In the first observation of the switching phenomenon, ultrananocrystalline diamond (UNCD) films were coated on a 1x1 cm2 Si flat and a 3 mm diameter Si3N4
velocity, a wear track was created in a N2 environment with relative humidity (RH)
starting above 4.0%. The friction coefficient as a function of cycles was recorded, and RH levels were documented by hand every ~25 cycles. After 350 sliding cycles, the RH was lowered by flushing the system with only dry N2 (Fig. 4.1).
Fig. 4.1: Friction data from UNCD track exhibiting first instance of switching behavior.
At cycle 865 the friction coefficient was 0.009. As the RH dropped below 1.61%, the friction coefficient rose to 0.353 by cycle 890. This is over an order of magnitude increase in the friction coefficient for a RH drop of ~0.05%, occurring in just ~25 sliding cycles. The friction coefficient was high and erratic, reaching a maximum of ~0.53, and the RH eventually bottoms out at ~1.43%. Humidified N2 was then introduced back into
the chamber. At cycle 1546 the friction coefficient was at 0.266, and the RH had increased to 1.87%. By cycle 1619, the RH had increased to 2.02% and the friction coefficient had dropped back down to 0.008. This cycle is reproduced four times in the data set, where a period of high friction with low RH is followed by the low friction value as the RH is increased again. There is some hysteresis, as the transition from low to high friction occurs at ~1.6% RH, and the transition from high to low friction is at ~1.9% RH. The surface switches sharply between high and low friction with small changes in RH, and is henceforth referred to as UNCD 'switching' behavior. In each switching instance the friction becomes high and erratic at low humidity, and then at higher humidity recovers to almost exactly the same initial value. The friction during the higher RH sections is also quite stable.
Further sets of similar experiments using UNCD films (grown in different runs) saw the same behavior. Fig. 4.2 shows another self-mated UNCD friction plot. In addition to varying the RH, the load is also changed at specific intervals during the test. The track started out at 500 mN, again with a combination of dry N2 and humidified N2
providing RH control, 1 mm/s sliding velocity, and a 500 µm track length. RH was still be recorded by hand, and in this case only 20 measurements were recorded over the 12000 cycles. Fig. 4.1 gives a good indication about the pattern of RH changes. As sliding begins (Fig. 4.2) and the RH is lowered, the friction does not sharply increase, but instead slowly rises from its minimum value of ~0.0035 up to ~0.02 before spiking at ~0.0375. The RH is increased again until lower friction is achieved, but the system does not recover to the lower value and instead is at ~0.01. It is possible that the system
reached some sort of new steady-state behavior around cycle 5000 that is different than at cycle 1000. At cycle ~6000 the load was increased from 500 mN to 750 mN. Again the RH was lowered, and again the friction slowly trended upward (from ~0.01 to ~0.023) before switching to the higher friction state. Interestingly, the high friction state for this system is only ~0.04 (and spiky) whereas it was an ~order of magnitude higher (0.2-0.5) for the data in Fig. 4.1. This fact, combined with the lack of sharp switching behavior, implies there is something different about this second system. Finally, after the system recovers (more sharply) after cycle 8000, the load was increased from 750 mN to 1.0 N at cycle ~9200. A last, sharper switching transition is observed, showing that the ability to switch is not fixed at a certain load.
Results from this second test suggested sharp transition behavior is more likely at high loads. With this in mind, a final test was performed using a 1.0 N load, and
otherwise identical parameters, to determine if the switching behavior further evolved with number of switching occurrences. Fig. 4.3 shows a test where the system switches a total of 10 times between the low and high friction state. The friction behavior to this third preliminary test is more similar to the first test (Fig. 4.1) where the change between the low and high friction state covers almost two orders of magnitude, and the switching is sharp. The behavior does evolve with number of switches, which could be due to the accumulated wear. As more time is spent in the high friction state, the system is not able to return to the previously low friction value (cycle 3500). As the RH decreases, the friction slowly increases until the debris is cleared from the contact and a transition occurs (cycles 3500 to 5500).
Fig. 4.3: UNCD switching friction plot showing a total of 10 switches. Friction behavior evolves starting at cycle 3500 (arrow) and is not able to return to the low
friction value until cycle 5500.
Fig. 4.4: UNCD friction data as a function of sliding velocity with a 1.0 N load, RH between 0.7-0.9%.
Fig. 4.4a is a plot of friction coefficient as a function of cycle for changing sliding velocities. The load was kept constant at 1.0 N and the RH was kept between 0.7% and 0.9%. Here the friction is low (~0.005) even with only 0.9% RH. The difference between this test and previous tests (Chapter 3, and Fig. 4.1, Fig. 4.2, and Fig. 4.3) is that the sliding velocity is only 60 μm/s, compared to the usual 1000 μm/s. The increase in friction with increased sliding velocity further supports the passivation mechanism by showing that the reduced exposure time prevents sufficient passivation of the surface,
which increases dangling bond interaction across the interface. This also shows the transition behavior can be triggered by changing the exposure time instead of the amount of water.
Next we describe an experiment designed to elucidate the mechanism behind the switching behavior for UNCD. The same sequence of environmentally controlled
tribometry followed by PEEM measurements determine friction and chemical changes as UNCD interfaces run in, experience a switching transition, and recover. Recent density functional theory (DFT) work provides a possible explanation as to the driving
mechanism.