**3.1 Characterization of uncoated and plasma sprayed HA-C reinforced coating**

**Figure 8a** revealed the microstructure study of surgical stainless steel SS-316 L, as equiaxed austenite grains which are as per the grade of this steel [32]. Annealing twins are also present in the structure which reveals the resemblance of the structure discussed in the handbook. SS-316 L is austenitic stainless steel where 'L' denotes low carbon content which is limited to 0.03%. A carbide-free austenitic and strain-free microstructure is produced in this steel that retains in the soluble carbon at an annealing temperature of 1010–1065°C (1700–1850°F) due to rapid quenching. **Figure 8b** depicts that there exists a hexagonal close-packed and face-centered cubic crystalline structure with CoCrMo alloy. Typically, at normal temperature, the face-centered cubic phase is predominant causing fcc to hcp transformation. The microstructural grain consists of bigger Co dendrites having an hcp structure with the smaller surrounding zones correspond to the embedded carbides which are recognized as M23C6 type. The analysis suggests that there is uniformity in porosity with fine grain structure. The microstructure of Ti6Al4V **Figure 8c** is compared with the standard microstructures from Metals Handbook (1975) and ASM Handbook (1992, 2001). The microstructure consists of equiaxed α grains (light) and intergranular β (gray), which corresponds to the grade of this alloy. EDAX analysis confirms the CA/P ratio in between 1.67–2.4 which is the desired ratio for long-term applications of the implant [33]. CNT–HA composites can generate bonelike apatite on the surface of the sample due to greater calcium ion concentration in SBF and higher negative charge, as well as more accessible nucleation sites.

It has been reported in the earlier research work [34] that thicker coatings have been more durable when implanted in the body environment. But thickness

**Figure 8.** *Optical micrograph of HA-C coating on (a) SS 316L, (b) CoCrMo, (c) Ti6Al4V.*

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

more than the critical limit leads to self-disintegration of the coatings and this would lead to the problem of increased mechanical competence of coating [35]. High crystalline coating with higher thickness may lead to the formation of brittle material causing cracking under shear or bending forces. So it was aimed to produce coatings having thickness 0f 150–200 μm. Some randomly selected coated samples were measured along the cross-section for determining the coating thickness and reported in **Table 2**.

The porosity of the coating in bio implant application has an important role. Earlier literature proposed that the surface of the coating should be free from porosity and coating should act as a railing between the substrate and body fluid environment to avoid metal ion release in the body [36]. A slight decrease in porosity is observed with the addition of reinforcement of 10 wt% and 5 wt%. This decrease in porosity is due to the small size of reinforcement. The slight reduction is in agreement with the findings of Morks, [37, 38]. APS coated Ti samples have 0.40%–1.35% porosity, and in contrast to the higher side, the SS & CO samples exhibited marginally more porosity of the range 0.45%–1.40%. The surface of the bio implant must be rough enough for easy cell growth which improves the fixation of the bio implant in a physiological environment. Both Ra and Rz values of the reinforced HA-C coating on ally substrates are measured. Average values of surface roughness (i.e. Ra and Rz) of HA-C coatings on SS-316 L are 6.32 μm and 33.83 μm respectively, whereas for Ti6Al4V these are 6.06 and 39.41 μm respectively. It can be seen from the results that the average value of surface roughness (Ra) for uncoated samples is slightly lower than that of reinforced coatings on all the alloy substrates.

**Figure 9** represents XRD spectra of HA-C coatings of samples tested under wet (SBF) conditions. All peaks in the case of coating correspond to HA. One small broad peak of CaO is observed in all reinforced HA coatings which confirms the presence of amorphous CaO in a very minute amount. An important observation shows the presence of tricalcium phosphate (α-TCP, β-TCP) and tetra calcium phosphate (TTCP) peaks between 29° to 32° angle range on all as-sprayed coatings. As the original powders do not have these phases. The severe failure and destruction of the implant were caused due to the rapid solubility of the amorphous phase in the human body's physiological environment. The XRD analysis also depicts the sharp HA peaks which turn to be broader after coatings indicating the corrosion of crystalline materials to the amorphous phase during spray coating.

Coating Crystallinity was calculated at a 20° to 90° angle. Random HA-C coated samples were analyzed for average values and presented in **Table 3**. The fraction percentage of the amorphous (TCP & TTCP) phase was also shown.


**Table 2.**

*Average coating thickness of plasma sprayed HA-C coatings.*

#### **Figure 9.**

*XRD pattern for plasma sprayed reinforced HA-C coated wet samples. (a- 10 wt% & b- 5 wt% wet) HA, a-αTCP, b-βTCP, t-TTCP, c-CaO, g-graphene, #-Al2O3, \$-TiO2.*


#### **Table 3.**

*Crystallinity, TCP and TTCP phases in reinforced HA-C coatings.*
