Why Should Vacuum Oils be changed at specified intervals?

In the present time of cut throat competition, users are under great pressure to reduce the cost of production. To achieve this objective, many times vacuum oils are not changed at specified intervals to get more “life” from them.

This is a dangerous practice and can damage the vacuum system badly. Let us understand why-

Oils degenerate on account of oil oxidation. In Oxidation, oxygen reacts with the oil to form Hydro-peroxides. Hydro-peroxides are unstable and break down into Ketones. Ketones further oxidize to form Aldehydes and Carboxylic acid. Now Carboxylic acid attacks the pump metal parts as well as O rings, Oil seals etc. causing acid corrosion. That is not all. By products of oil oxidation in the presence of acid further react and combine to form larger molecular species which polymerize to form sludge.

 
Flow chart no. 1 - Degeneration of oil. 

Sludge makes oil very viscous and can cause –

·         Jamming and breakage of vanes in the Rotary Vacuum Pump.

·        Overheating and damage to the pump interiors in the Diffusion Pump.

What is even more damaging is that this process is like a nuclear chain reaction that keeps speeding up with time. With oxidation, oil thickens and this reduces lubrication, which in turn results in a higher temperature. At increased temperature the rate of oxidation further increases (Rate of oxidation doubles with every 10⁰C rise of temperature as per Arrhenius rate rule).  This damages costly vacuum pumps in a short time.

Flow chart no. 2 - Oil oxidation: chain cycle.

Oils therefore must be changed as per specified schedule.

Note: In this blog post I have explained the oxidation process of hydrocarbon/ mineral oils. In a different post, I shall explain how silicone oil degenerates.

                                                           

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Written by: Anshuman Punj.

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Evaporation Boat: How it is made

GENERAL PROPERTIES

Evaporation Boats are ceramic components required for vacuum deposition of various types of metals e.g. aluminum, copper, silver etc. over various substrates.

ALFA – Evaporation Boat (Supervac Industries)

They are of two types –

A) - 2 Component Boat – This type of boat is made with Titanium Diboride (Conductor) and Boron Nitride (Insulator).

B) - 3 Component Boat – This is made with Titanium Diboride (Conductor), Boron Nitride (Insulator) and Aluminum Nitride (Insulator).

Two Component boat is longer lasting and of better quality. In three component boat, Aluminum Nitride is added to bring down the cost. But consequently, it is lower in life and quality as well. Hence, users prefer 2 component boats to 3 component ones.

Production of Evaporation Boats is very expensive, involving heavy, costly machinery, raw materials and running energy costs. Also it is protected by patent laws. Therefore, very few companies in the world are producing it.

THE PROCESS

There are 5 basic steps in the production of Evaporation Boats:

1.     Preparing the powders – First of all, the powder particles are brought down to the desired size through milling.

2.     Blending of the raw materials – The powders are mixed together in the desired ratio and blended thoroughly for the sake of uniformity in the raw material.

3.     Hot-pressing – The blended raw material is then hot-pressed at a very high temperature (More than 1500⁰C) with a pressure of more than 150 tons. This delivers a big block of ceramic.

 

4.     Machining – The hot-pressed ceramic block is then cut into boats with a diamond cutter. Each boat is ground for a smooth finish and dimensional accuracy.  A cavity is ground into each boat.  

5.     Quality control – Each and every boat is measured for resistivity. After final inspection, boats are packed for shipping.

ALFA Boats come in various sizes and resistivity.

Supervac Industries make Evaporation Boats known with the trade name – ALFA ONE - are made in various sizes and resistivity as per the customer’s requirement. ALFA ONE boats come with Supervac Industries’ stamp of world- class quality and reliability.



                                                                                                       Written by: Anshuman Punj.

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Tips on Getting More Life Out of Your Diffusion Pump Oil

Diffusion Pump oil is of two types – Silicone and Hydrocarbon.

Most people these days prefer silicone DP oil because of the following reasons.

* Good quality silicone diffusion pump oil will have a high vapor point and therefore deliver a higher vacuum level.

* Will be thermally stable.

* Compared to Hydrocarbon oil, silicone DP oil will perform better over long periods of time with little to no degradation.

* This oil is also relatively safe from the standpoint of toxicity and flammability.

Diffusion pump oil is one of the costly consumables in a Metallizer. With a few precautions, its life can be prolonged considerably and much money saved.

Dos and Don’ts:

1)   Never open the pump when the oil inside is still hot. Doing so will affect the viscosity of the oil – it thickens and eventually turns into gel form.

2)   Care should be taken to top up the oil in the pump in a timely manner. In the absence of topping up, the reduced quantity of oil will accidently over heat and in extreme circumstances, can even crystallize into blue-grey powder or crystals.

3)   To avoid having the oil migrate into the vacuum chamber, a cold cap should be fitted at the top of the nozzle assembly inside the diffusion pump. This cold cap condenses the vapor in that area.

4)   A concentric circular chevron baffle should be present at the mouth of the pump. It allows air molecules to wander in, but traps the heavier oil vapors as they try to escape.

5)   Faulty gauges for pressure/heater too, can cause accidental overheating of the oil or the oil not getting the required heating. Check that all the gauges are functioning properly.

6)   There should be no gross vacuum leaks as this will cause the pressure to drop. In a lay man’s language, this means that from some point air is getting inside the pump, not letting proper vacuum to form. Such leaks can be plugged by applying silicone vacuum grease to O rings and joints.

7)   Keep track of the water flow through the cooling lines. Check the inlet and outlet water temperature as well as the flow rate. Lines can become clogged or corroded over time.

8)   Often back streaming of contaminants from the vacuum chamber takes place causing the oil to form sludge. This can   also happen if the Rotary vacuum pump is letting its vapors to escape into the diffusion pump. This is preventable by installing filters above the chevron baffle of the diffusion pump as well as on the line in between the rotary and the diffusion pump.

9)   Silicone diffusion pump oil starts to decompose if it comes in contact with even trace elements of alkali metals (e.g., sodium, potassium, cesium etc.). Use of the latter should be avoided in the presence of silicone DP oil.  

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                                                                       Written by: Anshuman Punj  

Go to Supervac Industries Website

 

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