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Chromium carbide–CNT nanocomposites with enhanced mechanical properties
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Acta Materialia 57 (2009) 335–344 www.elsevier.com/locate/actamat
Chromium carbide–CNT nanocomposites with enhanced mechanical properties Virendra Singh a, Rene Diaz a, Kantesh Balani b, Arvind Agarwal b, Sudipta Seal a,* a Plasma Nanomanufacturing Facility, Advanced Materials Processing and Analysis Center, Mechanical Materials Aerospace Engineering, Nanoscience and Technology Center, Engineering Building, Room #381, P.O. Box 162455, University of Central Florida, Orlando, FL 32816, USA b Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, USA
Received 3 September 2008; accepted 7 September 2008 Available online 22 October 2008
Abstract Chromium carbide is widely used as a tribological coating material in high-temperature applications requiring high wear resistance and hardness. Herein, an attempt has been made to further enhance the mechanical and wear properties of chromium carbide coatings by reinforcing carbon nanotubes (CNTs) as a potential replacement of soft binder matrix using plasma spraying. The microstructures of the sprayed CNT-reinforced Cr3C2 coatings were characterized using transmission electron microscopy and scanning electron microscopy. The mechanical properties were assessed using micro-Vickers hardness, nanoindentation and wear measurements. CNT reinforcement improved the hardness of the coating by 40% and decreased the wear rate of the coating by almost 45–50%. Cr3C2 reinforced with 2 wt.% CNT had an elastic modulus 304.5 ± 29.2 GPa, hardness of 1175 ± 60 VH0.300 and a coefficient of friction of 0.654. It was concluded that the CNT reinforcement increased the wear resistance by forming intersplat bridges while the improvement in the hardness was attributed to the deformation resistance of CNTs under indentation. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Plasma spraying; Nanoindentation; Hardness test; Coatings; Wear
1. Introduction Carbides are high-strength materials. Excellent mechanical properties and durability of carbides have made them a material of choice in applications involving extreme temperature and pressure, e.g., rocket nozzles, drill bits, cutting tools, golf shoe spikes and snow tires [1]. Primarily refractory carbides of the transition metals of the fourth to sixth groups of the periodic table, including the carbides of tungsten, titanium, tantalum and chromium, are used in these demanding applications [2]. In particular, chromium carbides (Cr3C2) demonstrate excellent strength, hardness, anti-erosion and corrosion properties, permanent nonmagnetizability and surface illustriousness [2]. As a result, chromium carbide-type materials have been widely used *
Corresponding author. Tel.: +1 407 882 1119; fax: +1 407 882 1156. E-mail address: [email protected] (S. Seal).
in a variety of industrial applications, such as shaft bearings, seals, high-temperature furnaces, nozzles and metal machining molds [3]. It has the potential to be an environmentally friendly replacement for conventional electrochemical hard chrome plating [4]. Despite chromium carbide’s excellent wear- and corrosion-resistant properties, it is not used as a primary carbide in industry owing to its lower hardness in comparison to other carbides (such as tungsten carbide). Several attempts have been made to improve the hardness and wear property of chromium carbide coatings by varying the coating techniques, processing parameters and characteristics of the feedstock powder [5–8]. Popular techniques, such as detonation gun (D-gun) and high-velocity oxy-fuel (HVOF), use Ni–Cr as a binder and are primarily low-temperature, high-deposition velocity processes. These processes can increase the density (up to 98%) of the coating and improve the wear, adhesion and mechanical properties moderately [8–10].
1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.09.023
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Cr3C2–20–25% NiCr cermets are used for wear-resistant coatings, where the NiCr acts as a soft binder metal matrix for the hard chromium carbide particles. The binder phase often dissolves carbide phase during deposition, recrystallizes and oxidizes upon exposure to the high-temperature applications, subsequently lowering the hardness and wear resistance of the coating [11,12]. However, the mechanical properties of chromium carbide can be improved by reinforcement with a second phase, which can further improve the fracture toughness by crack bridging. One possible material that can be used for this is carbon nanotubes (CNTs). As stated in the literature, CNTs have remarkable mechanical, electrical and thermal properties [13–18], and can be a potential replacement of soft metal binders in carbide coatings. Depending on their length and diameter, chirality and orientations, CNTs exhibit almost five times the elastic modulus (1 TPa) and nearly 100 times the tensile strength (150 GPa) of high-strength steels [18–21]. Multi-walled CNTs have already shown successful inclusion in and enhancement of the mechanical properties of plasma-sprayed Al–Si nanocomposites [22], alumina (Al2O3) [23], magnesium nanocomposites [24] and plasma-sprayed hydroxyapatite ceramic coatings, where fracture toughness is enhanced by 50–60% [25]. It has been demonstrated that CNT-reinforced nanocomposites exhibit three ways of toughening: crack deflection at the CNT/matrix interface; crack bridging by CNTs; and CNT pullout on the fracture surfaces [26]. In the present study CNT-reinforced chromium carbide coating was prepared via the air plasma spray (APS) process. Chromium carbide feedstock was blended with 1–2 wt.% CNTs as a potential replacement for soft metal binder. These composite coatings are characterized in terms of their microstructures and mechanical properties. Transmission electron microscopy (TEM) studies confirm the retention of CNTs in the coating. Effects of CNT reinforcement on wear and the coefficient of friction (COF) are correlated to the enhanced hardness and elastic modulus of the CNT-reinforced nanocomposite. 2. Experimental details 2.1. Powder processing Pre-processed plasma spray grade 99.8% irregular chromium carbide powder (particle size 45–100 lm) was used for composite coating (from AAE, Bergenfield, NJ). Multi-walled CNTs (MWCNTs; outer diameter = 10– 15 nm, inner diameter = 2–6 nm, length = 0.1–10 lm, >90% purity) were obtained from Sigma–Aldrich. Cr3C2 powder was blended with the CNTs at varying weight percentages through jar mill for 7 days to obtain a homogeneous mix and break up any agglomerates of the CNTs. Three powder batches were used for experimentation: chromium carbide (Cr3C2) as control, Cr3C2–1 wt.% CNT and Cr3C2–2 wt.% CNT. Powder morphology before and after mixing with CNTs is shown in Fig. 1.
Fig. 1. SEM image of as-received Cr3C2 powder, showing its irregular shape and surface porosities (inset). The SEM image of blended Cr3C2 and CNTs powder shows CNTs inside the pores of Cr3C2 particles.
2.2. Plasma spraying Air plasma spraying (APS) of Cr3C2 with and without CNTs was carried out using a Praxair SG 100 gun. The spray processing parameters are listed in Table 1. Powders were injected internally in the plasma spray gun (SG100) using argon as a carrier gas to deposit coatings on a steel substrate. Hydrogen gas was used as a secondary gas to increase the enthalpy of the plasma plume. Optimum plasma parameters were obtained by varying the powder flow rate, carrier gas flow, primary and secondary gas flow rates, standoff distance and power input to increase the density of the coating. Prior to coating, steel substrates were grit-blasted with alumina particles (40 grit size) in order to increase the roughness of the surface to allow better adhesion of splats to form a dense coating. 2.3. Structural and microstructural characterization A comprehensive microstructural evaluation of the plasma spray-formed Cr3C2–CNT was carried out using scanning electron microscopy (SEM; JEOL 6400F) on the surface and cross-section of the coating. The porosity of the cross-section of the coatings was calculated using image analysis software (IQmaterials 2.0 Software). X-ray diffraction (XRD; Rigaku D-Max B diffractometer) patterns were recorded from both powders and the coated surfaces of chromium carbide and Cr3C2–CNT composites. The X-ray diffractometer was set at 40 kV and 30 mA with ˚ ). The diffraction pata Cu Ka radiation target (k 1.54 A terns were recorded at a speed of 1° min1. In order to investigate the retention of CNTs in the coating and nanocrystalline grain formation, a high-resolution transmission electron microscope (Tecnai F30) was used with a field emission gun (FEG) operating at a voltage of 300 kV. A TEM sample of the composite coating was prepared by conventional mechanical grinding of the coating to a thickness of 50 lm. Final thinning to electron transparency was
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Table 1 Optimized parameters of air plasma spray of the Cr3C2–CNTs coating Current (A)
Voltage (V)
Power (kW)
Primary gas (slm)
Secondary gas (slm)
Stand-off distance (mm)
Carrier gas (slm)
700
40
28
Ar (47)
H2 (3.8)
100
Ar (3.8)
accomplished by ion milling using a 6 kV Ar ion beam incident at 10°.
3. Results and discussion 3.1. CNT interaction with carbide phase
2.4. Microhardness and nanoindentation testing Microhardness on the transverse section of the plasmasprayed coatings was tested using a Shimadzu HMV-2000 Microhardness Tester at a load of 300 g load with a dwell time of 10 s. All the samples were grounded and subsequently polished down to 0.5 lm roughness using diamond paste before hardness measurements. An average of 15–20 indentations were performed on each sample cross-section and values are reported along with error bars. Distorted indents indicating entrapped porosity and the intersplat region were discarded from the measurements. Indents were made on the porosity-free areas. Mechanical properties of coatings were evaluated using nanoindentation. The load vs. depth curves can be analyzed to determine the elastic moduli and hardness. Nanoindentation tests were carried out using a HysitronÒ Triboindenter (Minneapolis, MN, USA) with a 100 nm radius Berkovich pyramidal tip. The load cycle involved loading for 10 s to reach the peak load of 2500 lN, followed by a 2 s dwell at the peak load and subsequent unloading for 10 s. Peak load was adjusted to keep the segment time constant for loading, dwell time and unloading. 2.5. Wear testing The wear and friction performance of the coating was evaluated at room temperature using pin-on-disc wear tester at CSM Instruments (MA, USA). Samples to be tested were cleaned ultrasonically with isopropanol, dried and weighed before testing. Tribo Tests were conducted according to the parameters given in Table 2 at 10 N load for varying sliding distances. Upon completion of testing the topography, the size of the resulting wear track and debris morphology were analyzed by SEM.
The particle size and morphology of the blended and unblended powders are shown in Fig. 1. To ensure the homogeneous mixing and dispersion of CNT inside the surface pores of the Cr3C2 particles (see Fig. 1), the CNT–Cr3C2 mix was blended at 50 rpm using steel balls for 7 days. Blending was conducted below 50 rpm as high speed blending (>50 rpm) fractures the brittle Cr3C2 particles and hinders the flow of the mixed powder in the plasma plume. Morsi and Esawi [27] have also reported the fragmentation and welding of CNT–aluminum milled powder, dependent on the speed and milling time. Blended powder was injected into the plasma jet (flow rate 7.5 g m1, with 3.8 slm carrier gas flow) to obtain high-density coatings. Semi-molten particles containing CNT impact the substrate at high velocity, forming splats (Fig. 2). The implanted CNT in the coating increases the hardness and stiffness of the coating, as explained in a later section. The deposition parameters were optimized to obtain a high-density Cr3C2 coating by varying the plasma current, carrier gas and primary gas flow rate. Splat morphology, surface cracks and porosity (10–15 vol.%) are shown in Fig. 2. Back-scattered SEM images of a polished transverse section and the axial surface of the coating are shown in Fig. 3a and b, representing closed spherical pores and the intersplat boundary. The porosity distribution in the axial (Perpendicular to the spray direction, Z, X–Y plane) and transverse (the spray direction, Y–Z plane) directions are
Table 2 Details of wear test conditions for coating Static counterpart Load (N) Radius (mm) Speed (cm s1) Atmosphere Temperature Lubrication Humidity
100Cr6 (6 mm diam) 10 5.44 15 Air 25 °C No 40%
Fig. 2. SEM image of as-sprayed Cr3C2 plasma-sprayed coating surface revealing the surface cracks (due to higher cooling), partially melted particles (the irregular shape changes to near spherical; points A and B) and flat splats.
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Fig. 3. Back-scattered SEM images of chromium carbide coating. (a) Cross-section, showing the lamellar structure of the chromium carbide deposit. Curved black lines show the interlamellar boundary, black spots show the porosity generated during deposition. (b) SEM image of the polished coating surface viewed from the top, showing fine cracks and the closed porosities revealed after removal of the surface irregularities.
shown in Fig. 4a and b, respectively. A higher concentration of high-aspect-ratio porosity was found in the transverse direction. This indicates that the entrapped interlamellar pores were formed due to the incomplete filling of the pores by successive molten layers along the axial direction and the intersplat boundary region. The measured porosity of the coating was 10–15 vol.%. This is attributed to the partial melting and decarburization of the chromium carbide particles in the high-temperature zone of the plasma. Furthermore, the present study does not use the binder metal. In general, 20–35% of NiCr is used as a binder phase in coatings of chromium carbide, which increases the density and toughness of the coating [9,28]. However, in the present study we have improved the hardness (by 40%) and abrasive wear properties as compared to that of unreinforced Cr3C2 coating using plasma spray by incorporating only 2 wt.% CNTs. This confirms that the high strength of MWCNTs can play a vital role in augmenting the mechanical properties of the coating without the addition of a high percentage of binder phases.
Fig. 4. Porosity distribution with aspect ratio in the coating (a) axial direction, i.e., perpendicular to spray direction, and (b) transverse direction, i.e., the spray direction, showing a higher aspect ratio because of the interlamellar boundary. The distribution of porosity is represented on a 180° spread with respect to aspect ratio.
3.1.1. Morphological evaluation: transmission electron microscopy (TEM) The high-resolution transmission electron microscopy (HRTEM) image of the Cr3C2–2 wt.% CNT coating (Fig. 5) and the SEM image of the fractured surface (Fig. 6) corroborate the presence of undamaged CNT in the Cr3C2 matrix after spraying. Fig. 5 depicts the undamaged CNT and splat bonding where the CNT acts as a bridge between the splats. The bridging between the splats restricts any intersplat sliding movement and improves the toughness of the coating [29]. The excellent coverage of the chromium carbide matrix with CNT enhances the load transfer properties from the Cr3C2 splat to the CNT. The HRTEM image shown in Fig. 7 shows the CNT–chromium carbide interface with nanograins of carbide matrix attached to the CNT. The adjoining chromium carbide ˚ (inset C), which nanograins have lattice spacing of 2.24 A corresponds to the interplaner spacing of the plane (2 0 3) of Cr3C2 [9,10]. The presence of nanograins is attributed to the extremely high cooling rate (106–108 K s1) during the solidification of the molten carbide. Points A and B in the HRTEM image (Fig. 7) show the fused layer of the CNT and the carbide matrix. Absence of the lattice
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Fig. 5. HRTEM image of a Cr3C2–CNT composite. (a) Undamaged CNTs anchoring the carbide matrix. (b) The CNT core.
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Fig. 7. HRTEM image of a Cr3C2–CNTs composite showing a CNT surrounded by the carbide grains. Points A and B show the CNT–matrix interface. Inset C shows an inverse fast Fourier transformed (FFT) image of the adjoining matrix corresponding to the carbide with lattice spacing ˚ (plane 2 0 3). 2.24 A
3.2. Phase analysis
Fig. 6. SEM image of the wear debris of the Cr3C2–2 wt.% CNT coating, depicting ultrafine carbide particles adhering to the CNTs. The inset image shows a pull out of a CNT from the matrix, with one end being coated with carbide matrix.
fringes at the interface of the CNT and the matrix confirms an amorphous interface (1.2 nm thick). The presence of an amorphous layer can accommodate imperfect surface termination and inhibit further dislocation movement [23]. Such an interface helps in the stress transfer from the carbide matrix to the CNTs when the coating is subjected to the shear stress during sliding wear. This interface can also incorporate the frictional and viscoplastic resistance and lower the interfacial stress [23]. The SAED pattern of the carbide matrix (Fig. 7 inset) confirms the polycrystalline nature of the carbide matrix. The large number of spotty diffraction rings suggests the presence of additional carbide phases of chromium. Further presence of Cr7C3 was confirmed by X-ray diffraction (XRD) studies.
XRD was performed to determine whether the composition of chromium carbide altered from the starting feedstock powder. Comparison of the diffraction patterns of the coating with the feedstock Cr3C2 powder showed a decrease in the relative intensity of the chromium carbide due to the presence of other compounds of chromium in the coating (Fig. 8). The presence of additional carbides is attributed to the decarburization of Cr3C2. Cr3C2 can react with oxygen (Eq. (1)) to produce Cr7C3 and CO upon exposure to high-temperature plasma and ambient air during flight. Thermodynamic calculations also confirm the decarburization of Cr3C2 near the melting point. It was shown that the free energy of formation of Cr7C3 (214.86 kJ mol1) is lower than that of Cr3C2 (111.95 kJ mol1) [30,31], which is consistent with our calculations predicting the formation of Cr3C7 at 2083 K. 14 6 Cr3 C2 þ O2 ! Cr7 C3 þ 2CO DG 2083K 5 5 ¼ 535:17 kJ mol1
ð1Þ
The microsecond exposure of the particles to the higher plasma temperature only allows the surface decarburization of particle and formation of chromium carbides with lower carbon content. Other causes of loss of carbon include rebounding off the large unmelted carbide particles in the binder (NiCr) matrix during coating deposition [9,32]. However, this mechanism is not prevalent in our case as the present study is confined to the Cr3C2 deposition without binder. A shift in the peak along with the peak broadening was observed in the XRD spectrum of the
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Fig. 8. XRD patterns of Cr3C2 feedstock powder and Cr3C2 coating with and without CNTs. The XRD confirms the formation of decarburized phases, typically Cr7C3, Cr23C6 and a small amount of Cr2O3.
coatings. This indicates the residual stress and the presence of nanocrystalline grains in the plasma-sprayed coatings, which undergo rapid cooling (106–107 K s1) at the substrate. The diffraction peaks of additional carbides of chromium, such as Cr23C6, Cr7C3, coincide with the diffraction peaks of Cr3C2 coating (with and without CNTs), resulting in an increase in the intensity between the 2h from 42° to 46°. In contrast, the diffraction pattern of chromium carbide powder shows sharp peaks at (2h 39.1, JCPDS # 892723), showing the purity of the initial feed stock. Wellner et al. [33] also reported the transformation of Cr3C2 into various other stable compounds of chromium, such as Cr7C3, Cr23C6, Cr2O3, CrC and Cr0.62C0.35N0.35, in the coating, and found that it is more prominent when hydrogen is used as a secondary gas in the plasma. In the present study Cr7C3 and Cr23C6 peaks were found, with a small Cr2O3 peak, as shown in Fig. 8. These additional phases in the coating are harder than Cr3C2 and do not exhibit any detrimental effect on the wear resistance of the coating [34].
ment upon the application of load; (ii) strengthening due to secondary phase dispersion, which prevents localized crack growth during indentation [35]. Micro-Vickers hardness data (at 300 g force) is shown in Fig. 9 for chromium carbide coatings with and without CNTs. A 40% increase in the hardness was observed (1175 ± 60 HV) as compared to the bare Cr3C2 coating. It is interesting to note that the hardness values for Cr3C2–2 wt.% CNT coating is 15% higher than the reported literature values (HV 543–952) [8–10,36,37]. The quality and hardness of the plasmasprayed coating is affected by various parameters, including the characteristic of the initial feedstock material used. The chromium carbide coating in the present study showed 10–15% porosity irrespective of the CNT addition. This may be due to the high-temperature decarburization of the carbide. Though the D-gun and HVOF coatings are well known for high-density carbide coating and depict higher hardness values without CNT addition [8], our group have attempted to augment mechanical properties with plasma spray techniques using CNTs. We previously demonstrated enhancement of the hardness and fracture toughness of Al–CNT plasma spray composite coating, where 10 wt.% CNT addition improved the hardness value by 60% [22]. 3.4. Nanomechanical properties of the coatings The nanomechanical properties of CNT-reinforced carbide coating were measured using nanoindentation. An Oliver and Pharr [38] analysis was performed to determine the elastic modulus and hardness from the unloading elastic segment of the load–displacement curve, as shown in Fig. 10. The elastic modulus and hardness values obtained from the nanoindentation tests are shown in Table 3. The nanoindentation experiment also confirmed the increase
3.3. Effect of CNT reinforcement on hardness During the hardness evaluation of plasma spray coatings, the material under the indenter experiences compaction and facilitates the release of weak links. These weak links could be splat boundaries, the matrix-reinforcement interface, plastically deformed matrix or closed porosity. In this study, CNTs reinforce the matrix, which can increase the hardness in two ways (i) the CNTs can hold two splats by a bridging mechanism [23,25], as shown in the TEM image in Fig. 5, and minimize intersplat move-
Fig. 9. Comparison of microhardness values of Cr3C2 coating with and without CNTs at 300 g load. Cr3C2–2 wt.% CNTs shows higher hardness values compared to the available literature values of Cr3C2–NiCr coating deposited by different processes.
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Fig. 10. Load vs. indenter displacement curves from experiments on the mechanically polished surface of Cr3C2 and Cr3C2–CNT composite. The curves show higher indenter displacement for the Cr3C2 coating than for the CNT-reinforced Cr3C2 coating at the same value of maximum load.
Table 3 Nanoindentation results of Cr3C2 with and without CNTs Coating material
E (GPa)
Hardness (GPa)
Cr3C2 matrix Cr3C2 + 1% CNT Cr3C2 + 2% CNT
245.7 ± 27.1 274.9 ± 27.9 304.5 ± 29.2
9.9 ± 1.2 10.0 ± 1.6 12.9 ± 1.3
in the hardness with 2 wt.% CNT reinforcement in the coating. Nanoindentation hardness values are higher when compared to microhardness values because deformation is highly localized to low indentation depths, whereas higher indentation depths in microindentation tests induce pores and cracks. Fig. 10 shows that there is a higher maximum displacement of the indenter with the Cr3C2 coating (170 nm) when compared to with the CNT-reinforced Cr3C2 coatings (115 nm) at maximum load. The reduction in maximum displacement with CNT reinforcement indicates further resistance to surface damage of Cr3C2– CNTs coatings upon loading by a Berkovich tip. The limited indentation depth translates to enhanced hardness of the plasma-sprayed Cr3C2–CNT coatings. Fig. 11 shows scanning probe microscopy images of the indents in Cr3C2, Cr3C2–1 wt.% CNT coating and Cr3C2– 2 wt.% CNT coating. The indent (Fig. 11a) on the Cr3C2 coating surface shows a negligible pile-up region, which is attributed to the lower degree of plastic deformation and elastic recovery. The low modulus (245.7 GPa) and hardness (9.9 GPa) associated with the Cr3C2 coating therefore provide limited resistance to wear of the coating. The higher hardness obtained for the CNT-reinforced coatings is attributed to the lower contact depth (100 nm) when compared to that of Cr3C2 without CNT addition (155 nm). The pile-up observed in the Cr3C2–1 wt.% CNT indicates the higher degree of plastic deformation (thereby work hardening) of the material. The inverse proportionality of the wear loss to the elastic modulus (and marginally to hardness) is expected to reduce the material loss of the Cr3C2–1 wt.% CNT coating (E = 274.9 GPa
Fig. 11. Nanoindentation on (a) Cr3C2 matrix, (b) Cr3C2–1 wt.% CNT and (c) Cr3C2–2 wt.% CNT. This shows the negligible pile-up for Cr3C2, revealing elastic recovery, whereas the enhanced pile-up for the CNTreinforced coatings indicate a high degree of plastic deformation.
and H = 10 GPa) in comparison to the Cr3C2 coating without CNT reinforcement. The enhanced pile-up observed for the 2 wt.% CNT-reinforced Cr3C2 coating (Fig. 11c) demonstrates strong accommodation of plasticity with enhanced modulus (304.5 GPa) arising from the excellent modulus of CNTs. The higher elastic modulus of the 2 wt.% CNT-reinforced composite coating indicates that the CNTs are implanted into the carbide matrix and are distributed inside the coating. Further enhancement in the hardness (12.9 GPa) is expected to render enhanced wear resistance to the 2 wt.% CNT-reinforced Cr3C2 coating. 3.5. Wear and friction behavior of the composite coatings The sliding wear and friction behavior of plasmasprayed CNT-reinforced composite coatings is evaluated
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using the pin-on-disc method. Fig. 12 shows the effect of CNTs reinforcement on the coefficient of friction of the Cr3C2 coating. Tests were conducted for different sliding distances (900–1500 m) to measure the effect of the CNT reinforcement on the COF, especially for CNT-reinforced composites. The 2 wt.% CNT incorporation enhanced the wear resistance of the coating and the average COF to
l 0.65. It is interesting to note the rapid initial rise in the COF followed by a marginal fall in CNT-reinforced coatings. The initial rise is clearly necessary to overcome the starting friction forces of the rough surface of the coating. It is also interesting to note that the 2 wt.% CNT coating might have a higher degree of adhesive forces, which increases the resistance to pin movement and increasing
Fig. 12. Effect of the weight fraction of CNT on the COF of Cr3C2, showing first an increase then a marginal decrease in the COF with addition of CNTs (top). SEM of the worn surfaces of (a) Cr3C2, (b) Cr3C2–1 wt.% CNT and (c) Cr3C2–2 wt.% CNT coating. (d) High-magnification image of the worn out surface of Cr3C2–2 wt.% CNT showing the protrusion of CNTs.
V. Singh et al. / Acta Materialia 57 (2009) 335–344 Table 4 Summary of the wear test results of Cr3C2 coating with and without CNTs Coating material
HV300
Wear rate (mg km1)
Average FOC
Cr3C2 Cr3C2–1% CNTs Cr3C2–2% CNTs
797 ± 45 1045 ± 60 1175 ± 60
6.7 5.5 3.6
0.50 0.48 0.65
friction as compared to the bare chromium carbide coating. The SEM micrograph in Fig. 6 shows that the wear debris comprises CNT and carbide particles. In the initial stage of the test, the CNTs interlocks the splat by bridging or anchoring [25], and increases the COF by protruding out from the coating surface. In the later stages (i.e., at longer sliding distance) it becomes a three-body abrasion case instead of two-body sliding, with the release of CNTs and carbide debris, as shown in Fig. 6 inset. Reduction in the COF is due to the reduction in the surface asperities with wear, leading to a smooth surface. The CNTs act as a lubricant and reduce the COF for the CNT-reinforced composite during the later stages. This is in good agreement with the literature, where CNTs provide a lubricating condition during the abrasion between the coating and the selected counterpart [39,40]. Table 4 shows the average wear rate of CNT-reinforced Cr3C2 coating in mg km1. The wear rate decreases with increasing CNT fraction in the coating. This is attributed to the fact that CNT reinforcement anchors the carbides splat and improves the wear resistance of the coating. Fig. 12(a–d) shows the SEM of the worn out surfaces of plasma-sprayed Cr3C2 with and without CNTs. The wear mechanism of the plasma-sprayed hard carbide coating is governed by abrasion, fracture, ploughing and chipping or delamination of the splats [28]. All worn out surfaces show the characteristics of typical abrasive wear. A smooth wear track of the 2 wt.% CNT was observed, which indicates improved wear resistance and good lubrication provided by CNTs. The obtained wear data are in good agreement with the hardness values, which show an increase in wear resistance with increasing hardness. It is worth noting that the wear and friction behavior of the plasma-sprayed coating is significantly affected by its microstructural constituents, such as splats, porosity and second-phase dispersion. In addition to the microstructure, the wear test conditions, such as the load applied, sliding velocity, sliding distance and temperature, also significantly affect the COF and wear. In the present scenario, a very small weight-percentage addition of CNTs (as a second phase) improved the wear resistance of the chromium carbide coating significantly, with a decrease in COF (0.65) at longer sliding distances. 4. Conclusion We have demonstrated that plasma spraying is a viable process to fabricate hard, wear-resistant CNT–Cr3C2 coatings. Plasma-sprayed Cr3C2 with 2 wt.% CNTs showed 40% enhancement in microhardness values. The enhancement of hardness is due to the minimized intersplat
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movement upon indentation, with CNTs anchoring adjacent splats and also acting as a second-phase dispersion to prevent the growth of localized cracks. A decrease in the wear rate (by 45%) was observed with 2 wt.% CNT reinforcement, which is attributed to intersplat anchoring and bridging by the CNTs. A decrease in the depth of the nanoindentation of CNT-reinforced coatings clearly indicates the improvement in hardness. The higher pileup for CNT-reinforced Cr3C2 coatings suggest strong accommodation of plasticity with enhanced elastic modulus (25% rise in Cr3C2–2 wt.% CNT over Cr3C2 alone) arising from the excellent modulus of the CNTs. Acknowledgements We would like to thank the CSM instruments for wear testing of the coatings. We acknowledge the financial support obtained from NSF-REU EEC: 0453436 Program at University of central Florida. Part of the interfacial engineering work was funded by NSF DMII: 0500268 and the Office of Naval Research Young Investigator Award program ONR: N000140210591. References [1] Oyama ST. The chemistry of transition metal carbides and nitrides. Springer; 1996. [2] Schwarzkopf P, Kieffer R, Leszynski W, Benesovsky F. Refractory hard metals: borides, carbides, nitrides, and silicides. New York; 1953. [3] Fu CT, Li AK, Lai CP, Duann JR. High performance ceramic composites containing tungsten carbide reinforced chromium carbide matrix. US Patent No. 5580833; 1994. [4] Wang D-Y, Weng K-W, Chang C-L, Ho W-Y. Surf Coat Technol 1999;120–121:622. [5] Wang J, Sun B, Guo Q, Nishio M, Ogawa H. J Therm Spray Technol 2002;11:261. [6] Murthy JKN, Venkataraman B. Surf Coat Technol 2006;200:2642. [7] Wolfe DE, Eden TJ, Potter JK, Jaroh AP. J Therm Spray Technol 2006;15:400. [8] Factor M, Roman I. J Therm Spray Technol 2002;11:468. [9] Ji G-C, Li C-J, Wang Y-Y, Li W-Y. Surf Coat Technol 2006;200:6749. [10] Murthy JKN, Bysakh S, Gopinath K, Venkataraman B. Surf Coat Technol 2007;202:1. [11] Lee CH, Min KO. Surf Coat Technol 2000;132:49. [12] Matthews S, Hyland M, James B. Acta Mater 2003;51:4267. [13] Mamalis AG, Vogtlander LOG, Markopoulos A. Precis Eng 2004;28:16. [14] Baxendale M. J Mater Sci Mater Electron 2003;14:657. [15] Thostenson ET, Ren Z, Chou T-W. Compos Sci Technol 2001;61:1899. [16] Lau AK-T, Hui D. Compos B Eng 2002;33:263. [17] Mauron P, Emmenegger C, Zuttel A, Nutzenadel C, Sudan P, Schlapbach L. Carbon 2002;40:1339. [18] Zhang XF, Zhang XB, Van Tendeloo G, Amelinckx S, Op de Beeck M, Van Landuyt J. J Cryst Growth 1993;130:368. [19] Pipes RB, Hubert P. Compos Sci Technol 2002;62:419. [20] Salvetat-Delmotte J-P, Rubio A. Carbon 2002;40:1729. [21] Saether E, Frankland SJV, Pipes RB. Compos Sci Technol 2003;63:1543. [22] Laha T, Agarwal A, McKechnie T, Seal S. Mater Sci Eng A 2004;381:249.
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[23] Balani K, Zhang T, Karakoti A, Li WZ, Seal S, Agarwal A. Acta Mater 2008;56:571. [24] Cha SI, Kim KT, Lee KH, Mo CB, Hong SH. Scripta Mater 2005;53:793. [25] Balani K, Anderson R, Laha T, Andara M, Tercero J, Crumpler E, et al. Biomaterials 2007;28:618. [26] Xia Z, Riester L, Curtin WA, Li H, Sheldon BW, Liang J, et al. Acta Mater 2004;52:931. [27] Morsi K, Esawi A. J Mater Sci 2007;42:4954. [28] Factor M, Roman I. J Therm Spray Technol 2002;11:482. [29] Fan J-P, Zhuang D-M, Zhao D-Q, Gong Zhang, Wu M-S. Appl Phys Lett 2006;89:121910. [30] Teng L, Lu K, Aune R, Seetharaman S. Metall Mater Trans A 2004;35:3649.
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Acta Materialia 57 (2009) 335–344 www.elsevier.com/locate/actamat
Chromium carbide–CNT nanocomposites with enhanced mechanical properties Virendra Singh a, Rene Diaz a, Kantesh Balani b, Arvind Agarwal b, Sudipta Seal a,* a Plasma Nanomanufacturing Facility, Advanced Materials Processing and Analysis Center, Mechanical Materials Aerospace Engineering, Nanoscience and Technology Center, Engineering Building, Room #381, P.O. Box 162455, University of Central Florida, Orlando, FL 32816, USA b Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, USA
Received 3 September 2008; accepted 7 September 2008 Available online 22 October 2008
Abstract Chromium carbide is widely used as a tribological coating material in high-temperature applications requiring high wear resistance and hardness. Herein, an attempt has been made to further enhance the mechanical and wear properties of chromium carbide coatings by reinforcing carbon nanotubes (CNTs) as a potential replacement of soft binder matrix using plasma spraying. The microstructures of the sprayed CNT-reinforced Cr3C2 coatings were characterized using transmission electron microscopy and scanning electron microscopy. The mechanical properties were assessed using micro-Vickers hardness, nanoindentation and wear measurements. CNT reinforcement improved the hardness of the coating by 40% and decreased the wear rate of the coating by almost 45–50%. Cr3C2 reinforced with 2 wt.% CNT had an elastic modulus 304.5 ± 29.2 GPa, hardness of 1175 ± 60 VH0.300 and a coefficient of friction of 0.654. It was concluded that the CNT reinforcement increased the wear resistance by forming intersplat bridges while the improvement in the hardness was attributed to the deformation resistance of CNTs under indentation. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Plasma spraying; Nanoindentation; Hardness test; Coatings; Wear
1. Introduction Carbides are high-strength materials. Excellent mechanical properties and durability of carbides have made them a material of choice in applications involving extreme temperature and pressure, e.g., rocket nozzles, drill bits, cutting tools, golf shoe spikes and snow tires [1]. Primarily refractory carbides of the transition metals of the fourth to sixth groups of the periodic table, including the carbides of tungsten, titanium, tantalum and chromium, are used in these demanding applications [2]. In particular, chromium carbides (Cr3C2) demonstrate excellent strength, hardness, anti-erosion and corrosion properties, permanent nonmagnetizability and surface illustriousness [2]. As a result, chromium carbide-type materials have been widely used *
Corresponding author. Tel.: +1 407 882 1119; fax: +1 407 882 1156. E-mail address: [email protected] (S. Seal).
in a variety of industrial applications, such as shaft bearings, seals, high-temperature furnaces, nozzles and metal machining molds [3]. It has the potential to be an environmentally friendly replacement for conventional electrochemical hard chrome plating [4]. Despite chromium carbide’s excellent wear- and corrosion-resistant properties, it is not used as a primary carbide in industry owing to its lower hardness in comparison to other carbides (such as tungsten carbide). Several attempts have been made to improve the hardness and wear property of chromium carbide coatings by varying the coating techniques, processing parameters and characteristics of the feedstock powder [5–8]. Popular techniques, such as detonation gun (D-gun) and high-velocity oxy-fuel (HVOF), use Ni–Cr as a binder and are primarily low-temperature, high-deposition velocity processes. These processes can increase the density (up to 98%) of the coating and improve the wear, adhesion and mechanical properties moderately [8–10].
1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.09.023
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Cr3C2–20–25% NiCr cermets are used for wear-resistant coatings, where the NiCr acts as a soft binder metal matrix for the hard chromium carbide particles. The binder phase often dissolves carbide phase during deposition, recrystallizes and oxidizes upon exposure to the high-temperature applications, subsequently lowering the hardness and wear resistance of the coating [11,12]. However, the mechanical properties of chromium carbide can be improved by reinforcement with a second phase, which can further improve the fracture toughness by crack bridging. One possible material that can be used for this is carbon nanotubes (CNTs). As stated in the literature, CNTs have remarkable mechanical, electrical and thermal properties [13–18], and can be a potential replacement of soft metal binders in carbide coatings. Depending on their length and diameter, chirality and orientations, CNTs exhibit almost five times the elastic modulus (1 TPa) and nearly 100 times the tensile strength (150 GPa) of high-strength steels [18–21]. Multi-walled CNTs have already shown successful inclusion in and enhancement of the mechanical properties of plasma-sprayed Al–Si nanocomposites [22], alumina (Al2O3) [23], magnesium nanocomposites [24] and plasma-sprayed hydroxyapatite ceramic coatings, where fracture toughness is enhanced by 50–60% [25]. It has been demonstrated that CNT-reinforced nanocomposites exhibit three ways of toughening: crack deflection at the CNT/matrix interface; crack bridging by CNTs; and CNT pullout on the fracture surfaces [26]. In the present study CNT-reinforced chromium carbide coating was prepared via the air plasma spray (APS) process. Chromium carbide feedstock was blended with 1–2 wt.% CNTs as a potential replacement for soft metal binder. These composite coatings are characterized in terms of their microstructures and mechanical properties. Transmission electron microscopy (TEM) studies confirm the retention of CNTs in the coating. Effects of CNT reinforcement on wear and the coefficient of friction (COF) are correlated to the enhanced hardness and elastic modulus of the CNT-reinforced nanocomposite. 2. Experimental details 2.1. Powder processing Pre-processed plasma spray grade 99.8% irregular chromium carbide powder (particle size 45–100 lm) was used for composite coating (from AAE, Bergenfield, NJ). Multi-walled CNTs (MWCNTs; outer diameter = 10– 15 nm, inner diameter = 2–6 nm, length = 0.1–10 lm, >90% purity) were obtained from Sigma–Aldrich. Cr3C2 powder was blended with the CNTs at varying weight percentages through jar mill for 7 days to obtain a homogeneous mix and break up any agglomerates of the CNTs. Three powder batches were used for experimentation: chromium carbide (Cr3C2) as control, Cr3C2–1 wt.% CNT and Cr3C2–2 wt.% CNT. Powder morphology before and after mixing with CNTs is shown in Fig. 1.
Fig. 1. SEM image of as-received Cr3C2 powder, showing its irregular shape and surface porosities (inset). The SEM image of blended Cr3C2 and CNTs powder shows CNTs inside the pores of Cr3C2 particles.
2.2. Plasma spraying Air plasma spraying (APS) of Cr3C2 with and without CNTs was carried out using a Praxair SG 100 gun. The spray processing parameters are listed in Table 1. Powders were injected internally in the plasma spray gun (SG100) using argon as a carrier gas to deposit coatings on a steel substrate. Hydrogen gas was used as a secondary gas to increase the enthalpy of the plasma plume. Optimum plasma parameters were obtained by varying the powder flow rate, carrier gas flow, primary and secondary gas flow rates, standoff distance and power input to increase the density of the coating. Prior to coating, steel substrates were grit-blasted with alumina particles (40 grit size) in order to increase the roughness of the surface to allow better adhesion of splats to form a dense coating. 2.3. Structural and microstructural characterization A comprehensive microstructural evaluation of the plasma spray-formed Cr3C2–CNT was carried out using scanning electron microscopy (SEM; JEOL 6400F) on the surface and cross-section of the coating. The porosity of the cross-section of the coatings was calculated using image analysis software (IQmaterials 2.0 Software). X-ray diffraction (XRD; Rigaku D-Max B diffractometer) patterns were recorded from both powders and the coated surfaces of chromium carbide and Cr3C2–CNT composites. The X-ray diffractometer was set at 40 kV and 30 mA with ˚ ). The diffraction pata Cu Ka radiation target (k 1.54 A terns were recorded at a speed of 1° min1. In order to investigate the retention of CNTs in the coating and nanocrystalline grain formation, a high-resolution transmission electron microscope (Tecnai F30) was used with a field emission gun (FEG) operating at a voltage of 300 kV. A TEM sample of the composite coating was prepared by conventional mechanical grinding of the coating to a thickness of 50 lm. Final thinning to electron transparency was
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Table 1 Optimized parameters of air plasma spray of the Cr3C2–CNTs coating Current (A)
Voltage (V)
Power (kW)
Primary gas (slm)
Secondary gas (slm)
Stand-off distance (mm)
Carrier gas (slm)
700
40
28
Ar (47)
H2 (3.8)
100
Ar (3.8)
accomplished by ion milling using a 6 kV Ar ion beam incident at 10°.
3. Results and discussion 3.1. CNT interaction with carbide phase
2.4. Microhardness and nanoindentation testing Microhardness on the transverse section of the plasmasprayed coatings was tested using a Shimadzu HMV-2000 Microhardness Tester at a load of 300 g load with a dwell time of 10 s. All the samples were grounded and subsequently polished down to 0.5 lm roughness using diamond paste before hardness measurements. An average of 15–20 indentations were performed on each sample cross-section and values are reported along with error bars. Distorted indents indicating entrapped porosity and the intersplat region were discarded from the measurements. Indents were made on the porosity-free areas. Mechanical properties of coatings were evaluated using nanoindentation. The load vs. depth curves can be analyzed to determine the elastic moduli and hardness. Nanoindentation tests were carried out using a HysitronÒ Triboindenter (Minneapolis, MN, USA) with a 100 nm radius Berkovich pyramidal tip. The load cycle involved loading for 10 s to reach the peak load of 2500 lN, followed by a 2 s dwell at the peak load and subsequent unloading for 10 s. Peak load was adjusted to keep the segment time constant for loading, dwell time and unloading. 2.5. Wear testing The wear and friction performance of the coating was evaluated at room temperature using pin-on-disc wear tester at CSM Instruments (MA, USA). Samples to be tested were cleaned ultrasonically with isopropanol, dried and weighed before testing. Tribo Tests were conducted according to the parameters given in Table 2 at 10 N load for varying sliding distances. Upon completion of testing the topography, the size of the resulting wear track and debris morphology were analyzed by SEM.
The particle size and morphology of the blended and unblended powders are shown in Fig. 1. To ensure the homogeneous mixing and dispersion of CNT inside the surface pores of the Cr3C2 particles (see Fig. 1), the CNT–Cr3C2 mix was blended at 50 rpm using steel balls for 7 days. Blending was conducted below 50 rpm as high speed blending (>50 rpm) fractures the brittle Cr3C2 particles and hinders the flow of the mixed powder in the plasma plume. Morsi and Esawi [27] have also reported the fragmentation and welding of CNT–aluminum milled powder, dependent on the speed and milling time. Blended powder was injected into the plasma jet (flow rate 7.5 g m1, with 3.8 slm carrier gas flow) to obtain high-density coatings. Semi-molten particles containing CNT impact the substrate at high velocity, forming splats (Fig. 2). The implanted CNT in the coating increases the hardness and stiffness of the coating, as explained in a later section. The deposition parameters were optimized to obtain a high-density Cr3C2 coating by varying the plasma current, carrier gas and primary gas flow rate. Splat morphology, surface cracks and porosity (10–15 vol.%) are shown in Fig. 2. Back-scattered SEM images of a polished transverse section and the axial surface of the coating are shown in Fig. 3a and b, representing closed spherical pores and the intersplat boundary. The porosity distribution in the axial (Perpendicular to the spray direction, Z, X–Y plane) and transverse (the spray direction, Y–Z plane) directions are
Table 2 Details of wear test conditions for coating Static counterpart Load (N) Radius (mm) Speed (cm s1) Atmosphere Temperature Lubrication Humidity
100Cr6 (6 mm diam) 10 5.44 15 Air 25 °C No 40%
Fig. 2. SEM image of as-sprayed Cr3C2 plasma-sprayed coating surface revealing the surface cracks (due to higher cooling), partially melted particles (the irregular shape changes to near spherical; points A and B) and flat splats.
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Fig. 3. Back-scattered SEM images of chromium carbide coating. (a) Cross-section, showing the lamellar structure of the chromium carbide deposit. Curved black lines show the interlamellar boundary, black spots show the porosity generated during deposition. (b) SEM image of the polished coating surface viewed from the top, showing fine cracks and the closed porosities revealed after removal of the surface irregularities.
shown in Fig. 4a and b, respectively. A higher concentration of high-aspect-ratio porosity was found in the transverse direction. This indicates that the entrapped interlamellar pores were formed due to the incomplete filling of the pores by successive molten layers along the axial direction and the intersplat boundary region. The measured porosity of the coating was 10–15 vol.%. This is attributed to the partial melting and decarburization of the chromium carbide particles in the high-temperature zone of the plasma. Furthermore, the present study does not use the binder metal. In general, 20–35% of NiCr is used as a binder phase in coatings of chromium carbide, which increases the density and toughness of the coating [9,28]. However, in the present study we have improved the hardness (by 40%) and abrasive wear properties as compared to that of unreinforced Cr3C2 coating using plasma spray by incorporating only 2 wt.% CNTs. This confirms that the high strength of MWCNTs can play a vital role in augmenting the mechanical properties of the coating without the addition of a high percentage of binder phases.
Fig. 4. Porosity distribution with aspect ratio in the coating (a) axial direction, i.e., perpendicular to spray direction, and (b) transverse direction, i.e., the spray direction, showing a higher aspect ratio because of the interlamellar boundary. The distribution of porosity is represented on a 180° spread with respect to aspect ratio.
3.1.1. Morphological evaluation: transmission electron microscopy (TEM) The high-resolution transmission electron microscopy (HRTEM) image of the Cr3C2–2 wt.% CNT coating (Fig. 5) and the SEM image of the fractured surface (Fig. 6) corroborate the presence of undamaged CNT in the Cr3C2 matrix after spraying. Fig. 5 depicts the undamaged CNT and splat bonding where the CNT acts as a bridge between the splats. The bridging between the splats restricts any intersplat sliding movement and improves the toughness of the coating [29]. The excellent coverage of the chromium carbide matrix with CNT enhances the load transfer properties from the Cr3C2 splat to the CNT. The HRTEM image shown in Fig. 7 shows the CNT–chromium carbide interface with nanograins of carbide matrix attached to the CNT. The adjoining chromium carbide ˚ (inset C), which nanograins have lattice spacing of 2.24 A corresponds to the interplaner spacing of the plane (2 0 3) of Cr3C2 [9,10]. The presence of nanograins is attributed to the extremely high cooling rate (106–108 K s1) during the solidification of the molten carbide. Points A and B in the HRTEM image (Fig. 7) show the fused layer of the CNT and the carbide matrix. Absence of the lattice
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Fig. 5. HRTEM image of a Cr3C2–CNT composite. (a) Undamaged CNTs anchoring the carbide matrix. (b) The CNT core.
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Fig. 7. HRTEM image of a Cr3C2–CNTs composite showing a CNT surrounded by the carbide grains. Points A and B show the CNT–matrix interface. Inset C shows an inverse fast Fourier transformed (FFT) image of the adjoining matrix corresponding to the carbide with lattice spacing ˚ (plane 2 0 3). 2.24 A
3.2. Phase analysis
Fig. 6. SEM image of the wear debris of the Cr3C2–2 wt.% CNT coating, depicting ultrafine carbide particles adhering to the CNTs. The inset image shows a pull out of a CNT from the matrix, with one end being coated with carbide matrix.
fringes at the interface of the CNT and the matrix confirms an amorphous interface (1.2 nm thick). The presence of an amorphous layer can accommodate imperfect surface termination and inhibit further dislocation movement [23]. Such an interface helps in the stress transfer from the carbide matrix to the CNTs when the coating is subjected to the shear stress during sliding wear. This interface can also incorporate the frictional and viscoplastic resistance and lower the interfacial stress [23]. The SAED pattern of the carbide matrix (Fig. 7 inset) confirms the polycrystalline nature of the carbide matrix. The large number of spotty diffraction rings suggests the presence of additional carbide phases of chromium. Further presence of Cr7C3 was confirmed by X-ray diffraction (XRD) studies.
XRD was performed to determine whether the composition of chromium carbide altered from the starting feedstock powder. Comparison of the diffraction patterns of the coating with the feedstock Cr3C2 powder showed a decrease in the relative intensity of the chromium carbide due to the presence of other compounds of chromium in the coating (Fig. 8). The presence of additional carbides is attributed to the decarburization of Cr3C2. Cr3C2 can react with oxygen (Eq. (1)) to produce Cr7C3 and CO upon exposure to high-temperature plasma and ambient air during flight. Thermodynamic calculations also confirm the decarburization of Cr3C2 near the melting point. It was shown that the free energy of formation of Cr7C3 (214.86 kJ mol1) is lower than that of Cr3C2 (111.95 kJ mol1) [30,31], which is consistent with our calculations predicting the formation of Cr3C7 at 2083 K. 14 6 Cr3 C2 þ O2 ! Cr7 C3 þ 2CO DG 2083K 5 5 ¼ 535:17 kJ mol1
ð1Þ
The microsecond exposure of the particles to the higher plasma temperature only allows the surface decarburization of particle and formation of chromium carbides with lower carbon content. Other causes of loss of carbon include rebounding off the large unmelted carbide particles in the binder (NiCr) matrix during coating deposition [9,32]. However, this mechanism is not prevalent in our case as the present study is confined to the Cr3C2 deposition without binder. A shift in the peak along with the peak broadening was observed in the XRD spectrum of the
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Fig. 8. XRD patterns of Cr3C2 feedstock powder and Cr3C2 coating with and without CNTs. The XRD confirms the formation of decarburized phases, typically Cr7C3, Cr23C6 and a small amount of Cr2O3.
coatings. This indicates the residual stress and the presence of nanocrystalline grains in the plasma-sprayed coatings, which undergo rapid cooling (106–107 K s1) at the substrate. The diffraction peaks of additional carbides of chromium, such as Cr23C6, Cr7C3, coincide with the diffraction peaks of Cr3C2 coating (with and without CNTs), resulting in an increase in the intensity between the 2h from 42° to 46°. In contrast, the diffraction pattern of chromium carbide powder shows sharp peaks at (2h 39.1, JCPDS # 892723), showing the purity of the initial feed stock. Wellner et al. [33] also reported the transformation of Cr3C2 into various other stable compounds of chromium, such as Cr7C3, Cr23C6, Cr2O3, CrC and Cr0.62C0.35N0.35, in the coating, and found that it is more prominent when hydrogen is used as a secondary gas in the plasma. In the present study Cr7C3 and Cr23C6 peaks were found, with a small Cr2O3 peak, as shown in Fig. 8. These additional phases in the coating are harder than Cr3C2 and do not exhibit any detrimental effect on the wear resistance of the coating [34].
ment upon the application of load; (ii) strengthening due to secondary phase dispersion, which prevents localized crack growth during indentation [35]. Micro-Vickers hardness data (at 300 g force) is shown in Fig. 9 for chromium carbide coatings with and without CNTs. A 40% increase in the hardness was observed (1175 ± 60 HV) as compared to the bare Cr3C2 coating. It is interesting to note that the hardness values for Cr3C2–2 wt.% CNT coating is 15% higher than the reported literature values (HV 543–952) [8–10,36,37]. The quality and hardness of the plasmasprayed coating is affected by various parameters, including the characteristic of the initial feedstock material used. The chromium carbide coating in the present study showed 10–15% porosity irrespective of the CNT addition. This may be due to the high-temperature decarburization of the carbide. Though the D-gun and HVOF coatings are well known for high-density carbide coating and depict higher hardness values without CNT addition [8], our group have attempted to augment mechanical properties with plasma spray techniques using CNTs. We previously demonstrated enhancement of the hardness and fracture toughness of Al–CNT plasma spray composite coating, where 10 wt.% CNT addition improved the hardness value by 60% [22]. 3.4. Nanomechanical properties of the coatings The nanomechanical properties of CNT-reinforced carbide coating were measured using nanoindentation. An Oliver and Pharr [38] analysis was performed to determine the elastic modulus and hardness from the unloading elastic segment of the load–displacement curve, as shown in Fig. 10. The elastic modulus and hardness values obtained from the nanoindentation tests are shown in Table 3. The nanoindentation experiment also confirmed the increase
3.3. Effect of CNT reinforcement on hardness During the hardness evaluation of plasma spray coatings, the material under the indenter experiences compaction and facilitates the release of weak links. These weak links could be splat boundaries, the matrix-reinforcement interface, plastically deformed matrix or closed porosity. In this study, CNTs reinforce the matrix, which can increase the hardness in two ways (i) the CNTs can hold two splats by a bridging mechanism [23,25], as shown in the TEM image in Fig. 5, and minimize intersplat move-
Fig. 9. Comparison of microhardness values of Cr3C2 coating with and without CNTs at 300 g load. Cr3C2–2 wt.% CNTs shows higher hardness values compared to the available literature values of Cr3C2–NiCr coating deposited by different processes.
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Fig. 10. Load vs. indenter displacement curves from experiments on the mechanically polished surface of Cr3C2 and Cr3C2–CNT composite. The curves show higher indenter displacement for the Cr3C2 coating than for the CNT-reinforced Cr3C2 coating at the same value of maximum load.
Table 3 Nanoindentation results of Cr3C2 with and without CNTs Coating material
E (GPa)
Hardness (GPa)
Cr3C2 matrix Cr3C2 + 1% CNT Cr3C2 + 2% CNT
245.7 ± 27.1 274.9 ± 27.9 304.5 ± 29.2
9.9 ± 1.2 10.0 ± 1.6 12.9 ± 1.3
in the hardness with 2 wt.% CNT reinforcement in the coating. Nanoindentation hardness values are higher when compared to microhardness values because deformation is highly localized to low indentation depths, whereas higher indentation depths in microindentation tests induce pores and cracks. Fig. 10 shows that there is a higher maximum displacement of the indenter with the Cr3C2 coating (170 nm) when compared to with the CNT-reinforced Cr3C2 coatings (115 nm) at maximum load. The reduction in maximum displacement with CNT reinforcement indicates further resistance to surface damage of Cr3C2– CNTs coatings upon loading by a Berkovich tip. The limited indentation depth translates to enhanced hardness of the plasma-sprayed Cr3C2–CNT coatings. Fig. 11 shows scanning probe microscopy images of the indents in Cr3C2, Cr3C2–1 wt.% CNT coating and Cr3C2– 2 wt.% CNT coating. The indent (Fig. 11a) on the Cr3C2 coating surface shows a negligible pile-up region, which is attributed to the lower degree of plastic deformation and elastic recovery. The low modulus (245.7 GPa) and hardness (9.9 GPa) associated with the Cr3C2 coating therefore provide limited resistance to wear of the coating. The higher hardness obtained for the CNT-reinforced coatings is attributed to the lower contact depth (100 nm) when compared to that of Cr3C2 without CNT addition (155 nm). The pile-up observed in the Cr3C2–1 wt.% CNT indicates the higher degree of plastic deformation (thereby work hardening) of the material. The inverse proportionality of the wear loss to the elastic modulus (and marginally to hardness) is expected to reduce the material loss of the Cr3C2–1 wt.% CNT coating (E = 274.9 GPa
Fig. 11. Nanoindentation on (a) Cr3C2 matrix, (b) Cr3C2–1 wt.% CNT and (c) Cr3C2–2 wt.% CNT. This shows the negligible pile-up for Cr3C2, revealing elastic recovery, whereas the enhanced pile-up for the CNTreinforced coatings indicate a high degree of plastic deformation.
and H = 10 GPa) in comparison to the Cr3C2 coating without CNT reinforcement. The enhanced pile-up observed for the 2 wt.% CNT-reinforced Cr3C2 coating (Fig. 11c) demonstrates strong accommodation of plasticity with enhanced modulus (304.5 GPa) arising from the excellent modulus of CNTs. The higher elastic modulus of the 2 wt.% CNT-reinforced composite coating indicates that the CNTs are implanted into the carbide matrix and are distributed inside the coating. Further enhancement in the hardness (12.9 GPa) is expected to render enhanced wear resistance to the 2 wt.% CNT-reinforced Cr3C2 coating. 3.5. Wear and friction behavior of the composite coatings The sliding wear and friction behavior of plasmasprayed CNT-reinforced composite coatings is evaluated
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using the pin-on-disc method. Fig. 12 shows the effect of CNTs reinforcement on the coefficient of friction of the Cr3C2 coating. Tests were conducted for different sliding distances (900–1500 m) to measure the effect of the CNT reinforcement on the COF, especially for CNT-reinforced composites. The 2 wt.% CNT incorporation enhanced the wear resistance of the coating and the average COF to
l 0.65. It is interesting to note the rapid initial rise in the COF followed by a marginal fall in CNT-reinforced coatings. The initial rise is clearly necessary to overcome the starting friction forces of the rough surface of the coating. It is also interesting to note that the 2 wt.% CNT coating might have a higher degree of adhesive forces, which increases the resistance to pin movement and increasing
Fig. 12. Effect of the weight fraction of CNT on the COF of Cr3C2, showing first an increase then a marginal decrease in the COF with addition of CNTs (top). SEM of the worn surfaces of (a) Cr3C2, (b) Cr3C2–1 wt.% CNT and (c) Cr3C2–2 wt.% CNT coating. (d) High-magnification image of the worn out surface of Cr3C2–2 wt.% CNT showing the protrusion of CNTs.
V. Singh et al. / Acta Materialia 57 (2009) 335–344 Table 4 Summary of the wear test results of Cr3C2 coating with and without CNTs Coating material
HV300
Wear rate (mg km1)
Average FOC
Cr3C2 Cr3C2–1% CNTs Cr3C2–2% CNTs
797 ± 45 1045 ± 60 1175 ± 60
6.7 5.5 3.6
0.50 0.48 0.65
friction as compared to the bare chromium carbide coating. The SEM micrograph in Fig. 6 shows that the wear debris comprises CNT and carbide particles. In the initial stage of the test, the CNTs interlocks the splat by bridging or anchoring [25], and increases the COF by protruding out from the coating surface. In the later stages (i.e., at longer sliding distance) it becomes a three-body abrasion case instead of two-body sliding, with the release of CNTs and carbide debris, as shown in Fig. 6 inset. Reduction in the COF is due to the reduction in the surface asperities with wear, leading to a smooth surface. The CNTs act as a lubricant and reduce the COF for the CNT-reinforced composite during the later stages. This is in good agreement with the literature, where CNTs provide a lubricating condition during the abrasion between the coating and the selected counterpart [39,40]. Table 4 shows the average wear rate of CNT-reinforced Cr3C2 coating in mg km1. The wear rate decreases with increasing CNT fraction in the coating. This is attributed to the fact that CNT reinforcement anchors the carbides splat and improves the wear resistance of the coating. Fig. 12(a–d) shows the SEM of the worn out surfaces of plasma-sprayed Cr3C2 with and without CNTs. The wear mechanism of the plasma-sprayed hard carbide coating is governed by abrasion, fracture, ploughing and chipping or delamination of the splats [28]. All worn out surfaces show the characteristics of typical abrasive wear. A smooth wear track of the 2 wt.% CNT was observed, which indicates improved wear resistance and good lubrication provided by CNTs. The obtained wear data are in good agreement with the hardness values, which show an increase in wear resistance with increasing hardness. It is worth noting that the wear and friction behavior of the plasma-sprayed coating is significantly affected by its microstructural constituents, such as splats, porosity and second-phase dispersion. In addition to the microstructure, the wear test conditions, such as the load applied, sliding velocity, sliding distance and temperature, also significantly affect the COF and wear. In the present scenario, a very small weight-percentage addition of CNTs (as a second phase) improved the wear resistance of the chromium carbide coating significantly, with a decrease in COF (0.65) at longer sliding distances. 4. Conclusion We have demonstrated that plasma spraying is a viable process to fabricate hard, wear-resistant CNT–Cr3C2 coatings. Plasma-sprayed Cr3C2 with 2 wt.% CNTs showed 40% enhancement in microhardness values. The enhancement of hardness is due to the minimized intersplat
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movement upon indentation, with CNTs anchoring adjacent splats and also acting as a second-phase dispersion to prevent the growth of localized cracks. A decrease in the wear rate (by 45%) was observed with 2 wt.% CNT reinforcement, which is attributed to intersplat anchoring and bridging by the CNTs. A decrease in the depth of the nanoindentation of CNT-reinforced coatings clearly indicates the improvement in hardness. The higher pileup for CNT-reinforced Cr3C2 coatings suggest strong accommodation of plasticity with enhanced elastic modulus (25% rise in Cr3C2–2 wt.% CNT over Cr3C2 alone) arising from the excellent modulus of the CNTs. Acknowledgements We would like to thank the CSM instruments for wear testing of the coatings. We acknowledge the financial support obtained from NSF-REU EEC: 0453436 Program at University of central Florida. Part of the interfacial engineering work was funded by NSF DMII: 0500268 and the Office of Naval Research Young Investigator Award program ONR: N000140210591. References [1] Oyama ST. The chemistry of transition metal carbides and nitrides. Springer; 1996. [2] Schwarzkopf P, Kieffer R, Leszynski W, Benesovsky F. Refractory hard metals: borides, carbides, nitrides, and silicides. New York; 1953. [3] Fu CT, Li AK, Lai CP, Duann JR. High performance ceramic composites containing tungsten carbide reinforced chromium carbide matrix. US Patent No. 5580833; 1994. [4] Wang D-Y, Weng K-W, Chang C-L, Ho W-Y. Surf Coat Technol 1999;120–121:622. [5] Wang J, Sun B, Guo Q, Nishio M, Ogawa H. J Therm Spray Technol 2002;11:261. [6] Murthy JKN, Venkataraman B. Surf Coat Technol 2006;200:2642. [7] Wolfe DE, Eden TJ, Potter JK, Jaroh AP. J Therm Spray Technol 2006;15:400. [8] Factor M, Roman I. J Therm Spray Technol 2002;11:468. [9] Ji G-C, Li C-J, Wang Y-Y, Li W-Y. Surf Coat Technol 2006;200:6749. [10] Murthy JKN, Bysakh S, Gopinath K, Venkataraman B. Surf Coat Technol 2007;202:1. [11] Lee CH, Min KO. Surf Coat Technol 2000;132:49. [12] Matthews S, Hyland M, James B. Acta Mater 2003;51:4267. [13] Mamalis AG, Vogtlander LOG, Markopoulos A. Precis Eng 2004;28:16. [14] Baxendale M. J Mater Sci Mater Electron 2003;14:657. [15] Thostenson ET, Ren Z, Chou T-W. Compos Sci Technol 2001;61:1899. [16] Lau AK-T, Hui D. Compos B Eng 2002;33:263. [17] Mauron P, Emmenegger C, Zuttel A, Nutzenadel C, Sudan P, Schlapbach L. Carbon 2002;40:1339. [18] Zhang XF, Zhang XB, Van Tendeloo G, Amelinckx S, Op de Beeck M, Van Landuyt J. J Cryst Growth 1993;130:368. [19] Pipes RB, Hubert P. Compos Sci Technol 2002;62:419. [20] Salvetat-Delmotte J-P, Rubio A. Carbon 2002;40:1729. [21] Saether E, Frankland SJV, Pipes RB. Compos Sci Technol 2003;63:1543. [22] Laha T, Agarwal A, McKechnie T, Seal S. Mater Sci Eng A 2004;381:249.
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