The original approach encompassed the development of new surface engineering procedures applying purpose-oriented phases to soft substrates. Particular emphasis is given to the in situ fluorination of the tribolayer formed in the piston-cylinder tribopair. Although chlorinated halocarbons (CFC) are efficient refrigerants from a thermodynamic point of view, they do have serious environmental implications that have forced the refrigeration industry to switch to more environmentally friendly hydrofluorocarbon (HFC) based refrigerants. The first and most successful alternative to the CFCs was tetrafluorocarbon, particularly tetrafluoroethane (CF3CH2F), aka R134a refrigerant. The current trend to downsizing mechanical systems, smaller clearances, and increased speeds leading to greater energy efficiencies associated with miscibility issues imposing the use of costly, fully synthetic lubricants with the R134a refrigerant gas led to the introduction of a new hermetic compressor design, the Wisemotion®, the first, and until now, unique, oil-free hermetic compressor on the world market. In this context different types of multi-layers, their thickness, substrate material, processing routes, etc., have been studied and optimized. Si-rich hydrogenated DLC (a:C-H) presented enhanced tribological properties when tested under fluorine-rich atmospheres, and semi-industrial scale tests have been carried out to understand this point further. A homemade tribological emulator was developed allowing close-to-real tribopair, atmosphere, and imposed mechanical conditions used in an oil-free commercial hermetic compressor. The tests were carried under different stroke frequencies (5, 20, and 40 Hz) and atmospheres (R134a, ambient air, and argon). Results showed a strong influence of both atmosphere and stroke frequencies. The friction coefficients were significantly lower (~3.8X) for the refrigerant gas atmosphere, attributed to the fluorine and highly disordered graphitic structures rich tribolayers. Under the high frequency (40 Hz), the energy input seems to be a deterrent to the formation of stable tribolayers, and the DLC coating shatters on the first few sliding meters.
In total, 23% of the world’s total energy consumption originates from tribological contacts, and the introduction of new technologies to improve the tribological efficiency of mechanical systems could amount to the annual saving of 1.4% of the Gross Domestic Product (GDP) on a global scale (Holmberg and Erdemir, 2017). The vast number (~3 billion) of heat pump systems, air-conditioning, and refrigerators operating worldwide makes the refrigeration industry substantially contribute to this figure (International Institute of Refrigeration, 2015). Furthermore, household refrigeration represents 7.3% (and 17% for air conditioning) of residential energy consumption in the USA (U.S. Energy Information Administration, 2015), 30-40% in China (Cheng et al., 2011) and 30% in Brazil (Boeng and Melo, 2014).
Several challenges require that the refrigeration industry keep a continuous innovation program by research and development to achieve high energy efficiency, high-reliability levels, low noise, and environmental regulations. For instance, the chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are excellent refrigerant fluids. Yet, depending on their high Global Warming Potential (GWP), the CFCs were banned from refrigeration applications with the signature of the Montreal Protocol by 197 countries in 1987. Even the HCFCs are in the process of being phased out. Table 1 presents details on some of the most common refrigerant fluids. The banishment of CFCs has driven humanity to the use of hydrofluorocarbons (HFCs), such as tetrafluoroethane (CF3CH2F), aka R134a refrigerant, and more recently, the environment-friendly hydrocarbon isobutene (R600a) (Cannaday and Polycarpou, 2005; de Mello et al., 2009). Tetrafluoroethane was the first and most successful alternative to the CFCs, and it’s still widely used by the refrigeration industry, with applications varying from domestic refrigeration to automotive air conditioning.
Moreover, HFCs’ incompatibilities with mineral oils have also induced a change from the mineral type to polyol ester (POE) or polyalkylene glycol (PAG) lubricants. Miniaturization and the worldwide tendency to reduce lubricant oils and their additives also affect the refrigeration industry. One of the major impacts is the increase in the severity of the tribological contacts in hermetic compressors. Besides, many compressor components operate in the boundary and mixed lubrication regimes (Pergande et al., 2004).
The regular hermetic compressor is a relatively complex mechanical system containing the electric motor, the mechanical kit, the lubricant, and the refrigerant in a hermetic chamber. It must have a very long lifetime (10 years warranty operating in extremely different conditions), stringent tolerances, and uses very low viscosity oils (in order to reduce viscous losses). The regular compressor, Figure 1A, works in an on-off cycle (high losses due to boundary and elastohydrodynamic lubrication regimes), circular motion, single speed, many tribological contacts, oil for lubrication.
The efficiency of hermetic compressors has increased incrementally for the last 40 years. In this context, solid lubrication and solid lubricants began to appear as potential choices for governing friction and wear in hermetic compressors. Additionally, since the thermodynamic efficiencies of refrigeration cycles increases, if the reduction in refrigerant flow is minimized or even eliminated, the interest in the evolution to oil-less compressors is undeniable (Solzak and Polycarpou, 2006). Therefore, in the mid-2000s, the scenario was opportune for developing an entirely new concept, which created a real paradigm shift, able to meet the latest environmental and tribological requirements, including a transition toward an oil-less compressor (Barbosa et al., 2015).
However, the classical tribological literature (Jost, 1990) clearly shows that a lubrication system modification is insufficient to produce real and relevant progress. To obtain a substantial and significant evolution we must consider all the multi/inter disciplinary aspects of tribology.
Development of Oil-Less Hermetic Compressors
Our research group participated intensively in a research and development program that culminated with introducing a new generation of oil-free hermetic compressors. Wisemotion® is the first and, until now, unique, oil-free hermetic compressor on the market, and its development led to around 80 patents (Embraco, 2019). In contrast with regular compressors, the Wisemotion® mechanism, Figure 1B, performs only linear reciprocating movement and has a single tribological contact, the piston-cylinder pair that compresses the refrigerant fluid.
The Wisemotion® is smaller, much smaller than the previous generations. It means that it is possible to free up to 20 liters of cabinet space and to introduce new designs and architectures for refrigerators. The Wisemotion® is a top efficiency compressor and complies with some of today’s strictest efficiency regulations. Additionally, it is very silent, about 10 dB quieter.
Our research group’s main contribution is related to the unique tribological contact (piston-cylinder), particularly the piston.
Diverse inorganic materials (e.g., transition-metal, graphite, hexagonal boron nitride, boric acid, polymers, Diamond Like Carbon–DLC, etc.) can provide excellent lubrication (Lancaster, 1984; Lansdown, 1999; Erdemir, 2001). Common sense suggests using coatings, ensuring that the surface properties are sufficient to meet the tribological requirements (low friction and high wear resistance) of the system, as an ideal solution. For hard substrates such as hardened steels and cermet (WC-Co), the use of hard solid lubricant coatings has been widespread with great success (Rechberger et al., 1993; Holmberg and Matthews, 1994). However, spalling induced by high stresses generally promotes low adhesion in hard coatings-soft substrate systems associated with small and cheap components needed to be manufactured on a large and economically efficient scale (Holmberg and Matthews, 1994).
We started by carrying out a very comprehensive screening of commercially available coatings applied onto various soft substrates, Table 2.
This resulted in developing a new methodology to characterize coatings (de Mello and Binder, 2006); a very interesting, unpublished, proprietary ranking of the available coatings encompassing the average steady-state friction coefficient within the lubricious regime (µ ≤ 0.2), the scuffing resistance, the micro-abrasion wear coefficient, and the characterization of the wear mechanisms.
Initially, a new methodology allowing short test time with good discrimination between multifunctional layers was developed (de Mello and Binder, 2006). The protocol is based on the incremental normal load technique. By increasing the normal load in increments at a constant time interval, the surface durability of both the hard layer and the solid lubricant coating is determined (de Mello and Binder, 2006). Besides, the average steady-state friction coefficient within the lubricious regime (µ <0.2) is obtained.
Again, the literature provides judicious advice. Donnet and Erdemir (2004) synthesizing more than 2,000 published papers, stated that: there exists no single solid lubricant that can provide both low friction and wear over broad substrate hardness, temperatures, and environments use conditions. Multifunctional surface engineering processing route combining customized layers applied to soft substrates allows the combination of high wear resistance, load support, and low friction coefficient (de Mello and Binder, 2006). Our main conclusions pointed to multilayer, multipurpose coatings, with hydrogenated DLC as the low friction top layer.
Diamond-like Carbon (DLC) coatings are defined as a metastable form of amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H) prepared by a wide variety of PVD and CVD techniques. The film structure and properties are determined by the Hydrogen content and the relative proportion of sp2 and sp3 carbon hybridizations.
With excellent wear resistance, DLC-based tribosystems have very low dry friction coefficients, among the lowest (0.1 < µ <0.2) reported in the literature (Brookes and Brookes, 1991; Feng and Field, 1991; Miyoshi, 1995; Erdemir et al., 2000a,b; Field, 2012). Values as low as 0.01, associated with extremely low wear rates (10−9-10−10 mm3/N m) are frequently reported (Erdemir et al., 2000a,b; Donnet and Erdemir, 2008) which are (Grill, 1997; Erdemir et al., 2000a, b; Donnet and Erdemir, 2008). Si addition to DLC (Si-DLC) besides improving surface roughness and adhesion strength (Oguri and Arai, 1991) induces low friction, high durability, and stability against humidity and temperature.
The nature of the substrate, DLC and load-bearing films, their respective thicknesses, the nature of the counter faces, together with the environmental conditions, are of paramount importance and play an essential role in friction and wear control of DLC-based tribosystems (Franks et al., 1990; Oguri and Arai, 1991; Donnet and Grill, 1997; Voevodin et al., 1999; Erdemir et al., 2000a, b; Erdemir, 2001; Ohana et al., 2004a, b; Vercammen et al., 2004; Cho et al., 2005; Donnet and Erdemir, 2008).
In this sense, the genesis, stability, and nature of a tribolayer on the sliding contact appears as the key for obtaining low friction and long wear life in most solid-lubricated systems. The nature of the counter faces can significantly influence the size and nature of the transfer layer (both of which sometimes differ from the initial composition of the film) (Erdemir, 2001).
In this context, our group has been dealing with different kinds of DLC as low friction top most layer, different nitrides as load bearing/wear resistant layer, the thickness of these layers, low carbon steel and cast iron (gray and nodular) as substrate, the nature and geometry of the counterbody and, of course, the surrounding atmosphere.
Layers Nature and Thicknesses
Lara et al. extensively analyzed the effect of the thickness of DLC and CrN layers on hardness, scratch resistance, interface adhesion and sliding wear of proprietary magnetron sputtered diode multifunctional CrN-Si-rich DLC coatings deposited onto soft (120 HV) AISI 1,020 carbon steel substrates (de Mello, 1983; Lara and De Mello, 2012; Lara et al., 2015). The sliding wear tests, in particular, used a 3D triboscopic maps-based original approach.
Averaging the measured variable after steady-state conditions are reached is the leading technic used for data evaluation of tribological tests. Triboscopy, initially proposed by Belin (1993) and recently enhanced by dos Santos et al. (2015) is another potential and versatile approach that allows the location of microscopic tribological events as well as to know their evolution while providing information with both local details and global evolution of the tribological phenomena (Zhu et al., 2010).
The thicknesses of both layers were varied and the specimens could be grouped into two families: thicker and thinner coatings. Previous works extensively characterize the main coatings characteristics, e.g., Raman spectroscopy, thickness, mechanical properties of the coating layers and substrate (Lara and De Mello, 2012; Lara et al., 2015). The ball-on-flat reciprocating sliding tests were carried out in a tribometer Plint TE 67 using a frequency of 2 Hz and a stroke length of 10 mm at room temperature conditions (Lara et al., 2015).
Incremental load (7 N every 10 min) 3D triboscopic maps are shown in Figure 2. No significant variation could be observed for the durability of the coatings (Lara et al., 2015). However, thinner coatings presented the lowest average friction coefficient (0.07 vs. 0.11 respectively) within the lubricious regime. For thicker coatings, the very first friction coefficient (0.07) progressively increases as the normal load is incremented up to 105 N when the lubricious regime ends (Figure 2A). In contrast, for thinner coatings, the friction coefficient starts around 0.1, stabilizes at ~0.07 up to 112 N (Figure 2B).
Contact potentials for both samples (Figures 2C, D) are initially high (around 50 mV), which is typical of DLC-metal insulated contacts, and then decrease to values around zero, when the friction coefficient rises above 0.2, indicating a non-insulating contact. Very good matching was found between the friction coefficient maps and the contact potential maps for all the tested samples.
Typical friction triboscopic maps for the constant load tests showed that the family of thinner coatings presented a reasonably stable friction coefficient throughout the test, with few oscillations. In contrast, the thicker coatings presented higher friction coefficients in the region where spalling of the DLC coating could be detected, Figure 3.
Wear rates on the thinner layers were up to five times lower than the thicker ones, on the samples and their associated counterbodies. Thus, even though both layers have presented similar friction coefficients and durabilities within the lubricious regime, the thinner coatings presented better overall tribological behaviour.
To analyze the influence of the coating thickness on the stress distribution, the results above were compared to, and are in accordance with, indentation tests and simulations made using the commercial software FilmDoctor (Lara and De Mello, 2015; Lara et al., 2015).
To further understand the influence of the nature of the load-bearing layer, we developed, using Plasma Enhanced Chemical Vapour Deposition (PECVD), an in house proprietary coating registered as CHИ® (Shioga et al., 2016). The multifunctional coating consisted of a load-bearing layer (plasma nitride layer), and a low friction layer (DLC) deposited onto soft, low carbon steel (AISI 1,020).
Plasma nitriding is a diffusive thermo-chemical treatment that increases the surface hardness in steels and metallic alloys (Spalvins, 1983; Maliska, 1995; Pinedo and Monteiro, 2001; de Mello et al., 2010; Zhu et al., 2010).
Plasma nitriding processes generate little waste, require few consumables, and are easier to control than other nitriding processes (Spalvins, 1983). Additionally, the costs of the process comply with the industry requirements (Spalvins, 1983; Maliska, 1995; Pinedo and Monteiro, 2001; Zhu et al., 2010). Different nitride phases can be obtained by tuning the control variables for the plasma nitriding. Literature indicates that different gas mixtures used in the process can originate different hardness profiles and case depths (Karakan et al., 2002).
Three distinct nitride layers were synthesized: one compound layer mostly formed by an e (Fe2−3N) phase, one compound layer predominantly g’ (Fe4N), and one diffusion layer. All three surfaces received the same DLC coating. Shioga et al. depict in detail the processes and characterization techniques used in the work (Shioga et al., 2016).
The adhesion of the coatings was determined using an improved version of the technique described by the German standard VDI3198 (VDI, 1992). DLC layers on top of the composite e and g’ layers were the most resistant to delamination. In contrast, the delamination of the DLC layers deposited on the diffusion layers was almost two times larger.
The tribological performance of multipurpose coatings (composed of a DLC and a nitride Layer) was analyzed via the durability test method proposed by de Mello and Binder (de Mello and Binder, 2006). All specimens behaved similarly in the lubricious regime. Nevertheless, a significant variation was found in the friction coefficient as well as in the extent of the lubricious regime. The DLC deposited over the diffusion layer performed the best, with the durability of 13.000 N.m and friction coefficients around 0.05 (the lowest). The g’ layer coated with DLC demonstrated the worst performance; tests performed on that system have failed in the first few minutes, presenting durability of 600 N.m. The average friction coefficient was the highest (0.2).
Regrettably, the topographical features of the samples are altered by the plasma nitriding process, which usually increases the roughness to the micrometer range (Jeong and Kim, 2001; Dalibon et al., 2013), directly affecting the mechanical support for the DLC (Karakan et al., 2002; Masuko et al., 2013), and is strongly influenced by the hollow cathode effect (HCE), as recently demonstrated by Lamim et al. (2019). The initial roughness also influenced the case depths of the nitride layers (Singh et al., 2006).
…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.