Recent Applications Of HVOF Coating Technology In the Aerospace Industry
John Bonar - Engelhard Surface TechnologiesHVOF stands for "High Velocity Oxygen Fuel" which is a very high velocity particle spray process in which the particles are driven by a high-speed gas jet. The device is somewhat similar to a small jet engine. The technology has many application in the particle coating field, but tonight we will concentrate mostly on aerospace and industrial gas turbine applications.
As an overview of applications, HVOF has been successfully applied to depositing aluminum, abradables, sealing gear teeth, and shroud protection.
Englehard as a corporation is involved in Precious Metals, Engineered Materials, and Environmental Solutions. Among their core technologies is an material science laboratory which has an Advanced Coating Group. It includes capabilities for advanced processes and diagnostics measurment in the gas plume of the HVOF device.
For the HVOF technology custom powders are usually developed. Englehard uses both powders it produces, and also powders which are purchased.
The use of HVOF as a coating technology has really taken off in the past 7 years. Many demonstrations have been conducted that establish HVOF coating to be at least equivalent to (and in some cases superior to) the D-Gun technology it is competing against. Many GE specifications now allow either coating method to be used.
HVOF is suitable for both high temp and low temp wear resistance applications
Chromium Carbide coatings are used in mid-spans where fatigue protection is partially provided by the coating. Boeing had a program to replace chrome with WC-Co 83-17 using HVOF application technology.
On engine shroud assemblies HVOF WC-Co-Cr coatings have been found to last double the time (through two inspection cycles) as the prior chromium coatings. This feature is very attractive to the airlines anxd other consumers, but not generally desired by the OEMs who prefer to keep initial costs low.
As a abradable seal material HVOF coating have been found to be cheaper to use than competing technologies. In diesel turbocharger applications, the nature of such coatings has allowed a 1 - 2% improvement in engine efficiency by allowing zero clearance sealing in the turbocharger.
Pure aluminum coatings (up to 0.125" thick) have been applied to magnesium gear cases. The coated aluminum is essentially fully dense and similar in properties to wrought aluminum.
CoNiCrAly type mateials can also be applied to industrial gas turbines up to 0.125" thick without defects in the coating, and with a virtually undetectable interface between coating and base material.
In the non-metallic realm, Aluminum Oxide has been succesfully applied using HVOF technology. Since applied material are never in tension, such coatings hold up without cracking.
In applications where chromium has been replaced with HVOF, the HVOF is more expensive to apply; however, the advantage of permitting thicker coating application, and the fact that the HVOF coating can outlast chromium by a full life cycle, makes the HVOF technology less expensive in the long run.