High-pressure turbine blades are the most difficult parts of turbofan engines, which not only withstand the centrifugal force and vibration load of the rotor blades themselves, but also withstand extreme thermal stress, which is very prone to damage.
The pre-turbine temperature of an engine is an important indicator of engine performance. Under other conditions remaining unchanged, for every 100 °C increase in the temperature in front of the engine turbine, the maximum thrust of the engine can be increased by 8%~10%.
Figure 1 shows the influence of the development of turbine blade cooling technology on the temperature before the turbine. In addition to cooling technology, another key factor affecting the temperature before the turbine is the high temperature resistance of the turbine blade material.
In order to improve the high temperature resistance of the blade, the material is generally selected from a single-crystal nickel alloy, and an additional thermal barrier coating (TBC) is applied to the blade matrix.
First, the composition of the thermal barrier coating
Thermal barrier coatings have excellent thermally insulating, oxidation-resistant, and anti-corrosion properties, and coating them on the surface of blades can significantly improve engine thermal efficiency and reduce energy consumption.
Generalized thermal barrier coatings are generally multi-layer structural systems composed of a ceramic top layer, a thermally GrownOxide (TGO) and a bonding layer.
The current advanced cooling technology can currently form a temperature reduction of about 300 °C, and the adhesive layer is generally MCrAlY (M is a transition metal, containing Ni, Co and other elements) or Pt-modified aluminide coating, which is mainly used to buffer the large difference in thermal expansion coefficient between the ceramic layer and the matrix, play the role of bonding transition, and have the effect of systematic oxidation corrosion protection (see Figure 2).
When working in a high temperature environment, the interface between the thermal barrier layer and the blade matrix will form a continuous and dense protective film (TGO layer) to protect the substrate. An important factor in determining the service life of a thermal barrier coating is the oxidation of the adhesive layer.
After a long period of use of turbine blades, the bonding strength of the thermal barrier layer usually decreases, and the inevitable thermal strain between the thermal barrier layer and the base material will overload its bond strength, resulting in coating peeling, which not only causes the substrate to overheat, but also causes damage to the hot parts downstream of the air flow.
Second, the failure mode of the thermal barrier coating
Thermal barrier coating failure of turbine blades is common and is an important factor in blade damage. Due to the complex and variable internal environment of the engine, the causes of coating failure are also complex (see Figure 3).
Several common failure modes are as follows
1. Ceramic layer sintering
After depositing, the ceramic layer is sintered rapidly when the operating temperature exceeds a certain temperature. The sintering process causes changes in the volume and material properties of the ceramic. Sintering causes the ceramic layer to shrink, resulting in in-plane tensile strain, causing cracks at the vertical interface, causing the thermal barrier coating to peel off.
2. Oxidation of the adhesive layer
At high temperatures, the adhesive layer oxidizes to form an oxide layer, causing its volume expansion near the interface. Due to the constraints of the surrounding material, the oxide layer is formed with residual compressive stress.
Oxidation of the adhesive layer deteriorates the bonding strength of the ceramic layer. Even with a thickness of only a few microns, thermal growth oxides between the bonded layer and the ceramic layer can cause local spalling of the ceramic layer and accelerate thermal fatigue in weak areas of the matrix material.
3. Marginal effect
At the corner positions of the turbine blades, stress singularities occur due to different material properties. The peeling of the thermal barrier coating often originates in these areas.
4. Thermal corrosion influence
The fuel oil used in aero engines generally contains impurities such as sodium and sulfur, so thermal barrier coatings often encounter thermal corrosion problems, including oxidation, nitridation and vulcanization.
5. Impact damage
Small particles in the engine air flow will impact and rub against the coating surface, causing damage to the microstructure of the coating and a decrease in thickness, thereby increasing the temperature of the metal matrix and accelerating the oxidation of the adhesive layer. Material shedding of engine ignition nozzles is a more common category.
In addition, the properties of various materials of the thermal barrier coating during the high-temperature thermal cycle, the thermal gradient, the preparation process and the residual stress generated in the cooling coating all affect its lifetime.
Third, the oxidative damage of leaves
Oxidation is a typical type of damage caused by high-temperature corrosion, which is caused by the participation of oxygen, and severe oxidation is also often called ablation. Generally, oxidation is defined as the various failure mechanisms that eventually lead to the formation of oxides, which can be regarded as the superior concept of various damages, which is a relatively broad concept.
Oxidation can occur through the direct reaction of the matrix with oxygen in the atmosphere, or through reaction chains and diffusion processes. If the oxides produced by these reactions are damaged by stress erosion, oxidation continues until it inevitably weakens the cross-section and causes the component to fail.
Normal oxidation resistant coatings that become thin or suffer considerable thermal damage can accelerate the erosion of the substrate material. If the blade is left above the design temperature for a long time, it may cause short cracks in the shape of "washout" and the loss of "wrinkled" surface material, often referred to as the "orange peel effect".
In extreme cases, when the temperature is in the liquid contact temperature range or severe oxidation, the blade will partially separate, and without an obvious identifiable cross-section, the separation surface will become rough.
If the temperature is slightly lower, parts of the blade will break in the solid-phase temperature range. This usually occurs at the tip of the blade and in the particularly thin posterior edge area.
Discoloration of turbine blades is an early typical feature of oxidative damage, but because its consequences are not serious, it is usually treated differently from oxidation during engine maintenance.
In addition to the characteristics of discoloration, oxidative damage is generally accompanied by some other morphological damage, such as cracks, surface wrinkles or material loss, which may cause more serious consequences. Figure 4 shows the typical blade oxidation damage of a certain type of engine, and the material form of the blade surface has different degrees of damage compared with simple discoloration.
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