Wear Reductions and Contact Interface Observations for Carbon Samples Sliding Against Wavy and Smooth Copper Surfaces

Wear tests are presented in which a carbon graphite slider, loaded by a constant 20 N force spring, slides against smooth and wavy copper rotors. Rotors were prepared by attaching 250 μm thick copper foil to underlying backings of steel or photoelastic polycarbonate. A “smooth” rotor surface firmly attached copper sheeting to a steel disk, machined with a flatness tolerance less than 0.051 mm around the outer circumference. Special “smooth” and “wavy” rotors were prepared by curing polycarbonate beneath 250 micron thick copper foil. Sliding tracks on “wavy” rotors possessed surface waves of tens of microns; here higher harmonic (5 and greater) surface wave components had spectral amplitudes larger than about 50 μm.

Carbon samples with various contact geometries given in table 2 were slid over the smooth and wavy rotors; wear rates (μgm/sec) were plotted versus rotor speed. Wear rates for the smooth and wavy rotors were identical at some speeds, but at other speeds the wear rates for the wavy rotors were slightly less than half those on the smooth rotors.

Friction, contact voltage drop across the sliding interface (due to a 5 volt source and 220 ohm resistor in series with the slider and rotor), and vibration amplitudes for motions of the slider perpendicular to the sliding surface were also measured versus rotor speed. On the wavy rotor the vibration amplitudes were noticeably larger (tens to hundreds of microns) at those speeds where the wear rates for the wavy rotor were most reduced. Traces of the friction coefficient were similar on the wavy and smooth rotors. The contact voltage drop never changed abruptly (from low to high), implying that the vibrations never separated the slider from the rotor.

The photoelastic behavior of polycarbonate backed rotors permitted visual observation and direct measurement of the real contact areaswithin the contact interface between the opaque copper rotor and the carbon sample. Phenomena such as friction and wear could be correlated with changes in real contact area. At higher speeds where wear rates on the smooth and wavy rotors were most different, photoelastic images revealed concentrated contact on the smooth polycarbonate rotor, but little or none on the wavy polycarbonate rotor.

Spectral analysis of slider motions measured perpendicular to the sliding surface show enhancement of vibration amplitudes on the wavy rotor at those speeds where the wear rates were most reduced. Calculations of resonant frequency suggest that certain waviness harmonics passed beneath the slider at rotor speeds that excited natural frequencies of the slider-spring-rotor dynamic system.

This study suggests that appreciable reductions (up to 50%) in wear rates can be achieved by designing small surface waves on a rotor, and running the rotor such that the surface waves cause the slider-spring-rotor system to resonate. Although these waves apparently alter the interfacial physics, friction and contact integrity appear unaffected.