Acoustic emission (AE) was recorded during tribological tests on 52100 steel specimens under different loads. AE signals were transformed to the frequency domain using a Fast Fourier Transform and parameters such as power, RMS amplitude, mean frequency, and energy were analyzed and compared with friction coefficient and wear volume measurements. Results show that certain acoustic frequencies reflect friction while others reflect wear. If frequencies are chosen optimally, AE and friction signals are highly correlated (Pearson coefficients >0.8). SEM and Raman analysis reveal how plastic deformation and oxide formation affect friction, wear and AE simultaneously. AE recordings contains more information than conventional friction and wear volume measurements and are more sensitive to changes in wear mechanism. This all demonstrates AE's potential as a tool to monitor tribological behavior.waveform can correlate with the quantity of wear particles, as does the AE energy [14]. Moreover, the amplitude and length of a train of AE waves can be associated with individual slip events during the stick-slip [15]. In addition to this, the AE event location can determin mode I crack propagation in fretting fatigue [16]. The instantaneous RMS amplitude has been found to correlate with CoF [17][18][19], distinguish between different stages in CoF evolution [20] and even predict CoF based on a power law relation [18]. AE RMS can also be sensitive to applied load, velocity and mechanical properties of sliding components [20]. The integrated AE RMS can be related to frictional work and wear under different sliding speeds [18] and loads [21] and can detect the sliding speed threshold for accelerated wear [18]. The integrated AE RMS [22,23] may also have a direct correlation with the wear volume and wear rate [24].In the frequency domain, AE parameters such as energy and median frequency (MDF) can be used, with the former being correlated to CoF [25,26] and wear volume [27] and the latter being correlated to CoF [26,28] and frictional work [29]. The amplitude of individual AE frequency components can also be excited by specific wear mechanisms, which can be determined based on power spectral density (PSD) and auto-covariance techniques [30]. In addition to this, it has been found that low and high frequencies are associated with different lubricant conditions [28]. Peak frequencies can vary depending on wear mechanism, with adhesive wear emission suggested to occur around 1.1 MHz and abrasive wear between 0.25-1 MHz [31,32], while stick slip results in dominant AE frequencies around 10 kHz [20].Time-frequency analysis can also be applied to AE signals, with evolving PSD spectra indicating different stages wear [19] and wavelet analysis detecting the onset of scuffing [33,34] and other wear states [35] [36].Reviewing this literature it is apparent that the instantaneous RMS value of the acquired AE signal has been the most widely used parameter to correlate with friction and wear [17-19, 22, 23, 37, 38]. However, recently questions h...