Hypersonic Plasma Setup for Oxidation Testing of Ultrahigh Temperature Ceramic Composites

In this study, a hypersonic plasma setup was constructed based on a vortex plasma heater with prenozzle gas-dynamic insertion. The prenozzle allows the improvement of the characteristics of the vacuum system according to the necessities of the experiments. The plasma setup produces a hypersonic thermal flow, which is capable to test the thermal oxidation of ultrahigh temperature ceramics (UHTC) composites, such as zirconium diboride (ZrB2). Thereby, ZrB2 samples were prepared with a variation of 10, 20, and 30% of silicon carbide (SiC) in volume, in order to investigate the oxidation mechanisms and microstructural properties of the samples tested under hypersonic thermal flow. The results of the oxidation tests showed that the samples with 10 and 30% of SiC undergo to the active oxidation and forms an unstable and fragile ZrO2 oxide. The formed ZrO2 does not withstand the drag force and the thermal flux of the hypersonic plasma jet, partially volatilizing the oxide layer, causing an accentuated loss of mass. For the oxidation tests of the sample with 20% of SiC, the gain of mass was observed due to the formation of ZrSiO4 passivation layer, which is a stable oxide and promotes mechanical resistance, and low degradation rate. These results can be associated with the variation of SiC, which demonstrates an ideal proportion of 20% of SiC in ZrB2, which influences the oxidation mechanisms and produce a protective layer.

The Effect of Heat Treatment on the Oxidation Resistance of Cobalt-Based Superalloys

Degradation of the mechanical integrity of cobalt-based superalloys can occur as the carbide network is progressively oxidised during high temperature service. In this study, a heat-treatment aimed at redistributing the carbides was tested on two similar commercial Co-based superalloys, one with high C content (Co-101) and one with low C content (Stellite-21), to determine its influence on oxidation resistance. It was found that the carbide phases in the lower C-containing alloy could be solutioned more readily than the higher C-containing alloy, enabling the continuity of the carbide network to be reduced. This resulted in a reduced attack of the carbides down the interdendritic channels during oxidation testing, but increased thickness of the oxide overscale.

Study of oxygen content in titanium alloys after exposure at elevated temperature

Isothermal oxidation testing of near α titanium alloys VT18U, VT20, Ti6Al7Nb and Ti6242S was performed in air at 560 °С for 1000 hours. Parameters of diffusion layer on the alloy surfaces were studied by microhardness indentations, optical microscopy, X-ray diffraction analysis and nuclear microanalysis. It was established that concentration of oxygen in diffusion layer of tested alloys after oxidation differs significantly. An approach was demonstrated and validated by nuclear microanalysis data that allows comparative evaluation of the total concentration of interstitial impurities in the diffusion layer by the X-ray diffraction method.

Oxidation Resistance and Modification Reaction Mechanism of Al Coating Sprayed on Pure Ti Substrate

An Al coating was deposited on the surface of pure Ti substrate by arc spray technology. In order to enable the modification reaction between the Al coating and Ti substrate, the specimen was heated to a temperature above the melting point of Al. Oxidation testing of the uncoated Ti and coated specimen was conducted at 1073 K under an air atmosphere. The microstructure, chemical composition, and phase determination of the coatings and interfaces, before and after modification treatment, were done using SEM, EDS, and XRD methods. The relationships between the modification results and time and temperature were discussed. The results showed that, after heating at 973 K for 5 hours, there was still sufficient Al on the surface of the specimen. Only intermetallic TiAl3 was formed in the diffusion region. After heating at 1073 K for 5 hours, all the Al elements diffused into the Ti substrate. Intermetallics TiAl2 and Ti3Al were also formed in the diffusion front of Al, in addition to TiAl3. After heating at 1173 K for 5 hours, a new intermetallic TiAl phase was formed at the interface of TiAl2 and Ti3Al. As the modification reaction time was prolonged at 1173 K, the formation of intermetallics TiAl2, TiAl, and Ti3Al were all increased. Among them, the formation amount of TiAl2 > Ti3Al > TiAl. The specimen after modification treatment had better high temperature oxidation resistance than the pure Ti substrate without coating.

High-Temperature Oxidation Resistance of NiAl Intermetallic Formed In Situ by Thermal Spraying

An Al/Ni composite coating was deposited on the surface of a pure Ti substrate by arc spray technology and plasma spray technology. In order to enable the in-situ reaction between the Al/Ni composite coating and the specimen, they were heated under different conditions. In addition, oxidation testing was conducted to test the oxidation-resistant property of the coating. The phase transition regulation of the coating after heating, the influence of heating at different temperatures and time on the reaction depth, and the correlated theory of the in-situ formation of the NiAl intermetallic compounds were studied and analyzed. The results showed that after the heat treatment, a ragged wave-like morphology was exhibited in the diffusion front of Al, and a small amount of the Ni in the diffusion region did not participate in the reaction. The growth of the NiAl intermetallic layer in the diffusion region of the Al/Ni/Ti specimen was obviously slower compared with the Al/Ni specimen.