or PAO-based based lubricants primarily because it offers the lowest kinematic viscosity at the cold end  of the temperature range and has a comparable vapor pressure.   

The well-documented Achilles’ heel of PFPE lubricants is their tendency to rapidly degrade due to  catalytic chemical breakdown of the oil’s polymeric structure when used with common bearing materials  (e.g., 440C stainless steel). All polymer molecules are subject to degradation under high mechanical  shear and in a lubricant, high shear loads will act to rip apart the molecular chains. PFPE fluids react with  bearing surfaces producing “a friction polymer, which, in turn, reacts with existing oxides to produce  metallic fluorides [that] are effective, in situ solid lubricants, which reduce friction and prevent catastrophic  surface damage” [4]. Unfortunately, these metallic fluorides act as catalysts that speed up the breakdown of PFPE oils – they are Lewis acids (electron acceptors) that catalytically attack and pull the polymer’s  molecular structure apart. It is the worst under bou ndary lubrication conditions where there is a large area  of metal-on-metal contact and more of these fluorides are created. PFPE Z-fluids, which perform better at  low temperatures, are even more vulnerable to this phenomenon since they possess a more linear  structure (straight polymer chains as opposed to branched). To effectively inhibit the degradation of PFPE  fluids one must “prevent the catalytic action of strong Lewis acids on the functional surfaces” [3]. There  are two ways to reduce the catalytic action. First, one can minimize or avoid operating in a boundary  contact regime by a combination of reducing contact stress, increasing operating speeds, keeping temperature down and avoiding lubricant starvation – the goal should be to guarantee the bearing will  operate in a elastohydrodynamic (EHD) regime with virtually no metal-on-metal contact. The second  option is to use alternative materials (e.g., ceramics, additives) that possess a lower reactivity with the PFPE fluids. However, this was not viable since the schedule could not be met.    

PFPE Lubrication Evaluation

Lubrication modes include EHD, boundary contact, or the mixed regime (see Table 4). For long bearing  life, especially for those lubricated with PFPE fluids, it is important that the bearing not be operated in the  boundary contact regime for significant amounts of time. The SRT grinding bit bearing lubricant film  parameter (/g513) was calculated using both COBRA EHL software and manual calculations to determine the  operating lubrication regime [5]. Figure 3 shows the lubricant film parameter for the most severe operating  mode (Grinding Difficult Rock).  

Table 4: Fluid Lubrication Regimes  Lubrication Regime Lubricant Film Parameter (/g513)  Full Separation / EHD /g513 > 3.0  Acceptable EHD /g513 > 1.5  Mixed 1.0 < /g513 < 1.5  Boundary Contact /g513 < 1.0 

Full ball-raceway separation occurs at /g513 > 3.0; however one can assume very little asperity contact, and therefore EHD, for /g513 > 1.5 [5]. As shown in Figure 3, the bearings will most likely be operating in a full  EHD regime at room temperature (20° C) assuming the bearings are well lubricated. As lubricant is  depleted and the contact surfaces become starved, the bearing will begin to operate in a mixed regime  and ultimately boundary contact will occur. The analysis showed if the bearing was operating in a starved  condition at temperatures exceeding ~20/g219 C, there was a high likelihood of boundary contact **.                                                         ** One exception was the brushing load case, which is run at 1500 rpm. The analysis showed that the  bearing would operate in the EHD regime across the temperature range.  73