Dry sliding wear of eutectic Al–Si

Abstract

The wear of as-cast eutectic Al–Si was studied using pin-on-disk tribotests in two different environments, air and dry argon. The counterface in all tests was yttria-stabilized zirconia. It was found that wear of the Al–Si was reduced by about 60% by the removal of oxygen from the test environment. The zirconia counterfaces showed measurable wear after tests performed in air, while there was very little wear of the zirconia for tests conducted under argon. The near-surface regions of the Al–Si pins were examined using a transmission electron microscope (TEM), using specimens produced by focussed ion beam milling. The specimens that had been worn in air were characterized by a near-surface mechanically mixed layer containing a considerable amount of both aluminum oxide and zirconium oxide—the aluminum oxide particles had evidently acted as abrasive agents to remove material from the zirconia counterface. In contrast, TEM analysis of the Al–Si tested in argon showed little zirconium oxide in the near-surface regions.

Appendix: Calculation of contact area and contact temperature

Contact of stationary Al–Si pin with moving Zirconia disk

Operating conditions:

Normal load w = 23 N	Sliding velocity V = 1 m/s
Friction coefficient (measured) μ = 0.4	Pin radius R pin = 4.75 mm

Material properties (* measured, remainder from [22])

	Al–Si (material 2)	Zirconia (material 1)
H Hardness (GPa)	0.462*	14
E Modulus of Elasticity (GPa)	70*	290
ρ Density (kg/m3)	2654*	6100
ν Poisson’s ratio	0.33	0.24
K Thermal Conductivity (W/m K)	140	1.8
C Specific heat (Nm/kg K)		630

Contact Geometry (assuming Hertzian contact [21])

$$ {\text{Radius\; of\; contact\; circle }}\quad b = \left( {{\frac{3wr}{{4E^{\prime } }}}} \right)^{1/3} $$

(1)

$$ {\text{Effective\; modulus }}\quad E^{\prime } = \left[ {{\frac{{1 - \nu_{1}^{2} }}{{E_{1} }}} + {\frac{{1 - \nu_{2}^{2} }}{{E_{2} }}}} \right]^{ - 1} = 62.5\,{\text{GPa}} $$

(2)

$$ {\text{Effective\; radius }}\quad r = \left[ {{\frac{1}{{R_{\text{pin}} }}} + {\frac{1}{{R_{\text{disk}} }}}} \right]^{ - 1} = 0.00475\,{\text{m}} $$

(3)

Using 2 and 3 and operating parameters in 1 find the contact radius

$$ b = 10 9 \mu {\text{m}} $$

(4)

Contact Temperature rise (following methodology of [24])

Assume stationary Al–Si pin (material 2) and moving flat Zirconia disk (material 2)

$$ \Updelta T_{\max } = {\frac{{2.733\bar{q}_{\text{total}} b}}{{2.32K_{2} + 1.178K_{1} \sqrt {\pi (1.234 + Pe_{1} )} }}} $$

(5)

$$ {\text{where\; Peclet\; number }}\;Pe_{1} = {\frac{{Vb\rho_{1} C_{1} }}{{2K_{1} }}} = 116 $$

(6)

and

$$ {\text{average\; total\; heat\; flux }}\bar{q}_{\text{total}} = \mu \left( {{\frac{w}{{\pi b^{2} }}}} \right)V = 246 \times 10^{6}\,{\text{W}}/{\text{m}}^{2} $$

(7)

Using 5, 6, 7, and the material properties (above) in 5, find the peak contact temperature rise (above room temperature) ΔT _max = 200 °C.

This is the temperature rise due to frictional heating for the Hertzian contact. Given that the tests were conducted at room temperature (about 25 °C), the surface temperature at the center of the Hertzian contact area was thus at least T _max = 225 °C.

It should be noted that the temperature analysis outlined above gives a relatively conservative estimate of the contact temperature at the interface between the hemispherically shaped Al–Si pin and the flat zirconia disk, since a single Hertzian (elastic) contact was assumed. The contacting material within the Hertzian contact region on the stationary Al–Si pin would remain at an elevated temperature for a substantial period of time, allowing plenty of time for oxidation to occur, even at 225 °C [8]. In the actual contact, it is probable that at any instant the real area of contact within the Hertzian region consisted of a finite number of concentrated asperity-level contacts. The peak contact temperature rise could be considerably higher at localized asperity contacts, but these would be of shorter duration (flash temperature rise).