Ultrafine Ceramic Grains Embedded in Metallic Glass Matrix

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Applications of Polymer, Composite, and Coating Materials

Ultrafine ceramic grains embedded in metallic glass matrix: Achieving superior wear resistance via both increase in hardness and toughness Lina Yang, Mao Wen, Xuan Dai, Gang Cheng, and Kan Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02338 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

Ultrafine Ceramic Grains Embedded in Metallic Glass Matrix: Achieving Superior Wear Resistance via Both Increase in Hardness and Toughness Lina Yang†, Mao Wen†, Xuan Dai†, Gang Cheng‡, Kan Zhang*, †

†

State Key Laboratory of Superhard Materials, Department of Materials Science and Key

Laboratory of Automobile Materials, MOE, Jilin University, Changchun 130012, People's Republic of China. ‡

Department of Materials, Imperial College London, London, SW7 2AZ, United Kingdom.

ABSTRACT: As structural materials, either crystalline or metallic glass materials have attracted the scientific and practical interests. However, some mechanisms involving critical size and shear bands apply adverse effect on their mechanical properties, respectively. Here, we counter these two effects by introducing a special structure with ultrafine ceramic grains (with a diameter of ~2.0 nm) embedded into metallic glass matrix, wherein the grains are mainly composed of Ta-W-N solid solution structure in nature, surrounded by a W-based amorphous matrix that contains Ta and N atoms. Such structure is in situ formed during preparation, which combines the merits from both phase to achieve simultaneous increase in hardness and toughness relative to references (pure TaN and W), and thus superior wear resistance. Even more remarkable, a favorable variation of increased hardness but reduced elasticity modulus can be induced by this structure. Intrinsically, ultrafine ceramic grains (free of dislocations), embedded in the metallic glass matrix, could prevent shear bands propagation within the glass matrix and further improve the hardness of matrix material. In return, such glass matrix allows for stiffness neutralization and structural relaxation to reduce the elasticity modulus of ceramic grains. This study will offer a new guidance to fabricate ultrahigh performance metal-based composites.

KEYWORDS: ultrafine ceramic grains, metallic glass matrix, hardness/elasticity modulus (H/E), toughness, wear 1

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1. INTRODUCTION For a long time, attaining both enhancement in hardness and toughness is a significant portion in the investigation of structural materials.1-5 Besides, the tribological property in terms of wear resistance is of equal importance to prevent failure so as to extend the service life of workpieces. Unfortunately, high hardness is always accompanied by high brittleness,6 leading to the embrittlement (failure) of material under high load, and finally deteriorating its durability during long-term friction. Such a steep price in the overall performance would significantly limit practical applications. Thereby, addressing above conflicts and developing a material exhibiting comprehensive properties are necessary in either scientific community or industry. Intrinsically, the elasticity modulus, related to the elastic strain to failure, is a key parameter for engineering design.7 According to Hooke’s law,8 if we want to obtain a material that allows for higher elastic deformation prior to a failure at a given load, its elasticity modulus must be reduced. It means that the material with the lowest value of elasticity modulus at a given hardness (constant load), i.e., higher hardness/elasticity modulus ratio, is highly desirable. Just as reported, this ratio is proportional to the resistance to wear and the elastic strain to rupture, and can be used as a suitable parameter enabling to predict tribological9, 10 and anti-crack behavior.11-13 In this sense, the material with higher hardness but lower elasticity modulus is expected to obtain highly hard yet tough wear-resistance materials. This is a simple solution but quite difficult task, because researchers tend to achieve high elasticity modulus following high hardness. Hence, designing a new material system with special structure to solve this question possesses a certain scientific significance. Nanocrystalline materials attract broad attention as a consequence of enhanced mechanical, soft magnetic, optical and electronic properties in comparison with coarse-grained materials according to the size effect.14-16 In which, superior mechanical properties, such as improved hardness and toughness, can be obtained due to small grain size coupled with abundant grain boundaries.17 To get stronger strengthening effect, a straightforward way is grain refinement, just as described by Hall-Petch relation.18, 19 Based on this point, continuous efforts have been made to 2


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develop either metal-based

20-23

or ceramic-based materials.24-26 However, a practical

critical size invalidating this relationship does exist in both metals and ceramics.27, 28 In addition, it is always the case that a high elasticity modulus occurs in suchlike nanocrystalline materials. Apart from the nanocrystalline materials, a type of amorphous materials (i.e., metallic glasses), are also proven to exhibit outstanding mechanical29, 30 based on disordered atomic structure without grain boundaries or dislocations. Besides, they are intrinsically free of crystal-slip mechanism caused by size limiting factor, just as in crystalline materials. However, a highly localized failure event, associated with shear bands, governs their fracture during unconfined loading.31, 32 For avoiding this, efforts to incorporate microstructural stabilization technology to hinder shear bands elongation and encourage their multiplication are necessary. To accomplish it, an effective method is designing metallic glass-based composites. Recently, great progresses in this aspect have been made through in situ formed or extrinsically-added crystalline phases,33-36 and it was suggested that the theory of embedding nanoparticles into metallic glasses matrix is practicable for preventing shear bands-induced failure. Despite these promising findings upon such microstructure, the embedded grains are often limited to metallic nature that is similar to their glasses matrix, but harder ceramic grains are expected to perform better in the improvement of mechanical and anti-wear properties. Besides, how does this special structure affect elasticity modulus remains quite rare. Particularly, the hardness/elasticity modulus ratio within such structure is almost overlooked, although it is of great importance to tailor mechanical properties. In this work, given that WNx compounds are more difficult to form based on much lower heat of formation among common transition metal nitrides,37 we employ Ta-N-W system by magnetron co-sputtering Ta and W targets in a low N2 flow, and succeed in preparing a structure with ultrafine Ta-W-N solid solution grains (with a diameter of only ~2.0 nm) embedded in W-based metallic glasses matrix. This structure is found to be stabilized over a wide composition range (13.9 at.% 0.1) occurs in the coating with special structure, which would represent a good indication of enhanced toughness and wear resistance. In contrast, pure TaN and W coating exhibit much lower H/E* ratio. 3.3.2 Toughness For the measurement of toughness, nanoindentation technology has been widely used. Figure 4 displayed the SEM micrographs of indentation for all samples, it is found that pure TaN and W (Figure 4(a, b)) reference exhibit severe radial cracks, characteristic of brittle failure. The latter of which conforms to the poor fracture toughness of W.63 Instead, a perfect indentation without any radial crack can be found in all TaNW coatings (Figure 4(c-e)). To quantitatively obtain the fracture toughness (Kc), cube-corner indenter64 causing larger stress was employed to develop obvious racial cracks on all samples (Figure S6). Here, a lower Kc value at 1.44 and 1.19 Mpa m1/2, respectively, occurs in pure TaN and W coating, while TaNW coatings exhibits much higher Kc at 2.48 (TaNW1), 2.10 (TaNW2) and 2.13 Mpa m1/2 (TaNW3). That is the introduction of microstructure with ultrafine ceramic grains embedded into the metallic glass matrix could enhance the crack resistance (toughness). Confirming the 13



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decisive role of structure in the coordination of the H/E* ratio, and thus toughness, and this improved property occurs in the case of H/E* ratio larger than 0.1.

Figure 4. The SEM micrographs of the indentations by Berkovitch indenter developed on the pure (a) TaN and (b) W coating, and TaNW coatings with N/A of (c) 0.77, (d) 0.50, (e) 0.31.

In fact, enhanced toughness can be obtained only in the case when the cracks initiation is hindered or at least their propagation is reduced. According to the theory that crack initiation is associated with the grain size, and smaller size can prevent the crack formation by referring following equation:65

1 2

(1)

in which, 2a=crack length, ρ=tip radius, and the σtip/σapplied reflects the ease or complexity of crack formation to some extent. Here, this value can be decreased by significantly refined grain size (~2.0 nm), thus the TaNW coatings should exhibit superior toughness. On the contrary, catastrophic radial cracks occur in the pure TaN and W coating due to much larger grain size (28.4 and 12.1 nm respectively). Beyond that reason, the compact degree for the coating can affect the cracks propagation,22 for instant, the columnar structure (Figure 2(a, b)) facilitates it, while it can be restrained in a denser coatings (Figure 2(c-e)). On the other hand, the substitution of Ta by W atoms (Figure 1(a)) with one more valence electron should change the charge distribution of the TaN, thus benefit to the toughness improvement.3 Therefore, we 14


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contribute the toughness enhancement in the series of TaNW coatings to a synergistic effects of higher H/E* (larger than 0.1), refined grains, denser microstructure and increased valence electron concentration. 3.4 Wear Resistance

Figure 5. The SEM micrographs of wear track for the (a) pure TaN and (b) W coating, and TaNW coatings with N/A of (c) 0.77, (d) 0.50, (e) 0.31 after 3,000 sliding cycles against steel ball.

To further investigate the effect of H/E* ratio regulated by microstructure design on the wear, wear testing was carried out. Firstly, the worn surfaces of all samples are compared, as shown in Figure 5. It is quite straightforward that both references (pure TaN and W coatings) in Figure 5(a, b) possess poor wear resistance due to, respectively, substantial wear products (bright wear debris) adhered to almost entire wear track and worn out surface morphology. Whereas less wear debris appears in the narrower wear track for TaNW coatings (Figure 5(c-e)), in spite of subtle difference in the width of wear track exists among the coatings with various N/A. Such variation in the wear debris must reflect the wear resistance for coatings. To quantitatively evaluate this performance, wear rates for pure TaN and TaNW coatings have been calculated and summarized in Figure 6, meanwhile, their cross-sections of worn profiles are given as inserts. Apparently, the wear volumes for the pure TaN coating (1.0 position of x-axis) is far beyond other coatings, indicating the most severe wear, just as the highest wear rate at 7.18 × 10-8 mm3/Nm shown. In contrast, other TaNW 15



coatings show decreased wear with significantly dropped wear rate. To be specific, this value decreases to 3.83×10-9 mm3/Nm at N/A=0.77, then it reaches the minimum value of 2.56×10-9 mm3/Nm as the N/A decreases to 0.50, subsequently, the wear rate mildly elevates to 4.02×10-9 mm3/Nm at N/A of 0.31, even so, it still much lower than pure TaN coating, let alone pure W. Thus, the wear rate can be decreased by more than one order of magnitude via combining ultrafine Ta-W-N grains with W-based amorphous matrix. In particular, such ultra-low wear behavior is superior to conventional bimetallic nitrides 1, 66, 67 to a large extent.

Figure 6. The average wear rates for the TaNW coatings as a function of N/A, for comparison, the data about pure TaN is also added.

Obviously, the concept of optimizing both H to a sufficiently high value and E to a relatively low level, reaching higher H/E* value (> 0.1), could ensure wear-resistant function effectively. While hardness alone unable dominant the wear-reduction mechanisms, since only moderate improvement in the hardness (from ~24.7 to ~27.8 GPa) from pure TaN coatings to TaNW coatings (Figure 3) can be obtained in this way. According to a general rule, wear can be determined by the interplay of two opposite properties: toughness and hardness. Based on above discussion, it is suggested that the highly hard yet tough character for TaNW coatings highlights the outstanding wear resistance, this is because in such combination, high hardness can prevent the coating surface from being scratched and worn at extreme external pressures, that is enhance the durability. While the superior toughness allows for 16


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greater deformation without embrittlement, thus plays a same role as hardness. Meanwhile, high toughness is often accompanied by high elastic recovery (Figure S7),68 which could make the deformation spring back into its original state as much as possible when removing external stress. Combining with both contributions, the coating under a certain load must exhibit desired wear resistance. Therefore, the excellent mechanical properties that determined by special microstructure dominant wear-reduction mechanism, on the contrary, similar friction products (Figure S8) indicate this behavior isn’t driven by surface chemistry.69

4. CONCLUSION Here, we succeed in preparing a structure with ultrafine Ta-W-N grains (at a diameter of ~2.0 nm) embedded into W-based amorphous (metallic glass) matrix by reactive magnetron co-sputtering Ta and W targets, and found that it can be stabilized over a large composition range (13.9 at.%

Ultrafine Ceramic Grains Embedded in Metallic Glass Matrix

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