Effect of particle size distribution of suspension feedstock on the microstructure and mechanical properties of suspension plasma spraying YSZ coatings

In this work, aqueous suspension feedstocks with different particle size distribution from submicron- and nano-sized YSZ powders were prepared. A previous colloidal characterisation allowed concentrated suspensions (~ 40 wt%) to be prepared. These suspensions were then successfully deposited by suspension plasma spraying (SPS) onto stainless steel substrates at different spraying distances. Coatings were characterised in terms of microstructure and mechanical properties (hardness and elastic modulus) by nanoindentation. All coatings displayed a two-zone microstructure formed by partially melted areas containing nanometre or submicrometre-sized particles surrounded by fully melted areas. These partially melted areas strongly increased as standoff distance lengthened. Mechanical properties worsened with spraying distance increase. The finer the feedstock particle size the more the spraying distance effect was. A clear correlation between mechanical properties and amount of partially melted areas has been proved. Thus the use of a mixture of nano/submicron-sized particles as a SPS feedstock can represent a good balance between suspension processability and final coating performance