The thermal barrier properties of zirconia ceramic coatings are a key indicator of their insulation effectiveness. Porosity and thickness, as key design parameters, jointly determine their thermal insulation capacity and service stability by influencing the heat conduction path and thermal stress distribution.
The impact of porosity on thermal barrier properties is primarily reflected in the alteration of the heat conduction path. Zirconia ceramic bulk materials have high thermal conductivity, but the introduction of porosity allows stagnant air (with extremely low thermal conductivity) to fill the pores, significantly reducing the coating's equivalent thermal conductivity. Specifically, increasing porosity lengthens the heat flow path through the coating, increasing thermal resistance and thus improving thermal insulation. For example, increasing the porosity from 12% to 28% reduces the coating's thermal conductivity at high temperatures to less than one-third that of the bulk material. However, higher porosity is not always better; excessive increases in porosity can lead to a decrease in the coating's mechanical properties. In high-porosity coatings, evenly distributed small pores shorten crack propagation paths, prevent through-hole cracks, and thus enhance thermal shock resistance. However, if the pores are too large or unevenly distributed, the coating is susceptible to spalling under thermal stress. Therefore, in practical preparation, a balance must be struck between porosity and mechanical properties. Porosity is typically controlled between 15% and 25% to achieve both thermal insulation and durability.
The effect of thickness on thermal barrier performance manifests itself as a linear increase in thermal resistance. According to thermal conduction theory, coating thermal resistance is proportional to thickness. A thicker coating reduces heat flux through the coating and enhances its thermal insulation effect. For example, increasing the thickness from 0.1 mm to 0.4 mm can reduce the substrate surface temperature by 50°C to 170°C. However, increasing thickness also presents new challenges: First, the thermal expansion coefficients of ceramic and metal substrates differ significantly, and increased thickness can lead to thermal stress accumulation at the interface, increasing the risk of spalling. Second, excessively thick coatings can crack due to their own weight or concentrated thermal stress. Currently, the most commonly used coating thickness for aircraft engine hot-end components is 0.2mm-0.5mm, which provides adequate insulation while ensuring structural stability.
The synergistic effect of porosity and thickness further optimizes thermal barrier performance. At the same thickness, high-porosity coatings achieve better insulation by reducing thermal conductivity; while at the same porosity, increasing thickness linearly increases thermal resistance. For example, a 0.3mm thick coating with a 20% porosity may provide better insulation than a 0.2mm thick coating with a 25% porosity, but the latter exhibits better thermal shock resistance. Therefore, in practical applications, the parameter combination must be adjusted according to the required operating conditions: for short-term high-temperature exposure, increasing thickness can be prioritized; for long-term thermal cycling, optimizing porosity is crucial to improving durability.
Control of porosity and thickness is crucial during the manufacturing process. Plasma spraying, due to the flattening properties of particles upon impacting the substrate, tends to form a lamellar structure with a porosity typically ranging from 8% to 15%. Electron beam physical vapor deposition (EB-PVD), on the other hand, achieves lower porosity through columnar crystal growth but offers improved thermal shock resistance. Porosity and thickness can be precisely controlled by adjusting spray power, distance, or powder particle size. For example, reducing spray power reduces particle melting, thereby increasing porosity; increasing spray distance reduces particle velocity, resulting in a looser structure.
The thermal barrier properties of zirconia ceramic coatings are a function of the combined effects of porosity and thickness. By optimizing the synergistic relationship between these two factors, a balance can be achieved between thermal insulation, mechanical properties, and durability, meeting the protection requirements of high-temperature components in aerospace, energy, and other fields.
