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This study focuses on the new technologies for production of medical implants using combined areas of robotics and microplasma coatings. This involves the robot assisted microplasma spraying of a multilayer surface structure on the biomedical implant. The robot motion design reassures a consistent and customized plasma coating operation. Based on the analytical models results, certain spraying modes were chosen to form the optimized composition and structure of the titanium/hydroxyapatite multilayer coatings. It is desirable that the titanium coated low layer offer a dense layer to provide the implant with suitable structural integrity and the titanium porous layer and hydroxyapatite top layer present a biocompatible layers which are suitable for implant and tissue integration. Scanning and Transmission Electron Microscopy, and X-ray diffraction have been used for analyzing the structure of coatings. The new robot assisted microplasma spraying technique resulted from our research provides a promising solution in the medical implant technology.
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www.technology.matthey.com
https://doi.org/10.1595/205651320X15737283268284 Johnson Matthey Technol. Rev., 2020, 64, (2), 180–191
180 © 2020 Johnson Matthey
D. Alontseva*
School of Engineering, D. Serikbayev East
Kazakhstan State Technical University, 69
Protozanov Street, Ust-Kamenogorsk, 070004,
Kazakhstan
E. Ghassemieh**
The Wolfson School of Mechanical, Electrical
and Manufacturing Engineering, Loughborough
University, Loughborough, Leicestershire, LE11
3TU, UK
S. Voinarovych, O. Kyslytsia, Y.
Polovetskyi
E.O.Paton Electric Welding Institute of NAS of
Ukraine, 11 Kazymyr Malevich Street, Kyiv,
03150, Ukraine
N. Prokhorenkova, A. Kadyroldina
D. Serikbayev East Kazakhstan State
Technical University, 69 Protozanov Street,
Ust-Kamenogorsk, 070004, Kazakhstan
Email: *dalontseva@mail.ru;
**E.Ghassemieh@gmail.com
This study focuses on new technologies for the
production of medical implants using a combination
of robotics and microplasma coatings. This involves
robot assisted microplasma spraying (MPS) of
a multilayer surface structure on a biomedical
implant. The robot motion design provides
a consistent and customised plasma coating
operation. Based on the analytical model results,
certain spraying modes were chosen to form
the optimised composition and structure of the
titanium/hydroxyapatite (HA) multilayer coatings.
It is desirable that the Ti coated lower layer oer
a dense layer to provide the implant with suitable
structural integrity and the Ti porous layer and
HA top layer present biocompatible layers which
are suitable for implant and tissue integration.
Scanning electron microscopy (SEM), transmission
electron microscopy (TEM) and X-ray diraction
(XRD) were used to analyse the structure of the
coatings. The new robot assisted MPS technique
resulting from this research provides a promising
solution for medical implant technology.
1. Introduction
Manufacturing technology for medical implants
undergoes constant improvement in order to
accelerate the patient’s recovery and increase the
service life of the implant. Following this trend,
special attention is paid to applying biocompatible
coatings on the surface of implants (1–7). There
is a huge clinical need for advanced biomaterials
with enhanced functionality to improve the quality
of life of patients and reduce the burden of health
care for the world’s ageing population. In recent
years, Ti and HA have been widely used in medical
devices due to their favourable biocompatibility
(1–8).
The research presented here oers robot assisted
MPS of Ti wires and HA powder onto Ti substrates
(9–12). Currently, MPS technology is highly
specialised with few research groups working
on its development. Yet it has shown promising
results in delivery of high quality and well-designed
coatings (1–3). Among existing plasma spraying
Manufacturing and Characterisation of Robot
Assisted Microplasma Multilayer Coating of
Titanium Implants
Biocompatible coatings for medical implants with improved density and
crystallinity
181 © 2020 Johnson Matthey
https://doi.org/10.1595/205651320X15737283268284 Johnson Matthey Technol. Rev., 2020, 64, (2)
processes, MPS is particularly characterised by
low plasma power (up to 4 kW), small diameter
of the spray deposition spot on the surface (up to
15 mm) and the possibility of forming a laminar
jet. This laminar jet can be up to 150 mm in
length. It heats the refractory material in a stream
of argon plasma and provides low heat input into
the substrate (2). The process provides deposition
of Ti or HA on small size parts and components,
including those with complex geometry. This is
normally unachievable with any other method.
The MPS generally provides a micro-rough surface
and a higher degree of porosity (~20%) that in
the case of biocompatible coatings facilitates bony
tissues ingrowth; in most cases the bond strength
of MPS coatings with substrates is acceptable (2).
However, there are still a number of challenges
remaining to be addressed. The most important
issue is the formation of coatings with specied
structure and properties.
Currently, endoprosthesis medical practice widely
uses metal implants coated with HA (1–8, 13, 14). HA
is the calcium phosphate mineral Ca10(PO4)6(OH)2
of the apatite group, which is chemically similar
to the apatite of the host bone and is a source of
Ca and P for the bone-HA interface (3, 4, 6, 8).
HA coatings improve osseointegration and can
signicantly reduce the duration of implantation of
the endoprosthesis. It provides a reliable connection
with the bone and increases the reliability of the
implant (1–8, 13, 14). In the case of thermal
spraying of HA powder, the chemical composition
of the nal HA coating is dependent on the thermal
decomposition occurring during spraying. The
high temperatures experienced by HA powder
particles in the plasma spraying process lead to the
dihydroxylation and decomposition of the particles.
At temperatures above 1050°C HA decomposes
to tricalcium phosphate (β-TCP, Ca3(PO4)2) and
tetracalcium phosphate (TTCP, Ca4(PO4)2O), and
above 1120°C β-TCP is converted to α-tricalcium
phosphate (α-TCP, Ca3(PO4)2) (13, 14). Thus,
the resultant coating phase composition depends
on the thermal history of the powder particles.
The higher the plasma jet temperature and the
longer the exposure of the particles to plasma,
the greater the degree of phase transformation.
According to the British Standards Institution
standard specication BS ISO 13779-2:2000 (15),
the maximum allowable content of non-HA phases
in a HA coating is 5%, and the minimum allowable
percentage of crystallinity is 50%. The degree of
crystallinity of the HA coating aects the process
of osseointegration (16). Amorphous calcium
phosphate (ACP) has a higher rate of dissolution,
which reduces the recovery time of the patient, but
at the same time some reduction of the reliability
of xation of the endoprosthesis in the bone is also
possible. Thus, increased crystallinity provides
reliable xation of the implant in the bone (2, 3).
It is assumed that to increase the biocompatibility
of the implants and rapid accretion with the bone,
the implant surface should be covered with a
biocompatible coating with an extensive surface
morphology, with recommended pore sizes in
the coating from 20 µm to 200 µm, and closed
porosity of at least 30% (2–4, 16, 17). At the same
time, the coating must be rmly connected to the
implant, without transversal porosity, so that the
implant material does not interact directly with
the human body. Therefore in this research, the
proposed composition and thickness of the coating
is designed to meet these requirements. In order
to characterise the coated layer structures, SEM,
TEM and XRD are used. Optimum modes of plasma
spraying are chosen on the basis of the results of
structural characterisation.
The aim of this study is to select the modes of
MPS of both Ti wire and HA powder to obtain a
suciently thick (up to 300 μm) multilayer
Ti/HA coating. The dense Ti sublayer is supposed
to provide good adhesion to the substrate and
the porous Ti middle layer and HA top layer can
accelerate the bone ingrowth. Another objective is
to clarify the relationship between various plasma
spray process parameters and the resultant
coating structure. This is achieved through the
development of process models that relate process
parameters to various coating structures.
2. Materials and Methods
The HA was synthesised in the laboratories of
East Kazakhstan State Technical University.
The process of synthesis of HA powder with the
ratio Ca:P of 1.65 by a chemical precipitation
method was described in our previous paper
(10). The purity of HA powder was 99.5%, which
met the purity requirement (not less than 95%)
set out in the ASTM International (USA) standard
ASTM F1185- 03(2014) (18). The purity of the
synthesised HA is determined using the XRD
results which are further explained below. Before
spraying, the powder was dried at a temperature
of 120°C for 6 h, in order to avoid clogging during
powder feeding. HA powder was used as a sprayed
coating material. The particles had irregular shape
with smooth edges (Figure 1).
182 © 2020 Johnson Matthey
https://doi.org/10.1595/205651320X15737283268284 Johnson Matthey Technol. Rev., 2020, 64, (2)
The shapes of the particles are particularly
designed with smooth edges to make sure that
they do not stick together. Moreover the HA
particles should not cling to each other in order
to provide the necessary owability of the powder.
The owability is essential in gradual supply of the
powder to the plasma jet.
According to previous studies (9, 10), the particle
size of the HA powder for MPS should be in the
range of 40 μm to 90 μm. Before MPS, the dried
and milled HA powder was sieved through mesh
diameters 40 µm and 90 µm to obtain only the
powder fraction within the desired range. The
ow rate of the HA powder is in the range of
120 s 50 g–1.
Samples of medical Ti alloy of Grade 5 extra
low interstitials (ELI) (Table I) were used as
substrates for MPS. For deposition of Ti coatings,
wires of VT1-00 commercially pure Ti with
a diameter of 0.3 mm were used (Table I).
The samples of medical Ti alloy of Grade 5 ELI
(Table I) were cut with thicknesses of 3 mm from
rods with a diameter of 50 mm and 30 mm on
CTX 510 ecoline computer numerical control (CNC)
machine (DMG MORI AG, Germany). Plates of size
15 mm × 15 mm × 2 mm were cut from large
sheet of Grade 5 ELI alloy.
MPS of the HA powders and Ti wires was carried
out by MPN-004 microplasmatron (produced
by E.O.Paton Institute of Electric Welding, Kiev,
Ukraine) (22). The microplasmatron was mounted
on an industrial robot arm (RS010L, Kawasaki Heavy
Industries, Japan). It is able to move horizontally
along a computed trajectory at set speed. The
thickness of the coatings was varied from 80 μm
to 300 μm by changing the MPS parameters. The
speed of linear movement of the plasmatron along
the substrate was chosen to be 50 mm s–1. The
choice of speed of the plasmatron was based on
preliminary estimates of the temperature of the
substrate when exposed to a plasma jet (12).
This was to make sure the temperatures remain
well below the melting temperature. Ar served as
a plasma-forming and transporting gas for MPS;
additional heating of the substrate was not carried
out. Using a robotic arm allows precise spraying of
coatings with a uniform speed of movement of the
plasmatron along the surface of the implant, as well
as moving the plasmatron along a predetermined
path.
Before MPS, the surfaces of the samples were
degreased with acetone and subjected to ultrasonic
cleaning. To ensure proper adhesion of the coatings,
it was important to pre-treat the surfaces of the
substrates to increase their roughness. For surface
activation, gas abrasive surface treatment was
carried out on a Contracor ECO abrasive blasting
machine (Comprag Group GmbH, Germany) using
normal grade A14 electrocorundum. The chemical
reactivity of the substrate’s surface rapidly falls due
to oxidation and the adsorption of chemical gases
from the atmosphere. Therefore it is important
Table I Chemical Composition of Ti-Based Materials
Materials
grade
Reference
composition
wt% of element
Al V O C N H Fe Si Ti
Grade 5 ELI
alloy (19), (20) 5.50–6.75 3.50–4.50 0.13–0.20 0.08 0.05 0.015 0.25–0.40 base
VT1-00
commercially
pure Ti
(21) 0.3 0.12 0.05 0.03 0.003 0.15 0.08 base
500 µm 50 µm
(a) (b)
40.45 µm
48.23 µm
48.73 µm
80.57 µm
Fig. 1. SEM
images of
HA particles
indicating particle
size
183 © 2020 Johnson Matthey
https://doi.org/10.1595/205651320X15737283268284 Johnson Matthey Technol. Rev., 2020, 64, (2)
that the time interval between the gas abrasive
treatment and the coating on the surface does not
exceed 2 h. Before coating, samples were stored in
a tightly closed container.
To evaluate the porosity of biocompatible
coatings, the images obtained by the scanning
electron microscope JSM-6390LV (JEOL Ltd, Japan)
were processed using MicroCapture (MustCam,
Hong Kong) and ATLAS.ti (ATLAS.ti Scientic
Software Development GmbH) computer-aided
programs. The measurements were carried out
on the polished cross-section of the coatings
according to ASTM E2109-01(2014) standard (23).
The surface roughness of the substrates and the
as-sprayed coatings was measured in accordance
with ISO 4287:1997 (24) using a MarSurf PS 10
mobile roughness measuring instrument (Mahr,
Germany). Four measurements were taken for
each sample and the average was determined. The
adhesion strength of coatings to the substrates
was measured in tension using a AG-X universal
testing machine (Shimadzu, Japan) in a static
experiment according to ASTM C633-13(2017)
standard (25).
The crystallinity and the structure-phase
compositions of HA powders and plasma sprayed
HA coatings were measured by X’Pert PRO
diractometer (PANalytical, The Netherlands).
The interpretation of the XRD patterns was carried
out using Rietveld method and powder diraction
data from the database of the International Centre
for Diraction Data (ICDD, USA, 2003), the ASTM
card le and X’Pert HighScore Plus software
(Malvern Panalytical, UK). The percentage of
crystallinity of the HA powder was calculated
using the area of crystalline peaks in the region
of 15° to 45° and the area of the amorphous
diuse background in this region. Diraction
scans of the HA powder and coatings were carried
out in accordance with ASTM F2024- 10(2016)
(26). The purity of HA powder and HA coatings
was evaluated by calculating the areas of all
non-HA peaks found in the diraction pattern.
The impurity area was determined by calculating
the area in the region where the highest impurity
phase peaks were present. The impurity peaks
that would be expected to be present in HA
powders and HA coatings were those of TTCP,
α-TCP and β-TCP. The Rietveld method was used
to quantitatively determine the percentage of
various phases of impurity in HA coatings. X’Pert
HighScore Plus software was used to calculate
the impurity. Three purity measurements were
carried out for each of the XRD patterns.
Electron diraction patterns of samples of HA
powder were obtained by TEM on JEM-2100 (JEOL).
The structural-phase composition data obtained
using TEM were compared with the data obtained
using XRD analysis. TEM sample preparation
techniques are described in detail in a previous
publication (11).
The particles of the sprayed Ti wire after
collision with the substrate were studied using
splat tests (2). MPS of Ti wire onto the plates
of polished Ti alloy was performed in the plane
perpendicular to the axis of the plasma jet.
Speed of the linear movement of plasma jet was
set at 50 mm s–1. As a result, single particles of
the sprayed material (splats) were xed on the
substrate and deformed upon contact with the
substrate surface. The splats’ visual analysis was
carried out by SEM; the splats were classied
according to their appearance and their spraying
modes (Table II).
Тable II MPS Deposition Parameters of Ti Wires, the Porosity and the Arithmetic Ra of
Sprayed Coatings
Spraying
mode
Parameters/settings Porosity of
sprayed Ti
coatings, %
Ra, μm
I, A Vpg, slpm H, mm Vw, m min–1
125 3.7 120 4.3 6.2 ± 0.74 12.0 ± 0.97
225 3.7 40 3.0 13.8 ± 1.90 45.6 ± 4.40
325 2.3 120 3.0 12.0 ± 1.50 44.4 ± 3.85
425 2.3 40 4,3 5.7 ± 0.45 12.0 ± 1.13
515 3.7 120 3.0 9.6 ± 0.35 31.5 ± 3.10
615 3.7 40 4.3 10.6 ± 1.32 34.8 ± 3.23
715 2.3 120 4.3 8.7 ± 2.32 30.2 ± 3.87
815 2.3 40 3.0 31.0 ± 3.87 >50
184 © 2020 Johnson Matthey
https://doi.org/10.1595/205651320X15737283268284 Johnson Matthey Technol. Rev., 2020, 64, (2)
3. Results and Discussion
The eect of parameters of MPS such as electric
arc current (I, A), plasma gas ow rate in
standard litre per minute (Vpg, slpm), spraying
distance (H, mm) and wire ow rate (Vw, m min– 1)
or powder consumption (Ppowder
, g min–1) on the
surface morphology, porosity and structural-phase
transformations of the coatings have been
studied. The coating experiments for MPS were
accomplished in a two level fractional factorial
design (24–1). The experimental conditions in
fractional factorial designs have been selected to
provide balanced design (27). The maximum and
minimum values of the parameters for feasible
processing of high-quality coatings were chosen
empirically.
MPS of Ti wires on Ti alloy gas-abrasive treated
substrates was carried out in various modes as
shown in Table II. The key characteristics of
the resultant coatings such as their porosity and
roughness were examined. The data in Table II
and Table III represent the averaged values for
three experimental runs.
Examination of the Ti particles splats obtained
in Modes 1, 4 and 8 (Table II) showed that the
samples are completely melted and have formed a
disk (Figures 2(a) and 2(b)). The thicker splats
are formed in Mode 8 (Table II, Figure 2(c)).
The beginning of the process of solidication of the
particles upon impact with the substrate is shown
in Figure 2(c). The increase in the thickness of the
splats in Mode 8 (Table II) is due to the minimum
velocity of the particles during their interaction
with the substrate.
As can be seen in Figure 2, the splats obtained
in dierent modes have similar area but dierent
thickness. The increase in the thickness of the
splats in Mode 8 (Table II) is due to the minimum
velocity of the particles during their interaction
with the substrate. The minimum particle
velocity in Mode 8 is caused by the large size of
the sprayed particles, low gas ow rate and low
spraying distance. This also leads to a decrease
in the speed of the sprayed particles. Particles
of Ti wire melted by a plasma jet move towards
the substrate during plasma spraying. On the one
hand, the size of these particles depends on the
Table III The Dependency of the СTE, Phase Composition and Crystallinity of HA Coating on
the Spraying Parameters
Mode I, A Vpg, slpm H, mm Ppowder, g min–1 СTE, % СTE, % estimated HAcryst. Aph β-TCP
145 2.0 160 1.2 54 58 92 5 3
245 2.0 80 0.4 64 71 98 0 2
345 1.0 160 0.4 89 89 93 2 5
445 1.0 80 1.2 69 69 96 0 4
535 2.0 160 0.4 29 29 93 4 3
635 2.0 80 1.2 48 48 94 3 3
735 1.0 160 1.2 40 47 88 7 5
835 1.0 80 0.4 56 60 98 0 2
940 1.5 120 0.8 60 59 90 6 4
Fig. 2. SEM images of Ti splats sprayed in: (a) Mode 1; (b) Mode 4; and (c) Mode 8 (according to Table II)
500 µm 500 µm 500 µm
Mode 1 Mode 4 Mode 8
(a) (b) (c)
185 © 2020 Johnson Matthey
https://doi.org/10.1595/205651320X15737283268284 Johnson Matthey Technol. Rev., 2020, 64, (2)
set spraying parameters. On the other hand, both
the set spraying parameters and the size of the
particles aect the speed and the degree of heating
of the particles in the plasma jet. Before interacting
with the substrate, the particles of molten metal
can either heat up, being in the high-temperature
zone of the plasma jet (the initial section of the
plasma), or, more likely, cool down by going to the
low-temperature zone (the end of the plasma jet).
The size of the sprayed particles and the degree
of their melting in the plasma jet can be varied
by spraying parameters. A porous coating with a
high surface roughness can be achieved by large
particles moving with low speed, as in Mode 8.
A dense coating with a relatively low surface
roughness can be obtained by high speed small
and completely melted particles, as in Mode 1 or in
Mode 4 (Table II).
The appearance and structure of the splats varies
depending on the interactions of the sprayed
particles with the substrate. The interaction is
generally dened by the velocity of the particles on
impact and also their degree of melting. It is possible
to correlate the temperature and velocity of the
particles before the collision to the substrate with
the resultant structure of the coating. The prole
and cracks on the surface of the splat correlate with
the stress state of the coating (2, 28).
The substrate average roughness (Ra) after gas
abrasive treatment was Ra = 7.0 ± 0.35 μm. The
analysis of the surface morphology of Ti coatings
showed the possibility of obtaining dense coatings
with relatively low roughness (Ra is about 12.0 μm)
using Mode 1 and Mode 4 (Table II), the remaining
modes provide a high surface roughness (above
30 μm). The maximum size of open pores (up to
300 µm) is observed on the surface of coatings
obtained in Mode 8 (Figure 3).
The analysis of the cross-sections of the Ti coatings
showed that the coatings sprayed in Mode 8 have the
highest average porosity of 31.0%, while Mode 4
makes it possible to obtain dense coatings with an
average porosity of 5.7% (Figure 3 and Table II).
The tensile strength test established the average
adhesion strength of the coating with a thickness of
100 µm sprayed in Mode 4 (Table II) to be 38.7 MPa.
This meets the requirements of ISO 13179-1:2014
(29). According to ISO 13179-1:2014 (29), the
average static tensile strength of a Ti coating
should be more than 22 MPa.
As can be seen from the results presented
in Table II, the arithmetic Ra of the coatings
surface correlates with the porosity of the sprayed
coatings. The condition of the presence of large,
incompletely molten particles with a low speed in
the plasma jet leads to a high surface roughness
and increased porosity of the coatings. Thus, the
roughness of the coating is aected by the size
of the particles involved in the formation of the
coating. The main purpose of plasma spraying of
the Ti layers at the initial stage of the coating is
to form a porous surface with high roughness in
order to spray fully molten HA particles onto it. It
was previously shown (30) that partially melted
HA particles were not able to form dense coatings.
This leads to poor adhesion of the HA coating to
Mode 4 Mode 8
Surface
Cross
section
500 µm
500 µm
200 µm 200 µm
Fig. 3. SEM images of
surfaces and cross-sections
of Ti coatings sprayed
in Mode 4 and Mode 8
(according to Table II)
186 © 2020 Johnson Matthey
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the substrate. A desirable HA coating shows good
adhesion and high surface roughness. The surface
roughness of the HA coating aects osteoblast cell
attachment and thus accelerates bone growth into
the implant. Whereas broblasts and epithelial
cells prefer smoother surfaces, osteoblasts attach
and proliferate better on rough surfaces (31, 32).
A relatively thin HA layer (not more than 100 μm)
sprayed on a porous surface with a high roughness
(above 50 μm) can ensure the reliable adhesion
of HA coating to the surface, while maintaining
high surface roughness and porosity. XRD analysis
conrmed that the phase composition of the initial
HA powder was fully crystalline Ca10(PO4)6(OH)2.
The TEM images (Figures 4(a) and 4(b)) of
the HA powder particle and the corresponding
microelectron diraction pattern (Figure 4(c)) are
shown in Figure 4. The results of TEM analysis
are in good agreement with the results of XRD
analysis (Figure 4(d)). X-ray phase analysis
showed that the main phase (99.5%) is HA with
the hexagonal crystal system P63/m. The electron
diraction pattern corresponds to the hexagonal
phase of HA with unit cell parameters a = 0.94 nm,
c = 0.68 nm.
The plasma spraying of HA powder was carried
out using nine dierent modes (Table III). The
key criteria were the phase composition, the degree
of crystallinity (the proportion of the amorphous
phase (Aph), the proportion of the crystalline phase
(HAcryst)) and the coating transfer eciency (СTE).
In powder coating, transfer eciency is the ratio of
the quantity of powder deposited on the part to the
quantity of powder directed at the part. Transfer
eciency is provided as a percentage, with 100%
being most desirable. An experiment was conducted
to determine the impact of process parameters
such as I, Vpg, H and Ppowder on the СTE.
The application of well-known methods of
fractional factorial design (27) and the design
of experiment method (33) for the analysis of
plasma spraying of HA powder are described
elsewhere (34, 35) and in our previous paper (10).
A factorial experimental design to investigate the
relationship between plasma spray parameters
and the microstructure of HA coatings was rst
used by Dyshlovenko et al. (35). Three responses
were examined (35). This included the fraction
of HA, the fraction of decomposition phases and
the amorphous content of the coatings. In this
study, the next three responses were examined:
the fraction of HA, the amorphous content of the
coatings and CTE. The rst two responses were
chosen to ensure purity and crystallinity of the HA
coating. CTE was selected in order to increase the
eciency of the plasma spraying process. Moreover
it allows interpretation by a linear regression model
which could quite easily and reliably be measured
in experimental runs. The linear regression model
was chosen to estimate CTE. The coecients in the
10 20 30 40 50 60
Degrees 2θ, °
Relative intensity,
arbitrary unit
111
131
331
100 110
331
311
010
111
(a) (b) (c)
(d)
Fig. 4. (a, b) TEM images of the HA powder particles; (b, c) the corresponding indexed microelectron
diraction pattern; and (d) the XRD patterns of HA powder
HA
200 nm 100 nm
187 © 2020 Johnson Matthey
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regression Equation (i) were calculated by assigning
the corresponding units of measure: 2.575 A–1;
–0.246 slpm–1; –0.203 mm–1; 4.06 min g–1;
–0.825.
СTE, % = 2.575 I – 0.246 Vpg – 0.203 H
+ 4.06 Ppowder – 0.825 (i)
The comparison of calculated and experimental
results indicates good agreement (Table III).
Therefore, Equation (i) can be used for preliminary
estimation of CTE when selecting MPS modes. A
more complex regression model that takes into
account the mutual inuence of factors should
be applied to determine the dependency of other
values presented in Table II and Table III on
the spraying parameters. This requires further
investigation using the obtained experimental
data.
The results of XRD analysis presented in
Table III show that the phase compositions of
all coatings comply with ISO 13779-2:2000 (15).
However, Mode 3 provides the highest CTE. Thus,
we consider Mode 3 to be the most cost-eective.
This mode allows a desired HA coating thickness
(about 100 µm) to be obtained in one pass of a
plasma jet.
The modes of application of multilayer coatings
of Ti/HA were selected on the basis of the analysis
of the tests of MPS of Ti wire and HA powders
with the measurement of thickness and porosity
of the deposited layers. The porosity and surface
roughness of Ti coatings and the high CTE value,
purity and crystallinity of the HA coating indicate
the optimum coating composition and MPS modes.
The coating thickness and the expected adhesion
to the substrate are other parameters indicating
the quality of the coating. The rst relatively thin
(up to 80 µm) and dense layer of Ti wire coating is
applied in Mode 4 (Table II), then another layer
of Ti wire up to 100 µm thick is sprayed in Mode 8
(Table II) to form a porous coating with a rough
surface, then the top layer of HA powder coating
is applied with thickness up to 100 µm in Mode 3
(Table III). The microstructure of microplasma
sprayed multilayer Ti/HА coatings under the above
modes is shown in Figures 5(a) and 5(b). The
XRD pattern for microplasma-sprayed HA coating
is presented in Figure 5(c).
The desired porosity was achieved in the Ti lower
layer (30 vol%) (Figure 5(b)). Pore sizes in both
the Ti middle layer and the HA top layer are in
the range 20–50 μm (Figure 5(b)). HA coating
Degrees 2θ, °
Amorphous region HA
β
100 µm 50 µm
(a) (b)
(c)
10 20 30 40 50 60
Relative intensity, arbitrary unit
Fig. 5. SEM images of the multilayer coating with the Ti lower and middle layers and the HA upper layer:
(a) the surface; (b) the cross-section; and (c) the XRD patterns of microplasma-sprayed HA coating
188 © 2020 Johnson Matthey
https://doi.org/10.1595/205651320X15737283268284 Johnson Matthey Technol. Rev., 2020, 64, (2)
porosity is about 20% (Figure 5(b)). It should
be noted that for biocompatible coatings, open
porosity is essential: that is the egress of the pore
on the surface of the coating, where the bone
grows. Therefore, measuring the diameters of the
pore craters on the surface of the coating is an
appropriate way to indicate the surface morphology
and prole. In our experiment, the maximum pore
diameter on the surface of the HA coating was
about 150 μm (Figure 5(a)).
It was established by XRD that the mode specied
for HA powder provides the required structure-
phase composition in the HA coating: 93% by
weight of the HAcryst, 5% by weight of β-TCP phase,
and 2% by weight of the Aph. The coating purity
was determined using the procedure outlined in
Materials and Methods section above. The highest
peaks of HA and β phases for HA coatings were
located at 32° 2θ and 31° 2θ respectively. The
purity of coatings was found to be 95.1%. This
shows that the purity meets the 95% purity
requirements of ISO 13779-2:2000 (15). The
measurement error was 0.06.
The loci of Aph have been found on the XRD
patterns between 18° and 38° (Figure 5(c)).
All the diraction patterns in the range of 37.3° 2θ
were thoroughly investigated, but even weak
peaks of calcium oxide (CaO) were not found
(Figure 5(с)). It conrms that no harmful СаО
compound is formed through MPS coating of HA
powder.
For a multilayer coating, the porosity and adhesion
of the top HA layer depends on the characteristics
of the lower Ti layers such as the roughness and
open porosity of the middle Ti layer. To determine
the dependency of the porosity and adhesion of
the HA coating on the parameters of MPS, further
research is needed.
This study proves that it is possible to obtain
coatings from biocompatible materials with the
desired level of porosity and satisfactory adhesion
to the substrate using MPS. A robot assisted MPS of
coatings from biocompatible materials of Ti and HA
onto Ti implants has been implemented. Also the
composition and modes of microplasma deposition
of multilayer coatings for Ti implants have been
identied. The next stage of the research includes
the study of the biocompatibility of microplasma-
sprayed coatings (in vitro tests) and MPS of
dierent materials such as tantalum and zirconium.
Among the number of works that have
demonstrated the advantages of thermal spraying
of biocompatible coatings for use in medical
applications, three recent papers (36–38) have
shown promising directions for further development
of the research presented here. Cizek et al. (36) have
reviewed the patents concerning thermal spraying
for biomedical applications for the period 2005 to
2018. They have also reported recent research
and development trends in this eld. Among the
materials recommended for bio-applications, they
have mentioned Ta. It can be noted that MPS of Ta
wire onto Ti alloys using the technology presented
in our paper is highly feasible. Our trials with Ta
have indicated great potential. This could open
the potential to apply the developed technology
for other materials. Fotovvati et al. (37) have
compared the results of obtaining biocompatible
coatings by cold and thermal spraying in favour
of thermal spraying. Fousova et al. (38) have
shown the benets of using thermal plasma
spray to prepare bulk Ti for bone enlargements.
However, despite the advantages and relative cost
eectiveness of thermal plasma spraying, its use
for the manufacture of medical implants has not
yet become widespread. This is mainly due to the
high temperatures of the bulk resulting from the
thermal spraying process. MPS avoids the issue of
overheating. It allows coatings to be obtained from
materials with a high melting point, such as Ti and
Ta, by a microplasma jet while introducing a very
small thermal impact into the substrate.
The use of robotic MPS could be considered
promising for the production of patient specic
implants. Three-dimensional scanning and rapid
prototyping technologies facilitate the manufacture
of specically designed complex geometry
implants and robot assisted plasma coating is
used for coating. This is more advantageous for
the production of small endoprostheses with
biocompatible coatings, such as vertebral cages
and dental implants (39, 40).
4. Conclusion
It has been established that the main parameters
controlling the porosity of microplasma sprayed
coatings are I and Vpg. MPS parameters for the
formation of porous coatings of Ti wire and HA
powders with rough surfaces have been determined
and are reported here. The advantages of applying
SEM, TEM and XRD to analyse the structure of
sprayed Ti and HA coatings to substantiate the
choice of plasma spraying modes of the coatings
were demonstrated. It is also proven that by using
the appropriate MPS process parameters, a layer
of HA with a high degree of crystallinity (93%) can
be obtained, controlled by changing the deposition
189 © 2020 Johnson Matthey
https://doi.org/10.1595/205651320X15737283268284 Johnson Matthey Technol. Rev., 2020, 64, (2)
mode. The small size of the spraying spot (up to
8 mm) provides a signicant reduction in Ppowder
when depositing on implants of small size compared
to conventional plasma spraying.
The composition and modes of microplasma
deposition of multilayer coatings for Ti implants,
including a dense Ti sublayer, porous Ti middle
layer and HA top layer have been established. The
total thickness of such coatings is about 300 µm.
The porous middle coating layer has a porosity
of 30% and a pore size varying from 20 µm to
50 µm. Moreover the upper layer of HA indicates
a thickness up to 100 µm with a 95% level of HA
phases and 93% crystallinity. The results of this
research are of signicance for a wide range of
researchers developing plasma spray technologies
for biocompatible coatings manufacture.
Acknowledgments
The study has been conducted with the nancial
support of the Science Committee of the Ministry of
Education and Science of the Republic of Kazakhstan
by the project AP05130525 “The intelligent robotic
system for plasma processing and cutting of
large-size products of complex shape”.
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The Authors
Darya Alontseva completed her PhD in Physics at East Kazakhstan State University in
2002. She completed her postdoctoral studies at Altai State Technical University, Russia,
in 2013 and received the degree of Doctor of Sciences in Physics and Mathematics. In
2016 she was awarded the academic title of Full Professor of Physics. She has 19 years of
research experience in developing new material and processes and management of funded
scientic projects. She is a lead researcher in her research area: physics of condensed
state, material science and surface engineering.
Elaheh Ghassemieh has 25 years of research experience in the areas of advanced
manufacturing including additive manufacturing, development of novel materials
especially composites using range of experimental and multiscale numerical methods.
After completing her PhD in simulation of micromechanics of composite materials,
she worked at several universities in the UK including The University of Sheeld,
Queen’s University Belfast and Loughborough University. She has obtained a number
of research funding grants from UK and international research councils and has also
secured and managed many industrially funded projects where she has transferred
novel research outcomes to relevant industrial users in the aerospace, automotive or
biomedical elds.
191 © 2020 Johnson Matthey
https://doi.org/10.1595/205651320X15737283268284 Johnson Matthey Technol. Rev., 2020, 64, (2)
Sergey Voinarovych completed his postgraduate study at E.O.Paton Electric Welding
Institute, Ukraine, in 2008 and received a PhD degree in the specialty “Welding and related
processes and technologies”. Since 2010 he has been working as a Senior Researcher at
E.O.Paton Electric Welding Institute. He has 20 years of research experience in developing
new processes, materials and equipment in the area of thermal spray coatings. He has
designed MPS equipment and technology for forming biocompatible coatings on parts of
endoprostheses. Currently he is researching new biocompatible materials and coatings.
Oleksandr Kyslytsia completed his postgraduate study at E.O.Paton Electric Welding
Institute in 2010 and received PhD degree in the specialty “Welding and related processes
and technologies”. Since 2011 he has been working as a Senior Researcher at E.O.Paton
Electric Welding Institute. For over 20 years he has been developing new processes,
materials and equipment for producing coatings by gas thermal spraying methods. He
developed equipment and technology for MPS from wire materials to obtain biocompatible
coatings on parts of endoprostheses. Currently he is researching new biocompatible
materials and coatings.
Yuri Polovetskyi graduated from the Chernihiv Technological Institute, Ukraine, in 1999
with a degree in Welding Technology and Equipment and entered the doctoral program
at the E.O.Paton Electric Welding Institute. After completing his doctorate in 2002 and to
the present, Polovetskyi is a senior researcher of the department of physical and chemical
research of materials. His area of scientic expertise is the structural characteristics,
chemical composition and mechanical properties of welded joints and plasma coatings for
various purposes.
Nadezhda Prokhorenkova received her PhD in Technical Physics in 2014. She has nine years
of research experience in developing new materials and processes. She is an accomplished
researcher in her research area: material science. Her area of scientic expertise is X-ray
analysis of coating structures. Currently Prokhorenkova is an associate professor at the
School of Engineering at D. Serikbayev East Kazakhstan State Technical University.
Albina Kadyroldina received her BS and MS degrees from D. Serikbayev East Kazakhstan
State Technical University. She is currently pursuing a PhD with the School of Engineering,
D. Serikbayev East Kazakhstan State Technical University. Her research interests include
automation, control and mathematical modelling.
... This speed was chosen experimentally to ensure plasma spraying of the coating with a uniform thickness. Experiments have shown that this speed of linear traveling of the microplasmatron does not lead to disturbances in the plasma jet flow due to air resistance and, therefore, ensures the stability of the spraying process with different parameters [21]. The coating thickness varied from 150 µm to 500 µm due to changing the plasma spraying parameters (amperage, spraying distance, plasma gas flow rate and powder flow rate) and change in the number of passes of the plasma jet. ...
... The main challenge for the development of thermal plasma spraying technologies is the formation of a coating with controlled microstructures and properties. The use of the intelligent robotic systems allowed maintaining the specified speed, spraying distance and angle, providing precision coating deposition on complex-shaped implant parts [21,22]. It was shown that the robotic microplasma spraying can produce biocompatible coatings on patient-specific medical titanium implants meeting the requirements of international standards of implants for surgery in terms of coating adhesive strength, crystallinity and purity [21]. ...
... The use of the intelligent robotic systems allowed maintaining the specified speed, spraying distance and angle, providing precision coating deposition on complex-shaped implant parts [21,22]. It was shown that the robotic microplasma spraying can produce biocompatible coatings on patient-specific medical titanium implants meeting the requirements of international standards of implants for surgery in terms of coating adhesive strength, crystallinity and purity [21]. The small size of the spraying spot on the plasma-coated surface (with a diameter of 5mm to 15mm) reduces the loss of spraying material when coating small-sized products, which include most parts of endoprostheses. ...
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Presently, the development of novel ceramic materials with improved biomedical functions is at the forefront of health-related issues in many countries. Arguably, research into bioceramics including coatings for endoprosthetic implants has reached a level of involvement and sophistication comparable only to developments ongoing in the realm of electronic ceramics [1]. Despite the fact that calcium phosphate-based coatings deposited on hip, knee and dental implants as well as bone screws and osteosynthetic devices have an impressive history of clinical success, the quest for improving the longevity of implants and to impart them with better physiological properties is high up on the agenda of numerous research groups around the world. The contributions in this topical issue of The Open Biomedical Engineering Journal attest to these developments.
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Electrophoretically deposited hydroxyapatite (HAP) coatings on type 316L SS was developed at the optimum coating parameters of 60 V and 3 min. Sintering of the coating enhances the metal–ceramic bond strength, but HAP structure is sensitive to temperature as it decomposes to other calcium phosphate phases. Sintering of HAP coatings in air at 900 °C for 1 h indicate the formation of a composite surface containing oxides of the alloy and decomposition products of HAP, mainly tricalcium phosphate. Open circuit potential–time measurements, potentiodynamic cyclic polarisation and electrochemical impedance experiments performed in Ringer’s solution indicate that the corrosion performance of HAP coatings were severely affected by the sintering atmosphere and temperature. Higher capacitance and low polarisation resistance values obtained from electrochemical impedance spectroscopic studies further indicate that the coatings are more prone to dissolution on comparison with the pristine type 316L SS. The sintering of the coatings in vacuum at 600, 800 and 900 °C for 1 h did not alter the phase purity of the coatings, and shifted the electrochemical parameters towards noble direction. Sintering of the coatings in vacuum lead to the formation of an adherent, stoichiometric HAP coating with enhanced corrosion resistance.
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Manual, acompañado por un disco compacto, que expone en 16 pasos el método desarrollado por el japonés Genechi Taguchi para el mejoramiento de los productos y los procesos. Contenido: 1. Diseño de experimentos y el acercamiento Taguchi. 2. Definición y medición de la calidad. 3. Experimentos comunes y métodos de análisis. 4. Diseño experimental mediante el uso de formaciones ortogonales. 5. Diseño experimental con factores de dos niveles. 6. Diseño experimental con factores de tres y cuatro niveles. 7. Análisis de varianza. 8. Diseño experimental para el estudio de la interacción de factores. 9. Diseño experimental con factores de niveles diversos. 10. Diseños de combinaciones. 11. Estrategias para un diseño sólido. 12. Análisis mediante el uso de relaciones signo-ruido. 13. Resultados que comprenden múltiples criterios de evaluaciones. 14. Cuantificación de la reducción de variaciones y desempeño. 15. Preparación y planeación de un experimento efectivo y 16. Estudios de caso.
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Tissue engineering in vitro and in vivo involves the interaction of cells with a material surface. The nature of the surface can directly influence cellular response, ultimately affecting the rate and quality of new tissue formation. Initial events at the surface include the orientated adsorption of molecules from the surrounding fluid, creating a conditioned interface to which the cell responds. The gross morphology, as well as the microtopography and chemistry of the surface, determine which molecules can adsorb and how cells will attach and align themselves. The focal attachments made by the cells with their substrate determine cell shape which, when transduced via the cytoskeleton to the nucleus, result in expression of specific phenotypes. Osteoblasts and chondrocytes are sensitive to subtle differences in surface roughness and surface chemistry. Studies comparing chondrocyte response to TiO2 of differing crystallinities show that cells can discriminate between surfaces at this level as well. Cellular response also depends on the local environmental and state of maturation of the responding cells. Optimizing surface structure for site-specific tissue engineering is one option; modifying surfaces with biologicals is another.
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E. J. Tobin, Adv. Drug Deliv. Rev., 2017, 112, 88
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