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Ti target series: research progress on ti - al alloy sputtering coating targets

        With the enhancement of people's awareness of environmental protection, the surface coating technology of materials has gradually transferred from the traditional electroplating and electroless plating technology to the vacuum sputtering coating technology.Vacuum magnetron sputtering coating technology has been widely used in metal decoration coating, surface protection coating, electronic product coating, electromagnetic shielding coating and automobile and building glass coating.Therefore, the need for coating target materials in this field is very urgent.

Ti - al alloy is a kind of alloy sputtering target for vacuum coating.Ti-al intermetallic compound is a kind of hard and brittle material, which has good wear resistance.If sputtering is carried out in combination with nitrogen discharge arc, surface film with high hardness and low friction coefficient can be obtained, which is especially suitable for surface coating of various tools, molds and other vulnerable parts. Therefore, it has a good application prospect in the mechanical processing industry. 

The preparation of ti - al alloy target is difficult.According to the phase diagram of ti-al alloy, a variety of metal compounds can be formed between ti-al alloy and al alloy, which leads to the brittleness of ti-al alloy.At the same time, exothermic expansion in the alloying process can easily lead to bubbles, shrinkage cavity and porosity, resulting in high porosity of the alloy and unable to meet the requirements of density of the target material.

Sputtering coating process

1.1 sputtering target

Sputtering target refers to sputtering source which forms various functional films on the substrate through magnetron sputtering, multi-arc ion plating or other types of coating equipment under appropriate technological conditions.Sputtering target materials should be widely used in decoration, mold, glass, electronic devices, semiconductor, magnetic recording, plane display, solar cells and many other fields, different fields need different target materials.According to the production method, sputtering target can be divided into powder target, melting target and spraying target.According to its shape, it can be divided into plane target and tubular target, and plane target can be divided into rectangular target and arc target.According to the classification can be divided into pure metal target, alloy target, oxide target, silicified target and many other varieties.

1.2 principle of sputtering coating

By bombarding the solid surface with accelerated ions, the ions and the solid surface exchange momentum with the atoms, so that the atoms on the solid surface leave the solid and deposit on the substrate surface. This process is called sputtering coating.The films deposited by sputtering with the target material have high density and good adhesion to the substrate.Therefore, sputtering deposition has become a widely used thin film preparation technology.

Sputtering target preparation technology of titanium aluminum alloy

Sputtering alloy target material needs to meet the requirements of purity, density, grain size and surface finish.The purity, density and grain size are directly related to the preparation process of the target material.Metal alloys are usually prepared by ordinary smelting.However, the preparation of ti-al alloy is not suitable for this method.

The main reasons are as follows :(1) various intermetallic compounds, such as Ti3A1, TiAl, TiAl2 and TiAl3, are easy to be formed in the melting process of titanium and aluminum alloy. The presence of these intermetallic compounds leads to brittleness in the processing of titanium and aluminum alloy.(2) during the process of smelting to prepare target materials of titanium and aluminum alloy, bubbles, porosity and segregation are likely to occur during pouring, resulting in uneven composition and structure of the alloy, resulting in unstable quality of the target materials. 

According to the research and report on the production process of ti-al alloy target at home and abroad, the main preparation technologies of ti-al alloy sputtering target at present include: intense current heating method, hot isostatic sintering method and hot pressing sintering method.

2.1 intense current heating method

Scholar yu peng invented a method to produce target materials of titanium and aluminum alloy by high intensity current.In this method, a device with high current can be used to heat titanium powder and aluminum powder, apply pressure and make the aluminum and titanium react to form a target material.The ti-al alloy target products were prepared by this method with the density of > 99%, grain size ≤100, TTM and purity of > 99%.The composition range of target materials is: titanium content 5% ~ 75% (atomic ratio), the rest is aluminum content.The method has low cost and high product density, which can fully meet the requirements of large-scale industrial production.The preparation method requires a device that can obtain a large current, and the positive and negative electrodes in the device need to have conductive and voltage transfer effects, as well as good conductivity, strong toughness and high temperature softening resistance.Therefore, the key to the preparation method is to have a large current device that meets the technological requirements and has stable performance.2.2 hot isostatic sintering

Scholar zhang fengge et al. invented a powder metallurgy method for preparing titanium aluminum alloy target materials.This method mixes titanium powder and aluminum powder, and then makes hot isostatic pressing and forming after charging powder and cold isostatic pressing and degassing process. Finally, the target material of titanium and aluminum alloy is obtained through sintering and processing.The titanium-aluminum alloy target prepared by this method has the characteristics of high density, no porosity, no porosity and segregation, uniform composition and fine grain.The density and purity of the target materials are more than 99%, the grain size is less than 100%, and the maximum size is up to 1000mm.The composition range of target materials is: titanium content 20% ~ 80% (atomic ratio), the rest is aluminum content.

Hot isostatic pressure preparation is the main method to prepare ti-al alloy sputtering target materials for coating industry.The target material prepared by this method has high density and wide size range.However, this method requires the use of a key pressing sintering equipment - hot isostatic pressure sintering furnace, which is expensive, resulting in the preparation of target material products, high cost, difficult to be industrialized promotion.7

2.3 hot-pressed sintering method

Scholar yu kun et al. invented a rapid hot pressing and sintering process for titanium and aluminum alloy target materials.The process makes the ti-al powder sintered and formed under the joint action of heat and force. By using the low melting point of aluminum in ti-al mixture, the titanium powder is bonded together to form alloy sputtering target material.

The alloy target prepared by this method has high density (≥ 95%), fine grain size (≤100 gm) and uniform composition.Target materials with different components of ti - al alloy can be prepared by this process.The composition range of target materials is: titanium content 25% ~ 75% (atomic ratio), the rest is aluminum.

The process is short, low cost and good coating effect, which can meet the requirements of large-scale industrialization.The process is characterized by the transfer of pressure through rigid mold in vacuum or protective atmosphere, so that the material can be sintered under the joint action of heat and force. 

The process requires high equipment, the key equipment is the hot press sintering furnace.The prepared target material is mainly circular alloy target material with diameter ≤100 mm.In addition, there are some problems such as difficult demoulding and low service life in the preparation of target materials with high aluminum content.Each target requires a set of mould to prepare, which increases the production cost of the target.Therefore, appropriate release agent should be selected to improve the rate of finished products and the utilization rate of the mold.

At present, hot pressing sintering is the most potential preparation technology of titanium aluminum alloy sputtering target.The technology has high efficiency and is suitable for industrial production.However, there are some problems in preparing ti-al alloy sputtering target by hot-press sintering, such as difficult demoulding and low service life of the die.Each target requires a set of mould to prepare, which increases the production cost of the target.

In the sintering process, titanium belongs to the hard phase with high melting point and aluminum belongs to the bonding phase.In the sintering process, the aluminum with low melting point gradually melts, and the titanium atoms can be bonded together to form the alloy target.However, the wetting Angle of aluminum liquid and graphite mold is obtuse Angle.Therefore, suitable release agent should be selected in the hot press sintering process of high-aluminum titanium aluminum alloy target to improve the rate of finished products and the utilization rate of molds. 

In addition, the sintering temperature, heating rate and hot pressing pressure in the process of hot-pressing sintering have a great influence on the final structure and properties of the product.The sintering temperature is too high, which will lead to the transition from solid phase sintering to liquid phase sintering, resulting in the collapse of the hard phase skeleton with high melting point, and waste of power energy, which is not conducive to reducing the cost.In addition, the sintering temperature is too high, which will lead to the grain growth and the increase and growth of the second phase, which will reduce the target material sputtering coating properties.If the target grain is too large, the sputtering surface will be consumed during sputtering, and the concave and convex surface on the erosion part of the sputtering target will be increased, which will lead to the decrease of the uniformity of the film thickness of the alloy film, the increase of the frequency of the abnormal discharge of the micro arc when large power is put in, and the resulting splash and other problems.At the same time, compared with the substrate, the second phase is easy to be residual due to poor sputtering efficiency, which will also affect the uniformity of sputtering film formation.Sintering temperature is too low, will lead to incomplete sintering, the target product density is low.In the process of hot pressing, if the heating rate is fast, the working efficiency can be improved. However, sintering in some areas leads to the flow of adhesive phase, resulting in uneven final microstructure and composition of the target material.The high pressure can also improve the efficiency, but at the same time make the mold loss serious, reduce the service life of the mold.

        In summary, hot-pressed sintering is the most potential preparation technology for ti-al alloy sputtering target.The technology has high efficiency and is suitable for industrial production.In order to prepare titanium aluminum alloy target materials with this technology, repeated tests are needed to select appropriate release agent to improve the rate of finished products and the utilization rate of molds. Meanwhile, the sintering hot-pressing process conditions (sintering temperature, heating rate and hot-pressing pressure) should be optimized to improve the production efficiency and reduce the cost.With the rapid growth of sputtering coating and the trend of gradual popularization in China, the market demand for target materials for sputtering coating increases rapidly every year.Therefore, the study on the preparation technology of ti - al alloy target has practical significance and great application prospect.


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