Surface defects repairing of sprayed Ca-P coating by the microwave-hydrothermal method

The increasing interest in decreasing the surface defects of sprayed Ca-P coating deposited on carbon/carbon (C/C) composites to enhance the bonding strength, bioactivity and corrosion resistance of the coating is justified by the growing evidence of its beneficial effect on the bone replacement fields. Microwave-hydrothermal (MH) method detailed in the previous study is successfully used to reduce the above coating defects and the MH mechanism is well studied here. Hence, five different treatment reagents involving calcium and phosphorus solution, sulfuric acid (H 2 SO 4 ) solution, ammonium hydroxide (NH 3 ·H 2 O) solution, only Ca 2+ solution and deionized water are selected as the precursor solution. The surface, cross-sectional morphologies, phase and composition of the coatings are characterized by the scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), microscopy Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) spectra. Elastic modulus and coating hardness are measured by nanoindentation. Results reveal that the presence of calcium and phosphorus ions, as well as the H 2 SO 4 in the precursor solution during the MH process, have a positive influence on the reduction of sprayed Ca-P coating surface defects. However, the coating treated by other three solutions cannot produce new phases on the basis of sprayed Ca-P coating and the surface defects of it are not decreased. Nevertheless, the elastic modulus and hardness of the coating treated by H 2 SO 4 solution are very weak. MH treated coating by calcium and phosphorus ions in the precursor solution and in NH 3 ·H 2 O solution, only Ca 2+ solution and deionized water own the similar elastic modulus and hardness to that of the sprayed Ca-P coating. To conclude, in the MH process, the surface defects of the sprayed Ca-P coating are only lowered in calcium and phosphorus precursor solution and the coating strength is not dropped, which demonstrates the promoting mechanism of MH process.

Carbon/carbon (C/C) composites recognized as one of the biomedical materials with unique combination properties for instance low density, excellent mechanical properties, high corrosion resistance and biocompatibility, are widely utilized in bone replacement fields [1][2][3][4]. Notably, their elastic moduli are equivalent to that of the human bone. However, numerous carbon particles and fragments released from C/C substrates would be found at the interface while they are directly implanted in the human body [5]. Besides, C/C composites are the biologically inert material with a hydrophobic surface and have no conduction or induction of bone regeneration [6,7].
Consequently, in order to avoid the generation of carbon fragments and obtain the bioactive substrates, it can be achieved by depositing a C/C substrate with a coating of calcium phosphates [8,9].
Currently, plasma spray (PS) has proved to be a considerable method for preparing Ca-P coating on the substrate [10]. Among these PS techniques, supersonic atmospheric plasma spray (SAPS) method is commonly considered as one of the ways as it owns the advantages of the environmentally-friendly, good controllability, high efficiency and small thermal damage to the substrates [11,12]. Generally, in the course of the SAPS process, thermal decomposition and bonding strength of the coating were the main concerns and got full attention as previously reported [13,14].
These impurity phases, as well as the surface defects in the coating, can form mechanically unstable bonds to the bone and diminish the corrosion resistance of the substrate. In present work, fabricated hydroxyapatite (Ca 10 (PO 4 ) 2 (OH) 6 , HA) coating is widely applied as the bioactive coating, thanks to its favorable bioactivity and biocompatibility [17][18][19][20]. Yang et al. [21] studied the micro-crack changes and phase transition of HA coating after the air calcination and the hydrothermal treatment. The results reflected that the micro-cracks on the coating surface increased after the calcined HA coating for a period, and CaO phase in coating still existed. But when HA coating was treated by hydrothermal method, surface micro-cracks were decreased and impurity phase (CaO) was converted into HA phase. It is indicated that the hydrothermal method can effectively repair the surface defects and homogeneous surface impurities existing in the sprayed HA coating, nevertheless, this process took 6 h and was time-consuming. Li et al. [22] can diminish the impurity phases and the 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 non-crystalline phases of the sprayed HA coating to achieve the effect of recrystallization by sprayed high-temperature steam for the coating. However, the high-temperature steam equipment is difficult to be controlled during the spraying process and this may lead to the secondary crystallizations. Cao et al. [23] post-processed sprayed HA coating using water vapor method. It is evident that the non-crystalline phase and impurity phase can transform into HA phase, but high-temperature treatment for a long time would cause a certain degree of damage to the HA coating. This is main due to the stress redistribution between the coatings formed and regenerated more secondary micro-cracks. It may reduce the durability of the implant into the human body. Sun et al. [24] used hollow HA powder as the sprayed powder to prepare HA coating. During the spraying process, only using lower spraying power can completely melt HA powder and reduce the thermal decomposition phenomenon of HA phase. Nevertheless, the surface defects appeared in sprayed coating including spherical particles, micro-cracks, holes and un-melted particles are a lack of well studying. In previous work, the related work using microwave-hydrothermal (MH) method to reduce the above defects has been given.
MH method is a rapid, simple, environmental-friendly, and it is a promising route to synthesize the crystals on substrates in recent years, such as metals and carbon/carbon (C/C) composites [25,26]. Meanwhile, microwave heating mainly occurs inside the materials and this heating method makes the materials heat evenly, therefore, having no temperature gradient, stimulating the budding of the crystal, accelerating the rate of the crystallization, lowering the crystallization temperature and shortening crystallization time [27]. Especially, MH treatment without using surfactants, templates, controlling pH values or emulsions, is able to obtain a well-distributed and dense structure [28]. As a result, MH is suggested as a method to repair sprayed Ca-P coating on C/C substrate to achieve the purpose of reducing defects.
To achieve long-term stability of sprayed Ca-P coating, the reduction of the surface defects and impurity phases is the critical factor. Furthermore, MH method is employed, incorporated calcium and phosphorus precursor solution by an in-situ growth of MH treated coating on sprayed Ca-P coating, to suppress the surface defects in coating formation [29]. The uniform morphology appeared on the surface of sprayed Ca-P coating. So far, MH reaction mechanism for using treatment for the repairing to the sprayed Ca-P coating is a lack of study. In this paper, on the basis of MH treated coating fabricated using calcium and phosphorus precursor solution on 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65   sprayed Ca-P coating, H 2 SO 4 solution, NH 3 ·H 2 O solution, only Ca 2+ solution and   deionized water are studied during the MH process to investigate the MH reaction   mechanism. For the surface defects in sprayed Ca-P coating, whether MH method   plays a role in aqueous of different pH values, only Ca 2+ solution or in deionized water, the morphology, phases and nanoindentation test are analyzed to justify the result. It is worth noting that only one sprayed Ca-P coating surface is used as the substrate for the subsequent reaction in the MH process. Herein, it is considered that the mechanism of the promoting inhibition in MH route has been determined by the calcium and phosphorus ions in solution.
2 Experimental procedure

Materials
C/C composites with the density of 1.40 g/cm 3 used in this study, were fabricated through chemical vapor infiltration (CVI) [30]. The preform of the material was two-dimensional needle punched carbon felt (consisting of no weft cloth, a tires layer, and a needle), and the initial density was 0.43 g/cm 3 . Subsequently, the mixed natural gas was composed of 96.1% CH 4 , 0.45% C 2 H 6 , 3.2% CO 2 , 0.10% other hydrocarbons.
A small amount of H 2 S and N 2 were used as the precursor. CVI deposition was performed between 900°C ~ 1100°C. C/C substrates were cut into specimens by wire-cutting with the dimension of 8 mm × 8 mm × 2 mm from the bulk. Thanks to the existence of the machine oil and carbon particles on C/C substrates, acetone, ethyl alcohol and deionized water were employed to wash them in an ultrasonic bath, then, dried in an oven for 24 h. The powders for the SAPS process were the commercial HA. Before starting to spray, HA powders were agglomerated to enhance the fluidity.
The morphology and phases of the agglomerated HA powders were reported elsewhere [29,31].

Deposition of sprayed Ca-P coating on C/C substrate
Sprayed Ca-P coating was manufactured via SAPS technology using HEPJ-100 sprayer on C/C substrate. The detailed parameters for spraying process including voltage, current, carrier gas, a second gas, spraying time, powder feeding rate and specimen position were mentioned in our previous work [29]. Besides, the sprayed Ca-P coating was only produced one surface for the further study. Finally, the obtained sprayed Ca-P coating was cleaned with alcohol to remove the remaining powder on the coating surface for subsequent use. Ammonium phosphate monobasic (NH 4 H 2 PO 4 ) was provided as the starting phosphorus ions. The two precursor materials were dissolved in 20 mL of deionized water, respectively. The molar ratio of calcium to phosphorus was 1.67. Then, the resulting solution as well as the specimens were added into a microwave reactor heated to 180°C and maintained at this temperature for 30 min. The MH treated coating could appear on the above sprayed Ca-P coating. The schematic illustration of the composite coating formation on C/C substrate was shown in Fig. 1

Morphology and phase analysis of the sprayed Ca-P coating
In our previous work, the surface morphologies of sprayed Ca-P coating have been described [25]. In order to expressly understand the typical micrographs of the coating, schematic illustration and the high-resolution micrographs are given in Fig. 2.
The main compositions of the sprayed Ca-P coating are clearly depicted as the general plan of defect structure in Fig. 2(a). Naturally, surface defects appeared on the sprayed Ca-P coating mainly include four kinds of morphologies represented in Fig.2 (d) and (e). As illustrated, it can be inferred that the spherical particles seem to be caused by the particle impacting on C/C substrate. Consequently, these melted particles are generated splashes. Because of the residual stress in the coating, the micro-cracks will appear when the coating cools to the room temperature. For the holes and un-melted particles forming on C/C substrate, while the hot particles are reached the cold substrate surface, some particles may not be melted before being encountered on the surface of C/C substrate, and some melted particles of HA quickly are solidified.
The XRD results of sprayed Ca-P coating was reported in our previous work [29].

Morphology and phase analysis of MH treated coating
In terms of surface macrocrack of the coating, the sprayed Ca-P coating (in Fig.   9a) has large amounts of macrocracks. Residual stress existing inside the coating after spraying is an important factor to lead to the surface macrocracks. MH treated coating can be observed from Fig. 4(a), and the microcracks are disappeared during MH treatment. Then, the surface macrocracks of MH-H-42, MH-OH-942, MH-167, and MH-0 are changed. In Fig. 9(b), fewer microcracks of MH-H-42 are generated than that of on the sprayed Ca-P coating, forming several holes on the coating surface.
About these holes, which are much larger than that of the sprayed Ca-P coating, their appearance may be caused by the coating reaction in solution and the porous nature of 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 C/C composites. It infers that the macrocracks found in Fig. 5(a) are reduced because of these new morphologies forming on the sprayed Ca-P coating. Additionally, the phase of these morphologies has been shown in XRD patterns that the sprayed Ca-P coating can react with the H 2 SO 4 solution. The same phenomenon is observed in Fig.   9(c), (d) and (e). The surface morphologies of the MH-OH-942, MH-167, and MH-0 are still smooth like that of sprayed Ca-P coating. However, these macrocracks can completely be vanished after being treated in the MH process with the calcium and phosphorus precursor solution [29]. The above results demonstrate that the surface defects of the coating are lowered and the holes may still exist in the MH-OH-942, MH-167, and MH-0. It is concluded that the growth of the particles in the sprayed Ca-P coating could expand and fill the macrocracks as well as the holes to achieve the goal. Nevertheless, the growth of particles could not be provided enough ability to repair the surface defects [36]. The macrocracks present in the coating greatly limit the application fields of the coating. In contrast, the MH treated coating not only has the dense structure but also vanishes the surface defects of sprayed Ca-P coating. It is reflected that the repairing defect need not only calcium ions, but also phosphorous ions. Under the circumstances, the calcium ions quickly react with the phosphorous ions forming Ca-P nucleation and then rapid penetration into the coating under the MH process. Although the cracks have few changes after being treated by MH method, in comparison to the high-temperature heat treatments [37,38], their treatment temperatures are lower and just reach to 180°C, and can achieve the same positive influence as that of the treatment in high temperatures. In addition, some reports about the hydrothermal crystallization on sprayed Ca-P coating are given, however, it would spend much more time than the MH treatment. MH method used in this work could drop the reaction efficiency [21].
The coating formation mechanism of promoting inhibition in the MH process is Thirdly, HA phase in the sprayed Ca-P coating has much rich hydroxy when the MH process is carried out in an aqueous environment. Consequently, the Ca 2+ can be concentration on the sprayed Ca-P coating and instantaneously saturate to form a Ca-P nucleation to generate the new layer. Next, it is schematically described the possible reactions and mechanism of the MH treated coating, MH-H-42, MH-OH-942, MH-167 and MH-0. As it is seen from Fig. 13, the detailed mechanism for the formation of products in the MH process is described. In the stage of forming MH treated coating, calcium and phosphorus precursor solution, which has a Ca-P molar ratio of 1.67, promotes the formation of the new layer and reduces the surface defects of sprayed Ca-P coating. The mechanism for defect repairing of sprayed Ca-P coating is reported in our previous work [29]. MH-H-42 is the reaction product of the sprayed  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 sprayed Ca-P coating cannot be repaired in NH 3 ·H 2 O solution, only Ca 2+ solution, and deionized water, respectively during the MH process. But a small number of (calcium and phosphorous) ions from precursor solution have a critical role in the repairing surface defects of sprayed Ca-P coating by the microwave-hydrothermal method.

Conclusions
Sprayed Ca-P coating is post-processed by the MH method to lower the surface defects (spherical particles, micro-cracks, holes and un-melted particles). The promotion inhibition of the MH process under different conditions is investigated.