Page 81 - IJAMD-1-3

International Journal of AI for
Materials and Design
Review of gas turbine blade failures by erosion

Table 10. Influence of coating porosity on erosion resistance
Porosity Coating Erosion resistance Failure mode Key observations
level (%) material
<10% Dense ceramic Very high: minimal Micro-cracking and Effective at high temperatures but
coatings particle penetration delamination prone to brittleness
10 – 15% YSZ Moderate: balances Surface pitting and Optimal balance for TBC
insulation and toughness. spalling performance 57
>15% Porous ceramic Low: High particle ingress Accelerated erosion High porosity reduces mechanical
coatings and reduced strength and chipping integrity 58
Abbreviations: TBC: Thermal barrier coating; YSZ: Yttria-stabilized zirconia.

rate and the impact of particle collisions on the material n and m are exponents typically determined through
surface is essential. 69,70 Here, we consider models such as experimentation.
Finnie’s erosion rate equation shown by Equation I, as well In gas turbines, particles with larger diameters and
as more advanced particle impact models derived from higher velocities cause greater erosion, while materials
fluid dynamics. Finnie’s equation provides a basic model with higher hardness (e.g., superalloys or coated surfaces)
for estimating the erosion rate of a material based on exhibit better erosion resistance.
particle velocity and impact angle. The erosion rate “E” is
expressed, as shown in Equation I. 3.2. Material degradation and fatigue theory

2
 ρ v  The second theoretical pillar explores how erosion-induced
θ
2
EC=  p  sin () (I) material degradation leads to fatigue failure in gas turbine
 2 
  blades. While the previous pillar focused on how particles
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cause surface damage, this section focuses on how that
where E is the erosion rate (material loss per unit
area); C is a constant dependent on material properties damage propagates into more severe material failures over
time, considering both mechanical and thermal stressors.
and particle characteristics; ρ is the particle density; v is
p
the velocity of the impacting particle; and θ is the angle of Material fatigue refers to the progressive weakening of
impact of the particle. a material due to repeated stress cycles, which eventually
This model shows that impact angle plays a leads to crack formation and structural failure. Alqallaf and
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significant role in erosion severity, with steep angles Teixeira demonstrated that in gas turbine blades, erosion
(close to 90 degrees) causing the highest erosion due to not only causes surface damage but also accelerates fatigue by
maximal energy transfer. The sin (θ) term captures the creating stress concentrators – localized areas where stress is
2
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influence of angle on erosion. Once particle impacts are intensified due to surface irregularities such as pits, scratches,
determined, empirical models such as Finnie’s erosion and micro-cracks. These stress concentrators initiate cracks
rate equation or more complex erosion models 48,49 are that grow over time, ultimately leading to catastrophic failure
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used to calculate the erosion rate at each impact site. For of the blade. The Paris–Erdogan law is frequently used to
instance, Finnie’s equation given by Equation I provides model crack growth, which relates the crack propagation rate
a direct method for estimating material removal due to to the applied stress intensity. Fatigue failure theory helps
each particle impact. in predicting how long a turbine blade can operate under
erosive conditions before failure is likely to occur.
In more complex models, the erosion rate is also a
function of particle size and material hardness. For example, In high-temperature environments, turbine blades
Anand and Parammasivam developed an empirical experience creep, a slow, time-dependent deformation
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model that accounts for particle size “d,” hardness “H,” and that occurs when materials are exposed to constant stress
velocity “v,” as given by Equation 2. at elevated temperatures. Erosion exacerbates creep by
thinning the blade material, reducing its ability to resist the
n
vd⋅ m mechanical stresses that it experiences during operation.
W = k (II) 76
H Rajabinezhad et al. highlighted that as erosion removes
protective coatings and material from the surface, it
where E is the erosion rate; k is an empirically exposes deeper layers to higher temperatures, accelerating
determined constant; v is the particle velocity; d is the creep deformation. This leads to reduced load-bearing
particle diameter; H is the hardness of the material; and capacity and increases the likelihood of blade rupture.

Volume 1 Issue 3 (2024) 75 doi: 10.36922/ijamd.5188