Understanding Wall Thickness Effect on Accelerated Oxidation

Abstract

The impact of high-temperature oxidation on component life in gas turbine engines is a critical area of study. One less-explored factor influencing this type of degradation is the component wall thickness, which necessitates further investigation and experimentation to understand its effects. Recent cyclic oxidation work on a Ni-based superalloy carried out at Cranfield has shown that a thinner wall thickness exhibits reduced degradation. This thesis aims to understand why this behaviour was observed. A simplified approach was taken to assess this effect by performing isothermal oxidation on cylindrical specimens of varying wall thickness (0.5, 1.0, 2.0, 3.0mm, and bulk). The isothermal experiment was executed using an AFCT rig for 100 hours under atmospheric conditions at 1000 °c. Weight change, OMI, SEM, XRD, and GIXRD were the characterisation techniques used to assess the oxides formed, degradation, and oxide residual stresses. Further assessment was made using FEA to understand the substrate behaviour during heating. The experiment showed that while all hollow specimens gained weight, bulk specimen lost weight, with the 0.5mm WT showing the slightest weight change per surface area (0.365, 0.658, 0.465, 0.670, -1.110 mg/cm2). SEM line scans on bulk and 0.5mm WT revealed no significant chemical differences, suggesting that chemical differences did not influence degradation performance. XRD results confirmed consistent phase composition on both specimens. GIXRD scans did not reveal clear trends in oxide scale stresses; however, the bulk sample displayed notably lower stress, likely due to fracture. The 0.5mm WT specimen exhibited the lowest stress among the hollow samples, likely due to creep effects. FEA indicate that thinner samples reach thermal equilibrium faster, facilitating stress relief. This, along with lower peak stresses attributed to reduced thermal gradients, likely contributes to decreased oxide spallation in thinner substrates.