Effects of Cobalt Content on the Microstructure, Mechanical Properties and Cavitation Erosion Resistance of HVOF Sprayed Coatings

Liu Liu,Bai Bai,Yuan Yuan,Chen Chen

doi:10.3390/coatings9090534

Abstract

Cobalt-based alloy coatings and WC-Co-based ceramic–metal (cermet) coatings have been widely used because of their desirable mechanical properties and corrosion resistance. In this work, the influence of Co content on the microstructure, mechanical properties and cavitation erosion (CE) resistance were investigated. A cobalt-based alloy coating, a WC-12Co coating, and a WC-17Co cermet coating were deposited by high-velocity oxygen fuel (HVOF) spraying on 1Cr18Ni9Ti substrates. Results indicate that the cobalt-based alloy coating had the largest surface roughness because surface-bonded particles of lower plastic deformation were flattened. The existence of WC particles had led to an increase in hardness and improved the fracture toughness due to inhibit crack propagation. The pore appeared at the interface between WC particles, and the matrix phase had introduced an increase in porosity. With the increase in Co content, the cohesion between matrix friction and WC particles increased and then decreased the porosity (from 0.99% to 0.84%) and surface roughness (Ra from 4.49 to 2.47 μm). It can be concluded that the hardness had decreased (from 1181 to 1120 HV0.3) with a decrease in WC hard phase content. On the contrary, the fracture toughness increased (from 4.57 to 4.64 MPa∙m1/2) due to higher energy absorption in the matrix phase. The WC-12Co and WC-17Co coatings with higher hardness and fracture toughness exhibited better CE resistance than the cobalt-based alloy coating, increasing more than 20% and 16%, respectively. Especially, the WC-12Co coating possessed the best CE resistance and is expected to be applicable in the hydraulic machineries.

Highlights

Lamana et al [13] demonstrated that an increase in Co content could lead to an increase in the fracture toughness and cavitation erosion (CE) resistance of WC-Co coatings, which resulted from the increase in compressive residual stress
A cobalt-based alloy and WC-Co-based cermet coatings were deposited by a high-velocity oxygen fuel (HVOF) spraying process
The existence of finer WC particles decreased the surface roughness, and it caused pores to appear at the interface between WC particles and the matrix phase

Summary

Introduction

A cobalt-based alloy coating is widely used to protect materials against wear, cavitation erosion (CE), and corrosion because of their higher hardness, strength, and better corrosion resistance.The cobalt-based alloys (such as Stellite 6 or Stellite 12) consist of fcc cobalt-based dendrites with interdendritic lamella of Cr-rich carbides, which provide the alloy with the hardness to resist CE [1].Multiphase materials composed of a metallic matrix reinforced by the distribution of hard particles are extensively used to enhance toughness and hardness [2,3] and to improve the resistance of CE [4], wear [5], and slurry erosion [6].To improve the CE resistance of material, high-velocity air fuel [7], high-velocity oxygen fuel (HVOF) [8], arc spraying [9], atmospheric plasma spraying [10], and other surface modification technologies had been developed. A cobalt-based alloy coating is widely used to protect materials against wear, cavitation erosion (CE), and corrosion because of their higher hardness, strength, and better corrosion resistance. Multiphase materials composed of a metallic matrix reinforced by the distribution of hard particles are extensively used to enhance toughness and hardness [2,3] and to improve the resistance of CE [4], wear [5], and slurry erosion [6]. Coatings 2019, 9, 534 and fracture toughness), HVOF-sprayed WC-Co-based ceramic–metal (cermet) composites coatings can improve CE resistance [11,12]. Chivavibul et al [14] prepared WC-Co coatings with different cobalt contents (8 wt.%, 12 wt.%, 17 wt.% and 25 wt.%) by HVOF spraying, and they found that the hardness and Young’s modulus decreased with an increase in cobalt content, but the fracture toughness did not show noticeable changes. The mechanical properties and CE resistance affected by the cobalt content are still needed to be further studied

Results

Discussion

Conclusion