| Austenitic 316L stainless steel is an attractive industrial material combining outstanding corrosion resistance,ductility and biocompatility,with promising structural applications and biomedical uses.However,316L has low strength and wear resistance,limiting its high-performance applicability.Adding ceramic particles to steel matrices,thereby forming steel-matrix composites,can overcome these problems,improving the performance and the applicability of 316L.SiC and Si3N4 particles are commonly used with stainless steel matrices owing to their high melting temperature,high hardness and superior wear resistance.Powder metallurgy(PM)is an attractive forming technique for preparing metal matrix composites due to the ability to produce complex net or near net-shaped parts,which can eliminate the operational and capital costs associated with complex machining operation.In this work,the PM method has been employed to prepare the aforementioned particles reinforced 316L composites,and SiC was added in the form of PCS,which is an excellent polymeric precursor of SiC nanoparticles.The main research contents and conclusions are listed as follows:(1)A new in-situ powder metallurgy route for preparing submicron particles reinforced 316L matrix composites from the polymer route has been developed in this work.Firstly,PCS was dissolved in n-hexane,followed by the addition of 316L powders.Then,PCS coated 316L powders were obtain by the evaporation of the n-hexane while stirring on a hotplate.Finally,PCS/316L composites were fabricated using the resultant composite powders through a SPS route.The in situ reinforcements were formed by solid-solid diffusion reaction:PCS decomposed within the 316L into SiC when heated in the 600-800 ? range;SiC dissociated into its constituents Si and C at the sintering temperature;Si dissolved in 316L and formed liquid phase,while C reacted with Fe and Cr and fromed Cr rich carbides M7C3.The SEM and TEM results indicated that submicro in-situ reinforcements were distributed at the grain boundaries and the particle-metal interfaces were well bonded after sintering at 1050 ? for 5 min.In the composite,fine ceramic particles pin the grain boundaries,preventing grain growth resulting in a refined microstructure.Moreover,the addition of PCS enhanced the sintering of 316L stainless steel powders,and led to a higher densification.(2)The effect of PCS addition on the hardness,mechanical properties andwear resistance of composites was investigated,and corresponding strengthening mechanism was also discussed.In general,by introducing PCS to 316L,the hardness,tensile strength and wear resistance of the composites were remarkably improved owing to the combined effects of grain refinement and grain-boundary strengthening.Composites with 3wt%PCS addition showed the best overall performance with a mirohardness value of 341 ± 15 HV0.1,yield strength of 526±7 MPa,ultimate tensile strength of 898±9 MPa,strain of 6.13%and wear rate of 3.966±10-5 mm3/Nm.Compared to that of the unreinforced 316L(mirohardness of 195±6 HVo.1,yield strength of 352±5 MPa,wear rate of 2.912×10-4 mm3/Nm),the hardness increased by 75%,yield strength by 50%,wear rate decreased by seven times.(3)Furthermore,powder injection molding(PIM)has been applied to prepare the PCS/316L composites by utilizing PCS as part of the backbone component of a water-soluble binder system.The composition of the water-soluble binder was 80:15:5wt%PEG:PVB:SA.A powder loading of 55vol%has been selected for PIM and no visible surface defects was found on the molded.parts after injection moulding.A combination of water and thermal debinding was adopted to remove the binder system from the mold parts.After pre-sintering at 900 ?,all samples maintained their shape with no visible distortion and had adequate strength for handing.Sintering at 1250 ? with 5wt%PCS addition(relative to the binder)was identified to be the optimum condition for PIM PCS/316L,which produced samples with a relative density of 96.5%,tensile strength of 530.79 Mpa,elongation of 19.53%.Further increase in the sintering temperature to 1350 ? or PCS content to 10wt%increased the density and tensile strength,but the elongation was below 5%,which was well below the criterion required for PIM 316L.(4)The sintering behavior of PIM Si3N4/316L was studied at different sintering temperatures range from 1150 to 1350 ? with varying Si3N4.Results showed that Si3N4 dissociated during sintering and Si formed liquid phase with Fe and caused rapid densification.Nearly full densification(99.1%)was achieved with 5wt%Si3N4 addition after sintering at 1350 ? for 1 h.At the same temperature,10wt%Si3N4 addition caused slumping of the samples.The mechanical properties and wear resistance was also measured.It was found that a maximum ultimate tensile strength of 496.81 MPa was obtained with 5%Si3N4 addition after sintering at 1350 ? for 1 h.The sliding wear test showed that the wear loss of the composites decreased with increasing Si3N4 content from 7.83×10-3mm3/Nm for PIM 316L to 1.36×10-3 mm3/Nm for 10wt%Si3N4 addition,which showed a decrease about 80%. |
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