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The 300 µm thick (Ti,Cr,V)N nanocomposite coatings were prepared by plasma spraying technique combined with combustion synthesis reaction of N and Ti-Cr-V reconstituted composite powders, and their microstructure and hardening mechanism were explored. The phases and microstructures were analysed by X-ray diffractometer, X-ray photoelectron spectroscopy and scanning electron microscopy, transmission electron microscopy, respectively, and a microhardness tester was employed to measure the microhardness. The main phase of the composite coating is (Ti,Cr,V)N solid solution, which has a multilayered structure of elongated columnar dendrites arranged in parallel, with different solidification positions resulting in crystal bundles possibly having different orientations. The composite coating has nanoscale fine crystals, as well as two typical strengthening models, "dislocation-TB" and "lattice distortion", which together contribute to the high hardness of the (Ti,Cr,V)N composite coating. The (Ti,Cr,V)N composite coating containing 10 wt% Cr has better surface morphology and the highest microhardness value (1844.80 HV 0.5). • Plasma spraying nano multi-nitride thick coatings in air. • The coating with multi-layer nanoclustered dendrites in various orientations. • Hardening mechanism: Besides Fine Crystal, "Dislocation-TB" and "Lattice Distortion". [ABSTRACT FROM AUTHOR]

FeAl 2 O 4 and AlFe 2 O 4 have similar spinel structures but different atomic arrangements, leading to different properties. In this study, mechanical properties and structures of the two spinel phases in composite coatings prepared by reactive plasma spraying of Fe 2 O 3 -Al composite powder were investigated by means of multi-mode atomic force microscopy, X-ray diffraction, and scanning electron microscopy. It was demonstrated that AlFe 2 O 4 was stronger than FeAl 2 O 4 with a higher electron work function (EWF). First-principles calculations were conducted to understand the difference in mechanical properties between the two phases. It was shown that FeAl 2 O 4 and AlFe 2 O 4 have “normal” and “inverse” spinel structures with different configurations of valence electrons: Fe 2+ tet (Al 3+ ) 2 O 4 and Fe 3+ tet (Fe 2+ Al 3+ ) oct O 4 . Compared to Fe 2+ tet (Al 3+ ) 2 O 4 , Fe 3+ tet (Fe 2+ Al 3+ ) oct O 4 has its O and Fe atoms in the octahedral sites forming covalent bonds, which increase the stability and strength of the crystal, corresponding to higher EWF and larger bulk, shear and Young's moduli. [ABSTRACT FROM AUTHOR]

Thick nano-grained titanium nitride (TiN) coatings fabricated by reactive plasma spraying of micrometer-sized Ti powders possess very desirable properties; however, high brittleness restricts their engineering applications. Annealing processes were carried out at different temperatures in an Ar atmosphere to improve the microstructure and properties of the coatings. The microstructure of the TiN coatings as-sprayed and annealed in Ar was observed, and their fracture toughness was calculated on the basis of diamond indentation tests. The highest fracture toughness was obtained from coating annealed in Ar at 800 °C. The wear resistance of these samples was enhanced by more than 25% compared with the as-sprayed coating. The reasons for the improvements in toughness and wear resistance are discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Plasma sprayed Ni60 coating can provide corrosion protect for the substrate. [•] Depositing ceramic coatings on metallic coating can improve the corrosion resistance. [•] The corrosion resistance of Al2O3 coating was better than that of ZrO2 coating. [•] The porosity had direct effect on the corrosion rate of the plasma sprayed coatings. [•] The top layer and the bond layer were treated as one coating in the EIS tests. [ABSTRACT FROM AUTHOR]

Abstract: In order to improve the corrosion resistance of metallic materials in molten zinc, ZrO2–Ni/Al gradient coatings were sprayed on the surface of the Fe–0.35–0.44 wt.% C steel. The corrosion behaviour and corrosion mechanism of the ZrO2–Ni/Al gradient coatings in molten zinc were studied. The ZrO2–Ni/Al gradient coatings on the surface of steels prolonged the lifetime of samples and changed the corrosion behaviour of the samples in molten zinc. The lifetime of the ZrO2–Ni/Al gradient coatings immersed in molten zinc at 620 °C is 28 days, which is 4 times as long as that of the general ZrO2 coatings. The ZrO2–Ni/Al gradient coatings were corroded in molten zinc at 620 °C, which was caused by zinc atom diffusing along the crystal boundary and pores of the ZrO2–Ni/Al gradient coatings, and reacting with Ni/Al particle in the ZrO2–Ni/Al gradient coatings. The corrosion mechanism of the coatings in molten zinc at 620 °C was crystal boundary corrosion, pitting corrosion and reaction corrosion. [Copyright &y& Elsevier]

Fe2O3–Al composite powders were prepared using ferric oxide agent powders, aluminum powders and polyvinyl alcohol. Self-reaction composite coatings containing ceramic and metal multi-phases were fabricated by plasma spraying Fe2O3–Al composite powders. This technology successfully combines self-propagating high-temperature synthesis with plasma spraying. The morphology of the composite powders was examined by scanning electron microscope (SEM). The chemical composition, microstructure, Vickers hardness, density, porosity and wear resistance of the composite coating were studied. The wear resistance of the ceramic and metal multi-phase composite coating was significantly improved under heavy load. [Copyright &y& Elsevier]

Abstract: Al2O3 ceramic coatings with different metallic bonding layers, i.e. Ni60, NiAl and FeAl, were prepared by the plasma spraying technique. To study the effect of bonding layer on corrosion behavior of plasma sprayed Al2O3 coatings, metallic coatings were also prepared for direct use as working layer. Phase analyses of the coatings were determined by X-ray diffraction (XRD). The corrosion behavior of the coated samples was evaluated by potentiodynamic polarization tests in simulated seawater. The results showed that Ni60 could make Al2O3 coating attain the best corrosion resistance amongst the three metallic bonding layers. The corrosion resistance of Al2O3 coatings first increased and then decreased with the increased Ni60 bonding layer thickness. Conclusively, it is found that chemical composition and thickness of bonding layers played an important role in the corrosion behavior of the plasma sprayed Al2O3 coatings. [Copyright &y& Elsevier]

For this study, NiCrBSi–TiN composite coatings were fabricated on Q235 steel substrates by reactive plasma spraying and the coatings were heat treated at 600 °C, 700 °C, 750 °C and 800 °C for 1 h. The microstructure evolution of these coatings untreated and treated at different temperatures was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The hardness of the coatings was characterised using the Weibull distribution while the wear and corrosion resistance were studied using a block-ring tribometer and an electrochemical workstation. The phase composition of coatings after heat treatment changed, the compactness of coatings was increased, and the interlayer bonding was significantly improved. Comprehensive experimental results illustrated that the performance of the NiCrBSi–TiN composite coatings was excellent at the heat treatment temperature of 700 °C. [ABSTRACT FROM AUTHOR]

Highlights: [•] TiN coatings were sealed using epoxy resin with small fraction of nanoparticles TiO2. [•] The corrosion potential of TiN coatings increased after sealing treatment. [•] The corrosion current decreased two orders of magnitude after sealing treatment. [•] Adding TiO2-nanoparticles to epoxy resin could improve sealing efficiency. [•] Modified epoxy resin sealing was more effective in enhancing the corrosion resistance. [ABSTRACT FROM AUTHOR]

Abstract: Nanostructured Al2O3–TiO2–ZrO2 composite powders for plasma spraying were prepared by spray drying granulation technology. The effects of processing parameters on the microstructure and properties of composite powders were investigated. The results show that with increasing the slurry solid content, the particle size of powders increases, and the bulk density of powders increases, and the flowability of powders increases firstly and then decreases. With increasing the binder content, the particle size of powders increases, and the bulk density of powders increases, and the flowability of powders increases firstly and then decreases. With increasing the spray drying temperature, the particle size of powders increases, and the bulk density and flowability of powders increases firstly and then decreases. The most appropriate spray drying parameters are the slurry solid content of 40 wt.%, the binder content of 2.0 wt.% and the spray drying temperature of 250 °C. The nanostructured composite coating was successfully prepared by using the as-prepared nanostructured Al2O3–TiO2–ZrO2 composite powders as feedstocks. The nanostructured coating possessed higher hardness and toughness compared with the conventional microstructured one, which was attributed to the use of the nanostructured composite powders feedstocks. [Copyright &y& Elsevier]

Abstract: Nanostructured ceramic matrix composite coating was prepared in-situ by reactive plasma spraying micro-sized Al-Fe2O3 composite powders. The microstructure, toughness and Vickers hardness of these coatings were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and mechanical tests. The results indicated that the coating exhibited nanostructures which consisted of FeAl2O4, Al2O3, Fe (or Fe solid solution) and a little FeAl. The composite coating showed significantly higher toughness and wear resistance than the conventional Al2O3 coating. [Copyright &y& Elsevier]

Abstract: Fe2O3–Al composite powders were deposited onto steel substrate by plasma spraying. The reaction products of the Fe2O3–Al composite powders in the plasma flame and their solidification process were investigated. The results showed that the reaction products of the Fe2O3–Al composite powders in the plasma flame were Fe–Al–O ceramic melt and Fe melt. Fe was not always formed in the reaction products of each composite particle, and the formation of Fe was dependent on the composition distribution and the reaction kinetics process of each composite particle. The composition inhomogeneity, discontinuity and porousness of the composite particles resulted in the difference of the reaction kinetics of each composite particle and the composition difference of the droplets. Hercynite solid solution, Fe, Al2O3 and FeAl phases were formed by non-equilibrium solidification of the Fe–Al–O ceramic melt. For the smaller size droplet, when it spread on the substrate, the spreading droplet was quickly chilled to form equiaxed grains with size of 100–200nm. Nano-sized or submicron equiaxed grains, nano-sized columnar grains and cellular structure were formed in the larger spreading droplet. Fe rich hercynite solid solution nucleated and grew preferentially and then Al rich hercynite solid solution grew, which form composition segregation in the columnar grains. The main phases in the composite coating were hercynite solid solution. [Copyright &y& Elsevier]

Abstract: Fe–Al intermetallic coating was prepared on steel substrate by plasma spraying. The corrosion behavior of the Fe–Al coating in molten zinc at 640°C was investigated in detail. The results showed that the plasma spraying Fe–Al coating was composed of FeAl and (Fe2Al5 +FeAl2) eutectic structure. In the molten zinc corrosion process, (Fe2Al5 +FeAl2) eutectic structure was corroded preferentially. FeAl2 disappeared in the initial stage of corrosion. The phase transition process of Fe2Al5 induced by molten zinc corrosion was Fe2Al5 →Fe2Al5Zn x (η)→ η +L (liquid phase)→ η + δ(FeZn7)+L→L. And the phase transition process of FeAl induced by molten zinc corrosion was FeAl→FeAlZn x → δ+FeAlZn x → δ + α + Γ(Fe3Zn10)→ δ + Γ → δ+L→L. The order of corrosion resistance of FeAl2, FeAl and Fe2Al5 against 640°C molten zinc was FeAl>Fe2Al5 >FeAl2. The results may provide us a useful guide to design Fe–Al intermetallic coating with high corrosion resistance. [Copyright &y& Elsevier]

Abstract: Nanostructured Al2O3–13wt.% TiO2 coating was fabricated by plasma spray with nanocrystalline powders and the microstructures of the feedstock and the coating were characterized by means of XRD, SEM and TEM. It was found that three forms of substructure existed in the coating: one evolving from the unmelted part of the feedstock and showing a round-shaped morphology; one resembling the liquid-phase-sintered structure consisting of the flattened partially melted region and fully melted region; another being of the particulate-reinforced-solid solution type with fine particles distributed in the matrix. The TEM analysis revealed that partially melted α-Al2O3 particles were in the size range of 20–70nm and were embedded in the TiO2-rich matrix. The mechanism of the substructure formation was also explained in terms of the melting and flattening behavior of the powders during plasma spray processing. [Copyright &y& Elsevier]

The effect of V concentration on the microstructure and phase composition of nano-(Ti, V)N composite ceramic coatings prepared by in-situ reactive plasma spraying of mechanically mixed Ti–V powders were investigated using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, X-ray energy dispersive spectroscopy, and transmission electron microscopy. The microhardness, toughness, wear resistance, and strengthening mechanism of the prepared nano-(Ti, V)N coatings were measured and analysed. The results showed that the nano-(Ti, V)N coating comprised a large proportion of nano-(Ti, V)N grains, which was the solid solution of TiN and VN. All the V atoms completely entered the TiN lattice and the solubility limit of V in TiN is approximately 25 wt%. The grains of the (Ti, V)N (25 wt% V) coating had a face-centred cubic structure and a large quantity of twins; they were primarily equiaxed grains morphology with a few columnar grains. From comparing the experimental statistics, the (Ti, V)N (25 wt% V) coating displayed the highest microhardness (1952 ± 78.5 Hv) and the most even dispersion but a slightly lower toughness compared with the (Ti, V)N (35 wt% V) coating. The (Ti, V)N (25 wt% V) coating with a dense microstructure obtained a high microhardness due to dislocation strengthening, fine grain strengthening, and solid solution strengthening (from the solid solution of VN in TiN). Furthermore, a lower friction coefficient and wear volume loss indicated that the (Ti, V)N (25 wt% V) coating had superior tribological properties and great potential as a wear resistant coating. [ABSTRACT FROM AUTHOR]

TiCN coatings were reactive plasma sprayed with different Ti/graphite powder ratios. Their crystallization behaviours were investigated. The results showed that the TiN coating consisted of TiN and TiN 0.3 O phases. With the decrease in the Ti/graphite ratios, the phases in the coatings changed from TiC 0.7 N 0.3 +TiC 0.3 N 0.7 +amorphous (Ti:graphite = (6-10):1) to TiC 0.3 N 0.7 + TiC + amorphous (Ti:graphite = 4:1). The surface of the TiN coating exhibited the columnar crystals. With the decrease in the Ti/graphite ratios, the columnar crystals gradually skewed towards the surface. The cross-section crystalline morphologies of all the coatings exhibited the layer-layer columnar crystals with better bonding interfaces between the layers. The micro-hardness of the sprayed TiN coating was 1000 HV 0.1. The micro-hardness of the TiCN coatings increased with the decrease in the Ti/graphite ratios. The maximum microhardness (1315 HV 0.1) was observed at a Ti/graphite ratio of 4:1, approximately 30% higher than that of the TiN coating. The increase in the micro-hardness was correlated with the phase composition and crystalline morphologies. [ABSTRACT FROM AUTHOR]

Abstract: Nanostructured FeAl2O4-based ceramic matrix composite coatings were prepared in-situ by reactive plasma spraying micro-sized Al–Fe2O3 and Al–Fe2O3–Cr2O3 powders. The microstructure, toughness, Vickers hardness, and adhesive strength of these coatings were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and mechanical tests. The results indicated that both the coatings exhibited a nanostructured microstructure. The grains of coating AFC sprayed with Al–Fe2O3–Cr2O3 powders are finer than those of the coating AF sprayed with Al–Fe2O3 powders. The composite nano-coating sprayed with Al–Fe2O3–Cr2O3 powders exhibited higher hardness and better wear resistance compared with those of the composite nano-coating sprayed with Al–Fe2O3 powders. The adhesive strength, toughness, and wear resistance of the composite coating sprayed with Al–Fe2O3–Cr2O3 powders were significantly enhanced compared with those of the composite coating sprayed with Al–Fe2O3 powders, which were attributed to the Cr2O3 addition. [Copyright &y& Elsevier]

Abstract: In situ nanostructured ceramic matrix composite coating was prepared by reactive plasma spraying micro-sized Al–Fe2O3 composite powders. The microstructure of the composite coating was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy, respectively. The results indicated that the composite coating exhibited dense and crack-free microstructure with a number of spherical α-Fe and γ-Al2O3 nano-grains embedded within equiaxed and columnar FeAl2O4 nano-grains matrix. The composite coating showed markedly higher toughness and wear resistance than the conventional Al2O3 coating. [ABSTRACT FROM AUTHOR]

Abstract: In the present study, Fe–Al2O3–FeAl2O4 composite coatings were successfully deposited by reactive plasma sprayed Al/Fe2O3 agglomerated powder. Phase composition and microstructure of the coatings were determined by XRD and SEM. The results indicated that the composite coatings were principally composed of three different phases, i.e. FeAl2O4 phase as main framework, dispersed ball-like Fe-rich phase, and small splats of Al2O3 phase, and it was thought that the in situ synthesized metal phase was helpful to toughen the coating matrix. According to the results of the indentation and frictional wear tests, the composite coating exhibited excellent toughness and anti-friction properties in comparison with conventional Al2O3 monophase coating, though its microhardness value was a little lower than that of Al2O3 coating. The formation mechanism and the toughening mechanism of the composite coating were clarified in detail. [Copyright &y& Elsevier]

Abstract: In the present study, the nanostructured TiN coatings are obtained by means of reactive plasma spraying in the air. The XRD analysis shows the coating was mainly composed of TiN (max. 86.3%) and a small quantity of Ti3O. Their microstructures were observed by SEM and TEM. The TEM pictures show the TiN coatings were composed of the nanoscaled grains (range from 70 to 90nm), and the Scherrer equation analysis on the average grain size corresponded with that result. The effects on the average grain size also were studied by changing the spraying power and the spraying distance. [Copyright &y& Elsevier]

Abstract: Aluminum coating was plasma sprayed on Fe-0.14–0.22 wt.% C steel substrate, and heat diffusion treatment at 923 °C for 4 h was preformed to the aluminum coating to form Fe2Al5 inter-metallic compound coating. The corrosion mechanism of the Fe2Al5 coating in molten zinc was investigated. SEM and EDS analysis results show that the corrosion process of the Fe2Al5 layer in molten zinc is as follows: Fe2Al5 →Fe2Al5Zn x (η)→ η + L(liquid phase)→ L + η + δ(FeZn7)→ L + δ → L. The η phase and the eutectic structure (η + δ) prevent the diffusion of zinc atoms efficiently. Therefore the Fe2Al5 coating delays the reaction between the substrate and molten zinc, promoting the corrosion resistance of the substrate. [Copyright &y& Elsevier]

The corrosion behavior of plasma-sprayed Ni/Al–Al2O3 and Ni/Al–Al2O3+13 wt.% TiO2 graded ceramic coatings in 5%HCl solution was investigated. It was shown that the corrosion resistance of a Ni/Al–Al2O3 graded coating was 130 times that of a single Al2O3 coating. The corrosion resistance of Ni/Al–Al2O3+13%TiO2 graded coatings was increased by 39.2% compared to a Ni/Al–Al2O3 graded coating with the same compositional gradient. The improvement in corrosion resistance of graded ceramic coatings mainly resulted from the change of corrosion behavior due to the change in structure of the coating. The bond layer alloy particles in the transition layers were distributed isolated in the ceramic matrix. Thus, the connected pores in the working layer were actually pseudo-connected pores. As a result, the electrochemical corrosion between the bond layer and the matrix was restrained. In addition, the pseudo-connected porosity in graded coatings lay on not only the open porosity but also the structure of the graded coating. [Copyright &y& Elsevier]

In this study, NiCrCoMo–TiC composite coatings were plasma sprayed using NiCrCoMo alloy powders and Ti–C powders. The microstructure, micro-hardness, and wear behaviours were investigated. NiCrCoMo coating consisted of Ni–Cr–Co–Mo phase. NiCrCoMo–TiC coatings consisted of Ni–Cr–Co–Mo, TiC, and TiO 2 phases. The average porosity of NiCrCoMo coating was about 5.5 ± 0.6% and it increased to 10.1 ± 0.9% after adding 25 wt% Ti–C powders. All the coatings had a thickness of ≥200 μm with a layered structure. The micro-hardness of NiCrCoMo–TiC coatings increased with the addition of Ti–C powders, and it increased by 39% to 854HV 0.2 as the added Ti–C powders was 25 wt%. The friction coefficient of NiCrCoMo coating was 0.43, and that of NiCrCoMo–TiC coatings was 0.60–0.64. The wear volume of NiCrCoMo–15TiC coating was 0.3099 mm3, which was just 28% of that of NiCrCoMo coating. The wear mechanism of NiCrCoMo–TiC coatings was adhesive wear, fatigue wear and abrasive wear. • In-situ synthesized TiC reinforced NiCrCoMo coatings were first prepared by reactive plasma spraying. • The in-situ synthetic mechanism of TiC in NiCrCoMo coatings was investigated. • NiCrCoMo–TiC coatings had enhanced hardness and improved wear resistance by in-situ TiC synthesis. [ABSTRACT FROM AUTHOR]