Insulation coatings are typically applied to metal substrate surfaces to block current flow or serve as a dielectric medium in capacitors. Commonly used insulation coating materials include organic polymers, ceramics, and composites. Polymer coatings suffer from drawbacks such as susceptibility to aging, poor mechanical rigidity, low breakdown strength, and decomposition at high temperatures, making them unsuitable for demanding service environments. In contrast, ceramic insulation materials offer high mechanical strength, resistance to environmental aging, a wide bandgap, and high-temperature tolerance, overcoming the limitations of polymer insulation in harsh conditions.
Thermal spraying is a primary technique for preparing ceramic insulation coatings, offering advantages such as high deposition efficiency, minimal restrictions on metal component size, and low production costs. The overall performance of thermally sprayed ceramic coatings depends on the choice of coating material, processing parameters, and post-treatment methods.
Selecting the appropriate coating material for a specific application environment is critical. Generally, ceramic insulation coating materials are required to possess high dielectric strength, high volume resistivity, moderate relative permittivity, low dielectric loss factor, high elastic modulus, impact resistance, and corrosion resistance. Commonly used insulation coating materials are as follows:
1. High-Purity Alumina
Al₂O₃ ceramics feature high mechanical strength, good thermal conductivity, high insulation strength and resistivity, low dielectric loss, and a relatively high dielectric constant. Notably, their dielectric properties remain stable with changes in temperature and voltage frequency, making Al₂O₃ the most widely used insulation coating material.
The purity of Al₂O₃ powder determines the impurity content in the coating, which in turn affects the coating's insulation performance. Common impurities in the powder include SiO₂, Na₂O, CaO, Fe₂O₃, and MgO. Among these, small-diameter ions such as Na⁺ and K⁺ have greater mobility within the coating and significantly reduce the insulation resistance of the Al₂O₃ coating.
Additionally, Al₂O₃ exists in multiple crystalline phases, including α, β, γ, δ, κ, and θ. The Al₂O₃ powder used for thermal spraying is typically high-temperature calcined pure α-phase material. However, during spraying, the powder partially or fully melts and impacts the substrate surface, cooling and depositing to form the coating. This process generates a substantial amount of metastable phases, primarily γ-phase along with a small amount of β-phase. Studies indicate that the unmelted portions of the powder retain the α-phase, while the melted portions preferentially form γ-phase columnar grains upon rapid solidification due to the lower critical nucleation work of the γ-phase. Therefore, the more complete the powder melting, the higher the γ-phase content in the coating. In summary, the inevitable presence of γ-phase in thermally sprayed Al₂O₃ coatings reduces their insulation performance. This can be mitigated by adjusting the spraying method, process parameters, or powder structure.
2. Doped Al₂O₃
Coatings made from high-purity Al₂O₃ powder exhibit high insulation performance but suffer from poor resistance to impact and vibration loads due to high accumulated stress. Additionally, high-purity Al₂O₃ is expensive, and its high melting point leads to low deposition efficiency and high spraying costs. Research shows that adding moderate amounts of other elements (e.g., Mg, Ti) can lower the powder's melting point and improve coating density, albeit at the cost of some insulation performance. Therefore, for applications requiring higher mechanical performance, doped Al₂O₃ materials may be chosen.
Magnesia-doped Alumina – MgO and Al₂O₃ can form spinel (MgAl₂O₄) with congruent melting characteristics. Adding a certain amount of MgO to Al₂O₃ powder lowers the initial melting temperature, resulting in a coating with fewer vertical cracks and higher density. Meanwhile, the presence of highly insulating MgAl₂O₄ spinel reduces the γ-Al₂O₃ content in the coating. Thus, doping Al₂O₃ with an appropriate amount of MgO can comprehensively improve both the insulation and mechanical properties of the coating.
Titania-doped Alumina – Adding TiO₂ to Al₂O₃ lowers the powder's melting point, improves coating density and deposition efficiency, and forms a TiAl₂O₅ phase that enhances impact toughness. TiO₂ is also relatively inexpensive. Moreover, TiO₂ doping partially suppresses the transformation from α-Al₂O₃ to metastable γ-Al₂O₃, reducing the γ-phase content in the coating. However, Ti ions in the doped coating significantly degrade its insulation performance. Therefore, when a balance between insulation and mechanical properties is required, lower-cost TiO₂-doped Al₂O₃ coating materials may be considered. The TiO₂ content significantly affects both insulation and mechanical properties. Common doping levels include 13 wt.% and ≤0.2 wt.%. Coatings with 13 wt.% TiO₂ exhibit higher toughness and wear resistance but lower insulation performance. A doping level of 3 wt.% TiO₂ provides a combination of insulation and impact resistance. Experimental studies show that after 10 thermal shock cycles from room temperature to liquid nitrogen, the coating showed no cracks, although its resistivity decreased. This indicates that a 3 wt.% TiO₂-doped Al₂O₃ coating, due to its high density, avoids a significant drop in insulation performance, while higher TiO₂ contents substantially reduce insulation.
Tetragonal Zirconia Toughening – In Zirconia-Toughened Alumina (ZTA), tetragonal ZrO₂ (partially stabilized zirconia) can be retained at room temperature. Under external stress, it undergoes a martensitic transformation to monoclinic phase, accompanied by volume expansion and shear stress that hinder crack propagation, thereby enhancing the flexural strength and fracture toughness of the ceramic. Below 1650°C, both Al₂O₃ and ZrO₂ are stable and exhibit little solid solution; only above 1650°C does limited solid solution occur, with no chemical reaction across the composition range. Partially stabilized ZrO₂ has relatively high oxygen ion mobility, so its doping level greatly affects the coating's insulation performance.
3. Other Materials
In recent years, Y₂O₃ has been widely studied as a protective coating in the semiconductor industry due to its high insulation and corrosion resistance. Additionally, research on MgO-doped Al₂O₃ insulation coatings has revealed that MgAl₂O₄ exhibits excellent insulation properties.
Y₂O₃ – Yttria has a room-temperature bandgap of 5.5 eV and is a common insulation coating material. Compared to Al₂O₃, Y₂O₃ has lower mechanical strength but similar relative permittivity (~10) and loss factor (<1×10⁻⁴). A study compared the AC breakdown strength at 50 Hz of plasma-sprayed Al₂O₃, Y₂O₃, and YSZ coatings. It found that Y₂O₃ coatings had a slightly higher breakdown strength (17.3 kV/mm) than Al₂O₃ (16.6 kV/mm), both significantly higher than YSZ (11.1 kV/mm). These results indicate that Y₂O₃ offers comparable insulation performance to Al₂O₃ but with lower mechanical strength, making it suitable for non-load-bearing insulation coatings.
MgAl₂O₄ – Magnesium aluminate spinel is an equimolar compound of MgO and Al₂O₃ formed at high temperatures. It possesses high mechanical strength, chemical stability, and insulation properties, making it suitable for insulation coatings. Thermal spray MgAl₂O₄ powder is typically produced by reaction sintering of AlO(OH) and Mg(OH)₂, followed by spray drying and secondary densification sintering. The phase structure of the thermally sprayed MgAl₂O₄ coating remains the same as that of the powder, ensuring compositional uniformity.