Polymer derived oxidation barrier coatings for Mo-Si-B alloys

Thermodynamic analysis was carried out to predict the phase composition of a perhydridopolysilazane-type polymer derived ceramic coating on Mo₅SiB₂ matrix particles after heat treatment. The most probable chemical reactions between these constituents and resulting phases were calculated. The feasibility of PHPS/Mo₅SiB₂ chemical reactions was proved experimentally. An amorphous SiO_xN_y phase and free Si were found by X-ray diffraction analysis and Raman spectroscopy. The presence of these two phases explain the improvement in oxidation resistance of the Mo₅SiB₂ particles, which was found to be as twice as high at 800 °C and 1100 °C in air, compared to the unprotected plain Mo₅SiB₂ phase. The oxidation of the free silicon provided by the PHPS conversion was addressed as an oxygen trap.     

Corrosion resistant polymer derived ceramic composite environmental barrier coatings

Corrosion resistant coatings are a promising solution to protect structural metals in harsh environments. Ceramic composite coatings made from polymer-derived ceramics are highly attractive due to the ease of their processing and the ability to work in various environments. This paper is focused on the performance of a TiSi₂-filled SiOC ceramic composite coating system on 316 stainless steel (SS) substrates as a corrosion resistant coating. The best-performing quadruple-dip coatings were shown to be able to reduce the weight loss due to hot sulfuric acid (95+%, 104–107 °C) corrosion by 85% over a 30-day period. Coatings from the same system were also examined under 800 °C static (100 h) and cyclic (10 cycles) oxidation. Our results indicate that the coatings perform well under both conditions of prolonged high temperature oxidation and thermal cycling, suggesting the strong potential of this system as an environmental barrier coating (EBC).     

Polymer derived silicon oxycarbide ceramic monoliths: Microstructure development and associated materials properties

Polymer derived SiOC and SiCN ceramics (PDCs) are interesting candidates for additive manufacturing techniques to develop micro sized ceramics with the highest precision. PDCs are obtained by the pyrolysis of crosslinked polymer precursors at elevated temperatures. Within this work, we are investigating PDC SiOC ceramic monoliths synthesized from liquid polysiloxane precursor crosslinked with divinylbenzene for fabrication of conductive electromechanical devices. Microstructure of the final ceramics was found to be greatly influenced by the pyrolysis temperature. Crystallization in SiOC ceramics starts above 1200?°C due to the onset of carbothermal reduction leading to the formation of SiC and SiO₂ rich phases. Microstructural characterisation using ex-situ X-ray diffraction, FTIR, Raman spectra and microscopy imaging confirms the formation of nano crystalline SiC ceramics at 1400?°C. The electrical and mechanical properties of the ceramics are found to be significantly influenced by the phase separation with samples becoming more electrically conducting but with reduced strength at 1400?°C. A maximum electrical conductivity of 10¹ S?cm^?1 is observed for the 1400?°C samples due to enhancement in the ordering of the free carbon network. Mechanical testing using the ball on 3 balls (B3B) method revealed a characteristic flexural strength of 922?MPa for 1000?°C amorphous samples and at a higher pyrolysis temperature, materials become weaker with reduced strength.     

Thermodynamic assessment of the group IV,V and VI oxides for the design of oxidation resistant multi-principal component materials

Multi-principal component materials (MPCMs) are currently being investigated for use in high and ultra-high temperature environments. The design of oxidation resistant multi-component materials requires as input the oxidation behavior of each of the components. FactSage free energy minimization software and databases were used to calculate the equilibrium oxide phases and free energies of formation for the oxides of the Group IV, V and VI refractory metals, and their carbides, nitrides and borides. The results are summarized in Ellingham diagrams. Periodic trends were noted; Group IV elements form the most stable oxides with the highest melting temperatures (T_m), Group V elements form oxides with low T_m, and Group VI elements form gaseous oxide species. Oxygen diffusion data from literature for some of these oxides were also reviewed and summarized. The results are utilized to identify strategies for optimizing oxidation resistance of MPCMs for service at temperatures above 1700°C.     

Corrosion products evolution and hot corrosion mechanisms of REPO4 (RE= Gd,Nd, La) in the presence of V2O5 + Na2SO4 molten salt

REPO₄ (RE = Gd, Nd, La) ceramics with a monazite structure were fabricated by a chemical co-precipitation and calcination method. Hot corrosion tests were carried out in V₂O₅+Na₂SO₄ molten salt at 800 °C, 900 °C and 1000 °C for 2 h and 10 h. The temperature and heat duration had little effect on the type of corrosion products in this study. However, GdPO₄ and REPO₄ (RE = Nd, La) revealed different hot corrosion behavior. Exposed to the molten salt, GdVO₄ and Gd₄(P₂O₇)₃ formed as the corrosion products for the GdPO₄ case, while an RE(P,V)O₄ (RE = Nd, La) solid solution was generated for NdPO₄ and LaPO₄ cases. The formation of the solid solution had less damage to the original microstructure, which benefited the hot corrosion resistance of the ceramics. From the crystallographic characteristics of rare earth phosphates/vanadates and a thermodynamics perspective, the hot corrosion mechanisms of REPO₄ (RE = Gd, Nd, La) are discussed.     

Corrosion of RE2Si2O7 (RE=Y,Yb, and Lu) environmental barrier coating materials by molten calcium-magnesium-alumino-silicate glass at high temperatures

With the increased demand for high operating temperature of gas turbine engines, corrosion by molten calcium-magnesium-alumino-silicate (CMAS) exhibits a significant challenge to the development of durable environmental barrier coatings (EBCs). EBC candidates, γ-Y₂Si₂O₇, β-Yb₂Si₂O₇, and β-Lu₂Si₂O₇ were explored on their corrosion resistance to CMAS melts at 1300 °C and 1500 °C for 50 h. Interaction and degradation mechanisms were investigated and the corrosion behaviors showed different trends at high temperatures. At 1300 °C, RE₂Si₂O₇ dissolves into CMAS melts and apatite phases reprecipitate forming a thick recession layer. However, when the temperature increases to 1500 °C, CMAS melts vigorously penetrate through the grain boundary of RE₂Si₂O₇ and ‘blister’ cracks form throughout the samples. The reduced grain boundary stability at 1500 °C promotes the penetration of CMAS melts in RE₂Si₂O₇. Grain boundary engineering is critically demanded to optimize CMAS corrosion at high temperatures.     

Shrinkage and dimensional accuracy of porous ceramics derived from capillary suspensions

Shrinkage and dimensional accuracy are of particular importance for industrial material production. High dimensional accuracy directly lowers finishing works and cost. Capillary suspension processing is a novel, easy method to produce highly porous ceramic materials. Their shrinkage and shape accuracy is investigated during processing using a laser microscope. The total shrinkage is reduced by 20% for capillary suspension compared to pure suspension. This results from an increase in linear shape accuracy for top radii as well as height. The linear shape accuracy is increased by 6% in top radius and by 16% in height by using the capillary suspension phenomenon. We also show that this capillary suspension method is applicable for continuous shaping processes, like extrusion. The combination of an easy, robust processing route with known dimensional accuracy and applicability for continuous shaping processes makes this capillary suspension processing route highly desirable for industrial processes.     

Processing,characterization and properties of novel gradient Si3N4/SiC fibers derived from polycarbosilanes

In this work, we report a novel kind of Si₃N₄/SiC composite fibers, which exhibit a controlled gradient Si₃N₄(shell)/SiC(core) structure. These composite fibers are fabricated through a controlled nitridation and pyrolysis process on electron irradiation-cured polycarbosilane fibers. Structural and chemical analysis based on Elemental Analyzer, FT-IR, Raman spectroscopy, electron probe micro-analyzer, X-ray photoelectron spectroscopy, and X-ray diffraction confirm the gradient structure of obtained fibers, which consist a shell with high Si₃N₄ content and a SiC core. The as-fabricated fibers exhibit dense and smooth surfaces, and no microscopic holes or defects were observed. The effects of nitridation temperature on mechanical properties and electrical resistivity were also investigated. Combined with high mechanical properties and lightweight, the present gradient Si₃N₄/SiC fibers open a new strategy to fabricate multifunctional and electromagnetic wave absorbing materials.     

Immobilization of tannery industrial sludge in ceramic membrane preparation and hydrophobic surface modification for application in atrazine remediation from water

Chromium laden waste produced from tannery industry was immobilized in ceramic matrix for fabrication of the tubular single channel microfiltration membranes by extrusion. The presence of chromia resulted in substitutional solid solution formation with alumina and catalyzed mullite phase growth, hence increasing the mechanical and chemical stability of the membranes. The structural, morphological and water permeation characteristics of the membranes were studied to analyze their formation mechanism and effect of different parameters, viz. the sintering temperature, amount of waste added, presence of organics and extent of chromium immobilization. The surface of the macroporous membrane was hydrophobically modified, by polydimethylsiloxane (PDMS), producing contact angle of 141°. The process efficiency of the hydrophobic membrane was assessed in terms of the removal of atrazine, a contaminant of emerging concern, following the principle of hydrophobic interaction. Effect of different operating parameters affecting atrazine removal, viz. transmembrane pressure, cross flow velocity and filtration time was studied in cross flow filtration mode. High atrazine removal of >95% was obtained along with the maintenance of high flux during the filtration operation. The prepared cost-effective microfiltration membranes can thus be further modified for efficient water treatment applications.