High‐Temperature Tribological Behavior of CrN‐Ag Self‐lubricating Coatings

Abstract

Transition metal nitride hard coatings like CrN or TiN are widely used in several applications due to their excellent wear, corrosion and oxidation resistance. However, the friction coefficient of most transition metal nitride coatings is fairly high (0.6-0.8) and the tribological effectiveness, especially at elevated temperatures, is insufficient. [1,2] Therefore, a lot of effort has been made in recent years to decrease the friction coefficient at room as well as at elevated temperatures. [3][4][5] Adding layered solid lubricants like DLC, MoS 2 or WS 2 to hard coatings by forming nanocomposite structures has been established as a suitable approach to reduce friction. [6][7][8][9] Nevertheless, these lubricating phases fail due to degradation and oxidation at temperatures > 350°C. [5] The incorporation of V as a high temperature lubricant into transition metal nitride coatings forming a solid solution (Ti-Al-V-N) [10] or superlattice structures of hard and lubricating layers (TiAlN/ VN, TiAlN/TiAlVN) [11,12] led to a significant enhancement of the tribological performance in high-temperature applications. At temperatures between 500 and 700°C, so-called Magneli phase oxides (V-O) are formed on the coating surface. Some of them, in particular V 2 O 5 , show low shear strength in the solid state as well as low melting temperatures, thus providing lubrication by solid and/or liquid oxide phases. [13,14] However, these oxides are only formed under certain environmental conditions (high temperatures and oxygen-containing atmosphere). Apart from metal oxide lubricants, soft noble metal phases (Ag, Au) have been reported as promising candidates for lubrication over a wide temperature range (e.g. synthesized as nanocomposites TiC/Ag, [15] yttrium-stabilized zirconia YSZ/Au, [16,17] CrN/Ag [18] ). The benefit of these nanocomposite systems is that noble metals may act as lubricants at room temperature (RT) as well as high temperatures due to their low shear strength and stable thermochemistry. Due to their low sensitivity to the environment, such coatings are applicable in ambient air as well as under vacuum conditions in a wide temperature range.We have recently shown that the incorporation of Ag as a solid lubricant in hard wear-resistant CrN coatings decreases friction and wear at RT. [18] In addition, we found that vacuum annealing at 600°C leads to the formation of Ag agglomerates on the coating surface. In this study, special emphasis is laid on the influence of surface reactions, like the formation of these agglomerates and/or surface oxidation, on the hightemperature tribological behavior at 600°C. Characterization of the friction curves and the wear tracks provides an understanding of the relation between the out-diffusion of Ag (forming agglomerates) and its lubricating effect. ExperimentalThe substrates were ground and polished 304 stainless steel samples that were cleaned in ultrasonic baths of trichloroethane, acetone, methanol, and deionized water, respectively and blown dry with N 2 . The ∼ 3 lm thick coa...

Adaptive Mo2N/MoS2/Ag Tribological Nanocomposite Coatings for Aerospace Applications

“…This was close to that measured in the current work for the Mo 2 N/Ag coating ( Table 2, coating S-27-0). Also, Kutchej et al [41] reported CoF values greater than 0.4 for the CrN/Ag system with different Ag contents when tested at 600°C against Al 2 O 3 . Unfortunately, neither of these literature reports provide data for tests against Si 3 N 4 balls.…”

CrCuAgN PVD nanocomposite coatings: Effects of annealing on coating morphology and nanostructure

“…Ag or Cu), as a solid lubrication phase, embedded in a hard wear-resistant matrix, such as a transition metal nitride [2][3][4][6][7][8][9], carbide [10,13] or oxide [14][15][16], and mixtures of these ceramics (in ternary/quaternary/nanocomposite coating systems), have all been extensively studied, with the promise of improved tribological performance during transient and/or cyclic temperature changes [17][18][19]. In particular, coatings based on Cr-Ag-N [7,8,[17][18][19][20][21] and Cr-Cu-N [6,22,23] (as two typical coating systems), have been studied. For coatings in the Cr-Ag-N system, it is revealed that Ag precipitates often tend to exhibit a lamellar shape (height/width: ~ 1/2 to 1/3), with a uniform, but isolated distribution in the ceramic nitride matrix of the deposited coating [24,25].…”

Tribological Properties of TiN/Ag Nanocomposite Coatings

“…These soft metal phases give rise to improved tribological properties over a wide temperature range due to their low shear strength. For this purpose, preferably Au and Ag, due to their chemical inertness, are used in combination with oxides [10][11][12][13], nitrides [6,[14][15][16], and carbides [17], where reduction of friction and/or wear have been reported. The addition of a soft metal phase into a hard coating offers also the possibility to improve coating toughness by influencing e.g., intrinsic stresses and structure [18][19][20][21][22].…”

“…The addition of a soft metal phase into a hard coating offers also the possibility to improve coating toughness by influencing e.g., intrinsic stresses and structure [18][19][20][21][22]. In previous investigations, it was found that the addition of Ag to transition metal nitride coatings only reduces the friction coefficient at room temperature (RT) when a relatively high Ag content is used [14][15][16]. For high Ag contents, Endrino et al [17] reported about a considerably reduced friction coefficient in vacuum also for the systems WC/Ag and TiC/Ag.…”