April_AMP_Digital

iTSSe TSS A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 0 4 5 iTSSe TSS (c) FEATURE was developed to produce extremely high spray velocity. Vari- ous HVOF guns use different methods to achieve high velocity spraying. Jet Kote uses a relatively simple system comprised of a high-pressure combustion nozzle and air cap. Fuel gas (hydrogen, propane, or propylene) and oxygen are supplied at high pressure and combustion occurs outside of the nozzle but within an air cap supplied with compressed air. The com- pressed air pinches and accelerates the flame and acts as a coolant for theHVOF gun. Powder is fed axially at highpressure from the center of the nozzle. PTI conducted extensive wear testing of Triboglide coat- ings and compared them to NASA’s PS 304 class coatings. PS 304 is a self-lubricating composite coating developed by NASA. It is based on a nichrome binder (80%Ni and 20% Cr by weight) and hard chrome oxide (Cr 2 O 3 ) wear-resistant phase, with the addition of silver and eutectic BaF 2 /CaF 2 as low and high temperature solid lubricants, respectively. Figure 2 shows themicrostructure of the Triboglide coatings with and without silver along with the microstructures of the PS 304 coatings. The PS 304 coating appears to possess a unique microstruc- ture with a lamellar structure and interfacial defects. It also exhibits a wide distribution of the metallic component within the oxide matrix. WEAR TESTING OF TRIBOGLIDE COATINGS PTI’s new blend of proprietary HVOF Triboglide CrC-NiCr- based coatings exhibit good adhesion and hardness. Two coat- ings with and without silver, deposited by the HVOF process, were sent to Argonne National Laboratory for high tempera- ture wear testing (Figs. 3-6). As can be seen from the wear data in Fig. 3a, the friction coefficient of the uncoated substrate is 0.6. With the coating, it is reduced to ~ 0.3 (Fig. 3b). Comparing the CrC-NiCr + 5% (CaF 2 + BaF 2 ) coatings with and without silver shows that the addi- tion of silver deteriorates the overall high temperature wear behavior of the coating (compare Figs. 3b and 3c). The heavy noise indicates adhesive galling at the initial stages and at the end of the pin-on-disc wear test. The good candidate shows very little noise, indicating excellent low wear characteristics with no galling and a low friction coefficient (Fig. 3b). Based on this study, the Triboglide coating without silver was chosen as the candidate for testing on the Honeywell foil bearing shaft. Figure 3d shows the wear graph for the PS 304 coating, which exhibits periodic spikes that indicate a buildup of transfer film and possibly enhanced wear due to this buildup. Such wear 11 Fig. 3 — Wear resistance of (a) substrate; (b) CrC-NiCr + 5% (CaF 2 + BaF 2 ); (c) CrC-NiCr + 5% (CaF 2 + BaF 2 + Ag); and (d) NASA PS 304 coating. (d) (a) (b)