3.1. Hardness and young modulus measurements
Figure 1 shows the mechanical properties (hardness and young modulus measurements) between the bare and coated foils. As can be seen in Fig. 1, bare foil has relatively lower hardness and young modulus values. In consideration of an increment of both hardness and young modulus values from coated foils, coordinated nano-composites therein could be able to provide better Tribological properties [13–17].
3.2. Tribology measurements (Wear and friction)
To determine the quality of novel bare and coated foils, in terms of their cushion of hydro-dynamic oxide layers which act as a buffer between the shaft of high-speed machinery and the AFB, tribology measurements were analysed from various underpinnings which include: load and angles’ variation, change of distance and progression of time. A deep scrutiny was on stability of coating from running-in periods to the end of test.
3.2.1. Specific wear and wear indexes rates
Wear rates were conducted from the equations explained under Sect. 2.4.2. Figure 2 shows SWRs for both bare and coated foils, while AWIs are shown in Fig. 3. As can be seen in Fig. 2, bare foil had a higher SWR, while relatively low from coated foils. In particular, F2 possesed lowest SWR. For relative AWIs on coated foils, F3 possessed lowest AWI as can be seen in Fig. 3. Overall, AWIs followed the following ascending order: bare < F3 < F1 < F2. Based on criteria following SWR and AWI, all coating materials show an effectiveness in terms of lowering wear rates under engaged tribological conditions [18–23].
3.2.2. Frictional coefficient
Frictional coefficients (µ) on both bare and coated foils under different exposural conditions are shown from Fig. 4 to 8. As can be seen from the Figs, µ from all coated samples is fairly low ralative to the bare foil. However, the bare foil is still having remarkably low frictional coeffient relative to some of the reported bare foils µ [24–30]. An improvement observed from the coated foils is attributed to a likelihood formation of wear debris which act as buffer (protective layer) detering excessive wear on the surface of bare foil [31–36]. Most importantly, this underpins the point made by [37] that coating foil with soft material lowers shear stress, thermal damage and coating delaminations, particularly when it’s coated with soft-solid coating. In comparison to the bare foil, coated foils are able to provide significant liftoff (hydrodynamic lubrication) during harsh conditions operation.
3.3. SEM Analyses
SEM analyses conducted post tribological measurements are shown from Fig. 9 to 13. As can be seen in Fig. 9 (under low load conditions), bare foil possessed some scratches and ploughing grooves as can be seen in Fig. 9 (a). On the other hand, F3 coated sample showed shallow inclusions and voids as can be evidenced in Fig. 9 (d). Contrarily, micrographs demonstrating shallow fine grooves and uniformly distributed ploughing can be seen in Fig. 9 (b) and (c), respectively. Under higher load conditions, aggressive surface was recognized on the morphology of the bare foil. In fact, large amounts of wear debris, grooves and cracks were discovered as can be seen in Fig. 10 (a). Uncooperatively, smaller chips can be seen in Fig. 10 (b) for F1 coated foil. Most noticeably, smooth surface can be seen on micrograph corresponding to the F2 as shown in Fig. 10 (c). In contrast, scars and shallow wear can be seen in Fig. 10 (d)
In consideration of the variation of contact angle during the measurements, different features were furthermore observed from the micrographs. Eliminated layers were observed on the surface of the bare foil as can be seen in Fig. 11 (a), while small scale cracks and chips were discovered on F3 micrograph demonstrated in Fig. 11 (d). However, shallow inclusions, voids and pores can be noticed in Fig. 11 (b). Furthermore, a smooth surface micrograph (with no cracks and pits) can be seen in Fig. 11 (c).
In progression of the time during the measurements, large scale cracks and chips were noticed from the surface of the bare foil as can be seen in Fig. 12 (a). Nonetheless, inconsiderable wear products from the oxidation are straightforwardly observed in Fig. 12 (b). Likewise, smooth surface with negligible accumulation of wear products were noted from the micrograph of the F2 as shown in Fig. 12 (c). Defiantly, superficial pits and rapid wear debris can be realized in Fig. 12 (d).
On the variation of distance during Tribological measurements, micrograph composed of spalling can be seen in Fig. 13 (a) for the bare foil. Conversely, uniformly distributed shallow scratches can be seen in Fig. 13 (b). Similarly, shallow and fine grooves can be noticed in Fig. 13 (c). On the other hand, the micrograph consists of grooves; ploughing, dimples and voids can be seen in Fig. 13 (d). Overall, a slight difference in features of morphologies can be noticed from all coated foils relative to the bare foil. This shows that novel nanocomposites applied on foils differ slightly. In particular, slight difference between F1 and F2 could be an indication of a strong intactness of the applied nano-coatings [38]. Although F3 relatively showed inferior Tribological properties, aggressions on its micrographs are much lesser than those that were found on the bare foil, and that evidences effectiveness of all applied nano-coatings against dynamic Tribological conditions [39–50].