Tribological Properties of Si3N4-hBN Composite Ceramics Bearing on GCr15 under Seawater Lubrication

This paper concerns a comparative study on the tribological properties of Si₃N₄-10 vol% hBN bearing on GCr15 steel under seawater lubrication and dry friction and fresh-water lubrication by using a pin-on-disc tribometer. The results showed that the lower friction coefficient (around 0.03) and wear rate (10⁻⁶ mm/Nm) of SN10/GCr15 tribopair were obtained under seawater condition. This might be caused by the comprehensive effects of hydrodynamics and boundary lubrication of surface films formed after the tribo-chemical reaction. Despite SN10/GCr15 tribopair having 0.07 friction coefficient in the pure-water environment, the wear mechanismsits were dominated by the adhesive wear and abrasive wear under the dry friction conditions, and delamination, plowing, and plastic deformation occured on the worn surface. The X-ray photoelectron spectroscopy analysis indicated that the products formed after tribo-chemaical reaction were Fe₂O₃, SiO₂, and B₂O₃ and small amounts of salts from the seawater, and it was these deposits on the worn surface under seawater lubrication conditions that, served to lubricate and protect the wear surface.

[Preparation processes of 3% yttria-stabilized tetragonal zirconia polycrystalline ceramic composite powders coated with nano-sized Al₂O₃-boron nitride]

OBJECTIVE

In this work, we aim to determine the optimum pH value for the preparation of 3% yttria-stabilized tetragonal zirconia polycrystalline (3Y-TZP)/3%Al₂O₃ and optimum calcination temperature of Al₂O₃ precursor and amorphous boron nitride (BN) for Al₂O₃-BN coating 3Y-TZP powders.

METHODS

The 3Y-TZP/3%Al₂O₃ composite powders were prepared through the heterogeneous precipitation method under different pH values (6.5, 7.5, 8.5, and 9.5) and analyzed through transmission electron microscopy (TEM) to determine the optimum coating morphology. Al₂O₃ precursor, which was prepared under the optimum pH value, was calcined at different temperatures (800, 1 000, 1 200, and 1 400 ℃). The amorphous BN coating 3Y-TZP/3%Al₂O₃ powder was prepared via in situ reaction with boric acid and urea (calcined with N₂ at 850 ℃ for 5 and 3 h and 800 ℃ for 5 and 3 h). TEM, energy dispersive spectrometry (EDS), and X-ray diffraction (XRD) were performed to characterize the results.

RESULTS

The optimum coating morphology was obtained at 8.5 pH. When the Al₂O₃ precursor was calcined at 1 200 ℃ and coated with BN (calcined with N₂ at 800 ℃ for 5 h), the Al-B-O compound was less, and t-ZrO₂ was more represented.

CONCLUSIONS

The optimum pH value for 3Y-TZP/3%Al₂O₃ composite preparation is 8.5. For the preparation of Al₂O₃-BN coating 3Y-TZP powders, the optimum calcination temperature of the Al₂O₃ precursor and amorphous BN are 1 200 and 800 ℃, respectively.