3.1. Characterization of Stellite 6 powder
The commercially procured Stellite 6 powder is characterized by Scanning electron microscopy (SEM) and the morphology of the powder is shown in Fig. 1(a). The micrograph shows that most of the particles are spherical with regular shapes with little agglomeration seen for smaller size particles. Agglomeration of particles sometimes takes place leading to a variation in shape. More ever, very few fragmented particles are also existing. Therefore, with the presence of regular shape particles, the coating produced by Stellite 6 powder is generally smooth and homogenous. Figure 1(b) shows the particles size distribution and the size concentration. It is found that the size range of particles lies between 18 µm to 34 µm. It has a peak value of 25 µm with a Gaussian distribution of 8.18. This value indicates around 25µm is the film thickness that will be obtained in a single pass at the time of coating. X-Ray Diffraction pattern of Stellite 6 powder is shown in Fig. 1(c). It is evident from the diffraction pattern that the powder consists of a Co-based solid solution with prominent peaks. It has very low peaks with uniform distributions of prominently Cr7C3 carbide phase. Cobalt possesses a hexagonal closed pack structure at room temperature [21]. The Stellite 6 powder is normally produced by spray drying and the atomization process. During the atomization process, a high cooling rate is experienced, due to which a low amount of Cr7C3 carbide is observed. This finding is also corroborated [22].
3.2. Calculation of Coating thickness and Adhesion Strength
Coating of Stellite 6 powder on the vertically held steel substrate is carried out in a layered approach through passes with longitudinal and transverse direction and a 25 µm thickness of the coating is obtained in every pass. An optical micrograph image of the plasma-spray coated Stellite 6 on AISI 304 Stainless steel substrate is shown in Fig. 2. From the micrograph three distinct layers are observed i.e., the first layer (marked 1) is the bottom layer depicts SS 304 steel substrate, intermittent layer (marked 2) represents the coated surface, and the top layer (marked 3) is the resin. The Stellite 6 powder coating was deposited in a lamellar texture. It is found that sample 3 depicts the most uniform coating thickness whereas sample 1 has an irregular coating thickness as compared with all three micrographs. Average coating thickness as measured from the micrographs is 74 µm, 128 µm, and 215 µm for 1st, 2nd, and 3rd samples and the variation in coating thickness ranges between 38.43%, 20.23%, and 7.69% respectively. It can be inferred that without changing the process parameters for atmospheric plasma spray deposition technology larger coated products exhibit a sound and smooth coating. Large particles associated with the powders are melted instantly, incur rapid solidification, and get deposited uniformly in the wall of the substrate. This phenomenon could not be witnessed while depositing the coating material on the lower coating thickness products.
The adhesion tests were performed on the 100 mm x 60 mm sample size with an automatic adhesion tester. The Adhesion dolly with a circular contact surface of 10 mm diameter has adhered with sample with adhesive HTK ultra-bond 100 after proper surface cleaning with emery and acetone. Assembled system is then put into an oven at 150˚C for 80 minutes for the curing of the adhesive and to strengthen the bond of the dolly with the coated surface. The parting of the coating has taken place at the interface (coalition) of the coating and substrate which is shown in Fig. 3. In all three samples, adhesion pull-up strength has a magnitude varying between 26.5 ± 2MPa corresponding to strong intra-bonding in Stellite particles. Also evident from the fractured surface, a negligible amount of coating has been pulled out from the coating. In the case of sample 3 having 215µmcoatings thickness, the adhesion strength value is beyond 28 MPa, showing an appreciable amount of visible coating pull-out. The strength obtained in adhesion testing is considered good because of more no of touchpoints between the well-flattened splats.
3.3. Density, Porosity, and Surface Roughness of Stellite 6 coating
The coating density is calculated by peeling a small portion from the surface of the substrate by mechanical means. Archimedes principle [23] as shown in Eq. (1) is used for the determination of density.
$$Density of the coating={d}_{w}\times \frac{{M}_{a}}{\left({M}_{a}-{M}_{w}\right)}$$
1
…………………………………………………..
where \({d}_{w}\) is the density of water, \({M}_{a}\) is the weight of the coated sample in the air (gm), \({M}_{w}\) is the weight of the coated sample in water (gm). Material with higher coating density indicates a superior and homogeneous coating justifying a superior load requirement for its delamination and removal leading to failure.
Physical parameters such as density, porosity, and surface roughness are depicted in Table3. The density of Stellite 6 is reported as 8.69 g/cm3. The calculated value of the density of the coating was 8.44 g/cm3. Optical microscopic image analysis (OMIA) techniques were used for the determination of the homogeneity of the coating. The porosity of coated samples was calculated by Image J processing and analysis software. Ten different observations are taken at different locations and the porosity of the coated surface is calculated using the statistical average method. The porosity value for the plasma sprayed coating is found to be in the range of 2.9 ± 0.05%. Sidhu et al. [24] observed the porosity of plasma-sprayed Stellite 6 coating lies between 2-3.5%. From the table, it is observed that sample 2 has the lowest porosity value.
The surface roughness of the coated product was measured using a surface roughness instrument. Five roughness readings are taken on each sample and their average value is taken as the surface roughness. The coated product surface roughness value for all the samples is measured using a surface roughness meter and its roughness values vary between 4.8 ± 0.02 µm. The average value of surface roughness for HVOF sprayed Stellite 6 coating on various grades of steel substrates lies in the range of 4.892 ± 0.38 µm [25]. The atmospheric plasma spray technique has resulted in a better surface finish as compared to the HVOF process in depositing Stellite 6 powder on stainless steel substrates.
Table 3
Density, porosity, and surface roughness of the Stellite 6 coated samples
Samples | Coating thickness | Density | Porosity | Surface roughness |
Sample-1 | 74 µm | 8.44gm/cm3 | 2.95% | 4.78 µm |
Sample-2 | 128 µm | 8.44gm/cm3 | 2.85% | 4.8 µm |
Sample-3 | 215 µm | 8.44gm/cm3 | 2.90% | 4.82 µm |
3.4. Phase analysis of Stellite 6 coating
Figure 4 shows the XRD pattern of the Stellite 6 coating. From the XRD plot, it is observed that the structure of the coating consists of cobalt solid solution along with Cr23C6, Cr7C3andCrO. At the highest peak intensity, the carbides of Chromium (Cr23C6 and Cr7C3), coincides with the Co-based matrix. Due to instant cooling after deposition of the powder on SS 304 steel plate during atmospheric plasma spray process, cobalt fails to undergo the alteration of equilibrium phase, thus permitting to remain in FCC structure consistent with the temperature exceeding4170C [26]. There is also a small peak of CrO from the XRD pattern. The presence of CrO indicates that some oxidation of chromium might have taken place during the plasma spraying process.
3.5. Analysis of coating microstructure
Figure 5 shows the SEM image of the plasma sprayed Stellite 6 coating on the AISI304 substrate. It is observed that two different phases are visible from the micrograph. The dark grey phase corresponds to structure 1 enriched with Chromium which is also called as inter-dendritic eutectic phase and the light grey region corresponds to structure 2 enriched with Cobalt called a dendritic phase. This interdendritic eutectic phase consists of Cobalt with Chromium carbide [27, 28] and the dendritic phase consists of Cobalt solid solution (FCC structure). Solid solution hardening and precipitates of carbides play a great role to provide strength to the cobalt-based superalloy. It is also observed from XRD that Chromium is the main element forming carbide during the coating process. It also improves the solid solution hardening along with resistance to corrosion and oxidation. The carbides likeCr23C6 and Cr7C3are formed by Cr. But due to the metastable nature of Cr7C3 sometimes it converts into Cr23C6during a higher temperature working environment [29].
The corresponding EDS analysis of different structures (marked as 1 and 2 in micrograph) in all the samples is presented in Fig. 7 and Table 4. From this table, it is found that Co, Cr, and W are the dominant element in the dendritic as well as the interdendritic region. Oxygen is present in the interdendritic region in an abundant manner mostly due to SiO2. Also, it is observed that there is the presence of splats, partially melted and un-melted particles in the form of voids, pores, or globules in the coatings which are the characteristic feature of plasma spray coatings. These pores or voids which are marked from SEM and optical micrographs are black. Some oxides of Cr appear as thin dark (black) phases and are located as a marginal line between the laminas. These are formed instantly after the deposition process due to the reaction of lamellae with the adjacent air [30, 31]. When W and Mo are present in the microstructure of Stellite 6 they provide strength by forming carbides or intermetallic compounds [27, 29]. But the presence of W from EDS elemental mapping analysis is very less as compared with Co and Cr. So, there is no presence of white phase which is enriched with tungsten as seen from PTA coated surfaces [32, 33]. Hence it is assumed that tungsten is embedded in the cobalt matrix within the interdendritic phase and provides additional strength employing solid solution hardening.
Table 4
EDS analysis of Stellite 6 coatings
Samples | Marked region in the micrograph | Co | Cr | W | Fe | C | O | Si |
Sample 1 | 1 | 56.71 | 29.64 | 6.58 | 2.32 | 4.75 | --- | --- |
2 | 21.02 | 57.02 | 2.16 | ---- | --- | 18.53 | 1.27 |
Sample 2 | 1 | 62.74 | 23.07 | 5.48 | 1.91 | 6.18 | 0.61 | --- |
2 | 16.26 | 51.54 | 2.21 | --- | 6.09 | 22.45 | 1.45 |
Sample 3 | 1 | 57.29 | 30.82 | 3.89 | 1.35 | 4.94 | 0.3 | 1.4 |
2 | 16.45 | 57.11 | 3.36 | --- | | 23.08 | --- |
3.6. Analysis of microhardness
Figure 7 shows the microhardness values of the coated materials undertaken on SS 304 substrate. The substrate material has an average microhardness value of 163 HV0.3. Sample1 exhibits the lowest hardness of 315 HV0.3 as compared to the other samples. The Microhardness of sample 2, sample 3 is nearly identical. As the coating thickness of sample 1 is very small so it gets punctured thus reflecting the hardness of the substrate. Yu et al., [34], reported that the hardness value of (402.6 ± 20.9 HV (2.94 N) was obtained for the Stellite6 materials produced through sand casting.