Abstract
In the thermal spraying process, the porosity of ceramic coatings contributes directly to the efficiency of the thermal insulation. The size, shape, and distribution of the pores determine the level of both thermal and sintering resistance. In this work, three different atmospheric plasma sprayed (APS) alumina coatings were fabricated with the same spraying parameters using alumina powders with fine, medium, and coarse particle size. The microstructure of the obtained coatings was analyzed regarding the obtained total porosity, pore size, and pore shape. It was found that it is expedient to divide the pore size range into fine, medium, and large sizes. The shape was characterized with regard to the circularity aspect. In this way, all types of cracks can be considered as oblate pores and were included in the calculation of the total porosity. In the case of using fine feedstock powder, the densest coatings were produced among all coatings, and the fraction of fine pores and cracks are thereby substantially higher. However, the total porosity increases with increasing feedstock powder size. A connection was also made between thermal insulation and porosity fraction which includes fine pores and cracks.
Highlights
Thermal spraying technique is designed to add additional thermal and mechanical properties to base substrate materials
Cernuschi et al [22] modeled the thermal conductivity of plasma sprayed thermal barrier coatings and determined shape and orientation of the pores as well as the porosity fractions
Wang et al [23] have reported that sizes and shapes of pores affect significantly the effective thermal conductivity of ceramic coatings
Summary
Thermal spraying technique is designed to add additional thermal and mechanical properties to base substrate materials. Cernuschi et al [22] modeled the thermal conductivity of plasma sprayed thermal barrier coatings and determined shape and orientation of the pores as well as the porosity fractions. Wang et al [23] have reported that sizes and shapes of pores affect significantly the effective thermal conductivity of ceramic coatings. Several studies have reported the effect of pores and cracks on thermal properties of thermally sprayed ceramic coatings through porosity characterization. The work focuses on the accurate measurement of porosity fractions based on a clear criterion that distinguishes each porosity component Coating properties such as hardness, phase composition, and thermal insulation were evaluated and related to the quantified microstructural features
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