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Preparation Method for Thermal Barrier Coatings
Release time:
2020-08-30 21:59
Source:
Thermal Barrier Coating Preparation Methods
Plasma spraying
Plasma spraying involves feeding metal or ceramic powders into a high-temperature plasma flame, where the plasma jet heats the spray material to a molten or highly plastic state. Under the guidance of the high-speed plasma jet, these particles rapidly impact the surface of the workpiece. During the spraying process, the spray material first undergoes heating, reaching either a fully molten or partially molten state; then, it enters a flight phase, propelled forward by the gas stream; finally, it strikes the substrate surface with a certain kinetic energy, producing intense collisions that flatten the particles into a thin, planar layer and cause them to solidify instantaneously. The resulting spray coating consists of an intricate layered microstructure formed by countless deformed particles interlaced and stacked in a wavy pattern. Between the particles, there are inevitably some voids or pores, with porosity typically ranging from 4% to 20%. The coating also contains oxides and inclusions. By employing a high-temperature plasma arc heat source, supersonic velocities, and low-pressure or protective-atmosphere spraying techniques, these defects—such as voids and pores—can be significantly reduced.
Since the coating has a layered structure, its performance exhibits certain directional characteristics. The bonding between the coating and the substrate surface is generally considered to occur in two ways: mechanical bonding, in which flattened particles formed by impact follow the irregularities on the substrate surface and interlock with its uneven features, thereby achieving mechanical anchoring; and metallurgical bonding, which occurs when diffusion and alloying take place at the interface between the coating and the substrate, resulting in the formation of intermetallic compounds or solid-solution phases at the bonding interface. Among these bonding mechanisms, plasma-sprayed coatings primarily rely on mechanical bonding.
The performance of a coating is closely related to the quality of the spray powder and the spraying process itself. The treatment of the powder used for spraying is therefore highly important. The spraying process also significantly influences coating performance; if the power during spraying is too high, the coating becomes overly dense, or the substrate temperature is improperly controlled, residual stresses may develop, leading to coating delamination and failure.
Electron Beam Physical Vapor Deposition (EB-PVD)
The EB-PVD thermal barrier coatings, which have emerged in recent years, are fabricated by heating and vaporizing ceramic sources using a high-energy electron beam. The ceramic vapor is deposited onto the substrate atom by atom. In the preparation of gradient thermal barrier coatings, a continuous transition in both structure and composition is achieved between the metallic bond coat and the ceramic layer. Following high-temperature post-processing, diffusion occurs between the bond coat and the ceramic layer, effectively eliminating the interfacial boundary. The coating microstructure consists of columnar crystals oriented perpendicular to the substrate surface. The metallurgical bonding between the columns and the substrate ensures excellent stability, and at elevated temperatures, the columnar microstructure exhibits outstanding strain tolerance, significantly enhancing the coating’s resistance to thermal fatigue. In thermal cycling tests, coating failure is primarily attributed to cracking within the Al2O3 layer. Moreover, the coating surface is smooth and requires no further machining; its process parameters are easy to control, and the coating is repairable—all of which represent significant advantages over plasma-sprayed thermal barrier coatings. However, drawbacks such as uncontrollable coating thickness, complex surface cleaning procedures, sophisticated and expensive equipment, relatively low deposition rates, and cumbersome processing procedures urgently call for further research and improvement.
Schematic diagram of the EB-PVD principle
EB-PVD columnar grain structure
Liquid Injection Plasma Thermal Spraying
Liquid-injection plasma spraying is a promising coating preparation method that has emerged in recent years. Although there are virtually no domestic reports on this technique, some exploratory studies have been conducted abroad. The principle behind using liquid-injection plasma spraying to produce thermal barrier coatings involves drawing a zirconium salt solution through a motor-driven pump. Under the action of a carrier gas, the solution is atomized by a nozzle and then introduced into the plasma. In the hot plasma environment, physicochemical reactions occur, leading to the deposition of the coating onto the metallic substrate. Conventional thermal barrier coatings prepared by powder injection can typically withstand around 400 thermal cycles, whereas thermal barrier coatings produced by electron beam physical vapor deposition (EB-PVD) can endure approximately 780 cycles. By contrast, the new thermal barrier coatings prepared via liquid-injection plasma spraying can withstand an average of 1,018 thermal cycles, demonstrating significantly improved thermal cycling performance. The phase composition of the coating is predominantly tetragonal, a non-transformative phase, and no phase transformation occurs during thermal cycling at 1,121°C. The width of cracks tends to increase with the number of thermal cycles. Initially, the hardness of the coating shows a slight increase during thermal cycling. Moreover, the columnar crystal structure of the coating remains intact throughout the thermal cycling process. The penetration depth of the liquid into the plasma spray nozzle has a significant impact on the coating’s deposition efficiency. Finally, coating failure primarily occurs within the ceramic top layer, near the interface between the ceramic top layer and the bond coat.
Overall, thermal barrier coatings prepared by plasma spraying using solution injection exhibit the following characteristics: (1) A unique microstructure: The coating features grain sizes ranging from 10 to 30 nm, uniform nanoporous and microporous structures, longitudinal microcracks, and the absence of layered particles or lamellar grain boundaries; (2) The growth of nanocrystals is effectively inhibited; (3) The coating demonstrates excellent resistance to thermal shock.
Preparation of Ceramic Coatings by the Hot-Press Filtration Method Using Sol-Gel Composite Slurry
By using the sol-gel composite slurry hot-pressing filtration method to prepare an Al2O3-ZrO2-Y2O3 composite coating featuring YPSZ particles embedded in an Al2O3-Y2O3 spatial network membrane structure, it is possible to combine the advantages of both Al2O3-Y2O3 and ZrO2-Y2O3 coatings, thereby achieving superior overall performance.
The PYSZ coating prepared by the hot-pressing filtration method features a nano/micro/microporous composite structure, which can effectively reduce phonon thermal conduction and convective heat transfer, thereby endowing the coating with superior thermal barrier performance. The thermal barrier effectiveness of the coating increases as the sol content in the slurry rises.
In the Al2O3-ZrO2-Y2O3 composite coating, the Al2O3-Y2O3 network film can block the transport of oxygen ions. The embedded YPSZ phase helps to match the thermal expansion coefficients between the coating and the substrate. Meanwhile, the nano/micro/microporous composite structure of the coating facilitates stress relaxation. Consequently, the Al2O3-ZrO2-Y2O3 composite coating exhibits excellent resistance to high-temperature oxidation and resistance to oxidant-induced spalling.
Materials - Equipment - Processes - Solutions
We have accumulated extensive experience in coating applications, and we are now replicating these successful case studies. We’ll guide you through the entire coating manufacturing transformation process, ensuring: rapid production start-up; a reliable supply solution that covers everything—from materials and equipment to processes; coating trials conducted either at your site or at our technology center; and consistently high coating quality and efficiency. Start collaborating with us today and reap success tomorrow!
Relevant Information
The complete set of plasma-sprayed equipment has been shipped to the customer’s site in Thailand.
Core spraying equipment: SX-80 plasma spray main unit, with a 15,000℃ flame suitable for multiple materials including zirconia and silicon carbide; powder feeding accuracy of ±1%, and coating thickness tolerance ≤ ±5 μm. Environmental protection support system: Customized soundproof booth + pulse dust collector—effectively addressing spray noise pollution while achieving highly efficient dust recovery, compliant with Thai environmental standards. Automated equipment: 6-axis industrial robot + intelligent spray cart, enabling precise spraying of complex workpieces and automated loading/unloading, boosting production efficiency by 40%.
On-site engineers provide hands-on training, focusing on the core operations of plasma spraying equipment such as the SX-80. Sanxin has customized a complete solution for its customers, including an automated spraying robotic arm, a soundproof booth, and a dust removal system—ensuring both high spraying efficiency and compliance with environmental standards.
Guangzhou Sanxin Thermal Spraying Turntable Shipment
Guangzhou Sanxin Company manufactures single-station and multi-station thermal spray turntables, which can be paired with plasma equipment, supersonic equipment, and arc equipment. We offer rapid, customized production tailored to customer requirements and product dimensions. A thermal spray turntable is a high-precision rotary worktable specifically designed for thermal spraying processes—including flame spraying, plasma spraying, and arc spraying. By providing controllable rotational speed and exceptional stability, it ensures uniform coating thickness on the workpiece surface, thereby enhancing spraying efficiency and quality.
SX-80 plasma equipment is being used at a customer site in Russia.
SX-80 plasma equipment is being used at a customer site in Russia.
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