Investigation of anti-wear performance of automobile lubricants using thin layer activation analysis technique

Nuclear Instruments and Methods in Physics Research B 399 (2017) 69–73

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Investigation of anti-wear performance of automobile lubricants using thin layer activation analysis technique Jayashree Biswal a, G.D. Thakre b, H.J. Pant a,⇑, J.S. Samantray a, P.K. Arya b, S.C. Sharma c, A.K. Gupta c a

Isotope and Radiation Application Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Tribology and Combustion Division, Indian Institute of Petroleum, Dehradun 248005, Uttarakhand, India c Nuclear Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India b

a r t i c l e

i n f o

Article history: Received 27 January 2017 Received in revised form 21 March 2017 Accepted 28 March 2017

Keywords: Thin layer activation Wear Proton beam Cobalt-56 Lubricant Tribometer

a b s t r a c t An investigation was carried out to examine the anti-wear behavior of automobile lubricants using thin layer activation analysis technique. For this study disc gears made of EN 31 steel were labeled with a small amount of radioactivity by irradiating with 13 MeV proton beam from a particle accelerator. Experiments on wear rate measurement of the gear were carried out by mounting the irradiated disc gear on a twin-disc tribometer under lubricated condition. The activity loss was monitored by using a NaI(Tl) scintillation detector integrated with a multichannel analyzer. The relative remnant activity was correlated with thickness loss by generating a calibration curve. The wear measurements were carried out for four different types of lubricants, named as, L1, L2, L3 and L4. At lower load L1 and L4 were found to exhibit better anti-wear properties than L2 and L3, whereas, L4 exhibited the best anti-wear performance behavior than other three lubricants at all the loads and speeds investigated. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction The reliability of industrial equipments, transportation systems and other machine parts can be significantly influenced by wear. The wear of these equipments can be controlled and minimizing by applying suitable gear lubricants. A gearbox is a vital component of many industrial applications. The gear lubricants are formulations, which are applied to prevent premature component failure, assure reliable operation, reduce operating cost, and increase service life. The important objectives accomplished by these lubricants include: reduction of friction and wear, corrosion prevention, reduction of operating noise, improvement in heat transfer, and removal of foreign or wear particles from the critical contact areas of the gear surfaces. The need of suitable technique for monitoring wear is usually based on three main factors, i.e., economics, safety and energy conservation. The wear of industrial components causes economic losses to the industry; it can compromise the safety of operating equipment by causing failure of parts. Wear monitoring can help in designing engineered surfaces, thereby increasing the working life of components, resulting in saving large sums of money, thus leading to conservation of material, energy and the environment. The conventional techniques

⇑ Corresponding author. E-mail address: [email protected] (H.J. Pant). http://dx.doi.org/10.1016/j.nimb.2017.03.143 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.

such as gravimetric, micrometry, profilography, replica method are used for wear measurements in industry but these techniques have poor accuracy, low sensitivity and cannot be applied in all situations due to non-accessibility. Thin layer activation (TLA) analysis is a highly sensitive nuclear technique used for monitoring wear and corrosion phenomena employing radioactive tracer [1–21]. In this technique gamma emitting radioisotopes are introduced insitu and distributed in a small area on the surface of interest of an engineering component [4,7,14,18]. The radioisotopes are removed from the surface along with the base element of the sample during the wear process. The material loss can be monitored either by monitoring the remaining radioactivity on the sample or by measuring the removed radioactivity from the sample using specific radioactive counting equipment [7]. The TLA technique has a number of advantages over other conventional techniques, such as high sensitivity in monitoring slow degradation process, offline as well as online measurement, simultaneous measurement of surface degradation of several components in the same machine, working with relatively low level of activity and quicker measurements [22]. Due to these advantages, the TLA technique is considered as a versatile technique for monitoring wear processes in several industrial components. The objective of the present study was to compare four different types of mineral oil-based automobile lubricants [23] for their anti-wear behavior and to find out the best one, which can be

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J. Biswal et al. / Nuclear Instruments and Methods in Physics Research B 399 (2017) 69–73

applicable at different load and speed conditions. For this purpose, using TLA technique, the wear rates of disc gears made of EN31 steel were measured in presence of each lubricant at different experimental conditions. 2. Experimental 2.1. Proton irradiation The disc gears samples made of EN31 steel (composition:1.1 wt % C, 0.52 wt% Mn, 0.22 wt% Si, 1.3 wt% Cr, 0.04 wt% S and 0.04 wt% P and 96.78 wt% Fe and dimension: outer diameter = 35 mm, thickness = 10 mm) (Fig 1) were irradiated with a 13 MeV proton beam using BARC-TIFR Pelletron accelerator facility, Mumbai. Each disc was irradiated under vacuum for 4 h with a collimated proton beam of 3 mm diameter having beam current of 200 nA. The radioactivity was confined to a circular zone of 3 mm diameter on the surface of the disc gear. After the completion of the irradiation, the disc samples were cooled for 10–15 days to allow decay of short lived radioisotopes, if any. The major element in the disc gear is iron (Fe), which under goes nuclear reaction, when irradiated with proton beam of suitable energy. The nuclear reaction involved in this case is 56Fe(p, n)56Co. The nuclear data for this reaction is given in Table 1. The nuclear reaction has threshold energy of 5.446 MeV and cross section of 392 mb for 13 MeV proton. The product 56Co has a half life of 77.3 days and gamma energies 846.77 keV (100%) and 1238.28 keV (67%). Throughout the experiment the count rate was measured from the area of 846.77 keV photopeak. The gamma energy spectrum of the activated disc gear was measured using an HPGe spectrometer and is shown in Fig. 2. All the observed peaks in the spectrum are the characteristic energies of 56Co [24]. This indicates that, there was no other radionuclide byproduct produced in the process of irradiation. The proton energy chosen for irradiation has been decided on two factors; i.e., threshold energy and the cross section of the reaction involved. Both the Coulomb

Fig. 1. Counter-rotating motion of two disc gears.

barrier and the Q-value of the nuclear reaction are deciding factor for the threshold energy of the nuclear reaction. Hence in such type of particle induced nuclear reaction, the kinetic energy of the projectile particle should be chosen in such a way that, it should be greater than the threshold energy and also the reaction cross section should be maximum at this energy. So that the irradiation time required will be less for producing certain amount of activity. The cross section value for 56Fe(p,n)56Co reaction is maximum for 13 MeV proton (Table 1), hence this energy of proton beam has been chosen for irradiation. The activities of the irradiated disc gears were measured using an HPGe gamma spectrometer and were found to be in the range 2.1–2.2 MBq. 2.2. Determination of experimental calibration curve In actual wear tests, the loss in radioactivity of investigated samples of interest is measured as a function of test time. There is a need to correlate the loss of radioactivity to material degradation as a consequence of wear in order to estimate the wear rate. For this purpose depth profiling was carried out by irradiating a stack of iron foils (Fe foils) of known thickness under identical beam parameters and geometry as used for irradiating the disc gears [1,18]. After completion of the irradiation, the radioactivity of the stacked foils was measured by removing one by one from the surface layer, by using NaI(Tl) gamma spectrometer. The calibration curve was obtained by plotting the relative remnant activity versus thickness removed. The detail description of estimation of relative remnant activity has been given in the Section 2.3. Fig. 3 shows a calibration curve generated by irradiating a stack of fifteen Fe foils, each having thickness 0.025 mm with a proton beam of 13 MeV energy and having 200 nA current for 4 h. 2.3. Wear measurements The wear tests were carried out in an Amsler-type twin-disc tribometer, in which two identical disc gears were mounted on shafts located one above other. This type of tribometer is a standard type of machine, in which the real time gear contact can be simulated for wear and friction measurements. In the wear test, two discs rotate against each other on their cylindrical surface in counterdirection with contact width of 10 mm (Fig. 1). The proton irradiated disc was mounted on the upper shaft with the activated zone located at the contact point, as shown in Fig. 4a and Fig. 4b. To carryout wear experiments under steady-state conditions, initial running-in of the disc was carried out for 15 min at 10 kgf load and 200 rpm speed. Running-in is an initial subsurface conditioning process that often occurs when sliding or rolling contact is established between two solid bodies. Before mounting the disc on tribometer for actual wear test the radioactivity of the disc (counts/15 min) was measured for 15 min using the NaI(Tl) gamma spectrometer. Four different mineral oil based automobile lubricants equivalent to SAE 80W90 grades (L1, L2, L3, L4) were produced by an oil company and they were tested for their anti-wear behavior. The properties of the lubricants are specified in Table 2. The antiwear performance of different lubricants at various loads and rotating speeds were evaluated by measuring the extent of wear of the irradiated disc (upper) in presence of individual lubricant. At predetermined intervals the radioactivity labeled disc was removed from the tribometer, washed thoroughly with acetone, dried and the radioactivity was measured for 15 min using the gamma spectrometer (counts/15 min). Each measurement was repeated five times and the average count rate was estimated. The counting of disc gear samples and the stack foils were conducted with same instrument under identical detection geometries. The decrease in remnant activity on the disc with

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J. Biswal et al. / Nuclear Instruments and Methods in Physics Research B 399 (2017) 69–73 Table 1 Nuclear reaction involved in proton irradiation of iron based materials and measurement data of product radioisotope. Nuclear reaction

Threshold energy (MeV)

Cross section at 13 MeV (mb)

Product half life (d)

Major gamma energy/relative intensity; (keV)/ (%)

Yield (Bq lA 1 h 1 lm

56

5.44

392

77.3

846.77 (100), 1238.28 (67)

2  104

Fe(p,n)56Co

7000

847keV

Counts/1000 s

511keV

4000 1238keV

3000 2000

1038keV

1000

1776keV

2599keV

0 500

1000

1500

2000

2500

Enegy (keV) Fig. 2. Gamma energy spectrum of

56

Co generated on the disc gear surface.

1.0

Relative remnant activity

)

Usefull irradiation thickness (lm) 240

different test time is an indication of material loss, i.e. wear of the material. The wear tests were performed under constant load and rotation speed for a particular duration and again the wear behavior with different rotation speed was also studied at a particular lubricant temperature and for two different loads. These experiments were performed in presence of each lubricant. For a specific lubricant a fresh radioactivity labeled disc sample was used. The thickness loss was estimated from the loss in radioactivity using the following procedure:

6000 5000

1

0.8

(i) The initial activity of the disc gear before starting the wear test and after running-in was denoted as A0. (ii) Measured activity at each time interval (A) was divided with the initial activity (A0) to make the measurement independent of activity induced in the sample. This is known as relative remnant activity (A/A0) (Fig. 5). (iii) The thickness loss of the material was calculated by correlating the relative remnant activity with thickness removed with the help of the calibration curve. (iv) The thickness loss (referred as wear depth) was plotted against time and the wear rate was obtained from the slope of this curve as shown in Fig. 5. (v) In case of a single measurement for certain test duration, the wear rate was obtained by dividing time duration of the test with the corresponding thickness loss. 3. Results and discussion

0.6

Materials made up of EN-31 alloy steel are most extensively used in automotive industry such as heavy duty gear, shaft, pinion, camshafts, gudgeon pins and machining components. This type of steel has high resisting nature against wear and can be used for components which are subjected to severe abrasion, wear or high surface loading.

0.4

0.2

0

50

100

150

200

Thickness removed (µm) Fig. 3. Calibration curve generated from stacked Fe foil irradiation.

3.1. Estimation of wear rate The wear behavior of the disc gear was studied for 4 h at room temperature at constant load (30 kgf) and rotating speed (200 rpm)

Fig. 4. (a) Mounting of irradiated disc at the upper position in tribometer set up, (b) wear test of disc gear in presence of gear lubricant.

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J. Biswal et al. / Nuclear Instruments and Methods in Physics Research B 399 (2017) 69–73

Table 2 Characteristic properties of automobile lubricants. Lubricant samples L1

Relative remnant activity (a.u.)

Density @ 15 °C Kinematic viscosity At 40 °C At 100 °C Flash point Pour point

L2

0.89 g cm

3

142 mm2 s 1 14 mm2 s 1 234 °C 30 °C

1.000

(a)

(b)

6

4 0.990 0.985

2

0 0.975 100

150

200

3

0.89 g cm

143 mm2 s 1 15 mm2 s 1 222 °C 26 °C

3

144 mm2 s 1 14 mm2 s 1 208 °C 24 °C

Table 3 wear rate at steady state condition as obtained from slope of wear curve in Fig. 6.

0.980

50

L4

0.88 g cm

8

0.995

0

3

146 mm2 s 1 15 mm2 s 1 175 °C 27 °C

(a) decrease in relative remnant activity (b) increase in wear depth

1.005

L3

0.9 g cm

Wear depth (µm)

Properties

250

Test time (minute) Fig. 5. The plot of relative remnant activity versus test time and the corresponding wear depth versus test time (Experimental condition: Lubricant: L2, Load: 30 kgf, Speed: 200 rpm).

over an extended duration of test time in presence of each lubricant. The relative remnant activity was measured intermittently and the wear depth with test time was found with the help of the calibration curve. Fig. 6 shows the variation of wear depth with test time for four lubricants (L1, L2, L3, L4). It was observed that, initially up to 60 min of test time, there was no significant difference in the wear rate in all the four cases, whereas, beyond this time, the wear rate was lesser in the case of L1. The steady state wear rate (after 90 min of wear test) was calculated from the slope of different wear curves as shown in Fig. 6 and is given in Table 3. The order of wear rate in all the four lubricants is L1 < L4 < L2 < L3.

Lubricant

Wear rate (nm/h)

L1 L2 L3 L4

478 ± 38 990 ± 79 1176 ± 94 654 ± 52

Experimental condition: 30 kgf load and 200 rpm speed.

The results showed that, the wear rate is relatively less in the case of L1 and L4 at low load and low speed, where as the L3 did not show good anti-wear property as compared to other lubricants at this experimental condition. 3.2. Effect of load and rotating speed on wear rate Factors, such as temperature, load and rotating speed can influence the wear process of the gear, thereby can affect the performance of the lubricant. Experiments were carried out to see the effect of rotating speed at two different loads at 60 °C on the wear rate of disc gear in presence of each lubricant. The plot of variation of wear depth with speed at 30 kgf and 40 kgf loads are shown in Figs. 7 and 8, respectively. Under current experimental condition, the following observations were made from Figs. 7 and 8. (i) At lower load (30 kgf), the wear rate significantly increases with increase in rotation speed in case of L2 and L3. But in case of L1 and L4, there is little increase in the wear rate with increase in rotation speed. Hence, at lower load (30 kgf) L1 and L4 have better anti-wear properties than L2 and L3.

7 700

6

(b)

600

(c)

4

(d) Wear rate (nm/minute)

Wear depth ( µm)

5

(a)

3 2 1 0

500

(b)

(c)

400 300 200 (d) 100

(a)

0

-1 0

50

100

150

200

250

Test time (minute) Fig. 6. Variation of wear depth with test time for four different lubricants at 30 kgf load and 200 rpm speed: (a) L1, (b) L2, (c) L3, (d) L4.

0

100

200

300

400

Speed (rpm) Fig. 7. Effect of speed on wear rate for four different lubricants at 30 kgf load and 30 min of test duration at each speed: (a) L1, (b) L2, (c) L3, (d) L4.

J. Biswal et al. / Nuclear Instruments and Methods in Physics Research B 399 (2017) 69–73

carrying out the sample irradiation. The authors also thank Mr. S. A. Patil, Isotope and Radiation Application Division for technical cooperation provided by him in conducting the study.

800

Wear rate (nm/minute)

700

73

(a)

600

References

500

(c)

400 (b)

300 200

(d)

100 0 0

100

200

300

400

Speed(rpm) Fig. 8. Effect of speed on wear rate for four different lubricants at 40 kgf load and 30 min of test duration at each speed: (a) L1, (b) L2, (c) L3, (d) L4.

(ii) At higher load (40 kgf), the wear rate increases significantly with increase in rotation speed in case of L1, L2 and L3, although the extent of increase is different for different lubricants. At this load, the wear loss is lowest in L4, highest in case of L1 and intermediate in L2 and L3. (iii) The wear loss is lower and almost steady in the case of L4 at all loads and all speeds investigated. The above observations suggest that, L4 has the best anti-wear performance behavior for all the load and speeds considered. 4. Conclusions The wear measurements of disc gears under simulated conditions were studied using thin layer activation technique in presence of four different lubricants (L1, L2, L3, L4). TLA method has been proved to be a highly sensitive, relatively quicker than conventional method and it is an effective method to measure wear process in industrial and automotive components. In the current study, it was inferred that at lower load (30 kgf) L1 and L4 have better anti-wear properties than L2 and L3 and at higher load (40 kgf) L4 has best anti-wear property than other three lubricants. By using TLA method, out of the four lubricants studied, L4 was identified as the lubricant having best anti-wear behavior for all the load and rotation speeds considered. Acknowledgements The authors wish to express their sincere gratitude to Mr. K. S. S. Sarma, Head, Isotope and Radiation Application Division for his guidance and support during this study. The authors wish to thank staffs working at Pelletron accelerator facility for their help in

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