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Adhesion is the strength of the bond of the coating to the substrate. Adhesion can be measured in several ways, such as: Daimler-Benz Adhesion test (see), Scratch Adhesion test (see), Tape test (see), and Stud-pull-off test (see).
Cohesion refers to the strength of the material to support itself. As opposed to adhesion (see). Adhesion failure happens when one layer delaminates from another, cohesion failure is when the delamination occurs within a material.
Chemical Vapor Deposition. A coating deposition process that is characterized by a chemical reaction producing the desired film material. CVD processes are usually performed in vacuum, but not always. Vacuum CVD processes generally are not performed in high vacuum conditions (as used in PVD processes). Many CVD processes are performed at elevated temperatures. Hard coatings such as TiC, TiN, and TiCN are CVD- coated above the tempering temperature of steels and require a re-heat treat step after coating. BryCoat provides CVD deposition of TiC for bearing balls.
- Daimler-Benz Adhesion Test
A method of checking the adhesion of PVD coatings that relies on observation of a Rockwell C indent. The stresses around the rim of a normal Rockwell C indent can cause microcracking and delamination of the coating. Because these stresses are greatest at the rim of the crater and then taper off radially outward, the size of the affected area gives an indication of the adhesion of the coating. A set of 6 standard drawings is used to classify the indent as a measurement value. Unfortunately, this test is not a pure adhesion test. It is affected by substrate material and hardness, the size of the indent, and coating stresses, thickness, and material.
Delamination refers to the separation of the coating from the substrate. It can also refer to separation between multiple layers of coating or even to cohesive failure of a coated part.
- Elastic Deformation or Elastic Hardness
Refers to a temporary deformation of the material in response to an applied force. As the force is removed the elastic deformation recovers or springs back. As opposed to “plastic deformation or hardness” (see). I
Wear caused by low amplitude motion between parts. Usually occurs in parts that are not designed to be moving relative to each other, but move because of vibration.
The ability of a material to resist deformation. Generally “plastic deformation” (see) is the measured property, but in some cases “elastic deformation” (see) is also of interest.
- Plastic Deformation or Plastic Hardness
Refers to a permanent deformation of the material in response to an applied force. The part will be deformed and the deformation will remain after the load is removed. As opposed to “elastic deformation or hardness” (see) where the material springs back to it’s original shape. In general, a material can have both plastic and elastic characteristics.
The measurement of the force exerted by a medium on the objects in the medium. In vacuum coating, pressure refers to the pressure of the gases in the vacuum chamber. Pressure is normally measured in Torr. Other units are psi, bars, atmospheres, pascals, etc.
Physical Vapor Deposition. A type of vacuum deposition process where a material is vaporized in a vacuum chamber, transported atom by atom across the chamber to the substrate, and condensed into a film at the substrate’s surface. The concept has been stretched to include reactive processes that form a coating compound by bombardment of different species on the substrate’s surface, and processes that rely on low pressure plasma interactions for greater energy levels and film density.
There are actually several different PVD processes, including: thermal evaporation, electron beam gun, ion plating, sputtering, cathodic arc, and laser ablation.
- Scratch Adhesion Test
A coating adhesion test that uses a stylus (usually diamond) applied to the coating surface at an applied load and moving relative to the coating at a specified velocity. The observed damage to the coated surface is observed and recorded. Commonly, the reported value is the critical load where the coating is fully removed from the substrate. In general, however, this is only one simplistic view of a complex set of results. As with other adhesion tests, this test unfortunately does not measure only the adhesion of the coating. It is affected by many other parameters such as the substrate material and hardness, the coating material, thickness and hardness, and the geometry of the part. But, it is repeatable if these variables are kept fairly consistent.
Strength is the ability of a part to resist bending (stiffness) or pulling (tensile strength). Strength is generally a bulk material property. Coatings improve surface properties, not bulk properties. Coatings generally do not make parts stronger.
- Stud-Pull-Off Adhesion Test
A coating adhesion test that uses a small stud whose head has a specific surface area and which is epoxy-glued to the coated surface. A machine then pulls the stud with increasing force until the stud pulls off. The measured force divided by the surface area then gives the coating adhesion. This technique is successful for measuring adhesion of paint and some platings. However, the adhesion of PVD coatings is generally stronger than the holding power of the glue to the coating. In fact, the adhesion of PVD coatings often exceeds the cohesion of the substrate material. Also, the PVD coatings are generally good non-stick surfaces, creating excellent release of the epoxy from the coating surface, further reducing the effective range of this type of measurement..
The material or product that is to be coated. BryCoat is a coating service that applies a range of coatings to customer supplied substrates.
- Tape Adhesion Test
A coating adhesion test where a specific certified tape is applied to the coated part and then removed. Any loose coating that comes off with the tape is cause for rejection. In general, this test is not adequate for testing PVD coatings because the tape adhesive force is much lower than the coating adhesion. However, it is a reasonable assurance that the adhesion is not terrible. If any coating comes off on the tape, it is a sign of a serious adhesion problem in the PVD film.
- Thermal Spray Coatings
Thermal Spray is a process where coating material is deposited to a substrate by heating it and accelerating it to very high velocities and temperatures. The coating material is usually in a fine powder form and forms a coating as it impinges onto the substrate.
- Thin Film
A coating layer that is so thin that only it’s surface properties are used. That is, no bulk material properties can be observed. The material behaves as a 2-dimensional rather than a 3-dimensional object.
The science and technology concerned with interacting surfaces in relative motion, including friction, lubrication, wear and erosion. (–ASTM Committee G-2 on Wear and Erosion)
The absence of matter such as air. A perfect vacuum is a condition that does not exist. High-vacuum coating chambers pump down from an atmospheric pressure of 760 Torr to a pressure of 1 x 10-5 Torr. This means that for every 100,000,000 molecules of air in the chamber before pumpdown, only about 1 remains after.
- Vacuum Coating
A coating process in which the coating material is applied to the substrate in a vacuum chamber. Generally, the coating material is vaporized, transported across the chamber, and then condensed on the substrate parts.