Issues Related to Qualification of HVOF Coatings on Landing Gear For fatigue and hydrogen embrittlement, the issue is the impact of the coating deposition on the base material. For wear and corrosion, the issue is the performance of the coatings themselves (or the performance of the coating/substrate as a system). Fatigue: Recognize that application of most coatings will degrade fatigue performance of base material. So issue is whether the application of the HVOF coatings degrades fatigue to lesser, equal, or greater degree than hard chrome, not the fatigue performance of the coatings themselves. Evidence from very large amount of testing is that on average, application of HVOF coatings degrades fatigue to lesser degree than hard chrome. However, coating integrity problem (delamination and spalling) first observed during fatigue testing. Obviously, if coating spalls during the cyclic stressing of a component in service, it is no longer providing the protection that is required (wear and corrosion).

Qualification of HVOF Coatings on LG But even when spalling occurs, engineers can retain confidence that the fatigue life of the component has not been compromised Corrosion: Very large amount of corrosion test data accumulated by various organizations. It is clear that hard chrome, WC/Co, and WC/CoCr provide some level of protection to high-strength steel. Problem is establishing baseline performance of hard chrome. From latest work of Steve Gaydos, it appears that, for whatever reason, hard chrome from Southwest United demonstrates superior corrosion performance to other hard chrome. So what do we use as baseline performance?

Qualification of HVOF Coatings on LG Additional problem is inconsistent results on HVOF coatings. B117 testing of WC/Co and WC/CoCr on rods by HCAT showed evidence of undercutting and liftoff; testing on rods by Boeing St. Louis showed no evidence of undercutting. HCAT B117 testing showed inferior performance of HVOF coatings compared to SU chrome; Boeing B117 showed inferior performance of HVOF coatings compared to SU chrome and superior performance compared to other qualified chrome. HCAT three-year atmospheric beach testing showed much superior performance of HVOF WC/Co compared to depot chrome. HCAT GM9540 cyclic testing for 1000 hours showed virtually no corrosion on either SU chrome or WC/Co. In-service examples showing superior or equivalent performance of WC/Co and W/CoCr compared to previously used hard chrome Conclusion: Corrosion is not an issue for qualification of WC/Co and WC/CoCr to replace hard chrome

Qualification of HVOF Coatings on LG Wear: Substantial amount of sliding, abrasive, and impact wear testing generally shows superior performance of WC/Co and WC/CoCr to hard chrome. Other than dimensional restoration, wear protection is reason for using hard chrome. Thus, for principal application of hard chrome, HVOF coatings are superior. Hydrogen Embrittlement: Significant issue for hard chrome; not an issue for HVOF coatings Stress-corrosion cracking: HCAT test results on notched specimens showed far superior performance of HVOF WC/Co over hard chrome; Eun Lee tests on bars showed somewhat superior performance of hard chrome

Qualification of HVOF Coatings on LG Question: Do we need to develop additional more complicated laboratory tests to (hopefully) better simulate real-life environments to assess relative performance of HVOF compared to chrome? Examples: –Corrosion followed by cyclic stress –Cyclic stress followed by corrosion –fretting corrosion Answer: No. Most likely, the more complicated the test, the less definitive the results will be (i.e., they will be all over the map--see corrosion test results for example). Forget more complicated lab tests and do component tests. After all, rig tests (LG, engine, transmissions, prop hubs, etc.) that are used to qualify new processes, materials, etc. and put them into service have no environmental factors built into them.

Qualification of HVOF Coatings on LG Coating Integrity: Problem first observed in standard fatigue testing on small coupons (0.25-inch gage section). Stress level where spalling observed depends on geometry of specimen. Occurs at lower stress levels on smooth gage samples compared to hourglass samples. (Latter can go to very high stress levels with no spalling). On larger specimens, stress level where spalling occurs different if stress applied in bending as opposed to axially (higher for former). Stress level where spalling occurs lower for R = -1 than for less than fully reversed stresses. On all specimens, stress level for spalling decreases with increasing coating thickness. How to address all of these issues to satisfy approval authorities to put these coatings on their LG? Can’t realistically have variety of usage “rules” for different LG components on one aircraft.

Qualification of HVOF Coatings on LG Bottom line comes down to philosophy of organization that would implement HVOF technology. If you are inclined not to use it, then you can seize on any number of results as a reason for rejecting it (or there will always be one more test that has to be done), while at the same time ignoring the poor and inconsistent results on hard chrome. If you are inclined to want to implement HVOF technology, then you examine all materials test results, design your own specific tests and if they are executed successfully, then implement technology on most if not all LG components on all aircraft (just as Hill AFB is doing). HCAT will continue, for a finite length of time, conducting work to optimize WC/Co and WC/CoCr deposition parameters for integrity, explore integrity of alternative coatings on which there exists some test data (e.g., Tribaloy 400), and explore a duplex coating for thick (> 10 mil) applications.