| As the TiB2exhibits a high hardness, excellent oxidation resistance up to1000℃and good adhesion-resistance with Fe-based material, thermally sprayedTiB2-M coating has been considered as one of the best candidate to replace thetraditional Cr3C2-NiCr coating being used in450℃~900℃high-temperaturewear-resistant filed recently. However, as compared with the traditional cermetsystem such as WC-Co, Cr3C2-NiCr, etc, the study on the thermally sprayed TiB2-Mcoating is still limited. The preparation of TiB2-M feedstock powder, the formationprocess and microstructure characteristics of TiB2-M coating, the relationshipbetween the microstructure and properties of the coating has not been understoodand revealed sufficiently, which limited the further development and application ofthe TiB2-M cermet coating.In this work, Ni was selected as the main metallic binder phase for its bestwettability with TiB2. Then, a new kind of TiB2-Ni cermet coating was prepared byplasma spraying technique with the agglomerated and sintered powder. Themicrostructure characteristics of the TiB2-Ni cermet coating, as well as the effectfactors on it had been analyzed and investigated systematically. The mechanicalproperties and sliding wear performance of the coating were also estimated.According to this work, a new route and a scientific basis for the development ofhigh-performance TiB2-Ni cermet coating is provided.Firstly, the fine TiB2and Ni particles were agglomerated by agglomeration andsintering method. Then, the TiB2-Ni cermet powder with uniform structure, highapparent and appropriate diameter was prepared with the optimizing composition ofslurry and parameter of spray drying and heat-treatment. The primary condition forimproving the apparent density of powder is adding appropriate dispersant in slurry,which is benefit for homogeneous and dense agglomeration of TiB2and Ni particle.Based on the optimization of dispersant, the solid spray-dried powder could beobtained by improving the solid content and decreasing the spray-drying temperature,and its apparent density was increased from1.02g/cm3to1.33g/cm3. Finally,according to improving the heat-treatment temperature for the abundant liquidsintering, the apparent density of powder could be further increased to1.79g/cm3.The diameter of powder with dense structure could be controlled by adjusting therotation speed of the atomizer. According to fitting experiment data, the diameter ofpowder is roughly inversely proportional to the0.74power of the rotation speed of the atomizer. When the rotation speed decreased to10800r/min~9000r/min, theTiB2-Ni powder for plasma spraying with the medium diameter of49.28μm~55.92μm was obtained.To ensure more molten powder during the plasma spraying process and thussuccessfully prepared the dense TiB2-Ni cermet coating using the agglomerated andsintered powder, the plasma spray parameter was optimized. By analyzing the heattransfer process of single TiB2-Ni powder in plasma jet and simplifying thedescription of heating capacity of plasma jet, the melting fraction of powder underdifferent spray parameters was calculated. As a result, the melting fraction of powderhad a good linear relationship with porosity of the coating. To increase the meltingfraction of powder and obtained coating with denser structure, improving the heatingcapacity of plasma jet to powder and heating time of powder by optimizing sprayparameter is crucial. Finally, the porosity of coating had been controlled to4.16%orless.The microstructure and phase of TiB2-Ni coating was analyzed after the densestructure was obtained. The Ni20Ti3B6brittle phase was found in the coating as thedissolution of TiB2in binder phase during spraying process. To decrease theNi20Ti3B6content, parameters of plasma spray and structure of powder wasoptimized. The Ni20Ti3B6content in coating was decreased from43.64%to35.19%through adjusting the spray parameter to reduce the over-heating degree of powderwhen its melting fraction was also considered. Based on the optimization of sprayparameter, the Ni20Ti3B6content in coating was further decreased to27.27%or19.57%by improving the densification of agglomerated and sintered powder orusing the clad powder with a larger TiB2particle size. However, the rebounding oflarge TiB2particle may be occurred during the deposition process while using theclad powder, leading the content of TiB2decreased too. Therefore, as compared tothe clad powder, the coating deposited from agglomerated and sintered TiB2-Ni withhigh-densification has a more homogeneous and density microstructure.The hardness and fracture toughness of the coatings deposited from differentpowder and spray parameter were also tested. The unmolten powder in the coatingwould decrease the cohesive strength of lamellar, which further influenced themechanical properties of coating largely. However, for the coating with densestructure, the hardness and fracture toughness of the coatings is dependent on theTiB2and Ni20Ti3B6content in the coating respectively. Under the optimized spray parameter, the TiB2-Ni coating with higher hardness and fracture toughness would beobtained from agglomerated and sintered powder with high-densification, whosehardness is up to1230kgf/mm2and fracture toughness is up to2.2MPa·m1/2.The sliding wear performance of the coatings was also test. By the analysis ofthe friction and wear mechanism of the coating, it was found TiB2has a goodantifriction and wear-resistance effect when sliding with Fe-based material and thecoating with more TiB2has a lower friction coefficient and wear volume. The mainwear mechanism of TiB2-Ni coating is the flaking of brittle phase in fatigue state andthe third-body wear with flaking off debris. As the TiB2-Ni coating deposited fromagglomerated and sintered powder with high-densification has fewer Ni20Ti3B6brittle phase and smaller TiB2particle, the flaking and third-body wear would bealleviated, the wear volume of the coating is37.68%and26.98%lower than thecoating deposited from porous powder and clad powder. |
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