Characterization of thermal sprayed Si on sintered SiC for space optical applications

ABSTRACT

Silicon carbide (SiC) ceramic exhibits several outstanding properties, such as superior chemical and thermal stabilities, high radiation resistance and low thermal expansion, that qualify it as a prime candidate for space applications. Optical quality with surface figure accuracy of the order of 60 nm peak-to-valley (PV) and micro-roughness (root-mean-square, RMS) less than 2 nm is necessary for space optical applications. Sintered SiC (α-HCP) is inherently porous and challenging to make high-grade optics. Hence, it calls for a surface modification of the sintered SiC by means of a clad layer that has superior adhesion to the substrate and facilitates optical polishing. This paper describes a typical method of synthesis and characterization of a thick silicon coating on sintered SiC substrate, directly machinable to aspheric for optical telescope applications. Present approach describes the quantitative opto-mechanical characterization of silicon film over SiC by thermal spray coatings, eventually leading to its space flight-worthiness.

KEYWORDS:

• Si
• sintering
• flame-spray
• interferometry
• morphology
• micro-roughness
• SiC
• nanoindentation

Acknowledgements

The authors are thankful for the support rendered by M/s. ARCI team for their preparation and supply of defect-free sintered silicon carbide substrate for this activity. The team of M/s. Sai Surface Coating Technologies have immensely supported the Si coating process for several SiC samples. Authors are also grateful to Dr H.C. Barshilia from CSIR-NAL, Bengaluru, for providing the facility to carryout nanoindentation tests under ISRO rate contract.

Disclosure statement

No potential conflict of interest was reported by the author(s).