Atmospheric effects of friction, friction noise and wear with silicon and diamond. Part III. SEM tribometry of polycrystalline diamond in vacuum and hydrogen

In this part III of a multi-part paper series, the results of additional SE M tribometric experiments are described, performed with polished, mostly C( 100)-oriented polycrystalline CVD diamond film [PCDC(100) vs. PCDC(100)] co unterfaces sliding in similar to 1 x 10(-5) Torr and in 0.1-0.3 Torr partia l pressures of pure hydrogen gas. These tests were completed under a 28 g ( 0.27 N) normal load, under standard and slow thermal ramping conditions at temperatures ranging from room temperature to 1000 degrees C. The friction data were examined per the computer logging and analysis techniques describ ed in part I. The treatment of the data is similar to that of Si in part II : the maximum and the average coefficients of friction (MAX.COF and COF) an d their ratios (the friction noise FN) are employed to measure possible lub ricative interaction of the diamond surfaces with rarefied hydrogen. The re sults indicate that excited species of molecular hydrogen enter into tribot hermally catalyzed reactions not only with Si but with PCDC(100) surfaces a s well. Similar to the behavior of Si, the most beneficial friction-reducin g regime occurs in a temperature range just before the thermal desorption o f adsorbates. The general magnitudes of MAX.COF, COF and the FN are signifi cantly lower than those of the Si crystallinities, in both vacuum and P-H2. The wear rate of the PCDC(100) film characteristic of the standard thermal ramping test procedure performed mostly in P-H2 is around 4 x 10(-16) m(3) /(N m), in good agreement with the wear rate previously measured in vacuum for unpolished, fine-cauliflowered diamond films. The data indicate that sm ooth polycrystalline diamond is a significantly better bearing material for miniaturized moving mechanical assembly applications than Si.