In Superhard Nanolayered Composites

CNx/MeN (Ti, Zr) The idea of this type of nanolayered composite originated from numerous attempts to synthesize carbon nitride (¡-C3N4) in the crystalline state, which was predicted to have a hardness higher than that of diamond [4, 5]. Pseudomorphic stabilization between two nanolayers having a similar structure to ¡-C3N4 was proposed [535-537]. The faces of TiN(111) and ZrN(111) were expected to be the best stabilizers because of the good lattice match between TiN(111) or ZrN(111) and ¡-C3N4 (00.1). The mismatch is less than 0.5% between ZrN(111) and ¡3-C3N4(00.1) and approximately 0.7% between TiN(111) and ¡-C3N4(00.1) [538].

Dual-cathode unbalanced magnetron sputtering with two targets of Zr/Ti and graphite was used for deposition in an Ar + N2 gas atmosphere at about 1 Pa [537-539]. The substrates used were Si(100) wafer [536-540], M2 steel [536-539], and high-speed steel [541]. The microhardness of the ZrN/CNx nanolayered coatings in relation to process parameters such as nitrogen partial pressure, target power, substrate bias, and substrate turntable rotation speed has also been investigated [539]. In particular, the thickness of ZrN and CNx-nanolayers was controlled by the rotation speed of the holder.

A maximum microhardness of 45-50 GPa for the CNx/ZrN nanolayered composite coatings, where the bilayer repeat periods were 1.5 nm [539] and 1.39 nm [536], was achieved. The bilayer repeat period corresponding to 20-24 rpm was 1.5 nm [539]. The intermediate nitrogen partial pressure (53-67 Pa) and the high substrate bias (-180 V) were applied to obtain the maximum hardness [539]. The experimental evidence demonstrates the good correlation between nanostructure and microhardness of the ZrN/CNx nanolayered composites. The ZrN/CNx nanolayered composite coatings have a polycrystalline-type structure. Both high-resolution transmission electron microscopy of the cross-sectional ZrN/CNx imaging (Fig. 9a) and selected-area electron diffraction (SAED) (Fig. 9b) confirm the polycrys-talline nature of the nanocomposites. Some nanocrystalline regions denoted by an arrow go through the nanolayers (Fig. 9a). The intensive diffuse halo at the middle of the SAED presents clear evidence that the ZrN/CNx nanolay-ered composites contain a certain portion of amorphous state, which is common for the carbon nitride phase [542].

TiN/CNx nanolayered composite coatings have been deposited on high-speed tool steel substrates as well [535, 541, 543-545]. The microhardness of the coatings was found to be in the range of 35-47 GPa [541]. A maximum hardness of approximately 48 GPa for the CNx/TiN nanocrystalline composites was obtained at a speed of 6 rpm and a substrate bias voltage of -150 V [536]. The TiN/CNx thickness ratio for maximum hardness should be hTiN/hCNx = 2.5 [546].

c-AlN/MeN (Me = Ti, V) Usually, the stable forms of the TiN phase and the AlN phase have a crystal lattice of NaCl type and hexagonal wurtzite type, respectively [547]. The idea of synthesis of these nanolayered composite coatings is to stabilize the cubic form of the aluminum nitride phase. The point is that TiN nanolayers can take part as stabilizers of cubic-type AlN because the AlN phase of NaCl type exists only under high pressure. As expected, the stabilized cubic AlN phase should have a high bulk modulus and high hardness.

Superhard nanolayered composites of AlN/TiN were deposited using reactive magnetron sputtering [548-551], ion-beam-assisted processing [552-555], and pulsed laser deposition [556]. It was established [552] that AlN nanolay-ers are crystallized in a NaCl-type lattice for a bilayer repeat period of A < 3 nm. The AlN/TiN nanolayered composite coatings reach a microhardness of approximately 40 GPa at a repeat period of 2.5 nm [551, 552, 556]. Close to the bilayer repeat period of 2.5 nm, formation of the cubic AlN phase similar to the TiN nanolayers was observed [557]. The cubic AlN phase strongly affects the hardness of the nanolayered composite coating [552]. Selected-area electron diffraction

Figure 9. (a) Cross-sectional image of high-resolution transmission electron microscopy of the ZrN/CNx nanolayered composite coating deposited at a substrate rotation speed of 7 rpm, a nitrogen partial pressure of 0.05 Pa, and a substrate bias of -120 V. (b) Selected-area electron diffraction of the superhard nc-ZrN/nc-CNx nanolayered coatings. Reprinted with permission from [539], M. L. Wu et al., Thin Solid Films 308-309, 113 (1997). © 1997, Elsevier Science.

Figure 9. (a) Cross-sectional image of high-resolution transmission electron microscopy of the ZrN/CNx nanolayered composite coating deposited at a substrate rotation speed of 7 rpm, a nitrogen partial pressure of 0.05 Pa, and a substrate bias of -120 V. (b) Selected-area electron diffraction of the superhard nc-ZrN/nc-CNx nanolayered coatings. Reprinted with permission from [539], M. L. Wu et al., Thin Solid Films 308-309, 113 (1997). © 1997, Elsevier Science.

of the AlN/TiN coatings provided evidence that the coatings are nanocrystalline type with a (111) growth texture.

The cubic AlN phase was also stabilized in an AlN/TiN(001) nanolayered composite template [558]. Aluminum nitride included in the AlN/TiN(001) nanocompos-ite of the monocrystal-type structure was stabilized for a c-AlN nanolayer thickness of less than 2 nm as a NaCl-type structure with a lattice parameter of 0.408 ± 0.002 nm. The formation of c-AlN at nanolayer thickness was provided by lower c-AlN/TiN interfacial energy. The lattice mismatch of TiN (aTiN = 0.424 nm) with c-AlN (ac-AlN = 0.408 nm) was 4.8%. The c-AlN/TiN nanolayered composite coatings with A = 1.8-8 nm were grown on Mg0(001) using an ultrahigh vacuum direct-current magnetron sputtering system with Al and Ti targets in a gas mixture of Ar + N2.

Other attempts to synthesize superhard c-AlN/TiN nanocomposite coatings with high hardness employing magnetron sputtering were not successful [559, 560]. A microhardness of only 23 GPa was obtained at a bilayer repeat period of 1.1 nm.

VN/c-AlN nanolayered composite coatings were developed to improve the wear resistance of cutting tools [561, 562]. The cubic form of aluminum nitride (c-AlN) was stabilized with the help of NaCl-type VN. A maximum hardness of 59-60 GPa was reached when the bilayer repeat period was 3.6 nm with hVN = 1.8 nm and hc-AlN = 1.8 nm. The transformation of the AlN phase from a NaCl structure to a hexagonal wurtzite-type structure for the AlN/VN architecture occurs when the AlN nanolayers are thicker than 4 nm [563].

Nanolayered composite cermets of AlN/W with a bilayer repeat period of 3.5-7 nm were grown by magnetron sputtering on a Mg0(001) [564] and A! [565] substrates. The zinc-blended phase (zb-AlN) was formed when hAlN < 1.5 nm, and the stabilization was explained as a result of good interfacial matching between W(100) and zb-AlN(011) [564]. Formation of a wurtzite-type structure (w-AlN) was detected when the thickness of the AlN nanolayers was above 1.5 nm [564]. A decrease in the longitudinal elastic response in the direction of the nanocoating growth as a function of bilayer repeat period was observed in AlN/ZrN [565]. The nanocomposite coatings of AlN/NbN did not exhibit any anomalies in dependence on a bilayer repeat period in the range of 3-100 nm [567].

C-BN/TiN Unbalanced magnetron sputtering was used to template cubic boron nitride (c-BN) nanolayers between TiN nanolayers [568] because synthesis of one-phase c-BN coatings is difficult. As in the case of CNx and c-AlN, the c-BN/TiN nanolayered composite structure was a good template for trapping the metastable c-BN phase between the TiN-nanolayers.

c-BN/TiN nanolayered composites with a hardness of 40-45 GPa were obtained [568]. The bilayer repeat period depended on the substrate bias, sputtering rate of each compound, and nitrogen partial pressure. The nanocomposite coatings remained stable and hard up to 700 °C.

An attempt to stabilize c-BN structures when sandwiched between aluminum nitride nanolayers was undertaken [569]. The nanolayered composite coatings of c-BN/AlN were prepared using reactive direct-current magnetron sputtering of a boron carbide (B4C) target in argon-nitrogen plasma.

ß-WC1-x /MeN (Me = Ti, Ti1-y Aly) WC/TiN nanolayered composites were deposited on Si(100) wafer and cemented carbide (WC-Co 3 at%) using the arc ion-plating system [570]. Although the single-phase WC coatings deposited separately were identified as a mixture of carbon-deficient metastable phases of a-WC (trigonal) and of ß-WC1—x (NaCl type), only ß-WC1—x was recognized in the TiN/WC nanolayers; that is, the nanolayered composite coatings can be classified as having ß-WC1—x/TiN composition. In this case, the nanolayers of TiN (a = 0.4242 nm, NaCl type) serve as a trap for the stabilization of the metastable phase of ß-WC1—x (a = 0.4235 nm, NaCl type), that is, stabilization of the metastable structure owing to the templating effect. These phases were matched with a small distortion.

The cross-sectional transmission electron microscopy investigation of ß-WC1—x/TiN nanolayered composites provided evidence that the coatings consist of TiN and ß-WC1—x-nanolayers, which are nanocrystalline conglomerates. This was also verified by selected-area electron diffraction [570]. The residual stress of the as-deposited ß-WC1—x/TiN nanolayered coatings was measured to be up to 7.9 GPa. To decrease this high level of residual stress, three types of intermediate layers, namely WC, Ti, and Ti-WC, were incorporated into the ß-WC1—x/TiN nanolayered coatings. Significant reduction of the residual stress was achieved with the Ti intermediate layer (up to 2.3 GPa) and the Ti-WC intermediate layer (up to 2.2 GPa).

The microhardness of the ß-WC1—x/TiN nanolayered coatings was measured to be in the range of 38-40 GPa. The maximum hardness was achieved when the bilayer repeat period had an optimum value of 7 nm. The Ti-WC inter-layer did not have any effect on stress, and the ß-WC1—x/TiN had the same hardness (38-40 GPa) before the interlayer was incorporated.

Nanolayered composite coatings of ß-WC1—x/Ti1—yAlyN were deposited using the multicathode arc ion-plating evaporation of Ti, Al, and WC targets [571]. The approach was similar to the synthesis of ß-WC1—x/TiN nanolayered composite coatings [570]. Silicon wafers of Si(100) and WC-Co(3 at%) served as substrates for the coatings. Steering the arc power density for each target controlled the composition of the coatings. To reduce the residual stress, interlayers of WC, Ti, and Ti-WC were periodically incorporated.

The nanostructure of the coatings is defined by the Al content (y). X-ray diffraction demonstrated that the coatings with 0.35 < y < 0.57 Al content and with a bilayer repeat period of 10 nm consisted of Ti1—yAlyN (NaCl type) and of cubic ß-WC1—x (NaCl type). The template effect of the Ti1—yAlyN nanolayers trapped the metastable phase of ß-WC1—x. The hardness of the ß-WC1—x/Ti1—yAlyN nanolayered composite coatings reached a maximum of 50 ± 5 GPa when y = 0.57. Titanium-aluminum nitride nanolayers had a (111) texture. Increasing the Al content up to y = 0.57, the nanolayered composite coatings transformed from a monocrystalline-type structure to a nanocrystalline-type structure with an average grain size of about 10 nm. Increasing the bilayer repeat period up to 20 nm led to the transformation of ß-WC1—x nanolayers into a-W2C nanolayers where the a-W2C phase possesses a trigonal lattice.

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