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Fracture toughness and wear behavior of NiAl-based nanocomposite HVOF coatings
Surface & Coatings Technology 235 (2013) 212–219
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Fracture toughness and wear behavior of NiAl-based nanocomposite HVOF coatings B. Movahedi ⁎ Department of Nanotechnology Engineering, Faculty of Advanced Sciences and Technologies, University of Isfahan, Isfahan, 81746-73441, Iran
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Article history: Received 27 April 2013 Accepted in revised form 12 July 2013 Available online 23 July 2013 Keywords: Intermetallics Nanocomposite coating HVOF Fracture toughness
a b s t r a c t The fracture toughness and wear behavior as well as the microstructural evolutions of the NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) reinforced nanocomposite HVOF coatings were studied. Depending on their microstructures, the characteristics of the coatings were examined in detail by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) methods. The results indicated that the microhardness increased by adding Al2O3–13% TiO2 nanoparticles serving as reinforcing materials, which must be due to the interaction of nanoparticles with mobile dislocations during plastic deformation. It should be that the NiAl-15 wt% (Al2O3–13% TiO2) nanocomposite coating was tougher (7.12 MPa m1/2) than the pure NiAl intermetallic coating (4.28 MPa m1/2). It seems to be a consequence of grain boundary and nanoparticles pinning mechanism to limit the initial crack propagation over the course of indention test. The wear test studies also showed improved wear resistance of the nanocomposite coating compared with the NiAl intermetallic coating. On the other hand, the average specific wear rate for NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating (0.78 ± 0.33 × 10−15 m3/N⋅m) is much less than that for NiAl intermetallic coating (4.11 ± 1.98 ×10−15 m3/N⋅m). Consequently, the changes in wear resistance between both coatings were attributed to the changes in the susceptibility to crack propagation by adding Al2O3–13% TiO2 reinforcing nanoparticles. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Owing to their low density (5.86 g/cm3), high melting point (1911 K) and excellent oxidation resistance up to 1573 K as well as good thermal conductivity of four to eight times higher than that of Ni-based superalloys [1], NiAl-based intermetallic compounds are known as potential high temperature structural materials. These properties have made NiAl-based intermetallic compounds suitable materials for protective coatings and structural applications. However, they suffer from poor ductility and fracture toughness, that is, 5 MPa m1/2 at room temperature and low strength as well as low creep resistance at elevated temperatures, resulting in severe limitations of their practical use. Therefore, extensive efforts have been made to overcome these difficulties in the mechanical properties of nickel aluminates [2,3]. The effective approach is to utilize NiAl-based intermetallic compounds as a composite matrix reinforced with boride, carbide or oxide in the form of particulates, whiskers or fibers [4–8]. Previous studies concluded that the toughness and strength of NiAl at room-temperature can be improved by adding ceramics inclusions. For the NiAl-Al2O3 system, the presence of Al2O3 inclusions limits the grain growth of NiAl; thus, the strength of NiAl is increased. The NiAl/ Al2O3 interface in the composites is weak. The weak interface digresses the propagation of cracks such that the toughness of NiAl is enhanced. ⁎ Tel.: +98 3117934404; fax: +98 3117932342. E-mail address: [email protected]. 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.07.035
Therefore, the NiAl/Al2O3 composites are potential candidates for protective coatings [9]. The addition of Al2O3 whiskers into NiAl by Kumar et al. [10] resulted in an increase in the fracture toughness of NiAl coming up to 9 MPa m1/2 at 15 vol.% Al2O3. It has been demonstrated that the addition of rare earth elements can increase the wear and corrosion resistance of NiAl-based coatings due to improved hardness, toughness, bond strength and thermal shock resistance [5,11–16]. It is worth noting that many researchers have tried to use the intermetallic compounds specially nickel aluminates as protective coatings for use in wear applications in aggressive environments [17]. The highvelocity oxy fuel (HVOF) is the most versatile processes for sprayed intermetallic and nanocomposite compounds [5,13–15,18]. Most of the reported literature has discussed the synthesis and properties of NiAl– Al2O3 composite materials and coatings containing mostly α-alumina Table 1 HVOF spraying parameters for intermetallic and nanocomposite coatings. Parameter
Value
O2 gas flow rate (SLPM) Fuel (kerosene) flow rate (SLPM) Nozzle length (mm) Powder feed rate (g/min) Standoff distance (mm) Scanning velocity (mm/s) Cooling (argon gas)
560 0.14 100 35 250 10 yes
SLPM: standard liter per minute.
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particles [4,9,19–22]. To the best of my knowledge, there are no reports on the comparative study of the fracture toughness and wear-resistant properties of NiAl-based nanocomposite thermal spray coatings. It is interesting to show the effect of nanoparticles as reinforcing materials on the mechanical properties of NiAl-based intermetallic nanocomposite coatings. In the present study, the new composition of NiAl nanocomposite coatings reinforced by Al2O3–13% TiO2 nanoparticles were fabricated using the HVOF spraying of mechanically alloyed powders. The aim of this study is to investigate and compare in detail the wear behavior, microhardness and fracture toughness of both NiAl intermetallic and NiAl–15 wt%(Al2O3–13% TiO2) reinforced nanocomposite HVOF coatings. 2. Experimental procedure 2.1. Feedstock materials The feedstock NiAl intermetallic and nanocomposite NiAl–15 wt% (Al2O3–13% TiO2) powders were prepared in the planetary highenergy ball mill (PM100) in an argon gas atmosphere over the course of 8 and 10 hours of milling, respectively. The elemental powders were blended to give a nominal composition of NiAl and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite. The details of feedstock materials synthesis and characterizations have been published elsewhere [18]. 2.2. HVOF spraying experiments
Fig. 1. SEM images of cross-sectional microstructure of HVOF coatings (a) NiAl intermetallic and (b) NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings.
HVOF spraying was performed using Metallization Met JET III system. The modified spraying conditions for NiAl intermetallic and nanocomposite coatings reported elsewhere [18,23] has been summarized in Table 1. The mild steel (BS080A15) substrate (50 × 50 × 5 mm) surfaces were cleaned prior to spraying by sand-blast with alumina grit on the one side followed by ultrasonic cleaning in acetone to remove any contaminations. The substrates were mounted on a vertical axis of a turntable and the rotation speed was set to impart a surface velocity
Fig. 2. XRD patterns of (a) NiAl intermetallic and (b) NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite HVOF coatings.
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Table 2 The measured parameters used for computing crystallite size of NiAl phase of both HVOF coatings based on Williamson–Hall formula. 2θ
31.175 44.425 64.975
Cos θ
0.963221 0.925788 0.843086
Sin θ
0.268709 0.378042 0.537115
FWHM (radian) NiAl intermetallic coating
NiAl–15 wt% (Al2O3–13% TiO2) coating
0.001459 0.001512 0.001652
0.002895 0.003082 0.003393
to the substrates of approximately 1.1 m/s across the spray path. The HVOF gun was positioned in front of them, aligned to give a spray path perpendicular to the axis and set to the required standoff from the surface of the substrates. It was then scanned vertically up and down at 10 mm/s to build up a coating of the required thickness. To
Fig. 3. (a) Bright and (b) dark filed TEM images of NiAl intermetallic HVOF coating.
avoid significant heating during the progress of the process and maintain the substrate temperature below 200 ° C, the substrates were cooled on their backside using compressed argon gas. Several passes (8–10) of the torch were done to gain 250–350 μm thick coatings. 2.3. Characterization methods X-ray diffraction (XRD: Philips XPERT-MPD) was performed to study the structural evolutions of the coatings. All XRD experiments were carried out in the continuous scanning mode using Cu-Kα radiation (λ = 0.1542 nm), time per step of 1 second and 0.03º step size. The Williamson–Hall method [24] was used to evaluate the crystallite size of the different phases. The instrumental peak broadening was corrected using a coarse grain strain-free sample of pure annealed Ni.
Fig. 4. (a) Bright and (b) dark filed TEM images of the NiAl–15 wt% ( Al2O3–13% TiO2) nanocomposite HVOF coating.
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The cross-sectional microstructures of the coatings were investigated by scanning electron microscopy (SEM: Philips XL30SERIES). Transmission electron microscopy (TEM) was performed using JeolJEM-2010 TEM at an accelerating voltage of 200 kV and resolution of 0.19 nm. To prepare the sample, the coating was first sectioned with a diamond saw and then mechanically thinned to about 100 μm with emery paper. In the second step, the thickness of the thinned layer was decreased down to 5 μm using a dimpling machine (Gatan dimple grinder 656). The final step was ion-milling at low incident angle (Gatan ion polishing 691) to obtain a very thin layer of less than 100 nm. 2.4. Mechanical properties measurement The microhardness values and standard errors were calculated as an average of 10 different area measurements recorded on polished cross sections of the coatings under a 25-gf load and a 15-s dwell time using a digital microhardness tester (Matsuzawa MAXT70). The fracture toughness of HVOF coatings was determined by the indentation method. A Vickers indenter was used on metallographically prepared cross sections of coatings with a 20-N load. The indenter was positioned such that the two indent diagonals were parallel and perpendicular to the substrate/coating interface respectively. Following the indentation, no cracking was seen in the plane perpendicular to the coating/substrate interface but was generally, though not always, seen to be parallel to the coating/substrate interface. The lengths of the latter cracks (where present) were measured from scanning electron micrographs imaged in the secondary electrons (SE) mode. At least 20 indentations from each coating were examined. The fracture toughness (Kc) values were calculated using cracks parallel to the substrate/coating interface. The equation for the indentation fracture toughness has been selected for the current work due to its applicability to systems generating short cracks as given by Evans and Wilshaw [25], is:
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Table 3 Summary of different HVOF coating properties. Coating designation
NiAl intermetallic NiAl–15 wt% (Al2O3–13% TiO2)
Microhardness (GPa)
Fracture toughness (MPa m1/2)
Median value
Standard deviation
Median value
Standard deviation
5.51 8.35
0.25 0.61
4.28 7.12
0.39 0.53
as well as a dense microstructure due to the deposition and resolidification of the molten or semi-molten droplets. The light gray layers were Ni-rich and dark layers were interlamellar oxidation as well as porosities consistent with the work reported by Hearley et al. [20]. As mentioned earlier, HVOF deposits were seen to be very dense with very few interlamellar pores compared to other thermal spray techniques. The higher particle velocities and lower particle temperatures in flight lead to a decrease in the volume fraction in the melted phase thereby resulting in a lower amount of coating oxidations [26,27]. There are some microcracks developed in the pure NiAl intermetallic coating seemingly as a result of the NiAl brittle phase (indicated by arrows in Fig. 1a). The addition of the Al2O3–13% TiO2 nanoparticles significantly improved the quality of the coatings (Fig. 1b), for example, fewer microcracks without inter-splat fracture were present in the coating layers, and the overall coating microstructure was homogeneous and uniform with little closed porosity along the lamella boundaries. The porosity content of NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings was computed using the image-analyzing method as an average of 15 different measurements at about 3.24 ± 0.23 and 2.05 ± 0.12 percent respectively.
3=2 logð4:5a=cÞ K c ¼ 0:079 P=a where P is the applied indentation load (N), a is the indentation half diagonal (m) and c is the crack length from the centre of the indent (m). The recommended c/a ratio for use in this equation is 0.6 ≤ c/a b 4.5. Wear tests were performed using a ball-on-disc type sliding wear CSEM tribometer apparatus under ASTM G99-95a. The tests were performed at a normal load of 10 N, sliding speed of 10 cm/s and sliding distance of 1000 m against a single crystal alumina ball (6 mm in diameter) in an air atmosphere inside an environmental chamber. Prior to wear tests, the samples were mechanically polished such that the initial surface roughness was kept around 0.8 μm (Ra). The specific wear rate is defined as the worn volume per unit of the normal load and sliding distance. The cross-sectional area of the wear track was obtained through a KLA-Tencor Alpha Step IQ profilometer on completion of 1000 m sliding distance. At least four positions along the wear track were measured. The total worn volume can be obtained by multiplying the cross-sectional area of the wear track by the perimeter of the sliding track. The worn surfaces and debris were characterized using SEM method. All the tribological tests were carried out at least three times to ensure reproducibility of the experimental results under equal conditions, and then the average results were reported. 3. Results and discussion 3.1. Nanocomposite coatings Fig. 1a and b show the cross-sectional SEM images of HVOF coatings produced using two different feedstock powders: (a) NiAl intermetallic and (b) NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite. It was observed that the coatings showed a typically very fine lamellar morphology
Fig. 5. Vickers indentation mark at applied load of 17.5 N of (a) NiAl intermetallic and (b) NiAl–15 wt%(Al2O3–13% TiO2) nanocomposite HVOF coatings.
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The XRD patterns of both NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) as-sprayed HVOF coatings are shown in Fig. 2. The crystallite size of NiAl phase in both coatings was obtained from XRD analysis using the Williamson–Hall method. The 2θ, full-width of halfheight maximum (FWHM) and corresponding Cos θ and Sin θ values obtained from XRD peaks are shown in Table 2. The results showed that the NiAl crystallite size is about 100 nm in pure NiAl and about 52 nm in NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings. Figs. 3 and 4 shows bright and dark-field TEM general view images as well as selected area diffraction pattern (SADP) of HVOF coatings. In accordance with the XRD patterns in Fig. 2, it is clearly reveals that the crystallite sizes in the pure NiAl HVOF coating are in the submicron range (more than 100 nm), but in the NiAl–15 wt% (Al2O3–13% TiO2), nanocomposite coating are less than 50 nm for NiAl grains and Al2O3–13% TiO2 particles as well as more than 50 nm for Al2O3–13% TiO2 clusters, indicated by circles in the dark field image (Fig. 4b). It seems that Al2O3– 13% TiO2 as the reinforcing nanoparticles are located at the grain boundaries. These nanoparticles exert a pinning force on the grain boundaries of NiAl such that the grain growth of NiAl is thus prevented during HVOF spraying. This is in good accordance with the previous studies [9].The presence of rings in the SADP (inserts in Figs. 3 and 4) indicate the presence of a polycrystalline material with a very fine grain size. 3.2. Microhardness and fracture toughness The results of microhardness and fracture toughness measurements performed on polished HVOF coating cross sections are presented
in Table 3. As seen, the hardness values for NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings were about 5.51 and 8.35 GPa respectively. Hu et al. [28] measured the average value of microhardness for nanostructured NiAl coating deposited using HVOF technique to be at 7.6 GPa. It was observed that the microhardness increased by adding of Al2O3–13% TiO2 nanoparticles as the reinforcing materials. The high hardness values obtained for the nanocomposite coating are attributed to the Hall–Petch strengthening effect. This mechanism is based on dislocation pileup or slip hindered at grain boundaries and/or reinforcing nanoparticles [29]. As shown in TEM images of NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating (Fig. 4a and b), the nanoparticles of less than 50 nm are distributed evenly throughout the NiAl matrix. These features are thus expected to contribute significantly to increase hardness. Thus, the extra hardening of about 2.5 GPa in NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite rather than the pure NiAl intermetallic must be due to the presence of Al2O3–13% TiO2 nanoparticles and their interaction with mobile dislocations during plastic deformation. The fracture toughness values were calculated from measurements of Vickers indentations (and the associated cracks) on the coating cross sections at an applied load of 20 N. It was found that not all indentations gave rise to cracks parallel to the coating/substrate interface. However, when cracking did occur, c/a values always fell within the range of 0.6 ≤ c/a b 4.5 for which the Evans and Wilshaw equation is valid. It is also evident that the NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating was tougher (7.12 MPa m1/2) than the NiAl intermetallic coating (4.28 MPa m1/2).
Fig. 6. Friction coefficient versus the sliding distance curve under an applied load of 10 N and sliding speed of 10 cm/s for (a) NiAl intermetallic and (b) NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings.
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Fig. 5a and b show the cross-sectional SEM micrographs of both HVOF coatings with Vickers indentation marks at an applied load of about 17.5 N. It can be seen that cracks with a length of about 21.74 μm were created in the NiAl intermetallic (Fig. 5a), while no cracks occurred in the NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating (Fig. 5b) at this load. As a result, the brittle mechanism should dominate for the NiAl intermetallic coating, as shown in Fig. 5a. It goes without saying that the individual lamellae in the pure NiAl are cracked and then chipped before removal. As expected, the coating with Al2O3–13% TiO2 nanoparticles as the reinforcing materials exhibited the highest fracture toughness value. On the other hand, no cracks were observed around the indentations in the nanocomposite coating at an applied load of 17.5 N (Fig. 5b) in comparison with the pure NiAl intermetallic coating (Fig. 5a), which suggests that the significant plastic deformation can be accommodated. The increase in toughness, however, seems to be a consequence of the grain boundary and nanoparticles pinning mechanism to limit the initial crack propagation over the course of indention test. 3.3. Wear behavior The friction coefficient versus the total sliding distance curve for the (a) NiAl intermetallic and (b) NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings against alumina single crystal ball under an applied load of 10 N and a sliding speed of 10 cm/s at room temperature are given in Fig. 6a and b. These curves indicate two friction regimes. At the beginning, the friction coefficient increased till it reached the maximum value (μs: statically friction force). The maximum value is followed by a gradual decrease down to a lower steady-state value (μd: dynamically
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friction force). The static friction coefficients of the NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings were found to be 0.354 ± 0.150 and 0.463 ± 0.038, respectively. Furthermore, the dynamic friction coefficients of the NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings were 0.236 ± 0.150 and 0.189 ± 0.038, respectively. It can be seen that for the NiAl intermetallic coating, the friction coefficient gradually decreases after the initial transient running time. However, as the sliding distance increases, it provides fluctuation values with an upward trend of up to 0.765 ± 0.150. Yet, for the NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating, the dynamic friction coefficient has a sudden rise after the initial transient running time and then a steady drop with the mean value of about 0.288 ± 0.038. Over the course of the sliding distance, the friction coefficient slightly increases up to 0.374 ± 0.038. The friction between the sliding surfaces is due to the combined effects of adhesion between the flat surfaces, ploughing by wear debris and hard asperities and asperity deformation [30]. The SEM micrographs of the wear track and debris for the NiAl intermetallic coating are depicted in Fig. 7. Detailed observation of the worn surface tested under dry conditions can be characterized by local plastic deformations as revealed by the splat delaminations, grooves, scratches and adhesive junctions. An important aspect of wear in this coating was the crack propagation and fracture of the splats along the sliding distance. Fig. 7c shows the wear scar profile with the deepest and widest wear track. When surface cracking and fragmentation of brittle NiAl intermetallic splats occur, the large fragments present in the wear debris (Fig. 7d) are produced. These fragments are able to abrade the surface so that deep wear ploughs are observed. This leads to surface fatigue and a rapid increase in the contact area as well as specific wear rates
Fig. 7. (a and b) SEM micrographs at different magnifications of the wear track, (c) surface profile of worn surface and (d) wear debris of the NiAl intermetallic HVOF coating.
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[31]. The wear track of NiAl intermetallic coating clearly seen that micro-cutting, micro-ploughing and adhesive are major mechanisms. The delamination wear mechanism could also be suggested to take place after propagation of the cracks around the splats. Based on the delamination theory [32], delamination wear is caused by subsurface plastic deformation, crack nucleation and crack propagation. However, all the wear debris can accelerate the delamination process by increasing the frictional force and surface fatigue when they are entrapped between the sliding surfaces. The average specific wear rate for NiAl intermetallic coating was calculated at about 4.11 ± 1.98 × 10−15 m3/N⋅m. Fig. 8 shows the SEM images of the wear track and debris as well as the surface profile for NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating. For the nanocomposite coating, scratches as well as severe
plastic deformation can be observed on the surface, showing a typical abrasive wear mode in which the grooves are formed along the sliding direction. It indicates that surface plastic deformation and subsequent tearing are the dominant material removing mechanisms. However, in contrast with the NiAl intermetallic coating, the depth of the scratches is decreased and no long cracks are observed. This wear behavior is suggested and attributed to the high fracture toughness of the nanocomposite coating rather than to the brittle NiAl intermetallic coating. Similar results were published elsewhere [9]. Fig. 8b is a representative wear track profile of the nanocomposite coating as measured by profilometry. The surface is rather smooth, and the depth of the wear track is about 2 μm. The debris generated was loose and powdery having a sub-micrometer size, as shown in Fig. 8c. The average specific wear rate for NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating was computed at about 0.78 ± 0.33 × 10−15 m3/N⋅m, much less than that for NiAl intermetallic coating. Consequently, the changes in the wear resistance between NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings were attributed to the changes in the susceptibility to crack propagation by adding Al2O3–13% TiO2 nanoparticles as the reinforcing materials. 4. Conclusions To summarize the approach, the following conclusions can be drawn from the above discussion: 1. The addition of Al2O3–13% TiO2 nanoparticles as the reinforcing materials significantly improved the quality of the HVOF coatings and the overall coating microstructure was found to be homogeneous and uniform with little closed porosity as well as fewer microcracks along the lamella boundaries. 2. The microhardness increased by adding Al2O3–13% TiO2 nanoparticles as the reinforcing materials. The high hardness values obtained for the nanocomposite coating are attributed to the Hall–Petch strengthening effect as the nanoparticles of less than 50 nm distributed evenly throughout the NiAl matrix. 3. Fracture toughness value evidently indicates that the NiAl–15 wt% TiO2) nanocomposite coating was tougher (Al2O3–13% (7.12 MPa m1/2) than the NiAl intermetallic coating (4.28 MPa m1/2) and that the brittle mechanism dominates in the NiAl intermetallic coating. 4. In contrast with the NiAl intermetallic coating, the depth of the scratches was decreased, and no long cracks were observed in the NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating. This wear behavior is attributed to the high microhardness together with the high fracture toughness for nanocomposite coating. The average specific wear rate for the nanocomposite coating is much less than that for NiAl intermetallic coating attributed to the change in the susceptibility to crack propagation by adding Al2O3–13% TiO2 nanoparticles serving as reinforcing materials. Acknowledgment The author would like to acknowledge the Iran National Science Foundation: INSF for funding this work under grant no. 90002285. References
Fig. 8. (a) SEM micrographs of the wear track, (b) surface profile of worn surface and (c) wear debris of the NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite HVOF coating.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Fracture toughness and wear behavior of NiAl-based nanocomposite HVOF coatings B. Movahedi ⁎ Department of Nanotechnology Engineering, Faculty of Advanced Sciences and Technologies, University of Isfahan, Isfahan, 81746-73441, Iran
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Article history: Received 27 April 2013 Accepted in revised form 12 July 2013 Available online 23 July 2013 Keywords: Intermetallics Nanocomposite coating HVOF Fracture toughness
a b s t r a c t The fracture toughness and wear behavior as well as the microstructural evolutions of the NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) reinforced nanocomposite HVOF coatings were studied. Depending on their microstructures, the characteristics of the coatings were examined in detail by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) methods. The results indicated that the microhardness increased by adding Al2O3–13% TiO2 nanoparticles serving as reinforcing materials, which must be due to the interaction of nanoparticles with mobile dislocations during plastic deformation. It should be that the NiAl-15 wt% (Al2O3–13% TiO2) nanocomposite coating was tougher (7.12 MPa m1/2) than the pure NiAl intermetallic coating (4.28 MPa m1/2). It seems to be a consequence of grain boundary and nanoparticles pinning mechanism to limit the initial crack propagation over the course of indention test. The wear test studies also showed improved wear resistance of the nanocomposite coating compared with the NiAl intermetallic coating. On the other hand, the average specific wear rate for NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating (0.78 ± 0.33 × 10−15 m3/N⋅m) is much less than that for NiAl intermetallic coating (4.11 ± 1.98 ×10−15 m3/N⋅m). Consequently, the changes in wear resistance between both coatings were attributed to the changes in the susceptibility to crack propagation by adding Al2O3–13% TiO2 reinforcing nanoparticles. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Owing to their low density (5.86 g/cm3), high melting point (1911 K) and excellent oxidation resistance up to 1573 K as well as good thermal conductivity of four to eight times higher than that of Ni-based superalloys [1], NiAl-based intermetallic compounds are known as potential high temperature structural materials. These properties have made NiAl-based intermetallic compounds suitable materials for protective coatings and structural applications. However, they suffer from poor ductility and fracture toughness, that is, 5 MPa m1/2 at room temperature and low strength as well as low creep resistance at elevated temperatures, resulting in severe limitations of their practical use. Therefore, extensive efforts have been made to overcome these difficulties in the mechanical properties of nickel aluminates [2,3]. The effective approach is to utilize NiAl-based intermetallic compounds as a composite matrix reinforced with boride, carbide or oxide in the form of particulates, whiskers or fibers [4–8]. Previous studies concluded that the toughness and strength of NiAl at room-temperature can be improved by adding ceramics inclusions. For the NiAl-Al2O3 system, the presence of Al2O3 inclusions limits the grain growth of NiAl; thus, the strength of NiAl is increased. The NiAl/ Al2O3 interface in the composites is weak. The weak interface digresses the propagation of cracks such that the toughness of NiAl is enhanced. ⁎ Tel.: +98 3117934404; fax: +98 3117932342. E-mail address: [email protected]. 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.07.035
Therefore, the NiAl/Al2O3 composites are potential candidates for protective coatings [9]. The addition of Al2O3 whiskers into NiAl by Kumar et al. [10] resulted in an increase in the fracture toughness of NiAl coming up to 9 MPa m1/2 at 15 vol.% Al2O3. It has been demonstrated that the addition of rare earth elements can increase the wear and corrosion resistance of NiAl-based coatings due to improved hardness, toughness, bond strength and thermal shock resistance [5,11–16]. It is worth noting that many researchers have tried to use the intermetallic compounds specially nickel aluminates as protective coatings for use in wear applications in aggressive environments [17]. The highvelocity oxy fuel (HVOF) is the most versatile processes for sprayed intermetallic and nanocomposite compounds [5,13–15,18]. Most of the reported literature has discussed the synthesis and properties of NiAl– Al2O3 composite materials and coatings containing mostly α-alumina Table 1 HVOF spraying parameters for intermetallic and nanocomposite coatings. Parameter
Value
O2 gas flow rate (SLPM) Fuel (kerosene) flow rate (SLPM) Nozzle length (mm) Powder feed rate (g/min) Standoff distance (mm) Scanning velocity (mm/s) Cooling (argon gas)
560 0.14 100 35 250 10 yes
SLPM: standard liter per minute.
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particles [4,9,19–22]. To the best of my knowledge, there are no reports on the comparative study of the fracture toughness and wear-resistant properties of NiAl-based nanocomposite thermal spray coatings. It is interesting to show the effect of nanoparticles as reinforcing materials on the mechanical properties of NiAl-based intermetallic nanocomposite coatings. In the present study, the new composition of NiAl nanocomposite coatings reinforced by Al2O3–13% TiO2 nanoparticles were fabricated using the HVOF spraying of mechanically alloyed powders. The aim of this study is to investigate and compare in detail the wear behavior, microhardness and fracture toughness of both NiAl intermetallic and NiAl–15 wt%(Al2O3–13% TiO2) reinforced nanocomposite HVOF coatings. 2. Experimental procedure 2.1. Feedstock materials The feedstock NiAl intermetallic and nanocomposite NiAl–15 wt% (Al2O3–13% TiO2) powders were prepared in the planetary highenergy ball mill (PM100) in an argon gas atmosphere over the course of 8 and 10 hours of milling, respectively. The elemental powders were blended to give a nominal composition of NiAl and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite. The details of feedstock materials synthesis and characterizations have been published elsewhere [18]. 2.2. HVOF spraying experiments
Fig. 1. SEM images of cross-sectional microstructure of HVOF coatings (a) NiAl intermetallic and (b) NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings.
HVOF spraying was performed using Metallization Met JET III system. The modified spraying conditions for NiAl intermetallic and nanocomposite coatings reported elsewhere [18,23] has been summarized in Table 1. The mild steel (BS080A15) substrate (50 × 50 × 5 mm) surfaces were cleaned prior to spraying by sand-blast with alumina grit on the one side followed by ultrasonic cleaning in acetone to remove any contaminations. The substrates were mounted on a vertical axis of a turntable and the rotation speed was set to impart a surface velocity
Fig. 2. XRD patterns of (a) NiAl intermetallic and (b) NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite HVOF coatings.
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Table 2 The measured parameters used for computing crystallite size of NiAl phase of both HVOF coatings based on Williamson–Hall formula. 2θ
31.175 44.425 64.975
Cos θ
0.963221 0.925788 0.843086
Sin θ
0.268709 0.378042 0.537115
FWHM (radian) NiAl intermetallic coating
NiAl–15 wt% (Al2O3–13% TiO2) coating
0.001459 0.001512 0.001652
0.002895 0.003082 0.003393
to the substrates of approximately 1.1 m/s across the spray path. The HVOF gun was positioned in front of them, aligned to give a spray path perpendicular to the axis and set to the required standoff from the surface of the substrates. It was then scanned vertically up and down at 10 mm/s to build up a coating of the required thickness. To
Fig. 3. (a) Bright and (b) dark filed TEM images of NiAl intermetallic HVOF coating.
avoid significant heating during the progress of the process and maintain the substrate temperature below 200 ° C, the substrates were cooled on their backside using compressed argon gas. Several passes (8–10) of the torch were done to gain 250–350 μm thick coatings. 2.3. Characterization methods X-ray diffraction (XRD: Philips XPERT-MPD) was performed to study the structural evolutions of the coatings. All XRD experiments were carried out in the continuous scanning mode using Cu-Kα radiation (λ = 0.1542 nm), time per step of 1 second and 0.03º step size. The Williamson–Hall method [24] was used to evaluate the crystallite size of the different phases. The instrumental peak broadening was corrected using a coarse grain strain-free sample of pure annealed Ni.
Fig. 4. (a) Bright and (b) dark filed TEM images of the NiAl–15 wt% ( Al2O3–13% TiO2) nanocomposite HVOF coating.
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The cross-sectional microstructures of the coatings were investigated by scanning electron microscopy (SEM: Philips XL30SERIES). Transmission electron microscopy (TEM) was performed using JeolJEM-2010 TEM at an accelerating voltage of 200 kV and resolution of 0.19 nm. To prepare the sample, the coating was first sectioned with a diamond saw and then mechanically thinned to about 100 μm with emery paper. In the second step, the thickness of the thinned layer was decreased down to 5 μm using a dimpling machine (Gatan dimple grinder 656). The final step was ion-milling at low incident angle (Gatan ion polishing 691) to obtain a very thin layer of less than 100 nm. 2.4. Mechanical properties measurement The microhardness values and standard errors were calculated as an average of 10 different area measurements recorded on polished cross sections of the coatings under a 25-gf load and a 15-s dwell time using a digital microhardness tester (Matsuzawa MAXT70). The fracture toughness of HVOF coatings was determined by the indentation method. A Vickers indenter was used on metallographically prepared cross sections of coatings with a 20-N load. The indenter was positioned such that the two indent diagonals were parallel and perpendicular to the substrate/coating interface respectively. Following the indentation, no cracking was seen in the plane perpendicular to the coating/substrate interface but was generally, though not always, seen to be parallel to the coating/substrate interface. The lengths of the latter cracks (where present) were measured from scanning electron micrographs imaged in the secondary electrons (SE) mode. At least 20 indentations from each coating were examined. The fracture toughness (Kc) values were calculated using cracks parallel to the substrate/coating interface. The equation for the indentation fracture toughness has been selected for the current work due to its applicability to systems generating short cracks as given by Evans and Wilshaw [25], is:
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Table 3 Summary of different HVOF coating properties. Coating designation
NiAl intermetallic NiAl–15 wt% (Al2O3–13% TiO2)
Microhardness (GPa)
Fracture toughness (MPa m1/2)
Median value
Standard deviation
Median value
Standard deviation
5.51 8.35
0.25 0.61
4.28 7.12
0.39 0.53
as well as a dense microstructure due to the deposition and resolidification of the molten or semi-molten droplets. The light gray layers were Ni-rich and dark layers were interlamellar oxidation as well as porosities consistent with the work reported by Hearley et al. [20]. As mentioned earlier, HVOF deposits were seen to be very dense with very few interlamellar pores compared to other thermal spray techniques. The higher particle velocities and lower particle temperatures in flight lead to a decrease in the volume fraction in the melted phase thereby resulting in a lower amount of coating oxidations [26,27]. There are some microcracks developed in the pure NiAl intermetallic coating seemingly as a result of the NiAl brittle phase (indicated by arrows in Fig. 1a). The addition of the Al2O3–13% TiO2 nanoparticles significantly improved the quality of the coatings (Fig. 1b), for example, fewer microcracks without inter-splat fracture were present in the coating layers, and the overall coating microstructure was homogeneous and uniform with little closed porosity along the lamella boundaries. The porosity content of NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings was computed using the image-analyzing method as an average of 15 different measurements at about 3.24 ± 0.23 and 2.05 ± 0.12 percent respectively.
3=2 logð4:5a=cÞ K c ¼ 0:079 P=a where P is the applied indentation load (N), a is the indentation half diagonal (m) and c is the crack length from the centre of the indent (m). The recommended c/a ratio for use in this equation is 0.6 ≤ c/a b 4.5. Wear tests were performed using a ball-on-disc type sliding wear CSEM tribometer apparatus under ASTM G99-95a. The tests were performed at a normal load of 10 N, sliding speed of 10 cm/s and sliding distance of 1000 m against a single crystal alumina ball (6 mm in diameter) in an air atmosphere inside an environmental chamber. Prior to wear tests, the samples were mechanically polished such that the initial surface roughness was kept around 0.8 μm (Ra). The specific wear rate is defined as the worn volume per unit of the normal load and sliding distance. The cross-sectional area of the wear track was obtained through a KLA-Tencor Alpha Step IQ profilometer on completion of 1000 m sliding distance. At least four positions along the wear track were measured. The total worn volume can be obtained by multiplying the cross-sectional area of the wear track by the perimeter of the sliding track. The worn surfaces and debris were characterized using SEM method. All the tribological tests were carried out at least three times to ensure reproducibility of the experimental results under equal conditions, and then the average results were reported. 3. Results and discussion 3.1. Nanocomposite coatings Fig. 1a and b show the cross-sectional SEM images of HVOF coatings produced using two different feedstock powders: (a) NiAl intermetallic and (b) NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite. It was observed that the coatings showed a typically very fine lamellar morphology
Fig. 5. Vickers indentation mark at applied load of 17.5 N of (a) NiAl intermetallic and (b) NiAl–15 wt%(Al2O3–13% TiO2) nanocomposite HVOF coatings.
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The XRD patterns of both NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) as-sprayed HVOF coatings are shown in Fig. 2. The crystallite size of NiAl phase in both coatings was obtained from XRD analysis using the Williamson–Hall method. The 2θ, full-width of halfheight maximum (FWHM) and corresponding Cos θ and Sin θ values obtained from XRD peaks are shown in Table 2. The results showed that the NiAl crystallite size is about 100 nm in pure NiAl and about 52 nm in NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings. Figs. 3 and 4 shows bright and dark-field TEM general view images as well as selected area diffraction pattern (SADP) of HVOF coatings. In accordance with the XRD patterns in Fig. 2, it is clearly reveals that the crystallite sizes in the pure NiAl HVOF coating are in the submicron range (more than 100 nm), but in the NiAl–15 wt% (Al2O3–13% TiO2), nanocomposite coating are less than 50 nm for NiAl grains and Al2O3–13% TiO2 particles as well as more than 50 nm for Al2O3–13% TiO2 clusters, indicated by circles in the dark field image (Fig. 4b). It seems that Al2O3– 13% TiO2 as the reinforcing nanoparticles are located at the grain boundaries. These nanoparticles exert a pinning force on the grain boundaries of NiAl such that the grain growth of NiAl is thus prevented during HVOF spraying. This is in good accordance with the previous studies [9].The presence of rings in the SADP (inserts in Figs. 3 and 4) indicate the presence of a polycrystalline material with a very fine grain size. 3.2. Microhardness and fracture toughness The results of microhardness and fracture toughness measurements performed on polished HVOF coating cross sections are presented
in Table 3. As seen, the hardness values for NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings were about 5.51 and 8.35 GPa respectively. Hu et al. [28] measured the average value of microhardness for nanostructured NiAl coating deposited using HVOF technique to be at 7.6 GPa. It was observed that the microhardness increased by adding of Al2O3–13% TiO2 nanoparticles as the reinforcing materials. The high hardness values obtained for the nanocomposite coating are attributed to the Hall–Petch strengthening effect. This mechanism is based on dislocation pileup or slip hindered at grain boundaries and/or reinforcing nanoparticles [29]. As shown in TEM images of NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating (Fig. 4a and b), the nanoparticles of less than 50 nm are distributed evenly throughout the NiAl matrix. These features are thus expected to contribute significantly to increase hardness. Thus, the extra hardening of about 2.5 GPa in NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite rather than the pure NiAl intermetallic must be due to the presence of Al2O3–13% TiO2 nanoparticles and their interaction with mobile dislocations during plastic deformation. The fracture toughness values were calculated from measurements of Vickers indentations (and the associated cracks) on the coating cross sections at an applied load of 20 N. It was found that not all indentations gave rise to cracks parallel to the coating/substrate interface. However, when cracking did occur, c/a values always fell within the range of 0.6 ≤ c/a b 4.5 for which the Evans and Wilshaw equation is valid. It is also evident that the NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating was tougher (7.12 MPa m1/2) than the NiAl intermetallic coating (4.28 MPa m1/2).
Fig. 6. Friction coefficient versus the sliding distance curve under an applied load of 10 N and sliding speed of 10 cm/s for (a) NiAl intermetallic and (b) NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings.
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Fig. 5a and b show the cross-sectional SEM micrographs of both HVOF coatings with Vickers indentation marks at an applied load of about 17.5 N. It can be seen that cracks with a length of about 21.74 μm were created in the NiAl intermetallic (Fig. 5a), while no cracks occurred in the NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating (Fig. 5b) at this load. As a result, the brittle mechanism should dominate for the NiAl intermetallic coating, as shown in Fig. 5a. It goes without saying that the individual lamellae in the pure NiAl are cracked and then chipped before removal. As expected, the coating with Al2O3–13% TiO2 nanoparticles as the reinforcing materials exhibited the highest fracture toughness value. On the other hand, no cracks were observed around the indentations in the nanocomposite coating at an applied load of 17.5 N (Fig. 5b) in comparison with the pure NiAl intermetallic coating (Fig. 5a), which suggests that the significant plastic deformation can be accommodated. The increase in toughness, however, seems to be a consequence of the grain boundary and nanoparticles pinning mechanism to limit the initial crack propagation over the course of indention test. 3.3. Wear behavior The friction coefficient versus the total sliding distance curve for the (a) NiAl intermetallic and (b) NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings against alumina single crystal ball under an applied load of 10 N and a sliding speed of 10 cm/s at room temperature are given in Fig. 6a and b. These curves indicate two friction regimes. At the beginning, the friction coefficient increased till it reached the maximum value (μs: statically friction force). The maximum value is followed by a gradual decrease down to a lower steady-state value (μd: dynamically
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friction force). The static friction coefficients of the NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings were found to be 0.354 ± 0.150 and 0.463 ± 0.038, respectively. Furthermore, the dynamic friction coefficients of the NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings were 0.236 ± 0.150 and 0.189 ± 0.038, respectively. It can be seen that for the NiAl intermetallic coating, the friction coefficient gradually decreases after the initial transient running time. However, as the sliding distance increases, it provides fluctuation values with an upward trend of up to 0.765 ± 0.150. Yet, for the NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating, the dynamic friction coefficient has a sudden rise after the initial transient running time and then a steady drop with the mean value of about 0.288 ± 0.038. Over the course of the sliding distance, the friction coefficient slightly increases up to 0.374 ± 0.038. The friction between the sliding surfaces is due to the combined effects of adhesion between the flat surfaces, ploughing by wear debris and hard asperities and asperity deformation [30]. The SEM micrographs of the wear track and debris for the NiAl intermetallic coating are depicted in Fig. 7. Detailed observation of the worn surface tested under dry conditions can be characterized by local plastic deformations as revealed by the splat delaminations, grooves, scratches and adhesive junctions. An important aspect of wear in this coating was the crack propagation and fracture of the splats along the sliding distance. Fig. 7c shows the wear scar profile with the deepest and widest wear track. When surface cracking and fragmentation of brittle NiAl intermetallic splats occur, the large fragments present in the wear debris (Fig. 7d) are produced. These fragments are able to abrade the surface so that deep wear ploughs are observed. This leads to surface fatigue and a rapid increase in the contact area as well as specific wear rates
Fig. 7. (a and b) SEM micrographs at different magnifications of the wear track, (c) surface profile of worn surface and (d) wear debris of the NiAl intermetallic HVOF coating.
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[31]. The wear track of NiAl intermetallic coating clearly seen that micro-cutting, micro-ploughing and adhesive are major mechanisms. The delamination wear mechanism could also be suggested to take place after propagation of the cracks around the splats. Based on the delamination theory [32], delamination wear is caused by subsurface plastic deformation, crack nucleation and crack propagation. However, all the wear debris can accelerate the delamination process by increasing the frictional force and surface fatigue when they are entrapped between the sliding surfaces. The average specific wear rate for NiAl intermetallic coating was calculated at about 4.11 ± 1.98 × 10−15 m3/N⋅m. Fig. 8 shows the SEM images of the wear track and debris as well as the surface profile for NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating. For the nanocomposite coating, scratches as well as severe
plastic deformation can be observed on the surface, showing a typical abrasive wear mode in which the grooves are formed along the sliding direction. It indicates that surface plastic deformation and subsequent tearing are the dominant material removing mechanisms. However, in contrast with the NiAl intermetallic coating, the depth of the scratches is decreased and no long cracks are observed. This wear behavior is suggested and attributed to the high fracture toughness of the nanocomposite coating rather than to the brittle NiAl intermetallic coating. Similar results were published elsewhere [9]. Fig. 8b is a representative wear track profile of the nanocomposite coating as measured by profilometry. The surface is rather smooth, and the depth of the wear track is about 2 μm. The debris generated was loose and powdery having a sub-micrometer size, as shown in Fig. 8c. The average specific wear rate for NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating was computed at about 0.78 ± 0.33 × 10−15 m3/N⋅m, much less than that for NiAl intermetallic coating. Consequently, the changes in the wear resistance between NiAl intermetallic and NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coatings were attributed to the changes in the susceptibility to crack propagation by adding Al2O3–13% TiO2 nanoparticles as the reinforcing materials. 4. Conclusions To summarize the approach, the following conclusions can be drawn from the above discussion: 1. The addition of Al2O3–13% TiO2 nanoparticles as the reinforcing materials significantly improved the quality of the HVOF coatings and the overall coating microstructure was found to be homogeneous and uniform with little closed porosity as well as fewer microcracks along the lamella boundaries. 2. The microhardness increased by adding Al2O3–13% TiO2 nanoparticles as the reinforcing materials. The high hardness values obtained for the nanocomposite coating are attributed to the Hall–Petch strengthening effect as the nanoparticles of less than 50 nm distributed evenly throughout the NiAl matrix. 3. Fracture toughness value evidently indicates that the NiAl–15 wt% TiO2) nanocomposite coating was tougher (Al2O3–13% (7.12 MPa m1/2) than the NiAl intermetallic coating (4.28 MPa m1/2) and that the brittle mechanism dominates in the NiAl intermetallic coating. 4. In contrast with the NiAl intermetallic coating, the depth of the scratches was decreased, and no long cracks were observed in the NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite coating. This wear behavior is attributed to the high microhardness together with the high fracture toughness for nanocomposite coating. The average specific wear rate for the nanocomposite coating is much less than that for NiAl intermetallic coating attributed to the change in the susceptibility to crack propagation by adding Al2O3–13% TiO2 nanoparticles serving as reinforcing materials. Acknowledgment The author would like to acknowledge the Iran National Science Foundation: INSF for funding this work under grant no. 90002285. References
Fig. 8. (a) SEM micrographs of the wear track, (b) surface profile of worn surface and (c) wear debris of the NiAl–15 wt% (Al2O3–13% TiO2) nanocomposite HVOF coating.
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