Qualitative analysis on a large number of metal particles in the virgin and re-used state was accomplished by means of SEM and EDS analysis. Representative surface morphologies of the virgin powder are presented in Figures 2(a) through (c). Generally, the virgin powder exhibits a surface that is visually free from secondary phases and contamination species. As already described in Reference 3, nonmetallic inclusions with a size of a few µm were occasionally observed on the powder surface and inside the powder. In most cases, their blocky shape and yellow tinge in the optical microscope identified them as Ti-rich nitride particles. This was confirmed from EDS analysis which gave rise to significantly intense peaks from Ti and N, corresponding to a concentration of close to 50 at. pct each for both elements. Similarly, others were identified as Al-rich oxide inclusions. Figures 2(d) through (l) show typical surface morphologies of powder after varying number of re-use cycles, as observed at different magnifications in the SEM. In Figures 2(d) through (f), a sample collected after the first 70 hours of processing shows that already after the first build cycle, a significant amount of nanosized particulates has formed on the powder surface. Powder from the following build cycles [up to cycle number 14, see Figures 2(g) to (i)] indicates that the average number and size of these features tend to increase with increasing number of re-use cycles. Furthermore, on occasion, the recycled powder surface also exhibits areas covered with thick and porous products, as shown in Figures 2(j) through (l). In reference to the matrix, EDS point analysis spectra obtained from a large number of these features exhibit intense peaks of Al and O, which indicates that it consists of Al-rich oxide. Compositional information on the observed features obtained from EDS point analysis are listed in Table II. Regarding the data in Table II, however, it should be noted that the interaction volume from where the EDS signal is generated is generally much larger than the size of the particulates features, which means that a significant contribution of the signal originates from the surrounding matrix material. It should also be pointed out that the accuracy of EDS is generally limited when it comes to detection of light elements. Therefore, the figures in Table II should mainly be considered as a preliminary qualitative assessment of the powder surface chemistry, preceding the much more accurate analysis by XPS and AES, presented in Sections III–B and III–C below.
On the cross section of the powder after 14 re-use cycles, see Figure 3, it can be seen that the Al-rich oxide particulates are present only within a region very close to the original surface. Usually, their size is in the range of 50 to 200 nm.
The oxygen levels in the virgin and the re-used powders as measured by inert gas fusion (combustion analysis) are presented in Table III. In addition to the build cycle number, the total (accumulated) process time of all preceding cycles has been included as additional information. However, it should be noted that both build cycle number and process time serve only as rough indicators of the powder condition.
Nevertheless, as seen in Table III, the increasing amount of oxide during powder re-use is confirmed by a continuously increasing oxygen level in the powder samples collected throughout the powder recycling study. The nitrogen level on the other hand is relatively stable at around 150 ppm in all powder samples irrespective to the condition of the powder.
Figure 4 shows the XPS survey spectra measured on powder in virgin state and after 14 re-use cycles. The spectra shown are recorded after slight ion etching, corresponding to around 1 nm from the as-received surface, to remove surface absorbed species. As indicated in the figure, photoelectron peaks corresponding to nickel and the major alloying elements (Ni2p, Cr2p, Fe3p, Nb3d, Ti2p, Al2p, Mo3d) together with carbon (C1s), nitrogen (N1s), and a strong oxygen (O1s) signal were registered for both powder conditions. The nitrogen peak corresponds to the presence of TiN, as also seen in SEM. As previously reported,[20,21] segregation of sulfur to the metal–gas interface during high-temperature exposure explains the sulfur peak (S2p) in the spectrum obtained from the as-received surface of the re-used powder.
Figure 5 illustrates the elemental concentration depth profiles for virgin and recycled powder, respectively, as measured by XPS analysis. For improved visualization, data recorded in the region closest to the as-received surface are presented separately in Figures 5(b) and (d).
As shown in Figure 4, it is evident that the surface of both kinds of powder have similar chemistry. However, a significant difference in element distribution through the etch depth is observed. Both powder conditions have a high oxygen content at the as-received surface, decreasing directly after initial etching, which implies that they are both covered by a thin oxide layer. The major differences between the two powder conditions arise as a consequence of the much thicker oxide particulates on the re-used powder, in agreement with the observations by SEM. This is shown by the oxygen level in the recycled powder which is considerably higher down to larger etch depths compared to that of the virgin powder. Also, enrichment of Al on the surface of the recycled powder is more pronounced as compared to the virgin powder, which further indicates the selective oxidation of Al on the powder surface. Mainly, the strong carbon signal on the original surface corresponds to adsorbed hydrocarbons and is removed already after slight ion etching. The remaining carbon signal after ion etching most likely originates from presence of MC carbides, which is known to form directly from the melt during solidification,[4,22] but is not considered further in this study.
Curve fitting of the XPS peaks from the high-resolution narrow scans for oxygen and most of the oxide-forming elements was performed to determine the chemical state evolution along the depth from the as-received surface. As seen in Figures 6(a) and (b), there is a clear difference between the peaks for Al in the virgin and re-used powder. For the virgin powder, the Al peak is positioned at binding energy levels which could be an indication of Al in a spinel type oxide such as NiAl2O4, as reported in Reference 23. In the case of the recycled powder, the Al peak in Figure 6(b) is closer to values for Al2O3 positions reported in many references. Also, due to a stronger oxidation of Al to the surface of the recycled powder, the Al oxide peak is more pronounced compared to the virgin powder, both at the as-received surface and after ion etching. A clear difference is also seen in the oxygen peaks after initial ion etching, Figures 6(c) and (d). A considerable portion of the oxygen signal of the virgin powder originates from the transition metal oxides at around 530 eV (Cr2O3, NiO, Fe2O3, TiO2, Nb2O5), while the recycled powder shows a strong shift towards 532 eV binding energy, corresponding to Al2O3. As noted above, this signal is present to much larger etch depths in the recycled powder compared to the virgin powder, indicating that the thickness of the oxidized region/features is much greater in the recycled powder, and that it is not connected to any transition metal hydroxide, which would have been removed after initial ion etching. Hence, the detection of Al oxide to greater etch depth in combination with the O2− peak indicative for Al2O3 for the O1s-region suggest that the oxide appearing in particulate form on the surface of re-used powder is Al-rich oxide, possibly Al2O3. As indicated in Figures 6(e) and (f), Ni hydroxide is possibly present on the outermost surface of both virgin and re-used powder. The presence of a weak Ni metal peak in the signal from the as-received surface and its instant rise in intensity after initial ion etching indicate that Ni mainly exists in hydroxide/oxide state in a region close to the as-received surface. Furthermore, this thin layer may also be related to presence of spinel oxide NiAl2O4 whose peaks for both Ni2p and O1s cannot be easily distinguished from those of Ni hydroxide; both the spinel and Ni(OH)2 have similar characteristic peak positions at around 856.2 eV. For both kinds of powder, basically all other metallic elements are mainly present in oxide state in a region close to the as-received surface. Analysis of the oxygen peak indicates decreasing contributions from both transition metal oxide (Ni, Fe, Cr, Ti, Nb-base) at around 530 eV and Al2O3 oxide at approximately 532 eV with increasing etch depths. The changes in relative amounts of oxide and metal states for other oxide-forming elements as well as the shift from oxide (or hydroxide) reflect the similar distribution in depth for their surface oxide products (Figures 6(a) to (j)). In the case of Ti as shown in Figures 6(i) and (j), the position of the peaks obtained close to the original surface indicates the presence of a mixed type of oxide and/or the presence of several different Ti-oxides. In accordance with the above SEM observations, besides Ti-oxide, peaks in Figures 6(i) and (j) correspond also to TiN and possibly some Ti-metal after extended ion etching. Since the Ti-oxide is mainly removed already after 3 to 5 nm, it can be assumed the TiN is the main Ti-compound present with greater thickness on the surface.
Curve fitting of the presented peaks above provides further indications on the difference in oxidation between the two powder conditions. Figures 7(a) and (b) show the concentrations of elements in oxide or hydroxide state (cation concentration) along the first 100 nm from the original surface of the virgin and recycled powder, respectively. The insets show the elements constituting the oxide/hydroxide closest to the as-received surface. The steep decrease in cation concentration of all elements except that of Al shows that they are present as oxide/hydroxide state in a very thin region close to the surface of both kinds of powder, rather than in thicker reaction products as in the case of Al, for which the decay in cation content is much lower with increasing etch depth. As expected, the largest difference between the two powder conditions lies in the Al cation concentration due to the large amount of Al-rich oxide particulates observed on the surface of the re-used powder. Consequently, the amount of Al in oxide state is considerably larger in the re-used powder down to an etch depth of around 100 nm, where it stabilizes at a level which is similar for both powder conditions.
Areas of variations in the surface oxide layer morphology of the virgin powder surface, as shown in Figures 8(a) and (b), were analyzed by AES depth profiling, see Figure 9. In agreement with the overall chemical analyses by XPS (Figure 7), the high O-level reflects the oxide character of the surface and hence, alloying elements on the as-received surface can be depicted as being present in their oxide state. Furthermore, as seen in the XPS results, the high surface content of Al is shown in both spectra. Hence, Figure 9(a) shows that the outermost region of the visually homogeneous areas in Figure 8(a) (point A) consist of a Ni-Al-rich oxide, together with Cr, Ti, and Fe oxide mainly present a small depth below the original surface. The relatively high Ni/Al atomic ratio in the outermost surface suggests that this part of the surface mainly consist of Ni-based hydroxide/oxide, but still with potential presence of Al in oxidized state. As shown in Figure 9(b), the flake-like areas in Figure 8(b) (point B) has a higher aluminum content and a Ni/Al atomic ratio that is closer to that of NiAl2O4. Together these two compounds coincide with the overall XPS composition profile shown in Figure 7(a), for which Al in oxide state is present as a major element across the whole surface layer, together with Ni in oxide state closest to the as-received surface. Furthermore, it shows that the difference in surface oxide visible in SEM relates to the extent of aluminum oxide on the powder surface. Based on the oxygen depth profiles in Figures 9(a) and (b), the Ni-rich oxide layer is estimated to have a thickness of around 2 nm in Ta2O5 units.
The AES analysis of the powder after 14 re-use cycles is shown in Figure 10. Significant peaks from Al and O in the spectra obtained from the bright particulate features (point 1, Figure 10(a)) confirms high concentrations of Al-rich oxide phase. Compared to the AES analysis of the virgin powder in Figure 9, there is a low Al concentration in the spectrum obtained from the as-received surface in between the oxide particulates of the recycled powder (point 6, Figure 10(a)). This suggests that growth of the oxide particulates has resulted in a depletion of Al in adjacent areas.
Instead, a transition metal oxide/hydroxide layer is found in these regions, as compared to the previously observed Ni and Al-rich oxide/hydroxide layer as seen on the virgin powder. The fact that the recycled powder is partly covered by a thin transition oxide layer is also expected from Figure 6(d) since the oxygen signal at 530 eV is reduced after initial ion sputtering.