Tribological testing was carried out using a pin-on-disk tribometer (K-SST, KTmfk, Erlangen, Germany) as illustrated in Figure 1 in rotational sliding mode. Thereby, the disk was clamped in the stainless-steel lubricant reservoir with a screw and was driven by a three-jaw chuck.

Photograph of the pin-on-disk tribometer inside the climatic chamber (a) and schematic representation of the experimental setup (b).

To lubricate the contact, the reservoir was filled with 25 mL of a substitute SF (Laboratory for Biomechanics and Implant Research, Clinic for Orthopedics and Trauma Surgery, Heidelberg University Hospital, Heidelberg, Germany). Therefore, BCS (Biochrom, Berlin, Germany) with a composition according to Table 2 was used and diluted to a protein content of 20 ± 1 g/L according to ISO 14234-1 [47] by adding deionized water. For an anti-microbiological function and to reduce calcium phosphate layers on the material surfaces, 1.85 g/L sodium azide (NaN3) as well as 5.85 g/L ethylenediaminetetraacetate (EDTA) were added. Since the fluid substantially influences the tribological performance, a detailed characterization of the respective rheological properties and the suitability for mimicking human SF was already shown and discussed in [77]. The fluid was pre-cooled to 4 °C and subsequently deep-frozen at −20 °C. Prior to testing, the BS was naturally thawed to 8 °C.

BCS composition as specified by the supplier [77].

The pin was fixed to a cantilever, and a normal force F of 10 N was applied by means of a weight. The friction force was determined with strain gauges based upon the deflection of the cantilever with a scanning rate of 3 Hz. Thus, the coefficient of friction (COF) could also be determined from the quotient of the frictional force and the normal load. With a disk rotational speed of 1 s−1 and a wear track radius of 15.92 mm, the sliding velocity u was set to 0.1 m/s. The tribometer was placed on a vibration damping frame and in an environmental chamber (3436/15, Feutron, Langenwetzendorf, Germany). A constant temperature of 37 ± 0.2 °C could thus be ensured in the lubricant bath and an ambient relative humidity of 50% in the surrounding environmental chamber. Uncoated references as well as coated pairings were tested, more specifically the following four disk|pin combinations:

UHMWPE|CoCr

UHMWPE|Ti64

UHMWPE:H|CoCr:W

UHMWPE:H|Ti64:W

For each pairing, three repetitions with new specimens were performed for statistical purposes. The experimental duration covered an overall sliding distance s of 2 × 104 m, corresponding to 2 × 105 cycles. To examine the temporal wear evolution as well, the tests were divided into 10 intervals of 2 × 103 m or 2 × 104 cycles, respectively. After each interval, the specimens were cleaned non-tactilely with distilled water and characterized regarding wear-related changes as described in Section 2.3. Subsequently, the evaporated or absorbed substitute SF was refilled to a level of 25 mL and the next interval was pursued.

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