Publication

Abstract : Hot air erosion studies were carried out on niobium (Nb) alloy, C-103 coated with yttria stabilized zirconia (YSZ) with a prior bond coat of NiCrAlY alloy by plasma-spraying method. Erosion behavior of coated alloys is compared with that of bare alloy. The test specimens were subjected to hot-air at a temperature of ∼600 °C with a supersonic flow of Mach 2 using a Scramjet testing facility. Combustion of a mixture of compressed air and hydrogen was used as a heat source in the erosion studies. It was observed that the YSZ coated alloys with a bond coat showed lower back wall temperature as compared to the uncoated alloy, which is attributed to the lower thermal conductivity of the coating material in comparison to the Nb alloy. The erosion testing is accompanied with a negligible weight loss in bare alloy specimen; while a significant weight loss is observed in coated alloys. The numerical transient thermal analysis, carried out using ANSYS software, was employed to measure the temperature variations on the specimen surfaces with respect to time. The back wall temperature obtained from experiments on various specimens are found to be in excellent agreement with the computationally predicted temperature profiles. Computational simulations further suggests that the temperature gradients of ∼20 °C, 150 °C and 265 °C across the front and the back wall surface are found for bare alloy and 300 μm and 600 μm YSZ coated alloys, respectively with a progressive decrease in temperature from the front to back wall surface. The simulations furthermore reveal that the Von Mises stresses around most regions of the Nb alloy, bond coat and YSZ coated alloys were below their respective yield stresses. However, high stresses are observed at the interfaces of top and bond coats, which caused de-bonding of the YSZ top coat from the bond coat. The back wall temperatures of ∼578 °C, ∼483 °C and ∼448 °C were experimentally obtained for bare alloy and 300 μm and 600 μm YSZ coated alloys. X-ray diffraction studies suggest that YSZ has undergone a marginal phase-transformation from tetragonal to cubic phase due to hot-air erosion. More number of microcracks is observed in thicker YSZ coated alloys as a result of hot-air erosion as revealed by scanning electron microscopy.