The present investigations are being carried out to encapsulate 316L SS with a Ti-based alloy coating fabricated by using PVD sputtering method. TiN, TiO2, and TiCoCr powders with prescribed process parameters are consecutively coated on 316L SS through PVD sputtering process with coating durations of 30, 60, and 90 minutes respectively. Further microhardness, surface roughness, microabrasion and adhesion strength tests were also carried out to determine the impact of the coating material and the influence of coating time on the coated surfaces. Comparing to uncoated substrate merely a 60% improvement in abrasion resistance was observed in TiCoCr coated samples. The XRD results confirmed the optimal formation of Ti alloy coatings with corresponding orientation over the SS substrates. Moreover, TiCoCr with a 90 minutes coating duration has much better surface characteristics than TiO2 and TiN. Thus it is preferable that the 90 min coating duration is optimal for coating in steel for bio implants.
Research Article
Wear performance of Ti based alloy coatings on 316L SS fabricated by sputtering method – Relevance to Biomedical Implants
https://doi.org/10.21203/rs.3.rs-1565129/v1
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posted 19 Apr, 2022
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The biomaterial is a material that can be implanted to associate with the biological system of a human body as far as medical devices particularly artificial joints, hip implants, dental implants, and so on. The implant biocompatibility plays a key role owing to in vivo environment of humans may give hazardous products impacting on the tissues and organs (Hallab & Jacobs 2013). Moreover, the medical implant generally utilizes chrome steel, Co-Cr alloys, and Ti alloys (Sahasrabudhe et al. 2016). For biocompatibility nature, strength, and invaluable corrosion resistance, stainless steel (SS) is perhaps the most recognized materials used as biomaterials including iron, chromium, and nickel in loftier in extent, SS is referred to be a partner especially the iron-based alloys fundamentally will rust or corrode in a limited capacity to focus (Cao et al. 2014). For biocompatibility nature, strength, and invaluable corrosion resistance, stainless steel (SS) is perhaps the most recognized materials used as biomaterials including iron, chromium, and nickel in loftier in extent, SS is referred to be a partner. Martensitic SS are not as corrosion resistant as ferritic or austenitic SS because of their lower substance of chromium (Holland et al. 1991). Authors were reported SS for biomaterial implants for the sake of resistance towards human fluids due to the existence of molybdenum (Mo) in the alloy (Sivakumar et al. 1994). Regardless of this fact, 316L has a downside in that it holds unsatisfactory performance in the wake of tribological behavior and mechanical strength. This is due to the fact that the hardness value stands low which directed the experimenters to concentrate on various surface engineering processes so as to enhance its tribological behavior retaining the distinguishable corrosion resistant property (Sieradzki et al. 1986). With an end goal to enhance the properties of the biological implant materials such as tribological, electrical, optical, biological and more, surface coating is employed (Saravanan et al. 2018). As well as that it exhibits dreadful behavior in wear (Sun and Bell 2002), friction and abrasion (Rahman et al. 2005), micro-cracks(Li et al. 2019) and severe plastic deformation (Li& Bell 2004). Hence, there is a need for surface modification over the base material 316L SS so as to be utilized as an incomparable biological implant. (Uddin et al. 2019) hypothesized the tribological, mechanical, and chemical characteristics of TiN coatings on Ti substrate for biomedical applications. Authors reported the better adhesion strength, hardness, wear range, and corrosion rate in TiN coated samples at different parameters (Caha et al. 2019) sprayed TiN coatings using sputtering process. The results showed that TiN coatings deposited over 80 minutes had superior overall behavior than bare metal and TiN coatings deposited over 30 minutes. The CAE-PVD TiN film deposited on 316LVM SS and Ti6Al4V alloy was examined by (Lepicka et al. 2019). Author reported the mechanical characteristics, adhesion, and anti-wear performance, durability of TiN coated by using PVD deposition technique (Verissimo et al. 2015) and energized ions to be accelerated to employed higher corrosion resistance, wear resistance (Makhlouf 2011). Microabrasion is a minimally invasive procedure used to renew overall coating quality and texture. The microabrasive test helps in simulating the wear conduct of materials under various conditions (Li &Leroux 2016). The covered and uncoated substrates were permitted to go through wear instrument with the guide of a micro abrasion tester utilizing a block-on-ring technique, in light of standard ASTM G77 systems as demonstrated in the schematic representation of micro-abrasive Apparatus (MS Priyan &Hariharan 2014). The geometry of the wear scar, thus formed is proportional to that of the rotating ball, Thereupon the wear volume can be estimated from either the crater diameter (b) or the depth (t) of the coating material as follows:
Here, radius of the rotating ball is expressed as R, imposed wear crater depth for the abrasive wear mechanism as‘t’.
AFM can map out the topography of any surface and represents various physical parameters of the surface including friction(Gutierrez et al. 2017; Rao and Costa 2014) and 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 (1-100 µA) (Pankaew et al. 2012). The goal of this study is aimed to produce Ti-based alloy coatings on bio implantable material 316L SS utilizing a PVD sputtering procedure with a flexible time frame. Surface features such as abrasion resistance, micro hardness, coating thickness, and adhesion strength are planned to study evaluate for the specimen. Coating materials including TiN, TiO2, and TiCoCr are also being considered for usage in the development process. The coated 316L SS substrates are being studied in order to define their coating behavior for applying the bio implants application.
The 316 stainless steel chemical composition mentioned in the Table 1. The commercially available TiN and TiO2 powder is used for coating process. The proposed novel TiCoCr combination of Ti-55%, Co-25% and Cr-20% was mechanically alloyed using high energy planetary ball mill (retsch pm 200, Germany).
| Element | C | Mn | Si | P | S | Cr | Mo | Ni | N |
|---|---|---|---|---|---|---|---|---|---|
| Wt. % | 0.03 | 2.0 | 0.75 | 0.045 | 0.03 | 17 | 2.5 | 12 | 0.1 |
The substrate material for this experiment is a cylindrical 316 L grade stainless steel with a diameter of 25 mm and a thickness of 1mm, which was cut using the wire-cut EDM (Electrical Discharge Machining) process. The deposition of Ti based alloy coating target over the base material was performed by sputter deposition type of physical vapour deposition process. The coating time has been fixed for all samples at 30 min, 60 min, and 90min separately. Further, the worn surface morphology of various coating materials on substrate was analyzed using a scanning electron microscope (Hitachi S-3400, Japan). The crystal structure analysis of various coating materials on substrate was done using an XRD analyzer (Match phase analyzer, Germany). The radiation source used was Cu-Kα 1 and the wave length was 1.54 ͦA. Moreover, the diffracted X-rays were captured using an X-ray film and thus 2θ peaks were plotted. The observed 2θ peaks were compared with reference 2θ peaks from [JCDPS] files. Further, the percentage of each chemical element examined on coated specimen by using energy dispersive X-ray analysis method. The microabrasion tester using a block-on-ring method, based on standard ASTM G77 procedures as shown in the schematic representation of micro abrasive apparatus in Fig. 1. The test parameters are exemplified in Table 2. The geometry of the wear scar, thus formed was proportional to that of the rotating ball, there-upon the wear volume was estimated from either the crater diameter (b) or the depth (t) of the coating material as follows in accordance to Equations.
Here, radius of the rotating ball is expressed as R, imposed wear crater depth for the abrasive wear mechanism as‘t’. Where, R is the radius of the rotating ball, and‘t’ is the imposed wear crater depth for the abrasive wear mechanism as shown in Fig. 1.
| Parameter | Value |
|---|---|
| Load range | 0.5N, 1N, 1.5N |
| Diamond ball | High carbon – high chromium material |
| Sliding distance | 150 rev |
| Ball diameter | 25.4 mm |
| Metal wheel | 50 mm |
| Rubber wheel | 50 mm |
| Abrasive paste | Dry slurry |
| Motor | 0.15/150 kW/rpm |
The Vicker’s microhardness measurements were conducted in accordance to ASTM E384. The adhesion strength between various titanium alloy coating and steel substrate was investigating by the direct pull-off method. In pull-off tests, the coated substrates were glued with epoxy resin of known adhesive strength to the uncoated steel. The pull-off test was carrying out on an electronic tensile testing machine FIE UTE-20, India. The test was carried out in compliance with ASTM D5868 guidelines.
Figure 2 (a), (b) and (c) shows that 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 proves the formation of Titanium at 35.93, 40.142 and 53.01, Cobalt at 62.925, 75.867 and Chromium at 44.244, 38.386. The crystal plane index was found for Titanium, Cobalt and Chromium such as (101), (110) and (100) and it was evident in JCPDS file number 653362, 897373, 892871, respectively. Owing to their minimum weight percentage proportion, the intensity of diffraction peaks was low for Co and Cr corresponds with titanium. Similarly, Fig. 2 (b) shows the characterization of sharp peaks and proves the formation of TiN located at 36.675, 42.797, 61.857 and 74.445. The crystal plane index was found for TiN such as (111), (200), (220), (311) and it was evident in JCPDS file number 870633. Similar indications were noted in Fig. 2 (c) the XRD graph of TiO2 coated surface on 316L SS. The strong 2 theta peaks at 25.102, 36.485 and 48.912 equal to the miller indices plane (101), (004) and (200) were confirms the presence of Ti and O molecules in the coated surface. Thus the coating on 316L SS is effective, which produce considerable amount of dopants on the substrate. Figure 3 (a), (b) and (c) shows the EDAX spectrum of TiCoCr, TiN and TiO2 coated 316L steel specimens. It is noted that the spectrum for TiCoCr gives the atoms of Co, Cr and Ti with different keV values.
Table 3 shows the micro abrasion properties such as wear volume and specific wear rate of various coating done on stainless steel substrate. The incremental in wear volume of 71% and 261.8% was noted for 1 N and 1.5N loading conditions. This notably higher wear volume and specific wear rate was the reason for adhesion and three body abrasion wear loss mechanism. Thus higher wear loss is evidenced at higher loading condition. Figure 4 shows the SEM worn surface tracks of uncoated 316L SS specimens at different loading conditions.
| 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 with different loading conditions and coating time. It is noted that the wear volume of 16.5, 12.8 and 10x10− 5 mm3 was observe for 1.5N 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 is significantly higher for TiO2 coating by 30 and 60 min. This increment in the wear volume and specific wear rate is the reason for no strengthening mechanism of TiO2 on steel surface. Moreover, there is no evidence for pitting marks and dimple, which confirms the high wear resistance behavior of material even after the shear force is applied.
Table 6 shows the wear volume and war rate is significantly reduced for TiCoCr coating by 30, 60 and 90 min. On comparing to the uncoated surface the TiCoCr coated steel gives very high wear resistance. A very lowest wear volume of 7.372×10− 5m3 is observed for coating done for 90 min at 0.5N loading condition. Moreover in 90 min coating time all the loading values gives improved abrasion resistance. This is because of higher penetration time of Co and Cr atoms onto the surface of steel. The addition of Cr and Co may produce fine intermetallic on the substrate’s surface, thus giving higher wear resistance. Both the Co and Cr may reinforce into the surfaces of steel and produced high surface rigidity and also giving lesser 3 body abraded particle and wavy structure.
| 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 | ||
| 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 |
| Coating time ( min) | Load (N) | Wear volume ×10− 5m3 | 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 |
Figure 5 (a), (b) and (c) observed COF of 0.09, 0.053, 0.05 and 0.041 in 0.5 N load, 0.118, 0.082, 0.091 and 0.077 in 1N and 0.311, 0.169, 0.178 and 0.123 in 1.5N were noted for coating time 30 min at different loads. At higher loading condition such as 1.5N the uncoated 316L SS substrate gives higher COF. This is because of higher affinity of uncoated surface to the abrasion ball and lesser sliding velocity. On comparing to all coating time the 90 min coating time gives typically lesser COF, which indicates the higher wear resistance of 90 min coated substrates.
Figure 6 shows the micro hardness values of coated surfaces with respect to the loads. It is noted that the uncoated surfaces have the surface hardness of 210HV at 5N, 191HV at 10N, 179HV at 20N, 175HV at 30N and 172HV at 40N. This nominally lesser hardness is the reason for coarse grain structure in the substrate material. However the doping of reinforcement such as TiN, TiO2 and TiCoCr onto the 316L SS surface the surface hardness is increased. This is because of adding Ti atoms to the surface along with N, Co and Cr. Figure 7 shows the AFM resulting of TiN, TiO2 and TiCoCr coated on the 316L SS substrate at 30, 60 and 90 min respectively. Moreover, the coated grains were coarser and flatter, which increased the plastic deformation rate. The AFM image of TiCoCr coated 316L SS at 30, 60 and 90 min coating time. The thickness of 110nm, 170nm and 260nm were achieved, which is eventually higher than TiN and TiO2 coatings on the substrate.
Figure 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. The coated surfaces at the lesser coating time (30 min), the surface roughness is typically higher and the same decreases with increase of coating time up to 90 min. It is noted that the surface roughness of 0.31, 0.38 and 0.28µm were noted for TiN, TiO2 and TiCoCr coating on 316L SS substrate at the lesser coating time 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, 0.17µm for TiN, TiO2 and TiCoCr coating at 60 and 90 min respectively. 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 at the aged 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 gives marginal adhesion strength of 26.7, 32.4 and 34.2 MPa. However when the coating time increases the there is a marginal shift in adhesion strength. The coating time 60 min yield improved adhesion strength of 28.8, 35.5 and 36.4MPa, 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 increases, which resulted higher adhesion strength when the shear force is applied. It is further noted that the prolonged coating time 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 respectively. The observed adhesion strength of 28.1, 34.8 and 35.8MPa were noted for 90 min coating time in 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.
It is noted that on comparing TiN, TiO2 and TiCoCr the higher adhesion strength of 36.4MPa was observed for 60 min coated TiCoCr on 316L SS substrate. This higher adhesion strength for TiCoCr is the reason for higher cohesive force of atoms with each other than the 90 min coated substrate. Moreover it is noted that by comparing all the TiCoCr coating gave improved results in 60 min coating time. This is the reason for high cohesion strength of Ti, Co and Cr atoms and their penetration ability. Thus the 60 min coating time yields higher adhesion strength in all coated substrates.
The specific conclusions made from this present investigation are as follows.
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The sputtering process successfully yields significant coating thickness over the stipulated coating time. The results of coating were confirmed using XRD and EDAX analysis.
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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. A lesser specific wear rate of 0.105×10–14 mm3/Nm with COF of 0.023. However the TiN and TiO2 coating on substrate yield lower value.
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The microhardness of TiCoCr 90 min coated 316L SS implants gave highest Vicker’s hardness number. The Vicker’s hardness number of 682 HV was observed for 90 min coated TiCoCr 316L SS implant. The other coating time yields lesser microhardness.
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The surface roughness of 90 min coating time on 316L SS substrate with TiO2, TiN and TiCoCr gave highly smooth surfaces whereas at 30 and 60 min the surface roughness marked higher.
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The adhesion strength of 60 min coated samples yields improved results. Highest adhesion strength of 36.2MPa was observed for 60 min coated TiCoCr on 316L SS implant. However the 30 and 90 min coatings gave marginal dip on the adhesions strength.
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Thus according to the findings the titanium alloy TiCoCr with 90 min coating time gave much improved surface properties than oxide state of TiO2 and nitrate state of TiN. Thus it is preferable that the 90 min coating time is idle for coating titanium based alloys in steel based bio implants. Moreover instead of the oxide and nitrate state of titanium the alloy state of TiCoCr could be preferable in order to achieve desirable results on the bio implants.
The authors declare no competing interests
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