Superhardness Phenomenon

in Nanolayered Composites TiN/MeN (Me = V, Nb, Ta, TaxW1-x, VxNb1-x, Cr, Si)

TiN/VN This type of nanolayered composite coating was one of the first subjects of systematic investigation of the superhardness effect. Monocrystal Mg0(100) [377, 397, 429], M2 tool steel [430, 431], high-speed steel [432], cemented carbide (WC, 10wt% Co) inserts [433, 434], and Si wafers [345] served as substrates.

TiN/VN nanolayered coatings with a total thickness of about 2.5 ¡m were deposited by reactive magnetron sputtering from two V and Ti targets in nitrogen atmosphere [377]. Deposition of the coatings started with titanium nitride [377, 397] where the TiN nanolayer thickness (hTiN) was varied between 0.75 and 16 nm and the ratio of hTiN/A ranged from 0 to 1.

The TiN/VN interfaces were free of misfit dislocations, but dislocation loops with diameter of 8-10 nm extending across a few nanolayers were observed [377]. The TiN/VN nanolayered composite coatings themselves represent a monocrystal-type structure with the alternation of TiN and VN nanolayers [377, 397]. The lattice mismatch of these two nitrides having a NaCl-type crystal structure was estimated to be 2.4% [377]. Equally spaced satellite reflexes located above and below the main Bragg reflection in the [100] direction characterized the bilayer repeat period.

It was found [377] that the microhardness of the TiN/VN nanolayered coating depends on both A (Fig. 6) and the ratio of hTiN/A (Fig. 7). The highest microhardness of such nanolayered composite coatings was determined to be 5556 GPa [377, 431, 435] when the bilayer repeat period and thickness of the TiN and VN nanolayers were optimum, that is, hTiN/A = 0.5 and A ~ 5.2 nm. The microhardness of TiN/VN was found to be significantly higher than the micro-hardness of each polycrystalline TiN and VN phase as shown on the hardness axis (Fig. 6). It was assumed [377] that the

Figure 6. Microhardness of the TiN/VN nanolayered coatings (hTiN/hVN = 1) with orientation (100) perpendicular to the substrate versus bilayer repeat period. Reprinted with permission from [377], U. Helmersson et al., J. Appl. Phys. 62, 481 (1987). © 1987, American Institute of Physics.

Figure 7. Dependence of the microhardness of the TiN/VN nanolayered composite coatings on the ratio of hTiN/A when A = 6.5 nm and the total coating thickness was 2.5 ¡¡m. Reprinted with permission from [377], U. Helmersson et al., J. Appl. Phys. 62, 481 (1987). © 1987, American Institute of Physics.

Figure 7. Dependence of the microhardness of the TiN/VN nanolayered composite coatings on the ratio of hTiN/A when A = 6.5 nm and the total coating thickness was 2.5 ¡¡m. Reprinted with permission from [377], U. Helmersson et al., J. Appl. Phys. 62, 481 (1987). © 1987, American Institute of Physics.

increase in microhardness is based on a decrease in dislocation mobility.

TiN/NbN A wide range of bilayer repeat period from 1.6 to 450 nm was investigated for TiN/NbN nanolayered composite coatings [397, 436-447], which were grown on different sorts of substrates, for example, monocrystal Mg0(100) substrate [397], cemented carbide [434], M2 tool steel [431, 448], and high-speed steel [432], using ultrahigh vacuum reactive magnetron sputtering with a total pressure of about 2 Pa of Ar + N2 gas mixture [397, 430] and unbalanced magnetron sputtering [432] with the same gas mixture.

The microhardness of the TiN/NbN nanocomposites of nanocrystalline-type structure is a function of the bilayer repeat period (A) [430]. The maximum microhardness of 55-57 GPa was achieved when the bilayer repeat period was between 4 and 8 nm [431]. This microhardness exceeds the microhardness of each polycrystalline TiN and VN phase by more than 2.5 times. A compressive residual stress for the TiN/NbN nanolayered composite coatings was evaluated as -1.2 GPa [433, 434]. The interfaces between the nanolayers of the nanocrystalline-type structure [430] were not so perfectly planar and abrupt as they were between nanolayers in the monocrystalline-type nanocomposites [397].

The maximum hardness of 48-49 GPa for the TiN/NbN nanocomposites of monocrystalline type with a thickness of approximately 2 ¡m was achieved when the bilayer repeat period was A = 4-7 nm and the average layer thickness ratio hTiN/A = 0.3 ± 0.5 [397, 432]. The lattice mismatch of TiN and NbN reached 3.6% [397], which is larger than that for TiN/VN. X-ray diffraction indicated [397] that the TiN/NbN interfaces were relatively sharp. No misfit dislocations were observed for the optimal bilayer repeat period.

The electron diffraction pattern consisting of points (Fig. 8a) reaffirmed that the TiN/NbN nanolayered coatings have a monocrystal structure. The satellite reflexes characterized the long-range ordering of the TiN/NbN nanolayered composite coatings. The long-range ordering corresponds to the bilayer repeat period where the dependence is as follows:

• The satellite reflexes are closer to the central peaks if the bilayer repeat period is increased.

• The central peaks have the same intensity if the bilayer repeat period and ratio hTiN/A are maintained severely; that is, the nanolayered composite structure is well ordered.

The typical high-resolution transmission electron microscopy image, represented in Figure 8b, verifies that interfacial misfit dislocations were not observed [397]. The NbN nanolay-ers appear darker than the TiN nanolayers because of the higher atomic number of Nb.

Using a reactive hybrid deposition process consisting of a combination of electron beam evaporation and direct current magnetron sputtering [449], the TiN/NbN coatings with variation of TiN and NbN layers in nanoscale range thickness were obtained. The maximum microhardness was 34 GPa for hTiN/hNbN = 2 with approximately 33% NbN content. The hardness was found to increase with an increase in the relative thickness of the NbN nanolayer.

Figure 8. Cross section of the TiN/NbN nanolayered coatings: (a) conventional transmission electron microscopy image and selected-area electron diffraction and (b) high-resolution transmission electron microscopy of the {200} lattice fringe images. Reprinted with permission from [397], M. Shinn et al., J. Mater. Res. 7, 901 (1992). © 1992, Materials Research Society.

Figure 8. Cross section of the TiN/NbN nanolayered coatings: (a) conventional transmission electron microscopy image and selected-area electron diffraction and (b) high-resolution transmission electron microscopy of the {200} lattice fringe images. Reprinted with permission from [397], M. Shinn et al., J. Mater. Res. 7, 901 (1992). © 1992, Materials Research Society.

TiN/TaN The TiN/TaN nanocomposites, having a small lattice mismatch of 2.1%, were grown on cemented carbide inserts [433, 434, 450] and high-speed steel substrate [451, 452]. A microhardness of 39-40 GPa was reported [345, 433, 434] when the bilayer repeat period was 9-12 nm with a ratio of hTiN/A = 0.64 and the cubic TaN phase was formed. If the tantalum nitride lamella was thicker than 15 nm, hexagonal phases of TaN08 and Ta2N formed [451]. Pseudomorphic stabilization of the cubic TaN phase in nanolayers thinner than 6 nm was reported [451]. The com-pressive residual stress for the TiN/TaN system was relatively low—3.1 GPa [433, 434].

TiN/TaxW1-x N The TiN/TaxW1-xN nanocomposites with NaCl structure of Tax W1-xN were deposited [345] from Ti and Ta09W01 targets. A maximum microhardness value of 50 GPa for the TiN/Ta09W01N nanolayered composite coatings was found to conform to A = 5.6 nm and hTiN/A = 0.64.

TiN/VxNb1-xN Nanocomposites of TiN/V0 6Nb0.4N composition with monocrystal-type structure were deposited on Mg0(001) substrates using ultrahigh vacuum reactive sputtering with a computer-controlled shutter for two targets [378]. A maximum microhardness of approximately 41 GPa was achieved when the bilayer repeat period had an optimal value of 10-12 nm [378, 453]. The nanolayers had flat and abrupt interfaces. The composition of the targets was chosen so that the V0 6Nb0 4N and TiN nanolayers did not have a lattice mismatch, which was approximately 0.3%. These results indicate that coherency strains do not play major role in increasing the microhardness of the TiN/V0.6Nb0.4N nanolayered composite coatings.

Later [400], the composition of V0.3Nb07N for the TiN/V03Nb07N nanolayered composite coatings was chosen in order to increase the lattice mismatch, which was 1.7% between TiN and V0.3Nb07N. In this case, the hardness increased rapidly and reached a maximum of approximately 51 GPa when the bilayer repeat period was 6 nm. It was concluded [400] that coherency strains, which are larger for TiN and V0.3Nb0 7N, are responsible for the increase in hardness.

Attempts to obtain superhard nanolayered composite coatings of TiN nanolayers in combination with TiAlN and CrN were undertaken [454-459], but the hardness of these coatings was not so impressive. TiN/TiAlN and TiN/CrN nanolayered composite coatings had a microhardness of 23.5 GPa and 29-32 GPa, respectively. It is important to note that coating hardnesses demonstrated strong sensitivity to the nanolayer thickness and to their bilayer repeat period as well.

TiN/CrN Polycrystalline nanolayered composites of TiN/CrN were deposited [460-462], in particular, on M1 tool steel [463]. Nanoindentation of 2-^m-thick TiN/CrN nanolayered composites fixed the maximum hardness of 35 GPa at the bilayer repeat period of 2.3 nm [463].

TiN/a-SiNx Nanolayered composites of TiN/a-SiNx were deposited with dual-cathode unbalanced reactive magnetron sputtering [464, 465]. Amorphous nanolayers of SiNx periodically interrupted the growth of TiN in order to suppress the columnar structure of TiN [465]. A maximum hardness of approximately 45 GPa for the nanolayered composites of TiN/a-SiNx was achieved [465] for the deposition under optimum conditions, that is, ATiN = 2.0 nm, Aa-SiNx = 0.5 nm, and substrate bias ranging from -80 to -90 V

Nanolayered composite coatings of TiN/FeN [466], TiN/ZrN [467], TiN/CrAlN [468], and TiN/Al2O3 [469] composition were synthesized. These nanocomposites did not display a superhardness effect.

VN/MeN (Me = TixAl1-x, TixAla96-xY004) A computer-controlled vacuum system combining two physical vapor deposition processes, namely, cathodic arc and unbalanced magnetron with two pairs of TiAl (50: 50 at%) and V (99.8%) targets, was used to deposit the VN/TixAl1-xN nanolayered composites [470, 471]. For deposition of the VN/TixAl0 96-x Y0 04N nanolayered composites, the TiAl targets were replaced with the TiAl targets containing 4 at% Y. An Ar + N2 gas atmosphere was used for sputtering the foregoing targets and the synthesis was carried out under a total pressure of 0.3 Pa.

The VN/TixAl1-xN and VN/TixAl0 96-xY0 04N nanolayers were deposited on a thin (0.1-0.2 ^m) base layer of vanadium nitride for the enhancement of the adhesion of the VN/TixAl1-xN and VN/TixAl0 96-xY0.04N nanolayered coatings to substrates. Mirror-polished hardened M2 high-speed steel, cemented carbides, and 304L stainless steel served as substrates for measuring the hardness, adhesion, high-temperature tribology, corrosion resistance, and structure of the nanolayered composite coatings [471].

The VN/TixAl1-xN nanolayered composites had a {111} texture at -75 V of bias voltage. The incorporation of small amounts of yttrium into the TixAl1-xN nanolayers transformed the {111} texture of the VN/Tix Al1-xN nanolayered coatings into the {110} texture of the VN/TixAl0 96-x Y0 04N nanocomposites [471]. Moreover, increasing the bias voltage higher than -75 V leads to domination of the {111} texture in the VN/TixAl0 96-xY0 04N nanolayered coatings.

The highest hardness was approximately 39 GPa and 78 GPa for the VN/TixAl1-xN and VN/TixAl0.96-xYa04N nanolayered composites having a bilayer repeat period of 3.2 nm and 3.5-4 nm, respectively [170, 471]. The range of the residual compressive stress for both VN/TixAl1-xN and VN/TixAl0 96-xY0 04N was between -3.3 and -8.5 GPa [471]. Alloying with yttrium increases the compressive residual stress.

CrN/NbN Nanolayered coatings of CrN/NbN composition were deposited using combined cathodic-arc and unbalanced magnetron sputtering of Cr and Nb targets on an M2 high-speed steel substrate under a working pressure of 0.3-0.36 Pa [472-478] and on knife blades produced from martensite 420 SS and from 1% carbon steel [479]. The flow rate was selected so that single-phase (NaCl-type) structures of CrN and NbN were co-deposited [472]. The low-angle X-ray diffraction gave evidence [472] that the CrN/NbN nanolayered coatings with a bilayer repeat period in the range of 2.76-7.4 nm were obtained.

The microhardness of the CrN/NbN nanolayered coatings as a function of the nitrogen flow rate was investigated [472]. The CrN/NbN nanolayered coatings had the dominant texture of {200} crystallographic orientation. Both stoichiomet-ric and substoichiometric CrN/NbN nanolayered coatings were formed with the highest microhardness of approximately 56 GPa [170].

The residual compressive stress for the CrN/NbN nanolayered coatings having stoichiometric composition (N/Me~1) was as low as 1 GPa [472], while the compressive stress for the substoichiometric composition (N/Me~0.45) reached 7 GPa.

CrN/TixAl1-xN The CrN/TixAl1-xN nanolayered composite coatings reached the highest microhardness of 60 GPa [170]. The highest value of a compressive residual stress reached -10 GPa for the CrN/TixAl1-xN nanolayered composites [170]. In other cases [480-485], the microhard-ness of CrN/TixAl1-xN nanolayered composite coatings was only 32-35 GPa. Hardness as a function of the bilayer repeat period demonstrated strong dependence. The optimal bilayer repeat period was found to be 3-4 nm for the highest hardness.

Other nanolayered composite coatings with chromium nitride such as CrN/Cr2N [486, 487] and CrN/Si3N4 were reported [488]. The hardness measured by nanoindentation was approximately 22 GPa for CrN/Cr2N. The superhard-ness effect was not observed in CrN/Si3N4.

ZrN/TixAl1-xN The ZrN/TixAl1-xN nanolayered composites were deposited using combined unbalanced magnetron sputtering and steered-arc evaporation [170]. The micro-hardness of the ZrN/TixAl1-xN nanolayered composites was very high and reached a maximum of 55 GPa at a bilayer repeat period of 2.6 nm [170]. In the case of ZrN/TixAl1-xN nanolayered composites, the compressive residual stress was as high as 10 GPa. The ZrN/TixAl1-x N coatings demonstrated a well-defined (100) growth texture.

NbN/TaN Nanocomposite coatings of alternating NbN and TaN nanolayers were prepared, employing the magnetron sputtering technique on stainless-steel wafers [401, 489, 490]. This system of nanolayers was found to be attractive because the NbN (NaCl-type structure) and TaN phases (hexagonal structure) possess different types of crystal lattices and, therefore, different slip systems of dislocation. Transmission electron microscopy of NbN/TaN cross-sections showed that the lattice planes of {111}NbN were coherent with the {11.0}TaN planes.

It has been shown [401] that X-ray diffraction (XRD) of the NbN/TaN nanolayered composites with a small bilayer repeat period is similar to the XRD pattern of single-phase tantalum nitride. The hexagonal niobium nitride structure, as expected [401], was stabilized by the hexagonal TaN nanolayers. Overlapping of the X-ray diffraction lines of niobium nitride and tantalum nitride was also possible [401]. The (200) reflex of cubic NbN on selected-area electron diffraction patterns was present when A = 73.2 nm.

The effect of the thickness of the bilayer repeat period (A) on the microhardness of NbN/TaN nanolayered composites has been investigated from A = 0 nm to A = 73.2 nm [401]. The influence of the bilayer repeat period on the microhardness of the coatings exists in a wide range of A = 2. 3-17 nm, and the microhardness reaches a maximum of 51 GPa [401, 490]. This effect differs from the earlier investigated ones where extreme microhardness existed within a narrow band of bilayer repeat period. The coherent stresses and the structural barriers (NbN, NaCl-type structure; TaN, hexagonal structure) for dislocation movement through the NbN-TaN interface are believed [401] to be the main reasons for the extreme microhardness in the broad range of the bilayer repeat period.

Other nanolayered composites of NbN/Si3N4 [491] and NbN/Fe4N [492] have been investigated. Niobium nitride, being a superconducting material, demonstrated superconductivity at a NbN nanolayer thickness of approximately 10 nm. Below this thickness, ferromagnetism of the Fe4N nanolayers was detected.

Si3N4/SiC Nanolayered composites of Si3N4/SiC were deposited on Si substrates and cemented carbide inserts by radio frequency (rf) magnetron sputtering [493]. It was determined that the microhardness of Si3N4/SiC nanocom-posite coatings reaches a maximum of 3700 HV0.01 when the bilayer repeat period equals 11.6 nm. The residual stress of the nanocomposite was as low as 2-2.45 GPa.

MeN/Me (Me = Ti, Hf, W, Mo) Nanolayered composites of TiN/Ti [494-506], HfN/Hf [494], (Ti, Al)N/Mo [507, 508], and WN/W [494] were deposited. A radio frequency diode sputtering system [188] was used. The total pressure was about 1.3 Pa for the preparation of TiN/Ti and HfN/Hf and about 2.7 Pa for the WN/W nanolayers. The nitrogen content in the sputtering gas mixture was 20%, 25%, and 50% for the TiN, HfN, and WN nanolayers respectively. The nanocomposite coatings were grown with total thickness varying from 3 to 8 ^m.

For TiN/Ti nanolayered composite coatings, a maximum hardness of approximately 37 GPa was reported [494] when the bilayer repeat period was A = 16-18 nm and the individual nanolayers of TiN and Ti were equivalent to each other, that is, hTiN = hTi = 8-9 nm. The TiN/Ti nanocom-posite coatings possess a very narrow range of bilayer repeat period where the hardness has the maximum value. Further reduction of the bilayer repeat period leads to the hardness droping sharply to approximately 22 GPa when the thickness of each nanolayer is below 7 nm. Later attempts [496, 509] to obtain TiN/Ti nanolayered composite coatings with larger bilayer repeat period [496] and with various ratios of hTiN/A [509] gave hardnesses of about 29 GPa and 23 GPa, respectively.

The hardness of HfN/Hf nanolayered composites was also measured [494]. When the bilayer repeat period was equal to 8 nm, the hardness of the HfN/Hf nanocomposite reached a maximum of 50 GPa and remained constant within the range of A = 4-8 nm. The thickness of the HfN and Hf individual nanolayers remained equal to each other during the experiments (hHfN = hHf).

The hardness of the WN/W nanolayered composite coatings reached a value of about 35 GPa [494], which is significantly higher than that of W and WN coarse-grained one-phase coatings. The bilayer repeat period for the maximum hardness varied in the range of A = 16-40 nm. The individual W and WN nanolayers were equal to each other (hWN = hW) during the experiments.

Nanolayered composites of (Ti, Al)N/Mo with a bilayer repeat period of approximately 6 nm have a hardness value of 51 GPa [507], which was significantly improved in comparison with the previous attempt [508].

Epitaxial nanolayered composites of NbN/Mo [510, 511] and NbN/W [512, 513] with a bilayer repeat period between 1.3 and 120 nm were deposited on Mg0(001) substrate by direct-current (dc) reactive magnetron sputtering. The orientation relationships were (001)Me||(001)NbN and [110]Me||[100]NbN. A maximum hardness of approximately 32 GPa was detected at a bilayer repeat period of 2-3 nm. The hardness follows the Hall-Petch relationship: H = 10.3 + 26.7A-038 GPa for NbN/Mo and H = 12.88 + 22.1A-0 3 GPa for NbN/W Other hybrid nanolayered composites of nitrides in combination with metals, namely, AlN/Mo [514], TiN/Pt [515, 516], TiN/Ni [517], TiN/Ag [501], and CrN/Cr [518, 519], were obtained.

TiC/Me (Me = Fe, Mo) Nanolayered coatings of TiC/Me as a composition of TiC with both soft metals (Fe, Al, Cu, Ti) and hard metals (W, Mo) were synthesized using the ion beam sputtering technique [271, 272, 520-525] in order to investigate the mechanical properties of the ceramic-metal nanolayered composite coatings. The microhardness of the TiC/Me system has been determined [62, 272] and was found to have a great dependence on the bilayer repeat period; in particular, the TiC/Fe nanolayered composite coatings showed the superhardness effect.

Fe/TiC nanolayered composite coatings were deposited on Si wafer using Ar+ ion beam sputtering of the composite (Ti + C) and Fe targets under a pressure of 7 x 10-3 Pa [62, 272]. The Ti/C ratio of the composite target was selected so that the formation of stoichiometric TiC was realized.

The maximum microhardness of the Fe/TiC nanolayered composite coatings was 42 GPa [272, 522] when the thickness of the Fe nanolayers was about 0.43 of the total thickness of the bilayer repeat period. Simultaneously, the maximum toughness of the Fe/TiC nanolayered composite coatings reached and surpassed TiC over 3 times [62]. In this case, the bilayer repeat period had a value of 14 nm and the components were distributed with hTiC = 8 nm and hFe = 6 nm. It was found [272, 522] that every particular TiC and Fe nanolayer, in turn, had a nanocrystalline structure. Within the nanograins, the crystallographic coherent arrangement of TiC(200)||Fe(110) was commonly observed [62].

A maximum microhardness of 47.62 GPa for TiC/Mo was reported at A = 8 nm [520]. The bilayer repeat period was investigated in the range of 2-14 nm [520, 521], and the layer thickness was maintained to be equal (hTiC = hMo).

Attempts to synthesize nanolayered composite coatings of Mo/TiC were undertaken [526]. Other nanocomposite coatings demonstrated a microhardness dependence on the bilayer repeat period as well. But the microhardness was significantly lower, namely, approximately 21 GPa for Mo/TiC at A = 2.5 nm [526], approximately 24 GPa for W/TiC [272], and approximately 16 GPa for Al/TiC [272].

Nanolayered composites of carbides with other materials, such as TiC/Al2O3 [526, 527], TiC/DLC [528], TiC/B4C [524], W/B4C [529, 530], TiC/SiC [531], TiC/VC [532, 533], and TiC/Ni3Al [534], have been obtained. The microhard-ness of the nanolayered composites demonstrated a strong dependence on the sharpness of the nanolayers and the bilayer repeat period. But their microhardness was moderate and below the conventional superhardness definition or had not been reported.

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