Mechanical, Tribological and Hydrophobic Properties Evaluation of Atmospheric Plasma Sprayed NiTi Coating

Abstract

In the current investigation, the atmospheric plasma spray technique was implemented to develop a protective coating of NiTi alloy on the mild steel substrate using different primary gas flow rates and plasma arc currents. For this purpose, an elemental mixture of equiatomic Ni and Ti powder was considered as the feedstock material. After successful deposition, the coatings were characterized using various characterization methods. The physical, mechanical, and tribological properties of the coatings were determined. Furthermore, the plasma spray process parameters were optimized using different optimization tools to achieve the best product. The physical and mechanical properties include the investigation of phase, microstructure, porosity, deposition efficiency, surface roughness, microhardness, adhesion strength, and correlation. The phase analysis revealed the presence of various phases in the coatings, such as NiTi-B2, Ni3Ti, Ti2Ni, Ni, Ti, NiO, TiO, and Ni4Ti3. From the microstructural analysis, various surface and interface defects such as surface microcracks, unmelted/partially melted particles, surface pores, splat fracture, interface porosity, inter-lamellar cracks, vertical cracks, inter-splat cracks, etc. were observed in the coatings developed at lower primary gas flow rates and plasma arc currents. Furthermore, the molten fraction of the powders, thermal pinching effect, enthalpy effect, and air diffusion into the plasma plume influenced the microhardness values of the plasma sprayed NiTi coating. The mechanical interlocking phenomenon of the coating is mainly responsible for the adhesion strength variation of the coatings. The adhesion strength analysis of the coatings depicted various failure modes such as adhesive failure, cohesive failure, mixed-mode failure, glue joint failure, etc. The tribological behavior of the NiTi plasma sprayed coatings was analyzed by the solid particle erosion test performed at two different erodent impingement angles, i.e., 45˚ and 90˚. The results revealed that with the increase in primary gas flow rate and plasma arc current, the erosion rate of the samples eroded with 90˚ erodent impingement angle increase due to the rise in brittle nature of the surface. The lack of edge strength and increase in stress concentration of the coatings having more porosity percentage lead to more damage by the erodents at both the angles of impingements. Furthermore, the surface area of the roughness peaks and the gap between the roughness peaks also influenced the erosion rate of the coatings. Again, due to the difficulty in penetration of the erodent in the surface of the coatings having a higher hardness, the erosion rate is less in those samples. Various wear mechanisms have been observed in the eroded samples, such as plastic deformation, ploughing, microcutting, lip formation, scratches, groove formation, splat fracture, splat fragmentation, splat delamination, pit formation, etc. To optimize the process parameters of the plasma spray coating, in the current work, two optimization techniques were considered. Initially, using the genetic algorithm technique, the process parameters were optimized, and then to validate the obtained result the fuzzy-TOPSIS technique was adopted. From the genetic algorithm, the obtained results revealed the optimized parameters as plasma arc current 550 A and primary gas flow rate as 45 lpm, and the fuzzy-TOPSIS technique also confirmed the same. Also, the primary gas flow rate was the major contributing factor proved by the ANOVA technique. The coating developed at optimized process parameters revealed the superhydrophobic characteristic ensured by the water contact angle 165º and sliding angles 8±10. The surface profile of the coating supports the theory. The coating loses its superhydrophobic characteristics after 90 passes of abrasion in sandpaper. Furthermore, from the annealing, it was observed that the coating retained its superhydrophobic characteristics up to 400 ˚C. The pH test disclosed the sensitiveness of the coatings in an acidic (pH=10) and basic (pH=2) environment. The dropwise evaporation analysis ensured the lower heat absorption of the coating. The coating also has interchanging characteristics in the presence of an electric field.