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Research Status and Prospects of Plasma-Sprayed Alumina Ceramic Coatings
Release time:
2020-08-11 21:33
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Research Status and Prospects of Plasma-Sprayed Alumina Ceramic Coatings
1. Research on Plasma-Sprayed Alumina Coatings
Alumina ceramic coatings have roughly gone through several stages, including alumina coatings, alumina-titanium oxide coatings, and nano-alumina coatings. The powder particle size has been refined from the micrometer level to the nanometer level, and the coating composition has evolved from single-component systems to composite materials. Moreover, the coating microstructure has transitioned from single-layer structures to multi-layer or gradient-variation structures. The preparation of structural composite coatings and functionally graded coatings using plasma spraying of alumina is one of the current hot research areas both domestically and internationally, focusing on the microstructural characteristics as well as the wear resistance, corrosion resistance, and high-temperature oxidation resistance of ceramic coatings.
1.1 Microstructure and Properties of Conventional Alumina Coatings
Early-stage research indicates that the alumina ceramic coatings produced by plasma spraying exhibit a lamellar microstructure with a small number of pores, microcracks, and impurities. The typical crystal structure of alumina is the stable phase α-Al2O3; however, after plasma spraying, the proportion of α-Al2O3 in the coating decreases significantly, with the dominant phase becoming the metastable γ-Al2O3. Alumina coatings can be used as low-stress abrasive wear-resistant coatings at room temperature, as well as coatings resistant to hard-surface wear, corrosion by various chemical media and chemical gases, cavitation erosion, and impingement erosion. They are also suitable for high-temperature applications, including resistance to gas-phase cavitation, thermal barrier coatings, high-temperature wearable coatings, and high-temperature emissive coatings.
Alumina ceramic materials have inherent weaknesses, such as brittleness, sensitivity to stress concentrations and cracks, and poor thermal shock resistance. They also differ significantly from metallic materials in terms of thermophysical properties (e.g., thermal expansion coefficient, elastic modulus, thermal conductivity). Moreover, conventional plasma coatings themselves exhibit low bonding strength and high porosity; under conditions of large temperature gradients, these coatings tend to crack or even peel off easily. To address these issues, gradient coatings have been designed. A gradient coating refers to a composite structure in which the material composition, microstructure, density, and functional properties gradually vary from the substrate to the coating surface. Alumina gradient coatings feature no abrupt changes in microstructure or macroscopic interfacial boundaries between layers. Instead, the coating’s microstructure displays both macroscopic heterogeneity and microscopic continuity. The graded composition of the coating effectively mitigates thermal stresses arising from differences in thermophysical properties between the constituent materials. Compared with conventional two-layer alumina ceramic coatings, alumina gradient coatings demonstrate improved bonding strength, wear resistance, and thermal shock resistance, while their porosity is reduced.
1.2 Microstructure and Properties of Alumina-TiO2 Coatings
Because TiO2 has a lower melting point and better wettability than Al2O3, TiO2 ceramic coatings exhibit very low porosity, excellent wear resistance, resistance to chemical reactions, high toughness, and ease of machining. They can be ground to achieve exceptionally high surface finish and are resistant to corrosion by most acids, salts, and solvents. As a result, TiO2 coatings serve as crucial protective layers against both corrosion and wear, making them particularly well-suited for spraying high-wear-resistant coatings onto titanium and titanium alloys, as well as aluminum and magnesium alloys. Precisely because TiO2 possesses these advantageous properties, Al2O3-TiO2 coatings demonstrate improved performance compared to single Al2O3 coatings. Currently, research is focused on ceramic coatings composed of Al2O3 with 3% to 50 wt% TiO2—especially the Al2O3-13 wt% TiO2 coating (hereafter referred to as AT13)—which exhibits outstanding combined properties such as wear resistance, corrosion resistance, and electrical insulation below 540 ℃.
Literature reports that Al2O3-TiO2 coatings are prepared by plasma spraying. The ceramic coating primarily consists of rutile-type TiO2, anatase-type TiO2, Magneli phase, and γ-Al2O3, with minor amounts of α-Al2O3 as well as microcrystalline or amorphous phases. Compared to Al2O3 coatings, the addition of TiO2 to the AT13 coating reduces porosity in the ceramic layer, making the coating more dense.
Compared to Al2O3 coatings, AT13 coatings have lower hardness but exhibit less variability in hardness distribution, resulting in better coating uniformity. Under the same friction and wear test conditions, AT13 coatings demonstrate superior wear resistance compared to Al2O3 coatings. Gradient coatings prepared by spraying exhibit better thermal shock resistance than non-gradient coatings; the gradient distribution of coating composition helps relieve thermal stresses and enhances the coating's resistance to thermal shock failure.
1.3 Microstructure and Properties of Nano-Aluminum Oxide Coatings
Traditional ceramic materials suffer from drawbacks such as high brittleness and poor toughness, making them highly susceptible to cracking and brittle failure when subjected to impact by high-speed particles. Nanocrystallization of ceramics is one effective approach to addressing the brittleness issues inherent in conventional ceramics. Nanoceramic materials exhibit outstanding properties, including superior strength, toughness, oxidation resistance, corrosion resistance, and superplasticity similar to that of metals. Compared with conventional coatings, plasma-sprayed nanostructured coatings demonstrate significant improvements in strength, toughness, corrosion resistance, wear resistance, thermal barrier performance, and resistance to thermal fatigue; moreover, some of these coatings can simultaneously possess multiple of the aforementioned properties.
Literature reports that conventional composite ceramic coatings exhibit a layered microstructure. The nano-ceramic layers consist of a partially melted zone as well as a fully melted, lamellar zone similar to that formed by conventional plasma spraying; however, the lamellar structure is not particularly pronounced, and the number of coating cracks is significantly reduced. In the nanostructured composite ceramic coatings, the partially melted zone can be further divided into two regions: a liquid-phase sintered region characterized by a three-dimensional network or skeletal structure in which sub-micron Al2O3 particles are embedded within the TiO2 matrix, and a solid-phase sintered region featuring residual nanoparticles that have grown to some extent but still remain at the nanoscale. These distinct partially melted microstructures arise from the difference in melting points between Al2O3 and TiO2 in the composite ceramic powder. The microstructural changes in the nano-ceramic coatings improve both the porosity and toughness of the coatings, resulting in significantly enhanced microhardness and bonding strength compared to conventional coatings. During erosion testing, the surface of conventional ceramic coatings exhibits severe flaking, whereas the nano-ceramic coatings show much lower mass loss due to erosion. Moreover, the nano-AT13 coating demonstrates a markedly higher number of thermal-shock resistance cycles than conventional alumina coatings, with this advantage becoming more pronounced at higher thermal-shock temperatures. Flame-spray tests reveal that, upon failure, the nano-AT13 coating suffers less burn-through area and exhibits a longer resistance time against ablation compared to conventional coatings.
2. Study on Laser Remelting of Plasma-Sprayed Al2O3 Coatings
Alumina coatings produced by plasma spraying have already been widely adopted in industry. However, the plasma-spraying process imposes limitations on coating quality. Laser remelting offers a new approach to overcoming these technical challenges. Laser remelting can address defects inherent in plasma-sprayed coatings, such as lamellar structure, high porosity, numerous cracks, and poor mechanical bonding between the coating and the substrate. Both domestic and international researchers have combined laser remelting technology with plasma-spraying techniques to fabricate alumina ceramic composite coatings, and have been investigating the effects of laser remelting on the microstructure and properties of these ceramic coatings.
2.1 Laser Remelting Technology
Laser remelting technology involves continuously irradiating and scanning a coating with a focused laser beam under inert gas protection, rapidly heating the coating surface to its melting point. During subsequent cooling, heat is quickly transferred to the substrate metal, resulting in rapid solidification at a high cooling rate and yielding a ceramic coating with a uniformly dense microstructure and refined grain size on the surface of the sprayed ceramic layer.
2.2 Microstructure and Properties of Alumina Coatings Prepared by Laser Remelting of Plasma-Sprayed Alumina Coatings
Laser remelting is a rapid heating and cooling process. The mass transfer occurring within the coating inevitably leads to changes in its microstructure, thereby causing varying degrees of alteration in the performance of the ceramic coating. According to published literature, laser remelting has been applied to Al2O3 coatings, AT13 coatings, and nano-AT13 coatings prepared by plasma spraying. After remelting, the grains within the coating become finer, more uniform, and denser; the layered structure transforms into an equiaxed grain layer and a columnar dendritic structure. Moreover, the Al2O3 undergoes a phase transformation: γ-Al2O3 and β-Al2O3 completely disappear, and all of the alumina is converted into α-Al2O3. Additionally, the bonding mechanism between the coating and the substrate shifts from mechanical bonding to metallurgical bonding. After extensive experimentation, researchers generally agree that, compared to plasma-sprayed ceramic coatings, laser remelted ceramic coatings exhibit improved bonding strength between the ceramic coating and the metal substrate, as well as enhanced density, hardness, wear resistance, thermal shock resistance, and erosion resistance.
2.3 Laser Remelting Defects
Due to the differences in melting point, thermal expansion coefficient, elastic modulus, and thermal conductivity between the coating material used and the metallic substrate, coupled with the significant temperature gradient in the molten pool formed during laser remelting, the resulting thermal stresses can easily lead to problems such as cracking and coating spalling. Currently, laser remelting of plasma-sprayed alumina ceramic coatings is still at the experimental stage, and further in-depth research is needed on rapid solidification theory and specific laser process parameters.
3 Component Modification Based on Alumina Coating Additives
3.1 Add a low-melting-point buffer phase
Adding a small amount of certain components to coating materials can improve the microstructure of the coating and produce coatings with excellent performance. To address cracking in pure ceramic coatings and achieve strong bonding with the metallic substrate, incorporating buffer phases such as CaO, SiO2, and TiO2—powders with low melting points and high thermal expansion coefficients—can help relieve stress, reduce crack formation, enhance powder wettability, increase coating toughness, and improve its friction and wear resistance.
3.2 Adding Rare Earth Elements
Adding a small amount of rare-earth elements or rare-earth oxides to ceramic coatings can enhance the density of metal-ceramic coatings, increase their toughness, and promote the uniformity of the coating microstructure by dispersing hard ceramic phases. This also reduces the adverse effects of impurities and gases in composite coatings, thereby improving the density of the coating microstructure. Furthermore, it slows down the initiation and propagation of microcracks, enhancing the coating's bonding strength, tribological performance, and resistance to thermal shock.
3.3 Adding Carbon Nanotubes
Carbon nanotubes (CNTs), as a novel electromagnetic material, possess unique topological structures, exceptional electromagnetic properties, outstanding mechanical performance, and stable physicochemical characteristics. They represent one of the most promising candidates for next-generation high-temperature microwave absorbers. Adding carbon nanotubes to alumina ceramic powders and studying the microstructure and properties of the resulting coatings has become one of the latest research directions in thermal spraying, both domestically and internationally. Literature reports indicate that scholars at home and abroad have investigated the strengthening mechanisms and grain growth behaviors of plasma-sprayed alumina ceramic coatings reinforced with varying concentrations of CNTs, as well as the effects of plasma-spraying CNTs/Al2O3-TiO2 composite coatings on improving the microstructure and performance of these coatings.
3.4 Preparation of Special Functional Coatings
As equipment continues to be upgraded, there is an increasing demand for high-performance coatings that can withstand harsh operating conditions, prompting ongoing development of new functional coatings. Currently, smart coatings—such as self-lubricating, self-healing, or microcapsule-based self-repairing coatings—are beginning to emerge, and more and more researchers are devoting their efforts to this field. According to published literature, by adding an appropriate amount of solid lubricants (such as graphite, MoS2, WS2, etc.) with particle sizes comparable to those of alumina ceramic powders, self-lubricating or self-healing coatings can be prepared via plasma spraying. At high temperatures, these lubricants fill and seal cracks and pores within the coating, thereby achieving both high-temperature lubrication and self-healing effects.
4 Conclusion and Outlook
This article provides a comprehensive review of plasma-sprayed composite coatings made from alumina, Al2O3-TiO2, and nano-alumina. It briefly discusses the effects of laser remelting on plasma-sprayed alumina coatings and highlights the valuable insights it offers for the study of other ceramic materials. By selectively adding various components to alumina ceramic coatings, the quality of these coatings can be significantly improved, providing reliable process methodologies for preparing special-function coatings using plasma spraying and laser remelting technologies. As research into nanomaterials and laser remelting continues to deepen, these advancements hold great promise for further enhancing the microstructure and performance of plasma-sprayed alumina coatings. It is anticipated that such coatings will find increasingly widespread applications in industrial sectors including aerospace, mechanical and chemical engineering, and iron and steel metallurgy.
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