As can be known from the result of the crystal phase test, the foreign impurity particles can seriously affect the surface roughness of CVD tungsten tube. If the impurity particle is larger, the impact is more significant.

The deposited layer where impurity particles exist is presenting radial columnar crystals.

When there is impurity particles deposited substrate surface, due to the presence of foreign particles, the gas flow state of the portion of the reaction will change, resulting in a faster growth rate of the site to obtain a lateral growth space, and this advantage will be further expanded. And it will eventually lead to the deposition surface roughness increases.

Mutual interference among the deposition grain growths will also affects the surface roughness of CVD tungsten tube. In order to improve the smoothness of the surface of the tungsten tube, WF₆ can be discontinuously passed over the reaction chamber during deposition process.

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The surface roughness of CVD tungsten tube affected by the deposition temperature is significant, and its roughness increases with the increasing of deposition temperature.

When the deposition temperature is raised from 600 ℃to700 ℃, the surface roughness of the tungsten tube has significantly increased. Therefore, the deposition temperature should be controlled at about 600 ℃, in order to obtain deposit tungsten tube with smooth surface and good flatness.

The surface roughness of tungsten tube is mainly influenced by the growth of the film surface morphology and deposits organizations. If the deposit growth interface (Presentation of columnar crystals) is of good stability, therefore, the flatness of the surface of the film is the best. As the temperature rises to a certain extent, reduced W atoms will form nucleation again in the deposition growth interface and continue to growing, that is to say, high-temperature resulting in the change of the microstructure of the deposited layer and deposition growth interface, leading to an increase of the roughness of tungsten tube surface ultimately.


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Compare fluoride (WF6) CVD (Chemical Vapor Deposition) to Chloride CVD findings, which shows that fluoride CVD tungsten tube has advantages as follows:

Low impurity content. Most metal fluoride having a boiling point over 1000 ℃. Therefore, at a constant temperature (about 20-28 ℃), in WF₆ evaporation stage, WF₆ is able to separate with most of other associated impurities.

Reduction reaction: WF₆ + H₂ → W + 6HF (125Kj/mol)

At a standard atmospheric pressure, the reaction can be achieved when the reaction temperature is above 300 ℃; at the temperature of 600 ℃, the equilibrium constant of the reaction is close to 1.

Because of the low reaction temperature for the reaction, it is possible to use ordinary heating equipment (such as stainless steel equipment, etc.), which makes the production of the heating equipment and actual operation easier. That is to say, the choice of fluoride CVD method for preparing tungsten tube is relatively plentiful.


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Reactant gas: WF₆

Reducing gas: H₂

Deposition matrix: copper tube;

Atmospheric conditions: at atmospheric pressure;

CVD (Chemical Vapor Deposition) tungsten tube productive process:

First, at a constant temperature, heating WF₆ and wait for gasification, then pass WF₆ into the mixing gas chamber until it mixes with hydrogen fully, and then pass them into the reaction chamber.

Heating the copper tube until the temperature is up to the desired deposition temperature.

WF₆ and H₂ set off a chemical reaction on the surface of copper tube; WF₆ is reduced to W atom. And the W atom will gather nucleation and grow at the copper surface, eventually forming tungsten deposits---tungsten tube.

Chemical reaction formula of WF₆ and H₂ reaction is as follows:

WF₆+ H₂→ W + 6HF (125Kj / mol)

Using gas absorption device to absorb the residual gas of WF₆ and H₂, and HF discharged from reaction chamber.


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Because the tungsten tube prepared by CVD (chemical vapor deposition) owns many advantages, so it has attracted many metallurgical industry and other stakeholders’ attentions.

Advantages of CVD:

Compared to the powder metallurgy process, CVD process is more simplified and stable, and the tungsten tube can be achieved by one-time forming, thus the sintering, pressing, forging and other processes can be avoided.

With fast deposition speed and high efficiency. Because the refractory metal owns large atomic radius, high melting point and other characteristics, so it is not improper to adopt PVD (Physical Vapor Deposition) and other preparation technology ---with low deposition efficiency.

Good rolling and sputtering plating. Thus the deposited product can be obtained with complex shape, compact microstructure or large size. It is also easy to implement the film layer deposition on the pipe inwall or container inwall. It can even achieve the multilayer film deposition and multi-component alloy layer deposition.

The deposited film is of high purity, good density and multi-component alloy layer ingredient can be precisely controlled.


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Great advances have been made in the production of X-rays, chiefly by the employment of very heavy currents. The exposures necessary for producing radiographs of the thorax have been reduced from minutes to fractions of a second.

To make this possible, much attention has been devoted to the target or anti-kathode, which is the critical part of the tube, for here it is that the focus of the kathode stream strikes, and the energy of the bombarding electrons is transformed into X-radiation.

A special target of tungsten has been introduced, that is being largely used by manufacturers of X-ray tubes. The tungsten is in the form of a thick button brazed into a solid block of copper, in some cases weighing as much as half a pound; this forms a lasting and efficient target, even when heavy currents are used for considerable periods of time, as is often necessary when using X-rays for therapeutic purposes.

The adaptation of tungsten for this purpose is an example of the great value that lies hidden in the rare and little-known elements, and doubtless other instances of a similar nature will develop as the metals become available.

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In order to accommodate applications where the X-ray flux is too high for both the detector and the electronics that process the X-ray spectrum, a “Collimator Kit” has been developed to collimate the primary X-ray beam. This system is comprised of standard 1.5 inch extender box which slides inside collimator housing.

The collimator housing can accommodate up to two tungsten collimator disks (tungsten alloy shielding, 90% W, 6% Ni, 4% Cu) that are placed inside a bayonet holder in front of the detector. By selecting the appropriate tungsten collimator disks, the user can reduce the incoming X-ray flux and allow the detector and electronics to process the X-ray spectrum. Seven different tungsten collimator disks are provided with different size holes in order to allow for a wide range of applications.

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After tungsten carbide button sintering process, there can be some pores inside button which will affect its property. Thus reducing pores in the button is an effective way to promote its property. In 1960s, Kennametel Corp in America firstly applied hot isostatic pressing method into producing tungsten carbide button. This method reduces a lot of pores in the tungsten carbide button; enhance its density and tenacity, prolong its service life.

However, hot isostatic pressing method requires expensive equipment; it is of low efficiency which increases producing cost. In order to solve these problems, some researchers combine vacuum sintering with hot isostatic pressing which is called low pressure hot isostatic sintering method. It can fully improve property of tungsten carbide button which decreases pores in the button and refined tungsten carbide grain size. It eliminates the coarse tungsten carbide grain on the surface of button and prevents gradients from alternating on the surface of tungsten carbide button. Thus increase its mechanical property.


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Tungsten carbide buttons are widely applied in geological prospecting, coal mining and oil well boring. Because of its wear resistance and hardness, tungsten carbide button is ideally suited for wear parts, other machine parts and dies which are subject to severe service conditions, such as high temperatures, corrosion and abrasion.

Some manufactures in China and from abroad add a trace of TaC into tungsten carbide button in order to improve tungsten carbide button binder structure, promote its wear resistance, resistance to heat shocks and thermal plastic deformation. But TaC can reduce tungsten carbide button hardness and toughness, increase the producing cost.

Some other researchers add Ta and Mo into tungsten carbide button and it turns out the button is of high tenacity and high hardness. This is because of a complex layer formed in the tungsten carbide grain with Ta and Mo. Thus tungsten carbide button performs a great impact toughness and high wear resistance.

 
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Development of tungsten carbide button is mainly concentrated on microelement of tungsten carbide materials, gradient structure optimization, nano crystallization of tungsten carbide button, improvement of producing method and also designation of tungsten carbide button shape. Tungsten carbide button future marketing competition will be focused on technology content instead of producing quantity.

The following proposals need to be considered in the future development of tungsten carbide button:

1.Nano techniques shows great characters in producing tungsten carbide button. Researchers should speed up development progress, fully apply nano materials in producing tungsten carbide button.

2.Although research on functionally gradient tungsten carbide button has made certain achievement, the property of tungsten carbide button does not reach the anticipated result. During the producing process the quality is hard to control. So there is still a lot of study to be done in this field.

3.Apart from low pressure hot isostatic sintering producing method, other methods like micro sintering and laser sintering can also be applied.


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