Influence of feedstock powder characteristics and spray processes on microstructure and properties of WC–(W,Cr)2C–Ni hardmetal coatings

doi:10.1016/j.surfcoat.2010.07.032

Surface and Coatings Technology

Volume 205, Issue 4, 15 November 2010, Pages 1080-1087

https://doi.org/10.1016/j.surfcoat.2010.07.032 Get rights and content

Abstract

The composition WC–(W,Cr)₂C–Ni (commercial designations WC–‘CrC’–Ni, WC–Cr₃C₂–Ni and WC–NiCr) is unique among the WC-based materials used for the preparation of thermally sprayed hardmetal coatings. These coatings show a significantly higher oxidation resistance and high-temperature sliding wear resistance than WC–Co and WC–CoCr coatings do. Unlike WC–Co and Cr₃C₂–NiCr, WC–(W,Cr)₂C–Ni is not a simple binary hard phase–binder metal composite as it is composed of two hard phases: WC and (W,Cr)₂C. Surprisingly this composition has been poorly investigated in the past.

In this paper coating microstructures and properties obtained from five commercial feedstock powders of different origins using two different liquid-fuelled high velocity oxy-fuel (HVOF) systems (K2 and JP-5000) were investigated. Additional experiments were performed with one powder using atmospheric and vacuum plasma spraying (APS and VPS, respectively). The microstructures and phase compositions of the powders and the coatings were studied. Focus was on the appearance, composition and distribution of the (W,Cr)₂C phase which might form or might change its Cr/W ratio during the spray process. The composition of the (W,Cr)₂C phase was estimated from the lattice parameters. Hardness HV0.3 was measured for all coatings. The density, Young's modulus and abrasion wear resistance of HVOF-sprayed coatings were studied.

Introduction

The composition WC–(W,Cr)₂C–Ni is widely used today in industrial practice for the preparation of thermally sprayed hardmetal coatings by high velocity oxy-fuel (HVOF) spraying. It was invented in 1958 for coatings prepared by detonation gun spraying (DGS) [1]. As opposed to studies on WC–Co and WC–CoCr coatings, investigations of this composition have been rare in the past; most of the studies were performed by Japanese researchers, e.g. [2], [3], [4], [5]. A more detailed literature survey is given elsewhere [6]. The authors' previous investigations showed an oxidation resistance that was much superior to that of other commercial WC-based coatings [7]. The coatings show low wear rates and low coefficients of friction in dry sliding wear conditions when mated with alumina at 800 °C. At this temperature other WC-based coatings cannot be applied at all [8].

Frequent use of the commercial designations WC–‘CrC’–Ni, WC–Cr₃C₂–Ni and WC–NiCr indicates the insufficient knowledge about the phase compositions, microstructures and properties of the coatings as well as on the corresponding feedstock powders. This was the backdrop of the author's previous analysis of the state of knowledge on the microstructure, phase composition and coating properties of this composition [6]. A special feature of this composition is the appearance of the second hard phase (W,Cr)₂C. This phase has the structure of W₂C with different amounts of chromium dissolved in it. The dependence of the lattice parameters on the chromium content was published by Stecher et al. [9] (see Fig. 1). However, no information about the properties of this phase is available; only recently were new results on its formation published [10], [11]. Unlike WC–Co and Cr₃C₂–NiCr, WC–(W,Cr)₂C–Ni is not a simple binary hard phase–binder metal composite due to the appearance of the second hard phase [6]. Surprisingly, despite the low content of the metallic binder and its inhomogeneous distribution in the feedstock powders, the WC–(W,Cr)₂C–7%Ni composition can be sprayed with a high deposition efficiency.

In this paper the processability of five commercial WC–(W,Cr)₂C–Ni feedstock powders was studied. These feedstock powders were sprayed with two different liquid-fuelled HVOF systems (K2 and JP-5000). Additional experiments were performed with one powder using atmospheric and vacuum plasma spraying (APS and VPS, respectively). The microstructures and phase compositions of the powders and the coatings were investigated. Focus was on the appearance, Cr/W ratio and distribution of the (W,Cr)₂C phase. Preliminary results of this work were published in earlier conference proceedings [12].

Section snippets

Materials and processes

Five different commercially available WC–(W,Cr)₂C–Ni powders were used in this study. The suppliers, trade names, compositions as traded, manufacturing methods and particle sizes are given in Table 1. Three powders were prepared by agglomeration and sintering (a&s), one by sintering and crushing (s&c) and one by agglomeration and plasma densification (a&pd). All powders were deposited by HVOF spraying using a K2 spray gun (GTV mbH, Germany) at the Fraunhofer Institute of Materials and Beam

Powder characterization

Each of the five WC–(W,Cr)₂C–Ni powders exhibited a characteristic microstructure and phase composition. The particles of the a&s and the a&pd powders were more porous than those of the s&c powder were. Fig. 2 shows selected cross sections of the a&s powders 1 and 2, the s&c powder 4 and the a&pd powder 5. The WC grains appeared bright in the powder cross sections. The WC contents of powders 4 and 5 were lower than those of the a&s powders. Analysis of the s&c powder 4 revealed areas of WC and

Summary and conclusions

Coatings prepared by HVOF spraying with two liquid-fuelled systems from five commercial WC–(W,Cr)₂C–Ni powders were investigated. Despite the fact that at 7 mass % the binder content in this composition is low compared with other commercial hardmetal coating compositions, the processability and coating properties were excellent. One selected a&s powder was also processed by APS and VPS.

Major differences in the feedstock powder phase compositions and microstructures were observed between the

Acknowledgements

The authors wish to thank Mr C. Jordan for spraying of HVOF samples with the K2 process, Ms B. Wolf (Fh-IWS) for metallographic preparation and Ms A. Richter, Ms S. Raschke and Dr J. Bretschneider (Fh-IWS) for SEM investigations. The stay of M. Kašparova at Fh-IWS was supported by the Erasmus program of the European Community.

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Influence of feedstock powder characteristics and spray processes on microstructure and properties of WC–(W,Cr)2C–Ni hardmetal coatings

Abstract

Introduction

Section snippets

Materials and processes

Powder characterization

Summary and conclusions

Acknowledgements

Surf. Coatings Technol.

Wear

Tribol. Int.

J. Thermal Spray Technol.

J. Thermal Spray Technol.

Proc. Int. Thermal Spray Conf. ITSC 2005, May 2–4 (Basel, Switzerland), (DVS-Verlag, Düsseldorf) CD

Influence of feedstock powder characteristics and spray processes on microstructure and properties of WC–(W,Cr)₂C–Ni hardmetal coatings