sputter-deposited on steel test coupons and their tribological performance was studied in humid air at two  temperatures. The objective of this study was to determine the wear rates of the coatings and the wear  rates of the uncoated, steel counter-faces.   Experimental Procedure  Deposition  Coatings were deposited onto AISI 52100 steel specimens in a high-vacuum, magnetron sputtering system.  Ti and Sb 2O3/Au targets were co-sputtered with MoS 2, a description of which can be found elsewhere [4],  [6]. Singh et al. [4] described the deposition procedure and parameters in detail for the Ti-MoS 2 coating. All  three coating depositions followed the same basic procedure. After ultrasonically cleaning the substrates  in isopropyl alcohol, the substrates for the Ti-MoS 2 and MoS 2 coatings were placed onto a stationary fixture  that faced two 50-mm-diameter magnetrons. First, the substrates were sputter etched with Ar ions for about  30 minutes. This removed surface contaminants and some of the native oxide from the steel specimens.  Next, about 100 nm of Ti was deposited onto the substrates to form metallurgical bonds to the steel as well  as to the functional coating. Different combinations of target materials were used in the deposition of the  functional top layers of the three coating architectures. Only MoS 2 targets were sputtered to form the  functional top layer of the undoped MoS 2 coating specimens, while both Ti and MoS 2 targets were sputtered  simultaneously to form the functional top layer of the Ti-doped MoS 2 coating specimens. The functional top  layer of the Sb 2O3/Au-doped MoS 2 coating was deposited by simultaneously sputtering MoS 2 and Sb 2O3/Au  targets.  Characterizations  All three coatings were determined to be 1.1 ± 0.2 µm thick by means of the calotest procedure. Adhesion  strengths of the coatings to their substrates were qualitatively determined to be excellent according to the  standard Rockwell C indentation tests (DIN CEN/TS 1071-18). Whereas the Ti-MoS 2 and Sb 2O3/Au-MoS 2 functional top layers have been determined previously to be amorphous by transmission electron  microscopy, the undoped MoS 2 functional top layer was found to comprise randomly oriented, microcrystallites of MoS 2. Compositional measurements performed by x-ray photoelectron spectroscopy  indicated that all three functional top layers were slightly sub-stoichiometric in sulfur, and the dopant  concentrations were about 18% and 15% for the Ti-MoS 2 and Sb 2O3/Au-MoS 2 top layers, respectively.   Surface roughness measurements of the coatings were performed by 3D optical interferometry. The rms  roughness values for the MoS 2, Sb 2O3/Au-MoS 2, and Ti-MoS 2 coatings were determined to be 9 ± 1 nm,  7 ± 2 nm, and 8 ± 2 nm, respectively. The average rms roughness values of the uncoated AISI 52100 balls  was 598 ± 19 nm.  Tribological Test Parameters  Dry reciprocating sliding contact experiments were performed on a ball on disk tribometer at a contact  stress of 660 MPa (96 ksi). These tests were conducted with a sliding amplitude of 1 mm, a frequency of  20 Hz, and a normal load of 1 N. Tests were conducted in laboratory air (17% RH) and at 30°C and 100°C.  Coefficients of friction were determined in situ, and at least 3 different durations (sliding distances) were  performed for each temperature. Subsequent to each test, wear scars in the coatings and on the uncoated  balls were measured using 3D optical profilometry. Wear volumes for the steel balls and coated disks were  determined as a function of dissipated energy ( Ed), where Ed is the product of the frictional force and the  sliding distance. Although wear volumes of the coating were determined directly using 3D optical  interferometry, wear volumes of the balls were calculated from a measurement of the radius of the wear  scar   = (ℎ+ 3) (1)  where  ℎ =−− , (2)  Rb is the radius of the ball [ µm], and r is the radius of the wear scar [ µm].  NASA/CP—2018-219887 142