DISCUSSION G. F. Schmitt, Jr., Wright-Patterson Air Force Base: Was there some particular reason for settling for a coating thickness of 10 microns or less? Admittedly you have pretty close tolerances on the thicknesses of coatings that can be put on these blades because of aerodynamics, etc. The coating thickness is certainly not limited to anything as small as that. Is there not some reason to believe that even better protection might be obtained with a somewhat thicker coating? K. Gentner: The reason I used only up to 10 microns was that I wanted to see definitely any erosion damage on the films so I could learn something. It showed that on the back sides of the blades, the film was eroded away before we came to the end. By using a sputtering process I can coat selective coatings over the area. That means I can give the front side, which needs very little protection, a very light coating, and give the back side a real heavy coating. It has been shown that the back side erodes at least 10 times more severely than the front. Engine people are interested in a minimum added weight from a coating. With sputtering, a light coating can be put on the front side and a heavy coating on the back side which keeps the added weight to a minimum. J. E. Newhart: The thicker the coating, the greater are the fatigue losses in the substrate. At about a 0.0015 in. thickness and with certain types of high temperature coatings, a 40% loss in the fatigue strength of the substrate can be expected. In some turbine engines there isn't much of a pad to play with. G. F. Schmitt, Jr.: You just answered my next question which was about the effect on the fatigue life. I recognize that sputtering is a pretty low temperature process as opposed to vapor deposition. K. Gentner: Actually, to have a high sputtering rate, the sputtering electrode is chilled. The hotter the electrode, the lower is the sputtering rate. If a film is deposited, it should be annealed on the plate. In any other process a high temperature is used which is not good for the base plate, but in sputtering the energy comes from the high speed of the incoming particles. That means the blade can be kept relatively cold and all the energy to anneal the film comes from the incoming particles. G. F. Schmitt, Jr.: What about using it on titaniun? You said you sputtered titanium carbide on titanium. What about anything else and what sort of adhesion or lack thereof would you expect? K. Gentner: High adhesion is obtained if all the oxides are removed first. How much material should be removed? I have in many cases not removed more than 1. micron. I went as high as 4 microns in one case but it was not necessary. R. L. Johnson, NASA, Lewis Research Center: We've used the processes of sputtering and ion plating primarily for lubricating coatings for sometime. A year and a half ago there was a conference at Lewis that dealt with both sputtering and ion plating and if anyone is interested in it I can provide copies of this. I think there is a point here that needs to be raised, though. The question was brought out in the discussion of the effects on fatigue. The usual concepts of the adverse influence of coatings on fatigue don't apply with these types of coatings. We have been doing bending fatigue studies with metal specimens with coated surfaces, and we find in some of the surface analyses that sputtered or ion plated materials put on with reasonable processes will, in fact, immobilize surface defects, rather than create them which occurs with some other kinds of coatings. We've shown an increase of 10-50% in endurance life in bending fatigue studies on thin flat sheets. This is with only about 2000 angstrom units thickness of metal coated on another metal. I think this matter of fatigue effects can be advantageous. I mentioned earlier that these coatings will, in effect, tie down surface defects. Ion plated metals will penetrate grain boundaries, and although we haven't experimented with them, in fluid erosion studies, I think that they can mitigate against some of the mechanisms that have been discussed here earlier where slip is occuring at grain boundaries, etc. We had some erosion problems with a fluid in a hydraulic system pump with a bronze surface. The problem was associated with the fluid itself, but we tried to salvage some parts using sputtering of monel. There was erosion damage on the operating parts of a pump similar to the hydraulic pump, the piston pump that we saw here earlier, in a matter of a 1/2 hour with the bronze. With the monel coating, which was a few thousandths of an inch thick, much thicker than we usually use, it took at least 10 hours before we saw any sign of damage. So the coatings can be used in this area without going to the extremely hard coatings that are being talked about. The process is a very flexible one. RAIN DROPLET EROSION MECHANISMS IN TRANSPARENT PLASTIC MATERIALS George F. Schmitt, Jr. Elastomers & Coatings Branch The damaging effects of rain erosion on optically transparent materials such as are used for windshields and infrared, television and laser guided missiles have become an increasingly severe problem as the number of systems using this type of guidance have increased. The rain erosion of these materials is a twofold problem; first of all, subsonic erosion and runway debris damage during captive carry beneath the wings on the aircraft, and secondly supersonic erosion during free flight after being launched. On glass materials the subsonic erosion damage takes the form of isolated pits which eventually join together with subsequent chunking of larger pieces. At supersonic speeds catastrophic fracture occurs with large scale cracking and material loss. All materials suffer a significant deterioration (up to 80% loss in transmission) after rain erosion expo sure even when no measurable weight loss has occurred. The front end covers for optically guided missiles called ''domes" are typically constructed in hemispherical shapes to enable optimum transmission through them. However, this shape with its large normal (90°) or near normal incidence angles is the worst geometry possible from an erosion standpoint. Reduction in impingement angle is always desirable to reduce erosion effects but this is not compatible with the tracking requirements. A potential important use of transparent plastic materials which possess some degree of subsonic rain erosion resistance and a moderate degree of thermal capability for withstanding aerodynamic heating during captive carry on super sonic aircraft and after launch is for guided missile optical covers. Polycarbonate and polysulfone plastics have improved thermal stability over acrylics, lower cost than glass, excellent strength properties, and good visual transmission characteristics. The purpose of this investigation was to determine the effects of subsonic rain erosion exposure on the transmission of radiation at a single wavelength (1.06u) through these plastics. |