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

Abstract

Reactively sputtered Mo 2 N/MoS 2 /Ag nanocomposite coatings were deposited from three individual Mo, MoS 2 , and Ag targets in a nitrogen environment onto Si (111), 440C grade stainless steel, and inconel 600 substrates. The power to the Mo target was kept constant, while power to the MoS 2 and Ag targets was varied to obtain different coating compositions. The coatings consisted of Mo 2 N, with silver and/or sulfur additions of up to approximately 24 at%. Coating chemistry and crystal structure were evaluated using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), which showed the presence of tetragonal Mo 2 N and cubic Ag phases. The MoS 2 phase was detected from XPS analysis and was likely present as an amorphous inclusion based on the absence of characteristic XRD peaks. The tribological properties of the coatings were investigated in dry sliding at room temperature against Si 3 N 4 , 440C stainless steel, and Al 2 O 3 . Tribological testing was also conducted at 350 and 600°C against Si 3 N 4 . The coatings and respective wear tracks were examined using scanning electron microscopy (SEM), optical microscopy, profilometry, energy dispersive X-ray spectroscopy (EDX), and microRaman spectroscopy. During room temperature tests, the coefficients of friction (CoF) were relatively high (0.5-1.0) for all coating compositions, and particularly high against Si 3 N 4 counterfaces. During high-temperature tests, the CoF of single-phase Mo 2 N coatings remained high, but much lower CoFs were observed for composite coatings with both Ag and S additions. CoF values were maintained as low as 0.1 over 10,000 cycles for samples with Ag content in excess of 16 at% and with sulfur content in the 5-14 at% range. The chemistry and phase analysis of coating contact surfaces showed temperature-adaptive behavior with the formation of metallic silver at 350°C and silver molybdate compounds at 600°C tests. These adaptive Mo 2 N/MoS 2 /Ag coatings exhibited wear rates that were two orders of magnitude lower compared to Mo 2 N and Mo 2 N/Ag coatings, hence providing a high potential for lubrication and wear prevention of hightemperature sliding contacts.

Molybdenum Nitride Films: Crystal Structures, Synthesis, Mechanical, Electrical and Some Other Properties

“…The results obtained by Aouadi et al [5] are consistent with the previous ones. The authors synthesized Mo2N/MoS2/Ag nanocomposites about 2.8 μm thick on hardened 440C grade stainless steels and on inconel 600 substrates from individual targets of Mo, MoS2 and Ag in (Ar-N2) gas mixture with partial pressures of 0.3 and 0.04 Pa, respectively, using an unbalanced magnetron sputtering apparatus.…”

“…They also have a good chemical resistance. Thus, they can be used in various applications such as hard coatings, corrosion and abrasion resistant layers to protect the tools (few microns thick), or as adaptive tribological coatings with low friction coefficient and wear [5][6][7][8] and for micromechanical applications [9]. They are also very interesting as gate electrodes and barriers of diffusion [10][11][12][13][14].…”

“…Among transition metal nitrides such as Ti, Zr, W, or Cr nitrides, Mo nitrides have been much less studied although they exhibit hardness values ranging from 28 to 34 GPa, whereas those for Ti or Cr nitrides range between 18 and 24 GPa [5,6]. Because of their strong covalent bonding, the calculated values of the bulk modulus of the stoichiometric MoN phase of hexagonal or cubic structure are comparable with those of c-BN and even diamond.…”

Nanocomposite Coatings: Preparation, Characterization, Properties, and Applications

“…This method is used generally for producing of the I/I nanocomposite coating [41][42][43][44][45][92][93][94][95][96][97], which includes the inorganic matrices and inorganic nanoparticles. For these coatings, PVD method includes the following: laser ablation [98], thermal evaporation [99], ion beam deposition [100], ion implantation [101,102], laser-assisted deposition [103], and atom beam cosputtering technique.…”

A Thermochemical Process Using Expanding Plasma for Nitriding Thin Molybdenum Films at Low Temperature

“…They also have high melting point, good chemical stability, low electrical resistivity; they can be used as gate electrode materials for high-k gate dielectrics [1][2][3]. Their surface oxides (MO x ) of lamellar structure offer good lubricating properties at high temperature.…”