Abstract
The Cr3C2-NiCr coating,NiAl coating and NiAl/Cr3C2-NiCr composite coating were prepared by plasma spraying on the cast iron roll.The plasma spraying processing parameters were optimized by the orthogonal test with the bonding strength between the coating with the substrate as an indicator,and then Cr3C2-NiCr coating and NiAl/Cr3C2-NiCr composite coating were prepared based on optimization of the optimum parameters.The surface morphology and the organizational structure were studied by SEM,EDS,XRD and so on.The Microhardness of Cr3C2-NiCr coating and NiAl/Cr3C2-NiCr composite coating were tested,and analyzed the distribution regularity by statistical methods.After thermal shock test,we analyzed the merits of thermal fatigue resistance.
Above studies,results are as follows:The factors of Cr3C2-NiCr coating and the substrate bonding strength from main to second:Power>spraying distance>primary gas flow>powder feeding rate.The bonding strength of Coating prepared by optimized process parameters is 64.33 MPa,and porosity is 4.47%.The factors of NiAl coating and the substrate bonding strength from main to second:powder feeding rate>spraying distance>primary gas flow>Power.The bonding strength of Coating prepared by optimized process parameters is 75.89MPa,and porosity is 4.37%.The factors of NiAl/Cr3C2-NiCr composite coating and the substrate bonding strength from main to second:powder feeding rate>Power>primary gas flow>spraying distance.The bonding strength of Coating prepared by optimized process parameters is 74.11MPa,and porosity is 1.33%.
The Cr3C2-NiCr coating and the substrate are well bonded,the combination between coating and substrate is mechanical bonded.There is exothermic reaction between Ni and Al during the process of plasma spraying the NiAl coating，and various kinds of intermetallics such as Al4Ni3,Al3Ni2,AlNi3 and so on are formed at the same time.Thus there is a micro-metallurgical bonding in some micro-zone between the coating and substrate,but the bonding was mainly mechanical bonded.Both in the parallel and perpendicular directions compared with substrate,the micro-hardness of Cr3C2-NiCr coating and NiAl coating has greatly dispersion.But in the parallel direction to substrate,they significantly conform to normal distribution.
Cr3C2-NiCr coating and NiAl/Cr3C2-NiCr coating have good thermal shock resistance and high thermal fatigue life which reaches higher number of macro-crack initiation and failure times.Thermal shock resistance of Cr3C2-NiCr coating is better than that of NiAl/Cr3C2-NiCr coating.Its number of macro-crack initiation and failure times is also more than NiAl/Cr3C2-NiCr coatings’under the same conditions.The coating failure in two ways:coating cracks initiation along with the bonding interface,expansion to peeling;coating cracks initiation and small pieces fall off at the interface of the coating and substrate,eventually peel off over 1/3 and failure.And the coating thermal shock failure is due to thermal shock temperature.
Key words：plasma spraying;cast iron roll;orthogonal test;microstructure and properties;microhardness;thermal shock test
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