Hot Sliding Wear of 88 wt.% TiB-Ti Composite from SHS Produced Powders

Abstract

Titanium alloys and composites are of great interest for a wide variety of industrial applications; however, most of them suffer from poor tribological performance, especially at elevated temperatures. In this study, spark plasma sintering was utilized to produce a fully dense and thermodynamically stable TiB-Ti composite with a high content of ceramic phase (88 wt.%) from self-propagating high temperature synthesized (SHS) powders of commercially available Ti and B. Microstructural examination, thermodynamic assessments, and XRD analysis revealed the in situ formation of titanium borides with a relatively broad grain size distribution and elongated shapes of different aspect ratio. The composite exhibits a considerable hardness of 1550 HV30 combined with a good indentation fracture toughness of 8.2 MPa·m^1/2. Dry sliding wear tests were performed at room and elevated temperature (800 °C) under 5 and 20 N sliding loads with the sliding speed of 0.1 m·s^-1 and the sliding distance of 1000 m. A considerable decline in the coefficient of friction and wear rate was demonstrated at elevated temperature sliding. Apart from the protective nature of generated tribo-oxide layer, the development of lubricious boric acid on the surface of the composite was wholly responsible for this phenomenon. A high load bearing capacity of tribo-layer was demonstrated at 800 °C test.

Keywords: SHS; SPS; TiB-based composite; high temperature; sliding wear.

Figure 1

(a) SEM image of initial Ti and B powder mixture; (b) combustion temperature and pressure evolution; (c) high temperature synthesized (SHS) bulk product; (d,e) typical examples of SHSed fragments after manual crushing; and (f) composite powder after disintegrator milling.

Figure 2

(a) SEM image of SPSed (spark plasma sintering) composite; and (b) XRD patterns of SHSed powder and SPSed composite.

Figure 3

SEM images of fractured cross-sections: (a) Backscattered electron (BE); and (b) secondary electron (SE).

Figure 4

The effect of temperature and load on wear rate of Ti–TiB specimen and alumina counterbody (logarithmic scale).

Figure 5

Effect of temperature and load on evolution of coefficient of friction.

Figure 6

SEM images of composite surfaces with different magnification taken after sliding test. The test conditions are indicated in the figure.

Figure 7

XRD pattern of composite surface after sliding at 800 °C, 20 N.

Figure 8

(a,b) SEM images of a central part of wear track cross-section of the composites tested at 800 °C with load of 5 or 20 N; (c) effect of oxidation temperature on weight gain and microhardness of composite.

Figure 9

(a,b) SEM images of the counterbody ball worn surface; and (c,d) energy dispersive spectrometer (EDS) mapping of the worn area of the counterbody tested at 20 °C and 800 °C with load of 5 N.