To view recommended Pretreatment Process Flows for most dry film lubricant systems, based on substrate, please select from the charts below:
The marriage of pre-treatments and dry film lubrication is a bit like the Chinese concept of the forms Ying and Yang. Each can perform independent of the other, but a single half lacks the symmetry of the whole, and therefore, lacks optimum function. Combined together they form the two halves of the perfect circle. The corollary in dry films are the functions of adhesion, cohesion, wear, corrosion resistance, and coefficient of friction for any dry film lubricant can all be markedly improved by a proper pretreatment. The optimum pretreatment can make a dramatic difference. The level of difference can be quantified by test for any specific application, but is nearly impossible to predict precisely without testing a specific application. Any generalized prediction is subject to many factors such as specific alloy, temper, initial surface condition, specific conditions of use, etc.
Most dry film coating manufacturers such as Acheson Colloids, DuPont Corporation, Everlube Products, Whitford Corporation, Sandstrom Corporation, and others will have a chart with information very similar to the one available below, stipulating the recommended pre-treatments for military specification coatings. These Mil. Spec. suggestions, proven over many years, are generally applicable to any resin bonded dry film lubricant. Be careful as cure temperatures above 600° F precludes some specific plating or conversion processes from being used as a pre-treatment. Click to view the “Recommended Process Flow for Most Dry Film Lubricant Systems” charts.
The information presented here is not intended to supplant the expert knowledge and recommendations of your plating, or conversion coating sources for ordinary uses. The information here is structured to provide a generalized view of various platings, electro-chemical conversions, and straight chemical conversion coatings specifically deemed useful in improving dry film lubricant, coating, and painting applications. Much of the information is specifically directed to special needs encouraging polymeric adhesion to various base materials with consideration of unique time/temperature/humidity, etc., involved in thermoset coatings. Discussions with our application engineers prior to final print/specification formulation are strongly recommended.
The general “rules of thumb” for dry film lubricant films, platings and conversion coatings apply. Of course nothing replaces actual test data. The general rules are:
For the preponderance of parts the following pre-treatments are the norm:
STEEL PARTS:
There are four basic pre-treatments utilized. The processes are iron phosphates, heavy zinc phosphate, fine grain phosphate (Bonderize type), and grit blast. Below they are listed in order of their typical overall efficacy with the lowest performer listed first.
1Employing grit blast prior to a phosphate substantially improves the performance of the lubricated coating. The grit blast induces more points of initiation for the phosphate conversion providing increased, and improved crystal structure plus a profile below the metal surface for additional reservoir of lubricated coating. The total corrosion and wear improvement can be measured in at least a full magnitude or more. DOD-P-16232 for zinc and manganese phosphates requires a grit blast prior to the phosphate immersion process.
2An iron or zinc phosphate passivates the metal surface offering less potential for atmospheric or handling corrosion prior to the coating operation.
The most effective sealers for phosphates are those incorporating chromate. If not specifically instructed to eliminate this process, many phosphate sources use chrome sealers as a first choice. Be sure to be fully aware of your end customers’ hazardous materials purchasing policies, and specifications. Most automotive customers have established, or are promulgating requirements to eliminate usage of chrome additives in coatings and coating pre-treatments.
In order to understand the benefit of a phosphate crystal used with a wear coating, visualize the benefit of a steel belted tire vs. a non-belted tire. With steel belted tires, due to improved cohesion, you get reduced squirm of the rubber against the road. This is especially true at high speed / heat. The matrix formed by the coating and phosphate crystal operate somewhat analogous to that vision.
Be aware that cure temperatures can affect phosphate quality. Thermogravimetric, and differential thermal analysis have shown that zinc phosphate crystals lose the first two molecules of water at 180° C (350° F). This transforms the zinc phosphate crystal from a tetrahydrate crystal to a more firmly attached, smaller dihydrate crystal, thus markedly improving both adhesion and corrosion resistance. Typically this works best with the bake done prior to applying a coating, however; a curing bake at 180° C to 200° C (350° F to 390° F) after coating will provide positive benefit of corrosion resistance in addition to releasing most molecular hydrogen at the part surfaces. Do not cure phosphated parts at temperature levels above 320° C (600° F) as the second two molecules of water are lost, and phosphate quality decreases markedly due to severe crystal shrinkage. Grit blast only is the recommended pretreatment for cure requirements above 600° F.
CHROMATE CONVERSION PROCESSES:
These processes are commonly used for improving the corrosion resistance of some metals, and to promote adhesion of coatings, and paints. These chrome-based processes are a science unto themselves. This precludes any depth of coverage here. We are only going to provide a few rudimentary precautions in their use with dry film lubricants.
STAINLESS STEEL:
3Passivation for stainless steels should be an integral aspect of the above treatments. This will add to total cost.
ALUMINUM:
In this we are only going to discuss aluminum anodizing processes which are the most broadly utilized. We will refer to those processes covered by Mil-A-8625, a standard reference. A great deal of literature is available.
Three basic types of anodic conversion are commonly used. These are the chromic acid processes, sulfuric acid at approximately ambient temperature, and sulfuric acid at lower temperatures, which is called ‘Hardcoat’. The essential difference in the anodic film character, as induced by the acid and applied power variables, is the anodic film density. All three films produced are amorphous alumina (trioxide of aluminum, or Al2O3). Their performance differences result from variations in density, thickness caused by process environment, pre & post treatments, and the voltage or current density applied.
Chromic acid and standard sulfuric acid films are formed by constant voltage. ‘Hard Anodize’ (Hardcoat) is formed with constant current density, sometimes with a superimposed AC current over the DC current to prevent process initiation “hot spots”. This is especially useful with high copper content alloys. When these processes are used with dry film lubrication coatings they should either be left unsealed, or only hydrated with hot water. Salts precipitation from seal operations such as nickel acetate inhibits adhesion of applied coatings.
Dimensional changes resulting from the anodize processes and subsequently applied dry film coatings must allow for all OD, ID and thread pitch diameters. The typical film thickness of the various anodize film’s are .00004″ to .00008″ for chromic acid coatings, .0004″ to .0008″ for sulfuric anodize, and Hard Anodize is typically formed between .0005″ to .004″ thickness. With chromic and standard sulfuric anodize about 70% of the film is buildup, and with hard anodize only one half the total coating thickness is in dimensional buildup. The other portion of the coating thickness results from reduction of the original basis material consumed in the conversion process. The thicker the film that is formed, or the higher the heat of the acid bath, the more porous the coating will be at the outer surface due to increased exposure to the acid, which causes degradation of the film. The film at the interface between the metal alloy and anodic film interface is always the highest density (hardest). Often a burnish is used to get to this layer when the anodic film is used as the single treatment for wear, or friction reduction. Of course, a burnish would reduce the adhesion of any applied lubricated coating by damaging the surface porosity. We highly recommend discussing any pretreatment processes being considered with your dry lubricant applications engineer.
Aluminum & Anodize References used:
ZINC AND CADMIUM PLATINGS:
The use of zinc or cadmium platings over steel, and under organic coatings can improve corrosion resistance by several magnitudes over either’s individual capacity to inhibit corrosion. For example, a commercial zinc plate (.00015″ thick) with a chromate conversion and a thermoset organic film topcoat (.0004″) can yield 240 hr’s resistance to red rust. Minor amounts of white corrosion may appear earlier. This is in accordance to electromotive scales where zinc sacrifices to steel. This can be significantly enhanced by the use of an aluminum-filled organic topcoat in addition to the above process. In this scenario, the aluminum sacrifices to the zinc, and the zinc sacrifices to the steel. In fact, more often than not, hardened substrates >40 Rockwell C, a zinc-filled organic coating is used in place of zinc plating. By controlling coating thickness, types, and volume of organic fill, 5% neutral salt spray results of 1,000 hr’s or more can be shown.
Cadmium performs equal to or superior to zinc, and exhibits slightly better friction characteristics. However, many countries legally preclude use, or import of cadmium as it is a heavy metal. We understand US multi-national corporations, such as the US headquartered automobile companies, will not allow use of cadmium after January 2003. This precludes the use of cadmium in large measure. Cadmium has been off & on the US’s banned list throughout the years. It appears cadmium may not be used after January 2003.
NICKEL AND CHROME PLATINGS:
A grit blast is the recommended process for these platings. They must be adherent enough, and thick enough to tolerate the resultant profile. The profile will vary dependent on the mesh selected, the blast pressure, nozzle angle, and nozzle distance from the work piece.
COPPER OR COPPER ALLOYS:
Adhesion to copper (or copper alloys) is difficult. Grit blast helps in providing increased adhesion through increased profile; however, a copper surface should then be made passive after the blasting operation. A chromate works well with copper. Bronze should be coated without delay after the grit blast, so ambient oxidation doesn’t occur. Ambient oxidation will decrease adhesion markedly.
TITANIUM:
Typically a grit blast is employed, however, a titanium anodize process can be useful.
MAGNESIUM:
The process flow referenced above suggests grit blast, dichromate, or anodizing. Special chemicals for these processes are available from major manufacturers. Be careful when working with magnesium, it is highly flammable!
SPECIAL COMMENTS:
The information contained here is general in character. Any use of pre-treatments in conjunction with dry films should be discussed with your dry film lubricant application engineer, and thoroughly evaluated and tested prior to final use.