Wear, 162-164 (1993) 597-604
Short Communication Sliding wear studies using acoustic emission S. Lingard,
C. W. Yu and C. F. Yau
The University of Hong Kong, Deportment of Mechanical Engineering, Pokfulam Road, Hong Kong (Hong Kong)
associated with metal cutting and tool wear, but an increasing interest in basic tribological studies has emerged in recent years [2-4]. The present paper reports experimental investigations aimed at gathering information on acoustic emissions in relation to fundamental friction and wear parameters and mechanisms.
2. Experimental methods Abstract Deformation processes in solids, such as dislocation movements under plastic flow, crack propagation and void crushing, produce stress waves at ultrasonic frequencies, usually described as acoustic emission (AE), which can be detected by sensitive instruments and which are related to the severity and nature of the deformations. The paper discusses the characteristics of the stress waves and their variation with wear rates, wear regimes and friction forces, as determined during laboratory experiments on metallic specimens in relative sliding motion, both unlubricated and with elastohydrodynamic lubrication. It is shown that there are systematic relationships between the acoustic emissions, the wear rates, the frictional work inputs and established tribological contact variables. The predominant frequencies of the emissions are also evaluated and considered in relation to the materials and wear conditions.
1. Introduction Acoustic emission (AE) in the sense used here refers to elastic waves produced by microscopic deformations occurring in materials as they are stressed and which comprise part of the elastic energy released during deformation. Sometimes called stress wave emissions, acoustic emissions are associated with dislocation movements, with crack growth, with deformation of inclusions and with other mechanisms. Detectable emission may be observed in a wide variety of materials, in different modes of deformation, and can be detected at solid surfaces with appropriate instrumentation [l]. Emission frequencies of more than 10 MHz have been recorded, but perhaps the most useful range is 50 kHz to 2 MHz. Our concern, therefore, is with the ultrasonic regime of wave transmission as distinct from low frequency sound or noise. The amplitudes are usually small, so appropriate instruments may be required to respond to surface displacements as low as lo-l4 m. For friction and wear processes the main thrust of AE research has been
The experiments were conducted on metal specimens in pure sliding motion using a two-disc machine. A piezoelectric transducer attached to the stationary disc was used to detect the AE signals which were amplified and processed using a preamplifier,, a conditioning amplifier (B&K 2638), a pulse analyser (B&K 4429), a 1 MHz A/D converter (Keithley Metrabyte DAS-50) and a microcomputer (Fig. 1). Most of the results reported are from experiments which involved a 40 mm diameter stationary specimen of 2011 series aluminium alloy (composition copper 5.5, lead, bismuth, balance aluminium), 70/30 brass or low carbon mild steel and a 40 mm diameter rotating specimen of low carbon mild steel or of hardened medium carbon nickel-chrome steel to specification En 24 (composition carbon 0.35-0.45, silicon 0.1-0.35, manganese 0.45-0.7, nickel 1.3-1.8, chromium 0.9-1.4, molybdenum 0.2-0.35). In all cases the discs were 10 mm wide and were machined from bar stock to an initial nominal surface roughness of 0.25 pm R,. The diamond pyramid hardnesses of the aluminium, brass, steel and hardened medium carbon steel were 150 kgf mm-*, 140 kgf mm-*, 180 kgf mm-* and 540 kgf mm-* respectively. Specimen preparation was completed by 5 min degreasing in acetone in an ultrasonic cleaning tank followed by air drying. Consistent mounting and coupling arrangements for the AE transducers had load
Fig. 1. Experimental arrangement.
S. Lingard et al. I Acoustic
already been established during earlier work aimed at ascertaining the conditions for optimum signal output . They included specified clamping torques for the transducer mounting plate and for the transducer itself, and the application of a petroleum grease couplant at the specimen-mounting-plate and mounting-platetransducer interfaces. Typically, a test was run for a period, depending on the wear rate, from about 20 min to 60 min, during which the following data were recorded at regular intervals, usually of 100 s: sliding distance, friction force, total frictional work, wear volume, AF count rate, and cumulative AE count. Samples of the amplified raw AE output signal were also collected and digitized for subsequent analysis.
3. Nature of the acoustic emission Two main types of acoustic emission are usually recognized. For a discrete short-term event such as an increment of crack propagation in a brittle material,’ an emission burst lasting typically a fraction of a millisecond can be identified. For deformation proceeding steadily at a lower level, as in large-scale yielding of a ductile material, multiple individual emission waves of low amplitude are thought to merge and overlap producing continuous emission which appears on initial observation similar to electronic noise. A continuous signal can be defined as one in which the average time between emissions of similar amplitude is less than the duration of the emission. Since the source signal amplitudes are so small it is necessary to use high gain, high signal-to-noise ratio amplifiers. The choice of transducer tends to hinge on whether waveform information is required, in which case a wide band sensor is usually chosen, or whether maximum sensitivity is needed, when a narrow band resonance sensor can improve the signal-noise-ratio. In the current work a wide band transducer (100 kHz-l MHz) and two resonance transducers (200 kI-Iz and 800 kHz) were used. Quantitative data are most frequently expressed in terms of the root mean square (rms) value of the signal (or amplified signal} or in terms of measurements known as ringdown counts which record the number of times the amplified acoustic emission signal exceeds a preset trigger voltage level (or a weighted count based on a number of pre-set trigger voltage levels). The former is held to be more appropriate for continuous emissions and the latter more suited to burst emissions. In each case the measurement is closely related to the energy of AE activity. In steady wear tests the emission is of a pseudocontinuous type with continuous output plus bursts due to fast high energy events, as would be expected on the basis of the idea of rapidly occurring multiple
emission of s&&g
asperity contacts of varied severities. Frequency distributions are found to be broad band (Fig. 2), here shown as weighted count rates in different frequency ranges detected by a wide band transducer using a number of band pass filters. Substantial emission can be observed at frequencies from below 10 kHZ, where mechanical system resonances are probably predominant, up to more than 1 MHz for which the sources can be of microst~ctural origin. It is necessary to recognize that possible explanations of the frequency distribution must include attenuation and modulation in specimens, mountings and transducers as well as the nature and mode of deformation at the source. However, the detection system was periodically investigated for reproducibili~ of sensor response by pencil lead break tests conducted in accordance with ASTM E976 and PAC-RTM-2-8/91 (Physical Acoustics Corporation).
4. Wear and friction characte~sties For the experimental configuration and material pairs used it has been found that, whilst the processes of wear and friction are inherently unsteady in the short term (of the order of 10e3 s), the wear rate and coefficient of friction averaged over a few seconds remain essentially constant for the duration of a test. On the basis of microscopic examination of the surfaces and debris, it is thought this is because potential large-scale material transfer associated with the adhesive wear mechanism is inhibited by oxidative effects [S]. Typical results for wear volume as a function of time are shown in Fig. 3, for AE weighted count rate against frictional work in Fig. 4, and for AE weighted count rate against wear volume in Fig. 5. Power-law expressions have been advanced in previous investigations 12, 31 to describe the rms voltage or count rate dependence on frictional work and wear rates. It can be reasonably
Fig. 2. 100 N, 0,405 ms-‘, aluminium transducer, ~plifi~tion 10 dB.
on steel, wide band
S. Lingard et al. /Acoustic
Fig. 3. 25 N, 0.405 ma-‘, aluminium tests.
on steel, six independent
emission of sliding wear
(Fig. 6). The AE output therefore appears to be more sensitive to the contact conditions than either the wear rate or the friction force. A running-in period, of l-10 min depending on operating conditions, was always observed. That the acoustic emission technique is able to detect changes in wear regimes has also been shown by the investigations of Jiaa and Dornfeld  and of Boness et al.  for quite different experimental situations from those discussed here. There are clear indications that the Al3 is intimately related to the fundamental material and surface actions in the wear zone so that explanations of some of the physical phenomena therein can be reasonably expected to emerge from related work in due course. When sliding is interrupted the AE activity is resumed, on restarting, at its earlier quasi-steady level rather than at the run-in levels observed with new surfaces (Fig. 7), which is similar in some ways to the Kaiser effect [l] observed in materials under bulk deformations, where a relaxation of stress causes the emission to cease, and not to reappear until the stress exceeds its previous maximum value. Such behaviour has also been
50 kHz-2 MHz
200 kHz-2 MHz
100 kHz-2 MHz
400 kHz-2 MHz
Fig. 4.50 N, 0.405 ms-‘, aluminium on steel, wide band transducer, amplification 10 dB, filter pass 100 kHz-2MHz, four independent tests.
1000, 3 E 2
7 o 5
2 Z ‘Z S i
; n _0
Fig. 6.25 N, 0.405 ms-‘, aluminium on steel. wide band transducer, amplification 10 dB.
Wear vol. (mm”3) 120
Fig. 5.25 N, 0.405 ms-‘, aluminium on steel, wide band transducer, amplification 10 dB, filter pass 50 kHz-2MHz.
concluded that systematic relationships between AE parameters and sliding contact variables have been established, at least approximately, for typical wear tests. Taken as a function of time, the AE count rate shows a pattern of steady increase during the first few minutes running followed by a levelling off to a steady value for the remainder of the test. Similar behaviour applies for a number of high-pass filtered frequency bands
2 :, =.
Fig. 7.25 N, 0.405 ms-‘, aluminium on steel, wide band transducer.
et al. I Acoustic
reported by Boness ef al.  for a steel-on-steel system.
5. Fhquency domain The amplified AE signals were sampled at a frequency of 1 MHz during each of the tests. Each sample comprised 32 768 data points which were buffer stored and subsequently analysed using a fast Fourier transform algorithm. Frequency spectra for a number of different experiments are shown in Figs. 8-14. The frequency characteristics of the wear-generated acoustic emissions look fairly similar, in a broad sense, for all the test conditions investigated. They show large numbers of frequency peaks which reflect the complex nature of the signals. The dominant frequencies seem to occur in emission bands clustered around 100 kHz, 200 kHz and 300 kHz (Figs. 8 and 9 show the results of the same test plotted using linear and logarithmic voltage scales respectively). Whilst AE is produced at frequencies both lower and higher than the range 50 kI-Iz to 400 kHz, which can be clearly seen in Fig. 9, this appears to be the region in which the most significant emissions usually occur, providing it is accepted that the lower frequencies, less than 50 kHz say, be disregarded as their sources may be relatively large-scale mechanical vibrations. It should be mentioned in this context that care is needed in interpretation of the amplitudes associated with the peak frequencies because they are naturally dependent on the sensor and in-
Pig. 8. 75 N. 0.809 ms-‘, ahninium
emission of sliding wear
strument response characteristics. In the present case a 200 kHz resonant transducer was used (with a wide band pass filter) for many of the experiments to obtain greater sensitivity in the appropriate range. Frequencies around 200 kHz are thereby accentuated in Figs. 8-13 and are not necessarily more significant than those around 100 kHz and 300 kHz. This was confirmed in similar tests using the wide band sensor (Fig. 14). Looking at the frequency spectra in more detail reveals that in most cases there are prominent peaks at many frequencies, particularly 55, 85, 110, 147, 170, 180, 188, 206, 220, 225 and 290 kHz, all of which have been identified consistently by approximate measurements of the spectral plots. Clear harmonic relationships are evident between a number of the frequencies mentioned (but not all) and that, coupled with the fact that the general features of the spectra appear quite similar, tends to suggest that the results may arise from low amplitude harmonics of natural mechanical system vibrations unrelated to the fundamental actions in the wear contact zone. It remains possible that this is so. However, other investigators  have measured dominant peaks in the regions of 100 MIZ, 200 kHz and 300 kHz in the acoustic emission emanating from steady state metal cutting with a single point tool. Du et al. , who used a carbide cutter insert and an AISI 1040 ysteel workpiece, attributed their 100 kH.z peaks to material shearing and the 300 kHz peaks to flank and rake face friction. In addition, it has been found in the disc machine test discussed here, where, incidentally,
on steel, 200 kHz resonant transducer, amplification 15 dB, filter pass 0.1 Hz-2 MHz.
S. Lingard et al. t Acoustic emission
Fig. 9. 75 N, 0.809 ms-‘,
on steel, 200 kHz resonant
of sliding wear
15 dB, filter pass 0.1 Hz-2 MHz.
0.0021 0.002 0.0019 0.0018 o.oon 0.0016 0.0015 g
ii OXI: 0.000* 3
0.0007 0.0006 0.0009 0.0004 0.0003 0.0002 0.0001 0 0
Fig. 10. 25 N, 0.405 ms-l,
steel on hardened
steel, 200 kHz resonant
the highest fundamental natural frequencies of the test assembly are in the region of 5 kHz, that changes to wear specimen dimensions and sliding speed make little difference to the frequencies of the principal spectrum peaks (Figs. 10 and 11). It is argued, on that basis, that the overall spectral features noted are characteristic of the sliding friction and wear phenomena.
15 dB, filter pass 0.1 Hz-2 MHz.
The effects of the different wearing materials on the spectra are quite subtle, and, if significant, they are apparently overwhelmed by the strong frequency pattern associated with the predominant wear regime, the presence of steel as a common feature of all the tests and/ or the test arrangement. Clearly, a much more exhaustive analysis of the frequency patterns is needed and a
et al. I Acoustic
emission of sliding wear
Fig. 11. 75 N, 0.405 ms-‘, brass (44 mm diameter) 15 dB, filter pass 0.1 Hz-2 MHz.
steel (50 mm diameter).
200 kHz resonant
Fig. 12. 75 N, 0.809 ms-‘, brass (40 mm diameter) 15 dB, filter pass 0.1 Hz-2 MHz.
steel (40 mm diameter),
200 kHz resonant
system for doing this is under development. However, strong indications exist that the differences in material behaviour and structure may be evident in the frequencies observed. Experiments involving a stationary aluminium alloy slider gave a high peak at 140 kHz,
which did not appear with other material combinations. Similarly, the frequency of 195 kHz seemed to be exclusively associated with tests using soft/hard steels, and the peaks at 131 kHz and 202 kHz with the use of a brass slider.
S. Lingard et al. / Acoustic emission of sliding wear 0.00013
I 1 1
0.00009 :, ?
Fig. 13. 50 N, 0.809 ms-‘, aluminium on steel, 200 kHz resonant with light petroleum oil of viscosity 0.022 Ns m-’ at 25 “C.
15 dB, filter pass 0.1 Hz-2 MHZ, lubricated
I ; d
Fig. 14. 75 N, 0.809 ms-‘,
on steel, wide band transducer,
6. Lubricated sliding A number of experiments was conducted using the same test arrangements but with the introduction of a light mineral oil fed under gravity to the confluence
10 dB, filter pass 50 kHz-2 MHz.
of the discs at contact entry, giving initial elastohydrodynamic film thicknesses of 0.1-0.5 pm. Results are shown in Figs. 13 and 15. The principal spectrum frequencies were generally the same as in the unlubricated experiments although the signal output levels
S. Lingard et al. / Acoustic emission of sliding wear
log count rate coefft. of friction
Fig. 15. Aluminium on steel, lubricated of viscosity 0.022 Ns m-* at 25 ‘C.
with light petroleum
were considerably lower, reflecting the much reduced wear rates. The acoustic emission also proves to be a very sensitive indicator of the lubrication conditions when the weighted count rate is shown in relation to the film parameter A where A = h,,l(U12 +
with hmin the theoretical minimum film thickness, and u the initial rms surface roughness. Figure 15 indicates the variation of coefficient of friction with A (in the form of a Stribeck-Hersey curve), and shows how the weighted count rate can provide a measure of the severity of surface interaction in a manner like the lubrication factor-A correlation employed in rolling bearing theory. 7. Conclusion Exploratory work on the acoustic emission generated by the sliding contact of metals has confirmed that the
signals are intimately related to the deformations in the contact zones, and that their levels are functions of overall contact variables, particularly friction forces and wear rates. The energies of the emissions are probably a measure of the volume of material under deformation and the strain rates. The frequencies range widely and intensities vary in a complicated manner with changes in test conditions and with time but, for the wear conditions investigated, were consistently strongest in a number of peaks grouped around approximate frequencies of 100 kHz, 200 kHz and 300 kHz, respectively. Certain peak frequencies were associated with different rubbing materials. Further information on the frequency content of the AE is needed for a variety of wear regimes to enable a theoretical background to be established.
References J. Kaiser, Ph.D. Thesis, Technische Hochschule Munich, 1950. C. L. Jiaa and D. A. Dornfeld, Experimental studies of sliding friction and wear via acoustic emission signal analysis, Wear, 139 (1990) 403424. S. Lingard and K. K. Ng, An investigation of acoustic emission in sliding friction and wear of metals, Wear, 130 (1989) 367-379. R. J. Boness, S. L. McBride and M. Sobczyk, Wear studies using acoustic emission techniques, Tribal. Znt., 23 (5) (1990) 291-295. S. Lingard, K. H. Fu and K. H. Cheung, Some observations on the wear of aluminium rubbing on steel, Wear, 96 (1984) 75-85. R. Du, D. Yan and M. A. Elbestawi, Time-frequency distribution of acoustic emission signals for tool wear detection in turning, 4th World Meet. on Acoustic Emission and 1st Znt. Conf on Acoustic Em&ion in Manufacturing The American Society for Nondestructive Testing, Columbus, OH, 1991, pp. 269-285.