Ultra-high Temperature Ceramic Coatings Research Articles

Microstructure development of plasma sprayed dual-phase high entropy ceramic coating derived from spray dried and induction plasma spheroidized powder

Dual phase high entropy ceramics (DPHECs) exhibit excellent high temperature mechanical properties and oxidation resistance. Nevertheless, they cannot be applied in the structural components on large scale due to their intrinsic brittleness. Hence, DPHEC coating is a necessary development direction for industrial applications. In this study, we first fabricated micron-order DPHEC powder with high quality by precursor synthesis, spray drying (SD), and induction plasma spheroidization (IPS). The DPHEC coating was prepared by supersonic atmospheric plasma spraying (SAPS). The microstructure, phase compositions, and mechanical properties of the DPHEC powder and coating are investigated in detail. Results show that the SD&IPS powder and SAPS coating are all composed of HEB and HEC phases, showing slight composition segregation and typical global eutectic microstructure. The nanohardness and Young's modulus are 22.17 ± 2.76 GPa and 309.79 ± 35.71 GPa, respectively. The fracture toughness was calculated to be 2.23 ± 0.17 MPa m1/2 from the energy analysis method. The mechanical properties are enhanced by the solid solution and fine grain strengthening effects and are weakened by the intrinsic defects of plasma sprayed coating. This work provides a new method and a potential material for thermal spraying ultra-high temperature ceramic coatings.

High speed laser cladding as a new approach to prepare ultra-high temperature ceramic coatings

High speed laser cladding as a new approach to prepare ultra-high temperature ceramic coatings

Exceptionally low ablation rates realized in the cellular-structured MCMB@WC composites via biomimetic design

Thermal protection materials (TPMs) with extra-low ablation rate in long-term are ungently required in reusable spacecrafts. However, the ablation property and operation life of conventional graphite/carbon-based composites with ultra-high temperature ceramic coatings are rather limited. Inspired by the self-healing behavior of plants, we designed and fabricated cellular-structured tungsten carbide (WC) reinforced graphite composites via the combination of molten salt coating and spark plasma sintering. Owing to the intelligent biomimetic structure and regeneration of WC during the ablation process, the composites exhibited not only excellent thermal shock resistance but also exceptionally low ablation rates. In the cyclic ablation test under 4.18 MW·m−2 for 360 s, the linear and mass ablation rates of the composite at the last ablation period decreased to 0.33 mg·s−1 and 0.17 μm·s−1, respectively. The biomimetic structural design offers new insights for fabricating graphite-based composites with exceptionally low ablation rates, which are promisingly applied in reusable spacecrafts.

Reactive synthesis of ZrC-ZrSi2 composite coating by atmospheric plasma spraying

In-situ reaction between zirconium and silicon carbide was introduced to atmospheric plasma spraying process to prepare ZrC-ZrSi2 composite coating. The in-situ fabricated ZrC-ZrSi2 composite coating was featured with dense microstructure and promising mechanical properties. Well-arranged whiskers and nanosized spheric grains were observed in the as-prepared composite coating. Water-quenching experiment was conducted to investigate the formation mechanism of the target phase (ZrC and ZrSi2) and the final coating. This work validated the feasibility of preparing ZrC composite coating using Zr-SiC reactive system and furthermore, gave references for preparing ultra-high temperature ceramic coatings by reactive synthesis method.

Ultra-high hardness induced by W precipitation within Ta-Hf-W-C ultra-high temperature ceramic coatings

Aging strengthening of traditional alloys is of great practical significance in industry, and the nano second phase precipitated from supersaturated solid solution alloys effectively prevent the movement of grain boundaries and dislocations, so as to improve the hardness of alloys. However, the concept is difficult to be realized in ultra-high temperature ceramics (UHTCs) by the existing sintering process, it is still a challenge to obtain large-scale high concentration supersaturated UHTCs. To achieve this, we prepared a supersaturated Ta-Hf-W-C solid solution ceramic coating by a step-by-step reactive plasma spraying, inducing a high-density precipitation of W by spark plasma sintering and finally obtained an ultra-high temperature Ta-Hf-W-C nano-composite ceramic coating with grid interlocking structure. The average hardness and elastic modulus of the Ta-Hf-W-C composite ceramic coating exceeded 47 GPa and 580 GPa due to fine grain strengthening, solid solution strengthening and ordered grain boundary strengthening.

Development of SiC-ZrC-based ultra-high temperature ceramic coatings via composite method of polymer precursor pyrolysis plus gaseous reactive infiltration

SiC-ZrC-based composite coatings on carbon/carbon composites via ZrC polymer precursor pyrolysis (PPP) plus gaseous reactive infiltration (GRI) of Si or ZrSi 2 , separately denoted as GSIC and GZSIC, were comparatively studied to explore the feasibility and effectiveness of GRI of ZrSi 2 . This composite method by applying ZrSi 2 or Si was proven to similarly produce dense coatings with enhanced coating/substrate interfacial bonding strength for the formed zig-zag transition layer. Furthermore, the GRI of ZrSi 2 instead of Si further increased the UHTC (ZrC) content in the as-deposited coatings resulted from the reaction of infiltrated ZrSi 2 and pre-introduced graphite, enhancing the ablation resistance of the coatings for carbon/carbon composites due to the formed stable Zr-rich Zr-Si-O multiphase oxide. The average mass and linear ablation rates were −0.12 mg/s and 1.56 μm/s for GSIC and −0.3 mg/s and −0.53 μm/s for GZSIC specimens, respectively. The average linear ablation rate of the GZSIC specimen was decreased by ~134% when compared to the GSIC specimen. These results further indicate that the PPP + GRI method with metal silicide MeSi 2 (Me = Zr, Ti, Hf, Cr, etc.) will contribute to the optimal fabrication of dense composite coatings with controllable microstructure, adequate evenly-distributed ultra-high temperature ceramic (UHTC) phases, enhanced interfacial bonding strength and wide circumstance suitability for potential applications. The related work is ongoing in our laboratory. • SiC-ZrC-based coatings were made by precursor pyrolysis plus gaseous reactive infiltration. • Pre-introduced graphite reacted with infiltrated ZrSi 2 further improved ZrC content in the coatings. • The gaseous infiltrated ZrSi 2 instead of Si enhanced ablation resistance of the coatings. • The coating's linear ablation rate was reduced by ~134% as gaseous infiltrated with ZrSi 2 .

Boride-based ultra-high temperature ceramic coatings deposited via controlled atmosphere plasma spray

This study investigates the capability of a custom-built controlled atmosphere plasma spray (CAPS) system for deposition of zirconium boride (ZrB2) and hafnium boride (HfB2) thermal protection system (TPS) coatings. Due to poor oxidation resistance these materials must be deposited in the absence of oxygen to avoid high inclusions of oxides that detrimentally effect TPS coating properties. A low cost, simple CAPS system was developed to allow for the deposition of these coatings within an inert atmosphere, in a typical lab environment. The CAPS system has demonstrated a 46% reduction in oxide inclusions for ZrB2 coatings with a 40% reduction in elemental oxygen and facilitated the deposition of a HfB2 coating with low oxide inclusions. Weibull analysis on the variability in the hardness of ZrB2 samples revealed a tri-modal distribution of hardness that was attributed to regions of differing oxide inclusions. The CAPS deposition of HfB2 did not achieve a deposited microstructure that was considered effective as a TPS. A Weibull analysis of the hardness of the HfB2 coating demonstrated a bi-modal distribution that corresponded to low oxide regions and locations composed of a similar mix of boride and monoclinic oxide. The heterogeneous microstructure was attributed to the pre-processing of the HfB2 powder.

Ablation behavior of SiC/ZrB2 ultra-high temperature ceramic coatings by solid shielding shrouded plasma spray for high-temperature applications (temperature above 2000 °C)

In this paper, ablation behavior of C/SiC/ZrB2 ultra-high temperature ceramic coatings applied by solid shielding shrouded plasma spray method was studied for as nose tip, leading edges, solid-fueled rocket nozzles, propulsion systems, re-entry missile nozzles throat applications. For this purpose, SiC inner layer coating was applied by reactive melt infiltration (RMI) under argon atmosphere. Then, ZrB2 outer layer coating was deposited on SiC coated graphite by solid shielding shrouded plasma spraying (SSPS). To evaluate the performance of the coating, SiC and SiC/ZrB2 coatings were exposed to oxy-propane and oxy-acetylene flames at 90o angle. The results of ablation test showed that mass ablation rates of SiC/ZrB2 coating under oxy-propane flame after 300 s and under oxy-acetylene flame after 120 s is 0.426 × 10−4 g.cm−2.s−1 and 4.8 × 10−4 g.cm−2.s−1 respectively. It seems that ablation behavior of SiC/ZrB2 coatings are controlled by the formation of the ZrO2 dense layer.

Selective laser melting for the preparation of an ultra-high temperature ceramic coating

Selective laser melting of a ZrB2, ZrC, and B4C powder blend was chosen to explore the use of the additive manufacturing technology to prepare dense ceramic coatings on a metallic tungsten substrate. Laser powers between 200 and 400 W were used for the application of the ceramic coatings. Adherent coatings were fabricated; however, a thermal expansion mismatch between the coating and substrate proved to be detrimental to coating integrity after cross-sectioning. Analysis of the microstructure and composition of the resulting coatings was used to elucidate the effects of thermal gradients and the effects of tungsten and SiC impurities on the solidification of the ceramic coatings. To the authors’ knowledge, these are the first ultra-high temperature ceramic coatings fabricated with a selective laser melting process.

Ablation Behaviors of ZrC-20vol.%MoSi<sub>2</sub> Composite Coating under Different Heat Fluxes

ZrC-20vol.%MoSi2 (ZM) composite coating was fabricated by vacuum plasma spray and the ablation resistance was assessed using plasma flame under low (1.94 MW/m2) and high (3.01 MW/m2) heat fluxes, respectively. Results showed that the ultimate surface temperatures of ZM coating were about 2100 °C and 2400 °C, respectively. ZM coating exhibited good ablation resistance at low heat flux, which benefited from the low evaporation of SiO2 and the diffusion of Si derived from MoSi2 decomposition. However, bubble-burst event took place under high heat flux. The different ablation behaviors of ZrC-MoSi2 coating were analyzed, which might contribute to the application of ultra-high temperature ceramic coatings.

Ablation Properties of ZrB<SUB>2</SUB>-SiC Ultra-high Temperature Ceramic Coatings

为了提高C/C复合材料的抗氧化烧蚀性能, 采用浆料浸涂与原位反应复合工艺在材料表面制备了ZrB 2 -SiC超高温陶瓷涂层, 利用氧-丙烷火焰测试了涂层的抗烧蚀性能。结果表明: 采用复合工艺所制备的ZrB 2 -SiC超高温陶瓷涂层与基材具有较高的结合强度; 在氧-丙烷火焰冲刷条件下, 涂层具有良好的抗烧蚀性能, 涂层经1500℃下烧蚀600 s, ZrB 2 -SiC涂层无明显烧蚀, C/C复合材料保持完好。微观结构观察表明: 烧蚀测试后, 涂层中存在ZrO 2 和大量超高温陶瓷相, 涂层抗烧蚀形式主要表现为热化学烧蚀和机械剥蚀。

In-situ synthesis of functional silica nanoparticles for enhancement the corrosion resistance of TBCs

Ultra high temperature ceramic coatings (UHTCs) have a potential use as thermal protection systems in rocket exhaust cones, insulating tiles for the space shuttle, and engine components. The porous ceramic coatings require a sealing strategy with ceramic nanoparticles to enhance the corrosion resistance and prevent the formation of thermally grown oxides (TGO) at the ceramic/metallic interface. In this context, we report a simple, yet highly versatile sol–gel process to seal the pores in alumina coating with silica nanoparticles. Tetraethylorthosilicate (TEOS) was used as sol–gel precursor. Polysilicate molecules are likely formed inside the coating pores during the sol–gel process. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests were performed to measure the corrosion resistance of alumina coatings before and after treatment with TEOS. A significant enhancement of the corrosion resistance of alumina coating was performed via post-treatment with sol–gel silica which perfectly sealed the open pores and voids in alumina coatings.

Effects of heat treatments on oxidation resistance and mechanical properties of ultra high temperature ceramic coatings

A critical point in the development of future generation space vehicles is the availability of materials able to withstand the extreme temperatures generated during orbital re-entry. UHTC (Ultra High Temperature Ceramics) materials, such as Zirconium diboride based ceramic composites, exhibit outstanding oxidation and thermal shock resistance, high emissivity and very high melting temperature: all these properties make ZrB 2 based materials possible candidates in designing thermal protection systems for the hottest structures of re-entry vehicles such as wing leading edges and nose-tip. An innovative, proprietary way to deposit ZrB 2 based coatings by plasma spraying has been developed. In the present work, the influence of high temperature exposure on oxidation resistance and mechanical properties has been investigated. Several coatings constituted by different amounts of SiC particles dispersed in a ZrB 2 matrix were produced; some samples were tested in as sprayed conditions, other ones were submitted to a high temperature heat treatment before testing. Oxidation resistance was determined by exposing samples in air at 2073 K for 180, 1800 and 2520 s. Oxidation kinetics were analyzed and modelled. Evaluation of flexural strength and elastic modulus by means of four-points bending tests were carried out on samples showing better oxidation behaviour; mechanical tests were performed also on samples exposed at a typical operative temperature (1800 s at 2073 K). Results showed that heat treatment does not affect oxidation resistance, whilst it significantly improves mechanical properties.