Example 3

Reciprocating tests were used to characterize both friction and wear behavior of the ester blends at 25° C. and 40° C. under boundary lubrication. As mentioned prior, each ester was blended at a concentration of 1% by weight. Neat oil served as the control. The testing device is a custom ball-on-flat microtribometer as seen in FIG. 3 and was operated in conjunction with a temperature-controlled stage. In brief, precise normal loading of a probe onto the sample substrate is performed via software controlled linear stages. The sample substrate is forced to slide against the probe and subsequent lateral or frictional forces are measured. The temperature-controlled stage consists of an aluminum block that contains an oil reservoir. The block's temperature is monitored and controlled via an adhesive thermocouple connected to a PID controller. In addition, the oil temperature is monitored with a thermistor. Prior to testing, an equal volume filled the reservoir and the oil temperature was equilibrated. The oil level sits just above the substrate surface so there is a constant supply of oil into the contact zone.

Reciprocating tests were carried out using a SiC-steel interface: a 4 mm diameter silicon carbide ball on an AISI 8620 steel substrate. The ceramic was chosen for its superior hardness relative to the substrate in order to isolate the majority of the wear to the substrate and preserve the probes geometry. In this way, a consistent contact pressure can be maintained. A constant normal load of 3.4 N (maximum Hertzian pressure of 1.5 GPa) was applied as the substrate was translated at a rate of 10 mm/s over a 8 mm stroke length for 4500 cycles. The load was chosen after initial tests with the PEs at 1.0 GPa were not sufficient to generate measureable wear scars (wear depths were on the same order as the surface roughness). The substrate was isotropically polished to a finish of 0.043 μm Ra determined from a scan area of 1.41 mm×1.88 mm using a Zygo optical profilometer. Based on EHL theory, the roughness, load, and viscosity parameters placed this study well within the boundary lubrication regime as the estimated λ ratio was much less than one.

After test completion, the substrate and probes were wiped with isopropyl alcohol before undergoing SEM and EDS analysis. In addition, the substrate wear scars were scanned using the Zygo optical profilometer. Nine to eleven unique scan areas were gathered to capture the entire length of each scar. All topographic and force data was then imported into MATLAB where the average wear depth and coefficient of friction was calculated. Three replicate tests were completed for each treatment.

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