SiC/SiC–YAG–YSZ oxidation protective coatings for carbon/carbon composites

SiC/SiC–YAG–YSZ coatings were prepared by pack cementation, chemical vapor deposition and slurry painting on carbon/carbon (C/C) composites. The microstructures and oxidation behavior of coatings were investigated. The results show that the coatings displayed good oxidation and thermal shock resistance due to a dense glassy layer with silicates formed on the coating of SiC–YAG–YSZ. The weight gain rate of coated C/C composites was 1.77% after oxidation for 150 h at 1773 K. SiC in outer coating can promote the formation of oxygen diffusion barrier and lead to the optimum oxidation resistance for the coatings, compared with YSZ and YAG.     

Cracking investigation of W and W(C) films deposited by physical vapor deposition on steel substrates

This paper presents the investigation of the cracking of coatings deposited on steel substrates. The coating on substrate systems consisted on pure tungsten films (W) and films of solid solutions of carbon in tungsten [W(C)], which were deposited by direct current reactive magnetron sputtering on stainless steel substrates. The systems were strained uniaxially with a microtensile device adapted to a scanning electron microscope. The mechanical response was analyzed from the experimental results: the straining of the samples showed an evolution of the density of cracks in the coating, which was described trough an empirical equation based on the Weibull distribution function. The density of cracks, which corresponds to the crack saturation of the coating, appeared to vary inversely with coating thickness. Critical parameters relative to their mechanical stability were also determined from the experimental results: the strain energy release rate for crack extension through the film, G^f_c, and the fracture toughness, K^f_Ic, of the coatings. These values are included between 0.2 and 14 J m⁻², and between 0.1 and 2.5 MPa m^−1/2. The fracture resistance of W and W(C) coatings was found to be correlated to their thickness and microstructure.     

Synthesis and characterization of organically modified silicate/NiZn ferrite hybrid coatings

Hybrid coatings based on organically modified silicate (Ormosil)/Ni_0.5Zn_0.5Fe₂O₄ (10–30 wt.%) were synthesized through a sol–gel technique. Tetraethylenepentamine, 3-glycidoxypropyltrimethoxysilane, tetraethoxysilane and Ni_0.5Zn_0.5Fe₂O₄ were used as precursors for the hybrid coatings. These hybrid films were deposited via spin coating onto an aluminum alloy in order to improve the corrosion protection and to act as infrared stealth coatings. The effects induced by the NiZn ferrite content on the chain dynamic, ferromagnetic behavior, infrared stealth and corrosion performances of the coated samples were investigated. The rotating-frame spin–lattice relaxation times and scale of the spin-diffusion path length indicated that the configuration of the hybrid films was highly cross-linked, dense and adhered to the aluminum alloy substrates. The magnetic properties of the resulting hybrids showed super-paramagnetic behavior, such as zero coercive force (coercivity = 0 G) and a low blocking temperature (∼75 K). The thermal extinction of the hybrid coatings increased with the increase in the NiZn ferrite content. Potentio-dynamic and salt-spray analysis revealed that the hybrid films provided an exceptional barrier and corrosion protection in comparison with untreated aluminum alloy substrates.     

Magnetron-sputtered cobalt-based protective coatings on ferritic steels for solid oxide fuel cell interconnect applications

Co, Co–Mn (67:33 at.%) and Co–Cu (67:33 at.%) coatings were fabricated using magnetron sputtering on two kinds of ferritic stainless steels (Crofer22APU and F17TNb) in order to form spinel protective coatings on metallic interconnects for solid oxide fuel cells. Despite the thickness unevenness at different regions, dense metallic coatings were successfully applied onto all necessary surfaces of the channelled interconnect substrates. Upon oxidation, spinel oxide coatings with very low Cr content were formed, reducing effectively the Cr release. Among the three protective coatings, Co–Cu coating showed the lowest area specific resistance (<15 mΩ cm² at 800 °C).     

The role of beam dispersion in Raman and photo-stimulated luminescence piezo-spectroscopy of yttria-stabilized zirconia in multi-layered coatings

Laser beam dispersion affects the resolution of Raman and photo-stimulated luminescence piezo-spectroscopy measurements of transparent materials. In this paper, we investigate the lateral spreading of the laser beam and the axial sampling depth of Raman spectroscopy measurements within thermal sprayed yttria-stabilized zirconia (YSZ) thin coatings. The lateral diameters of the laser beams (λ = 632.8 nm and 514 nm) reach approximately ～160 μm after travelling through a thickness of 200 μm of air plasma sprayed (APS) YSZ and ～80 μm after travelling through 120 μm of electron beam physical vapour deposited YSZ. The Raman spectroscopy sampling depth was found to be between 30 and 40 μm in APS YSZ. The beam dispersions within these two coatings were simulated using the ray-tracing software ZEMAX to understand the observed scattering patterns. The results are discussed with respect to the application of these two spectroscopic techniques in multi-layered thermal barrier coating systems.     

Low-temperature transformation kinetics of electron-beam deposited 5 wt.% yttria-stabilized zirconia

The transformation kinetics of ZrO₂ coatings stabilized with 5.6 mol% YO_1.5 (5YSZ), and deposited by electron-beam physical vapor deposition, were studied between room temperature and 600 °C using in situ Raman spectroscopy, and are described in the form of a transformation-time–temperature diagram. The coatings were found to be monoclinic (m) at temperatures below 375 °C, while above 400 °C they transformed to the tetragonal (t) phase. On cooling, the coatings transformed back to monoclinic below ∼375 °C. Between 375 and 400 °C, the transformation rates approached zero, indicating that the thermodynamic driving force for the transformation also approaches zero in this temperature range. This provides a direct measurement of the T₀(m–t) temperature for the 5YSZ composition.     

Erosion performance and characterization of nanolayer (Ti,Cr)N hard coatings for gas turbine engine compressor blade applications

(Ti,Cr)N nanolayer coatings were deposited on Ti–6Al–4V, 17-4PH and Inconel 718 substrates using cathodic arc physical vapor deposition for improved erosion and corrosion resistance. Coating corrosion performance was highly dependent on the coating thickness and packing factors and correlated with increased chromium content within the (Ti,Cr)N nanolayer coatings. The change in cathode current predominantly affected coating thickness and the bias affected the packing factor. Erosion tests of the coated and uncoated substrates at both 30° and 90° erodent impingement angles were conducted using angular aluminum oxide media at particle velocities up to 145 m/s. Chromium evaporator current and substrate bias were varied to change film stoichiometry and microstructure for erosion performance evaluation. When chromium evaporator current was varied, the increase in chromium content led to an increase in binary CrN phase volume and a decrease in TiN phase volume. The increase in CrN phase volume decreased both hardness and erosion performance at both impingement angles. Lower bias values resulted in better erosion performance. At 30° erodent impingement, all coated samples outperformed the uncoated substrate; whereas, for 90° impingement, only coatings deposited at low bias values (? 25 V, ? 50 V, and ? 100 V) and high Ti:Cr ratios (> 2.4) outperformed the uncoated substrate. The primary coating failure mechanism was microchipping.     

Effect of C2H2 gas flow rate on synthesis and characteristics of Ti–Si–C–N coating by cathodic arc plasma evaporation

The influences of C₂H₂ gas flow rate on the synthesis, microstructure, and mechanical properties of the Ti–Si–C–N films were investigated. Quaternary Ti–Si–C–N coatings were deposited on WC-Co substrates using Ti and TiSi (80:20 at.%) alloy target on a dual cathodic arc plasma evaporation system. The Ti–Si–C–N coatings were designed with Ti/TiN/TiSiN as an interlayer to enhance the adhesion strength between the top coating and substrate. The Ti–Si–C–N coatings were deposited under the mixture flow of N₂ and C₂H₂. Composition analysis showed that as the C₂H₂ gas flow increased, the Ti, Si and N contents decreased and the carbon content increased in the coatings. The results showed the maximum nanohardness of approximately 40 GPa with a friction coefficient of 0.7 was obtained at the carbon content of 28 at.% (C₂H₂ = 15 sccm). However, as the C₂H₂ gas flow rate increased from 15 to 40 sccm (carbon content from 25.2 to 56.3 at.%), both the hardness and friction coefficient reduced to 20 GPa and 0.3, respectively. Raman analysis indicated the microstructure of the deposited coating transformed from Ti–Si–C–N film to TiSi-containing diamond-like carbon films structure, which was strongly influenced by the C₂H₂ flow rate and is demarcated at a C₂H₂ flow of 20 sccm. The TiSi-containing diamond-like carbon films reveal low-friction and wear-resistant nature with an average friction coefficient between 0.3 and 0.4, lower than both TiSiN and Ti–Si–C–N films.     

Long-term atmospheric corrosion properties of thermally sprayed Zn,Al and Zn–Al coatings exposed in a coastal area

The long-term atmospheric corrosion properties of thermally sprayed Zn, Al and Zn–Al coatings have been evaluated using an electrochemical impedance measurement and several analytical techniques. All the thermal-sprayed specimens with 100 μm coating thickness have protected the steel substrates. In case of the Zn–Al coating, the red rust was not observed regardless of the coating thickness. The corrosion products were identified by the XRD analysis. Preferential dissolution of zinc was observed on the Zn–Al coating by EPMA analysis. The electrochemical impedance results provided an insight about the surface structures of each thermally sprayed coating.     

Cyclic oxidation of Pt/Pd-modified aluminide coating on a nickel-based superalloy at 1150 °C

Pt-, Pd-, and Pt/Pd-modified aluminide coatings were prepared on Inconel 738LC by pack aluminizing at 1034 °C. During pack aluminizing, Pt-modified aluminide coating formed a two-phase β-NiAl + PtAl₂ layer and a β-NiAl layer on an interdiffusion zone, whereas Pd- and Pt/Pd-modified aluminide coatings formed only the thicker β-NiAl layer. However, Pd-modified aluminide coating had many pores. During cyclic oxidation, Pt/Pd-modified aluminide coating had a surface that was less rumpled than that of Pt-modified aluminide coating due to its thicker thickness. Pt/Pd-modified aluminide coating had a 22% greater Al-uptake than Pt-modified aluminide coating. Cyclic oxidation tests at 1150 °C showed that Pt/Pd-modified aluminide coating had the best cyclic oxidation resistance. After the cyclic oxidation, an additional γ-Ni phase was seen beneath the outermost alumina scale on the the γ′-Ni₃Al phase in Pt/Pd-modified aluminide coating. The γ-Ni phase, which had a higher Cr content, increased the adhesion and stability of the alumina.     

Effect of high-temperature aging on the thermal conductivity of nanocrystalline tetragonal yttria-stabilized zirconia

The thermal conductivity of yttria-stabilized zirconia (YSZ) thermal barrier coatings increases with high-temperature aging. This common observation has been attributed to the densification of the coatings as porosity sinters out and pores and cracks spheroidize to minimize their surface energy. We show that the thermal conductivity of fully-dense 3 mol.% Y₂O₃ stabilized zirconia (3YSZ) also increases with high-temperature aging, indicating that densification and pore shape changes alone are not responsible for all the observed increase in thermal conductivity of coatings. Instead, there are also increases due to a combination of phase separation and grain growth. The increase in thermal conductivity can be described by a Larson–Miller parameter. It is also found that the increase in thermal conductivity with aging is greatest when measured at room temperature and decreases with increasing measurement temperature. Measured at 1000 °C, the thermal conductivity of zirconia is almost temperature independent and the changes in thermal conductivity with aging are less than 15%, even after aging for 50 h at 1400 °C.     

A comparative study of two kinds of Y and Al modified silicide coatings on an Nb–Ti–Si based alloy prepared by pack cementation technique

Two kinds of Y and Al modified silicide coatings on an Nb–Ti–Si based alloy were prepared by pack cementation technique. The microstructure and oxidation behavior of both coatings were studied. Both coatings had a multiple layer structure, but the outer layers were composed of either Y- and Al-doped (Nb,X)Si₂ or Y-doped (Nb,X)₃Si₅Al₂ + (Nb,X)Si₂ phases, respectively. The former coating could protect the substrate alloy from oxidation at 1250 °C for 100 h, but the latter coating could only endure for less than 20 h. The scale formation mechanisms and microstructural changes of both coatings upon oxidation have been illustrated.     

Chromium carbide–CNT nanocomposites with enhanced mechanical properties

Chromium carbide is widely used as a tribological coating material in high-temperature applications requiring high wear resistance and hardness. Herein, an attempt has been made to further enhance the mechanical and wear properties of chromium carbide coatings by reinforcing carbon nanotubes (CNTs) as a potential replacement of soft binder matrix using plasma spraying. The microstructures of the sprayed CNT-reinforced Cr₃C₂ coatings were characterized using transmission electron microscopy and scanning electron microscopy. The mechanical properties were assessed using micro-Vickers hardness, nanoindentation and wear measurements. CNT reinforcement improved the hardness of the coating by 40% and decreased the wear rate of the coating by almost 45–50%. Cr₃C₂ reinforced with 2 wt.% CNT had an elastic modulus 304.5 ± 29.2 GPa, hardness of 1175 ± 60 VH_0.300 and a coefficient of friction of 0.654. It was concluded that the CNT reinforcement increased the wear resistance by forming intersplat bridges while the improvement in the hardness was attributed to the deformation resistance of CNTs under indentation.     

A methodology,based on sintering-induced stiffening,for prediction of the spallation lifetime of plasma-sprayed coatings

This study concerns the effect of sintering-induced stiffening in promoting spallation of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings. Coatings with thickness in the range 350–800 μm were sprayed onto dense alumina substrates. In order to ensure a tough interface, the surface of the alumina substrates were first roughened by laser treatment. Specimens were heat treated at 1500 °C and periodically quenched to ～100 °C, using nitrogen jets. During cooling, specimens were monitored for spallation via a webcam. Spallation lifetimes were observed to be shorter for thicker coatings. Using a simple fracture mechanics approach, with the strain energy release rate obtained using measured coating stiffness values, the behaviour was found to be consistent with an approximately constant interfacial fracture energy value of the order of 300 J m^?2. If this interfacial toughness had been known beforehand, then the rationale presented here could have been used for prediction of coating lifetime. While the experiments are based on use of a ceramic substrate, the approach could be applied to conventional metallic substrate systems.     

Dry sliding wear behaviour of nickel aluminides coatings produced by self-propagating high-temperature synthesis

Nickel aluminides coatings have been produced by self-propagating high-temperature synthesis using concentrated solar energy, with nickel composition of coatings ranging from 45 to 75 at.%. The dry sliding wear behaviour of coatings has been performed in a pin-on-disk tribometer. NiAl coatings (50 at.% Ni) have been tested against Al₂O₃ and WC–Co balls, while other coatings have been tested against Al₂O₃ balls. In all the coatings a three-body abrasion is produced by the particles detached from the coating surface and then oxidized, which remain between the ball and the coating. NiAl coatings exhibit the lowest wear coefficient while coatings with the highest Ni content have the highest wear coefficients. Wear coefficients show that NiAl coatings or coatings composed mainly of NiAl have a high wear resistance.     

Effect of Y2O3 content in the pack on microstructure and hot corrosion resistance of Y–Co-modified aluminide coating

Y–Co-modified aluminide coatings on nickel base superalloys were prepared by pack cementation method. Effect of Y₂O₃ content in the pack mixture on microstructure and hot corrosion resistance of the coatings was investigated. The results show that with the increase in Y₂O₃ content, the content of Co in the coatings increases. The mass gain of the coatings with Y₂O₃ addition of 1, 2 and 3 wt.% is 0.6, 0.55 and 0.42 mg/cm² after hot corrosion at 1173 K for 100 h, respectively. Y₂O₃ addition accelerates the diffusion of Co and thus increases the hot corrosion resistance of the coating.     

Structural and mechanical properties of corundum and cubic (AlxCr1−x)2+yO3−y coatings grown by reactive cathodic arc evaporation in as-deposited and annealed states

Coatings of (Al_xCr_1?x)_2+yO_3?y with 0.51 ? x ? 0.84 and 0.1 ? y ? 0.5 were deposited on hard cemented carbide substrates in an industrial cathodic arc evaporation system from powder-metallurgy-prepared Cr/Al targets in pure O₂ and O₂ + N₂ atmospheres. The substrate temperature and bias in all the deposition runs were 575 °C and ?120 V, respectively. The composition of the coatings measured by energy dispersive X-ray spectroscopy and elastic recoil detection analysis differed from that of the facing targets by up to 11%. Microstructure analyses performed by symmetrical X-ray diffraction and transmission electron microscopy showed that corundum, cubic or mixed-phase coatings formed, depending on the Cr/Al ratio of the coatings and O₂ flow per active target during deposition. The corundum phase was promoted by high Cr content and high O₂ flow per target, while the cubic phase was observed mostly for high Al content and low O₂ flow per active target. In-situ annealing of the cubic coatings resulted in phase transformation from cubic to corundum, completed in the temperature range of 900–1100 °C, while corundum coatings retained their structure in the same range of annealing temperatures. Nanoindentation hardness of the coatings with Cr/Al ratio <0.4 was 26–28 GPa, regardless of the structure. Increasing the Cr content of the coatings resulted in increased hardness of 28–30 GPa for corundum coatings. Wear resistance testing in a turning operation showed that coatings of Al–Cr–O have improved resistance to crater wear at the cost of flank wear compared with TiAlN coatings.     

Corrosion resistance of Ti–6Al–4V alloy with nitride coatings in Ringer’s solution

The corrosion behaviour of Ti–6Al–4V alloy with nitride coatings was investigated in Ringer’s solution at 36 and 40 °С. Nitride coatings of different composition, thickness and surface quality were formed because of changing nitrogen partial pressure from 1 to 10⁵ Ра and nitriding temperature from 850 to 900 °С. Results shown that nitride coatings improve anticorrosion properties of alloy at both solution temperatures. Corrosion resistance of alloy increases with the content increase of TiN phase in nitride coating. With increase of temperature from 36 to 40 °С the corrosion resistance of alloy is determined significantly by quality of nitride coating.     

Annealing effects on the intermetallic compound formation of cold sprayed Ni,Al coatings

The annealing of Ni and Al coatings under various conditions on substrates fabricated by a cold gas dynamic spray process (CDSP) were investigated. The powder particles were accelerated through a standard De Laval-type nozzle with air used as the main carrying gas. The coatings were annealed at 450–550 °C in either argon or air atmospheres for 4 h. In the case of Ni coatings during annealing both in argon and air atmospheres, intermetallic compound layers such as Al₃Ni and Al₃Ni₂ were observed at the interfaces between the Ni coating and Al substrate. Also, the intermetallic layer formation of Al₃Ni and Al₃Ni₂ at the interfaces depended on the solid-state diffusion and the annealing temperature. The intermetallic compound AlNi was obtained at the interface of Al coating on a Ni substrate by low-temperature annealing under the melting temperature.     

Polyaniline-lignosulfonate/epoxy coating for corrosion protection of AA2024-T3

Corrosion protection arising from epoxy coatings incorporating lignosulfonate-doped polyaniline (Pani-LGS) upon AA2024-T3 was studied in 0.6 M NaCl. Synthesized Pani-LGS particles were investigated using TEM, FTIR, TGA and conductivity, whilst coatings were also physically examined using SEM. The coating performance was studied using a combination of potentiodynamic polarisation, EIS, FTIR spectroscopy and X-ray photoelectron spectroscopy. The performance of Pani-LGS/epoxy blends is discussed more generally, with tests revealing that on exposure to 0.6 M NaCl solution for 30 days, a 5 wt% Pani-LGS/epoxy coating resulted in low levels of corrosion. A mechanism for the postulated mode of corrosion protection is presented.