Deposition of thick Si coating with low residual stress on SiC ceramics by fabricating multilayer with compressive/tensile stress layer-pairs

•Si multilayer coatings with alternate compressive/tensile stress layer pairs exhibit low residual stresses.•The thickness of Si multilayer coatings can reach 20 μm.•Si-modified SiC ceramics exhibit ultra-flat mirror surface with surface Roughness of 0.69 nm after polishing.

Surface modification can significantly improve surface properties of lightweight SiC ceramics as optical component for Large Space Optical Telescope. Usually, thick Si-modified coatings can be applied to cover surface defects and decrease polishing cost/time of SiC ceramics. However, it is one great challenge for depositing thick Si-coating with low residual stress on large-scale SiC ceramics by physical vapor deposition (PVD) method because too large residual stress often leads to the stress cracking of thick coatings. Here, silicon multilayer with alternate compressive/tensile stress layer pairs was designed and deposited alternately onto SiC substrates in order to prepare thick Si-coatings with low residual stresses. The silicon multilayer with compressive/tensile stress pairs were obtained by plasma ion assisted deposition and electron beam physical vapor deposition, respectively. The total thickness of as-deposited Si-multilayer can reach as much as 20 μm. Remarkably, the residual stress of silicon-multilayer tends to be negligible. Meanwhile, thick Si-coating bonds tightly with SiC substrate, and shows ultra-flat mirror surface with surface roughness of 0.69 nm RMS after polishing. It is demonstrated that silicon multilayer with low residual stress is a promising way to realize surface modification of SiC ceramics for optical mirror application.

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