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Mar 15, 2018 - HA crystals both in long bones and in tooth enamel have preferred .... did not have an apparent lamellar structure with columnar grains.
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The Influence of Process Parameters on the Structure, Phase Composition, and Texture of Micro-Plasma Sprayed Hydroxyapatite Coatings Xiaomei Liu 1, *, Dingyong He 1,2, *, Zheng Zhou 1,2 , Zengjie Wang 2 and Guohong Wang 2 1 2

*

College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China; [email protected] Beijing Engineering Research Center of Eco-Materials and LCA, Beijing University of Technology, Beijing 100124, China; [email protected] (Z.W.); [email protected] (G.W.) Correspondence: [email protected] (X.L.); [email protected] (D.H.); Tel.: +86-10-6739-6188 (X.L.)

Received: 8 February 2018; Accepted: 12 March 2018; Published: 15 March 2018

Abstract: In this study, hydroxyapatite (HA) coatings were deposited on Ti-6Al-4V by micro-plasma spraying (MPS). The influence of the process parameters on the microstructure of HA coatings was investigated. The splat morphology and spreading behavior were examined to understand the influence of process parameters on the coating. The texture strength of HA coatings was characterized by X-ray diffraction (XRD). The texture coefficients were all applied to characterize the variation in texture. The morphology of splats and coatings were characterized by scanning electron microscopy (SEM). XRD pattern shows that the texture intensity of the c-axis of HA was greatly influenced by spraying distance and spraying current. SEM reveals the different texture strength of HA coatings with different ratios of columnar grains. The strongest c-axis texture was found in the coating by 60 mm spraying distance with a spraying current of 40 A. In the cross-section SEM images of the coating with the strongest c-axis texture, uniform distribution columnar grains were observed in the upper part (~100 µm). The investigation of splats indicates that columnar grain growth occurs after fully melted particles impact the heated substrate. By controlling the melting state prior to in-flight particle impacts, columnar grain growth can be achieved during slow solidification of the disk shape splat during MPS. Keywords: hydroxyapatite; micro-plasma spraying; texture; splat

1. Introduction Hydroxyapatite (HA) coatings have been used as surface coatings on metallic implants because they have a chemical composition similar to that of teeth and bone [1,2], and can stimulate bone growth at the bone implant interface [3]. However, HA has poor bending strength and fracture toughness. It is unsuitable for use in load bearing applications. However, with the rapid development of surface technology, HA coatings deposited by using surface technology on a metal substrate provide excellent biocompatibility and good mechanical properties. The plasma spraying technique is a common method for depositing HA coating on metal implants [4–6]. However, the overheating of HA particles in the plasma jet can cause the decomposition of the HA powders. This leads to the decomposition of HA into tricalcium phosphate (TCP) or tetracalcium phosphate (TTCP) or even toxic calcium oxide (CaO) [7,8]. The rapid cooling rate of the particles on the substrate results in the formation of amorphous calcium phosphate (ACP) [9]. These phases exhibit a fast dissolution rate in the body fluid [10]. The fast dissolution rate of these phases can accelerate implant failure. In order to achieve long-term service, HA coatings with high crystallinity and phase purity is pursued.

Coatings 2018, 8, 106; doi:10.3390/coatings8030106

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Recently, micro-plasma spraying (MPS) has been used to produce HA coatings. MPS, due to its special characteristics, is a feasible thermal spray process for depositing HA coatings on small-scale medical implants [11]. As we know, the phase composition and the microstructure of HA coatings are important factors that influence the properties of these coatings [12]. In turn, these microstructure features depend on the MPS process parameters such as spraying current, plasma gas flow rate, and spraying distance. There have been several reports about the effect of the MPS parameters on the structure and properties of HA coatings [13]. For example, increased spraying power and spraying distance lead to an increase in the crystallinity and phase purity of such coatings. Previous studies also showed that MPS can produce a higher crystallinity (>90%) of HA coatings [14,15]. Many factors influencing the biological and mechanical properties of HA coatings, such as phase composition, crystallinity, surface topography, and grain size [12,16]. Another essential microstructure feature responsible for cell adhesion behavior and the mechanical properties of HA is the crystallographic texture. Several studies on the behavior of a protein adsorbed on different HA crystal surfaces suggested that calcium phosphate materials with texture surfaces may enable control over cellular behavior [17,18]. HA crystals both in long bones and in tooth enamel have preferred orientation in the direction of the c-axis. It was reported that hydroxyapatite coatings, compared with randomly oriented coatings, with high c-axis orientations have a higher chemical stability [19]. This crystal orientation in HA coatings is promising for long-term and safe applications. In addition, the HA coatings with high c-axis orientation have high hardness and Young’s modus values [20]. Therefore, the preparation of HA coatings with c-axis orientation might be highly reliable in medical applications. There are several techniques such as radio-frequency thermal plasma spraying (TPS) [20–22] and micro-plasma spray (MPS) [9,14,19] that have been reported to prepare HA coatings with strong (002) crystallographic textures. Compared with radio-frequency (RF)-TPS, MPS can be used to obtain HA coatings with (002) crystallographic textures and high HA content. Although a few studies have investigated the microstructure and properties of MPS coatings [15], the effect of different MPS parameters on texture is still not firmly established. In the present study, the variation in the phase composition and microstructure of HA coatings with different spray parameters were evaluated. The influence of process parameters on the c-axis texture of the coatings was studied. Moreover, through calculation of the texture coefficient, the preferred orientation of crystallites in HA coatings was evaluated. Furthermore, in order to understand the influence of spray parameters on coatings, the splat morphology and spreading behavior were analyzed. This study has important scientific significance and practical value since the coming results will provide a theoretical basis for the controlled preparation of HA coatings that can satisfy the clinical application requirements by MPS. 2. Materials and Methods HA coatings were deposited by micro-plasma spraying (MPS, WDP-1, Beijing University of Technology, Beijing, China) on Ti-6Al-4V plates with dimensions of Φ15 mm × 2.5 mm. They were grit-blasted to roughen and clean the surface before deposition. Hydroxyapatite powder (Medicoat AG, Mägenwil, Switzerland) with particle sizes of 45–63 µm was employed as a raw powder for depositing the coatings. The MPS process parameters are shown in Table 1. To study the effect of spraying distance, the spraying current was kept constant at 40 A for all experiments done at different spraying distances. At different currents, the spraying distance was kept constant at 60 mm. The morphologies of raw powders and coatings were observed by scanning electron microscopy (SEM, SU8020, Hitachi, Tokyo, Japan) with an acceleration voltage of 10 kV. SEM observation of the raw powders shows that the powders exhibit a spherical shape (Figure 1a). The high magnification SEM of powder cross-section analysis indicated a porous structure of the raw powder (Figure 1b). The polished and polished-etched cross sections of the coatings were prepared before observation. All samples were coated with a thin gold film using a gold ion sputtering system to make them electrically conductive prior to SEM observation. The phase composition of raw powder and as-sprayed coatings

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was qualitatively determined through X-ray diffraction (XRD, D8 ADVANCE, Bruker AXS GMBH, Bruker, Karlsruhe, The operating conditions were 40 kV and 30 mA using Cu Kα radiation Coatings 2018, 8, x FOR Germany). PEER REVIEW 3 of 12 ◦ ◦ source. The scattering angle ranges (2θ) from 20 to 60 with a step size of 0.02 . The infrared using Cu Kα radiation source. The scattering angle ranges (2θ)Hong fromKong, 20 to 60° withwas a step size ofto 0.02°. thermometer (Smart sensor AR882A, ARCO electronics Ltd., China) utilized test Thesubstrate infrared temperature. thermometer (Smart sensor AR882A, ARCO electronics Ltd., Hong Kong, China) was the utilized test the substrate temperature. Theto texture coefficients of as-sprayed coatings were calculated as follows [23,24]: The texture coefficients of as-sprayed coatings were calculated as follows [23,24]: I ( abcd)/I ( abcd) TC( abcd) = 1 N𝐼(𝑎𝑏𝑐𝑑)/𝐼ICDD 𝐼𝐶𝐷𝐷 (𝑎𝑏𝑐𝑑) TC(𝑎𝑏𝑐𝑑) =N ∑(hkil )−1 I (hkil )/I ICDD (hkil ) 1 𝑁 ∑ 𝐼(ℎ𝑘𝑖𝑙)/𝐼𝐼𝐶𝐷𝐷 (ℎ𝑘𝑖𝑙) 𝑁 (ℎ𝑘𝑖𝑙)−1 where I(abcd) is the measured intensity of the (abcd)-peak, IICDD is the standard intensity for HA raw where I(abcd) is the measured intensity of the (abcd)-peak, IICDD is the standard intensity for HA raw powders in the International Centre for Diffraction Data (ICDD), and N is the number of XRD peaks powders in the International Centre for Diffraction Data (ICDD), and N is the number of XRD peaks considered—in our case, (0002), (2131), (1122), and (0004), so N = 4. considered—in our case, (0002), (213̅1), (112̅2), and (0004), so N = 4. Table 1. Micro-plasma process parameters. Table 1. Micro-plasma process parameters. Current Current 20–50 20–50AA

Voltage Voltage 25 V V 25

Flow Rate of Argon Spraying Distance Flow Rate of Argon Spraying Distance 1.3 L/min 60–110 mm 1.3 L/min 60–110 mm

Figure 1. Hydroxyapatite (HA) powder morphology: (a) surface morphology; (b) cross-section Figure 1. Hydroxyapatite (HA) powder morphology: (a) surface morphology; (b) cross-section morphology. morphology.

3. Results 3. Results 3.1. Microstructure of As-Sprayed Coatings at Different Spraying Distances 3.1. Microstructure of As-Sprayed Coatings at Different Spraying Distances The XRD patterns of as-sprayed HA coatings are shown in Figure 2. The peaks of TTCP and The XRD patterns of as-sprayed HA coatings are shown in Figure 2. The peaks of TTCP and αα-TCP were found in as-sprayed coatings, and the phase CaO could not be identified in the patterns. TCP were found in as-sprayed coatings, and the phase CaO could not be identified in the patterns. The peak intensity of TTCP and α-TCP decreased appreciably when the spraying distance increased The peak intensity of TTCP and α-TCP decreased appreciably when the spraying distance increased from 40 to 60 mm. However, the peaks of TTCP and α-TCP increased and the amorphous hump from 40 to 60 mm. However, the peaks of TTCP and α-TCP increased and the amorphous hump became significant when the spraying distance increased from 60 to 110 mm. It should be noted that became significant when the spraying distance increased from 60 to 110 mm. It should be noted that the XRD patterns of raw powders (Figure 2) show that all diffraction peaks correspond to a standard the XRD patterns of raw powders (Figure 2) show that all diffraction peaks correspond to a standard HA powder pattern (ICDD 09-0432), which indicated that the raw powder exhibits random crystal HA powder pattern (ICDD 09-0432), which indicated that the raw powder exhibits random crystal orientation. However, the intensity of the peaks (0002) and (0004) of as-sprayed coatings increased at orientation. However, the intensity of the peaks (0002) and (0004) of as-sprayed coatings increased at short spraying distances (40–80 mm). These results indicate that a preferential orientation of crystals short spraying distances (40–80 mm). These results indicate that a preferential orientation of crystals with the (0002) and (0004) planes for the c-axis texture is formed during MPS deposition. with the (0002) and (0004) planes for the c-axis texture is formed during MPS deposition. Figure 3 shows the effect of spraying distance on the preferred c-axis orientation of the as-sprayed Figure 3 shows the effect of spraying distance on the preferred c-axis orientation of the asHA coatings. From Figure 3, it is clear that TC (0002) and TC (0004) increased when the spraying sprayed HA coatings. From Figure 3, it is clear that TC (0002) and TC (0004) increased when the distance increased from 40 to 60 mm. This illustrates that the c-axis texture improved by increasing spraying distance increased from 40 to 60 mm. This illustrates that the c-axis texture improved by the spraying distance. However, TC (0002) and TC (0004) decreased significantly when the spraying increasing the spraying distance. However, TC (0002) and TC (0004) decreased significantly when the distance increased from 60 to 110 mm. Thus, the texture intensity greatly decreases in this case. spraying distance increased from 60 to 110 mm. Thus, the texture intensity greatly decreases in this These results indicated that the spraying distance had a significant influence on the texture intensity of case. These results indicated that the spraying distance had a significant influence on the texture HA coatings. intensity of HA coatings.

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Texture Texturecoefficient coefficient

Figure 2. XRD patterns of HA coatings with different spraying distances. Figure 2. XRD patterns of HA coatings with different spraying distances. 2.0 2.0 1.8 1.8 1.6 1.6 1.4 1.4 1.2 1.2 1.0 1.0 0.8 0.8 0.6 0.6 30 30

(0002) (0002) (0004) (0004)

40 40

50 50

60 70 80 90 100 110 120 60 70 80 90 100 110 120 Spraying distance (mm) Spraying distance (mm)

Figure 3. Variations of the texture coefficient for HA coatings deposited at different spraying Figure 3.3.Variations Variationsof of texture coefficient forcoatings HA coatings deposited at spraying different distances. spraying Figure thethe texture coefficient for HA deposited at different distances. distances.

Figure 4 shows the SEM micrographs of cross sections of HA coatings at different spraying Figure 4 shows the SEM micrographs of cross sections of HA coatings at different spraying distances. Samples Samples sprayed sprayed at at 40 40 mm mm showed showed that partially melted particles exhibited columnar distances. Samples sprayed at 40 mm showed that partially melted particles exhibited columnar crystals. The The columnar columnar grains grains with with hexagons hexagons in in the the coating coating were in a non-uniform non-uniform distribution crystals. The columnar grains with hexagons in the coating were in a non-uniform distribution (Figure 4a). In addition, columnar grains extend from the bottom to the top of the HA splat. In some (Figure 4a). In addition, columnar grains extend from the bottom to the top of the HA splat. In some splats, columnar grains are not completely perpendicular to the coating surface (Figure 4b). From the splats, columnar grains are not completely perpendicular to the coating surface (Figure 4b). From the cross-section thethe coating deposited under 60 mm, uniform distribution columnar grains cross-section SEM SEMimages imagesofof coating deposited under 60 mm, uniform distribution columnar cross-section SEM images of the coating deposited under 60 mm, uniform distribution columnar with hexagon sections were observed in the upper part (~100 µm) (Figure 4c). The SEM grains with hexagon sections were observed in the upper part (~100 μm) (Figure 4c).cross-section The cross-section grains with hexagon sections were observed in the upper part (~100 μm) (Figure 4c). The cross-section image showsshows that the columnar grains were parallel to the deposition direction direction (Figure 4d). However, SEM image that the columnar grains were parallel to the deposition (Figure 4d). SEM image shows that the columnar grains were parallel to the deposition direction (Figure 4d). the morphology of the coatings deposited under 80 andunder 110 mm coatings However, the morphology of the coatings deposited 80 changed and 110 significantly. mm changedThese significantly. However, the morphology of the coatings deposited under 80 and 110 mm changed significantly. did notcoatings have an did apparent lamellar structurelamellar with columnar grains. addition,grains. spherical particles These not have an apparent structure with In columnar In addition, These coatings did not have an apparent lamellar structure with columnar grains. In addition, were observed deposited at 110 mm. The variation in theThe columnar grain ratio is in agreement with spherical particles were observed deposited at 110 mm. variation in the columnar grain ratio is spherical particles were observed deposited at 110 mm. The variation in the columnar grain ratio is the evolution with of thethe XRD pattern.of the XRD pattern. in agreement evolution in agreement with the evolution of the XRD pattern.

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Figure 4. Cross-sectional morphology of HA coatings with different spraying distances: (a,b) 40 A, 40 Figure 4. Cross-sectional morphology of HA coatings with different spraying distances: (a,b) 40 A, mm; (c,d) 40 A, 60 mm; (e,f) 40 A, 80 mm; (g,h) 40 A, 110 mm. 40 mm; (c,d) 40 A, 60 mm; (e,f) 40 A, 80 mm; (g,h) 40 A, 110 mm.

3.2. Microstructure of As-Sprayed Coatings at Different Spraying Currents 3.2. Microstructure of As-Sprayed Coatings at Different Spraying Currents Figure 5 shows the XRD patterns of HA coatings sprayed at different spraying currents. Figure 6 Figure shows thevalues XRD patterns of HA coatings sprayed at different spraying shows the5calculated of TC (0002) and TC (0004) for different currents. XRDcurrents. patternsFigure show 6 shows the coating calculated values (0002) andtoTC different currents. patterns that that the sprayed atof 20TC A is similar that(0004) of thefor raw powders, which XRD illustrates thatshow the HA thecoating coating sprayed at at20 20AAexhibits is similar to that of the raw powders, which illustrates that the HA coating deposited a random crystalline orientation. TC (0002) and TC (0004) are below deposited at 20the A intensity exhibits aofrandom crystalline orientation. TC coatings (0002) and TCA,(0004) 1. However, the peaks (0002) and (0004) of HA at 30 40 A,are andbelow 50 A 1. However, theFor intensity the peaks (0002) and (0004) of HAofcoatings at 30 A,and 40 A, and 50 A increased. increased. coatingofsprayed at 40 A, the decomposition TTCP, α-TCP, ACP decreased. The intensity peaks (0002) (0004) increased compared the coating sprayed at 30 A. Meanwhile, For coatingofsprayed at 40 and A, the decomposition of TTCP,with α-TCP, and ACP decreased. The intensity of TC (0002) (0004) of the HA coating at 40 A increased. This illustrates the growth the peaks (0002)and andTC (0004) increased compared with the coating sprayed at 30 A.that Meanwhile, TCof(0002) (0002) and (0004) preferred orientations obvious. On the contrary, that the intensity of peaks and TC (0004) of the HA coating at 40 A is increased. This illustrates the growth of the(0002) (0002)and and (0004) of the HA coating sprayed at 50On A decreased, and (0002) and TC (0004) (0004) preferred orientations is obvious. the contrary, theTCintensity of peaks (0002)decreased. and (0004) the decomposition α-TCP, and increased. wasdecreased. also observed in this of Meanwhile, the HA coating sprayed at 50 of A TTCP, decreased, and TCACP (0002) and TCCaO (0004) Meanwhile, case. the decomposition of TTCP, α-TCP, and ACP increased. CaO was also observed in this case.

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Figure 5. XRD patterns of HA coatings with different spraying currents. Figure XRD patterns HA coatings with different spraying currents. Figure 5. 5. XRD patterns ofof HA coatings with different spraying currents. 2.4 2.4 2.2 2.2 2.0 2.0 1.8 1.8 1.6 1.6 1.4 1.4 1.2 1.2 1.0 1.0 0.8 0.8 0.6 0.6

Texture coefficient Texture coefficient

(0002) (0002) (0004) (0004)

20

20

25

30 35 40 45 50 25 30 35 40 45 50 Spraying current (A) Spraying current (A)

Figure 6. Variations of the texture coefficient for HA coatings deposited at different currents. Figure Variations texture coefficient HA coatings deposited different currents. Figure 6. 6. Variations ofof thethe texture coefficient forfor HA coatings deposited at at different currents.

SEM micrographics of HA coatings sprayed at different current are shown in Figure 7. The SEM micrographics of HA coatings sprayed at different current are shown in Figure 7. The SEM micrographics HA coatings sprayed atHA different current are shown in Figure 7. The coating coating sprayed at 20 Aofshows many unmelted particles (Figure 7a). On the other hand, the coating sprayed at 20 A shows many unmelted HA particles (Figure 7a). On the other hand, the sprayed atgrains 20 A shows many unmelted HAcoating particles (Figure othersprayed hand, the columnar were not observed in this (Figure 7b).7a). ForOn thethe coating at columnar 30 A, the columnar grains were not observed in this coating (Figure 7b). For the coating sprayed at 30 A, the grains were not observed in thiswere coating (Figure the coating sprayed at 30 A, the columnar columnar grains with hexagons observed in 7b). someFor splats (Figure 7d), and the distribution in the columnar grains with hexagons were observed in some splats (Figure 7d), and the distribution in the grains with observed in some splats (Figure and thetodistribution in the coating is coating is nothexagons uniform were (Figure 7c). When the spraying current7d), increased 40 A, the ratio of columnar coating is not uniform (Figure 7c). When the spraying current increased to 40 A, the ratio of columnar not uniform (Figure 7c). When the spraying current to 40 A, current the ratioincreased, of columnar in grains in coatings increased appreciably (Figure 7f).increased As the spraying thegrains ratio of grains in coatings increased appreciably (Figure 7f). As the spraying current increased, the ratio of coatings increased appreciably (Figure 7f). variation As the spraying current increased, of columnar columnar grains decreased (Figure 7h). The in the columnar grain ratiothe is inratio agreement with columnar grains decreased (Figure 7h). The variation in the columnar grain ratio is in agreement with grains decreased (Figure 7h). The variation in the columnar grain ratio is in agreement with the the evolution of XRD pattern. the evolution of XRD pattern. evolution of XRD pattern.

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Figure 7. Cross-sectional morphology of HA coatings with different spraying currents: (a,b) 20 A, 60 Figure 7. Cross-sectional morphology of HA mm; (c,b) 30 A, 60 mm; (e,f) 40 A, 60 mm; (g,h)coatings 50 A, 60 with mm. different spraying currents: (a,b) 20 A, 60 mm; (c,b) 30 A, 60 mm; (e,f) 40 A, 60 mm; (g,h) 50 A, 60 mm.

4. Discussion 4. Discussion MPS can be used to obtain HA coatings with strong c-axis textures. Through an examination of can be usedand to obtain HA coatings withtexture strongcoefficient, c-axis textures. an examination theMPS microstructure calculation of the c-axis it wasThrough recognized that sprayingof thedistance microstructure andhave calculation of the c-axis texture coefficient, it was recognized that spraying and current a significant influence on texture. Asand we current know, the final coating is built up byon numberless distance have a significant influence texture. single splats. In this investigation, the splat were determined by the particle and substrate In this the MPS process, Astypes we know, the final coating is builtmelting up by state numberless singlecondition. splats. In investigation, substrate temperature is significantly higher whenstate the spraying distance is below thethe splat types were determined by the particle melting and substrate condition. In 60 themm MPS (Figure 8). As a result, the fully melted particles that strike onto the substrate are able to spread out process, the substrate temperature is significantly higher when the spraying distance is below 60 mm and develop a diskthe shape splat. The particles heat flux of thestrike disk shape splats was perpendicular the (Figure 8). As into a result, fully melted that onto the substrate are able to to spread substrate surface, hence promoting the (002) growth deposition direction during out and develop into a disk shape splat. The crystal heat flux of thealong disk the shape splats was perpendicular MPS. During solidification, the columnar grains preferred to grow within disk shape splats. At short to the substrate surface, hence promoting the (002) crystal growth along the deposition direction spraying distances, the shorter dwell time in the plasma jet is insufficient to melt all the in-flight during MPS. During solidification, the columnar grains preferred to grow within disk shape splats. The partially melted particles impact the substrate. They are not able to spread out and Atparticles. short spraying distances, the shorter dwell time in the plasma jet is insufficient to melt all the develop into hemispherical shape splat or disk shape splat. The partially melted particles with lower

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in-flight particles. The partially melted particles impact the substrate. They are not able to spread out and develop into hemispherical shape splat or disk shape splat. The partially melted particles with Coatings 2018, 8, x FOR PEER REVIEW 8 of 12 lower melting degrees impact the substrate and develop into hemispherical shape splats (Figure 9a). degrees impact the substrate and develop into hemispherical splats (Figure 9a). However, melting partially melted particles with higher melting degrees aftershape impacting the substrate will However, partially melted particles with higher melting degrees after impacting the substrate will tend to form disk shape splats (Figure 9b). Columnar grains are not able to form within hemispherical tend to form disk shape splats (Figure 9b). Columnar grains are not able to form within hemispherical splats. As splats. a result, columnar grains with the coatings deposited short spraying As athe result, the columnar grains withhexagons hexagons ininthe coatings deposited at shortat spraying distances are in a non-uniform distribution. distances are in a non-uniform distribution.

Figure 8. Evaluation of substrate temperatures with different spraying distances.

Figure 8. Evaluation of substrate temperatures with different spraying distances. However, columnar grains are not completely perpendicular to the coating surface in some splats. This can be attributed to the fully melted particles striking the hemispherical shape splat, However, columnar grains are not completely perpendicular to the coating surface in some which leads to the reduction of the flattening ratio of fully melted particles. In these splats, the heat splats. This can becompletely attributed to the fully melted striking the hemispherical shape splat, flow is not perpendicular to the surfaceparticles of the substrate. Columnar grains grow parallel to heat flow. Consequently, columnar grainsratio are notofcompletely perpendicular to the which leads to the reduction of the flattening fully melted particles. Incoating these surface splats, the heat these splats.perpendicular As spraying distance increased, of in-flightColumnar particles decreased slightlyparallel to flow is notincompletely to the surfacetheofvelocity the substrate. grains grow [25], which led to an increase in the dwell time of the in-flight particles in the plasma jet. Therefore, heat flow. Consequently, columnar grains are not completely perpendicular to the coating surface in the in-flight particles had been fully melted. Fully melted particles tend to form disk shape splats or these splats. As spraying distance increased, the velocity of in-flight particles decreased [25], splashed shape splats, depending on the substrate temperature. In this study, the plasma jet can slightly act source andin heat the dwell substrate. As the distance particles decreased, the temperature which led astoa heat an increase the time ofspraying the in-flight in substrate the plasma jet. Therefore, improved. Thehad fullybeen melted particles struckFully the heated substrate, whichtend resulted disk disk shape shape splats. splats or the in-flight particles fully melted. melted particles to in form As spraying distance increased, substrate temperature decreased because the substrate was further splashed shape splats, depending on the substrate temperature. In this study, the plasma jet can act as from the plasma jet (Figure 10). In this condition, the lower substrate temperature resulted in a heat source and heat substrate. the spraying distance decreased, theunder substrate temperature splashed shape the splats (Figure 9c).As The in-flight particles had been fully melted a spraying of 60 mm. Inparticles this condition, thethe splats are dominated a disk resulted shape splat limited improved.distance The fully melted struck heated substrate,bywhich inordisk shape splats. splashing at its rim. As a result, a dense and uniform coating consisting of columnar grains was As spraying distance increased, substrate temperature decreased because the substrate was further formed (Figure 11b), which corresponds to an increased texture coefficient. As spraying further from the plasma jet (Figure 10). In this condition, the lower substrate temperature resulted in splashed increased, the dwell time of the in-flight particles in the plasma jet increased. On the one hand, shape splats (Figure The had been fully melted under a spraying distance of increasing the9c). dwell timein-flight of in-flightparticles particles caused more HA decomposition and dehydroxylation. thecondition, other hand, the as the spraying increased, substrate temperature due to 60 mm. InOn this splats aredistance dominated by the a disk shape splat or decreased limited splashing at its the factathat the substrate was further fromconsisting the plasma of jet.columnar In this case,grains the splat on formed the substrate rim. As a result, dense and uniform coating was (Figure 11b), changed from a disk shape to a splashed shape at a rapid cooling rate and thus caused the formation which corresponds to an increased texture coefficient. As spraying further increased, the dwell time of ACP, but the columnar grains of the HA coatings deposited at 110 mm were not obvious of the in-flight particles ininvestigation, the plasmawhen jet increased. On theexceeded one hand, increasing dwell time of (Figure 11c). In this the spraying distance 80 mm, the in-flightthe particles moved out of the plasma of MPS (Figure 10).and The in-flight particles tended to cool in-flight particles caused more flame HA decomposition dehydroxylation. On the down otherafter hand, as the moving out the plasma flame. Hence, the fine particles surface re-solidified. These particles, which spraying distance increased, the substrate temperature decreased due to the fact that the substrate impinge on the substrate, led to the formation of hemispherical shape splats (Figure 9e). As a result, rewas further from the plasma jet. In this case, the splat on the substrate changed from a disk shape solidified particles with a spherical shape were observed in the coating during spraying (Figure 11c).

to a splashed shape at a rapid cooling rate and thus caused the formation of ACP, but the columnar grains of the HA coatings deposited at 110 mm were not obvious (Figure 11c). In this investigation, when the spraying distance exceeded 80 mm, the in-flight particles moved out of the plasma flame of MPS (Figure 10). The in-flight particles tended to cool down after moving out the plasma flame. Hence, the fine particles surface re-solidified. These particles, which impinge on the substrate, led to the formation of hemispherical shape splats (Figure 9e). As a result, re-solidified particles with a spherical shape were observed in the coating during spraying (Figure 11c).

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Figure Schematicofofsplat splattype type formation formation of the single particles during (a)(a) hemispherical shape Figure 9. 9.Schematic duringMPS: MPS: hemispherical shape Figure 9. Schematic of splat type formation ofthe thesingle single particles particles during MPS: (a) hemispherical shape splat;(b)(b)oblate oblatespheroidal spheroidalsplat; splat; (c) (c) disk disk shape splat; (d) splashed shape splat; (e)(e)hemispherical splat; splat; (d) splashed shape splat; hemispherical splat; (b) oblate spheroidal splat; (c) disk shape splat; (d) splashed shape splat; (e) hemispherical shape splat. shape splat. shape splat.

Figure 10.10.(a)(a)Schematic and (b) (b)the theactual actualexperimental experimental conditions during Figure Schematicdiagram diagramof of an an MPS MPS torch and conditions during Figure 10. (a) Schematic diagram of an MPS torch and (b) the actual experimental conditions during thethe MPS ofof HA mm; 40 40 A, A,80 80mm; mm;40 40A, A,110 110mm. mm. MPS HApowder: powder:40 40AA40 40mm; mm; 40 40 A, A, 60 mm; the MPS of HA powder: 40 A 40 mm; 40 A, 60 mm; 40 A, 80 mm; 40 A, 110 mm.

The coatingsprayed sprayedatat20 20 A contains many many partially melted particles the plasma power The coating particlesbecause because the plasma power The coating sprayed at 20AAcontains contains manypartially partially melted particles because the plasma power is insufficient to melt the in-flight particles. The molten shell of the partially melted particle spread is insufficient to melt the in-flight particles. The molten shell of the partially melted particle spread is insufficient to melt the in-flight particles. The molten shell the partially melted particle spread out. The unmelted core of the partially melted particles is still present in the splat center (Figure 11a). unmelted core thepartially partiallymelted meltedparticles particles is still (Figure 11a). out.out. TheThe unmelted core ofofthe still present presentin inthe thesplat splatcenter center (Figure 11a). The size of HA grains located in the unmelted part is much larger than that of those rod-like of grains HA grains located the unmelted part islarger muchthan larger that rod-like of thosenanograins rod-like TheThe sizesize of HA located in thein unmelted part is much thatthan of those

in raw powder (Figure 7a,b), which indicates the growth of grains during spraying. Meanwhile,

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columnar grains be formed in the unmelted part (Figure With the increase spraying nanograins in cannot raw powder (Figure 7a,b), which indicates the 11b). growth of grains duringinspraying. current, the temperature of the plasma jet increased. The previous studies showed that the temperature Meanwhile, columnar grains cannot be formed in the unmelted part (Figure 11b). With the increase of the plasma jet generally increased as spraying increased, and studies the velocity didthat not in spraying current, the temperature of the plasma jet current increased. The previous showed the temperature thewhich plasma jetnot generally increased as thethe spraying current increased, and thein increase significantlyof[9], did significantly decrease dwell time of in-flight particles didso not increase significantly [9],in-flight which did not significantly the current dwell time of inthe velocity plasma jet, the melting degree of the particles increased decrease as spraying increased. flight particles in the plasma jet, so the melting degree of the in-flight particles increased as spraying When the spraying current increased to 40 A, more in-flight particles melted. As a result, more When the spraying increased to 40 A, the more in-flight particles melted.grains As a diskcurrent shapeincreased. splats were formed on the current substrate. Meanwhile, proportion of columnar result, more disk shape splats were formed on the substrate. Meanwhile, the proportion of columnar was improved, corresponding to the increased c-axis orientation intensity of the HA coating in improved, However, corresponding to the the spraying increased current c-axis orientation the HA coating the grains presentwas experiment. when exceededintensity 40 A, theofdehydroxylation in the present experiment. However, when the spraying current exceeded 40 A, the dehydroxylation and decomposition of in-flight particles increased as the plasma temperature increased at a higher and decomposition of in-flight particles increased as the plasma temperature increased at a higher spraying power, which enhanced the formation of the impurity phase after the substrate was impacted. spraying power, which enhanced the formation of the impurity phase after the substrate was Therefore, more HA decomposed. More dehydroxylation and decomposition of HA does not contribute impacted. Therefore, more HA decomposed. More dehydroxylation and decomposition of HA does to the formation of columnar grains in the HA coating. In addition, the higher plasma temperature at not contribute to the formation of columnar grains in the HA coating. In addition, the higher plasma higher power caused the formation of CaO. temperature at higher power caused the formation of CaO.

Figure Schematic of the formation threerepresentation representationHA HAcoatings coatings during MPS: Figure 11. 11. Schematic of the formation ofofthree MPS:(a) (a)HA HAcoating coating contains many partially melted particles; HAcoating coatingwith withcolumnar columnar structure; structure; (c) contains many partially melted particles; (b)(b) HA (c) HA HAcoating coatingwith with higher amorphous calcium phosphate. higher amorphous calcium phosphate.

This investigation has shown that columnar grains preferred to grow within disk shape splats. This investigation has shown that columnar grains preferred to grow within disk shape splats. In order to understand the mechanism of this phenomenon, further investigation, such as the crossIn order to understand the mechanism of this phenomenon, further investigation, such as the section of splat, will be conducted. In addition, further studies will be focused on the investigation of cross-section of splat, will beof conducted. In addition, further studies will be focused on the investigation the biological properties HA coatings with a c-axis texture. of the biological properties of HA coatings with a c-axis texture. 5. Conclusions 5. Conclusions In this paper, we investigated the influence of the process parameters on the texture intensity of In this paper, we influence of the process parameters on the MPS-deposited HAinvestigated coatings. Thethe microstructure of the HA coatings sprayed in texture differentintensity sprayingof MPS-deposited coatings. The microstructure of the HA coatingsFrom sprayed in different spraying distances andHA currents was systematically compared and analyzed. the results, the following distances and currents was systematically compared and analyzed. From the results, the following can can be concluded: be concluded:  The process parameters had a significant influence on the phase composition of the coatings. • TheMeanwhile, process parameters a significant influence on the phase composition of the distance coatings. the texturehad intensity of the c-axis of HA was greatly influenced by spraying Meanwhile, the texture of thec-axis c-axistexture of HA was wasfound greatly distance and spraying current.intensity The strongest ininfluenced the coatingby byspraying 60 mm spraying with a spraying current ofc-axis 40 A.texture Uniform distribution columnar anddistance spraying current. The strongest was found in the coatinggrains by 60with mmhexagon spraying sections were observed current in the upper of the coating (~100 μm). distance with a spraying of 40part A. Uniform distribution columnar grains with hexagon sections were observed in the upper part of the coating (~100 µm).

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An increase in the number of fully melted particles was connected to an increase in columnar grain ratio. This can be attributed to the fact that the disk shape splats were formed by fully melted particles and that columnar grains preferred to grow within disk shape splats. All the texture intensity variation was ascribed to the growth probability of columnar grains during MPS spraying. The texture intensity in MPS-sprayed HA coatings can be controlled by adjusting the melting state of the in-flight particles and the splat type.

Acknowledgments: The authors gratefully appreciate the financial support of the National Natural Science Foundation of China (51471010). Author Contributions: Xiaomei Liu and Dingyong He conceived and designed the experiments; Xiaomei Liu performed the experiments; Xiaomei Liu, Zheng Zhou, Zengjie Wang, and Guohong Wang analyzed the data; Xiaomei Liu and Dingyong He wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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