Vapor-Pressure-Curves wsWhen performing any type of vacuum heat treatment it is always important to be aware of the possibility of evaporation and/or sublimation of elements, which can be present in the material being processed, introduced into the vacuum system with the workload, inherent in the equipment design or introduced during maintenance, repair or rebuilds. In cases where evaporation may be a concern, the vaporization rate is of prime importance and is directly related to the furnace pressure (the higher the pressure, the more frequent the collision of gas molecules and correspondingly, the few metal atoms leave the metal’s surface).

What is Evaporation? Vaporization is the process that occurs when a chemical or element is converted from a liquid (or a solid) to a gas. When a liquid is converted to a gas, the process is called evaporation or boiling; when a solid is converted to a gas, the process is called sublimation. The pressure exerted by the vapor of a liquid in a confined space is called its vapor pressure. As the temperature increases so too does its vapor pressure. Conversely, the vapor pressure decreases as the temperature decreases. By Dan Herring


Figure-4 wsVacuum deposition is a generic term used to describe a type of surface engineering treatment used to deposit layers of material onto a substrate. The types of coatings include metals (e.g., cadmium, chromium, copper, nickel, titanium) and nonmetals (e.g., ceramic matrix composites of carbon/carbon, carbon/silicon carbide, etc.), deposited in thin layers (i.e. atom by atom or molecule by molecule) on the surface.

Vapor deposition technologies include processes that put materials into a vapor state via condensation, chemical reaction, or conversion. When the vapor phase is produced by condensation from a liquid or solid source, the process is called physical vapor deposition (PVD). When produced from a chemical reaction, the process is known as chemical vapor deposition (CVD). These processes are typically conducted in a vacuum environment with or without the use of plasma (i.e., ionized gas from which particles can be extracted), which adds kinetic energy to the surface (rather than thermal energy) and allows for reduced processing temperature. By Dan Herring


vacuum-furnace-wsTungsten is used in vacuum furnaces when there is a need for structural integrity at elevated temperature and/or in situations where other materials may degrade, such as when lower melting point eutectics are a concern. One example of its use in is roller rail assemblies in which graphite wheels are positioned between molybdenum rails using tungsten axles.

Tungsten (chemical symbol W) is a member of the family of refractory metal (Mo, Nb, Re, Ta, W) and has the highest melting point and vapor pressure of this group. Due to this unique property, it is commonly used as a material of construction in specific areas of vacuum furnace hot zones operating above 1315ºC (2400ºF). Tungsten can also be used for heating elements given that it has the highest duty temperature, typically 2800°C (5075°F). In practice, this rating is often downgraded as it is for all heating element material choices. Tungsten will become brittle, however, if exposed to oxygen or water vapor and is sensitive to changes in emissivity. In general, tungsten is resistant to corrosion below 60% relative humidity. By Dan Herring 

moly-hot-zone wsVacuum furnace hot zones are manufactured using materials that can withstand temperatures in the range of 1315ºC (2400ºF) and higher. Of the various types of refractory metals in use, none is more common than molybdenum.

The popularity and widespread use of molybdenum in vacuum furnaces is due to the wide range of properties that it exhibits, namely: high melting point, 2620ºC (4748ºF), low vapor pressure, high strength at elevated temperature, low thermal expansion, high thermal conductivity, high elastic modulus, high corrosion resistance, and elevated recrystallization temperature, between 800º - 1200ºC (1470º - 2190ºF). Mechanical properties of molybdenum are influenced by purity, type and composition of any alloying elements and by microstructure. Properties such as strength, ductility, creep resistance and machinability are enhanced by additions of alloys such as titanium, zirconium, hafnium, carbon and potassium along with rare earth element (La, Y, Ce) oxides. By Dan Herring 

Figure-1 wsLubricants in vacuum applications include wet and dry lubricant types (Table 1), greases and oils. So-called “wet” lubricants tend to stay wet on the surface to which they are applied, while dry lubricants go on wet but dry as they are applied. In general solid particulates do not stick to dry lubricants but they do not tend to last as long as wet lubricants and as such need to be reapplied. By contrast, greases adhere better than oils and tend to last longer. Oil is preferred where the lubricant needs to be circulated.

The major disadvantage of conventional liquid lubricants is that they have relatively high vapor pressures (= 1.3 x 10-4 Pa at room temperature) and surface diffusion coefficients (= 1 x 10-8 cm2/s) with low surface tensions (in the order of 18 – 30 dyne/cm) and can volatilize or creep away from areas of mechanical contact resulting in high friction, wear or mechanical seizure. In addition, their volatility can cause issue with achieving proper vacuum levels and/or depositing on component part surfaces. The presence of other gaseous species in a vacuum environment (e.g., water vapor, oxygen, carbonaceous gases) can cause the force of adhesion between metal surfaces joined by liquid lubricants to be so strong that the joined areas can only be separated by fracture. By Dan Herring 

Installation-wsWe continue our discussion on the factors that must go into the decision making process during the acquisition phase of a vacuum furnace. Part One focused on how one goes about choosing the right furnace for the job and talked about the various choices for hot zones (e.g. insulation, heating elements, etc.). Part Two discussed pumping systems, controls and ancillary support items (e.g., grids/baskets/fixtures, water systems, features & options). It is now time to understand how your supplier partner will handle the project once an order is received, including project management, codes & standards, approvals, installation, commissioning and long-term support.

Order Processing; Okay, you have selected a vendor partner to supply your vacuum furnace, so what type of support should you now expect? To begin with, it is important to understand how your order will be handled internally by the company you have selected. The first task they face is to transfer the order from their sales team to their engineering team and, ultimately, to the manufacturing and service/support teams. After receipt of an order, your supplier partner will typically schedule an engineering “kickoff” meeting where the project is given to engineering and a project manager (or project engineer) is assigned. Out of that meeting will be generated the final equipment specification and this should be provided to the purchaser shortly (e.g., 1 – 2 weeks) after receipt of the purchase order for approval. By Dan Herring

Tips for Selecting Vacuum Furnace Equipment – Part One

Tips for Selecting Vacuum Furnace Equipment – Part Two


Figure-1 wsWe continue our discussion on the many factors that must go into the decision making process during the acquisition phase of a vacuum furnace. Part One focused on how one goes about choosing the right furnace for the job and talked about the various choices for hot zones (e.g. insulation, heating elements, etc.).

 We now continue this discussion by looking at other common vacuum furnace features and options. Recall that the four common elements of any vacuum furnace are; Hot zone (c.f. Part One), Heating elements (c.f. Part One), Pumps and Controls. Once decisions have been made in these areas, other ancillary items (e.g. partial pressure control, loaders, etc.) must also be considered and will be talked about here as well. By Dan Herring

Tips for Selecting Vacuum Furnace Equipment – Part One

Tips for Selecting Vacuum Furnace Equipment – Part Three