257 GATR test conditions   The GATR experiments  are started at –20°C in >99% N 2 and run for a duration of 1h at 8050 rpm at a  nominal braking torque of 0.8 Nm. Every 5 minutes, the motor is stopped for 10 seconds before ramping  the speed back up to 8050 rpm at 1000 rpm/s. This test is repeat ed twice with the second repetition at  1.0 Nm braking torque, giving a total of 1.5 M input pinion revolutions (~30 000 at output). In total, five  actuator units are evaluated: 2 units are lubricated with MAC, 2 with P -SiSO, and 1 with PFPE. Units number  2 are used for repetition of the first test with MAC and P -SiSO.  

Post-test damage analysis   After subjecting the gearbox to 1.5 million input cycles , post test damage analysis is performed in three  scales; (1) surface macro scale by optical inspection  and digital microscop y, (2) surface micro scale by 3 D  surface profilometry  and scanning electron microscopy with electron dispersive x -ray spectroscopy (SEMEDS), and finally (3) sub- surface microscale analysis by x -ray micro tomography (XMT) . After initial  inspection of MAC and P -SiSO lubricated gears, two gear teeth are cut out of a  Stage 3 ( S3) planet gear  using electric spark erosion to be further analyzed. The thin tribofilms are analyzed by SEM in low voltage  high contrast detector mode (vCD) at 3 kV, usi ng a Magellan 400 FEG -SEM (FEI Company, Eindhoven,  The Netherlands) . EDS was performed using an X -Max 80 mm2 X-ray detector (Oxford Instruments,  Abingdon, UK)  operated at 3- 5 kV,  which is  just enough to detect the elements C, O, Fe, and Si. Finally t he  gear teeth are scanned with XMT using a Zeiss Xradia 510 Versa (Carl Zeiss X -ray Microscopy, Pleasanton,  CA, USA) , with a resolution of 4 µm per voxel  (volume pixel) . Tiff stacking and a Canny method edgedetection algorithm was employed to quantify sub surface damage from XMT data.  

Results and Discussion  

Results I –  Efficiency and Temperature   The actuator efficiency and temperature over a 1 hour test cycle is shown in Figure 1 . Between the three  lubricants, a clear trend in both efficiency and temperatures c an be distinguished with efficiency ranking of  P-SiSO>MAC>PFPE. As expected, efficiency and temperature are inversely correlated, with high efficiency  corresponding to low temperature increase and vice versa. This corresponds to previous model scale  tribot ests where variants of P -SiSO has been shown to reduce friction and wear compared to neat PFPEs  as well as formulated lubricants [5], [6].  

Figure 2. (a) Measured efficiency of actuator setup when lubricated with MAC, P -SiSO, and PFPE  respectively. lubricants. (b) Gearbox housing temperature (T 1) and chamber interior temperature (T 2) over  1 h test (500 000 input revolutions). As expected, efficiency is inversely related to increasing gear house  temperature (T 2). 

Results II –  Macroscale surface inspection   The gearboxes wer e disassembled and inspected after 3h of test . PFPE showed evidence of heavy wear,  with large amount of wear particles. Therefore we focused on MAC and P -SiSO. Inspection of MAC Stage  3, Figure 3(a) , revealed dark particles and discolouring of the separator disc (B) . EDS analysis confirmed  a layer rich in carbon, indicating indicating decomposition of the hydrocarbon lubricant . Microscopy image  of the MAC driven gear show a clear wear pattern, with particle build up towards the root. Inspection of P - SiSO Stage 3,  Figure 3(b), did not show any obvious sign of degradation., but microscopy images show a