The Research On The In-situ Synthesis Of ZrB₂ Reinforced Iron-based Composite Coating

Zirconium diboride has been widely used in many applications, because of its high melting point, good chemical stability, high hardness, good electrical and thermal conductivity as well as its excellent oxidation and thermal shock resistance. Mild steel is viewed as the most favorable substrate due to its low cost, wide application and obvious performance improvement. Considering the great differences between ZrB₂ ceramic and iron substrate, energy density of heat resource is of vital importance for the in-situ synthesis of ZrB₂ reinforced iron-based composite coating.In this investigation, argon arc, plasma transferred arc and laser beam were used as heat resources in the fabrication of ZrB₂/Fe composite coating. The coatings formed by plasma arc cladding are smooth, and severe defects like pores and granulated spatters are avoided. Moreover, ZrB₂ grains with fine size and high content appear in the coatings formed by plasma arc cladding. But these ZrB₂ reinforced coatings cannot spread well on Q235 steel matrix due to the bad wettability between ZrB₂ and Fe. However, the surface condition and uneven distribution of ZrB₂ in the coating can be significantly improved by optimizing the processing parameters and pre-coating’s composition. Plasma arc cladding, by contrast, is considered to be the most suitable technology for the in-situ synthesis of ZrB₂/Fe composite coatings by assessing preparation cost, production efficiency, coating formation and properties.The shape and size of ZrB₂ phases in plasma arc surfacing coatings are diverse. Based on the ratio of length and width of ZrB₂, they can be divided into three categories, including acicular shape, clubbed shape and blocky shape. It is interesting to found that acicular shaped ZrB₂ tends to appear in these regions, such as the upper part, the left and right edges as well as the bottom of the composite coating, while blocky formed ZrB₂ often appears in the middle and lower part of the surface coating. The factors of cooling condition, nucleation mode, growth rate and concentration of ZrB₂ could have great impacts on the appearance of zirconium diboride.With a precursor of Zr and B4C, gradient distribution of ZrB₂ grains appears in the coating’s cross-section from both the vertical and horizontal direction. The content of ZrB₂ decreases with the increase of coating depth, which is consistent with its trend of microhardness. The appearance of ZrB₂ changes from coarse acicular to fine lump as the observing point shifts from the coating’s upper part to its bottom. And a similar shape change happens as the observing point transfers from the coating’s edge to its center. However, the horizontal transformation of ZrB₂ grains on both shape and size is unapparent in contrast with their vertical dispersion.The interaction between Zr and B4C is considered to be the most favorable in Fe-Zr-B4C reaction system thermodynamically. And ZrB₂ is regarded as the most stable resultant. But there are still four pathes for the in-situ synthesis of ZrB₂ in melting pool, which may have significant influences on the characterization of ZrB₂ phases. The liquid to liquid mode mainly happened in the middle of cladding process, is of vital importance on the content and shape of ZrB₂ grains.