Structural Evolution And Multifunctionality Of Metastable Cr-C Coatings Prepared By Magnetron Sputtering

Cr-rich films are in high demand for protecting metal materials,which benefits from their adequate decorative characteristics,mechanical properties,and corrosion resistance.Recently,Cr-rich films have been considered a promising protective coating on fuel-clad Zr alloys for good resistance to high-temperature steam oxidation.This study aims to prepare a high-quality thick film with multifunctionality.However,vapor-deposited coatings,prepared under a rapid cooling rate,have various metastable structures.Yet it lacks a method to describe and predict the formation and evolution of metastable phases.Besides,it remains a major challenge to prepare a thick highquality(ultra-smooth and compact)coating;the corresponding evolution of its growth morphology is rarely studied.The Cr-C system is ideally suited for such structural evolution studies.In this work,a series of Cr-C coatings with low carbon content(C content<30 at.%)were deposited by low-temperature magnetron sputtering.We investigated the phase evolution of representative coatings at different temperatures and times.On the other hand,this work took supersaturated solid solution coating as an example,investigated the evolution of film morphology under different growth times,and expounded the microscopic mechanism of the morphological evolution of ultrathick films.Finally,this work evaluated various properties of ultra-thick films.The main findings and conclusions are as follows:(1)This work has developed a method to describe a series of metastable equilibrium using metastable phase diagrams.The metastable phase diagram with a T0 line can illustrate the phase composition of as-deposited magnetron sputtered coatings,such as supersaturated solid solution region or single-phase amorphous region.For the sequence of phase evolution,the difficulty of transformation depends on the range of composition change,the atomic mobility,and the complexity of topological transformation.A series of metastable phase diagrams modified from the equilibrium phase diagram can illustrate the corresponding metastable equilibriums.(2)This thesis enables sustainable growth of high-quality ultra-thick films.Specifically,we report the growth of Cr-C films without introducing additional interfaces;the film presents dense(95%relative to the theoretical density),ultrasmooth(0.5 nm/micron),and up to～10 microns thick;the film also has a fibrous structure(high aspect ratio 50:1),insignificant lattice distortion,and relatively low coarsening kinetics(growth exponent β～0.6).Based on the relevant roughening features,this work studied the atomic mechanism of the sustainable growth.(3)This research opens the possibility of injecting functions into a single-layer coating.In fact,the method of introducing small atoms,such as the formation of supersaturated Cr(C)solid solution,is used to strengthen the mechanical properties;the ultra-thick high-quality coatings dominated by favorable elements with a limited content of small atoms were prepared in sustainable growth.Finally,the coating can be applied to decorative and high-temperature protective applications and displays a combination of high hardness(21 GPa,more than 2 times that of pure Cr coatings),high optical reflectance(close to pure Cr coatings),and excellent resistance to hightemperature steam oxidation(>8 h at 1473 K for protecting Zr alloys,much longer than 2 h for pure Cr coatings).Three approaches have been developed in this work to achieve multiple functions in a single-phase coating,which relieves the limitation of thickness on applications and breaks through the single application of a single-layer coating.This work provides a solution for designing multi-application coatings.