Vacaero

Manufacturers of heat treating and brazing vacuum furnaces and controls, complete hot zone and vacuum furnace retrofits, thermal spray coatings, plasma, HVOF and paint coating services.

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Getter Materials

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Experience has shown us that sensitive materials in the presence of minute quantities of unwanted gaseous contaminates can destroy the integrity and shorten the life expectancy of components. It is natural to ask ourselves what can be done to further protect the work in a vacuum environment after the pumps have done their part in reducing the chamber pressure to as low as is economically feasible in a production environment? This task falls on getter materials.

A getter is simply a reactive material that is deliberately placed inside a vacuum system for the purpose of improving the efficiency of that vacuum by scavenging unwanted contaminates. Essentially, when gas molecules strike the getter material, they combine with it chemically or by adsorption so as to be removed from the environment. In other words, a getter eliminates even minute amounts of unwanted gases from the evacuated space. 

Intergranular Oxidation (IGO) / Intergranular Attack (IGA)

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IGO is a surface phenomenon that is most often associated with atmosphere gas carburizing. The consequence of IGO (and the concentration gradients that develop during oxide formation) is that the material adjacent to the oxides has modified transformational behavior. Instead of forming martensite on quenching, steels with this condition develop non-martensitic transformation products (e.g. bainite, pearlite), which adversely affect mechanical properties (e.g. hardness, residual stress, bending fatigue).

The rate of diffusion of oxygen into a steel surface is dependent on the oxygen potential of the furnace atmosphere and the process variables (i.e. the depth of oxide penetration is influenced by case depth, time at carburizing temperature, carbon potential and the chemical composition of the steel). During the carburization process, the oxygen atoms (which are about 35% smaller than the iron atoms) are released as a direct result of the presence of water vapor and carbon dioxide in the gas. Oxygen diffuses slowly into the steel surface (as does carbon and hydrogen, albeit more quickly) and migrates to the grain boundaries. Once in the steel, oxygen combines chemically with the elements already present (e.g. chromium, titanium, manganese) that have an affinity for oxygen.

A Brief Discussion of Pressure and Vacuum Levels

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It is not uncommon in the heat treatment industry to hear one talk about high-pressure gas quenching and in doing so refer to terms such as 2 bar, 6 bar or even higher pressures. In scientific terms, a bar is defined as a unit of pressure equivalent to 100 kilopascals. A bar can also be thought of as roughly equal to atmospheric pressure (the amount of force air exerts on the Earth at sea level). To be technically correct, one atmosphere of pressure is 1.01325 bar or to put it another way, one bar is equal to 14.5 psia. Conversion calculators and tables are available to change bar into other units. Another common unit you might come across when talking about (negative pressure) vacuum levels in a vacuum furnace is millibar (mbar), which is 1 x 10-3 bar.

When talking about high-pressure gas quenching you might also hear one say that that they are quenching at 6 bar, to which they might quickly add, “5 bar absolute”. Just what are they trying to say here? Absolute pressure is referenced against a so-called “perfect vacuum” and as such is equal to gauge pressure plus atmospheric pressure. By contrast, gauge pressure is referenced against ambient pressure (14.7 psia), so it is equal to absolute pressure minus atmospheric pressure. So, when one talks about 5 bar absolute, one must add 1 bar of pressure (think of this as going from negative pressure to atmospheric) to arrive at total pressure, in this case 6 bar. As an example, measurements in the English system that relate the pressure of a system to a reference pressure are given by specifying the pressure in terms of pounds per square inch absolute (psia) or pounds per square inch gauge (psig).

Vacuum Pump Maintenance

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To maximize your investment and produce repeatable high-quality process results, it is mandatory to institute a thorough pump preventative maintenance program. In a pump’s normal operating life, nearly all unexpected vacuum pump failures can be prevented, and when carefully maintained, a vacuum pump will provide years of reliable service.

Be Organized and Document Everything. Start by generating a checklist of routine maintenance activities. The pump manufacturer typically provides this as part of the operating manual. Second, incorporate a maintenance log documenting all routine maintenance, repairs and component replacement. The log will play a critical role in diagnosing future problems, scheduling various maintenance activities, and stocking spare parts. Install a running hour meter to document the number of operating hours on the pump between service activities and enter this into the log. If possible, measure and electronically record the vacuum before and after the pump during every operating cycle, as well as the pumpdown times and ultimate vacuum level achieved in the furnace. This information is often collected and stored as data points in the furnace’s data acquisition system for process reasons. It can also be useful for planning and scheduling maintenance. Changes in these variables can be programmed into the furnace control system to notify the operator when a maintenance inspection or a specific maintenance activity is necessary.

Roots Blowers (aka Booster Pumps)

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Booster pumps (aka Roots blowers or intermediate stage vacuum pumps) fall into the class of dry, gas transfer pumps. As a dry pump, they do not introduce oil or water into the pumped gas stream. Gas transfer is accomplished by mechanically means, that is the transfer of pumped gas molecules, rather than their collection, as with, for example, cryogenic or turbomolecular pumps. 

The words “Roots blower” are synonymous with these pumps and the reason they are also commonly referred to as booster pumps is that they are mounted at the inlet of a primary/backing pump (such as a rotary vane, claw pump, or screw pump). They “boost” the performance of the primary pump improving pumpdown speed (much as a relay race in which the baton is passed from one runner to the next). The combination of Root blower and primary pump provides roughly a seven-fold increase in pumping speed and a ten-fold increase in pressure, in comparison to a primary pump alone.

Vacuum Pump Oil: The “Circulatory System” of the Vacuum Furnace

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Just as vacuum pumps can be considered the heart of the vacuum furnace, so too can the oil be thought of as its circulatory system. The selection and properties of the oil are critical to proper furnace operation. Pump oil serves different purposes in different types of pumps, and even has different functions within the same pump. In addition to lubrication, it helps provide the seal on rotary vane and other wet pumps, and serves as the media to propel the pumped gas via kinetic action in diffusion pumps.

Oil Formulations – Different pump oil formulations are specifically designed for different pump applications and careful consideration must be given to the oil selection. Typical motor oil, for example, is not sufficiently refined for use in a vacuum pump, has insufficient resistance to chemical attack, and contains additives that may be detrimental to the process being performed in the vacuum furnace. In addition, the viscosity must be considered. Lower viscosity oils are used for lower operating temperatures, and for smaller pumps, and medium viscosity oils are used for medium to large pumps. Temperature resistance is also critical, as many pumps operate at high temperatures, and the oil must be rated for these temperatures. Many of the oils used in vacuum pumps are not traditional oils at all, but made of silicone or other non-hydrocarbon fluids.

Coating Gas Turbine Engine Blades Using HVOF

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In recent years, the operating temperatures of land-based gas turbine engines have increased to improve efficiency.  As a result, greater demands are placed on the materials used in the manufacture of the engine components.

In particular, hot section turbine blades must function in a very severe operating environment.  The blades are usually manufactured from advanced nickel-based superalloys but these materials on their own are still not durable enough.  To enhance their durability, they are protected from hot corrosion and high-temperature oxidation through the use of special coatings.  The coatings form adherent oxide layers that inhibit the blade material from directly interacting with potentially damaging elements within the combustion gases like oxygen, sulphur, and other contaminants.  A popular approach involves coating the blades with an MCrAlY bond coat topped with a thermal barrier coating (TBC) overlay. BY JEFF PRITCHARD