Nannan Zhang

Tribological properties of FeCoCrNiAlBx high-entropy alloys coating prepared by laser cladding

FeCoCrNiAlBx (x = 0, 0.25, 0.50, 0.75) coatings were prepared by laser cladding to study the effects of boron on the structure and properties of high-entropy alloys coatings. The microstructure, microhardness, and wear resistance properties of the samples were investigated by scanning electron microscopy, X-ray diffraction, metallographic micro-hardness test, and friction-wear test, respectively, and the mechanism of the wear behavior was also analyzed. The results showed that the high-entropy alloys consisted of BCC phase and eutectic structure, which contained FCC phase and M2B. With boron addition, the content of BCC phase increased while that of eutectic structure decreased. The wear resistance of the high-entropy coatings was considerably improved with increasing addition of boron, and accordingly, the FeCoCrNiAlB0.75 coating showed the best wear resistance.

EFFECT OF OXYACETYLENE FLAME REMELTING ON WEAR BEHAVIOUR OF SUPERSONIC AIR-PLASMA SPRAYED NiCrBSi/h-BN COMPOSITE COATINGS

NiCrBSi alloy coatings are widely used in wear and corrosion protection at higher temperature. As a primary hard phase forming element, B element can effectively improve the coating hardness. In this study, the low coefficient of friction of BN with three ratios (10%, 20%, and 30%) was added in order to reduce the wear rate and provide additional B element. The NiCrBSi/h-BN composite coatings were successfully prepared on a cast-iron substrate using supersonic air-plasma spray technology. The phase constitution, microstructure characterization, and microhardness of the coatings before and after oxyacetylene flame remelting were investigated by means of scanning electron microscope (SEM), X-ray diffraction, and energy dispersive analysis of X-ray techniques, respectively. The wear resistance of composite coatings was also tested in this paper. It was found that the microstructure was well refined by remelting treatment and this was beneficial for the adherence between the coating and the substrate, which was nearly 33MPa. The wear resistance of the NiCrBSi alloy coating was also improved with the increasing component of h-BN in remelted samples. When the h-BN content reached 30%, the friction coefficient decreased to 0.38 for the remelted coating. The effect of the remelting process on the anti-abrasive property and extension of the material’s wear life was quite important.

THERMAL SHOCK PROPERTIES OF YTTRIA-STABILIZED ZIRCONIA COATINGS DEPOSITED USING LOW-ENERGY VERY LOW PRESSURE PLASMA SPRAYING

Yttria-stabilized zirconia (YSZ) coatings have been frequently used as a thermal protective layer on the metal or alloy component surfaces. In the present study, ZrO2-7%Y2O3 thermal barrier coatings (TBCs) were successfully deposited by DC (direct current) plasma spray process under very low pressure conditions (less than 1 mbar) using low-energy plasma guns F4-VB and F100. The experiments were performed to evaluate the thermal shock resistance of different TBC specimens which were heated to 1373 K at a high-temperature cycling furnace and held for 0.5 h, followed by air cooling at room temperature for 0.2 h. For comparison, a corresponding atmospheric plasma spray (APS) counterpart was also elaborated to carry out the similar experiments. The results indicated that the very low pressure plasma spray (VLPPS) coatings displayed better thermal shock resistance. Moreover, the failure mechanism of the coatings was elucidated.

In-flight particle characterization and coating formation under low pressure plasma spray condition

Yttria-stabilized zirconia (YSZ) coatings were deposited by low pressure plasma spray (LPPS) in 1.0×104 Pa, 1.5×104 Pa, and 2.5×104 Pa. Both in-flight particle diagnostic detected by DPV-2000 system and ANSYS-FLUENT software were used to study the connection between the parameters of flying particles and the coating formation, which might help to recognize the relationship between the operation parameters and the coatings quality. The results of simulation showed that particles in a lower spray pressure could achieve a higher velocity. The particle velocity was around 380 m/s at a distance of 35 cm from the nozzle at 1.0×104 Pa while only 300 m/s at 2.5×104 Pa in actual measurement. The results showed that the velocity of particles increased with decreasing the spray pressure, which might enhance the flattening rate of coatings and thereby decreased the porosity. The deposited YSZ coating with the lowest porosity can be gained under 1.0×104 Pa condition.

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.

Influence of laser re-melting and vacuum heat treatment on plasma-sprayed FeCoCrNiAl alloy coatings

FeCoCrNiAl high entropy alloy coatings were prepared by supersonic air-plasma spraying. The coatings were post-treated by vacuum heat treatment at 600 and 900 °C, and laser re-melting with 300 W, respectively, to study the influence of different treatments on the structure and properties of the coatings. The phase constitution, microstructure and microhardness of the coatings after treatments were investigated using X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. Results showed that the as-sprayed coatings consisted of pure metal and Fe-Cr. The AlNi3 phase was obtained after the vacuum heat treatment process. A body-centered cubic structure with less AlNi3 could be found in the coating after the laser re-melting process. The average hardness values of the as-sprayed coating and the coatings with two different temperature vacuum heat treatments and with laser re-melting were 177, 227, 266 and 682 HV, respectively. This suggests that the vacuum heat treatment promoted the alloying process of the coatings, and contributed to the enhancement of the coating wear resistance. The laser re-melted coating showed the best wear resistance.