, Camilla Schulze, Matthias Schulze, and Uwe Wagener for their support in the presented work. This work was funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG).
- 1.A.P. Alkhimov, V.F. Kosarev, and A.N. Papyrin, A Method of Cold Gas-Dynamic Deposition, Sov. Phys. Dokl., 1990, 35(12), p 1047-1049 (in Russian, Transl: American Inst. of Phys., 1991)Google Scholar
- 8.C.J. Li, G.J. Yang, X.C. Huang, W.Y. Li, and A. Ohmori, Formation of TiO2 Photocatalyst Through Cold Spraying, Thermal Spray 2004: Advances in Technology and Application, May 10-12, 2004 (Osaka, Japan), ASM International, 2004, p 315-319Google Scholar
- 14.H. Gutzmann, J.-O. Kliemann, R. Albrecht, F. Gärtner, T. Klassen, F.-L. Toma, L.-M. Berger, and B. Leupolt, Evaluation of the Photocatalytic Activity of TiO2-Coatings Prepared by Different Thermal Spray Techniques, Proc. ITSC 2010, Thermal Spray: Global Solutions for Future Applications, May 3-5, 2010 (Singapore), DVS-Berichte 264, DVS Media GmbH, Düsseldorf, Germany (on CD), 2010, p 187-191Google Scholar
- 15.Matweb.com (August 13th 2010)Google Scholar
- 16.M. Yamada, H. Isago, K. Shima, H. Nakano, M. Fukumoto, and J. Toyohashi, Deposition of TiO2 Ceramic Particles on Cold Spray Process, Proc. ITSC 2010, Thermal Spray: Global Solutions for Future Applications, May 3-5, 2010 (Singapore), DVS-Berichte 264, DVS Media GmbH, Düsseldorf, Germany (on CD), 2010, p 187-191Google Scholar