F. W. Bach, Leibniz Universität Hannover, Garbsen, Germany; K. Moehwald, M. Erne, D. Kolar, Leibniz Universität Hannover, Witten, Germany
A couple of non stoichiometric phases in the system Ti-O show a reduction of their coefficient of friction under dry sliding conditions with rising temperature, being therefore sometimes called solid lubricants. But these homologous series tend to transform towards the rutile structure losing their good friction behavior.
To avoid this effect, the aim of this work was the stabilization of these phases by substitution of titania cations by several mono- and heterovalent cations. The resulting phases are homologous to TinO2n-1, but have the advantages of a better thermal stability and a broader area of formation in the resulting three-component system compared to the Ti-O-system. The nanosized feedstock powders where suspension plasma sprayed, as it allows achieving target stoichiometries by giving two precursors with the equivalent molar masses into the suspension and mixing them in the plasma. Furthermore the achievable coatings are very dense and thin and therefore allow an operation in fields where high demands towards the adhesion, cohesion and near net shape are made.
Summary: To use the ability of adjusting the desired phase content in suspension sprayed coatings by mixing precursor nanosized feedstock powders several points have to be kept in mind. The grain size and therefore the surface/volume-ratio of commercially available nanosized powders often vary from the given specifications. Furthermore it is sometimes difficult to get powders in the same range both resulting in an unequal melting and mixing behavior disturbing the formation of the desired phase. Besides the discussion of methods determining the size distribution of powders down to about 10 nm the consideration of using the unique features of the Triplex torch for suspension plasma spraying shows how to reach high deposition rates per pass. To overcome the problem of blocking of the injection nozzles and to alter the injection conditions the use of sophisticated two phase nozzles with convertible inner diameter is beneficial.
