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Interfacial Microstructure of Chromium Oxide Coatings

Chromium oxide coatings are chemically inert, have high mechanical strength, hardness and good optical characteristics; therefore, they have been widely used in many applications including corrosion protection, wear resistance, electronics, and optics. Cr2O3 is well suited for wear resistance applications, as it is one of the hardest oxides with 29.5 GPa hardness. Several deposition techniques have been tried for making these coatings. Cr2O3 coating hardness can vary substantially due to compositional and microstructural variations, depending on the deposition method. Hardness of a plasma-sprayed Cr2O3 coating, 50 lm thick was about 14.7 GPa, while a 200 nm thick RF-sputtered chromium oxide coating, stoichiometrically close to Cr2O3, exhibited 30 GPa hardness combined with good scratch resistance. Even for the bulk Cr2O3, hardness values reported were from 9 GPa to 29.5 GPa. Hones et al. investigated a correlation between the hardness and the sputtering deposition parameters, i.e. oxygen partial pressure and substrate temperature, and found favorable deposition conditions with an oxygen partial pressure of about 15–20 % of the total sputtering gas pressure at substrate temperatures exceeding 500 K. Good coating adhesion is required for wear and corrosion resistance applications. Premature failure can occur for many reasons including coating delamination, cracking and plastic deformation. In addition to this, thin ceramic PVD coatings usually have columnar grain structure with micro cracks, pinholes, transient grain boundaries and often high throughcoating porosity, which all lead to accelerated pitting corrosion and failure at the coating/substrate interface, especially in hostile environments. On the other hand, several studies showed that coating thickness plays an important role in enhancing both PVD-coated tool cutting performance and resistance to abrasive and erosive wear. Graded systems have been employed to obtain thicker coatings without losing performance in terms of coating adhesion and toughness. It is likely that thicker coatings will improve corrosion resistance in aqueous environments by eliminating through-thickness pin-hole defects. Coating mechanical, adhesion and wear properties are strongly affected by microstructure. Interfaces with high adhesion are known to ensure prolonged coating life and good wear resistance. Sputtered coating microstructure and physical characteristics depend on the deposition parameters. Also, substrate surface conditions prior to deposition, characterized by surface roughness, stress and oxidation state, play an important role in controlling coating properties. In this paper SEM and TEM techniques were used to characterize thicker chromium oxide coating interfacial microstructure as a step towards developing a unique method for depositing thicker coatings with small grains, smooth surface and low residual stress.