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Johnson Matthey Technol. Rev., 2020, 64, (2), 180

doi:10.1595/205651320x15737283268284

Manufacturing and Characterisation of Robot Assisted Microplasma Multilayer Coating of Titanium Implants

Biocompatible coatings for medical implants with improved density and crystallinity

    • 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
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Article Synopsis

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 offer 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 diffraction (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 (17). 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 (18).

The research presented here offers robot assisted MPS of Ti wires and HA powder onto Ti substrates (912). 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 (13). Among existing plasma spraying 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 specified structure and properties.

Currently, endoprosthesis medical practice widely uses metal implants coated with HA (18, 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 significantly reduce the duration of implantation of the endoprosthesis. It provides a reliable connection with the bone and increases the reliability of the implant (18, 13, 14). In the case of thermal spraying of HA powder, the chemical composition of the final 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 specification 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 affects 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 fixation of the endoprosthesis in the bone is also possible. Thus, increased crystallinity provides reliable fixation 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% (24, 16, 17). At the same time, the coating must be firmly 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 sufficiently 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).

Fig. 1.

SEM images of HA particles indicating particle size

SEM images of HA particles indicating particle size

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 flowability of the powder. The flowability 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 flow 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.

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

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 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 Scientific 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 diffractometer (PANalytical, The Netherlands). The interpretation of the XRD patterns was carried out using Rietveld method and powder diffraction data from the database of the International Centre for Diffraction Data (ICDD, USA, 2003), the ASTM card file 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° 2θ and the area of the amorphous diffuse background in this region. Diffraction 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 diffraction 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 diffraction 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 fixed on the substrate and deformed upon contact with the substrate surface. The splats’ visual analysis was carried out by SEM; the splats were classified according to their appearance and their spraying modes (Table II).

Table 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
1 25 3.7 120 4.3 6.2 ± 0.74 12.0 ± 0.97
2 25 3.7 40 3.0 13.8 ± 1.90 45.6 ± 4.40
3 25 2.3 120 3.0 12.0 ± 1.50 44.4 ± 3.85
4 25 2.3 40 4,3 5.7 ± 0.45 12.0 ± 1.13
5 15 3.7 120 3.0 9.6 ± 0.35 31.5 ± 3.10
6 15 3.7 40 4.3 10.6 ± 1.32 34.8 ± 3.23
7 15 2.3 120 4.3 8.7 ± 2.32 30.2 ± 3.87
8 15 2.3 40 3.0 31.0 ± 3.87 >50

3. Results and Discussion

The effect of parameters of MPS such as electric arc current (I, A), plasma gas flow rate in standard litre per minute (Vpg, slpm), spraying distance (H, mm) and wire flow 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.

Table III

The Dependency of the CTE, Phase Composition and Crystallinity of HA Coating on the Spraying Parameters

Mode I, A Vpg, slpm H, mm Ppowder, g min–1 CTE, % CTE, % estimated HAcryst. Aph β-TCP
1 45 2.0 160 1.2 54 58 92 5 3
2 45 2.0 80 0.4 64 71 98 0 2
3 45 1.0 160 0.4 89 89 93 2 5
4 45 1.0 80 1.2 69 69 96 0 4
5 35 2.0 160 0.4 29 29 93 4 3
6 35 2.0 80 1.2 48 48 94 3 3
7 35 1.0 160 1.2 40 47 88 7 5
8 35 1.0 80 0.4 56 60 98 0 2
9 40 1.5 120 0.8 60 59 90 6 4

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 solidification 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.

Fig. 2.

SEM images of Ti splats sprayed in: (a) Mode 1; (b) Mode 4; and (c) Mode 8 (according to Table II)

SEM images of Ti splats sprayed in: (a) Mode 1; (b) Mode 4; and (c) Mode 8 (according to Table II)

As can be seen in Figure 2, the splats obtained in different modes have similar area but different 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 flow 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 set spraying parameters. On the other hand, both the set spraying parameters and the size of the particles affect 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 defined 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 profile 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).

Fig. 3.

SEM images of surfaces and cross-sections of Ti coatings sprayed in Mode 4 and Mode 8 (according to Table II)

SEM images of surfaces and cross-sections of Ti coatings sprayed in Mode 4 and Mode 8 (according to Table II)

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 affected 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 the substrate. A desirable HA coating shows good adhesion and high surface roughness. The surface roughness of the HA coating affects osteoblast cell attachment and thus accelerates bone growth into the implant. Whereas fibroblasts 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 confirmed 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 diffraction 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 diffraction 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 different 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 efficiency (CTE). In powder coating, transfer efficiency is the ratio of the quantity of powder deposited on the part to the quantity of powder directed at the part. Transfer efficiency 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 CTE.

Fig. 4.

(a, b) TEM images of the HA powder particles; (b, c) the corresponding indexed microelectron diffraction pattern; and (d) the XRD patterns of HA powder

(a, b) TEM images of the HA powder particles; (b, c) the corresponding indexed microelectron diffraction pattern; and (d) the XRD patterns of HA powder

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 first 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 first two responses were chosen to ensure purity and crystallinity of the HA coating. CTE was selected in order to increase the efficiency 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 coefficients in the 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.

 

(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 influence 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-effective. 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 first 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/HÀ 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).

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

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

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 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 profile. 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 specified 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° 2θ (Figure 5(c)). All the diffraction patterns in the range of 37.3° 2θ were thoroughly investigated, but even weak peaks of calcium oxide (CaO) were not found (Figure 5(c)). It confirms that no harmful CaO 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 identified. The next stage of the research includes the study of the biocompatibility of microplasma-sprayed coatings (in vitro tests) and MPS of different 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 (3638) 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 field. 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 benefits of using thermal plasma spray to prepare bulk Ti for bone enlargements. However, despite the advantages and relative cost effectiveness 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 specific implants. Three-dimensional scanning and rapid prototyping technologies facilitate the manufacture of specifically 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 mode. The small size of the spraying spot (up to 8 mm) provides a significant 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 significance for a wide range of researchers developing plasma spray technologies for biocompatible coatings manufacture.

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Acknowledgements

The study has been conducted with the financial 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”.

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 scientific 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 Sheffield, 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 fields.


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 scientific 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 scientific 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.

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