Study On In Situ Synthesized Fe-based Borides Reinforced Coating By PTA

Fe-Ti, Fe–B, B4C, Ti and Zr powders were used as raw materials and two powdersystems of Fe-Ti+B4C, Ti+B4C, Ti+Fe-B and Zr+B4C, Zr+Fe-B were designed in thisdissertation. Thus TiB2/Fe and ZrB2/Fe cladding coatings were respectively in situsynthesized on Q235steel substrate by Plasma Transferred Arc (PTA) cladding. Theoptimal powder formulation and cladding procedures for preparing TiB2/Fe andZrB2/Fe cladding coatings were determined after a large number of experiments. Thephases and microstructures of the cladding specimens were analyzed by X-raydiffraction, optical microscope, scanning electron microscopy and energy dispersivespectrum. And the properties of the cladding coatings were carried out bymicrohardness, wear resistance and high temperature oxidation resistance tests.The results show that TiB2/Fe and ZrB2/Fe cladding coatings are successfullyprepared with PTA powder cladding. Ti+B4C and Zr+B4C powder systems are fittestto precursor powders for the cladding coatings. It is better to control the dilution ratesto about65%for cladding coatings with good forming and excellent performances.From the top to the bottom of the cladding coatings, the contents of TiB2and ZrB2decrease gradually and the morphologies of reinforcement phases transit from coarseacicular to fine block shapes, which leading to gradual microhardness decreasing withgradient distribution. Main defects in Fe-based cladding coatings reinforced by in situsynthesized TiB2and ZrB2are incomplete fusion, surface imperfection, cracks,composition segregation, gas cavities and inclusions.Within Fe-Ti-B4C and Fe-Zr-B4C reaction systems, it was found by thermodynamicanalysis that the metallurgical reaction between Me (Ti, Zr) and B4C is carried outwith the highest driving force. Also, the reaction resultants of TiB2and ZrB2phaseshave the highest chemical stability. There are several paths for the forming modes ofTiB2and ZrB2reinforcement phases inside the PTA molten pool. And they havesignificant influence on the morphology characterization of TiB2and ZrB2phasesinside coatings. The crystallization mode of the cladding coatings is that the upperpart and the bottom of the molten pool solidify first and the middle solidifiesafterwards. Thus in situ synthesized reinforcement phases of TiB2and ZrB2exhibit faceted nature. The faster cooling by PTA cladding does not lead to transformationfrom smooth to rough growth interface for the reinforcement phases.It was shown by dry sliding wear test at room temperature that the wear resistanceof in situ synthesized TiB2/Fe cladding coatings was16–25times higher than that ofthe Q235steel substrate. Their wear mechanism was a comprehensive action ofmicrocutting, surface flaking, adhesive wear, slight abrasion and surface fatigue wear.But the wear resistance of in situ synthesized ZrB2/Fe cladding coatings was only8–10times higher than that of the Q235steel substrate and the wear mechanism wassuperficial flaking caused by fatigue stress and slight abrasive wear.It was shown by high temperature oxidation tests that the oxidation rates of TiB2/Feand ZrB2/Fe cladding coatings comply with parabolic rules at constanttemperature environment from673K to1073K. The reinforcement phases can protectthe matrix metals from oxidation by reducing diffusion of O2-and preventing thegeneration of loose FeO. The oxide layer tightly attaches to the surface of the claddingcoating, which can effectively isolate the cladding coating from air. The oxidationmechanism of Fe-based cladding coatings reinforced by in situ synthesized borideceramic phases of TiB2and ZrB2is as follows. Firstly, the matrix metals and ceramicreinforcement phases in cladding coatings are oxidized and simple binary oxides ofMeaOb, B2O3and FemOnformed. Subsequently, several complex mixed oxides ofMexFeyOzand FehBiOjformed within the oxidation scale.