THERMAL SPRAY FOR STEEL INDUSTRY – Part I: Furnace Rolls

The steel production process exposes much of its hardware to very arduous environments.  High wear, corrosion and/or high temperatures are often encountered in many segments of steel production. Since the early 1970s, thermal spray coatings have apparently been used to minimize maintenance, extend service life, and to enhance performance/quality of steel facilities.  Although there are many references to different thermal spray applications in steel mills , there is limited mention of the specifics relating to these applications beyond patents.  The exception to this is the thermal spray protection of processing rolls which is known to be widely used around the world .  This and the next two blogs will discuss thermal spray applications of furnace (Part I), continuous galvanizing (Part II), and cold forming (Part III) rolls.

Furnace rolls are used in continuous annealing lines where the steel strip is heat-treated to enhance ductility .  Ideally, these rolls should resist high-temperature corrosion, thermal cycling, wear, and foreign particle pick-up.  In addition, a certain degree of surface roughness is required to mitigate slip and strip meandering.  For working temperatures of up to 850°C, CrC-NiCr coatings are often used; however, higher temperature environments incorporate materials often seen in gas turbine engine applications - namely, MCrAlYs.  At temperatures up to 1100 °C, MCrAlYs are combined with oxides such as Al2O3 to provide simultaneous wear, hot corrosion, and oxidation resistance .  For even greater temperatures, monolithic oxide ceramic top coats are applied over an MCrAlY bond coat.  The top coats are composed of one or more combinations of ZrO2, SiO2, Cr2O3, TiO2, Al2O3, Y2O3, CeO2, etc.

There is an obvious need to develop more reliable and high performing coatings for the harsh environment found in the steel industry.  Unless new materials that can handle these harsh conditions are developed, advanced coating materials and coating processes will be required to meet the challenge.

The steel production process exposes much of its hardware to very arduous environments.  High wear, corrosion and/or high temperatures are often encountered in many segments of steel production. Since the early 1970s, thermal spray coatings have apparently been used to minimize maintenance, extend service life, and to enhance performance/quality of steel facilities.  Although there are many references to different thermal spray applications in steel mills , there is limited mention of the specifics relating to these applications beyond patents.  The exception to this is the thermal spray protection of processing rolls which is known to be widely used around the world .  This and the next two blogs will discuss thermal spray applications of furnace (Part I), continuous galvanizing (Part II), and cold forming (Part III) rolls.

Furnace rolls are used in continuous annealing lines where the steel strip is heat-treated to enhance ductility .  Ideally, these rolls should resist high-temperature corrosion, thermal cycling, wear, and foreign particle pick-up.  In addition, a certain degree of surface roughness is required to mitigate slip and strip meandering.  For working temperatures of up to 850°C, CrC-NiCr coatings are often used; however, higher temperature environments incorporate materials often seen in gas turbine engine applications - namely, MCrAlYs.  At temperatures up to 1100 °C, MCrAlYs are combined with oxides such as Al2O3 to provide simultaneous wear, hot corrosion, and oxidation resistance .  For even greater temperatures, monolithic oxide ceramic top coats are applied over an MCrAlY bond coat.  The top coats are composed of one or more combinations of ZrO2, SiO2, Cr2O3, TiO2, Al2O3, Y2O3, CeO2, etc.

There is an obvious need to develop more reliable and high performing coatings for the harsh environment found in the steel industry.  Unless new materials that can handle these harsh conditions are developed, advanced coating materials and coating processes will be required to meet the challenge.

Coated Hearth Roll

REFERENCES

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[i] Introduction to Thermal Spray Processing, Handbook of Thermal Spray Technology, 2004, pp. 3-13.

[ii] R.L. Hao, Thermal Spraying Technology and Its Applications in the Iron & Steel Industry in China, International Thermal Spray Conference and Exposition (May 14 - 16, 2007) Beijing.

[iii] S. Matthews, B. James, Review of Thermal Spray Coating Applications in the Steel Industry: Part 1—Hardware in Steel Making to the Continuous Annealing Process, Journal of Thermal Spray Technology Volume 19(6) December 2010, pp. 1267-1276.

[iv] T.-S. Huang, C.-S. Yu, J.-R. Wu, H.-Y. Liou, S.-H. Hsieh, T.-K. Huang, Effect of Mn on the Formation of Pickups on Thermal Spray Coatings, thermal Spray 2009: Proceedings of the International Thermal Spray Conference, pp. 595-600.

[v] Bender US website - http://www.benderus.com/Steel%20Coatings/SteelCoat.html.

MANUAL AND AUTOMATED THERMAL SPRAYING

In most cases, thermal spraying is carried out in spray booths either through a manual (handheld) or automated (robotic, x-y manipulators, etc.) method.  There are several key reasons for using automated systems.  These include: health and safety; quality and reproducibility; and productivity and economics.

For obvious reasons, the use of automated methods can notably reduce risks to workers by limiting exposures[i]^{,[ii],[iii],[iv]} to:

• ·                Gases and vapors;
• ·                Dust/powder;
• ·                Noise;
• ·                Radiation;
• ·                Heat;
• ·                Electric Shock.

Among the numerous dangers associated with thermal spray application, respiratory issues relating to exposure to gases and vapors, dust particles, and toxic metals are one of the most significant concernsⁱⁱ.  By contrast, the principal danger associated with robots is that a robot does not have any detection capability and a collision with a worker in its pre-defined trajectory can occurⁱⁱ. 

Automation can also help enhance quality and reproducibility through its ability to accurately control several aspects of the gun/torch positioning and movement, i.e., working distance, translational speed, pitch/increment between passes, and spray angle^i,iv^,[v],[vi].  This is most important for complex-shaped components that require high thermal and mass transfer tolerances^{iv,v,vi,[vii],[viii]}.  McDonald et al.[ix] provide helpful general information on robot programming for thermal spraying. 

Example of F.W. Gartner automated thermal spray application.

In addition to enhancing coating quality and reproducibility, incorporating automated thermal spraying can also increase productivity and makes good economic sense, especially in high-volume production^i,iv,[x].  Kutay[xi] at Carnegie Mellon University provides some insight into the economic impact of automation technology.

Even with the advantages of incorporating automation into a thermal spray process, numerous applications require manual spraying with protective personal gear, i.e., bridges, boilers, artwork, architecture, etc.  Also, an automated system can require considerable preplanning and programming which can take time and affect profitability.  Hence, it is not always feasible to remove the human factor from all applications.

Example of F.W. Gartner manual thermal spray application.

REFERENCES

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[i] D.E. Crawmer, Process Control Equipment, Handbook of Thermal Spray Technology, J.R. Davis-editor, ASM International and the Thermal Spray Society, 2004, pp. 85-98

[ii] H. Heriaud-Kraemer, G. Montavon, S. Hertert, H. Robin, C. Coddet, Harmful Risks for Workers in Thermal Spraying: A Review Completed by a Survey in a French Company, Journal of Thermal Spray Technology Volume 12(4) December 2003, pp. 542-554

[iii] C.P. Howes, Thermal spray safety and OSHA compliance, THEFABRICATOR.COM, July 2001, http://www.irmaassociates.com/articles/Thermal_Spray_Safety_and_OSHA_Compliance.pdf

[iv] Z. He, B. Lu, J. Hong, Y. Wang, Y. Tang, A novel arc-spraying robot for rapid tooling, Int J Adv Manuf Technol (2007) 31: 1012-1020

[v] A. Kutay, L. Weiss, Economic Impact of Automation: The Case of Robotic Thermal Spraying, The Robotics Institute – Carnegie Mellon University, March 1990, http://www.cs.cmu.edu/~lew/PUBLICATION%20PDFs/TECH%20REPORTS/CMU-RI-TR-90-07.pdf

[vi] A. Frutos, Numerical analysis of the temperature distribution and Offline programming of industrial robot for thermal spraying, Final Thesis, University of Stuttgart, January 2009

[vii] P.D.A. Jones, S.R. Duncan, R. Rayment, P.S. Grant, Optimal Robot Path for Minimizing Thermal Variations in a Spray Deposition  Process, IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 15, NO. 1, JANUARY 2007, pp. 1-11

[viii] W. Xia, H. Zhang, G.-L. Wang, Y. Yang, G. Han, H. Zou, Integrated Robotic Plasma Spraying System for Advanced Materials Processing, PIERS Online, Vol. 4, No. 8, 2008, pp. 876-880

[ix] A. McDonald, K. Schoof, B. Harvey, Thermal Spraying Training Module: For As-sprayed (un-polished) Coatings, http://www.ualberta.ca/~andre2/files/McDonald_Training%20Module.pdf

[x] D. Breen, Thermal Robotic Arm Controlled Spraying via Robotic Arm and Vision System, Doctoral Thesis, Dublin Institute of Technology, Ireland, January 2010

[xi] A. Kutay, The Economic Impact of Automation Technology, The Robotics Institute – Carnegie Mellon University, July 1989, http://www.ri.cmu.edu/pub_files/pub3/kutay_aydan_1989_1/kutay_aydan_1989_1.pdf

Effect of carbide cermet powder morphology on coating characteristics

For novices to the field of thermal spray, the importance of selecting not only the proper particle size distribution and composition, but also the powder morphology, may be difficult to fully appreciate.  However, a brief literature search reveals several studies showing that there are a number of ways in which powder morphology may affect important coating properties.  Since the specific thermal spray process and parameters used in each study will also influence coating properties, the results presented in this blog were selected from sources where effort was made to isolate the effects of powder morphology.

Some of the coating properties affected by powder morphology include: hardness; carbide decomposition; microstructure (porosity, carbide particle retention, eta-phase formation); abrasive and erosive wear; and residual stress. Although agglomerated/sintered and sintered/crushed powders are more common types of carbide powder, some studies included clad as well as casted-crushed/fused and blended manufacturing methods. A study by Li et al.[i] studied the coatings derived from Jet-Kote and APS application of different WC-Co powder types (sintered/crushed, agglomerated/sintered, clad, casted-crushed/fused).  The results were as follows:

• The degree of carbide decomposition within the coatings seemed to be directly influenced by the level of decomposition in the spray powder - the greater the decomposition in the powder, the greater the decomposition in the coating.
• Metallic cobalt found in some of the starting powder was lost following spraying, which meant that all coatings experienced a certain level of carbide decomposition and reaction with the cobalt matrix.
• Dense coatings were formed using all but the casted-crushed/fused powder.
• Coatings formed using sintered/crushed, agglomerated/sintered, and clad powder with no carbide degradation retained some stoichiometric WC particles within the coating.
• The coating formed using clad powder had smaller carbide particles due to rebounding of larger carbide particles.
• The coating formed from sintered/crushed powder retained the most WC particles.

Li et al. [ii] studied different types of CrC-NiCr powders and their effects on abrasive and erosive wear performance of their respective coatings sprayed via HVOF.  The different types of powder in the study included: sintered/crushed; agglomerated/sintered; mechanically blended; and sintered.  The study produced the following findings, among others:

• The coating deposited with the blended powder fared poorest against abrasive and erosive wear.
• Sintered/crushed and agglomerated/sintered powders had similar abrasive and erosive wear performance.
• The sintered powder (Diamalloy 3003 NS) produced the most erosion resistant coating.
• All coatings were dense; however the content, size and distribution of carbide particles within the coatings varied.
• The wear performance of the coating is significantly influenced by carbide size and content.

In a study performed by Legoux et al. [iii]^{, [iv]}, sintered/crushed (SM 5843) and agglomerated/sintered (SM 5847) WC-CoCr powder were sprayed using HVOF processes.  The study produced the following results, among others:

• All coatings were dense, with uniformly distributed carbides and fine pores.
• The sintered/crushed powder produced a coating with slightly larger pores.
• The coating formed using the agglomerated/sintered powder had lower porosity and higher carbide degradation, which led to the presence of stringers (reaction of W with Co and Cr matrix).
• The coating formed using the agglomerated/sintered powder had superior Coriolis erosion and dry erosion resistance, as well as higher compressive residual stress and lower hardness.

WC-CoCr powder: sintered & crushed (a)+(b) and agglomerated & sintered (c)+(d)ⁱⁱⁱ

Wirojanupatump et al. [v] studied three types of commercially available CrC-NiCr powder to form HVOF coatings and compare their abrasive wear resistance.  The three types of powder were blended, sintered/crushed, and carbide activation (a Praxair technology that forms fine grained carbides), and their findings included:

• The coating deposited using sintered/crushed powder had the highest hardness, followed by the carbide activation and blended powder in that order.
• The abrasive wear resistance of the coatings increased in the following order: blended, sintered/crushed, and carbide activation.

As these studies demonstrate, powder morphology has a strong influence on coating microstructure and chemistry, which in turn dictates the coating properties.  Thus, morphology is an important factor to consider in selecting a powder for thermal spray processes.

REFERENCES

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[i] C.-J. Li, A. Ohmori, Y. Harada, Effect of powder structure on the structure of thermally sprayed WC-Co, JOURNAL OF MATERIALS SCIENCE  31 (1996) 785-794.

[ii] C.J. Li, K. Sonoya, G.-C. Ji, Y.-Y. Wang, EFFECT OF POWDER TYPE ON THE RELATIONSHIP BETWEEN SPRAY PARAMETERS AND PROPERTIES OF HVOF SPRAYED Cr3C2-NiCr COATINGS, Proceedings of the 15th International Thermal Sray Conference, 25 ·29 May 1998, Nice, France.

[iii] J.-G. Legoux, B. Arsenault, H. Hawthorne, J.-P. Immarigeon, Erosion Behavior of WC-10Co-4Cr HVOF Coatings, in Thermal Spray 2003: Advancing the Science and Applying the Technology, Proceedings of the International Thermal Spray Conference, 5-8 May 2003, B.R. Marple and C. Moreau eds., Published by ASM International, Materials Park Ohio, USA, 2003, pp 405-410.

[iv] J.-G. Legoux, S. Bouaricha, Evaluation of starting material and process parameters for HVOF WC-10Co4Cr coatings, Thermal Spray 2002: International Thermal Spray Conference (DVS-ASM), 2002, pp. 289 – 294 

[v] S. Wirojanupatump, P.H. Shipway, D.G. McCartney, The influence of HVOF powder feedstock characteristics on the abrasive wear behaviour of CrxCy-NiCr coatings, Wear 249 (2001) 829-837.

POTENTIAL FOR NANOSTRUCTURED COATINGS FOR BIOMEDICAL APPLICATIONS – SURFACE NANOTOPOGRAPHY

This blog is a continuation of a topic that was discussed in an earlier blog published in March 2010[i]. It will provide additional information pertaining to the enhanced bioactivity of thermal spray nanostructured coatings, especially with respect to surface nanotopography.

There have been several studies that show promising results relating to thermal spray nanostructured coatings for biomedical applications. Some of the key attributes of these nanostructured coatings include:

• ·       greater bioactivity;
• ·       enhanced adhesion to implant surface; and,
• ·       superior wear resistance and toughness

In addition to the nanostructured titania coating mentioned in the prior blogⁱ, research on atmospheric plasma sprayed (APS) nanostructured calcia-stabilized zirconia has revealed that the presence of nano-scale topography/roughness formed by nanogrooves, nanopits and/or nanoislands provided higher cell proliferation.  This work also showed that the formation of a nanostructured surface, in addition to nano-scale topography, further enhanced bioactivity[ii]^, [iii].  Wang et al. [iv] mentioned how zirconia coatings in nanostructure form improved hardness and toughness, leading to superior wear resistance.  They also reported that the introduction of carbon nanotubes has been shown to improve the mechanical properties of hydroxyapatite (HA), and that irradiating oxide coatings such as nanostructured zirconia can further enhance bioactivity.  A separate study revealed that APS-applied nanostructured zirconia coating was more sensitive to hydrothermal aging than bulk zirconia; hence, care must be taken when this coating is used in an environment with the presence of water and/or water vapors[v].

The enhanced bioactivity on nano-scale topography may be due to adsorption and interaction of nanosized protein such as fibronectin, which plays a major role in cell adhesion, growth, migration and differentiation[vi].  The fibronectin then acts as an anchor site for osteoblast cells.  This phenomenon is observed on both ceramic and metallic materials such as alumina, titania, zirconia, titanium and cobalt alloys.

A different approach towards attaining a nanostructured coating surface was proposed by scientists at the Shanghai Institute of Ceramics[vii]^, [viii].  Vacuum plasma spray (VPS) was used to deposit commercially pure titanium coatings with a thickness of around 100 µm.  The coated specimen were then alkali and heat treated to instill a nanotexture with open-cell, fibrous net-like nanostructure (see figure below).  Following the alkali treatment, the passive layer thickened from less than 20 nm to around 150 nm.  In addition, sodium was detected at the surface; hence, the nanotextured surface may have converted into Na₂Ti₅O₁₁.  A bond-implant contact test revealed a value of 60.5±6.2% for the alkali-treated coating and 20.2±8.5% for the as-sprayed Ti coating. This approach has demonstrated the ability to chemically alter the surface of Ti coatings from microtopographic to nanotopographic structures which can encourage cellular proliferation at the critical early stages following implant fixation.

It is clear from these studies that nano-scale surface structure/texture/roughness of nanostructured coatings can have a notable influence on osseointegration and thus on the performance of implants.

SEM photographs of the surface of plasma-sprayed titanium coatings: (a) as-sprayed, (b) after alkali modification and (c) higher magnification of (b)^viii.

REFERENCES

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[i] George Kim, http://blog.fwgartner.com/blog/2010/3/16/potential-for-nanostructured-coatings-for-biomedical-applica.html, March 16, 2010.

[ii] Guocheng Wang, Xuanyong Liu*, Chuanxian Ding, Effect of Surface Topography of Zirconia Coating

on its Bioactivity and Osteoblast Response, Nanoelectronics Conference (INEC), 2010 3rd International, Jan. 2010, pp. 844 – 845.

[iii] Chuanxian Ding, and Xuanyong Liu, Microstructure and Properties of Plasma Sprayed

Nanostructural ZrO2 Coatings, Nanoelectronics Conference (INEC), 2010 3rd , March 2010, pp. 12 – 13.

[iv] Guocheng Wang and Hala Zreiqat, Functional Coatings or Films for Hard-Tissue Applications,

 Materials 2010, 3 , pp. 3994-4050.

[v] Guocheng Wang, Xuanyong Liu, Jianhua Gao, Chuanxian Ding, In vitro bioactivity and phase stability of plasma-sprayed nanostructured 3Y-TZP coatings, Acta Biomaterialia 5 (2009) pp. 2270–2278.

[vi] T.J. Webster, R.W. Siegel, and R. Bizios, Osteoblast Adhesion on Nanophase Ceramics, Biomaterials, 20, 1221, 1999.

[vii] Youtao Xie, Fei Yang, Kerong Dai, Xuebin Zheng, Chuanxian Ding, Nano-Structured Titanium Coating for Improving Its Biological Performance, Nanoelectronics Conference (INEC), 2010 3rd International, Jan. 2010, pp. 1423 - 1424.

[viii] Weichang Xue, Xuanyong Liu, XueBin Zheng, Chuanxian Ding, In vivo evaluation of plasma-sprayed titanium coating after alkali modification, Biomaterials 26 (2005) pp. 3029–3037.

BIOMEDICAL APPLICATIONS OF THERMAL SPRAY COATINGS

Thermal spray coatings have been instrumental in extending the life and performance of orthopedic and dental implants.  Two of the more common coating biomaterials include hydroxyapatite (HA) and titanium.  The role of the coatings may include:

• ·      wear protection;
• ·      corrosion protection; and/or
• ·      implant/bone bond enhancement.

Hydroxyapatite (HA) is a ceramic that is found in bones and teeth of mammals and is bioactive, thereby permitting tissue growth through the porous coating[i] [ii] [iii].  The interaction is a derivative of an ion-exchange reaction between the HA coating and surrounding body fluid resulting in the formation of a biologically active carbonate apatite (CHAp) layer on the implant that is chemically and crystallographically equivalent to the mineral phase in bone[iv]. 

The challenge in thermal spraying HA is in laying down a well-bonded coating without excessive decomposition of the HA into less bioactive alternate phases and/or amorphous HA[v].  Several approaches have been used to address the issue of HA decomposition during the spray process.  These include: spraying of larger size particles^v (good for porous coatings but not so for dense structures); applying post-spray heat treatment to retrieve crystalline structure; and selecting different thermal spray process (i.e., APS^v, microplasma spray[vi], VPS[vii], HVOF[viii]).

VPS applied hydroxyapatite on titanium alloy hemispherical femoral cup^vii.

Titanium is frequently used as a dense bioinert coating to provide corrosion resistance to the implant against body fluids.  Ti coatings can also be applied for wear resistance and for promoting biological ingrowth.  These coatings have most often been applied via APS[ix] or VPS[x]; however, newer spray methods such as cold[xi] and warm[xii] spray processes are being evaluated for their ability to deposit metals with little or no oxidation and without the need for a vacuum chamber.  Although in the past titanium coatings were not considered to be bioactive, this has changed with recent developments permitted by the use of thermal spray to produce well-adhered sponge-like or very porous Ti coatings[xiii]. Thus, there is evidence that this approach could also be applied for osseointegration^xi.

Eurocoating’s VPS applied porous Ti coating^x.

Not surprisingly, studies investigating the possibility of introducing nanotechnology into biomedical coatings have been initiated.  In an upcoming blog, we will provide some insight into these efforts.

REFERENCES

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[i] Y. Wang, K.A. Khor, and P. Cheang, Thermal Spraying of Functionally Graded Calcium Phosphate Coatings for Biomedical Implants, Journal of Thermal Spray Technology Volume 7(1) March 1998, pp. 50-57

[ii] A. Dey, A.K. Mukhopadhyay, S. Gangadharan, M.K. Sinha, and D. Basu, Characterization of Microplasma Sprayed Hydroxyapatite Coating, Journal of Thermal Spray Technology Volume 18(4) December 2009, pp. 578-592

[iii] F. N. Oktar, M. Yetmez, S. Agathopoulos, T. M. Lopez Goerne, G. Goller, I. Ipeker, J. M. F. Ferreira, Bond-coating in plasma-sprayed calcium-phosphate coatings, J Mater Sci: Mater Med (2006) 17:1161–1171

[iv] G. Heness and B. Ben-Nissan, Biomaterials - Classifications and Behaviour of Different Types of Biomaterials, Abstracted from “Innovative Bioceramics” in Materials Forum, Vol. 27, 2004.

[v] V. Deram , C. Minichielloa, R.-N. Vanniera, A. Le Maguera, L. Pawlowskia, D. Muranob,  Microstructural characterizations of plasma sprayed hydroxyapatite Coatings, Surface and Coatings Technology 166 (2003) 153–159

[vi] A. Dey, A.K. Mukhopadhyay, S. Gangadharan, M.K. Sinha, and D. Basu, Characterization of Microplasma Sprayed Hydroxyapatite Coating, Journal of Thermal Spray Technology Volume 18(4) December 2009, pp. 578-592

[vii] A. Tonino, C. Oosterbos, A. Rahmy, M. Therin, AND C. Doyle, Hydroxyapatite-Coated Acetabular Components, The Journal of Bone & Joint Surgery · JBJS.ORG Volume 83-A · Number 6 · June 2001, pp. 817-825

[viii] J.D. Haman, A.A. Boulware, L.C. Lucas, and D.E. Crawmer, High-Velocity Oxyfuel Thermal Spray Coatings for Biomedical Applications, Journal of Thermal Spray Technology Volume 4(2) June 1995, pp. 179-184

[ix] The Role of Plasma Spray Coatings in Medical Devices, http://www.themedicines.info/2009/07/role-of-plasma-spray-coatings-in.html

[x] Eurocoating website: http://www.eurocoating.it/plasma_spray_coatings/vacuum_plasma_spray_titanium_coatings/vacuum_plasma_spray_titanium_coatings_1.aspx

[xi] T.S. Price, P.H. Shipway, and D.G. McCartney, Effect of Cold Spray Deposition of a Titanium Coating on Fatigue Behavior of a Titanium Alloy, Journal of Thermal Spray Technology Volume 15(4) December 2006, pp. 507-512

[xii] J. Kawakita, S. Kuroda, S. Krebs, H. Katanoda, In-situ densification of Ti coatings by the warm spray (two-stage HVOF) process, Materials Transactions Volume 47, Issue 7, July 2006, Pages 1631-1637

[xiii] Eurocoating website: http://www.eurocoating.it/plasma_spray_coatings/growth/default.aspx