TEMPERATURE EFFECTS ON MECHANICAL PROPERTIES OF ZINC DITHIOPHOSPHATE TRIBOFILMS

Transcription

1 TMPRATUR FFCTS ON MCHANICAL PROPRTIS OF ZINC DITHIOPHOSPHAT TRIBOFILMS Karim DMMOU, Sandrine BC, Jean-Lu LOUBT & Jean-Mihel MARTIN Laboratoire de Tribologie & Dynamique des Systèmes - UMR 5513 CNRS/CL/NIS ole Centrale de Lyon ; 36 Av. Guy de Collongue ully Cedex - FRANC Abstrat: Nanoindentation tests were performed at several temperatures (24 to 8 C) on one antiwear zin dialkyl-dithiophosphate (ZDTP) tribofilm using a Nanoindenter XP entirely set into a limati hamber. Mehanial properties of the tribofilm were determined using a simple model. AFM observations were onduted to estimate the order of magnitude of the film s thikness. The effet of applied pressure on the elasti properties was demonstrated and taken into aount in the present analysis. The use of the F 2 ) parameter, independent of ontat geometry, revealed a hardness dependeny upon temperature. Furthermore, areful AFM observations of the residual pile-up produed by plasti flow around the indents pointed out the evolution of the film deformation proess with temperature. Keywords: Nanoindentation, Zin Dialkyl-ditihiophosphate (ZDTP) Tribofilm, Temperature effets : Corresponding author. -mail address: karim.demmou@e-lyon.fr 1

2 1. Introdution Sine the 4 s, Zin dialkyl-dithiophosphates (ZDTP) are high performane lubriant additives whih are extensively used beause of their good antioxidant and antiwear properties. Many researhes have been arried out on these additives in order to better understand how tribofilms are formed and how they at inside ontats. Nevertheless, faed with more and more restritive rules onerning sulphur and phosphorous ontents in lubriants, the aim is now to redue the use of ZDTP and to develop less pollutant new antiwear additives [1]. Most of these studies deal with the hemial aspet of the formation and ation of ZDTP tribofilms [2-5] but few are onneted with their own mehanial behaviour [6-9]. The main reason is that mehanial measurements are diffiult to perform on suh films: ZDTP tribofilms are very thin, with a pathy pattern whih results in a great inhomogeneity in shape and thikness. Pads are about 1 to 1 µm long and less than 1 µm wide. Their thikness ranges from a few tens to a few hundreds of nanometers. Nanoindentation has revealed itself to be an effiient method for measuring near surfae mehanial properties and partiularly thin films ones. However, in the ase of thin films, omposite mehanial properties, whih take into aount both the film s and the substrate s properties, are measured. In order to overome this diffiulty, a simple model was used to extrat film s modulus from the omposite one [1]. Furthermore, it is known that pressure influenes elasti properties of polymer-like materials [11]. Suh behaviour has been evidened for ZDTP tribofilms [6] and has been taken into aount in this study. It is also worth noting that, as far as we know, all published results onern measurements made at ambient temperature although servie onditions are around 8 C. In this study, nanoindentation tests were performed on a Nanoindenter XP, entirely set into a limati hamber, whih permits measurements between -5 and 1 C. So, mehanial properties of ZDTP films have been measured from ambient temperature to 8 C. 2

3 Post indentation AFM observations at room temperature have also been performed in order to estimate the thikness of the indented pads and to quantify pile-up around the residual indents. 2. xperimental details 2.1. Sample preparation The tribofilm was formed on a Cameron-Plint tribometer using a ylinder (6 mm long and 6 mm in diameter) sliding against a flat, both in AISI 521 steel. The sliding distane was 7 mm long at a frequeny of 7 Hz. The lubriant was a syntheti base oil poly alpha olefin (PAO) with 3% in weight ZDTP. The applied normal load was progressively inreased to 35 N during the frition test and then maintained onstant (total test duration: 1 hour). The test temperature was kept onstant at 8 C. Optial mirosopy observations onfirmed the pathy struture of the ZDTP tribofilm Nanoindentation tests Nanoindentation tests were performed with a Nanoindenter XP using the ontinuous stiffness measurement method [12-14]. This method onsists in superimposing a small displaement osillation (small enough to generate only an elasti strain) at a given frequeny during the indentation test. So, the sample endures small loading-unloading yles. The simultaneous measurement of the normal fore and the ontat stiffness allows us to san mehanial properties, hardness, H, and Young s modulus,, ontinuously along the whole indentation period. Thanks to the limati hamber (Fig. 1 a & b), nanoindentation tests have been performed at 24, 4, 6 & 8 C. The maximum applied load was 1 mn, at a onstant strain rate, df Fdt, of s -1, where F is the normal load. The optial mirosope integrated in the devie allowed us to preisely define the indentation loation and to verify the position of the residual indents. 3

4 The small superimposed displaement had an amplitude of 1.5 nm at a frequeny of 32 Hz. The blowing fan frequeny of the heating system has been set at 12 Hz during experiments in order to minimize noise. Corretions have been made to prevent errors due to the geometry of the ontat area. Preliminary indents were done on fused silia in order to haraterize the tip defet of the Berkovih tip (three side pyramid with an angle of 115,12 between edges). AFM observations were used to obtain topographi information onerning pile-up around the indents, neessary to determine atual ontat area. More than 1 indents have been performed for eah temperature at different plaes on the tribofilm. Some indents were made at different temperatures on the same pads (example on Fig. 2) Film s mehanial properties The hardness, H, is defined as a ontat pressure by q. 1. F H = (q. 1) A Where F is the normal fore applied to the sample, aount pile-up around the indent and tip defet. A = Ψ ( h + h ) (q. 2) 2 2 r ' A is the atual ontat area taking into A is defined by q. 2 [1]. where hr ' is the ontat depth, h is the equivalent height of the tip defet and 2 Ψ is the ratio between atual ontat area (taking into aount pile-up) versus theoretial triangular area. The order of magnitude of 2 Ψ was estimated using AFM observations. The ontat radius, a, orresponding to the flat punh whih has the same ontat area A is defined by A π a 2 =. lasti properties are diretly related to the measured normal ontat stiffness, S, and to the equivalent ontat radius, a (q. 3 & 4). S = (q. 3) 2a 4

5 1 ν 1 ν = + (q. 4) i i where is the apparent redued modulus,, ν, i & ν i are, respetively, the Young s modulus and the Poisson s ratio for sample () and indenter (i) i = ν i GPa. In the ase of thin films deposited on a substrate, both the film and the substrate elasti properties and of the film thikness. is a omposite modulus. It is a funtion of Several models an be used to extrat the film elasti property from the omposite one [15-18]. The model developed by S. Be & al. [1] has the advantage of being simple and it does not need the use of an adjustable parameter nor the knowledge of the Poisson s ratio of the film and the substrate. S. Be & al. onsider that the film + substrate system an be modelled by two springs onneted in series. Considering suitable boundary onditions, the omposite modulus an be expressed from q. 5: 1 2a t 1 = ( ) t π a π a f a s (q. 5) Where t is the film s thikness, f and Young s moduli. Thus, the knowledge of the two parameters, t and steel substrate), allows us to determine the redued Young s modulus, s are respetively the film s and substrate s redued s (22 GPa for the AISI 521 f, of the film Anvil effet It is known that hydrostati pressure influenes elasti properties of polymers and polymer-like materials [11, 19]. As the film is trapped between the diamond indenter and the steel substrate and 5

6 annot flow, the ontat pressure i.e. the hardness, has the same order of magnitude as the hydrostati pressure. This is alled Anvil effet: the elasti modulus inreases with hydrostati pressure. For ZDTP tribofilms, similar effet was observed takes plae from a threshold hardness value, H. Thus, for ontat pressure lower than H, the film s modulus is onstant. For ontat pressure higher than H, the film s modulus inreases linearly with hardness. This was interpreted like an aommodation proess to the severity of ontat onditions [6] modulus, This effet was taken into aount in the model used to determine the film s redued Young s f AFM observations A VCO CP-II Atomi Fore Mirosope was used to perform observations of the indents and partiularly of the pile-up at their periphery at room temperature. Beause of the very low load, the residual indents were about 2 or 3 µm in diameter. ven if we ould observe them with optial mirosopy, AFM permitted high three dimensional resolution and topography measurements. Thereby, pads height ould be estimated and pile-up around the indents ould be quantified in order to obtain reliable hardness values onsidering atual ontat area. AFM was used in ontat mode with sharp silion probes oated with gold. For eah indent, large images of the whole pad, images of the indent itself and detailed image of the pile-up around the indent were arried out. 3. Results and disussion 3.1. AFM observations 6

7 Fig. 3 shows an error mode AFM image of two residual indentation prints made on the same pad at 24 C (Fig. 3a) and 8 C (Fig. 3b). The error observation mode is similar to a derivative image of the topography image. Therefore, hanges in olour are linked to hanges in surfae slope. That is why suh images were used to measure atual ontat areas and thus to aess to 2 Ψ value. If pile-up is not taken into onsideration, this results in overestimating modulus and hardness values. Beause of the irregular stairs-like shape of the pile-up around the indents, it was diffiult to measure the atual ontat area. Neverthless, taking Ψ =1.2, whih is the standard value used in similar onditions, gives a good estimation of the ontat area. In the following the same value of Ψ was used for all tests, at all temperatures. On the other hand, topography AFM images were used in order to estimate the order of magnitude of the pad height. For the indents made on the same pad at 24 and 8 C, the height of the pad was estimated around 25±5 nm. Moreover, it an be notie a reproduible differene between the pile-up formed at 24 C (Fig. 3a) and 8 C (Fig. 3b). Pile-up, whih is stable with time, seems to have been formed by suessive pronouned steps at 24 C whereas, at 8 C it seems to result from a more ontinuous flow of matter. This may denote different deformation proesses between these two temperatures. Additional studies are in progress to further investigate this phenomenon (strain rate dependeny, larger temperature range, ) Film s modulus AFM topography measurements allowed us to estimate the film thikness. This value is one of the parameter neessary to alulate the film redued modulus, f, from the measured omposite modulus with our model. The pads height was estimated at about 25±5 nm. This value was refined by fitting the alulated omposite modulus urve with the measured one. The AISI 521 steel substrate redued modulus value, s, was taken to 22 GPa. 7

8 Fig. 4 shows the evolution of the film modulus and hardness versus plasti depth for one indent at 24 C. The film modulus and the hardness are first onstant at respetive values and H, until a f depth hr ' and then inrease. Similar urves were obtained for all indents at all temperatures. For hardness values higher than the threshold value H, the film s modulus urve an be fitted, as expeted from [6], by a linear funtion of the hardness, H (q. 6) f = f H H q. 6 As an illustration, Fig. 5 shows the evolution of the film modulus versus hardness for two indents performed on the same pad at 24 C and 8 C. For all tests the film modulus,, was found f to be independent of temperature, at a value f 39±4 GPa. On the opposite, the threshold ontat pressure, H was found to be temperature dependent. The threshold value of ontat pressure at 24 C is about twie the threshold value at 8 C. Fig. 6a & 6b show the evolution of the omposite modulus, h r ' alulated from using f, versus the indentation depth, value in our model. From the blak urve (symbol ) on Fig. 6, an inrease of the omposite Young s modulus is observed as the ontat depth inreases. For ontat pressure under the threshold value, this inrease is only due to the influene of the substrate. That is why, for shallow indentation depth, the omposite modulus alulated with our model and a onstant film modulus fits well the measured one (red urves, symbol ). f For deeper indentation, the alulated urve deviates from the measured omposite modulus. The effet of hydrostati pressure beomes signifiant. From the threshold pressure H, whih orresponds to the indentation depth h r ' where the model deviates from the experimental data, the film s modulus is onsidered to inrease with the pressure as defined in q. 6. Using this inreasing value for the film modulus gives a better fit of the 8

9 alulated omposite modulus urve with the measured one for indentation depth deeper than (blue urves, symbol ). h r ' 3.3. Temperature effets In order to better evidene the temperature effets on mehanial properties, the F 2 ) parameter [normal load divided by ontat stiffness squared] versus stiffness was hosen. It was plotted versus the ontat stiffness, S, whih is proportional to indentation depth, beause it is a diret measurement not affeted by ontat geometry onsiderations. Thus, S is more aurate than indentation depth for measurements made on the same pads. F 2 ) parameter is independent of geometrial onsiderations inherent to nanoindentation ontat modelling [2]. Moreover, F 2 ) parameter is proportional to ( 2 ) H ratio. As it was just shown that the Young s modulus of the ZDTP film is onstant in the studied temperature range, variations in this parameter are due to variations in hardness, H. The influene of temperature on F 2 ) parameter versus ontat stiffness, S, is shown on Fig. 6 & 7. From Fig. 7, it an be observed, for two indentations made on the same pad at 24 C and 8 C, that the F 2 ) parameter dereases rapidly for small stiffness values (orresponding to shallow indentation depths). For high stiffness values, it tends to an asymptoti value orresponding to a greater influene of the substrate on stiffness. For low stiffness, the F 2 ) parameter measured at 24 C is signifiantly higher than at 8 C. As the film modulus is the same at both temperatures, this orresponds to a signifiant differene in hardness values: ZDTP film s hardness is about twie lower at 8 C than at 24 C. Fig. 8 shows F 2 ) parameter for several indentations made at different temperatures on different pads. The derease in hardness with inreasing temperature is onfirmed independently of the 9

10 hosen pad. Sattering of values might be due to thikness differene between pads. It is more pronouned at high temperature where experiments are more deliate to ondut. 4. Conlusions and prospets First, nanoindentation tests were made on a pathy ZDTP tribofilm at several temperatures, up to 8 C. The highest temperature, 8 C, is near servie temperature. Careful analysis of the obtained data allowed us to extrat the film s Young s modulus. Its independene with temperature variations in the studied range whih extends from 24 to 8 C was shown. On the other hand, the study of the F 2 ) parameter versus stiffness allowed us to point out an important derease in hardness while the temperature inreases. Seond, AFM observations have been used to quantify the pile-up around indentation print and to estimate the height of the pad. These parameters were neessary to extrat the Young s modulus of the film from omposite properties. Moreover, the observation of the pile-up around the indents revealed a differene between deformation proesses depending on temperature. Thus, the next step of this study will be the investigation of the visous-plasti properties of these films against temperature in order to better understand the different behaviours highlighted by AFM observations and to try to relate them to physial hanges due to temperature inrease. 1

11 Referenes 1. SPIKS, H., Tribology Letters, Vol. 17(3), 24: p MARTIN, J.M., et al., Tribology International, Vol. 34, 21: p MINFRAY, C., et al., Thin Solid Films, Vol , 24: p BANCROFT, G.M., et al., Tribology Letters, Vol. 3, 1997: p WILLRMT, P.A., et al., Tribology International, Vol. 28(3), 1995: p BC, S., et al., Royal Soiety, Vol. 455, 1999: p NICHOLLS, M.A., et al., Tribology Letters, Vol. 15(3), 23: p WARRN, O.L., et al., Tribology Letters, Vol. 4, 1998: p GRAHAM, J.F., et al., Tribology Letters, Vol. 6, 1999: p BC, S., et al., Philosophial Magazine A, Vol. 74(5), 1996: p TSCHOGL, N.W., et al., Mehanis of Time-Dependent Materials, Vol. 6, 22: p OLIVR, W.C. and PHARR, G.M., Journal of Materials Researh, Vol. 7(6), 1992: p PTHICA, J.B. and OLIVR, W.C., Mat. Res. So. Symp. Pro., Vol. 13, 1989: p HOCHSTTTR, G., et al., Journal of maromoleular siene - physis, Vol. B38(5 & 6), 1999: p KING, R.B., International Journal of Solids Strutures, Vol. 23(12), 1987: p RAR, A., et al., Mat. Res. So. Symp. Pro., Vol. 695, PRRIOT, A. and BARTHL,., Journal of Materials Researh, Vol. 19(2), 24: p GAO, H., et al., International Journal of Solids Strutures, Vol. 29(2), 1992: p ROCH, S., et al., Mat. Res. So. Symp. Pro., Vol. 778, 23: p SAHA, R. and NIX, W.D., Ata Materialia, Vol. 5, 22: p

12 Figure Captions Fig. 1: Nanoindenter XP a) Climati hamber. On the right side stands the heating-ooling system. This system an reah temperatures from -5 to 1 C. Liquid Nitrogen is used to ool the hamber. b) Integrated in its limati hamber, it stands on a pneumati table in order to avoid vibrations. During experiments air flow is made as small as possible and is blown at low veloity in order to minimize perturbations near the devie. Blowing fan frequeny is tuneable. Fig. 2: Optial observation of two indents on same pad at 24 & 8 C. Color homogeneity orresponds to thikness homogeneity. Fig. 3: AFM error mode images of two indents on the same pad. a) indent made at 24 C (4x4µm) b) indent made at 8 C (3x3µm). Fig. 4: Representative urves of Young s modulus and hardness of the film versus plasti depth. Film modulus and hardness are first onstant then inrease from a threshold value of plasti depth h. r ' Fig.5: Film modulus versus hardness for two indents performed at 24 C and 8 C on the same pad Fig. 6: Composite modulus taking into aount film and substrate ones as a funtion of indentation depth, a) at 24 C & b) at 8 C. Indentations were performed on the same pad. xperimental omposite modulus Composite modulus alulated with Be s model Composite modulus alulated with Be s model and taking into aount a film s modulus growing linearly with applied pressure from a threshold value Fig. 7: Load divided by stiffness squared versus stiffness for two indents on the same pad. For shallow indentation depth, the F/S² parameter is twie lower at 8 C than at 24. Fig. 8: Load divided by stiffness squared versus stiffness for several indentation made at different temperatures (24 (Blak), 4 (Blue) 6 (Brown) & 8 C (Red)) on different pads. 12

13 Fig. 1 13

14 Fig. 2 a b Fig. 3 14

15 5, 4,5 Indent 24 C 9 8 4, H (GPa) f (GPa) 7 3,5 6 H v (GPa) 3, eee 5 f (GPa) 2,5 4 2, 1,5 1, e e 3 h r' h r' (nm) Fig. 4 f (GPa) C 8 C 24 C 8 C Linear 24 C Linear 8 C 4 3 e rr e re H (GPa) Fig. 5 15

16 2 18 = f(h' r 24 C (GPa) Measured Be's model Model + Pressure effet from h r' h' r (nm) a) = f(h' r 8 C (GPa) Measured Be's model Model + Pressure effet from h r' h' r (nm) b) Fig. 6 16

17 5,x1-13 4,x C 8 C F/S² (m²/n) 3,x1-13 2,x1-13 1,x1-13, S (N/m) Fig. 7 5,x1-13 F/S² (m²/n) 4,x1-13 3,x1-13 2,x C-1 24 C-2 4 C-1 4 C-2 6 C-1 6 C-2 8 C-1 8 C-2 1,x S (N/m) Fig. 8 ar 17