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

Abstract

To protect the structural components of a power generating unit from the corrosive environment, thermal spray coatings are applied to the components. In the present work, four different types of thermal barrier coating (TBC) viz. partially stabilized zirconia (8YSZ), zirconia-20% alumina (ZA) composite coating without carbon nanotube (CNT) reinforcement, and ZA with 1% and 3% CNT reinforcement have been developed. The coating was deposited on NiCrAlY coated P91 steel using a plasma spraying process. The coating microstructure and phases were characterized using field emission 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 characterized by potentiodynamic polarization testing and electrochemical impedance measurements. The thermal conductivity of the composite coating was found to increase 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 the 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 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 residual stress of the coatings. A finite element (FE) model was developed and validated the model was then used to establish a relationship 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 (CNT) had a negligible effect on the residual stress on the surface of the coating. The further addition of CNT (3% wt.) introduced a tensile residual stress in the coating due to the agglomeration of CNT. The preheating of the substrate reduced the magnitude of the compressive stress along with the thickness of the coating. Hot corrosion behavior of air plasma sprayed 8% yttria-stabilized zirconia (8YSZ) -alumina composite TBC was evaluated. The investigation reveals improved isothermal hot corrosion behavior of 1% CNT reinforced coating. The dominating effect of the hot corrosion 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% CNT reinforced coating exhibited monoclinic phase percentage of around 9% and 34%, respectively. Nano-indentation 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 topcoat in retarding the infiltration of molten salt. The bond coat of 8YA1C coating exhibited the lowest modulus value (m=8.55), indicating non-uniform infiltration of detrimental species. The 1% CNT reinforced TBC system was more resistant to degradation than the conventional 8YSZ and ZA composite coatings. In the 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 properties were analyzed to compare the infiltration resistance of the topcoats. The formation of YVO4 and tetragonal to monoclinic zirconia phase transformation was one of the degrading mechanisms of the coatings. The difference in CTE also proved to be detrimental to the performance of the coating. The monoclinic phase for 8YSZ, 8YA, 8YA1C, and 8YA3C was found to be around 56 %, 79 %, 92 %, and 63 % respectively after cyclic hot corrosion. The “m” for Youngs modulus of bond coat was found to be lowest for 8YA3C coating indication higher heterogeneity. The addition of CNT was found to be detrimental in cyclic hot corrosion as it leads to the cracking of the coatings. The life prediction model for coatings was developed using Weibull modulus of the bond coat.