Research On Powder-feeding System In Hydroxyapatite Cold Spraying Equipment

Because of good biocompatibility and strength, HAP (Hydroxyapatite, HA or HAP) is widely applied to some fields such as prosthetic implants and bone repair etc. However, the traditional preparation methods for thermal spraying are easy to produce phase transition matter, which is easy to dissolve in the human body, resulting in coatings flaking. This paper proposed a method for making HA coatings using high-speed gas jet. This method is simple and with no phase transition matter produced. Because powder feeding system needs to meet the requirement of feeding powder with uniform speed and micro quantity when producing HA coatings by cold spray, the paper developed a new powder feeding system based on the theory of feeding powder using ultrasonic wave. In the system, the smallest feeding rate for HA powder is about 0.002g/s, and the maximum rate is about 0.25g/s.1. The powder feeding system is composed of piezoelectric ceramics, PMMA tube, drive power and the mixing device. The principles for the powder feeding are as follows:the annular piezoelectric ceramics polarized axially generates radial contraction vibration with the frequency of supersonic range when inspired by the high-frequency sinusoidal alternating current. During propagating, vibration wave becomes attenuation traveling wave, because the wall of the PMMA tube and the powder can absorb vibration leading to the energy loss. In this case, the particles on the wall of the PMMA tube moves with oval shape. The moving direction is opposite to the one of wave propagation. So the powder is droved along the direction from the feeding cup to the piezoelectric. The powder fluids out of the discharge hole is mixed high-pressure air in the mixing device and injected through Laval nozzle.2. Based on the theoretical analysis of the powder process using ultrasonic, some conclusion are obtained. The feeding capacity of the system is mainly related to the voltage and frequency of the drive power, the performance and structural parameters of the piezoelectric ceramics, the length and damping coefficients of the PMMA tube.3. The simulation results of the annular piezoelectric ceramics show that the resonant frequency of the radial telescopic mode decreases with the increase of the size of the inner or the outer diameter and has a little relationship with the thickness of the annular piezoelectric ceramics. When the driving voltage is constant, the more the drive frequency approaches the resonant frequency, the bigger the amplitude of the radial vibration of the particle on the inner ring becomes. The amplitude reaches the maximum at the resonant frequency. When the driving frequency is equal to the resonant frequency, the amplitude of the radial vibration increases with the rise of the drive voltage.4. The finite element simulation results of the PMMA tube indicate that the PMMA tube has multiple-order radial contraction vibration modes near the resonant frequency of the piezoelectric ceramic. The attenuation of the amplitude of the radial vibration along the wall of the PMMA tube becomes more obvious with the increase of the damping coefficient of the PMMA tube. The amplitude of the radial vibration of each point on the wall decreases with the increase of the distance from the point the vibration source.5. Some trials about feeding HAP powder using the feeding powder system with different structure parameters have been done. The contrast tests of the single factor show that the feeding rate increases with the rise of the drive voltage. The feeding rate can be adjusted by changing the driving voltage. The curve of the relationship between the drive frequency and the feeding rate is similar with the one of the relationship between the piezoelectric and the feeding rate. The feeding rate gets the maximum at the resonant frequency of the piezoelectric ceramic. The curve has better linearity at the right side of the resonant frequency, in which the feeding rate can be adjusted by changing drive frequency. The feeding rate decreases with the increase of the length of the feeding pipe and has little relationship with the height of the powder in the cup. The particle size of the HAP powder has little effect on the feeding rate in the available transport.6. Under the same experimental conditions, some feeding tests using different types of powder has been done. The test results show that the feeding powder system has good feeding capacity when the powder has good mobility, rough surface and large density.