| 1. IntroductionHydroxyapatite (HA) has a chemical composition similar to that of the inorganic part of human bones and has excellent biocompatibility and bioactivity in comparison with other implant materials. Bioactive hydroxyapatite ceramics implant is not only safe, non-toxic, but also can induce the growth of mature bone. And now it is widely used as the replacement or repair material in bone surgery and dental surgery. However, HA's mechanical properties, especially fracture toughness and strength are not high enough for application in human bone replacement. So in most situations, HA coating is sprayed on a titanium (Ti) substrate in replacement implant. In recent years more and more accidents of loosening of implant have been reported after some years of implantation, which is due to the shedding coatings. Therefore the adhesion strength between Ti alloy substrate and HA coatings is one of the most important factors which contribute to implant loosening. After several years of study of HA coating formation, our research group has found that the temperature of Ti alloy influences the adhesion strength between coating and substrate. Based on the above study, the paper carries out the research on HA coating formation on heated Ti alloy substrate.2. Powder spraying test system developmentAfter several rounds of demonstrations and tests, we develop a whole set of test equipment for HA coating formation (Fig.1.Schematic Diagram of the system).The system consists of gas source, processing chamber, powder feeder, heater, and control system. The whole system can realize the accurate control of powder feeding, spraying distance, incident angle, pressure, time and the heating temperature.In the above devices, we choose induction heating device as the heater. Induction heating is a clean, fast heating method, which does not make heat damage to the surrounding environment, so it is widely used in mangy fields. There are many factors which contribute to induction heating including frequency, current, magnetic permeability of material, coil turn, area of toroid. Through simulation research we find out:1. The effect of frequency on induction heating is not only the induction electromotive force, but also the depth of induced current density, which is called skin effect. Higher frequency generates smaller depth of induced current density.2. When in the absence of magnetizer, the magnetic strength will be very low. Therefore it is very important for the effect of heating to choose the right magnetizer.Taking all the factors into account, we choose the frequency of 40 kHz.In addition, in order to meet the different needs of induction heating, we have designed two kinds of coil, one is the open wide coil, which is used while the object is processing; the other is toroid, which is used for the irregular shape object (Fig.2.Coil). The application of IGBT is the key factor of the development of high-frequency induction heating. In the applied circuits except for the main circuit, the other circuits is used to support IGBT or protect IGBT, including the overvoltage protection circuit, synchronous circuit, current checking circuit. The design of high-frequency circuit is based on the design of induction heater, and we just make a small change to meet our application needs.3. Study on temperature controlPresent temperature control method contains fixed value switch control, PID control and intelligent control. Draw the temperature curves based on the experiment, and analyze the characteristics of heating system. The heating system has temperature rising unidirectivity, nonlinearity, hysteretic nature, inertia and so on. When we use the fixed value switch temperature control, we can make use of the high temperature or the low temperature when it stops to do the experiment. At that point it always stops for 20s, and the deviations are not more than 4℃in the area of 40℃of the high temperature or the low temperature. However, the method is just the open-loop control, and it can't adjust the temperature, correct the error. So it can't meet the need of accurate temperature control.PID control is based on classical control theory, and proportion value, integral value, differentiation value decide the control quality. Simulating the whole control process of PID with Matlab software, we can get the good result of temperature control. But the inertia of heating system is related to the fixed value, when the value is very low, the inertia becomes high. So when we use PID control, high inertia and big overshoot also exist. We can resolve this problem through sectional control, iterative adjustment, which will bring a lot of troubles. So a new effective control technology is needed in many big projects. Fuzzy control is one of advanced control technologies, which contains PID parameters self-learning, self-tuning, and it is the future of control theory.4. Experimental study of HA coating formation on heated substrateChoose HA powder particle of 25μm as the experimental material, then do verification experiment at different temperature in three different spray distances. The experiment results show that the temperature of substrate affects spray result deeply. With the temperature rises between 0 and 200℃, the effect of spray has a clear improvement. And the experiment further verifies the influence of spray distance on coating formation. 5. Main conclusion1) Analyze different heating method and make attempts in this field, then choose induction heating as my research direction.2) Introduce related theories of induction heating and related studies of electromagnetism, and analyze the factors which contribute to induction heating including frequency, current, magnetic permeability of material, coil turn, area of toroid.3) Simulate the relationship between skin depth and frequency of the open wide coil in ideal state, choose right frequency, and design the coil. Toroid's magnetic field is distributed better than open wide coil, so it has higher magnetic strength. Open wide coil must be added magnetizer if it wants to receive good heating performance.4) The application of IGBT is the key factor of the development of high-frequency induction heating. In the applied circuits except for the main circuit, the other circuits is used to support IGBT or protect IGBT, including the overvoltage protection circuit, synchronous circuit, current checking circuit. The design of high-frequency circuit is based on the design of induction heater, and we just make a small change to meet our application needs.5) The selection of control method is very important to the heating performance. If the requirement of constant temperature time is not very high, we can choose fixed value switch control to meet the experiment's need. Otherwise, we can choose PID control, which has well heating performance.6) Do the experiment of HA coating formation on heated substrate, and verify the effect of temperature of substrate on adhesion strength between coating and substrate. |
PDF Full Text Request