| Particle reinforced iron matrix composites (PRMMC) have been attracted great attention by researchers for their high strength, high hardness, high stiffness and excellent wear resistant ability, which have a wide range of potential applications under the conditions of elevated temperatures, high speed and high wearing. Electroless metal plating technology was used to coat nickel and copper on the SiC particles and the optimum processing has been investigated to obtain a well coverage coating layer. Using metal-coated SiC particles as reinforcements, the SiCp/Fe composites were manufactured by using the dynamic temperature resistant hot press technology (DTRHP) and the effect of metal coating on their mechanical properties has been studied. A high-energy ball milling technology was used to avoid the reinforcing particles clustering and improve the uniformity of SiC particle distribution in the composites. The effect of the high-energy ball milling technology combined with the metal coating processing together on the properties of SiCp/Fe composites was further investigated, associated with different volume fraction, different particle size and size mixture of the SiC reinforcements.The conventional electroless nickel plating process was adapted to perform nickel coating on SiC particle surface. The effect of nickel coating on mechanical properties of the composites was studied when the volume fraction of SiC particles was 10% and 15% and the particle size was 21μm,45μm and 85μm, respectively. The results show that when the molar ratio of the nickel salt versus the reducing agent (sodium hypophosphite) was 1:3, the reduction of nickel salt approached near 100%, with the temperature of 91℃～93℃ and pH value of 4.6～5.0. When the particle size was 45 μm and the volume fraction was 10%, the tensile strength of the nickel-coated SiCp/Fe composites was mostly increased by 22.9% compared with that of uncoated one. Meanwhile, the final elongation is also improved significantly. For example, it increased up to 30.0% with the particle size of 21μm and the volume fraction of 15%.A further study about the process of the electroless copper coating on SiC particle surface was carried out. The effect of copper coating on mechanical properties of the composites was investigated, associated with the volume fraction of SiC particles of 10% and 20% and the particle size of 21μm, respectively. The results show that when the molar ratio of the copper salt and the reducing agent (sodium hypophosphite or formaldehyde) was 1:4 or 1:2, the reduction of copper salt approached near 100%, with the temperature of 65℃ and pH value of 9.0-11.0. The sodium hypophosphite can result in a better coating surface morphology than the formaldehyde but it requires a double overdose to obtain the thickness. It is also found that the copper coating layer can be better smooth and coverage over the SiC particles if a mixed complexing agent of sodium citrate and DL-malic was used with a proper mole ratio of 2:1. Compared with mechanical properties of the composites with and without copper coating, it was shown that the copper coating improved the tensile strength by 12.6% and the final elongation by 18.1% with 20vol.% SiC, compared with that of 17.3% and 11.5% by nickel coating, respectively. Thus the copper coating has a same effect on the tensile strength improvement as a similar nickel coating process. But it has a significantly better effect on improvement of the final elongation than nickel coating, because it could cover the SiC particle surfaces better and reduce interfacial defects more effectively, resulted in a better plasticity behavior. So it is suggested that using copper coating treatment substitute the conventional and more expensive nickel coating treatment.Effects of the high-energy ball milling process and copper coating on mechanical properties of SiCp/Fe composites were investigated. The results show that SiC particles could be well distributed in iron powder when the ball-to-powder ratio was 3:1, the rotational speed was 200r/min and the milling time was 3 hr in the high-energy ball milling process. Compared with the conventional powder blending method, the high-energy ball milling process improved the tensile strength of 19.7% and the final elongation of 37.9% when the composites reinforced by the uncoated SiC particles with the volume fraction of 30% and the particle size of 21 μm. The tensile strength and final elongation was also improved by 7.2% and 3.8% after copper coating. The high-energy ball milling process can improve the uniformity of the particle distribution inside and the copper coating can reduce the interfacial defects, so the improvement of mechanical properties of the composites comes from both contributions. For the 30vol.% copper-coated SiCp/Fe composites, the tensile strength was increased by 7.2% from improving the uniformity of particle distribution and increased by 12.5% from reducing interfacial defects. It indicates that the effect of reducing the interfacial defects on the properties of the composites is more evident.Effects of volume fraction, particle size of reinforcing SiC particles on the mechanical properties of the SiC/Fe composites made by using the high-energy ball milling and copper coating processes were further studied. When the volume fraction of SiC particles increasing from 5%,10%,15% to 20% with a constant particle size of 21 μm, the tensile strength, relative density and hardness of the composites firstly increased and then decreased, whereas the final elongation decreased monotonically. The improvement of the composite properties becomes more significantly with increasing volume fraction after copper coating. When the particle size increasing from 3.5μm、10μm、21μm to 45μm with the volume fraction of 20%, the tensile strength, final elongation, relative density and hardness of the composites increased at first and then decreased. The improvement of the tensile strength and final elongation becomes more significantly with decreasing particle size by copper coating, whereas the improvement on the relative density and hardness is very small. Microstructure analysis reveals that the clustering of SiC particle becomes severer with increasing its volume fraction and the coating effect becomes more evident due to reducing the interfacial defects and preventing the direct contact of particles. Effect on the mechanical property of the composites reinforced by a mixture of copper-coated SiC particles with two sizes was studied. The results show that the mechanical property of the composites reinforced by the mixture of SiC particles of 10μm and 21μm with each half of the volume fraction of 30% were better than that of the corresponding composites reinforced by the individual-sized SiC particles. Compared with that reinforced by 10μm and 21μm SiC particles individually, the improvement of tensile strength of the mixture-sized composites was 4.9% and 5.6% and that of final elongation was 11.9% and 19.0%, respectively. Nevertheless, the wear loss of the composites reinforced by the mixture of SiC particles of 21μm and 45μm with 20vol.% was 1.6mg under the friction wearing condition, whereas that of the composites reinforced by 21μm and 45 μm SiC particles individually was 6.3mg and 5.1mg, respectively. Moreover, microstructure analysis indicates that the smaller-sized SiC particles intend to improve the strength of the matrix and the larger-sized SiC particles carry and transfer more load so that the mixture-sized particle reinforcements can improve the mechanical properties of the composites more effectively. |
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