*3.2.2 Wear (Abrasive wear) and friction mechanism of bare and APS-coated specimens against steel*

Morphological structure and hardness are the two main parameters affecting the wear resistance of any coatings. **Figure 10a** and **b** represents the micrograph image of the wear mechanism of the Ti sample during uniform directional sliding of HA-C coated specimen. SEM micrograph after wear test exposure shows uniform and continuous wear. It can be observed that wear tracks are visible due to abrasive

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


**Table 4.**

*Result of Tafel polarization test of reinforced HA-C coating on SS 316L, CoCrMo and TTTi6Al4V in SBF solution.*

**Figure 10.**

*Wear scar micrograph of APS- sprayed HA-C coatings against steel ball (a- wear mechanism & b- wear path).*

action on the both uncoated and coated surface. The micrograph also shows the small debris by abrasion. SEM images revealed the presence of areas of cracks and fractures due to plastic deformation, which is attributed to abrasive wear [40]. In the HA-CNT wear track, the image depicts smaller craters and a huge area of abraded surface. Fracture and chipping form the crater, while abrasive wear results in the rough surface.

Average Weight loss (kg/m2 ) of uncoated samples was 0.14, 0.12 and 0.09 kg/m2 respectively whereas for reinforced HA-C coating it was 0.24, 0.22 and 0.18 kg/m2 respectively. It may be observed from the wear results that weight loss of metallic substrates is comparatively lower than that of all as-sprayed coatings on alloy substrates. It can be observed from the plotted results from **Figure 11** that, the wear volume was reduced up to 75% with the improvement in the wear resistance of reinforced HA-C coatings with 10 wt% and 5 wt% reinforcement. This improvement is because of the increased fracture toughness and elastic modulus.

The average wear rate and friction coefficient fall into a near mild wear regime, with values ranging from 10 to 7 to 10–4 mm3 /Nm [41]. The tribological test parameters were chosen with the conditions of medical implants working inside the human body, specifically the failure of the femoral head within the acetabular cup of the hip joint. Normally 0.8–2.5 MPa of stress is considered in a hip joint while walking [42]. The HA-C coating has to withstand a maximum frictional force for a minimum period of 20 years [43]. To compensate the entire working life a high value of 10 N load is kept during the entire test.

Surface hardness, toughness, spraying technique, and contact pressure are considered as the significant effecting factors on the coefficient of friction value. **Figure 12** represents the accumulated mean CoF from 0.35 to 0.42 due to the

#### **Figure 11.**

*Variation in wear loss (kg/m<sup>2</sup> ) of uncoated, 10 wt% and 5 wt% HA-C coatings alloy in dry and wet (SBF) condition.*

#### **Figure 12.**

*Average CoF curves for APS coating against steel ball SAE 52100 at contact pressure of 2000 MPa. (a- sliding distance vs CoF & b- time vs CoF curve).*

varying contact pressure of the steel ball. The effect of the spray technique is the drop in mean CoF up to 0.37 for the Ti coated samples in wet (SBF) conditions. The total mass loss study demonstrated that the presence of PBS in the electrolyte solution tended to exacerbate the deterioration of the alloy under the circumstances examined, which was also verified by the 3D analysis of the wear track geometry [44]. As hardness of counter body governs the tribological failure, so it primarily influences the specific wear rate. Ti6Al4V alloy has shown maximum wear resistance among all uncoated and coated specimens. Improvement in wear resistance may be observed with the incorporation of reinforcement in the case of HA coatings. There is a slight decrease in CoF from 0.8 to 0. 6 with the addition of reinforced CNT into the HA matrix. As the graphene layer is peeled off from the CNT surface it offers lubrication causing the decrement in CoF value.

A tensile force ≥11 GPa is required to remove the single graphite layer from multiwalled CNT along its axial direction [39]. The evaluated tensile stress in the wear track in the current study was ~12 GPa, which was found to be sufficient for the removal of

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

the graphene layer from CNT. Similar results were seen when the test was carried out under SBF immersed solution. As the body fluid would offer additional lubrication on the implant surface it would decrease the amount of wear debris thus, expecting better performance of coating inside the human body. Tribo Mechanical wear is the most common wear mechanism, as seen by abrasion, adhesion, and cracking [45].

**Figure 13** represents the wear depth profile and 3D topography of HA-C coating against SAE 52100 steel. The wear volume against the track was calculated (using average value of cross-section area from three variable experimental sets with track diameter) from the profile and found to be 0.25 mm3 for HA-CNT coating. With CNT addition it decreases up to 75% resulting in a "reduced probability of disturbance in the biological environment around the implant". The 15.70 μm of wear depth is recorded which is maximum for both types of coatings versus a steel ball.

#### **3.3 Bio-compatibility evaluation of HA-CNT coatings**

MTT assay was used to determine the mitochondrial activity and cellular viability of 3T3-L1 viable cells on uncoated and reinforced HA-CNT coatings. When incubated with viable cells, the reduction reaction of MTT reagent causes it to be reduced into purple formazan crystal. The cell viability is indirectly reflected by the absorbed formazan crystal. As a positive control, cells seeded on tissue culture plates are used.

#### *3.3.1 Cell viability study*

Morphology of cell-seeded on bare SS-316 L, CoCrMo, and Ti6Al4V substrates after 2 days were analyzed. Cell viability (%) is shown in the histogram presented in **Figure 14**, after 2, 15, and 30 days of incubation. It may be observed that cell viability on Ti6Al4V substrate was 72%, 81%, and 88% respectively whereas it was 68%, 78%, and 85% on CoCrMo and 66%, 78%, and 84% respectively on SS-316 L. Further, cells were comparatively less viable initially on all the three alloy substrates than that of the grown cells on the tissue-cultured plate (p,0.05), but viability increased with exposure time duration.

The morphology of 3T3-L1 cells seeded on reinforced HA-CNT with 10 wt% and 5 wt% coatings (as-sprayed) on all Ti6Al4V alloy samples is shown in micrographs presented in **Figure 15a–d**, respectively. According to the viability results, cell viability of 5 wt% reinforced HA-CNT coatings is lower than that of 10 wt%

**Figure 13.** *(a) Wear depth profile and (b) 3D topography for APS coated sample.*

**Figure 14.** *Viability of 3T3-L1 cells on Ti6Al4V, CoCrMo and SS-316L alloy from 2 to 30 days.*

**Figure 15.** *SEM (BSE) image of 3T3-L1 cells seeded on (a) control (b) bare Ti6Al4V (c) 5 wt% Ti6Al4V (d) 10 wt% Ti6Al4V.*

coatings after 2 and 15 days of exposure, respectively. However, after 30 days of exposure, the viability of all as-sprayed coatings was comparable.
