Porous alumina coatings with a final thickness in the order of 0.1 mm were produced by spraying ethanolic dispersions of particle mixtures onto steel substrates, followed by drying and heating at 500°C for 60 min. The coatings consisting only of a mixture of coarse and sub-µm sized alumina particles are generally poor in adherence and in abrasion resistance. By including a colloid of nanoscaled zirconia (d ~7 nm, calculated from the specific surface area) to the dispersion, coatings with clearly improved mechanical properties can be obtained. The zirconia fraction of 8 vol.% (related to the ceramic content) causes strong bonding between the alumina particles and the substrate. This bonding effect is quantified by bending tests, which have been performed after spray drying the dispersions, pressing the powders to form mechanical testing beams and applying a heat treatment at 500°C. The bending tests reveal that strengths are improved at least by a factor of 5 when the nanoparticles are present in the mixture. The described method to establish ceramic bonds in ceramic structures at temperatures, which are far below usual sintering temperatures, is a typical effect which arises in the nano size range, as the nanoparticles high reactivity based on their large specific surface area is deployed.
Materiały Ceramiczne / Ceramic materials , 2010, 62 (4), 510-515.