| Micro arc oxidation(MAO)is an important surface modification process that is used to coat valve metals to improve corrosion and wear resistance as well as to create thermal and electrical barriers.However,there are some limitations with respect to the equipment capabilities,efficiency,and economy in the treatment of large workpieces and field repairs.Based on this fact,a modification of the MAO process,termed scanning micro arc oxidation(SMAO),is presented.In the present work,the preparation process,discharge characteristics,growth mechanism and service performance of SMAO ceramic coating were studied systematicallyBased on the working principle and technological process of the brush plating technology,SMAO equipment and process method for controlling ceramic coating growth trajectory were developed.The effect of key process parameters such as:inter-electrode gap,electrode orientation,scanning speed and scanning passes were investigated in order to reveal the influence of process parameters on coating characteristics.Results show that a)a shorter inter-electrode gap tends to result in higher coating forming efficiency;b)the coating forming efficiency is highest when electrode orientation is 0°;c)for ceramic coating thickness less than 40?m,single pass mode is more efficient whereas for ceramic coating thickness greater than 40?m,multiple passes tend to be more efficient.The influence of process parameters on the discharge characteristics of A356 alloy SMAO was studied,and SMAO composite discharge model of A356 was established.Results show that shortening inter-electrode gap will effectively reduce the breakdown voltage,and decreasing scanning speed will increase the energy of single discharge spark.The basic conditions for occurrence of SMAO A-type discharge is that voltage reaches the critical breakdown voltage,the basic conditions of B-type discharge occurs as the energy of single discharge spark and voltage reaches a critical value at same time.The analysis of evolution mechanism of SiC particles in the SMAO process confirmed that the D-type discharge occurred in ceramic coating during the SMAO process.Based on the characteristics of surface morphology and microstructure of SMAO ceramic coating,a growth model of SMAO ceramic coating was established.In the outer layer of SMAO ceramic coating,there is a discontinuous "dense" remelting layer.The growth of the ceramic coating is the result of multiple cycles consisting of:coating formation?in situ growth of coating and surface layer remelting?coating formation.The composition and formation mechanism of SMAO ceramic coating are obviously different from that of fixed nozzle spraying mode treatment and conventional MAO treatment.Analysis of the service performance of SMAO ceramic coating on A356 Alloy show that ceramic coating micro-hardness is about 1000?1400 HV0.1,and the strength of coating/substrate is up to 80 MPa.Electrochemical analysis show that the corrosion current density of the ceramic coating is 4?5 orders less than un-coated A3 5 6 alloy such that SMAO ceramic coating has excellent corrosion resistance.Under dry friction condition,SMAO ceramic coating has good tribological properties.When the coating thickness is more than 50?m the wear mechanism is micro mechanical polishing of brittle fracture whereas when the coating thickness is less than 20?m wear mechanism,abrasive wear due to ceramic coating extrusion occurs.Scanning micro arc oxidation technology is the supplement of traditional micro arc oxidation technology.The research work of this paper has important theoretical significance and engineering application value for the popularization and application of SMAO technology. |
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