COMPARISION OF COLD SPRAY GASES, PART 3 of 3

Although the cost of helium can be an order of magnitude higher than that of nitrogen, in a production setting, the option of installing a helium recovery system (HRS) can greatly reduce this cost[i].  Several presenters at the 1^st North American Cold Spray Conference revealed very enlightening facts regarding the ramifications of gas selection.  For instance, one speaker from a company that designs and manufactures products and services related to process control and flow measurement provided cost analyses of using helium (with HRS) and nitrogen for a specific application, namely spraying titanium for sputtering target application[ii].  This speaker indicated there is a lower gas cost when using helium with a recovery system ($150/h) compared to using nitrogen ($200/h).  He also identified the following advantages obtained when using helium: 

• ·      4.5 Times feed rate when spraying Ti for same coating density.
• ·      > 75 % higher parts processing rate;
• ·      Higher spray temperature capability, thereby reducing the flow rate;
• ·      Lower Porosity Deposits.

Moreover, according to this presenter, the cost of spraying sputtering target with He was calculated to be $360/piece versus $ 2160/piece with N2. In addition, the He process had the advantage of reductions in labor and equipment use.

 

Filtration & gas separation unit of a helium recovery system[iii].

During the same conference, ARL also provided some valuable findings and recommendations based on their extensive experience using cold spray processing[iv].  These include the following:

.           Nitrogen should be used when spraying low cost materials;

.           Helium should be used when spraying high value materials;

.           The highest possible feed rate will produce improved results;

.           Powder characteristics (sizing, morphology etc) are the factor with the largest influence on cost;

.           Powders with small diameter particles are preferable;

.           A narrow particle size distribution (small SD) is preferable;

.           Irregularly shaped particles perform better than spherical;

.           Overall coating costs are composed mainly of gas and powder costs;

.           These cost calculations do not consider coating quality;

.           Helium recycle, not considered here, would lower costs.

REFERENCES

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[i] J.-G. Legoux, E. Irissou, s. Desaulniers, J. Bobyn, B. Harvey, W. Wong, E. Gagnon, S. Yue, Characterization and performance evaluation of a helium recovery system designed for cold spraying, International Thermal Spray Conference and Exposition 2010, Thermal Spray: Global Solutions for Future Application (DVS-ASM), pp. 560-565

[ii] S. Desaulniers, Economics of spraying with light gas, North American Cold Spray Conference 2011 – oral presentation

[iii] S. Desaulniers, Helium Recovery System designed for Cold Spraying, 2^nd Canadian Cold Spray Conference 2010 – oral presentation

[iv] D. Helfrtch, M. Trexler, How Operating Parameters and Powder Characteristics Affect Cold Spray Costs, North American Cold Spray Conference 2011 – oral presentation

COMPARISION OF COLD SPRAY GASES, PART 2 of 3

Corrosion studies on cold sprayed coatings using helium and nitrogen yielded some interesting results.  In one of the studies, 1100 Al was cold sprayed onto 1100 Al substrates using helium-only and helium-nitrogen mixture^i[i].  As expected, the coating derived using helium-only gas possessed lower porosity and greater plastic deformation. Ironically, this denser coating possessed lower immersion corrosion resistance compared to the coating derived using helium-nitrogen gas.  The authors attribute this lower corrosion resistance to higher residual stress instilled within the coating structure, when forming the dense coating, leading to more active corrosion.  In addition, cold sprayed coatings, irrespective of which gas was used, fared better against corrosion when compared to that of 1100 Al substrate.

In a separate study[ii], salt fog testing (35 ºC with 5% salt solution) of cold sprayed Al on Mg using helium and nitrogen led to performances exceeding the minimum acceptance requirement for the application of 336 h.  Due to experimental development challenges, this study could not provide a direct corrosion resistance comparison between coatings derived using helium and nitrogen.  However, this study did reveal that adherent, dense coatings that provide adequate corrosion protection for some applications can be achieved by cold spray process.

  Al-coated Mg panels after exposure in salt fog chamber.

In another study[iii] conducting preliminary work on the mechanical properties of cold sprayed CP-Al on Al 7075-T6 using helium and nitrogen gases, it was shown that the ultimate tensile strengths of both as-sprayed coatings were similar to the strength of the substrate, although the samples derived using nitrogen possessed slightly higher values.  The researchers also noted that there may be a velocity regime where the cold spray process imparts beneficial compressive stresses but above which surface damage may occur, thereby reducing the fatigue performance of the coated samples.  The bend test results showed that cold spray samples using nitrogen did not result in coating cracks, whereas, cracks were observed on the samples using helium. The preliminary results from this study suggest caution in assuming that all coatings properties will benefit from increased particle velocities.  More work in this area is needed to substantiate or refine the interesting results observed in this study.

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[i] K. Balani, T. Laha, A. Agarwal, J. Karthikeyan, N. Munroe, Effect of carrier gases on microstructural and electrochemical behavior of cold-sprayed 1100 aluminum coating, Surface & Coatings Technology 195 (2005) pp. 272– 279

[ii] V. K. Champagne, P.F. Leyman, and D. J. Helfritch, Magnesium Repair by Cold Spray, Army Research Laboratory Technical Report 4438 (ARL-TR-4438) May 2008

[iii] J. Barnes, V. Champagne, D. Ballard, T.J. Eden, B. Shoffner, J.K. Potter, and D.E. Wolfe, MECHANICAL AND MICROSTRUCTURAL EFFECTS OF COLD SPRAY ALUMINUM ON Al  7075 USING KINETIC METALLIZATION AND COLD SPRAY PROCESSES, AFRL-ML-WP-TP-2007-431

COMPARISION OF COLD SPRAY GASES, Part 1 of 3

By now most people who have looked into cold spray technology know that the type of propellant/accelerating/process/deposition/carrier gas used in cold spray has an influence on deposit quality.  In addition, it is common knowledge that there is a large difference in cost for the two more popular types of process gas used in cold spray, namely nitrogen and helium.  This blog will provide insight into some obvious and not-so-obvious differences resulting from the use of these two gases.

Since cold spray relies so heavily on the kinetic energy of the impinging particles, it is important to identify the different means of adjusting this feature.  To increase impact velocities of impinging particles, the following measures can be taken: reduce gas molecular weight, increase gas temperature, use longer nozzles for longer stay of particles, and use less-dense and smaller spray particles.  Hence, selecting the lower molecular weight helium is an obvious means of attaining higher particle velocities.

 

Comparison of CP titanium particle velocity profile, in respect to the cold spray nozzle axis, as a function of distance from nozzle exit[i].

Increased particle impingement velocities have been shown to increase hardness[ii]^{, [iii]}, deposition efficiency[iv], adhesive strength[v]^{, [vi]}, and coating density^{ii, iii, [vii], [viii], [ix], [x]}.  These increases are more notable when spraying harder, higher melting temperature powder, such as titanium, which can only be densely cold spray processed using helium gas and not nitrogen^{iv, viii, ix, x}.  For more ductile, lower melting materials, these benefits derived from increased particle velocity are not as prominent.  For instance, work carried out by Army Research Laboratory (ARL) on magnesium repair by cold spraying Al alloy^vi shows that spraying with nitrogen gas provides adequate quality for this application.

REFERENCES

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[i] S.H. Zahiri, W. Yang, M. Jahedi, Characterization of Cold Spray Titanium Supersonic Jet, Journal of Thermal Spray Technology Volume 18(1) March 2009, pp. 110-117

[ii] K. Balani, T. Laha, A. Agarwal, J. Karthikeyan, N. Munroe, Effect of carrier gases on microstructural and electrochemical behavior of cold-sprayed 1100 aluminum coating, Surface & Coatings Technology 195 (2005) pp. 272– 279

[iii] J. Barnes, V. Champagne, D. Ballard, T.J. Eden, B. Shoffner, J.K. Potter, and D.E. Wolfe, MECHANICAL AND MICROSTRUCTURAL EFFECTS OF COLD SPRAY ALUMINUM ON Al  7075 USING KINETIC METALLIZATION AND COLD SPRAY PROCESSES, AFRL-ML-WP-TP-2007-431

[iv] W. Wong, S. Yue, E. Irissou, J.-G. Legoux, Optimization of Cold Sprayed Pure Titanium, North American Cold Spray Conference 2011 – oral presentation

[v] H. Fukanuma, N. Ohno, A study of adhesive strength of cold spray coatings, Thermal Spray 2004: Advances in Technology and Applications (ASM International) 2004, pp. 329 - 334

[vi]  V. K. Champagne, P.F. Leyman, and D. J. Helfritch, Magnesium Repair by Cold Spray, Army Research Laboratory Technical Report 4438 (ARL-TR-4438) May 2008

[vii] S.V. Klinkov, V.F. Kosarev, M. Rein, Cold spray deposition: Significance of particle impact phenomena, Aerospace Science and Technology 9 (2005) pp. 582–591

[viii] S.H. Zahiri, C.I. Antonio, M. Jahedi, Elimination of porosity in directly fabricated titanium via cold gas dynamic spraying, journal of materials processing technology 2 0 9 (2009) pp. 922–929

[ix] W. Wong, E. Irissou, J.-G. Legoux, S. Yue, Influence of helium and nitrogen gases on the properties of cold gas dynamic sprayed pure titanium coatings, http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=16169245&lang=en

[x] J.-G. Legoux, E. Irissou, s. Desaulniers, J. Bobyn, B. Harvey, W. Wong, E. Gagnon, S. Yue, Characterization and performance evaluation of a helium recovery system designed for cold spraying, International Thermal Spray Conference and Exposition 2010, Thermal Spray: Global Solutions for Future Application (DVS-ASM), pp. 560-565

EFFECTS OF CARBIDE SIZE ON CERMET MATERIALS AND COATINGS Part 2: From Bulk to Coating

There have been numerous attempts to transfer these very attractive properties observed in bulk nanostructured WC-Co to coatings.  Thermal spray technology has been most commonly used for these attempts.  In the first study of its kind, the US Navy work on “Thermal Spray Processing of Nanostructured Coatings”[i] revealed the challenges involved in thermal spraying dense nanostructured WC-Co coatings without excessively decarburizing the nano-scale hard particles.  To meet the objective of attaining coatings with intrinsic properties similar to bulk WC-Co, one will have to closely mimic the physical and compositional features of the bulk samples, including reducing excessive decarburization and limiting porosity in the coating.  Control and optimization of feedstock (size and structure), selection of thermal spray process, and optimization of process parameters are key components to controlling the microstructure and chemistry of the coating.  The following list indicates some of the attributes of thermal sprayed nanostructured carbide cermet coatings compared to their conventional counterparts. (Please note that the author is presenting observed trends; exceptions to the statements can be found on occasion.)

Thermal spray processing of nanostructured carbide cermet feedstock resulted in:

• ·       a coating with greater decarburization[ii]^{, [iii], [iv], [v], [vi], [vii], [viii], [ix]}
• ·       a coating with lower porosity^{v, vi, [x]}
• ·       a coating with more uniform distribution of the hard particles within the metal matrix with a lower mean free path^{viii, [xi], [xii], [xiii]}
• ·       a coating with higher hardness^{ii, iii, iv, v, vi, vii, viii, x, xi, xii, [xiv], [xv]}
• ·       a coating with higher fatigue resistance^xiv
• ·       a coating with higher fracture toughness^{xii, xv}
• ·       a coating with higher abrasive wear resistance (Fig. 1)^{iii, iv, v, vi, xii, [xvi]}
• ·       a coating with higher erosive wear resistance^{vi, x}
• ·       a coating with higher sliding wear resistance^{viii, xi}
• ·       a coating with higher corrosion resistance^viii
• ·       a coating with lower and more consistent friction^{xiii, [xvii]}

Fig. 1: Dependence abrasion resistance on microhardness of conventional and nanostructured WC-Co coatings.^iv 

A study by Li et al.^xvi revealed some interesting and very pertinent results.  Their findings seemed to indicate that although finer carbides undergo greater levels of decarburization, more of the carbides (whether in original or oxidized stoichiometry) were retained in the coating. The larger carbide particles (> 5 µm) tended to bounce off the surface instead of embedding themselves into the matrix.  They also observed that even amongst the larger carbide particles that remained in the splats, the particles were not very well anchored by the matrix.  One can then deduce that spraying with finer carbides can lead to improved cohesive and adhesive strength in coating due to enhanced splat-to-splat and splat-to-substrate surface bonding.

Recent studies using cold spray hold promise for advancing the properties of nanostructured carbide cermet coatings by further reducing the process temperature (thereby reducing decarburization) while maintaining the high kinetic energy (thereby decreasing porosity) of the impinging feedstock powder.  Kim et al.[xviii] demonstrated the ability to deposit dense nanostructured WC-Co coatings with no signs of thermal degradation (i.e., decarburization) by using cold spray technology. Unfortunately the study did not include any wear test results; however, the coating possessed very high hardness (approx. 2050 HV).

FW Gartner and Perpetual Technologies have carried out their own collaborative work on the development and evaluation of nanostructured WC-Co(Cr) coatings.  Syncrude Canada, with their extensive in-house experience in evaluating surface engineered samples, was approached to carry out the abrasion wear test.  Their results showed that the nanostructured WC-CoCr coating out-performed all other thermal sprayed WC-Co(Cr) coatings in their database and approached the values observed for welded (plasma transferred arc) deposits with similar chemistries (Fig. 2).

Fig.2: Syncrude Canada’s in-house dry sand rubber wheel abrasion test results comparing nanostructured “multimodal” WC-Co coating against existing coatings of same or similar composition.

 REFERENCES

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[i] Lawrence T. Kabacoff, “Nanoceramic Coatings Exhibit Much Higher Toughness and Wear Resistance than Conventional Coatings”, The AMPTIAC Newsletter, Spring 2002, Volume 6, Number 1

[ii] D.A. Stewart, P.H. Shipway, and D.G. McCartney, Abrasive wear behavior of conventional and nanocomposite HVOF-sprayed WC-Co coatings, Wear 225-229 (1999) pp. 789-798

[iii] A. Ghabchi, T. Varis, E. Turunen, T. Suhonen, X. Liu, and S.-P. Hannula, Behaviour of HVOF WC-10Co4Cr Coatings with Different Carbide Size in Fine and Coarse Particle Abrasion, Journal of Thermal Spray Technology, Volume 19(1-2) January 2010, pp. 368-377

[iv] M. F. Morks, M. A. Shoeib, A.Ibrahim, Comparative Study of Nanostructured and Conventional WC-Co Coatings, Thermal Spray 2004: Advances in Technology and Applications (ASM International), 2004, pp. 857 – 860

[v] Q. Wang , Z.H. Chen, Z.X. Ding, Performance of abrasive wear of WC-12Co coatings sprayed by HVOF, Tribology International 42 (2009) pp. 1046–1051

[vi] Q. Wang, Z.H. Chen, Z.X. Ding, Z.L. Liu, Performance Study of Abrasive Wear and Erosive Wear of WC-12Co Coatings Sprayed by HVOF, 2008 2^nd IEEE International Nanoelectronics Conference (INEC 2008) pp. 340-344

[vii] P.H. Shipway, D.G. McCartney, T. Sudaprasert, Sliding wear behaviour of conventional and nanostructured HVOF sprayed WC-Co coatings, Wear 259 (2005) pp. 820–827

[viii] J.M. Guilemany, S. Dosta, J. Nin, and J.R. Miguel, Study of the Properties of WC-C Nanostructured Coatings Sprayed by High-Velocity Oxyfuel, Journal of Thermal Spray Technology Volume 14(3) September 2005, pp. 405-413

[ix] J. He, M. Ice, S. Dallek, and E.J. Lavernia, Synthesis of Nanostructured WC-12 Pct Co Coating Using Mechanical Milling and High Velocity Oxygen Fuel Thermal Spraying, METALLURGICAL AND MATERIALS TRANSACTIONS A, VOLUME 31A, FEBRUARY 2000, pp. 541-553

[x] B. Zha, H. Wang, X. Su, Nano Structured WC-12Co Coatings Sprayed by HVO/AF, Thermal Spray 2004: Advances in Technology and Applications (ASM International) 2004, pp. 881-883

[xi] X-Q Zhao, H-D Zhou, J-M Chen, Comparative study of the friction and wear behavior of plasma sprayed conventional and nanostructured WC–12%Co coatings on stainless steel, Materials Science and Engineering A 431 (2006) pp. 290–297

[xii] C. A. da Cunha, N. B. de Lima, J. R. Martinelli, A. H. de Almeida Bressiani, A. G. F. Padial, L. V. Ramanathan,  Microstructure and Mechanical Properties of Thermal Sprayed Nanostructured Cr3C2-Ni20Cr Coatings, Materials Research, Vol. 11, No. 2 (2008) pp. 137-143

[xiii] N.I. Smirnov, M.V. Prozhega, N.N. Smirnov, Study of Tribological Properties of Detonation Nanostructured WC-Co-Based Coatings, Journal of Friction and Wear, Vol. 28, No.2 (2007) pp. 200-205

[xiv] A. Ibrahim, C.C. Berndt, Fatigue and Mechanical Properties of Nanostructured WC-Co Coatings, Thermal Spray 2004: Advances in Technology and Applications (ASM International) 2004, pp. 878 – 880

[xv] A. Padial, C.A. Cunha, N.B.Lima, J.R. Martinelli, O.V.Correa and L.V.Ramanathan,  STRUCTURE AND PROPERTIES OF THERMAL SPRAYED NANOSTRUCTURED Cr3C2-25(Ni20Cr) COATINGS, 18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.

[xvi] C.-J. Li, Y.-Y. Wang, G.-J. Yang, A. Ohmori and K. A. Khor, Effect of solid carbide particle size on deposition behaviour, microstructure and wear performance of HVOF cermet coatings, Materials Science and Technology September 2004 Vol. 20, pp. 1087-1096

[xvii] X-Q Zhao, H-D Zhou, J-M Chen, Comparative study of the friction and wear behavior of plasma sprayed conventional and nanostructured WC–12%Co coatings on stainless steel, Materials Science and Engineering A 431 (2006) 290–297

[xviii] H-J Kim, C-H Lee, S-Y Hwang, Fabrication of WC–Co coatings by cold spray deposition, Surface & Coatings Technology 191 (2005) pp. 335– 340

EFFECTS OF CARBIDE SIZE ON CERMET MATERIALS AND COATINGS Part 1

Bulk Carbide Cermets

The following blog is a continuation of the blog dated April 21, 2010 entitled, “NANOSTRUCTURED CARBIDE COATINGS - WHY AND HOW?”.  Emphasis will be placed on the understanding of how carbide particle size affects the intrinsic properties of bulk nanostructured carbide cermet coatings such as hardness and toughness.  Some of the results from the numerous studies attempting to transfer these attractive properties to coating form will be highlighted.

Carbide cermets are also referred to as cemented carbides and represent composite materials that combine very hard carbide particles (e.g., WC, Cr3C2, SiC, TiC, etc.) with a tough metal matrix (e.g., Co, CoCr, Ni, NiCr, etc.).  This combination of hardness or strength with toughness makes for a material that is resistant to numerous types of wear.  Of the various possible compositions of carbide cermets, WC-Co exceeds all others in terms of application.  In bulk form, WC-Co has proven itself to be very valuable for mining and machining tool applications.  Since the late 1970s, the trend has been to decrease the carbide size to harness superior hardness and wear resistance.

It is widely accepted that within conventional materials, hardness and toughness are inversely proportional.  It is also common knowledge that wear resistance of a material is largely dependent on its hardness and fracture toughness.  For conventional carbide cermets, the fracture toughness is a function of mean free path between the carbide particles where the metal binder plastically can deform and tear when subjected to sufficient stress.  Hence, one would deduce that uniformly distributed finer carbide particle size should lead to reduced mean free path, thereby reducing the plastic zone and the fracture toughness.  On this basis, one would expect the fracture toughness of nanostructured WC-Co to be very low; however, due to the unique physical nature and mechanical properties of nanostructured materials, scientists have observed simultaneous increases in hardness and toughness (Fig. 1) as the carbide particle size is reduced into the nano-scale regime^{[i], [ii]}.  The improved toughness is likely related to reduction in flaw size, increase in the amount of carbide-matrix interface, and change in the deformation mechanism.

Fig. 1: Surface crack resistance versus hardness of WC-Co materials made using nano, conventional submicron, and commercial WC powders.[iii] REFERENCES [i] K. Jia, T.E. Fischer, B. Gallois, Microstructure, hardness and toughness of nanostructured and conventional WC-Co composites, Nanostructured Materials, Volume 10, Issue 5, July 1998, pp. 875-891 [ii] L. Bartha, P. Atato, A.L. Toth, R. Porat, S. Berger and A. Rosen, Investigation of hip-sintering of nanocrystalline WC/Co powder. Journal of Advanced Materials,  32, 3 (2000), pp. 23–26 [iii] Z. Fang and J.W. Eason, Study of Nanostructured WC-Co Composites, International Journal of Refractory Metals & Hard Materials 13 (1995) pp. 297-303