In the current study, surface modification of austenitic stainless steel, SS316L, was done using friction stir processing. The strain rates during processing were varied over a wide range by using two different tool rotational speeds of 388 rpm and 1800 rpm. The processing was done at two different conditions of: (i) ambient cooling and (ii) submerging in liquid bath maintained at 0 °C. Friction stir processing resulted in significant grain refinement from 22 μm for the as-received alloy to 0.67 μm for the sample processed at 388 rpm under submerged cooling condition. A relationship between the final recrystallized grain size and Zener-Holloman parameter was developed as Z=72.84/d. Deformation texture for processed specimens showed strong strain rate and temperature dependence. Sample processed at high temperature and low strain-rate showed copper-type texture while low temperature and high strain-rate promoted brass-type texture. Hardness of processed specimens showed an increase while elastic modulus decreased after processing which is explained by texture evolution during processing. Tribological properties of processed specimens were evaluated using reciprocating wear tests and slurry erosion tests. The processed samples showed significant improvement in the wear and erosion resistance and were found to have strong correlation with the flow work given as hardness to elastic modulus ratio. © 2017 Elsevier B.V.

Harpreet Singh Grewal

Mechanical Engineering

(SoE) School of Engineering

Selvam K.

Prakash A.

Arora H.S.

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Shiv Nadar University

Materials Chemistry and Physics

Owing to its good biocompatibility and low cost, stainless steel is one of the most widely utilized biomaterial. However, longtime assessment of stainless steel has shown problems related to material degradation, especially localized corrosion and bio-film formation. In addition, the leaching of toxic nickel and chromium ions from stainless steel leads to additional health complications. Here, we utilized submerged friction stir processing, a severe surface deformation technique for significantly enhancing its durability, bio-activity as well as antibacterial resistance. The processing was done with a wide variation in strain rates to produce tunable surface microstructure. High strain-rate processing resulted in nearly single-phase fine-grained microstructure, while slow strain-rate processing developed a dual-phase fine-grained microstructure. The bio-corrosion rate of processed steel was reduced by more than 60 \% along with significant enhancement in the pitting resistance. The processed steel showed nearly no bacterial adhesion/biofilm formation, evaluated using S. aureus and E. coli bacterial strains. Further, the processed stainless steel surface demonstrated minimum leaching of the toxic elements, significantly enhancing its appeal for bio-implant applications. The observed behavior was explained based on the formation of a stable passive layer, rich in Cr2O3, as determined using x-ray photoelectron microscopy (XPS) and increased hydrophilicity. © 2020 Elsevier B.V.

Colloids and Surfaces B: Biointerfaces

Enhanced durability, bio-activity and corrosion resistance of stainless steel through severe surface deformation

Here, an excellent cavitation erosion-corrosion resistance of stainless steel using a novel severe surface deformation technique, reciprocating friction processing (RFP) was demonstrated. The RFP treated stainless steel showed nano-grained, nano-twinned microstructure along with high dislocation density. The RFP treated steel significantly surpassed the performance of advanced structural materials including high entropy alloy (Al0.1CoCrFeNi) and complex concentrated alloy (SiC reinforced Al0.1CoCrFeNi), demonstrating the lowest cavitation erosion and cavitation erosion-corrosion rates for this class of materials. The RFP treated steel demonstrated outstanding corrosion behavior in 3.5\% NaCl solution as well. The unusual performance of RFP treated steel is attributed to its excellent mechanical strength and strong passivating behavior. © 2020 Elsevier B.V.

Wear

Cavitation erosion-corrosion resilient surfaces through reciprocating friction processing

Thermal spray coatings are widely used for addressing erosion problems in fluid machineries. However, the presence of splat boundaries, non-homogeneous microstructure and element segregation limits their performance. In this study, we developed Ni-Cr-40Al2O3 coatings on stainless steel (SS316L) substrate using high-velocity flame spray followed by post-processing using friction stir processing. The performance of as-sprayed and processed coatings was evaluated in slurry erosion, erosion–corrosion and pure corrosion in 3.5\% NaCl solution. As-sprayed coating showed lower erosion and erosion–corrosion resistance compared to stainless steel. Friction stir processing resulted in significant microstructure refinement concurrent with enhanced hardness and fracture toughness of the developed coating. At oblique impingement angle, the processed coating showed nearly 30\% to twofold higher erosion and erosion–corrosion resistance compared to stainless steel substrate. During pure erosion at normal impingement, both as-sprayed and processed coating showed higher volume loss. However, during erosion–corrosion at normal impingement, the processed coating was able to demonstrate similar volume loss as that of the substrate. In addition, the processed coating showed significant improvement in the corrosion performance as well. The study showed that friction stir processing could be a promising pathway to unravel the deleterious effects of inherent non-homogeneities in thermal spray coatings. © 2019, ASM International.

Journal of Thermal Spray Technology

Post-processing of Ni-Cr-Al2O3 Thermal Spray Coatings Through Friction Stir Processing for Enhanced Erosion–Corrosion Performance

<p>Biofilm related bacterial infection is one of the primary causes of implant failure. Limiting bacterial adhesion and colonization of pathogenic bacteria is a challenging task in health care. Here, a highly simplistic processing technique for imparting antibacterial properties on a biomedical grade stainless steel is demonstrated. Low-temperature high strain-rate deformation achieved using submerged friction stir processing resulted in a nearly single phase ultra-fine grain structure. The processed stainless steel demonstrated improved antibacterial properties for both Gram-positive and Gram-negative bacteria, significantly impeding biofilm formation during the in vitro study. Also, the processed stainless steel showed better compatibility with human fibroblasts manifested through apparent cell spreading and proliferation. The substantial antibacterial properties of the processed steel are explained in terms of the favorable electronic characteristics of the metal-oxide and by using classical Derjaguin–Landau–Verwey–Overbeek (DLVO) and the extended DLVO (XDLVO) approach at the cell–substrate interface. © 2019, © 2019 Informa UK Limited, trading as Taylor &amp; Francis Group.</p>

Biofouling

Enhanced antibacterial properties and the cellular response of stainless steel through friction stir processing

Substrate-cell interactions for a bioimplant are driven by substrate's surface characteristics. In addition, the performance of an implant and resistance to degradation are primarily governed by its surface properties. A bioimplant typically degrades by wear and corrosion in the physiological environment, resulting in metallosis. Surface engineering strategies for limiting degradation of implants and enhancing their performance may reduce or eliminate the need for implant removal surgeries and the associated cost. In the current study, we tailored the surface properties of stainless steel using submerged friction stir processing (FSP), a severe plastic deformation technique. FSP resulted in significant microstructural refinement from 22 μm grain size for the as-received alloy to 0.8 μm grain size for the processed sample with increase in hardness by nearly 1.5 times. The wear and corrosion behavior of the processed alloy was evaluated in simulated body fluid. The processed sample demonstrated remarkable improvement in both wear and corrosion resistance, which is explained by surface strengthening and formation of a highly stable passive layer. The methylthiazol tetrazolium assay demonstrated that the processed sample is better in supporting cell attachment, proliferation with minimal toxicity, and hemolysis. The athrombogenic characteristic of the as-received and processed samples was evaluated by fibrinogen adsorption and platelet adhesion via the enzyme-linked immunosorbent assay and lactate dehydrogenase assay, respectively. The processed sample showed less platelet and fibrinogen adhesion compared with the as-received alloy, signifying its high thromboresistance. The current study suggests friction stir processing to be a versatile toolbox for enhancing the performance and reliability of currently used bioimplant materials. © 2017 American Chemical Society.

ACS Applied Materials and Interfaces

Friction Stir Processing of Stainless Steel for Ascertaining Its Superlative Performance in Bioimplant Applications

Metallurgical and Materials Transactions A

Revisiting Stacking Fault Energy of Steels

Materials & Design

Microstructure and mechanical properties of friction stir processed AISI 316L stainless steel

Applied Surface Science

Improvement in cavitation erosion resistance of AISI 316L stainless steel by friction stir processing

CORROSION

Corrosion and Cavitation Erosion Behaviors of Friction Stir Processed Ni-Al Bronze: Effect of Processing Parameters and Position in the Stirred Zone

Surface modification of hydroturbine steel using friction stir processing

Structural refinement in austenitic stainless steel by submerged friction stir processing