**3. Results and Discussion**

### *3.1. Powders Morphology and Coatings Microstructure*

CP (commercially pure)-Al (in the size range of 9–40 μm), and CP-Ti (in the size range of 10–45 μm) powders all possess spherical morphology as shown in Figure 1a,b respectively. Surface morphology of CS Ti and Al coatings (in as-sprayed condition) is shown in Figure 2b,d, respectively. *R*a of coatings (in as-sprayed condition) was 2.816 ± 0.7 μm and

2.182 ± 0.7 μm for Al and Ti coatings, respectively. Lower *R*a for Ti coating was due to intense plastic deformation of Ti particles during CS process. Figures 2a,c and 3 demonstrate the microstructure of polished cross section of the coatings on the AZ31B substrate.

**Figure 1.** SEM images of feedstock powders (**a**) commercially pure-Al, and (**b**) commercially pure-Ti powders.

The local deformation of Ti powder particles was obvious (Figure 3c–e). Moreover, relatively dense microstructure (Figure 3a,b and Figure 2a) along with very limited micropores with porosity level of about 0.40 ± 0.20% was observed for titanium coating (in this research work). On the contrary, higher level of porosities were detected in atmospheric plasma sprayed (APS) Ti coating (with 10.2% porosity level) and CS Ti coating (with 2.7% porosity level) as well [31,39]. Different thermal spray methods have been employed to deposit Ti coatings with low level of porosities and high purity as well. However, thermal degradation of the deposited Ti powder particle occurs during HVOF spray process. This could be related to the temperature range of spray powder particles which is about 1227–2427 ◦C. In fact, the probability of a hard and brittle oxygen enriched layer formation (in the case of Ti) which is also known as "α-case" is expected and could cause the notable loss of plasticity, ductility, etc. [31]. Another method is LPPS (low pressure plasma spray) process that could restrict oxidation during spray process. This is attributed to the vacuum environment which makes this technique costly. Instead, in warm spray (as modification of HVOF spray system [31]) method, supersonic gas flow temperature is adjusted by injecting N2 into the mixing chamber. Formation of a relatively dense coating with limited oxidation of powder particles is anticipated using this method. Nevertheless, higher level of porosities was reported in the deposited Ti coatings (with porosity level of about 3.8–5.5%) on Mg alloys by WS method under different N2 flow rates [31,39]. Higher level of porosities in warm sprayed Ti coatings substantially declined their corrosion resistance in 3.5 wt % NaCl solution and caused quick degradation of Mg substrate during long term corrosion (after 24h of immersion) [31].

**Figure 2.** *Cont*.

**Figure 2.** SEM images from (**a**) cross section of Ti-coated AZ31B, (**b**) Ti coating surface, (**c**) cross section of Al-coated AZ31B, (**d**) Al coating surface, EDS analysis of 1 (**e**), 2 (**f**), and EDS analysis of 3 (**g**), 4 (**h**).

**Figure 3.** *Cont*.

**Figure 3.** Photomicrographs from polished crossed section of (**<sup>a</sup>**–**<sup>e</sup>**) Ti-coated AZ31B at different magnifications, and (**f**–**j**) Al-coated AZ31B at different magnifications before and after etching.

In this study, CP-Al coating with porosity level of about 1.00 ± 0.20% depicted higher porosity level than as-sprayed Ti coatings (Figure 3f–j). The presence of considerable number of micro-pores and even (worse) micro-cracks in cold sprayed CP-Al coating microstructure (at inter-particle boundaries) on AZ91D Mg alloy was also reported by Y. Tao, et al. [22]. In fact, lower degree of localized plastic deformation (localized heating, stresses) [22] resulted in an extensive formation of micro-defects at inter-particle boundaries. Cold sprayed Al coating with high denseness and having sub-micron sized grains considerably improved the corrosion resistance compared to CP-Al bulk substrates [22,25].

Figure 4 shows the XRD spectra of bare Mg alloy, feedstock powders and as-cold sprayed coatings. XRD spectrum (Figure 4a) shows that bare AZ31B is mostly comprised of α-Mg phase. Powder particles and CS coatings displayed similar phase structure and crystal planes (Figure 4b,c). Phase transformation and oxidation weren't evidently observed in the CS coatings (Figure 4b,c). The broadened peaks in the XRD pattern of Ti (Figure 4c) coating are primarily related to the intense plastic deformation of powder particles in the coatings compared to Al coating (Figure 4b) during cold spray process [40,41]. Low processing temperature and sizable peening effect of the powder particles (during cold spray process) could lead to the retention of primary phase and crystal planes of powder particles in CS coatings. On the contrary, wire flamed sprayed Ti coatings were mainly constituted by oxides, nitrides, and carbides phases due to the nature of the flame spray process. This resulted in the inferior corrosion protection performance of the sprayed Ti coatings. Hence, these coatings had to be sealed with epoxy or Si resin for usage in the chloride containing solutions [31], so it is anticipated that a Ti coating (in this research) without any post-spray treatments could significantly decrease the corrosion rate of magnesium alloy and make AZ31B Mg alloy usable in chloride containing solutions for long periods of time.

Titanium coating considerably raised average micro-hardness (HV0.025) of AZ31B Mg alloy surface (Figure 5a), while aluminum coating lowered average micro-hardness (HV0.025) of substrate surface (Figure 5a). Higher micro-hardness in Ti coating may implies severe plastic deformation (high dislocation density) mostly at exterior region of powder particles (or inter-particle boundaries) that caused the increase in the coating denseness [13,25].

**Figure 4.** XRD patterns of (**a**) bare AZ31B, (**b**) as-received Al powder particles and as-sprayed Al coating, and (**c**) as-received Ti powder particles and as-sprayed Ti coating.

**Figure 5.** (**a**) Average micro-hardness (HV0.025); and (**b**) wear rate of bare and coated AZ31B Mg substrates.

The wear depth on Ti-coated Mg alloy was around 7 μm in which Ti surface was in contact with the steel ball whereas the Al-coated Mg alloy displayed the highest wear depth of ~70 μm. The wear rate of the entire track calculated from the wear depth was plotted in Figure 5b. These results were also compared with bare AZ31BMg alloy samples that showed lower wear rate compared to Al coating surface but higher wear rate than Ti coating surfaces. In fact, a surface with higher hardness showed lower wear rate than a surface with lower hardness. This proves that CS Ti coating substantially raises surface hardness and lowers the wear rate of Mg alloys compared to CS Al coatings.

The mechanical and tribological characteristics of CS pure Ti coatings on Ti-6Al-4V substrates were studied by Khun et al. [36]. The results indicated that wear resistance of the CS Ti coating (experimented against steel balls) was noticeably higher than that of Ti-6Al-4V alloy. This was related to the cold work hardening (strain hardening) during spray process and interestingly highly wear-resistant oxide layers formation on wear tracks of CS Ti coatings (during wear tests). In fact, cold sprayed pure Ti coatings with higher compactness and lower porosities showed higher hardness and thus improved wear resistance on the Ti64 alloy as substrate [36]. Astarita, et al. also reported that CS Ti coatings can improve the wear performance of bare AA2024 alloy [42].
