Improvement in Tribological Properties of Plasma-Sprayed Alumina Coating upon Carbon Nanotube Reinforcement

Crushed alumina powder heterocoagulated with 1 or 2 wt.% carbon nanotube (CNT) was used to produce plasma-sprayed coatings with improved tribological properties. Heterocoagulation is a colloidal processing technique, and it was used to produce a homogenous mixture of CNT in alumina. These coatings were compared with a pristine alumina coating using a pin-on-disk setup under various speed-load conditions. A 6-mm-diameter WC-Co ball was used as counterbody. The wear resistance of the coatings increased in the following order: pristine alumina, alumina with 1 wt.% CNT, and alumina with 2 wt.% CNT. Wear progressed by low cycle fatigue, microfracture, and finally pulverization of the already dislodged wear debris on repeated rubbing by the counterbody. The higher wear resistance of the reinforced coatings is attributed to their higher hardness and fracture toughness. The pristine alumina coating was found to crumble owing to diffuse intergranular fracture beneath the surface during rubbing under higher speed and load. Upon crumbling, the pulverized debris assumed a paste-like form and was ejected at the top under the compressive action exerted by the counterbody at its leading edge. The high wear of the pristine alumina coating is attributed to this effect.