Plasma spray coatings

The plasma spray process is widely used to produce thick coatings by the successive pilling of particles deposited in a molten or semi-molten state on a prepared substrate. However, this process includes time-dependent phenomena that affect the reliability of the process and reproducibility of coating. These phenomena are principally linked to the continuous movement of the electric arc root on the anode wall in the plasma gun. Such a movement leads to arc length variations resulting in fluctuations in arc voltage, enthalpy input to the flow and instabilities in the plasma jet. This paper presents an attempt to develop a time-dependent and 3-D model of the plasma spray process that can provide a useful insight in the time-evolution of the performance of the process. The effect of the transient behaviour of the arc on the gas flow is modelled with a time dependant heat source located inside the nozzle and evolving with the arc voltage. The first stage of the study consisted in the validation of the flow model thanks to the comparison of steady-state computed results with experimental data. The second dealt the time-dependant simulation of the flow.

The clinical use of plasma-sprayed hydroxyapatite (HA) coatings on metal implants has aroused as many controversies as interests over the last decade. Although faster and stronger fixation and more bone growth have been revealed, the performance of HA-coated implants has been doubted. This article will initially address the fundamentals of the material selection, design, and processing of the HA coating and show how the coating microstructure and properties can be a good predictor of the expected behavior in the body. Further discussion will clarify the major concerns with the clinical use of HA coatings and introduce a comprehensive review concerning the outcomes experienced with respect to clinical practice over the past 5 years. A reflection on the results indicates that HA coatings can promote earlier and stronger fixation but exhibit a durability that can be related to the coating quality. Specific relationships between coating quality and clinical performance are being established as characterization methods disclose more information about the coating. © 2001 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 58: 570–592, 2001

The most advanced thermal barrier coating (TBC) systems for aircraft engine and power generation hot section components consist of electron beam physical vapor deposition (EBPVD) applied yttria-stabilized zirconia and platinum modified diffusion aluminide bond coating. Thermally sprayed ceramic and MCrAlY bond coatings, however, are still used extensively for combustors and power generation blades and vanes. This article highlights the key features of plasma spray and HVOF, diffusion aluminizing, and EBPVD coating processes. The coating characteristics of thermally sprayed MCrAlY bond coat as well as low density and dense vertically cracked (DVC) Zircoat TBC are described. Essential features of a typical EBPVD TBC coating system, consisting of a diffusion aluminide and a columnar TBC, are also presented. The major coating cost elements such as material, equipment and processing are explained for the different technologies, with a performance and cost comparison given for selected examples.

The purpose of this research work was to develop a nozzle capable of depositing dense CoNiCrAlY coatings via cold gas dynamic spray (CGDS) as well as compare the oxidation performance of bond coats manufactured by CGDS, high-velocity oxy-fuel (HVOF) and air plasma spray (APS) at temperatures of 1000°C and 1100°C. The work was divided in two sections, the design and manufacturing of a CGDS nozzle with an optimal profile for the deposition of CoNiCrAlY powders and the comparison of the oxidation performance of CoNiCrAlY bond coats. Throughout this work, it was shown that the quality of coatings deposited via CGDS can be increased by the use of a nozzle of optimal profile and that early formation of protective α-Al2O3 due to an oxidation temperature of 1100°C as opposed to 1000°C is beneficial to the overall oxidation performance of CoNiCrAlY coatings.

To protect the superalloy blades in industrial gas turbine engines from failure Atmospheric plasma-sprayed YSZ (yttria-stabilized zirconia) thermal barrier coatings (TBCs) are widely used. The spalling of YSZ coating is the main failure of TBCs. This failure usually occurs within the YSZ coating near the YSZ/bond coat interface. In the present feasibility study, a novel durable multilayer TBC was fabricated using atmospheric plasma spray (APS) on Ni718 superalloy substrate consisting of a NiCrAlY-20%YSZ-MoSi 2 top coat. The developed system introduces a protective top layer of MoSi 2 for preventing diffusion of oxygen, oxidation of the bond coating, provides thermal insulation and protection against corrosion and high temperature erosion. The results of hot gas flow exposure test (H 2 +O 2) to the triple layered coating, a kinetic heating study, showed that a decreased temperature 400 o C for heating of 145 second achieved without any visible damage or delamination in such drastic combustion condition exposure even after 27 cycles of such repeated exposure.

A key aspect of the operation of conventional non-transferred direct current (dc) plasma torches is the random motion of the arc inside the nozzle. Various plasma gun designs have been developed to limit the arc fluctuations without increasing the heat load to the anode wall that results in surface erosion and anode wear. However, construction of these plasma torches is highly complex, while the conventional dc plasma torch consists of a small number of elements and is simple to manufacture and maintain. A better understanding of the behavior of the arc-anode attachment and how it depends on operating conditions may help in the design and operation of conventional plasma torches so that the fluctuation of the time-voltage, and therefore the time-enthalpy variation, is as low as possible with a fluctuation frequency adapted to the time characteristic of the powder particles in the plasma jet. This study deals with a three-dimensional (3D) time-dependent modeling of the arc and plasma generation in such a torch operating under the so-called “restrike” mode. The latter is characterized by rather large voltage fluctuations, corresponding to a broad range of conditions used in the manufacturing of plasma coatings. The mathematical model is based on the simultaneous solution of the conservation equations of mass, momentum, energy, electric current, and electromagnetic equations. These make it possible to predict the effect of operating parameters of the plasma torch on the motion of the anode root attachment over the anode surface and the time-evolution of arc voltage and flow fields in the nozzle.

Structural engineering components in plant made of mild steel and copper are affected by severe erosive wear due to presence of particulates in its surrounding. To protect these costly structural elements, we have taken atmospheric plasma spray process. This process is better for mild steel and copper due to several advance properties such as high temperature stability, coating efficiency, wear and corrosion protection. Coating by this process can be applied onto all suitable base materials with the widest variety of powders. In this investigation we have taken industrial waste like fly-ash, quartz and ilmenite powder as coating composite material and deposited on Mild Steel and Copper substrates with different weight proportions and different power levels of the plasma torch. Erosion type and their mechanism extensively investigated using scanning electron microscopy. Erosion properties have been studied using Air Jet erosion test Reg. with Silica erodent. Two significant parameters i.e. Erosion rate and Avg. microhardness have been measured by varying input i.e. power lever, velocity of erodent and time of erosion exposure. It is observed that, maximum erosion occurs at normal impact angle that indicates about brittle erosion condition . Different graphs are being plotted between mass loss-rate versus time period/impact pressure/impact angle, which gives good correlation with surface features observed. Based on these observations artificial neural network (ANN) models are developed to predict the result in various parameter setup.

Zirconia-based thermal barrier coatings (TBCs) of nominal chemical composition 8 wt.% Y2O3–ZrO2 and 25.5 wt.% CeO2–2.5 Y2O3–ZrO2 were prepared by atmospheric plasma spray and low-pressure plasma spray by selecting different deposition parameters. The surface chemical composition has been investigated by X-ray photoelectron spectroscopy in order to study the variation of surface chemical composition induced by the plasma-spraying process as a function of deposition parameters. The results reveal the occurrence of chemical–physical reactions such as stabilizing oxide depletion and enrichment, reduction to lower valence states, impurity segregation phenomena and the formation of new species. The chemical information was confirmed by differential thermal analysis measurements, which indicates that chemical aspects in plasma spraying are relevant and should be considered in designing reliable TBCs for maximum performance in aerospace applications.

The nanostructured and two conventional alumina–titania coatings were deposited by atmospheric plasma spray. The presented studies show that nanoparticles are the predominant component of nanostructured Al2O3–13TiO2 powder grains. The microstructure of coating consisted of two distinct regions: fully melted and unmelted or partially melted nanostructured areas, which were comprised of components of a starting powder. Microhardness and modulus of these coatings were significantly lower than the values obtained for nanostructured coating. It was found that the plasma-sprayed nanostructured Al2O3–13TiO2 coating possessed better tribological properties than those of conventional alumina–titania coatings. The lower coefficient of friction and wear of the nanostructured Al2O3–13TiO2 coating is attributed to the bimodal microstructure and the enhanced mechanical properties in comparison to conventional alumina–titania coatings.