| Surface coatings is a new surface modification technology that uses varied methods such as magnetron sputtering technique to generate a surface layer to improve the performance of mould and subsequently prolong service life of mold. In industry, most of failures of machine parts begin from their surface, so it is important to explore new surface-strengthening technologies so as to improve their properties and durability, so surface coating has been regarded as one of the most promising surface modification technologies.In this work, DC-magnetron sputtering technique was selected to deposit a chromium layer on the surface of ion-nitrided/nitrocarburized/ origin H13 steel, the growth mechanism and types of crystals were investigated as well as the influence of pre-treatment on properties of coating. Microhardness, adheres strength, thermal cracks resistance, oxidation resistance, and mechanical properties were measured, using X-ray diffractometer (XRD), optical microscope (OM), field-emission scanning slectron microscope (FESEM), atomic force microscope (AFM), and so on. The conclusions are summarized as follows:(1) The chromium layer prepared on the surface of ion-nitrided/ nitrocarburized H13 steel grew through homologically/ heterologically structural denotative mode. The performance of chromium layer was controlled by the microstructure, texture and roughness of the surface of pre-treated H13 steel, which dominated the nucleation, growth mode and types of crystals.(2) All layers provided with excellent adhere strength, and adhere strength of origin, ion-nitrided and nitrocarburized H13 steel, which increased in the sequence of. The adhere strength of layer and matrix was influenced by the differences in the microstructure and texture of films and matrix, the status of interface, the characteristics of layer, the coefficient of thermal expansion, and so on. However, the properties of coating and surface of H13 steel after pre-treatment acted an primary affect on the adhere strength.(3) At temperature of 700℃, the nitrocarburized H13 steel exhibited better thermal crack resistance owing to the lower grad of hardness and tougher of pervasion layer. However, the coatings on origin/ nitrocarburized H13 steel, as a result of larger grads between hardness of layer and matrix and lower antiplasticization; ion-nitrided H13 steel, which had hard and brittle compound layer, and the Cr coated ion-nitrided H13 steel, exhibited higher grad of hardness and different coefficiency of thermal expansion between layer and matrix, and subsequently possessed inferior thermal crack resistance.(4) The oxidation resistance of layer was of remarkable difference due to the surface microstructure, texture and chemical composition of H13 steel after pre-treatment at 700℃: the layer on ion-nitrided/ origin H13 steel held inferior oxidation resistance because of the diffusion of C atoms into the Cr layer from the compound layer of ion-nitrided and origin H13 steel. However, the layer on nitrocarburized H13 steel provided with more excellent oxidation resistance.(5) After the process of annealing at 700℃for 100 hours, the surface hardness of coating on nitrocarburized H13 steel was enhanced because the mutual diffusion resulted in solid-solution strengthening, while the hardness of layer on ion-nitrided/ origin H13 steel decreased drastically on account of the cracked layer.
|
PDF Full Text Request