Study On Growth Model And Properties Of Niobium Carbide Coating By TD

The niobium carbide coating prepared by the TD method has the characteristics of high hardness,good adhesion,uniformity and density,outstanding wear resistance,and low equipment requirements.At present,most of the researches on TD salt bath niobium carbide coatings are focused on friction performance,coating thickness experimental solution,and corrosion performance in a single environment.There are rarely researches on theoretical solution for coating thickness considering the substrate composition,and the comprehensive corrosion resistance of the coating in acid,alkali and neutral environments.In this paper,niobium carbide coatings on 10 type steel surfaces were prepared by TD salt bath technology at the temperature range of 850?950?.The morphology and microstructure of the coating were observed,and the phase composition,hardness and thickness of the coating were tested.The classical dynamic model of coating thickness and the theoretical model based on regular solution sublattice theory and considering the influence of substrate composition were established.Thermodynamic analysis of the formation of niobium carbide coating was conducted.According to the morphology and thickness model of the coating in different periods,the formation process and mechanism of the coating were analyzed.The Tafel curves and EIS impedance spectrums of three typical substrate-NbC samples under acid,neutral and alkali conditions were tested.The main conclusions are followed:1.As-received coatings with density,uniformity,light yellow colour,and straight interface are all composed of single phase of NbC which grew preferentially in the direction of?111?and?200?.2.Thermodynamic calculations show that aluminum powder as a reducing agent is more suitable to prepare niobium carbide coatings at 850?950?than boron carbide.3.The diffusion activation energies Q and diffusion constants K₀ derived from the classic dynamic model of coating thickness for 10 steels range from 142.08 to179.32kJ/mol,and from 0.1987 to 0.9574 cm²/s,respectively.4.The theoretical model of coating thickness is:l²Aa_C^?D _C,TD^efft.Q is almost constant?233±4 kJ/mol?,and the diffusion coefficient factor D₀ varies greatly from9.85 to 36.2 cm²/s.The main factors affecting coating thickness are:TD processing time and temperature,and substrate composition.Refining grain size is more effective method to increase coating thickness than extending TD processing time and raising TD processing temperature.5.The growth of niobium carbide coating is divided into two stages:nucleation and growth.The niobium carbide coating is composed of ultrafine crystal,columnar crystal,equiaxed crystal and cell crystal.The coating growth is mainly controlled by reaction within 1 hour and carbon diffusion in coating by grain boundary after 1 hour.6.The hardness of coating is about 3.5 times higher than that of substrate.In terms of electrochemical corrosion resistance,the self-corrosion potentials and current density of coatings under acid,alkali and neutral environments are higher and smaller than those of related substrates,respectively.As a whole,the corrosion resistances of coatings are better than those of related substrate.7.The corrosive rules for different coatings are quite different reflecting in porosity of coatings in different corrosive environments because of different corrosive mechanisms.In general,the corrosion rate of all samples in acidic,alkaline,and neutral environments is ranked as:acidic>neutral>alkaline.