Yufeng Li

High Temperature Tribology and Solid Lubrication of Advanced Ceramics

The high-temperature friction and wear characteristics of different ceramics and ceramic matrix composites (CMCs) incorporated with various solid lubricants have been investigated from room temper- ature to 1000oC. The solid lubricants considered in this paper include representative precious metals, hexagonal boron nitride, graphite, fluorides, soft oxides, chromates, sulfates, and combinations of various solid lubricants. General design considerations relevant to solid lubrication were proposed on the basis of friction and wear data of self-lubricating CMCs. The self-lubricating composites incorporated with SrSO4 or/and CaSiO3 exhibits low and stable friction coefficients of 0.2 to 0.3 and small wear rates in the order of 10-6 mm3/Nm from room temperature to 800oC. The optimized composites appear to be promising can- didates for long-duration, extreme environment applications with low friction and small wear rate.

Influences of SrSO4 and Ag on High Temperature Tribological Properties of Spark-Plasma-Sintered ZrO2(Y2O3)-Al2O3 Composites

ZrO2(Y2O3)-Al2O3-SrSO4 nanocomposites incorporated with and without Ag addition have been fabricated by spark plasma sintering (SPS) to evaluate their friction and wear properties in sliding against alumina ball from room temperature to 600 oC. X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy were used to investigate microstructure and self-lubrication mechanisms of nanocomposites after wear tests at different temperatures. The ZrO2(Y2O3)-Al2O3-SrSO4 nanocomposite exhibits low and stable friction coefficients of 0.2 to 0.3 and wear rates in the order of 10-6 mm3/Nm at high temperatures. With the addition of Ag into the composite, the intermediate temperature lubricating property is greatly improved. Plastic deformation of SrSO4 and sliver during sliding plays an important role in formation of lubricating films on worn surfaces of nanocomposites. These lubricating films reduce the friction and wear of the ZrO2(Y2O3)-Al2O3 matrix composites.

HIGH TEMPERATURE TRIBOLOGICAL PROPERTIES OF SPARK-PLASMA-SINTERED Al2O3-SrSO4 SELF-LUBRICATING NANOCOMPOSITES INCORPORATED WITH AND WITHOUT Ag ADDITION

Spark plasma sintering (SPS) is employed to fabricate self-lubricating Al2O3-SrSO4 nanocomposites incorporated with and without Ag addition. The friction and wear properties have been evaluated using a high temperature friction and wear tester from room temperature to 600°C in dry sliding against alumina ball. X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) analyzer were used to investigate microstructure and self-lubrication mechanisms of nanocomposites after wear tests at different temperatures. Al2O3-SrSO4 nanocomposites with optimum compositional combinations exhibit low and stable friction coefficients of 0.22 and wear rates in the order of 10-5 mm3/Nm at high temperatures. At low temperature, the Ag additives in the composite form a discontinuous lubricating film to effectively reduce friction and wear. With increasing test temperature, plastic deformation of SrSO4 during sliding plays an important role in formation of lubricating films on worn surfaces to reduce the friction and wear.

Tribological Properties of Spark-Plasma-Sintered Al2O3-SrSO4 Self-Lubricating Nanocomposites at Elevated Temperatures

Spark plasma sintering (SPS) is employed to fabricate the self-lubricating Al2O3-SrSO4 nanocomposites by tailoring the chemical compositions. Their friction and wear properties have been evaluated using a high temperature friction and wear tester from room temperature to 600°C in dry sliding against alumina ball. Nanosized SrSO4 powders were synthesized by chemical precipitation process at room temperature and atmospheric pressure. For a comparative study, SrSO4 solid lubricants with different particle sizes are intentionally designed to investigate the influence of size effect on self-lubricating properties of Al2O3-SrSO4 nanocomposites under the identical test conditions. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate microstructure and self-lubrication mechanisms of nanocomposites after wear tests at different temperatures. Al2O3-SrSO4 nanocomposites with optimum compositional combinations exhibit low and stable friction coefficients of 0.18 to 0.29 and wear rates in the order of 10−3 to 10−5 mm3/Nm at high temperatures. With increasing test temperature, plastic deformation of SrSO4 during sliding plays an important role in formation of lubricating films on worn surfaces of nanocomposites. These lubricating films reduce the friction and wear of the Al2O3-SrSO4 nanocomposites.