On using of gas detonation for spraying of biocompatible films onto the carbon nanocomposites

The paper presents the experimental study of the using of gas detonation for spraying of hydroxyapapite (HOA) particles onto the carbon nanocomposites. The characteristic parameters of the heterophasic detonation flows were defined. The prepared films (with a thickness of 80…100 μm) were polycrystalline with Ca and P concentration ratio of ≈1.67 as for raw HOA. It was noted about the prospects of gas detonation for HOA spraying.


Introduction
The one of main problems of the modern engineering is designing of biocompatible materials for the bone substitutes [1]. Nowadays the films of hydroxyapatite (HOA) Ca 10 (PO 4 ) 6 (OH) 2 onto SiC and Ti substrates are under consideration [2]. Carbone nanoimplants (CNI) [3] are more perspective due to the structure features.
The commercial technologies [4] of HOA spraying are plasma and high velocity oxygen fuel spraying. The using of such methods is limited [2] because of poor adhesion, imperfection of structures etc. Therefore nowadays the application of gas detonation [5] for this aim is under studying [2]. The detonation provides pulsed action onto a substrate for minimization of abrasive wear and increasing of the adhesion.

Experimental set
Based on commercial CCDS 2000 equipment [5] the experimental set was designed (see figure 1). Gas detonation generated within a cylindrical barrel (1) with a diameter d b of 1.6 cm in a stoichiometric acetylene-oxygen mixture by a spark (2). The mixture was supplied via controlled solenoid valves by a gas feed system (3). A portion of the mixture η within the barrel was varied from 0.6 to 0.95. Nitrogen flow was used for the blowdown among shots. The injection of fine (d p ≤100 μm) HOA particles was realized into the direction normal to the barrel axe by a pneumatic feeder (4). A volume flow rate of HOA was about of 0.3…1.0 mm 3 per a shot. Piezometric sensors were used for control of the pressure dynamics ( ) p t and estimation of average velocity D at different distances from the spark. The one (5) was built in the barrel at the injection zone at the distance of a=56 cm from the spark and the other (6) was embedded into a substrate (7). A distance δ between the muzzle and the substrate was varied from 2.5 up to 23.0 cm. The length of the particle acceleration zone was of l=40 cm. The recording of data from the sensors was realized by a digital oscilloscope (8). A system of laser Schlieren photography was used for the visualization of the detonation flow (9). This system consisted from a Nd:YAG laser (10) and a high-speed camera (11). The red (λ=670 nm) and the green (λ=532 nm) lasers were used. The polarizing filter (12) regulated of an intensity of the laser radiation. The system consisting of an interference filter (13) with λ max =532 nm and ∆λ=11 nm for the green laser and with λ max =670 nm and ∆λ=11 nm for the red laser and a diaphragm (14) was used for cutoff of the flow (9) radiation and visualization of flow disturbances. The laser beam was collimated by lenses (15) and (16). Synchronizing unit BNC 575 (17) delayed a picture capturing by the camera relativity to the spark (2). The study of the detonation dynamics was done both for the non-dusty flows and heterophasic flows with HOA particles with an average size of d p =70 nm и 100 μm.

Results and discussion
The main features of the dynamics of non-dusty and heterophasic flows were defined. In figure 2  p   and a further rarefaction (2). The main energy releasing occurred in a deflagration zone (3). The following pressure decreasing (4) was bound with the product cooling. The wide pressure peaks (5) were caused by a deflagration afterburning of the mixture. Further the rarefaction (6) caused by leaving of products from the barrel was recorded. Characteristic velocities D between the spark and the injection zone were varied from 0.7 up to 1.5 km/s. Such dispersion is explained by an irregularity of the valve operation, inhomogeneity of the mixture etc. The impulse amplitude recorded by the sensor (6) and the flow structure were depended on δ (see figure 3). At δ≈2.5 cm the substrate was undergone by an action of strong shock wave (1) . On the other hand HOA injection caused a increasing of the transmitted on to the substrate impulse in ≈1.5…3.5 times than of non-dusty flows. This fact was linked with action of the dispersed phase. substrate (2). At the periphery of the muzzle (4) the vortexes (5) and rarefaction waves (6) were registered. Further (t d ≈680 μs) the detonation deceleration occurred. The formation of high density shadowed region (7) and wave interference (8) between the substrate and the barrel were detected. At the larger times (t d ≈720 μs) the vortex growth was detected. This phenomenon resulted to the further «spreading» of the flow and stabilization of high pressure field over the substrate during of 5070 μs. This fact was recorded by the piezometric sensor too (see figure 2). Study of the HOA films on CNI allowed to define the main features. The films (see figure 5) had a non porous structure without visible defects. It was showed that the coatings are polycrystalline with Ca and P concentration ratio of ≈1.67 that corresponds to raw HOA. The adhesion of the films depended non-monotonically on η and  p respectively (see figure 6).
Here  max / A A A  is dimensionless adhesion and A max is maximum measured adhesion at l≈5 cm and d p ≈70 nm. Initially the increasing of  p led to  A growth due to effective HOA acceleration but further increasing of  p (higher than of 0.85) caused the CNI micro crashing and  A decreasing.

Conclusion
Gas detonation flows are suitable for spraying of HOA onto CNI. The prepared films (with a thickness of 80…100 μm) have the close packed structure as raw HOA. It was demonstrated that the flow dynamics has an effect on the adhesion. The optimal conditions of the film spraying were defined.