The Influence of Temperature on the Structure and Phase Composition of Yb2Si2O7 Coatings Applied on Kh20N80 Alloy and Sitall Substrates by Atmospheric Plasma Spraying

Abstract—The phase composition and structure of Yb2Si2O7 coatings applied by atmospheric plasma spraying on substrates made of Kh20N80 alloy and ST-32-11 sitall have been investigated by high-temperature X-ray diffraction at 200–1000°C. In the temperature interval 200–900°C, coatings of identical phase composition have been found to form on both substrates. The phase composition includes amorphous and crystalline monoclinic Yb2Si2O7 phases, the fraction of the amorphous phase reaching 53% in the coating on Kh20N80 and 69% in that on sitall. In the same temperature interval, the linear thermal expansion coefficient (LTEC) in different directions of the Yb2Si2O7 lattice far exceeds that of the bulk material due to tensile stresses arising at higher LTEC substrates. In the temperature range 900–1000°C, the amorphous phases completely crystallizes with the formation of a one-phase coating. As the temperature rises to 1000°C, the coating completely separates from the Kh20N80 substrate. The result is a sharp decrease of all lattice periods in the monoclinic phase down to values typical of the purely thermal expansion of Yb2Si2O7. This makes it possible to estimate the LTECs of the coating as αa = 4.5 × 10–6, αb = 2.5 × 10–6, and αc = 2.6 × 10–6 K–1.