Herein, bimodal-reinforced AlCoCrFeNi complex concentrated alloy composite cladding on stainless steel 316L substrate using microwave hybrid processing is synthesized. Detailed microstructural analysis of the claddings shows the presence of cellular structure A2 (disordered BCC) with intermetallics (B2 (ordered BCC) and σ phases) at the intercellular regions. The presence of Cr23C6 is also observed for the reinforced claddings due to the dissociation of the SiC particles. The bimodal-reinforced cladding shows the highest hardness (in excess of 800 HV) and fracture toughness (≈12 MPa√m), followed by micro- and nanocomposite claddings. Under slurry erosion condition, the bimodal composite cladding exhibits 10 times better erosion resistance than stainless steel 316L and nonreinforced cladding (1.5–2 times), respectively, at oblique angle followed by nano- and microreinforced unimodal counterparts. The outstanding performance of the bimodal composite cladding under both test conditions is related to higher hardness, fracture toughness, and resistance to plastic deformation. The detailed analysis of the eroded samples shows microcutting and ploughing as a dominant mechanism at an oblique angle while platelets, cracks, and microindents being the contributing factors for erosion at the normal impact angle. © 2020 Wiley-VCH GmbH

Harpreet Singh

Mechanical Engineering

(SoE) School of Engineering

Harpreet Singh Grewal

Nair R.B.

<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

Advanced Engineering Materials

In this current study, we developed SiC (10 wt\%) reinforced AlCoCrFeNi complex concentrated alloy composite claddings with different particle sizes (micro, nano and bimodal) on stainless steel 316L substrate using microwave irradiation. Microstructural analysis showed cellular structured claddings with intermetallic phases occupying the intercellular regions along with low porosity (&lt;1\%). The claddings were mainly composed of A2 (disordered BCC) and B2 (ordered BCC) along with Cr23C6. The bimodal (mixture of nano and micro) composite cladding showed highest hardness and fracture toughness (810 HV and ~12.2 MPa√m) followed by nano and micro composite claddings. Under cavitation erosion (distilled water) condition, bimodal cladding showed highest incubation period (IP) (~13 h) with extremely low mean depth erosion rate (MDER) (~0.089 μm/h) with 16 and 27 times lower than stainless steel 316L and WC-based coating, respectively. The cavitation erosion resistance observed for the bimodal composite cladding is among the highest demonstrated by the CCAs at present. However, under erosion-corrosion conditions, non-reinforced and micro-reinforced claddings showed higher degradation resistance with lower material removal rates than that observed for bimodal cladding. Standalone electrochemical corrosion studies also showed highest corrosion and passivation resistance for non-reinforced cladding. These results were explained on the basis of formation of Cr-depleted micro-galvanic cells due to high negative enthalpy of Cr with C dissociated from SiC particle. The detailed morphological analysis of the tested samples showed presence of tearing top surface as the primary degrading mechanism along with fracture of intermetallic phases. The results show that bimodal composite CCA cladding provides a potential surface engineering solution for improvising the serviceability of the many engineering systems. © 2020 Elsevier B.V.

Surface and Coatings Technology

Complex concentrated alloy bimodal composite claddings with enhanced cavitation erosion resistance

In this study, a novel processing technique for microstructural refinement of Ni-Cr-5Al2O3 composite coating developed using high velocity oxy-fuel (HVOF) technique is demonstrated. The processing technique, known as stationary friction processing (SFP) is an adaptation of the well-known friction stir processing (FSP). The as-sprayed coating showed a typical lamellar microstructure along with non-homogeneous elemental distribution. The SFP treatment resulted in significant microstructural refinement with complete elimination of splat boundaries and pores together with fully homogeneous elemental distribution. The performance of as-sprayed and processed coatings was evaluated in slurry erosion, erosion-corrosion and pure corrosion in 3.5\% NaCl solution. At oblique impingement angle, the SFP treated sample showed minimum erosion rate of nearly 0.1 mm3/h which is 3–5 times lower than the as-sprayed coating and the substrate. For normal impingement, the erosion rate of SFP specimen was nearly 30\% lower compared to the as-sprayed coating and the substrate. This is attributed to higher hardness as well as fracture toughness of the SFP treated coating as a result of microstructural refinement. In addition, the SFP treated coating was able to demonstrate superior resistance under erosion-corrosion conditions as well. Further, the processed sample showed lowest corrosion rate of 0.079 μA/cm2, 5–6 times lower than the as-sprayed coating (0.39 μA/cm2). The enhancement in the corrosion resistance of the coating after processing is attributed to complete homogenization of the coating with removal of all splats, splat boundaries, pores and regions of elemental segregation. © 2020 Elsevier B.V.

Structural rejuvenation of thermal spray coating through stationary friction processing

In present work, we investigated the tribological behavior of the high entropy alloy (HEA) claddings under different degrading environments viz, slurry erosion-corrosion, liquid impingement erosion, electrochemical corrosion, and sliding wear. The HEA claddings were developed by varying Al fraction (AlxCoCrFeNi) (x = 0.1 to 3) using microwave processing. The claddings were composed of cellular structure with intermetallic phases segregated in the intercellular region. Variation in Al content showed significant influence on the microstructure and mechanical properties. The hardness of the claddings increased with an increase in Al fraction with a maximum value of 624 HV observed for the cladding containing three molar Al. Correspondingly, the fracture toughness showed an opposing trend. All claddings showed better wear resistance than SS316L with equimolar composition outperforming other counterparts. The equimolar HEA cladding showed 3 to 23 times higher wear resistance compared to SS316L steel depending upon test conditions. Inimitable wear resilience of the equimolar composition was attributed to its high hardness and strain-hardening ability. Surface morphologies of the tested claddings indicated the presence of ductile-brittle mixed damage modes. The tested surfaces showed numerous pits and cracks with delaminated secondary phase. Presence of groves and signs of delamination were also observed in case of sliding wear. The results of the present work show that microwave synthesized equimolar HEAs can effectively counter different tribological forms. © 2019 Elsevier B.V.

Wear

Tribological behavior of microwave synthesized high entropy alloy claddings

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.

Journal of Thermal Spray Technology

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

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.