Wettability, Thermal and Sliding Behavior of Thermally Sprayed Fly Ash Premixed Red Mud Coatings on Mild Steel

The present experimental work reveals the surface characteristics like wettability, thermal and sliding wear behaviour of plasma-sprayed red mud (RM) coatings premixed with fly ash (FA). Varying weight % of FA (10, 20, 30 and 40)—RM composite powder is used as precursor for coating. Atmospheric plasma-sprayed coatings are developed at different operating power like 5 kW, 10 kW, 15 kW and 20 kW separately on mild steel substrate. Tribological behaviour viz. sliding wear properties are studied at distinct operating load (10N, 15N, 20N, 25N), speed (40 rpm, 50 rpm, 60 rpm, 70 rpm) and track diameter of 100 mm using a pin on disc tribometer for duration of 30 minutes with 3 minute gap period for each experiment. The DSC and TGA experiments of the coatings are performed to understand the high temperature application areas. The contact angle result signifies the wettability of the prepared coatings is principally a function of composition. The reaction of surface roughness and spraying power is insignificant on water contact angle (WCA). In conclusion, the sliding wear experiments are optimized by Taguchi method to ascertain the influencing parameter on wear.


Introduction
Plasma Spray Technology (PST) has been widely used in industrial practises for its versatility like high deposition rate, capacity to coat complex shapes, and abil-ity to process materials with high melting temperatures [1]. In general, PST uses feed materials in powder form with particles sizes ranging from 10 microns to 100 microns. The feed powder is entrained in a gas stream and injected into the plasma plume [2]. Development of water repellent coatings by PST is in focus for the researchers worldwide [3]. Thermal spray coatings find application to resist corrosion and wear. Investigation on surface behaviour like wettability prepared by PST using ceramic materials is limited. Changes in fluid contact angle in the interaction with materials surface play an important role in hydro machine components [4]. The wetting behaviour of coatings defines the efficiency and service life time of many engineering equipments.
Depending on water contact angle (WCA), the surface of all the materials and coatings is divided into hydrophobic or hydrophilic. WCA is an important factor for various practical applications, such as self-cleaning, antireflection, antifogging and antisticking [5] [6]. Superhydrophilic, hydrophilic, hydrophobic, and superhydrophobic surfaces are shown schematically in Figure 1. The wettability of the surfaces is controlled by surface energy, which in turn governed by surface chemical composition and surface roughness [7]. The contact angle formed between small liquid droplet and a perfectly smooth surface is expressed by Young [8]. The Equation (1) stated below is governed by Young's model.
where Y θ is the apparent contact angle in Young's model. sv γ , sl γ and lv γ are the interfacial surface energies (surface tension) associated with the solid-vapor, solid-liquid and liquid-vapor interfaces respectively. In practical, all real surfaces are rough. The roughness enhances the wettability. The surface models for wetting on rough surfaces are governed by either Wenzel or Cassie-Baxter state [9] [10]. In Wenzel model, a liquid droplet fully penetrates the rough surface and increases the solid-liquid interfacial area, as shown in Figure   2(a). On the other hand, in Cassie-Baxter model, the droplet rests on the top, trapping air within the grooves as illustrated in Figure 2(b). Equation (2) where θ w is Wenzel apparent contact angle where θ CB is Cassie and Baxter apparent contact angle.  Surface area wetted by the liquid Projected area below the droplet f = , f is the area function and always less than unity. When 1 f = , Young's equation is recovered. But there have been discussions and disagreements over Wenzel and Cassie-Baxter equations by several surface engineers [11] [12] [13] [14].
The generation of red mud during alumina production and its disposal is a matter of concern. A broad literature review on red mud is reported [15]. The potential uses of red mud as reinforcing material to metals is widely accepted [16]- [21]. But over a decade researchers found it as suitable coating material [22] [23] [24] [25]. Tribological properties like erosion wear [26] [27], sliding wear [28] at different operating conditions are reported in literature. Thermal behaviour of red mud composite coatings at elevated temperature on mild steel is presented [29].
In the present paper, an attempt has been made to evaluate the wettability of plasma-sprayed red mud-fly ash composite coatings with respect to water. Surface chemistry of the prevailing coatings towards high temperature is observed

Collection of Coating Powder
The principal raw material used in the present study is the industrial wastes like particles. Fly ash is reinforced to red mud to prepare the composite powder having 10%, 20%, 30% and 40% of fly ash by weight. Each powder mixture is blended continuously using a V-shaped blender to achieve homogeneity. Here after the material sent for coating.

Plasma Coating
Atmospheric Plasma Spraying (APS) technology is adopted to coat the prepared whereas Nitrogen as secondary gas agent. Plasma spraying was conducted at 90˚ to base by feeding the powder material as external to gun. The detailed operating parameters of the coating deposition process are shown in Table 1.
In order to study the sliding wear, coating is made on one side cross section of cylindrical shaped mild steel substrate (see Figure

Dry Sliding Wear Behaviour
A pin on disc type friction and wear monitor (DUCOM; TR-20-M100) is adopted to study the sliding wear behaviour under un-lubricated condition. The tribometer with data acquisition system has a hardened ground steel disc

Design of Experiment (DOE)
We have used Taguchi optimization method [30] to design the experimental data corresponding to wear behaviour. Mass loss in gram is considered as response. Optimization is conducted using MINITAB-16 software. The factors considered in the study are fly ash content (composition), operating power, speed and applied load with four levels each. The operating condition implemented in the analysis is reported in Table 2.
Taguchi optimization is conducted in accordance with L 16 (4 4 ) orthogonal array (OA). The S/N ratios for minimum mass loss (in gram) is calculated "under   ( ) where "n" is the repeated number trial conditions and "y" is the sliding wear data.

Study of Thermal Behaviour
The coatings thermal behaviour is analysed using a DSC (Perkin-Elmer DSC 7,

Results and Discussion
The static water contact angle of the as sprayed coatings is measured using a      Table 4. The mean of the S/N ratio is found to be 31.38785 dB. Figure 9 shows graphically the effect of control factors on wear rate. According to process parameter settings the higher will be the S/N ratio the minimum will be the variance and optimum will be the quality. From Figure 9, it is observed that the factor combination of P 1 , C 1 , S 1 and L 4 gives minimum wear rate. The minimum wear rate for the developed composite coatings is obtained when the power (P), composition (C) and speed (S) are at the lowest level and load (L) at the highest level. Figure 9(b) reveals the mean effect of control factors on mass loss. It is noted that, there is continuous increase in mass loss up to 15 kW operating power. Afterwards there is a marginal fall at 20 kW. The trend is similar for the factor composition (C).
In our experiment the wear rate is continuously increasing with speed. The results are obvious because, increase in sliding velocity results in increased rubbing action at the coating and steel disc interface and consequently the wear rate increases. However, the mass loss shows both increasing and decreasing trends with load rate. With increase in load, there will be thermal softening of the coating surface and loosening of the coating material and hence increase in wear rate. But at higher load there will be dislodging of the coating and wear debris forms and whiskers at the interface. These whiskers take over the load and reduce the wear rate. The S/N ratio response is given in Table 5. It is concluded from     to justify the insignificant factor. Table 6 shows the results of the ANOVA for mass loss. The analysis is carried out with 5% level of significance (95% level of confidence). "P" value, less than 0.05 for a particular parameter, indicates that it has the major effect on the responses. The 6 th column of Table 6 designates the order of significance of all factors. The ANOVA results signify the factor power has the greatest static influence (P = 0.006) followed by composition, speed and load. The correlation between mass loss (ML, non-variable factor) and variable factors (power, composition, speed, and load) is developed by multiple linear regressions and is given by Equation (5).  Table 7, and the average improvement is found to be 1.2%.

Conclusion
The above experimental research concludes salient features of the prepared ceramic coatings. The plasma-sprayed coatings of red mud and fly ash composites may find suitable tribological applications. Fly ash can be a beneficial reinforcing agent to red mud. There is significant improvement in thermal properties like stability towards high temperature. These coatings are fit to use in elevated temperature areas with excellent thermal resistance. The experimental findings remark  the wettability of the coatings. An improvement in wetting property is seen with fly ash augmentation indicating it to be a function of primarily the fly ash content. But the wetting characteristic is almost independent on spraying power and surface roughness. In this study, the Taguchi optimization result shows that the control factor power is the most influential factor on wear. Although factors like composition, speed and load are less influential, but cannot be neglected. The present work may be extended to find many other coating properties like erosion wear, mechanical strength. The chemical stability of the coatings may be evaluated by the study of corrosion wear properties to recognize definite application areas.