NANOSTRUCTURED CARBIDE COATINGS - WHY AND HOW?

 In 1997, Jia and Fischer ^[[i]] carried out sliding wear tests of bulk cemented carbides with varying carbide particle sizes.  The conventional samples had carbide particles between 0.7 and 2.5 μm, whereas, the nanostructured samples had carbide particles of around 70 nm.  Their findings showed that unlike the previous results where sliding wear resistance of cemented carbides increased with reduced cobalt content and increased grain size, the wear of the nanostructured carbide cermet with the same cobalt content was only 60% of the conventional cermet.  Yao et al. ^[[ii]] also mention the merits of nanostructured bulk WC-Co material; however, they also touched on the challenge in thermal spraying carbide cermets powder.  It is common knowledge that decarburization of the carbide particles occurs readily in conventional coatings and, therefore, must be closely monitored.  However, for very fine, nanosized carbides, the issue of decarburization is considerably greater.  The table below chronologically highlights findings related to thermal spraying of nanostructured carbide coatings.  

Reference	Coating material	Spray process	Test	Highlights
[iii]	WC-Co	VPS	- Sliding against alumina	- nano more wear resistant - VPS produced n-coatings with reduced decomposition - less carbide degradation led to higher wear resistance
[iv]	WC-Co	HVOF	- Microhardness	- deposition efficiencies in the range of 60-70% can be achieved with nano powder - coating hardness increased with increase in particle temperature and velocity - nano coating using H2 was 200HV higher than that using propylene - more degradation using propylene than H2
[v]	WC-Co	HVOF	- Microhardness	- nano has similar hardness compared to conventional coating
[vi]	CrC-NiCr	HVOF	- Microhardness - Scratch test	- nano was 20.5% harder at 1020 HV0.3 than conventional - the nano coating exhibits twice the scratch resistance and lower COF than conventional coating - heat treatment in air at ~ 900 K resulted in further increase in hardness to 1240 HV0.3 for nano coating but not for conventional - HTed nano coating showed improved wear and COF over as-sprayed nano
[vii]	WC-Co	HVOF	- Sliding wear against Si3N4 - Abrasive wear - Microhardness	- nanostructured WC-Co coatings can be deposited with superior hardness and toughness - nanostructured WC-Co inflict less wear against the opposing Si3N4 surface - reducing the flame temperature to reduce decarburization does not yield better wear resistance - hardest and toughest coatings were obtained using a hot, neutral flame and powders that minimize decarburization
[viii]	WC-Co	HVOF	- Abrasive wear	- high flame temperature led to superior bonding but greater decarburization - the wear resulted from plastic deformation and plowing of matrix and spalling of carbide particles - low flame temperature led to reduced bonding but lower decarburization resulting in failure at splat boundaries - higher temperature conditions led to highest abrasive wear resistance in this study
[ix]	WC-Co	HVOF	- Microhardness - Fatigue strength - Abrasive wear	- increased fatigue, hardness, and wear resistance were obtained in nanostructured coatings
[x]	WC-Co	HVOF, HVAF & HVO-AF	- Microhardness	- HVOF nanostructured coatings showed signs of carbide degradation and a decrease in hardness - both HVAF and HVO-AF showed little signs of decarburization; however, the latter’s higher particle velocity gave it higher hardness - average microhardness of the nanostructured coating was about 1.4 to 1.5 times that of conventional coating
[xi]	WC-Co	HVOF	- Microstructure	- to optimize the application of nano coating, one needs to minimize decomposition and use low ratio of fuel-oxygen chemistry - in addition to decreasing temperature, one can add grain size inhibitors to reduce decomposition - propylene over hydrogen seemed better with reduced decomposition and lower porosity
[xii]	WC-Co	HVOF	- Sliding wear against alumina	- nano coating exhibited greater degree of decomposition - nano coating had higher wear rate
[xiii]	WC-Co	HVOF	- Ball-on-disk sliding test - Dry sand rubber wheel abrasion test - Corrosion	- nano coating suffer more degradation - nano coating were around 30% harder - nano coating had 30% lower COF - nano and conventional coatings had similar abrasive wear resistance - nano coating showed 3.5 times higher corrosion protection for the substrate in salt fog test – likely due to better sealing properties - nano coatings were sprayed without complete optimization of spray parameters
[xiv]	WC-Co	Cold spray	- Microhardness	- nano coating showed no signs of decarburization - nano coating had low porosity and very high hardness (2050 HV0.5)
[xv]	WC-Co	HVOF	- Microhardness - Dry abrasion	- nano (multimodal) coatings tend to have higher hardness; however, there is little difference in abrasion resistance when compared to conventional - particle temperature between 1,750 to 1,950 C at the point of impact with the substrate produced the most wear resistant nano coatings; too low particle temperature resulted in low cohesive strength and too high led to excessive degradation of carbide - nano coating deposited at high efficiency (approx. 70%)
[xvi]	WC-Co	APS	- Ball on disk sliding wear against Si3N4	- nano coating underwent greater degree of decomposition - nano coating was harder, with lower COF and higher wear resistance
[xvii]	CrC-NiCr	HVOF	- Micro indentation	- nano coating was 20% harder, smoother, and had similar fracture toughness and elastic modulus compared to the conventional coating - nano coating showed no change of phases compared to that of the powder

The following are some interesting characteristics of thermal sprayed nanostructured cermet coatings derived from the literature review:

• Unlike conventional carbide cermet coatings where sliding and abrasive wear resistance improves with carbide size, nanostructured coatings with limited carbide degradation seem to possess very good wear resistance, even with very fine carbides.
• The mechanism of wear is different for nanostructured carbide coatings; the wear scars resemble plastic deformation even though they the coating is as hard or harder than its conventional counterpart.
• Deposition technique and spray parameters play a critical role in the quality, i.e., structure, hardness, toughness, of all carbide coatings, but even more so for nanostructured carbide cermets. 
• Any means of attaining dense, well-adhered nanostructured coating with limited decomposition will likely result in a harder and tougher coating.
• Nanostructured carbide powder with dense structure will likely result in decreased decarburization as opposed to porous agglomerates
• Although there are mixed results indicating whether nanostructured carbide coatings with a higher degree of decomposition will be harder and/or more wear resistant than the conventional carbide coatings, all results with well-bonded nanostructured coatings with limited or no decarburization indicate superior microhardness and/or superior wear resistance and toughness.
• It is important to note however that there is no wear data for the cold spray application of these materials which have led to very high hardness values.

The data to date on nanostructured carbide cermet coatings indicate the possibility of attaining a notably superior wear resistant and tougher coating by limiting compositional degradation within a well-adhered, dense coating.  To attain this quality, a balance between thermal and kinetic energies will be required through proper thermal spray process selection and optimization of the spray parameters.  One must also select a feedstock material that will best suit the thermal or cold spray process. 

REFERENCES

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[i] K. Jia, T.E. Fischer, “Sliding wear of conventional and nanostructured cemented carbides”, wear 203-204 (1997) 310-318.

[ii] Z. Yao, J.J. Stiglich, T.S. Sudarshan, “Nanosized WC-Co holds promise for the future”, MPR March

1998, 26-33.

[iii] Y-C Zhu, K. Yukimura, C-X Ding, P-Y Zhang, “Tribological properties of nanostructured and conventional WC-Co coatings deposited by plasma spraying”, Thin Solid Films 388 (2001) 277-282.

[iv] B.R. Marple, J. Voyer, J.-F. Bisson, C. Moreau, “Thermal spraying of nanostructured cermet coatings”, Journal of Materials Processing Technology 117 (2001) 418-423.

[v] Z.-G. Ban, L. L. Shaw, “Synthesis and processing of nanostructured WC-Co materials”, Journal of Materials Science 37 (2002) 3397-3403.

[vi] J. He, E.J. Lavernia, “Precipitation phenomenon in nanostructured Cr3C2–NiCr coatings”, Materials Science and Engineering A301 (2001) 69–79.

[vii] Y. Qiao, T.E. Fischer, A. Dent, “The effects of fuel chemistry and feedstock powder structure on the mechanical and tribological properties of HVOF thermal-sprayed WC–Co coatings with very fine structures”, Surface and Coatings Technology 172 (2003) 24-41.

[viii] Y-Y Wang, C-J Li, J. Ma, G-J Yang, “Effect of flame conditions on abrasive wear performance of HVOF sprayed nanostructured WC-12Co coatings”, Trans. Nonferrous Met. Soc. China, Vol.14, Special 2, 2004.

[ix] A. Ibrahim, C.C. Berndt, “Fatigue and Mechanical Properties of Nanostructured WC-Co Coatings”, Thermal Spray 2004: Advances in technology and application: Proceedings of the International Thermal Spray Conference, Osaka, Japan, 10-12 May 2004, pp. 878-880.

[x] B. Zha, H. Wang, “Nano Structured WC-12Co Coatings Sprayed by HVO/AF”, Thermal Spray 2004: Advances in technology and application: Proceedings of the International Thermal Spray Conference, Osaka, Japan, 10-12 May 2004, pp. 907-909.

[xi] M. Josephson, A. Holmbom Larsen, F. Larsson, “Nanostructured WC-Co Coating”, Project for the course ”Introduction to Nanotechnology” at Malmö University Sweden 2005. Advisor: Liu-Ying Wei.

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

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

[xiv] H-J Kim, C-H Lee, S-Y Hwang, “Superhard nano WC-12%Co coating by cold spray deposition”, Materials Science and Engineering A 391 (2005) 243-248.

[xv]B.R. Marple, R.S. Lima, “Process Temperature/Velocity-Hardness-Wear Relationships for High-Velocity Oxyfuel Sprayed Nanostructured and Conventional Cermet Coatings”, Journal of Thermal Spray Technology Volume 14(1) March 2005, pp. 67-76.

[xvi] 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.

[xvii] M. Roy, A. Pauschitz, J. Bernardi, T. Koch, and F. Franek, “Microstructure and Mechanical Properties of HVOF Sprayed Nanocrystalline Cr3C2-25(Ni20Cr) Coating”, Journal of Thermal Spray Technology, Volume 15(3) September 2006, pp. 372-381.

George E. Kim, Ph.D.

F.W. Gartner

Perpetual Technologies, Inc.

email: gkim@perpetualtech.ca

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