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Figure 11. Average nanocrystal size (d) and hardness (Hv) of the nc-W2N/a-Si3N4 nanocomposite versus a-Si3N4 content. Reprinted with permission from [615], S. Veprek et al., Surf. Coat. Technol. 108-109, 138 (1998). © 1998, Elsevier Science.

The same nc-TiN/a-Si3N4 nanocrystalline composite coatings have been deposited using the unbalanced magnetron sputtering technique [166, 605, 606, 616] where two separate Ti and Si targets were utilized. As expected, this physical vapor deposition technique should eliminate any weaknesses associated with plasma-assisted chemical vapor deposition and the use of the gas mixture TiCl4+SiCl4/SiH4, which is hazardous and flammable, as the precursor. The incorporation of chloride into the deposits may induce interface corrosion at elevated temperatures.

As in the case of plasma-assisted chemical vapor deposition, coatings deposited by PVD exhibit a similar structure; namely, the TiN nanocrystallites are surrounded by an amorphous matrix of a-Si3N4. The microhardness of the nc-TiN/SiNx coatings, where 0 < x < 1.3, reaches 38 GPa when the optimal content of silicon is 4-6 at%.

The presence of a-Si3N4 has been detected using X-ray photoelectron spectroscopy, whereas selected-area electron diffraction and X-ray diffraction do not provide any evidence of a crystal form of Si3N4. However, both X-ray diffraction and the selected-area electron diffraction show the presence of a crystalline form of TiN. The characteristic size of nc-TiN is approximately 20 nm.

The advantages of the nc-MeBN/a-Si3N4 nanocrystalline composite coatings include [208]:

• Thermodynamical stability

• Resistance against oxidation in air at elevated temperatures

• Low-temperature deposition (500-550 °C)

• Compatibility with nonplanar substrates

The weakness of the nc-Me„N/a-Si3N4 superhard nano-crystalline coating is that the solubility of silicon is high in many metal substrates [615]. Thus, the similar disadvantage of diamond-like carbon is not overcome. Replacement of the a-Si3N4 phase by boron nitride or aluminum nitride is likely to be one of the possible ways to eliminate the weakness of the nc-Me„N/a-Si3N4 coatings. Such superhard nano-structured coatings, where the amorphous silicon nitride is replaced by BN and AlN, are reviewed in the following sections.

nc-Ti1-xAlxN/a-AlN As has been noted [615], the disadvantage of nc-MeN/a-Si3N4 nanocomposite coatings could be the high solubility of silicon in many metals and alloys. Replacement of the a-Si3N4 phase with a-AlN can bring new advantages to the modified coatings.

Superhard nanocrystalline coatings of nc-Ti1-xAlxN/a-AlN composition have been physically vapor deposited from the alloyed target TiAl (60/40 at%) in an Ar+N2 gas mixture and at constant total pressure (0.5 Pa) using the unbalanced magnetron technique [617, 618]. It had been found earlier [618] that the Ti1-xAlxN nanocrystalline composite coatings can be formed in the range of 0.52 < x < 0.59. The nc-Ti1-xAlx N/a-AlN nanocomposite coatings with maximum microhardness up to 47 GPa have been obtained for a stoichiometry of x = 0.562 and a substrate temperature of 200 °C. The coatings consist of nanograins (with an average size of ~30 nm) of Ti1-xAlxN phase bordered by the amorphous AlN phase. This nanocomposite coating exhibits high elastic recovery up to 74%.

The microhardness as a function of the partial pressure of nitrogen is a complicated relationship that is connected with the changes in structure and chemical composition of the Ti1-xAlxN deposits [617]. A maximum hardness of about 27 GPa and 33 GPa for a 5% and for 37% nitrogen partial pressure, respectively, was obtained when the substrate temperature corresponds to room temperature.

The influence of the substrate temperature was investigated [617]. The highest hardness (~47 GPa) of the nc-Ti1-xAlxN/a-AlN is achieved around 200 °C. The maximum possible hardness (~47 GPa) of the nc-Ti1-xAlxN/a-AlN nanocomposite coatings is optimized for an average nanograin size of about 18 nm, although the single-phase coatings of TiN, AlN, and Ti1-xAlxN have hardness of 21 GPa, approximately 13 GPa, and approximately 30 GPa, respectively, which is significantly lower than the approximate 47 GPa for the nc-Ti1-xAlxN/a-AlN nanocomposite coatings.

The experiments demonstrated [617] that nanograin size and grain orientation are of fundamental importance for the synthesis of such superhard nanocrystalline coatings. The maximum hardness can be realized if the grains have a suitable average size and the deposits have the appropriate texture. The (111) crystal planes of the nc-Ti1-xAlxN were parallel to the substrate surface. The structure of the superhard nc-Ti1-xAlxN/a-AlN nanocomposite coatings characterized by X-ray diffraction can be generalized as nanocomposites, which constitute highly oriented nanograins of TiAlN embedded into an amorphous matrix of AlN.

Nanocomposite coatings of (Ti, Al, Si)N with a Si content of 9.5 at% displayed maximum hardness values of approximately 60 GPa [619].

nc-Mo2C/(a-C + a-Mo2N) Superhard nanocomposites of nc-Mo2C/(a-C+a-Mo2N) composition have been synthesized during plasma nitriding with a hollow cathode discharge [620]. The conventional direct-current glow discharge [621] was modified with an auxiliary cathode [622] so that the sputtering of molybdenum and graphite into the plasma of a N2 +H2 +Ar gas mixture under 665 Pa pressure was accomplished.

The incorporation of molybdenum and carbon simultaneously into the surface of nitrided steel has enabled the formation of superhard nanocrystalline composite coatings consisting of Mo2C nanocrystals encircled with an amorphous mixture of Mo2N and carbon [620]. The structure of such superhard nanocomposite coatings can be expressed as nc-Mo2C/(a-C+a-Mo2N).

The highest hardness of 52.5 GPa is achieved when the nanocrystals have (1) predominant orientation of Mo2C(110) parallel to the substrate surface, that is, a (110) texture, and (2) an average nanocrystal size of 48 nm. It has been explained [620] that such a high microhardness is a result of both the deposition of the nanocrystalline-amorphous composite and the orientation of the hard nc-Mo2C nanograins only in one direction. For comparison, the hardness of polycrystalline material of Mo2C is only 16.6 GPa [623].

nc-TiC/DLC It was expected [609, 624-628] that hydro-genated amorphous carbon (a-C:H), having both limited temperature stability up to 400 °C and low hardness of 15-25 GPa, can be improved by adding a nanocrystalline TiC fraction. Generally, amorphous carbon coatings containing transitional metals (Ti, Zr, W) can be obtained with improved tribological properties [629-636]. The combination of a closed-field unbalanced magnetron sputtering of the Ti target in the presence of argon (Ar) and acetylene (C2H2) or methane (CH4) was used for the deposition of various compositions of nc-TiC/a-C : H nanocrystalline composite coatings on single-crystal Si(100) or Si(111) wafers and steel substrates [624-627].

Both the hardness and the toughness [637] of nc-TiC/ a-C:H and nc-TiC/a-C [469] nanocomposites are remarkably increased with the incorporation of amorphous non-hydrogenated/hydrogenated carbon into the TiC coatings. The coating hardness in relation to the Ti content is illustrated in Figure 12. The average nanocrystalline size varies from 3-5 nm for Ti content below 43 at% to 20 nm for Ti content higher than 43 at% (Fig. 13). The maximum microhardness of 35 GPa is achieved approximately at a composition of 80% TiC and 20% a-C: H. The average TiC

Figure 13. Average nanocrystal size of TiC versus Ti content. Reprinted with permission from [624], T. Zehnder and J. Patscheider, Surf. Coat. Technol. 133-134, 138 (2000). © 2000, Elsevier Science.

grain size remains within the limits of 3-5 nm until the maximum microhardness is reached. This maximum of 3540 GPa corresponds to an average nanograin size of 4-8 nm [469, 624]. The coatings qualified [624] as nanocomposites consisting of nc-TiC embedded in an amorphous matrix of a-C : H, that is, nc-TiC/a-C: H (Fig. 14).

The Ti concentration of approximately 40% is a crucial point for other properties of the nc-TiC/a-C : H nanocrys-talline composites. For example, the coefficient of friction is rapidly increased up to 0.6.

Thus, nc-TiC/a-C : H nanocrystalline composite coatings provide high hardness (~35 GPa) and low coefficient of friction (0.25-0.3) at the optimal nanostructure (an average nanograin size of ~5 nm) and composition (80% TiC and 20% a-C:H).

Vacuum-deposited nanocomposite coatings of ^-WC nanograins (1-10 nm) embedded in a nonhydrogenated diamond-like carbon matrix were obtained with carbon content between 30 and 90 at% [634]. The nanostructure of jS-WC/DLC results in a nanohardness [634] that is twice as hard as that of metal-doped DLC coatings. The distribution

Figure 12. Microhardness of nc-TiC/a-C : H versus Ti content for negative bias of -240 and -91 V Reprinted with permission from [624], T. Zehnder and J. Patscheider, Surf. Coat. Technol. 133-134,138 (2000). © 2000, Elsevier Science.

Figure 14. Dependence of coefficient of friction on Ti concentration for Ubias = -240 and -90 V. Reprinted with permission from [624], T. Zehnder and J. Patscheider, Surf. Coat. Technol. 133-134, 138 (2000). © 2000, Elsevier Science.

Figure 12. Microhardness of nc-TiC/a-C : H versus Ti content for negative bias of -240 and -91 V Reprinted with permission from [624], T. Zehnder and J. Patscheider, Surf. Coat. Technol. 133-134,138 (2000). © 2000, Elsevier Science.

Figure 14. Dependence of coefficient of friction on Ti concentration for Ubias = -240 and -90 V. Reprinted with permission from [624], T. Zehnder and J. Patscheider, Surf. Coat. Technol. 133-134, 138 (2000). © 2000, Elsevier Science.

and density of nanograins in metal-containing DLC strongly influence the mechanical properties of Me-DLC nanocom-posites [638].

Me-B-X (Me = Ti, Hf; X = C, N) This kind of super-hard nanocrystalline composite coating was prepared under an optimal ratio and composition from TiB2 +TiC and TiB2+TiN targets [281, 639-653] as well as from TiB2, Ti+h-BN, and Hf+h-BN targets using magnetron sputtering [654-656].

Analysis of the Ti-B-N phase diagram provides evidence [639, 640] that nanocomposite coatings possessing both high hardness and high toughness may be obtained where TiB2 and TiN coexist and form coherent interfaces [623, 640]. The optimal elemental composition of Ti04B04N02 with a concentration ratio of Ti:B:N = 2:2:1 can provide a combination of high hardness with high toughness. The phases of TiB2 and TiN can overcome the superhard mark if the microstructure of the TiB2/TiN and TiB2/TiC composites is optimized [639].

A hardness value of up to 40 GPa was obtained for Ti-B-N based coatings containing both TiB2 and c-BN phases [657]. The highest hardness of approximately 57 GPa for TiBN05 has been obtained using magnetron sputtering of the heterogeneous Ti/h-BN target for a bias substrate of -150 V and temperature of 400 °C [658]. If the substrate temperature is around 25 ° C, the hardness of the as-deposited TiB2/TiN coatings is about 10 GPa, but the highest hardness is achieved by annealing for 6 h at 800 °C. The highest hardness of the TiB2/TiN coatings can be realized using the deposition of TiN/h-BN nanolayered coatings subject to annealing at 400 ° C for 40-200 h [659] depending on the thickness of the Ti/h-BN nanolayers. Annealing transforms the soft nanolayers to a superhard (~57 GPa) nanocrys-talline composite of TiB2/TiN.

Further investigation of the nanostructure has demonstrated [281, 649] that the nanograin sizes of the TiB2/TiN and TiB2/TiC nanocrystalline coatings with compositions of about 35-45 at% Ti, 18-44 at% B, and 20-36 at% N or 22-40 at% C vary between 3 and 5 nm. At low boron content, the coatings represent nanocrystals of TiN or TiC surrounded by a quasi-amorphous TiB2 phase, that is, nc-TiN/a-TiB2 or nc-TiC/a-TiB2. Increasing the concentration of boron reverses the nanocrystalline composite structure. At higher boron content, the coatings are nanocrystals of TiB2 embedded by the quasi-amorphous phase of TiN or TiC, that is, nc-TiB2/a-TiN and nc-TiB2/a-TiC. A microhardness of approximately 68 GPa was obtained for the TiB2 nanocrys-talline coatings having an average nanograin size of 11 nm. It was expected [281] that such a high hardness results from the high intrinsic stress achieved under intense ion bombardment conditions.

The highest hardness of up to 60 GPa has been obtained for nanocomposite coatings and for the stoichiometry of

TiB^N 45 and HfB0.85N0.3 [655]. The hardness of the coatings sputtered with bias voltage demonstrates a strong maximum, which corresponds to the lowest deposition rate. Without the bias voltage, a hardness of approximately 30 GPa can only obtained. The bias voltage (-30 V) causes a decrease in the average nanograin size from 8 to 3.5 nm for TiBxN1-x and from 10.5 to 3 nm for HfBxN1-x. The stress of the superhard nanocomposite coatings was negligible.

Nanocomposite coatings of Ti/BN with the presence of a c-BN phase as determined by XRD were deposited by plasma vapor deposition (PVD) and combined PVD/plasma-activated chemical vapor deposition (PACVD) techniques [660].

nc-TiN/a-MeNx (Me = W, Mo) Nanocomposites of Ti-W-[661, 662] and Ti-Mo-N composition [663] have been deposited using the unbalanced magnetron sputtering technique from W-Ti (30 at%) and Ti-Mo (10 at%) targets in a gas mixture of Ar+N2 on steel and Si substrates.

The influence of the grain size, chemical composition, and relative content of individual phases on the microhard-ness of the nanocomposite coatings has been studied. For a nitrogen partial pressure of more than 0.1 Pa, only the single body-centered cubic phase of TiN was identified. The maximum microhardness of approximately 60-66 GPa for the nc-TiN/a-WNx nanocomposite coatings was achieved for 25 at% nitrogen content and a ratio of Ti/W = 0.32 [661]. The result was interpreted [661] that when the nitrogen concentration in the coatings reaches approximately 30 at%, tungsten becomes amorphous and nanograins of TiN are formed. The formation of a-WNx and TiN nanograins of 1020 nm creates the nc-TiN/a-WNx nanocomposite with a high hardness [661, 664].

Nanocomposite coatings of the Ti-Mo-N system have a maximum microhardness of approximately 45 GPa, which is achieved when the (111) or (200) texture reveals itself. The presence of the TiN phase only was fixed by X-ray diffraction. As has been stated [663], the coatings of the Ti-Mo-N system behave like nanocomposite coatings of Zr-Cu-N, Ti-Al-N, and Zr-Y-N where the amorphous phase plays the role of a binder. Therefore, the structural composition of the coatings could be classified as nc-TiN/a-MoNx.

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