Search

For advanced thermal barrier coating (TBC) system in a modern aero and land based engine, there is pressing needs to impede the uncontrolled growth of thermally grown oxide (TGO) layers as well as the weight reduction of overall TBC system. To address both the issues, present study introduces a ‘bondcoat less’ TBC. Selective laser melting (SLM) technique has been used to fabricate two different substrates of composition NiCrAlY and NiCrAlY + 1 wt% carbon nanotube (CNT). Lanthanum zirconate (LZ) coating was deposited over the prepared substrate by plasma spraying technique. SLM processed NiCrAlY + 1 wt% CNT substrate showed the relatively higher increase in hardness and elastic modulus by 1.6 and 2.7 times, respectively. Top coat LZ coating and reinforced CNTs in NiCrAlY significantly suppressed the growth of the TGO layer at 1800 °C. It is shown that a fine microstructure, absence of pores in the coating and addition of CNTs in conjunction prevent the diffusion of the reactive species Al, Cr, Y from the NiCrAlY substrate towards the coating interface. Further, LZ coating despotised over NiCrAlY + 1 wt% CNT substrate was intact even after 48 cycles during the thermal shock resistance performed at 1800 °C.

Silicon carbide (SiC) reinforced nickel–tungsten (Ni–W) coatings were successfully fabricated on steel substrate by pulse electrodeposition method (PED) and the amount of SiC was varied as 0 g/l, 2 g/l, and 5 g/l in Ni–W coating. Effect of subsequent addition of SiC on microstructures, phases and on corrosion property of the coating was investigated. Field emission scanning electron microscopy (FE-SEM) image of the surface morphology of the coating showed the transformation from the dome like structure to turtle shell like structure. X-ray diffraction (XRD) of Ni–W–5 g/l SiC showed the disappearance of (220) plane of Ni(W), peak splitting in major peak of Ni(W) and formation of distinct peak of W(Ni) solid solution. Absence of (220) plane, peak splitting and presence of W(Ni) solid solution was explained by the high resolution transmission electron microscopy (HR-TEM) images. Tafel polarization plot was used to study the corrosion property of the coatings in 0.5 M NaCl solution. Ni–W–5 g/l SiC coating was showed higher corrosion resistance (i.e. ∼21% increase in corrosion potential, Ecorr) compared to Ni–W coating. Two simultaneous phenomena have been identified for the enhanced corrosion resistance of Ni–W–5 g/l SiC coating. (a) Presence of crystallographic texture (b) formation of continuous double barrier layer of NiWO4 and SiO2.

Electroless Ni–P coating was deposited on steel substrate and the effect of intermediate temperature oxidation on crystallization mechanism and corrosion properties of the coating was investigated. Ni–P coatings were annealed at three different temperatures, viz. 200 °C, 400 °C and 600 °C for 2 h in air. Formation of nickel oxide (NiO) was observed in the coating upon annealing beyond the crystallization temperature (330 °C). Crystallization mechanism provided insight about the step by step formation of long range ordered Ni,Ni 3 P and NiO phases.Improvement in the corrosion resistance of Ni–P coating compared to bare steel was found to be ∼21% on annealing at 400 °C in air which gradually increased to ∼31% on annealing the coating at 600 °C in air. Increasedcorrosion resistance at 400 °C annealed coating was attributed to the formation of crystalline Ni and Ni 3 P phases. Two simultaneously effects have been identified for the increased corrosion resistance of the coating annealed at 600 °C in air. (a) Formation of NiO layer which acts as a passivation layer and protects the underlying P enriched layer and (b) absence of an interdiffusion layer from substrate to coating.