Characterization of a flat superpolished mandrel prototype with hard (TiN / SiC) overcoating to enhance the surface durability

TL;DR

This paper presents the development and characterization of superpolished flat mandrel prototypes coated with TiN and SiC to enhance surface durability for multilayer X-ray mirror fabrication.

Contribution

It demonstrates that TiN and SiC overcoatings can be superpolished to low roughness levels comparable to nickel, suitable for multilayer optics.

Findings

TiN and SiC coatings achieve superpolished surface quality

Surface roughness remains low after coating and polishing

Characterization confirms suitability for X-ray mirror production

Abstract

A number of hard X-Ray (10 - 100 KeV) astronomical missions of near future will make use of multilayer-coated focusing mirrors. The technology based on Nickel electroformed replication is suitable for the multilayer optics realization, since multi-modular telescopes are foreseen. For example, for the Constellation-X mission there is the need of realizing up to 14 identical modules (12 flight modules plus two spares) which can be replicated by the same series of mandrels. The Ni replication approach is derived from the method already successfully used for making the Au coated soft X-ray mirrors with good imaging performances of the missions BeppoSAX, XMM-Newton and Swift. In the technological extension of the process to the multilayer optics fabrication, it would be convenient to overcoat the external surface of mandrels (normally in Kanigen) with a layer made of a very hard material. This would help to maintain the very low roughness level requested by the application (typically less than a couple of Angstroms for a 1 micrometer scan length with AFM) also after many replications and successive cleaning of the mandrel. Good material candidate are at this regard TiN and SiC, both characterized by a very high hardness. We have proven that flat prototypes with TiN and SiC overcoating can be superpolished at a level comparable to the traditional electroless Nickel coating. In this paper we will present a characterization by topographic measurement (AFM and WYKO) and by X-Ray scattering of two of these samples.