Effect of solidification cooling rate on microstructure and tribology characteristics of Zn–4Si alloy

The main objective of this work was to modify the microstructure and enhance the tribological properties of a new Zn–4Si alloy through a high solidification cooling rate (SCR). According to the results, by increasing the SCR from 2.0 to 59.5°C/s the average size of primary Si particles and that of the grains reduced from 76.1 and 3780 μm to less than about 14.6 and 460 μm, respectively. Augmenting the SCR also enhanced the microstructural homogeneity, decreased the porosity content (by 50%), and increased the matrix hardness (by 36%). These microstructural changes enhanced the tribological behavior. For instance, under the applied pressure of 0.5 MPa, an increase in the SCR from 2.0 to 59.5°C/s decreased the wear rate and the average friction coefficient of the alloy by 57% and 23%, respectively. The wear mechanism was also changed from the severe delamination, adhesion, and abrasion in the slowly-cooled alloy to the mild tribolayer delamination/abrasion in the high-cooling-rate-solidified sample.