After the cessation of RT/Duroid 5813, manufacturing tests were performed by CNES and ESA/ESTL in order to find an alternative material. Although PGM-HT was selected as the best candidate, limitations about its tribological capabilities to replace RT/Duroid 5813 were later pointed out. Today, the predictability of the tribological behaviour of those materials is not fully overcome. The motivation to this work is to complement studies of self-lubricating materials by coupling experimental analyses with numerical modelling, in order to predict their tribological behaviour. A Discrete Element Method is chosen to construct the numerical material, because it allows to represent wear and the third body generation at the scale of the ball/retainer contact. An underlying role of the adhesion between components in controlling the tribological properties of the transfer film has been observed.

Predicting the tribological behaviour of dry lubricants remains difficult because it greatly depends on their mechanical and physicochemical environment. While it is difficult to analytically model dry lubrication, Discrete Element Method (DEM)-based modelling has been able to provide valuable insight into the tribological behaviour of dry lubricated contacts. The present study aims to experimentally define interactions between the discrete elements used for simulating different materials in contact, in order to accurately model and predict the tribological behaviour of dry lubricants. Those interactions are here defined by using the work of adhesion (W) between engineering materials: AISI440C, pristine MoS2 coating, as well as the related transfer film. A method was developed and applied on regular laboratory tribological test samples and ball bearings from the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument of the James Webb Space Telescope. Measured W values were consistent between all worn surfaces. The first DEM modelling results exhibit behaviours similar to those observed experimentally including surface plasticization and transfer.

INTRODUCTION: Articular cartilage is a connective tissue, composed of chondrocytes and an extracellularmatrix, rich in collagens and proteoglycans 1. The main biological function of articular cartilage is to permitfrictionless movements of the connected bones while facilitating force transmission. Cartilage thereforeexhibits a sufficient rigidity to resist mechanical loading and absorbs a part of the contact energy between therelated bones. However, articular cartilage is a non-vascularized tissue with limited self-healing and repaircapacities. With aging and disease, articular cartilage fails to respond to biomechanical stimuli resulting inimpaired capacity of regeneration. Better understanding the processes of mechanotransduction and cartilagegrowth will speed up the improvement of tissue engineering approaches. The aim of this study is to set up acompression device to measure biomechanical properties of cartilage micropellets in different conditions tohave a modelisation of cartilage growth.METHODS: In the present study, we used the in vitro model of cartilage micropellets obtained from thedifferentiation of human mesenchymal stem cells (MSC) into chondrocytes by 3D-culture in presence of theinducing factor TGFβ3 2. These cartilage micropellets were submitted to mechanical loading (i.e., compressiontests by using a home-made compression device) and biochemical analysis (i.e., RT-qPCR andimmunocytochemistry) after respectively 7, 14, 21, 29 and 35 days of cell differentiation.RESULTS: This cross analysis showed that micropellets generated with TGFβ3 exhibit a significant higherYoung’s modulus and disspated energy than undifferentiated micropellets (generated without TGFβ3). Thesedata reflected the visco-elastic mechanical properties of micropellets that are close to those of human nativearticular cartilage. In particular, the elasticity modulus felt into the range of values obtained by using AFM onnon-degraded articular cartilage at nanometer scale (i.e., actually 150 kPa vs 83 kPa3). Interestingly, we couldnot measure the mechanical properties of micropellets during the two first weeks of culture where productionof the cartilage matrix components was low. The Young’s modulus and the dissipated energy both increasedfrom day 14 to day 29 in parallel to the increase of chondrocyte gene expression. Finally, the mechanicalproperties were highly correlated with gene expression levels of type II collagen and LINK protein.DISCUSSION & CONCLUSIONS: The present results confirm that MSC-derived cartilage micropelletsare a relevant in vitro model devoted to mechanobiological and biomechanical studies of cartilage growth.