A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Nitrides with addition of silver <s> J. Mayer, J. Microscopy 1998, 194, 71. [21] J. F. Ziegler, J. P. Biersack, U. Littmark, The Stopping Range of Ions in Matter (Pergamon Press, New York, 1985). [22] H. Gao, Y. Huang, W. D. Nix, J. W. Hutchinson, J. Mech. Phys. Solids 1999, 47, 1239. [23] H. Mughrabi, Mater. Sci. Eng. A 2004, 387–389, 209. [24] B. von Blanckenhagen, P. Gumbsch, E. Arzt, Phil. Mag. Lett. 2003, 83, 1. [25] R. Sedlacek, Mater. Sci. Eng. A 2005, 393, 387. [26] D. Hull, D. Bacon, Introduction to Dislocations Elsevier Butterworth-Heinemann, Oxford, 2004. [27] L. H. Friedman, D. C. Chrzan, Phil. Mag. A 1998, 77, 1185. [28] E. O. Hall, Proc. R. Soc. Lond. 1951, B64, 747. [29] N. J. Petch, J. Iron Steel 1953, 174, 25. [30] E. Arzt, G. Dehm, P. Gumbsch, O. Kraft, D. Weiss, Prog. Mater. Sci. 2001, 46, 283. [31] S. P. Baker, A. Kretschmann, E. Arzt, Acta Mater. 2001, 49, 2145. [32] N. Kalyanasundaram, M. C. Moore, J. B. Freund, H. T. Johnson, Acta Mater. 2006, 54, 483. <s> BIB001 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Nitrides with addition of silver <s> Abstract This paper reports on the investigation of the mechanical and tribological properties of reactively sputtered CrAlN-Ag nanocomposite films that were co-sputtered from three individual targets of Cr, Al, and Ag onto Si (111) and stainless steel SS-440C substrates. The deposition parameters included: power to the Cr and Al targets of P Cr = 200 W and P Al = 160 W; substrate temperature T = 300 °C; substrate bias voltage V b = − 130 V; and, power to the Ag target P Ag = 0 to 20 W. The elemental composition of the films was deduced from X-ray Photoelectron Spectroscopy and the Ag content was found to increase with the increase in P Ag . The crystal structure and film architecture were evaluated using X-ray diffraction and transmission electron microscopy. The films were found to consist of nanocrystals of Ag embedded in a solid solution of CrAlN with a pattern typical of the NaCl structure. The CrAlN grain size was found to decrease with increasing Ag content in the film. The hardness and elastic modulus of each sample were measured by nanoindentation. The hardness increased initially for a very small increase in Ag content ( P Ag = 5 W) and then decreased substantially with further increases in P Ag . Finally, the coatings were rubbed against alumina balls using a ball-on-disk tribotester. Characterization of the wear tracks were performed by optical profilometry. A significant decrease in the friction and wear coefficients was achieved with CrAlN single and multi-phase films compared to the CrN-based counterpart. For example, a low friction coefficient of 0.23 and a wear rate of 3.0 × 10 − 8 mm 3 /N m. were obtained when the silver content was in the 8% range. <s> BIB002 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Nitrides with addition of silver <s> Abstract 5 µm thick CrN–Ag composite layers with 22 at.% Ag were deposited by reactive magnetron co-sputtering on 440C stainless steel substrates. Increasing the growth temperature from T s = 500 to 600 to 700 °C leads to Ag segregation within the CrN matrix and the formation of embedded lamellar Ag aggregates with increasing size, 5 , 9 × 10 6 , and 7 × 10 7 nm 3 , respectively. Ball-on-disk tests against 100Cr6 steel, followed by optical profilometry and energy dispersive spectroscopy, indicate that the Ag grains for T s = 500 °C are too small to facilitate an effective lubricious surface layer, resulting in a friction coefficient μ = 0.58 and a composite coating wear rate of 3.8 × 10 − 6 mm 3 /Nm that are nearly identical to those measured for pure CrN with μ = 0.64 and 3.6 × 10 − 6 mm 3 /Nm. The T s = 600 °C coating exhibits a Ag concentration which is 15% higher within than outside the wear track, and acts as a lubricious layer that reduces μ to 0.47 and yields a 16× and 2.4× lower wear rate for coating and counterface, respectively. T s = 700 °C leads to a dramatic increase in surface roughness and an associated increase in friction, μ = 0.85, and wear, 9.9 × 10 − 6 mm 3 /Nm. Replacing the steel counterface with an alumina ball results in the lowest μ = 0.50 for T s = 500 °C, attributed to the presence of Ag and the relatively low hardness of 16.5 GPa for this particular coating. In contrast, friction and wear increase dramatically for T s = 600 °C, which is attributed to a breakdown of the lubricious Ag layer by the harder counterface. The transient friction coefficient μ t during experiments with continuously increasing testing temperature T t = 25–700 °C initially decreases for all samples, attributed to drying of the environment and an effective softening of both coating and counterface. For the T s = 500 °C coating, a temperature activated solid lubricant transport yields a lubricious Ag surface layer and a very low μ t = 0.05 at T t ~ 500 °C. All coatings exhibit an increasing μ t for T t > 500 °C, which is attributed to oxidative degradation. <s> BIB003 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Nitrides with addition of silver <s> Abstract A two-phase nanocomposite coating that consists of inclusions of silver in a vanadium nitride matrix (VN/Ag) was investigated as a potential adaptive coating with a reduced friction coefficient from 25 to 1000 °C. This nanocomposite structure was selected based on the premise that silver and silver vanadate phases would form on the surface of these coatings, reducing their friction coefficient in the (i) room to mid-range and (ii) mid-range to high temperatures, respectively. Silver and vanadium were expected to react with oxygen at high temperatures and create a lubricious silver vanadate film on the coating. The VN/Ag coatings were deposited using unbalanced magnetron sputtering and their elemental composition was evaluated using X-ray photoelectron spectroscopy. The tribological properties of the materials against Si 3 N 4 balls were investigated at different temperatures. The lowest friction coefficients recorded for samples with identical compositions were 0.35, 0.30, 0.10 and 0.20 at 25, 350, 700 and 1000 °C, respectively. Post-wear testing Raman spectroscopy and X-ray diffraction (XRD) measurements revealed the formation of silver vanadate compounds on the surface of these coatings. In addition, real time Raman spectroscopy and high temperature XRD revealed that silver vanadate, vanadium oxide and elemental silver formed on the surface of these coatings upon heating to 1000 °C. Upon cooling, silver and vanadium oxide were found to combine at about 400 °C, leading predominantly to the formation of silver vanadate phases on the surface of these materials. <s> BIB004 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Nitrides with addition of silver <s> article i nfo Nanocomposite films that consist of niobium nitride with silver nanoinclusions were created using unbal- anced magnetron sputtering to investigate their potential as adaptive, friction reducing coatings. The coatings were tribotested against a Si3N4 counterface in the 25 to 1000 °C temperature range. The coatings displayed coefficients of friction in the 0.15 to 0.30 range at T>700 °C. Post-wear testing structural and chemical char- acterization revealed that, in the low to mid-temperature range, silver migrated to the surface to reduce friction. At higher temperatures, oxygen, silver and the transition metal reacted to form lubricious binary metal oxide phases (silver niobate) in addition to pure silver. In situ Raman spectroscopy measurements were taken during heating and wear testing at 750 °C to identify the evolution of phases in the coatings surface and in the wear track. The analysis of the in situ Raman spectroscopy data revealed the various stages of formation of these binary metal oxides. The coatings were subsequently doped with MoS2 to investigate the effect of the introduction of a low temperature lubricant. The addition of MoS2 did not appreciably reduce the room temperature coefficient of friction, likely due to the miscibility of this compound with the transition metal nitride. However, the coefficient of friction was significantly reduced at high temperatures because of the synergistic lubricious effect of silver niobates and molybdates. <s> BIB005 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Nitrides with addition of silver <s> Abstract Silver tantalate was investigated as a potential lubricious material for moving assemblies in high temperature tribological applications. Three different approaches were explored for the creation of such materials on Inconel substrates: (1) powders produced using a solid state which were burnished on the surface; (2) monolithic silver tantalate thin films deposited by magnetron sputtering; and, (3) an adaptive tantalum nitride/silver nanocomposite sputter-deposited coating that forms a lubricious silver tantalate oxide on its surface when operated at elevated temperatures. Dry sliding wear tests of the coatings against Si 3 N 4 counterfaces revealed friction coefficients in the 0.06–0.15 range at T ~ 750 °C. Reduced friction coefficients were found in nanocomposite materials that contained primarily a AgTaO 3 phase with a small amount of segregated Ag phase, as suggested by structural characterization using X-ray diffraction. Furthermore, cross-sectional transmission electron microscopy techniques determined that the reduced coefficient of friction at T ~ 750 °C was primarily the result of the formation of a lubricious AgTaO 3 phase that reconstructs during the wear process into a mechanically mixed layer of AgTaO 3 , Ta 2 O 5 , and Ag nanoparticles. <s> BIB006
Ag and Au possess sufficiently low shear strength over a wide temperature range as well as stable thermal chemistry allowing them to be used in both ambient air and vacuum. Monolithic solid lubricants have a limited working range and wear relatively quickly due to their intrinsically soft nature. Silver (Ag) nanoparticles are easily embedded in carbide, oxide and nitride coatings, forming a composite structure. This multiphase structure, consisting of a solid lubricant dispersed in a hard matrix, can be designed so that their surface chemical composition and structure changes to minimise the coefficient of friction and wear rate in a broad range of temperatures. In this way, the soft noble metals with low shear strength and good ductile properties, acts as solid lubricants, and the hard coatings improve the abrasion resistance. In our research, silver and vanadium were simultaneously deposited on the surface of CM247 superalloy in an active screen plasma nitriding atmosphere (ASPN-V/Ag), and the CoF of the treated surface against Al 2 O 3 ball was reduced from 0.45 at room temperature to 0.38 at 300°C and 0.35 at 600°C (Figure 3) . Nitride based nanocomposites consisting of hard (wear resistant) phases and lubricious noble metals have been reported extensively, for example, Mo 2 N/Ag  , Mo 2 N/Cu  , CrN/Ag BIB003 , CrAlN/Ag BIB002 , VN/Ag BIB003 and NbN/ Ag BIB005 . A summary of the CoF of these systems at different temperatures is shown in Figure 3 . NbN/Ag nanocomposite coatings were deposited using unbalanced magnetron sputtering BIB005 . The inclusion of silver in NbN significantly reduces the CoF against Si 3 N 4 counterface to below 0.35 in the temperature range between 25 and 1000°C ( Figure 3) . The structural and chemical characterisation after wear testing revealed that, in the low to mid-temperature range, silver migrated to the surface to reduce friction. At higher temperatures, oxygen, silver and the transition metal reacted to form lubricious binary metal oxide phases (silver niobate) in addition to pure silver. Similarly, silver additions can reduce the CoF of TaN and VN significantly, but the amount of silver needs to be carefully controlled and optimised as suggested in Figure 4 BIB006 BIB004 . The introduction of silver into Mo 2 N thin films (Mo 2 N/ Ag), deposited by magnetron sputtering, had a complex effect. The addition of silver led to a relative low CoF at high temperature, up to 400°C. Moreover, the formation of the MoO 3 lamellar oxide played a key role in the lubricating mechanism at high temperature. However, the CoF at room temperature was much higher than that of Mo 2 N thin film without silver  . Copper additions to Mo 2 N (Mo 2 N/Cu) up to 22 at.% only had a marginal impact on the tribological behaviour  . Mulligan et al. added varying contents of Ag up to 22 at.% into CrN, via magnetron sputtering deposition. The CoF against alumina counterface dropped to 0.1 at 500°C during ball-on-disc wear tests conducted in the temperature range from 25 to 700°C. However the CoF was persistently high (0.5-0.90) in the temperature range from 500°C to 700°C BIB003 . The low CoF observed at the beginning of the test was strongly influenced by the size and shape of Ag agglomerates formed during heating on the coating surface, as Ag is highly mobile in CrN matrix at elevated temperature (see Figure 5 ). After the soft phase was worn away, the supply for the Ag from the coating ''bulk'' either was too low or the Ag reservoir was exhausted due to quick outward diffusion of Ag BIB001 . CrN, CrAlN and CrAlN/9 at.% Ag coatings were deposited on stainless steel substrates by unbalanced dc magnetron sputtering BIB002 . The CoF of these coatings sliding against an alumina ball in air at room temperature were 0.55, 0.37 and 0.23 respectively. The addition of Ag reduced the coefficient of friction of CrAlN. But it was strongly dependent on the content of Ag.