ICMCTF1998 Session E3-2: Coatings Resistant to Erosion, Corrosion and Severe Environments Including Biomedical Environments (2)

Under the extreme working conditions of the Al high pressure die casting process it is included the corrosion of the die surface, provoked by the die dressing sprayed on the hot die surface for better ejection of the casting piece. The major failure mechanism of the die is originated by the cyclic temperature change on the die surface, causing thermal fatigue on it. Although this failure is mainly caused by the temperature gradient between die surface and its bulk volume, it is expected a reduction on heat checking ocurrence for a die when its surface has good corrosion resitance, which results a smother surface.

PVD-coatings were deposited on hot work steel and corrosion tested with die dressing. Polarization curves, as current density potential, were taken for Titanium Aluminium Nitride and Cr based coatings, deposited by Magnetron Sputtering and Cathodic Arc processes. The coated and uncoated substrates were also tested after an oxidation in air at 720 C, which has shown better results due to the oxide formation on the coatings. SEM analysis was used for a better comparison of the results, which has confirmed the improvements achieved for the coated substrates.

The synergistic, simultaneous action of corrosion and mechanical wear processes may incur more severe damage of components than that invoked by corrosion or wear alone. Such corrosive wear environments occur in many engineering applications. In spite of their large potential in such corrosive wear applications, little research has been conducted on chromium boride coatings obtained by physical vapour deposition. To redress this situation, CrB₂ coatings were deposited on polished AISI 316L substrates by non-reactive unbalanced magnetron sputter deposition from a sintered CrB₂ ceramic target. The substrate temperature during deposition remained below 200 degrees C.

X-ray diffraction studies showed that all of the coatings were crystalline with strong preferential orientation. In a previous study by the present authors an amorphous or quasi-crystalline structure was observed for coatings deposited from a ceramic CrB target. Nanoindentation hardness testing of the coatings showed that in all cases the coating hardness was greater than 43GPa (corresponding to approximately 4100HV) and exceeded the microhardness reported for bulk CrB₂. The CrB₂ coatings dramatically increased both the localised corrosion resistance (in NaCl, HCl and FeCl₃ solutions) and general corrosion resistance (in H₂SO₄ solutions) of the substrates. Pin-on-disc wear testing showed that the wear resistance of the CrB₂-coated samples may be limited by fracture of the brittle ceramic coating.

Zirconium like other value metals (I.e., Ti. Al, Hf, Ta), forms a stable anodic oxide film which resists attach by mineral acids in a reducing atmosphere. In highly oxidising conditions, chlorides attach the zirconium oxide film.

Aggressive ions such as Cl, Br and F activate pits although the actual mechanisms are not fully understood. One mechanism suggests that Cl- ions chemisorb on cracks or pores in the passive layer and dissolve the film. Unaggressive ions such as So₄--, NO₃- and OH- sometimes decrease the attack by competitively absorbing on the metal surface.

The results of this investigation have been used to compare the corrosion of Zr with other metals. The pit nucleation potential (Ep) and the repassivation potential (Er) are the electro chemical parameters used to compare the pitting characteristics of Zr. Er has been determined primarily by the scratch method but limited results have also been acquired by using the potentiodynamic and potentiostatic methods.