PERFORMANCE STUDY AND LIFE PREDICATION MODEL FOR COATING ON HEAT RESISTANT STEELS

Abstract

To protect the structural components of a power generating unit from the corrosive environment, thermal spraycoatings are applied to the components. In the present work, four different types of thermal barrier coating (TBC)viz. partially stabilised zirconia (8YSZ), zirconia-20% alumina (ZA) composite coating without carbon nanotube(CNT) reinforcement, and ZA with 1% and 3% CNT reinforcement. The coating was deposited on NiCrAlY coated P91 steel using a plasma spraying process. The coating microstructure and phases were characterised using fieldemission scanning electron microscope (FE-SEM) with energy dispersive spectroscopy (EDS). The phases of the coating were analyzed using X-ray diffraction technique. The effect of CNT reinforcement on the thermal conductivity, porosity, and hardness of the composite coatings was investigated. The protective behavior of the coatings was characterised by potentiodynamic polarization testing and electrochemical impedance measurements. The thermal conductivity of the composite coating was found to be increased with increasing CNT content. Hardness was found to be highest for 3% CNT reinforcement and the thermal conductivity was found to increase with decreasing porosity. The electrochemical measurements indicate that reinforcement of CNT in zirconia alumina composite coating improved its corrosion resistance. The CNT reinforced composite coating showed superior mechanical properties as compared to the conventional 8YSZ coating. The addition of alumina and 1%CNTs increased the Young's modulus by around 25% and 40%, respectively. However, the increase in CNT content decreased the Young's modulus as a result of CNT agglomeration. The hardness, fracture toughness increased with addition of alumina and CNT. The fracture toughness increased from 0.55±0.26 MPa m1/2 for 8YSZ to 1.76±0.65 MPa m1/2 for 17%alumina-3%CNT reinforcement due to various toughening mechanisms like crack deflection, bridging etc. The maximum displacement of indenter was found to decrease from ~ 176 nm for conventional YSZ coating to ~ 120 nm for 3% CNT reinforced coating. The surface roughness (Ra) was found to be decrease with alumina addition and CNT reinforced coating. The CNT reinforced coating showed surface roughness of around 6 μm. The adhesion strength of coatings was determined using tensile adhesion test, the XRD technique was used to determine residual stress in the surface of the coating. A stress relaxation technique using a slow-speed diamond cutter has been used to determine through-thickness RS of the coatings. A finite element (FE) model was developed and validated the model was then used to establish a relationship viii between applied stress and relaxed strain. The effect of substrate preheating (200 °C and 400 °C) on residual stress variation along the depth of the coating was determined using a novel stress relaxation technique. The addition of alumina increased the compressive residual stress on the surface of the coating by 42%, addition of 1% multi-walled carbon nanotube (MWCNT) had a negligible effect on the RS on the surface of the coating. The further addition of MWCNT (3% wt.) introduced a tensile RS in the coating due to agglomeration of MWCNT. The preheating of the substrate reduced the magnitude of the compressive stress along with the thickness of the coating. Hot corrosion (HC) behavior of air plasma sprayed 8% yttria stabilized zirconia (8YSZ) -alumina (YA) composite thermal barrier coating was evaluated. The investigation reveals improved isothermal HC behavior of 1% MCNT reinforced coating. The dominating effect of the HC was recognized as depletion of yttria leading to destabilization of 8YSZ. The formation of YVO4 was the corrosion product containing the depleted Y2O3 of YSZ. The 1% and 3% MCNT reinforced coating exhibited monoclinic phase percentage of around 9% and 34% respectively. Nanoindentation was carried out along the cross-section before and after the isothermal HC. The Youngs modulus after HC increased by 46%, 42%, 12.5% and 38% for 8Y, 8YA, 8YA1C and 8YA3C coating, respectively. Weibull modulus of Youngs modulus of bond coats was used to identify the efficiency of top coat in retarding the infiltration of molten salt. The bond coat of 8YA1C coating exhibited lowest modulus value (m=8.55) indicated non-uniform infiltration of detrimental species. The 1% MCNT reinforced thermal barrier coating (TBC) system was more resistant to degradation than the conventional 8YSZ and YA composite coatings. In case of cyclic hot corrosion the coating cross-section properties were determined by the nano-indentation test, and Weibull modulus (m) of the bond coat (BC) properties were analyzed to compare the infiltration resistance of the topcoats. The formation of YVO4 Life prediction model for coatings was developed using Weibull modulus of the bond coat