**2.2 Wear analysis**

Ball/Pin on disc tribo tester (Ducom Instruments, model TR-20LE) was used for bare and HA-CNT coated samples under dry and wet (SBF) sliding conditions for analyzing the abrasive wear. The samples were prepared as per G95-99a. The effect of the counter body of Steel Ball SAE52100, H = 9.46 GPa was studied to analyze the counterparts. The roughness (Ra) values of testing samples were made in the range of 0.8 μm or less. A varying speed of 100 RPM with a 25 mm circular track radius with a total traveling distance of 90–200 m was used to study the wear at a macro level. A specific linear speed of the tribo probe was set to 10 mm/s. The counter


#### **Figure 1.**

*Scanning electron micrograph showing morphology of 10 wt% reinforced HA-CNT powder with element composition from EDX analysis.*



#### **Figure 2.**

*Scanning electron micrograph showing morphology of 5 wt% reinforced HA-CNT powder with element composition from EDX analysis.*

*Tribological Behavior of Atmospheric Plasma Sprayed HA-CNT Coatings of Biomaterials DOI: http://dx.doi.org/10.5772/intechopen.103860*

body (probe) used was an 8 mm diametric steel ball. The inbuilt LVDT sensor gives the value of linear force in between the coated surface and steel probe under the depth of wear track. The coefficient of frictional data is acquired at the 17 Hz frequency level. Alicona 3D profilometer was used to obtain the profiles of wear track on tested samples. The value of wear volume is computed using the wear track depth profile.

**Figure 3** shows the pictorial view of wear (tribological) study in physiological conditions. The tests were performed with samples immersed in externally supplied simulated body fluid (SBF), in the tribo tester with the same test parameters and conditions as was used during the dry wear test. The SBF is prepared and purchased from KET's Scientific Research Centre Mumbai, India using Takadama Hiroaki [26] with the same ion concentration of chemicals as compared with human blood. The chloride ions are the cause of corrosion biomaterials, so these fluids are suitable for analyzing the corrosion resistance [27].

#### **2.3 Coating characterization**

The critical evaluation of SS 316 L, CoCrMo, and Ti6Al4V substrates coated by 10 wt% and 5 wt% CNT and worn-out surfaces are done. The metallographic study of as-sprayed coatings on these substrates is examined under ZEISS Gemini field emission scanning electron microscope (FE-SEM), operating at 18 kV. To have highresolution imaging, gold is used as the sputter coating material before analyzing in SEM. It can be seen from the optical micrograph **Figure 4** that the top surface of the coating is free from cracks and macro-level porosity. Most of the splats are wellformed without any sign of disintegration. Some melted grains are also visible on the surface of coatings in most of the micrographs. Further, it can be observed from the SEM/EDX analysis that a whitish appearance somewhere in the coating in SEM micrographs indicates Ca-rich HA particles. In reinforced HA coatings some streaks are detected whose EDX analysis confirms the presence of reinforcing content which is distributed in the matrix of the HA-C coatings [28, 29]. EDS spectroscopy indicated a Ca/p ratio of 1.62 in calcium phosphate deposits, which is similar to the ratio of 1.64 in natural bone [30].

**Figure 5** shows an insight on the FTIR analysis of HA-CNT reinforced coated samples in the range of 4000 cm−1 to 500 cm−1. OH– group at 3568 cm−1 and 652 cm−1 can be seen with the presence of PO4 3− group at 960 cm−1, 1033 cm−1 and

#### **Figure 3.**

*Experimental set-ups for tribological wear test (a) wear test mechanism under SBF (b) wear test sample in physiological condition.*

#### *Tribology of Machine Elements - Fundamentals and Applications*

558 cm−1. CO2 molecules which were not visible in the original powder are now can be observed at 2360 cm−1. The intensity of OH− group has become comparatively weaker after coatings in the case of reinforced HA-C coatings in all alloy substrates.

The basic mechanical property of HA-C coating is microhardness, which may play an important role in bio implant application. At the coating interface, the microhardness of coatings on alloy substrates was measured. Profiles for microhardness with distance from the coating substrate interface are shown in **Figure 6**. The hardness of both (10 wt% & 5 wt%) reinforced HA-C coating is lowest (310 Hv, 340 Hv, and 402 Hv in case of uncoated SS 316 l, CoCrMo, and Ti6Al4V, whereas 322 Hv, 352 Hv, and 418 Hv in case of reinforced HA-C respectively) at the interface in case of all the three alloy substrates. Slight improvement in hardness was observed with


**Figure 4.**

*Optical micrograph of reinforced HA-C coating on Ti6Al4V alloy with element composition from EDX analysis.*

**Figure 5.** *FTIR spectroscopy for plasma sprayed reinforced HA-C coating powder (a) 10 wt% CNT.*

*Tribological Behavior of Atmospheric Plasma Sprayed HA-CNT Coatings of Biomaterials DOI: http://dx.doi.org/10.5772/intechopen.103860*

**Figure 6.**

*Micro hardness profiles of reinforced HA-C coatings on SS 316L, CoCrMo, and Ti6Al4V alloy along the cross-section.*

the reinforcement content. Enhanced Vickers hardness resulted in improved hydrophobicity for a 5% HAP/MWCNT coating [31]. The impression of Vickers indent **Figure 7** was observed under SEM for an accurate radial crack length measurement.
