Porous TiAl3 intermetallcis are of great interest due to its excellent properties and widely applied in filtering apparatus, separation material and catalyst carrier. In this study, interconnected ...pore-structures have been synthesized by diffusion or thermal explosion (TE) reaction sintering with different heating rates. The thermal characteristics such as temperature-time curves, exothermic change and visual images indicate that the sample experienced a significant TE reaction at higher heating rates. Results shown that the sample was ignited at 672 °C and then rapidly increased to combustion temperature of 1169, 1110 and 933 °C in tens of seconds with the heating rate of 15, 10 and 5 °C∙min−1 respectively. Meanwhile, TE represented the uniformity of volume combustion, instantaneous reaction and rapid cooling to furnace temperature, the amount of heat released during TE reaction dropped from 1303 to 963 J g−1. This indicates that the entire sintering process was controlled by TE and the pre-diffusion reaction before the melting temperature of Al atom, which would affect the subsequent combustion reaction. Thermodynamic data explained that the reaction mechanism is mainly step-controlled diffusion reaction at a low heating rate (1 °C∙min−1), while the energy gradually accumulated and thermal explosion (TE) reaction become obvious with the increasing of heating rate (from 2 to 15 °C∙min−1).
•High porosity of TiAl3 intermetallics were prepared and formed interconnected pore channels.•Obvious thermal explosion reaction can be observed by naked eyes and actual sample temperature.•The reaction mechanism was clarified by the thermal behaviors at different heating rates.
A brand new HfB2-SiC-MoSi2 coating was fabricated to protect carbon/carbon (C/C) composites with inner SiC coating from oxidation, which was prepared by in-situ synthesis. In this paper, the C/C ...substrate with the protection of the HfB2-SiC-MoSi2/SiC coating could resist oxidation in 1773 K air for 408 h. The double coating also presented expected oxidation protection performance at dynamic oxidation environment. In the test process, the surface coating was oxidized to form a self-sealing silicate glass layer containing HfO2 and HfSiO4, which could hinder crack propagation in coating.
•The UHTCs HfB2-SiC-MoSi2 coating was fabricated through in-situ synthesis.•The coating could protect C/C for 408 h with only 0.76% mass loss in 1773 K air.•The SiO2 glass layer offered the expected self-cure ability.•The Hf-Si-O glass layer was responsible for the good oxidation resistance.
The formation mechanism of porous TiAl
3
intermetallics from Ti-75 at.% Al powder compact was explored through a continuous sintering process. The exothermic behavior, temperature changes and open ...porosity were investigated by ranging the sintering temperature from 600°C to 700°C. Results show that a thermal explosion (TE) occurs once the sample temperature reaches to 675°C, upon which it rapidly climbs to 1148°C within 10 s. Visual observation was achieved and revealed the exothermic state at every step, showing the overall heating and instantaneous reaction of TE. The open porosity increased significantly from 24.8% to 58.7% by increasing the sintering temperature from 670°C to 675°C. The reaction mechanism of porous TiAl
3
materials in the Ti-75 at.% Al system indicates that sintering at 600°C, 650°C, and 670°C was controlled by atomic diffusion, diffusion-induced TE, and liquid-induced TE reaction, respectively. The diffusion reaction activation energy was 161.5 kJ mol
−1
for diffusion-controlled growth of the TiAl
3
reaction layer.
ZrB2-SiC coatings with different ZrB2 contents were prepared by liquid phase sintering. The oxidation processes of coatings were explained according to TG results of ZrB2-SiC coatings and powders ...tested from 298 K to 1773 K. Results show that, increasing ZrB2 content made the weight of the samples changed from weight-loss of 10.04% to weight-gain of 0.14%, while the fastest weight-loss regions were narrowed, whose inflection points reduced from 1310℃ to 1050℃. Increasing ZrB2 content made the relative oxygen permeability of the ZrB2-SiC/SiC coatings reduced from 40%–60% to -10%-5%. Increasing ZrB2 content enhanced high-temperature stability of coatings, making final weight of samples changed from weight-loss of 0.16% to weight-gain of 0.11% after oxidation at 1773 K for 200 h. The peeling and dispersion of Zr-oxides formed Zr-B-Si-O compound glass layer, presenting enhanced stability, dispersion strengthening and pinning effect of Zr-oxides, which were responsible for the excellent anti-oxidation protective effect of coatings in a broad temperature region.
The HfB2‐MoSi2‐SiC oxygen blocking coatings were prepared by the spark plasma sintering (SPS) technique, whose oxidation inhibition ability was further strengthened by the pre‐oxidation treatment. ...The effect of MoSi2 content and pre‐oxidation treatment process on the oxygen blocking ability of the HfB2‐MoSi2‐SiC coating at 1973 K were conducted. After SPS, for the HfB2‐MoSi2‐SiC coatings with 20 wt%, 40 wt%, and 60 wt% MoSi2, the relative density of the coatings are 92.6%, 93.9%, and 85.6%, respectively. Owing to the enhanced compactness of the coatings, increasing MoSi2 content can significantly improve the protection efficiency of the coatings during the activation oxidation stage. However, due to the increased formation of gaseous by‐products during the inerting oxidation stage, excessive MoSi2 weaken the oxidation inhibition ability of the coatings. The sufficient dispersion of Hf‐oxides nanocrystals in the glass layer conduces to enhance the oxygen blocking ability of the glass layer, making the 40HfB2‐40MoSi2‐20SiC coating present the best oxidation protective ability. The pre‐oxidation treatment at 1773 K conduces to form the steady glass layer with fewer defects at the cost of a lower oxidation consumption of the coating, which enhanced the protection efficiency of the coating from 96.9% to 99.8% and reduced the oxygen permeability from 0.13% to 0.028%.
MoSi2-MoB-ZrO2 composite powders are one-step synthesized prepared by self-propagating high-temperature synthesis (SHS), which are further utilized to prepare the oxidation protective coatings by ...spark plasma sintering (SPS) method on molybdenum substrate. Microstructure and properties of the coatings were investigated and determined. Oxidation behavior of the coatings at high temperature was also studied. The results showed that the composite coating is dense and crack-free due to the addition of ZrO2, which is beneficial for the decrease of mismatch in coefficient of thermal expansion, and no void exists in the diffusion layer owing to the diffusion of Si, which is inhibited by adding MoB. Compared with the pure MoSi2 coating, the high hardness of the composite coating is enhanced by about 20% while the fracture toughness of the composite coating is improved by about 25%, which makes the occurrence of intergranular fracture, crack deflection and crack bridging. After oxidation at 1400 °C for 80 h, MoSi2 coating failed while xMoSi2-MoB-3ZrO2 (x = 3, 4, 5) composite coating still kept its protective effect. Only a little substrate is consumed during oxidation and improved oxidation resistance is gained by MoSi2-MoB-ZrO2 composite coating. 3MoSi2-MoB-3ZrO2 coating shows the relatively best oxidation resistance due to the formation of crack-free oxide film on the surface of the coating.
•MoB and ZrO2 are added as reinforced phases to improve properties of MoSi2 coating.•One-step synthesis of MoSi2-MoB-ZrO2 composite powders is prepared by SHS.•Preparation of MoSi2-MoB-ZrO2 coating by SPS is conducted by using powders of SHS.•Cracks are decreased and fracture toughness is improved by adding ZrO2 and MoB.•Oxidation resistance is improved by the reduction of cracks and formation of ZrSiO4.
Spent MoSi2 and MoB were used as raw materials to prepare multilayer MoSi2/MoB coating on molybdenum by the two‐step method of slurry deposition and spark plasma sintering. The results showed dense ...MoSi2/MoB coating after sintering while penetrated cracks appeared in MoSi2 coating due to coefficient of thermal expansion mismatch between the Mo substrate and coating. After the sintering of MoSi2/MoB coatings, MoB and Mo2B diffusion layers were formed between MoB transition layer and Mo substrate without defects, exhibiting good metallurgical bonding. The high‐temperature oxidation behavior of coatings (1500°C) was also explored. After oxidation of 50 h at 1500°C, lowest mass gain (0.035 mg/cm2) was obtained for MoSi2/MoB coating, and the oxide scale was dense and complete without voids, making the oxygen diffusion at elevated temperature inhibited. Compared with MoSi2 coating under the same oxidation conditions, relatively thinner silica oxide scale was acquired by MoSi2/MoB coating because of the reduction of cracks, and the multilayer coating exhibits better anti‐oxidation properties at high temperature.
Two‐step method of slurry deposition and spark plasma sintering was applied to prepare MoSi2/MoB multilayer coating on Mo. The multilayer coating possessed improved anti‐oxidation properties with dense and complete oxide scale after 1500°C oxidation.
HfB2-20%wtSiC composite coating was prepared by liquid phase sintering method. After modification of HfB2 phase, the initial oxidation consumptions of the SiC coated samples were delayed from 500 ℃ ...to 800 ℃. Due to the higher oxidation activity of HfB2, the sufficient generated B2O3 is capable of inhibiting oxidation consumption of carbon substrate in oxidation activation region (800 ℃-1000 ℃) and fastest oxidation region (1000 ℃-1280 ℃). The enhanced oxidation activity of SiC above 1000℃ leads to the increased generation of SiO2, inhibiting the evaporation of B2O3 through the formation of Hf-B-Si-O glass layer, improving its stability and oxidation resistance above 1000℃. The heterogeneous refractory Hf-Oxides embedded in Hf-B-Si-O glass layer play role of reinforcement phases, restricting generation and spread of cracks. The inerting effect of Hf-B-Si-O glass layer strengthened with the increase of thermogravimetric analysis (TG) recycle oxidation times, indicating promising oxidation inhibition potential of the coating in dynamic aerobic environment.
PDF
