Processing-Microstructure-Properties of Columns in Thermal Barrier Coatings: A Study of Thermo-Chemico-Mechanical Durability.

Contemporary gas turbine engines rely on thermal barrier coatings (TBCs), which protect the structural components of the engine against degradation at extremely high operating temperatures (1300-1500 °C). The operational efficiencies of aircraft engines have seen significant improvement in recent years, primarily through the increase in operating temperatures; however, the longevity of TBCs can be potentially impacted by several types of degradation mechanisms. In this comprehensive study, a wide range of novel columnar suspension plasma sprayed (SPS) coatings were developed for their erosion, calcium-magnesium-aluminum-silicate (CMAS), and furnace cycling test (FCT) performance. Through a comprehensive investigation, the first of its kind, we achieved a range of SPS microstructures by modifying the spray parameters and measuring their microhardness, fracture toughness, column densities, and residual stresses using Raman spectroscopy. We were able to produce dendritic, lateral, branched, and columnar microstructures with a unique set of processing parameters. Coatings enhanced with a refined columnar microstructure, achieved by modulating the distance from the plasma torch, exhibited superior thermal cycling resilience. Conversely, the development of a columnar microstructure with dendritic branches, obtained by decreasing the robot's traversal speed during deposition, bolstered resistance to erosion and minimized damage from molten CMAS infiltration, thereby notably augmenting the coating's lifespan and robustness. The pursuit of the optimal columnar microstructure led to the conclusion that for each SPS coating, a general framework of optimization needs to be conducted to achieve their desired thermo-chemico-mechanical resistance as the properties required for TBCs are intertwined.

Advanced Open-Celled Structures from Low-Temperature Sintering of a Crystallization-Resistant Bioactive Glass.

Most materials for bone tissue engineering are in form of highly porous open-celled components (porosity >70%) developed by means of an adequate coupling of formulations and manufacturing technologies. This paper is dedicated to porous components from BGMS10 bioactive glass, originally designed to undergo viscous flow sintering without crystallization, which is generally known to degrade the bioactivity of 45S5 bioglass. The adopted manufacturing technologies were specifically conceived to avoid any contamination and give excellent control on the microstructures by simple operations. More precisely, 'green' components were obtained by digital light processing and direct foaming of glass powders suspended in a photosensitive organic binder or in an aqueous solution, activated with an organic base, respectively. Owing to characteristic quite large sintering window of BGMS10 glass, sintering at 750 °C caused the consolidation of the structures generated at room temperature, without any evidence of viscous collapse.

Glass-Ceramic Foams from 'Weak Alkali Activation' and Gel-Casting of Waste Glass/Fly Ash Mixtures.

A 'weak alkali activation' was applied to aqueous suspensions based on soda lime glass and coal fly ash. Unlike in actual geopolymers, an extensive formation of zeolite-like gels was not expected, due to the low molarity of the alkali activator (NaOH) used. In any case, the suspension underwent gelation and presented a marked pseudoplastic behavior. A significant foaming could be achieved by air incorporation, in turn resulting from intensive mechanical stirring (with the help of a surfactant), before complete hardening. Dried foams were later subjected to heat treatment at 700⁻900 °C. The interactions between glass and fly ash, upon firing, determined the formation of new crystal phases, particularly nepheline (sodium alumino⁻silicate), with remarkable crushing strength (~6 MPa, with a porosity of about 70%). The fired materials, finally, demonstrated a successful stabilization of pollutants from fly ash and a low thermal conductivity that could be exploited for building applications.

Extension of the 'Inorganic Gel Casting' Process to the Manufacturing of Boro-Alumino-Silicate Glass Foams.

A new technique for the production of glass foams, based on alkali activation and gel casting, previously applied to soda-lime glass, was successfully extended to boro-alumino-silicate glass, recovered from the recycling of pharmaceutical vials. A weak alkali activation (2.5 M NaOH or NaOH/KOH aqueous solutions) of fine glass powders (below 70 µm) allowed for the obtainment of well-dispersed concentrated aqueous suspensions, undergoing gelation by treatment at low temperature (75 °C). Unlike soda-lime glass, the progressive hardening could not be attributed to the formation of calcium-rich silicate hydrates. The gelation was provided considering the chemical formulation of pharmaceutical glass (CaO-free) to the formation of hydrated sodium alumino-silicate (N-A-S-H) gel. An extensive direct foaming was achieved by vigorous mechanical stirring of partially gelified suspensions, comprising also a surfactant. A sintering treatment at 700 °C, was finally applied to stabilize the cellular structures.

Bioactive Glass-Ceramic Foam Scaffolds from 'Inorganic Gel Casting' and Sinter-Crystallization.

Highly porous bioactive glass-ceramic scaffolds were effectively fabricated by an inorganic gel casting technique, based on alkali activation and gelification, followed by viscous flow sintering. Glass powders, already known to yield a bioactive sintered glass-ceramic (CEL2) were dispersed in an alkaline solution, with partial dissolution of glass powders. The obtained glass suspensions underwent progressive hardening, by curing at low temperature (40 °C), owing to the formation of a C-S-H (calcium silicate hydrate) gel. As successful direct foaming was achieved by vigorous mechanical stirring of gelified suspensions, comprising also a surfactant. The developed cellular structures were later heat-treated at 900-1000 °C, to form CEL2 glass-ceramic foams, featuring an abundant total porosity (from 60% to 80%) and well-interconnected macro- and micro-sized cells. The developed foams possessed a compressive strength from 2.5 to 5 MPa, which is in the range of human trabecular bone strength. Therefore, CEL2 glass-ceramics can be proposed for bone substitutions.

Bioactive Glass-Ceramic Scaffolds from Novel 'Inorganic Gel Casting' and Sinter-Crystallization.

Highly porous wollastonite-diopside glass-ceramics have been successfully obtained by a new gel-casting technique. The gelation of an aqueous slurry of glass powders was not achieved according to the polymerization of an organic monomer, but as the result of alkali activation. The alkali activation of a Ca-Mg silicate glass (with a composition close to 50 mol % wollastonite-50 mol % diopside, with minor amounts of Na2O and P2O5) allowed for the obtainment of well-dispersed concentrated suspensions, undergoing progressive hardening by curing at low temperature (40 °C), owing to the formation of a C-S-H (calcium silicate hydrate) gel. An extensive direct foaming was achieved by vigorous mechanical stirring of partially gelified suspensions, comprising also a surfactant. The open-celled structure resulting from mechanical foaming could be 'frozen' by the subsequent sintering treatment, at 900-1000 °C, causing substantial crystallization. A total porosity exceeding 80%, comprising both well-interconnected macro-pores and micro-pores on cell walls, was accompanied by an excellent compressive strength, even above 5 MPa.