Arc ion plating technology on composite surface
Composite structure function integration parts with light weight, high specific strength, than high modulus, good corrosion resistance, fatigue resistance performance is good, is of strong design and a series of unique advantages and is more and more become the important parts in the field of aviation, aerospace and other defense, to achieve certain function, need to deal with composite material surface, which is widely used is a means of processing surface metallization. There are many ways to metallize the surface of composite materials, including chemical plating, electroplating, thermal spraying, vacuum coating, etc. Vacuum coating can be subdivided into evaporation coating, magnetron sputtering, arc ion coating, etc.
As the dimension of structural and functional integrative composite material components may be large and the curved surface may be complex, this brings two outstanding problems for the realization of surface metallization of composite materials by the vacuum coating method. Arc ion plating technology is a better choice. Its advantages include high energy distance of deposition particles, good diffraction range, high metal film binding force and fast deposition speed, etc.
1. Brief introduction of arc ion plating technology
Arc Ion Plating, (AIP) is the technology used in the vacuum coating, Arc at the cathode material under vacuum and vacuum chamber formed between anode Arc discharge, use of Arc discharge evaporation of target material, and deposit to the surface coating process. Arc ion plating technology got great development in the 20th century, especially in the late 1970 s, scientists in the former Soviet union and the United States for industrial application has carried out extensive research to its, and realized the arc ion plating technology in the 1980 s in the field of hard film, decorative film industrialization, at present, the application in these two areas has considerable scale. In the last 20 years, arc ion plating technology has begun to develop in optical, electrical thin films and other applications.
Arc discharge will form arc spots that move randomly on the surface of cathode target. The current density of arc spots is as high as 1012A/m2 and the energy density is as high as 1013W/m2. The high energy density directly leads to the transformation of target material from solid phase to metal vapor plasma at arc spot.
(1) it can be plated with a variety of metals, especially refractory metals (such as tungsten and tantalum), which is difficult to achieve by other physical vapor deposition (PVD) technologies;
(2) unlike the sediments of other PVD technologies, which are mainly neutral particles, cathode arc spots can generate a large number of charged (single charge or multiple charge) particles, and ions can be accelerated, restrained and move in a specific direction, and they are deposited on the surface of the parts.
(3) the initial energy ions generated by the cathode arc spots between 20 and 200 ev, in the process of thin film deposition, bombarding effect happens, enhances the sedimentary particle diffusion ability and the nucleation density, and stripped off the thin film surface loose particles, partly eliminate the internal stress of columnar crystal and films, reason has increased membrane surface activity, such as thin film densification effect;
(4) when the cathode arc spot produces charged particles to form plasma, it also generates a large number of droplets and fragments, while other PVD technologies generate few large particles.
Therefore, arc ion plating technology has the advantages of compact film and high binding force. In addition, it can realize thin film deposition on complex surface. The biggest disadvantage of arc ion plating technology is that larger droplets will also be deposited on the surface of the film, resulting in rough surface of the film and affecting the film performance. But with the continuous development of magnetic filter technology, already is a good way to control a larger drop, avoid the deposition to the membrane surface, the arc ion plating technology deposition of thin film surface quality is close to other PVD technology, the electric arc ion plating technology applied to film in optics, electricity and other fields.
2. Two problems of vacuum coating on composite surface
Compared with metal or semiconductor materials, composite materials with integrated structure and function cannot withstand high temperature, and some functional parts require high precision of profile. Therefore, high temperature is not allowed in vacuum coating of composite materials, so as to avoid material damage and profile accuracy reduction. However, high deposition temperature is very favorable for obtaining solid and dense metal films. From the perspective of obtaining high quality films, high temperature is desired. A feasible way to resolve this contradiction is to use ionic source cleaning and other technical means to conduct in-situ activation treatment on the surface of the composite before the film deposition, so as to increase the surface activity to increase the binding force between the metal film and the substrate.
Composite materials for large size parts, in the process of vacuum coating material outgassing is a key problem, the outgassing rate of higher material may cause series of problems such as the oxidation of metal film, which can influence the apparent quality and electrical properties of the metal film, so you need to pre vent and other technical means to deal with composite materials.
3. Examples of applications of arc ion coating on composite surface
The arc ion plating technology has been successfully used to deposit Al film on the surface of large composite materials. The resulting film has uniform color and luster, uniform thickness, close electrical conductivity to the block aluminum, and solid bond with the base.
The test procedure for depositing Al film on the surface of composite materials using arc ion plating technology is as follows:
(1) clean the surface of the composite material, wipe it with a dust-free cloth dipped in ethanol for 3-5 times, and dry it naturally;
(2) the composite parts are connected and fixed on the coating tooling of the vacuum chamber;
(3) pump the background vacuum to better than 5 x 10-3pa;
(4) the surface of composite material is cleaned by ion source;
(5) arc ion plating film;
(6) vacuum chamber is opened to the atmosphere, and composite materials are taken out.
The technological parameters of arc ion coating are as follows:
(1) the gas pressure is 1.1~1.5 x 10-1pa;
(2) the discharge arc pressure is 45~50V;
(3) discharge arc current is 50~55A;
(4) the distance between the composite surface and the arc source is 0.3-1m.
The local appearance of the Al coating on the composite surface after arc ion plating is shown in figure 1. As can be seen from figure 1, the Al membrane is uniform and compact with almost no droplet, and the surface topography of the film is the mapping of the surface morphology of the composite matrix. The surface morphology of Al film was observed by using Wyko NT9300 optical profiler, as shown in figure 2. The surface of the film was flat and uniform in thickness. and its surface roughness Ra is 0.145 μm.
The thickness of Al film on the surface of composite material was tested by using a stepmeter. Samples were taken from slides pasted at different parts of the part. The test results are shown in table 1.
The strength of the binding force between the film and the substrate is an important index to evaluate the quality of the film. There are many methods to test the binding force, including tension method, strip method, scratch method, friction method, ultrasonic method and centrifugal force method. Considering that the Al film coated on the surface of composite material is a typical soft film, and the force applied in the practical application of this film is mainly shear force, we use the pull band method to evaluate the binding force between Al film and the base of composite material.  taking method of the concrete implementation method for the peel strength adhesive tape for 7 n/cm, dense and uniform in the thin film surface and away from the edge of not less than 3 mm, with the hand pull tape one end and the adhesive tape and film surface into 90 °, uniform will slowly (about 5 mm/s) tape pull away from the membrane surface, watch film with and without peeling or damaged. When the film is intact without shedding, it shall be deemed that the film is well combined with the composite material and meets the requirements for use. When the film falls off, it is deemed that the binding force does not meet the requirements. The adhesion test was carried out at different positions on the surface of composite materials by the method of tension.
A multimeter was used to test at any two points on the surface of the composites, and the results were all conductive. Four point probe method is used to test the paste in the composite parts with different parts of the sample on the surface of the Al film resistivity, the results are shown in table 2, and Al block material resistivity of 2. 66 x 10-8 Ω m. for the same order of magnitude.
Arc ion plating technology can realize the surface metallization of complex composite material components with high quality, uniform density, firm binding and controllable thickness, which has a broad application prospect. However, this technology also has limitations such as large equipment, high cost and long cycle.
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