*3.1. Frictional Behaviour*

The average duration of an experiment until reaching the stop criterion was 14.5 ± 2.6 h, corresponding to roughly 44,170 ± 7400 cycles.

Although the temporal evolution of the measured signals varied between the experiments, a few characteristic features were observed throughout the experiments. Figure 5 shows the time series of coefficient of friction, temperature, and contact pressure of experiment 4 as an example for characteristic features observed during the series of experiments. For the coefficient of friction, the arithmetic means of the absolute values of the 10% and the 90% quantiles are displayed. The 10% quantile gives a characteristic value for the coefficient of friction in negative stroke direction, whereas the 90% quantile was used for the positive direction.

After the run-in, the system was operating in a stable condition at a mean coefficient of friction of around 0.06, with the temperature steadily increasing close to 90 ◦C. After typically 15,000 to 30,000 cycles, a region of pre-critical and critical operation was observed. This manifests itself in a sudden increase in the coefficient of friction and the temperature exceeding 100 ◦C. This may be attributed to locally inferior lubrication and consequently short-time metal-to-metal contact and adhesion. After a few minutes, the system was able to loosen the adhesive contact spot or to tear off a machining chip from the edge of a lubricant macrodepot. After that, the system remained in the pre-critical state, self-healed, and thus slowly returning to steady operation, albeit at a slightly higher coefficient of friction in most cases, typically between 0.07 and 0.10. The eventual steady increase of the coefficient of friction can be attributed to the gradually deteriorating lubricant supply due to capillary forces, which reduce due to the increasing number and depth of abrasive grooves caused by wear particles. The short-time critical states with subsequent stabilisation of the system due to self-recovery could be observed repeatedly in all experiments.

**Figure 5.** Temporal evolution of the coefficient of friction (blue), the sample temperature (red), and the applied contact pressure (green) from one selected experiment. The annotation of the observed features is based on a tribologist's expert opinion.

During the experiment, spikes in the friction curves were observed. In order to investigate the origin of these spikes, additional experiments were carried out and stopped manually when the first spike occurred. Investigation of the bearing revealed that these spikes were most likely caused by wear debris in the form of tiny machining chips detached from the edge of a lubricant macrodepot and subsequently transported further in the contact zone to be either embedded within another lubricant macrodepot or transported out of the contact at the edges of the sliding element; see Figure 6a. Figure 6b indicates the large extent of the clearly visible wear area on the self-lubricating journal bearing after the experiment.

**Figure 6.** (**a**) Macro image showing deposition of wear debris on the surface of a lubricant macrodepot, (**b**) Macro image of the wear area of a journal bearing after the experiment illustrating typically occurring grooves and shifting deposit material onto the bronze base structure and vice versa.

Prior to reaching the set threshold criteria of the system by reaching a given lateral force, four experiments exhibited an extended instable state, which lasted for up to several thousand cycles. However, in the other five experiments, the stop criterion was reached almost instantaneously, with instable operation of less than 10 min before termination of the experiment. Experiment 4, as shown in Figure 5, belongs to the latter category. In experiment 6, no intermediary critical operation was observed. The system remained steady for about 10 h, with a sudden increase of the lateral force in the end, exceeding the stop criterion. Experiment 3 was manually terminated after about 2 h of pre-critical operation, before reaching the stop criterion.
