Preparation and characterization of MAO-Si3N4 composite coating on AZ31B magnesium alloy

Micro arc oxidation process was carried out on AZ 31 B magnesium alloy using alkaline silicate based bath at a constant current density of 0.04 A/cm2. Nano size silicon nitride (Si3N4) particles were added in the bath to obtain MAO- Si3N4 composite coatings. Plain oxide coatings were also prepared for comparison. The developed coatings were characterised for their surface morphology, composition, structure, roughness, nanohardness and wear resistance properties. Field Emission Scanning Electron Microscopy (FE-SEM) analysis of the coating exhibited the irregular porous structure with cracked morphology. Energy Dispersive Analysis of X-ray (EDX) over the surface of the composite coating showed the presence of O (42.8 wt.%), Si (13.2 wt.%), F (4.8 wt.%), Al (0.63 wt.%) and N (7.8 wt.%) with balance Mg respectively. XRD pattern obtained for composite coating revealed the characteristic peaks corresponding to Mg, MgO and Mg2SiO4. Apart from these peaks the presence of a low intensity peak corresponding to Si3N4 was also observed. Composite coating exhibited about 56% increase in nanohardness value (387 HV) compared to plain oxide coating (167 HV). Dry reciprocating wear test experiment was carried out for composite, plain oxide and substrate materials against alumina ball. Wear loss obtained for the composite is 3 times less (10 µms) compared to plain oxide coating which indicated improved wear resistance of the MAO-Si3N4 composite

XPS Characterization and Microhardness of Heat treated Co-W Coatings Electrodeposited with Gluconate Bath

Thermal stability and effect of heat treatment on electronic structure and microhardness of electrodeposited CoW alloy coatings using gluconate bath was characterized by DSC and XPS. XPS studies demonstrate that as-deposited alloy coating has significant amount of Co and W metals as well as Co2+ and W6+ species. There is a decrease in Co metal concentration in the alloy heated at 600 C and Co is in fully oxidized form when it is heat treated at 800 C. Marginal decrease in W metal concentration and presence of both W6+ and W5+ species are observed when the coating is heated at 600 C, whereas mostly W6+ species along with a little amount of W5+ could be seen in the coating heated at 800 C. Microhardness values of 1017 and 1336 HK are observed when Co−W coatings are heated at 500 and 600 C, respectively and they are comparable with as-deposited hard ch

Chromate and HF free pretreatment for MAO/electroless nickel plating on AZ31B magnesium alloy

: Microarc oxidation (MAO) coating was developed as an interlayer for the electroless nickel (EN) top coat to improve the corrosion resistance of Mg alloy. Prior to the electroless nickel coating, oxide layer was activated by using NaBH4 solution as a replacement for traditional chromate and HF activation process. The EN coatings were prepared from the alkaline carbonate bath. The prepared coatings were characterized for the surface morphology and composition using Field emission scanning electron microscopy (FE-SEM) attached with Energy dispersive analysis of X-ray (EDX). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies were carried out in non-deaerated 3.5% NaCl solution to find out the corrosion resistance of the coatings. The MAO coating showed porous morphology with micro cracks whereas, MAO/Ni-P coating exhibited dense nodular structure. The composition analysis on the surface of MAO/Ni-P showed 10 wt.% P indicated the high P coating. The cross- sectional images showed good adhesion between MAO and Ni-P layers. This clearly indicates that the present activation process results in dense with uniform pores of MAO coating which supply excellent bonding interface for Ni–P coat. The MAO/EN coating combination showed about 97 times improved corrosion resistance as compared with the substrate and similar behaviour was observed by EIS studies

STUDIES ON THE MECHANISM OF BIO CORROSION IN AIRCRAFT FUEL TANKS

Bio-corrosion is a major problem faced in aircraft fuel tanks. Microbes attach to the surface by forming biofilm. AA2024-T3, the structural component used in aircraft fuel tanks was immersed in aviation fuel and aviation fuel+biofuel (1:1) to understand the mechanism of bio-corrosion. Anodised AA2024-T3 and Ormosil coated coupons were also evaluated for their corrosion protection efficiency. Kinetics of pH, viability and protein concentration were studied and accelerated corrosion tests performed to evaluate corrosion protection. The results indicated an indirect correlation between pH and microbial growth. Anodized AA2024-T3 (Icorr-0.075* E-6) and Ormosil + Benzotriazole coating (Icorr- 0.29* E-6) showed better corrosion protection than that of untreated AA2024-T3 immersed in fuel for 60 days. Interestingly, addition of biofuel to aviation fuel offered enhanced corrosion protection compared to fuel alone

Role of powder metallurgical processing and TiB reinforcement on mechanical response of Ti-TiB composites

In this work, titanium–titanium boride (Ti–TiB) composites were synthesized by three different powder metallurgical techniques, namely, spark plasma sintering (SPS), hot iso-static pressing (HIP) and vacuum sintering (VS). The mechanical properties of the composites were determined using the nanoindentation technique. The role of the material processing route and TiB reinforcement employed on the mechanical properties of the composites was investigated. The results revealed that the composites processed by SPS possessed improved mechanical properties relative to those of the composites prepared by the HIP and VS techniques. Furthermore, reinforcement of the composites with TiB enhanced the hardness, elastic modulus and contact stiffness, whereas it reduced the fracture toughness and indentation creep

Effect of chemical treatment on the corrosion and bioactivity of equiatomic NiTi alloy

NiTi alloy in its equiatomic concentration is widely used in biomedical industry owing to its shape memory and superelasticity properties. The main problem facing for using it as implant materials is the possibility of elution of nickel, which is a known carcinogen. However, the elution rate can be reduced from the toxic limit to a reasonably safe level by adopting suitable surface modification techniques. In the present study, the advantage of using ferric chloride, which is a common etchant for nickel was explored to modify the surface of NiTi alloy. NiTi alloy was chemically treated using acidified ferric chloride solution and post treated by annealing at 400 oC and passivation in nitric acid. The alloy surface after chemical treatment resulted in a nanogrid structure with a combination of one dimensional channel and two dimensional network-like patterns. The undulations formed after chemical treatment remained unaltered after annealing, while after passivation process the undulated surface was found to be filled with titanium oxides. XPS analysis revealed that the surface of passivated sample was enriched with oxides of titanium, predominantly as TiO2. The corrosion potential, corrosion current density and breakdown potential were less noble for chemically treated NiTi alloy. The breakdown potential obtained for annealed surface was almost 200 mV higher than the passivated surface (0.8 V). Due to the decrease in surface nickel content and formation of compact titanium oxide, the overall resistance was in the range of mega ohms for passivated surfaces and the amount of nickel released after 14 days of immersion is almost half when compared to untreated NiTi alloy. Thus a combination of chemical treatment, annealing and passivation processes on NiTi alloy exhibits good bioactivity and corrosion resistance and may be considered suitable for biomedical implant applications

Preparation and characterization of adherent autocatalytically deposited nickel coating on carbon fiber

In the present study carbon fibers were successfully coated with nanocrystalline nickel using an acidic bath by electroless plating method. Coating thickness obtained was about 1.5 microns beyond that there was coating delamination. Coated fibers were characterized for various properties such as morphology, composition, structure, phase transformation temperature, resistivity and tensile strength. Field emission scanning electron microscope (FESEM) studies revealed that the coating showed nodular morphology. Energy dispersive analysis of X-ray (EDAX) showed that the coating containing about 10.5 wt.% P with balance Ni. Structural studies carried out on these coated fibers exhibited two major diffraction peaks and were assigned as C (002) and Ni (111). Differential scanning calorimetry (DSC) studies on these coated fibers exhibited a single exothermic peak at 3510C. Activation energy obtained for the crystallization process of high P deposit is about 215.9 kJ/mol. Bulk resistivity was measured using four-probe technique over a single coated fiber and the obtained value was around 3.2 μΩ-m. Tensile strength of these coated fibers were also carried out and observed that not much variation found in the strength of coated and uncoated fibers

Investigation on wear and corrosion behavior of equal channel angular pressed aluminium 2014 alloy

Aluminium 2014 alloy solutionized at 495°C, aged at 195°C was subjected to Equal Channel Angular Pressing (ECAP). Dry sliding wear tests were conducted using pin on disc tribometer system under nominal loads of 10N and 30N with constant speed 2m/s for 2000m in order to investigate their wear behavior after ECAP. The Co-efficient of friction and loss in volume were decreased after ECAP. The dominant wear mechanism observed was adhesion, delamination in addition to these wear mechanisms, oxidation and transfer of Fe from the counter surface to the Al 2014 pin were observed at higher loading condition. The corrosion behavior was evaluated by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution. The results obtained from PDP showed higher corrosion potential and lower corrosion density after ECAP than base. Electrochemical impedance spectroscopy (EIS) showed higher charge transfer resistance after ECAP. Surface morphology showed decreased pit size and increased oxygen content in ECAP sample than base after PDP

Electroless deposition and characterization of high phosphorus13; Ni-P-Si3N4 composite coatings

Composite coatings were prepared using hypophosphite reduced electroless nickel bath containing 1g/L submicron silicon nitride particles at pH 4.6 +/- 0.2 and temperature 85 +/- 2 degrees C. Deposition rate was 6-8 mu m/hour for both plain Ni-P and composite coatings. The amount of silicon nitride particles codeposited in the Ni-P matrix was around 3.5 wt.%. As-deposited coating surface composition analysis, carried out by Energy Dispersive Analysis of X-ray (EDX), results showed that plain Ni-P and Ni-P-Si3N4 deposits were having around 10 wt.% phosphorus. The X-ray diffraction (XRD) pattern of Ni-P-Si3N4 coating was very similar to that of plain electroless Ni-P coating in as-deposited condition. Presence of a single, broad peak around 45 degrees 2 which corresponds to Ni (111) peak was seen in both deposits. The calculated grain size by Debye-Scherrer method for both deposits was around 1.2 nm. Optical micrograph of the deposit cross-section revealed that the particles incorporation was uniform throughout the thickness of the coating. Phase transformation behavior studied by Differential Scanning Calorimeter (DSC) indicates that the particle incorporation had not influenced the crystallization temperature of the composite coatings. Presence of metastable phases like NiP2 and Ni5P4 were observed for both coatings annealed at crystallization temperature. Increase in grain size of the deposits from 1.2 nm to 21 nm was also observed due to the annealing at crystallization temperature. Microhardness measurements made on the as-deposited and annealed (400 degrees C) cross-sectional coatings showed that there was about 10% and 22% increase in hardness values respectively with the codeposition of silicon nitride particles

Assessment of chromate and chromate-free conversion coatings for the corrosion protection of aerospace aluminum alloy

Aerospace aluminum alloys, especially the most commonly used AA 2024 T3 alloy is often susceptible to corrosion in chloride containing environment. These alloys are phase separated, highly complex metal-in-metal composites and tend to have weakness towards local galvanic corrosion because of this microstructure. At present the coating system used for aerospace aluminum alloys involves a multi-layer system such as a pretreatment layer at the primer/Al-alloy substrate interface, primer and topcoat. Chromate conversion coatings (CCCs) are usually applied as pretreatment layer to aluminum and its alloys. Despite its efficiency and versatility, the use and emission of hexavalent chromium (Cr6+) has been under increasingly strict regulation because of its high toxicity and carcinogenic effect. For the past two decades concentrated efforts were made to develop an ecofriendly conversion coating system as an alternative to CCC and also chromate free additives in primer layer