Tribological properties of plasma sprayed Cr₂O₃, Cr₂O₃–TiO₂, Cr₂O₃–Al₂O₃ and Cr₂O₃–ZrO₂ coatings

Abstract

<p>Plasma sprayed Cr<sub>2</sub>O<sub>3</sub> is widely used to protect industrial components against wear. The present study seeks to clarify how its properties can be modified by alloying with other oxides. Therefore, pure Cr<sub>2</sub>O<sub>3</sub> and Cr<sub>2</sub>O<sub>3</sub>–25%TiO<sub>2</sub>, Cr<sub>2</sub>O<sub>3</sub>–16%Al<sub>2</sub>O<sub>3</sub>, Cr<sub>2</sub>O<sub>3</sub>–35%Al<sub>2</sub>O<sub>3</sub>, Cr<sub>2</sub>O<sub>3</sub>–10%ZrO<sub>2</sub> and Cr<sub>2</sub>O<sub>3</sub>–20%ZrO<sub>2</sub> coatings were studied. All samples were obtained from pre-alloyed feedstock, resulting in rather homogeneous solid solutions. Compared with pure Cr<sub>2</sub>O<sub>3</sub> and Cr<sub>2</sub>O<sub>3</sub>–Al<sub>2</sub>O<sub>3</sub> coatings, the Cr<sub>2</sub>O<sub>3</sub>–25%TiO<sub>2</sub> and Cr<sub>2</sub>O<sub>3</sub>–ZrO<sub>2</sub> ones exhibit lower indentation hardness (H<sub>IT</sub>) but higher toughness, qualitatively assessed by scratch testing. Cr<sub>2</sub>O<sub>3</sub> and Cr<sub>2</sub>O<sub>3</sub>–16%Al<sub>2</sub>O<sub>3</sub> also exhibit higher hardness/elastic modulus ratios (H<sub>IT</sub>/E*, H<sub>IT</sub><sup>3</sup>/E*<sup>2</sup>) than all other samples. The sliding wear resistance of the coatings against Al<sub>2</sub>O<sub>3</sub> and ZrO<sub>2</sub> balls is most closely correlated to indentation hardness and, secondarily, to the hardness/modulus ratios. Pure Cr<sub>2</sub>O<sub>3</sub> is therefore the most sliding wear resistant of all samples, whilst Cr<sub>2</sub>O<sub>3</sub>–25%TiO<sub>2</sub> suffers very severe wear. However, ZrO<sub>2</sub> counterparts cause systematically more severe wear than do Al<sub>2</sub>O<sub>3</sub> ones. Dry particles' abrasion, which proceeds through flake formation, is controlled by toughness. The resistance to abrasive wear is, therefore, predicted by scratch testing. The various coatings rank almost the opposite as they did in sliding wear tests, with comparatively lower wear losses for Cr<sub>2</sub>O<sub>3</sub>–25%TiO<sub>2</sub> and (most of all) Cr<sub>2</sub>O<sub>3</sub>–ZrO<sub>2</sub> samples.</p>