**4. Conclusions**

The HA-CNT coatings with different wt.% was processed by air plasma spray on alloy samples. The characterization and tribological properties comparatively were investigated:

HA-CNT composites could be successfully synthesized using the air plasma spraying method. Reinforced CNT has not shown any adverse effect on densification. The uniform and improved densification of hydroxyapatite microstructure are because of the high electrical and thermal conductivity of nanotubes.

The computed tensile stress in the wear track was ~12 GPa which, was achieved because of the well retained peeled off graphene layers. Both the 10 wt% and 5 wt% HA-C coatings form dense morphological structures. Due to high interstitial bonding of HA with CNT matrix the cracks were properly bridged and fracture energy was fully absorbed causing a better hardening mechanism.

The wear mechanisms revealed by APS-sprayed HA-C coating on SS 316 L and CoCrMo that, due to higher contact stress a small amount of carbon adhesion with minor plastic deformation of austenitic grains took place. However, in the case of Ti6Al4V coatings exhibited brittle cracking, spalling and abrasive grooves.

An improved wear resistance by 75% with the lesser volume of wear debris generation was seen as the 10 wt% and 5 wt% CNT reinforcement was added to plasma spray HA coating. The lubrication provided by peeled off graphene layer from CNT decreases the CoF on the coated surface. CNTs content reduces the wear particle size (HA-CNT: 0.15–3.4 μm).

Contact pressure, hardness of counter body, spraying technique, and coating reinforcement with Hydroxyapatite are the major influencing parameters from the investigation.

Biocompatibility of all HA-CNT coatings was improved and is relatively higher than that of bare alloys.

Control > as sprayed reinforced 10 wt% HA-CNT coatings > as sprayed reinforced 5 wt% HA-CNT coatings > Bare Ti6Al4V > CoCrMo > SS-316L.

Therefore, the outcome of the present research work could be used to improve the efficacy with enhanced service life of medical implants and other such applied components. A very attractive solution to existing problem of HA with different reinforcement material could be established by HA-CNT composites and coatings. The process of HA-CNT coatings with different wt% compositions are user friendly and capable to provide alternative solutions to other high end coating techniques and reinforcement materials.
