Synthesis and characterization of hard metal coatings by electro-plasma technology

Surface & Coatings Technology 200 (2005) 1587 – 1594 www.elsevier.com/locate/surfcoat

Synthesis and characterization of hard metal coatings by electro-plasma technology P. Gupta a,*, G. Tenhundfeld a, E.O. Daigle a, P.J. Schilling b a

CAP Technologies, LLC, LBTC, Louisiana State University, South Stadium Drive, Baton Rouge, LA 70803-6100, United States b Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, United States Available online 4 October 2005

Abstract Electro-plasma technology (EPT) is a cathodic atmospheric plasma process which has shown great promise for the deposition of metal coatings exhibiting excellent adhesion to the substrate along with high deposition rates. The present study involves synthesis and characterization of molybdenum coatings using EPT processing. The morphology, composition and structure of Mo-coated surfaces were characterized. The results indicate the successful deposition of molybdenum, with molybdenum alloyed into the surface of both the 4330V steel and Inconel 718 substrates. The surface morphology and roughness of the coated samples reflect unique EPT-induced micro-roughness features. Superposition of the EPT-induced micro-roughness profile on the macro-roughness profile of the substrate surface provides the potential to improve adhesion characteristics of surface. An increase in hardness observed on Mo-coated steel indicates potential to produce hard surfaces by EPT, leading to many advanced applications. D 2005 Elsevier B.V. All rights reserved. Keywords: Electrolytic plasma; Surface modification; Alloying; Mo coating; XRD

1. Introduction Electro-plasma technology (EPT) is a hybrid of conventional electrolysis and atmospheric plasma processing [1]. Formation of stable plasma, at atmospheric pressure, on the surface of conductive materials during electrolysis of various aqueous electrolytes gives the capability to conduct various surface treatments including cleaning [2,3], coating [4,5] and surface texturing [6]. The presence of plasma micro-discharges over the work piece (generally the cathode) leads to melting of localized micro-zones on the surface. Subsequent quenching by the liquid electrolyte and mechanical effects produced by imploding plasma bubbles has shown to form a surface with unique characteristics [4– 6]. Furthermore, surface interactions during the non-equilibrium EPT processing provide a great potential to alloy various elements into metal surfaces. * Corresponding author. Tel.: +1 225 578 7455. E-mail address: [email protected] (P. Gupta). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.08.030

Mo coatings have gained attention due to their application in various industries such as aerospace, chemical, pulpand-paper, etc. Mo coatings are also potential candidates for replacement of Cr as a protective coating. The most common deposition method for Mo coatings is thermal plasma spray [7 –9]. Thermally sprayed coatings are usually inhomogeneous and discontinuous, characterized by pores, oxide lamellas or partly molten spray particles [8]. Mo coatings generally exhibit a layered structure [7,8]. It has been reported that the oxide phase MoO2 is present between the layers, which may reduce the interlameller strength [7]. There is a need for new technologies for the deposition of Mo, and EPT processing may provide such capability. It is interesting to note that the thermal spray process, prior to coating, requires grit blasting in order to clean and obtain necessary surface roughness. EPT has shown to clean metal surfaces with the creation of uniform micro-roughness and morphology that provides better coating adhesion as compared to grit blasting [2,10]. Electrodeposition of Mo coatings from aqueous electrolytes has not been very successful, but its co-deposition

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has been investigated in the past [11,12]. To the best knowledge of the authors, there is no literature on the study of deposition of Mo coatings by electro-plasma processes. In the present study, Mo coatings were deposited on 4330V steel and Inconel 718, using EPT. Surface characterization and structure analysis of the Mo coatings are presented.

2. Experimental details The present work involved deposition of Mo coatings by EPT on 4330V and Inconel 718 substrate materials. The same coating procedure was used for all samples. Experimental details associated with the EPT process have been described in detail elsewhere [4,6]. Mo coatings were deposited using a solution of sodium molybdate (Na2MoO4), with additional Mo supplied to the solution in the form of Mo powder. The processing conditions were set to achieve a power density of 139.5 W/cm2 and a processing time of 5 min was used for all specimens. The differences among the samples lay in the substrate material and the surface preparation prior to coating. The following substrates were used —4330V steel (samples Mo-S1 and Mo-S2) and Inconel 718 (samples Mo-I1 and Mo-I2). Test coupons were cut in rectangular pieces with dimensions of 10 cm  2.5 cm  2.5 cm. Surface preparation options included polishing by metallography and cleaning by EPT processing. Samples Mo-S1, Mo-S2 and Mo-I1 were polished to a mirror finish (to 0.05 Am), prior to coating. Samples Mo-S1, Mo-I1, and Mo-I2 were cleaned by EPT processing for ¨ 20 sec using sodium bicarbonate based electrolyte prior to coating. One sample of In718 (Mo-I2) was coated with Mo in as-received state (without polishing) after EPT cleaning. The sample preparations are summarized in Table 1. The surface morphology and composition were analyzed using a Hitachi S-4500II field emission scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDX). Surface roughness and profile of the samples was measured using an optical profilometer (Wyko NT3300). Average roughness (R a), rms value of roughness (R q) and total roughness (R t, i.e. the sum of vertical distances from the deepest valley and to the highest peak) values are reported. Roughness parameters reported represent the average of three scans taken at different places on the processed sample surfaces.

Table 1 Surface preparation steps prior to Mo coatings Sample

Substrate

Polished

EPT-cleaned

Mo-S1 Mo-S2 Mo-I1 Mo-I2

4330V 4330V In718 In718

Yes Yes Yes –

Yes – Yes Yes

X-ray diffraction measurements were performed using a Panalytical X’Pert Pro system using Cu Ka radiation with an operating voltage of 45 kV and current of 40 mA. Continuous scans were performed from 30- to 90- 2h with a 0.03- step size and a counting time of 4 sec/step. Scans were made in a standard Bragg –Brentano (h – 2h) mode, as well as in grazing incidence mode with a fixed incidence angle of 2- for enhanced surface sensitivity. This results in a sub-micron X-ray attenuation length for 8.04 keV (Cu Ka) radiation (¨ 0.14 Am for pure Fe; 0.22 Am for pure Mo), where attenuation length is defined as the depth into the material measured along the surface normal where the intensity of X-rays falls to 1/e of its value at the surface [13]. The hardness of Mo-coated surface was determined with a Zeiss microhardness tester. Multiple indentations were carried out at a load of 50 g using a Knoop indenter. The microhardness reported is the average of at least five indentations.

3. Results 3.1. Surface morphology Fig. 1 presents micrographs obtained from the two coatings deposited on 4330V steel substrates —Mo-S1 and Mo-S2. These images reveal surfaces that are relatively flat, reflecting the use of a polished substrate, but exhibit a morphology characteristic of electro-plasma processing [2 – 6]. This morphology is distinguished by the formation of micro-craters and spheroid-shaped elevations, which results in creation of the micro-roughness. This morphology is uniformly distributed across the surface of sample Mo-S1, as shown in Fig. 1(a) and (b). In specimen Mo-S2, this EPT morphology is interspersed with flat featureless regions, as shown Fig. 1(c) and (d). The difference between these two specimens is that the specimen MoS1 was cleaned using the EPT process prior to the EPT coating step, while the specimen Mo-S2 was directly subjected to the coating step. This indicates that the inclusion of the cleaning step (or increased total EPT processing time) provided a more homogeneous morphology of the EPT-modified surface. The scale of the EPTproduced morphological features can be estimated from these images. In both the Mo-S1 and Mo-S2 surfaces, the typical diameter of the micro-craters is estimated to be of the order of 3– 5 Am. Typical SEM micrographs obtained from the two coatings deposited on Inconel 718 substrates – Mo-I1 and MoI2 – are presented in Fig. 2, along with the images obtained from the as-received substrate material. The SEM images of Mo-I1 (polished and EPT-cleaned prior to coating) shown in Fig. 2(a) and (b) reveal the characteristic EPT induced morphology evenly distributed over an otherwise flat surface. The uniform distribution of the surface morphology is similar to that observed for sample Mo-S1, which was

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(a) (a)

Mo-S1

60 µm m

(b)

Mo-S1

30 µm

(c) (c)

Mo-S2

60 µm m

(d) (d)

Mo-S2

30 µm m

Fig. 1. SEM micrographs of Mo-S1 and Mo-S2 at different magnifications: (a) Mo-S1, lower mag.; (b) Mo-S1, higher mag.; (c) Mo-S2, lower mag.; (d) Mo-S2, higher mag.

also polished and EPT-cleaned prior to coating. However, the scale of the EPT-induced surface features is larger for the Inconel substrate. The typical micro-crater diameter is of the order of 5 –10 Am. The surface morphology of the asreceived, unpolished Inconel substrate is shown in Fig. 2(c) and (d). Some surface roughness due to grit-blasting is observed. The difference between the samples Mo-I1 and Mo-I2 is that the latter was not polished prior to processing. For the Mo-I2 specimen, the coarse roughness of the substrate can still be observed (Fig. 2(e) and (f)) which can be described as a superposition of the EPT-induced micro-roughness on the coarser features of the substrate. In addition, two distinct morphologies can be observed. In Fig. 2(f), the lower, right corner of the image shows the combination of micro-craters and elevated spheroidal shapes. The region in the upper right corner has a denser, less porous morphology. These two types of morphologies are interspersed throughout the surface.

addition, significant oxygen content is detected on all four surfaces. For samples Mo-S1, Mo-S2, and Mo-I1, spot analysis by EDX produced basically the same results throughout the surfaces, reflecting the overall compositions shown in Table 2. This indicates a relatively homogenous composition across the surface. All three of these samples were polished before processing, and all three have a uniform surface composition. The only specimen in which spot analyses revealed an inhomogeneous distribution of elemental surface composition was Mo-I2. The two morphologies described in the previous section produced markedly different compositions. The regions dominated by microcraters have a composition similar to the overall surface. In contrast, the denser regions are mainly composed of Mo and O.

3.2. Surface composition

The surface roughness for the four coated samples and the as-received parent Inconel 718 substrate material was analyzed in more detail using optical profilometry were measured. Surface roughness parameters R a, R q, and R t are summarized in Table 3. Fig. 3 shows the surface profile of Mo-S1 and Mo-S2 deposited on 4330V. The surface profiles are characterized by the presence of micro-roughness created by EPT. Other than the micro-roughness, both have flat, horizontal profiles — a reflection of the polished substrates. The surface roughness of Mo-S1 is observed to be greater than that of Mo-S2 (see Table 3). Both the samples were polished

A summary of the results of semi-quantitative EDX analysis is presented in Table 2. Surface composition reported for each specimen represent the average of 3 area scans performed at different magnifications. This data is not intended to represent an accurate description of the surface coating layer compositions since the sampling volume for EDS penetrates through the coating into the substrate material. The data do serve to demonstrate the presence of a significant amount of Mo on all the coated specimens, indicating successful deposition by the EPT process. In

3.3. Surface roughness

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(a) (a

Mo-I1

300 µm

(b)

Mo-I1

60 µm

(c)

In 718

300 µm

(d)

In 718

60 µm

(e)

Mo-I2

300 µm

(f)

60 µm

Mo-I2

Fig. 2. SEM micrographs of Mo-I1, the as-received Inconel 718 substrate, and Mo-S2 at different magnifications: (a) Mo-I1, lower mag.; (b) Mo-I1, higher. mag.; (c) In 718, lower mag.; (d) In 718, higher mag.; (e) Mo-I2, lower mag.; (f) Mo-I2, higher mag.

before deposition, but Mo-S1 was cleaned by EPT whereas Mo-S2 was not. Mo-S1 was exposed to a longer total EPT processing time, which resulted in higher roughness as compared to Mo-S2. This is consistent with the qualitative assessment based on SEM results. The values of R t (5.5 Am for Mo-S1, 3.8 Am for Mo-S2) are consistent with the scale of features observed by SEM. Fig. 4 shows depth profiles of the surfaces after Mo coating on In718 — samples Mo-I1 and Mo-I2. The asreceived, unpolished, parent substrate surface is also

Table 3 Surface roughness of coated samples

Table 2 Surface composition of coated samples Sample

Mo-S1 Mo-S2 Mo-I1 Mo-I2

included for comparison. The surface roughness parameters are given in Table 3. Fig. 4(a) shows the depth profile of sample Mo-I1, which was polished before deposition. Uniform micro-roughness created by EPT can be distinctly seen, superimposed on the flat horizontal background. This surface profile is similar to that in the MoS1 sample, but the scale for the former is greater. This can be observed in the roughness values reported in Table 3 as well as in the magnitude of the peaks and valleys in

Sample

Surface roughness (Am) Ra

Rq

Rt

Parent In718 Mo-S1 Mo-S2 Mo-I1 Mo-I2

4.3 0.24 0.17 0.7 5.1

5.3 0.4 0.23 0.9 6.5

31.4 5.5 3.8 8.4 48.3

Composition (wt.%) Ni

Fe

Cr

Mo

Na

O

2.1 2.6 29.7 27.4

61.2 72.4 10.7 10.1

1.0 1.5 10.4 7.2

26.1 20.1 37.4 41.4

1.5 – 1.2 1.5

7.8 3.4 10.5 12.3

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20

(a)

Depth of Profile, µm

15

Mo-S1

Mo-S1

10 5 595.3

um -3.2 452.7

0 -5 -10

0.0 0.0 um

-15

X

Y

-20 0

100

200 300 400 500 Length in X direction, µm

600

20

(b)

Depth of Profile, µm

15

Mo-S2

Mo-S2

10 5 um -1.8 452.7

0 -5 -10

595.3

0.0 0.0 um

-15

Y

X

-20 0

100

200 300 400 500 Length in X direction, µm

600

Fig. 3. Surface profile (left) and 3-D surface image (right) for (a) Mo-S1 and (b) Mo-S2.

Fig. 4(a). This is consistent with SEM observations of the EPT-induced morphology (on the Mo-I1 sample) with a larger length scale, compared to the Mo-S1 and Mo-S2 samples. In contrast to the overall flat profile of the polished MoI1 sample, the depth profile of as-received In718 shown in Fig. 4(b) indicates a roughness on a very different scale, with much higher wavelength or distance between consecutive valleys. This distinction between the uniformly microrough EPT surface and the parent In718 surface can be clearly seen in 3-D surface images, Fig. 4(a) and (b), respectively. Fig. 4(c) shows the surface profile and 3-D image of Mo-I2, which was deposited without polishing. It is interesting to note that Mo-I2 surface retains profile of parent In718 (grit-blasted surface) with creation of microroughness (EPT profile), leading to an increase in overall surface roughness as compared to parent In718. The profile in Fig. 4(c) can be viewed as a superposition of the EPT micro-roughness profile in Fig. 4(a) and the macro-roughness profile of the substrate in Fig. 4(b). Again, this is consistent with the qualitative observations made by SEM. 3.4. Structure The results of XRD measurements for sample Mo-S1 are presented in Fig. 5. Fig. 5(a) shows data for the uncoated

4330V substrate material. The diffraction peaks can be associated with a body-centered cubic structure with a lattice parameter very close to that of a-iron. The data in Fig. 5(b) was collected by performing a standard (h – 2h) diffraction scan. The a-Fe peaks from the substrate dominate, but additional peaks due to the coating are observed. The GI XRD data are presented in Fig. 5(c). The a-Fe peaks remain, but the relative intensity of new peaks due to the surface coating is greatly enhanced. Among these peaks, there are several which match standard patterns for molybdenum, and several which match an iron –molybdenum intermetallic phase Fe0.54Mo0.73 (PDF reference code 41-1223). These phase assignments are shown as peak labels in Fig. 5. This clearly demonstrates the successful deposition of Mo and the alloying of Mo into the substrate material. It is notable that no XRD evidence of the presence of oxides was observed, despite the indication of oxygen content in the EDS analysis. XRD results for the Mo-S2 sample were nearly identical to those for Mo-S1. The results of XRD measurements for sample Mo-I1 are presented in Fig. 6. Fig. 6(a) shows data for the uncoated Inconel 718 substrate material. The main diffraction peaks can be assigned as reflections associated with a face-centered cubic structure similar to nickel (space group Fm3m), but ˚ . These peaks are with a lattice parameter of ¨ 3.60 A labeled Inconel in Fig. 6. Two lower intensity peaks

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(a)

Depth of Profile, µm

15

Mo-I1

Mo-I1

10 5

-5 -10

0.0 0.0 um

-15 -20

595.3

um -4.0 452.7

0

0

100

200

300

400

500

Y

X

600

Length in X direction, µm 20

(b)

Depth of Profile, µm

15

In 718 In 718

10 5 0 -5

um -18.1 452.7

-10

595.3

-15 -20

0

100 200 300 400 500 Length in X direction, µm

0.0 0.0 um

600

Y

X

20

(c)

Depth of Profile, µm

15

Mo-I2

Mo-I2

10 5

-5 -10

0.0 0.0 um

-15 -20

597.1

um -18.9 452.3

0

0

100 200 300 400 500 Length in X direction, µm

Y

X

600

Fig. 4. Surface profile (left) and 3-D surface image (right) for (a) Mo-I1, (b) as-received In 718, and (c) Mo-I2.

appear at 2h values of 35.0- and 40.6-. These peaks match two prominent peaks for a niobium carbide phase Nb6C5 (PDF reference code 37-1201). EDS spot analysis of precipitates in Inconel 718 cross-sections indicated a high niobium content, with some titanium. The XRD peaks are attributed to these precipitates and labeled as Nb – C in the figure. The data in Fig. 6(b) were collected by performing a standard (h – 2h) diffraction scan on the Mo-I1-coated sample. The Inconel peaks from the substrate dominate, but additional shoulders appear to the lower 2h (higher d-spacing) side of each Inconel peak. This indicates the presence of another phase with the same structure, but a larger lattice parameter. Based

on the peak position, this lattice parameter is estimated ˚ . This observation, combined with the to be ¨ 3.65 A high Mo-content observed by EDS, suggests that these peaks may be assigned to a phase formed by alloying of significant amounts of Mo into the Inconel structure. These peaks are therefore labeled Mo – Inconel. In the surface-sensitive grazing incidence (GI) XRD scan presented in Fig. 6(c), the Mo – Inconel peaks dominate. Also in the GI XRD scan, peaks representing Mo metal can be identified. As was observed for the coated steel, the data clearly demonstrate the successful deposition of Mo and the alloying of Mo into the substrate material. Also, no XRD evidence of the presence of oxides was

4000

α-Fe

P. Gupta et al. / Surface & Coatings Technology 200 (2005) 1587 – 1594

(a)

4330 V

α-Fe

α-Fe

2000

Table 4 Microhardness of coated samples Sample

HK0.05

Uncoated 4330V Mo-S1 Mo-S2

490 T 47 882 T 109 694 T 155

α-Fe

α-Fe

α-Fe, FeMo

α-Fe

500

Mo

1000

Mo-S1 GI FeMo, Mo

(c)

α-Fe, FeMo

2500

0 1500

Mo-S1

FeMo, Mo FeMo FeMo

(b)

FeMo, Mo FeMo FeMo α-Fe

Intensity [counts]

0 5000

0 40

50

60

70

80

2θ [deg] Fig. 5. Diffraction data representing (a) standard XRD scan of 4330V steel substrate, (b) standard XRD scan of Mo-S1-coated sample, and (c) grazingincidence XRD scan of Mo-S1-coated sample.

Nb-C

2500 Nb-C

In718 Inconel

(a)

Inconel

5000

Inconel

observed, despite the indication of oxygen content in the EDX analysis. XRD results for the Mo-I2 sample were nearly identical to those for Mo-I1.

Inconel

Mo-Inconel

Inconel

Mo-Inconel

(c)

2500

Inconel

(b)

Mo-Inconel

5000

Mo-Inconel

Mo-I1

0 30

40

50

60

70

Inconel

Mo

2000

Mo-Inconel

4000

Mo-I1 GI Inconel

6000

Mo-Inconel

0 Inconel

Intensity [counts]

0

80

2θ [deg] Fig. 6. Diffraction data representing (a) standard XRD scan of In 718 substrate, (b) standard XRD scan of Mo-I1-coated sample, and (c) grazingincidence XRD scan of Mo-I1-coated sample.

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3.5. Microhardness testing The results of microhardness of uncoated 4330V, Mo-S1 and Mo-S2 are given in Table 4. Mo-S1 exhibited 80 T 10% increase, whereas Mo-S2 exhibited 42 T 10% increase in hardness as compared to the uncoated 4330V substrate.

4. Discussion The SEM and optical profilometry results provide great insight into the nature of the surface morphology and roughness produced by the EPT process. The unique EPTinduced surface morphology reported previously (e.g. [2,3]) is observed in all samples, regardless of substrate and surface preparation. This morphology is characterized by micro-roughness generated by the presence of micro-craters and spheroidal or bulbous elevations. For the polished samples Mo-S1, Mo-S2, and Mo-I1, this micro-roughness clearly appears in the surface profiles as peaks and valleys above and below a horizontal base line. A comparison of samples Mo-S1 (which was cleaned by EPT processing) and Mo-S2 (which was not EPT-cleaned) indicates a higher surface roughness for Mo-S1. This is consistent with SEM observations of relatively flat, featureless regions on the surface of Mo-S2. Sample Mo-S1 underwent a longer total EPT processing time. A recent study on steel cord showed that longer processing time resulted in increased roughness [6]. However, the total time difference is 320 sec versus 300 sec. This suggests that the 20-sec cleaning step (using sodium bicarbonate-based electrolyte) is very efficient in producing the EPT-modified surface morphology. This is consistent with the steel cord results, which showed a significant difference in surface roughness after treatment with different electrolytes [6]. Samples Mo-S1 and Mo-I1 were both polished and EPTcleaned prior to coating. The only difference is the substrate material. In both cases, a homogeneous EPT-induced morphology is observed across the surface. However, the scale of features is larger on the Mo-I1 surface as observed by SEM, and in the roughness profiles. In the steel cord study, different chemical reactions between the substrate and different electrolytes were found to affect the surface roughness induced by EPT treatment [6]. Here we observe that with the same electrolyte, but different substrates, different roughness values are produced. The electrolyte may be reacting differently with the In718 and 4330V substrates. It is not just the electrolyte, but the electrolyte/ substrate combination that is critical in controlling the

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surface roughness. The reactions involved in two substrates need detailed study. In view of the observations above, EPT processing offers the potential to tailor surface roughness to desired levels, within limits. In EPT processing, interactions take place through the electrode – plasma –electrolyte system. The plasma is mostly formed in discrete gas bubbles on the surface of the work piece that leads to localized melting of the processed surface. The liquid electrolyte near the work piece quenches the surface, thereby leading to the formation of a spheroidor bulbous-type morphology (e.g. [4]). Simultaneously, implosion of plasma bubbles has been reported to form the crater-type morphology. The size of plasma bubbles, which mainly depends upon processing parameters including type of electrolyte, is critical to produce the scale of features observed after EPT processing [6]. Also, in view of the discussion, it can be seen EPT imparts electrical, mechanical, thermal and chemical treatment to the surface. Thus, more than likely, properties of the treated material also play a role in producing the size of EPT morphology as observed with 4330V and In718 substrates. The SEM and optical profilometry results for Mo-I2, when compared to Mo-I1, clearly indicate that the final surface roughness of EPT treated material depends on the initial surface roughness of the substrate. The surface profile obtained can be described as a superposition of the EPT micro-roughness profile on the profile of the substrate material. The Mo-I2 surface retains the profile of the parent In718 (grit-blasted surface) with creation of micro-roughness (EPT profile) leading to an increase in surface roughness as compared to the parent In718. Retention of grit-blasted morphology in Mo-I2 also indicates that EPT is a localized surface treatment process as discussed earlier. Previous studies have shown that an EPT-cleaned steel surface exhibited superior adhesion properties as compared to a grit-blasted surface [2,10]. The profile of Mo-I2 suggests the potential to improve the adhesive properties of the surface, even more, by having a combination of the grit-blasted macro-profile and the EPT micro-profile. In contrast, sample Mo-I2, which had a rough surface, a uniform surface is not produced. In the SEM images, two surface morphologies can be distinguished. One is characterized by the typical EPT-induced morphology containing micro-craters. The other is more dense, with few or no micro-craters. The presence of these two surface types can be assumed to be related to the substrate surface roughness —the only difference between Mo-I2 and Mo-I1 is the substrate surface roughness. A more detailed study of this phenomenon is necessary. EDS and XRD data indicate that Mo is successfully deposited and alloyed into the substrate materials in all samples. Alloying of Mo into the substrate may be a result of enhanced diffusion taking place due to localized melting by plasma bubbles in conjunction with the high electrical potential at the surface. Previous studies conducted on similar processes resulted in high diffusion rates during

carburizing or nitriding as compared to the conventional processes [14,15]. Microhardness results show an increase in hardness for Mo-S1 and Mo-S2 as compared to uncoated 4330V. This may be attributed to the phases alloyed by Mo that were formed on the surface due to EPT processing as revealed by XRD. Exact reason for higher hardness of Mo-S1 as compared to Mo-S2 is not known, but it may be due to higher Mo content as determined by EDS. The ability to create hardened surfaces in a controlled process presents a great promise for tribological applications.

5. Conclusions The study represents for the first time the EPT processing has been used to deposit Mo coatings. EDS and XRD results indicate the successful deposition of Mo and alloying of Mo into both 4330V and In718 substrates. The final surface roughness obtained by the EPT process was found to depend on the initial roughness of the substrate. The wellcharacterized EPT-induced micro-roughness profile is superimposed on the macro-roughness profile of the substrate surface. Increased hardness was observed due to Mo coating, which opens up avenues for advanced tribological applications.

Acknowledgments The authors would like to acknowledge Dr. V. Singh at CAMD, LSU for help in SEM and surface profile measurements.

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