Microstructural Evolution in Single Splat

Studying the mechanism of microstructure evolution is essential as it gives knowledge about clustering. A thorough understanding of the clustering phenomena is

essential for creating new strategies to avoid it. Plasma sprayed coatings are formed by a layer by layer deposition of splats. Splats are formed when the molten/semi-molten droplets impact the substrate and solidify. Two kinds of splat morphologies namely fingered and disc shaped splats are observed for the SD Al-5CNT powder as shown in Figures 4.19 and 4.20 respectively.

Fig. 4.19: SEM images showing a) Fingered splat from SD Al-5CNT powder, b) splat finger showing CNT cluster infiltrated with metal marked by rectangles, c) and d) high magnification images of the CNT infiltrated cluster

Fig. 4.20: SEM images showing a) Disc splat of Al-5CNT powder showing CNT rich cluster, and b) CNT rich cluster with poor metal infiltration

Splats were mostly irregular disc type for Al-10CNT powders as shown in Fig.

4.21. A CNT rich cluster that has not been infiltrated with metal is also seen.

Fig. 4.21: SEM images showing a) Disc splat from SD Al-10CNT powder showing CNT rich cluster, and b) high magnification image of the CNT rich cluster with poor metal infiltration

It was observed that the fingered splats are smaller than the disc splats indicating that they are formed from smaller powder particles. This could be due to the fact that small particles undergo complete melting and contain low CNT content due to which the viscosity is low. Larger particles could exhibit higher viscosity due to higher CNT content which inhibits spreading and splashing. Viscosity of the droplet from composite particles can be given as [231]





e 1 1R /R



  Equation 4.1

Where e is the effective dynamic viscosity,  is the dynamic viscosity of the liquid phase, RC is the radius of CNT clusters and the RP is the droplet size and  is the CNT fraction. Considering RC/RP<<1, equation 4.1 predicts 6.5% and 13.8% increase in viscosity of the molten Al-Si alloy due to addition of 5 wt.% and 10 wt.% of CNTs respectively. Most of the splats in Al-10CNT are disc shaped which is due to the increase in the viscosity of the melt caused by increased CNT content. The splat sizes ranged from 70 – 190 m for Al-5CNT and 50 – 160 m for Al-10CNT powder. This is in accordance with the starting powder sizes. The fingers represent the last material that solidifies during splat formation. The CNTs flow in radial outward direction along with the molten particle on impact. It is seen from Fig. 4.19b that CNTs are found in the finger area indicating that they retain the thermal energy and keep the melt molten. This is deduced from the fact that the specific heat capacity of C (Graphite) and Al-12%Si alloy at 2300K is equal to 2.145 J.g^-1.K^-1 and 1.152 J.g^-1.K^-1 (taken from thermo-chemical software and database FactSage) respectively. It is concluded from the magnified images (Fig. 4.19c and 4.19d) that the CNT clusters in the fingers have been infiltrated by the molten alloy.

Distribution of CNTs in each of the fingers is very uniform. Figure 4.20 shows a disc splat of SD Al-5CNT and a CNT rich cluster (Fig. 4.20b) which has been partially infiltrated with molten Al-Si alloy. The disc splat is larger (150 µm) compared with fingered splat (75 µm) for reasons explained earlier. Figure 4.21 shows a disc splat from SD Al-10CNT particle showing CNT clusters which are partially infiltrated with metal.

The size of the CNT clusters observed within a splat is between 10 -30 µm which is same as the size of CNT clusters observed in the coating cross sections Fig. 4.8). A few clusters of size up to 50 µm are observed in the Al-10CNT coating cross section, which form due to contiguity of the CNT clusters from individual splats.

Landry et al. [232] have shown that the wetting of Al-Si alloys on graphite improves with time i.e., the contact angle diminishes from 160 to 40 degrees with time due to formation of interface carbide layer over a period of 10⁵ s at a temperature of 1190K. This indicates that reaction products forming at the interface promote wetting and hence infiltration. The pressure induced infiltration of porous compacts is governed by the Darcy’s law given by [233]

  ^

^

where h is the infiltrated depth of the metal into the porous compact, kp is the intrinsic permeability of the compact,  is the viscosity of melt, t is the infiltration time, Vp is the particulate volume fraction and P is the applied pressure. P0 is the threshold pressure for infiltration to occur and is governed by the capillary forces and is given by [234]





where  is a parameter dependent on the particulate shape, LV is the liquid vapor surface tension of the infiltrating liquid and  is the contact angle. From eq. 4.2 and 4.3 it can be seen that lower the viscosity and surface tension of the infiltrating liquid, larger will be the infiltration depth. During plasma spraying, the impact of molten particles on the substrate could also lead to pressure infiltration of the CNT clusters. P0 was computed ~ 67-200 MPa with following values used for computation: CNT diameter (D) = 60 nm, 

= 1 for spherical shape particle,  = 40^o [232], Vp assumed ~ 0.50-0.75 and LV = 0.889 N.m^-1 (for A356 alloy [235]). Assuming that an Al-Si droplet of 50 m diameter travelling at 200 m.s^-1 comes to rest in 1 s forming a splat of 100 m, the pressure generated due to impact is found to be ~4 MPa, which is orders of magnitude smaller than P0. Hence, it is concluded that infiltration of the CNT clusters during impact is rather unlikely. Table 4.1 showed the powder properties along with the in-flight temperature and velocity. Considering the stand-off distance as 100 mm, the flight time has been calculated and shown in Table 1. The particle flight time is less than 1 ms. The measured temperature of the particles during flight is of the order of 2300 K. It is anticipated that infiltration would be limited due to the small interaction time between the CNT cluster and the molten metal. But under such high superheated conditions, the viscosity and surface tensions would be very low which will promote infiltration. Since in Al-10CNT a larger amount of CNT clusters are observed (due to higher CNT content), it is expected that there would be more clusters which have not been infiltrated with metal (Figure 4.21b). Also it is observed that there is lot of porosity within each CNT cluster. This accounts for the low density of the coatings.

So in order to improve the dispersion of CNTs in the coatings, CNTs must wet the molten alloy and remain dispersed during the melting stage. This can be possible if there is a limited reaction between the alloy and the CNT so that the CNTs wet the alloy.

Plasma spraying can be used for obtaining densified powders. However, air plasma spraying is not suitable as it will lead to oxidation the powders.