Heji Huang

Ultrafast thermal plasma physical vapor deposition of yttria-stabilized zirconia for novel thermal barrier coatings

This research aims to develop advanced thermal plasma spraying technology for the next-generation thermal barrier coatings (TBCs) with a high power hybrid plasma spraying system. By using thermal plasma physical vapor deposition (TP-PVD), various functional structured yttria-stabilized zirconia (YSZ) coatings were deposited. Parameters, such as powder feeding rate, hydrogen gas concentration, and total mass flow rate of the plasma gas, were optimized, and their influences on the evaporation of YSZ powder were investigated. Ultrafast deposition of a thick coating was achieved at a rate of over 150 μm/min. The deposited porous coating has a low thermal conductivity of 0.7W/mK and the dense coating with interlaced t′ domains possesses a high nanohardness of 27.85 GPa and a high reflectance. These characteristics show that the TP-PVD technique is a very valuable process for manufacturing novel TBCs.

Novel structured yttria-stabilized zirconia coatings fabricated by hybrid thermal plasma spraying

Abstract Yttria-stabilized ZrO2 powders with initial sizes of 5–22 mm were chsosen as feedstock for hybrid thermal plasma deposition. At 100 kW RF input power, the microstructures of the deposited coatings varied from mostly sprayed splats to physical-vapor-deposited nanostructures when the powder feeding rate was reduced from 4 to 1 g/min. At a powder feeding rate of 2 g/min, a peculiar layered coating consisting of both structures was deposited at a rate over 50 mm/min, which is promising for the fabrication of next-generation novel thermal barrier coatings.

Laminar/Turbulent Plasma Jets Generated at Reduced Pressure

Laminar DC plasma jets are attractive for precisely controlled plasma-material processing. The design of a novel nontransferred plasma torch enabled the switching between turbulent and laminar plasma flows by simply changing the plasma generation parameters. Images of the plasma flows generated at different conditions are presented.

High-power hybrid plasma spraying of large yttria-stabilized zirconia powder

To testify to the advantage of large ceramic powder spraying, numerical simulations and experimental studies on the behavior of large yttria-stabilized zirconia (YSZ) powder in a high-power hybrid plasma spraying process have been carried out. Numeric predictions and experimental results showed that, with the high radio frequency (RF) input power of 100 kW, the most refractory YSZ powder with particle sizes as large as 88 μm could be fully melted and well-flattened splats could be formed. A large degree of flattening (ξ) of 4.7 has been achieved. The improved adhesive strength between the large splat and the substrate was confirmed based on the measurement of the crack density inside of the splats. A thick YSZ coating >300 μm was successfully deposited on a large CoNiCrAlY-coated Inconel substrate (50×50×4 mm in size). The ultradense microstructure without clear boundaries between the splats and the clean and crack-free interface between the top-coat and the bond-coat also indicate the good adhesion. These results showed that highpower hybrid plasma spraying of large ceramic powder is a very promising process for deposition of highquality coatings, especially in the application of thermal barrier coatings (TBCs).

Instabilities in a non-transferred direct current plasma torch operated at reduced pressure

Using an oscilloscope, a high-speed video camera and a double-electrostatic probe system, the periodicity and amplitude of the fluctuations in arc voltage, jet luminance and ion saturation current of a plasma jet were monitored to investigate various sources of instabilities and their effects in a non-transferred dc plasma torch operated at reduced pressure. The results show that besides a 300 Hz main fluctuation inherited from the power supply, arc voltage fluctuation of 3–4 kHz with an amplitude less than 5% of the mean voltage was mainly affected by the total gas flow rate. The arc voltage fluctuation can affect the energy distribution of the plasma jet which is detectable by electrostatic probes and a high-speed video camera. The steadiness of energy transfer is also affected by the laminar or turbulent flow state of the plasma.

Arc Root Motion in an Argon-Hydrogen DC Plasma Torch

Arc root motion on the anode surface of a DC nontransferred plasma torch was observed. Adding hydrogen changes the arc root attachment from a diffused type to a constricted type, and the arc root of Ar-H2 plasma suddenly jumps from one spot to another irregularly. Images of the arc root motions taken by a high-speed video camera are presented.

Arcjet Thruster Operated With Different Propellants

As a simple and reliable propulsion system, arcjet thrusters have been used in multiple satellite missions. The performance of an arcjet thruster is controlled by the characteristics of the arc-discharge behavior and the arc-electrode interactions to a large extent. The nozzle of an arcjet thruster is where the input electric energy is converted into kinetic energy. The nozzle images of an arcjet thruster operated with different propellants are presented in this paper.

Energy fluctuations in a direct current plasma torch with inter-electrode inserts operated at reduced pressure

Direct current (dc) plasma torch with inter-electrode inserts has the merits of fixed arc length, relative high enthalpy and may show advantages in future plasma processes where stability and controllability are must-have. Energy fluctuations in the plasma may result from power supply ripple, arc length variation, and/or acoustic oscillation. Using an improved power supply with a flat waveform, the characteristics of an argon plasma energy instabilities under reduced pressure were studied by means of simultaneously monitoring the arc voltage and arc current spectrum. Dependence of the arc fluctuation behavior on the plasma generating parameters, such as the current intensity, the plasma gas flow rates and the vacuum chamber pressure were investigated and discussed. Results show that the plasma torch has a typical U-shaped voltage-ampere characteristic (VAC). The correlation between the VAC and the probability of energy distributions was studied. Through pressure measurements at the cathode cavity and the vacuum chamber, the existence of sonic flow in the inter-electrode insert channel was confirmed.

Supersonic Plasma Jets of Different Gases in Low-Pressure Environment

A low-power dc arc plasma generator with regeneratively cooled or natural-radiation-cooled anode is used to produce supersonic plasma jets of pure helium, argon, hydrogen, nitrogen, hydrogen-nitrogen mixture, and helium-nitrogen mixture in a vacuum chamber. Plasma plume images of different characteristics were observed, and are discussed briefly along with the measurement and analysis results.

Underexpanded Supersonic Plasma Jet Generated by a Small Arc-Heated Facility

As a flexible and efficient heat source to generate high enthalpy flows, a small arc-heated facility is unique in its ability to reproduce local aerodynamic heating environments for a long time period, to be used for validating the survivability of thermal protection materials used in hypersonic flights. Images of an underexpanded supersonic plasma jet generated by a small arc-heated facility and its interaction with some typical material samples are presented.