1.1        INTRODUCTION Hypereutectic Al–Si alloys areattractive materials for numerous commercial applications such as liner-lesscylinder blocks, cylinder head, piston etc. owing to their low density, lowthermal expansion coefficient and high wear resistance. Theyare perfect piston materials and are ideal for tribological applications 58–60. However, theyhave low ductility and poor machining properties due to the formation offaceted and blocky primary Si during conventional processing, which greatlyconfine their application in potential fields.

It is necessary to refineprimary silicon for improving the properties60.Several researches have beencarried out for studying the wear behaviour of different Al-Si alloys (e.g.hypoeutectic, eutectic and hypereutectic).For example, it has been reportedthat the wear resistance of Al-Si alloys increased up to eutectic composition61–64.

The minimum wear at the eutecticpoint was the result of balance between two opposing factors, the increasedhardness and decreased aluminium matrix 65. Unstable subsurface caused by theformation of shear bands and cracks was considered to be responsible forincreased wear in hypereutectic alloys 66. In contrast, the wear resistanceof Al-Si alloys was found to increase constantly with the increase of thesilicon content under several experimental conditions66–68. Extra silicon content in alloysamplified the load-bearing capacity of the material, decreased the matrixdeformation  and in turn improved thewear resistance 67,68. Sand casted hypereutectic Al-18Si(wt.

%) is widely used as anti-wear component of engines ,in the automotiveindustry, and its wear mechanism has been well analysed and developed 6970.Therefore, this composition waschosen in this work to analyze the dry wear and friction characteristics of Nirich piston alloys.  This study aims toinvestigate the effect of Ni on the mechanical and tribological characteristicsofhypereutectic Al-18Si-5Cu-(2-2.6wt.%)Ni piston alloys and compare with thealready obtained wear result of eutectic Al-12.6Si-5Cu-2.3Ni piston alloydiscussed in section 6.3.6.

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