Synthesis of Thin Titania Coatings onto the Inner Surface of Quartz Tubes and Their Photoactivity in Decomposition of Methylene Blue and Rhodamine B

An evaporation-deposition coating method for coating the inner surface of long (>1 m) quartz tubes of small diameter has been studied by the introduction of two-phase (gas-liquid) flow with the gas core flowing in the middle and a thin liquid film of synthesis sol flowing near the hot tube wall. The operational window for the deposition of continuous titania coatings has been obtained. The temperature range for the deposition of continuous titania coatings is limited to 105–120 °C and the gas flow rate is limited to the range of 0.4‒1.0 L min−1. The liquid flow rate in the annular flow regime allows to control the coating thickness between 3 and 10 mm and the coating porosity between 10% and 20%. By increasing the liquid flow rate, the coating porosity can be substantially reduced. The coatings were characterized by X-ray diffraction, N2 chemisorption, thermogravimetric analysis, and scanning electron microscopy. The coatings were tested in the photocatalytic decomposition of methylene blue and rhodamine B under UV-light and their activity was similar to that of a commercial P25 titania catalyst.

Fatigue behaviour of Plasma Sprayed Titania and Chromia Coatings

Cr2O3 and TiO2 powders were deposited by atmospheric plasma spray (APS) on steel substrates. Microstructural analysis of the coatings showed typical lamellar structure with good coating quality. Fatigue strength was studied by using cyclic testing (measuring with an inhouse-built apparatus the strength of the coated systems under a wide range of impact cycles) and static loading tests (Vickers tests standards with 600N and 1500N) measuring the adhesion properties of the coatings. In low cycles (1x103) Titania coatings exhibited better strength, while at intermediate (4.5x105) and high (1x106) number of impact cycles, both Chromia and Titania coatings exhibited quite similar strength characteristics. At low impact force the thickness of the coatings plays critical role with better performance obtained by Chromia coatings. During static loading both coatings exhibited similar characteristics at the crater diameter but with larger crater depth for Titania. Chromia coatings exhibited higher strength resistance than Titania coatings with better mechanical properties and coating structure.

INFLUENCE OF CARBON NANOTUBES REINFORCEMENT ON CHARACTERISTICS OF THERMALLY SPRAYED CERAMIC COATINGS

The influence of reinforcement of carbon nanotubes (CNTs) on microstructural features and mechanical properties of thermally sprayed Al2O3–3[Formula: see text]wt.%TiO2 and WC–20[Formula: see text]wt.%Co coatings was investigated. Alumina–Titania coatings were deposited by Air Plasma Spraying (APS) and Tungsten Carbide–Cobalt coatings were deposited by High-Velocity Oxy-Fuel (HVOF) spraying process. The coatings obtained with reinforcement of CNTs were characterized to interpret the microstructural changes and also to evaluate the variation in their mechanical properties. The percentage composition of CNTs in both APS and HVOF coatings systems were varied in the order of 2, 4, and 6[Formula: see text]wt.%. It has been found that homogenous dispersion of carbon nanotubes in the coating systems results in increased microhardness and reduced surface roughness. Also, the microstructural features of the coating systems clearly showed that the coatings are denser with fewer pores due to the presence of CNTs.

ROLE OF FRICTION ENERGY AND TRIBO-REDUCTION IN ABRASIVE WEAR OF ALUMINA–TITANIA COATINGS

This paper addresses friction energy-induced reduction of TiO2 during abrasive wear and friction of alumina–titania coatings with different TiO2 content. Alumina–13% titania and alumina–40% titania coatings were deposited by atmospheric plasma spray technique. An improvement in fracture toughness and better densification of coatings was observed with an increase in titania content. Alumina–40% titania coating showed lower friction coefficient and higher abrasive wear resistance than alumina–13% titania coating. An increase in friction energy was associated with enhanced reduction of TiO2 to Ti2O3 and an increase in the extent of gamma alumina to alpha alumina phase conversion. These observations along with structural and mechanical properties of coatings could explain differences in tribological response of these coatings. Friction energy parameter (FEP) was used to identify different regimes of wear and degradation mode of coatings.