Our results show, unequivocally, the great potential of in situ tribo-fluorination in situ as friction and wear control agent in DLC-steel pairs when in a fluorine-rich atmosphere, the object of this article.
In situ Tribo-Fluorination
This study evaluated the role of a fluorine-rich atmosphere on the tribological behaviour of a tribopair formed by a DLC-coated piston sliding against a stainless-steel cylinder of an oil-free hermetic compressor. To simulate close-to-real operation conditions, it was used a homemade tribological emulator. Tribological tests were performed with real mechanical components as tribopairs. With such a kind of apparatus, it was possible to achieve frequencies closer to those of a hermetic compressor while also controlling the testing atmosphere.
Materials and Methods
A homemade reciprocating tribological emulator was used to test Si-rich DLC-coated AISI 1020 pistons vs. ¼ AISI 316 cylinders. Both components originate from a hermetic oil-less compressor assembly. Figure 7 presents a schematic diagram of the tribological emulator and highlights the piston-cylinder tribopair. The emulator is programmed in such a way that the transversal sliding only starts when the normal load has been fully applied and the test chamber is above atmospheric pressure. The transversal motor achieves frequencies up to 60 Hz in <1 s and the torque remains constant.
Tribological dry sliding reciprocating tests were performed using the parameters shown in Table 3. The atmosphere and frequencies are presented in Table 4. Five equal tests, monitoring the friction coefficient, were performed for each condition. An automatic vacuum system was used to purge the testing chamber before pressurizing it with gas (ambient air or R134a). Before and after tests, the DLC-coated pistons and cylinders were cleaned with acetone were cleaned in an ultrasonic bath for 15 min and then dried in a warm airflow.
Topographical data of DLC-coated pistons and stainless-steel cylinders were obtained using White-Light Interferometry (WLI) (Zygo NewView 7300) and then processed using the MountainsMap Universal 7.1 software. Sa and Sq surface roughness parameters and texture isotropy were acquired by the following operation sequence:
- Filling in non-measured points (<1%).
- Remove the form.
- Standard Gaussian filter with an 80 µm cut-off to separate roughness and waviness.
Five samples of 0.05 mm2 each were analyzed for each material, and the average value and standard deviation were obtained. The same analyzes were carried out on the wear marks.
Scanning Electron Microscopy (SEM) (Tescan Vega 3) was used to analyze morphological characteristics of the wear marks on the DLC-coated pistons and cylinders using 10 kV of acceleration voltage. Chemical analyzes were performed on the surfaces using Energy Dispersive Spectroscopy (EDS) (Oxford Instruments AZtecOne) with 10 and 5 kV acceleration voltages. 5 kV was used to detect the signal as close as possible from the surface. To characterize the tribolayers and the coating, Raman analysis (Renishaw inVia micro-Raman with a 514 nm Ar laser) was performed on the DLC and various wear marks zones. A widely adopted procedure to analyze a-C:H Raman spectra was used: the Raman spectra were obtained by firstly removing the baseline from the Raman signal, and secondly, a Gaussian fitting was used to determine D and G bands positions and intensities. From these values, the ID/IG ratio was calculated (Robertson, 2002; Sánchez-López et al., 2003; Casiraghi et al., 2005). The cross-section of a piston before tests was metallographically prepared via grinding (up to #1200), polishing (alumina 1 µm), and etching (Nital 2%) to evaluate the coating microstructure and measure its thickness using an optical microscope (Leica DM6).
Results and Discussion
Figure 8 shows the axonometric projection of the topography of the original cylinders and DLC-coated pistons. It is important to notice the one order of magnitude difference between the scales, which is lower for the pistons. The arrows point the sliding direction. In both cases, transversal to the sliding direction, it is possible to see grooves originated from the machining processes, whereas the surfaces of the pistons have a much smoother aspect. On the cylinder, they are parallel among one another while being crossed on the coated piston surface.
Roughness parameters of the original surfaces are presented in Table 5. As already qualitatively observed in Figure 8, Sa and Sq parameters are approximately three times higher for the cylinder. Furthermore, both surfaces are strongly anisotropic, as the low texture isotropy values demonstrate.
Figure 9 presents the typical microstructure and Raman spectrum results of the coating before tribological testing. In the coating cross-section (Figure 9A) is possible to observe the DLC film and the chromium nitride layer, which provides mechanical support to the DLC. The DLC coating has an average thickness of 1.40 ± 0.04 µm and the compound layer, 1.8± 0.02 µm. Figure 9B shows the Raman spectrum of the DLC, which is a typical spectrum of an amorphous hydrogenated DLC (a-C:H) with an ID/ IG ratio of 0.57 ± 0.01 and the D and G bands centered on 1,381 ± 4 cm−1 and 1555.95 ± 0.42 cm−1, respectively.
The typical evolution of the friction coefficient vs. sliding distance is shown in Figure 10. For the 5 Hz frequency and ambient air atmosphere (red curve), the friction coefficient starts at around 0.07. It steadily increases until about 30 m of sliding distance, when there is a discontinuity, possibly due to a detachment of the film, which was confirmed after the test. Then, probably due to DLC debris on the contact, the friction coefficient drops again to around 0.15 (40 m of sliding distance) and stays steady until the end of the test.
When the testing atmosphere is changed to R134a refrigerant gas, and the frequency is kept constant at 5 Hz (black curve), the friction coefficient behavior is drastically distinct, starting at 0.04 and maintaining this steady-state through the whole test (lubricious regime). For the frequency of 20 Hz and R134a atmosphere (blue curve), the typical friction coefficient does not vary much from the previous condition (5 Hz and R134a atmosphere), and it is steady at around 0.05. The average friction coefficient is statistically the same for 5 and 20 Hz under the refrigerant gas atmosphere, as shown in Figure 5, which indicates that similar lubricity mechanisms govern the friction under these conditions. The friction coefficient of the tests carried out at a higher frequency of 40 Hz (green curve) starts as low as the other tests performed under lower frequencies (5 and 20 Hz and R134a atmosphere). Still, it starts to rapidly increase in the very first meters (~2.5 m) of sliding, reaching values as high as 0.40. In these cases, due to the high temperatures of the emulator system, the DLC failed, and the tests had to be interrupted at ~50 m of sliding distance.
…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.
Copyright © 2021 Borges, Salvaro, Binder, Binder, Klein and de Mello.
Correspondence: Jose D. B. de Mello, firstname.lastname@example.org
The original article was published in www.frontiersin.org