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Thermal Spray of Superfine Wc/Co Feedstocks with Improved Coating Properties

INTRODUCTION. Thermal sprayed WC/Co coatings have been widely applied to machine components. For decades, the engine industry has been developing thermal spray deposition technologies for applying protective coatings to engine parts that experience abrasive, erosive, and corrosive wear. Recently, considerable interests are directed toward the development of thermal spray techniques to deposit dense nanostructured and superfine metal-ceramic composites, e.g., tungsten carbide/cobalt (WC/Co). The extraordinary performance properties of nanocoatings and superfine coatings [1-2] include high hardness and wear properties without concomitant loss of ductility or fracture toughness. For example, thermal spray of nanostructured and superfine WC/Co materials shows many benefits over the conventional coating material. Commercially available superfine and nanostructured WC/Co materials are reprocessed with grain growth inhibitors and alloying additions into the thermal spray feedstocks which are uniquely suited to thermal spray.

POWDER FEEDSTOCK. Inframat’s reconstituted InfralloyTM WC/12CO powder feedstocks have a near spherical morphology with particle size distribution ranging from 5 to 45 mm in diameter. Each individual agglomerate is an assemblage of millions of superfine particles of 0.2 – 0.4 mm WC grains uniformly distributed inside the Co matrix. The as reconstituted WC/Co particles exhibited some degrees of porosity. These porosities are eliminated after post-sintered at elevated temperatures. Fig. 1 shows typical SEM micrographs of the as-reconstituted superfine WC/12Co feedstock powder.

The measured tapping density of the powder feedstock is 6.5 g/cc. The x-ray theoretical density of bulk WC/12Co is about 14.5 g/cc. Assuming a perfect spherical shape, the theoretical packing density of any powder can reach up to 70% of its bulk density. Thus it can be concluded that the superfine WC/12Co powder feedstock has reached to 64% of its theoretical packing density. The measured powder Hall flow rate of the reprocessed superfine WC/12Co is approximately 180 g/min.


Coating microstructure and hardness. Cross-section SEM examination revealed that all these coatings are very dense. Typical SEM micrographs of the coating cross-sections are showing in Fig. 2. Visual examination of the SEM micrographs revealed less than 3% coating porosity. Particle sizes of the coating are relatively small, range from 100 nm to 400 1mm, with average particle size being ~200 nm. Optical microscopy examination revealed that the as-deposited superfine WC/12Co coatings are very dense, with porosities of less than 1%. The coating density is comparable to conventional micrometer sized WC/Co coatings of similar compositions.

Bond strength: The bond strength tests were performed using the ASTM standard pull test (ASTM C633-79). All the samples resulted in glue failure. This glue usually fails at a load of ~ 12,000 psi. Since all of our samples resulted in glue failure, we cannot quantify the exact bond strength of thermal sprayed superfine WC/12Co coatings. Typical thermal-sprayed WC/Co coatings using conventional micrometer sized WC/Co have a bond strength from 9,000 - 10,000 psi.


Abrasive wear resistance: The abrasive wear tests were performed on a grinding/polishing machine at room temperature (Struers grinding/polishing system composed of RotoPol-22 controlled grinding and polishing discs equipped with a RotoForce-4 specimen mover and a Multidoser with a RotoCom memory unit). After modification, the system is similar to a computer controlled pin-on-disk wear tester. Prior to wear tests, coating surfaces were polished to 1 mm after 9 mm and 3 mm. The tests were conducted under the normal load of 45 N. Coated samples were used as pins (cylindrical samples) with a flat surface of 32 mm in diameter. Before wear testing, all samples were cleaned with acetone. A 40 mm diamond abrasive pad with a diameter of 170 mm was used as disk. It was dressed and cleaned before each experiment, and lubricated with water during the tests.

Fig. 3 compares the wear volumes of various grades of thermal sprayed WC/Co coatings. It reveals that the wear volume of InfralloyTM 7412 superfine WC/12Co coated specimens is the lowest. This means that the wear resistance of this coating is likely to be better than Metco Diamalloy 2004 powder. The reason for the good wear resistance is that this material can prove high cross-section microhardness (close to Metco 2004 coatings, 1,000 VHN). This coating also exhibited high surface hardness (higher than Metco 2004, 1,200 VHN) due to the preferential crystal orientation formation in the coating during thermal spray and high toughness due to their smaller grain size.


HARDNESS AND TOUGHNESS. In conventional materials, materials hardness is always inversely proportional to toughness. That is, increased hardness will result in decreased toughness, and vice versa. As shown in Table 1, Inframat’s InfralloyTM 7412 superfine hardcoating exhibited increased hardness and toughness at low cobalt content level.

Table 1. Hardness and toughness values (results obtained from Prof. ONR DUST Report, SUNY, Stony Brook [1])

Various Materials Microhardness (HV300g) Fracture Toughness Indentation (MPam1/2)
Hardness Standard Deviation Lawn Evans
Metco 2004 (WC/12Co) 1068 37 3.14 5.37
Metco 2005 (WC/17Co) 1334 110 2.25 2.31
SX-432 (WC/18Co) 1203 69 2.10 2.16
InfralloyTM7412 (WC/12Co) 1200 116 4.47 6.47


DEVELOPMENT STATUS. Inframat is currently making large quantities of InfralloyTM thermal spray feedstocks at competitive cost. Orders can be placed through web inquiries or contact us at (860) 432-3155, Ms. Alison Quatro.


[1]. Sanjay Sampath, “Evaluation of conventional and nanostructured Coatings produced by HVOF and HVAF Thermal Spray Processing,” Navy ONR report, Contract No. 000149910405, April, 2000.
[2]. P.R. Strutt, B.H. Kear, and R. Boland, “Nanostructured Feeds for Thermal Spray Systems, Method of Manufacture, and Coatings Formed Therefrom,” U.S. Patent No. 6,025,034 (2.23.00).
[3]. B.H. Kear and P.R. Strutt, “Nanostructures, The Next Generation of High Performance Bulk Materials and Coatings,” Naval Reviews, XLVI, 4 (1994).
[4]. Contract No. N00014-98-3-0005 (ONR) “Thermal Sprayed Nanostructured Coatings for Dual Use Applications.”


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