2. Thesis and Dissertations
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Item Development and Characterization of Metal Injection Moulded Components in Improving Resistance to High Temperature Wear and Oxidation(National Institute of Technology Karnataka, Surathkal, 2019) C, Veeresh Nayak.; Ramesh, M. R.; Desai, Vijay.Metal injection moulding (MIM) is a near-net shape manufacturing technology for producing intricate parts, cost-effectively. MIM comprises combined techniques of plastic injection moulding and powder metallurgy. A wax-based binder system consisting of paraffin wax (PW), low-density polyethylene (LDPE), polyethylene glycol (PEG-600) and stearic acid is established for MIM of powder systems of Cr3C2- NiCr (30% Wt.) +NiCrSiB (70% Wt.), SS316L (70% Wt.) +WC-CrC-Ni (30% Wt.) and Tool Steel. The excellence of the MIM product depends on feedstock characteristics, process parameters of the injection moulding stage as well as debinding and sintering stage. Injection stage is most important as many defects such as phase separation, weld line, voids etc. may occur during injection stage due to improper selection of injection moulding parameters and these defects cannot be repaired in the subsequent debinding and sintering stages. The feedstock was characterized by rheological properties at different temperatures. Injection temperature was determined by the rheological investigation of the feedstock having the 56 % powder loading and 44% binder by volume. The solvent debinding temperature is optimized and defect-free MIM component is obtained at a temperature of 48°C. Sintering process was carried out with the temperature cycle in the range of 1150–1200 °C under hydrogen purged atmosphere. The sintering density achieved was 96%. The MIM components showed good and acceptable shrinkage in linear dimensions. Material behaviour at elevated temperature is becoming an increasing technological importance. Components working at higher temperatures like in land-based gas turbines, power generation boiler tubes, hot sections of aero engine, gas and steam turbine, propulsion bearings, materials processing, and internal combustion engines are subjected to surface friction, wear, oxidation conditions. Service conditions of such components in elevated temperature environments may compromise their mechanical properties resulting in a reduced life cycle. Components working in such adverse conditions demand suitable components processed through near net shape techniques. The proposed MIM compacts are investigated for their resistance to wear and oxidation under laboratory conditions.vi Three types of MIM specimens namely Cr3C2-NiCr+NiCrSiB, SS316L+WC-CrC-Ni, and Tool Steel are characterized using Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD). Further, microstructure and mechanical properties were characterized to evaluate their potential for hightemperature application. Dry sliding wear behaviour of MIM specimens are evaluated using a high-temperature pin on disc tribometer. The SS316L+WC-CrC-Ni and Tool Steel MIM specimens displayed a lower coefficient of friction and wear rate in comparison with Cr3C2- NiCr+NiCrSiB. Excellent wear resistance of the MIM specimens is attributed to the solid lubricants effect. Based on the wear rate data, the relative wear resistance of the MIM specimens under dry sliding conditions is arranged in the following sequence: (Tool Steel) > (SS316L+WC-CrC-Ni) > (Cr3C2-NiCr+NiCrSiB) Higher wear resistance of MIM Tool Steel specimens is attributed to the high hardness of Cr3Ni2 phase formed during the sintering process. Thermo cyclic oxidation behaviour of MIM specimens was carried out at 700 °C for 20 cycles. Each cycle consisted of heating at 700 °C for 1 hour, followed by 20 minutes of cooling in the air. The thermogravimetric technique is used to approximate the oxidation kinetics of MIM specimens. The Cr3C2-NiCr+NiCrSiB and SS316L+WCCrC-Ni MIM specimens reported lower weight gain as compared to the Tool steel. Cr3C2-NiCr+NiCrSiB MIM specimens registered less weight gain as compared to SS316L+WC-CrC-Ni which is attributed to the excellent oxidation resistance of NiCrSiB and formation of NiCrO4 along with NiO and Cr2O3 oxides on the surface of MIM specimens. In the present study, the powders of Cr3C2-NiCr+NiCrSiB, SS316L+WC-CrC-Ni, and Tool Steel are successfully metal injection moulded and sintered to achieve 96% density. Developed MIM components exhibit resistance to high-temperature wear and oxidation which is suitable for components subjected to elevated temperature service conditions.Item Studies on Elevated Temperature Tribological Behavior of Fly Ash Based Plasma Spray Coatings(National Institute of Technology Karnataka, Surathkal, 2018) Mathapati, Mahantayya; M R, Ramesh; Doddamani, MrityunjayMaterial 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.Item Synthesis and Development of Sm2SrAl2O7 Based Air Plasma Sprayed Ceramic Thermal Barrier Coatings: Oxidation, Hot Corrosion and High Temperature Erosion Study(National Institute of Technology Karnataka, Surathkal, 2018) T, Baskaran; Arya, Shashi BhushanSamarium Stronium Aluminate (SSA) ceramic oxide was synthesized by molten salt method and coated on a bond coat (NiCrAlY)/Inconel 718 superalloy by air plasma spray (APS) process. The pre-oxidation, oxidation, hot corrosion and high temperature erosion tests were conducted at elevated temperatures. The pre-oxidation study was carried out to examine the role of thermally grown oxide (TGO) thickness on the oxidation kinetics of thermal barrier coatings (TBCs). TGO thicknesses were controlled at different pre-oxidation times of 10, 20 and 30 h at 1050 ºC in the argon atmosphere, the highest TGO thickness being found for 30 h pre-oxidized samples. Among these three pre-oxidation times, 20 h pre-oxidized SSA TBCs showed 65% higher oxidation resistance as compared to conventional Yttria stabilized zirconia (YSZ) TBCs (20 h) after oxidation at 1100 ºC in air mainly due to the presence of highly enriched α-Al2O3 at the interface. Electrochemical impedance spectroscopy was carried out and it was noticed that the highest charge transfer resistance and lowest capacitance about 0.48 × 106 Ohm cm2 and 1.1 nF cm-2 respectively were observed for 20 h of pre-oxidation. The impedance response was reduced significantly after oxidation at 1100 ºC for 10 h pre-oxidized specimen as compared to 20 and 30 h due to the compositional change of pure α-Al2O3 based TGO into more conductive NiCr2O4. The lowest diffusion coefficient, DCr3+ in the NiO lattice which reduced the formation of metal ion vacancies at the TGO-top coat interface caused to exhibit higher TGO resistance for 20 h pre-oxidized specimens over 10 and 30 h after oxidation at 1100 ºC. The hot corrosion resistance of SSA TBCs showed approximately 8-39% lower than conventional YSZ in hot corrosion environments of (i) 50 wt.% Na2SO4 + 50 wt.% V2O5 and (ii) 90 wt.% Na2SO4 + 5 wt.% V2O5 + 5 wt.% NaCl at 700 and 900˚C. The lower hot corrosion life of SSA TBCs was mainly due to the basicity of SrO. It is much higher than Sm2O3, Y2O3 and Al2O3, which indicates SrO has higher tendency followed by Sm2O3, Y2O3 and Al2O3 to react with molten salt at 700 and 900 ˚C. The high temperature erosion test was conducted in air jet erosion tester at an impingement angle of 30 and 90˚ for different testing temperatures of 200, 500, and 800 ˚C. The test was performed on pre-oxidized SSA and YSZ TBCs at 1050 ˚C for 10 h. The SSA TBCs exhibited 50% lower erosion resistance than conventional YSZ TBCs. The observed lower resistance is mainly due to the formation of mud-cracks during pre-oxidation treatment.