Surface Engineering

The present investigation has been carried out in order to study the erosion wear behaviour of WC–Co base thermal spray coatings. WC–12Co and WC–10Co–4Cr coatings were deposited by means of high velocity oxygen fuel (HVOF) thermal spraying. The erosion tests were conducted at impact angles of 30 and 90° using SiC particles of ∼50 μm in diameter as erodent, at a velocity of 83·4 m s⁻¹. It has been found that the erosion rate for both coated systems was higher when the test was carried out at an angle of 90°. The through-thickness residual stresses of the coatings, as well as the microstructural characterisation, allowed an explanation of the results and the erosion mechanisms in each case. It has been found that, under the experimental conditions carried out in the present study, the WC–10Co–4Cr coating exhibited a higher erosive wear resistance as compared to the WC–12Co coating.

Yttrium silicate coating for SiC precoated C/C composite was prepared by a hydrothermal electrophoretic deposition process. The yttrium silicate coating was sealed with a borosilicate glass outer layer. Phase compositions, surface and cross-section microstructures of the as prepared multilayer coatings were characterised by X-ray diffractometer and scanning electron microscopy. The influences of hydrothermal electrophoretic deposition factors on the phase, microstructure and oxidation resistance of the multilayer coated C/C composites were particularly investigated. Results show that the hydrothermal electrophoretic resulting coating is composed of crystallites with a main phase of Y₂Si₂O₇ and Y₂SiO₅. The thickness and density of the coatings are improved with the increase in deposition voltage and hydrothermal temperature. Compared with the pack cementation SiC monolayer coating, the as prepared multilayer coatings exhibit better antioxidation property. When oxidation in air and combustion atmosphere, the coated C/C composites lose weight by a linear and a parabolic law respectively. The prepared multilayer coating can effectively protect the C/C composites in combustion atmosphere at 1773 K for 85 h. The flexural strength of the coated specimen arrives at the lowest value at about 1123 K in combustion atmosphere.

The high temperature oxidation behaviour of nanostructured Cr/Co–Al coatings on Superni-718 substrate, deposited by sputtering, has been investigated. Cyclic high temperature oxidation tests were conducted on bare and coated samples at 900°C for up to 100 cycles. The results showed that a dense scale formed on the coated samples during thermal cycling at the peak temperature of 900°C. The external scale exhibited good spallation resistance during cyclic oxidation testing. The improvement in oxide scale spallation resistance is believed to be the fine grained structure of the coating. The Cr/Co–Al coating on the superalloy substrate showed better performance of cyclic high temperature oxidation resistance due to its possession of β-CoAl phase as Al reservoir and the formation of Al₂O₃ and spinel phases, such as CoCr₂O₄ and CoAl₂O₄, in the scale.

A Si–Mo–Al coating for protecting carbon/carbon (C/C) composites against oxidation at elevated temperature was prepared on the surface of the C/SiC coated C/C composites by slurry method. The phase composition, microstructure and oxidation resistance of the coating at 1773 K in air were investigated. The results show that the as prepared coating was mainly composed of SiC, MoSi₂, Mo(Si,Al)₂ and Si. The coating provided excellent oxidation protective ability at 1773 K in air and can effectively protect the C/C composites from oxidation for 150 h. The failure of the coating at 1873 K in air is attributed to the spalling of the glass layer during the oxidation tests.

High temperature sulphidation is one of the most deleterious forms of surface degradation. In the present work, the high temperature sulphidation behaviours of high velocity oxyfuel sprayed NiCrAlY and NiCrAlYCe coatings on high pressure steels in H₂–H₂S mixtures were investigated. Results showed that the NiCrAlYCe coating provided much better sulphidation resistance than the NiCrAlY coating in the given environment. After adding minor CeO₂, the outer layer of scales became compact and showed good adhesion, which decreased the sulphur vapour pressure inside the scale and promoted the formation of more protective inner layer of sulphospinels (Cr, Ni)Al₂S₂.

In the present study, NiCrAlY coatings were applied on steel substrate using high velocity oxygen fuel spraying process. The spray parameters were varied in order to study their influence on the coating properties. Powder and coating microstructures were investigated by a combination of X-ray diffraction, optical and scanning electron microscopy as well as energy dispersive spectroscopy analysis. Bond strength and microhardness tests were also performed on the coatings. It was found that the propane flowrate, the oxygen flowrate, the spray distance and powder feedrate have a significant effect on the coating characteristics. Finally, with the aim of minimising oxygen content and maximising bond strength, the optimum parameters were selected as the preferred spray parameters.

X-ray diffraction (XRD) techniques including stress measurement were applied to untreated, low temperature plasma nitrided and low temperature plasma carburised AISI 304 austenitic stainless steels treated at 425°C for 12 h in H₂/N₂ and H₂/CH₄ gases respectively. Relationships between surface microhardness and XRD peak broadening were established. The results also showed that both surface treated layers were under a compressive residual stress. The compressive residual stresses of the low temperature plasma nitrided and the low temperature plasma carburised layers were 2·19 and 1·58 GPa.

Nanostructured alumina coatings have been deposited on SS 304 substrate using atmospheric plasma spray process. In the present work, nanostructured feed powder is obtained by manual granulation of nano-Al₂O₃ powder particles followed by exhaustive sieving, which is helpful in their proper consolidation. The tribological performance of the nano-alumina coatings, such as dry sliding, slurry erosion and cavitation erosion, has been investigated and compared with conventional coatings. The superior wear and erosion resistance of nanostructured coatings as compared to conventional coatings are due to its higher hardness and effective hindrance to crack propagation. Wear mechanism was explained based on their microstructure and worn surface morphologies. The microhardness and porosity of the two coatings were also experimentally investigated.

Nanostructured Cr/Co–Al coatings on Superni-718 superalloy substrate are deposited by direct current/radio frequency magnetron sputtering. The microstructure and hot corrosion behaviour of sputtered Cr/Co–Al coatings on Superni-718 superalloy have been investigated in molten salt of 40 wt-%Na₂SO₄+60 wt-%V₂O₅ at 900°C. The results showed that a dense scale formed on the coated samples during thermal cycling. The spinel phases of CoCr₂O₄, CoAl₂O₄ and NaAlO₂ are present in the scale of the corroded coating, resulting in an effective inhibition of O and S diffusion. The sputtered coatings showed better performance of hot corrosion due to its possession of β-CoAl phases (as Al reservoir) in the outer layer of the coating and Cr as an intermediate layer. The relevant corrosion mechanisms are discussed.

Sol–gel derived organic–inorganic hybrid coatings have been investigated as a potential replacement for the environmentally hazardous chromate conversion coatings for corrosion protection of aerospace aluminium alloy AA 2024. Sol–gel hybrid coatings were prepared using a sol mixture (1∶1 volume ratio) of 3-glycidoxypropyl trimethoxysilane and tetraethoxysilane and doped with cerium nitrate and deposited on AA 2024 by dip coating and spray coating techniques. The effect of surface pretreatment on the corrosion protection efficiency of the sol–gel hybrid coatings was studied. The corrosion resistance of the coatings in 3·5% sodium chloride solution was evaluated by potentiodynamic polarisation measurements, electrochemical impedance spectroscopy and salt spray tests. The results showed that a sol–gel coating system with a simple silanisation as surface pretreatment exhibited improved corrosion protection for AA 2024.

High velocity arc spraying and cold gas dynamic spraying were applied to prepare the aluminium coating, and electrochemical impedance spectroscopy measurement in combination with scanning electron microscopy analysis of corrosion surface and X-ray diffraction results of corrosion product were used to compare the corrosion behaviour of these two coatings. It was found that there was sparse and porous corrosion product covering on the surface of the arc sprayed aluminium coating through which the Cl⁻ can penetrate into the inner coating interface. In contrast, the surface of the cold sprayed aluminium coating after immersion was less porous and tended to seal the pores in the coating. This kind of structure can obstruct the penetration of corrosion medium into the inner coating surface and reduce the corrosion rate to a certain extent. In addition, the diffusion impedance O was introduced in the electrochemical impedance spectroscopy modelling for the purpose of getting good fitting results.

Aging of plasma surface modification of inkjet printing polyester fabric was investigated. Woven polyester fabrics were exposed to atmospheric pressure air plasma and then stored at the standard condition of 20°C and 65% relative humidity to perform the aging experiments. The surface morphology and chemical compositions of the control and treated samples were analysed by scanning electron microscopy and X-ray photoelectron spectroscopy. Wetting time was used to evaluate the surface hydrophilicity on each sample. The aging effect could be detected only in the surface chemical compositions, whereas the morphology remained stable for 45 days. Research on the antibleeding property and colour strength showed that the inkjet printing property of polyester fabrics decreased with increasing aging time until a plateau value was reached. The modified fabrics should be printed within 24 h after plasma treatment in order to get fresher colour and better antibleeding performance.