Cavitation erosion is a huge problem in engineering structures working under hydrodynamic conditions. The synergistic effect of erosion and corrosion can aggravate the material removal by several orders of magnitude. Surface coatings are widely used to address material degradation by erosion–corrosion. However, non-homogenous microstructure and presence of defects leads to premature coating failure under erosion–corrosion conditions. Thus, it is imperative to identify plausible solutions to address cavitation-related material degradation. In this study, Ni-Cr-5Al2O3 coatings were deposited on stainless steel substrate using high-velocity-oxy-fuel technique. The as-sprayed coating showed highly non-homogeneous microstructure comprising splats, pores, intermetallic compounds and elemental segregation. A new thermo-mechanical processing technique, stationary friction processing (SFP), was utilized for achieving through-thickness microstructural refinement in as-sprayed coating. As-sprayed and SFP-treated coatings were tested in pure cavitation erosion, corrosion in 3.5\% NaCl solution and erosion–corrosion. SFP treatment resulted in 5-times enhancement in the erosion and corrosion resistance compared to as-sprayed coating. The remarkable performance of Ni-Cr-5Al2O3 coatings after SFP treatment is attributed to significant enhancement in the mechanical properties including hardness and fracture toughness which is the consequence of complete removal of splat boundaries, pores, intermetallic compounds and uniform element distribution up to the coating–substrate interface. © 2020, ASM International.

Harpreet Singh

Mechanical Engineering

(SoE) School of Engineering

Harpreet Singh Grewal

Rani M.

Perumal G.

Singh H.

<p>Shiv Nadar University is a comprehensive, multidisciplinary, research-focused, and student-centric University offering a full range of academic programs at the undergraduate, postgraduate and doctoral level. With state-of-the-art infrastructure, the University comprises of academic wings, sophisticated labs, international standard sports facilities, amphitheaters, auditorium, conference rooms and smart classrooms. The University&rsquo;s goal is to become internationally recognized for the quality of its research and creative endeavors and their applicability to improving quality of life, generating new insights and expanding the boundaries of human knowledge creativity. Committed to excellence in teaching, research and service, the University aims to serve the higher education needs of India and the world beyond.</p>

Shiv Nadar University

Journal of Thermal Spray Technology

Slurry and cavitation erosion often limits the durability of many fluid machines such as hydroturbines, pumps, and propellers. Despite good resistance against slurry erosion, thermal spray coatings generally exhibit poor resistance against cavitation erosion. The performance of thermal spray coatings under cavitation erosion is limited by the presence of defects such as pores, splat boundaries, and limited adhesion with the substrate. In the present work, we performed post-processing of the WC-10Co-4Cr and Ni coatings deposited using the detonation gun process. For post-processing, microwave technique was used owing to its capability for atomic-level heating. The microstructural characterization of the as-sprayed and post-processed coatings showed significant homogenization for the latter. Compared to the typical lamellar structure of as-sprayed, the processed samples exhibited a columnar structure with metallurgical bonding with the substrate. The mechanical properties of the coatings were significantly improved after post-processing owing to the elimination of splat boundaries and pores which significantly enhanced the cavitation and slurry erosion resistance. Post-processed coatings showed at least ten times higher resistance to cavitation erosion. The slurry erosion resistance of the coatings also improved up to three times depending upon the impingement angle. A significant difference in the erosion mechanism was also observed after post-processing. © 2019, ASM International.

Microwave-Assisted Post-processing of Detonation Gun-Sprayed Coatings for Better Slurry and Cavitation Erosion Resistance

In this study, we demonstrate a novel pathway to engineer the properties of metallic alloys for limiting their cavitation erosion-corrosion. A facile single-step processing technique was used to develop bimodal grain structure in stainless steel. The bimodal steel was tested in cavitation erosion and erosion-corrosion conditions. In both the cases, bimodal steel showed exceptionally high degradation resistance, nearly 7 times higher compared to as-received steel. The remarkable cavitation erosion resistance demonstrated by bimodal steel is attributed to its high yield strength along with high work-hardening rate. In addition, the bimodal steel showed significantly low corrosion rate of 0.001 mm/year in 3.5 wt \% NaCl solution compared to 0.088 mm/year for as-received stainless steel. © 2019 Elsevier Ltd

Tribology International

Exceptional cavitation erosion-corrosion behavior of dual-phase bimodal structure in austenitic stainless steel

In the current work we developed Ni-SiC based composite claddings on SS316L through microwave irradiation using different reinforcement weight fractions and particle sizes (micron to nano scale). A bimodal derivative using equal proportions of both micro and nano scale particles was also developed. Microstructural characterization showed claddings were composed of columnar structure with precipitated intermetallic phases segregated at the grain boundaries. The Ni-SiC bimodal particle reinforced claddings showed highest hardness and fracture toughness. All the developed claddings showed significantly high cavitation erosion resistance compared to SS316L steel. The Ni-10 wt\% SiC bimodal particle reinforcement cladding showed highest erosion resistance with around 5 times lower erosion rates than SS316L steel along with highest incubation period. The investigated bimodal particle reinforced cladding also outperformed the thermal sprayed WC10Co4Cr and Stellite 6 coatings and CA6NM hydroturbine steel used for comparison. Detailed structure-property correlation analysis showed that the exquisite performance of the investigated bimodal particle reinforcement cladding can be ascertained to high hardness and H/E ratio (inversely related with plasticity index). Compared to brittle failure mode exhibited by thermal spray coatings due to splat delamination, the Ni-SiC claddings showed mixed signatures of both ductile and brittle failure modes. Examination of the eroded surfaces of the claddings showed presence of significant number of micro pits surrounded by extruded lips and dislodged intracellular phase. Detailed SEM analysis of the samples near incubation period showed damage accumulation initiated around these intermetallic phases in the form of cracks and pits. It was shown that Ni-SiC based microwave-derived bimodal particle reinforced claddings are an effective and efficient solution for countering the cavitation induced erosion in fluid machinery. © 2019 Elsevier B.V.

Wear

Microwave synthesized composite claddings with enhanced cavitation erosion resistance

Surface phenomenon such as cavitation erosion-corrosion limits the working life and durability of the fluid machines through significantly altering the efficiency. Surface modification is an apparent and economical route for improving the sustainability of these components. Recently developed complex concentrated alloys (CCAs) or high entropy alloys (HEAs) possess exceptional properties owing to high configurational entropy. We developed CCA coatings on the stainless steel using a facial and effective microwave processing technique. The effect of Al molar fraction in AlxCoCrFeNi (x = 0.1–3) CCAs on ultrasonic cavitation erosion-corrosion was investigated in 3.5\% NaCl solution. For comparison, cavitation erosion and electrochemical corrosion behavior of the pre- and post-tested samples was also performed. Detailed microstructure and mechanical characterization of the developed coatings were also preformed using different analytical techniques. The equimolar CCA coating showed apical degradation resistance under both pure erosion and erosion-corrosion conditions. The observed behavior is attributed to high strain hardening, optimal hardness, fracture toughness, and utmost stability of the passive layer. The phenomenal conjugation of these properties was associated with highest configurational entropy for equimolar composition resulting in sluggish diffusion, and severe lattice straining. Compared to pits, striations and cracks characterizing the morphology of the degraded stainless steel, the equimolar and Al0.1CoCrFeNi CCAs showed TTS (tearing topograph surface) as the dominant failure mode characterized by presence of microplastic deformation. The degradation of the Al3CoCrFeNi CCA occurred mainly through brittle failure mode. The difference in failure mechanism is related to the mechanical properties and underlying microstructure. © 2018 Elsevier B.V.

Fulltext

Ultrasonics Sonochemistry

Microwave synthesized complex concentrated alloy coatings: Plausible solution to cavitation induced erosion-corrosion

Cavitation erosion remains the primary cause of material degradation in fluid machinery components operating at high speed. Micro-jets/shock waves caused by implosion of bubbles on material surface results in significant material loss and premature failure of the components. The presence of corrosive medium further exuberates this effect, causing rapid degradation. Here, we demonstrate a novel pathway to control cavitation erosion-corrosion by tailoring the surface properties using submerged friction stir processing (FSP), a severe plastic deformation process. FSP parameters were varied over wide range of strain-rates to generate tailored microstructures. High strain-rate processing resulted in nearly single phase fine grained structure while low strain-rate processing resulted in phase transformation in addition to grain refinement. As-received and processed samples were subjected to ultrasonic cavitation in distilled water as well as in corrosive environment of 3.5\% NaCl solution. Individual roles of cavitation erosion, corrosion and their synergistic effects were analyzed. Depending on the microstructure, processed samples showed nearly 4–6 times higher cavitation erosion resistance compared to as-received alloy. Superior cavitation erosion-corrosion resistance of processed samples was attributed to surface strengthening, higher strain-hardening ability and quick passivation kinetics. The results of current study could be potentially transformative in designing robust materials for hydro-dynamic applications. © 2018 Elsevier B.V.