Conventional titanium alloys have poor tribological properties because they are chemically active and exhibit strong adhesion when in contact with themselves and other materials (ref. 1). This adhesion causes high friction and ultimately galling, which results in heavy surface damage--such as the Galileo spacecraft’s high-gain antenna (which could not deploy because the titanium alloy component on the antenna was stuck to a nickel-base superalloy component) and severe galling and fretting on titanium-based alloy fan blade-disk couples in fan engine aircraft propulsion systems (ref. 2).
High-resolution transmission electron micrographs of dispersed MWNTs. Left: Overview. Right: Details.
Long description of figure 1.
The NASA Glenn Research Center and the Air Force Research Laboratory have been working to reduce fretting wear and fatigue damage in titanium alloys, particularly Ti-6Al-4V. The tribological properties of monolithic Ti-6Al-4V alloy can be improved by the application of an advanced solid-film lubricant, such as a multiwalled carbon nanotube (MWNT) coating (see the photomicrographs). Such advanced solid-film lubricants may decrease adhesion and subsequent galling and may increase wear resistance under fretting conditions. This study investigated whether or not solid-film lubricants could decrease the wear and galling of Ti-6Al-4V in cyclic relative motion.
Fretting experiments in cyclic relative motion were conducted with 10-wt% TiC/Ti-6Al-4V hemispherical pins in contact with solid-film lubricants--MWNTs, diamondlike carbon (DLC), MoS2, and graphitelike carbon (GLC)--coated on Ti-6Al-4V disks and also in contact with bare alloy disks of Ti-6Al-4V, Ti-48Al-2Cr-2Nb, and nickel-based superalloy 718. All experiments were conducted at a load of 1.0 N, a frequency of 80 Hz, and a slip amplitude of 50 μm for 1 hr at 296, 423, and 523 K in air. The TiC/Ti-6Al-4V composite was used for the counterpart pin specimens because uniformly distributed particle-strengthened titanium matrix composites can now be produced at a cost lower than for many continuous-fiber composites (refs. 3 and 4).
The bar graphs show the pin and disk wear volume losses. All the solid-film lubricant coatings dramatically improved the wear of both the pin and disk. The MWNT coating reduced pin and disk wear so much that they were almost not measurable. When an MWNT-coated Ti-6Al-4V substrate disk was brought into contact with a composite TiC/Ti-6Al-4V pin under fretting, strong bonds were formed between the MWNTs and the composite, protecting both the pin and disk from wear. Scanning electron microscopy indicated that the entire contact area of the pin contained transferred MWNTs, and the contact area of the MWNT-coated Ti-6Al-4V disk contained residual MWNT coating. The wear volume losses for both the pin and disk were much lower than 10-6 mm3.
Fretting wear volume loss at 296 K in air for TiC/Ti-6Al-4V pins in cyclic relative motion contact with different disk materials: Ti-6Al-4V coated with MWNTs, DLC, MoS2, and GLC, and bare Ti-6Al-4V, Ti-48Al-2Cr-2Nb, and nickel-base superalloy 718. Top: Volume loss of pins. Bottom: Volume loss of disks.
The wear volume loss of a composite TiC/Ti-6Al-4V pin fretted against DLC/Cr-, MoS2/Ti-, and GLC/Cr-coated Ti6Al-4V disks was 1/274th, 1/251st, and 1/247th, respectively, of that fretted against bare Ti-6Al-4V. The wear volume loss of the pin fretted against DLC-coated Ti-6Al-4V was 2.8×10-6 mm3, whereas that of the pin slid against bare Ti-6Al-4V was 7.8×10-4 mm3. The wear volume loss of DLC/Cr-, MoS2/Ti-, and GLC/Cr-coated Ti-6Al-4V disks was 1/88th, 1/64th, and 1/10th, respectively, of that of bare Ti-6Al-4V.
When solid-film lubricants were applied to Ti-6Al-4V, alloy-to-alloy contact was avoided, and no galling occurred in the contacts during fretting. When bare alloy disks were brought into contact with TiC/T-6Al-4V, however, strong adhesion, galling, and severe fretting damage occurred in the contacts. This investigation indicates that solid-film lubricants may be suitable antigalling and antiwear films for aerospace applications under fretting conditions.
Last updated: October 11, 2006
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