Applications Of Superhard Nanocomposites

Superhard nanocomposites in the form of coatings have a great potential in applications for the improvement of the performance of working surfaces [828-833]. The current manufacturing industry is in need of cutting tools [834] that can

• Be used without a cutting fluid, that is, green manufacturing

• Reduce frictional energy, which accounts for some 25%-35% of the total cutting energy

Superhard nanocomposite coatings have been used as thin wear-resistant protection of cutting tools against wear [561, 562, 835-837].

Superhard Nanolayered Composite Coatings The mechanism of wear of superhard nanolayered coatings differs from that of conventional coatings having the same chemical composition [334, 408, 838]. Conventional TiAl-CrN coatings are plastically deformed, resulting in delami-nation and crack formation beneath the surface at a depth of 50-75 nm [839]. Novel CrN/TixAl1-xN nanolayered composite coatings have been observed to be deformed plastically with the formation of cracks beneath the surface at a depth of 6-8 nm [471, 483, 839]. Figure 17 summarizes the results of wear resistance of such coatings.

Nanolayered composites of TiN/CrN, TiN/NbN, and TiN/TaN composition have been deposited on cemented carbide cutting inserts [433, 434]. They demonstrated 2-5 times higher wear resistance in comparison with TiN coatings and 9-20 times higher wear resistance than cemented carbide (WC + 9 wt% Co). The cutting performance of the cemented carbide cutting inserts, coated with TiN/TaN nanolayered composite coatings, has been estimated for the machining of austenitic stainless steel (AISI/SAE 303/304) [433], which is more difficult to machine than carbon low-alloy steels [840].

TiN/TaN nanolayered composite coatings have demonstrated the best cutting performance [433]. The TiN/TaN nanolayered composite coating was compared with commercially available TiCN and TiAlN as well as with the chemically deposited trilayer coating of TiCN + Al2O3 + TiN. The cemented carbide inserts (WC + 11 wt% Co) with TiCN coatings exhibited a high flank wear resistance as well.

Figure 17. Mechanical failure in conventional (a) and nanolayered composite (b) coatings. Reprinted with permission from [170], W.-D. Munz et al., Surf. Eng. 17, 15 (2001). © 2001, Institute of Materials.

VN/TixAl0 96-xY0 04N nanolayered composite coatings have displayed excellent performance in milling tests [170]. Two fluteed ball-nosed cutters were coated by VN/Ti, Al0 96_x Y0 04N nanolayered composite coatings. Comparisons with conventional TiCN coatings demonstrated that VN/Ti,Al0 96-,Y0 04N nanolayered composite coatings were responsible for tripling the lifetime of the cutters [170].

The superiority of the novel CrN/NbN nanolayered composite coating is demonstrated in Figure 18 [170]. The nanocomposite coating can provide a good replacement for electroplated hard chromium, which is widely used in industry. The 3- to 5-^m CrN/NbN thin nanolayered composite coating exceeds the 18-^m electroplated hard chromium in wear resistance by a factor of 10 [170]. Currently, CrN/NbN nanolayered composite coatings are successfully applied in forming tools in the manufacturing industry [170], combing roll blades in the textile industry [479], and knife plates in the food industry [170, 479]. Replacing a conventional TiN coating with CrN/NbN increases the lifetime of the forming tools by a factor of 2.5 [170]. The lifetime of combing rolls coated with the 25-^m Ni electroless was 6500 hours, whereas the combing rolls with the 5-^m CrN/NbN nanolay-ered composite coating had a lifetime of more than 11,000 hours [170]. For polyester fiber cutting, scissors coated with the 5-^m CrN/NbN composite coatings made 3 x 106 cuts in comparison to 1.3 x 106 cuts in the uncoated ones [170]. The CrN/NbN nanocomposite coatings on automotive engine valves increased the lifetime of the valves up to 8000 working hours in comparison with 3000 hours in the uncoated ones [170]. The corrosion behavior of the CrN/NbN nanolayered composite was investigated as well [841].

The CrN/NbN [472] and TiN/AlN [555] nanolayered composites have excellent oxidation resistance at elevated temperatures. The temperature at which the CrN/NbN coatings start to oxidize increases with an increase in the nitrogen flow rate (Fig. 19). The near-stoichiometric composition of the coatings gives rise to temperatures on the order of 820850 °C.

Drills coated with ZrN/CN, nanolayered composite coatings have been tested for their cutting life [541]. The ZrN/ CN, nanolayered composite coatings prolong the lifetime of

Figure 18. Chart of wear depth for various coatings in pin-on-disk test: load of 5 N; Al2O3 ball; 50,000 revolutions. Reprinted with permission from [170], W.-D. Munz et al., Surf. Eng. 17,15 (2001). © 2001, Institute of Materials.

Figure 17. Mechanical failure in conventional (a) and nanolayered composite (b) coatings. Reprinted with permission from [170], W.-D. Munz et al., Surf. Eng. 17, 15 (2001). © 2001, Institute of Materials.

Figure 18. Chart of wear depth for various coatings in pin-on-disk test: load of 5 N; Al2O3 ball; 50,000 revolutions. Reprinted with permission from [170], W.-D. Munz et al., Surf. Eng. 17,15 (2001). © 2001, Institute of Materials.

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