Studies on Elevated Temperature Tribological Behavior of Fly Ash Based Plasma Spray Coatings

Abstract

Material behavior at elevated temperature is becoming an increasing technological importance. Components working at higher temperatures like in land based gas and steam turbines, power generation boiler tubes, hot sections of aero engine, propulsion bearings, materials processing and internal combustion engines are subjected to surface friction, wear, oxidation and hot corrosion conditions. Service conditions of such components in elevated temperature environments may compromise their mechanical properties resulting in the reduced life cycle. Components working in such adverse conditions demand suitable surface modification techniques like thermal spray coatings that are widely adopted in similar situations. Plasma spray coating processes belong to the family of thermal spraying techniques and are widely used in many industries to protect the components against erosion, oxidation and wear. Thermal energy is utilized in this process to deposit a wide variety of materials including finely divided metallic and non-metallic materials. Higher temperatures utilized in these processes enable the use of coating materials with very high melting points like ceramics, cermets, and refractory alloys. The present work explores the possibility of using fly ash based plasma spray coatings for high temperature applications. The proposed coatings are investigated for their resistance to erosion, oxidation and wear under laboratory conditions. Commercially available Cr3C2-25NiCr, NiCrAlY, WC-Co, fly ash cenospheres, MoS2, CaF2 and CaSO4 are used as coating feedstock in the present investigation. Six types of coatings namely Cr3C2-NiCr/Cenosphere, NiCrAlY/WC-Co/Cenosphere, Cr3C2-NiCr/Cenosphere/MoS2/CaF2, Cr3C2-NiCr/Cenosphere/MoS2/CaSO4, NiCrAlY/WC-Co /Cenosphere/MoS2/CaF2 and NiCrAlY/WC-Co /Cenosphere/MoS2/CaSO4 are deposited on MDN 321 steel substrate (Midhani Grade). Coatings are characterized using Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD). Further, microstructure and mechanical properties (microhardness, adhesion strength, erosion, oxidation, and wear) have been characterized to evaluate their potential for hightemperature application. For the chosen spray parameters, seemingly dense laminar structured coatings (six types as mentioned earlier) with a thickness in the range of 350-400 m having porosity lower than 5 % has been achieved. Erosion behavior of MDN 321 steel, Cr3C2-NiCr/Cenosphere, and NiCrAlY/WCCo/Cenosphere coatings are investigated at elevated temperatures using solid particle erosion test (ASTM G76-13) set up at 200, 400, 600 °C with 30 and 90° impact angles using alumina erodent. Erosion resistance of both the coatings is observed to be higher than the substrate for the test temperatures chosen and noted to be more prominent at lower impact angle and higher temperature. Both the coatings exhibited a brittle mode of material removal through brittle cracking and chipping. NiCrAlY/WC-Co/Cenosphere coating reported better erosion resistance as compared to Cr3C2-NiCr/Cenosphere coating which may be attributed to plastic deformation of the NiCrAlY matrix due to the ductility of the matrix and hard WC-Co reinforcement to resist the matrix plow thereby reduces the erosion loss. Cyclic oxidation behavior of MDN 321 steel, Cr3C2-NiCr/Cenosphere and NiCrAlY/WC-Co/Cenosphere coatings are further carried out at 600 °C for 20 cycles. Each cycle consisted of heating at 600 °C for 1 hour, followed by 20 minutes of cooling in air. The thermogravimetric technique is used to approximate the kinetics of oxidation of substrate and coatings. Both the coatings reported lower weight gain as compared to the substrate. NiCrAlY/WC-Co/Cenosphere coating registered less weight gain as compared to Cr3C2-NiCr/Cenosphere coating which is attributed to the excellent oxidation resistance of NiCrAlY and formation of CoWO4 along with NiO and Cr2O3 oxides on the coating surface. Influence of solid lubricants on Cr3C2-NiCr/Cenosphere and NiCrAlY/WCCo/Cenosphere coatings is dealt next for tribological response. Dry sliding wear behavior of MDN 321 steel, Cr3C2-NiCr/Cenosphere/MoS2/CaF2, Cr3C2- NiCr/Cenosphere/MoS2/CaSO4, NiCrAlY/WC-Co/Cenosphere/MoS2/CaF2 and NiCrAlY/WC-Co/Cenosphere/MoS2/CaSO4 is carried out using high temperature pin on disc tribometer as outlined in ASTM G99-05 standard. All the four coatingsdisplayed a lower coefficient of friction and wear rate in comparison with the substrate. Excellent wear resistance of the coatings is attributed to the solid lubricants effect. Based on the wear rate data, the relative wear resistance of the coatings under dry sliding conditions is arranged in the following sequence: (Cr3C2-NiCr/Cenosphere/MoS2/CaSO4) > (Cr3C2-NiCr/Cenosphere/MoS2/CaF2) > (NiCrAlY/WC-Co/Cenosphere/MoS2/CaSO4) > (NiCrAlY/WC-Co/Cenosphere/MoS2/CaF2) Higher wear resistance of Cr3C2-NiCr/Cenosphere/solid lubricant coatings is attributed to the high hardness of Cr3C2-NiCr which is incorporated in the coatings. Developed coatings in the present study exhibit higher temperature resistance to erosion, oxidation and wear as compared to MDN321 steel making them suitable for components subjected to elevated temperature service conditions.