Hydroxyapatite Coatings on High Nitrogen Stainless Steel by Laser Rapid Manufacturing
Hydroxyapatite Coatings on High Nitrogen Stainless Steel by Laser Rapid Manufacturing
ASHISH DAS 0
MUKUL SHUKLA 0
0 1.-Department of Mechanical Engineering , MNNIT, Allahabad , India. 2.-School of Mechanical and Aerospace Engineering, Queen's University , Belfast, Northern Ireland , UK. 3.-Department of Mechanical Engineering Technology, University of Johannesburg , Johannesburg , South Africa. 4.-School of Mechanical Engineering, KIIT University , Bhubaneswar , India. 5.-
In this research, the laser rapid manufacturing (LRM) additive manufacturing process was used to deposit multifunctional hydroxyapatite (HAP) coatings on high nitrogen stainless steel. LRM overcomes the limitations of conventional coating processes by producing coatings with metallurgical bond, osseointegration, and infection inhibition properties. The microstructure, microhardness, antibacterial efficacy, and bioactivity of the coatings were investigated. The microstructure studies established that the coatings consist of austenite dendrites with HAP and some reaction products primarily occurring in the inter-dendritic regions. A Vickers microhardness test confirmed the hardness values of deposited HAP coatings to be higher than those of the bare 254SS samples, while a fluorescence activated cell sorting test confirmed their superior antibacterial properties as compared with pristine samples. The coated samples immersed in simulated body fluid showed rapid apatite forming ability. The results obtained in this research signify the potential application of the LRM process in synthesizing multifunctional orthopaedic coatings.
For joint replacements, orthopedic implants are
frequently used and expected to rise for the next 15–
20 years.1 As reported previously, 18% of implants
fail due to aseptic loosening while 20% fail because of
infection.2,3 Hence, strong osseointegration and
antibacterial properties both need to be taken into
consideration while developing an implant material.4
Internal fixation devices are still being made of
austenitic stainless steels (largely SS 316L) in spite
of their lower corrosion resistance compared with
titanium. This is a result of their excellent
mechanical properties and low cost.5,6 Lately, a newer
generation of steel UNS S31254 SS (254SS) has been
studied. As reported in the literature, despite 254SS
not being bioactive and lacking strong
osseointegration, the absence of toxic effects and high nitrogen
content of 254SS have made it a candidate for
scientific investigation, in the hope of developing a
futuristic orthopedic implant material.7,8
Nevertheless, the literature confirms that these
problems can be mitigated by depositing
hydroxyapatite (HAP) coatings on metallic implants.9–17
Hydroxyapatite (HAP) holds a significant position
as an inorganic biomaterial, but bulk HAP ceramics
possess poor mechanical properties restricting its
use for non-load-bearing implant applications. To
overcome this limitation, HAP is being coated on
metals and their alloys to derive the advantages of
both the bioactivity of HAP and the mechanical
performance of metals.18 HAP is a widely used
coating material for orthopedic implants because its
mechanical properties closely match with those of
human bone mineral, and it is also biocompatible
with human bone tissues.19 HAP coating of
orthopedic implants can be an effective method of
improving their physiological response leading to
an overall improved performance of the implants.20
HAP coatings can be deposited on a metallic
substrate using techniques such as direct laser
melting, sol–gel, and vapor deposition processes,
including plasma spraying, which is a commercially
viable technique for clinical applications.20–27
Nevertheless, HAP coatings produced by plasma-spray
bond poorly with metal substrate and are a
candidate to easy wear and tear, which results in
undesirable debris formation.28 Synthesizing a
composite layer with HAP on the implant surface is one
way to solve these problems. This can be achieved
by melting the preplaced HAP on the implant
surface using a laser.24 The primary aim of this
research is to assess the feasibility of synthesizing
multifunctional orthopedic coatings on 254SS
implant surfaces using laser rapid manufacturing
(LRM), which can prevent infection while still
promoting osseointegration. LRM is an additive
manufacturing (AM) process, which can be used not
only for preparing customized implants but also for
treating their surfaces in several ways.24,29,30
An LRM AM system equipped with a 2.0-kW
continuous wave, Ytterbium-doped, fiber laser with
a beam size of 2.5 mm was used. For direct laser
melting of the preplaced HAP powder bed on the
surface of a high nitrogen stainless steel (254SS)
substrate (100 9 50 9 2 mm3 sheet), several coating
samples, over a 10 9 10 mm2 area, were prepared at
1000-W laser power and 1-m/min scan speed. These
process parameter settings were finalized based on
Refs. 24 and 25, and preliminary trials were
conducted. At 800-W and 900-W laser power with 0.8-m/
min, 0.9-m/min, and 1.0-m/min scan speeds,
discontinuous tracks with weak bonding were produced. At
1000-W power with 0.8-m/min and 0.9-m/min scan
speeds, discontinuous tracks were produced while
1200-W power with 0.8-m/min, 0.9-m/min, and
1.0m/min scan speeds resulted in red hotness. These
outcomes are undesirable for orthopedic coatings,
and hence, the laser power and scan speed were
finally fixed at 1000 W and 1 m/min, respectively.
The HAP-coated 254SS samples were sectioned and
metallographically prepared for microstructural
studies according to ASTM E3-11. Microstructure and
surface morphology studies were conducted using an
optical microscope and a scanning electron microscope
(SEM, ZEISS EVO 50) equipped with an
energydispersive spectrometer (EDS), respectively. The
phase composition of the coated samples was
examined using an x-ray diffraction (XRD; RIGAKU Model:
Smart lab 3 KW). The microhardness of pristine and
coated 254 stainless steel was determined using a
Vickers microhardness tester. The antibacterial
efficacy of pristine and coated 254 stainless steel samples
was determined using the fluorescence activated cell
sorting (FACS) technique, which is one of the best,
modern technologies for cell sorting.31 Finally, the
apatite-forming ability of the coated samples was
determined by the simulated body fluid (SBF)
immersion test. The samples were taken out from SBF after
2 days of immersion, washed with distilled water,
dried at 100 C, and then examined under SEM for
apatite layer formation. It is believed that forming
bone-like apatite in vitro is critical because apatite is
the dominant inorganic phase of hard tissue. Thus,
bone-like apatite formation indicates good
osteoconductivity in biomaterials.32 Nevertheless, extensive
in vivo studies are required to confirm the
osseointegration, cytotoxicity, and biocompatibility of UNS
S31254 stainless steel.
RESULTS AND DISCUSSION
Surface Morphology Characterization
In Figs. S1, S2 and S3 of the supplementary
material, the cross sections of clad, heat-affected
zone (HAZ), substrate, and successful deposition of
coating on the substrate are clearly visible.
Figure 1 presents the SEM micrographs of the
cross-sectional microstructure of the coated sample.
Austenite dendrites and some interdendritic phases
are clearly visible in the coated samples as shown in
Fig. 1. The SEM micrograph of Fig. 1 confirms that
the interface between HAP coating and the 254SS
substrate is sound and diffused, which are the
characteristics of the laser deposited
coatings.29,30,33–35 Contrarily, HAP coatings deposited
by other methods such as plasma spraying have a
sharp interface, which is unacceptable for their
A typical SEM microstructure image of the
HAPcoated 254SS sample is shown in Fig. 2. Calcium is
present in interdendritic phases, chromium is
present in austenite dendrites phases, and iron is
present in both austenite dendrites and
interdendritic phases as clearly demonstrated in Fig. 2.
To acquire information about the composition and
phase of the elements present in the deposited HAP
coatings, EDS elemental mapping is carried out in
this research. The EDS elemental mapping images
of the HAP-coated 254SS sample are shown in
Figs. S4, S5 and S6 of the supplementary material.
The consistent surface morphology of the
LRMcoated 254SS sample at 2000 times magnification is
shown in Fig. 3.
The XRD diffractographs of the substrate, HAP
powder, and HAP coating are presented in Fig. 4.
The XRD results confirm the presence of HAP in
crystalline format 2h (31.66 , 34.48 , 35.98 , 39.04 ,
47.26 , 56.58 , 62.74 , 64.44 ) with minor traces of
Fe3P at 2h (41.64 ).
Microhardness tests were conducted to confirm
the coatings’ resistance to plastic deformation at a
load of 300 g applied for 30 s as per Ref. 36. The
hardness of 504 ± 6 HV of HAP-coated 254SS
substrate was found to be significantly higher than
the hardness of the bare substrate 265 ± 14 HV, as
confirmed by the difference in size of the indents in
the optical micrograph of Fig. 5.
The in vitro antibacterial efficacy of the bare and
HAP-coated 254SS samples was investigated
against Escherichia coli by using the previously
used FACS technique.37 This is to confirm the
reattainment of the antibacterial property in HAP
coatings synthesized by LRM. Compared with the
bare 254SS implant surface, the surface morphology
of the composite layer of HAP produced on the
implant surface helps in preventing bacterial
attachment. This results in reduced bacterial
infection, as confirmed by the FACS results and the
literature.1 Figures S7 to S10 in the supplementary
material show the FACS graphs that represent the
relative data of E. coli death, i.e., 32.2% for the
HAP-coated 254SS sample, with respect to the bare
The main influencing factor in reducing bacterial
infection is the increased surface roughness. As
previously reported in the literature,38 the
antibacterial effect can be put down to the scale of surface
irregularities, which are comparable with the size of
bacteria. The higher roughness seems to limit the
anchoring points for bacteria and reduces the area
in contact with their membrane.38
The bioactivity of HAP-coated 254SS substrate was
examined by conducting an immersion test in SBF.
Figure 6 shows the surface morphology of the
HAPcoated substrate after immersion in SBF. The growth
of apatite layers is clearly visible after the immersion
test, resulting in an average increase of nearly 0.16%
in the weight of HAP-coated substrate.39
The overall results of the present research
confirm that coating of HAP on 254SS by LRM is useful
for orthopedic implant applications. It overcomes
the limitations of plasma-sprayed HAP coatings,
such as poor bonding strength and wear resistance.
In this research, a successful attempt has been
made to treat the surface of 254SS, a high nitrogen
stainless steel, with HAP using an LRM system.
The coatings showed a composite microstructure
with austenite dendrites and interdendritic HAP
with small traces of Fe3P phase. A Vickers
microhardness test confirms the higher hardness of
HAPcoated 254SS substrate than that of the bare 254SS
substrate. FACS tests confirm that the coated
samples have a superior antibacterial property
when compared with the pristine samples. An SBF
immersion test confirms the rapid formation of
apatite on the HAP-coated surface. The present
research ably demonstrates that it is feasible to
synthesize multifunctional orthopedic coatings that
can prevent infection while still promoting
osseointegration on 254SS implant surfaces using the LRM
additive manufacturing process.
The use of synthesis, testing, and characterization
facilities of RRCAT, Indore (through Dr. C. P. Paul),
IIT Kanpur, CIR (through Dr. Naresh Kumar),
CMDR, Biotechnology and Applied Mechanics
Departments, MNNIT Allahabad, are gratefully
acknowledged. The authors would like to thank the
Ministry of Human Resource Development,
Government of India, and the University of Johannesburg,
South Africa, for providing financial support.
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The online version of this article (doi:10.1007/
s11837-017-2529-x) contains supplementary
material, which is available to authorized users.
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