Effect of MoSi2 addition on ablation behavior of ZrC coating fabricated by vacuum plasma spray

To improve the ablation resistance of ZrC coating, MoSi₂ incorporated ZrC composite coatings were fabricated by vacuum plasma spray. The ablation resistance of the composite coatings was evaluated using a plasma jet with a heat flux of 1.94?MW/m². The phase compositions and microstructures of the coatings before and after ablation were investigated, and the ablation mechanisms and effect of MoSi₂ were analyzed based on thermal dynamics and microstructure changes. Results showed that MoSi₂ addition could improve the ablation resistance of ZrC coating by means of decreasing the surface temperature and changing the microstructure of the oxidation layer. Si derived from the decomposition of MoSi₂, which occurred within coating, was beneficial to maintain the thickness and integrity of the SiO₂ layer and reduce the oxygen pressure beneath. The thickness of the SiO₂ layer was related to the formation rate (V_f) and the consumption rate (V_c) of SiO₂. The diffusion of Si was in favor of increasing the value of V_f. MoSi₂ could be one choice to improve the ablation resistance of the ZrC coating.     

Ablation resistance and mechanism of ZrC-SiC-Yb2O3 ternary composite coatings fabricated by vacuum plasma spray

In this work, ZrC-SiC-Yb₂O₃ ternary composite coatings with different contents of Yb₂O₃ (0, 5, 15 and 30 wt.%) were fabricated by vacuum plasma spray and the anti-ablation property above 2000 °C was evaluated. The results showed that the microstructures of the oxide scales were changed with different Yb₂O₃ contents and then greatly modified the ablation resistant property of the composite coatings. The composite coating with 15 wt.% Yb₂O₃ exhibited the best ablation resistance. The disappearance of SiC-depleted layer and formation of ZrC_xO_y were observed. The mechanism of Yb₂O₃ on the oxidation products and ablation behaviors of ZrC-SiC system was analyzed.     

Deposition of graded SiO2/SiC coatings using high-velocity solution plasma spray

Carbon/carbon (C/C) composites are widely used in structural components, particularly in the aerospace and aeronautics sectors. However, the application of C/C composites is limited by low oxidation resistance at high temperatures. In order to overcome this problem, graded SiO₂/SiC coatings were deposited on C/C composites by a high-velocity solution plasma spray (HVSPS) process. Graded coatings were formed by reactions between the Si(OH)₄ sprayed liquid precursor and the C/C substrate; these reactions were promoted by the high temperature of the plasma torch. The morphologies, microstructures, and chemical compositions of the coatings were investigated by X-ray diffraction, Raman spectroscopy, Fourier-transform infrared spectroscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy. By altering the deposition time, the coating thickness was controlled, therefore demonstrating SiC formation and realizing graded SiO₂/SiC coatings.     

Polycrystalline infrared-transparent MgO fabricated by spark plasma sintering

In the present research, polycrystalline magnesium oxide (MgO) bodies were fabricated using spark plasma sintering (SPS) at different temperatures and times from MgO nanopowder. Microstructural development, densification, and optical properties were investigated during SPS. The critical pressure of plastic deformation of the MgO compacts during sintering was also analyzed. The results showed that the plastic deformation phenomenon had a profound effect on the grain size and optical properties. In addition, the optical properties and microstructure of MgO bodies were strongly dependent on sintering temperature and time. Full-dense infrared-transparent magnesium oxide with a relative density of 99.99% was prepared at 1200 °C for 5 min under the pressure of 80 MPa. The spark plasma sintered MgO demonstrated the highest infrared transmittance of 72% in the 3–7 μm wavelength range, which was comparable with the values reported for MgO single crystal.     

Oxidation behaviors and mechanisms of Yb2O3-doped silicon coatings fabricated by vacuum plasma spray

Environmental barrier coatings (EBCs) have been widely studied for the protection of ceramic matrix composites (CMCs). The phase transition of silica thermal growth oxide (TGO) has been proved to be an important factor for the durability of EBCs. Yb₂O₃ could react with SiO₂ TGO and form silicate which may improve the stability of TGO and prolong the service life of EBCs. In the present work, Si coatings doped with different contents of Yb₂O₃ were fabricated by vacuum plasma spray. The oxidation behaviors of the composite coatings were evaluated at 1350 °C and compared with the pure Si coating. The evolution of phase composition and microstructure of mixed thermal growth oxide (mTGO) was characterized in detail. The results showed that the newly formed oxidation product, namely Yb₂Si₂O₇, could reduce the vertical cracks in mTGO layer and the mTGO/coating interface cracks, leading to a better binding performance of the mTGO layer. The oxidation mechanisms of the Yb₂O₃-doped Si coatings were analyzed based on microstructure and phase composition observations.     

Induction plasma spheroidization of ZrB2-SiC powders for plasma-spray coating

To improve the microstructure and properties of ZrB₂-SiC (ZrB₂-30?vol%SiC, Z7S3) coatings, a facile synthesis route involving induction plasma spheroidization (IPS) has been proposed, and the morphologies, particle size distributions, and phase compositions of the feedstock powders were analysed by scanning electron microscopy and focused ion beam techniques. The obtained results showed that the surfaces of the produced powders contained eutectic-like and granular zones. Owing to the existence of a temperature gradient, the internal microstructure of the IPS-treated powder exhibited a three-layered structure consisting of a ′surface shell′, a ′transition layer′, and a ′porous core′. Additionally, the properties of the IPS-treated samples were compared with those of the spray-dried (SD) powders. The former exhibited good plastic deformation properties, and their single splats contained flattened structures, while the single splats of the SD powder melted only partially retaining some of their original characteristics.     

Ablation behavior of Mo/MSZ thick alternate multilayer fabricated by combinative processes of plasma spray and hot isostatic pressing

Thermal barrier and anti-ablation coatings play essential roles in protecting jet nozzles, gas turbine engines and other machines from high temperatures. In this work, a novel curved structure of Mo/MSZ alternate multilayer was designed and fabricated by air plasma spraying. Two steps of the hot isostatic pressing were applied to improve the mechanical strength and interlayer bonding. The alternate structure enabled thick MSZ (Magnesium oxide stabilized Zirconia) layers and show an excellent thermal insulation ability under high-temperature ablation (4.3 MW/m²). Due to the curved surface, the thermal and residual stress is larger at the outer layer and edge, leading to cracks at the most out layer during the ablation. Moreover, during the ablation, the surface suffered steps in the order of hot flush damaging, MSZ and Mo melting, mixing, and Mo oxidizing. This work provided design, reinforcing method and ablation behavior analysis of alternate multilayer of TBC, supporting a solution to increase the performance of thermal insulation and anti-ablation.     

Environmental barrier coatings using low pressure plasma spray process

Yb₂Si₂O₇/Si bilayer environmental barrier coatings (EBCs) on SiC ceramic substrate were produced by low pressure plasma spray (LPPS) process. Phase composition, microstructure, and thermal durability of LPPS Yb₂Si₂O₇/Si coating were investigated. XRD analysis indicated that the coating is mainly composed of Yb₂Si₂O₇ with ~15.5v% Yb₂SiO₅ phases. The LPPS EBCs have a dense microstructure with porosity less than 4%. Adhesion strength measurement indicated the LPPS EBCs have an average adhesion strength of 29.1 ± 0.8 MPa. Furnace cycle test (FCT) on the coatings in air at 1316°C was performed and the test ran for 900 cycles and there was no coating spallation/failure for LPPS Yb₂Si₂O₇/Si EBCs. The FCT results demonstrated the excellent thermal cycle durability of LPPS EBCs. Oxidation kinetics investigation of LPPS EBCs in flowing 90% H₂O (g)+10% air at 1316°C showed that the thermally grown oxide (TGO) growth rate is close to the oxidation rate of pure Si in dry air and is significantly lower than that in water vapor environment. The LPPS process is promising in making highly durable Yb2Si2O7-based dense EBCs by impeding diffusion and ingression of water vapor/O₂.     

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.     

Influence of phase composition on microstructure and thermal properties of ytterbium silicate coatings deposited by atmospheric plasma spray

In this study, ytterbium silicate coatings with different compositions were designed by controlling the Yb₂O₃/ SiO₂ ratio and fabricated by atmospheric plasma spray. The microstructure and thermal properties of these coatings were characterized. Results showed that the Yb₂O₃-rich coatings contained Yb₂O₃ and Yb₂SiO₅ phases, which were characterized by Yb₂O₃ columnar grains, obvious interfaces between splats and many microcracks. The SiO₂-rich coatings were composed of Yb₂SiO₅ and Yb₂Si₂O₇ phases, which were composed of well bonded splats with many spherical pores. The Yb₂O₃-rich coatings had higher coefficient of thermal expansion values and lower thermal conductivities than the SiO₂-rich coatings. The SiO₂-rich coatings presented much better thermal cycling resistance than the Yb₂O₃-rich coatings. The relationship among phase composition, microstructure and thermal properties of ytterbium silicate coatings was analyzed. The results of this study may provide some clues for designs and applications of rare-earth silicates as environmental barrier coatings.     

Challenging zircon coatings by suspension plasma spraying

Zircon is a ceramic material that decomposes at high temperature, limiting its use by conventional thermal spraying. In this work, it is intended to use thermal spraying from concentrated aqueous suspensions to evaluate the possibility of obtaining coatings in which a significant proportion of zircon could be preserved. For this purpose, stable concentrated suspensions of zircon have been prepared, which have been subsequently sprayed at two different spraying distances. The coatings were characterised in terms of microstructural features and the amount of zircon present in the coatings was quantified. All the coatings obtained display the typical microstructure derived from the deposition of liquid feedstocks by plasma spraying. In all cases, the XRD analysis demonstrates the partial decomposition of zircon into zirconia and residual silica, but also that a significant percentage (about 20%) is preserved without decomposing, which marks a strong difference with respect to reported data for atmospheric plasma spraying.     

Effect of suspension characteristics on the performance of thermal barrier coatings deposited by suspension plasma spray

This paper investigates the influence of suspension characteristics on microstructure and performance of suspensions plasma sprayed (SPS) thermal barrier coatings (TBCs). Five suspensions were produced using various suspension characteristics, namely, type of solvent and solid load content, and the resultant suspensions were utilized to deposit five different TBCs under identical processing conditions. The produced TBCs were evaluated for their performance i.e. thermal conductivity, thermal cyclic fatigue (TCF) and thermal shock (TS) lifetime. This experimental study revealed that the differences in the microstructure of SPS TBCs produced using varied suspensions resulted in a wide-ranging overall TBC performance. All TBCs exhibited thermal conductivity lower than 1 W/(m. K) except water-ethanol mixed suspension produced TBC. The TS lifetime was also affected to a large extent where 10 wt % solid loaded ethanol and 25 wt % solid loaded water suspensions produced TBCs exhibited the highest and the lowest lifetime, respectively. On the contrary, TCF lifetime was not as significantly affected as thermal conductivity and TS lifetime, and all ethanol suspensions showed marginally better TCF lifetime than water and ethanol-water mixed suspensions deposited TBCs.     

溶液注入等离子喷涂——一种制备热障涂层的新技术

介绍了溶液注入等离子喷涂技术(SPPS)在制备热障涂层领域的应用,分析了SPPS热障涂层的结构和性能特点,将该技术与其它工艺过程进行比较,指出了该技术的优势和未来发展方向。     

Preparation of spherical WC-Co powder by spray granulation combined with radio frequency induction plasma spheroidization

In this paper, spray granulation and radio frequency plasma spheroidization were used to obtain spherical WC-Co powder for laser powder bed fusion from mixed WC and Co powder. The effects of solid content and polyvinyl alcohol content of the slurry on granulated powder were studied. Then the spheroidization effects of different granulated WC-Co powders were investigated and compared. Results show that the granulated powder obtained from the slurry with solid content of 65 wt% and polyvinyl alcohol content of 2.5 wt% has the best performance after plasma spheroidization, whose flowability and apparent density are 10.20 s/50g and 6.76 g/cm³, respectively. Moreover, W and C distribute uniformly in the spheroidized WC-Co powder while the Co element is mainly distributed in the gaps of tungsten carbide. It is also noted that W₂C, free carbon and Co₃W₃C appear in the spheroidized WC-Co powders due to the decomposition of WC during the plasma spheroidization process. Furthermore, the decomposition of WC-Co powder particles with smaller size is more serious, which leads to the presence of black and white particles with significantly different carbon content distribution in the spheroidized powder.     

Structure and mechanical properties of hydroxyapatite coatings produced on titanium using plasma spraying with induction preheating

Coatings of hydroxyapatite (HAp) were prepared by plasma spraying with induction preheating of titanium substrate from 200 to 1000 °C. The combination of conventional plasma spraying and induction preheating ensured high mechanical properties of HAp coatings. The coatings produced in the temperature range 400–600 °C were characterized by homogeneous nanostructure of splats with an average grain size of 12–31 nm. According to the results of nanoindentation HAp coatings with high hardness 0.9–1.2 GPa and elastic modulus 7–16 GPa were formed on the titanium.     

Reactive plasma spraying of supersaturated tungsten super-hard Ta-Hf-W-C solid solution coating

A supersaturated tungsten Ta-Hf-W-C solid solution coating is successfully prepared by a reactive plasma spraying. The reaction process and solution behavior of TaC-HfC-WC-C composite powder during induction plasma spheroidization (IPS) and vacuum plasma spraying (VPS) are described. Compared with the direct spraying, the coating made by IPS-VPS multi-stage plasma heating process has lower porosity and better composition uniformity. The highest hardness obtained by nanoindentation can reach up to 43.3 GPa due to solid solution strengthening. This work provides a new method and a potential material for thermal spraying super-hard coatings.     

Thermal shock resistance of thermal barrier coatings for nickel-based superalloy by supersonic plasma spraying

Double-layer thermal barrier coatings (TBCs), including a top ZrO₂ layer and an inner CoNiCrAlY layer, were deposited on nickel-based superalloy using supersonic atmospheric plasma spraying (SAPS). Thermal shock resistance of the TBCs between 1200 °C and room temperature was investigated. After thermal shock test, the adhesive strength of the coatings was evaluated through scratch test. The SAPS–TBCs present good thermal shock resistance, exhibiting only 0.26% mass gain up to 150-time thermal cycling. Before thermal cyclic treatment, SAPS–TBCs exhibited a strong adhesion with the absence of the thermally grown oxide (TGO) between out and inner layer. With the increasing of thermal cycles, the TGO layer was formed and its thickness firstly increased and then dropped down. The critical load fell down by about 32% for topcoat–bondcoat adhesion (up to 50 cycles) and 35% or so for TBCs–substrate adhesion (up to 150 cycles) compared to the counterpart of as-sprayed specimens. The strain introduced by the existence of TGO and mixed oxides resulted in a varied adhesion for TBCs on nickel-based alloy during thermal cycling.     

Friction behavior of PTFE-coated Si3N4/TiC ceramics fabricated by spray technique under dry friction

PTFE coatings were deposited on the Si₃N₄/TiC ceramic substrate by using spray technology. The surface and cross-section micrographs, adhesive force of coatings with substrate, surface roughness and micro-hardness of the coated ceramics were examined. The friction and wear behaviors of ceramic samples with and without coatings were investigated through carrying out dry sliding friction tests against WC/Co ball. The test results indicated that the coated ceramics exhibited rougher surface and lower micro-hardness, and the PTFE coatings can significantly reduce the surface friction and adhesive wear of ceramics. The friction performance of PTFE-coated sample was affected by applied load due to the lower surface hardness and shear strength of coatings, and the main wear failure mechanisms were abrasion wear, coating delamination and flaking. It can be considered that deposition of PTFE coatings is a promising approach to improve the friction and wear behavior of ceramic substrate.     

Microstructure and thermal properties of inflight rare-earth doped thermal barriers prepared by suspension plasma spray

Rare-earth doped yttria-stabilized zirconia (YSZ) coatings with lower thermal conductivity have been fabricated via suspension plasma spray by dissolving rare-earth nitrates into YSZ powder-ethanol suspensions prior to plasma spraying. The effect of dopant concentration and dopant type on properties of the coatings was determined by comparing two coatings containing different concentrations of the same dopant pair (Nd₂O₃/Yb₂O₃), and three coatings having similar concentrations of different dopant pairs (Nd₂O₃/Yb₂O₃, Nd₂O₃/Gd₂O₃, and Gd₂O₃/Yb₂O₃). The porosity content of the coating was found to increase with increased total rare-earth dopant concentration but did not significantly change with dopant pairs. The cross-sectional morphology of every coating displayed a cauliflower-like structure. However, the most heavily doped coating exhibited a larger surface roughness and feathery features in the columnar structures. The thermal conductivity measurement showed that the thermal conductivity decreased with increased Nd₂O₃/Yb₂O₃ concentration. Among coatings containing different dopant pairs, the Gd₂O₃/Yb₂O₃ doped coating exhibited lowest conductivity.     

Deposition and ablation resistance of HfC-based coatings prepared on SiC-coated C/C composites by supersonic atmospheric plasma spraying

In order to improve the ablation properties of C/C composites, HfC-based coatings with different mass ratios of SiC were deposited on the surface of SiC-coated carbon/carbon composites by supersonic atmospheric plasma spraying. The morphologies and microstructures of the HfC-based coatings were characterised. The ablation resistance test was carried out by oxyacetylene torch. The results show that the as-prepared coatings are multiphase coatings consisting of HfC, HfO₂, SiC and SiO₂. The structure of different coatings is dense. After ablation for 60?s, the ablation centre region of coating is smooth without obvious microcrack and pinhole, and no interlaminar crack can be observed at the cross-section. An Hf–Si–O compound oxide layer is generated on the surface of coating, which is beneficial for protecting the C/C composites from being ablated. Meanwhile, the further generated HfSiO₄ can play a pinning effect, which can prevent crack extension.