Innovative Coating Developments Using a New, User Friendly HVOF and Metal Based Powders

G. R. Heath*, A. Tremblay, P. Andersson

Castolin Eutectic, Switzerland *E-mail: gheath@castolin.com

A. Arizmendi Morquecho Advanced Materials Research Center, Monterrey, Mexico

Abstract

The practical experience with the new Castolin Eutectic CJK5 High Velocity Oxy Fuel thermal spray unit has grown since its successful launch in 2011. It has many design features that simplify the user interface and servicing, but maintaining outstanding coating quality. The technical and practical features are described with detailed microstructural characterization of the resulting carbide based coatings using an established powder composition from several powder sources. With the recent development of a state-of-the-art metal powder manufacturing plant in Europe, highest quality “satellite-free” metal powders can be manufactured. New self-fluxing chemistries have been developed that are also low in Nickel and are designed to have good wear resistant properties. With the growing practical experience and parameter optimization, several innovative powders and coatings have been developed and are presented. Microstructural and mechanical properties and their relationship to processing parameters are presented.

Introduction Since the launch of the TAFA JP5000 the High Pressure oxygen/kerosene HVOF system has grown in popularity and became an established technology in the thermal spray industry (Ref 1) for delivering high quality coatings and a reasonable operating cost. Over the years, a few other related systems have entered the market (Ref 2). These range from entry level units (which have expanded the market but are limited on quality and reproducibility) or higher end units with more focus on sophisticated control for OEM and aero applications. The later models have traded size and complexity for reproducibility, reliability and monitoring. However, there is a market demand for a high quality kerosene HVOF system, which has the advantages of portability, improved ease of use, lower operating costs (deposition efficiency, capital costs, reliability, reproducibility, consumables, etc.). HVOF portability has long been an important requirement and units

have been launched on the market (Ref 3). The newly launched Castolin Eutectic CJK5 unit meets these market demands and this publication will present the latest results of development work with the unit for a range of materials. In the end, it is the coating quality which is the important product resulting from the HVOF unit, optimized powder and optimized spray parameters. This paper will highlight the sensitivity of the coating characteristics to powder source, powder type and spray parameters. In the field of HVOF technology, several popular coatings have evolved, for example in the area of hard chrome replacement (Ref 4, 5) and these will be tested/optimized to establish the quality of CJK5 in delivering equivalent or better quality than the current technology. In addition to the established HVOF powders and coatings, there is a constant demand to develop new coatings with improved performance that also meet the changing commercial and environmental needs. Two such needs that have developed are to reduce the amount of nickel that is used in certain alloys (for cost, environmental and health issues), and also to develop alternative alloys that rely less on high carbides content (WC, CrC) to give good coating wear properties. One option is to use metallic-based self-fluxing alloys which are available most commonly with Ni-Cr-B-Si compositions (Ref 6) and have a hardness range typically between 35-60 HRC. There is a long tradition of successful applications of self-fluxing alloys in tough wear and corrosion environments, including boiler coating (Ref 7), oil, etc. applied and fused with a variety of technologies. Corrosion resistance of these alloys is particularly good due to the high nickel, chrome and closed porosity (Ref 8). Recently, iron-based self-fluxing alloys have been developed but are not widely known or used in the HVOF industry. Several compositions of self-fluxing alloys were manufactured in a new powder production plant using water-water and dry gas atomization. The latter has an advanced system to produce satellite-free particles.

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Table 2: Fixed test conditions in PART A spray trials

Powder WC/10Co/4Cr Robot movement Traverse: 300 mm/s Pitch: 5 mm Substrate S235JR (300 x 200 x 5 mm) Spray Time 1.2 mins Powder Feed 80 g/min

Powder Samples All of the WC/10Co/4Cr powders that were sprayed were purchased from established manufacturers and suppliers of tungsten carbide HVOF powders. The metallic powders sprayed were manufactured at Castolin Eutectic Ireland Limited, in Dublin, Ireland which possesses a state-of-the-art gas-gas powder atomization tower, incorporating a novel design that can produce “satellite free” metallic powders. These have better flow parameters, a lower specific surface area and have been shown to give better quality coatings and deposition characteristics over conventional metal powders. Scanning Electron Microscope (SEM) images of the powders are given in Figure 3, to show the difference between conventional and a satellite-free, metallic powder.

Figure 3: SEM photos of metallic powders produced with conventional and satellite-free powder atomizer towers. Sample Preparation and Testing In Table 3 is an overview of the parameters used for analyzing the various coatings sprayed. Specific details are below. Sample preparation: Standard 300 x 200 x 5 mm S235JR mild steel plates were used for all tests. Prior to spraying they were grit blasted to remove all rust, oxide and any dirt. The sample was then weighed before and after spraying, and the amount of powder sprayed was also weighed before and after. Microscopy: After spraying the samples were sectioned using a Presi Mecatone T260 cut off machine, mounted and polished on a Presi Mecapol P230 polishing machine. All samples were studied in cross-section on a Leica DM4000M optical microscope. The samples were not etched. Porosity measurement: Coating porosity was measured by two distinctly different methods. One was with the polished cross section in the optical microscope using the Motic Images

Advanced 3.2 image analysis software. The other method used was the water absorption method (Ref 9) performed at the Swedish research institute Swerea IVF AB. Microhardness: Microhardness on WC/Co/Cr samples was measured on polished cross sections on a Vickers hardness machine with a 300 g load (HV0.3). For the NiCrBSi and Fe-based self-fluxing coatings hardness was measured using a Vickers hardness machine with a 2 kg load (HV2). All values quoted are average values. Bond strength measurement: Bond strength was measured at the Swedish research institute Swerea IVF AB using the EN 582 procedure. Surface roughness measurement: The surface roughness of the samples in PART A trials was measured and standard Ra and Ry values were given in microns. Table 3: Test procedure overview

Porosity Water absorption + image analysis

Bond strength EN 582 Coating thickness Micrometer and in optical

microscope Hardness HV0.3 and HV2 Flow ability Hall flow Powder chemistry Induction Coupled Plasma Particle size distribution JEL sieves DE% EN ISO 17836 Powder feeding Weighing and ocular from

gun Parameter optimization Varying parameters Temperature influence Temperature measurement Surface roughness Ra and Ry

Results PART A An overview of the PART A trial results are given in Table 4, with the number of the sample corresponding to the spray conditions given in Table 1. What is clear is the large variation in the values of DE (%), hardness and surface roughness, and quite a poor correlation between the parameter in terms of quality. The best all-rounder appears to be sample 16, which offers a HV0.3 value of 1229 and a DE of 42% with one of the lowest Ra/Ry values. However even higher DE values were measured at 46% in 2 samples but at the expense of lower hardness values. The highest HV0.3 values of 1423 and 1351 were obtained but both at a DE of 32%.

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The importancessentially idepecification.

observed in thbigger differenwere optimizedNo. 4. Here it around 2% doparameters. Atwith increasingbetween hardnystems and

correlation canand hardness However, the where a directDE.

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The new CJKproducing highof the powder n detail and mportant was

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chemical/hardnprovide an adwear and corroalloys were development oexcellent mechpreliminary wopowders. The program to coystem. Futur

hardness and w

Di

eral interestingw CJK5 HVOhigh quality, re00 HV0.3 andof up to 46%this. The d

duced with thquid fuel HVOrosity and hard

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n be found. Ais found for opposite was

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onfirm sensitivre work on wear results of

iscussion

g things that caF unit has proeproducible cod porosity le

% for the WC/direct comparhe same powOF system, shdness.

source was aders in termsences were measurement. erved when thfor a single po to reduce thepurely with thme the hardn

However, if a osity for the rboth A and

A stronger corWC/10Co/4Cfound for the

ardness was a

i- and Fe-bases of their very

%.

nclusions

t has proven roducible coatie one alloy (Whave an impof parameter e first results oing metallic p

osity below 1%y of these self

conventionaltions. The resu

and could t, more ecolorties. The abov

w system and be repeated invities of coatithe sensitivit

f these alloys a

ame out of thioven that it is coatings. The hevels of 0.3%/10Co/4Cr chrison with idwders on a hows superior

lso demonstras of chemist

most signif However, ahe CJK5 parawder, that of Pe porosity levhe changing oness reduced correlation is range of para

B trials, norrelation betwe

Cr powders (ine self-fluxing achieved with

ed self-fluxingy low porosity

that it is capings. The imp

WC/10Co/4Cr) portant effect.

optimization of HVOF sprapowders show%. The high Df-fluxing alloyl HVOF coatults for the Fe

be the basogical coatingve results are jalso with som

n the on-goining quality wty of DE, poand others.

is work. capable ardness

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r results

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References 1. J.R. Davis, Introduction to HVOF Thermal Spray

Processing Handbook of Thermal Spray Technology, ASMI and TSS 2004, p 3-13

2. R. Schwetzke and H. Kreye, Microstructure and Properties of Tungsten Carbide Coatings Sprayed with Various High-Velocity Oxygen Fuel Spray Systems, Journal of Thermal Spray Technology, Volume 8, Number 3, 433-439, September 1999

3. G.R. Heath, R.J.Dumola, M. Dvorak and R. Torday, Practical Experience with the New Generation of Low-Cost, Portable HVOFs, International Thermal Spray Society 1998, poster.

4. HCAT, “The Hard Chrome Alternatives Team Introduction to Thermal Spray Processing” http://www.hcat.org/about.html, Feb. 25, 2008

5. G. Bolelli, R. Giovanardi, L. Lusvarghi and T. Manfredini, Corrosion Resistance of HVOF-Sprayed Coatings for Hard Chrome Replacement, Corrosion Science, 2006, Vol. 48, Issue 11, p 3375-3397

6. M.P. Planche,, H. Liao, B. Normand and C. Coddet, Relationships between NiCrBSi Particle Characteristics and Corresponding Coating Properties Using Different Thermal Spraying Processes, Surface and Coatings Technology, Volume 200, Issue 7, 21 December 2005, Pages 2465–2473

7. G.R. Heath, R. Polak, T. Tsujita and S. Gustafsson, Role of Thermal Spray Coatings in Boilers; Development of Reliable Spray and Fuse Coatings for Boiler Protection, National Thermal Spray Society, 1996, p 1-8

8. G.R. Heath, P. Heimgartner, G. Irons, R. Miller and S. Gustafsson, An Assessment of Thermal Spray Coating Technologies for High Temperature Corrosion Protection, 4th International Conference on High Temperature Corrosion, Les Embiers (1996)

9. J. Skogsmo and P. Andersson, Comparison of Different Techniques for Porosity Measurements of Thermally Sprayed Coatings, MC International Metallography Conference, Colmar, France, 10-12 May 1995