Figure 7 displays the wear volumes for the Ti-MoS 2 coating and mated steel ball obtained from tests  performed at 100°C. The wear coefficients for the ball and the Ti-MoS 2 coating were determined to be 1,438  ± 16 mm3/J and 7,941 ± 222 mm3/J, respectively. The optical image of the ball shows a moderate wear  scar radius with complete coverage of transferred material, which is consistent with the small value of the  wear coefficient. The wear scar produced on the Ti-MoS 2 coating is relatively shallow and does not exhibit  a large debris field. This indicates that the majority of the coating that was worn in the test wound up as a  transfer film on the ball, and an indication that the Ti-MoS 2 coating also formed an effective VAL during the  100°C test.  Discussion  Ball and coating wear coefficients determined from analysis of the 30°C and 100°C data are gathered in  Table 1. It is convenient to assign wear regimes as low ( αL < 1000 µm3/J), moderate (1000 µm3/J < αM <  10,000 µm3/J), and high ( αH  > 10,000 µm3/J). According to this convention, low ball wear rates were  obtained for the 30°C measurements from the Ti-MoS 2 and MoS 2 coatings, with the lowest ball wear rate  produced by the Ti-MoS 2 coating. An extraordinarily high ball wear rate was produced by the Sb 2O3/AuMoS 2 coating during the 30°C test. Whereas the 30°C measurements generated moderate wear rates on  the Sb 2O3/Au-MoS 2 and MoS 2 coatings, the wear rate of the Ti-MoS 2 just falls in the high wear category.  The MoS 2 coating had the lowest wear rate at 30°C.   The MoS 2 coating produced the lowest ball wear rate of the 100°C tests, while the Ti-MoS 2 and the  Sb2O3/Au-MoS 2 coatings generated moderate ball wear rates. Of the coating wear rates at 100°C, the TiMoS 2 and the Sb 2O3/Au-MoS 2 coatings had moderate wear rates while the MoS 2 coating had a high wear  rate.   The Sb 2O3/Au-MoS 2 coating exhibited a high degree of abrasiveness to the steel ball. Incorporation of the  Sb2O3 in the MoS 2 is believed to disrupt the ability of the material to achieve long range crystallographic  order, making the coating less susceptible to degradation from moisture [10]. The proposed mechanism  through which this coating can achieve a sustainable VAL is by the thermally driven diffusion of Ag to the  surface of the coating [6]. It is proposed that the average friction coefficient of µavg = 0.2 measured in the  30°C experiments was insufficient to generate enough local heating of the coating to initiate Ag diffusion.  Furthermore, it is also proposed that the Sb 2O3 content in the coating was responsible for the abrasiveness  exhibited during the 30°C testing. On the other hand, the coating was much less abrasive in the 100°C tests  and the pairing with the steel ball yielded a very small friction coefficient of m avg = 0.04. This appears to  indicate that the elevated temperature of 100°C was sufficient to establish a beneficial VAL between the  steel and this coating.   An effective VAL is one that inhibits high amounts of wear of the coating and the counterface over the  temperature range and relative motion experienced by a specific application. Although Ti-MoS 2 has been  previously shown to perform exceptionally well in rolling contact [4,9,12], based upon the results of these  measurements, it can be concluded that the Ti-MoS 2 coating would meet the VAL requirements better than  the tested MoS 2 and Sb 2O3/Au-MoS 2 coatings when in contact with reciprocating sliding steel counterfaces  over a temperature range of 30°C to 100°C.   It is important to point out that although the experiments were performed in laboratory air, the environment  had a very low humidity (17% RH) during the testing. Although it is expected that the wear rates of all three  coatings will increase with increasing relative humidity, undoped MoS 2 tends to experience the greatest  increase [3].  NASA/CP—2018-219887 147