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.     

Comparison of plasma sprayed NbB2-NbC coatings obtained by ex-situ and in-situ approaches

NbB₂-NbC composite coatings were fabricated on the surface of TC4 by plasma spraying NbB₂-NbC and Nb-B₄C composite powders. The microstructure and properties of the as-prepared coatings were investigated, and the reaction mechanism of the Nb-B₄C composite powder in the plasma jet and the formation mechanism of the NbB₂-NbC coatings were revealed. During the plasma spraying process, NbB₂-NbC composite powder underwent melting-depositing and no phase transformation occurred. The formation mechanism of the coating by plasma spraying Nb-B₄C composite powder was solid phase diffusion-reaction-melting-deposition, and NbB₂ and NbC were formed in situ by the solid phase diffusion reaction between Nb and B₄C in the plasma jet. The coating obtained by reactive plasma spraying Nb-B₄C composite powder has obvious lamellar structure, low porosity, high density, higher microhardness, good toughness, and better wear resistance compared with the coating prepared by NbB₂-NbC composite powder, which is attributed to the exothermic reaction between Nb and B₄C.     

Structure-property relationship and design of plasma-sprayed La2Ce2O7/8YSZ composite coatings for gas turbine blades

Novel lanthanum-cerium oxide/8?wt% yttria partially stabilized zirconia (LC/8YSZ) thermal barrier coatings (TBCs) were deposited by supersonic atmospheric plasma spraying. The thermal insulation temperature and thermal shock resistance of LC/8YSZ double-ceramic-layer TBCs (DCL-TBCs) were quantitatively evaluated by a burner rig test. The results showed that the thermal insulation temperature increased with the increase of LC layer thickness in DCL-TBCs. When the thickness ratio between LC layer and 8YSZ layer was close to 1:1, the DCL-TBCs had the highest thermal shock resistance. LC/8YSZ thickness ratio significantly affected the energy release rate and the stress induced by thermal gradient or sintering. The sintering stress was found to be the main reason that caused the delamination of LC layer, however, the stress induced by thermal gradient resulted in the spallation of YSZ layer.     

Ablation behavior and thermal protection performance of TaSi2 coating for SiC coated carbon/carbon composites

For extending application of TaSi₂ in complex coating system, the ablation behavior and thermal protection performance of TaSi₂ coating is studied to evaluate its potential applications for anti-ablation protection of C/C composites. TaSi₂ coating is prepared by supersonic atmospheric plasma spraying (SAPS) on the surface of SiC coated carbon/carbon (C/C) composites. Phase variation and microstructure are characterized by XRD and SEM, respectively. During the ablation process, the coating is quickly oxidized to SiO₂ and Ta₂O₅ accompanied by a lot of heat consumption. The linear and mass ablation rates are 0.9?µm?s^?1 and ??0.4?mg?s^?1 after ablation for 80?s, respectively Results show that the prepared coating possesses optimal ablation performance under the heat flux of 2.4?MW/m². Moreover, the TaSi₂ coating and SiC inner coating have good chemical and physical compatibility during the ablation process. Therefore, the excellent performance of TaSi₂ coating during the ablation process makes it a candidate for anti-ablation protection for C/C composites.     

Preliminary study on suspension plasma sprayed ZrO2 + 8 wt.% Y2O3 coatings

ZrO₂ + 8 wt.% Y₂O₃ powder of a mean diameter d_VS = 38 μm was milled to obtain fine particles having mean size of d_VS = 1 μm. The fine powder was used to formulate a suspension with water, ethanol and their mixtures. The zeta potential of obtained suspensions was measured and found out to be in the range from −22 to −2 mV depending on suspension formulation. The suspension was injected through a nozzle into plasma jet and sprayed onto stainless steel substrates. The plasma spray experimental parameters included two variables: (i) spray distance varying from 40 to 60 mm and (ii) torch linear speed varying from 300 to 500 mm/s. The microstructure of obtained coatings was characterized with scanning electron microscope (SEM) and X-ray diffraction (XRD). The coatings had porosity in the range from 10% to 17% and the main crystal phase was tetragonal zirconium oxide. The scratch test enabled to find the critical load in the range of 9-11 N. Finally, thermal diffusivity of the samples at room temperature, determined by thermographic method, was in the range from 2.95 × 10⁻⁷ to 3.79 × 10⁻⁷ m²/s what corresponds to thermal conductivities of 0.69 W/(mK) and 0.97 W/(mK) respectively.     

Effect of powder particle size on vibration damping behaviour of plasma sprayed alumina (Al2O3) coating on AISI 304 stainless steel substrate

The damping capacity of plasma sprayed alumina (Al₂O₃) coatings on AISI 304 stainless steel was investigated in this study as a function of particle size of the starting alumina powder. The coatings were prepared from different sizes alumina powder using commercial air plasma spraying (APS) technique. The damping properties of coated samples were characterized by damping capacity (Q^?1) measured experimentally using dynamic mechanical analyzer (DMA). The surface morphology of the coatings was studied using scanning electron microscope (SEM). The results revealed that the coating was porous and was able to improve the damping capacity of bare substrate. It was also observed that the powder particle size had a significant effect on the damping characteristics of the coatings. The damping values were found to be increased with the increase in particle size in the measured strain range. This behaviour was correlated with the microstructure investigated by SEM.     

Cf/SiC 复合材料的 ZrB2-SiC/SiC 超高温陶瓷涂层研究

采用两步包埋法在Cf/SiC复合材料表面制备了Zr B_2-SiC/SiC超高温陶瓷涂层。借助SEM、XRD对涂层的微观结构及物相组成进行了分析研究,并进行了高温静态氧化和热震测试。研究表明,1500°C氧化5 h后,涂层表面覆盖有平整的玻璃相氧化层,氧化失重率为6.4%;热震测试10次后涂层的氧化失重率为14%。Zr B_2-SiC/SiC涂层能有效提高Cf/SiC复合材料的高温抗氧化性能。     

Bimodal microstructure ZrB2-MoSi2 coating prepared by atmospheric plasma spraying for carbon/carbon composites against long-term ablation

To protect carbon/carbon composites against long-term ablation, a bimodal microstructure ZrB₂-MoSi₂ coating, consisting of an outer ZrB₂-MoSi₂ layer modified by Y₂O₃ and an inner basal ZrB₂-MoSi₂ layer, was prepared by atmospheric plasma spraying. The microstructure, phase composition and ablation resistance of the proposed coating were investigated in detail. Results showed that the bimodal coating maintained integrity in structure except for phase composition. There was no visible interlayer between the inner ZrB₂-MiSi₂ layer and the outer modified one. Mass ablation rate of the bimodal microstructure ZrB₂-MoSi₂ coated C/C composites was −2.02 × 10⁻³ g/s under an oxyacetylene flame ablation at 1873 K for 600 s, which exhibited better ablation resistance than a single ZrB₂-MoSi₂ coating. The excellent ablation resistance was ascribed to the positive effect of Y₂O₃, which not only pined in the glassy phase and alleviated the volatilization of SiO₂ glass phase by reacting with SiO₂ to form high viscosity of Y₂SiO₅, but also stabilized ZrO₂ and promoted its recrystallization and growth.     

Effect of pyrolytic carbon coating on the microstructure and fracture behavior of the Cf/ZrB2-SiC composite

The high sintering temperature and interface interaction seriously degraded the toughening effects of continuous carbon fiber in ZrB₂-SiC ceramic. The pyrolytic carbon coated carbon fiber reinforced ZrB₂-SiC composite (C_f-PyC/ZrB₂-SiC) with desirable properties was successfully achieved via brushing nano ZrB₂-SiC slurry followed by spark plasma sintering at relatively low sintering temperature. The fabricated C_f-PyC/ZrB₂-SiC composite presented a non-brittle fracture feature and a remarkable enhancement in comparison with the ZrB₂-SiC composite reinforced by the as-received carbon fiber (C_f-AS/ZrB₂-SiC). The fracture toughness and critical crack size were increased from 5.97?±?0.18–7.66?±?0.24?MPa?m^1/2 and from 91.6 to 164.5?µm, respectively. A high work of fracture of 1915?J/m² for C_f-PyC/ZrB₂-SiC composite was achieved, almost four times higher than that of the C_f-AS/ZrB₂-SiC composite (463?J/m²). Multiple toughening mechanisms contributed to such enhancement, such as crack deflection, fiber bridging, fiber pull-out and crack branching. This work provides a feasible approach to fabricate high-performance fiber reinforced ceramic composites having a high work of fracture.     

Oxidation behavior of ZrB2-MoSi2-SiC composites in air at 1500 °C

We investigated the oxidation behavior and the effect of the amount of SiC added on oxidation resistance in both hot-pressed ZrB₂-MoSi₂-SiC composites, 55ZrB₂-40MoSi₂-5SiC and 40ZrB₂-40MoSi₂-20SiC (vol.%), exposed to dry air at 1500 °C for up to 10 h. Quantitative electron microprobe analysis characterizations of the chemical compounds of post-oxidized composites were carried out. Parabolic oxidation behavior was observed for both composites. The addition of SiC improved the oxidation resistance of ZrB₂-MoSi₂-SiC composites, and the improvement enhanced with amount of SiC added. The microstructure of the post-oxidized composites consisted of two characteristic regions: oxidized reactive region and unreactive bulk material region. The oxidized reactive region divided into an outermost dense silica-rich scale layer and oxidized reactive mixture layer. The improvement of oxidation resistance with SiC addition is associated with the presence of a thicker dense outermost scale layer which inhibited inward diffusion of oxygen through it.     

Effects of modification of hBN by nickel plating on coating structure and properties of supersonic plasma spraying NiCr-Cr3C2-hBN@Ni coatings

Hexagonal boron nitride (hBN) with excellent self-lubrication performance is expected to relieve the friction resistance and wear of NiCr–Cr₃C₂ coatings. However, the poor wettability of hBN with most materials makes it difficult to fabricate NiCr–Cr₃C₂-hBN composite coating with good cohesion strength. In this study, hBN was firstly pretreated through magnetron-sputtering aided Ni plating to form hBN@Ni particles. Then, NiCr–Cr₃C₂-hBN@Ni powder was prepared by spray granulation. Next, corresponding coatings were prepared through supersonic atmosphere plasma spraying. It was found that in comparison with NiCr–Cr₃C₂-hBN coating, the NiCr–Cr₃C₂-hBN@Ni coating exhibited a decreased porosity (from 3.6% to 0.3%), elevated cohesion (from 52.78 N to 62.11 N), and the wear rate decreased by an order of magnitude. It was concluded that hBN@Ni can effectively improve the component interface inside powder, enhance the cohesion of molten in-flight particles, and make the internal structure of the coating denser.     

Evaluation of hot corrosion behavior of plasma sprayed ceria and yttria stabilized zirconia thermal barrier coatings in the presence of Na2SO4+V2O5 molten salt

In this study, substrates of Inconel 738 LC superalloy coupons were first sprayed with a NiCoCrAlY bondcoat and then with a ceria and yttria stabilized zirconia (CYSZ; ZrO₂−25 wt%CeO₂−2.5 wt%Y₂O₃) topcoat by air plasma spraying (APS). Hot corrosion studies of plasma sprayed thermal barrier coatings (TBCs) were conducted in 45 wt%Na₂SO₄+55 wt%V₂O₅ molten salt at 1000 °C for 30 h. The results showed that the coating defects, such as pores and microcracks play important roles as effective paths for the salt penetration in hot corrosion. Based on the results, the reaction between molten salt and stabilizers of zirconia (Y₂O₃ and CeO₂), the formation of YVO₄, CeVO₄ and CeO₂ crystals, the detrimental phase transformation of zirconia from tetragonal to monoclinic due to the depletion of stabilizers and finally, the creation of stresses were recognized to be in the degradation mechanism of CYSZ ceramic coatings in the presence of molten sulfate–vanadate salt.     

Electrolytic production of NF3 with a LiNiO2 coated nickel sheet anode prepared by atmospheric plasma spraying technique

A nickel sheet coated with LiNiO₂ powder having average particle sizes of 40 and 50 μm in diameter by atmospheric plasma spraying technique was employed as the anode for electrolytic production of NF₃. In electrolysis of a molten NH₄F·2HF at 100 °C and 25 mA cm⁻², the anode gas generated at the LiNiO₂ coated Ni sheet anode was composed of N₂, O₂, NF₃, N₂F₂, N₂F₄, and N₂O, and its composition was almost the same as that at the Ni sheet anode. The current efficiency for the NF₃ formation on the LiNiO₂ coated Ni sheet anode was increased to reach the constant value of ca. 55% during electrolysis for 100 h, and it was almost the same as that on the Ni sheet anode. The anode consumption of the LiNiO₂ coated Ni sheet was small compared with that of the Ni sheet. Also, the oxygen content in the oxidized layer formed on the LiNiO₂ coated Ni sheet anode was high compared with that on the Ni sheet anode, and the surface of the LiNiO₂ coated Ni sheet anode was covered with a compact and adhesive film having some defects. Although the bottom of the hollow was covered with a thinner layer, no pore penetrated through the oxidized layer. Hence, the LiNiO₂ coated Ni sheet anode is favorable for the electrolytic production of NF₃, and the oxidized layer on the LiNiO₂ coated Ni sheet anode has the higher resistance to corrosion, because of the compact and adhesive film containing the higher content of oxygen formed on the anode.     

Conducting polyaniline-nano-TiO2 composites for smart corrosion resistant coatings

Coatings prepared from polyaniline-nano-TiO₂ particles synthesized by in situ polymerization were found to exhibit excellent corrosion resistance much superior to polyaniline (PANI) in aggressive environments. The corrosion studies were carried out on steel plates coated with these formulations containing 10 wt% polyaniline prepared with different concentrations of nano-TiO₂. The electrochemical impedance spectroscopy was studied at periodic intervals during exposure to hot saline (65 °C) conditions for prolonged durations over a period of 90 h. The open circuit potential (OCP) was found to shift with time from −0.38 V SCE to more anodic side (−0.2 V SCE) much above that of bare steel (−0.5 V SCE). The presence of nano-TiO₂ was found to be vital in the prevention of corrosion and the shift of OCP to anodic side. From these data, one could envisage more than 100 times improvement in the corrosion resistance especially for polyaniline prepared with 4.18 wt% nano-TiO₂. The exceptional improvement of performance of these coatings has been associated with the increase in barrier to diffusion, prevention of charge transport by the nano-size TiO₂, redox properties of polyaniline as well as very large surface area available for the liberation of dopant due to nano-size additive.     

Joining of Cf-SiC composites and ZrB2-SiC based composites for ultra high temperature applications

Gas tungsten arc welding (GTAW) of Cf–SiC composites to themselves and to ZrB₂-SiC based composites have been carried out with a filler material of (ZrB₂-SiC-B₄C-Y₂O₃-Al₂O₃) composite. The weld interfaces of joints of composites were clean and free from porosity and cracks. Penetration of filler material into voids and pores existing in the Cf-SiC composites was observed. An average shear strength of 25.7?MPa was achieved. The ZrB₂-SiC based composite joined to Cf-SiC (CVD) composite was exposed for 300?s to the oxy-propane flame at 2300?°C. The joint and interfaces between the filler material and parent composites were found to be unaffected by thermal cycling and oxidation during the exposure to the oxy-propane flame.     

ZrO2 removing reactions of Groups IV-VI transition metal carbides in ZrB2 based composites

In this paper, ZrO₂ removing reactions of Groups IV-VI transition metal carbides (MCs, M = Hf, Nb, Ta, W, Ti and V) in ZrB₂ based ultra-high temperature ceramics (UHTCs) are investigated. Distinct roles of various MCs were observed during this process. According to thermodynamic analysis and experiment verifications, the sequence for oxide removing ability of different MCs is WC > VC > NbC > TaC > HfC > TiC. The importance of this study concerns the establishment of a map of reactivity of the transition metal carbides against ZrO₂, in order to choose the proper additive for the densification of ZrB₂-composites.Utilizing right reactions and controlling proper sintering atmospheres, high density ZrB₂-SiC ceramic could be eventually obtained by pressureless sintering at temperatures (around 2000-2200 °C).     

Effect of arc current on microstructure,texturing and wear behavior of plasma sprayed CaZrO3 coatings

In order to enhance the biocompatibility of metallic implants, various ceramic coatings are currently in vogue. CaZrO₃, a promising candidate material, was deposited through plasma spraying on stainless steel (316L) substrates at arc currents of 400, 500 and 600 A. The coatings were characterized using a SEM, XRD, surface profilometers and a tribometer. It was found that the arc current had profound effects on the thickness, microstructure, phase evolution, crystallinity and wear behavior of the coatings. The cross-sectional images and fractographic analysis showed that a denser coating with better inter-splat fusion was produced at arc current of 600 A. The average roughness (Ra) of the coatings increased from 3.62 to 6.68 μm as the arc current was increased from 400 to 600 A. The feedstock (powder) and the coatings were predominantly composed of CaZrO₃ along with a minor amount of CaZr₄O₉ phase. The rise in the arc current resulted in a slight increase in the relative proportion of the CaZrO₃ phase. Also, the coating produced at arc current of 600 A exhibited highest crystallinity. The detailed XRD analysis of (002) and (200) reflections of the ferroelectric CaZrO₃ revealed the preferred orientation of crystals in the coatings. The presence of this texture is explained on the basis of shifting the unstable Zr⁴⁺ ion in oxygen octahedral cage preferably in one direction. The increase in the arc current decreased the coefficient of friction and, as a result, relatively better wear resistance was observed for the coating produced using higher arc current. Moreover, the coating fabricated using arc current of 600 A reduced the volumetric weight loss by 13 times during the wear test as compared to the substrate. Plasma sprayed CaZrO₃ coating not only enhanced the wear resistance of the stainless steel but also showed the potential to furnish a bioactive surface.     

Gradient HfB2-SiC multilayer oxidation resistant coating for C/C composites

The gradient HfB₂ modified SiC coating was prepared on the surface of SiC-coated C/C composites by in-situ synthesis. Anti-oxidation behaviors of the coated C/C samples at 1773, 1873 and 1973?K were investigated. The results show that the gradient HfB₂ modified SiC coatings possess excellent oxidation resistance, which can protect C/C substrates from oxidation for 800, 305 and 100?h at 1773, 1873 and 1973?K, respectively. In addition, with the oxidation temperature increasing, the evaporation of the Hf-Si-O glass layer and the active oxidation of SiC were accelerated, which is the reason for the worst oxidation resistance of the sample at 1973?K among the three temperatures.     

Mullite-Al2O3-SiC oxidation protective coating for carbon/carbon composites

In order to exploit the unique high temperature mechanical properties of carbon/carbon (C/C) composites, a new type of oxidation protective coating has been produced by a two-step pack cementation technique in an argon atmosphere. XRD analysis showed that the internal coating obtained from the first step was a gradient SiC layer that acts as a buffer layer, and the multi-layer coating formed in the second step was an Al₂O₃-mullite layer. It was found that the as-received coating characterized by excellent thermal shock resistance on the surface of C/C composites during exposure to an oxidizing atmosphere at 1873 K, could effectively protect the C/C composites from oxidation for 45 h. The failure of the coating is due to the formation of bubble holes on the coating surface.     

Water vapor corrosion behavior and failure mechanism of plasma sprayed mullite/Lu2Si2O7-Lu2SiO5 coatings

A new coating with mullite as the inner layer and Lu₂Si₂O₇-Lu₂SiO₅ (lutetium disilicate-lutetium monosilicate, LuDS-LuMS) composite as the top layer was designed and fabricated on the surface of porous SiC substrate by atmospheric plasma spraying (APS). The microstructure evolution, phase transformation, inter-diffusion and failure mechanism of the coated samples during steam cycling at 1450?°C were systemically investigated. The results indicated that the coated samples maintain weight gain and the average weight gain rate is 1.12?×?10^?1 mg/cm² h. The thermal expansion coefficient (CTE) mismatches between the coating and the substrate as well as between the two ceramic layers have produced a thermal mismatch stress. In addition, the chemical reaction induced the sintering of LuDS-LuMS layer, the amorphous-crystalline phase transformation and the transformation between LuDS and LuMS phases could result in aging stress. With the accumulation of thermal mismatch stress and aging stress, the through and horizontal cracks have appeared in the coating, leading to the coating failure.