K.P. Sreekumar

Microstructure, adhesion, microhardness, abrasive wear resistance and electrical resistivity of the plasma sprayed alumina and alumina-titania coatings

Abstract Al 2 O 3 and its mixtures with TiO 2 powders were prepared by a fused and crushed method. Prepared powders were sprayed at 10, 12 and 16 kW power in atmospheric plasma spraying. Microstructure of the coatings was analysed by SEM. Alumina coatings showed better quality than composite coatings. Different types of pores were observed in alumina–titania (AT4016) coating. Pore forming mechanisms are reported. Adhesion strength, microhardness and abrasive wear resistance of alumina and alumina–titania coatings were measured and results are explained. The influence of the power to the plasma torch, powder particle size and its distribution and feed rate of the powder on the abovesaid properties of the coatings are explained. Electrical resistivity was measured in composite coatings. The electrical conductivity increases with increased titania content in the coating.

Co-spraying of alumina–titania: correlation of coating composition and properties with particle behaviour in the plasma jet

Abstract Plasma-sprayed coatings of alumina–titania (60:40 by weight) have been prepared under different operating parameters. The coatings have been characterised with respect to chemical composition, phase composition, wear resistance and microhardness. It was observed that the chemical composition of coatings prepared at low input power is significantly different from that of the feedstock powder. The coating composition progressively approaches that of the feedstock powder at higher power input. Numerical process simulation has been carried out using a one-dimensional model to illustrate the importance of plasma–particle interaction during co-spraying of alumina–titania powder blend. Experimental results compare very well with the results predicted by the model. At lower power levels, titania particles are melted, whereas alumina particles remain unmelted. This is expected to give a coating composition quite different from that of the feedstock powder. With an increase in the plasma power, alumina particles are also heated to their melting point, and by choosing the proper combination of power and particle size, the spray process can be optimised to obtain a targeted coating composition.

Three-Dimensional Numerical Modeling of an Ar-N2 Plasma Arc Inside a Non-Transferred Torch

A three-dimensional numerical model is developed to study the behaviour of an argon-nitrogen plasma arc inside a non-transferred torch. In this model, both the entire cathode and anode nozzle are considered to simulate the plasma arc. The argon-nitrogen plasma arc is simulated for different arc currents and gas flow rates of argon. Various combinations of arc core radius and arc length, which correspond to a given torch power, are predicted. A most feasible combination of the same, which corresponds to an actual physical situation of the arc inside the torch, is identified using the thermodynamic principle of minimum entropy production for a particular torch power. The effect of the arc current and gas flow rate on the plasma arc characteristics and torch efficiency is explained. The effect of the nitrogen content in the plasma gas on the torch power and efficiency is clearly detected. Predicted torch efficiencies are comparable to the measured ones and the effect of the arc current and gas flow rate on predicted and measured efficiencies is almost similar. The efficiency of the torch, cathode and anode losses and core temperature and velocity at the nozzle exit are reported for five different cases.

Simulation studies to optimize the process of plasma spray deposition of yttrium oxide

Simulation studies on the thermal behaviour of yttrium oxide particles in a thermal plasma jet was carried out with the objective of controlling and optimization of the plasma spray process. The in-flight behaviour of yttrium oxide particles in the plasma jet was studied by solving the heat transfer and momentum transfer equations using the velocity and temperature distribution in the plasma jet obtained from a two-dimensional model. In particular, the effect of particle size, thermal power of the torch and the torch operating parameters like gas flow rates were considered to calculate the heat transfer and momentum transfer to the particle. Results of simulation studies agree quite well with the experimental results on variation of deposition efficiency with power and particle size. The complete description of the model with the results obtained for the typical operating parameters of our plasma spray torch is presented in the paper.

Effect of Spraying Parameters on Deposition Efficiency and Wear Behavior of Plasma Sprayed Alumina-Titania Composite Coatings

The effects of parameters, in the process of plasma-sprayed ceramic coating, upon the deposition efficiency of alumina-13 wt.% titania composite coatings are reported. The coatings were prepared by the atmospheric plasma spray process. The plasma torch input power, flow rates of primary, secondary and carrier gas, powder feed rate and spraying distance were considered as variables. The results show that the variations in all the selected spraying parameters strongly affect the deposition efficiency. The micro-hardness, as well as erosive and sliding wear rates of the coating are also affected by these parameters. Especially the input power strongly affects the phase and microstructure of the coatings.

Effect of oxygen pick-up on the properties of plasma sprayed titanium diboride coatings

Abstract Plasma sprayed titanium diboride (TiB2) coatings (50–200 μm thick) have been prepared on alumina substrates. X-ray diffraction, oxygen analysis and electrical conductivity measurements on the sprayed specimens indicate substantial oxygen pick-up during the spray process, resulting in the formation of titanium oxide and boron trioxide, which is subsequently converted to H3BO3 by absorbing moisture. The above reactions adversely effect the electrical conductivity of the coatings as evidenced by conductivity values which are found to be 100–1000 times less than that of sintered TiB2. The use of argon as a shield gas during spraying has been found to minimise this oxidation problem. The effects of carbon addition have also been studied, as well as the use of hydrogen as a secondary plasma gas in controlling the oxidation problem. The addition of carbon did not have much influence on the oxygen pick up. Although an argon-hydrogen (Ar-H2) plasma substantially brought down the oxygen-content in coatings, the oxidation problem could not be avoided totally. In order to completely overcome this problem, the spray process will have to be carried out in a controlled environment chamber.

In-Flight Formation of Nano-Crystalline Titanium Dioxide Powder in a Plasma Jet and Its Characterization

Nanocrystalline titanium dioxide powder was synthesized by in-flight oxidation of titanium dihydride (TiH2) powder in a thermal plasma jet. TiH2 powder was injected into the thermal plasma jet and allowed to react with oxygen injected downstream the jet. Characterization of the powder by various analytical tools indicated that the powder consisted of nano-sized titanium dioxide particles consisting predominantly of the anatase phase. It is suggested that the thermo-chemistry of the oxidation process contributes significantly to the formation of nano-sized titania. The large energy released during the oxidation process dissociates the TiO2 particles into TiO(g) and titanium vapour, which recombine downstream with oxygen and form nano particles of TiO2.

Studies on spray efficiency and chemical analysis and density of the plasma sprayed A12O3 and its mixtures with TiO2 coatings

Abstract Al2O3 and its mixture with 3% TiO2 powder has been prepared by the fused and crushed method. Al2O3 + 13% TiO2 and A12O3 +40% TiO2 powders are mixtures of Al2O3 + 3% TiO2 and 10% and 37% of blend TiO2 respectively. The size distributions of the powders are analysed and the median diameters of the powders are measured. Coatings were prepared using plasma spraying at 10, 12 and 16 KW power inputs from all powders. The spray efficiencies of the powders were measured and are discussed. Efficiency is found to inerease with inereases in power input and % of TiO2 added in the powder. Chemical composition of the powder and coatings are measured and results are discussed. The composition of the composite coatings was found to be different than that in the powder. It is found to match almost with the powder composition at higher power. The bulk density of the coatings prepared at different power inputs are studied. In Al2O3 the coating density inereases with inerease in power. In composite coatings it decre...

Modelling of non- transferred argon-nitrogen plasma arc and plasma jet

One of the challenging problems in the plasma spray technique is reproducibility of the coating quality. This problem is mainly associated with arc fluctuations, which affect the plasma jet temperature and velocity inside the plasma torch. In this study, three- dimensional numerical models are developed to study the behaviour of Ar-N2 plasma arc inside a non-transferred torch and plasma jet. Ar-N2 plasma arc is simulated for given arc current and gas flow rate. Different arc sizes, which correspond to given torch power, are predicted. A most feasible arc size, which corresponds to an actual physical situation of the arc inside the torch, is identified using thermodynamic principle of minimum entropy production for particular torch power. Predicted torch efficiency is comparable with measured one. Cathode and anode losses are reported. Predicted temperature and velocity profiles at the nozzle exit are used to simulate the plasma jet. Since plasma gas is mixed with cold air, plasma jet is diffused in the radial direction. Three-dimensional effect on plasma jet temperature and velocity is diminishes along the axial direction.