Spraytime

Introduction

For decades, chrome plating and, more recently, high-velocity oxygen fuel (HVOF) applied coatings have been used on key components in the oil and gas industries to give enhanced protection in aggressive environments. Today, there is a newer application method that is challenging the HVOF status quo, and that is high-velocity air fuel (HVAF). HVAF application methods can produce denser coatings than HVOF methods, allowing for the same performance to be achieved in thinner coatings (Refs. 1, 2). HVOF applications are in the range of 200–250 µm, while HVAF can achieve the same level of performance with 50–100 µm. This is driven by HVAF’s higher particle velocities, which reach up to 1300 m/s (Ref. 3).

These flash carbide coatings require finer sieve-sized powder as a feedstock. However, this trend toward the use of flash carbides can produce issues. Very fine particle size distribution (PSD) powders may lead to more irregularly shaped particles due to the inhomogeneous structure of the individual particle impairing the flowability of the powder. This can be partially compensated by using smaller primary particles. Also, the production of finer PSD powders in the standard powder manufacturing processes can result in lower available volumes compared to the growth in demand and, going forward, could disrupt the security of supply and add price pressure, eroding the cost benefit of applying the thinner coatings. WC10Co4Cr has an annual market size of $200 million for HVOF applications (Ref. 4). Methods to produce fine HVAF-suitable powders in larger volumes are required to be able to take full advantage of the benefits of HVAF flash carbide applications.

Höganäs has developed a manufacturing method to produce WC10Co4Cr powders in fine PSD but with the capability of large volumes. The assessment of this new powder, named Amperit® 658.067, on key properties is the focus of this work.

Experiment

The new powder was trialed with other existing established Höganäs fine PSD WC10Co4Cr powders (Amperit® 558.052 and 554.067). These WC10Co4Cr powders were manufactured by various methods giving differing morphologies, as identified in Table 1. A scanning electron microscope (SEM) was used to observe the morphology of the powders — Fig. 1.

The three different powders were applied by HVAF, and the deposit efficiency (DE), roughness, hardness, and wear properties were then characterized. The testing was carried out with the support of University West, Trollhättan, Sweden, using their UniqueCoat M3™ HVAF system. Using four different application conditions gave variations of hotter and colder spray conditions. The four parameter sets were designated N1, N2, N3, and N4 — Fig. 2. (The details of the parameter sets are not given here as the focus of this article is on the observations in behaviors and properties between the different powder types.)

During spraying, the deposit rate in terms of thickness per pass was monitored. The powder P3 (Amperit 658.067) was observed to be more consistent and in most cases gave a higher DE across the various spray conditions evaluated, except for the N1 spray condition — Fig. 2.

The fine PSD used in the HVAF process led to a very smooth as-sprayed coating surface. The measured values for the average surface roughness (Ra) are shown in Table 2 and range from 1.56 to 2.4 μm. The three powders showed minor differences, while the spray condition had a significant influence.

Metallographic examinations of the samples were also carried out. In all cases, dense coatings were achieved as desired. Typical structures for each are shown in Fig. 3.

The as-sprayed samples were evaluated by microhardness using a Struers Duramin-40 tester with ten indentations at a normal load of 2.94 N (300 gf) and a dwell of 10 seconds — Fig. 4.

The results showed that the hardness achieved per spray condition for the three different powders was similar — Fig. 4. The N1 spray condition showed lower hardness and considerable variation. N2 to N4 conditions gave a hardness of 1300–1500 HV0.3.

Sliding wear was evaluated using a ball-on-disc test on a tribometer Anton Paar TRB3. The linear speed was 20 cm/s, at a 20 N, and a traverse length of 5000 m.

The sliding wear of all samples was low, and results were comparable within the variability of measurement results across all powders and spray conditions — Fig. 5.

Conclusion

The work carried out with these three WC10Co4Cr powders showed that for the key properties of as-sprayed roughness, hardness, and wear, the parameters had more influence on the results observed than the powder feedstock. Results across these powders were similar. The new Amperit® 658.067 produced from the new manufacturing route gave improved DE and an equivalent performance to existing HVAF powders but with higher-volume capability.

The Höganäs 600 series of powders are the designation assigned for sustainable powder solutions. These powders are designated as such because they remove key elements, such as cobalt and nickel. They also actively reduce the use of material required for a given application. The new method of manufacture allows for larger volumes of fine PSD powders to be achieved and is also applicable to compositions other than just WC-Co-Cr. This higher-volume production process enables wider use of flash carbides and thin HVAF coatings, which increases the sustainability for such applications as more can be done with less.

References

1. Verstak, A. A., and Gries, B. 2018. HVAF flash carbide as economical alternative to electroplated hard chrome. ITSC 2018 International Thermal Spray Conference and Exposition — 2018 Conference Proceedings. Materials Park, Ohio: ASM International.
2. Lanz, O., and Gries, B. 2019. HVAF — Chance and challenge for users and for powder producers. International Thermal Spray Conference and Expositon (ITSC 2019). Materials Park, Ohio: ASM International. DOI: 10.31399/asm.cp.itsc2019p0015
3. Kumar, R. K., Kamaraj, M., Seetharamu, S., Pramod, T., and Sampathkumaran, P. 2016. Effect of spray particle velocity on cavitation erosion resistance characteristics of HVOF and HVAF processed 86WC-10Co4Cr hydro turbine coatings. Journal of Thermal Spray Technology 25(6): 1217–1230. DOI: 10.1007/s11666-016-0427-3
4. Thermal Spray Coatings Market Size Report 2022–2030. Grand View Research.

David Sansom (david.sansom@hoganas.com) is vice president of technology, North American Höganäs Co., USA; Alexander Barth (alexander.barth@hoganas.com) is director technology EMEA, Höganäs GmbH, Germany; and Arashk Memarpour (arashk.memarpour@hoganas.com) is vice president of product development, Höganäs AB, Sweden