Friction reduction in MoS2 DLC coatings by cyclic plasma quenching - Part 1 - Experiments with 3D-printed substrates

Plasma quenching for the formation of nanoparticles uses a cyclical increase in the prevailing pressure in the vacuum chamber in sputtering processes. In the case of carbon (C), using a typical industrial magnetron sputtering system and cathode sputtering of molybdenum disulphide targets (_MoS2) in an acetylene atmosphere (_C2H2) as well as a technically simple reduction in the suction power of the high vacuum pump via a slide valve, a significant reduction in the friction coefficients on smooth, polished stainless steel substrates (CrNi-18-10) was achieved, polished stainless steel substrates (CrNi-18-10) and rough, wavy 3D printed polyamide 12 substrates (PA12). Despite locally very high surface pressures with only a partial contact area of the aluminum oxide (_Al2O3) mating body in the tribological tests, which are accompanied by strong plastic deformations of the rough laser-sintered PA12, significantly lower coefficients of friction are measured during the investigated contact cycles than for contact with uncoated PA12 or _MoS2-C coatings produced conventionally on PA12 without the use of plasma quenching. Part 1 presents the preliminary considerations.

Introduction

Based on decades of use as a rapid prototyping process for the production of design and product samples, generative manufacturing processes ("3D printing", additive manufacturing, AM) of plastic parts are increasingly replacing established conventional manufacturing processes such as injection molding and CNC machining - especially in areas of smaller, customer-specific component series (i.e. in the range of up to 3000-5000 pcs.) with complex geometry. In particular, the potential for high mechanical properties and very good manufacturing accuracy (< 0.05 mm) currently shows the greatest possible industrial application potential for selective laser sintering (SLS) of semi-crystalline plastic powders into highly stressable technical components.

Post-processing opens up additional application possibilities for AM production, e.g. by applying wear-resistant coatings to replace aluminum components. As surface treatments for increased wear resistance, but without reducing friction, painting with mostly epoxies and (vinyl) acrylates or sol-gel systems to increase hardness dominates [12]. Frequently used galvanic and electrochemical processes (Cu-Ni-Cr, electroless nickel plating, sometimes with particle reinforcement [15]) often result in low coating adhesion. Vacuum coatings of plastics are currently mainly used for decorative purposes in thin layers with prior lacquering to smooth the surface. It is very difficult to produce hard, wear-resistant coatings, as most industrial manufacturing processes - such as thermal and plasma spraying processes and PVD/PACVD processes in a vacuum - lead to high temperature stress on the substrate and thus to deformation and/or degeneration or outgassing of the plastic [4, 10]. Layer adhesion problems also occur in many cases due to the very different material properties of hard, wear-resistant coatings with a high modulus of elasticity or flexible plastics. This results in a lack of load-bearing capacity in thin coatings [14], which also have low toughness with high hardness. Even slight elastic deformation of the plastic (e.g. ~ 2% elongation), e.g. under point loads, then triggers cracking (failure) in ceramic coatings [5-7]. Weight-optimized thin-walled components are therefore often not coatable.

Ceramic abrasion-resistant fillers are not an alternative, as although they increase wear resistance, they often lead to extremely brittle material behavior of the plastic itself at the contents required for abrasion resistance [8]. At the same time, high coefficients of friction are the result, as detached particles have a highly abrasive effect on plastics - similar to particles from hard wear protection coatings (i.e. nitride-, carbide- and oxide-based hard materials (TiN, _Al2O3, CrN, TiCN, etc.)). This can be avoided by using solid lubricants [16, 17]. In general, _MoS2 and graphite with a lamellar structure [13, 3] exhibit very low friction coefficients (_MoS2: min. 0.002, WS2: graphite: min 0.07 under laboratory conditions) due to sliding of planes aligning under load and transfer layer formation [9]. However, humidity in particular increases the friction [2, 9] for _MoS2 by up to 150 times (i.e. 0.25), for carbon up to 0.5. In a humid environment, electrostatic and capillary forces to the water interlayer occur again [3]. In non-lamellar-structured diamond-like carbon (DLC), the low friction is due to chemical inertness and transfer layer formation. The influence of moisture is significantly lower, especially for H-containing DLC (a-C:H). The combination of both reduces high fluctuations.

Fig. 1: 4-phase model for the in-situ formation of (nano)particles during plasma quenching of "unbalanced" magnetron sputter plasmas (according to [Zi17-2])Traditionally, solid lubricants are applied as powder, aerosol and colloid [3], but this has the disadvantages of low thickness homogeneity, porosity and, above all, low adhesion to the surfaces to be lubricated. Solid lubricant particles as fillers in plastics or in paint or electroplating coatings are also widely used, but have disadvantages for small (precision) components in terms of significantly reduced brittleness. The use of vacuum plasma coating processes, on the other hand, leads to very dense, nanocrystalline coatings without adhesion problems, whereby the content of chemical species in the layer (C and _MoS2) can also be controlled very well as a gradient with depth-dependent mechanical properties and intrinsic residual stresses [Th16/19]. Thick layers >10 µm can be easily realized, but coatings with a thickness of ~5 µm, i.e. far below the occurring roughness values (e.g. 35% of Ra), already show low friction and wear rates due to ablation of roughness peaks (i.e. PA12 base polymer with MoS2 _DLC layer) and attachment of submicrometer-sized particles in the adjacent roughness trenches ("self-healing effect"). Cracking in these transfer layers between polymer and coating particles is prevented by the high deformability of the layers, good bonding between all components due to very similar surface energy of polymer and _MoS2.

The development goals of the research project are (1) higher coating rates with low heating and at the same time (2) low coefficients of friction between 0.15 and 0.2 for plain bearings (gear flanks, bearing sleeves) under dry friction conditions (emergency running properties). It is possible to increase the coating rates in sputtering processes for DLC coatings in particular by combining them with PACVD coating (i.e. with a higher flow of hydrocarbon reactive gas, e.g. _C2H2 or CH4) [18], which results in a higher H content in the a-C:H coatings and thus lower coating hardness. Friction can be reduced by incorporating reservoirs of _MoS2 or graphite into the coatings. Reservoirs with sizes in the range of 100 nm diameter can be achieved by plasma quenching, which has, however, only been scientifically investigated in very few cases for magnetron sputtering processes [21]. The "quenching" of the plasma, which is also used in (PA)CVD processes for nanoparticle synthesis, is based on the introduction of e.g. He into the existing plasma, which is composed of Ar ions (sputter gas) and sputtered ions and atoms of high kinetic energy (e.g. C in [20]). In magnetron sputtering, plasma extending far from the sputtering cathodes into the recipient is decisive ("unbalanced" magnetic field configuration).

Collisions result in energy transfer to the He atoms (reduced mean free path due to locally higher particle density in the plasma). Slower dispersion of the sputtered species increases the tendency of their mutual physical combination ("seeding phase" in Fig. 1) [11] and subsequent chemical, e.g. covalent bonding. In addition, these small clusters or particles collect electrons from the plasma (coalescence), whereby the negative charge contributes to their enlargement by combining with other sputtered ions [1]. The shape of the particles depends on the kinetic energy and the Coulomb forces [19]. In the C plasma, for example, mainly ^sp2-bonded graphite particles are formed [11], in _MoS2(PA) CVD plasmas stoichiometric particles with a round to cauliflower-like and brittle structure. In addition to the species and the flow of e.g. He as quenching gas, the duration of the inlet has a significant influence on the extent of particle formation, i.e. especially the particle size, which is between 40 and 150 nm in the "coalescence" phase.

Unwanted agglomerates with a size of several 100 nm to µm are formed especially if the inlet time is too long and ultimately lead to granular layers [21]. Zia et al [20] report an increase in coating hardness with a simultaneous decrease in friction coefficient (~ 0.1) and wear rate (factor 3) when low submicron particle contents are realized in or on the coatings. The tribological improvement in particular is associated with the formation of a graphite film on the contact surface. However, higher particle contents or agglomerates have a negative effect.