Corrosion behavior of volcanic ash on sintered mullite for environmental barrier coatings

The high-temperature corrosion behavior of volcanic ash(VA) in attacking sintered mullite was investigated, and the corrosion resistibility of mullite environmental barrier coatings (EBCs) to VA was predicted. Sintered mullite specimens were prepared by using the spark plasma sintering method. These specimens were subjected to a hot corrosive environment—molten Icelandic VA at 1400 °C—for three different duration times (2, 12, and 48 h). The microstructure and phase of the specimens were analyzed by using a scanning electron microscope equipped with an accessory system for energy dispersive spectroscopy and X-ray diffraction. In addition, in-situ high-temperature X-ray diffraction was carried out to identify the dynamics of phase evaluation in the volcanic ash and mullite mixture powders. Results show that a reaction layer was generated and continuously dissolved into the melted volcanic ash. The primary incursive component is iron; however, a minimal amount of sodium plays a more important role in disintegrating sintered mullite.     

Interaction of Gd2Si2O7 with CMAS melts for environmental barrier coatings

Environmental barrier coatings (EBCs) prevent the oxidation of ceramic matrix composites (CMC), which are used as components in gas turbines. However, EBCs deteriorate more rapidly in real environments, molten silicate deposits accelerate the deterioration of EBCs. In this study, high-temperature behavior sintered Gd₂Si₂O₇ with calcia-magnesia-alumina-silica (CMAS) melt at 1400 °C for 0.5, 2, 12, 48, and 100 h was investigated. HT-XRD results showed that at 1300 °C, CMAS and Gd₂Si₂O₇ chemically reacted to form Ca₂Gd₈(SiO₄)₆O₂ (apatite). The reaction layer became thicker as the heat-treatment time increased, and the thickness of the reaction layer has increased following a parabolic curve. With the extension of the reaction time from 0.5 to 100 h, the thickness of the reaction layer increased from approximately 98 to 315 µm. It was confirmed that Ca₂Gd₈(SiO₄)₆O₂ grew vertically on the Gd₂Si₂O₇ surface. Vertical and horizontal cracks were found after reacting at 1400 °C for 100 h, but no interfacial delamination occurred in this study. In addition, the effects of CaO:SiO₂ molar ratios, monosilicates (RE₂SiO₅) and disilicates (RE₂Si₂O₇), heat-treatment time, and cation size were determined and compared with the results of previous studies (Gd₂SiO_5, Yb₂SiO_5, and Er₂Si₂O₇).     

Novel structured spark plasma sintered thermal barrier coatings with high strain tolerance and oxidation resistance

Titanium alloys play an important role in lightweight aircraft engines owing to their low densities and high specific strengths. However, an increase in the thrust-to-weight ratio causes the engine operating temperature to be much higher than the service temperature, which deteriorates the oxidation resistance and mechanical properties. In this study, yttria-partially stabilised zirconia (8YSZ)/NiCrAlY thermal barrier coatings (TBCs) with a bimodal structure were prepared on Ti–6Al–4V by using spark plasma sintering (SPS) to improve the service temperature. The distinctive bimodal structure possessed dense particle contacts and a uniform distribution of porous nanoparticles, resulting in higher strain tolerance, sintering resistance, and lower thermal conductivity. Therefore, the bimodal structure prepared by lowering the SPS preparation temperature increased the high-temperature service time of TBCs on titanium alloy. The ceramic top coating (TC) and bond coating (BC) were well connected after isothermal oxidation at 800 °C for 100 h. The TBCs only shed 6% of their surface area at high temperature and large-angle bending. In addition, the bimodal-structured TBCs effectively improved the oxidation resistance of the Ti–6Al–4V substrate. The Ti–6Al–4V substrate with bimodal-structured TBCs only gained 0.51 times the mass gained by the bare Ti–6Al–4V after 100 h of isothermal oxidation.     

Mechanical properties and microstructure of Yb2SiO5 environmental barrier coatings under isothermal heat treatment

Yb₂SiO₅ (ytterbium monosilicate) top coatings and Si bond coat layer were deposited by air plasma spray method as a protection layer on SiC substrates for environmental barrier coatings (EBCs) application. The Yb₂SiO₅-coated specimens were subjected to isothermal heat treatment at 1400 °C on air for 0, 1, 10, and 50 h. The Yb₂SiO₅ phase of the top coat layer reacted with Si from the bonding layer and O₂ from atmosphere formed to the Yb₂Si₂O₇ phase upon heat treatment at 1400 °C. The oxygen penetrated into the cracks to form SiO₂ phase of thermally grown oxide (TGO) in the bond coat and the interface of specimens during heat treatment. Horizontal cracks were also observed, due to a mismatch of the coefficient of thermal expansion (CTE) between the top coat and bond coat. The isothermal heat treatment improves the hardness and elastic modulus of Yb₂SiO₅ coatings; however, these properties in the Si bond coat were a little bit decreased.     

High-temperature performances of Si-HfO2-based environmental barrier coatings via atmospheric plasma spraying

To improve high-temperature bearing capability of coatings, novel agglomerated Si-HfO₂ powders were prepared by adding HfO₂ powders into original Si powders by spray drying method. Three-layer environmental barrier coatings (EBCs) with Si-HfO₂ bond layer, Yb₂Si₂O₇ intermediate layer and Yb₂SiO₅ surface layer were prepared on SiC ceramic substrates by atmospheric plasma spraying (APS). The high temperature properties of coatings were systematically investigated. The results indicated that the coatings had good high temperature oxidation resistance, and remained intact after being oxidized or steam corrosion at 1400 °C for 500 h, so the addition of HfO₂ improved the thermal cycling performances of the coating. The HfO₂ in Si bond coating could effectively inhibit the growth of thermal grown oxide at high temperatures. This work indicates that the high temperature properties of the coatings are improved by this novel EBCs using the novel agglomerated Si-HfO₂ powders.     

Sintering behavior and mechanical properties of spark plasma sintering SiO2-MgO ceramics

SiO₂-MgO ceramics containing different weight fractions (0, 0.5, 1, 2, and 4 wt%) of SiO₂ powder were prepared by mixing nano MgO powder, and the powder mixtures were densified by spark plasma sintering (SPS). The effect of SiO₂ addition and SPS method on the sintering behavior, microstructure and mechanical properties were investigated. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The highest relative density, flexural strength and hardness of 2 wt% SiO₂-MgO ceramics reached 99.98%, 253.99 ± 7.47 MPa and 7.56 ± 0.21 GPa when sintered at 1400 °C by SPS, respectively. The observed improvement in the sintering behavior and mechanical properties are mainly attributed to grain boundary "strengthening" and intragranular "weakening" of the MgO matrix. Furthermore, the spark plasma sintering temperature could be decreased by more than 100 °C as compared with the HP method, SPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.     

Reaction synthesis of spark plasma sintered MoSi2-B4C coatings for oxidation protection of Nb alloy

MoSi₂-B₄C coatings with different B₄C contents were prepared on Nb alloy by spark plasma sintering (SPS) process. Powder mixtures of Mo, Si and B₄C were used as the coating starting materials. Besides MoSi₂ and B₄C phases, small amounts of SiC and MoB are also found in the coatings because of the reactions of Mo, Si and B₄C powders during sintering. Compared with single MoSi₂ coating, the MoSi₂-B₄C coatings show better oxidation resistance at 1450?℃, and dense B₂O₃-SiO₂ oxide scales form after 100?h oxidation. The B₄C or MoB in the MoSi₂-B₄C coatings can serve as the B donor for the formation of B₂O₃. A slight degradation in the microstructure of the MoSi₂-B₄C coatings after oxidation is observed, which can be attributed to the presence of an NbB layer in the inter-diffusion zone of the coatings that retards the inward diffusion of Si from the coating into the substrate alloy. The microstructure development and oxidation behavior of the MoSi₂-B₄C coatings have been discussed.     

Oxidation and self-healing behaviour of spark plasma sintered Ta2AlC

Self-healing and oxidation of spark plasma sintered Ta₂AlC was investigated using a newly developed wedge loaded compact specimen to determine strength recovery in a single specimen. Previous work had predicted dominant Al oxidation leading to dense and strong reaction products to result in favourable healing properties. However, crack-gap filling and strength recovery of Ta₂AlC were not achieved by oxidation at 600 °C. Oxidation below 900 °C in synthetic and atmospheric air resulted in porous Ta-oxides, with no Al₂O₃ formation. DTA up to 1200 °C revealed a two-step reaction process with the final products Ta₂O₅ and TaAlO₄. The study shows that the kinetics may overrule the self-healing MAX-phase design criteria based on thermodynamics.     

Microstructure and nanomechanical properties of plasma-sprayed nanostructured Yb2SiO5 environmental barrier coatings

In this study, nanostructured and conventional Yb₂SiO₅ coatings were prepared by atmospheric plasma. The microstructure and nanomechanical properties of these coatings were compared before and after heat treatment. The results show that the nanostructured Yb₂SiO₅ coatings have a mono-modal distribution, and the conventional Yb₂SiO₅ coatings have a bimodal distribution. Both types of coatings had improved nanomechanical properties after heat treatment. However, the increased elastic modulus and nanohardness of the nanostructured Yb₂SiO₅ coating were more apparent than those of the conventional Yb₂SiO₅ coatings. The nanostructured Yb₂SiO₅ coating had a higher elastic modulus than the conventional Yb₂SiO₅ coating, reflecting its high density. Subsequently, the microscopic morphology and micromechanical properties of the coatings were analyzed after heat treatment. Defects in the coatings, including pores, and microcracks, were significantly reduced with grain growth after thermal treatment, and the nanostructured Yb₂SiO₅ coatings had improved healing ability and micro-mechanical properties.     

Hot corrosion behavior of multialkaline-earth aluminosilicate for environmental barrier coatings

Multialkaline-earth aluminosilicate Ba_1/3Sr_1/3Ca_1/3Al₂Si₂O₈ (BSCAS) were synthesized to serve as new environment barrier coatings. Their hot corrosion behavior in an Na₂SO₄ environment was studied in the temperature range of 900–1100 °C over a period of 100 h. The phase and cross-sectional morphology evolutions of the corroded samples were characterized via X-ray diffraction and scanning electron microscopy. Combined with the thermodynamic analysis of the possible reactions occurring during hot corrosion, the competitive out-diffusion of the alkaline-earth elements to react with Na₂SO₄ is believed to have a considerable influence on the hot corrosion behavior of BSCAS. The sluggish diffusion and the dense Ca₂Al₂SiO₇ layer, which originate from the competitive reactions of the multialkaline earth elements, lead to an improvement in the hot corrosion resistance of BSCAS. A model is proposed to describe the hot corrosion process.     

Reaction between environmental barrier coatings material Er2Si2O7 and a calcia-magnesia-alumina-silica melt

The suitability of sintered erbium disilicate (Er₂Si₂O₇) as an environmental barrier coatings (EBCs) for gas turbine applications was assessed by characterizing its high-temperature corrosion behavior in contact with a synthetic calcia-magnesia-alumina-silica (CMAS) melt. Er₂Si₂O₇ was fabricated using spark plasma sintering at 1400 °C for 20 min. Corrosion tests were performed by coating sintered Er₂Si₂O₇ pellets with CMAS and heating them to 1400 °C for 2, 12, and 48 h. High-temperature X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis were used to identify and analyze the reaction products. The two materials were found to react chemically to form an apatite phase, Ca₂Er₈(SiO₄)₆O₂, at their interface. The Ca₂Er₈(SiO₄)₆O₂ grains were observed to have shard-like morphologies oriented perpendicular to the Er₂Si₂O₇ surface; the reaction layer thickened with increasing heat-treatment time, with the thickness after exposure for 48 h approximately three times the thickness after 2 h.     

Densification behavior and related phenomena of spark plasma sintered boron carbide

In this work, boron carbide ceramics were sintered in the temperature range of 1400–1600 °C by spark plasma sintering (SPS). The influence of sintering temperature, heating rate, and holding time on the microstructure, densification process and physical property was studied. The heating rate was found to have greater influence than that of the holding time on the microstructure and the densification of boron carbide. The optimal sintering temperature was 1600 °C under the heating rate higher than 100 °C/min. The relative density, flexural strength, Vickers hardness and fracture toughness of the sample synthesized at 1600 °C were 98.33%, 828 MPa, 31 GPa and 2.66±0.29 MPa m^1/2, respectively. The densification mechanism was also investigated.     

Preparation of crystalline ytterbium disilicate environmental barrier coatings using suspension plasma spray

In high-speed modern industries, high-temperature stability of materials is essential. A promising high-temperature material currently attracting attention is silicon carbide (SiC)-based ceramic matrix composites (CMC). However, a disadvantage of these materials is their reduced lifetime in an oxidizing atmosphere. To overcome this, environmental barrier coating can be employed. In this study, we aimed to fabricate an environmental barrier coating using suspension plasma spray with Yb₂Si₂O₇, which exhibits excellent oxidation resistance and a similar thermal expansion coefficient to SiC. To prepare the crystalline Yb₂Si₂O₇ coating layer, the gas concentration of the plasma spray was adjusted, and then the suspension manufacturing solvent was adjusted and sprayed. The prepared coating samples were analyzed by X-ray diffraction, scanning electron microscope, transmission electron microscopes, and energy dispersive X-ray spectroscopy to determine phase and microstructure changes. Highly crystalline ytterbium disilicate was observed at low plasma enthalpy with no hydrogen and 20% addition of water.     

Wear and friction properties of spark plasma sintered SiC/Cu nanocomposites

In this study, in order to determine the effect of SiC nanoparticles on tribological properties of nanostructured copper, the dry sliding wear and friction behaviors of nanostructured copper and copper reinforced with silicon carbide nanoparticles, produced by high energy ball milling and spark plasma sintering, were investigated by using an oscillating friction and wear tester under different normal loads. To determine the dominant wear mechanism, the worn surfaces and obtained debris after wear tests were analyzed by scanning electron microscope (SEM). The results showed that the addition of 4 vol% silicon carbide to copper matrix reduced the wear track depth and the coefficient of friction. Investigation of the worn surfaces revealed that SiC nanoparticles on the top of worn surface decreases the plastic deformation in subsurface region and alleviate severe wear. Lower plastic deformation during dry sliding wear test was attributed to high hardness of the nanocomposite that has been resulted from grain growth inhibiting and reinforcing effects of the nanoparticles. Plastic deformation and delamination were determined as major wear mechanisms in both materials.     

Crystallization behavior of air-plasma-sprayed ytterbium-silicate-based environmental barrier coatings

A combination of characterization techniques has been used to provide new understanding of the complex crystallization behavior of as-sprayed amorphous Yb₂Si₂O₇-based air-plasma-sprayed environmental barrier coatings (EBCs). During crystallization heat-treatment, initially a mixture of metastable α-Yb₂Si₂O₇ and X1-Yb₂SiO₅ phases form, along with stable β-Yb₂Si₂O₇ and X2-Yb₂SiO₅ phases. Eventually the metastable phases transform to the stable β-Yb₂Si₂O₇ (major) and X2-Yb₂SiO₅ (minor) phases. The significant volume expansion associated with these transformations partially contributes towards the anomalous expansion measured in these EBCs after crystallization, but it does not account for all the measured expansion. In this context, in similar EBCs, it is also observed that the porosity increases upon crystallization heat-treatment, primarily in the form of thin, interconnected pores, which also contributes to the measured anomalous expansion. Based on this understanding, guidelines are provided for ‘near-net-shape’ crystallization of phase-pure, dense β-Yb₂Si₂O₇ EBCs that are free of vertical cracks.     

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).     

Effect of Yb2SiO5 addition on the physical and mechanical properties of sintered mullite ceramic as an environmental barrier coating material

In this study, ytterbium monosilicate (Yb₂SiO₅)-added sintered mullite ceramics are prepared as candidate materials for environmental barrier coatings (EBCs). The effect of adding Yb₂SiO₅ on the physical and mechanical properties of the sintered mullite ceramics is investigated. The Yb₂O₃–SiO₂–Al₂O₃ ternary phase diagram indicates that adding Yb₂SiO₅ to the mullite goes beyond simply mixing; instead, liquid sintering occurs. Therefore, when we add Yb₂SiO₅ to the mullite, the sintered body possesses a denser microstructure and faster densification rate than does pure mullite. The density rapidly increases with the addition of 6 wt% Yb₂SiO₅ in the mullite, and almost full densifications are achieved with the addition of 12 wt% and 18 wt% Yb₂SiO₅. In this study, mullite ceramic that contains 12 wt% Yb₂SiO₅ exhibits the smallest plastic deformation and the highest elastic modulus among ceramics containing 6, 12, and 18 wt% Yb₂SiO₅, according to Hertzian indentation results. The results suggest that 12 wt% Yb₂SiO₅-doped mullite may be expected to act as a potential EBC material based on its excellent elastic properties, dense microstructure, and appropriate coefficient of thermal expansion.     

CMAS corrosion behavior of dual-phase composite Gd2Si2O7/Sc2Si2O7 as a promising EBC material

Environmental barrier coatings (EBCs) applied to gas-turbine components require excellent corrosion resistance to molten siliceous debris such as sand or volcanic ash in high-temperature environments while maintaining mechanical integrity. To date, most research has focused on single-phase rare-earth (RE) disilicates as candidate EBC materials, but here we report the superior corrosion resistance of a dual-phase disilicate composite, namely Gd₂Si₂O₇/Sc₂Si₂O₇ (70/30 vol%). EBSD measurements of cross-sections of the EBC after exposure to a calcium magnesium alumino-silicate (CMAS) for 0.5, 2, 12, and 48 h at 1400 °C reveal that, unlike in single-phase systems, the CMAS reaction layer consists of two distinct sublayers. The inner sublayer consists of a mixture of Ca₂Gd₈(SiO₄)₆O₂ and Sc₂Si₂O₇ crystals in a Ca-depleted glassy matrix, whereas the thinner outer region contains larger, elongated Ca₂Gd₈(SiO₄)₆O₂ crystals oriented perpendicular to the composite surface and devoid of any Sc₂Si₂O₇ crystals. The total thickness of the reaction layer is found to be about 20% less compared to that of single-phase Gd₂Si₂O₇ under the same conditions, indicating that dual-phase RE-disilicate composites are a promising materials system for increasing the lifetime performance of EBCs.     

Plasma spray deposition of tri-layer environmental barrier coatings

An air plasma spray process has been used to deposit tri-layer environmental barrier coatings consisting of a silicon bond coat, a mullite inter-diffusion barrier, and a Yb₂SiO₅ top coat on SiC substrates. Solidified droplets in as-deposited Yb₂SiO₅ and mullite layers were discovered to be depleted in silicon. This led to the formation of an Yb₂SiO₅ + Yb₂O₃ two-phase top coat and 2:1 mullite (2Al₂O₃*SiO₂) coat deposited from 3:2 mullite powder. The compositions were consistent with preferential silicon evaporation during transient plasma heating; a consequence of the high vapor pressure of silicon species at plasma temperatures. Annealing at 1300 °C resulted in internal bond coat oxidation of pore and splat surfaces, precipitation of Yb₂O₃ in the top coat, and transformation of 2:1 mullite to 3:2 mullite + Al₂O₃. Mud-cracks were found in the Yb₂SiO₅ layer and in precipitated Al₂O₃ due to the thermal expansion mismatch between these coating phases and the substrate.     

Microstructure and mechanical properties of spark plasma sintered TiB2 ceramics combined with a high-entropy alloy sintering aid

A high-entropy alloy (HEA), CoCrFeNiMn_0.5Ti_0.5, is used as a sintering aid for the densification of TiB₂ sintered by spark plasma sintering. The HEA content in the starting TiB₂-HEA mixture is varied from 0 to 10?wt-%. The microstructure and mechanical properties of the sintered samples are analysed and the optimum HEA content of 10% is found for the preparation of the TiB₂-HEA ceramics, allowing combining high mechanical properties (Vickers hardness of 2174.64?HV and flexural strength of 427.69?MPa) and high relative density of 99.1%.