INTERACTION OF POWDER-AND COATING QUALITY
The quality of thermal spray coatings is a complex of problems since there are various interdepengdencis between powder properties,spray process and coating quality.
The most important interdependencies are shown in fig.2.(略).The powder type with its production-specific characteristics such as morphology,constitution and composition causes certain thermophysical properties.In interaction with the spray parameters the melting behavior of the powder particales with the peripheral spray conditions,such as spraying distance and substrate,the depositon of droplets and the final coating quality are being determined.It is quite obvious that powder quality and spray parameters have to be adjusted.
There is a number of powder of same chemical composition and granulation in the market.For many a user these may nominally be equivalent powders which,however,differ obviunsly in some production-specific properties.Figure 3 shows a selection of powder types schematically.Even if the majority of the coating properties is determined by the processing conditions.as shown in fig 2 certain powder types do have remarkable specific characteristics which should be considered and utilized.For example,the powder particle's porosity and microstructure and the granulation can be used in order to achieve a vertain melting behaviour and by this a deposition and coating formation of a desired microstructure and porosity.
Of course,all these are only possibilties.Both with porous and with dense spray powders a dense and a porous spray coating can be achieved of correspongding spray parameters are chosen.
With different powder manufacturing processes different powder types with certain powder characteristics will be achieved.Some of these characteristics are marked in property fields in fig4 and fig5,thus making clear that some powder properties are producing-specific and that at the same time controllable variations are still possible within individual powder production processes.In the production of spray powders the quality shall,of course,remain as constant as possible.In order to check and ensure this a comprehensive powder characterisation is necessary,too.
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