Ultra High Vacuum Technology Section

Technology Development

1. Development of Titanium Coating for Alumina Ceramic Chamber

Alumina ceramic UHV chambers is used for fast pulsed pinger magnets in Indus-2 to mitigate the eddy current issue. Internal surface of this chamber requires Titanium coating to provide conducting path to image current. Technology of UHV compatible uniform thickness (0.5 micron±10%) coating of Titanium for alumina ceramic chamber (81mm x 17mm) aperture was developed in UHV Lab using the DC Magnetron Horizontal Sputter Coating system. Process parameters, for Ti coating, were optimized by successive trials on glass and ceramic samples. Repeatability of coating on was also confirmed with optimized process parameters. Adhesion of coating was also checked by tape test as per ASTM-D3359. No peel off was observed after coated sample was subjected to multiple (05 times) thermal cycling (room temperature to 150°C to room temperature). Optimized process parameters are shown in Table-1.

Table-1: Optimized Coating Process Parameters
Coating methodCylindrical D.C. magnetron sputtering
TargetØ2 mm Titanium wire
Cathode configurationTwo wire axially placed
Krypton gas pressure ~5.0E-2 mbar
Magnetic Field 200 Gauss
Voltage (D.C.) 500 to 600 V
Current (mA) :140 to 150 mA
Duration of the coating~10 hours

Measured thickness of titanium coating in coated ceramic tube was found to be ~0.55 micron. Post coating, an ultimate vacuum of ~8.0x10-10 mbar was achieved. Coated ceramic tube was further subjected to characterization with pinger magnet. Photograph of ceramic chamber coated with Ti is shown in Figure-1.

Fig 1:  Ti caoted Alumina Ceramic Chamber

2. DC Magnetron Horizontal Sputter Coating System for Development of NEG Coated Vacuum Chamber

Non-evaporable getter (NEG) coating technology for stainless steel vacuum chambers was developed in UHV Section for its application in vacuum systems of particle accelerator and storage ring. Ti-Zr-V NEG coating is a novel room temperature sorption pump for achieving very low pressure down to extreme high vacuum (< 10-11 mbar). It pumps active gases by chemisorption and hydrogen by diffusion. Salient features of Ti-Zr-V NEG thin film coating are: (i) It is a passive pump which doesn’t require any power for its pumping action. (ii) It acts as a diffusion barrier & hence reduces the specific thermal outgassing rate (iii) It does not require any extra space hence very useful for conductance limited long vacuum chambers of particle accelerators (iv) It reduces photo desorption yield (PID) and (v) It facilitates reduction of the Secondary Electron Yield (SEY) of a vacuum chamber surface.
A UHV compatible cylindrical DC magnetron horizontal sputtering system was developed for this purpose and the NEG coating parameters were optimized for stainless steel (SS) 316L vacuum chambers. Fig 1.shows the DC Magnetron Sputter Coating Set-up in UHV Lab. Many vacuum chambers (Size: 100mm diameter, 400mm length and material grade SS-316L) were coated with Ti-Zr-V NEG film at different coating parameters. Ti-Zr-V getter alloy with a well-defined composition ranges to provide low activation temperature. The vacuum performance and activation of coated chambers were studied by pumping speed measurement and ultimate vacuum testing of these chambers.

Fig 2. DC magnetron Horizontal sputter system details

Some of the important results achieved for coated SS chambers are as follows: The lowest ultimate vacuum achieved: 4x10-12 mbar, Thickness of coating: ~ 2.8 µm, Coating composition: Ti- 32%, Zr-23%& V-45%, Activation temperature & time: 250°C for 24 hrs. This development has paved the way for the indigenous development of NEG coating of conductance limited vacuum chambers to be used in particle accelerators and storage rings.

3. Development of Alumina to Titanium Brazed Joint

To overcome the issue of eddy current effects, alumina ceramic chambers are required for pulsed kicker magnets of accumulator ring of IFSR. Titanium as end flange for UHV sealing of alumina ceramic chamber is preferred due to: its compatibility with respect to co-efficient of thermal expansion of alumina, non-magnetic nature and extremely short radioactive half-life. Technological breakthrough has been accomplished by successful vacuum brazing and testing of UHV compatible un-metalized alumina (99.7% pure) to titanium-Gr-5 joints. Brazed specimen as per standard ASTM F19 is shown in Figure 3a. Brazed joints showed helium leak rate better than 2 x 10-10 mbar l/s even after 6 nos. of baking cycles at 150 °C for 8 hour soaking time. Brazing parameters were: brazing alloy- BVAg-8, vacuum ~ 4 X 10-04 mbar, temperature: 830 °C, soaking time: 3 minutes.

Fig. 3a: ASTM F19 Brazed Aluimna-Ti Specimen.

Microscopic examination of alumina/titanium braze joints revealed sound interface between the two dissimilar parts as shown in Figure 3b. The brazed joint exhibited a two-phase microstructure comprising of an Ag-rich light phase and Cu-Ti-V grey phase. Formation of a thin continuous Ti-Cu-Al-V film on the alumina surface appears to be responsible for satisfactory wetting of alumina by the brazing alloy. Mechanical characterization of brazed joints was also carried out. 4-point bend test of brazed sample as per ASTM C1161 showed bending strength > 50 MPa, an international benchmark of bend strength of ceramic to metal joint.

Fig. 3b: Micrograph of brazed joint (Top-Al2O3; bottom–titanium).

Best viewed in 1024x768 resolution