Wear performance of Ti-based alloy coatings on 316L SS fabricated with the sputtering method: Relevance to biomedical implants

Abstract

BACKGROUND:

This investigation was conducted to encapsulate 316L SS with a Ti-based alloy coating.

OBJECTIVE:

The aim was to fabricate a coating using TiN, TiO₂, and TiCoCr powders on 316L SS through the physical vapor deposition (PVD) sputtering process.

METHODS:

The powders were consecutively coated on 316L SS through the PVD sputtering process with coating durations of 30, 60, and 90 min. Further microhardness, surface roughness, microabrasion, and adhesion strength tests were also carried out.

RESULTS:

A 60% improvement in abrasion resistance was observed in TiCoCr-coated samples compared to the uncoated substrate. The X-ray diffraction results confirmed the optimal formation of Ti alloy coatings with corresponding orientation over the SS substrates. Moreover, TiCoCr with a 90 min coating duration had much better surface characteristics than TiO₂ and TiN.

CONCLUSION:

The 90 min coating duration should be optimal for coating in steel for bio-implants.

Keywords

Roughness micro-abrasion microhardness adhesion

1. Introduction

A material that is biologically compatible and is associated with the human body in the form of medical devices such as artificial joints, hip implants, and dental implants is known as biomaterial. Biocompatibility plays a key role in the vivo environment of humans, giving hazardous products an impact on the tissues and organs [1]. Moreover, medical implants generally use chrome steel, Co-Cr alloys, and Ti alloys [2]. Stainless steel (SS) is the most recognized material used as biomaterials, including iron, chromium, and nickel. Hence, it is preferred to be a companion, whereas the iron-based alloys will rust or corrode in a limited capacity of application [3]. Martensitic SS is not as wear-resistant as ferritic or austenitic SS because of their lower chromium content [4]. Regardless of this fact, 316L has a downside in that it shows unsatisfactory performance in the wake of tribological behavior and mechanical strength. This is because the fact that the hardness value stands low which directed the experimenters to concentrate on various surface engineering processes to enhance its tribological behavior retaining the distinguishable wear-resistant property [5]. With an end goal to enhance the properties of the implant materials such as tribological, and biological, surface coating is used [6]. Also it exhibits undesirable behavior in wear [7], friction and abrasion [8], micro-cracks [9] and severe plastic deformation [10]. Hence, there is a need for surface modification over the base material 316L SS to be used as an incomparable biological implant [11]. Researchers reported better adhesion strength, hardness, and wear range in TiN-coated samples when applying different parameters for sprayed TiN coatings using a sputtering process [12]. The PVD thin film of TiN and Ti6Al4V was deposited on 316L SS. The results showed that TiN coatings deposited over 80 min had superior behavior than bare metal [13]. On the basis of their outcomes, it can be said that the important element determining the durability of TiN-coated alloys is the elastic modulus mismatch between the coatings over the substrate when using the PVD deposition technique [14]. Sputtering is a plasma-based PVD technique where a thin film is developed in which the highly energized ions are allowed to be accelerated toward the target material. Hence the resultant film has higher wear resistance [15]. Micro-abrasion is a minimally invasive procedure used to assess overall coating quality and texture. The micro-abrasive test helps us in simulating the wear conduct of materials under various conditions [16]. The covered and uncoated substrates were permitted to go through wear instruments with the guidance of a micro-abrasion tester using a block-on-ring technique, following ASTM standard G77 [17].

Atomic force microscopy (AFM) can map out the topography of any surface and represents various physical parameters of the surface including friction [18]. To produce the diffraction pattern, X-rays are used because their wavelength, 𝜆, is often the same order of magnitude as the spacing, d, between the crystal planes (1100 μA) [19]. The X-ray source takes advantage of graphite monochromatic to generate Cu-K α radiation with a wavelength of 1.54056 Å. A distinctively devised holder is a manipulator designed to fasten the thin film precisely so that the X-rays are made to reach the film surface [20]. The study aimed to produce Ti-based alloy coatings on 316L SS using a PVD sputtering procedure with a flexible time frame. Surface features such as abrasion resistance, microhardness, and adhesion strength were considered to evaluate the coating behavior of the specimen.

2. Methods

The chemical composition of 316 SS is listed in Table 1.

Table 1
List of chemical elements in 316L SS and their respective weight percentages

Element C Mn Si P S Cr Mo Ni N Fe

wt. % 0.03 2.0 0.75 0.045 0.03 17 2.5 12 0.1 Bal.

Element	C	Mn	Si	P	S	Cr	Mo	Ni	N	Fe
wt. %	0.03	2.0	0.75	0.045	0.03	17	2.5	12	0.1	Bal.

The pellet was prepared using the proposed novel TiCoCr combination of Ti-55%, Co-25% and Cr-20% and was mechanically alloyed using a high-energy planetary ball mill (PM 200 model; Retsch, Germany). The ball material used in this process was zirconium dioxide balls with a diameter of 10 mm in a polyurethane jar. All three powders were equally maintained at approximately about 500 nm. To ensure careful milling, the mass of the powder to the ball ratio was selected as 1:3.5. The milling speed was maintained at 300 rpm throughout the process for 30 min. The alloy of TiCoCr was filled in a 2-inch diameter split-up die, for compression using a compression molding machine under the operational condition of 500 kN for 3 min. Hence, the pellet with a weight of 20 g, width of 50 mm and thickness of 10 mm was prepared. Similar preparation was done for commercially available TiN, and TiO₂ powder up to 400 kN about 1 min 5 s. However, the sintering process was carried out using a Carbo lite muffle furnace for hardening the pellet of TiCoCr, TiO₂, and TiN. The pellets were placed in a crucible which was filled with fine river sand and kept in the furnace by maintaining the sintering temperature of 950 °C for about 30 min. Finally, the pellets were prepared and ready for the coating process.

The cylindrical 316L SS substrate with a diameter of 25 mm and a thickness of 1mm, was cut into 30 pieces using the wire-cut EDM process. The deposition of Ti-based alloy coating target over the base material was performed by sputter deposition type of PVD process via thin films utilizing an RF magnetron sputtering technique. The coating time WAS fixed for all samples at 30, 60, and 90 min. However, using the same sputtering time for different target materials is based on process standardization or specific experimental considerations. The experiment focused on achieving consistent film thickness, maintaining deposition conditions, or optimizing sputtering parameters. The coating thickness and worn surface morphology of the coated substrate were analyzed using an SEM (S-3400; Hitachi). The crystal structure analysis of various coating materials on the substrate was done using an X-ray diffraction (XRD) analyzer (Match phase analyzer). The radiation source used was Cu-Kα 1 and the wave length was 1.54 Å. The observed 2θ peaks were compared with reference 2θ peaks from JCDPS files. Further, the percentage of each chemical element was examined on the coated specimen by using the EDX analysis method. The micro-abrasion tester uses a block-on-ring method, based on ASTM Standard G77. The test parameters are shown in Table 2.

Fig. 1.

Schematic diagram of abrasion tester and its mechanism.

Figure 1 shows the geometry of the wear scar, which is proportional to that of the rotating ball, Therefore, the wear volume (V) was estimated from the radius (R) of the rotating ball, using either the crater diameter (b) or the depth (t) of the coating material, as specified by Eqs (1) and (2). $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle V={\displaystyle \frac{{\pi}b^{4}}{64R}}\quad \text{for}∼b\ll R\end{eqnarray}$ (1) $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle V={\pi}t^{2}R\quad \text{for}∼t\ll R.\end{eqnarray}$ (2)

The Vicker’s microhardness (model HM-200A) measurements were conducted in accordance with ASTM Standard E384. The adhesion strength between various titanium alloy coatings and substrates was investigated by the direct pull-off method, as specified by ASTM Standard D 5868. In this test, the coated substrates were bonded to the uncoated steel using epoxy resin with a known adhesive strength. These pull-off tests were carried out using the FIE UTE-20 apparatus.

Table 2

Micro- abrasion test parameters

Parameter	Value
Load range	0.5 N, 1 N, 1.5 N
Diamond ball	High carbon –, high chromium material
Sliding distance	150 rev.
Ball diameter	25.4 mm
Abrasive paste	Dry slurry
Motor	0.15/150 kW/rpm

3. Results and discussion

Figures 2(a), (b) and (c) show the XRD results of coated TiCoCr, TiN, and TiO₂ on 316L SS specimens. The samples were analyzed over a range from 10° to 80°. Figure 2-(a) shows the sharp peaks and confirms the formation of titanium at 35.93, 40.142, and 53.01; cobalt at 62.925,- and 75.867 and chromium at 44.244, and 38.386. The crystal plane index was found for titanium, cobalt and chromium as (101), (110) and (100), respectively, were consistent with JCPDS file numbers 653362, 897373, 892871. Owing to their minimum weight percentage proportion, the diffraction peaks for Co and Cr were less intense than titanium. Similarly, Figure 2-(b) shows the sharp peaks and confirms the formation of TiN at angles 36.675°, 42.797°, 61.857° and 74.445°. The crystal plane indices for TiN were identifed as (111), (200), (220), (311) as evident in JCPDS file no 870633. In Fig. 2(c), the XRD graph of the TiO₂-coated surface on 316L SS, shows similar findings. The strong 2θ peaks at 25.102, 36.485, and 48.912 equal correspond to the miller indices plane (101), (004), and (200), respectively, confirming the presence of Ti and O molecules in the coated surface. Thus, the coating on 316L SS is effective and introduces a significant amount of dopant onto the substrate. Figures 3-(a), (b), and (c) show the EDAX spectra of TiCoCr, TiN, and TiO₂-coated 316L SS specimens. Notably the spectrum for TiCoCr reveals the presence of atoms from Co, Cr and Ti, each with distinct keV values.

Fig. 2.

XRD graph of (a) TiCoCr, (b) TiN, (c) TiO₂ coating on 316L SS.

Fig. 3.

SEM fractograph with EDX of (a) TiCoCr, (b) TiN, (c) TiO₂-316L SS substrate.

Table 3 shows the micro-abrasion properties such as wear volume and specific wear rate of various coatings done applied to a stainless steel substrate. The increment in wear volume of 71% and 261.8% was noted for 1 and 1.5 N loading conditions. This higher wear volume and specific wear rate can be attributed to adhesion and three-body abrasion wear loss mechanisms. Thus, higher wear loss was observed at higher loading conditions.

Fig. 4a.

SEM- worn surface tracks of uncoated 316L SS at 0.5 N.

Fig. 4b.

SEM -worn surface tracks of uncoated 316L SS at 1 N.

Fig. 4c.

SEM- worn surface tracks of uncoated 316L SS at 1.5 N.

Fig. 4d.

SEM- worn surface tracks of TiN coated 316L SS at 0.5 N.

Fig. 4e.

SEM- worn surface tracks of TiN coated 316L SS at 1.0 N.

Fig. 4f.

SEM-worn surface tracks of TiN coated 316L SS at 1.5 N.

Fig. 4g.

SEM-worn surface tracks of TiO₂ coated 316L SS at 1.5 N with 30 min coating time.

Fig. 4h.

SEM-worn surface tracks of TiO₂ coated 316L SS at 1.5 N with 90 min coating time.

Fig. 4i.

SEM-worn surface tracks of TiCoCr coated 316L SS at 1.5 N with 30 min coating time.

Fig. 4j.

SEM-worn surface tracks of TiCoCr coated 316L SS at 1.5 N with 60 min coating time.

Fig. 4k.

SEM-worn surface tracks of TiCoCr coated 316L SS at 1.5 N with 90 min coating time.

Figures 4a, (b), and (c) show the SEM-worn surface tracks of uncoated 316L SS specimens at different loading conditions. The worn surfaces are present with scars and clear evidence of abrasion disc. There are pit marks and dimples present on the worn surface, which indicates the ductile nature of 316L SS and the adhesive wear loss phenomenon of the material. This also indicates a higher adhesion phenomenon of ductile uncoated 316L SS substrate with abrasion disc.

Table 3

Wear behavior of uncoated 316L SS substrate

Coating time (min)	Load (N)	Wear volume ×10⁻⁵ m³	Specific wear rate × 10⁻¹⁴ mm³/Nm
Uncoated	0.5	18.54	1.027
	1	31.72	0.881
	1.5	67.08	0.931

Table 4 shows the wear volume and specific wear rate of TiN-coated 316L SS substrate under different loading conditions and coating time. It is noted that the wear volume of 16.5, 12.8, and 10 × 10⁻⁵ mm³ was observed for 1.5 N load at 30, 60, and 90 min coating time respectively. Table 5 shows the TiO₂ coating on 316L SS at 0.5, 1.0, and 1.5N loads and 30, 60 and 90 min coating time. The wear volume and wear rate are significantly higher for TiO₂ coating for 30 and 60 min. This increment in the wear volume and specific wear rate is the reason for the lack of a strengthening mechanism of TiO₂ on the steel surfaces. Moreover, there is no evidence for pit marks and dimples, which confirms the high wear resistance behavior of the material even after the shear force is applied.

Table 6 shows, that wear volume and war rate is significantly reduced for TiCoCr coating for 30, 60 and 90 min. In comparison to the uncoated surface the TiCoCr-coated steel shows very high wear resistance. A very low wear volume of 7.372 × 10⁻⁵ m³ is observed for coating done for 90 min at 0.5 N loading condition. Moreover, with a 90 min coating time, all the loading values show improved abrasion resistance. This is because of the higher penetration time of Co and Cr atoms onto the surface of the steel. The addition of Cr and Co may produce fine intermetallic on the substrate’s surface, thus providing higher wear resistance. Both Co and Cr may reinforce the surfaces of steel and produce high surface rigidity and also result in lesser three-body abraded particles and a wavy structure.

Table 4

Wear behavior of TiN-coated 316L SS substrate

Coating time (min)	Load (N)	Wear volume ×10⁻⁵ m³	Specific wear rate × 10⁻¹⁴ mm³/Nm
30	0.5	15.9	0.883
	1	16.3	0.452
	1.5	16.5	0.229
60	0.5	9.7	0.538
	1	9.9	0.275
	1.5	12.8	0.177
90	0.5	9.4	0.522
	1	9.5	0.263
	1.5	10	0.138

Table 5

Wear behavior of TiO₂-coated 316L SS substrate

Coating time (min)	Load (N)	Wear volume ×10⁻⁵ m³	Specific wear rate × 10⁻¹⁴ mm³/Nm
30	0.5	21.94	1.219
	1	22.79	0.633
	1.5	23.38	0.324
60	0.5	18.84	1.047
	1	20.25	0.562
	1.5	20.94	0.290
90	0.5	13.51	0.750
	1	13.66	0.379
	1.5	14.15	0.196

Table 6

Wear behavior of TiCoCr-coated 316L SS substrates with different load and coating time

Coating time (min)	Load (N)	Wear volume ×10⁻⁵ m³	Specific wear rate × 10⁻¹⁴ mm³/Nm
30	0.5	13.017	0.722
	1	13.360	0.371
	1.5	13.54	0.188
60	0.5	8.212	0.456
	1	8.405	0.233
	1.5	9.074	0.126
90	0.5	7.372	0.409
	1	7.524	0.208
	1.5	7.600	0.105

Figures 5(a), (b) and (c) show COF of 0.09, 0.053, 0.05 and 0.041at 0.5 N load;- 0.118, 0.082, 0.091 and 0.077 at 1 N and 0.311, 0.169, 0.178, and 0.123 in 1.5 N for coating time 30 min. At higher loading conditions such as 1.5 N, the uncoated 316L SS substrate show a higher COF. This is because of the higher affinity of the uncoated surface to the abrasion ball and lesser sliding velocity. However, the various coating materials such as TiO₂, TiN, and TiCoCr show lower COF at various loading conditions. In all coating materials, the TiCoCr-coated material has a much lower COF of 0.041. It is further observed that the COF of 0.09, 0.043, 0.039, and 0.026 at 0.5 N; 0.118, 0.058, 0.053, and 0.033 at 1 N and 0.311, 0.061, 0.072, and 0.05 at 1.5 N were noted for coating time 60 min. Similarly, the COF of 0.09, 0.03, 0.038 and 0.012 at 0.5 N load; 0.118, 0.036, 0.046 and 0.023 at 1N; and 0.311, 0.061, 0.072, and 0.05 at 1.5 N were noted for coating time 90 min. When comparing all coating times, the 90 min coating time gives typically lower COF, indicating the higher wear resistance of 90 min coated substrates.

Fig. 5.

COF of (a) 30 min, (b) 60 min, (c) 90 min coated substrates.

Figure 6 shows the micro-hardness values of coated surfaces concerning the loads. It is noted that the uncoated surfaces have the surface hardness of 210 HV at 5 N, 191 HV at 10 N, 179 HV at 20 N, 175 HV at 30 N and 172 HV at 40 N. This nominally lesser hardness is the reason for plastic deformation in the substrate material. However, the reinforcement such as TiN, TiO₂ and TiCoCr onto the 316L SS surface increases the surface hardness. This is because of addition of Ti atoms to the surface along with N, Co and Cr. The addition of hard-structured atoms such as Ti and Co onto the steel surface and the formation of intermetallics created higher penetration resistance against mechanical loading. This improvement is because of the addition of nitrogen atoms to the steel surface. This nitrogen penetrates and likely forms fine intermetallics on the steel surface, which eventually gives higher microhardness. Moreover the fine deposition of TiN on the substrate produced hindrance to the larger plastic deformation. The increased load induces more extensive deformation, allowing greater accommodation of the stress, thereby diminishing the material’s hardness, Fig. 7 shows the AFM resulting for TiN, TiO₂ and TiCoCr coated on the 316L SS substrate at 30, 60 and 90 min respectively.

Fig. 6.

Micro hardness values of coated and uncoated 316L SS substrates.

The coatings were finer at the initial time with a 100 nm thicknesses and when the coating time increases the coating grain size marginally increase up to 169 nm for 60 min and 256 nm for 90 min respectively. At 90 min coating time the coatings are a bit coarser. However, due to higher coating density and the effective filling pores the surface hardness remains higher. However, the addition of TiO₂ onto the surface gives lesser micro-hardness. A highest micro-hardness of 467 HV was observed for substrates coated with TiO₂ for 90 min. This is significantly lower than the TiN-coated substrate at the same time. Fig. 7 shows the 30, 60 and 90 min coated TiO₂ on the substrate’s surface. The coated grains are coarser and flatter, which increase the plastic deformation rate. The figure shows the AFM image of TiCoCr-coated 316L SS at 30, 60 and 90 min coating time. The thickness of 110, 170, and 260 nm was achieved, which is eventually higher than TiN and TiO₂ coatings on the substrate.

Fig. 7.

AFM images of Ti based coating on 316L SS at (a) 30 min (b) 60 min (c) 90 min.

Figures 8 and 9 shows the graph of surface roughness (Ra) and the adhesion strength of various coating done using 316L SS substrate with different coating time. It is observed that the surface roughness of 30, 60 and 90 min coated surfaces show significant changes in the surface roughness. At coating times of 30 min or less, the coated surfaces exhibit higher surface. However, as the coating time increases to 90 min, the surface roughness decreases. It is noted that the surface roughness value of 0.31, 0.38, and 0.28 μm were noted for TiN, TiO₂ and TiCoCr coating on 316L SS substrate at the lesser coating time of 30 min. However the aged coating time up to 60 and 90 min led lesser surface roughness of 0.25, 0.33, 0.22 and 0.19, 0.26, 0.17 μm for TiN, TiO₂ and TiCoCr coating. The values showed reduction trend in surface roughness with respect to the coating time. This reduction in the coating roughness is the reason for uniform filling of atom during the aging coating process.

Figure 9 shows the adhesion strength of various coatings on 316L SS substrate with various coating time. It is noted that the 30 min coating time in TiO₂, TiN and TiCoCr yields marginal adhesion strength of 26.7, 32.4, and 34.2 MPa respectively. However when the coating time increases the there is a marginal shift in adhesion strength. The coating time of 60 min yields improved adhesion strength of 28.8, 35.5 and 36.4 MPa, which is equal to 7.86%, 9.56% and 6.43% on compare to the 30 min coating time. This improvement is the reason for effective layer building and high cohesive strength of particle. When the coating time is higher, the particle settling and the cohesive force between the TiO₂, TiN, and TiCoCr increase, which results higher adhesion strength when the shear force is applied. It is further noted that the prolonged coating time of up to 90 min significantly reduced the adhesion strength. There is a marginal dip in the strength is observed in all coating materials such as TiO₂, TiN, and TiCoCr. The observed adhesion strength of 28.1, 34.8, and 35.8 MPa were noted for 90 min coating time for TiO₂, TiN, and TiCoCr, respectively. This is about 2.43%, 1.97% and 1.64% of decrement on compare with the 60 min coating on the substrate. This marginal decrement is the reason for higher rate of deposition and typically weaker cohesive force when the shear force is applied perpendicularly. This phenomenon produced higher pull force in the deposited atom, and this force would be higher than the cohesive force. Thus, the cohesive force collapse will happen, thereby achieving lesser adhesive strength.

Fig. 8.

Surface roughness of coatings on substrate.

Fig. 9.

Adhesion strength of coatings on substrate.

When comparing TiN, TiO2, and TiCoCr, a higher adhesion strength of 36.4 MPa was observed for 60 min coated TiCoCr on 316L SS substrate. This enhanced adhesion strength for TiCoCr contributes to the higher cohesive force of atoms compared to the 90 min coated substrate. Moreover when comparing all the TiCoCr coatings improved results were obtained at the 60 min coating time. This could be attributed to strong cohesion between Ti, Co, and Cr atoms, enhancing their penetration ability.

4. Conclusions

The following conclusions were obtained from this investigation:

The titanium alloy TiCoCr coating at 90 min on 316L SS gave higher abrasion resistance among other. Approximately 60% improvement was observed than that of uncoated substrate. The specific wear rate was significantly low at 0.105 × 10⁻¹⁴ mm³/Nm with COF of 0.023. However the TiN and TiO₂ coating on substrate yielded lower values.

The Vicker’s hardness number of 682 HV was observed for 90-min-coated TiCoCr 316L SS implant. The other coating times yielded lower microhardness.

The adhesion strength of 60-min-coated samples showed improved results. A superior adhesion strength of 36.2 MPa was observed for the 60-min coated TiCoCr on the 316L SS implant. However, the 30 and 90-min coatings showed a marginal decrease in adhesion strength.

On the basis of research findings, the titanium alloy TiCoCr with 90 min coating time showed siginificantly improved surface properties than oxide state of TiO₂ and nitrate state of TiN. Thus it is preferable that the 90-min coating time is recommended for coating titanium based alloys in steel based bio-implants. Moreover, opting for alloy state of TiCoCr instead of the oxide and nitrate state of titanium can led to desirable results in bio-implant applications.

The combination of these elements in the coating (Ti, Co, Cr) and the substrate (mainly Cr and Fe in SS) creates a synergistic effect at the interface. This effect arises from chemical interactions; inter diffusion, and the formation of strong intermetallic compounds and oxide layers. As a result, the adhesion between the TiCoCr coating and the stainless steel substrate is significantly enhanced compared to coatings with different compositions.

Footnotes

Conflict of interest

The authors declare that they have no conflicts of interest related to the research presented in this manuscript.

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