Preparation method of large-thickness high-density ceramic coating

Abstract

The invention discloses a preparation method of a large-thickness high-density ceramic coating, and relates to the technical field of light metal surface treatment. The invention aims to solve the problems of low density, small thickness, inapplicability to complex special shapes and high cost of the high-density defect-free coating prepared by the magnetron sputtering technology in the prior art. According to the method, the large-thickness high-density ceramic coating is prepared on the surface of the aluminum alloy, the high frequency / ultrahigh frequency is set to be larger than or equal to 10000 Hz, the negative voltage is set to be-40 V to-120 V, arc suppression ultralow-temperature plasma discharge is regulated and controlled, the coating grows evenly and slowly, and the aluminum oxide coating with the density larger than 95% and the high-density ceramic coating with the thickness reaching 50-80 microns on the surface of the aluminum alloy are obtained; and the alloy has the advantages of high bonding strength, high hardness and high corrosion resistance (resistance to salt spray corrosion for 3000 hours).

Description

Technical Field

[0001] The invention relates to the technical field of light metal surface treatment, and in particular to a method for preparing a thick and high-density ceramic coating. Background Art

[0002] The technology of rapidly preparing high-density ceramic coatings at low cost is of great significance for future marine corrosion protection, aerospace high-precision wear resistance, and electronic packaging. However, at present, high-density and defect-free coatings are mainly produced by magnetron sputtering technology, which has problems such as small thickness, unsuitability for complex and special shapes, and high cost. Micro-arc oxidation technology can produce thick ceramic coatings. However, due to the high plasma discharge temperature, the traditional micro-arc oxidation coatings have violent discharges, loose and porous coatings, and it is difficult to uniformly prepare high-density ceramic coatings. Therefore, it is urgent to develop new micro-arc oxidation technologies to reduce the micro-area plasma discharge temperature, form arc-suppressing ultra-low temperature plasma, induce the coating to achieve high-density growth, and replace part of the expensive magnetron sputtering technology. If the uniform preparation of thick and high-density ceramic coatings is achieved, and high bonding strength, high hardness, high and high corrosion resistance are exhibited, this will be applied to harsh marine and aerospace service environments. Summary of the invention

[0003] The purpose of the present invention is to solve the problems of low density, small thickness, unsuitability for complex shapes and high cost of the existing high-density, defect-free coatings prepared by magnetron sputtering technology, and to provide a method for preparing a thick and high-density ceramic coating.

[0004] A method for preparing a thick and dense ceramic coating is specifically completed by the following steps:

[0005] 1. Aluminum alloy pretreatment:

[0006] The surface of the aluminum alloy is polished using sandpaper, and then ultrasonically cleaned and dried to obtain a pretreated aluminum alloy;

[0007] 2. Preparation of arc suppression electrolyte:

[0008] dissolving hydrogen oxalate and arc-suppressing molecules in deionized water to obtain an arc-suppressing electrolyte;

[0009] 3. Add arc-suppressing electrolyte into the electrolytic cell, immerse the pretreated aluminum alloy and stainless steel plate into the arc-suppressing electrolyte, wherein the stainless steel plate serves as the cathode and the pretreated aluminum alloy serves as the anode; connect the cathode and the anode to the power supply of the ultra-low temperature plasma induced ultra-high density coating equipment;

[0010] Fourth, the ultra-low temperature plasma induced ultra-high density coating equipment is adjusted to an asymmetric pulse output with alternating positive and negative pulses, forming a new mode combining high frequency / ultra-high frequency with cathode discharge; start the power supply, induce uniform and dense ultra-low temperature plasma sparks in the plasma discharge micro-area, and the high-density arc-suppressing ultra-low temperature plasma discharge heals the holes, promotes the high-density growth of the coating, and oxidizes for a period of time to obtain a thick and high-density ceramic coating;

[0011] The high frequency / ultra-high frequency described in step 4 is a plasma discharge frequency ≥ 10000 Hz.

[0012] The present invention realizes asymmetric pulse output of alternating positive and negative pulses by optimizing the main circuit topology structure, forming a new mode of "high frequency ≥ 10000Hz (achievable by using the ultra-low temperature plasma induction ultra-high density coating equipment in the present invention) + cathode discharge -40~-120V", combined with weakly acidic arc suppression electrolyte, to induce ultra-low temperature plasma spark discharge;

[0013] The present invention reduces the micro-area plasma discharge temperature from the traditional 10000K to 3000K through the above power supply mode and the new weakly acidic electrolyte composition, and increases the plasma discharge density by one order of magnitude;

[0014] The present invention utilizes high-density arc-suppressing ultra-low temperature plasma discharge to heal the holes generated in the early stage, promotes the high-density growth of the coating, and further realizes the uniform preparation of the thick and high-density ceramic coating.

[0015] Principle of the present invention:

[0016] The ultra-low temperature plasma induced ultra-high density coating equipment in the present invention is optimized through the main circuit topology structure. It is input by three-phase mains power, and after transformation, rectification and filtering in sequence, it is converted into a DC voltage and input into the plasma induction circuit. The controller controls the switch tube in the plasma induction circuit to continuously switch operations to generate a high-frequency pulse signal, and then the sampling circuit collects the voltage and current signals in real time and feeds them back to the controller. The controller adjusts the switching frequency and duty cycle (conduction time) of the switch tube, thereby achieving stable control of the output voltage and current and maintaining the stability of high-frequency discharge. Therefore, the present invention uses the controller to alternately control the plasma induction circuit to generate alternating positive and negative pulses, so that the positive and negative duty cycles are alternated (the positive duty cycle is 5%, the negative duty cycle is 5%, and the positive and negative frequency ratio is 1:1), and the frequency of the high-frequency / ultra-high frequency boost is ≥10000Hz; the present invention designs an asymmetric bipolar pulse soft switching converter composed of an alternating inductor and capacitor auxiliary network to achieve positive and negative pulses. The invention uses an alternating asymmetric pulse output to induce arc-suppressing ultra-low temperature plasma discharge technology by using an asymmetric bipolar pulse to perform low-temperature plasma electrolytic oxidation on the surface of the pretreated aluminum alloy to in-situ grow a high-density ceramic layer; specifically, the arc-suppressing ultra-low temperature plasma discharge is induced by a negative pulse voltage, and the high-frequency self-feedback "arc-suppressing ultra-low temperature plasma" spark dynamic control is realized by optimizing the main circuit topology structure design, so that the micro-area plasma discharge temperature is reduced from 10000K to 4000K, and the plasma discharge density is increased by 1 order of magnitude, so that the electron temperature in the plasma is in a thermal equilibrium state with the temperature of the gas / ion, the plasma discharge energy is stabilized, and uniform and fine "ultra-low temperature plasma spark" discharge is induced in the plasma discharge micro-area, forming uniform and dense arc-suppressing ultra-low temperature plasma healing micropores, promoting the uniform growth of the high-density ceramic coating, and then realizing the uniform preparation of a thick and high-density ceramic coating with a thickness of 50-80μm and a density of more than 95%.

[0017] Compared with the prior art, the method for preparing a thick and dense ceramic coating provided by the present invention has the following advantages:

[0018] (1) The present invention optimizes the main circuit structure through topology, eliminates the residual energy anodized carrier, forms a self-balanced discharge impedance, and realizes asymmetric pulse output with alternating positive and negative pulses. At the same time, the negative pulse voltage induces uniform and dense "ultra-low temperature plasma" discharge in the plasma discharge area to achieve continuous bombardment densification without damaging the surface flatness. The low-temperature plasma discharge heals the micropores. The above-mentioned power supply mode is supplemented by the arc-suppressing electrolyte to coordinately control the plasma energy density technology, so that the electron temperature in the plasma and the temperature of the gas / ion are in a thermal equilibrium state, the plasma discharge energy is stabilized, and a more uniform and dense arc-suppressing ultra-low temperature plasma is formed, which is conducive to the in-situ growth of a thick and dense alumina ceramic coating on the surface of the aluminum alloy, with a density of more than 95%;

[0019] (2) The present invention uses aluminum alloy as the substrate material (marine ships, aerospace bearing structures, key components of electronic packaging heat dissipation substrates, etc.), and on this basis, uses asymmetric bipolar pulse discharge assisted by arc suppression solution to coordinately control plasma energy technology, so that the electron temperature in the plasma and the temperature of the gas / ion are in a thermal equilibrium state, stabilize the plasma discharge energy, form a more uniform and dense arc suppression ultra-low temperature plasma, and continuously and uniformly bombard the substrate in situ to form an ultra-dense ceramic coating with uniform structure and high surface quality. Compared with magnetron sputtering, it has lower cost and has broad application prospects in marine ships, aerospace, electronic packaging, etc.;

[0020] (3) The present invention proposes for the first time a method for inducing and realizing arc-suppressing ultra-low temperature plasma, which can be expanded and applied to different fields. In addition to valve metals such as aluminum, magnesium, and titanium, it also includes the uniform preparation of dense coatings on the surfaces of non-valve metals (steel, copper and other alloys). The preparation method provided has a simple process, low production cost, strong design, and can be applied to large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the principle of an ultra-low temperature plasma-induced ultra-high density coating device;

[0022] Figure 2 Circuit diagram of the rectification and filtering circuit and plasma induction circuit for ultra-low temperature plasma induced ultra-high density coating equipment;

[0023] Figure 3 The actual pulse voltage output waveform diagram under different negative pulse insertions;

[0024] Figure 4 This is a comparison chart of the discharge temperature of micro-area of ​​micro-arc and arc-free ultra-low temperature plasma spark;

[0025] Figure 5 This is the pulse output waveform of the micro-arc oxidation bipolar pulse power supply. In the figure, t' on is the positive pulse on time, t' off is the positive pulse off time, t' o ' n is the negative pulse on time, t' o ' ff is the negative pulse off time, T P is the number of positive pulse working pulses, T N is the number of negative pulse working pulses, J' m is the positive average current density, J' m ' is the negative average current density, U P is the positive voltage amplitude, U N is the negative voltage amplitude, U P =U N;

[0026] Figure 6 The SEM images of the cross section of the conventional coating and the surface and cross section of the thick / high-density ceramic coating on the aluminum alloy substrate in Example 1 of the present invention;

[0027] Figure 7 This is a SEM image of the surface of the thick / high-density ceramic coating on the surface of the magnesium alloy substrate in Example 2 of the present invention;

[0028] Figure 8 The bonding strength of the thick / high-density ceramic coating on the surface of the aluminum alloy substrate in Example 1 of the present invention;

[0029] Fig. 9 is the hardness of the thick / high-density ceramic coating on the surface of the aluminum alloy substrate in Example 1 of the present invention;

[0030] Fig.10 The corrosion resistance of the thick / high-density ceramic coating on the surface of the aluminum alloy substrate in Example 1 of the present invention. DETAILED DESCRIPTION

[0031] Without departing from the essence of the present invention, modifications and substitutions made to the methods, steps or conditions of the present invention are all within the scope of the present invention.

[0032] Specific implementation method 1: This implementation method is a method for preparing a thick and dense ceramic coating, which is specifically completed by the following steps:

[0033] 1. Aluminum alloy pretreatment:

[0034] The surface of the aluminum alloy is polished using sandpaper, and then ultrasonically cleaned and dried to obtain a pretreated aluminum alloy;

[0035] 2. Preparation of arc suppression electrolyte:

[0036] dissolving hydrogen oxalate and arc-suppressing molecules in deionized water to obtain an arc-suppressing electrolyte;

[0037] 3. Add arc-suppressing electrolyte into the electrolytic cell, immerse the pretreated aluminum alloy and stainless steel plate into the arc-suppressing electrolyte, wherein the stainless steel plate serves as the cathode and the pretreated aluminum alloy serves as the anode; connect the cathode and the anode to the power supply of the ultra-low temperature plasma induced ultra-high density coating equipment;

[0038] Fourth, the ultra-low temperature plasma induced ultra-high density coating equipment is adjusted to an asymmetric pulse output with alternating positive and negative pulses, forming a new mode combining high frequency / ultra-high frequency with cathode discharge; start the power supply, induce uniform and dense ultra-low temperature plasma sparks in the plasma discharge micro-area, and the high-density arc-suppressing ultra-low temperature plasma discharge heals the holes, promotes the high-density growth of the coating, and oxidizes for a period of time to obtain a thick and high-density ceramic coating;

[0039] The high frequency / ultra-high frequency described in step 4 is a plasma discharge frequency ≥ 10000 Hz.

[0040] Specific implementation method 2: This implementation method is different from specific implementation method 1 in that the aluminum alloy described in step 1 is 1050, 2024, 6061, aluminum silicon alloy or aluminum copper alloy. The other steps are the same as those in specific implementation method 1.

[0041] Specific implementation method 3: This implementation method is different from specific implementation method 1 or 2 in that: in step 1, the surface of the aluminum alloy is polished with 800#, 1000# and 1200# sandpaper in sequence to remove pollutants on the surface of the aluminum alloy, and then ultrasonically cleaned with deionized water and dried to obtain the pretreated aluminum alloy. The other steps are the same as those of specific implementation method 1 or 2.

[0042] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that: the concentration of hydrogen oxalate in the arc-suppressing electrolyte described in step 2 is 1 g / L to 20 g / L, the concentration of arc-suppressing molecules is 1 g / L to 5 g / L; the pH value of the arc-suppressing electrolyte is 5 to 7. The other steps are the same as those of specific embodiments 1 to 3.

[0043] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that: the hydrogen oxalate described in step 2 is sodium hydrogen oxalate or calcium hydrogen oxalate; the arc-suppressing molecule described in step 2 is glycerol, C6H5Na3O7, glycerol, isopropyl alcohol or ethylene glycol. The other steps are the same as those of specific embodiments 1 to 4.

[0044] Specific embodiment 6: This embodiment differs from specific embodiments 1 to 5 in that the temperature of the arc-suppressing electrolyte in step 3 is 10° C. to 30° C. The other steps are the same as those in specific embodiments 1 to 5.

[0045] Specific embodiment 7: The difference between this embodiment and specific embodiments 1 to 6 is that in step 4, the ultra-low temperature plasma induced ultra-high density coating equipment is adjusted to an asymmetric pulse output with alternating positive and negative pulses, a forward voltage of 500V to 600V, a negative voltage of -40V to -120V, a positive duty cycle of 5%, a negative duty cycle of 5%, and a positive to negative frequency ratio of 1:1. The other steps are the same as specific embodiments 1 to 6.

[0046] Specific embodiment 8: This embodiment differs from specific embodiments 1 to 7 in that the oxidation reaction time in step 4 is 5 to 30 minutes. The other steps are the same as those in specific embodiments 1 to 7.

[0047] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that the high-density ceramic coating described in step 4 has a density of >95%, a thickness of 50 μm to 80 μm, a bonding strength of 50 MPa to 60 MPa, a hardness of 1200 HV to 1500 HV, and is resistant to 3000 h salt spray corrosion. The other steps are the same as specific embodiments 1 to 8.

[0048] Specific implementation method ten: The difference between this implementation method and specific implementation methods one to nine is that the other steps are the same as those of specific implementation methods one to nine.

[0049] The ultra-low temperature plasma induced ultra-high density coating equipment described in step 4 includes two rectification and filtering circuits, a plasma induction circuit, a controller and a driving circuit;

[0050] Two rectifying and filtering circuits are used to transform, rectify and filter the three-phase power supply in sequence, and obtain positive pulse DC power supply and negative pulse DC power supply respectively, which are input to the plasma inducing circuit;

[0051] A controller, used for alternately sending a positive pulse driving signal and a negative pulse driving signal to the plasma inducing circuit through a driving circuit;

[0052] The plasma inducing circuit is used to output positive pulses to the load according to receiving a positive pulse DC power supply and a positive pulse driving signal, and output negative pulses to the load according to receiving a negative pulse DC power supply and a negative pulse driving signal, and the alternating output of positive and negative pulses causes plasma discharge to be generated on the surface of the aluminum alloy in the load, thereby forming a plasma micro-region between the aluminum alloy in the load and the arc-suppressing electrolyte, and finally growing a thick and high-density ceramic coating on the surface of the aluminum alloy.

[0053] Preferably, the plasma inducing circuit includes switching tubes Q11 to Q1i, switching tubes Q21 to Q2i, switching tubes Q31 to Q3i, switching tubes Q41 to Q4i, capacitors C1 to C2, blocking diodes D1 to Di and a resonant circuit;

[0054] The positive output end of the positive pulse DC power supply is simultaneously connected to the collectors of the switch tubes Q11 to Q1i and one end of the capacitor C1; the negative output end of the positive pulse DC power supply is simultaneously connected to the other end of the capacitor C1, the emitters of the switch tubes Q21-Q2i, one end of the resonant circuit, the emitters of the switch tubes Q31 to Q3i, one end of the capacitor C2, the negative output end of the negative pulse DC power supply and the power ground;

[0055] The emitters of the switch tubes Q11 to Q1i are simultaneously connected to the collectors of the switch tubes Q21 to Q2i, the other end of the resonant circuit and one end of the load; the other end of the load is simultaneously connected to the collectors of the switch tubes Q31 to Q3i and the emitters of the switch tubes Q41-Q4i; the collectors of the switch tubes Q41 to Q4i are connected to the cathodes of the blocking diodes D1 to Di; the anodes of the blocking diodes D1 to Di are simultaneously connected to the positive output end of the negative pulse DC power supply and the other end of the capacitor C2.

[0056] Preferably, the device further comprises a sampling circuit;

[0057] A sampling circuit, used for collecting the voltage and load current of the plasma inducing circuit in real time and sending them to the controller;

[0058] A controller is used to reduce the conduction angle of the switch tube to a preset conduction angle when the received voltage is higher than the preset voltage, and to reduce the positive and negative pulse widths of the switch tube to a preset width or reduce the number of positive and negative pulse widths of the switch tube to a preset value when the received current is higher than the preset current, and to control the switching state of the switch tube according to the adjusted conduction angle and positive and negative pulse widths.

[0059] The following examples are used to verify the beneficial effects of the present invention:

[0060] Embodiment 1: A method for preparing a thick and dense ceramic coating is specifically completed by the following steps:

[0061] 1. Aluminum alloy pretreatment:

[0062] The surface of the aluminum alloy is polished using 800#, 1000# and 1200# sandpaper in sequence to remove pollutants on the surface of the aluminum alloy, and then ultrasonically cleaned and dried using deionized water to obtain a pretreated aluminum alloy;

[0063] The aluminum alloy described in step 1 is 2024;

[0064] 2. Preparation of arc suppression electrolyte:

[0065] dissolving hydrogen oxalate and arc-suppressing molecules in deionized water to obtain an arc-suppressing electrolyte;

[0066] The concentration of hydrogen oxalate in the arc-suppressing electrolyte described in step 2 is 20 g / L, and the concentration of arc-suppressing molecules is 3 g / L;

[0067] The hydrogen oxalate described in step 2 is sodium hydrogen oxalate;

[0068] The arc-suppressing molecule described in step 2 is C6H5Na3O7;

[0069] 3. Add arc-suppressing electrolyte into the electrolytic cell, immerse the pretreated aluminum alloy and stainless steel plate into the arc-suppressing electrolyte, wherein the stainless steel plate serves as the cathode and the pretreated aluminum alloy serves as the anode; connect the cathode and the anode to the power supply of the ultra-low temperature plasma induced ultra-high density coating equipment;

[0070] The temperature of the arc-suppressing electrolyte described in step 3 is 30° C.

[0071] Fourth, the ultra-low temperature plasma induced ultra-high density coating equipment is adjusted to an asymmetric pulse output with alternating positive and negative pulses, forming a new mode combining high frequency / ultra-high frequency with cathode discharge; start the power supply, induce uniform and dense ultra-low temperature plasma sparks in the plasma discharge micro-area, and the high-density arc-suppressing ultra-low temperature plasma discharge heals the holes, promotes the high-density growth of the coating, and oxidizes for a period of time to obtain a thick and high-density ceramic coating;

[0072] In step 4, the ultra-low temperature plasma induced ultra-high density coating equipment is adjusted to an asymmetric pulse output with alternating positive and negative pulses, applying a forward voltage of 550V, a negative voltage of -80V, a positive duty cycle of 5%, a negative duty cycle of 5%, and a positive-negative frequency ratio of 1:1.

[0073] The oxidation reaction time in step 4 is 10 min;

[0074] The high frequency / ultra high frequency described in step 4 is a plasma discharge frequency of 10000 Hz;

[0075] The ultra-low temperature plasma induced ultra-high density coating equipment described in step 4 includes two rectification and filtering circuits, a plasma induction circuit, a controller and a driving circuit;

[0076] Two rectifying and filtering circuits are used to transform, rectify and filter the three-phase power supply in sequence, and obtain positive pulse DC power supply and negative pulse DC power supply respectively, which are input to the plasma inducing circuit;

[0077] A controller, used for alternately sending a positive pulse driving signal and a negative pulse driving signal to the plasma inducing circuit through a driving circuit;

[0078] The plasma inducing circuit is used to output positive pulses to the load according to receiving a positive pulse DC power supply and a positive pulse driving signal, and output negative pulses to the load according to receiving a negative pulse DC power supply and a negative pulse driving signal, and the alternating output of positive and negative pulses causes plasma discharge to be generated on the surface of the aluminum alloy in the load, thereby forming a plasma micro-region between the aluminum alloy in the load and the arc-suppressing electrolyte, and finally growing a thick and high-density ceramic coating on the surface of the aluminum alloy.

[0079] The plasma induction circuit is further defined as:

[0080] The plasma inducing circuit includes switch tubes Q11 to Q1i, switch tubes Q21 to Q2i, switch tubes Q31 to Q3i, switch tubes Q41 to Q4i, capacitors C1 to C2, blocking diodes D1 to Di and a resonant circuit;

[0081] The positive output end of the positive pulse DC power supply is simultaneously connected to the collectors of the switch tubes Q11 to Q1i and one end of the capacitor C1; the negative output end of the positive pulse DC power supply is simultaneously connected to the other end of the capacitor C1, the emitters of the switch tubes Q21-Q2i, one end of the resonant circuit, the emitters of the switch tubes Q31 to Q3i, one end of the capacitor C2, the negative output end of the negative pulse DC power supply and the power ground;

[0082] The emitters of the switch tubes Q11 to Q1i are simultaneously connected to the collectors of the switch tubes Q21 to Q2i, the other end of the resonant circuit and one end of the load; the other end of the load is simultaneously connected to the collectors of the switch tubes Q31 to Q3i and the emitters of the switch tubes Q41-Q4i; the collectors of the switch tubes Q41 to Q4i are connected to the cathodes of the blocking diodes D1 to Di; the anodes of the blocking diodes D1 to Di are simultaneously connected to the positive output end of the negative pulse DC power supply and the other end of the capacitor C2.

[0083] Specifically, the switch tubes Q11 to Q1i are connected in parallel, the switch tubes Q21 to Q2i are connected in parallel, the switch tubes Q31 to Q3i are connected in parallel, and the switch tubes Q41 to Q4i are connected in parallel to increase the current carrying capacity, while reducing the conduction loss, making heat dissipation easier, and playing a redundant role when some switch tubes fail, thereby enhancing reliability.

[0084] The working principle of the plasma induction circuit is:

[0085] Figure 1 The two DC power supplies Usp and Usn are obtained by three-phase AC input and thyristor half-bridge rectification. Since there is always a relationship of Usp>Usn, a group of blocking diodes D1 to Di need to be connected in series on the output side of the DC power supply Usn to avoid circulating current between the two DC sources.

[0086] When two groups of IGBT tubes (switch tubes) Q11 to Q1i and Q31 to Q3i are turned on at the same time, the current flows from Usp+ through Q11 to Q1i from left to right through the load, and returns to GND through Q31 to Q3i, thereby generating a positive pulse; similarly, when Q21 to Q2i and Q41 to Q4i are turned on at the same time, the current flows from Usn+ through Q41 to Q4i from right to left through the load, and returns to GND through Q21 to Q2i, thereby generating a negative pulse.

[0087] The resonant circuit composed of L13 and C7 will charge and discharge the junction capacitance of the IGBT during the dead time of switching from Q11 to Q1i and from Q21 to Q2i, thereby achieving ZVS (soft switching) conditions, avoiding the hard switching of traditional circuits, and reducing switching losses.

[0088] Figure 2 Capacitive load is used to represent the characteristics of the load, and other characteristic loads can also be used to represent the load. VT1_Gate to VT6_Gate are connected to the controller, and the controller drives the gate of the thyristor; Q11_G to Q1i_G, Q21_G to Q2i_G, Q31_G to Q3i_G, and Q41_G to Q4i_G are connected to the controller.

[0089] In order to continuously ensure ultra-low temperature and stable plasma discharge, the present embodiment further needs to utilize a sampling circuit to collect the voltage and load current of the plasma induction circuit in real time, and the controller adjusts the switch state of the switch tube in real time according to the voltage and current to maintain stable plasma discharge. Specifically, the device further includes a sampling circuit;

[0090] A sampling circuit, used for collecting the voltage and load current of the plasma inducing circuit in real time and sending them to the controller;

[0091] The controller is used to reduce the conduction angle of the switch tube to a preset conduction angle when it detects that the received voltage is higher than the preset voltage, and when the received load current is higher than the preset current, reduce the positive and negative pulse widths of the switch tube to a preset width or reduce the number of positive and negative pulse widths of the switch tube to a preset value, and control the switching state of the switch tube according to the adjusted conduction angle and positive and negative pulse widths.

[0092] Specifically, the sampling circuit is implemented using a Hall current sensor and a voltage sensor.

[0093] This embodiment adopts variable frequency high voltage (small current) + thyristor half-wave rectification + ultra-high frequency boosting to improve the power factor and stabilize the plasma energy density; wherein, the frequency of high frequency / ultra-high frequency boosting is ≥10000Hz. At the same time, the controller adopts full digital signal fast self-feedback control, combined with multiple groups of switch tubes in parallel to form a low-temperature plasma induction control system, so that the accuracy of the switch tube control synchronous disconnection reaches the microsecond level, thereby meeting the synchronization requirements of sub-microsecond precision disconnection, and realizing the precise control of "low-temperature plasma" energy;

[0094] Through the plasma induction circuit, the asymmetric pulse output of alternating positive and negative pulses is realized (the amplitude and pulse width of the two polarity pulses can be adjusted independently), and then the random insertion of negative pulses is carried out to achieve one positive to multiple negative or one negative to multiple positive unequal pressure insertion, such as Figure 5As shown, the dominant anodization process is adjusted to anode-cathode cooperative discharge, so that the coating growth quickly crosses the Faraday region with high energy consumption and low film formation rate before arcing, while improving the coating growth efficiency and density, and eliminating the residual energy anodization carrier, forming a large field strength ionized gas and small field strength conduction micro-area plasma discharge environment.

[0095] Embodiment 2: This embodiment is different from Embodiment 1 in that the negative voltage in step 4 is -60 V. The other steps and parameters are the same as those in Embodiment 1.

[0096] The negative voltage in step 4 of Example 1 was changed to 0V, -40V, and -120V, respectively, to study the influence of 0V, -40V, -80V, and -120V on the micro-arc spark discharge process. The other steps and parameters were the same as those in Example 1. Figure 3 As shown;

[0097] Figure 3 The actual pulse voltage output waveform diagram under different negative pulse insertions;

[0098] Based on the developed high-frequency self-feedback "ultra-low temperature plasma" coating densification equipment, the influence of negative voltage (0V, -40V, -80V, -120V) on the micro-arc spark discharge process was studied, numbered N0, N40, N80 and N120. The results show the output waveform of the pulse voltage output of a pulse cycle, which affects the subsequent coating densification growth, especially the negative pulse of 80V, which can produce an ultra-dense coating structure.

[0099] Figure 4 This is a comparison chart of the discharge temperature of micro-area of ​​micro-arc and arc-free ultra-low temperature plasma spark;

[0100] Figure 4 The upper curve in the middle is no arc, and the lower curve is a slight arc;

[0101] A high-frequency self-feedback "ultra-low temperature plasma" coating densification equipment has been developed, which has achieved the reduction of the micro-area plasma discharge temperature from the traditional micro-arc discharge 10000K to the arc-free discharge 3000K, and the plasma discharge density has been increased by 2 orders of magnitude. The ultra-low temperature plasma discharge enables the coating to grow uniformly without defects.

[0102] Figure 6 The SEM images of the cross section of the conventional coating and the surface and cross section of the thick / high-density ceramic coating on the aluminum alloy substrate in Example 1 of the present invention;

[0103] The ultra-high frequency 10000Hz + negative voltage -80V mode is adopted, assisted by the above-mentioned arc-suppressing electrolyte coordinated regulation of plasma energy technology, so that the electron temperature in the plasma and the temperature of the gas / ion are in a thermal equilibrium state, the plasma discharge energy is stabilized, and a uniform and dense arc-suppressing ultra-low temperature plasma is formed, inducing the formation of a high-density (≥95%) aluminum oxide coating on the surface of the aluminum alloy, and the density is increased from 75% of the traditional micro-arc oxidation to a high density of 95% (no holes are observed on the surface / cross-section) ( Figure 6 ), is the first time that ultra-high frequency asymmetric bipolar pulse arc suppression ultra-low temperature plasma discharge technology has been used to prepare thick and dense coatings.

[0104] Figure 7 This is a SEM image of the surface of the thick / high-density ceramic coating on the surface of the magnesium alloy substrate in Example 1 of the present invention;

[0105] The ultra-high frequency 10000Hz + negative voltage -60V mode is adopted, and the above arc-suppressing solution additives are used to coordinately control the plasma energy technology to form a uniform and dense arc-suppressing ultra-low temperature plasma, and the micropores are healed at low temperature to form a high-density magnesium oxide ceramic coating ( Figure 7 ), the surface morphology shows that the formed pores are healed and sealed, showing a high density of more than 90%.

[0106] Figure 8 The bonding strength of the thick / high-density ceramic coating on the surface of the aluminum alloy substrate in Example 1 of the present invention;

[0107] At present, the highest bonding strength of traditional micro-arc oxidation coatings is about 40MPa. The bonding strength of alumina ceramic coatings prepared by weak acid electrolyte + ultra-low temperature plasma discharge reaches 50-60MPa, showing ultra-high bonding strength, which is significantly higher than the bonding strength of coatings prepared by traditional micro-arc oxidation, magnetron sputtering and plasma spraying.

[0108] Fig. 9 is the hardness of the thick / high-density ceramic coating on the surface of the aluminum alloy substrate in Example 1 of the present invention;

[0109] The Vickers hardness of the alumina ceramic coating prepared in weakly acidic electrolyte + ultra-low temperature plasma discharge reaches 1500Hv, which is the hardest coating currently prepared using plasma discharge oxidation induced ceramic coating technology. It is expected that the hardness value will be further improved by controlling the process parameters to reach the hardness value of alumina ceramics.

[0110] Fig.10 The corrosion resistance of the thick / high-density ceramic coating on the surface of the aluminum alloy substrate in Example 1 of the present invention.

[0111] The dense alumina ceramic coating was placed in a salt spray chamber (ACE-60, Dongguan Kuaijie Measuring Instrument Co., Ltd.) according to the neutral salt spray test standard GB / T 10125-2012, and the changes of the coating in the salt spray chamber were observed. The results showed that there was no change in the macro and micro morphology of the coating after 3000 hours, showing excellent resistance to salt spray corrosion. This is attributed to the nanocrystalline layer formed in the coating induced self-passivation film and the in-situ sealing self-repair mechanism of insoluble salt, which significantly improved the salt spray corrosion resistance for 3000 hours and can be used in harsh marine corrosion protection fields.

[0112] Coatings prepared by existing micro-arc oxidation technology (high voltage, high current + high frequency ≤ 1000Hz, low power factor): density ~ 75% ( Figure 6 ); Maximum bonding strength ~40MPa; Hardness <1000HV.

Claims

1. A method for preparing a thick and dense ceramic coating, characterized in that The preparation method is specifically completed according to the following steps:

1. Aluminum alloy pretreatment: The surface of the aluminum alloy is polished using sandpaper, and then ultrasonically cleaned and dried to obtain a pretreated aluminum alloy; 2. Preparation of arc suppression electrolyte: dissolving hydrogen oxalate and arc-suppressing molecules in deionized water to obtain an arc-suppressing electrolyte; 3. Add arc-suppressing electrolyte into the electrolytic cell, immerse the pretreated aluminum alloy and stainless steel plate into the arc-suppressing electrolyte, wherein the stainless steel plate serves as the cathode and the pretreated aluminum alloy serves as the anode; connect the cathode and the anode to the power supply of the ultra-low temperature plasma induced ultra-high density coating equipment; Fourth, the ultra-high density coating equipment induced by ultra-low temperature plasma is regulated to have an asymmetric pulse output with alternating positive and negative pulses, forming a new mode combining high frequency / ultra-high frequency with cathode discharge; Start the power supply to induce uniform and dense ultra-low temperature plasma sparks in the plasma discharge micro-area. The high-density arc-suppressing ultra-low temperature plasma discharge heals the holes, promotes the high-density growth of the coating, and undergoes oxidation reaction for a period of time to obtain a thick and high-density ceramic coating. The high frequency / ultra-high frequency described in step 4 is a plasma discharge frequency ≥ 10000 Hz.

2. The method for preparing a thick and dense ceramic coating according to claim 1, characterized in that The aluminum alloy described in step 1 is 1050, 2024, 6061, aluminum silicon alloy or aluminum copper alloy.

3. The method for preparing a thick and dense ceramic coating according to claim 1, characterized in that In step 1, the surface of the aluminum alloy is polished using 800#, 1000# and 1200# sandpaper in sequence to remove pollutants on the surface of the aluminum alloy, and then ultrasonically cleaned with deionized water and dried to obtain the pretreated aluminum alloy.

4. The method for preparing a thick and dense ceramic coating according to claim 1, characterized in that The concentration of hydrogen oxalate in the arc-suppressing electrolyte described in step 2 is 1g / L-20g / L, and the concentration of arc-suppressing molecules is 1g / L-5g / L; the pH value of the arc-suppressing electrolyte is 5-7.

5. The method for preparing a thick and dense ceramic coating according to claim 1, characterized in that The hydrogen oxalate described in step 2 is sodium hydrogen oxalate or calcium hydrogen oxalate; the arc-suppressing molecule described in step 2 is glycerol, C6H5Na3O7, glycerol, isopropanol or ethylene glycol.

6. The method for preparing a thick and dense ceramic coating according to claim 1, characterized in that The temperature of the arc-suppressing electrolyte described in step 3 is 10°C to 30°C.

7. The method for preparing a thick and dense ceramic coating according to claim 1, characterized in that In step 4, the ultra-low temperature plasma induced ultra-high density coating equipment is adjusted to an asymmetric pulse output with alternating positive and negative pulses, applying a forward voltage of 500V to 600V, a negative voltage of -40V to -120V, a positive duty cycle of 5%, a negative duty cycle of 5%, and a positive to negative frequency ratio of 1:

1.

8. The method for preparing a thick and dense ceramic coating according to claim 1, characterized in that The oxidation reaction time in step 4 is 5 min to 30 min.

9. The method for preparing a thick and dense ceramic coating according to claim 1, characterized in that The high-density ceramic coating of large thickness described in step 4 has a density of >95%, a thickness of 50 μm to 80 μm, a bonding strength of 50 MPa to 60 MPa, a hardness of 1200 HV to 1500 HV, and is resistant to 3000 h of salt spray corrosion.

10. The method for preparing a thick and dense ceramic coating according to claim 1, characterized in that The ultra-low temperature plasma induced ultra-high density coating equipment described in step 4 includes two rectification and filtering circuits, a plasma induction circuit, a controller and a driving circuit; Two rectifying and filtering circuits are used to transform, rectify and filter the three-phase power supply in sequence, and obtain positive pulse DC power supply and negative pulse DC power supply respectively, which are input to the plasma inducing circuit; A controller, used for alternately sending a positive pulse driving signal and a negative pulse driving signal to the plasma inducing circuit through a driving circuit; The plasma inducing circuit is used to output positive pulses to the load according to receiving a positive pulse DC power supply and a positive pulse driving signal, and output negative pulses to the load according to receiving a negative pulse DC power supply and a negative pulse driving signal, and the alternating output of positive and negative pulses causes plasma discharge to be generated on the surface of the aluminum alloy in the load, thereby forming a plasma micro-region between the aluminum alloy in the load and the arc-suppressing electrolyte, and finally growing a thick and high-density ceramic coating on the surface of the aluminum alloy.

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