Structure and water-lubricated tribological properties of Cr/a-C coatings with different Cr contents

• The chromium carbides were formed in high Cr containing a-C coatings(>4.9 at%).
• After Cr incorporation, the critical loads of a-C coatings increased.
• The Cr/a-C coatings (Cr≤4.9 at%) decreased the initial friction coefficient of a-C coatings.
• The Cr/a-C coatings (Cr≤4.9 at%) shorten the running-in distance of a-C coatings.
• The Cr/a-C coatings (Cr≤4.9 at%) decreased the wear rate of SUS440C balls.

The Cr containing amorphous carbon coatings (Cr/a-C) with varying Cr content were deposited using unbalanced magnetron sputtering. The results revealed that the chromium carbide nano-clusters were formed when the Cr content exceeded 4.9 at%. The critical load increased while the hardness decreased after the Cr element incorporation. Although the low Cr containing Cr/a-C coatings (≤4.9 at%) exhibited similar friction coefficient with a-C coatings, but the initial friction coefficient, running-in distance and wear rate of SUS440C balls all decreased. However, the Cr/a-C coatings with high Cr content (11.98–14.09 at%) would worsen the tribological properties because chromium carbides acted as abrasive wear particles during tribotests.