Damage-Tolerant, Hard Nanocomposite Coatings Enabled by a

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Damage-Tolerant, Hard Nanocomposite Coatings Enabled by a Hierarchical Structure Jiang Xu,*,†,‡ XiangZhen Mao,† Zong-Han. Xie,‡ and Paul Munroe§ †

Department of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, People's Republic of China ‡ School of Engineering, Edith Cowan University, Joondalup, WA6027, Australia § School of Materials Science and Engineering, University of New South Wales, NSW 2052, Australia ABSTRACT: A novel hierarchical structured coating was prepared on a pure titanium substrate by a double cathode glow discharge technique. This coating consisted of a 1 μm thick C40-structured MoSi2 outer layer with an average grain size of 5 nm and an underlying MoSi2/Mo5Si3 layer having a gradient, bimodal microstructure, in which C40structured MoSi2, with an average grain size of 5 nm, is embedded within an intergranular region of D8m-structured Mo5Si3, with an average grain size of 40 nm. The ratio of Mo5Si3 to MoSi2 increased gradually away from the outer surface. Compared with a monolithic MoSi2 coating, the hierarchical structured coating exhibited a marked increase in toughness, while maintaining its hardness.

’ INTRODUCTION Molybdenum disilicide (MoSi2) is a potential candidate for high-temperature structural applications, primarily due to its high melting temperature, relatively low density, good oxidation resistance, and metal-like thermal and electrical conductivities.1 Unfortunately, as with many other intermetallic materials, its low toughness at ambient temperature remains a major obstacle to a wide range of practical applications. To address this problem, different toughening strategies have been proposed to improve the toughness of MoSi2. For example, the introduction of a second phase to form a composite microstructure and alloying with other silicides have been applied to enhance the damage resistance of monolithic MoSi2, since MoSi2 is thermodynamically stable with a wide variety of ceramics and high melting point silicide reinforcements.2,3 From the standpoint of fracture mechanics, a reduction in grain size down to nanoscale levels is another potential avenue to toughen these materials.4 Indeed, there has been considerable effort to understand and control the mechanical behavior of nanostructured metals and alloys,5
7 but less so for intermetallic compounds.8 Our previous studies have shown evidence that nanoscrystalline Cr3Si and Mo5Si3, with an average grain size smaller than 10 nm, have superior hardness and toughness compared with their coarse-grained counterparts.9,10 In this paper, we present a novel hierarchical structured coating that consists of a nanostructured MoSi2 outer layer supported by a compositionally graded, bimodal MoSi2/Mo5Si3 nanocomposite layer. This coating exhibits a marked improvement in the room-temperature fracture toughness with a hardness comparable to that of a monolithic MoSi2 coating having an average grain r 2011 American Chemical Society

size of 5 nm. Such a hierarchical microstructure is formed in situ through uphill diffusion of Mo during a glow discharge deposition process.

’ EXPERIMENTAL SECTION The bilayer and monolithic MoSi2 nanocrystalline coatings were engineered onto commericially pure titanium substrates by a double cathode glow discharge technique using two targets with different stoichiometric ratios (i.e., Mo25Si75 and Mo35Si65). Inside the chamber, one cathode is used as the target, and the other as the substrate, as described in our previous papers.9,10 The glow discharge sputtering conditions are as follows: base pressure, 4  10
4 Pa; target electrode bias voltage,
900 V; substrate bias voltage,
300 V; substrate temperature, 800 °C; working pressure, 35 Pa; parallel distance between the source electrode and the substrate, 15 mm; and treatment time of 3 h. The sputtering targets are fabricated from ball-milled Mo (99.99% purity) and Si powders (99.99% purity) by employing cold compacting technology under a pressure of 600 MPa. Substrate discs, 40 mm in diameter and 3 mm in thickness, were cut from a commercially pure titanium rod (0.003% N, 0.010% C, and 0.074% O). Prior to coating deposition, the substrates were ground using SiC grinding papers of 400, 800, 1200 grades and, finally, polished using diamond paste. For the sake of brevity, we Received: May 3, 2011 Revised: August 24, 2011 Published: August 24, 2011 18977

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The Journal of Physical Chemistry C denote the coating prepared using the Mo25Si75 and that using Mo35Si65 targets as MM coating and GC coating, respectively. The phase composition of the as-received coatings was characterized with X-ray diffractometry (XRD, D8ADVANCE) operating at 35 kV and 40 mA. X-ray data were collected using a 0.1° step scan with a count time of 1 s. The etching of the asdeposited coatings was accomplished with the use of Kroll’s reagent (10 mL of HNO3, 4 mL of HF, and 86 mL of distilled water) for 20
30 s. The cross-sectional morphology and chemical composition of the as-deposited coatings were studied by scanning electron microscopy (SEM, Quanta200, FEI Company) with an X-ray energy-dispersive spectroscopy (EDS, EDAX Inc.) analyzer attachment and field emission transmission electron microscopy (FEGTEM, Phlips CM 200, Eindhoven, Netherlands).

Figure 1. XRD patterns obtained for the as-prepared coatings grown on a pure Ti substrate: curve (a), the MM coating; curve (b), the GC coating; and curve (c), X-ray diffraction patterns recorded from 5 μm beneath the top surface of the GC coating.

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Plan-view samples for TEM observation were prepared using a single-jet electrochemical polishing technique from the untreated side of the substrate. The cross-sectional TEM specimens were prepared using a focused ion beam (FIB) microscope (FEI  P200, FEI Company, Hillsboro, OR).The damage tolerance of the as-received coatings was evaluated by a Vickers indenter. The hardness of the as-prepared films was measured by the nanoindentation tester (NHT) equipped with a Berkovich tip. The standard analysis procedure proposed by Oliver and Pharr11 was used to determine the hardness of the specimens from the unloading curve.

’ RESULTS Figure 1 shows the XRD patterns obtained from the asprepared coatings. For the MM coating, diffraction peaks arising from the hexagonal C40-structured MoSi2 phase (JCPDS card No. 81-0167) are observed (curve (a) in Figure 1). Dominance of the C40-structured MoSi2(111) peak at 41.36° suggests that a preferred grain orientation exists in the MM coating. With increasing Mo content in the target (Mo35Si65), additional diffraction peaks related to the D8m-structured Mo5Si3 phase appear (curve (b) in Figure 1), and their intensities of reflections from this phase further increase after removal of the outer 5 μm of the coating by grinding (curve (c) in Figure 1), indicating that the volume fraction of the Mo5Si3 phase is higher away from the outer layer. It is noteworthy, however, that the C40-structured phase is metastable with respect to the C11b-structured MoSi2 phase at temperatures below the congruent melting point. The reasons for the formation of metastable C40-structured MoSi2 phase are two-fold:12,13 (1) the coatings during the glow discharge deposition grow under nonequilibrium conditions, which are similar to other thin film deposition processes, such as PVD, and (2) the activation energy for nucleation of the C40structured MoSi2 phase is lower than that of C11b-structured MoSi2, implying that the nucleation of C11b MoSi2 is necessary to surmount the greater energy barrier. Figure 2 shows the SEM

Figure 2. SEM images of the etched cross section of the GC coating and MM coating. The insets show the concentration profiles of the elements across the entire thickness of two coatings as measured by an EDS line scan. 18978

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Figure 3. (a) Bright-field TEM image showing the overall coating structure of the GC coating. (b) Higher-magnification image of the area adjacent to the interface between the top layer and the intermediate layer. (c) A typical plan-view bright-field TEM image of the topmost layer of the GC coating and the corresponding SAED patterns (lower left inset). (d) HRTEM images of the topmost layer of the GC coating. (e) A plan-view bright-field TEM image taken from the GC coating at 10 μm below the top surface, with corresponding SAED patterns shown in the lower left inset. (f) HRTEM images taken from the GC coating at 10 μm below the top surface.

images of the etched cross sections of the MM coating and the GC coating. Both the as-prepared coatings have uniform thicknesses and fairly dense and homogeneous microstructures and are well bonded to the substrates. The insets in Figure 2a,b display the concentration profiles of the elements from the EDS line scan across the entire thickness of two coatings. For the GC coating, the 1 μm thick top layer contains mostly Mo and Si elements with a Si/Mo atomic ratio of about 2, consistent with being MoSi2. Beneath this layer, the concentration of Mo is gradually increased further away from the surface and reaches a maximum at a distance of 17 μm from the surface, characterized by uphill diffusion up its own concentration gradient. By combining the XRD and EDS analyses, the intermediate layer consists of the mixture of the MoSi2 and Mo5Si3 phases, which develops in situ in the Ti versus MoSi2 diffusion couples, accompanied by a progressive increase in the Mo5Si3 concentration toward the Ti substrate. The phenomenon of uphill Mo diffusion is presumably due to the fact that Ti decreases the chemical activity of Mo to the extent that the Mo activity gradient in the interdiffusion zone is opposite in sign to the Mo concentration gradient. Uphill diffusion of Mo in the diffusion zone was also observed in the Me versus MoSi2 diffusion couples where

Figure 4. Load
displacement curves of the MM coating and the GC coating under a maximum load of 20 mN.

Me = W, Re, Nb, or Ta.14 A transition zone lies beneath the intermediate layer, which is a composite of β-Ti, α00 , and 18979

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Figure 5. Optical micrographs of Vickers indentation in the MM coating (a) and the GC coating (b) at loads ranging from 100 to 1000 g. (c) Overlap of Vickers indentation in the MM coating with a 300 g load. Overlap of Vickers indentation in the GC coating with a 300 (d), 500 (e), and 1000 g (f) load.

α0 layers, as described in our previous paper.9 The gradient distribution of alloying elements in the transition zone offers a smooth transition of mechanical properties, which is suitable to improve the adhesion strength of the films on the commercially pure Ti substrate and to relieve the stress concentration between the films and the substrate when the films are subjected to external stress. Other than the former, the MM coating has a 15 μm thick MoSi2 layer and a mixture layer of MoSi2/Mo5Si3 of negligible thickness. A bright-field cross-sectional transmission electron micrograph of the GC coating (Figure 3a) shows a 1 μm thick outer MoSi2 layer with a typical columnar structure perpendicular to the surface. The distinct nonplanar interface morphologies are visible at the interface between the outer layer and the intermediate layer, which may reflect anisotropy in diffusion (Figure 3b). This interfacial pattern presumably serves to enhance the adhesion strength by maximizing the contact area,

minimizing local stress concentration, and deflecting lateral cracks. Figure 3c shows a1 μm thick outer MoSi2 layer with a typical plan-view bright-field TEM image of the outermost layer of the GC coating (the lower left inset is its corresponding selected area electron diffraction (SAED) pattern). The microstructure of the outer layer consists of nearly rounded grains with an average grain size of ∼5 nm. The high intensity in the MoSi2(111) reflections confirms that the coating has a strong (111) oriented texture. The lattice fringes of 0.218 nm (marked with circles) correspond to a d spacing of the (111) plane of MoSi2 (Figure 3d). A very similar microstructure feature is also observed for the MM coating within the 15 μm thick outer layer on (not shown here). A plan-view bright-field TEM image and high-resolution TEM image taken from the GC coating about 10 μm below the outer surface are shown in Figure 3e,f. A bimodal microstructure composed of equiaxed “coarse” grains 18980

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Figure 6. Schematic diagram of the major toughening mechanisms for the GC coating.

(indicated by blue arrows) and clusters of rounded fine grains (indicated by red arrows) is clearly observed. The “coarse” grains with an average grain size of 40 nm are identified as Mo5Si3 with well-defined high-angle boundaries. The fine grains with an average grain size of 5 nm, which are embedded between the coarse grains, are resolved as MoSi2. Figure 4 shows the representative load
displacement (P
h) curves of the MM coating and the GC coating under a maximum load of 20 mN. The nanoindentation tests indicate that hardness values of the GC coatings (25 GPa) are slightly higher than that of the MM coating (23 GPa). Figure 5 shows optical micrographs of Vickers indentations in the MM and GC coatings. As shown in Figure 5a, b, no cracks are observed to propagate radially from the corners of indentation at loads ranging from 100 to 1000 g. According to the literature, radial cracks emanating from indentations were observed at the loads less than or equal to 1000 g, in both the coarse-grained monolithic MoSi2 and the MoSi2
SiC composite materials.15,16 Cracking was even noted in nanocrystalline MoSi2
SiC composites with the average grain size of 60 nm.2 Thus, both the MM and the GC coatings are superior in terms of damage tolerance as compared to the above-mentioned materials. A qualitative method of damage analysis, in which cracks are induced by the interactions of stress fields from neighboring indents, is applied to evaluate the toughness of the MM and GC coatings. When the intervals between two neighboring indentations (D) are set as about twice the Vickers indentations diagonal length (L), radial cracks are clearly observed in the MM coating under a load of 300 g (Figure 5c), whereas for the GC coating, radial cracks do not occur until the indentation load reaches 1000 g, implying that the bimodal microstructure plays an important role in improving the toughness of the GC coating.

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’ DISCUSSION According to the simulations and experimental observations, when the grain size of conventional materials decreases from the micrometer scale down to the nanometer scale, a shift occurs in the governing deformation mechanism from a dislocation-mediated process in the coarse-grained materials to a grain boundarymediated process in the nanocrystalline materials.17
20 The beneficial effects of grain boundary sliding on the toughness of nanocrystalline intermetallics and ceramics has been observed by other researchers,4,8,21 which is in agreement with the experimental results from the MM coating. However, numerous investigations on single-phase nanocrystalline metals indicated that a grain boundary-mediated process is insufficient, compared to that for a dislocation-controlled process, which results in low toughness at room temperature.22
24 The reduced toughness in nanostructured metals is presumably due to their inability to resist excessively large local strains responsible for plastic flow instabilities, such as necking and shear banding.25,26 Interestingly, nature has provided excellent examples for the design of hard, yet tough, materials. Structural hierarchy is a common feature in materials, such as seashells, tooth enamel, and bone. Toughening processes take place at the multiple length scales, leading to remarkably high toughness in these materials.27 Similarly, in the case of the GC coating, multiscale plastic deformations occur through shear sliding between the fine MoSi2 grains and dislocations motions within the coarse Mo5Si3 grains.28,29 The fine MoSi2 grains are uniformly distributed among the coarse Mo5Si3 grains, which act as obstacles to restrict highly localized shear-banding to isolated small regions, alleviating the propensity for plastic instabilities.30 Moreover, the gradient, bimodal microstructure causes a large strain gradient on the nanometer scale, thus resulting in an improved strain hardening.31,32 The other possible mechanism that may contribute to improved toughness is the generation of residual stresses between the coarse Mo5Si3 grains and the finer MoSi2 grains during cooling from the deposition temperature. This residual stress results from the differences in the elastic moduli and the linear thermal expansion coefficients between the Mo5Si3 grains (Mo5Si3 = 5.2  10
6/K) and MoSi2 grains (9.4  10
6/K). As the GC coating is cooled from the deposition temperature, the MoSi2 grains, having a higher thermal expansion coefficient than that of the matrix, contract more than the Mo5Si3 grains, leading to the compressive stresses around MoSi2 grains. The compressive stresses can either suppress the formation of microcracks or generate crack shielding stress.33 Although we have not measured the residual stress in the asdeposited coatings, a preliminary analysis of the magnitude of the residual stress can be obtained from the preparation method of the coating and XRD data. From the view of sputtering deposition conditions, the as-deposited coatings are prepared at a temperature of 800 °C and, after a treatment time of 3 h, cooled slowly to room temperature. Compared with the other sputtering techniques, the double cathode glow discharge technique uses high deposition temperatures, which help to reduce the residual stresses. Besides, a slow cooling rate avoids the stress between the coatings and the substrate during the cooling stage. We also judge the magnitude of the residual stresses by measuring the deviation of the diffraction peak position between the as-deposited MoSi2 and the standard hexagonal C40-structured MoSi2 given in the JCPDS card file (No. 81-0167). When the XRD results (shown in Figure 1) and data reported on the JCPDS card file are compared, it can be seen that the deviation is negligible, suggesting that the residual stress in the as-deposited coatings is low. A schematic diagram of the major 18981

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The Journal of Physical Chemistry C toughening mechanisms for the GC coating is shown in Figure 6. Note that these bimodal structures documented in the literature were created on the nanosubmicroscale, which sacrifices some of the strengths gained from nanostructuring.32,34,35 By forming nanocomposite coatings having a bimodal microstructure, we obtain a higher toughness compared with that of monolithic nanostructured MoSi2, while maintaining high hardness.

’ CONCLUSION In summary, a novel hierarchical structured coating was successfully prepared on commercially pure titanium substrates by a double cathode glow discharge technique. The coating consisted of a 1 μm thick C40-structured MoSi2 outer layer, supported by a gradient nanocomposite MoSi2/Mo5Si3 layer with a bimodal grain distribution, which is created by uphill diffusion of Mo. As discussed above, the bimodal microstructure can activate multiscale toughening processes, shear sliding among the fine grains and dislocations within the larger grains, thus imparting a much higher toughness than that of the monolithic MoSi2 coating with the average grain size of 5 nm and that of coarse-grained MoSi2 and its composite reported in the literature. By doing so, we have introduced novel strategies to toughen the MoSi2/Mo5Si3-based coatings enabled by its hierarchical structure. The novel MoSi2/Mo5Si3-based coatings, equipped with high hardness and good toughness, have the potential for largescale industrial applications. The materials design principles revealed in this study will also provide guidance for improving the toughness of other brittle materials, such as ceramic and intermetallics. Further research will focus on the relationship of the ratio of MoSi2/Mo5Si3, distributions of the grain size in the MoSi2/Mo5Si3 nanocomposites, and mechanical properties, which would lead to optimization of the microstructure. ’ AUTHOR INFORMATION Corresponding Author

*Phone: 086-02552112626. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors acknowledge the financial support of the National Natural Science Foundation of China under Grant No. 51175245 and the Key Program of Jiangsu Province Natural Science Foundation of China under project No. BK2010073. Z.-H.X. and P.M. also acknowledge the financial support provided by Edith Cowan University (ECU) through the ECUIndustry Collaborative Scheme.

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