INFLUENCE OF NORMAL FORCE AND HUMIDITY ON FRICTION AND WEAR OF UNLUBRICATED STEEL/ STEEL COUPLES D. KLAFFKE Federal Institute for Materials Research and Testing (BAM), Lab. VIII.2, Unter den Eichen 87, 22 GERMANY; e-mail: dieter.klaffke@bam.de SUMMARY The friction and wear behaviour of different steel/steel couples was investigated in laboratory tests with unlubricated reciprocating sliding motion. Two different steel balls were tested against two different steel disks in dry, normal and moist air at room temperature. The influence of normal force on friction and wear was studied in the range from N to N on all three levels of relative humidity (R.H.). A strong influence of R.H. on wear behaviour was found for all 4 couples, while the friction behaviour was less effected by R.H. For all couples an influence of normal force on wear rate was found with the tendency of increasing wear rate for decreasing normal force. The friction shows also the trend of increasing coefficient of friction for decreasing load, but to a much smaller extend than evaluated for the wear rate. Keywords: reciprocating sliding, stainless steel, role reversal, normal force, relative humidity INTRODUCTION Basic tribological investigations are often done with a ball-on-disk configuration, mainly with balls of bearing steel (Cr6) used as counter body. A normal force of N is typical is such tests. In many cases tests run in air of ambient temperature with not controlled humidity. Load and humidity, however, can affect friction and wear results considerably. Especially, when regarding the actual trends towards micro- and nano- problems, the affect of reduced load on wear and frictional behaviour gets some importance. 2 OBJECTIVES The goal of this study is the evaluation of effects of counter body material and relative humidity on the friction and wear results of unlubricated reciprocating sliding tests with different steel/steel couples. Furthermore, the effect of reduced load on friction and wear results obtained from laboratory testing shall be investigated. 3 EXPERIMENTAL 3. Materials The materials investigated in this study are commercially available balls of bearing steel and stainless steel, the disk specimens were rings with 42 mm diameter and 3 mm thickness, made of bearing steel and disks with 42 mm diameter and 5 mm thickness, all with polished surfaces, see Table. Sample Material Type of steel Ball Cr6 Bearing XCrNiMoNb8 Stainless Disk Cr6 Bearing X5CrNi89 Stainless 3.2 Tribo testing 3.2. Tribometer Table : Materials of investigation A tribometer was used for this investigation that is described in more detail in []. A ball with mm diameter is pressed (by a dead weight) against a plane specimen that is fixed on a horizontal table which is actuated to rotational vibrations by a frequency controlled DC motor with an excenter on its shaft. The ball-on-flat contact is loaded with a normal force F n by a dead weight. The tribometer is enclosed by an acrylic glass chamber. Inside this chamber the relative humidity can be set to 5 % R.H., 45.. 55 % R.H. or % RH., respectively. The test parameters stroke ( x), frequency (ν) and the normal force (F n ) can be varied by one order of magnitude, but are kept constant in each test. 3.2.2 Test conditions All tests were performed with a ball-on-flat arrangement with unlubricated reciprocating sliding motion at room temperature according to the conditions compiled in Table 2 Upper body Ball Lower body Plane disk Stroke x,2 mm Frequency ν 2 Hz Normal force F n / 2.5 / 5 / N Number of cycles n Lubricant ### without Temperature T 25 C Rel. Humidity R.H. 5 % / 5 % / % Total sliding distance s 2 m Mean sliding speed v.8 m/s Test duration t.39 h Table 2: Test conditions During each test the coefficient of friction and the total linear wear are measured and stored by a PC system. After each test the volumetric wear of both specimens in contact is determined on the base of wear scar dimensions and additional profiles of the wear scars on the disk, see Chapter 3.3.2.
3.3 Tribological quantities 3.3. Friction The friction behaviour is described by the coefficient of friction (COF), f, which is defined as the friction force F r divided by the normal force F n. For comparison of the results of different tests the average value of the second half of each test is used in order to eliminate running-in effects. A typical plot of coefficient of friction versus number of cycles is shown in Figure 3. 3.3.2 Wear In ball-on-flat tests the linear wear can be easily measured. The linear wear shows usually a square root like curvature, see Figure 3, that is typical for a ball-on-flat geometry. If the loss of material is proportional to the sliding distance (constant wear rate), the linear wear shows a square root like curvature. Therefore, the linear wear should not be used as a direct base for the determination of wear rates. However, this signal can be used for the calculation of evolution of volumetric wear and allows the detection of changes in wear rate that occur in some tests. The comparative description of wear behaviour is based on the volumetric amount of wear. Two typical profiles of wear scars on steel disks are shown in Figures and 2. Figure shows the wear scar and the profile on a Cr6 disk after a test against a stainless steel ball. Profile depth [µm] 5-5 - -5-2 gz2-8 5 5 Lateral position [µm] Figure 2: Profile of the wear scar on a X5CrNi89 disk after test against Cr6 ball in normal air The volumetric wear at ball and disk wear is calculated on the base of the diameters of the wear scar and the profilometric wear W q at the disk according to equation () and (2) from [2]: W v, Ball = π d 2 2 d 2 (/R') /64 () W v, Disk = π d 2 d 2 2 (/R-/R') / 64+ x * W q (2) Profile depth [µm] 5-5 - -5-2 aw9-5 5 5 Lateral position [µm] Figure : Optical micrograph and profile of a wear scar on a Cr6 disk after a test against XCrNiMoNb8 ball in normal air A deep wear scar with a contour similar to that of the ball indicates that much wear occurs at the disk and only small wear at the ball, Figure. The wear scar on the stainless steel disk after a test against a Cr6 ball, Figure 2, has only a small depth. This indicates that the wear is high at the ball and is small at the disk. with: R' = d 3 / (2 W q ) (3) d diameter of the wear scar, perpendicular to the sliding direction, d 2 : diameter of the wear scar, parallel to the sliding direction, R: Radius of the ball, R': Radius of the curved wear scar at the ball after the test, W q : planimetric wear, determined at the disk. For the comparison of wear results derived from tests with different conditions the coefficient of wear, k, is used, describing the loss of material (volumetric wear W v ) per unit of sliding distance s and load F n, Eqn. (4): k = W v /(s F n ) = W v /(2 x n F n ) (4) 3.3.3 Electrical contact resistance The tribometer is equipped with an electrical circuit for a qualitative determination of the electrical contact resistance R. The electrical loading of the contact is below. Watt. The contact resistance provides some information about wear mechanisms: If the contact is
metallic, the resistance is close to zero, if tribooxidation leads to a layer of insulating particles the resistance is close to infinity. 3.3.4 Data acquisition The following quantities are measured and recorded by means of stain gauges on thin metal strips, amplifier and chart recorder: Linear wear W l Friction force F f The electrical contact resistance R (measured by a bridge circuit). The evolution of these three signals is shown exemplary in Figure 3. Volum. wear [ -3 mm 3 ]. Cr6 Cr6 XCrNiMo... Cr6 5 48 Relative humidity R.H. [%] Figure 4: Volumetric wear at ball and disk vs. R.H. for tests with different ball materials against Cr6 disk Linear wear, Wl [ µm ] 5 4 3 2 R f W l,8,6,4,2 Coefficient of friction, f The volumetric wear at ball and disk in tests against stainless steel disk is shown in Figure 5. Volum. wear [ -3 mm 3 ] Cr6 X5CrNi89 XCrNiMo... X5CrNi89,5, Time, t [ h ] Figure 3: Evolution of friction coefficient f, linear wear W l and electrical resistance R in a test with XCrNiMoNb8/X5CrNi89 in normal air 4 RESULTS 4. Influence of humidity 4.. Wear behaviour The volumetric wear at the ball and at the disk is shown in Figure 4 for tests in dry, normal against Cr6 disk. For both balls the wear decreases for increasing R.H. For Cr6/Cr6 the wear at the ball is nearly the same as at the disk in dry air but considerably smaller at the disk in normal and in moist air. The wear at the stainless steel ball is much smaller than that at the Cr6 disk for all values of R.H.. 49 56 Relative humidity R.H. [%] Figure 5: Volumetric wear at ball and disk vs. R.H. for tests with different ball materials against X5CrNi89 disk In the tests with Cr6 ball against stainless steel disk the wear at the ball decreases, while the wear at the disk increases with increasing R.H. The wear at the ball is, however, much greater than at the disk. In the tests with stainless steel/stainless steel the wear at the ball and at the disk is of similar magnitude, the wear at the disk is much higher than against the Cr6 ball. The total wear of the system, quantified by the coefficient of wear, k, is shown in Figure 6. The coefficient of wear decreases for all couples with increasing humidity, most significantly in the range from dry to normal air. In dry and normal air the smallest wear is found for the stainless steel couple, while in moist air only marginal differences occur between the different couples of steel.
Total wear coeff. k [ -6 mm 3 /Nm] 5 5 Ball / Disk Cr6 / Cr6 XCrNi./ Cr6 Cr6 / X5CrNi.. XCrNi./ X5CrNi. 2 4 6 8 Relative humidity R.H.[%] Figure 6: Coefficient of total wear vs. R.H. for different steel couples 4..2 Friction behaviour The coefficient of friction, Figure 7, is in the range from.7 to.8 for all couples in dry and in normal air. In moist air the coefficient of friction drops to.5 for all couples with at least one partner made of Cr6. For the stainless steel couple, however, the friction remains high (.8) also in moist air...8.6.4.2. Ball / Disk Cr6 / Cr6 XCrNi./ Cr6 Cr6 / X5CrNi.. XCrNi./ X5CrNi. 2 4 6 8 Relative humidity R.H.[%] Figure 7: Coefficient of friction vs. R.H. for tests with different ball materials against different steel disks 4.2 Influence of normal force 4.2. Wear behaviour In order to compare the results of tests with different normal force the coefficient of wear, k, is used. This quantity is calculated according to Eqn. (3) and describes the wear of the system when the total wear (sum of wear at the ball and the disk) is used. Figure 8 shows the coefficient of wear, k, versus normal force, F n, for tests with Cr6/Cr6. Total wear coeff. k [ -6 mm 3 /Nm] Ball: Cr6 Disk: Cr6 k=57*f n -.5 k= 83*F n -.25 k= 38*F n -.23 R.H. = 5 % R.H = %. Figure 8: Coefficient of total wear vs. normal force for tests with Cr6/Cr6 in dry, normal For all values of R.H. a small decrease of k is found for increasing normal force. The k-values for tests with N are roughly twice that of tests with N load. The tendencies can be approximated fairly well by exponential laws (straight lines with negative slope in a log-log plot). The k-values are the highest in dry air by a factor of 4 higher than in moist air. When a stainless steel ball is used as counter body against Cr6, the wear behaviour results as is shown in Figure 9. Total wear coeff. k [ -6 mm 3 /Nm] Ball: XCrNiMo.. Disk: Cr6 k=27*f n -.4 k = 76*F n -.42 k = 44*F n -.3 R.H. = 5 % R.H = %. Figure 9: Coefficient of total wear vs. normal force for tests with XCrNiMoNb8/Cr6 in dry, normal The tendency of decreasing wear with increasing normal force is also valid for this couple, however, the scatter of experimental results is higher. For all values of normal force the wear rate is highest in dry air and the smallest in moist air (factor 3 to 4). Figure shows the results for tests with Cr6 ball against stainless steel disk. The wear rate is nearly independent of normal force in dry air, but shows a similar decrease as described above in normal and in moist air.
Total wear coeff. k [ -6 mm 3 /Nm] Ball: Cr6 Disk: X5CrNi89 k=47*f n -.3 k = 8*F n -.27 k= 54*F n -.39 R.H. = 5 % R.H = %..8.6.4 Ball: Cr6 Disk: Cr6.2 R.H. = 5 % R.H = %. Normal force Fn [N] Figure : Coefficient of total wear vs. normal force for tests with Cr6/X5CrNi89 in dry, normal and moist air A different wear behaviour is found for the stainless steel couple, Figure. In dry air the strongest effect of normal force is found (exponent -.42), while in normal and in moist air no influence of load is detectable (exponent.2). For this couple the wear rate in dry air is by a factor of higher than in normal for N, but only by a factor of 3 for N load. Total wear coeff. k [ -6 mm 3 /Nm] Ball: XCrNiMo.. Disk: X5CrNi89 k=39*fn -.42 k = 32*F n.2 k= 35*F n.2. R.H. = 5 % R.H. = % Figure : Coefficient of total wear vs. normal force for tests with XCrNiMoNb8/X5CrNi89 in dry, normal 4.2.2 Friction behaviour The coefficient of friction shows a relative small effect of normal force. The COF decreases slightly for increasing load for the Cr6/Cr6 couple, Figure 2. The friction values are about 2 % lower for N load than for N. The values are nearly the same for dry and for normal air, but reduced by about 25 % in moist air. Figure 2: Coefficient of friction vs. normal force for tests with Cr6/Cr6 in dry, normal The friction behaviour of the couple XCrNi../Cr6, Figure 3, is similar to that of Cr6/Cr6, Figure 2. The influence of load is more pronounced for the tests in moist air. In dry and in normal air the coefficients of friction are practically identical. Tests against the stainless steel disk reveal similar tendencies as found in tests against Cr6 disk. Figure 4 shows the COF versus normal force for the couple Cr6/X5CrNi89. The trend of decreasing COF in moist air is based on an excessive high value for (.95) for N load, for higher loads the COF remains nearly constant (.5....6).. The difference of COF for normal and dry air are small, in moist air the smallest friction values are found..8.6.4.2 Ball: XCrNiMo.. Disk: Cr6 R.H. = 5 % R.H = %. Figure 3: Coefficient of friction vs. normal force for tests with XCrNiMoNb8/Cr6 in dry, normal
.8.6.4 Ball: Cr6 Disk: X5CrNi89.2 R.H. = 5 % R.H. = %. Figure 4: Coefficient of friction vs. normal force for tests with Cr6/X5CrNi89 in dry, normal and moist air For the of stainless steel couple a slightly different behaviour is detected, Figure 5..8.6.4.2 Ball: XCrNiMo.. R.H. = 5 % Disk: X5CrNi89 R.H. = %. Normal force Fn [N] Figure 5: Coefficient of friction vs. normal force for tests with XCrNiMoNb8/X5CrNi89 in dry, normal The COF is practically independent of normal force in normal, while in dry air the COF tends to increase for increasing load. For this couple nearly no influence of normal force is found on all levels of R.H.. This couple is the only one that shows the smallest friction values in dry air. 5 DISCUSSION The friction and wear behaviour of steel/steel couples, tested at room temperature in ambient air depends sensitively on relative humidity. The wear rate is for all couples of investigation the highest in dry air, This behaviour can probably attributed to the formation of protective layers of oxidised wear particles, that can form more effective in the presence of higher contents of water vapour. The influence of normal force on wear behaviour shows the tendency of increasing wear rate for reduced load. This finding is probably due to the effective loading of the asperities: If the Hertzian pressure is believed to be responsible for the removal of material one should expect an increase of volumetric wear for increasing load according to a power law (5): /3 W v = c * F n (5) and consequently, since k ~W v /F n k = c' * F -2/3 n (6) Indeed, negative exponents are found for most conditions, however, the exponents are smaller than according to equation (6). This finding indicates that the stresses acting at the asperities are smaller than according to the Hertz equation. Various sources can contribute to the deviation from the "Hertzian behaviour" (plastic deformation of the asperities, size, shape and distribution of asperities, properties of oxidised material in the micro contacts,...). The friction coefficient is only slightly affected by humidity and normal force. In most cases the friction tends to decrease with increasing load and to decrease with increasing humidity. The affect of load and humidity on friction is too small that it is probably not of technical relevance and is also too small as to be attributed to different mechanisms. 6 CONCLUSIOS Friction and wear results in oscillating sliding tests at room temperature are affected seriously by the water vapour content of surrounding air and by the applied normal force. The general trend of increasing wear for decreasing relative humidity is found. Homogenous steel/steel couples show comparable amounts of wear at ball and disk, while heterogeneous couples (stainless steel/bearing steel) always show significantly lower wear at the stainless steel sample. The friction behaviour is affected to a much smaller extend by humidity and load than the wear behaviour is. For the comparison of laboratory tests at room temperature the affects of humidity and load have to be considered in order to obtain repeatable and reproducible results. 7 ACKNOWLEDGEMENTS Thanks are due to Mrs. Ch. Neumann for tribo testing and Mr. J. Schwenzien for profilometric measurements. 8 REFERENCES [] Klaffke, D.: On the Influence of Test Parameters on Friction and Wear of Ceramics in Oscillating Sliding Contacts. Tribotest journal (995) 4, 3-32 [2] Klaffke, D.: Fretting wear of ceramics. Tribology international, 22 (989) 2, S. 89-