Interface Characteristics Of SiC_p/Al Composite And Its Corrosion Behavior In Cl Ion Environment

Due to its unique properties such as high specific strength,specific stiffness,low density,thermal expansion coefficient,good thermal conductivity,SiC_p/Al composites as one of the preferred material for novel electronic packaging material,have been extensively applied in field of aerospace,shipbuilding and electronic component industries.In views of the using environment in marine climate,the corrosion resistance of SiC_p/Al composites is one of the important factors to satisfy its practical value.In this paper,taking SiC_p/Al composites prepared by using a pressureless infiltration technique as research object,surface modification of SiC_p particle such as oxidation,nickel plating and copper plating was carried out.The influences of surface modification of SiC_p particle,adding of Mg/Cu in Al matrix and SiC_p particle size on microstructure related to interface characteristics of the composites and corrosion behavior of the composites were investigated.The effects of Cl^- ion and pH on the corrosion behavior of the composites were studied.In order to reveal the pitting mechanism of SiC_p/Al composites in Cl^- ion-containing solutions,the potential and Cl^- ion concentration distribution on micro-area surface of the composites in 3.5% NaCl solution were in-situ examined by the scanning micro-reference electrode?SMRE? technique.In this thesis,the effects of SiC_p particle conditions on interface characteristics and corrosion behavior of the composites have been systematically studied.The results show that when SiC_p particles size is too large,infiltration dead-zone will occur,while infiltration become difficult because of increasing of specific surface area,when SiC_p particles size is too small.The feasible SiC_p particles size is of 53?m as viewed from obtaining ideal interface bonding and dense microstructure of the composites.At the early stages,decreasing SiC_p particles size leads to increase of pitting active points and corrosion current I_corr,decrease of corrosion potential E_corr of the composites,however,corrosion rate of the composites decrease on the contrary at the middle-later stages of corrosion because of the reason that presence of SiC_p particles make matrix channels get narrower or become completely closed.The oxidation temperature of 1100? and the oxidation time of 10h are preferable for pre-oxidation of SiC.Nickel coating with the thickness of 3?m and copper coating with the thickness of 4?m are continuous,uniform and perfect.By pre-oxidation,nickel plating and copper plating of SiC particles,corrosion resistance of SiC_p/Al composites are increased,due to inhibition of the interface phase Al₄C₃ of the composites,improvement of wettability between SiC_p and Al matrix,and reduction of interface defects of the composites,despite of the formation of Al3Ni and CuAl₂ phases at the interface.Galvanic effect between aluminum matrix and Al₃Ni/CuAl₂ phases will occur,which is unfavorable to corrosion resistance of the composite,but the adverse effect is lower than the beneficial effect induced by the SiC_p/Al inhibition of Al₄C₃ phase and the decrease of interface defects.The addition of alloying element to Al matrix has an important effect on the interface of SiC_p/Al composites.when the Mg element is added to Al matrix,the fluidity of molten aluminum and wettability of matrix and SiC particles are increased,resulting in obvious reduction of defects on the interface of the composites with more than 4% Mg.The interface phase MgA1₂O₄,Mg₂Si and Si as a result of the interfacial reaction occur at interface of the composites with the addition of Mg element.Heat treatment of T1,T4,T6 and annealing promotes the precipitation of interface phase MgA1₂O₄,Mg₂Si and Si.The precipitates contents?f?of composites are correlated with heat treatment,and present the order of f_T1<f_T4<f_T6<f_annealing.After adding Mg element to the matrix,the corrosion tendency and corrosion rate of the composites increase at early stage,while corrosion rate of the composites decrease due to hindering matrix corrosion channels by the interface reaction phase MgA1₂O₄,Mg₂Si and Si.The addition of Cu inhibits the formation of Al₄C₃ phase of the composites in some extent.When Al matrix contains 2%-6% Cu,CuAl₂ phases at the interface and grain boundary of Al matrix occur,and the precipitation contents increase with the increase of Cu addition.When Cu addition content is lower than 4%,the fluidity of molten Al-Cu alloy decreases,which is unfavorable to interface performance of the composites.When the Cu addition content of the matrix is about 6%,the wettability between SiC_p and Al significantly increases,and interface performance of the composites is improved.When Cu element is added to Al matrix,galvanic effect will appear between interface phase CuAl₂ as the cathode and Al matrix,resulting in increasing corrosion tendency.When addition content of Cu element reaches 6%,corrosion rate of the composites will decrease,thanking to the inhibition of interface reaction Al₄C₃ and of large area hydrolysis of the composites.The effects of Cl^- ion and pH on the corrosion behavior of SiC_p/Al composites were testified.With increasing Cl^- concentration,corrosion current increases,and corrosion potential decrease,which lead to increase of the local activity of the composites.When Cl^- concentration exceeds 7%,passivation of the composites is not obvious,the stability of passivation film is poor,and local active sites on the surface appear mutual connection.Cl^- is involved in the dissolving process of passivation film.With increasing Cl^- concentration,the pitting potential and passivation potential decrease,leading to the decrease of growth rate of passivation films and increase of corrosion rate.The Nyquist curves of the composites appear a unique capacitance arcs type in 0⁶% NaCl aqueous solution,and a mixed type including a capacitance arcs and Warburg type impedance in higher NaCl aqueous solution concentration of 6%.High pH value will promote the pitting of the composites and low pH value will promote the uniform corrosion of the composite in the Cl ion environment.After micro-arc oxidation?MAO? on SiC_p/Al composites,the MAO coating with rough surface exists subcircular,volcano-type cavity,consisting of?-Al₂O₃,?-Al₂O₃ and mullite?3Al₂O₃.2SiO₂?.After electroless Ni-P plating on SiC_p/Al composites,Ni-P coating exhibits dense,uniform,and bonds firmly with matrix.Both micro-arc oxidation coating and Ni-P coating can prevent the corrosion of the composites in Cl^- solution,improving the corrosion resistance of the composites.The corrosion potential?-0.58V? of ceramic coatings prepared for 30min was higher than that of substrate?-1.05V?,and the corrosion current density(7.94×10^-7A/cm²) had decreased 2 orders than substrate(4.31×10^-5 A/cm²).The corrosion potential?-0.75V?of Ni-P coatings prepared for 30min was higher than that of substrate?-1.03V?,and the corrosion current density(3.98×10^-7A/cm²) had decreased 2 orders than substrate(2.51×10^-5A/?m²).In order to expose pitting mechanism of the composites in Cl^- ion containing solutions,the potential-distribution and Cl^- ion concentration distribution of the surface of SiC_p/Al composites in an aqueous solution of 3.5 wt.% NaCl was in-situ investigated.A slight potential drop on the localized region of the composite surface was observed due to the instability of the passive film at early stage.At middle stage,the height of the potential drops increases and the site of potential drops changes.At latest stage,the stable potential drops will be appeared,and the height and the size of the potential drop increase with time.The potential of Z direction on the micro-surface is a function of the distance.For the active point,with the decrease of the surface distance,the potential decreases slowly at first and then decreases rapidly,which presents a "U" shape.For the non-active point,the potential decreases monotonously.The non-uniform adsorption of Cl^- ions on passive film with a much higher concentration at the local sites results in segregation of Cl^- ions at these sites.With increasing the immersion time,the accumulation peak of Cl^- ions concentration will remain or disappear.The Cl^- segregation effect at the repaired metastable pitting disappears,while the Cl^- ions at the stable pitting continuously segregates.Formation mechanisms of stable pits of SiC_p/Al composites consist of four stages:the formation of passive film,the breakdown of passive film due to Al₄C₃ hydrolysis,formation of metastable pits due to non-uniform adsorption of Cl^- ions on the breakdown passive film resulting in locally thinned passive film through local dissolution,metastable pit is either re-repaired or becomes stable pit.Some tiny metastable pit may prefer to be re-passivated via the formation of insoluble compounds of Al?OH?₂Cl and Al?OH?Cl₂ that cover across the metastable pit wall to prevent the adsorption of chloride ion on the metastable pit sites.large-scale metastable pits may undergo propagation to change into stable pits because of the formation of soluble salt films of AlCl₃.