**4.1.1 Crystallographic aspects**

In the system Titanium-Oxygen different non stoichiometric phases are known, which show the ability for deformation under mechanical stress due to a shearing of crystal lattice planes. These phases show a reduction of the frictional coefficient in dry sliding conditions under elevated temperatures of some 100 degrees Celsius. The beneficial effect was linked to the shearing processes being temperature induced (Gardos, 1988), the fundamental mechanism of the shearing processes are discussed elsewhere (Anderson, S. and Tilley, R. J. D., 1972). As the phases are expected to be not thermodynamically stable (for a discussion of redistribution effects of titanium and oxygen see Wood, G. J. et al., 1982), another approach was intended for these workings. By addition of a second cation besides Ti4+, phases can be obtained which are homologues to the nonstoichiometric titaniumoxides. Those so called Andersson-phases were first described for the system Ti-Cr-O (Andersson, S., Sundholm, A. & Magnéli, A., 1959) showing a composition of Tin-2Cr2O2n-1.

Up to now no coating systems are marketable in the field of metal forming like the direct hot extrusion process, which provide both surface protection of the parts being in contact to the billet (i.e. container and die), and a significant reduction of the frictional losses being induced by the billet passing along the container walls. To dispense the use of lubricants and to enhance the usable forming capacity of the process, different oxide ceramics were given in one suspension and plasma sprayed. The aim is to reach a mixing of the feedstock to obtain deterministic solid solutions of the oxide phases which show a reduction of their coefficient of friction under dry sliding conditions. To reach this goal the high surface-tovolume ratio of feedstock with primary particle sizes below 100 nm was used. By means of x-ray diffraction it could be proven, that the desired phases could be synthesized. The coatings showed a considerable lowering of their frictional coefficient in tribological testing against steel 100Cr6 in the region of the operation temperatures for the hot extrusion of aluminium alloys. Besides the experimental work the fundamentals of the mixing process of

Thermal sprayed coatings are not commonly used in the field of massive forming due to the high demands concerning the cohesion and adhesion of tool coatings. The cause is adhesive wear being induced by the elevated temperatures of operation and high relative velocities between the work piece and toolings resulting in high tensile and shear stresses. Nevertheless, there is the challenge to establish coatings to reduce both the wear of tools and frictional losses in the processes. For example in the case of direct hot extrusion, up to

of the forming force have to be applied to counterbalance frictional losses. To come up against that losses different lubricants and material separating agents are used, but with the disadvantages of a higher degree of reworking of the semifinished extruded product and a limited thermal stability of the substances. To overcome these disadvantages, the usability of specific oxide ceramic phases basing on titania was tested, which show a reduction of their frictional coefficient under tribological operation and elevated temperatures. The desired phases should be synthesized in the suspension plasma spraying process by mixing

In the system Titanium-Oxygen different non stoichiometric phases are known, which show the ability for deformation under mechanical stress due to a shearing of crystal lattice planes. These phases show a reduction of the frictional coefficient in dry sliding conditions under elevated temperatures of some 100 degrees Celsius. The beneficial effect was linked to the shearing processes being temperature induced (Gardos, 1988), the fundamental mechanism of the shearing processes are discussed elsewhere (Anderson, S. and Tilley, R. J. D., 1972). As the phases are expected to be not thermodynamically stable (for a discussion of redistribution effects of titanium and oxygen see Wood, G. J. et al., 1982), another approach was intended for these workings. By addition of a second cation besides Ti4+, phases can be obtained which are homologues to the nonstoichiometric titaniumoxides. Those so called Andersson-phases were first described for the system Ti-Cr-O (Andersson, S., Sundholm, A.

**4.1 Suspension plasma spraying of triboactive coatings** 

different oxides regarding crystallographic aspects are discussed.

different oxide feedstock with titania in one suspension.

& Magnéli, A., 1959) showing a composition of Tin-2Cr2O2n-1.

**4.1.1 Crystallographic aspects** 

60%

As chromium exhibits a high steam pressure with rising temperature and therefore may tend to evaporate out of the lattice, the homovalent substitution of the Ti4+-cation in the rutile base lattice was aspired. Several cations where chosen based on the rules for substitution processes stated by V. M. Goldschmidt (Goldschmidt, V. M., 1926), besides Cr3+ primary Ni3+, Co3+ and Zr4+ by considering the ionic radii and coordination given in (Shannon, R.D., 1976). The goal is to reach phases with a similar composition compared to the Andersson-type phases on the one hand and a sufficient stability in temperature ranges up to 800° C on the other, which are commonly used for hot extrusion of aluminum and copper based alloys.

The assumption that the applicability of substitution processes may lead to the formation of solid solutions of the desired stoichiometry can be proven by means of the Inorganic Crystal Structure Database. In **Figure 9** for example the structures of the cubic Co(II)-oxide and of tetragonal rutile (i.e. Ti(IV)-oxide) are shown on top, where the oxygen is represented by the larger balls.

Fig. 9. Structures of Co- and Ti-oxide (top) and of the "mixed" solid solution oxide (bottom)

From the structure it can be inferred, that both cations have similar radii, which is – besides the valence and the coordination by the surrounding ions – the key requirement for the dissolution of the oxides. When both oxides are mixed, a structure of lower symmetry (orthorombic) is formed with a composition of Co2Ti4O10. The difference compared to the aspired composition of Co2Ti4O11 for n = 6 is due to the fact, that the divalent cobalt is incorporated in the structure instead of the trivalent ion. Like the most structures being crystallographic possible solid solution of rutile with the named oxides, the cobalt-titaniumoxide with trivalent Co-ions is not refined yet. Without the feasibility to refine the structures, the full quantitative Rietveld analysis by means of X-ray diffraction of the sprayed coatings is not possible.

Thermal Spraying of Oxide Ceramic and Ceramic Metallic Coatings 183

Sample Ti-Oxides Ni, Co-Oxides, Cr Ti-(Ni,Co,Cr)-Oxides Borates Traces TiO2xNi2O3 55 7 26 10 2 % Ni TiO2xCo2O3 39 0 29 33 - TiO2xCr2O3 13 10 75 0 2 % Cr2O3

In case of the Ni- and Co-containing coatings, significant amounts of Ti(IV)-oxides were measured, of which approximately one third is anatase. As stated in (Bolelli, G., et al., 2009), in case of rutile feedstock the phase content of anatase especially in suspension sprayed coatings can be explained by slow cooling due to re-solidification of molten droplets in the process, compared to formation of rutile in rapid quenching on the substrate. Considering this explanation another assumption might be the influence of elevated substrate temperatures in the SPS process leading to a more slowly cooling of molten titania particles after impinging on the substrate. To distinguish both possible mechanisms further investigations will be conducted considering the thermodynamics of the phase changes of both titania species. In the case that the anatase content correlates well with the content of re-solidificated particles in the coating, the anatase-to-rutile ratio can be used to optimize the

For the coatings containing nickel, about 7% percent of Ni(II)-oxide were found, whereas in titania-cobalt-oxide systems no remains of the Co-feedstock was detected. The employed trivalent oxides of both cations decompose towards the divalent oxide at temperatures above approximately 600° in case of the Ni-oxide and 1910° C for the Co2O3. Otherwise the contents of borates formed by reactions of the boron oxide with the feedstock oxides is three times higher for the Co-based system compared to the titania-nickel-oxide coating. As the absolute value of the enthalpy of formation of the cobalt-borate is higher than that of the Niborate (Hawk, D. and Müller, F.; 1980; Paul, A., 1975), the Co-oxide feedstock is diluted in the boron oxide to a much higher extent compared to the Ni-containing system, and no remaining Co2O3 is embedded in the coating. In contrary to that the contents of Ni-borates are small in the titania-Ni-oxide coating, and remains of the Ni(II)-oxide are recorded. The phase contents of the aspired solid solutions are below 30 % for both coatings systems.

Compared to the Ni- and Co-containing coatings the mixing of titania with chromia leeds to different phase compositions. Due to the marginal miscibility of chromia with boron oxide (Tombs, N. C.; Croft, W. J. & Mattraw, H. C.; 1963), no borates and also just small amounts of the feedstock powders are found. The Andersson-phases with the mentioned stoichiometry of Tin-2Cr2O2n-1 amount to three quarters of the total coatings composition. Therefore it can be concluded, that the degree of mixing of the feedstock is significantly higher for the titania-chromia system. If the melted phase of the boron oxide supports the mixture process of the both oxide ceramics without further reaction cannot be clarified. Possibly the heat of the process is better transferred to the coarser feedstock of approximatly 100 nm median crystallite size compared to 30 to 60 nm of the feedstock of the Ni- and Cocontaining coatings. As the heat transfer degreases drastically when the agglomerate size of the feedstock particles falls below a critical limit (so called Knudsen effect, Fauchais, P. et al., 2008), this might be a supposable explanation of the higher degree of feedstock mixing in

Table 6. Phase contents of the three coating systems

injection and spraying parameters.

the case of the titania-chromia system.

Phase Contents (atomic percent)
