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

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.

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)

Heating target to a preset temperature

Ambient air cooling with heater OFF

Air knife cooling with heater ON

SST – steady state tempera-ture indicating the heat balance between the 6 kW heater and the cooling medium tested

PART 1:

EXPERIMENTAL DETERMINATION OF COOLING CAPACITY OF COMPRESSED AIR AND CRYO-JET FLUIDS, APPARATUS AND PROCEDURE

F Tf

Ctrl

Dac

X

F

Nozzle

Tc Tf AC

Target

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 Tf – face thermocouple embedded in wall Tc – temperature control thermocouple X – spray-cooling distance Ctrl – power controller maintaining preset target temperature, e.g. 300oF Dac – data acquisition system

APPARATUS CALIBRATION RUNS – EXAMPLES OF COMPLETE THERMAL PROFILE RECORDS

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

WC-10Co-4Cr Stellite/Jet-Kote WC-10Co-4Cr Sulzer-Metco

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

The Stellite/Jet-Kote powder is less sensitive to oxidation in air and CO2 atmospheres. It is selected for the further, spray-deposition studies to explore a more ‘conservative’ scenario.

No oxidative weight gain was found during TGA measurements under N2

Temperature sensing

FLIR A320 Thermal Imaging Camera

Wireless Ethernet Bridges

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)

JetKote-II gun Air knives (4) Thermal camera

Substrate – steel pipe Chuck rotating substrate LIN-GAN and/or

LCO2 nozzles

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)

Temperature control system used in the LIN-GAN spray-cooling tests maintains substrate temperature precisely at the operator’s preselected level.

DE evaluated per ISO 17836, 2004 (E), Annex A.

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0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 Ave

rage

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rfac

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em

pe

ratu

re, T

avg,

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t, d

eg.

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Spray-coating operation time, minutes

Multi-point, time-average substrate temperature measured during spray-coating,

left-middle-right control areas (Tavg) at component surface, 100 sec. averaging Air: 45%DE Tavg = 386oF Std.Dev.= 9.7%

LCO2-Air: 49% DE, Tavg = 318oF, T Std.Dev.= 10.1%

LIN-GAN-Air: 55% DE, Tavg = 308oF, T Std.Dev.= 6.8%

- Air only cooling (4 compressed air knives) - LCO2 combined with compressed air cooling - LIN-GAN combined with air cooling (automated T-control mode)

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.

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

SEM images of as-sprayed coating surfaces (original magn. x 4,000)

1. Air knives only cooled, 45.4% DE 2. LCO2-2.2 lbs/min and Air knives cooled, 49.3% DE 3. LCO2-2.2 lbs/min, 49.5% DE 4. LIN-5.0 lbs/min, 50.2% DE 5. LIN+GAN - 1.7 lbs/min, 54.3% DE 6. LIN-5.5 lbs/min and Air knives , 54.5% DE 7. LIN-GAN - 1.8 lbs/min and Air knives, Ctrl., 54.8% DE

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4.47

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Air-knives LCO2-2.2/Air LIN-1.8GAN/Air JK120H powder

DE –

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SEM1 pores %

C (wt%)

O (wt%)

Cr (wt%)

DE

DE

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)

DE evaluated per ISO 17836, 2004 (E), Annex A.

Parameters Value

Fuel Gas (H2) 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) 0.125

Spray distance (in) 8

Target powder feeder rate (g/min) 45

Part rotation (rpm) 80

Number of passes 40

Substrate preheat temp. (°C) N/A

Substrate material AISI 1018 steel cylinders

Substrate dimensions 200 mm long x 150 mm Ø x 3 mm wall

thickness, as per ISO 17836, 2004 (E),

Effect of cooling method on phase composition of deposited coatings

coating

substrate

X-ray diffraction surface

Overlay of XRD scans on different coatings

Conclusions: 1. LIN and LIN-GAN spray cooling are somewhat more effective on the mass flowrate basis than LCO2 cooling within the temperature range of interest (100oF-350oF). LIN, LIN-GAN, and LCO2 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 CO2. 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 LCO2 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.