• If you are interested in our group for collaboration or PhD research, feel free to email Prof. Santosh Hosmani (Email: sshosmani [AT] iiti.ac.in). We are open to institutions for collaborative studies. We look forward to collaborative opportunities that drive innovation in surface engineering. 

Glimpse of Recent Research: 

The SEHt Group, headed by Dr. Hosmani, is dedicated to understanding the severe surface deformation and surface alloying of various ferrous and nonferrous alloys. Our group attempts to study the effect of surface engineering on the microstructures and properties (mechanical, tribological, and corrosion) of the alloys. For such a study, our group frequently uses the characterization tools like SEM, EBSD, XRD, GDOES, EPMA, Raman spectroscopy, microhardness, nanoindentation, pin-on-disk tribometer (dry, lubricated, and high temperature), etc. Our group also focuses on coatings to enhance the tribological properties of alloys. 

Surface Mechanical Attrition Treatment (SMAT) of Stainless Steels and Mg-alloy: 

In this work, AISI 304L steel was used to investigate the effect of surface mechanical attrition treatment (SMAT) parameters on the hardness and microstructure. The SMAT-modified surface was examined using an optical microscope, SEM, electron backscattered diffraction, X-ray diffraction, and microhardness measurements. The surface hardness of specimens was more than two times the hardness of the non-treated core for all SMAT parameters. The high density of shear bands (which formed nanosize rhombic blocks) was observed near the treated specimen surface. The shear bands' density was reliant on the size and number of balls. Deformation-induced martensite was formed in the surface mechanical attrition-treated region. The volume percentage of martensite was enhanced with an increase in SMAT duration and number of balls. However, it was reduced with an increase in the gap between the vibrating plate and the specimen surface. Microstructure evolution at various SMAT parameters was effectively explained using newly introduced terminology called ‘peening intensity’.

In another study, the effect of surface mechanical attrition treatment (SMAT) on corrosion resistance and plasma nitriding behaviour of AISI 304L stainless steel (SS) was investigated. Mechanical twins and deformation-induced martensite phase were observed in the SMAT-affected region. SMAT improved the corrosion resistance and nitriding kinetics of AISI 304L SS. Effective nitriding time and hence, the thickness of the nitrided layer was increased with an increase in the duration of chemical etching and a decrease in the stability of the passive layer on the SMATed specimens. The surface hardness of the nitrided specimens was dependent on the formation of expanded austenite (γN) and its decomposition (especially, at higher effective nitriding time). 

Mg-alloy also responded very well to the SMAT treatment. A thick layer (~500 μm) with improved surface hardness (more than 2 times the as-received alloy) was successfully obtained on the AZ91D alloy surface. SMAT process had induced compressive residual stress in the SMATed layer. Nanoscale grains (~125 nm) in the near-surface region and multiple deformation twin-modes were observed in the SMATed layer. 

(Ph.D. Students: Digvijay Singh, Manoj Joshi, Vikesh Kumar, Nilesh Kumbhar, Mohit Tiwari, Sheetal);

(M.Tech. Students: Darshan Dange, Dheeraj Kumar, Akanksha Sharma, Rohan Jain, Mohit Tiwari)

Boronizing and Coatings of Steels and Ti-Alloys: 

In this study, pack-boronizing of AISI 4140 steel was done. Three mild-steel containers of different sizes labelled as the small-size container (SC), medium-size container (MC), and big-size container (BC) were used. Steel specimens were boronized at 950 degree-C for 2 or 3 h. Preconditioning of the containers was observed to be essential before doing the final pack-boronizing of steel. MC was found suitable for forming a monophase Fe2B layer, while BC produced an unwanted FeB phase along with the Fe2B layer. The Fe2B phase showed the columnar morphology. A continuous decrease in the hardness from the surface to the non-boronized core was observed. A maximum surface hardness of about 1367 HV0.1 was obtained (the hardness of non-boronized core was about 252 HV0.1) for the boronized steel. As compared to the normalized and hardened–tempered (HT) AISI 4140 steel pins, the boronized steel pin showed the lowest ‘‘specific wear rate’’ (0.89 x 10-6 mm3/Nm) and coefficient of friction (COF) (0.55).

In another study, AISI 4140 steel was pack-boronized at 950 degree-C for 3 h and gas-nitrided at 550 degree-C for 72 h. All specimens used in this work were prepared from the same steel bar. A 3 mm thick diamond-like carbon (DLC) coating (a-C:H) was deposited on the AISI D2 high-carbon, high-chromium, cold-worked tool steel by a plasma-assisted chemical vapour deposition technique. Normalized, boronized, and nitride steel pins were tested against DLC-coated AISI D2 steel at various normal loads (15, 30, 60, and 80 N) for 1,000 and 3,000 m sliding distance in ambient air. The specific wear rate of all pins decreased with increasing load, and a similar trend was observed for the coefficient of friction (COF). Microscopic and energy-dispersive spectroscopic (EDS) analysis confirmed the role of the transfer layer for a low COF with increasing load. At all loads, the specific wear rate of boronized pins was lower than that of the nitrided and normalized pin specimens. Boronized pins showed a specific wear rate in the range of 0.27 x 10-8 to 0.44 x 10-8 mm3/Nm, and the COF was about 0.1. 

(Ph.D. Students: Aditya Litoria, Shubham Parihar, Mohit Tiwari); 

(M.Tech. Students: Garima Dixit, Darshan Dange, Sahil Kumar)

More Information: Click the following Lecture link => https://www.youtube.com/watch?v=oHWEnevTgy0 (Resource Person: S.S. Hosmani, Online Lecture on “Boronizing of Steels” organized by ASM International India Chapter, India. (April 03, 2021)). 

Response of 3D Printed Stainless Steel to Severe Surface Deformation:

This study investigates the effect of surface mechanical attrition treatment (SMAT) (which is a severe surface-deformation process) on the microstructure and tribological behaviour of AISI 316L stainless steel (which is one of the promising biomaterials) manufactured using the selective laser melting (SLM) technique. The specimens are built in different directions (0°, 45°, and 90°). The microstructure of annealed SLM samples shows the non-uniform distribution and random orientation of grains. It contains high-angle grain boundaries and a high density of dislocations. The average grain size is about 63, 51, and 41 lm for 0°, 45°, and 90° build direction, respectively. SMAT is beneficial for SLM steel to reduce surface roughness (by ~87%) and eliminate internal porosity. The deformed layer of SLM steel shows a highly dense network of slip bands, distortion of grains, and hardness gradient (up to the depth of about 600 microns). The increase in surface hardness due to SMAT is maximum (~54%) for the sample having a 90° build direction. Typical observation of deformation-induced martensite (DIM) is absent for the SMAT-processed SLM steel. Under the higher load (especially, 20 N), the tribological response of the sample manufactured in the 90° direction is superior amongst the non-treated samples. Severe surface deformation enhances wear resistance and reduces SLM steel's coefficient of friction (COF).

(Ph.D. Students: Vikesh Kumar)

Polymer/Composite Coated AISI 316L Steel:

The current work investigates the reciprocating wear behaviour of surface mechanical attrition treated (SMAT) AISI 316L stainless steel with and without polymer and composite coatings. The high-performance aromatic thermoset polymer (ATSP) and its composite with MoS2 are deposited on non-SMATed and SMATed steel using the spin coating technique. Further, using different loads and reciprocation cycles, a reciprocating wear test is performed on non-SMATed, SMATed, and coated surfaces against the counter surface (alumina ball). Enhanced surface hardness (~89%), presence of deformation-inducted martensite (bcc Fe) phase, and lower oxidation during reciprocation cause a decrease in wear loss and COF of SMATed surface by ~37%, ~40% and ~20%, respectively, under 15 N load and most extended reciprocation cycle. The wear loss and COF of the steel are further reduced by depositing ATSP polymer and ATSP-MoS2 composite coatings. The adhesion and durability of coatings are enhanced by the SMAT process. This enhanced durability lowers the wear loss and COF of ATSP and ATSP-MoS2 composite coatings when deposited on the SMATed substrate. Polymer coating reduces the COF of non-SMATed and SMATed steel by ~68% and ~85%, respectively. Depending on the concentration of MoS2 in the composite coating, the COF ranges from 0.04-0.32 and 0.02-0.07 for non-SMATed and SMATed specimens, respectively. The best performance of a composite coating was observed at ~10 wt.% MoS2. The COF of composite-coated steel is ~96% lower than non-coated and non-treated steel surfaces. The SMAT processing of substrate and ATSP-MoS2 composite coating considerably enhances the tribological performance of AISI 316L steel. 

(Ph.D. Students: Manoj Joshi, Sheetal)

Tribological Behaviour of Surface Engineered Alloys:

The maximum wear resistance of the boronized surface is ~46 times the wear resistance of non-boronized surface (studied using the same wear parameters) under the dry sliding conditions. A steeper drop in the tribological advantage gained by the boronizing treatment is observed with an increase in load, especially under higher sliding speed. Amongst the investigated wear parameters, an improvement in the wear resistance under 60 N load is the lowest (i.e., about 8 times the wear resistance of non-boronized surface) for the boronized specimens. The CoF of boronized surface is higher than the non-boronized specimen under higher sliding speed. Oxidative wear is the resultant wear phenomenon for non-boronized specimens. The worn surfaces of non-boronized specimens show delamination, surface creasing, and grooves. However, the boronized specimens experience damage during the wear study due to surface oxidation, abrasion, fatigue, and brittle fracture. The Raman spectroscopy confirms the presence of Fe2O3, Fe3O4, B2O3, and H3BO3 in the tribo-oxide scale formed on the worn surface of boronized specimens. 

The SMATed surface of AISI 316L SS has a lower coefficient of friction (COF: 0.32-0.41) than the non-SMATed surface (COF: 0.43-0.51) during reciprocating wear study (under 15 N load against 10 mm diameter alumina ball). XPS analysis of the worn surfaces suggests more oxidation of non-SMATed specimens during the reciprocating wear. The oxide layer formed on the wear tracks contains Fe2O3, FeO, Cr2O3, and FeCr2O4. High surface hardness and lower oxidation during wear result in a lower and more stable COF versus the time profile of the SMATed specimen. The COF and material loss of specimens during the reciprocating wear study are lower when the coatings (ATSP polymer and ATSP-MoS2 composite) are deposited on the SMATed surface rather than the non-SMATed surface. The material loss of composite-coated specimens is substantially lower than that of non-coated and polymer-coated specimens. The average COF of NS+P and S+P specimens at 15 N load ranges from 0.35-0.42 and 0.08-0.12, respectively. Depending on the concentration of MoS2 in ATSP, the COF of NS+C and S+C specimens under 15 N load ranges from 0.04-0.32 and 0.02-0.07, respectively. The best performance of a composite coating is observed at ~10 wt.% MoS2. The durability and adhesion of the coating are higher on the SMATed surface. 

(Ph.D. Students: Manoj Joshi, Aditya Litoria, Vikesh Kumar, Nilesh Kumbhar, Shubham Parihar, Mohit Tiwari);

(M.Tech. Students: Darshan Dange, Dheeraj Kumar, Mohit Tiwari)

Corrosion Behaviour of Surface Engineered Alloys:

SMATed AISI 304 stainless steel specimens have superior corrosion properties than non-SMATed specimens. SMATed specimens demonstrate a considerably higher pitting potential than the non-SMATed specimen. The extent of overall corrosion improvement diminishes due to the SMAT processing of steel using the larger diameter (6 mm) balls. XPS is utilized to understand the passivation mechanism of SMATed steel comprehensively. 

A corrosion study of non-treated and surface-treated Mg alloy (AZ91D) is performed. After 24 h of immersion of Mg-alloy in 0.9% NaCl solution, the average corrosion rate of 11.0 ± 0.7 mm/year is observed for the SMATed specimen, which is ~3 times higher than that of the non-SMATed specimen (3.8 ± 0.7 mm/year). The corrosion product on non-SMATed specimens has densely packed nano-flacks morphology. However, the corrosion product of the SMATed specimen shows two different morphologies: (i) sparse nanowires and (ii) porous honeycomb-like structure. The inferior corrosion resistance of the SMATed AZ91D alloy is attributed to the combined effect of the high density of defects, high surface roughness, and the small volume fraction of β phase at the surface.

(Ph.D. Students: Digvijay Singh, Manoj Joshi, Vikesh Kumar, Nilesh Kumbhar, Shubham Parihar, Sheetal)

Severe Surface Deformation of Mg-alloys

Passivation and Corrosion Behaviour of 3D-Printed Steel

Role of Passivation on Plasma-Nitriding Behaviour of Stainless Steel

Tribological Behaviour of Coatings

Microstructure and Properties of Surface Mechanical Attrition Treated Gradient Nanostructured Alloys (Steel and Mg-alloy)

Tribological Behaviour of Boronized Steels

Microstructure Evolution and Properties of Boronized Low Alloy Steels

Surface Engineering of Stainless Steels: Role of Surface Mechanical Attrition Treatment (SMAT) and Nitriding