Plasma spraying involves myriad of process, equipment and powder parameters which has a direct effect on the coating properties [19, 20, 69-73]. Hence, there is a need of optimizing the plasma process parameters in order to achieve “dense” coating. The objective of the optimization of plasma process parameters is to develop the process maps to understand the influence of “key processing parameters” on the thermal and kinetic energy of the particle that affect “porosity” and hence “mechanical properties” of the coating. Effect of CNT content on the thermal and kinetic history of the particle in the plume is also investigated to obtain the coating with the lowest porosity. Development of process map for all three powders (A-SD, A4C-SD, and A8C-SD) has been discussed below.
5.1 Process Map for A-SD Powder
Figure 5.1 is the integrated process map for A-SD powder at 75 mm stand-off which correlates the plasma processing parameters (plasma power, feed rate and primary gas flow) with the particle state and effect of particle state on the coating’s porosity and microhardness. Figures 5.1a and b show the temperature and velocity of the A-SD particles at the 75 mm stand-off. The relative size of the legends in Figures 5.1-5.4 is proportional to the plotted characteristic i.e. temperature, velocity, porosity and microhardness. It is observed that both particle temperature and velocity increased with the increasing plasma power. An increase in the primary (argon) gas flow rate showed
velocity. As powder feed rate was increased, temperature and velocity of the powder particle dropped down. The highest temperature (2745 K) and velocity (338 ms-1) of the A-SD powder was achieved at the highest plasma power (34 kW), highest primary gas
Figure 5.1: Process map showing (a) temperature and (b) velocity distribution of in flight A-SD powder particle at 75 mm stand-off distance for various plasma processing parameters (c) Porosity and (d) Microhardness distribution of A-SD coating at 75 mm stand-off at optimized plasma processing parameters. The size of the circle and diamond legend is proportional to the numerical value
(a)
(b)
flow rate (56.6 slpm) and at the lowest feed rate (3 g/min). Porosity of the A-SD coating measured is shown in Figure 5.1(c). The lowest porosity of 4.2% was achieved at highest plasma power (34 kW), highest primary gas flow rate (56.6 slpm) and lowest feed rate (3 g/min.). The reverse is true for the highest porosity (9.1%) at the lowest plasma power (30 kW), lowest primary gas flow rate (42.5 slpm) and the highest feed rate (6.2 g/min). Microhardness of all the free-standing coatings samples were measured and shown in Figure 5.1(d). It should be noted that marginal difference in porosity of A-SD coating at 30 kW (~4.2%) and 34 kW (~4.4%) at 75 mm stand-off distance (as shown in Figure 5.1c) leads to ~12% improvement in hardness at higher plasma power. An increase in the hardness at higher power is attributed to (i) its exponential dependence on the porosity as shown in equation 5.1 [137] and (ii) improved intersplat bonding as a result of enhanced degree of melting of powder particle at higher plasma power.
H H0exp(aP) (5.1) In equation 5,1,p is porosity, H0is the hardness of fully dense material, H is the overall
hardness of the porous material and a is the arbitrary constant. Further, a direct correlation between the highest hardness (1498 VHN) and the lowest porosity (4.2%) was found. A similar correlation exists for the lowest hardness (913 VHN) obtained at the highest porosity (9.1%) suggesting one-to-one mapping.
A similar trend in the temperature and velocity of the in-flight particle was observed for A-SD powder at 100 mm stand-off, as seen in Figures 5.2a and b. It shows that stand-off distance has a significant effect on the particle state. Table 5.1 shows the
optimized processing parameter (plasma power: 34 kW, primary gas: 56.6 slpm, feed rate: 3 g/min) for stand-off distance of 75 and 100 mm. Temperature and velocity of the powder particle achieved at the 100 mm stand-off is ~3% and ~8% lower respectively, than temperature and velocity of the powder particle at 75 mm stand-off. With increasing stand-off distance, porosity of the coating increased resulting in low hardness.
Figure 5.2: Process map showing (a) temperature and (b) velocity distribution of in flight A-SD powder particle at 100 mm stand-off for various plasma processing parameters (c)) Porosity and (d) Microhardness distribution of A-SD coatings at 100 mm stand-off at optimized plasma processing parameters. The size of the circle and diamond legend is proportional to the numerical value.
(d)
(c)
Table 5.1: Particle characteristics and coating porosity for A-SD powder at optimized plasma process parameters for different stand-off distances.
* Error in porosity data was found to be ±0.3
5.2 Process Map for A4C-SD Powder
As earlier explained for A-SD powder, same methodology has been adopted for developing the process map for A4C-SD powder. Figures 5.3a and b illustrate the temperature and velocity of the particle respectively, at 75 mm stand-off whereas Figures 5.3c and d show the temperature and velocity of the particle at 100 mm stand-off. As the plasma power increased from 30 kW to 34 kW, both temperature and velocity of the particle increased. An increasing primary gas flow rate also increased the temperature and velocity of the in-flight particle. The higher feed rate of 6.2 g/min leads to lowering of the temperature and velocity due to larger mass in the plasma plume. The highest temperature and velocity of the particle achieved at 75 mm stand-off and 3 g/min feed rate is 2423 K and 319 m/s respectively. The temperature and velocity of the in-flight A4C-SD particle reduced to 2378 K and 281 m/s respectively for the higher stand-off (100 mm) at the same feed rate of 3 g/min. Figure 5.3 (e) and (f) represents the porosity
Temperature
(K) Velocity (m/s) *Porosity % Hardness (VHN)
Stand off: 75 mm 2745±9 338±0.6 4.2 1498±49
Figure 5.3: Process map showing (a) temperature and (b) velocity distribution of in flight A4C-SD powder particle at 75 mm stand-off for various plasma processing parameter (c) temperature and (d) velocity distribution of in flight A4C-SD powder particle at 100 mm stand-off for various plasma processing parameter (e) Porosity and (f) Microhardness distribution of A4C-SD coatings at both stand-off (75 mm & 100 mm) distances.