Remarkably Adherent, Thick And Tough Chromia (Cr2 O3) Coatings On Steel Made By A Laser Process

Hard materials research was focused-during the last decades-on searching for new crystal phases. The hope now for finding practical, yet unknown phases seems to be slim, especially after beta-C3N4 failure. The practical task can be rather experimental on how to fabricate both hard and tough materials. The granular structure and its architecture is an important factor in such an undertaking. This report is about Cr2O3 thick coating on steel fabricated with the help of a laser process. The uniqueness of this process is that it runs at the evaporation-solidification border of Cr2O3 and produces an inhomogeneous, in particle size, microstructure. Beside chromium carbide and chromium nitride, chromium sesquioxide Cr2O3 is a new candidate for hard and protective coatings. The reported bulk microhardness of Cr2O3 is 29.5 GPa according to Samsonov (1982) and 28GPa according to Robertson and Manning (1990), making it second among all oxides, even above sapphire. The Mohs hardness is 8. The hardest oxide is the high pressure form of SiO2, stishovite, with 35 GPa (Haines and Lager (1998)). Chromium sesquioxide Cr2O3 possesses Al2O3 sapphire type structure and belongs to the large group of chromium oxides which differ by chemical composition and valence states (Wells (1984)). The interest in fabrication of chromia Cr2O3 (named in analogy to alumina Al2O3) coatings was driven by physical and chemical properties of this crystalline phase. Beside hardness, low friction, high melting point, 2300 degrees C or 2435 degrees C were reported, chemical inertness in some environments, optical transparency in 600-11,000 nm, refractive index 2.45 at 500 nm have been the subject of consideration. The following practical applications: hard coatings (Zang et al. (1998)), corrosion resistance, wear resistance for the advanced digital recording systems (Sourty et al. (2002); Bijker et al. (2003)), and solar absorber material (Hutchins (1983)) have been suggested. The experiments on deposition of chromia coating started with rf sputtering more than 30 years ago (Bhushan (1979, 1980); Kao et al. (1989); Bhushan et al. (1997); Hones et al. (1999)). Later plasma spray and thermal spray of Cr2O3 have been worked out (Abass and Jaboori (1989); Ahn and Kwon (1999)) and such coatings are offered recently by Gordon and White Engineering Companies. Plasma sprayed coatings were treated by Nd:YAG laser to improve porosity, microstructure and adherence (Cuetos (1993)). There is growing interest in chromia coatings for a variety of applications. This report follows this trend. We describe application of simultaneous use of 3 lasers for fabrication of chromia coatings, chemical and structural characterization, and discussion on the microstructure-toughness relationship.