Surface Characterization of Three-Layer Organic Coating Applied on AISI 4130 Steel

Resin-bonded molybdenum disulfide (MoS2) is widely applied as a solid lubricant. However, multiple coatings are usually required to meet other requirements in mechanical systems. In this study, a quenched and tempered AISI 4130 steel was used as the substrate, being shot blasted. Furthermore, three layers were successively deposited: a zinc phosphate layer, a phenolic resin (basecoat), and a topcoat based onMoS2.)e thicknesses of different layers were obtained by scanning electronmicroscope and by the ball-cratering method. 3D surface roughness parameters were determined for each step of manufacturing, following three approaches: average values, isotropy level, and distribution of heights. )e ball-cratering method was successfully applied for determining the thickness of the zinc phosphate but presented a relative deviation for the others layers. )e phosphating step was decisive for the final surface topography of resin-bonded coating in terms of distribution of heights. On the other hand, the isotropy level imposed by the shot blasting of steel was practically unaltered by all manufacturing processes.


Introduction
Molybdenum disulfide (MoS 2 ) is a well-known solid lubricant, and its tribological properties are associated with its lamellar morphology, which facilitates the sliding among surfaces in contact [1][2][3].
Different processes have been applied to obtain coatings based on MoS 2 .Amongst them are burnishing, sputtering, and spraying [4].
e last one is used to produce resinbonded MoS 2 , which is perhaps the most common product for achieving a self-lubricating film on metal surfaces.
However, to meet the requirements needed in offshore devices exposed to a saline environment, the protective coating should perform multiple functions [5].e use of a solid lubricant as a filler cannot be enough to guarantee all required functions, especially the corrosion resistance.To check the performance, several tests and analysis may be done, such as those described by Momber et al. [6] for a dual-layer organic coating.Most of these characterizations are related to the surface integrity, which in turn are dependent on the surface roughness.Some investigations deserved attention only for a limited characterization of surface roughness, although it plays a key role in the manufacturing of coatings.Lin and Yan Guu [7] evaluated only the bidimensional average roughness Ra for three primary processes (shot blasting, phosphatizing, and salt-bath soft nitriding) previously applied onto a ground surface of steel before the final treatment to obtain a topcoat of MoS 2 .In the same way, Roberts and Williams [8] investigated the effect of the surface roughness on the tribological performance of sputtered MoS 2 films, but their approach was again limited to two-dimensional characterization and restricted to the average roughness.
In this context, this manuscript aims to bring a complete description of surface characterization of multiple-layered coatings, especially for the changes in surface roughness at each step of manufacturing, using a 3D approach to measurement.In addition, the usefulness of the use of the ball-cratering method for a three-layer organic coating is described.e manufacturing of a resin-bonded MoS 2 constituted three more steps.Firstly, a layer of zinc phosphate was deposited onto the substrate, composed mainly by ZnP 4 and ZnO, as presented elsewhere [9].e zinc phosphate layer was prepared in a bath.Furthermore, an epoxy phenolic resin was added, called here as the basecoat (primer).With this layer, besides the improvement of corrosion resistance, it is expected a correction on the waviness caused by the shot-blasting process previously applied on the substrate.Finally, a thermostable phenolic resin based on MoS 2 composition (called as the topcoat) was added.e application method used for both layers (top-and basecoat) was the spray.en, the curing process was carried out at 80-90 °C.

Materials and Methods
e topcoat contains a signi cant amount of Sb 2 O 3 (diantimony trioxide), as already described in [9]. is oxide can act positively in a tribosystem, acting against tribo-oxidation and providing a mechanical support tribo lm for MoS 2 [10].

Characterization.
Coatings were characterized considering their morphology and surface topography.Crosssectional and super cial views were used for that.Both sections were prepared to reach a surface roughness equivalent to that obtained in a polishing process, through metallographic techniques.
e morphology and thickness of coatings were evaluated using scanning electron microscope (SEM).
e top section was preferred to describe the morphology, while the measurement of thickness was made at the cross-sectional ones.Regarding the chemical composition, the energy dispersive X-ray spectroscopy (EDS) coupled to an SEM was used for this purpose.
Additionally, the thickness was evaluated with a ballcratering method.e Calotest ® is an equipment used to measure the coating thickness between 0.1 and 50 µm.e main reason for its use is to provide faster results, without a need to prepare a metallographic section of the coating.
e dimensions of the crater, a depression with the shape of a spherical cap, can be obtained by using optical microscopy (OM) or SEM.
With the dimensions of the crater, the thickness of the coating (t) can be calculated, following (1), according to ISO-26423 : 2009 [11].For multiple layers, Figure 1 shows a schematic representation used here for the resin-bonded MoS 2 .
where X and Y are the depressions of the projected surfaces of the coating and substrate sections and ϕ ball is the diameter of the ball.
Table 1 shows the parameters selected for the determination of the thickness of coating in the ball-cratering method.We have opted by using a standard solution supplied by CSM Instruments in the smallest available size range (0.05-0.1 μm), following the recommendation made in [12].In addition, a dilution during the tests was performed using distilled water, dripping water every 5 minutes on the ball surface.
Another important aspect done for the ball-cratering method here was the preconditioning of the ball. is variable was investigated by Allsopp et al. [13], and they veri ed a di culty in the use of a polished new ball for relatively soft specimens, which would be our case.To avoid this situation, at every hour, the ball was exposed to a manual agitation in a recipient full of a dry standard sand (A100) for 3 to 5 minutes.is operation led the ball surface roughness Ra to values of 0.20 ± 0.06 μm, guaranteeing a better particle entrainment during the tests [13].
e technique of coherence correlation interferometry (CCI) was employed to obtain 3D asperity information of each step of deposition. is equipment operates with a vertical  resolution of 0.01 nm and 1.63 μm for the lateral one.Four 3D parameters were selected for further analysis: the height parameter Sq (root-mean-square), the height distribution parameters Ssk (skewness) and Sku (kurtosis), besides the spatial parameter Str (texture aspect ratio).e sampling area used for the 3D characterization was 0.83 mm 2 , obtained from a combination between the lens and the resolution.e magni cation of 20 times and 512 pixels of the resolution were used to achieve that purpose.Each average value of roughness parameters corresponds to a series of 6 measurements.e layer close to the substrate, zinc phosphate, is considered essential for marine environments, promoting a barrier against the corrosion and providing a good adhesion to backing coating together with mechanical anchoring [14].Furthermore, a basecoat with approximately 15 µm was added, and nally, the topcoat based on the MoS 2 was deposited.

Results and Discussions
Measured values of thickness are presented in Table 2.As the ball-cratering method is usually applied for hard coatings [15], we separate a single layer of zinc phosphate to test the adequacy of the method when applied to a soft coating (Figure 3).e thickness values determined for this coating using SEM and those observed through the ball-cratering method were the same.
e found values of the zinc phosphate layers presented in Table 2 showed the e ciency of the ball-cratering method to determine their thickness, as no di erence between the measurements made in SEM and the ball-cratering method was detected.
On the other hand, when the thickness of multiple layers is evaluated using the same parameters, a di erence between SEM and the ball-cratering method appeared.e problem was clearly associated with the deformation left by a layer over the successive ones.For the topcoat (MoS 2 ) and for the basecoat (primer resin), the di erence was about 37% between methods.When there is more than one coating on the same sample, it is important to note that the measurement of the thickness of the outermost coating is in uenced by the measurement of the innermost one, in a successive way. Figure 1 shows clearly that the values of the rst coating lead to the determination of the second one and so on.However, it is remarkable that the values of the same order of magnitude use di erent methods for determining the thickness, showing the adequacy of selected parameters for soft coatings in the ball-cratering method.
For the authors' knowledge, no-one investigation made use of Calotest for phenolic coatings higher than 10 μm thick.Rivero et al. [16] used this technique for measuring the thickness of furan and phenolic coatings but is limited to <3 μm.It is remarkable that the di erence of rotation speed used in their investigation (2500 rpm) was much higher than that used here.Besides this variable, we used the conditioning of the ball before tests and an abrasive suspension with smaller particle size.Advances in Materials Science and Engineering

Surface Changes.
Towards this more general view of the system, the surface changes caused by each of the processing steps can be described in detail, especially by means of 3D roughness parameters.For the rst step of manufacturing, the shot blasting of the steel substrate, it is expected a Gaussian distribution of heights and a highly isotropic surface [17]. is expectation can be con rmed looking at the histogram of height frequencies and the polar graph of texture directions presented in Figure 4, along with the average value of 0.77 ± 0.05 for the Str parameter.Values typically higher than 0.3 for Str mean a high isotropy of surfaces [17], which is the case of this surface.Lin and Yan Guu [7] identi ed only a slight increase in the surface roughness Ra when the shot-blasting process was applied onto a ground surface of the steel.ey could have given more attention for that point, once the use of shot blasting as the previous treatment for a MoS 2 deposition resulted in the lowest friction coe cient in a ball-on-disk testing apparatus, comparing it with other surface treatments (phosphatizing and salt-bath soft nitriding).e further step of manufacturing was a deposition of a zinc phosphate layer.Figure 5 reveals a structure in the form of needles for the zinc phosphate layer [18,19].Besides, some regions are uncovered revealed by EDS analysis.ese discontinuities can be associated with the cleaning process of the substrate, in uencing the nucleation and formation of zinc phosphate at the surface [20], and their amount can be responsible for a relatively low performance of this coating against corrosion [19].
Figure 6 shows the 3D surface images of the blasted surface (Figure 6(a)) and zinc phosphate (Figure 6(b)), where it is possible to observe some di erence in topography caused by the Zn phosphate deposition.
e deposition of zinc phosphate becomes the surface predominant in peaks, instead of the slight prevalence of valleys as observed for the shot-blasted surface. is surface roughness alteration is the most signi cant one observed in this investigation.
Following Zhang and Kapoor [21], the initial surface roughness plays a decisive role in the surface texture of a phosphate surface.
e main reason for that is that the concentration of solution surrounding peaks is always stronger than that at valleys.erefore, as the initial di erence between peaks and valleys is higher, that is, the higher would be the average roughness, there is a tendency that the phosphating process increases the average roughness itself.
Although the investigation of Zhang and Kapoor made use of a 3D approach of surface texture, they used a stylus pro lometer for measuring the surface roughness, which means that the current investigation helps to corroborate its results using another technique (interferometry) and even another approach in terms of surface parameters.For this, the comparison between the Sdq parameter (rootmean-square slope) is useful.is parameter was altered from 1.12 to 6.12 as an e ect of the phosphating, a much higher increase than that observed for Sq. e change in Sdq means that the phosphating was able to sharpen the asperities, a clear e ect of the greater reaction occurred at the peaks.
An additional layer of phenolic resin was added after the zinc phosphate deposition. is layer is red (Figure 7), being the color easily revealed after a single-point scratch.
Figure 8 shows the SEM image of the basecoat surface, where the secondary electron image (Figure 8(a)) helps to    ) presents two defined regions.e fraction of the white constituent is estimated to be around 12.3%.According to Skeist [22], zinc oxide constituents are added to the phenolic resin in order to improve its corrosion resistance.e EDS analysis made on a whole image (Figure 8(c)) shows the presence of iron, which agrees to the resin's color; thus, one can infer that any iron oxide was used as the pigment for this kind of resin.
Moreover, the EDS analysis shows a significant presence of Zn and P within the composition of the white constituent.
eir presence can mean a gradient in terms of chemical composition for the whole system, avoiding major variations from one layer to another.
e layer based on MoS 2 composition can be observed in Figure 9, where its surface containing microparticles is uniformly dispersed.is pattern was also described in other investigations [23,24].In fact, these microparticles of MoS 2 and Sb 2 O 3 are suspended in the solution of the phenolic resin diluted in an organic solvent [19].
Figure 10 shows the 3D surface image of the MoS 2 surface (Figure 10(a)) and the correspondent polar graph with texture directions (Figure 10(b)).
Comparing Figures 10(a) and 8(b), one can affirm that the pattern of topography imposed by the zinc phosphate deposition not much altered up to the MoS 2 layer.In terms of isotropy (Figure 10(b)), it is worthy to note that the main texture directions are the same as those described for the shot blasting (Figure 4(b)).
ese findings can be analyzed by means of a summary of surface roughness results, shown in Figure 11, as well as each effect caused by the different steps of manufacturing.
Considering the average roughness in terms of the Sq parameter, the shot-blasted pattern (Figure 4) was significantly modified by the basecoat deposition, as previously discussed.On the other hand, the deposition of MoS 2 diminished the average roughness.A probable reason for that is because the MoS 2 layer was able to fill the spaces left after the basecoat deposition.If one considers the height distribution, the MoS 2 layer brings back the Gaussian values of Ssk and Sku with relatively low deviations, which supports the previous reasoning.
Considering the symmetry of surface finishing, significant changes can be described taking into account the Ssk parameter.After the shot-blasting process, although a Gaussian distribution has been described, both zinc phosphate and primer layer depositions increased the skewness.It means that the number of peaks was sufficiently high in comparison to the valleys, and even the average value of heights has been reduced.

Conclusions
is investigation presented a detailed analysis of surface changes along the different steps of manufacturing of a three-layer coating applied on AISI 4130 steel.Based on them, the following conclusions can be presented: (i) e ball-cratering method was successfully used to determine the thickness of the zinc phosphate layer, for single-and multiple-layer systems, using a diluted solution with smaller particles and a preconditioning of the ball.(ii) For the multiple-layer system, the thicknesses measured by the ball-cratering method were 37% higher than those values determined by SEM, for the basecoat (primer resin) and topcoat (MoS 2 layer).(iii) Phosphating promotes a significant change in the number of peaks, revealed by the skewness.e surface analysis corroborates the correlation between the surface roughness and phosphating reactions described by Zhang and Kapoor [21].(iv) e surface isotropy is kept practically the same along all manufacturing processes, meaning that the shot blasting imposed the final texture for that.Besides, few alterations observed in the Str parameter along the manufacturing processes corroborate this observation.

2. 1 .
Materials.A quenched and tempered steel (AISI 4130) for 233 HV was used as the substrate.It was subject to the shot-blasting process for cleaning purposes, performed with brown aluminum oxide (Al 2 O 3 ) particles of 35-70 mesh (212-600 µm).

Figure 1 :
Figure 1: Schematic representation of ISO-26423:2009 to thickness determination for coating with multiple layers.

3. 1 .
Overview of Deposited Layers and ickness Determination. Figure 2(a) shows the transversal section obtained by SEM, and Figure 2(b) shows the crater image obtained by the ball-cratering method to the same studied coating.ree layers were observed in both Figures 2(a) and 2(b).

Figure 2 :
Figure 2: (a) SEM image of cross-sectional areas of the MoS 2 coating system and (b) the surface after the ball-cratering method.

Figure 3 :
Figure 3: Surface of a single zinc phosphate coating after the ballcratering method.

Figure 5 :
Figure 5: Top surface of the zinc phosphate layer on shot-blasted steel.

Figure 4 :
Figure 4: (a) Histogram of height distribution of a representative shot-blasting steel surface and (b) the polar graph with texture directions.

Figure 7 :
Figure 7: Optical image of a scratched surface of the resin-bonded MoS 2 coating.Red regions represent the basecoat.

Figure 6 : 8
Figure 6: 3D surface images of the (a) shot-blasted surface and (b) zinc phosphate layer.

Figure 9 :Figure 8 :Figure 10 :Figure 11 :
Figure 9: SEM surface image of the MoS 2 surface (a and b) and EDS analysis of the global surface.

Table 1 :
Parameters used in the ball-cratering method.
2Advances in Materials Science and Engineering

Table 2 :
ickness values (μm) obtained in SEM (transversal section) and the ball-cratering method and the respective di erences (%) between them.