Investigation Microstructure And Properties Of Plasma Sprayed Coating On Cast Iron Roller Surface

The Cr₃C₂-NiCr coating, NiAl coating and NiAl/Cr₃C₂-NiCr composite coating wereprepared by plasma spraying on the cast iron roll. The plasma spraying processingparameters were optimized by the orthogonal test with the bonding strength between thecoating with the substrate as an indicator, and then Cr₃C₂-NiCr coating andNiAl/Cr₃C₂-NiCr composite coating were prepared based on optimization of the optimumparameters. 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-NiCrcomposite coating were tested, and analyzed the distribution regularity by statisticalmethods. 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 tosecond: Power> spraying distance> primary gas flow> powder feeding rate. The bondingstrength of Coating prepared by optimized process parameters is64.33MPa, and porosityis4.47%. The factors of NiAl coating and the substrate bonding strength from main tosecond: powder feeding rate> spraying distance> primary gas flow> Power. The bondingstrength of Coating prepared by optimized process parameters is75.89MPa, and porosity is4.37%. The factors of NiAl/Cr₃C₂-NiCr composite coating and the substrate bondingstrength from main to second: powder feeding rate> Power> primary gas flow> sprayingdistance. The bonding strength of Coating prepared by optimized process parameters is74.11MPa, and porosity is1.33%.The Cr₃C₂-NiCr coating and the substrate are well bonded, the combination betweencoating and substrate is mechanical bonded. There is exothermic reaction between Ni andAl during the process of plasma spraying the NiAl coating，and various kinds ofintermetallics such as Al₄Ni₃,Al₃Ni₂,AlNi₃and so on are formed at the same time. Thusthere is a micro-metallurgical bonding in some micro-zone between the coating andsubstrate, but the bonding was mainly mechanical bonded. Both in the parallel andperpendicular directions compared with substrate, the micro-hardness of Cr₃C₂-NiCrcoating 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 andfailure times. Thermal shock resistance of Cr₃C₂-NiCr coating is better than that ofNiAl/Cr₃C₂-NiCr coating. Its number of macro-crack initiation and failure times is alsomore than NiAl/Cr₃C₂-NiCr coatings’ under the same conditions. The coating failure in twoways: 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 andsubstrate, eventually peel off over1/3and failure. And the coating thermal shock failure isdue to thermal shock temperature.