Materials Science Forum Vols. 830-831

Abstract: Alumina coating is given to a 07Х16Н6M stainless steel substrate, to withstand high frictional loads at elevated temperatures. Properties like high hardness, good adhesion and low thermal mismatch are essential for the coating. An 80%Ni-20%Cr bond coat is given for better adhesion of the alumina coating to the substrate. Air plasma spray technique was used for bond coat as well as the alumina coating.The substrate, bond coat and the alumina coating were characterized using optical microscopy, scanning electron microscopy (SEM) and nanoindentation. Hardness was evaluated using nanoindentation in all the three regions at loads varying from 2 mN in the bond coat to 50 mN in the alumina coating. Alumina coating showed the highest hardness of 11.5 GPa, followed by the bond coat – substrate interface having a hardness of 7.4 GPa. Substrate exhibited the least hardness of 6.3 GPa.

Abstract: In this study, the deposition of Manganese (Mn) on Cu-Cr-Zr-Ti alloy was carried out using electro plating route. The purpose of this coating is to join copper alloys utilizing eutectic reaction between Cu and deposited Mn. Mn coating through electro deposition is difficult due to very high electronegativity. Two different bath compositions have been studied, with and without the addition of sodium selenate (Na₂SeO₄), where base composition remains same (MnSO₄ & (NH₄)₂SO₄)_. The influence of the bath composition and deposition parameters such as pH, current density, time on the deposition rate of Mn coating was studied. Higher deposition rate of Mn was obtained with lower pH and higher current density (~6 amps/dm²). Time of deposition was optimized to obtain varying thickness, ranging from 10 to 30 microns. Detailed characterization was carried on the Mn coated samples using OM, SEM-EDS and AFM. XRD phase analysis on the coated surface ensures the presence of α-Mn at room temperature. Subsequently, DSC analysis was carried out on the Cu alloy/Mn deposit pair to assess the brazeability temperature window.

Abstract: Low carbon stainless maraging steel (0.03%C-12%Cr-10Ni-0.6Mo-0.2Ti) is being used widely for various components of the aerospace engines. To improve the wear resistance of the steel various surface hardening processes are being utilized to improve the surface hardness above 900HV. In this present research, plasma nitriding was carried out at two different temperatures of 450 °C and 475 °C for the holding duration of 10 hrs. Temperature of the nitrding process was ensured below the ageing temperature (500 °C) of the steel to avoid lowering of mechanical properties. Effect of plasma nitriding parameters on the surface hardness, case depth, microstructure and phases present in the nitrided layer were investigated in detail using microhardness analysis across the nitrided layer, X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). It was observed that increase in nitriding temperature increased the surface hardness and case depth. In addition, the presence of Fe₃N and Fe₄N phases in the nitrided layer were observed using X-ray diffraction technique.

Abstract: The Mg-Al-Zn alloy is used in the coolant systems of automobile engines. Corrosion of Mg-Al-Zn engine components by coolants, which consist of 30% aqueous ethylene glycol as a coolant composition, is an important issue in automobile industries. With this preview, in the present study it is intended to study the effect of environmental contaminant sulphate ions on the corrosion behavior of Mg-Al-Zn alloy in 30% (v/v) aqueous ethylene glycol solution. Electrochemical investigations were carried out by potentiodynamic polarization and electrochemical impedance spectroscopy techniques. Scanning electron microscopy (SEM) and energy dispersion X-ray analysis (EDX) were used to study the surface morphology and composition of the alloy. Activation parameters for the corrosion process were calculated by conducting the corrosion studies at different temperatures. It was observed that corrosion rate of the alloy in 30% (v/v) aqueous ethylene glycol increases with the increase in the concentration of sulphate ions and also with the increase in temperature.

Abstract: The main aim of the present work is to develop an oxidation resistant Nb-Si coating on Nb alloys. For this purpose electroless molten salt - method was used, where a selective reaction occurs between NaCl–KCl–NaF–Na₂SiF₆ and the Si powder. Due to this reaction, oxidation resistant Nb-Si phases forms on the surface of substrate. This paper reports the thickness variation of Nb-Si coating as a function of deposition time. The surface morphology was studied by using scanning electron microscopy (SEM). The various Nb-Si based phases formed on the Nb substrate have been identified using X-ray diffraction (XRD) analysis, supplemented with energy dispersive X-ray spectroscopy results. Based on these results mechanism of silicide coating on Nb alloys has been discussed in detail.

Abstract: Electroless nickel coating is one of the versatile methods to improve the physical properties of aluminium alloys. The increasing demand for high corrosion resistant surface with improved performance leads to the development of poly-alloy and composite coatings with the aid of ultrasonication. In the present study Ni-B alloy and Ni-B-CeO₂ composite coatings were made on 356 aluminium alloy surfaces. The variation in structural and mechanical properties of coatings formed under both ultrasonication and magnetic stirring were investigated. Surface analysis by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) has shown the formation of more uniform coating under ultrasonication. The electrochemical polarization tests show enhanced corrosion resistance in ultrasonic assisted coatings.

Abstract: Yttria Stabilized Zirconia (YSZ) coating was developed using electron beam physical vapour deposition as a top coat material for thermal barrier application. In the present study, attempt to develop YSZ films by evaporating ZrO₂ stabilized with 8 wt% Y₂O₃ powder using EBPVD technique. YSZ films were deposited on Inconel-718 (Ni-Cr-Fe) substrate by varying the substrate temperature in the range of 673 K to 1073 K. The deposited films have shown good adherence to the substrate and exhibit uniform dense microstructure. The effect of substrate temperature, phase and surface morphology were investigated by XRD, AFM and SEM. The film deposited at 673 K shows crystalline nature and the crystallinity of the deposited films increase with increasing substrate temperature. The surface roughness of the films increases with increase in the deposition temperature due to the increase in crystallinity of the films. SEM images shows that the films are grown with triangular shaped morphology. Nanomechanical characteristics of the tetragonal phase of the YSZ films were investigated by nanointendation technique and the young’s modulus of the coating was found to be 118 GPa.

Abstract: Double perovskites, pyrochlores are having matching thermal expansion co-efficient with bond coat and may act as candidate materials for an intermediate layer between bond coat and top coat in Thermal Barrier Coatings. Hot corrosion stability of double perovskites and pyrochlores is also important for enhanced thermal cycle life. Based on these issues, systematic studies were conducted to find the hot corrosion stability of double perovskites and pyrochlores. The results revealed that, double perovskites and pyrochlores were undergone destabilization in the Na₂SO₄+50 wt.% V₂O₅ and Na₂SO₄+10 wt.% NaCl environments at 900 oC. The implications of these findings addresses the key issues related hot corrosion mechanisms and give a pathway to developing newer materials. This study clearly indicates the destabilization of both double perovskites and pyrochlores in vanadium and chloride environments at 900 oC.

Abstract: Polysiloxane resin copolymer was synthesized through acid catalyzed hydrolysis of methyl triethoxysilane (MTEOS) and diethoxytetramethyldisiloxane (DEOTMDS). The effect of reaction time on the properties of the polymer was studied and this copolymer was characterized by GPC, ²⁹Si NMR, IR, TGA, viscosity, refractive index, specific gravity and solid content. ²⁹Si NMR and IR showed characteristic signals of Si-O-Si linkage which confirmed the formation of the polymer. GPC and solid content analysis showed an increasing trend in molecular weight with reaction time. Thermo gravimetric analysis showed that the polymer was thermally stable upto ≈ 260°C and all the polymers gave a ceramic residue in the range of 77-80% at 900°C. Siloxane prepared inhouse and methyl phenyl silsequioxane (control) were used as coating materials and atomic oxygen (AO) resistance was evaluated on Al-Kapton, carbon polyimide composite and glass polyimide composite. The mass loss and surface morphology of the coated samples were measured at different time intervals. It is observed that mass loss of polysiloxane coated samples was very less, compared to coated control samples. The morphology of all the samples were studied using FESEM. Erosion kinetics and surface morphology investigation indicate that the polysiloxane coating possesses excellent AO resistance, and displays better cracking resistance on AO exposure.

Abstract: AISI 302 stainless steel is used for making compression springs for launch vehicle programmes. One such AISI 302 stainless steel compression spring used in flush and purge valve of liquid engine of a satellite launch vehicle failed during testing. The failure was at the second round of spring and it failed in a slanted type fracture. Detailed metallurgical analysis indicated that the failure was due to fatigue.