Nanocrystalline LowDimensional Matrix Nanocomposites

nc-W3C/nc-Carbynes It was demonstrated [45, 680] that tungsten carbide coatings can be synthesized by low-temperature chemical vapor deposition in a hot-wall reactor using the precursors of WF6, C3H8, H2, and Ar as the carrier gas. The process is carried out at atmospheric pressure and within a temperature range of 400-900 °C. The results of such experiments showed [45, 680] that the deposition of tungsten carbide phases depends on the concentration of propane in the gas mixture as well as catalytic pretreatment of the propane.

Equally with the conventional tungsten carbide phases of WC with a simple hexagonal lattice (a = 0.2907 nm and c = 0.2837 nm) and ^2C with a hexagonal close-packed lattice (a = 0.30008 nm and c = 0.47357 nm), a novel tungsten carbide phase (~0.8 wt% C) was synthesized [45, 680]. On the basis of chemical analysis and X-ray diffraction, the structure of the chemically deposited novel tungsten carbide was identified [680] as a cubic structure of A15 type with a lattice parameter a0 = 0.5041 nm. Later, the novel tungsten carbide phase was classified as the W3C phase [21, 175]. The A15 lattice has a cubic-type structure with a space group of Pm3n(#223) [681]. Among tungsten compounds, only three phases with the A15 lattice [i.e., W3O(8-W), W3Si, and W3Re] were known. Because this novel tungsten carbide phase has an unusual lattice of the A15 type, which is typical of superconductors [681], the critical temperature transition to superconductivity was defined experimentally as being equal to 2.55 K [680], which is different from 2.74 K for W2C and 1.28 K for WC. This novel tungsten carbide phase decomposes to tungsten and W2C carbide above 1000 °C [680].

This novel W3C phase differs from the very well established WC and W2C in terms of properties and is characterized by a very high hardness of approximately 35 GPa [680]. The deposited W3C coatings have been identified by X-ray diffraction as the single tungsten carbide phase [680].

Selected-area electron diffraction (SAED) indicated the presence of additional diffraction rings [682, 683], which were not fixed by X-ray diffraction. Comparison with the very well known carbon structures [684, 685], and carbides WC and W2C [299, 686, 687], showed that the extra electron diffraction rings belong to a- and S-carbynes and their mixture—chaoite [22]. Carbines are little known carbon compounds [22, 23, 685], that is, a- and S-carbynes, chaoite, with sp1 covalent bonds. The sp1 hybridized carbon atoms settle down along a straight line, provide symmetry, and form linear chainlike arrangements of carbon atoms.

The lamellar structure of the W3C deposits was found [21, 175, 682, 683, 688, 689] to be bound up with the codeposition of carbynes. The coatings represent a composite of the two phases: one being the W3C phase and the other the a/S-carbynes. This provided a hint on the existence of very fine grain sizes in the deposits. Calculations from extended X-ray peaks using the Scherrer formula [690] showed that the average grain size was 70 nm. Furthermore, direct TEM investigations using the dark and bright fields of the TEM images confirmed (Fig. 15) that (1) the composite deposits have a nanocrystalline structure and (2) they are compounds of nanoscale size of both nc-W3C phase and nc-carbynes. The W3C grains measuring in the range of 50100 nm, in turn, formed microclusters of size 2-5 nm embedded within a very fine (up to 1 nm) carbyne matrix. Carbines, which were present in the deposits throughout the volume of coatings, usually had size measuring between 2 and 5 nm.

Figure 15. Typical transmission electron microscopy images of the nc-W3C/nc-carbyne nanocomposite: (a) bright-field imaging, (b) selected-area electron diffraction, (c) dark-field imaging of W3 C nanograins, and (d) dark-field imaging of nc-carbyne nanograins. Reprinted with permission from [21], G. M. Demyashev et al., Nano Lett. 1, 183 (2001). © 2001, American Chemical Society.

Figure 15. Typical transmission electron microscopy images of the nc-W3C/nc-carbyne nanocomposite: (a) bright-field imaging, (b) selected-area electron diffraction, (c) dark-field imaging of W3 C nanograins, and (d) dark-field imaging of nc-carbyne nanograins. Reprinted with permission from [21], G. M. Demyashev et al., Nano Lett. 1, 183 (2001). © 2001, American Chemical Society.

Two kinds of alternating layers were regularly formed in the composite deposits: on the one hand, layers with higher carbine content, larger sized carbyne crystallites (~5 nm), and smaller sized W3C clusters (up to 50 nm); on the other hand, layers with lower carbyne content, smaller sized car-byne crystallites (~2 nm), and larger sized W3C clusters (up to 100 nm).

Thus, the nanocrystalline structure of the composite coatings in conjunction with the unusual crystal structure of both the W3C phase (A15-type lattice) and the carbynes (linear chainlike arrangements of carbon with double/triple bonds) was considered to be a major precondition for the high hardness (~35 GPa) of t he nc-W3C/nc-carbyne coatings. Repeated hardness measurements of cross sections demonstrated the gradual alternation of the microhardness between 35 and 40 GPa, corresponding to the change of car-bynes in the lamellae. These nanocomposite coatings exhibit low internal stress, high temperature stability, low coefficient of friction, good adhesion, and high corrosion protection [21, 175].

Investigation of the temperature stability of these coatings using an in-situ heating system of TEM showed [682, 683] that the deposits decompose into tungsten, W2C carbide, and amorphous carbon at about 1200 °C. This temperature is significantly higher than 1000 °C, as roughly evaluated before [680].

3.2.5. Summary

The main features of nanostructured composite coatings have been summarized as follows [579]:

• Boundaries in the form of an amorphous matrix play a decisive role in the formation and growth of the coatings.

• Superhardness of the nanocomposite coatings strongly depends on the size of the nanograins.

• Nanograins are free of dislocations.

One difference between the superhard nanocrystalline-hard-amorphous-matrix nanocomposite coatings (see Section 3.2.2) and the superhard nanocrystalline-soft-amorphous-matrix nanocomposite coatings (see Section 3.2.3) is that the first ones are brittle and the second ones have a higher toughness [580]. Both types of superhard nanocomposite coatings were represented in the "hardness-toughness" coordinate system [580].

A detailed investigation of the superhard nanocomposite coatings of the nc-MeN/soft-amorphous-phase type demonstrated the dependence of microhardness on [324, 580]:

• Content of the soft amorphous phase

• Kind of element that forms the hard nanocrystalline phase (nc-MeN)

Research demonstrated that the synthesis of ultrahard coatings requires at least three-phase nanocomposites [163, 324, 326]. For example [1], multiphase nanocomposite coatings with microhardness higher than 80 GPa consist of nc-TiN/a-Si3N4/a- and nc-TiSi2. Superhard nanocomposite coatings are characterized by a high elastic recovery (up to 85% for 70 GPa).

The nanocrystalline structure of the superhard coatings gives rise to other tribological properties of the coatings such as coefficient of friction and wear resistance. Incorporation of copper into TiN coatings reduces the coefficient of friction from 0.6-0.7 for TiN to 0.2-0.4 for the nc-TiN/a-Cu nanocomposite coatings [324]. The hardness of the nc-TiN/a-Cu nanocomposite coatings with a fric-tional coefficient of 0.2-3 ranges between 20 and 30 GPa [168, 324]. One potential disadvantage of the nc-MnN/a-Si3N4 nanocrystalline coatings may be the high solubility of silicon in many metals, including ferrous-based metals, aluminum alloys, superalloys, and others [615].

The nc-W3C/nc-carbyne superhard nanocomposite coatings do not follow a common scheme of formation of superhard nanocomposite coatings such as the nc-MeN/hard phase [615] or nc-MeN/soft phase [580]. First, carbynes are low-dimensional chainlike structures [691]. Second, the nc-W3C/nc-carbyne nanocomposite coatings achieve the superhardness threshold at a relatively large nanocrystal size (~70 nm). Third, the nc-W3C/nc-carbyne nanocomposite coatings with large grains (greater than 10 nm) have an unexpectedly low internal stress (0.5-1 GPa), as expected [328].

Currently, it is possible to state that superhard nanocrys-talline composite coatings can be synthesized as the following combinations:

• Nanocrystalline-hard-amorphous-matrix composite coatings (see Section 3.2.2.)

• Nanocrystalline-soft-amorphous-matrix composite coatings (see Section 3.2.3)

• Nanocrystalline-low-dimensional-matrix composite coatings (see Section 3.2.4)

The data in Table 3 were compressed as much as possible. Every line of Table 3 represents data from different sources. Table 3 was compiled on the basis of revised publications. The temperature stability is defined as the temperature recrystallization at which the grain growth occurs.

Table 4 presents some performance characteristics of superhard nanocrystalline composite coatings. First of all, attention was payed to their hardness. Data of other important tribomechanical properties are very limited, which means that the application potential of such coatings is still ahead.

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