Fabrication and mechanical properties of Al2O3–TiC ceramic composites synergistically reinforced with multi-walled carbon nanotubes and graphene nanoplates

In this study, Al₂O₃–TiC composites synergistically reinforced with multi-walled carbon nanotubes (MWCNTs) and graphene nanoplates (GNPs) were prepared via spark plasma sintering (SPS). The effects of the MWCNT and GNP contents on the phase composition, mechanical properties, fracture mode, and toughening mechanism of the composites were systematically investigated. The experimental results indicated that the composite grains became more refined with the addition of MWCNTs and GNPs. The nanocomposites presented high compactness and excellent mechanical properties. The composite with 0.8 wt% MWCNTs and 0.2 wt% GNPs presented the best properties of all analysed specimens, and its relative density, hardness, and fracture toughness were 97.3%, 18.38 ± 0.6 GPa, and 9.40 ± 1.6 MPa m^1/2, respectively. The crack deflection, bridging, branching, and drawing effects of MWCNTs and GNPs were the main toughening mechanisms of Al₂O₃–TiC composites synergistically reinforced with MWCNTs and GNPs.     

Influence of CeO2 addition on the microstructure and mechanical properties of Zirconia-toughened alumina (ZTA) composite prepared by spark plasma sintering

In this study, zirconia-toughened alumina (ZTA) samples with different amounts of CeO₂ were prepared by the spark plasma sintering method. The phase composition and microstructure of the samples were examined by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The addition of CeO₂ results in grain refinement and density increase; moreover, CeO₂ stabilises the high-temperature metastable phase. As the amount of CeO₂ reaches 7 wt%, a new CeAl₁₁O₁₈ phase appears. The Vickers hardness, modulus, and fracture toughness of the samples depend to a large extent on the grain size, relative density, and existence of the second phase. Among the composites, that with 5 wt% CeO₂ shows the best performance with the highest values of relative density, Vickers hardness, and fracture toughness: 96.51%, 1688 HV, and 9.91 MPa.√m, respectively.     

The effects of adding CNTs and GNPs on the microstructure and mechanical properties of hexagonal-boron nitride

This present work investigated the mechanical properties and microstructure of h-BN based ceramic composites reinforced with CNTs and GNPs. Accordingly, two different batches of pure h-BN, h-BN/0.1 wt%CNTs and h-BN/0.1 wt% GNPs were prepared through a high energy mixer mill to gain a uniform dispersion of reinforcement with the initial stable CNTs or GNPs solution in ethanol. After drying the mixtures, the pure h-BN and also, two different composite components were directly inserted into the graphite mold and the sintering process was performed with the initial and final pressure of 10 and 50 MPa, respectively, at 1900 °C, under the vacuum condition of 15–35 Pa. The relative density of the samples was calculated based on the Archimedes principle. The densification behavior of the samples showed the maximum amount of 98.31% for the theoretical density of the h-BN/GNPs composite. On the other hand, the minimum relative density of 96.41% was obtained for the h-BN/CNTs composite. The microstructure studies of the prepared sample showed the uniform distribution of GNPs in the h-BN layers; however, when the CNTs were added, some agglomerated area was found. Moreover, the fracture surface of all samples showed a laminar fracture as a result of the layer-by-layer structure of h-BN. The investigation of the mechanical properties of the prepared specimens also revealed the highest bending strength, fracture toughness and Vickers hardness of 199 MPa, 1.26 GPa and 3.62 MPa m^−1/2, respectively, which belonged to the h-BN/GNPs composite. In the case of CNTs, this trend exhibited lower amounts, probably due to the agglomeration of CNTs.     

Evolution mechanism of the microstructure and mechanical properties of plasma-sprayed yttria-stabilized hafnia thermal barrier coating at 1400 °C

Yttria stabilized hafnia (Hf_0.84Y_0.16O_1.92, YSH16) coatings were sprayed by atmospheric plasma spraying (APS). The effects of thermal aging at 1400 °C on the microstructures, mechanical properties and thermal conductivity of the coatings were studied. The results show that the as-sprayed coating was composed of the cubic phase, and the nano-sized monoclinic (M) phase was precipitated in the annealed coating. The presence of M phase effectively constrained the sintering of the coating due to its superior sintering-resistance. The Young's modulus kept at a nearly same level of ~78 GPa even after annealing, and the coating annealed for 6 h yielded a maximum value of hardness but revealed a declining tendency in the Vicker's hardness with prolonged sintering time. The thermal conductivity increased from 0.8-0.95 W m^-1 K^-1 at as-sprayed state to 1.6 W m^-1 K^-1 after annealing at 1400 °C for 96 h. The dual-phase coating is promising to serve at temperatures above 1400 °C due to its excellent thermal stability and mechanical properties.     

Beneficial role of carbon black on the properties of TiC ceramics

This investigation aimed to study the influence of carbon black on the qualifications of TiC-based materials. For this objective, two samples, namely monolithic TiC and TiC-5 wt% carbon black were sintered by spark plasma sintering (SPS) method at 1900 °C. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) were used to characterize the as-sintered samples. Introducing carbon black enhanced the relative density of TiC significantly, reaching a near fully dense substance. Phase analysis and microstructural studies manifested the formation of non-stoichiometric TiC_x in both ceramics. Although the introduction of carbonaceous additive considerably increased the thermal conductivity and flexural strength of TiC, standing at 25.1 W/mK and 658 MPa, respectively, its influence on the Vickers hardness was trivial (both ~ 3200 HV_0.1 kg). Finally, the composite specimen presented a lower coefficient of friction (~ 0.31) on average compared to the undoped TiC (~ 0.34).     

Microstructures,mechanical properties and corrosion resistance of sprayed Ca–P coating for micropatterning carbon/carbon substrate surface

Carbon/carbon (C/C) surface micropatterning is a method of modifying the surface into the complete and regular geometry. In this work, we introduce a positive effect on bonding strength between sprayed Ca–P coating and surface micropatterning C/C substrate. Interestingly, C/C substrate coated by Ca–P coating provides textured surface for a new bone ingrowth. The sprayed Ca–P coating is then subjected to microwave-hydrothermal (MH) treatment with the aim of eliminating surface defects and obtaining a uniform purity phase. These objectives were achieved in our previous study by the MH method. The molar ratio of Ca/P in the coatings is nearly close to 1, which is far below that of Ca/P for hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HA, 1.67). The purpose of this article is to transform the phases in the sprayed Ca–P coating, which owns the better bioactivity and high corrosion resistance. In order to raise the molar ratio of Ca/P, the coatings are treated under high-temperature (around 700 °C). They are analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and a fourier transform infrared spectra (FTIR). The bonding strength (coating/substrate), biological activity and corrosion resistance of the coatings are investigated. The resulting coatings own the different microstructures and phase compositions from the original sprayed Ca–P coating. Especially, results show that the shear strength of the sprayed Ca–P coating deposited on surface micropatterning C/C substrate increases by 61% which is more than that of the coating on non-surface micropatterning C/C substrate. Additionally, high-temperature treated coating presents a good biological activity and an excellent corrosion resistance of current density (1.3078 × 10^-6 A/cm²) and potential (−0.17 V_SCE).     

Present status and future prospects of plasma sprayed multilayered thermal barrier coating systems

Thermal barrier coatings (TBCs) play a pivotal role in protecting the hot structures of modern turbine engines in aerospace as well as utility applications. To meet the increasing efficiency of gas turbine technology, worldwide research is focused on designing new architecture of TBCs. These TBCs are mainly fabricated by atmospheric plasma spraying (APS) as it is more economical over the electron beam physical vapor deposition (EB-PVD) technology. Notably, bi-layered, multi-layered and functionally graded TBC structures are recognized as favorable designs to obtain adequate coating performance and durability. In this regard, an attempt has been made in this article to highlight the structure, characteristics, limitations and future prospects of bi-layered, multi-layered and functionally graded TBC systems fabricated using plasma spraying and its allied techniques like suspension plasma spray (SPS), solution precursor plasma spray (SPPS) and plasma spray –physical vapor deposition (PS-PVD).     

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.     

Characterization of a plasma polymer coating from an organophosphorus silane deposited at atmospheric pressure for fire-retardant purposes

Protective coatings from diethylphosphatoethyltriethoxysilane (DEPETS) have been deposited on different polymer substrates in a plasma discharge operated at atmospheric pressure. Plasma polymer chemistry and structure were characterized by means of Fourier transform infrared spectroscopy (FTIR), laser desorption ionization-mass spectrometry (LDI-MS), nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM). A chemical structure of the plasma polymer has been proposed based on the coating molecular characterization. Coatings were deposited on polycarbonate (PC) and polyamide 6 (PA6) substrates. The flame retardant properties of coated substrate samples were assessed using cone calorimetry and compared to those of bare substrates. A significant increase in the time to ignition (TTI), up to +143%, was recorded after coating deposition due to the formation of a high-performance barrier layer at the surface of both polymer substrates.