Influence of substrate cooling method on compositional shift and deposition efficiency of HVOF-sprayed WC-Co type coatings

Transcription

1 Influence of substrate cooling method on compositional shift and deposition efficiency of HVOF-sprayed WC-Co type coatings Z. Zurecki, L. A. Mercando and R. Ghosh, Air Products and Chemicals, Inc., Allentown, Pennsylvania; and R. Knight, Drexel University, Philadelphia, Pennsylvania Air Products and Chemicals, Inc., 2013

2 Abstract: Modern, high-throughput HVOF and plasma coating operations require an effective and precise control of substrate temperatures during deposition in order to achieve optimum performance of the final part and minimize operation time. Although frequently overlooked, substrate cooling method may influence interfacial stress-controlled coating adhesion, as well as deposition efficiency of powder, and oxidation resultant compositional shift in the deposited material. This study presents results of experimentation comparing cooling rates of selected media including compressed, room temperature air and nitrogen, cryogenically liquefied carbon dioxide (LCO2), and nitrogen (LIN). Oxidizing effect of these cooling media on two types of WC- CoCr feed powders is also reported. HVOF spraying tests show, that substrate cooling media may affect coating oxidation in a similar manner as feed powders. Chemical analyses of deposited coatings point to a close relationship between the extent of oxidation under cooling media and powder deposition efficiency. Cooler and more inert gases are found to maximize deposition efficiency. Presented results include SEM and XRD analysis.

3 Objective Determine effect of substrate cooling method on deposition efficiency and composition of coatings in HVOF spraying of WC-CoCr powders Outline PART 1: Experimental determination of cooling capacity of compressed air and cryogenic fluids PART 2: Oxidation kinetics-based selection of WC-10Co-4Cr powder for the present HVOF spray deposition study PART 3: Test set-up used to measure deposition efficiency and composition of WC-10Co-4Cr JK120 coatings as a function of substrate cooling method (LIN, LCO2, GAN, and compressed air)

4 PART 1: EXPERIMENTAL DETERMINATION OF COOLING CAPACITY OF COMPRESSED AIR AND CRYO-JET FLUIDS, APPARATUS AND PROCEDURE APPARATUS CALIBRATION RUNS EXAMPLES OF COMPLETE THERMAL PROFILE RECORDS X Ctrl AC Dac T c T f Target Nozzle F Heating target to a preset temperature Ambient air cooling with heater OFF Air knife cooling with heater ON T f Target for measuring jet cooling rates comprises stainless steel tube (6 dia x 8 long) with internal heater (6 kw), packed-bed copper powder filling and thermocouples embedded in the tube wall. F cooling fluid T f face thermocouple embedded in wall T c temperature control thermocouple X spray-cooling distance Ctrl power controller maintaining preset target temperature, e.g. 300 o F Dac data acquisition system F SST steady state temperature indicating the heat balance between the 6 kw heater and the cooling medium tested

5 TEST RESULTS: TARGET COOLING RATE AS A FUNCTION OF GAS TYPE, FLOWRATE, DISCHARGE PRESSURE, AND NOZZLE DISTANCE Temperature of electrically (6 kw) heated target surface during cryo-jet cooling as measured with embedded thermocouple

6 PART 2: Oxidation kinetics-based selection of WC-10Co-4Cr powder for the present HVOF spray deposition study [1] Sulzer-Metco 5847 powder Sprayed with DJ2600 gun, this powder has already shown significant deposition efficiency improvements in commercial operations when coating operation was combined with liquid nitrogen substrate cooling. [2] Stellite/Jet-Kote JK120H powder; in this study sprayed with Jet Kote II Nova gun WC-10Co-4Cr Sulzer-Metco WC-10Co-4Cr Stellite/Jet-Kote

7 No oxidative weight gain was found during TGA measurements under N 2 The Stellite/Jet-Kote powder is less sensitive to oxidation in air and CO 2 atmospheres. It is selected for the further, spray-deposition studies to explore a more conservative scenario.

8 PART 3: Test set-up used to measure deposition efficiency and composition of WC-10Co-4Cr JK120 coatings as a function of substrate cooling method (LIN, LCO 2, GAN, and compressed air) DE evaluated per ISO 17836, 2004 (E), Annex A. Thermal camera Air knives (4) JetKote-II gun Flow control panel used in the LIN-GAN spray-cooling tests (where liquid nitrogen is mixed with gaseous nitrogen in a patented spray atomizing nozzle) Chuck rotating substrate Substrate steel pipe LIN-GAN and/or LCO2 nozzles Temperature sensing Wireless Ethernet Bridges FLIR A320 Thermal Imaging Camera Temperature control system used in the LIN-GAN spraycooling tests maintains substrate temperature precisely at the operator s preselected level.

9 Average surface temperature, Tavg, of mponent, deg.f Temperatures recorded during spray deposition tests, where the LIN-GAN temperature was pre-selected by operator while the LCO2 and compressed air temperatures were the result of the maximum cooling capacity of these media. Multi-point, time-average substrate temperature measured during spray-coating, left-middle-right control areas (T avg ) at component surface, 100 sec. averaging 400 Air: 45%DE Tavg = 386 o F Std.Dev.= 9.7% LCO 2 -Air: 49% DE, T avg = 318 o F, T Std.Dev.= 10.1% LIN-GAN-Air: 55% DE, T avg = 308 o F, T Std.Dev.= 6.8% Air only cooling (4 compressed air knives) - LCO 2 combined with compressed air cooling - LIN-GAN combined with air cooling (automated T-control mode) Spray-coating operation time, minutes

10 Effect of cooling medium flowrate on average substrate temperature recorded for LIN and LCO2 during deposition efficiency runs.

11 SEM images of as-sprayed coating surfaces (original magn. x 4,000) 1. Air knives only cooled, 45.4% DE 2. LCO lbs/min and Air knives cooled, 49.3% DE 3. LCO lbs/min, 49.5% DE 4. LIN-5.0 lbs/min, 50.2% DE 5. LIN+GAN lbs/min, 54.3% DE 6. LIN-5.5 lbs/min and Air knives, 54.5% DE 7. LIN-GAN lbs/min and Air knives, Ctrl., 54.8% DE

12 Wt%: C, O, and Cr; vol% pores DE Deposition Efficiency wt% of powder sprayed at substrute Effect of cooling method on powder DE (deposition efficiency), and coating composition (C-carbon, O-oxygen, and Cr-chromium using Leco) and cross-section porosity (using SEM image analysis) Air-knives LCO2-2.2/Air LIN-1.8GAN/Air JK120H powder DE 54% 52% 50% 48% 46% 44% 42% 40% 38% 36% 34% 32% 30% 28% 26% 24% 22% 20% 18% 16% 14% 12% 10% 8% 6% 4% 2% 0% DE evaluated per ISO 17836, 2004 (E), Annex A. SEM1 pores % C (wt%) O (wt%) Cr (wt%) DE

13 Effect of cooling method on phase composition of deposited coatings X-ray diffraction surface coating substrate Parameters Value Fuel Gas (H 2 ) flow rate (scfh) 1300 Oxygen flow rate (scfh) 600 Powder carrier Ar flowrate (scfh) 60 Nozzle length/diameter 6 L x 1/4 ID HVOF torch traverse speed (in/s) 0.17 Step size (in) Spray distance (in) 8 Target powder feeder rate (g/min) 45 Part rotation (rpm) 80 Number of passes 40 Substrate preheat temp. ( C) Substrate material Substrate dimensions N/A AISI 1018 steel cylinders 200 mm long x 150 mm Ø x 3 mm wall thickness, as per ISO 17836, 2004 (E), Overlay of XRD scans on different coatings

14 Conclusions: 1. LIN and LIN-GAN spray cooling are somewhat more effective on the mass flowrate basis than LCO 2 cooling within the temperature range of interest (100 o F-350 o F). LIN, LIN-GAN, and LCO 2 methods are significantly more effective than the traditional compressed air cooling. 2. Popular WC-10Co-4Cr powders, Sulzer-Metco 5847 and Stellite/Jet-Kote JK120H, oxidize in air and, to a lesser extent, in CO 2. The JK120H powder is more oxidation resistant due to particle morphology. 3. Automated temperature control of the LIN-GAN system assures a complete thermal uniformity of substrate part during HVOF spraying. 4. LIN-GAN cooled HVOF coatings are less oxidized and less decarburized than coatings cooled by LCO 2 or compressed air and retain the highest amount of the desired, tough and hard WC phase. 5. LIN-GAN cooling offers the highest powder deposition efficiency and reduced coating porosity levels.