Sliding Wear of Conventional and Suspension Sprayed Nanocomposite WC-Co Coatings: An Invited Review

Abstract

The global thermal spray coatings market was valued at USD 10.1 billion in 2019 and is expected to grow at a compound annual growth rate of 3.9% from 2020 to 2027. Carbide coatings form an essential segment of this market and provide cost-effective and environmental friendly tribological solutions for applications in aerospace, industrial gas turbine, automotive, printing, oil and gas, steel, and pulp and paper industries. Almost 23% of the world’s total energy consumption originates from tribological contacts. Thermal spray WC-Co coatings provide excellent wear resistance for industrial applications in sliding and rolling contacts. Some of these applications in abrasive, sliding and erosive conditions include sink rolls in zinc pots, conveyor screws, pump housings, impeller shafts, aircraft flap tracks, cam followers and expansion joints. These coatings are considered as a replacement of the hazardous chrome plating for tribological applications. The microstructure of thermal spray coatings is however complex, and the wear mechanisms and wear rates vary significantly when compared to cemented WC-Co carbides or vapour deposition WC coatings. This paper provides an expert review of the tribological considerations that dictate the sliding wear performance of thermal spray WC-Co coatings. Structure–property relationships and failure modes are discussed to grasp the design aspects of WC-Co coatings for tribological applications. Recent developments of suspension sprayed nanocomposite coatings are compared with conventional coatings in terms of performance and failure mechanisms. The dependency of coating microstructure, binder material, carbide size, fracture toughness, post-treatment and hardness on sliding wear performance and test methodology is discussed. Semiempirical mathematical models of wear rate related to the influence of tribological test conditions and coating characteristics are analysed for sliding contacts. Finally, advances for numerical modelling of sliding wear rate are discussed.

Thermal Spray Coatings for Electromagnetic Wave Absorption and Interference Shielding: A Review and Future Challenges

“…Figure 2 shows some of the coating particle temperatures and velocity ranges achievable by thermal spray processes. [11] The particle velocity which can be achieved during the thermal spray process (Figure 2) ranges from %20 to %1050 m s À1 , which influences the mechanical, thermal, and EM properties due to the extent of the 1) designed phase transformation, for example, ratio of amorphous-tocrystalline phase for hydroxyapatite (Ca 5 (PO 4 ) 3 (OH)) coatings for biomedical applications [12,13] and controlled degree of eta phase (i.e., carbon-deficient form of WC that results in a harder or brittle cemented carbide) formation for sliding wear applications of WC-Co coatings, [9] etc., and 2) control of porosity during deposition. [14] Complex feedstock materials such as BaCoTiFe 10 O 9 , SrFe 12 O 9 , and hydroxyapatite [12,15,16] can be thermally sprayed using the correct powder chemistry, coating process, and coating process parameters.…”

“…[166] While there are numerous examples where material has been doped with REEs using techniques other than thermal spray to improve EM wave propagation characteristics, such as chemical coprecipitation [167] or hydrothermal synthesis, [156] it is also possible that doping certain REEs during thermally sprayed coatings could also help improve the EM propagation wave characteristics of materials. In an example, Bartuli, Cipri and Valente (2008) [70] used a range of complex ceramic-based composite coatings, which included lanthanum/La REE (e.g., Cr 2 O 3 þ La 0.5 Sr 0.5 MnO 3 40wt%, Cr 2 O 3 þ Al 20 wt% þ La 0.5 Sr 0.5 MnO 3 20wt%) fabricated by air plasma spraying to evaluate their tailored EM properties (essentially as absorbers in the microwave range (8)(9)(10)(11)(12)). However, thermal spraying can make REEs prone to oxidation during processing and operation, and oxidation can lead to brittle coatings and deterioration of magnetic properties.…”

“…This has enabled them to become an integral part of the aviation, transportation, power generation, chemical, and biomedical industry, which is currently worth 7.6 billion USD. [ 9 ] As will be seen later, the substrates in such thermally sprayed coatings varied from metals, alloys, glass, and ceramics to graphite; however, there is little or no evidence where lightweight polymeric substrates were considered for EM wave propagation applications. Though not a focus of current review, application of polymeric substrates (e.g., thermoplastics, which are excellent electrical and thermal insulators) is quite possible with some degree of limitations, as demonstrated by Bobzin, Wietheger, and Knoch (2021), [ 10 ] as such materials can well be combined with a high degree of design flexibility and economical mass production.…”

“…The lamella structure results in anisotropic mechanical properties, which are dependent upon the structure–property relationships, with major differences between the in‐plane and out‐of‐plane directions. [ 9 ]…”

Recent Research Advances in Plasma Spraying of Bulk-Like Dense Metal Coatings with Metallurgically Bonded Lamellae

“…However, despite the apparent dense microstructure of HVOF coatings, the electrochemical tests generally reveal that the as-sprayed metal coatings could not provide the substrate with effective and reliable protection from corrosion due to the existence of unbonded lamellar interfaces (Ref 25 , 26 ). Moreover, limited bonding also degrades the wear performance of hard alloy coatings since lamella spalling occurs easily through the propagation of sub-surface cracks along the lamellar interface at high stress conditions (Ref 24 , 27 ), especially as the coatings are subjected to dynamic loading (such as cavitation erosion) (Ref 28 ). Hiraga et al reported that compared with vacuum plasma-sprayed NiTi coating the laser remelting treatment leading to the formation of bulk-like NiTi presented the enhanced cavitation erosion resistance by a factor of magnitude from one order to over two orders (Ref 29 , 30 ).…”