In this study, Ti-6Al-4V coatings were deposited by low-pressure plasma spraying (LPPS) using different powder sizes and spray currents. The coatings were also heat treated at different temperatures, after which their microstructure and properties were assessed. The results show that finer powders are more conducive to the preparation of dense coatings and that porosity is reduced by increasing plasma current. As for the effects of heat treating temperature, at 870 °C, the lamellar structure of the coating disappeared and was found to be fully equiaxed with a grain size of 5-10 µm at 1100 °C. Hardness also increased, becoming significantly higher than that of forged TC4 alloy following treatment at 1100 °C.

Yttria stabilized zirconia with a composition of ZrO 2 -8wt%Y 2 O 2 typically serves as the topcoat in thermal barrier coating systems. It has been reported, however, that YSZ with lower yttria content is more resistant to thermal shock and the effects of high-temperature sintering. To investigate these reports, nano-agglomerated 5YSZ and 8YSZ powders were deposited on FeCrAl substrates by atmospheric plasma spraying and the coatings were heat treated at 1400 °C for 1, 5, and 20 h. The nanostructure content in the 5YSZ samples was found to be about 20% higher, the microhardness 11% lower, and the size of unmelted particles about 27% smaller, which shows that bimodal structured 5YSZ has higher sintering resistance than traditionally used 8YSZ.

A novel fully nano-equiaxed-structured thermal insulation Y 2 O 3 -stabilised ZrO 2 (YSZ) coating was first successfully deposited using very-low-pressure plasma spray (VLPPS) and the agglomerates feedstock from nanometer-sized powders. The microstructure of fully nano-equiaxed-structured coating is different from that of a “bimodal microstructure”, which is currently the dominant nanostructured coating. This desirable structure in the new TBCs appears to be responsible for their improved thermal cycling lifetimes and reduced thermal conductivity relative to those of conventional plasma-sprayed TBCs with lamellar structures or bimodal structures. Preliminary results from experiments aimed at understanding the VLPPS deposition mechanisms suggest that the nanometre-scale equiaxed in the coating are formed from original nano-particles, they underwent melted in the low pressure plasma plume and solidification onto the substrate. The VLPPS method, which offers some unique advantages over the conventional plasma-spray process, is generic in nature and can be potentially used to deposit a wide variety of ceramic coatings for diverse applications.

The equiaxed microstructure of 316L stainless steel coating was successfully deposited by low pressure plasma spraying (LPPS), which was different from the lamellar microstructure prepared by other thermal spraying technologies. In this article, the effect of substrate temperature during deposition process and post annealing treatment on the lamellar – equiaxed microstructural transition were investigated. The results indicated that the homogeneous equiaxed grains without lamellar boundaries coatings were observed when the deposition temperature was about 900 °C. Completely lamellar microstructural coatings were deposited at the substrate temperature of about 300 °C, and the lamellar microstructure can transform to equiaxed microstructure after annealing treatment. The hardness of equiaxed coating was lower than lamellar coating.

In this paper, the development of surface oxide scale and the evolvement of spallation mechanism of Fe-21Cr-5.6Al super alloy were investigated at 1200°C and 1300°C. The oxidation kinetic curves were obtained by isothermally measuring the weight gain of alloy oxidized with various time durations. The morphologies of oxide scale and grain structures were observed by SEM/EDX, and the phase structure was analyzed by XRD. The results show that the oxidation processes follow the parabolic law and the oxidation rate is higher at 1300°C than 1200°C. Though the FeCrAl alloy shows capabilities against oxidation even at a high temperature of 1300°C, the oxidation behavior and mechanism are distinct from those at moderate temperatures (<1000°C). Different morphologies and phase structure were found in oxide scales generated at different temperatures within the same time duration. Typical buckling was observed in the super alloy when it was subjected to 1200°C. Equiaixed grains with multiple voids were found near the alloy surface. At 1300°C, a flat and thicker oxide layer was formed. The grains were stretched vertically against the alloy and presented as coarse and compact near the interface. The vertically stretching of grain was triggered by fast element transportation inside the alloy. The differences in grain morphologies among the different test temperatures demonstrated that although the super alloy followed parabolic law at both test temperatures, the oxidation processes were different due to the evolvement of grain morphologies and oxide scale structures caused by exposure to high temperature.

The effects of the plasma gas composition (Ar-N 2 , Ar-H 2 ), arc current and voltage on the temperature and velocity of a low power (5 kW) plasma torch in the arc field free region have been investigated using an enthalpy probe. Coatings of Al 2 O 3 -13TiO 2 were deposited under different conditions. The results show that in the Ar-N 2 plasma, the enthalpy, temperature and velocity change little with arc current and voltage when regulating the nitrogen proportion in the plasma gas, and the hardness of the resulting coatings is 800-900 HV. For Ar-H 2 plasma, however, increasing the H 2 content in the gas mixture remarkably enhanced the velocity and heat transfer of the plasma jet with the result that the coatings showed the high hardness up to 1200 HV.

A mathematical model was used to describe the fluid flow, electromagnetic and heat flow of the plasma jet in the non-transferred model. A computer program has been developed to simulate the fluid properties of the low energy and high efficiency plasma jet. The numerical results approximate the measurements by using the enthalpy probe.

An innovative plasma spraying technology has been developed with low power consumption and high deposition efficiency. The plasma torch was operated at arc currents from 50 to 150A and voltages from 50 to 80V. The powder particles were injected into the arc region from the inside of nozzle entry by the carrier gas that then transferring through the plasma flame. The ceramic coatings of Cr 2 O 3 and ZrO 2 - Y 2 O 3 were created at power consumption less than 10kW and powder feed rate of 30-50g/min. The deposition efficiency could reach 70%, higher than that of 40kW APS with an external powder feed. In addition, the enthalpy, temperature and velocity of low power plasma jet were also measured by an enthalpy probe.