**1. Introduction**

The fundamental requirement of bio implant should be its biocompatibility with the biological system, high mechanical properties to withstand various stresses induced, and excellent corrosion resistance in the body fluid [1]. Materials used for bio-implants are categorized as Metals and alloys, Polymers, ceramics, and reinforced composites. Advanced materials like Surgical stainless steel, CoCrMo alloys, and different Titanium grades are the most commonly used implant material due to their good biomechanical properties. But their main limitation is that these materials may not always be biocompatible with the body fluids, and tissues may not grow on these materials after they are implanted in the body [2]. The recurring failure of traditional materials used in orthopedic implant manufacturing was due to a lack of or inadequate integration of implant materials to the juxtaposed bone & stress–strain imbalance between the interface of tissue and implant material [3].

Ceramics are commonly used for bio implant applications due to their superior biocompatibility in the body environment. Due to their similarity with human bone natural tissues easily grow on their surfaces [4]. Few bioceramic materials are HA, tri-calcium phosphates (TCP), and bioactive glasses. But uses of ceramics are limited due to their poor mechanical properties.

Polymers are soft materials that can be easily formed into complex shapes; however, they have poor mechanical properties and cannot be used in heavy loadbearing applications such as knee and hip prostheses [5]. Composite materials are typically created by joining two or more distinct material phases, such as metallicceramic, polymer-ceramic, and metallic-polymer [6]. These materials have good mechanical properties and biocompatibility, but it is extremely difficult to make composite parts biocompatible [7].

Hydroxyapatite (HA) coating is an excellent aspirant for bio-implants in order to improve their biocompatibility in body fluid [8]. Because the chemical structure of HA is very similar to the structure of natural bone, it can form new tissues on it in body fluids, which is critical for the fixation of bio-implants inside the body and also protects the body from any harmful metal ion released by the metallic implant [9]. It has lower mechanical properties, such as low bending strength, fracture toughness, and impact strength, and thus cannot be used in pure form for load-bearing applications [10]. The coating of HAP alters the dispersion behavior of MWCNTC in the PP matrix, causing variations in the tensile and thermomechanical characteristics of MWCNTs/PP composites [11].

Hence it is a good idea to overcome these mechanical limitations to coat this material on the metallic substrates which have good mechanical properties. From the previous work [12] it has been found that pure HA coating results in poor adhesion, bonding strength, and other mechanical properties. Hence to improve these properties reinforced materials like zirconia, silica, titania, alumina, carbon, and boron nanotubes are added in bulk HA. Particularly there must be sufficient adhesion strength between HA coating and metallic implants to avoid spalling, cracking and wearing of the coating [13]. Many techniques like electrophoretic, sol–gel, and thermal spraying (plasma and HVOF), etc. are used for HA deposition on metallic substrates. Among these, the air plasma technique is widely used because of its excellent adhesion strength, better process control, crystal structure, and thickness of the coating as compared with other such coating techniques [14]. This technique is recommended and approved by Food and Drug Administration (FDA) in the USA for clinical trials [15].

Despite the many advantages of hydroxyapatite coating on metallic substrates, the brittle nature and low strength of hydroxyapatite delay clinical trials under varying loading conditions [16]. Because HA is brittle, secondary reinforcement materials such as Zirconia, Ni3Al, Alumina (Al2O3), carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs), Silica, yttria-stabilized zirconia (YSZ), and Ti-grade-alloys are commonly used to improve mechanical properties such as fracture toughness and Young's modulus [17]. CNT reinforced composites outperformed other composites in terms of biological and tribo mechanical properties. The addition of CNTs increases the crystallinity, shape, and biological characteristics of HAP [18]. The COF of a composite can be reduced by varying the rate of CNT. The inclusion of CNTs boosts cell growth and adhesion. Wear-related mass loss is reduced by 13.33%, 66.67%, and 83.33% [19]. To understand the functionalization of CNTs to increase their hydrophobic qualities, the intrinsic nature of CNTs, including their physical and chemical properties, is explained. CNTs are often functionalized with different functional groups (such as-OH and –COOH) using covalent and non-covalent approaches to increase dispersity in aqueous conditions and limit toxicity [20]. The specimens' corrosion resistance reveals

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

the biocompatibility of Ti–Co–Cr as being more unique due to the presence of titanium metal [21]. Many corrosion, wear, and abrasion resistant applications exist for plasma sprayed Al2O3-TiO2 ceramics in industries such as aircraft, textile, and automotive [22]. Gell et al. [23] investigated plasma-sprayed nanostructured 13 wt% Al2O3-TiO2 coatings and reported that these coatings had been approved for submarine and shipboard applications by the US Navy.

It is well understood that with HA coating wear indicates the possibility of orthopedic implant degradation failure. According to previous research, the main issue that affects the total joint replacement longevity is "worn particle-induced osteolysis, particularly adjacent to acetabular components" [24]. Wear particles are generated by the articulating surface and migrate along the implant interface, causing osteolysis, implant loosening, and, eventually, implant failure. However, there was no detailed information on the mechanism of microstructural failure. As a result, investigating the tribological behavior of HA-CNT coated specimens at the microstructure level requires considerable effort. The effect of CNTs on the human body and the environment is not well established, owing to a lack of standardization of toxicological tests, which leads to discrepancies in the results [25]. The current work will investigate the microstructural properties of applied coatings and investigate the potential of HA-CNT coating by plasma spray for bio implant applications. In this work, an attempt was made to deposit HA with 10% and 5% CNT on surgical grade SS 316L, CoCrMo, and Ti6Al4V substrates and to establish their suitability for in vivo applications.
