Metal Oxide Films Reduce Friction in Ceramic Lubrication

Ceramic bearings, despite their high-performance potential, have long been plagued by excessive friction and wear, particularly under high-temperature conditions. New research reveals an unexpected solution that could finally overcome this persistent engineering challenge.

Ceramic materials boast exceptional properties including high melting points, chemical inertness, and low density, making them ideal candidates for demanding applications in diesel engines, turbine systems, and other high-temperature environments. However, their tribological performance in ceramic-ceramic pairings has remained disappointing.

Typical friction coefficients range between 0.5-1.0, accompanied by unacceptably high wear rates. Microscopic fracture, particularly along grain boundaries, emerges as the predominant wear mechanism. This stems from ceramics' inherent limitations: low tensile strength, minimal ductility, and high friction coefficients. During sliding contact, elevated tensile stresses initiate cracks that propagate along grain boundaries, ultimately causing grain detachment and wear particle formation.

The research focuses on comparing the tribological performance of ceramic-metal sliding pairs against traditional ceramic-ceramic configurations. Certain metal alloys, particularly nickel- and cobalt-based superalloys, demonstrate a remarkable capability to form lubricious oxide films at elevated temperatures, significantly reducing friction and wear.

Studies indicate that doping ceramics with metal ions that form lubricious oxides through ion implantation, or alloying ceramics with titanium carbide or nitride, can enhance high-temperature tribological performance in oxidizing environments. This suggests that specific ceramic-metal combinations may outperform conventional ceramic-ceramic pairings.

The research employed bench-scale experiments to measure friction and wear characteristics of ceramics sliding against themselves and against nickel-based alloy Inconel 718. Testing spanned temperatures from 25°C to 800°C (with some cases reaching 1200°C) in air environments.

• Inconel 718: A precipitation-hardened nickel-chromium alloy with hardness of Hv 520 kg/mm². Composition: Ni 53%, Cr 18.5%, Fe 18.5%, Nb 5%, Mo 3.1%, with minor elements.
• Oxide ceramics: Included mullite, alumina, alumina-SiC whisker composites, and partially stabilized zirconia.
• Silicon-based ceramics: Included silicon nitride and silicon carbide.

Pin-on-disk tribometer tests measured friction coefficients and wear factors (k), calculated from wear volume divided by load and sliding distance. Wear volume determination involved measuring wear scar diameters on pins and surface profilometry of disk tracks.

Alumina-alumina pairings showed friction coefficients starting at 0.60±0.10 at room temperature (highly unstable), increasing steadily with temperature to exceed 1.0 at 900°C. In contrast, alumina-Inconel pairings maintained similar room-temperature friction but demonstrated a dramatic reduction to 0.3±0.03 at 750-900°C, attributed to nickel-chromium oxide formation on the alloy surface.

Alumina-SiC composites showed less pronounced benefits, likely due to whiskers abrading the protective oxide films. However, three silicon-based ceramics and three oxide-based ceramics all demonstrated significant friction reduction when sliding against Inconel 718 at 800°C, suggesting the lubricious oxides' dominant effect at high temperatures.

Extended testing at constant temperatures revealed:

• Mullite showed lower friction (0.35-0.50) compared to alumina and alumina-SiC, but higher wear rates due to its lower hardness.
• Mullite-Inconel pairings showed 20% higher friction than mullite-mullite at 25°C, but lower friction at 800°C, with reduced wear in both configurations.
• Alumina-SiC composites showed configuration-dependent behavior: metal oxidation benefited ceramic pins on metal disks but not vice versa.
• Zirconia, SiC, and Si3N4 maintained high friction coefficients (0.7-0.8+) across all temperatures, with generally moderate-to-high wear rates.

1. Unlubricated ceramics generally exhibit high friction and wear under most sliding conditions.
2. In oxidizing environments, ceramic-metal pairings with nickel-chromium alloys demonstrate significantly lower friction and wear than ceramic-ceramic combinations.
3. Tough nickel-chromium oxide films provide effective lubrication at high temperatures, though SiC whiskers can compromise this effect through abrasive action.
4. The findings underscore the need for continued research into ceramic-specific lubrication solutions to fully exploit their high-temperature potential in bearing applications.