An investigation of the effect of processing conditions on the microstructure of vacuum plasma-sprayed Ti-Zr-Ni quasicrystal coatings.
Keywords Plasma spraying, Quasicrystals, Image analysis, Scanning electron microscopy (SEM), Transmission electron microscopy (TEM)
The discovery of quasicrystal (1) phases has been reported in 1984. Such phases offer outstanding material properties, e.g., high hardness, (2), (3) low friction coefficient, (4) low surface energy, (5) high thermo-electric power, (6) etc. Thus quasicrystals are potential candidates for many industrial applications. (7-9)
To date, quite a few alloys containing a quasicrystal phase have been identified. (10) Some of them are stable and worthy of investigations concerning their application potential in the engineering domain. The i-phase of T[i.sub.41.5]Z[r.sub.41.5]N[i.sub.17] constitutes one such system. (11) This phase reportedly is capable of storing hydrogen in large quantities (12) and hence is a highly potential candidate for fuel cell applications. The i-phase develops in the prior Laves and alpha phase (both HCP) eutectic matrix upon annealing at 570[degrees]C for seven days. (13) Further annealing at a lower temperature does not cause any transformation of the i-phase. It indicates that the QC phase in question is very stable. (11)
Quasicrystals, in general, are not produced by conventional fabrication techniques. They cannot be formed or cast readily. As stated, the growth of the i-phases in the T[i.sub.41.5]Z[r.sub.41.5]N[i.sub.17] alloy also involves prolonged annealing of the cast ingots. However, alloys having the composition of a QC can be reduced to powder and subsequently can be thermally sprayed to form a thick coating having the QC phase. (14-18) This phenomenon can be attributed to the rapid quenching associated with the thermal spraying processes.
This article deals with the vacuum plasma spraying of T[i.sub.41.5]Z[r.sub.41.5]N[i.sub.17] alloy. The coatings thus obtained have been tested for their integrity, microstructure, phases, and hardness. In a previous paper it has been shown that the polycrystalline phases of such alloys undergo a transformation i-phase during thermal spraying. (18) Such transformation has been attributed to the rapid quenching action associated with the thermal spraying process. This investigation has been undertaken to find out whether thermal spray process parameters, e.g., powder size, arc current affects a transformation from polycrystalline to i-phase.
T[i.sub.41.5]Z[r.sub.41.5]N[i.sub.17] powder has been produced by gas atomization of the alloy in argon environment. The powder has been received in two batches, having size fractions -116 + 37 [micro]m and -70 + 10[micro]m, respectively; referred to as batch 1 and batch 2 powder or coating. These powders have been vacuum plasma sprayed using a Medicoat 50 kW plasma spraying facility equipped with a 6-axes robot. Two different sets of parameters have been used for spraying the two batches of powder. Those parameters are listed in Table 1. The structural analysis of the powders and coatings has been carried out using a Siemens D 5000 X-ray diffractometer with MoK[alpha] radiation. The XRD data thus obtained has been represented as its CuK[alpha] equivalent in this article for comparison purposes. For metallographic investigation, the polished cross sections of the coatings and powders have been etched for 1-3 min using an etchant constituted by 100 mL distilled water, 2 mL concentrated hydrofluoric acid, and 5 mL concentrated nitric acid. The polished and etched cross sections have been subsequently examined under both optical microscopes and a Zeiss DSM 962 Scanning Electron Microscope (SEM) equipped with EDS. Samples for Transmission Electron Microscopy (TEM) have been prepared by pulverizing the coating. A small part of the coating has been ground in a mortar and pestle in alcohol suspension. After grinding, a drop of the suspension has been transferred to a copper grid. The TEM examination has been done using a Phillips CM 30 TEM. The porosity of the coatings has been estimated by image analysis using the Zeiss KS 400 software. The Vickers hardness of the coatings has been measured using a Leitz Wetzler hardness tester under a load of 100 g. An average of 10 readings has been reported.
Table 1: Parameters for vacuum plasma spraying of batch 1 and batch 2 powders S. No Parameter Unit Batch 1 Batch 2 1 Chamber pressure mbar 60 120 2 Standoff distance mm 275 400 3 Primary gas (Ar) flow rate sl/min 50 50 4 Secondary gas (He) flow rate sl/min - 10 5 Arc current ampere 800 700 6 Nozzle diameter mm 6 6
Results and discussion
Figures 1a and 1b show the secondary electron images of the polished and etched cross sections of powders from batch 1 and batch 2, respectively. The batch 1 powder shows a eutectic morphology similar to the one reported by Davis et al. (11) EDS study of the batch 1 powders indicates that the unetched phase has an Ni content of approximately 25 at.%. The composition corresponds to a Laves phase. (19) On the other hand, the etched part of the cross section is a lean Nickel phase. Possibly this is constituted by a low-temperature solid solution of Ti and Zr, which has only a small solubility for Ni. This phase is known as the x-phase. (19) However, the Ni content of the unetched phase in batch 2 powder is about 15 at.%. The batch 2 powder also contains these two phases as indicated from the results of the EDS examination of the etched and unetched part of the microstructure shown in Fig. 1b.
[FIGURE 1 OMITTED]
Figures 2a and 2b show the diffractograms of the batch 1 and batch 2 powders, respectively. It is not possible to identify the phases corresponding to most of the peaks in these figures, since a standard for comparison for phases other than i-phase is not available at this moment. However, it can be observed for Fig. 2a that the batch 1 powder does not contain any i-phase in it. (19) Figures 3a and 3b are the diffractograms of the as-sprayed coatings from the batch 1 and batch 2 powders, respectively. The figures show that the coatings obtained from both powders contain a significant amount of the quasicrystal (QC) i-phase. In addition, a comparison between Figs. 2b, 3a, and 3b indicates the presence of i-phase in the as-received gas atomized batch 2 powder. Hence it appears that the unetched part of the batch 2 powder is a mixture of Laves and i-phase.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Figures 4a and 4b show the optical micrographs of the as-polished cross sections of the coatings produced from the batch 1 and batch 2 powders, respectively. The coatings have a splat morphology with scattered pores, characteristic of thermally sprayed coatings. The individual particles are well molten and the coatings are well adherent to the substrate. The porosity of batch 1 coating is 4.9% and that of the batch 2 coating is 6.7%. The hardness values of the coatings are 631.5 HV and 559 HV, respectively. Figures 5a and 5b show the secondary electron images of the polished and etched cross sections of the coatings produced from powders of batch 1 and batch 2. respectively. In Fig. 5a, the unetched region has a composition approximately corresponding to that of the starting material. This possibly is the i-phase. A small fraction of the Laves phase is also likely to be present. EDS study of the microstructure reveals that the etched region is lean in Ni (3 at. %) and very rich in Zr (80 at.%). The composition is widely different from the [alpha]-phase present in the powder. In Fig. 5b the unetched region is restricted to smaller areas. Both the etched and unetched region have the same composition and correspond to the i-phase. It appears that a longer etching time in this case has resulted in the partial etching of the QC phase. Some scattered patches of the lean Ni (i.e., rich in Zr) phase are found in the microstructure. All of those patches have the morphology of small, round, elevated (i.e., unetched) islands surrounded by an annular etched area. A typical example of such a formation can be seen in the center of Fig. 5b wherein the aforementioned feature is shown by an arrow. Figure 6 shows a typical example of the bright field TEM images obtained from the coatings. It shows that the microstructure is constituted by extremely fine grains. Such morphology of the microstructure of this QC alloy has been reported by Majzoub.(19)
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
A closure understanding of the process is possible from the examination of the values of the spray parameters listed in Table 1 for both the powders. The batch 2 powders have been sprayed using a higher chamber pressure, higher stand off distance, and lower current. This indicates that spraying has been done using a shorter are (higher chamber pressure) and lower plume temperature (lower are current). A larger stand off also indicates a lower particle temperature during splat formation. Hence, the batch 2 particles have undergone quenching from a lower temperature as compared to batch 1 particles. This in turn speaks of a lower quenching rate for batch 2 powders. However, a smaller average particle diameter of the batch 2 powder is likely to offset the temperature effect to an extent. Overall, it appears that a change in quenching rate to an extent during splat formation in VPS does not qualitatively affect a phase transformation from polycrystalline to icosahedral phase.
T[i.sub.41.5]Z[r.sub.41.5]N[i.sub.17] powders having two different average sizes have been successfully vacuum plasma sprayed to yield coatings with good integrity. The starting powders consist mainly of [alpha] and Laves phase whereas the coating microstructure is constituted mainly by the QC i-phase along with some Laves and a high Zr-containing phase. TEM examination shows that the microstructure is constituted by very fine grains. A change in parameters effecting a change in quenching rate does not prevent a phase transformation from polycrystalline to quasicrystal phase.
Acknowledgments The authors would like to thank Mr C Schwendimann for his support in the preparation of metallographic samples and Mr M Aeberhard for the X-ray diffractometry work. The metallography team of EMPA Thun is acknowledged for the stimulating discussions and support for SEM work.
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P. P. Bandyopadhyay ([??])
Department of Mechanical Engineering, IIT Kharagpur.
Kharagpur 721302, India
EMPA, FeuerwektTstrassc 39. 3602 Thun. Switzerland
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|Title Annotation:||BRIEF COMMUNICATION|
|Author:||Bandyopadhyay, P.P.; Siegmann, St.|
|Date:||Sep 1, 2008|
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