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Article

Trade-Off Between Wear/Corrosion Performance and Mechanical Properties in D-AlNiCo Poly-Quasicrystals Through CNT Addition to the Microstructure

by
Seyedmehdi Hosseini
1,2,3,*,
Pavel Novák
2,
Mostafa Alishahi
4,
Zdeněk Kačenka
2 and
Petr Šittner
3
1
Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University London, Uxbridge UB8 3PH, UK
2
Department of Metals and Corrosion Engineering, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
3
Institute of Physics of the CAS, Na Slovance 1992/2, 18221 Prague, Czech Republic
4
Department of Materials and Polymers Engineering, Hakim Sabzevari University, Sabzevar 9617976487, Iran
*
Author to whom correspondence should be addressed.
Metals 2024, 14(11), 1269; https://doi.org/10.3390/met14111269
Submission received: 30 September 2024 / Revised: 1 November 2024 / Accepted: 5 November 2024 / Published: 8 November 2024
(This article belongs to the Special Issue Alloy Design and Its Performance Trade-Offs)
Browse Figures
Versions Notes

Abstract

:
An ultrafine-grained Al71Ni14.5Co14.5/CNT poly-quasicrystal (QC/CNT) composite was synthesized using spark plasma sintering of powder components developed through electroless Ni-P/CNT plating of Co particles and mechanical alloying. The performance of the synthesized samples was studied using various testing methods, such as room temperature/hot compression, wear, and corrosion tests. The results were compared to the properties of alloy samples fabricated from raw and coated powders (without CNTs). The wear rate and friction coefficient of the quasicrystalline samples improved significantly due to the contribution of the CNTs. The wear rate of the CNT-containing specimens was 0.992 × 10−4 mm3/N/m, which is 47.1% lower than that of the QC sample. The positive impact of the CNTs on the corrosion potential and current density was further validated by the potentiodynamic polarization tests in a saline solution. However, these improvements in surface properties came at the cost of a 21.5% reduction in compressive strength, although the compressive strength still remained above 1.1 GPa at 600 °C. The results highlight an interesting trade-off between surface properties and mechanical strength, pointing toward the development of materials suitable for extreme conditions.

1. Introduction

A high degree of long-range quasiperiodic and orientational order is the main feature of a quasicrystal (QC). Metastable quasicrystalline phases were first identified in rapidly solidified Al-Mn alloys by Shechtman et al. [1] in 1984. Since the discovery of QCs revolutionized crystallography, Shechtman was awarded the 2011 Nobel Prize in chemistry [2]. Over the past few decades, there has been significant motivation to develop new stable and metastable QCs [3]. As a result of their unique atomic arrangement and crystallographically disallowed rotational symmetry, QCs possess unique physical, chemical, and mechanical properties, including high hardness, low thermal/electrical conductivity, excellent oxidation resistance, good performance in corrosive environments, brittle-to-ductile transition at a high temperature, and temperature dependence of Young’s modulus [4,5,6]. However, a well-known drawback that limits the application of QCs is their intrinsic brittleness, which leads to plastic deformation only at high temperatures or under hydrostatic pressures [5]. Consequently, they are considered good candidates for coatings on metals, reinforcements in composites, and other applications at intermediate temperatures [4,7,8].
According to their rotational symmetries, QCs are mainly classified as icosahedral (i-), octagonal (o-), decagonal (d-), or dodecagonal (dd-) structures—fivefold in i-QCs, eightfold in o-QCs, tenfold in d-QCs, and twelvefold in dd-QCs—which are not found in periodic crystals, as already mentioned [9]. The i-QCs are the only class of these materials that are nonperiodic in all three spatial directions, while the d-QCs exhibit periodicity in one direction and quasiperiodicity in the other two directions [10]. Anisotropy in plastic deformation of d-QCs is the direct consequence of this structural feature. As a result, the research papers focusing on physical/mechanical properties and plasticity mechanisms of d-QCs have been mostly restricted to studies on decagonal single-quasicrystals [11,12,13,14]. Building on the authors’ previous research [10], this study focuses on the synthesis and characterization of a decagonal AlNiCo poly-quasicrystal containing a certain amount of carbon nanotubes (CNTs) to further improve the surface properties of the material.
CNTs have attracted significant scientific attention over the past few decades due to their unique mechanical, chemical, and electrical properties [15,16]. These cylindrical materials represent a high length-to-diameter ratio, exceptional strength, elastic modulus, flexibility, and stiffness, a large energy-absorbing capacity, and chemical stability, making them a valuable fiber for various composite materials. The uniform distribution of CNTs in the structure or on the surface of the materials can significantly enhance hardness, corrosion resistance, mechanical strength, and wear properties [16]. However, the application of CNTs in quasicrystals has been mostly limited to modifying the electrochemical hydrogen storage properties of Ti-based QC powders prepared by ball-milling and subsequent annealing processes [17,18].
To date, investigators have applied various techniques such as casting [19,20,21], mechanical alloying (MA) [22], sputtering [23], additive manufacturing [24,25], and melt spinning [26,27] to prepare QC samples in the forms of bulk, powder, coating, and ribbon. Sintering of the raw or mechanically alloyed powders is another effective approach to preparing a relatively dense bulk sample. Spark plasma sintering (SPS) is an advanced technique used to synthesize bulk materials from metallic powders in a short time due to its unique sintering mechanism [2]. This promising consolidation method has been widely used to produce QC-strengthened metal matrix composites [28,29,30]. However, only a few studies have been conducted to form bulk QCs by consolidating the mechanically alloyed powders using the SPS process. These studies have primarily focused on the powder metallurgy preparation of the bulk AlCuFe quasicrystals [2,31].
This research has two main aims. The first is to investigate the trade-off between the surface properties and mechanical performance of decagonal AlNiCo poly-QC bulk samples due to the addition of CNTs to the microstructure. The second is to explore the strengthening mechanism and positive temperature dependence of the mechanical properties of the Al71Ni14.5Co14.5/CNT poly-QC, which exhibits an abnormal behavior for a quasicrystalline phase under compression testing (up to 600 °C).

2. Experimental Procedure

2.1. Materials

The raw metallic powders used for MA were as follows: pure Al (99.9% purity, particle size < 20 µm; Strem Chemicals (Newburyport, MA, USA)), Co (99.9% purity, particle size < 5 µm; Sigma-Aldrich (St. Louis, MO, USA)), and Ni (99.9% purity, particle size < 5 µm; Sigma-Aldrich). The MWCNTs (96% < purity, outer diameter ≈ 18–28 nm, and length ≈ 2 µm) used for the electroless nickel–phosphorus (ENP) process were purchased from Nanografi (Ankara, Turkey).

2.2. ENP Process

The ENP process of Al and Co powders was carried out according to the procedure described for Bath No. 4 in Table 1 of reference [32]. The ENP bath components included nickel sulfate, sodium hypophosphite, lactic acid, and propionic acid. The temperature and pH were adjusted to 85 °C and 4.5, respectively.
Two pretreatment steps were applied to the metallic powders and CNTs prior to starting the ENP process. The surfaces of the Al and Co powders were cleaned with an acetone solution and then activated using a diluted mixture of nitric acid, hydrofluoric acid, and ammonium fluoride for 15 min under high-power sonication. The activated powders were rinsed several times with distilled water before being added to the ENP bath. The amounts of Al and Co powders used were 10 g and 7 g per 500 mL of bath solution, respectively.
A critical issue in the application of CNTs is how to homogeneously disperse them throughout the alloys. The ENP process has already been shown to effectively address this problem [15,16]. In this study, it was used not only to provide the required Ni (14.5 atomic percent) but also to uniformly disperse the CNTs over the surface of the Al/Co and Co powders, respectively (Figure 1). Based on the procedure described in [16], the as-received CNTs were subjected to an acid treatment using a mixture of HNO3 and H2SO4 (1:3 volumetric ratio) under high-frequency sonication and functionalized by the addition of a few drops of HCl for better dispersion in the electroless bath. After multiple washes, a neutralized water-based CNT solution containing 200 mg/L CTAB was prepared. For each ENP/CNT process, 50 mL of the water-based CNT solution (0.5 g/L) was added to the bath solution, corresponding to approximately 0.1 wt.% CNTs in the final QC.

2.3. MA and SPS Processes

According to Figure 1, three powder systems were defined for the MA step. The first system, hereafter referred to as QC, consists of a mixture of raw metallic powders with a nominal composition of Al71Ni14.5Co14.5. The second system, named QC-P, is a combination of Al and Co powders coated by the ENP process. The total amount of Ni required for the formation of the QC is supplied through the coating. The third system, designated as QC-P/CNT, involves mixing the ENP-coated Al powders and ENP/CNT-coated Co powders, so that if the P and CNT are excluded, the chemical composition remains identical to the first system.
Ball milling of all powder systems was carried out in a stainless-steel vial under an argon atmosphere. The rotational speed and milling time were set to 400 rpm and 60 min, respectively. The ball-to-powder ratio was 40:1. The resulting powders were consolidated in a cylindrical graphite die (inner diameter of 20 mm) using the SPS method under a pressure of 80 MPa and a temperature of 800–950 °C for 10 min, with a heating rate of 100 °C/min and a cooling rate of 50 °C/min.

2.4. Characterization and Testing

The compression test was performed on cubic samples, each with dimensions of 3 × 3 × 3 mm3, at room temperature and at 600 °C, with a strain rate of 0.001 s−1. The compression load was applied on the surface perpendicular to the thickness of the SPSed samples. The wear test was conducted using a pin-on-disc linear oscillating tribometer in accordance with ASTM G99-95a and G 133-95 (Tribotechnic, Paris, France). A reciprocating motion was applied to the fixed samples using an Al2O3 ball under dry conditions. A load of 5 N was selected, while the sliding distance, speed, and time were fixed at 5 mm, 10 mm/s, and 30 min, respectively. The electrochemical analysis was performed using potentiodynamic polarization (PDP) scanning with a Gamry Reference 600 potentiostat (Gamry Instruments, Inc., Warminster, PA, USA). A three-electrode system was employed, with either pure Al or QC samples as the working electrode (for comparison), Pt as the counter electrode, and a silver/silver chloride electrode (i.e., Ag/AgCl, 0.210 V vs. standard hydrogen electrode) as the reference electrode. Potentiodynamic polarization tests were carried out at a scanning rate of 1 mV/s over a potential range from −50 to 500 mV relative to the measured open circuit potential in a 3.5% NaCl solution. Phase formation was studied using X-ray diffraction (XRD) with a PANalytical X’Pert Pro device (PANalytical, Almelo, The Netherlands) using Cu–K radiation.
Microstructural analysis was carried out by SEM (Tescan VEGA 3-LMU, 20 kV, Brno, Czech Republic) and TEM (JEOL JEM 2200 FS, 200 kV, Tokyo, Japan).

3. Results and Discussion

3.1. Microstructural Analysis and XRD Data

Figure 2 shows the surface morphology and cross-section of the ENP- and ENP/CNT-coated Co and Al powders. The surface of the Co powders (Figure 2a) is uniformly coated with the Ni-P fine nodules, forming the characteristic “cauliflower-like” structure. The cross-section view and the EDS inset images of the Co powders in Figure 2b confirm that the ENP coating completely covers the surface, with a thickness ranging between 1 and 1.5 µm. EDS analysis across five different areas indicated that the weight percentages of Co, Ni, and P are 67%, 29%, and 4%, respectively.
To achieve the required amount of Ni for producing the Al71Co14.5Ni14.5 QC, the deposition parameters of the electroless bath were adjusted to ensure a minimal coating thickness on the Al powders (Figure 2c). As a result, the Ni-P deposition on the Al powders exhibits a “pea-like” structure, primarily forming a thin coating layer.
Figure 2d illustrates the Co powders coated using the ENP/CNT process. A network of numerous individual CNTs is visibly covering the surface of the Co powders. The uniform distribution of CNTs can be attributed to the role of the ENP process. Initially, the electroless reaction is triggered by the deposition of Ni nodules on the activated Co surface through an autocatalytic reaction. Once the water-based CNT solution is added to the ENP bath, the CNTs co-deposit on the powder surfaces, forming an adherent and uniform Ni–P/CNT coating [15,16].
Figure 3a illustrates the microstructural evolution of Al71Ni14.5Co14.5 QC (first alloying system) in terms of the SPS temperature. It is evident that by increasing the temperature from 800 °C to 950 °C, the size and density of pores reduce significantly. Although the presence of inter-particle space is one of the main features of SPSed products, no evidence of porosity is observed in the final image, revealing that a fully dense Al71Ni14.5Co14.5 QC sample can be achieved by SPS processing at 950 °C if an appropriate amount of pressure and holding time are employed. The mechanism of QC formation has been explained in reference [10].
Figure 3b shows the SEM image of the QC-P/CNT sample etched with Kroll’s reagent. This figure shows how CNTs are aligned in QC-P/CNT alloy and uniformly dispersed at grain boundaries. The TEM image from the sample in Figure 3c clearly confirms this statement. The bright region depicts a CNT lying along the boundary between two adjacent grains. Although no visible physical gap exists at the interface, the boundaries can be clearly distinguished in the inset image. This observation suggests that the bonding between the CNT and QC is relatively strong.
There are several reports on the fabrication of Al/CNT and Ni/CNT composites, which describe CNTs residing along the grain boundaries [33,34,35]. The formation of the Al4C3 compound is usually unavoidable in an Al/CNT composite due to the reaction between the CNT and Al at higher temperatures [33]. However, no Al4C3 phase was found at the interfacial areas in the QC-P/CNT sample. This absence can be attributed to the higher cooling rate and shorter holding time of the sintering process (SPS) compared to the conventional compaction methods, which prevent the formation of the Al4C3 phases at the interfaces [36].
The phases present in the QC systems are evaluated by XRD (Figure 4). The XRD patterns illustrate that the decagonal phase is present in all three QC systems. It has been previously shown that after 60 min of ball milling, all the diffraction peaks obtained from the milled raw powders correspond to those of the starting elemental powders (Al, Ni, and Co), indicating there is no deformation-induced transformation during the MA process [10]. In fact, through a sequence of solid-state transformations by heating and compacting the as-milled mixture during SPS, all the peaks related to the decagonal Al71Co14.5Ni14.5 phase can be recognized. Although the initial powder mixtures of the QC-P and QC-P/CNT systems contain P and semi-amorphous Ni [22], similar decagonal phase reflections are observed in the corresponding SPSed samples. Additional peaks are ascribed to the formation of aluminum phosphide (AlP) and aluminum phosphate (AlPO4) phases in the QC-P and QC-P/CNT specimens.
Before evaluating the further details of the QC-P/CNT microstructure, it is of utmost importance to discuss the observed features of the decagonal QC sample. The TEM image in Figure 5a exhibits the formation of ultrafine decagonal grains in the QC specimen with a mean grain size of around 1.5 μm. New insights into the orientation of the QC grains can be obtained from the selected area electron diffraction (SAED) patterns of two adjacent grains. The SAED pattern from grain (i) indicates that it contains basal planes with tenfold/fivefold symmetry. In the inverted image attached to the SAED pattern of grain (i), the fivefold symmetry is depicted. It is worth pointing out that the pattern spots reflect fivefold symmetrical positions in a nonperiodic mode with a deviation from τ, known as the golden ratio, which is calculated as (1 + 51/2)/2. In other words, the periodic arrangement of planes parallel to the tenfold axis leads to an equidistant arrangement of reflections in one direction. This pattern is reflected by the atomic planes that are perpendicular to the twofold axis. In poly-QCs, most of the grains or atomic planes are oriented at different angles relative to the twofold or tenfold zone axes. For instance, a tilt angle from the tenfold toward the twofold axis by 46° can be seen in the SAED pattern of grain (ii). This analysis is further supported using the calculated tilt angle provided in reference [37].
Figure 5b illustrates a higher-magnification TEM image of grains (i) and (ii), revealing the presence of many nanoparticles that are uniformly distributed inside the grains. The EDS point analyses of a nanoparticle (S4) and the d-QC matrix (S5) are shown in Figure 5c. Accordingly, Al, Ni, and Co are the main elements of the matrix (S5), which is close to the nominal composition of the QC. The light-grey particles are complex oxide nanoparticles with sizes ranging between 20 and 80 nm (S4). The oxygen source is likely due to the native oxide layer already formed on the micron-sized Al raw particles (the major element of the QC in this study), as both the ball-milling and SPS processes were performed under a protected atmosphere and vacuum, respectively.
Figure 6a shows a TEM image taken from the surface that is parallel to the thickness of the QC-P/CNT sample. The mean grain size is at least 50% less than that observed in the QC sample (Figure 5a). Two reasons may explain this deviation in mean grain sizes. It has been noted that after one hour of ball milling, the mean particle size of the Ni-P coated powders is remarkably lower than that of the milled raw powder mixture in the AlNiCo system, owing to the incorporation of the ENP coating as an anti-adhesive barrier compared to bare Al particles [22]. On the other hand, Chen et al. [33] stated that the CNTs at the grain boundaries could suppress the Al grain coarsening in Al/CNT composites at high temperatures. In the present study, a similar approach is expected to occur during the SPS at 950 °C.
Another important feature is the widespread existence of a new phase in the structure of the QC-P/CNT sample (bright areas in Figure 6a). To identify this phase, STEM analysis was performed, and EDS maps were measured (Figure 6b). The darker areas in Figure 6b, which range in size from fifty to five hundred nanometers, correspond to the bright regions in Figure 6a. According to the EDS maps, Co and Ni do not contribute to the formation of this phase. It is primarily composed of O, P, and Al. However, the amount of O and P varies in this phase, as evidenced by comparing the areas indicated by arrows in Figure 6b (O) and Figure 6b (P) with those in the upper right of the images (i.e., the shiny yellow parts). The XRD results (Figure 4) also demonstrated the formation of both aluminum phosphide and aluminum phosphate phases in the QC-P and QC-P/CNT specimens. The origin of the P and the increased amount of O are most likely ascribed to the water-based solution of the ENP bath. During SPS, and while heating/compacting the milled powders, the regions enriched with Al, Ni, and Co tend to expel impurities such as O, P, and CNT to achieve the nominal stoichiometry and form the QC grains via the diffusion mechanism. By comparing Figure 6b (O) and Figure 6b (P), it can be concluded that aluminum-based nanoparticles are formed inside the grains, while the aluminum phosphide and phosphate phases mainly appear along the grain boundaries. For instance, the bright spot inside the dark grain in Figure 6c is an oxide nanoparticle, while three aluminum phosphide/phosphate grains are located at grain boundaries. The SAED pattern showing tenfold symmetry confirms that the dark grain comprises the basal plane perpendicular to the incident electron beam.

3.2. Compressive Performance at Room Temperature and 600 °C

Figure 7 shows the compressive true stress–strain curves of the QC and QC-P/CNT samples at 25 °C and 600 °C. For the d-QC sample tested at 25 °C and 600 °C, yield strength (σY), rupture strain (εR), and ultimate compressive strength (σR) range between 240 and 372 MPa, 5.43 and 9.52%, and 860 and 1390 MPa, respectively. The results demonstrate that the σY, εR, and σR of the QC-P/CNT sample tested at 25 and 600 °C are within 205–225 MPa, 4.54–6.43%, and 835–1150 MPa, respectively. Several inferences can be drawn from the reported data. First of all, the strength and ductility values of both samples indicate a positive temperature dependence of the properties under compression testing. Secondly, the addition of the CNT to the structure appears to reduce the mechanical properties of the QC-P/CNT sample, probably due to the presence of the Al phosphide/phosphate phase. Thirdly, the negative effect of CNTs on the mechanical properties of the specimens varies between room temperature and 600 °C. Further discussions on the mentioned findings are presented below.
The increase in ductility with the rise in test temperature is typical behavior for QC alloys. However, previous results have shown that during the plastic deformation of QC alloys at high temperatures, their strength tends to reduce, and work-softening is observed upon the yield point [5]. In fact, QCs are brittle solids at room temperature but become more deformable at higher temperatures, typically above half of their melting point (563 °C for Al71Ni14.5Co14.5) [38,39].
Investigations on single-quasicrystal Al70Ni15Co15 demonstrated that deformation in grains with A45° planes (inclined by 45° to the tenfold symmetry axis) occurs through dislocation glide, primarily involving edge segments in the periodic direction. However, those with A∥ planes (parallel to the tenfold symmetry axis) undergo the climb process of edge dislocations, absorbing vacancies via mixed dislocations and leading to the expansion of edge-loops (Frank loops). In A⊥ planes (perpendicular to the tenfold symmetry axis), both glide and climb of dislocations with mixed and quasiperiodic characters contribute to deformation, generating numerous stacking faults [12,14]. A poly d-QC involves all these three types of grains, as well as those not aligned parallel to the specific angles (e.g., grain (ii) in Figure 5a). It is suggested that the grains not aligned parallel to the specific angles deform through a combination of the aforementioned mechanisms. It has already been proven that the anomalous increase in the strength in the d-QC specimen at 600 °C is attributed to the densely dispersed oxide nanoparticles in the decagonal poly-QC grains (Figure 5) with a complicated crystallographic arrangement utilizing both glide and climb mechanisms in periodic and quasiperiodic directions. It is highly probable that at higher temperatures, the typical plastic deformation of oxide dispersion-strengthened (ODS) d-QC is controlled by dislocation climbing/sliding over the unshearable nanoparticles, resulting in a yield strength anomaly and subsequent work-hardening in corresponding stress–strain curves [10]. It was shown that in the QC-P/CNT specimens, the crystallography of the grains with the dispersed oxide nanoparticles inside the grains is analogous to the QC alloy, but they differ in mean grain size, the presence of CNTs, and aluminum phosphide/phosphate phases at grain boundaries. Several mechanisms have been suggested to clarify the strengthening impact of CNTs in metallic composites, including load transfer from the matrix to CNTs, grain refinement by the pinning effect of CNTs, solution strengthening of carbon atoms, and strengthening by in situ-formed or precipitate carbide [33]. No evidence for the solution or precipitation strengthening due to the presence of CNTs was observed in the QC-P/CNT system owing to the short-term processing of the SPS and low solubility of carbon atoms in Al and, most probably, in Al-based QCs [40]. Although grain refinement and load transfer are proposed as the predominant mechanisms for strengthening, it appears that the formation of Al phosphide and phosphate precipitates at grain boundaries negatively impacts the overall compressive properties of the QC-P/CNT specimens.
The slight plastic deformation of both specimens can be ascribed to the limited active slip planes and the brittle nature of the QC and QC-P/CNT samples at room temperature. This is why the compressive properties of both specimens show no significant deviation at room temperature. As the test temperature increases to 600 °C, thereby activating slip planes in each grain based on its orientation to the load direction, nano-oxide particles play a significant role in improving the mechanical properties. It has been well established that the yield stress anomaly is dependent on the nature of obstacles inhibiting dislocation movements (obstacle efficiency) and how these obstacles are crossed to propagate moving dislocations [10]. A comparison of Figure 5 and Figure 6 shows that the formation of aluminum phosphate particles at grain boundaries, with oxygen consumption leads to fewer oxide nanoparticles inside the QC-P/CNT grains, thereby resulting in less effectivity of obstacles.

3.3. Tribological Behavior

The friction coefficient and wear rate values of the different QC systems recorded during the wear test under the described conditions are shown in Figure 8. A steady state was observed in the friction coefficient curves after a short sliding distance. Within this steady-state regime, the friction coefficient values of the QC, QC-P, and QC-P/CNT alloying systems are estimated to be 0.594, 0.589, and 0.53, respectively. Corresponding wear rates are 1.876 × 10−4, 1.659 × 10−4, and 0.992 × 10−4 mm3/N/m, respectively. The results reveal an improvement in the friction coefficient (10.7%) due to the incorporation of CNTs in the structure of d-QC, leading to a remarkable decrease in the wear rate (47.1%). These data are comparable to the results reported by Zhang et al. [41], who produced an Al-Fe-Cr QC composite using additive manufacturing. It has been reported that lower mechanical properties (fracture toughness) of a material compared to those of another with a similar composition might explain poorer wear resistance [42]. However, according to Figure 7a, the mechanical properties of the QC are slightly better than those of QC-P/CNT at room temperature.
SEM micrographs of the wear tracks and worn surfaces (Figure 9) were employed to analyze wear mechanisms and determine the cause of the improvement in wear resistance of the QC-P/CNT sample. In Figure 9a–c, scratches along the sliding direction of wear tracks are detected, indicating that the main mechanism leading to wear is abrasive. However, deep grooves, known as the main feature of severe abrasive wear regimes, are not observed. It is believed that in brittle materials such as QCs, wear debris particles are generated in the wear track when the locally applied load causes intensive fragmentation and exfoliation at the sliding interface. These debris particles act as a third body and are responsible for the (severe) abrasive wear damage [43]. On the other hand, Figure 9a (inset image) shows a more severe wear regime than the corresponding image in Figure 9c, as evidenced by the high wear rate displayed in Figure 8. Producing a wear-resistant QC is thus not an easy target due to the inherent brittleness [42,43]. If loads imposed on the sliding surface are adequately high to propagate defects such as cracks, pits, and pores (Figure 9a), the wear resistance will be extremely compromised. There are, however, some prospects for modifying the wear resistance in QCs, such as the incorporation of uniformly dispersed CNTs in the structure (Figure 9c). Trench profiles of all three QC systems were compared in Figure 9 (lower inset images) to find out whether the improvement in the wear rate in the QC-P/CNT is due to the incorporation of CNTs. The surface areas of trench profiles in the QC, QC-P, and QC-P/CNT were measured as 3753, 3382, and 1984 µm2, respectively, showing a lower amount for the QC sample containing CNT.
Yet other issues have to be taken into account to understand the reasons for the improvement in tribological behavior in the QC-P/CNT sample. Due to the presence of the CNTs, which may act as separators, close contact between the mating QC and ceramic surfaces is prohibited, leading to a sluggish wear rate of the QC-P/CNT sample. Moreover, during the movement of the contacting surfaces, the removed individual self-lubricating cylinders of the CNTs from the QC-P/CNT surface roll easily between the surfaces (shown in Figure 9c using arrows), thereby decreasing the friction coefficient and wear rate [16].

3.4. Corrosion Test

The PDP tests were performed to determine the corrosion potential (Ecorr) and the current density (icorr) of each sample immersed in the 3.5% NaCl solution. The corrosion resistance of QC samples was compared with the pure Al resistance since the synthesized specimens are Al-based QCs with 71 at.% of Al. Figure 10 shows the PDP curves for all studied samples. For each curve, the icorr value was derived from the intersection point of extrapolated anodic Tafel line at Ecorr. Table 1 summarizes the obtained Ecorr and icorr values for all studied samples.
The pure Al displays a significantly more negative Ecorr than the QC compounds. This result points toward the preferential dissolution of Al atoms and subsequent formation of a Ni/Co-rich passive layer on the surface of QC samples, indicating a higher chemical inertness of QC samples [44]. Such a passive layer provides high corrosion resistance in the QC sample, as the icorr of the QC sample (0.12 μA.cm−2) was measured to be more than three times lower than that of the pure Al sample (0.42 μA.cm−2). However, both pure Al and QC samples are susceptible to pitting corrosion at high polarization potentials, in which their protective passive layer is locally disrupted by aggressive ions (Cl−), and, therefore, small anodic sites are formed. It is noteworthy that the OSD d-QC produced in this work exhibits superior corrosion resistance in the saline solution compared to other types of QCs [45,46].
The results also show that the phosphor has a detrimental effect on the corrosion resistance of the Al-based QC compounds since the QC-P sample exhibits a higher icorr (0.38 μA.cm−2) and a more negative Ecorr (−471 mVAg/AgCl) compared to the QC sample. This can be attributed to the existence of the aluminum phosphide phase in the QC-P sample structure, already confirmed by the XRD and TEM analyses. The AlP phase can quickly decompose upon exposure to the saline solution and provide some surface defects suitable for pit nucleation. The multi-phase nature of the QC-P sample significantly decreased its passive potential range and transpassive dissolution potential. It has been reported that when a microstructure comprises multi phases, the risk of corrosion may increase by breaking the passive layer and providing an electron path [46].
Nevertheless, the incorporation of CNTs in the structure of the QC-P/CNT sample can partially compensate for the detrimental effect of phosphor, where it decreases the icorr value to 0.20 μA.cm−2, and increases the Ecorr value to −324 mVAg/AgC. Such an improvement can be attributed to the inertness of CNTs, which decreases the fraction of corrosion active surface area in contact with the corrosive solution [16]. This result is consistent with other studies that reported the beneficial influence of CNTs on the corrosion resistance of composite coatings and bulk Al and Mg composites [15,16,47,48,49].
Table 1. Corrosion parameters obtained from anodic potentiodynamic polarization curves for pure Al and various QC alloying systems.
Table 1. Corrosion parameters obtained from anodic potentiodynamic polarization curves for pure Al and various QC alloying systems.
SampleEcorr (mV)Icorr (µA.cm−2)
Pure Al−8610.42
AlNiCo QC−2700.12
AlNiCo/P QC−4710.38
AlNiCo/CNT QC−3240.2

4. Conclusions

Electroless Ni-P/CNT plating, mechanical alloying, and spark plasma sintering were employed to synthesize a decagonal AlNiCo poly-quasicrystal containing uniformly dispersed carbon nanotubes (QC-P/CNT). Simultaneously, QC-P specimens were produced by the same procedure without CNTs, and QC specimens were fabricated through SPS of the mechanically alloyed raw Al, Ni, and Co powders. Microstructural characterizations, XRD analysis, compressive properties, tribological behavior, and corrosion resistance of the alloying systems were mutually compared, leading to the following conclusions.
The QC-P/CNT sample comprised poly-quasicrystal grains, aluminum phosphate/phosphide phases attached to grain boundaries, CNTs at grain boundaries, and complex oxide nanoparticles in the grain interior.
The mean grain size of QC-P/CNT sample was 50% lower than that of the QC sample (1.5 µm).
Both QC and QC-P/CNT specimens showed positive temperature dependence of compressive properties at 600 °C. While the incorporation of CNTs could enhance compressive properties through grain refinement and load transfer mechanisms, the formation of aluminum phosphate and phosphide phases as a byproduct of CNT addition negatively impacted these properties.
The wear rate of the QC-P/CNT specimen that was achieved is 0.992 × 10−4 mm3/N/m, which is 47.1% lower than that of the QC sample.
The Icorr of the QC-P sample decreased from 0.38 to 0.2 µA.cm−2 with the incorporation of CNTs. However, the QC sample exhibited the lowest Icorr at 0.12 µA.cm−2, significantly lower than that of pure aluminum (0.42 µA.cm−2).
In conclusion, the incorporation of CNTs significantly improved the wear performance and, to some extent, the corrosion resistance of the decagonal poly-quasicrystals.

Author Contributions

Conceptualization, S.H. and P.N.; methodology, S.H. and P.N.; validation, S.H., M.A. and P.N.; formal analysis, S.H. and Z.K.; investigation, S.H.; resources, S.H., P.Š. and P.N.; data curation, S.H., M.A. and P.N.; writing—original draft preparation, S.H., M.A., Z.K., P.Š. and P.N.; writing—review and editing, S.H., M.A., Z.K., P.Š. and P.N.; visualization, S.H. and M.A.; supervision, S.H., P.Š. and P.N.; project administration, S.H.; funding acquisition, S.H., P.Š. and P.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the Ministry of Education, Youth and Sports of the Czech Republic, project No. CZ.02.2.69/0.0/0.0/16_027/0008351, under the ChemJets project. The authors also acknowledge the assistance with TEM sample preparation provided by the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth, and Sports of the Czech Republic under Project No. LM2015073.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic representation of the synthesis steps for the QC systems, detailing the processes of ENP, MA, and SPS.
Figure 1. Schematic representation of the synthesis steps for the QC systems, detailing the processes of ENP, MA, and SPS.
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Figure 2. SEM images of (a) Co powders coated with Ni-P electroless plating, (b) cross-section of the coated Co powders, (c) Al powders coated with Ni-P electroless plating, and (d) Co powders coated with Ni-P/CNT electroless plating.
Figure 2. SEM images of (a) Co powders coated with Ni-P electroless plating, (b) cross-section of the coated Co powders, (c) Al powders coated with Ni-P electroless plating, and (d) Co powders coated with Ni-P/CNT electroless plating.
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Figure 3. SEM image of mechanically alloyed (a) AlNiCo powders after the SPS process at various temperatures, (b) AlNiCo/CNT powders after the SPS process at 950 °C, indicating arrayed CNTs at grain boundaries (green arrows), and (c) a TEM image showing an individual CNT located at a grain boundary.
Figure 3. SEM image of mechanically alloyed (a) AlNiCo powders after the SPS process at various temperatures, (b) AlNiCo/CNT powders after the SPS process at 950 °C, indicating arrayed CNTs at grain boundaries (green arrows), and (c) a TEM image showing an individual CNT located at a grain boundary.
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Figure 4. XRD patterns demonstrating the formation of the decagonal phase in all three QC systems.
Figure 4. XRD patterns demonstrating the formation of the decagonal phase in all three QC systems.
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Figure 5. TEM images of (a) decagonal AlNiCo poly-quasicrystalline alloy showing the orientation of two adjacent grains, labeled as (i,ii), (b) higher magnification revealing a uniform distribution of oxide nanoparticles within the grains, and (c) EDS analyses of two points corresponding to S4 and S5 in (b).
Figure 5. TEM images of (a) decagonal AlNiCo poly-quasicrystalline alloy showing the orientation of two adjacent grains, labeled as (i,ii), (b) higher magnification revealing a uniform distribution of oxide nanoparticles within the grains, and (c) EDS analyses of two points corresponding to S4 and S5 in (b).
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Figure 6. TEM image of (a) the decagonal AlNiCo/CNT poly-quasicrystal, (b) corresponding STEM image of Figure (a) associated with elemental distribution maps, and (c) high-magnification view of a tenfold grain, showing the arrangement of oxide nanoparticles inside the grain and P-based phases at grain boundaries.
Figure 6. TEM image of (a) the decagonal AlNiCo/CNT poly-quasicrystal, (b) corresponding STEM image of Figure (a) associated with elemental distribution maps, and (c) high-magnification view of a tenfold grain, showing the arrangement of oxide nanoparticles inside the grain and P-based phases at grain boundaries.
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Figure 7. True compressive stress–strain curves of the QC and QC-P/CNT quasicrystalline alloys at (a) 25 °C and (b) 600 °C, representing the positive temperature dependence of compressive properties in both samples.
Figure 7. True compressive stress–strain curves of the QC and QC-P/CNT quasicrystalline alloys at (a) 25 °C and (b) 600 °C, representing the positive temperature dependence of compressive properties in both samples.
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Figure 8. Values of steady-state friction coefficient and wear rate in different QC systems under the dry sliding condition.
Figure 8. Values of steady-state friction coefficient and wear rate in different QC systems under the dry sliding condition.
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Figure 9. SEM images of wear tracks and worn surfaces, along with trench profiles of (a) QC, (b) QC-P, and (c) QC-P/CNT samples.
Figure 9. SEM images of wear tracks and worn surfaces, along with trench profiles of (a) QC, (b) QC-P, and (c) QC-P/CNT samples.
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Figure 10. Anodic potentiodynamic polarization curves of pure Al, QC, QC-P, and QC-P/CNT alloying systems.
Figure 10. Anodic potentiodynamic polarization curves of pure Al, QC, QC-P, and QC-P/CNT alloying systems.
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MDPI and ACS Style

Hosseini, S.; Novák, P.; Alishahi, M.; Kačenka, Z.; Šittner, P. Trade-Off Between Wear/Corrosion Performance and Mechanical Properties in D-AlNiCo Poly-Quasicrystals Through CNT Addition to the Microstructure. Metals 2024, 14, 1269. https://doi.org/10.3390/met14111269

AMA Style

Hosseini S, Novák P, Alishahi M, Kačenka Z, Šittner P. Trade-Off Between Wear/Corrosion Performance and Mechanical Properties in D-AlNiCo Poly-Quasicrystals Through CNT Addition to the Microstructure. Metals. 2024; 14(11):1269. https://doi.org/10.3390/met14111269

Chicago/Turabian Style

Hosseini, Seyedmehdi, Pavel Novák, Mostafa Alishahi, Zdeněk Kačenka, and Petr Šittner. 2024. "Trade-Off Between Wear/Corrosion Performance and Mechanical Properties in D-AlNiCo Poly-Quasicrystals Through CNT Addition to the Microstructure" Metals 14, no. 11: 1269. https://doi.org/10.3390/met14111269

APA Style

Hosseini, S., Novák, P., Alishahi, M., Kačenka, Z., & Šittner, P. (2024). Trade-Off Between Wear/Corrosion Performance and Mechanical Properties in D-AlNiCo Poly-Quasicrystals Through CNT Addition to the Microstructure. Metals, 14(11), 1269. https://doi.org/10.3390/met14111269

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