# Effect of Combined Grinding-Burnishing Process on Surface Integrity, Tribological, and Corrosion Performance of Laser-Clad Stellite 21 Alloys

Mohammad Uddin, R. Santifoller, C. Hall, T. Schlaefer

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Mohammad Uddin, R. Santifoller, C. Hall, T. Schlaefer. Effect of Combined Grinding-Burnishing Process on Surface Integrity, Tribological, and Corrosion Performance of Laser-Clad Stellite 21 Alloys. Advanced Engineering Materials, 2022, 25 (8), pp.2201332. 10.1002/adem.202201332. hal-04083035

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# Effect of Combined Grinding-Burnishing Process on Surface Integrity, Tribological, and Corrosion Performance of Laser-Clad Stellite 21 Alloys

Mohammad Uddin,* Remi Santifoller, Colin Hall, and Thomas Schlaefer

Herein, the influence of the grinding-burnishing on surface integrity, mechanical properties, and corrosion performance of Stellite 21 alloys coating deposited by laser cladding is investigated. The as-clad specimens are first ground followed by further modification by ball burnishing at forces of 424 N and 509 N. Results show that the grinding-burnishing enhances surface finish by lowering $R_a$ from 2.6 to 0.73 µm and $R_z$ from 13 to 4.9 µm, respectively. Surface porosity is found to decrease from 3.8% to 0.9%. Hardness is increased from 609 HV to 702 HV, with a surface alteration as deep as 250 µm, while wear resistance increases by reducing worn volume from 4.15 to 2.95 mm³. Because of high hardness, the grinding-burnishing increases impact resistance by lowering indent depth by 20%. Grains flatten and surface undulations are remarkably reduced due to burnishing. Finally, grinding-burnishing at 509 N improves the corrosion resistance by increasing positive corrosion potential from -0.41 to -0.14 V and lowering corrosion current density from $6.34 \times 10^{-4} \text{ A cm}^{-2}$ to $2.19 \times 10^{-5} \text{ A cm}^{-2}$, as compared to grinding. This synergistic grinding-burnishing can be a plausible post-treatment route for the laser-clad alloys.

## 1. Introduction

Cobalt-based hardfacing alloys, often termed “Stellite alloys” (a tradename of Kennametal), are widely used in wear applications, such as the impellers of slurry pumps used in the mining industry. They have high hardness at high temperature and high corrosion resistance under high pressure and abrasive medium. Among Stellite alloys, Stellite 21 is a popular candidate,[1] containing cobalt-rich dendrites with face-centered-cubic (FCC) crystal structure.

Recently, laser cladding-based additive manufacturing has been a trend to deposit such hardfacing metal alloy-based coating to repair/restore industrial components with an aim of improving their wear, corrosion, and fatigue performance.[2] Due to rapid cooling and solidification, the laser-cladded microstructure becomes inhomogeneous in the presence of internal residual

tensile stresses. Partially melted particles remaining in the microstructure form defects, hence facilitating voids/pore and cracking. Due to its inherent nature of layer-by-layer deposition, laser cladding generates very irregular or rough surface profiles, which impact surface integrity. Ganesh et al. (2010)[3] reported the deterioration of fracture resistance due to crack propagation in laser-clad Stellite 21 under tensile loading. A similar phenomenon was reported for clad Stellite 6 alloys.[4] This signals that the laser-clad surface often requires post-treatments to improve surface and mechanical properties for its functional applications.

In the past, surface modification strategies were adapted to tackle the surface integrity issue with laser cladding. Yang et al. (2020) studied milling of laser powder bed fusion (LPBF) Inconel 625 and suggested that an improved surface integrity can be attained when the cutter is oriented perpendicular to the build direction of the printing.[5] Uddin et al. (2017) demonstrated that the surface integrity enhancement by face milling increases the corrosion resistance.[6]

As a finishing process, grinding has also been attempted to improve surface integrity of various hard coatings.[7,8] While the grinding increases hardness, it can deteriorate surface topography by increasing microcracks due to the co-existence of brittle fracture and ductile flow, which consequently compromises corrosion and fatigue resistance. Grinding often results in microporosity within the subsurface of deposited hard coating due to thermal heating and cooling cycles.[8] Surface integrity issue in the subsequent

M. Uddin, R. Santifoller
UniSA STEM
University of South Australia
Mawson Lakes, SA 5095, Australia
E-mail: mohammad.uddin@unisa.edu.au

R. Santifoller
Ecole Centrale de Lyon - ENISE
LTDS
CNRS UMR 5513
University of Lyon
Saint-Etienne 42000, France

C. Hall
Future Industries Institute
University of South Australia
Mawson Lakes, SA 5095, Australia

T. Schlaefer
Laserbond Ltd.
Cavan, SA 5094, Australia

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adem.202201332.

© 2022 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adem.202201332
processing of additively manufactured (e.g., laser cladded) thick
coatings still remains a challenge. Therefore, it is of great impor-
tance to search for an effective alternative post-treatment route.

To address the above issues, ball burnishing (BB), as a
cold working process causing plastic deformation and is often
applied to improve the ultimate quality and properties.[9–12] Using
the high-pressure torsion technique, Mohd Yusuf et al. applied
severe plastic deformation (SPD) on selective laser melted 316 L
steel and reported that SPD can increase hardness and lower corro-
sion rate.[13] Turning followed by surface rolling on Cr–Ni alloys was
studied by other researchers.[14,15] Techniques, such as laser shock
peening,[16] high-temperature heat treatment,[2] and ultrasonic nano-
crystal surface modification (UNSM)[17] and laser polishing,[18] are
being studied and shown to improve the surface integrity.

However, the synergistic grinding and BB approach that can
address the surface integrity issues of the laser-clad hardfacing
Stellite 21 alloys has little been studied in the past, as a post-
treatment process. It is expected that the issues that arose from
the grinding of the clad components, as outlined earlier, can be
solved by applying further plasticity burnishing, which can
potentially increase the corrosion resistance.

Recently, Anirudh et al. studied cryogenic-assisted burnishing on laser-clad Stellite 6 and demonstrated higher hardness and residual stress profiles.[19] Burnishing has transformed detrimental tensile stress in C45 laser alloys into beneficial compressive stress by grain modification up to 30 µm thickness.[20] Similar observations on the efficacy of low-plasticity burnishing was reported in another study,[21] in which, turning followed by burnishing on laser-clad 17-4PH deposits showed a thin transformed layer close to the top surface and high surface integrity improvement. However, they reported relatively less effect of burnishing on cold-sprayed 17-4PH coating. So, the level and extent of burnishing effect depends on the intrinsic characteristics of the deposited coating microstructure.

More importantly, hardfacing Stellite 21 alloys with Co-rich matrix with high-carbide particle content are relatively newly being fabricated by AM for applications such as slurry pumps in extremely harsh mining environments. Unlike steel alloys which have favorable work-hardening effect as explained earlier, plastic deformation mechanisms of Stellite 21 alloys are still unknown.

Thus, in this article, the authors first time report a holistic
analysis and comparison in terms of surface integrity and

corrosion performance of laser-clad Stellite 21 alloys when
treated by additional burnishing. Though surface grinding has
been studied in previous research works, the main reason for
incorporating grinding before burnishing is to induce uniform
distribution of burnishing pressure on the clad surface, hence
potentially increasing further mechanical surface integrity by
their combined effect.

Moreover, understanding whether the intensity of burnish-
ing force influences deformation via grain modification or not
for Stellite 21 alloys is of high importance to design better post-
processing strategies for laser-clad components. In order to
evaluate surface alteration due to the grinding-burnishing, sur-
face roughness, microporosity, microhardness, microstructure,
wear, impact, and corrosion resistance of the treated surfaces
were measured and analyzed. Further, the influence of the bur-
nishing force on the performance metrics was compared with
that of the grinding process.

## 2. Experimental Section

2.1. Laser Cladding and Sample Preparation

**Figure 1** depicts an illustration of the proposed synergistic surface treatment approach presented in this article, with an evolution of surface profile change on the clad surface. Cladding of Stellite 21 was deposited on a rectangular flat substrate of mild steel G250 by a laser-cladding system with a diode laser system of 16 kW with a beam size of 4.8 mm in diameter. Cladding was performed in two passes. The substrate surface was preheated at about 200 °C before cladding to increase the bonding. Helium and argon as carrier gas and shielding gas, respectively, were used to prevent the melt pool from potential contamination and oxidation. The chemical compositions (%wt) of cladding (Stellite 21) and substrate (G250 steel) are shown in **Table 1**.

The thickness of as-clad layer on the substrate was about
2 mm. Its surface topography was found to be significantly rough
with R<sub>a</sub> of 29 µm and R<sub>z</sub> of 124 µm. This is quite obvious for any
additively manufactured surfaces. To remove the highly rough
surface layer, the clad surface was ground by a surface grinding
machine (BMT 4080 AH) with a diamond wheel at 1450 rpm in
five passes by removing a thin layer of 5 µm each pass, totaling a
30 µm-thick layer from the top surface. This was to minimize

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Schematic diagram of the synergistic grinding–burnishing on the laser-clad surface.</figcaption></figure>
<table><caption>Table 1. Chemical composition of the laser-clad (Stellite 21) and substrate (G250) materials.</caption><thead><tr><th>Material</th><th>Co [%]</th><th>Fe [%]</th><th>Cr [%]</th><th>Ni [%]</th><th>Mo [%]</th><th>Si [%]</th><th>Mn [%]</th><th>C [%]</th><th>P [%]</th><th>S [%]</th><th>Al [%]</th></tr></thead><tbody><tr><td>Stellite 21</td><td>Bal</td><td>&lt;2.0</td><td>28</td><td>3</td><td>5</td><td>&lt;2.0</td><td>-</td><td>&lt;1</td><td>-</td><td>-</td><td>-</td></tr><tr><td>G250</td><td>-</td><td>Bal</td><td>-</td><td>-</td><td>-</td><td>0.03</td><td>0.25</td><td>0.07</td><td>0.02</td><td>0.02</td><td>0.07</td></tr></tbody></table>

inducing further stress into the laser-clad surface before the additional burnishing process was applied. Grinding was carried out along the cladding direction in a cooling lubricant environment.

## 2.2. Burnishing Experiments

BB was carried out on the ground samples using a burnishing tool (HG6-9 E00° from Ecoroll), in which, a hydraulically pressurized ball deformed the material. The burnishing ball was made of SiC material and had a diameter of 6 mm. An illustration of burnishing experimental setup is shown in Figure 2. Burnishing was performed at a force of 424 N and 509 N, while a feedrate of 1200 mm min⁻¹ and a stepping distance of 2 mm were kept constant (Figure 2a). These two high forces were adopted to realize the obvious burnishing effect on hardfacing alloys like Stellite 21. Burnishing force was controlled by changing the hydraulic pressure of the pump and measured by a load cell mounted underneath the specimen. As shown in Figure 2b, a zigzag burnishing path was in a direction normal to the laser-cladding and grinding directions.

Burnished surface blocks were then cut into smaller specimens required for surface characterization and performance evaluation. As such, the specimens were categorized into three types: “ground,” “ground + BB 424 N,” and “ground + BB 509 N”. This allowed us to investigate the effect of burnishing force on surface integrity and mechanical properties.

## 2.3. Characterization

High-resolution Olympus Lext OLS 5000 confocal laser microscope was used to measure surface roughness and topography. Surface profiles were measured by laser scanning along the path perpendicular to the burnishing direction. Each surface roughness profile length was about 12 mm while the cutoff length was 4 mm. At least three profiles were measured on different

locations of the surface coupon and their average was taken as the final value. No digital filter was used in roughness measurement. The percentage of counts of porosity change on the surface due to plasticity burnishing was quantified by an Image J software. To observe microstructural change, samples were first ground with abrasives of 1200 grit and then polished with diamond particle paste of 9 µm and 6 µm, followed by an oxide polishing suspensions (OPS) for 15 min. Polished samples were etched with Kallang 2 solution consisting of 40 mL HCl, 2 g CuCl₂, and 80 mL ethanol. Etched samples were observed under an Olympus SC50 optical microscope to reveal the microstructure and grains.

Microhardness was measured using Nanovea's CB500 hardness tester at a load of 2 N for a dwell time of 12 s. Both surface hardness and cross-sectional hardness along the depth were measured.

Friction and wear behavior of the modified surfaces were studied by rotary pin-on-disc test (MicroTest Co). A constant vertical load of 10 N and the contact sliding speed of 0.18 m s⁻¹ were maintained until the ball traveled a sliding distance of 1000 m.

Low-velocity impact tests were performed at an impact energy of 5 Joules to determine the toughness. Permanent imprints of indents in terms of penetration width and depth were assessed by a laser confocal profilometer (Olympus Lext OLS 5000).

## 2.4. Corrosion Experiment

The electrochemical corrosion tests were conducted in 3.5% (wt) NaCl solution. The treated samples were cut into the specific sizes and fit to the potentiostat cell (Potentiostat of Pines Research), so that a surface area of about 3 cm² was exposed to the electrolytic medium. A three-electrode cell as specified in ASTM G1-03 (2017) e1 standard was used to conduct the experiment. The details of the electrodes and cell arrangement used in the experiment are presented in another study.[22]

<figure><img src="image_4.png" /><figcaption>Figure 2. a) Experimental setup for BB on the ground surface b) ground-burnished surface topography.</figcaption></figure>
Open-circuit potential (OCP) test was run for about 1.5 h to stabilize the potential of the electrolyte. Linear polarization resistance (LPR) was carried out at ±20 mV at a scan rate of 0.1 mV s⁻¹, followed by linear sweep voltammetry (LSV) from –600 to 0 mV at the same scanning rate of LPR. Further, Tafel plots were presented and analyzed to evaluate the corrosion characteristics. Corrosion experiments were repeated thrice for each modified specimen.

**3. Results**

**3.1. Surface Finish**

Shown in **Figure 3** is the surface topography and profile roughness R_a and R_z of the ground and ground-burnished surfaces. Roughness was measured along the profile perpendicular to burnishing direction. Initial roughness after grinding was measured to be R_a = 2.6 µm and R_z = 13 µm. It can be seen from Figure 3a, b that BB has a positive effect in reducing the values of R_a and R_z, as compared of the ground surface. When the burnishing force increased from 424 N to 509 N, the roughness reduction is very marginal. Because of its inherent nature of the process, the burnishing induces plastic deformation, causing displacement of materials from peaks to valleys, and hence smoothens further the ground surface. For example, compared to the ground

specimen, the burnishing at 424 N and 509 N resulted in about 70% and 72% decrease in R_a, and about 57% and 62% decrease in R_z, respectively. This indicates that higher the burnishing force, higher the roughness reduction (i.e., 2% and 4% extra jump in R_a and R_z, respectively). This phenomenon can be supported by observing a 2D surface profile and 3D topographies, as shown in Figure 3c,d. Ground surface was still relatively rough and machining marks were clearly visible. On the other hand, the ground-burnished surface appeared to be very smooth without traces of significant sharp peaks and valleys. In other words, surface peaks are flattened by severe plastic deformation in burnishing at higher force. Such superior surface quality is expected to increase wear and corrosion resistance.

**3.2. Surface Porosity**

**Figure 4** shows the images and porosity analysis of the modified surfaces. A sample area of 250 µm x 250 µm was imaged on at least three locations of the treated surface, and their average porosity was measured. Overall, a low porosity content was noticed for all the samples. The low level of porosity could be due to the high laser energy used, by which a high-density laser cladding layer was formed. Micropores were shown to vary in size and shape. Such defects in the laser-clad surfaces were well investigated before, and these unwanted micropores can be

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Comparison of surface roughness a) R_a and b) R_z and c) 2D profile and d) 3D topography of the ground and ground-burnished specimens at burnishing force of 424 N and 509 N.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Porosity analysis and optical images for a) the ground, b) the ground + BB 424 N, c) the ground + 509 N, and d) the porosity content comparison.</figcaption></figure>

attributed to entrapped gas and/or shrinkage pores within melted powder particles due to rapid heating and cooling cycles. As is shown in Figure 4a–c, compared to the ground surface, the ground-burnished surface exhibited a less number of pores, but, in places, a few pores were shown to be slightly larger in size. In all cases, no significant cracks and damages to the surface were observed. As shown in Figure 4d, grinding–burnishing at 424 N and 509 N caused a 30% and 78% decrease in porosity content, respectively, when compared with grinding. We can observe a higher reduction in microporosity at higher burnishing force. This would be because higher pressure-induced plastic deformation due to burnishing causes the voids/pores to be filled by the plastically deformed material, therefore strengthening it as well. Grinding is reported to reduce the micropores to some extent; however, it is clear that plasticity burnishing caused by the rolling ball can further minimize the pore density and size of the ground surface.

### 3.3. Microhardness

**Figure 5** exhibits the surface and cross-sectional microhardness of the ground and ground-burnished samples. As shown in Figure 5a, the grinding–burnishing increased surface hardness, compared with grounding. This is because burnishing caused plastic deformation into material, resulting in a work-hardening effect. For instance, grinding–burnishing at 424 N and 509 N resulted in about 13% and 16% increase in surface hardness, respectively, when compared with grinding. These results indicate that higher burnishing pressure led to higher hardness improvement.

We observed a similar phenomenon for hardness along the cross-sectional depth, as shown in Figure 5b. Hardness at 50 µm from the top surface (first points of the plots in Figure 5b) seemed very close to the surface hardness shown in Figure 5a.

Highest hardness increase was noticed at the top surface. Burnishing effect on increasing hardness gradually diminished at a depth up to 275 µm from the top surface, and afterward, the effect was negligible, that is, predominantly the hardness of the bulk as-clad layer prevails, followed by the heat-affected zone or dilution layer.

### 3.4. Friction and Wear

**Figure 6a** shows the evolution of coefficient of friction (CoF) of the modified surfaces. CoF profiles exhibited a high level of fluctuation as the sliding distance increased. This can be due to roughness of initial surfaces of all specimens. As the sliding continued with time, the peaks and troughs were removed due to combined actions of abrasion and plastic deformation. In other words, the contacting surface underwent a running-in phase of material removal.[23] It is seen that the CoF profile of the ground specimen was too abrupt after 500 m, while the ground-burnished specimens showed relatively a stable CoF profile. This may be attributed to highly rougher surface generation due to continuous sliding on the ground surface, as opposed to the work-hardened surface produced by BB, resulting in a slower, gradual, and smooth abrasion between the treated surface and the pin. Overall, grinding–burnishing decreased average CoF by 33% (lowering from 0.52 to 0.35), when compared to grinding, though the effect of burnishing force seemed very insignificant. It is evident that the increase in surface finish and hardness due to the plasticity burnishing reduced the COF.

An analysis of the wear track was performed to quantify evolution of track geometry and volume loss (see **Figure 7**). Figure 7a shows the change in wear track width and depth. It is seen from Figure 7b,c that grounding–burnishing at 424 N lowered track width by 4% (from 949 to 913 µm) and depth by 3% (from 34 to 33 µm), compared to the ground track. Reduction is just doubled
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Comparison of microhardness on a) the top surface and b) on the cross-section of the ground and ground-burnished samples.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 6.** a) Evolution of CoF against the sliding distance and b) the magnified view of CoF shown in the red rectangle of (a) for the ground and ground-burnished specimens at 424 N and 509 N.</figcaption></figure>

at a higher force of 509 N. Similarly, the ground-burnished
surface showed a decrease in volume loss, indicating an improve-
ment in wear resistance (see Figure 7d). The grinding-burnishing
at 424 N and 509 N exhibited a 19% and 29% reduction in volume
loss due to wear, respectively. Therefore, it is clear that burnishing
increases wear resistance, which is highly beneficial for improving
tribological performance in terms of extending fatigue life. The
results are again consistent with friction and hardness results pre-
sented in an earlier section. Note that volume loss on the spherical
steel ball surface as the pin was measured was found to be relatively

low (less than 20%) compared to wear on the specimens and there-
fore, was not presented.

**3.5. Impact Resistance**

Figure 8 shows the impact depth profiles for impact tests at kinetic energy of 5 Joules. A confocal laser microscope was used to locate the deepest point reached by the impact impression and the profilometer to obtain the profile curve. It can be seen from
<figure><img src="image_9.png" /><figcaption>**Figure 7.** a) Laser confocal microscopic imaging of the wear track, b) its width, c) depth and d) volume loss of the ground and ground-burnished specimens.</figcaption></figure>

Figure 8a that the ground-burnished samples showed a higher level of impact resistance as their profile's maximum depths were lower than that for the ground samples. For instance, grinding-burnishing at 424 N and 509 N resulted in a reduction in the profile's maximum depth by 12% and 20%, respectively, as compared to grinding. This improvement in impact resistance is due to higher hardness and cold-work hardening effect caused by plastic deformation in BB. The maximum depth for the burnished samples at 424 N and 509 N is 85 and 78 µm, respectively, which are found to be well below the burnished layer depth of 250 µm as observed in the hardness plots shown in Figure 5. So, higher burnishing force is beneficial in increasing the impact resistance. Figure 8b shows the gray color confocal microscopic images of the impact footprints at 5 J. Arrows in yellow color of Figure 8b indicate the scanning direction for the indent's depth profile measurement shown in Figure 8a. No cracks or crack initiations were observed. This may be due to the low-velocity impact tests considered in this study. High-velocity or repetitive impact

tests should be performed to evaluate rigorously the impact resistance and fracture toughness of the modified surfaces.

### 3.6. Microstructure Analysis

Figure 9 shows SEM imaging of microstructures of the cross-sectional laser-clad surface. It was found that the microstructure and grains are highly influenced by the laser-cladding process (Figure 9a). A typical dendritic structure of highly dense fine lathe-like grains with Co-rich matrix and interdendric carbides (Figure 9b) was observed in the top cladding layer which was formed due to rapid solidification and cooling rate. Below the dendritic structure, a heat-affected zone (HAZ) consisting of less finer grains (Figure 9c) was formed which was due to preheating of the substrate before the laser-cladding process. At the bottom is the substrate showing clearly the coarse grains of ferrites and austenite (Figure 9d).
<figure><img src="image_10.png" /><figcaption>**Figure 8.** a) Impact profile depth at kinetic energy of 5 Joules and b) the confocal microscopic imaging of the impact footprints for the ground and ground-burnished specimens.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 9.** a) Stellite microstructure of b) top-cladded layer, c) intermediate HAZ, and d) bottom substrate.</figcaption></figure>

**Figure 10** shows the microstructural topography of cross-sectional surfaces of the ground and ground-burnished specimens. As was revealed in Figure 10a, the ground surface showed relatively large cellular columnar eutectic structures consisting of Co-rich matrix dendritic (white areas) and interdendritic carbides (black/gray areas) phases. High surface undulations and roughnesses are clearly observed on the top surface edge. This could be due to the grinding marks left on the surface. Columnar dendritic cells are oriented along the vertical direction, that is, along the laser-clad build direction, and such microstructure and grain distribution can be formed due to the gradient thermal cycle

including rapid cooling and solidification in laser-cladding
process.[24]

By applying burnishing at 424 N, the top surface undulations and roughness have reduced, but no significant grain modification or refinement was noticed (Figure 10b). By increasing the burnishing force up to 509 N, the top surface undulations disappeared completely (Figure 10c). This again indicates that burnishing caused plastic deformation into material, flattening peaks and smoothening the surface, followed by the strain hardening effect resulting in an enhanced hardness of the material. Perhaps, the burnishing force together with room temperature-controlled
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Microstructural topography of cross-sectional surfaces for a) the ground, b) the ground-burnished specimen at 424 N, and c) the ground furnished at 509 N. Black/gray areas: interdendritic carbides, white areas: Co-rich matrix.</figcaption></figure>

burnishing experiments used in this study is not high enough to
trigger dynamic crystallization causing grain refinement.

**3.7. Corrosion Resistance**

**Figure 11** shows the OCP and potentiodynamic polarisation curves of the ground and ground-burnished samples. It can be seen from Figure 11a that within 1.5 h, OCP for all samples has stabilized, and the ground-burnished specimen showed higher OCP than the ground specimen, with a pronounced OCP for higher burnishing force. Higher OCP potential means the specimens are more resistant to corrosion attack. Further, Tafel plots presented in Figure 11b show that the polarization curves of the ground-burnished samples shifted to the right (higher noble corrosion potential) and bottom (lower corrosion current density) as compared to the ground sample. It is to note that the higher the potential and the lower the current density, the higher the corrosion resistance. For instance, the maximum corrosion potential of the ground-burnished specimen at 424 N and 509 N increased by 200 mV (from -0.408 to -0.204 V) and by 272 mV (from 0.408 to -0.136 V), compared with that of the ground sample. Higher burnishing force increased the potential by further shifting the curves to the right. Thus, it is clear that the burnishing has a positive influence in improving the corrosion resistance of the ground clad surface.

To quantify further the corrosion resistance, the Stern–Geary
equation was used to estimate the corrosion current (Icorr), as
shown in Equation (1).

<figure><img src="image_13.png" /><figcaption>**Figure 11.** a) OCP and b) potentiodynamic polarization plots of the ground and the ground-burnished samples measured in 3.5% (wt) NaCl solution.</figcaption></figure>
$$I_{\text{corr}} = \frac{\beta_a * \beta_c}{2.3 * R_p * (\beta_c + \beta_c) * \text{Area}} \quad (1)$$

The parameters $R_p$ is the polarisation resistance experimentally obtained from LPR tests, $\beta_a$ is the anodic Tafel slope, and $\beta_c$ is the cathodic Tafel slope, both obtained from LSV tests.

**Table 2** shows the corrosion results obtained from the potentiodynamic tests. The polarization resistance $R_p$ for the ground-burnished samples was higher than that for the ground sample. Higher burnishing force shows higher $R_p$. On the other hand, the corrosion current $I_{corr}$ for the ground-burnished specimens is lower than that for the ground surface. For instance, grinding-burnishing at 424 N and 509 N exhibited a reduction in $I_{corr}$ by about 95% (from $6.34 \text{ A m}^{-2}$ to $3.19 \times 10^{-1} \text{ A m}^{-2}$) and by about 97% (from $6.34 \text{ A m}^{-2}$ to $2.19 \times 10^{-1} \text{ A m}^{-2}$), respectively, when compared to grinding. Higher corrosion resistance attained by burnishing can be attributed to lower surface roughness, microporosity, and work-hardened microstructural change due to plastic deformation by burnishing. Our result is consistent with the work of Zhang et al.,[14] which reported a positive effect of synergistic surface treatment on the corrosion rate of additively manufactured Cr–Ni alloys.

## 4. Discussion

Despite the rapid growth of metal additive manufacturing for restoration of industrial parts, high surface roughness, microporosity, and microcracks are the major concerns, which necessitate the adaptation of post-treatment to enhance their functional usage in the practical field. This has been crucial for emerging hardfacing metal alloy coatings such as Stellite 21 deposited by laser cladding, where the material surface requires high wear, corrosion, and impact resistance. Corrosion resistance is generally closely connected to the surface finish, topography, and the passive layer, while wear resistance and impact toughness depend on the surface microhardness, grain structure, and microporosity.

In this work, we demonstrated the grinding-burnishing process as post-treatment can be applied to Stellite 21 alloys to 1) decrease surface roughness, 2) reduce microporosity, and 3) increase hardness. As a result of surface integrity improvement, wear, impact and corrosion resistance of alloys have significantly enhanced. While conventional grinding can reduce roughness and porosity to some extent, the hydrostatic burnishing tool caused a controlled significant plastic deformation into the material, thus flattening surface peaks/troughs and inducing work hardening. As shown in Figure 3, the grinding itself lowers

<table><caption>**Table 2.** Summary of the corrosion results obtained from Tafel plots.</caption><thead><tr><th>Corrosion parameters</th><th>Ground</th><th>Ground + BB 424 N</th><th>Ground + BB 509 N</th></tr></thead><tbody><tr><td>E<sub>corr</sub> (V) vs. (SCE)</td><td>-0.408</td><td>-0.204</td><td>-0.136</td></tr><tr><td>R<sub>p</sub> [Ω cm<sup>2</sup>]</td><td>56 680</td><td>344 000</td><td>727 600</td></tr><tr><td>I<sub>corr</sub> [A m<sup>-2</sup>]</td><td>6.34</td><td>3.19 × 10<sup>-1</sup></td><td>2.19 × 10<sup>-1</sup></td></tr><tr><td>β<sub>a</sub> [V]</td><td>1.29 E × 10<sup>-2</sup></td><td>7.20 × 10<sup>-3</sup></td><td>1.08 × 10<sup>-2</sup></td></tr><tr><td>β<sub>c</sub> [V]</td><td>1.19 × 10<sup>-2</sup></td><td>9.20 × 10<sup>-3</sup></td><td>1.19 × 10<sup>-2</sup></td></tr></tbody></table>

$R_a$ and $R_z$ of the as-cladded surface was reduced from 41 and 316 µm to 2.8 and 13 µm, but the burnishing decreases down to 0.75 µm and 4.5 µm, respectively. Such an encouraging effect is found to be more prominent at higher burnishing force, which amplifies the deformation intensity and depth, thus enhancing surface finish. Similar benefits are achieved when roller burnishing was performed on turned 41Cr4 steel, as reported in another study.[25]

Note the burnishing force higher than a threshold value may cause the deterioration of the surface by smearing, shearing, and flaking, hence increasing roughness.[26] As a result, the overarching advantage of post-treatment may be compromised. The degree of damage depends on the types of cladding materials to be treated, as new hard alloying materials are constantly being added into the matrix for developing superhard coating. Our results however clearly showed that burnishing at a force of as high as 509 N can improve the surface properties without any damage such as surface cracks (Figure 3).

As presented in Figure 4, we demonstrated that despite a very low porosity level in the as-clad Stellite 21 surface, applying burnishing on the ground surface reduced by up to 78% surface porosity. Two phenomena may partly explain this reduction. Frist, the plastic deformation due to burnishing displaces the material of the surface peaks into the existing micropores, thus closing the pore surface areas. Second, potential compressive residual stress induced by burnishing might have induced a compacting effect and stopped the initiation and propagation of potential inherent cracks within the clad surface/subsurface, thus limiting the appearance of new pores.[27] It is evident that only grinding is not adequate to address the porosity issues, as the grinding often causes further microcracks and dislodgement of hard particles from the binding matrix materials.[7] Therefore, the combined grinding-burnishing would easily be applied to eliminate/minimize the surface microporosity.

Noticeable microhardness increase at surface and along the cross section was observed due to the work-hardening effect caused by plastic strain resulting from both grinding and burnishing. Though we have not noticed any grain refinement within dendritic microstructures after burnishing even at higher force, the grains might have geometrically modified containing high-density dislocations due to plastic deformation. This would be the reason for hardness improvement. Such an argument can be reinforced by the presence of very low or no surface undulations of the ground burnished due to the flattening of surface peaks by burnishing (Figure 10). It is assumed that the modification effect has penetrated from the top surface through down up to about 275 µm (Figure 5). A fundamental mechanism for hardness improvement can refer to fine grains, as stated in the Hall-Patch theory and dense dislocation formations, as elaborated in Taylor theory on plastic deformation, outlining that microhardness increases with the increase of dislocation density.[15] Perhaps a burnishing force significantly higher than what we used in this study may be required to notice the grain refinement effect for hard-facing Co-rich matrix alloys, but note, as explained earlier, higher force may deteriorate surface integrity. However, our microstructural observation results presented in Figure 10 are quite consistent with the findings reported in other works.[17,19]
<figure><img src="image_14.png" /><figcaption>**Figure 12.** Polar diagram illustrating relative performance improvement for grinding and grinding-burnishing processes.</figcaption></figure>

The slight decrease in CoF and wear volume loss (Figure 7) underlines the supreme effect of burnishing in improving frictional resistance, and this can be attributed to higher surface hardness, surface finish, and reduced surface microporosity obtained by burnishing. It is reported that surface hardness is directly related to the evolution of friction as well as the wear volume lost.[28] We demonstrated that grinding-burnishing can enhance the impact resistance in terms of reducing indent depth by up to 20% at burnishing force of 509 N (Figure 8). The findings are aligned with the work reported in another study,[29] reporting that burnishing increased fracture toughness. Again, this can be attributed to the surface hardness improvement of the burnished specimen.

Electrochemical corrosion tests showed that grinding-burnishing increased the corrosion resistance significantly by increasing positive corrosion potential and lowering corrosion current density (Figure 11 and Table 2). Improved corrosion resistance may be ascribed to the cascade positive effect of the smoother surface, reduced porosity, adequate plastic straining within microstructure, and lower surface undulations induced by plasticity burnishing. Also, compressive residual stress promotes anodic current density by sealing pores, defects, and stops crack initiation/propagations within the surface/subsurface,[12] hence protecting the surface from the corrosive media and pitting corrosions. Therefore, it can be cautiously assumed that potential compressive stress generated by burnishing might be another factor responsible for surface integrity and corrosion resistance.[30]

Finally, six key performance characteristics, surface roughness ($R_a$), microporosity (area %), microhardness (surface hardness), wear resistance (volume loss), impact resistance (indent depth), and corrosion resistance (corrosion current), have been carefully chosen to determine the role of the combined grinding-burnishing. A six-axis polar diagram is plotted, as shown in **Figure 12**. The level of improvement for each characteristic parameter was determined by comparing with the results obtained for the ground sample. A normalized method was used to convert the varying values of those characteristic parameters

to a factor of 10. Higher the value of factor for a performance parameter, higher the improvement by the surface treatment strategy. It can be noted that, for all the characteristics, the grinding-burnishing showed a distinct performance improvement. Higher burnishing force stands to be more beneficial. This clearly indicates that sequential grinding and burnishing can be a viable route to further enhance surface integrity and corrosion resistance, hence extending the service life of the laser-clad repairable components. It is however to be noted that, depending on the laser-clad material and initial ground surface topography, an optimum combination of the key parameters such as burnishing force, feed rate, and stepping distance must be determined to achieve the best outcome of this combined surface treatment approach.

## 5. Conclusion

This article experimentally investigated the effect of grinding-burnishing on both mechanical properties and corrosion performance of the laser-clad Stellite 21 alloys. Effect of the burnishing force was evaluated. The following are a few major findings that can be drawn out from this study. 1) Compared to the grinding, the grinding-burnishing at a force 509 N has improved the surface finish and topography by decreasing $R_a$ and $R_z$ by up to 72% and 62%, respectively, while the surface porosity reduced by up to 78% due to the surface peak flattening and plastic deformation. 2) Accordingly, burnishing at 509 N increased the maximum surface microhardness by 16%, with a hardness diminishing effect at modification layer of up to 250 µm along the cross section from the top surface. 3) The grinding-burnishing significantly reduced surface undulations, indicating the presence of potential grain modifications and perhaps, dislocation movements within the top surface and subsurface. 4) In addition to a reduction in CoF, the grinding-burnishing improved wear resistance by decreasing specific volume loss by up to 29% at burnishing force of 509 N. Similarly, the combined treatment improved the impact resistance by lowering the maximum indent depth by 20%, when compared to grinding itself. 5) Finally, because of the improved
surface finish, porosity, and work-hardening effect as outlined
earlier, the grinding-burnishing increased the corrosion resis-
tance by lowering the corrosion current density by up to 97% N
(from 6.34 A m⁻² to 2.19 × 10⁻¹ A m⁻²) at a burnishing force
of 509 N.

Acknowledgements

The authors would like to acknowledge the use of the optical microscope, nanoindenter, Brilliant cutter provided by Microscopy Australia, a facility that is cofunded by the University of South Australia, the State Government of South Australia and the Australian Federal Government, and the use of laser confocal microscopy provided by the South Australian node of the Australian National Fabrication Facility (ANFF) under the National Collaborative Research Infrastructure Strategy. The authors like to thank the support of the ball burnishing experimental facility provided by UniSA STEM.

Open access publishing facilitated by University of South Australia, as part of the Wiley - University of South Australia agreement via the Council of Australian University Librarians.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

All authors contributed to the study conception and design. Material prep-
aration, data collection, and analysis were performed by Remi Santifoller
and Mohammad Uddin. The first draft was written by Mohammad Uddin
and Remi Santifoller and all the authors read, commented on, and
approved the manuscript.

Data Availability Statement

Research data are not shared.

Keywords

ball burnishing, corrosion resistance, grinding, laser cladding, Stellite 21 alloy, surface integrity

Received: September 13, 2022

Revised: November 9, 2022

Published online: January 12, 2023

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