Surface modification of Ti alloys with WC-TiB₂-reinforced laser composite coatings

3.1 SEM analysis

With the addition of YPSZ, the laser clad composite coating shows a fine microstructure (Figure 1A). As shown in (Figure 1B), a great number of fine piece-like or Ti-B stick-like precipitates are produced. It can be deduced that the growth of the Ti-B crystals may be retarded by those piece-like precipitates to a certain extent. In addition, tetragonal ZrO₂ was present in such coating (Figure 1C). A previous investigation [8] reported that as the stabilizer of ZrO₂, Y³+ in Y₂O₃ can suppress c→t→m transformation; furthermore, the Y³⁺ content had a different suppression effect on ZrO₂ transformation, which enabled metastable phase tetragonal ZrO₂ (t-ZrO₂) to exist. Meanwhile, t-ZrO₂ phase structure possesses stress-induced transformation character, which can produce a transformation toughness effect [9].

Figure 1:

SEM images of the TiAl/Fe+Co-coated WC/TiB₂+YPSZ laser clad coating.

(A) The clad zone, (B) TiB₂, and (C) t-ZrO₂.

3.2 XRD analysis

The composite coating mainly consists of γ-Co, Ti₃Al, Al₁₃Fe₄, M₁₂C (Co₆W₆C), α-W₂C, TiB₂, ZrO₂ and Al₁₆Co₇Zr₆, along with many amorphous alloys; these phases are produced through the in situ metallurgical reactions, as shown in Figure 2. Due to the addition of YPSZ, the diffraction peak of ZrO₂ can be observed in the XRD pattern, indicating that Fe reacts with Al in laser molten pool, which then leads to the formation of Al₁₃Fe₄. Moreover, a number of YPSZ show a very marked trend of floating on the surface of the molten pool due to its low density; thus, the diffraction peak of ZrO₂ can be detected.

Figure 2:

XRD pattern of the laser clad coating.

3.3 EDS/EPMA analysis

When the fine piece-like precipitates in the coating were scanned by electron probe micro-analyzer (EPMA), the diffraction peaks of Al, Co and Zr reached high value (Figures 3A and B). Combined with the XRD results, it can be deduced that there is a Al₁₆Co₇Zr₆ phase in the fine piece-like precipitates, reinforcing the fact that laser cladding is an effective method to prepare the amorphous alloy coatings on titanium alloys, and is a significant approach in improving the surface properties of materials. An earlier work [10] reported that the production of the amorphous alloys can help improve the wear resistance of metals.

Figure 3:

EPMA line scan results of the clad zone in the TiB₂/α-W₂C-reinforced composite coating.

(A) The clad zone and (B) Al, Co, Zr.

As shown in Figure 4A, the block-shape precipitate is produced. The EDS pattern indicates that C, Al, Ti, Fe, and W are in location 1 (Figures 4A and D). Combining the results of Table 1 and the XRD diagram, it can be deduced that the block-shape precipitates mainly consist of α-W₂C and Al₁₃Fe₄/Ti₃Al. The SEM image demonstrates that the nanocrystals are produced (Figure 4B). In general, the metallic liquid solidifies immediately into the crystalline phases due to the unstability of metallic liquid at temperatures below the ideal melting temperature. The stability of supercooled liquid plays an important role in the fabrication of the bulk metallic glass. Okai et al. [11] reported that the amorphous alloys have attracted increasing interest as a precursor to producing nanocrystalline alloys containing a remaining amorphous phase by partial crystallization, or alloys consisting of a nano crystalline single phase by full crystallization. These nanocrystalline alloys exhibit unique characteristics, such as good mechanical properties. The EDS pattern of location 2 indicates that there are O, Zr, Fe, Ti, and Al in the needle-like location (Figures 4C and E). According to this result, it can be assumed that m-ZrO₂ has been produced in the coating matrix. In accordance with the report of Chen et al. [12], the transformation from t-ZrO₂ to m-ZrO₂ (t-m) has proven to be nucleation controlled and martensitic in nature.

Figure 4:

SEM images of the the TiB₂/α-W₂C-einforced composite coating.

(A) The block-shape precipitate, (B) nanocrystals, and (C) the needle-like location; (D) Location 1 and (E) location 2.

3.4 Micro-hardness

The micro-hardness distribution of the composite coating is shown in Figure 5. As can be seen, the micro-hardness of such coating is in a range of 1700–1830 HV_0.2, which is approximately five times higher than that of the substrate (360 HV_0.2). Combined with the XRD analysis, it is known that the phase constituent of such coating is beneficial in increasing micro-hardness; moreover, the production of amorphous alloys and their fine microstructure are also beneficial in increasing micro-hardness. Owing to the rapid cooling rate of the molten pool, a small amount of elements, such as Al and Si, have had no time to be precipitated from the liquid and solution in γ-Co to form a super-solution. This strengthened the solution and improved micro-hardness.

Figure 5:

Micro-hardness distribution of the TiB₂/α-W₂C-reinforced composite coating.

3.5 Wear resistance

The wear test result indicates that the wear volume loss of the TiB₂/α-W₂C reinforced composite coating is about 1/4 of Ti-6Al-4V when the load is at 49 N (Figure 6). The high wear resistance of such coating is primarily attributed to the characteristics of coating with a phase constituent showing excellent wear resistance. It should be mentioned that the production of the amorphous alloy and t-ZrO₂/m-ZrO₂ play dominant roles in resisting wear attacks during the dry sliding wear process, preventing the alloy from serious deformation, such as micro-cutting or micro-plowing. Thus, the coating exhibits good wear resistance compared with Ti-6Al-4V.

Figure 6:

Wear volume loss of the TiB₂/α-W₂C-reinforced composite coating and Ti-6Al-4V.