Development and characterization of in-situ synthesized polymer-derived ceramic reinforced aluminum matrix composites

Abstract

Aluminum matrix composites (AMC) market propelled by increasing demand across key sectors such as aerospace and automotive is witnessing robust growth. This increase in demand is caused by the unique properties of AMCs such as high strength to weight ratio, low coefficient of thermal expansion, higher modulus of elasticity, tensile strength, creep resistance, fatigue resistance, cost-effectiveness, etc., which align with the growing emphasis on fuel efficiency and sustainability. The focus of the present work is to synthesize aluminum matrix composites having a uniformly distributed polymer-derived ceramic reinforcement with a good matrix-reinforcement interface. Two different polymer precursors were explored for in-situ synthesis of polymer-derived ceramics (PDCs) during the stir casting process to synthesize AMCs of interest. Commercially available allyl hydrido polycarbosilane (SMP10) and polyvinylsilazane (Durazane 1800) were cross-linked at 300 °C followed by in-situ pyrolysis at 800 °C in molten aluminum under nitrogen by stir casting route to synthesize SiC-reinforced AMCs and SiCN-reinforced AMCs, respectively, with different fractions of ceramic reinforcements. Structural characterization of developed AMCs were performed using FTIR, XRD, optical microscopy, FESEM and Raman spectroscopy. Mechanical properties were evaluated and contributing strengthening mechanisms of the synthesized composites were elucidated. In the case of SiC-reinforced AMCs, SiC particles (< 10 m) were found to be uniformly distributed in the aluminum matrix. The composites demonstrated significant improvements in hardness (by ~58%), ultimate tensile strength (by ~45%) and compressive strength (by ~58%) for 7 wt% in-situ formed SiC in aluminum matrix compared to pure as-cast aluminum. However, a loss of ductility was observed due to the increased defects in the composites which assist in the crack nucleation process. For the SiCN-reinforced AMCs, amorphous SiCN ceramic phase was segregated at grain boundaries. Enhancement in the investigated mechanical properties was found for the composites compared to pure as-cast aluminum, for instance, aluminum matrix composite reinforced with 4 wt% SiCN showed an improvement in hardness (by ~78%), UTS (by ~120%) and compressive strength (by ~172%). SiCN reinforcement phase segregation itself causes an increase in brittleness with an increase in the amount of reinforcement, thereby changing the mode of fracture to an intergranular type. Interaction of the polymers with aluminum was also investigated for understanding the interfacial (reinforcement/matrix) reaction in the temperature range of 600-900 °C. Allyl hydrido polycarbosilane and polyvinylsilazane mixed with aluminum revealed that aluminum undergoes a displacement reaction with the polymer-derived ceramics to form Al4C3 and Si at and above 700 °C. The reaction at the Al-ceramic interface leads to increased wetting of ceramic phases with aluminum matrix in the synthesized composites.