The effect of microstructure length scale on dry sliding wear behaviour of monotectic Al-Bi-Sn alloys

Highlights

• Droplets of self-lubricating Bi/Sn mixture typify the microstructure of Al-Bi-Sn alloys.

• The spacing between droplets, λ_G, has significant effect on the wear volume, V.

• Experimental equations V vs. λ_G are proposed for any Al-Bi-Sn alloy examined.

• More homogeneous distribution of Bi/Sn droplets (lower λ_G)→lower V.

• Lowest wear rates and V related to the monotectic composition: Al-3.2 wt%Bi-1wt%Sn.

Abstract

The addition of third elements to Al-based monotectic alloys can increase the alloy load capacity, and depending on the nature of the third element can also improve the tribological characteristics. In the present investigation 1.0 wt.%Sn is added to Al-Bi alloys of hypomonotectic, monotectic and hypermonotectic compositions, giving rise to microstructures typified by droplets of a self-lubricating mixture of Bi and Sn embedded in the Al-matrix. These alloys were directionally solidified with a view to permitting microstructures with a wide range of interphase spacings to be obtained. Micro-adhesive wear tests were carried out and experimental equations relating the wear volume, V, to the interphase spacing are proposed. A more homogeneous distribution of Bi/Sn droplets in the microstructure is shown to be conducive to lower V. The lowest wear rates and wear volumes are shown to be related to the monotectic composition (Al-3.2 wt.%Bi-1wt.%Sn alloy). The presence of higher fraction of Fe-oxide areas interrupting the Bi/Sn lubricant layer, is shown to induce inferior wear resistance for the hypermonotectic alloy (Al-7.0 wt.%Bi-1wt.%Sn) when compared with that of the monotectic alloy.

Introduction

Al-based alloys are extensively used for bearing components, in particular those alloyed with a soft phase such as Sn. The solid solubility limit of Sn in Al is below 0.09 wt.%Sn. In Al-Sn alloys, Sn particles spread over a continuous Al-rich matrix and the good tribological behaviour is due to the combination of the tough Al-matrix and the presence of Sn as solid lubricant [1], [2]. Designers and manufacturers are being pushed by pro-ecological demands to replace grease-lubricated with self-lubricated bearings [3]. Monotectic Al alloys such as Al-Pb, Al-Bi and Al-In are considered potential materials for advanced bearings in car engines with the immiscible self-lubricating soft element evenly distributed in the harder metal matrix [4], [5]. A low modulus of elasticity is required for bearing alloys, which is a characteristic of Al. From the aforementioned elements forming monotectic alloys, indium has the lowest modulus of elasticity followed by lead. Pb is also known to be more effective than Sn as a soft phase conferring appropriate antiscoring and antifrictional properties [6]. However, Pb is experiencing severe restrictions concerning commercial purposes due to environmental concerns, which is motivating its replacement in Al-based alloys for tribological applications with compatible soft alloying elements [7].

The length scale of the phases that constitute the microstructure of metallic alloys is known to affect significantly the resulting mechanical [8], [9], [10], [11], [12] and corrosion [13], [14] properties, in particular of Al-based alloys. The choice of Al-alloys for tribological applications demands not only suitable alloy compositions but also appropriate microstructural features, which are affected by both thermal parameters during solidification and by thermo-mechanical treatments. When intimate metallic surface contact occurs, that is, under unlubricated conditions; marginal use of lubricants; and eventual metallic contact between the moving surfaces even in the presence of lubricants, the effect of the microstructure can be significant [15], [16], [17], [18]. The morphology, continuity and distribution of the phases are important, and are generally synthesized by a length scale parameter. The effect of such parameter on the wear resistances of monotectic Al-In [19] and Al-Bi/Al-Pb [20] alloys has been demonstrated in recent studies, with the required strength provided by the matrix whereas the soft phases, having droplet morphologies, acted as a self-lubricant agent. The role of the interphase spacing was shown to be significant, but with opposite effects on the wear behaviour when considering Al-Pb/Al-In or Al-Bi alloys. For a same sliding distance, the wear volume (V) was shown to increase with the decrease in the microstructural spacing for Al/Pb-Al/In alloys, but for Al-Bi alloys V increased with the increase in spacing. The opposite behaviour has been attributed to lower hardnesses of Pb and In when compared with Bi and the apparent capability of coarser Pb/In droplets in providing more extensive and continuous film thickness during wear tests.

Despite the significant roles of microstructural features such as morphology, distribution of phases and length scale of the phases on application properties, studies on correlations between microstructure and tribological properties of multicomponent monotectic alloys are scarce in the literature. A recent study on Al-Bi-Sn alloys analysed the evolution of microstructure as a function of solidification thermal parameters and the corresponding effect on hardness [21]. The present investigation focuses on the effect of Sn addition to Al-Bi alloys of hypomonotectic, monotectic and hypermonotectic compositions on the resulting wear behaviour. Transient directional solidification experiments will be carried out under a wide range of cooling rates, thus permitting a wide spectrum of microstructural spacings (λ_G) to be obtained. Samples extracted from the directionally solidified (DS) castings, having different representative λ_G values will be subjected to wear tests. Experimental inter-relations of λ_G to both wear volume and wear rate for different sliding distances are envisaged.