Enlarge this picture Fig. 2. Soft spots found after shot peening on a low-pressure vacuum carburized gear due to cleaning issues

When vacuum furnaces were first introduced, many in the industry felt that the only acceptable part and fixture cleaning method was solvent vapor decreasing. Over the years, however, environmental and other factors have necessitated the use of aqueous systems. Therefore, it is important to understand how each method can successfully get the job done.

Cleaning is the application of time, temperature, chemistry and energy to remove contamination from the surface of a part to a level appropriate for the intended application. In other words, cleaning is simply moving contaminants from where they are not wanted (on the parts) to where they should be (in the waste disposal system). If all four aspects of the cleaning process are not working together, the parts will not be properly cleaned. Vacuum heat treating demands a high level of cleanliness compared to other methods; contamination left on parts can cause significant problems both in the equipment (Fig. 1) and on the parts themselves (Fig. 2).

Most vacuum systems are required to operate below 1 torr (1.33 mbar) and as such cannot contain or introduce any contaminants with a vapor pressure (at the process temperature) near the operating vacuum pressure. As is often the case, more than one contaminant is present, so the sum of the vapor pressures of each will be the limiting pressure of the system.

All cleaning systems depend on one or a combination of three basic actions:

• A physical action, i.e. a mechanical force, such as spray agitation, dunking, ultrasonics or even hand (abrasive) cleaning, to remove the contaminants from the part surface
• A thermal action to improve the activity of the cleaning solution and increase the kinetic energy of the system
• A chemical action to allow contaminants to be either desorbed from the part surfaces with the aid of surface active agents or dissolved by an action of absorption and dilution

Solvent Cleaning

Enlarge this picture Fig. 3. Typical vacuum vapor degreasing system used for cleaning parts prior to running in vacuum furnace (courtesy of HyperFlo, LLC)

Cleaning in a solvent offers a level of simplicity and forgiveness not seen in other cleaning methods. Solvent cleaning involves three basic steps: wash, rinse and dry. Washing is where the parts are immersed in or contact the solvent, which is typically boiling to help the removal process. The purpose of rinsing is to bring “fresh” or clean solvent in contact with the parts. The aim is to dilute the contaminated solvent present on the surface of the parts from washing. It is important to remember that the rinse solvent must be kept clean. Contaminated solvent is a very common problem and will only reintroduce contaminants back onto the surface. The drying step evaporates the solvent and separates the rinse solvent from the parts.

Solvent cleaning has a negative connotation in the heat-treating industry primarily due to environmental (VOC) concerns, safety and cost issues. The emergence of vacuum vapor degreasing in a sealed vessel (Fig. 3) offers an attractive alternative. It takes advantage of the best aspects of solvent cleaning – significantly reducing the size and amount of residual contaminants while meeting the most stringent cleaning requirements and avoiding the traditional problems of open degreasing systems.

Aqueous Cleaning

Enlarge this picture Fig. 4. Typical aqueous parts washer used for cleaning parts prior to running in a vacuum furnace (courtesy of ALD Thermal Treatment)

Aqueous cleaning (Figs. 4, 5) is the dominant approach used in the heat-treat industry. It’s simplicity, ease of use and overall flexibility is what makes it an attractive process. Aqueous cleaning uses detergents to lift contaminants from the surface of the parts; heat to make the detergents more compatible with the contaminants and to soften them; fluid force to dislodge the contaminants from the parts and to collect the insoluble contaminants in some removal systems; and time to allow the process to take effect. For cleaning parts and fixtures intended for vacuum operation, a clean water rinse followed by a forced hot-air drying system is highly recommended. Being absolutely dry prior to placing parts or fixtures into a vacuum furnace is mandatory.

Aqueous cleaning is not perfect, however. It often leaves a surface residue or “film” on the parts that may interfere with certain processes such as brazing or case hardening and thus requires subsequent removal. In general, aqueous cleaners don’t dry well and the solution is often difficult to remove from internal part surfaces such as holes, crevices and recessed areas. Finally, aqueous cleaners evaporate slowly, requiring large amounts of energy to dry parts, and they have been known to damage certain sensitive parts.

Other Types of Cleaning

Enlarge this picture Fig. 5. Aqueous parts washer used for cleaning parts prior to case hardening (courtesy of Aichelin USA)

Blast cleaning, pressure washing, steam cleaning, abrasive cleaning and other mechanical methods have all been used to clean parts and fixtures prior to vacuum heat treatment. Fluidized beds have also been used for years to remove contaminants on part surfaces. In cases where it is beneficial to remove imperfections such as stains or surface corrosion, heat discoloration, oxide films, weld marks, scratches and particles of all sizes, then electropolishing techniques can be used. These “nontraditional” cleaning approaches have value but present their own unique set of challenges.

Cleaning the Cleaning Equipment

One of the most overlooked aspects of successful parts cleaning is to schedule routine maintenance on the cleaning equipment. Batch chemistry, concentration and pH should be checked and, if necessary, adjusted daily. This not only includes replacing the cleaning chemistry (or distilling it in the case of solvent systems) but cleaning the parts washer thoroughly. Steam cleaning and scraping methods are often used on the inside of the tank to ensure that all areas are cleaned.

How Long and How Clean is Clean?

Enlarge this picture Fig. 6. Example of visual inspection

Cleaning time depends to a large extent on the system and the parts; typically, an aqueous-based process needs to run for 10-20 minutes while solvent-based techniques need only 5-10 minutes to complete the cleaning process. As a rule, it is more difficult to clean clean parts than dirty ones. A key question is always, How do we know when the parts are clean?

There are a large number of tests to measure cleaning effectiveness. The most common in the heat-treat industry include:

1. Visual inspection (Fig. 6)

Enlarge this picture Fig. 7. Example of a stereomicroscope inspection

2. Stereomicroscope (macroscopic, 5X - 50X) inspection (Fig. 7)

Fig. 8. Example of "white glove" inspection

3. "White glove" inspection (Fig. 8)

Fig. 9. Example of ultraviolet (black) light observation

4. Ultraviolet (black) light observation (Fig. 9)

Enlarge this picture Fig. 10. Example of tape sampling

5. Tape sampling (Fig. 10)

Enlarge this picture Fig. 11. Example of "water break" testing

6. "Water break" testing (Fig. 11)

Enlarge this picture Fig. 12. Example of surface tension testing

7. Surface tension test fluids (Fig. 12)

Enlarge this picture

8. Nordtest technique – a quantitative approach using low surface tension droplets spread onto the part surface; measure of the droplets' ability to wet the surface and form a film layer (Table 1)

Fig. 13. Example of Gravimetric Method

9. Gravimetric methods (Fig. 13)

Direct and indirect verification methods for part and fixture cleanliness can be classified into three main categories: gross verification, semi-precise verification and precise verification. The test choice is dependent on the needs of the specific vacuum process.

Gross verification looks for visible contamination, but it does not quantify them (for example, dirt is just visible at 0.1 gram/square inch and above). Examples include the use of stereomicroscopes, nonvolatile residue methods, tape tests, ultraviolet (uv) fluorescence observation, water break test and “white” glove test.

Semi-precise verification can be qualitative or quantitative to a moderate level of precision (0.001-0.1 gram/square inch). Examples include the use of contact angle measurements, the Millipore test and optical microscopy.

Precise verification is quantitative to extreme levels of accuracy (0.001 gram/square inch down to absolute zero). Examples include the use of Auger electron spectroscopy (AES), carbon coulometry, electron spectroscopy for chemical analysis (ESCA), Fourier transform infrared (FTIR), fluorometer, gas chromatography/mass spectrophotometry (GC/MS), ion chromatography, optically stimulated electron emission (OSEE), particle counting, scanning electron microscope (SEM) and secondary ion mass spectroscopy (SIMS).

Gross Verification Tests

Nonvolatile Residue (NVR)
The NVR test requires extraction of contamination from a dirty part into a volatile solvent, evaporating off the solvent and measuring the weight of the remaining residue using an analytical balance. Almost any clean volatile solvent can be used.

Tape Test
For polished or lapped parts, a strip of transparent (Scotch®) tape is affixed to the surface in question with firm pressure. The tape is removed and placed on a clean, white sheet of paper. The surface should appear as white as the original sheet of paper.

Ultraviolet (UV) Fluorescence
Fluorescence can provide a visual indication of where contamination remains on a surface since contaminants will fluoresce in the presence of UV light. The intensity of the radiation can also be measured via a registered signal on an instrument, which dictates the degree of contamination on a surface. This form of analysis is useful for locating contamination, but not identifying it.

Water Break Test
This is a simple test in which the part is immersed in clean water and upon removal the water film must continuously cling to the part surface for 30 seconds. If water beads the surface, it is considered to be contaminated with a film of oil or grease.

Semi-precise Verification Tests

Millipore (Patch) Test
The Millipore test consists of spraying a representative number of parts with filtered hexane, isopropyl alcohol or trichlorethylene at a pressure of 60 to 80 psi through a filter jet nozzle with a 1.2-micron membrane filter. The spray is collected and vacuum filtered onto a clean filter membrane, and the membrane is inspected for contaminants (placed under a microscope to measure, in microns, and count the number of dirt particles remaining). Weighing the membrane pad determines the total contaminant (in milligrams) that has been left behind.

Optical Microscopy
Optical microscopes use a beam of light and lenses to magnify objects. Optical microscopes are ideal for viewing residual oils and greases, flux residues, certain particles and surface anomalies.

Precise Verification Tests

Auger Electron Spectroscopy (AES)
AES is used for compositional analysis or determining which atoms are present on a surface. An argon (or other selected gas) stream directs electrons toward the surface, ionizing surface atoms by causing the removal of an electron from the atom’s inner shell. The atom now becomes excited and must release energy to "relax" and return to its original state. This is done by transferring the extra energy to an electron that can leave the atom. The exiting electron is known as the auger electron. The AES method of analysis measures the energy of the auger electron, which is unique to each particular atom. AES is used in the semiconductor field for corrosion, failure and thin-film analyses.

Carbon Coulometry
The technique employs in-situ direct oxidation of surface carbon to carbon dioxide, followed by automatic CO2 coulometric detection.

Electron Spectroscopy for Chemical Analysis (ESCA)
ESCA is a spectrophotometric technique in which X-ray bombardment of a surface results in the emission of an electron from a given atom. Knowing the energy of the X-ray and measuring the energy of the emitted electron can determine the binding energy of the electron. ESCA methods reveal chemical structure, bonding, and oxidation state. ESCA has the potential to be very useful in identifying organic compounds.

Gas Chromatography/Mass Spectrophotometry (GC/MS)
GC/MS is used to identify surface contamination by extracting contaminants into solvent and analyzing them. Organic compounds are separated via gas chromatography and are then identified, based on molecular weight, by mass spectrophotometry.

Ion Chromatography
Ion chromatography separates, identifies and quantifies ions. The analysis begins with a sample, typically a water matrix containing ions of interest. A portion is injected into the system and combined with a chemical stream that carries the sample to the analytical column. The analytical column separates the ions of interest in the sample into narrow bands within the stream of the chemicals. The chemical stream then sweeps these groups of ions into the suppressor device, which electrolytically transforms the chemical stream into pure water, leaving just the ions of interest in pure water to be swept downstream to the conductivity detector. The detector detects the ions based on their conductivity relative to the water.

A number of technical organizations, including ASTM, offer cleaning standards, often based on the type of material to be cleaned. Remember, clean is generally observed, not measured, and cleaning effectiveness is established by answering the question, Can we do what we need to do next? However, cleaning tests provide quantification of the nature of the part surface so that the influence of the remaining contaminants can be factored into the heat-treatment operation as well as subsequent manufacturing operations.

Final Thoughts

Cleaning is critical to the success of vacuum heat treating. The more we embrace the fact that parts and fixtures must be absolutely clean (and dry), the better the quality of our parts and the longer the life of our equipment.

It is also important to recognize that both solvent and aqueous cleaning processes can be made to clean almost all parts and fixtures requiring vacuum heat treatment. However, the choice should be made first by the degree of part cleanliness needed followed by other factors. The focus today is on improving physical action (force and volume) in combination with a chemistry choice balanced for the type of cleaning required.

Finally, take a systems approach – consider the manufacturer of the cleaning system and the supplier of the cleaning agent as partners in the long-term success of any cleaning operation.

Next Time: Part 14 of this series discusses diffusion bonding, eutectic melting, outgassing and other topics related to what can go wrong inside the vacuum furnace as we process parts.