Fly Ash Porous Ceramic/A356 Interpenetrating Composites Fabricated By Squeeze Casting And Their Properties

Coal fly ash,an abundantly available industrial waste generated during the combustion of raw coal in thermal power plants,is one of the main factors of haze sky in china.Developing high-value-added products from coal fly ash is the direct approach to promote the plants efficiently recycle the waste fly ash and decreases the air pollution.Therefore,porous mullite ceramics,which were obtained via replica method using coal fly ash as the main starting material,were used as reinforcement preforms to fabricate high-value-added ceramic/A356 interpenetrating composites?IPCs?by a three-stage schedule squeeze casting method.The entire procedure of the squeeze casting method consisted of three stages:infiltration stage,squeezing stage and maintaining stage.Different squeezing pressures?0,40,80 and 120 MPa?and pouring temperatures?700and 780??were applied during the fabricating processes of the IPCs.The effects of the pouring temperature,squeezing pressures and porous ceramic preforms on the microstructures of the IPCs were investigated.In addition,the effects of sintering temperature,solid loading of the slurry and pore size of the polyurethane sponge templates on the properties of porous ceramic preforms prepared by fly ash with different amounts of added Al₂O₃ were also investigated.To evaluate the wear resistance and corrosion resistance properties of the IPCs,the dry sliding wear tests and corrosion tests were carried out,and the wear mechanism and corrosion mechanism of the IPCs were also analyzed.The main phase of porous ceramic preforms derived from fly ash was Mullite.Using coal fly ash slurry samples supplemented with different amounts of Al₂O₃?0,23.34,38.12 and 45.76wt.%?,we fabricated mullite-based porous ceramics at different sintering temperature?1300,1400,1500 and 1600??via a dipping-polymer-replica approach,which is a popular method suitable for industrial application.As increasing of the sintering temperature,the porosities were decreased,and the skeleton densities,linear shrinkages and compressive pressures of the porous ceramics with different amounts of Al₂O₃ addition were increased.According to the SEM and XRD analysis results,the main phase of the porous ceramics sintering at 1600?was mullite,the microstructure and compressive strength of the porous ceramics was influenced dramatically by the content of Al₂O₃ addition.The compressive strength increased as the increased of Al₂O₃ addition from 0 to 38.12%,but then decreased with a further increase in Al₂O₃ content.When coal fly ash was used as a starting material,the mullite-based porous ceramic with 38.12%Al₂O₃ addition sintered at 1600?had a much higher compressive strength.The solid loading of the ceramic slurries and the pore size of the polyurethane sponge templates also influenced the properties of the porous ceramic preforms dramatically.Porous ceramic preforms were fabricated by slurries with solid loading of 50%,55%,60%and 65%,and sponge templates with pore sizes of 20,30 and 40PPI,respectively.The compressive pressure tests of the preforms were carried out.The results showed that the increased of the slurry solid loading,the compressive pressure increased;the increased of the template pore size,the compressive pressure decreased.the squeezing pressure had significant effects on the microstructure of the IPCs.As the squeezing pressure increased from 0 to 80 MPa,the primary aluminum grains became globular and small,the filling of the remaining pores of the preform struts with A356 was promoted,and the interfacial debonding gap size of the IPCs decreased.However,a further increase of the pressure?120 MPa?caused the destruction of fly-ash mullite preform.The composite fabricated by 80 MPa squeezing pressure achieved sound interfacial bonding and interpenetrating structure.The friction and wear behaviors of the IPCs were investigated by carrying out a pin-on-disc dry sliding test.Ceramic AlN was selected to be the counterface disc in the test due to its high hardness and wear resistance.The IPC pin samples were measured4.5 mm×11 mm?diameter×length?in size.The diameter of the sliding cycle on the disc was 36mm.The sliding speed was 100 n/min.Different loads?8,14 and 20 N?were applied in the tests.The results showed that the IPCs had not only better wear resistance,but also steadier and larger frication coefficient compared with the A356alloy.The wear rate of the IPCs was less than half of the one of A356 alloy at the load of 20N.The IPCs has good potential for use as a wear-resistant braking material.In addition,the wear mechanism of the IPCs was abrasion.As increased of the load,the wear rate and friction coefficient increased.As increased of the pore size?or volume friction?of the porous ceramic preform,the friction coefficient increased,and the wear rate decreased.The corrosion behaviors of the IPCs were investigated by the electrochemical corrosion tests and Immersion corrosion tests in the 3.5wt.%NaCl aqueous solution.The results showed that the bigger of the pore size and the volume friction of the porous ceramic preform,the better of the corrosion resistance of the IPCs.The corrosion surfaces and products of the samples were also analyzed by SEM and EDS.The analysis results showed that the main corrosion mechanisms of the IPCs were interfacial corrosion at interface and pit corrosion on A356 phase.The interfacial corrosion was caused by the stress?corrosion?at the interface,ant then developed to crevice gap corrosion.The pit corrosion on A356 phase was the galvanic corrosion between the the alloying elements?Fe and Cu?and Al on the surface of A356 phase.Therefore,the bigger pore size of the porous preform,the less of the interfaces and interfacial corrosions;and the more of ceramic volume fraction,the more of the A356phases and pit corrosions.