Investigation of Destruction Mechanisms for Heat-Resistant Coatings in Hypersonic Flows of Air Plasma

Abstract

The authors present the results of investigations of degradation processes that occur in the structure of heat-resistant coating of the Si-TiSi2-MoSi2-B-Y system in hypersonic flows of air plasma. It is found that coating operating capacity at surface temperatures Tw ≤ 1820÷1830°C is provided by the structural-phase state of its microcomposite main layer and formation on the coating surface of a heterogeneous passivating protective film. It is based on borosilicate glass reinforced by rutile microneedles. The mechanism of coating destruction at Tw ≥ 1850÷1860°C is erosion loss of oxide film as well as generation and growth of gas-filled cavities at the "coating main layer–oxide film" interface. As the pressure of saturated vapor of gaseous oxidation products (SiO, CO, MoO3 and B2O3) exceeds that of the ambient, the oxide film integrity is disrupted and oxidation process becomes active. The rates of erosion loss and sublimation grow as operating temperature increases and ambient pressure decreases.

Oxidation Resistance of a Si–TiSi2–MoSi2–TiB2–CaSi2 Coating on a Cf/C–SiC Substrate in High-Speed High-Enthalpy Air Plasma Flows

“…During oxidation of coatings in the Si-TiSi 2 -MoSi 2 -TiB 2 system, a heterogeneous oxide film is formed, consisting of borosilicate glass SiO 2 •B 2 O 3 and titanium oxide TiO 2 immiscible with it in the form of rutile [11,23]. The latter is a vast network of microscopic needles, located absolutely randomly, which leads to the appearance of the effect of "reinforcement" of the oxide film, and, consequently, to an additional increase in the resistance to erosion entrainment in flows.…”

“…The presence of eutectics between components in the composition of the coatings improves the adhesion between the layers and to the substrate, accelerates the regeneration of the oxide film, and also to some extent compensates for the differences in the coefficients of thermal expansion of coatings and protected materials. The effectiveness of the protective action of coatings was confirmed under static oxidation conditions in air at 1650 • C [13,25], in an oxidizing gas flow at 1900 • C [22], in high-speed high-enthalpy air plasma flows at 1850 • C [11,24], 1950 • C [23], 2000 • C [26], and 2200 • C [13], as well as under the conditions of an ablation fire experiment at 2400 • C [25].…”

“…The role of the healing phase in the inner layers of coatings of the MoSi 2 -Mo 5 Si 3 -HfSi 2 /TaSi 2 /TaC and WSi 2 -W 5 Si 3 -HfSi 2 /TaSi 2 /TaC systems [22], Si-TiSi 2 -MoSi 2 -TiB 2 [11,23], Si-TiSi 2 -MoSi 2 -TiB 2 -SiC w [24], ZrSi 2 /Si-ZrSi 2 /SiC [25] and ZrSi 2 -MoSi 2 -ZrB 2 [13,26] perform eutectics between silicides [22] or between silicon and silicides [11,13,[23][24][25][26]. The presence of eutectics between components in the composition of the coatings improves the adhesion between the layers and to the substrate, accelerates the regeneration of the oxide film, and also to some extent compensates for the differences in the coefficients of thermal expansion of coatings and protected materials.…”

“…Special attention should be paid to heat-resistant self-healing coatings based on the Si-TiSi 2 -MoSi 2 -TiB 2 system, including [11,23,27], wt%: titanium 15-40, molybdenum 5-30, boron 0.5-2.5 and silicon-the remainder. In the process of obtaining coatings, a dendritic-cellular structure is formed from disilicide phases (Ti x Mo 1−x Si 2 (0.1 < x < 0.87), TiSi 2 , MoSi 2 ).…”

“…The coatings have a low catalytic activity (the rate constant of heterogeneous recombination of atoms and ions at a flux K w = 3-5 m/s) and satisfactory characteristics of emissivity (the degree of blackness of the total normal radiation ε ≈ 0.7). Degradation of the structure of coatings, and, consequently, the loss of their operational properties, occurs mainly for two reasons [23]: Firstly, as the result of increases in erosional entrainment with increases in temperature; this is due to decreases in the number of skeleton-forming silicide phases due to their partial dissolution in the liquid phase upon reaching the solidus temperatures. Secondly, as a result of discontinuity of the oxide film due to formation at the "oxide film-coating" interface and the outflow of gaseous oxidation products (mainly SiO) under conditions of significant external rarefaction (P w ≤ 0.1 atm.).…”