| Ceramic particle reinforced iron matrix composites(PR-IMC)with the advantages of high hardness,high strength and excellent wear resistance havea great prospect in a wide range of applications under the conditions of elevated temperature,high speed,high wearing and etc.However,there are some problems limited the applications of PRIMC,such as a large amount of interface defects between particles and matrix during sintering and particle agglomeration at a high volume fraction which can greatly degrade the properties.Therefore,metal coating on the surface of ceramic particles through electroless plating technology was studied to reduce interface defectsand and hybrid mixed-type ceramic particles as reinforcements was employed to reduce defects and internal stress.The PR-MICs were fabricated by the dynamic temperature resistant hot press technology,which could reduce interfical reaction significantly.Taking the mixed-size,mixed-type of particles as hybrid reinforcement,the effects of monolithic and hybrid reinforcements with and without copper coating on mechanical properties of the PR-IMCs were investigated under different volume fraction of particles.Moreover,microstructure analysis and simulation study were also used to discuss the strengthening mechanisms in different conditions.The results showed that when the molar ratio of copper salt and the sodium hypophosphite as reducing agent was 1:4,the temperature was 65 ? and pH value was 9.0-11.0,the deposition rate of copper plating became faster with a smaller particle size and it could be controlled by the amount of complexing agent addition in the plating bath.When the controlled deposition rate of copper was 0.1g/min,the coating layer on the particle surface with acceptable quality was obtained.However,particle type had no significant effect on the the plating process.Mechanical properties of the SiCp/Fe,TiNp/Fe and TiCp/Fe composites containing reinforcing particulates with and without copper coating respectively were measured comparatively.The results showed that tensile strength of all three composites reinforcedby copper-coated particles were superior significantly to their counterparts.The highest improvement of tensile strength induced by copper coating was observed for the TiNp/Fe composite,up to 19%.It is clear thatthe defects at the interfaces should be a determining factor affecting the tensile strength of the composite.Therefore,copper coating can effectively improve low performance of composites.When the volume fraction of TiC was 10%,copper coating improved the tensile strength by 6.4%,while the volume fraction of 20%,the tensile strength increased by 14.5%after copper coating.A high particlefraction produces many interfacial defects that deterioratethe composite strength,but the copper coverage aroundthe particles could effectively eliminate the interfacial defects.The larger content of particles with copper coating in the composite was,the higher improvement in mechanical properties may be expected.When the volume fraction of particle was less than 20%,the tensile strength of SiCp/Fe composite was the highest among the three composites.The effect of hybrid mixed-type particles with and without copper coating on mechanical properties of iron matrix composites was investigated.The results showed that mechanical properties of the composite reinforced by hybrid particles without copper coating was between corresponding monolithic composite and closer to the stronger one.The tensile strength of the composite reinforced by hybrid particles with copper coating was higher than that of either of the corresponding monolithic composites.With the increase of the volume fraction,the hybrid strengthening effect became greater.When the volume fraction of hybrid(TiC+TiN)particles was 20%,with the volume ratio of 1:1 in each particle,the tensile strength of the(TiC+TiN)p/Fe composite was 50%and 31%higher than that of TiNp/Fe and TiCp/Fe,respectively.The difference of elastic modulus of hybrid particles caused an extra extensive stressgradient in the matrix,which improved the load transferring capacityfrom the matrix to particles and increased thecomposite strength.The elongation of the composite reinforced by hybrid particles with copper coating was higher than that of either of the corresponding monolithic composites,for along-range stress field of hybrid particles could compensateeach other and made slip lines plane connected easily,which led to a better plasticity.Hybrid particulate reinforcement strengthening effect hadno evident effect on hardness of composites,for the hardness of composite reinforced by(SiC+TiC)and(SiC+TiN)were both between their corresponding monolithic composite before and after copper coating,which indicated that hardness of composite was insensible to interface defects.The effect of mixed-size of hybrid mixed-type particles with copper coating on the mechanical properties of the PR-IMCs was studied under different volume fraction of the particles.The results showed that when the SiC particle size increased from 5?m to 15?m,mixed the TiN particles with the size of 15?m and the volume ratio of 1:1,the tensile strengthand elongation of composite firstly increased and then decreased,while the hardness of composites had nearly no changes.lt indicated that the mixture of particle sizewas also a factor to improve mechanical propertiesof the PR-IMCs.With the volume fraction of reinforcements increasing from 10%to20%,the tensile strength of the composites gradually decreased,while the hardness firstly increased and then decreased.When the volume fraction of the reinforcements increased,the probability of particle agglomeration increased,resulted in unevenness of particle distribution,which affected the strength of the composite significantly.Furthermore,with increasing volume fraction of reinforcements,the microstructural defects increased in the composite,which also reduced the strength of the composites.The wear resistance of the iron matrix composites reinforced by monolithic particles(SiC,TiC,TiN)and their hybrid particles mixture(TiC+TiN,SiC+TiN,SiC+TiC)underhigh volume fractionof 25%,30%and 35%were investigated.The results showed that thewear resistance of the composite had no direct correspondence to the hardness and tensile strength.When the volume fraction of the particles was 25%,the wear resistance of the TiNp/Fe composite was the best,the tensile strength ofthe TiCp/Fe was the highest.Althoughthe hardness of the(SiC+TiN)p/Fe was similart to that of the TiNp/Fe,the wear resistance of the former was much worse than that of the latter one.Unlike conventional materials,wear resistance of the PR-MICs was more sensitive to its fatigue property.Using monolithic particle as reinforcement,the wear resistance of the TiNp/Fe composite was the best among the three monolithic composites,that of the TiCp/Fe was second and that of the SiCp/Fe was the worst.With the increase of volume fraction of particle,the wear resistance of the TiNp/Fe and TiCp/Fe composites had no significantly change,whereas wear resistance of the SiCp/Fe composite sharply decreased.Using hybrid particle as reinforcement,the wear resistance of(SiC+TiC)p/Fe and(SiC+TiN)p/Fe composites were between that of the two corresponding monolithic composites,for the composites containing SiC particles could lead to a fatigue wear.However,wear resistance of the(TiC+TiN)p/Fe composite was better than that of the composites reinforced by corresponding monolithic particles,the(TiC+TiN)p/Fe composite exhibited a pursuit value of hybrid strengthening effect.The microstructure of worn surfaces showed that the wear mechanism of the(TiC+TiN)p/Fe composite was abrasive wear,in contrast to that of the(SiC+TiN)p/Fe and(SiC+TiC)p/Fe,which combined fatigue wear with abrasive wear.Molecular dynamic method was used to simulatethe stress-strain curves of SiC,TiN,and TiC particles and their types mixture reinforced iron matrix composites,aiming to explore hybrid particulate reinforcement strengthening mechanism.The results showed that the SiC particle had the best strengthening effect,the TiC had the second and the TiN had the worst,which was in a good agreement with the experimental observation.The stress of the(TiN+TiC)p/Fe composites was both higher than that of the TiNp/Fe and TiCp/Fe.When the applied strain was 19.5%,the failure mode in the TiNp/Fe composite was the fracture of the particles,but that in the TiCp/Fe was the fracture of the matrix at the interface.Therefore,it could be regarded that the hybrid strengthening effect maybe only occur when failure mode of the related monolithic type of particles reinforced composite(particle fracture and matrix fracture near the interface between the particle and matrix)was different,because part of the driving force of the local crack formation would be offset by a huge difference of stress field around different types of particles at microscale. 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