WO2019069322A1

WO2019069322A1 - Dynamic load sliding contact tribometer and method to simulate wear therewith

Info

Publication number: WO2019069322A1
Application number: PCT/IN2018/050628
Authority: WO
Inventors: Kodoor SREERAJ; Penchaliah RAMKUMAR
Original assignee: INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT Madras)
Priority date: 2017-10-03
Filing date: 2018-10-03
Publication date: 2019-04-11

Abstract

A dynamic load sliding contact pin-on-disc tribometer is provided. It includes a motor-powered disc sample configured to operate at predetermined speed of rotation. A pin holder is configured to hold a pin sample in sliding contact with the disc sample. A cyclic load generator is operably linked to the pin sample through a setup to transfer a load to the pin sample, the setup comprising a lever arm pivoted at a fixed hinge with one end of the lever arm connected to the pin holder and the other end of the lever arm connected to the cyclic load generator through one or more pulleys. A hydrogenating lubricant composition is added in between the disc sample and pin sample to initiate sliding, upon load transfer, between the pin sample in contact with the disc sample to simulate accelerated wear on a steel sample in 2.5 × 105 cycles or less.

Description

DYNAMIC LOAD SLIDING CONTACT TRIBOMETER AND METHOD TO

SIMULATE WEAR THEREWITH

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] The present applications claims priority to Indian Patent Application No. 201741035045, entitled "DYNAMIC LOAD SLIDING CONTACT TRIBOMETER AND METHOD TO SIMULATE WEAR THEREWITH," filed on October 3, 2017, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The disclosures relates generally to a device, a method and a composition for surface and sub- surface failure testing and in particular to a tribometer and a method for simulating accelerated wear.

DESCRIPTION OF THE RELATED ART

[0003] Appearance of white etching area (WEA) followed by white etching cracks (WECs) formation is observed as a dominant mode of premature failure in ball and rolling element bearings of dental drills, power transmission systems, wind turbine gearboxes, automobiles, vertical roller mills of cement industry and other components. According to recent statistics released by the National Renewable Energy Laboratory (NREL), 76% of the wind turbine gear box failures are caused by bearings. The L₁₀ life of bearing is the rolling contact fatigue (RCF) life of bearing for a given operating condition at which 90% of the bearings survive. But usually the RCF life exceeds the L₁₀ life and finally failed by spalling, which is inevitable. However, during transient operating conditions of wind turbine, bearings may fail prematurely due to the consequence of flaking involved subsurface micro structure changes. This type of failure is called white structure flaking (WSF) or white etching cracks (WECs). Due to the prevalent premature failure mode of roller bearings, the expected 20 year life of wind turbine gearbox reduced to 6-24 months (1-20% of Li₀ life). Many attempts have been made for replication of WECs at the component level. However, control of critical parameters is not achieved and the methods have not proven effective for WECs replication (Franke et al. 2017, Gutierrez Guzman et al. 2017).

[0004] Hydrogen release, from lubricant breakdown and water contamination, is proportional to wear rate. Further, the hydrogen diffusion in bearing steel during transient operating condition has been suggested as the driving force of WECs formation. In a system, bearing slip occurs during transient operations such as acceleration/deceleration, overloading and underloading and misalignment of rollers due to the gusty nature of wind. Slip increases the magnitude of cyclic shear stress acting in the subsurface region and damages the tribofilm; consequently, it releases and diffuses hydrogen into nascent steel surfaces. The hydrogen ingress into the bearing steel from the lubricant failure may create white etching regions which may be further enhanced by the presence of water contamination. Mechanical property deterioration involving hydrogen in steels is discussed under hydrogen enhanced decohesion and hydrogen enhanced local plasticity (HELP) theory (Song et al. 2014). Reduction of material yield stress and softening phenomenon due to hydrogen charging are widely interpreted using HELP mechanism. HELP is primarily supported by experimental evidences for enhanced dislocation motion and localised slip bands near the crack tip on hydrogen charged test samples. Postulates of HELP mechanisms are (i) hydrogen induced damage initiation is due to reduced dislocation-dislocation interaction (ii) material softening due to hydrogen induced higher dislocation mobility.

[0005] The failure analysis of WEAs associated WECs is typically conducted postmortem on the failed bearing samples which is limited and may not reveal the root cause of bearing failure and insight into its evolution. Further, existing rigs such as FE8 thrust ball bearing test rig to test at component level are limited and fail to provide a complete picture of the influencing factors. The major limitations of the available testing methods for simulating bearing wear are, lack of transient loading to reproduce service conditions of bearings, poor repeatability, large number of contact cycles required (more than 10 million cycles), difficulty in determining WEC in a large specimen.

[0006] US Patent No. 8402811B2 discloses a testing instrument that investigates failure behavior of items subjected to impact and sliding forces. The instrument disclosed in the '811 patent produces impact motion along with sliding in each testing cycle with maximum contact pressure similar to actual stresses applied to the items during real applications. However, the instrument is complex and fails to simulate accelerated wear. The Chinese utility patent CN202066765U discloses a simple pin-on-disk small sample abrasion test device made from composite material. The Chinese patent CN102809516A discloses a pneumatic variable-load friction wear testing machine and a testing method. The European patent EP0214687A2 discloses a device for tribological measurements. However, none of the machines enable cyclic loading during wear testing, which is typically encountered in bearing service.

[0007] There is therefore a need for a tribometer and method for testing bearing steel materials which overcomes the aforementioned problems and also provides a way of reproducing dynamic load and hydrogen associated damage in the steel specimen.

SUMMARY OF THE INVENTION

[0008] The disclosure relates to a tribometer and in particular to a dynamic load sliding contact tribometer for simulating accelerated wear.

[0009] In various embodiments, a dynamic load sliding contact tribometer which is configured to simulate accelerated wear includes a motor-powered rotatable disc sample configured to operate at predetermined speed of rotation. A pin holder configured to hold a pin sample in sliding contact with the disc sample. A cyclic load generator is configured to transfer a cyclic load to the pin sample via a lever arm and generate a contact pressure. The lever arm includes a first arm and a second arm pivoted at a fixed hinge located therebetween. The first end is connected to the pin holder and the second end is connected to the cyclic load generator. The wear of the disc or the pin sample is adjustable by varying a maximum load generated at the cyclic load generator.

[0010] In some embodiments, the pin holder is configured to at least partially rotate the pin sample about an axis normal to an axis of rotation of the rotating disc sample and an axis of the pin sample. In various embodiments the load cycling frequency is 1-30 Hz. In some embodiments the cyclic load generator generates maximum contact pressure in the range of 1-3 GPa.

[0011] In various embodiments the tribometer includes a linear variable differential transducer (LVDT) configured to measure the combined wear depth of the pin sample with reference to the disc sample. In various embodiments the pin holder is configured to control the temperature of the pin sample. In some embodiments the cyclic load generator includes a hydraulic cylinder, a pneumatic cylinder, a cam follower, an electromagnetic lifting mechanism, or a combination thereof.

[0012] In various embodiments a method is provided to simulate accelerated wear on a steel sample using a pin-on-disc tribometer. A disc sample and a pin sample for use in accelerated wear simulation are provided. A hydrogenating lubricant composition is fed in between the disc sample and the pin sample. The pin sample is placed in sliding contact with the disc shaped sample. A cyclic load is transferred from a load generator to the pin sample to simulate accelerated wear of the samples. The wear of the pin sample against the disc sample is monitored by measuring combined wear depth of the pin sample and the disc sample.

[0013] In some embodiments the lubricant composition comprises a mixture of NH₄SCN, water and polyethylene glycol (PEG) in a ratio of 1:2:2 by weight. In some embodiments, the test is configured to simulate contact wear between steel surfaces resulting in cementite dissolution in to matrix forming white etching area (WEA) in the subsurface of the pin or the disc sample.

[0014] In various embodiments, the accelerated wear is obtained in a test duration of 14 hours or less. In some embodiments the accelerated wear is obtained in 10⁶ contact cycles or less. In one embodiment the accelerated wear is obtained in 2.5xl0⁵ contact cycles or less. In some embodiments the hydrogenating lubricant composition for simulating accelerated wear of a steel sample in a tribometer, includes NH₄SCN, water and polyethylene glycol (PEG) in a ratio of 1:2:2 by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:

[0016] FIG. 1A depicts side view of a tribometer.

[0017] FIG. IB illustrates isometric view of a tribometer.

[0018] FIG. 2 illustrates a method of simulating accelerated wear on a steel sample using a tribometer.

[0019] FIG. 3A shows SEM image of wear scar diameter on the tested bearing ball and marking showing the cutting direction of wire-EDM

[0020] FIG. 3B shows two halves of the bearing ball in the mold

[0021] FIG. 3C depicts severe fatigue wear affected region marked in the molded specimen.

[0022] FIG. 4 depicts optical image of WEA formation due to cementite dissolution.

[0023] FIG. 5 depicts the SEM image of WEA formation due to cementite dissolution.

[0024] FIG. 6 depicts the EDS image of WEA formation due to cementite dissolution.

[0025] FIG. 7 illustrates SEM images of discontinuous micro-crack inception within WEA.

[0026] Referring to the drawings, like numbers refer to like parts throughout the views. DETAILED DESCRIPTION

[0027] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0028] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0029] The invention in its various embodiments provides for a pin on disc tribometer to generate cyclic load, simulating wear conditions and mechanisms typical in bearing service. Additionally a testing method of simulating hydrogen effect on wear mechanism in steel bearings under cyclic loading conditions is provided. A hydrogenating lubricant composition is added to accelerate the wear mechanism in a pin on disc tribometer. The method simulates accelerated hydrogen induced damage in industrial service conditions in a shorter duration and within fewer load contact cycles.

[0030] In some embodiments, a tribometer 100 that includes a motor-powered disc sample 110 configured to operate at predetermined speed of rotation is provided. The disc sample 110 is in sliding contact with a pin sample 120. The pin sample 120 is held by a motor-powered pin holder 130. A cyclic load generator 136 is operably linked to the pin sample 120 through a setup to transfer a load to the pin sample 120 via the pin holder 130. In one embodiment the pin sample 120 may comprise a spherical ball 121 affixed to the tip thereof. [0031] In various embodiments, the transfer of load from the cyclic load generator 136 includes a setup comprising a lever arm 138. The lever arm 138 is pivoted at a fixed hinge 132. The lever arm 138 is connected to the pin holder 130 on one side and the cyclic load generator 136 on the other side through one or more pulleys 134. The dynamic nature of load in the service condition is provided by a specially devised actuator 140 along with frequency controller 142.

[0032] In various embodiments the pin sample 120 is partially rotated or oscillated about a third axis normal to the axis of the pin and the axis of rotation of the disc as shown in FIG. IB in order to achieve enhanced or accelerated wear. The pin holder 130 may be powered by a servo motor 144 to partially rotate the pin sample over the rotating disc about a Z-axis 147 of the rotating disc.

[0033] In various embodiments a lubricant composition 146 is sprayed between the disc sample 110 and pin sample 120 to initiate sliding, upon load transfer, between the pin sample 120 in contact with the disc sample 110, as shown in FIG. IB.

[0034] In various embodiments the cyclic load generator 136 is powered by a hydraulic cylinder or a pneumatic cylinder, cam follower mechanism, electromagnetic cyclic lifting force or a combination thereof to generate a cyclic load in the range 1-30 Hz. In some embodiments the frequency of the tribometer 100 is varied based on the motor and number of lobes in the cam. The loading cycle is varied from low frequency (lHz) to higher frequency (30Hz). In some embodiments, the loading cycling is varied between 1-5 Hz.

[0035] In various embodiments the cyclic load generator 136 generates compressive load in the range between zero and a maximum. The maximum contact pressure in some embodiments is obtained in the range of 1-3 GPa. [0036] In various embodiments the tribometer 100 includes one or more load cells 148. The load cell 148 is configured to measure frictional forces or the tangential frictional forces obtained from the contact pair dynamic interaction between the contact surfaces.

[0037] In some embodiments the tribometer 100 includes a linear variable differential transducer (LVDT) 150 to measure the relative displacement of the pin sample 120 with reference to the disc sample 110, as shown in FIG. 1A. The combined material removal or the combined wear depth of the pin sample and the disc sample is measured in real time using LVDT displacement sensor 150.

[0038] In various embodiments the sample holder 130 is configured to control the temperature of the pin sample. The temperature generated is configured to be similar to service realistic conditions of the sample.

[0039] The invention in its various embodiments proposes a method 200 as shown in FIG. 2 for simulating accelerated wear on a steel sample using a pin-on-disc tribometer. In step 201, a disc sample and a pin sample are provided for use in the tribometer. In step 203, a hydrogenating lubricant composition is fed between the disc sample and pin sample. In step 205, a sliding contact between the pin sample and the disc shaped sample is established. A cyclic load from a load generator is transferred to a pin sample in step 207. The wear of the pin sample against the disc is monitored by measuring the combined wear depth or displacement of the pin and disc samples in step 209. Thereby, the accelerated wear on steel sample is simulated in step 211.

[0040] In various embodiments of the method the pin sample is configured to be partially rotated or oscillated during testing about a third axis normal to the axis of the pin or axis of rotation of the disc. The additional oscillating rotation is configured to accelerate contact between the mating surfaces and therefore the wear damage. [0041] In various embodiments a tangential frictional force is generated due to the relative motion between the disc sample and the pin sample.

[0042] In various embodiments the method 200 in step 203 involves applying a hydrogenating lubricant composition on the disc before the wear testing. The hydrogenating lubricant used during wear testing is configured to reproduce hydrogenating conditions prevailing during actual service of bearing-lubricant interaction. In some embodiments of method 200 the lubricant composition is a mixture of NH₄SCN, water and polyethylene glycol (PEG). The NH₄SCN, water and polyethylene glycol (PEG) in some embodiments is used in the ratio of 1:2:2 by weight. In various embodiments the hydrogenating conditions are maintained on the steel subsurface proportional to wear, thereby resulting in cementite dissolution in to matrix forming WEA in the subsurface of the sample. In various embodiments accelerated wear due to hydrogen damage is simulated on steel bearing samples at shortest duration with high repeatability.

[0043] In various embodiments the accelerated failure is obtained in a duration of 40 hours or less. In some embodiments the accelerated failure is obtained in a duration of 14 hours or less. In various embodiments the accelerated failure is obtained in 10⁶ contact cycles or less. In one embodiment the accelerated failure is obtained in 2.5 x 10⁵ contact cycles or less. In one embodiment the accelerated failure is obtained in 2.15 x 10⁵ contact cycles or less.

[0044] In various embodiments the tribometer 100 is used as an appropriate testing instrument for testing sliding contact fatigue failure studies of gears, rolling element bearings, micro-pitting, machine tools, new materials, advanced materials, lubricants, coatings, bio-implants, surface treated materials, surface coated parts, surface coated components, or a combination thereof. In some embodiments the sample is selected from gear, roller bearing element or a ball bearing element. The surface treatments may include carburizing, flame hardening, laser hardening, nitro-carburizing, nitriding, or quench hardening. In some embodiments the tribometer 100 is useful to test various parts or components by surface coated by a technique such as plasma spray coating, chemical vapor deposition (CVD), physical vapor deposition (PVD) or thermal spray coating and the like.

[0045] In various embodiments the tribometer 100 and method 200 are configured to simulate contact fatigue, hydrogen embrittlement, impact sliding wear, premature bearing failure, and sliding fatigue wear in steels suitable for accelerated evaluation.

[0046] Without being bound to any theory, it is stated herein that hydrogen ingress into steel due to the lubricant degradation is proportional to friction energy accumulation at the contact. The severe cyclic compressive and frictional shear stresses in the presence of hydrogen ingress from the lubricant induced local material softening and subsequently reduced flow stress within the bearing steel. This may cause various degrading microstructure changes called WEA/WECs. The atomic hydrogen kinetics and diffusion simulation within the tribo-contact bodies shows the strong relation between formation of WEC and shape of the rolling element. The geometry of the contact pair influences the penetration depth of atomic hydrogen in bearing steel. The surface area of the two contacting bodies other than the contact area (free surface area) acts as an escape route for diffused hydrogen from the bearing steel. But atomic hydrogen entry into steel is only through the contact location of the two surfaces.

[0047] The novel modified pin on disc tribometer 100 disclosed herein is configured to hold a spherical bearing ball specimen in a pin holder. Without being bound to any theory, it is suggested that the hydrogen outflow through the free surface area is less for the spherical bearing ball and leads to accelerate the formation of WEC in the novel testing method 200 due to the minimum surface area to volume ratio of the spherical shape. Smaller specimen sizes will make the metallographic examination of test specimen easier. The wear testing device 100 could also be used to test materials without the use of hydrogenating lubricant. The rate of hydrogen related damage on the material is comparatively lower without using hydrogenating lubricant than with the use of hydrogenating lubricant. The tribometer test results when used along with hydrogenating lubricant composition are obtained at least ten times faster than the presently available methods.

[0048] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material the teachings of the invention without departing from its scope. Further, the examples to follow are not to be construed as limiting the scope of the invention which will be as delineated in the claims appended hereto.

EXAMPLES

[0049] Example 1 - Accelerated WEC/WEA Testing in Bearing Steel

[0050] The conventional pin on disc tribometer is made up of a stationary pin (ball) in contact with a rotating disc under an applied load. The static loading arrangement of conventional pin on disc test rig is modified to dynamic (cyclic) load by using an additional cam and follower mechanism as illustrated in FIG. 1 for accelerated wear testing of bearing steel. The modified pin on disc tribometer was operated at under pure sliding condition between the ball and disc, in one instance at 90 loading cycles per minute. Both pin and disc were of SAE 52100 or AISI 52100 bearing steel of nominal composition as indicated in Table 1. The dimension of the disc in contact with the ball was of 165 mm diameter and 8 mm thickness. The wear track diameter varied from 60 mm to a maximum of 150 mm. The ball specimens used were 10 mm in diameter. The disc was used for multiple wear tracks based on a number of experiments carried out on the disc surface. The mechanical properties of the ball and disc material used in one instance are shown in Table 2.

[0051] The lubricant composition used was NH₄SCN, water and polyethylene glycol (PEG 600) in a ratio of 1:2:2 by weight. The kinematic viscosity of the newly formulated hydrogenating lubricant measured using cannon Fenske viscometer (ASTM method D445) is 29 cSt with density of 1.127 g/cm at 40 °C. The composition was supplied inside the wear track. Under severe boundary lubrication condition, the lubricant reacted with nascent steel surface and generated hydrogen. This new approach of accelerated hydrogen ingress shortened the duration of WEA formation in bearing steel. In one instance, the test condition was 0.2 m/s sliding velocity with maximum contact pressure of 2.03 GPa under severe boundary lubrication condition with a calculated lambda value of 0.10.

[0052] The tribometer maintained hydrogenation of steel subsurface proportional to wear. The experiments created WEAs consistently on SAE52100 bearing steel ball having effective mean contact pressure of about 1.3GPa within 40 hours test duration (2.15 x 10⁵ load cycles).

[0053] Multiple ball specimens were tested for different durations ranging from 1 xlO⁵ to 3 xlO⁵ cycles. Each tested specimen was analyzed after EDM cutting, metallographic polishing and microscopy to identify failure, as denoted by the appearance of white etching constituents.

TABLE 1: Nominal Composition of 52100 Bearing Steel

TABLE 2: Mechanical Properties of Ball and Disc Material

Properties Disc Ball Young's modulus 210 210

Density (kg/m³) 7800 7833

Poisson's ratio 0.3 0.3

Hardness (HV) 710 750

Surface roughness (μηι) 0.15 0.025

Dimensions (mm) 165 diameter x 8 thickness 10 diameter

[0054] Example 2 - Accelerated WEC/WEA Testing in Bearing Steel

[0055] The cam-follower mechanism of the dynamic load pin on disc tribometer was modified to achieve 4.5Hz frequency dynamic loading. The testing were carried out at lGPa, 1.45GPa and 2GPa contact pressures using the newly formulated real-time hydrogenating oil (water, PEG600 and NH4SCN in a ratio by weight 2:2: 1) and poly- alpha-olefins (PAO) base stock gear oil under boundary lubricating conditions.

[0056] Real-time hydrogenating oil with 4.5Hz dynamic load testing condition even reduce the total test duration to 13.3 hrs (with same 2.15x 10⁵ contact cycles) at lowest contact pressure of lGPa. PAO base oil is more stable synthetic gear oil against hydrogenation which replicate WEA within 13.3hrs (with same 2.15x 10⁵ contact cycles) but at 1.45GPa contact pressure.

[0057] The frequency of dynamic loading can be a positive catalyser to accelerate the process of sequential evolution of WEA formation in bearing steel. After all, the consequence of dynamic loading frequency is as severe as higher hydrogen ingress in bearing steel at a contact pressure under boundary lubrication. The simultaneous action of both dynamic load frequency and hydrogenation cumulatively add up the damage and radically reduce the critical safe/minimum contact pressure even less than lGPa within a test duration of 13.3 hours.

[0058] Various attempts have been made to replicate the WECs in laboratory level test rig for the detailed investigation. Test rig used in the various studies and the typical contact cycle to create WEA/WECs under identical test conditions is as given in Table 3. Overall, the fundamental RCF test requires longer test duration, higher number of contact cycles, tedious serial sectioning and metallographic examination of the test specimen. But the newly developed test rig along with the novel hydrogenating lubricant achieves WEA formation within 2.15xl0⁵ contact cycles, as demonstrated by testing 4 specimens at 40 h of testing time. The shortest number of contact cycles is achieved by the unique design with hydrogenating lubricant composition for simulating accelerated wear of a steel sample in a tribometer. Additionally, the method demonstrated very good repeatability.

TABLE 3: Comparison of Contact Cycles to Create WEA/WECs in Various Test Rigs

Under Identical Test Conditions

No Test rig Number of cycles to

form WEA/WECs

1 FE8 test rig 1 x 10^v

2 RDM (Roller disc 38 x 10⁷

machine)

3 Two roller test rig 4.2 x 10⁷

4 Pre-hydrogenation 40 x 10⁶

and two roller tester

5 This invention 2.15 x 10⁵

[0059] Example 3 - Characterization of the WEA/WEC formation

[0060] The bearing specimen wear scar diameters after 35 and 40h were about 2.6 and 3 mm, respectively as determined by SEM analysis. The SEM image of the 40 h tested bearing ball wear scar diameter is shown in FIG. 3A. Since the subsurface of wear scar area will be the most affected region in fatigue wear, the worn ball was sectioned along the wear scar diameter in to two semicircles as marked in FIG. 1(a) using wire cut- EDM technique. The two hemispherical specimens were moulded as shown in FIG. 3B for polishing and etching (Nital solution) purpose Metallographic inspection using the OM and SEM was conducted below the wear scar region for the identification of subsurface micro structural changes due to fatigue wear. After the detailed examination of prepared worn-out sample, most of the fatigue wear associated microstructural changes were observed in an around 3mm zone beneath the wear scar as shown in FIG. 3C. [0061] The micro structure changes were analyzed by serial sectioning using EDM and metallographic examination using optical microscope. The optical image of subsurface WEA of the bearing sample after testing is shown in FIG. 4. The WEA formed due to the cementite dissolution in to matrix was comparatively harder than normal bearing steel microstructure. Micro Vickers hardness of the WEA was observed in the range of 850-950Hv, compared to the fresh steel surface having hardness of 760Hv only. The microstructure changes were analyzed by serial sectioning of the multiple ball bearing specimens and metallographic examination using SEM to confirm WEAs formation as shown in FIG. 5. The energy dispersive spectrum (EDS) analysis from different spots from SEM images was obtained as shown in FIG. 6 and the composition analysis of the spots is shown in Table 4. The measurement of chemical composition of WEA with respect to matrix shows that chromium (Cr) concentration in the WEA is 400% higher than tempered martensite. Similarly, manganese (Mn) concentration of the WEA is 500% higher than the surrounding matrix. Since, Cr and Mn are substitutional hardeners; the higher concentration of these elements in the WEA, locally increases hardness of the white etching region. The stress waves generated due to cyclic loading trigger the movement of Cr and Mn from cementite to matrix and initiate the formation of localized hard white etching regions in the subsurface. Finally, it was found that 40 hours duration of dynamic loading is capable of generating inceptions of WEA. As shown in FIG. 7, a few locations of brittle WEA underwent discontinuous micro- cracking marked in arrows which resulted in formation of long continuous cracks as cyclic loading was continued.

TABLE 4: EDS analysis of WEA, matrix and cementite

Elements Fe Cr Mn Si P S

wt.% wt.% wt.% wt.% wt.% wt.%

WEA 95.51 1.07 3.05 0.29 0.03 0.03 Matrix 96.43 0.68 2.48 0.22 0.08 0.05

Cementite 89.82 5.98 2.40 0.13 0.8 0.44

Claims

We claim:

1. A dynamic load sliding contact tribometer (100) configured to simulate accelerated wear of mating steel components, comprising:

a motor-powered rotatable disc sample (110) configured to operate at a predetermined speed of rotation;

a pin holder (130) configured to hold a pin sample (120) in sliding contact with the disc sample; and

a cyclic load generator (136) configured to transfer a cyclic load to the pin sample via a lever arm and generate a contact pressure, the lever arm (138) having a first end and a second end, and pivoted at a fixed hinge (132) located therebetween, the first end connected to the pin holder and the second end connected to the cyclic load generator, wherein the wear of the disc or the pin sample is adjustable by varying a maximum load generated at the cyclic load generator.

2. The tribometer of claim 1, wherein the pin holder is further configured to at least partially rotate the pin sample about an axis (147) normal to an axis of rotation of the rotating disc sample and an axis of the pin sample.

3. The tribometer of claim 1, wherein the load cycling is variable in the range of 1- 30 Hz.

4. The tribometer of claim 1, wherein the cyclic load generator is configured to generate a maximum contact pressure in the range 1-3 GPa.

5. The tribometer of claim 1, further comprising a linear variable differential transducer (LVDT) configured to measure combined wear depth of the pin with reference to the disc sample.

6. The tribometer of claim 1, wherein the pin holder is configured to control the temperature of the pin sample.

7. The tribometer of claim 1, wherein the cyclic load generator comprises a hydraulic cylinder, a pneumatic cylinder, a cam follower, an electromagnetic lifting mechanism, or a combination thereof.

8. A method of simulating accelerated wear on a steel sample due to white etching area formation using a pin-on-disc tribometer, comprising:

providing a rotatable disc sample and a pin sample for use in the accelerated wear simulation;

feeding a hydrogenating lubricant composition between the disc sample and the pin sample;

placing the pin sample in sliding contact with the disc sample and rotating the disc sample;

transferring a cyclic load from a load generator to the pin in contact with the disc, thereby simulating accelerated wear of the samples; and

monitoring the wear of the pin sample against the disc sample by measuring the combined wear depth of the pin and disc samples.

9. The method of claim 8, wherein the lubricant composition comprises a mixture of NH₄SCN, water and polyethylene glycol (PEG) at a ratio of 1:2:2 by weight.

10. The method of claim 8, wherein the pin and the disc samples are of bearing steel and accelerated wear is indicated by cementite phase dissolution and formation of white etching area (WEA) in the subsurface of the pin sample or the disc sample.

11. The method of claim 10, wherein the WEA is obtained in a test duration of 14 h or less.

12. The method of claim 10, wherein the accelerated wear is obtained in 2.5xl0⁵ contact cycles or less.

13. The method of claim 10, wherein the accelerated wear is obtained in 10⁶ contact cycles or less.

14. The method of claim 10, wherein the sample is selected from a gear, roller bearing element, or a ball bearing element.