In this article Ni₅₀Ti₂₅Al₂₅ (at.%) powders, were prepared by mechanical alloying method by using a planetary ball mill (home-made) for plasma spraying process. Figure 1 shows the morphology of initial powders and Table 1 gives the characteristic of feedstock. In order to protect powders from oxidizing, the mechanically alloying process was conducted in an Ar atmosphere. Furthermore to protect feedstock materials against aggregation 3-wt% stearic acid should be added before putting the materials in a planetary ball mill. The planetary milling system was composed of a stainless steel container and 10-mm diameter steel balls. Ball-to-powder weight ratio (BRR) was 10:1 and the impeller was rotated at a speed of about 340 rpm. The Ni₅₀Ti₂₅Al₂₅ (at.%) powders were prepared at different mechanically alloying times to the most suitable powder was selected for plasma spray process.

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. SEM images of initial powders: (a) Nickel, (b) titanium, and (c) aluminum.

Table 1.

Table 1.

Table 1. Feedstock material specifications. . Feedstock powder Size/μm Purity Nickel 50–100 99% Titanium 100–300 99% Aluminum 50–100 99%

Table 1. Feedstock material specifications. .

In the present study, the plasma torch system that was used to deposit Ni₅₀Ti₂₅Al₂₅ (at.%) coating was developed in the Plasma Physics Research Centre. Figure 2 shows the schematic diagram of the plasma torch system. Argon gas was used to produce plasma and powder feed gas. In this research, a low carbon steel sheet (0.08% C, 0.017% Si, 0.017% P, 0.423% Mn, 0.025% Ni, 0.003% S, 0.006% Cr, balance Fe), was cut and used as substrate. To increase adhesion between the substrate and coating, all steel substrates were pre-treated by grit-blasting with aluminium oxide (Al₂O₃) particles. In all of the experiments, the powder was injected into the plasma jet In order to reduce the internal stress in the coating, all the samples were cooled to room temperature. The main parameters of the plasma spray system used in this study are presented in Table 2.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. (color online) Schematic diagram of plasma torch system.

Table 2.

Table 2.

Table 2. Technical specification of APS system. . Parameters Unit Working gas argon Arc current/A 500 Arc voltage/V 50 Stand-off distance/mm 120 Powder feeding rate/(g/min) 20 Plasma gas flow rate/(L/min) 40

Table 2. Technical specification of APS system. .

Phase compositions of powders and coatings are analyzed by x-ray diffraction (XRD, STADI-MP) with Cu Kα radiation (λ = 1.54056 Å). To evaluate the surface properties of the deposition layer, a scanning electron microscope (SEM, VEGAII TESCAN) equipped with energy dispersive x-ray analysis (EDX) was utilized and the porosity percentage of coating was calculated by using image analysis software (ImageJ).

In this research, Vickers microhardness testing device, (BUEHLER) was used to measure the microhardness values of coatings, according to the ISO/IEC17025 standard. Applied force was 500 grf for 10 s–15 s. For each sample, hardness test was performed five times on the polished cross section surfaces and the average of the results was recorded. Also, the microhardness values of samples in the substrate-coating interface were evaluated.

In addition, the adhesion strengths of coatings are measured by pull-off device (DEFELESCO) at three different points and their average was reported as the adhesion strength of each coating.

In this research, the electrochemical polarization test was used to investigate the corrosion behaviour of the coating. Test results are recorded at room temperature using an electro-chemical apparatus consisting of a platinum coil as the counter-electrode and calomel reference electrode in 3.5 wt% sodium chloride solution (NaCl), using an EG & G potentiostat (model 273 A). The test was started after 1-hour immersion with 1-mV/s scan rate. The corrosion potentials (E_corr) and corrosion current density (i_corr) were obtained with the aid of ‘Cview’ software.

The quality of plasma sprayed coating is influenced by the size and structure of the feedstock powder material.^[24] Figure 3 shows the XRD patterns of powder particles at different times of planetary milling. As the figure shows, after 16 h milling distinguishable phases are pure nickel, titanium and aluminum and no new inter-metallic phases are formed. A comparison among the XRD patterns at different milling times shows that the intensity of initial material peak decreases by increasing the milling time, slightly. This is because of reducing the crystallite size and increasing the dislocations caused by the internal strain which increases during mechanically alloying.^[25]

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. XRD pattern of feedstock powder particles at milling times of (a) 2 h, (b) 4 h, (c) 8 h, and (d) 16 h.

The SEM of feedstock particles after planetary milling is presented in Fig. 4. The morphologies of powders indicate that the lamellar structures are formed in all of the samples. When the lamellar structure exists in the structure, the formation of inter-metallic compounds increases during thermal plasma spraying due to the provision of large interfaces for exothermic reaction.^[26] When the mechanically alloying time is increased, more homogenous lamellar structure is produced. But, the particle size becomes very small and the small nano-particles move from the powder feeder device to plasma torch nozzle with difficulty. Furthermore, nano-particles more or less evaporate in the thermal plasma jet.^[27] So, in this study, Ni₅₀Ti₂₅Al₂₅ particles that are milled for 2 h are used. These particle sizes are between 20 μm and 70 μm.

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. SEM images of powder particles at milling times of ((a), (b)) 2 h, ((c), (d)) 4 h, ((e), (f)) 8 h, and ((g), (h)) 16 h.

Figure 5 presents the XRD patterns of coatings at substrate temperature from room temperature to 400 °C. The formation of an inter-metallic compound occurs at a suitable time and heating. With increasing substrate temperature, the probability of forming inter-metallic phases is higher. It shows the formation of different inter-metallic compounds such as Ni₃(Ti,Al) and Ni₂TiAl phases and also Ni and TiO, during spraying. This phenomenon is not unexpected. These phases can also be seen in other research.^[20,28] The Ni₃(Ti, Al) phase causes anti-corrosion properties at high temperature.^[29] In addition, the XRD pattern shows that at high substrate temperature, the intensity in the direction of the Ni peak is lower and the intensity in the direction of the Ni₃(Ti, Al) peak is higher than others.

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. XRD patterns of coatings at substrate temperatures of (a) room temperature, (b) 200 °C, and (c) 400 °C.

Figure 6 shows the SEM images of the cross section of deposition layers at different substrate temperatures. As can be seen in the figure, the Ni–Ti–Al coating without preheating contains some pores and voids. With the increase of the temperature of substrate, coatings each exhibit a uniform structure by minimizing the amount of porosity in the cross-sections of coatings. The porosity percentage of the cross-section of coating is calculated by ImageJ software. This calculation is repeated five times and the averages are about 8.4% for coating without preheating and 3.7% and 1.6% for coatings with preheating at 200 °C and 400 °C substrate temperature, respectively. When the surface is preheated prior to the coating, the deposition layer is more uniform with less porosity. This is because when the temperature of the substrate increases, the wettability increases. This provides better mechanical anchorage between the particles and substrate but in the surface without preheating, the heat of the particles is transferred to a cool substrate and the required wetting effect is not provided.^[30] Therefore, the porosity of coating without preheating is more than with preheating.

Fig. 6.

	Figure Option ViewDownloadNew Window
	Fig. 6. (color online) SEM cross section images of deposition layers at substrate temperatures of ((a), (b)) room temperature, ((c), (d)) 200 °C, and ((e), (f)) 400 °C.

One of the important mechanical properties of coating is microhardness. In all the coatings, inter-metallic phase greatly affects the microhardness of coating. In this research, it is found that with substrate temperature increasing from 200 °C to 400 °C, the microhardness value increases from 304 HV to 444 HV and the microhardness of coating without preheating is less, about 283 HV. This is due to higher flattening, less porosity and higher number of inter-metallic phases in the coatings with high substrate temperature.

Another important mechanical property of the coating is adhesion strength. As mentioned previously, when the temperature of the substrate increases, the wettability increases and this provides an excellent weld between the particles and substrate surface, thereby resulting in better adhesion between impacting particles and the substrate. So, with substrate temperature changing from room temperature to 200 °C and 400 °C, the adhesion strength increases from 13.7 MPa to 18.5 MPa and 20.4 MPa, respectively. Studies show that as the substrate temperature increases, the coated particles become disk shaped. In contrast, at low substrate temperature, the coated particles are irregular. Disk splats provide better substrate-coating adhesion strengths than irregular splats.^[23]

In this study, for better observing the adhesion between coating and substrate, interfacial indentation test is used. A square-based pyramidal-shaped diamond indenter is used at the interface between the substrate and surface. The load of the indenter is similar to the microhardness test for coating layer (500 grf). This experiment also shows the APS coating with 400 °C preheating temperature, and records the best adhesion. Figure 7 shows that there is no crack at the interface of the coating with 400 °C preheating temperature, after the interfacial indentation test.

Fig. 7.

	Figure Option ViewDownloadNew Window
	Fig. 7. Vickers test traces in the coating-substrate interface at substrate temperatures of (a) room temperature, (b) 200 °C, and (c) 400 °C.

The electrochemical polarization behaviors of the coatings and substrate are presented in Fig. 8. Polarization curves show that polarized coatings have higher corrosion potentials and lower current densities than carbon steel substrate. This means that substrate corrosion occurs earlier than coating. It is clear from Table 3 that the current density of coating decreases by increasing the substrate temperature. Therefore, the corrosion resistance increases due to the increase in preheating temperature, low porosity, and better coating-substrate adhesion. When the coating has a high number of micro cracks and pores, the corrosion rate of coating increases, because the electrolyte is driven to the substrate and prevention of a homogeneous passive layer form forming.^[28]

Fig. 8.

	Figure Option ViewDownloadNew Window
	Fig. 8. (color online) Tafel polarization plots of coating and carbon steel substrate.

Table 3.

Table 3.

Table 3. Technical specification of APS system. . Substrate temperature/°C ECorr/V ICorr/(μ A · cm−2) Room temperature −0.621 6.048 200 −0.599 5.695 400 −0.521 5.320

Table 3. Technical specification of APS system. .