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

Plasma sprayed Cr₂O₃ 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 Cr₂O₃ and Cr₂O₃–25%TiO₂, Cr₂O₃–16%Al₂O₃, Cr₂O₃–35%Al₂O₃, Cr₂O₃–10%ZrO₂ and Cr₂O₃–20%ZrO₂ coatings were studied. All samples were obtained from pre-alloyed feedstock, resulting in rather homogeneous solid solutions. Compared with pure Cr₂O₃ and Cr₂O₃–Al₂O₃ coatings, the Cr₂O₃–25%TiO₂ and Cr₂O₃–ZrO₂ ones exhibit lower indentation hardness (H_IT) but higher toughness, qualitatively assessed by scratch testing. Cr₂O₃ and Cr₂O₃–16%Al₂O₃ also exhibit higher hardness/elastic modulus ratios (H_IT/E*, H_IT³/E*²) than all other samples. The sliding wear resistance of the coatings against Al₂O₃ and ZrO₂ balls is most closely correlated to indentation hardness and, secondarily, to the hardness/modulus ratios. Pure Cr₂O₃ is therefore the most sliding wear resistant of all samples, whilst Cr₂O₃–25%TiO₂ suffers very severe wear. However, ZrO₂ counterparts cause systematically more severe wear than do Al₂O₃ 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 Cr₂O₃–25%TiO₂ and (most of all) Cr₂O₃–ZrO₂ samples.