Elif Temiz, 030070195, 7th Week Definitions

1-Tribo Charging 

 Previous Definiton

Tribocharging is the process of electricity generation when two different materials rub against each other. Some materials easily give up or accept electrons from other materials under friction. A triboelectric series lists materials that give up electrons in order from easiest to hardest. For many materials, the dielectric constant determines the position of that materialon the triboelectric series. Additional materials such as rabbit fur have been added based on experimentation. The further aart the two materials are that rub against each other, the more charge is transferred.

(Laurabce W. McKeen, Fluorinated Coatings and Finishes Handbook, p.150) 

New Definition (Better) (Coating Method) 

The basic principle of tribocharging relies on friction between the powder and the spray gun. The action of the powder flowing through the barrel of the gun generates a frictional charge on the powder. The charged powder is carried by air stream to the substrate where it deposits and sticks due to electrostatic attraction. Because there is no high-voltage system generating a field between the spray gun and the substrate, the electric field is substantially smaller and the powder tends to follow air currents rather than field lines. The smaller electric field results in a much reduced Faraday cage effect. Consequently, tribo guns produce smoother finishes, allow deposition of thicker films, and provide better coverage of intricately-shaped objects.

The frictional charge is generated because the powder and the gun have different dielectric constants. The tribocharging annular chamber is constructed of polytetrafluoroethylene (PTFE). Because PTFE has a low dielectric constant, a positive charge will be imparted to most powders. Sonic powders have low dielectric constants (e.g., mixtures of polyester and TUC) and do not pick up electrical charges readily. Therefore, attempts have been made to modify the powder composition. Small quantities of additives such as amines or quaternary ammonium phosphate salts increase the ability of the powder to accept positive charges. Additives, however, can modify the reactivity of the powder and also create a non-uniform composition. Uneven composition can lead to powder segregation and create problems with recycling. Steps are underway to produce a polyesteramide that has increased ability to accept frictional charge, reducing the need for additives (Loutz, et. al., 1993).

Tribocharging is less complex than traditional electrostatic powder coating systems because it does not use high-voltage transformers for applying the charge on the powder. Tribocharging guns do wear out faster than regular guns because of the abrasion of the powder on the PTFE surfaces. Because of the absence of electric field lines and a reduced dependence on leakage to ground office ions, tribo guns are more suitable for painting nonconductive surfaces.

(Williams, D., Randall, P., Guide to Cleaner Technologies: Organic Coating Replacements, p.31) 



2-Corona Coating 

 Previous Definition

Corona charging of the dielectric surface is realized using a corona discharge in air. A discharge is initiated by high DC voltage(5- 10kv) appliedto a corona electrode, which is a thin wire or a needle located about 1cm above the wafer. The ions created in this discharge are predominantly and (  for negative and positive polarity respectively. The very short mean free path ( cm) of the ions ensures collisiondominated transport with ions becoming thermalized before reaching the wafer. Corona charge deposition is non-destructive for charge doses up to few µC/ . For larger doses and especially for high corona fluxes, prolonged charging can lead to generation of traps at the interface and within a dielectric resulting in stress induced leakage current(SILC) in a process analogous to the effect of biasing stress in MOS capacitors. With further increasing of corona flux and dose,corona induced reactions can occur including corona induced oxide growth at room temperature. Corona charging utilized in this wor is limited to non-destructive range.

(S. Kar,R. Singh, Physics And Technology of High-K Gate Dielectrics II,p.426)

New Definiton (Better) (Coating Method)

A high-voltage power supply is attached to the spray gun in a corona-charging system. These power supplies provide adjustable voltage (typically 0- 100.000 volts) or controlled current. When the high voltage is applied to a charging electrode in the gun, a strong electric field is created between the charging electrode and the grounded attractor electrode, shown in Fig. 11.5. This strong electric field ionizes the air creating what is called a corona. Normally 30 kV will ionize clean dry air, but lower voltages can be used, especially when particles are present as in powder coating. Ions are created and electrons are emitted. The electrons interact with oxy-gen in the air to form negative ions. (Nitrogen molecules in air can produce positive ions.) The oxygen negative ions collide with the powder paint particles and transfer the electrons to those particles giving them a negative charge.

Once the powder particles arc charged they are blown out of the gun towards the substrate to be coated. If the substrate is grounded, the powder particles will be attracted along lines of electric force to the substrate as shown in Fig. 11.6.

(McKeen, L. W., Fluorinated coatings and finishes handbook: the definitive user's guide and databook, p.149) 



3- Alligatoring

Previous Definiton


Term describing the appearance of a film that is cracked into large segments resembling the hide of the alligator. When alligatoring is fine and incomplete, it is usually called checking.


Alligatoring may be caused by one coat being applied over another before the bottom coat is thoroughly hard and dry and/or having the material skinning over so that the lower portion of the film is still soft and elastic, or by less elastic material being applied over a more elastic undercoating. When these conditions are present and the finished article is exposed to actinic rays or changes in temperature and moisture content, expansion or contraction of the film cracks the hard outer crust while the softer core gives without breaking.


With excessively heavy coats of rather dilute materials, this cracking of the outer crust can take place without temperature change by the shrinking action of the bottom portion – much like clay mud is cracked under the summer sun.


Other causes of alligatoring include the too rapid evaporation of solvents or thinners and excessive air being forced into the film during spraying. 

(National Research Council (U.S.). Building Research Institute. Conferences, New Joint Sealants: Criteria, Design, and Material, page 175)






New Definition (Better) (Physical Defect Type)

A fracture situation of practical importance, although very rare, is the rolling defect of alligatoring. In the rolling of slabs, particularly of aluminium alloys, the work piece may split along a horizontal plane on exit, with the top and bottom parts following the rotation of their respective rolls. Figure 2 illustrates alligatoring in an A1-8%Mg alloy slab.

 Causes: Alligatoring occurs only in materials of limited ductility, such as Al-Mg alloys of higher Mg content and some of the Zn and Cu base alloys. It develops at some stage of hot rolling, in the production example quoted by Kasz and Varley [7] at h/l= 1. 33 (slab thickness to length of contact), by Meadows and Pearson [II] at h/l= 1.55 and in the hot rolling experiment of Sehey [5] on AI-8Mg billets at h/1=0.5-0.7. Alligatoring at high h/l ratios (say above I) is difficult to explain. Perhaps, as suggested by Kasz and Varley [7], inhomogeneous deformation leaves the centre layers weakened by porosity in the brittle as cast condition. Once a crack is initiated, it propagates very rapidly because the two halves of the billet now curve around the rolls, as though each half were deformed between the rolls and a central roll of infinite diameter. From consideration of material flow, it is much more likely that alligatoring should occur when deformation is more severe in the centre of the billet, that is, at low h/l ratios Meadows and Pearson [1 1] pointed out that the extrusion effect would result in tension normal to the rolling plane opening up the slab nose. Polakowski [12] suggested that in the initial stages of rolling (at high h/I ratios) existing defects are enlarged and perhaps cracks are initiated in the centre section of the as-cast structure. On further rolling, when the central plane of the slab is more heavily deformed, the tensile stresses generated on the surface and compressive stresses produced in the centre would form a torque couple that opens up the slab along its already weakened centre. The position of the pass line has a marked and often decisive influence on the occurrence of alligatoring. When the slab is entered into the roll gap with its centre plane above or below the centre line of the roll gap, the issuing material curls. In addition to the stresses imposed by curling, a shear stress develops also along the centre plane and this may contribute to opening up of the nose. Zhu and Avitzur [13] suggested that alligatoring occurs usually after severe successive reduction without anneal.

 Remedies: Alligatoring initiates at the leading end of the slab, and can be prevented even under otherwise damaging WI conditions. If splitting of the nose is to be prevented, one of the practical remedies is to taper the end of the slab so that the critical reduction is reached some distance away from the leading edge. The undeformed leading edge then acts as a clamp and in the course of further rolling the damaging stress pattern disappears. In the rolling of Al-Mg alloys the better centre quality of semi-continuously cast slab brought great improvement relative to chill cast ingots. The cross rolling of slabs cast with a pronounced V-edge is also effective as it neutralizes the tendency to the development of fish tail and eliminates the resulting stress concentration.

(Ghosh, S. K., Materials Processing Defects, pp.390-391) 



4- Ballistic Particle Manufacture  

Previous Definiton (Better)



Ballistic particle manufacturing was marketed as one of the most affordable RP processes at that time, with very inexpensive build materials.
The BPM utilizes ink-jet or droplet based manufacturing (DBM) techniques, where it builds models by firing micro-droplets of molten wax material from a moving nozzle or jet onto a stationary platform. The platform is then lowered and the process is repeated until a three-dimensional object is printed. Most parts are built as a hollow shell, although down to a 0.25inch internal cross hatch may be used.

(Rapid Prototyping Technology, Cooper, p.22) 


New Definition (Rapid Prototyping Method)

The BPM process involves a stream of molten droplets ejected from the piezoelec-tric ink-jet printing nozzles to deposit on the target substrate. The process still uses the 3D data of the solid model to position the stream of material on the substrate. Since the process is based on the melting of the material, it is particularly suited for the materials that easily melt and solidify such as thermoplastics and metals.

(Dahotre, N. B., Harimkar, S. P., Laser Fabrication and Machining of Materials, p.357)