Improvement on mechanical properties and wear resistance of HVOF sprayed WC-12Co coatings by optimizing feedstock structure

doi:10.1016/j.apsusc.2014.09.081

Applied Surface Science

Volume 320, 30 November 2014, Pages 364-371

https://doi.org/10.1016/j.apsusc.2014.09.081 Get rights and content

Highlights

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Four kinds of WC-12Co coatings with various WC size distributions were prepared by HVOF spraying.
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Bimodal coating exhibited both excellent hardness and fracture toughness among all the coatings.
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Compared with the other coatings, bimodal coating also exhibited the best wear resistance.

Abstract

WC-12Co powders with a bimodal size distributed WC particles were used to produce coating by high velocity oxy-fuel (HVOF) spraying (B coating), and HVOF sprayed WC-12Co coatings from microstructured, submicrostructured and nanostructured powders were also fabricated for comparison. The phase constitution, microstructure, mechanical properties and wear performance of the coatings were investigated. Decarburization occurred during coatings preparation, and the carbide retention of B coating was 0.934, higher than that of nanostructured coating. B coating exhibited typical multimodal microstructure, and had considerably high microhardness and the highest fracture toughness among the four coatings, with the values of 1291 HV_0.1 and 10.76 MPa m^1/2, respectively. When sliding against GCr15 ring in block-on-ring configuration, B coating exhibited the lowest wear rate and relatively lower friction coefficient compared with other coatings, with the average values of 0.94 × 10⁻⁷ mm³ N⁻¹ m⁻¹ and 0.63 at 245 N load, respectively, which could be attributed to the concrete-like structure.

Introduction

WC-Co coatings widely used in many industries can endow the components with excellent wear resistance without compromising other attributes [1], [2]. Commonly, air plasma spraying (APS) and high velocity oxy-fuel (HVOF) thermal spraying techniques are used to produce WC-Co coatings. Compared with APS WC-Co coatings, the coatings fabricated by HVOF spraying have high levels of retained WC, high density and excellent cohesive strength [3], [4], [5]. During HVOF spraying, less WC decomposition occurs due to high velocity and low temperature of powder particles, leading to reduced formation of detrimental reaction products such as W₂C, metallic W and brittle η(Co_xW_yC_z)/amorphous phase [6], [7], [8].

It has been reported that feedstock powder structure significantly affects the mechanical properties and wear resistance of WC-Co coating [9], [10], [11], [12], [13]. Small WC grains in WC-Co powders are beneficial for the mechanical properties and wear resistance of the coating [9], [10]. However, some authors reported that when WC grain size decreases to nanometer level, serious WC particles decomposition occurs during coating preparation, leading to reduced wear resistance of the coating [11], [12], [13]. Recently, Yang et al. proposed a WC-(nano WC-Co) concept for multimodal WC-Co materials, and obtained WC-Co cermet coating with both enhanced hardness and fracture toughness by cold spraying powders with combined micro- and nano-sized WC particles [14]. Ji et al. characterized the particles deposition behavior of cold-sprayed multimodal WC-12Co coatings, and investigated the microstructure and properties of the coatings, corroborating that the bimodal coatings had both high hardness and fracture toughness [15]. Although several previous studies reported HVOF-sprayed bimodal WC-Co coating exhibited excellent wear resistance compared to the conventional WC-Co coatings [16], [17], [18], [19], [20], the reports on whether WC-(nano WC-Co) coating sprayed by HVOF has both excellent hardness and fracture toughness like the cold-sprayed coating still can’t be obtained. Up to now, the investigations on sliding wear performance of WC-Co coatings mostly involved ceramics or cermets as the mating materials, studies relating wear behavior of WC-Co coatings against steel are limited [21], [22], [23]. To make more extensive use of WC-Co coatings for wear resistance in transmission parts, it is necessary to investigate the tribological behavior of HVOF sprayed WC-Co coatings against bearing steel at room temperature.

In the present research, WC-12Co powders with a bimodal size distributed WC particles were used to produce WC-Co coating by an HVOF spray system, and WC-Co coatings from nanostructured, submicrostructured and microstructured feedstock powders were also fabricated under the same spray conditions for comparison to investigate the effects of feedstock structure on mechanical properties of the coatings. The dry sliding wear tests of the four coatings were conducted against GCr15 bearing steel, the effects of the applied load on the friction coefficient and wear rate of the coatings were investigated.

Section snippets

Experimentation

All the feedstock powders were agglomerated and sintered WC-12Co powder, and provided by Ganzhou Zhangyuan Tungsten New Materials Co., Ltd., China. Fig. 1 shows the typical powder morphology. These powders are roughly spherical and porous. WC-12Co powders with a bimodal size distributed WC particles were specially made, the volume ratio between submicrometer-sized (0.5 ∼ 1.0 μm) and nanometer-sized (30 ∼ 100 nm) WC particles is 7:3. For convenience, the nanostructured powders, submicrostructured

Microstructure of feedstock powders

Fig. 2 shows the cross-sectional SE-BSE images of the four feedstock powders. WC grains are found to be embedded in Co binder phase. As can be seen in Fig. 2a, WC grains in B powders have a multimodal size distribution, and the volume ratio between submicrometer-sized and nanometer-sized WC particles is 7:3. In Fig. 2b, some big carbide particles can be observed, which could be attributed to the reaggregation of nano-sized WC grains during powders preparation. As shown in Fig. 2c and d,

Conclusions

B coating was produced by HVOF spraying, using WC-12Co powders with a bimodal size distributed WC particles as feedstock powders. N coating, S coating and M coating were prepared under the same spraying conditions for comparison from nanostructured, submicrostructured and microstructured powders, respectively. The phase constitution, microstructure, mechanical properties and wear performance of the coatings were investigated. Some conclusions can be drawn as follows:

(1)
Various extents of

Acknowledgments

The authors are grateful for the financial support by the National Natural Science Foundation of China (No. 51375332) and the Natural Science Foundation of Tianjin (No. 12JCYBJ12300).

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Citation Excerpt :
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Improvement on mechanical properties and wear resistance of HVOF sprayed WC-12Co coatings by optimizing feedstock structure

Highlights

Abstract

Introduction

Section snippets

Experimentation

Microstructure of feedstock powders

Conclusions

Acknowledgments

Int. J. Refract. Met. H.

Wear

Mater. Sci. Eng. A

Mater. Chem. Phys.

Wear

Mater. Sci. Eng. A Struct.

Wear

Scripta Mater.

Surf. Coat. Technol.

Surf. Coat. Technol.

Scripta Mater.

Surf. Coat. Technol.

Tribol. Int.

Trans. Nonferrous Met. Soc. China

Tribol. Int.

Acta Mater.