219   Raman spectroscopy of the samples revealed the coppery hue to be due to iron oxide, and EDX SEM  results showed increased concentrations of oxygen on the bearing races and ball surfaces. Both results  indicated the presence of corrosion. X -Ray Fluorescence (XRF) detected the presence of iron, chromium,  nickel, and copper in both grease samples. Iron, the Circular Spline material, and chromium and nickel,  constituent materials of the Flexspline, corresponded with the worn teeth observed on both via the SEM  inspection. The presence of copper was an unexpected discovery.  

Table 1.  Anion Content for Harmonic Drive Braycote Samples, Expressed as M icrogram Anion in the  Water Extract used in S onication, per gram of Braycote (ppm).   Sample  Amount (mg)  Fluoride (ppm)  Chloride (ppm)   Control – New Braycote  601EF  27.51  < 0.88  4.60  Braycote 601EF –  Bearing Sample  26.54  45.5 < 2.3   Braycote 601EF –  Tooth Sample  25.76  110 3.38 

Anion levels measured using Ion Chromatography in the blackened tooth grease sample and the glittery  bearing grease sample are shown in Table 1. Extremely high fluoride levels in both samples, especially in  the tooth sample, indicat ed Braycote degradation had occurred. The higher Fluoride level in the tooth  sample seemed due to the higher stress experienced by the grease at that interface, due to the sliding  interaction between the HD spline teeth.  

The Braycote had clearly polymeriz ed and the grease performance and appearance, with the wear patterns  along the ball paths of the splines and bearings, were reminiscent of failures described by Conley and  Bohner in their report on early perfluoropolyalkylether (PFPE) lubricant testing. [ 2] It seemed the continuous  operation of the HD at a load which was 45% of its maximum rated value, along with the dither -like motion  profile, were too much for the Braycote to handle. The severe degradation of the Braycote, especially in the  high-shear HD sliding gear mesh, and the presence of the notorious Lewis acids in the samples (higher at  the tooth interface) seemed consistent with the summary of PFPE breakdown under high stress by Herman  and Davis. [ 3] The conclusion was drawn that breakdown of Brayc ote occurred due to the high loads and  cycles, and polymerization of the grease caused lubricant starvation of the assembly, increasing the rate  of wear to the splines and bearing components. The rollercoaster effect apparent in the input torque and  output  speed test data appeared to demonstrate the effect of the generation of the friction polymer and its  reaction with oxides to produce metallic fluorides to act as in situ solid lubricants (yielding a momentary  improvement in performance), followed by the effect of these fluorides as catalysts to accelerate the PFPE  breakdown (resulting in subsequent drops in performance). [ 3] 

Path to Resolution  

Due to the status of the MAIA program, it was too late for load reductions, and the MDDA design similarly  could not be changed. The decision was made to change lubricants for the Drive Output Subassembly from  PFPE Braycote (Braycote 601EF and Brayco 815Z) to hydrocarbon- based Pennzane products (Rheolube  2000 and Nye Synthetic Oil 2001) to leverage its increased lifetime capability. While known to demonstrate  an increased lifetime and a decreased wear rate, use of Pennzane lubricants is also known to reduce cold  temperature performance, due to its increase in viscosity with reducing temperature. [ 4] The in- flight thermal  environment for the MDDA would be benign, operating nominally somewhere between 0°C to 30 °C, so  obtaining adequate performance in flight was not at risk, but demonstrating torque margin at the more  extreme cold temperatures required in ground testing was.  

Additionally, no experience could be referenced using Pennzane with ductile nodular cast iron for flight, so  material compatibility needed to be established. Furthermore, the Duplex Motors used for the MDDAs had