Diamond like carbon coating of ball bearings for raceway surface by triboadhesion

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 21, February 2014, pp Diamond like carbon coating of ball bearings for raceway surface by triboadhesion J A Rodríguez a *, J M Rodríguez b, J C García a, J A Arellano b & J L Maldonado a a Universidad Autónoma del Estado de Morelos CIICAP, Av. Universidad 1001, Col. Chamilpa, Cuernavaca, Mor, 62209, Mexico b Centro Nacional de Investigación y Desarrollo Tecnológico, Departamento de Ingeniería Mecánica Interior Internado Palmira S/N, Apdo. Postal , Cuernavaca, Mor., 62490, Mexico Received 1 February 2012; accepted 16 August 2013 This paper presents a comparison of performance of rolling bearings coated with diamond like carbon (DLC) by triboadhesion and without any coating. The bearings are coated only in the inner raceway. The performance evaluation is three fold: (i) an analysis of the vibration signal, (ii) an analysis of the relationship between tangential force and normal force determined like friction coefficient as an indication of the wear resistance of the surface, and (iii) a microscopic analysis of the rolling bearings surface. Four types of rolling bearings are tested: (a) lubricated and uncoated; (b) unlubricated and uncoated; (c) coated with DLC, no lubrication and normal load of 0.35 N and (d) coated with DLC, no lubrication and normal load of 0.5 N. It is found from the friction coefficient that bearings coated with DLC have a higher wear resistance than those without coating. Keywords: Friction, Surface, Triboadhesion, Wear Mechanical elements, such as bearings, experience metal to metal contact which causes adherence, abrasion and fatigue, i.e., wear 1. Friction and wear in rolling bearings can be diminished through surface treatments, in particular, by the use of coatings, e.g., carbides, nitrides, oxides, DLC or diamond among others. The surface properties on the rolling bearings, caused by the coatings, modify their wear resistance, as well as their stiffness and ultimately, their vibration signal. Diamond-like carbon (DLC) since its introduction by Aisenberg and Chabot 2 has drawn attention as a promising material owing to its characteristics such as high hardness, low friction coefficient and biocompatibility amongst others. Kalin et al. 3, compared the tribological performance of self-mated (DLC/DLC) non-doped and doped Ti, W, and Si DLC coatings, lubricated with paraffinic mineral oil and using conventional anti-wear (AW) and extreme-pressure (EP) additives. Among their contributions they found that the wear reduction due to the use of additives was much higher on the coated surfaces than on the steel surfaces, suggesting that the boundary-lubrication effect of the doped and non-doped DLC coatings could be achieved and/or improved also in self-mated contacts by the use of conventional additives that are used in metal/steel contacts. In *Corresponding author ( jarr@uaem.mx) biomechanics, in particular, some examples of application of DLC in prostheses are related to wear resistance and tribological behavior in general. Zeng et al. 4, compared the effect of sputtering coated DLC films on different substrates, and found out that sputtering DLC films on hard steel last less than those carried out in soft steel. On the other hand, Rodriguez-Lelis et al. 5 discuss the increment on wear resistance of prostheses coated with DLC by the triboadhesion process based on the analysis of the vibration signal. As described by Popp et al. 6 the reduction of friction is one of the keys for a successful application of hybrid or pure ceramic roller bearings under dry or nearly dry environmental conditions. They also mentioned that, in case of hybrid bearings, diamond-like thin film coatings deposited on the raceway seem to be an appropriate method for the improvement of its performance. Without any doubt, the performance of the coating depends on the deposition method. Stewart and Ahmed 7 published their findings about rolling contact fatigue (RCF) as a function of the type of coating process, in particular thermal spray, physical vapor deposition (PVD) and chemical vapor deposition (CVD). They observed that PVD coatings exhibited the highest resistance to RCF at room temperatures and elevated temperatures; this follows the performance of CVD and the thermal spray coatings. With the promising results of wear resistance of the coating by triboadhesion 8,9, this work is attempted to

2 RODRIGUEZ et al.: DIAMOND LIKE CARBON COATING OF BALL BEARINGS 105 evaluate qualitatively the wear resistance of ball bearings coated with DLC by the triboadhesion and without coating. Also, a comparison is included between uncoated ball bearings with and without lubrication. Microscopic analysis showed the spalling characteristic of wear due to rolling contact fatigue in all ball bearings, however, rolling bearings coated with DLC by the triboadhesion process showed an improvement on wear resistance. Following previous work 9, the ratio of shearing to normal forces, called friction coefficient, was used as an indication of the wear resistance of the surface. In addition vibration signals were obtained to evaluate the behavior of the ball bearings. Experimental Procedure Deposition process The triboadhesion coating was carried out only in the inner rolling surface of the bearing. A schematic diagram for the deposition process is shown in Fig. 1. This deposition process is composed by: (1) rotating wheel system, (2) force measurement system, (3) feeding system and (4) acquisition data system. The deposition process consists of passing the coating material, DLC, through the wheel, cotton mop, and the inner rolling surface to be coated. The wheel rotates at high velocity and exerts pressure on the inner rolling surface generating heat through friction. The feeding system ejects the DLC over the inner rolling. After it is deposited, its particles are embedded within the base material forming a thin film composite. This technology takes advantage of the heat generated by friction, because the surface voids available receive DLC particles are formed and disrupted through statistical fluctuations caused by increasing the temperature. Thus the steady state population of voids may be expressed by: by the sudden contact metal to metal may be found in the interval from 1 Hz to 10 khz. In this work the dynamic response of ball bearings without and with DLC coating by triboadhesion was assessed. For this purpose a set of four commercial bearings, all of the same type AISI 52100, were employed and labeled as: COMP1, COMP2, RE40S (for the last two bearings). The first two rolling bearings were employed as reference and the other two were coated by triboadhesion. The acronym 40S refer to the time employed by the triboadhesion coating process, that is, 40S refers to 40 seconds. The loads in these tests were at 0.35 N and 50 N. These bearings, for the dynamic test, were subjected to the same load and velocity conditions. Following Barkov et al. 11, the main analysis was focused on the interval of 2000 Hz to 7000 Hz, and as complement in accordance with Taylor 12, a spectral analysis was carried out to identify defects on the interval of 0 to 2000 Hz. As an overview of the behavior of the bearings COMP1, COMP2, RE40S, frequencies of the spectrum larger than g's are plotted (Figs 2-4). In Fig. 2, it can be observed a number of peaks with frequencies lower than 400 Hz, which can be attributed to unbalance, misalignment or mechanical instability, typical frequencies of bearing, etc. The component 1X normally is related to unbalance and exhibits amplitudes of g s and g s for bearings COMP1 and COMP2, respectively. The components 2X are also present and are attributed to misalignment. Beyond frequency of 400 Hz there are no components and only a N C * G = T N exp (1) K T Where T refers to the temperature, N and N C are the number of atoms and cavities per volume, K is the Boltzman constant and the free energy G T * can be obtained as shown in ref. 10 Vibration signal analysis As mentioned by Barkov et al. 11, bearings can exhibit vibration caused by six kinds of dynamic forces. Also, that the random vibration may be due to friction forces at frequencies around the 30 khz, exhibiting low amplitude compared to the vibration encountered in the interval from 2 khz to 10 khz. Vibration pulses caused Fig. 1 Deposition process: (1) rotating wheel system (a) high speed motor, (b) velocity control and (c) mop; (2) force measurement system (d) ring type load cell and (e) base; (3) feeding system (j) particles container, (k) pneumatic control system, (l) nozzle; (4) acquisition data system (f) amplifier, (g) volt-meter, (h) signal analyzer, and (i) PC

3 106 INDIAN J. ENG. MATER. SCI., FEBRUARY 2014 horizontal line is presented. Figure 2 shows too the spectrums for bearings RE40S. Here it is possible to observe the 1X component, where the larger amplitude of g s is presented by bearings RE40S. In this graph, the 3X component is related to misalignment. Figure 3 shows the spectrum of vibration in the interval of Hz, where it is observed that the frequencies amplitude of bearings COMP1 and COMP2 are lower than g s, with exception of two peaks near to 4000 Hz where the amplitude lies around of g s which corresponds to bearing COMP2. Figure 3, depicts the spectrums of bearings RE40S within the interval of Hz. Particularly, RE40S to 0.35 N and 0.5 N exhibits amplitudes around g s. The RE40S gives an indication of the change in stiffness in the contact zone and friction force. The differences between bearings RE40S to 0.5 N and RE40S to 0.35 N might be attributed to the amount of DLC deposited. The difference between RE40S and COMP1 bearings spectrums is shown in Fig. 4. In this figure the component 1X is also present and the amplitude for bearing RE40S is higher than that of COMP1. On the other hand, the amplitude of frequencies between 400 to 2000 Hz describes nearly a horizontal line. In Fig. 4 the spectrum in the interval of Hz is shown, it can be easily seen here that the amplitudes of RE40S is higher than those of COMP1. The frequencies of bearing RE40S are distributed between 3000 to 5000 Hz, whereas the frequencies of the bearing COMP1 are between 3700 and 4500 Hz. The comparison between the bearings COMP2 and RE40S is shown in Fig. 5. In Fig. 5 it can be Fig. 2 Spectra between 0 to 2000 Hz of COMP1, COMP2, RE40S at 0.35 N and 0.5 N Fig. 4 Spectra of bearing RE40S versus bearing COMP1 between 0 to 6000 Hz Fig. 3 Spectra between 2500 to 6000 Hz of COMP1, COMP2, RE40S at 0.35 N and 0.5 N Fig. 5 Spectra of bearing RE40S versus bearing COMP2 between 0 to 6000 Hz

4 RODRIGUEZ et al.: DIAMOND LIKE CARBON COATING OF BALL BEARINGS 107 appreciated that the amplitude of vibration is higher in COMP2 for the 1X component, which can mainly be related to shaft unbalance. Similarly, the RE40S bearing exhibits higher amplitudes compared to those of the COMP2 bearing as shown in Fig. 5. This can also be attributed to the different surface stiffness caused by the presence of the DLC coating. Evaluation of wear The schematic diagram for the wear evaluation is shown in Fig. 6. Here, the normal load is applied by a 10 tons hydraulic press. This press has at its end an aluminum load transducer with diameter of m where the load is measured. The load is applied directly to the outer ring of the ball bearing with a pressure of 61.4 MPa, which is then coupled to an electric motor. The electric current used by the motor is employed as an indirect measurement of the tangential force. The load and current signals are sent to a data acquisition card from where they are finally displayed and stored in the PC. The experimental procedure was performed by applying a load of N on the bearing mounted. It was determined the tangential force and the normal force as a parameter of friction and wear. Results and Discussion The following four tests were carried out: (i) a ball bearing with industrial lubrication (COMP1), (ii) a ball bearing without any lubrication (COMP2), (iii) a ball bearing coated at 0.35 N load and (RE40S) and (iv) a ball bearing coated at 0.5 N load (RE40S). The test conditions are shown in Table 1. For the purpose of this work the ratio of the tangential force between the normal force determine the friction coefficient. The results of Test 1 are shown in Fig. 7. The friction coefficient starts with an initial value of describing nearly a straight curve, and after 11 min increases to From this point onwards the friction coefficient remains nearly constant. After 2.4 h it reaches a value of At 2.6 h the slope of the curve becomes nearly exponential marking the end of the test. In Test 2 (Fig. 8) the behavior is similar as in Test 1, but it exhibits early deterioration of the surface Table 1 Wear test conditions Fig. 6 Shear and normal force experimental arrangement: (1) electric motor, (2) normal load transducer, (3) ball bearing, (4) acquisition data system and (5) PC Test Type Normal force, N Rotational speed, rpm Time o test, h 1 COMP COMP RE40S at 0.35 N RE40S at 0.5 N Fig.7 Test on ball bearing with lubrication (COMP1)

5 108 INDIAN J. ENG. MATER. SCI., FEBRUARY 2014 characteristics of the ball bearing surface. Its starting value is and the latest was The time taken between beginning and the end of test was 1.76 h. The oscillations presented in both tests were attributed to the deterioration of the rolling surface of the bearings. It should be noted in Test 2 that this deterioration initiated close to the beginning of the test. The results for the ball bearing coated with synthetic diamond at a load of 0.35 N are shown in Fig. 9. The coating was carried out only on the inner rolling surface of the bearing. In this case the friction coefficient curve starts with a value of and increases slowly and stable. During the time of the test the friction coefficient decreases three times probably caused by surface hardening of the rolling surface. The test lasted 5.16 h which is 115% larger than the bearings of test 1; and 218% respect to the bearing of test 2. The last test is shown in Fig. 10. For this test, the inner roll bearing surface was coated at a load of 0.5 N. In Fig. 10, the lowest friction coefficient is observed During the test the friction coefficient increases very slow and never reaches values above After 283 min of the test the friction coefficient increases rapidly and at 391 min the asymptotic slope is presented indicating the end of the test. In comparison with the previous tests, this lasted 150% more that the first one, 270% more than the second one and 16% more that the third one. Microscopy analysis Microscopy analysis of the ball bearing inner raceways was carried out with a Scanning Electron Microscope Leo The resolution was set up to 3.5 nm and 30 kv. Two samples were analyzed; the first one corresponds to a lubricated ball bearing (COM1) and the second one to a ball bearing with DLC coating (RE40S). The microscopy and chemical analysis for the lubricated ball bearing without any Fig. 8 Test on ball bearing without any lubrication and no coating (COMP2) Fig. 9 Test on ball bearing without any lubrication and coated at a load of 0.35 N (RE40S)

6 RODRIGUEZ et al.: DIAMOND LIKE CARBON COATING OF BALL BEARINGS 109 Fig. 10 Test on ball bearing without any lubrication and coated at a load of 0.5 N coating are shown in Figs 11 and 12. It may be seen from Fig.11 that large delamination is present with some flakes varying largely in size, that is, their averages sizes vary from a few microns or less up to sizes of the order of 30 microns. It can also be observed from this figure, a number of spots with loosing material, and specific zones of progressive wear where some cracks appear. Also, zones with high concentration of oxygen can be appreciated, which causes the initiation of spalling and in consequence wear. In addition, Fig. 12 shows the chemical analysis for the surface of the ball bearing. Here it may be seen the main elements of the ball bearing Fe, Cr, Si and C. It can also be appreciated a concentration of oxygen (Table 2) which could have given birth to corrosion fretting and in combination to the load imposed to the flakes formed on the surface. In contrast, Figs 13 and 14 show the surface micrograph and chemical analysis for the ball bearing with the DLC deposited by triboadhesion. Like the lubricated ball bearing, delamination is also present, but in a smaller scale, and the flakes present vary from nanometers to a maximum of 10 microns. In Fig. 13 it is possible to observe the high carbon concentration in the raceway surface as a consequence of the coating with synthetic diamond. The raceway surface has damaged conditions higher than the other bearings; this is because when raceway is broken, wear appears in a more rapid way than in other bearings. This is caused by the change of the mechanical properties. Similarly, Fig. 14 shows the chemical analysis which the same picks of the main elements of the ball bearing and the oxygen are present. A red line can be appreciated in Fig. 14 which shows a high pick which establishes a high Fig. 11 Inner roll bearing surface with lubrication Fig. 12 High content of Fe C in inner roll bearing surface with lubrication Table 2 Atomic percent of ball bearings Element COMP1 RE40S C O Si Cr Fe Total

7 110 INDIAN J. ENG. MATER. SCI., FEBRUARY 2014 limit of thickness coating for which there won t be significant increase in the amplitude of vibration. Fig. 13 Inner roll bearing surface coated at a load of 0.5 N Fig. 14 High content of C in inner roll bearing surface to coated at a load of 0.5 N carbon concentration. This concentration is related to the DLC deposited on the inner raceway. After analyzing all the spectra graphs it is observed that the primary zone or low frequency provides information that indicates the presence of frequencies due to unbalanced, misalignment, looseness, mechanical, bearing characteristic frequencies. The intermediate zone shows changes due to frictional forces, indicating a change in the contact zone which is significant for both bearings RE40S. The amplitude of vibration increase of the coated bearing may be attributed to the surface properties modification. It should be pointed out that the frequency lies within 3500 to 4500, where contact phenomena starts to appear. It could be envisaged, based on these results, that the higher the amount of DLC deposited, the higher is the amplitude increment. This is attributed to the stiffness increase caused by the DLC coating. It can also be argued that there is a Conclusions Synthetic diamond was successfully deposited on the inner rolling surface of ball bearings in accordance to the technology described in ref. 4. The dynamic response of ball bearings with and without DLC coating was assessed. An increase of the amplitude of vibration for the bearings with DLC coated by triboadhesion was detected. The increase of the amplitude of vibration was attributed to the change in stiffness and hardness of the rolling surface of the bearings once modified by the film DLC coating. Ball bearing coated at 0.5 N with diamond lasted from 150 up to 270% more than those bearings without coating, including the one with lubrication. Also, the coated bearings showed less damage on the surface after being tested. The larger flakes observed in the ball bearing with DLC coating were one third of those found in the case of the lubricated ball bearing. It may be anticipated that bearings coated with diamond and with lubrication will last even longer. Aknowledgements This work was financed by a CONACYTproject number U and DGEST References 1 Harker R G, et al., Trans ASME, J Eng Gas Turbines Power, 111 (1989) Aisenberg S & Chabot R, J Appl Phys, 42 (7) (1971) Kalin M et al., Tribol Lett, 17(2004) Zeng Q, Eryilmaz O, Erdemir A, Thin Solid Films, 519 (2011) Rodríguez-Lelis J M, Tolosa D, Vargas M, Navarro J, Piña G & Abundez A, J Appl Biomech, 26(3) (2010) Popp M, Sternagel R, Pfeiffer W, Blug B, Meier S, Wötting G & Frasseck L, Ceramics - Processing, Reliability, Tribology and Wear, 12 (2006) Ch Stewart S & Ahmed R, Wear, 253 (2002) Rodríguez J M et al., ASME J Manufact Sci Eng, 125 (3) (2003) Rodríguez J M, et al., Math Phys Astron, 44 (2001) Rabinowicz E, The Friction and Wear of Materials (Wiley, New York), Barkov A V, Barkova N A & Mitchell J S, Sound Vib, 29(6) (1995) Taylor J I, Trans ASME J Mech Des, 102 (1980)