Transition to low friction and wear regime in high speed silicon nitride/steel contacts

Stephen Berkebile¹, Mick Harris ^1,2, Nikhil Murthy¹, Jon-Erik Mogonye¹

¹US Army Research Laboratory, Aberdeen Proving Ground, MD, ²University of North Texas, Denton, TX

INTRODUCTION: Silicon nitride (Si₃N₄) has found increasing use in applications where high speed mechanical contacts between Si₃N₄ and metals are present. Higher fracture toughness, strength, and elastic limit than other ceramics,¹ coupled with favorable thermal properties led to its first broad acceptance in cutting tools. As advances in fabrication, machining, and testing have been made, silicon nitride has become an important material for mechanical components, particularly, rolling element bearings.² Si₃N₄ rolling elements are typically used with steel races due to issues of thermal expansion and strength in tension.

Sliding contacts between Si₃N₄ and steel (and cast iron) have been found to display a range of friction and wear characteristics at different contact conditions and rich tribochemical behavior that often results in overall lower wear than self-mated pairs of the same materials.^3-13 At low speeds (<0.5 m/s) and temperatures (<100 °C), tribochemically-encouraged abrasive wear of the Si₃N₄ is observed with little to no wear in the steel, despite a high coefficient of friction (COF) of 0.6 to 1.0. As speed or temperature are increased to 1 to 3 m/s or 100 to 400 °C, the wear rate of the steel increases while that of Si₃N₄ decreases, accompanied by a decrease in COF by up to a few tenths. The change is due to the onset of iron oxidation that spalls from the steel and protects the Si₃N₄, with oxide or metal adhesion to the Si₃N₄. As the speed and/or temperature are increased further, oxide films are found completely covering the Si₃N₄ but the wear and COF behavior diverges depending on the experimental conditions. When the steel specimen is held at high temperature (>500 °C) for moderate to low speeds, wear of Si₃N₄ continually decreases while wear of steel increases with either little change in COF,^{5, 11} or a drastic decrease in COF to 0.2.¹³ For room temperature experiments, the wear rate of steel and Si₃N₄ first increase around 4 to 6 m/s, then that of steel drops nearly to zero while Si₃N₄ remains constant with little change in COF above 6.5 m/s.⁶ The reduction of Si₃N₄ wear is attributed to the oxide film on the surface at both high temperature and at high speed, which is found to consist of mixtures of silicon and iron oxides of various compositions.

In this wear transition, lubrication mechanisms between tribofilms produced through ambient or instantaneous heat remains unclear, as does the behavior at intermediate stages. We previously reported on sliding-rolling contacts between steel and Si₃N₄.¹⁵ Our experiments showed similar behavior to the literature at moderate sliding speeds and, at high sliding speeds, displayed low steel and moderate Si₃N₄ wear but with a low COF similar to high temperature experiments in the literature. Here, we characterize the transition with sliding speed, compare to low speed measurements at temperatures from 22 to 1000 °C and examine the surface films.

METHODS: High speed sliding-rolling contact experiments were conducted with a ball-on-disk tribometer with AISI 9310 steel disks held at 120 °C and Si₃N₄ balls of 20.6 mm diameter at a load of 100 N (1.48 GPa Herztian contact stress) and monitored with thermal and optical imaging and ultrasonic microphone. Low speed experiments were conducted with a pin-on-flat tribometer in reciprocal motion at 5 Hz with 6.35 mm stroke length (max speed 0.1 m/s) on 51000 steel plates with Si₃N₄ balls of 6 mm diameter for a total distance of 200 m at a load of 5 N (1.2 GPa Hertzian contact pressure). Specimens were characterized with optical and electron microscopy, various spectroscopies, and X-ray diffraction.

RESULTS:  Fig. 1A shows the variation of COF with sliding speed in a sliding-rolling contact. For increasing sliding speed, the COF first increases, then decreases to a steady value around 9 m/s

with little change to 16 m/s. With decreasing sliding speed, the COF remains steady from 16 m/s, then increases smoothly below 9 m/s. In Fig. 1B, reciprocal motion shows a significant change in the appearance and size of the Si₃N₄ ball wear scar from 22 to 1000 °C with COF of 0.82, 0.73 and 0.44 from B1 to B3. A comparison between the films on the Si₃N₄ balls after high speed sliding (Fig. 1C) and high temperature (Fig. 1D) reveal cracking patterns in the surface films remaining on the Si₃N₄ balls.

Figure 1 – A) COF of sliding-rolling contact vs. sliding speed over 210 s. Average ball and disk speed maintained at 16 m/s. B) Optical images (1 mm wide) of Si₃N₄ ball wear scars of low speed at three temperatures. C) SEM image (150 µm wide) of Si₃N₄ ball after high sliding speed. D) Close-up of B3 (280 µm wide).

DISCUSSION: For high speed contacts, the transition in friction and tribochemical wear mode depends on immediate contact history, both during smooth variation of sliding speed (Fig 1A) and when changing between static sliding speeds (based on additional experiments). At static sliding speeds, the COF remains steady when coming from a higher speed/lower COF, but is less stable from a lower speed/higher COF. Such behavior indicates that the highly lubricious tribofilm can be maintained once formed to varying extent, but takes time to form in the first place from a higher friction state.

High speed specimen characterization reveals that the film on the steel surface is more homogenous and contains areas of Si/Fe/O, while the film on the Si₃N₄ surface has intermixed areas of Fe/O and Si/N. However, both steel and Si₃N₄ surfaces display significant plastic deformation in the surface film and fracture indicative of thermal stress upon cooling, both also visible on the high temperature specimens. Diffusion of Si into Fe oxide starts around 500 to 700 °C with the release of N₂ and formation of SiO₂ and Fe₂SiO₄ at 1000 °C.¹⁵ These temperatures can easily be met through flash heating at high sliding and explain the presence of Si, but not N on the steel surface film. The melting point of Fe₂SiO₄ is 1200 °C, several hundred °C below that of steel or other Fe oxides, which may provide a lower friction layer for sliding, however, no clear evidence of a particular ferrosilicate has yet been found.

REFERENCES: 1. Ashby, Materials selection in mechanical design (2005). 2. Wang, Wear (2000). 3. Silva, Wear (1991). 4. Gautier, Wear (1993). 5. Childs, Wear (1993). 6. Ravikiran, J. Amer. Cer. Soc. (1995). 7. Kalin, Mat. Sci. and Eng.: A (1996). 8. Kalin, Wear (1997). 9. Novak, Wear (1999). 10. Gomes, J. Amer. Cer. Soc. (1999). 11. Gomes, Wear (2001). 12. Carrasquero, Int. J. of Refr. Met. and Hard Mat. (2005). 13. Sutor, 8th Ann. Conf. on Comp. and Adv. Cer. Mat. (2008) 14. Berkebile, Trib. Front. (2015). 15. Kalin, Mat. Sci. and Eng.: A (2000).

Research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-16-2-0214. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.