TRIBOLOGICAL TOOLS FOR LUBRICANTS DEVELOPMENT FOR SPACE APLICATIONS
Polyana Alves Radi1, Lucia Vieira Santos2, Vladimir Jesus Trava-Airoldi3 Instituto Nacional de Pesquisas Espaciais – INPE C.P. 515, 12245-970, São José dos Campos, SP, Brasil
[email protected],
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Abstract: If a lubricant fails in space, generally, there is no means to repair the damage or to apply fresh lubricant. Furthermore if the damage occurs in a mechanism with critical importance to space device it could result in loss of mission. In the past the lubricants for space applications were chosen according to the legacy. As the missions had a short life time the lubrication was not a limiting factor because the electronic devices used to fail before the lubricant. Nowadays, the life time of the satellites increased and the problems with lubrication started to be very significant. Although the tribological components represent only a small fraction of the spacecraft’s cost, they are often single point failures that can render an expensive satellite totally useless. Consequently the research on space lubricants has gained increasing interest in recent years. One very promising material is the DLC (Diamond-Like Carbon) films. It presents unique properties and can be deposited in a wide number of substrates. Additionally its properties can be adjusted varying the deposition parameters. The DLC is already been used in some satellite parts, but more studies are needed to enlarge his application. The tribological studies deal with solutions to improve the products and process performance. In association with the growing films technology, tribology can help with the development of solid lubricants with controlled behavior during the use. So according to this needing, this paper presents some important tribological tools for space materials and lubricants testing, and some results for DLC films with different hydrogenation. Mapping of tribological material behavior was used to analyze friction, wear rate and wear volume as functions of load and sliding speed to show the DLC film potential for terrestrial and space applications. Keywords: tribology, DLC (Diamond-Like Carbon) films, tribologic maps, adherence.
1- INTRODUCTION For a long time in the history the lubricants for space mechanisms were chosen due to the long heritage and not based on the new technology or on best available materials. As the mission lifetime and duty cycles were minimal the spacecraft components as batteries, electronics and computers used to fail before the lubrication (Fleishauer and Hilton, 1991). However, in the last decade these ancillary mechanism components have been greatly improved and the lubricant has become the limiting factor in spacecraft reliability and performance. Although the tribological components represent only a small fraction of the spacecraft’s cost, they are often single point failures that can render an expensive satellite totally useless. One of the solid 1
lubricants of the new age is the DLC (Diamond-Like Carbon) films. They exhibit unique mechanical and tribological properties and have applications on terrestrial and spatial environments. Depending on the hydrogen concentration on the DLC films and on the environment they can present high or low friction coefficient and wear rates. The DLC films are already been used in some satellite parts but more studies are necessary to increase its applications taking in account each specific requirement of the device. For example, there are some parts in the satellite that is necessary to have high friction coefficient in atmosphere and low friction in space. Nowadays is possible to produce adaptive coatings for applications in cycling environmental conditions. According to the parameters deposition of DLC films, it is possible to control the wear resistance, friction coefficient, chemical inertness and it can be deposited in light substrates that are used for spatial applications (V.J. Trava-Airoldi, 2007). Additionally it is possible to compound another materials with DLC to produce adaptive coatings that changes their tribological behavior according to the atmosphere (Voevodin A. A. e Zabinski, J. S., 2000). Tribology is defined as the science and engineering of interacting surfaces in relative motion. It is a multidisciplinary subject that includes the study and application of the principles of friction, lubrication and wear obtained on physics, chemistry, mechanics and, material engineering. The tribological studies deal with solutions to improve the products and process performance of physic system subjected to wear. In association with the growing films technology, tribology can help with the development of solid lubricants with extremely controlled behavior during the use according to the necessities. The present work intends to demonstrate some tribological tools that are being used in our lab to study and develop DLC films for spatial applications. The knowledge presented here can be employed to study other solid lubricants for spatial or terrestrial applications. 1.1 - Tribological tools During the contact between two surfaces in relative motion, the tribological processes are very complex because they involve friction, wear and, deformation at different levels and kind. The friction and the wear are intrinsic related, but this relationship is not linear or comprehensible (Kato, K., 2000). Thus, each new tribologic par has to be tested to predict its behavior. Following, the main tribologic tools (friction studies, wear mechanism, adhesion, and tribologic mapping) are discussed. 1.1.1
- Friction Studies
Friction force (F) is the force that resists motion when the surface of one object comes into contact with the surface of another. The friction coefficient () is the non-dimensional rate between friction force (F) and normal load applied (L) (ASTM G40, 1999):
The efforts to define the exactly value, since XVII century, created a common sense that the friction force is an intrinsic material property. In fact, the friction force is the material property of the material in response to the conditions during the motion (Blau, 2001). So each sliding condition produces a different level of friction. During the motion a lot of changes can occur, as roughness changes, material oxidations, gas adsorptions and, tribochemical reactions on the materials removed from the surface (Ludema, 2001). 2
Thus, there are a lot of variables that influences the friction force as, contact geometry, physical and chemical lubricant properties, motion direction, applied load, third body, temperature and, vibration resistance. All these variables need to be taken in account when the friction force is studied. 1.1.2
Wear Studies
The wear is the mainly studies focus on tribology, and is defined as the progressive material loss due to mechanic action, or contact and movement of a solid body against a solid, liquid or gaseous body (Suski, 2004). The wear can be adhesive, abrasive, corrosive or fatigue. Traditionally there are four mainly wear mode as showed in figure 1.
Figure 1 - Schematic images of four representatives wear modes (Kato, K., Adachi, K., 2001). The wear mechanisms are described considering complex changes during the surfaces motion. In general, the wear occurs by more than one mechanism. So the comprehension of each one is important to define which one is more important (Kato, K., Adachi, K., 2001). 1.1.3
Adherence Studies
The adhesion is defined as the state where two surfaces are joined by interfacial forces, that con be constituted by chemical forces, physical forces or both (ASTM D907, 2004). Adhesion also is adherence of a film under a substrate. Thus, adhesion can be defined as the work to break atoms and molecules bonding. The presence contaminants on the surface substrates can produce local changing on the adhesive force. Thus, the experimental adhesion may be consider the mean adhesive force (Pulker, H.K., et al, 1981). There are some tests to measure the adhesion, for example scratching test, bending test, scraping test, and Rockwell imprint (Ollendorf, H.; Schneider, D., 1999), however sometimes they produce conflicting or just qualitative results. The scratching test is a semi-qualitative test that consist in scratch the sample, using a diamond tip increasing the load until the film cleavage or the substrate appear. The scratching test is monitored with a acoustic sensor that detects the moment when the film crack. The load where the film cracks is defined as critical load. The track can be observed on the optical microscope to determine the adhesion type (Bull, S.J.; Berasetegui, E.G., 2006). 3
Figure 2 - Schematic representation of scratching test (Bull, S.J.; Berasetegui, E.G., 2006).
The observed failure for a given coating on a substrate depends on the test load, coating thickness, residual stress and substrate properties, as well as, test parameters such as indenter tip radius, sliding distance and load application rate. So, the scratching test can be used just to compare films analyzed at the same conditions. The failure of the hard films can be divided in two main groups, trough-thickness cracking (or cohesive failure) and interfacial (or adhesive) failure (Bull, S. J., 1997), as showed in figure 3 and figure 4, respectively.
Figure 3 - Through-thickness cracking failure modes in the scratching test (Bull, S. J., 1997).
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Figure 4 - Interfacial failure modes in the scratching test (Bull, S. J., 1997). 1.1.4
Friction and Wear Mapping
A material behavior map is a 3-dimensional diagram where points have been calculated or experimentally obtained. The map defines regions where the material behavior is nearly the same. These regions are separated by transition lines or bands that are functions of two or more parameters (S. M. Hsu, 2001). Once the behavior of a given pair of materials is mapped, the maps can be used as material selection guides, as well as design guides for different applications. Only a few products have actually been tested and used for industrial applications, because the typical industrial practice of making actual components and putting them in actual field trials, results in high test production and excessive testing. Tribological maps may be a way to solve these problems, because the tribological tests can be performed on small samples. These maps could be used as material selection guides to decide which additional tests are required, thus reducing testing time and material costs. However, before discussing the behavior maps themselves some important considerations regarding the adequate deposition of thin films should be considered (Radi, P. A., 2008):
The substrate needs to be polished to avoid lumps that can damage the film performance during the tests.
The substrate hardness can be considered as a parameter that limits the load and speed ranges because a hard film cracks easier when deposited over an elastic substrate.
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The film properties, in relation to hardness, adherence and stress need to be established to guarantee the application of results.
Load and sliding speed range limits need to be determined before testing gathering, in order to guarantee that the film will not fail during the tests.
An adequate cycle number must be determined to achieve measurable wear in all tests. Environmental conditions have to be controlled to guarantee the same conditions for all tests. By following these guidelines, DLC films with high degree of hardness, high adherence, and low stress have been produced on metallic substrate as part of on-going research and testing.
2- EXPERIMENTAL The DLC films were deposited by using a new concept of low cost, pulsed-DC discharge under controlled conditions to obtain maximum hardness, minimum stress and a maximum deposition rate. DLC film with a 2 µm thickness was deposited on a titanium plate (6.0x6.0x0.5 cm3) under strictly controlled conditions (Bonetti L.F., 2006). To guarantee adherence and avoid bumps, the titanium (Ti6Al4V) substrate was polished and then cleaned in ultrasonic bath in isopropyl alcohol for 5 minutes and than cleaned in a vacuum chamber under 10 Pa pressure of argon discharge for 30 minutes. For tribological tests the analyzed pair was a 4-mm-diameter Ti6Al4V ball and a Ti6Al4V plate coated with DLC film. A new area on the plate and on the ball was used for each test. The tests were carried out by using a UMT-CETR ball-on-plate tribometer in the reciprocating mode, in a humidity of 26±2%RH and a temperature of 25±1ºC within a controlled environment at five different sliding speeds (2.0; 4.0; 6.0; 8.0 and 10.0mm.s-1) under 10 loads (varying from 1.0 up to 10.0N). The tests were run three times for each combination of load and speed, amounting to 150 tests. The friction coefficient was monitored by using a strain-gauge load cell and continually recorded for 250 cycles (or 500 pass) with 10mm displacement distance. To generate the friction map, the friction coefficient average was collected from the steady-state (equilibrium) region (A. Erdemir, 2004). After the friction measurements, the pair was analyzed through optical microscopy. No measurable wear was observed on the plate, so the wear scar diameter was measured on the ball. The ball wear volume (Vb) was calculated through the following equation from the G99-95 ASTM. Assuming that there was no significant wear on the plate:
Vb
( DW ) 4 64 RS
Where, DW is the wear scar diameter, and Rs is the sphere radius. The ball wear rate (Wb) was calculated by using the following equation:
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Wb
Vb L.D
Where Vb is the ball wear volume, L is the load, and D is the sliding distance. The friction coefficient for the Ti6Al4V ball/DLC film pair and the Ti6Al4V ball wear volume and wear rate were plotted in a function of load vs. speed. Microstructural analysis of the DLC film, before and after the tribological test was done by Raman scattering spectroscopy, using a Renishaw 2000 system with an Ar+-ion laser (=514 nm) in backscattering geometry. The Raman shift was calibrated in relation to the diamond peak. All measurements were carried out in air at room temperature, and humidity 20%.
3- RESULTS AND DISCUSSION The DLC film deposition rate was 1µm/h, the hardness was 18GPa for DLC20%H and 8GPa for DLC35%H, and the total stress was 1.5 GPa. The slope of the photoluminescence (PL) background in visible Raman spectra can be used to estimate the H content (Casiraghi, C., 2005). The figure 5 shows that Raman photoluminescence increases with the hydrogenation increasing.
Figure 5 - Raman spectra of DLC films with different hydrogen concentration (λ=514,5 nm).
Figure 5 shows that for low H content, the visible spectra do not exhibit PL background while for high H content the PL increases linearly. 7
Figure 6 shows some friction coefficient results for Ti6Al4V/ DLCH35% and Ti6Al4V/ DLCH20% pairs for reciprocating mode with 5N load and 1,0; 5,0 e 10 mm/s sliding speeds in ambient atmosphere. DLCH35%
Friction Coefficient
0,22
DLCH20%
0,18
1mms 5mms 0,16 10mms
0,21
1mms 5mms 10mms
0,20 0,14
0,19 0,12
0,18 0,10
0,17 0
50
100
150
0200
50
100
150
200
Sliding number Figure 5 - Friction Coefficient versus sliding number for Ti6Al4V/ DLCH35% and Ti6Al4V/ DLCH20% pairs for reciprocating mode with 5N load and 1,0; 5.0 e 10 mm/s sliding speeds in ambient atmosphere. The friction curves for DLCH35% is different form that observed for DLCH20%. For DLCH35% the friction coefficient is more dependent on the sliding speeds, and the friction coefficient in the steady-state region is higher than the initial friction coefficient. For DLCH20% the friction coefficient on the steady-state is almost independent on the sliding speed, and the final friction is lower than the initial. For both films the friction coefficient increased with the sliding speed. Figure 6 shows a typical graphic of scratching test where the acoustic emission, applied normal load and friction coefficient can be observed as a function of diamond tip sliding distance. This test shows that for DLCH35% film the critical load is 7N. The results show that the friction coefficient also can be used to determine de critical load.
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Figure 6 - Scratching test result for DLC35H% films showing acoustic emission (AE), normal force (Fz) and Friction coefficient as a function of sliding speed. The tip sliding speed was 0.1 mm/s and the load rate is 0.25N/s. Figure 7 shows the optical microscope image of the scratching test track. The figure shows that the failure has an adhesive nature. This result was very important to define the next step of the research - increase the adhesion of the film. Nowadays, we can produce very adherent and resistant DLC films with desired hydrogen composition in the top layer.
0,5mm
Figure 7 - Optical microscope image of the scratching test track for the film DLCH35%. The analyses of the track image concurrently with the test results is very important on the analysis of the film quality, because in some cases, after the first crack (critical load) the film keeps adhered on the substrate as showed in the figure 8.
Figure 8 - Optical microscope image of the scratching test track for the film DLCH35% with increased adherence. 9
Figure 9 shows the friction maps a function of load and sliding speed when the titanium ball slides against a titanium plate coated with DLC (DLCH35% and DLCH20%) films.
Figure 9 - 3D-friction map of DLCH35% and DLCH20% films as a function of load vs. speed in ambient atmosphere. Figure 9 shows that the highest friction coefficient for both DLC films is obtained at lower load and higher sliding speed. The friction coefficient of DLCH35% is higher than DLCH20% in all conditions. Figure 10 shows the wear rate map for the titanium ball as a function of load and sliding speed when it slides against a titanium plate coated with DLC films.
Figure 10 - 3D-wear map of DLCH35% and DLCH20% films as a function of load vs. speed in ambient atmosphere.
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The wear rate of the titanium ball under this set of conditions was quite independent on load for DLCH35% and dependent on load DLCH20%. This is related to the hardness of the DLCH35% that is lesser than DLCH20%. So the DLCH20% wears the ball more than DLCH35%.
4- CONCLUSION DLC films are very promising for space application and the tribologic studies, helps to increase its applications, and develop new hybrid films to attend the applications needing. The adhesion test helps to compare the adhesion of different films on different kind of substrates and indicates what kind of improvement is necessary to increase the adhesion of the films. The maps highlighted abrupt shifts in the tribological behavior of the films and gives an overall idea of the DLC films behavior in a determined range of sliding speed and normal load. By identifying the parameters that control the friction and wear, and determining the conditions which these films will be submitted may become DLC a more viable option in space and industrial applications.
5- REFERENCIAS ASTM D907-04. American adhesives.Philadelphia, 2004.
Society
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Testing
and
Materials:
Terminology
of
Casiraghi, C.; ferrari, A.C.; Robertson, J. “Raman spectroscopy of hydrogenated amorphous carbons”. Physical Review B. v. 72, p. 1-14, 2005. ASTM G40-99 - American Society for Testing and Materials. “Terminology relating to wear and erosion”. Philadelphia, 1999. Bonetti L.F., Capote G., Santos L.V., Corat E.J., Trava-Airoldi V.J., “Adhesion studies of diamond-like carbon films deposited on Ti6Al4V substrate with a silicon interlayer” Thin Solid Films 515, 375-379 (2006). Burnett, P. J. Rickerby, D. S. “The scratch adhesion test: an elastic-plastic indentation analysis”. Thin Solid Films, v. 137, p. 233. 1988. Bull, S. J., “Failure mode maps in the thin film scratch adhesion test”. Tribology International, Volume 30, Number 7, July 1997, pp. 491-498(8) Erdemir, A., “Genesis of superlow friction and wear in diamondlike carbon films”, Tribology International 37 (2004). 1005-1012. Fleishauer, P.D. and Hilton, M.R., “Assessment of the Tribological Requirements of Advanced Spacecraft Mechanisms,” Aerospace Corp., El Segundo, CA, Report No. TOF-0090 (5064)-1. 1991. Kato, K., “Wear in relation to friction – a review”. Wear. v. 241, p151-157. 2000. Kato, K., Adachi, K. Wear mechanisms. In: BHUSHAN, B. (Ed.). Modern Tribology Handbook: principles of tribology. [S.l.]: CRC Press, 2001. v. 1 Cap. 7. Ludema, K. C., “Friction”. In: Modern Tribology Handbook: Principles of Tribology. CRC Press LLC, 2001. 11
Ollendorf, H.; Schneider, D. “A comparative study of adhesion test methods for hard coatings”. Surface and Coatings Technology, v. 113, p. 86-102. 1999. P.A. Radi, L.V. Santos, L.F. Bonetti, G.C. Rodrigues and V.J. Trava-Airoldi, “Friction and wear maps of titanium alloy against a-C:H20% (DLC) film”, Surface and Coatings Technology. Volume 203, Issues 5-7, 25 December 2008, Pages 741-744 Pulker, H.K.; Perry, A.J.; Berger, R. Adhesion. Surface Technology, v.14, n.1, p. 25-39. 1981. S. M. Hsu, “Wear maps” in Modern Tribology Handbook, ed. by B. Bhushan, Vol. 1(CRC Press, 2001) chapter 9. Suski, C.A., “Estudo do efeito de tratamentos e revestimentos superficiais na vida de ferramentas de conformação mecânica à frio”. 2004. 88f. Dissertação (Mestrado) - Centro Tecnológico, Universidade Federal de Santa Catarina, Florianópolis, 2004. V.J. Trava-Airoldi, L.F. Bonetti, G. Capote, J.A. Fernandes, E. Blando, R. Hübler, P.A. Radi, L.V. Santos, E.J. Corat, “DLC film properties obtained by a low cost and modified pulsed-DC discharge”, Thin Solid Films 516 (2007)272. Voevodin A. A., Zabinski, J. S., “Supertough wear-resistant coatings with “chameleon” surface adaptation”. Thin Solid Films, v. 370, p. 223-231, 2000.
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