Substrate Surface Finishing
To understand how the surface finishing (five different conditions) of a soft substrate influences the tribological behaviour of hydrogenated amorphous carbon (a-C:H) films, DLC coating was applied on plasma-nitrided low carbon steel substrates. Both processes happened in a single PECVD batch (Soprano et al., 2018). The mechanical properties and bonding structures of the a-C:H films are negligibly affected by the substrate topography. Two groups of samples were originated from the deposition process, regarding their topography: (a) Rough (0.17 < Sq <0.23 µm) and (b) Smooth (0.08 < Sq <0.11 µm).
Incremental normal load reciprocating dry sliding tests in ambient air revealed friction coefficients of 0.09 (Rough) and 0.05 (Smooth), which suggests that the formation and stability of tribolayers are impaired on the rougher substrates, leading to higher friction coefficients. For smoother substrate surfaces, graphite-rich tribolayers were formed in the counter bodies yielding substantially lower friction coefficient values (~0.04) but higher wear of the coating. Since plasma nitriding treatment notably alters the surface topography of smooth surfaces, producing substrate surface finishing beyond Sq = 0.06 µm to obtain friction coefficients ranging from 0.04 to 0.06 is pointless.
Enabling the use of DLC multifunctional coatings on lower-cost substrates while preserving the tribological and mechanical performance of the system is undoubtedly a current challenge in the industry and demands additional research. Gray Cast Irons (GCI) and Nodular Cast Irons (NCI) arise as potential candidates to fulfill the demand due to their vast industrial applications such as large stamping tools (Corbella et al., 2004), automotive parts (Sánchez-López et al., 2003; Podgornik et al., 2008; Maurizi et al., 2014), household appliances industry (Agarwal et al., 2013), among others. Despite that, a small number of studies concerning GCI and NCI tribological behavior when used as a substrate for DLC coating was found in the literature (Andújar et al., 2003; Corbella et al., 2004; Podgornik et al., 2008).
To help bridge this gap, a tribological investigation was performed on a-C:H films deposited onto nitrided gray and nodular cast irons. Significant topographical differences between both substrate materials were induced mainly by the nitriding process, where wedges of more than 5 µm formed on GCI near-surface graphite flakes, Figure 4 (Giacomelli et al., 2017). These wedges played a paramount role in the tribological behaviour. They were created in regions where the graphite flakes were located near the surface, and they frequently covered entirely the graphite that was previously exposed (Figure 4e). After DLC deposition (2.5 µm thick), little modification was observed compared to the nitrided surface, as shown by the very similar topographic parameters.
Coated samples of both substrates presented similar scuffing resistance when tested under incremental load (reciprocating). However, coated GCI samples systematically showed an unexpected low friction coefficient during most of the lubricious regime (0.06). In contrast, the NCI presented a steady 0.15 friction coefficient during the whole extent of the durability tests. The presence of tribolayers covering a considerable extension of the wear marks governs the low friction coefficient of coated GCI. Such phenomenon is explained by the highly disordered graphitic structure of the tribolayers, confirmed via Raman analysis and reported to provide low shear strength. Another consequence of the formation of these tribolayers is the smoothening of the contact region, reducing the contact severity of the asperities found on coated GCI samples. This effect contributes to the maintenance of the system in a self-lubricating regime for more extended periods. On the other hand, NCI-LR interrupted test revealed that such tribolayers were not found since the few debris particles available were piled-up inside valleys, not effectively contributing to a reduction in friction coefficient in that case. Finally, the wear volume of NCI was 2.4 times lower once there was not a brittle wear mechanism like the one found for coated GCI.
As evidenced, graphite regions near the surface in both substrates play a key role in thin-film applications, especially considering these graphite sites represent mechanical voids regardless of their morphology and, thus, spalling of coatings such as DLC may appear in these regions during tribological contact (Salvaro et al., 2017). Due to the cruciality of this aspect, surface treatments were developed by different research groups to remove all near-surface graphite before the nitriding of similar materials (Karamiş and Yildizli, 2010; Zenker et al., 2013). Another critical issue is the relatively low hardness of cast iron metallic matrixes, which induced the necessity of the development of suitable mechanical support layers, being the nitriding of ferrous substrates a commonly used solution (Agarwal et al., 2013; Ebrahimi et al., 2015; Shioga et al., 2016; Giacomelli et al., 2017). However, as reported, the plasma nitriding of gray cast irons strongly affects its topography, generating wedge-like features (Rolinski et al., 2007, 2009). Factors such as graphite flakes orientation relative to the surface, volumetric expansion during the nitride layers formation, weak graphite-matrix adhesion and nitrided layer interruption by graphite would play a vital role in the formation of wedges (de Mello, 1983; Dalibon et al., 2013; Shioga et al., 2016). In Figure 5, a scheme for each graphite flake orientation is shown. The most critical case is when the graphite flake is skewed to the surface. Weak graphite-matrix interaction, interruption of the nitride layer and an intense local volumetric expansion happen simultaneously, systematically leading to the formation of wedges (Figure 5B). If the graphite flake is vertical, although there is an interruption of the nitride layer, no intense nitriding occurs since there is no barrier for the diffusion of nitrogen. Thus, less volumetric expansion occurs locally and no wedge formation was observed (Figure 5C). When the graphite flake is parallel to the surface, there is no interruption of the nitride layer, inhibiting the formation of wedges (Figure 5D). However, these locations can easily nucleate defects on the surface (de Mello, 1983).
Counterbody Geometry and Nature
Finally, the effect of the counterbody geometry and nature and the potentiality of using multifunctional coatings as an auxiliary mechanism in mixed lubrication were evaluated (Salvaro
et al., 2017).
A multifunctional coating strategy was applied using a nitride layer for mechanical support, followed by a silicon-rich interlayer to enhance the adhesion of the uppermost a-C:H coating. Cylinder-plane tribopair configurations with coated and uncoated GCI surfaces were evaluated under lubricated and dry conditions. The low viscosity (4.2 cSt), linear alkylbenzene oil associated with BTP anti-wear additive was used as a lubricant. Four distinct reciprocating cylinder-plane configurations (horizontal cylinder sliding without rolling against a plane) were tested in dry and lubricated conditions: DLC coated plane on a GCI cylinder, a DLC coated cylinder vs. a GCI plane, a DLC coated cylinder vs. a DLC coated plane, and a GCI uncoated cylinder against a GCI uncoated plane (Salvaro et al., 2017).
In general, GCI surfaces presented ductile abrasive wear, while the DLC surfaces exhibited brittle wear (spalling) regardless of the test condition (dry or oil-lubricated). For dry tests, the smoothening of the contacts generated wear debris on the contact, favouring the genesis of tribofilms that govern the tribological behaviour. Additionally, for both coated and uncoated horizontal cylinder surfaces, the DLC presence on the plane surface increases the dry friction coefficient. For lubricated tests, the formation of a BTP-rich on the uncoated surfaces played a crucial role in governing the wear rate.
As mentioned, the surrounding environment is a fundamental factor in the genesis and establishment of the tribolayer, which in turn, governs the tribological behaviour. Consequently, a great effort was dedicated to understanding how the environment (air, CO2, Argon, R134a, and R600a) affects the tribological performance of a proprietary, Si-rich multilayer DLC coating deposited on soft AISI 1020 steel.
Finely ground (Sq = 0.23 ± 0.025 µm) AISI 1,020 soft steel disks were coated with multifunctional CrN-Si-rich DLC by magnetron sputtering PVD. The tribological tests were conducted in a specially designed High-Pressure Tribometer (HPT) to simulate the hermetic refrigeration compressor condition. Yoon et al. (1998) present a detailed HPT description. In contrast, the tribological test procedures and specimens’ features were well detailed and explained in a previous paper (de Mello et al., 2009).
To assess the effect of a typical hermetic compressor refrigerant atmosphere, dry (unlubricated) tests were carried out in CO2 and R600a at 0.1MPa environmental pressure. As a reference, identical tribological tests were performed in unpressurized laboratory ambient air (45% relative humidity, 200C).
Tests performed with R600a presented the best wear performance of both specimens and counter-bodies, along with the lowest mean friction coefficient. In contrast, air atmosphere induced the highest ones (234, 52, and 266% higher, respectively). The wear mechanisms act mainly on the prominent asperities of the topography. There was a clear inverse correlation between the tribological behaviour and surface topography as evidenced by the reduced peak height (Spk) surface parameter (derived from the Abbot and Firestone curve) and the surface evolution to sharper and negatively asymmetrical (as compared to the Gaussian distribution), as observed in the morphological space (de Mello, 1983).
EDS and Raman analyzes do not present significant differences between wear marks on the specimens. Therefore, the authors assumed that the tribolayers on the counter bodies play a crucial role in the tribopair behaviour. All Raman spectra presented smaller bands at lower Raman shift. These bands can be related to the iron oxide formed due to the tribochemical reaction of the counterbody (steel pin) with the surrounding atmosphere.
The tests with R600a refrigerant gas presented typical G and D bands. In contrast, spectra related to CO2 and air have shown only a broad peak at around 1,300 cm-1; e.g., the G band was not detected. Since this peak could not be unambiguously assigned (de Mello et al., 2009), the superior tribological performance induced by the R600a atmosphere is likely to be due to the presence of the graphitic structure in the tribolayers as indicated by the presence of a strong G-band in the spectra.
In other studies (Silverio et al., 2010, 2016) using a different environmental chamber, we compared the influence of HFC-derived (R134-a) atmosphere on the tribological behaviour of the same multi-purpose coating. There was a clear increase in surface durability. This was attributed to incorporating fluorine (from the gas CH2-FCF3) into the tribolayer present in the counterbody, as evidenced by the EDS analysis.
This trend was confirmed by another more fundamental and in-depth study (Barbosa et al., 2015). We studied, using Scanning Electron Microscopy (SEM), micro-Raman spectroscopy, and glow discharge optical emission spectroscopy (GDOES), the physical-chemistry of the tribolayers formed on AISI 304 stainless steel rubbed against a 52,100 cylinder coated with DLC under a special (refrigerant R134a) atmosphere. The DLC coating was obtained via PECVD in a semi-industrial scale reactor. The DLC film was 2.5 µm thick and presented a silicon-rich interlayer to improve adhesion to the substrate. The tribological tests were performed in a tailor-made, high-stiffness tribometer containing a hermetic chamber to control the atmosphere, as well-described in a previous paper (Barbosa, 2014).
The noisy Raman spectra associated with tribolayer produced in tests carried out in the R134a atmosphere exhibit the G and D bands characteristic of DLC. Also, a low Raman shift (720 cm−1) can be observed. It was attributed to chromite, an iron chromium oxide (FeCr2O4). Besides, the spectra showed a sharp peak at around 1,120 cm−1 that does not coincide with any of the possible usually formed iron oxides (de Mello et al., 2009). A set of new tests were carried out using argon as the surrounding atmosphere to clarify this point. The results show a high similarity between the Raman signal obtained and those of the original DLC coating (Figure 6). Therefore, the peak at 11,20 cm−1 was not detected. According to the literature, the DLC fluorine doping increases the ID/IG ratio, i.e., the D band intensity increases relatively to G band, which is evident when compared the spectra (Yu et al., 2003; Marciano et al., 2010). Another result supporting this hypothesis is the higher friction coefficient presented by the tests carried out in the argon atmosphere compared to R134a tests (0.18 and 0.12, respectively), clearly indicating the potentiality of the in-situ fluorination of tribolayers formed in fluorine-rich atmospheres, such as R134a.
In synthesis, tribological properties of DLCs are usually governed by the formation, stability, and composition of tribolayers which, in turn, are a function of the contact parameters, especially pressures and temperatures (Holmberg et al., 1998; Barbosa et al., 2015; Salvaro et al., 2016, 2017; Wong and Tung, 2016). They result from continuous reactions between the surfaces in tribological contact and the surrounding atmosphere, lubricants, and even contaminants.
The tribolayers contain high disordered graphite (solid lubricant), which is derived from the hydrogen diffusion from the DLC matrix at around 4000C (temperature achieved due to the contact action), resulting in the local change from sp3 to sp2 carbon-carbon bonds structure (graphite) on the tribological contact (Liu et al., 1996; Liu and Meletis, 1997).
To better understand this crucial point, e.g., the genesis, stability, and composition of tribolayers, the long term stability of tribolayers were evaluated. The test duration influence (180, 500, 1,000, and 2,500 h) on the characteristics of tribolayers formed in DLC-stainless steel tribopairs tested under a controlled atmosphere (R134a refrigerant gas) was analyzed (Salvaro et al., 2016). The dry tests were conducted using a proprietary, high frequency (350 Hz), specially developed emulator which uses, instead of laboratory specimens, real components as tribopair (stainless steel cylinders and DLC coated pistons).
Tribolayers composed of elements originating from mutual transfer material transfers and oxides were found. Additionally, their thicknesses ranged from 100 to 500 nm and were more evident on the stainless steel surface. Thicker tribolayers were found on the stainless steel cylinder, and thinner ones on the DLC coated piston. From the chemical and dimensional stability point of view, the tribolayers reach a stable state after 1,000 h testing. It was suggested that a mutual transfer of certain elements between the two surfaces is at the origin of mutual destruction and formation of tribolayers until a stable state was eventually reached (Salvaro et al., 2016).
Doping DLCs with some elements, such as hydrogen, silicon, molybdenum, and fluorine, can further enhance their tribological properties. Silicon is one of de most common doping elements for diamond-like carbon coatings (Si-DLC), providing outstanding properties such as low friction coefficient, high scuffing resistance, and stability against humidity and temperature. Moreover, according to Oguri and Arai (1991), Si addition enhances surface roughness and improves the adhesion strength of the DLC. Fluorination is reported as a consequence of higher inertness and lubricity to induce lower friction coefficients and high wear resistance than other types of DLC, such as a-C:H (Sung et al., 2009). Such effect is promoted by the enhancement of the hydrophobic behaviour of the film and the decrease of its surface-free energy, which is a common way to evaluate the anti-stickiness behaviour of the materials (Donnet et al., 1994; Trojan et al., 1994). According to Wang et al. (2013) fluorine termination layers on the DLC film can provide lower friction coefficients due to the large filled electron density coverage. Thus, the friction between similar F-terminated DLC film surfaces is governed by repulsive electronic interactions.
…To be continued
Gabriel Borges (GB) is a Student / Intern at Federal University of Santa Catarina, Florianopolis, Brazil.
Diego Salvaro (DS) is a Researcher at Federal University of Santa Catarina, Florianopolis, Brazil.
Roberto Binder (RB) is a Senior Researcher at Embraco (Brazil), Joinville, Brazil.
Cristiano Binder (CB) is (Primary) an Adjunct Professor at Federal University of Santa Catarina, Florianopolis, Brazil and an Adjunct Professor at Materials Laboratory – LabMat: Federal University of Santa Catarina, Florianópolis-SC, Brazil.
Aloisio N. Klein (AK) is a Professor at Federal University of Santa Catarina, Florianopolis, Brazil.
Jose D. B. de Mello (JM) is a Professor, Federal University of Uberlandia, Uberlândia, Brazil.