Experimental investigation and study of HVOF sprayed WC-12Co, WC-10Co-4Cr and Cr₃C₂-25NiCr coating on its sliding wear behaviour

Highlights

• WC-12 Co coating shows better wear resistance than WC–10Co–4Cr and Cr₃C₂-25NiCr coatings.

• The stability of the transfer layer governs the wear behaviour of coatings.

• The transfer layer stabilized after critical sliding distance.

• Tribo oxide layer counterbalances to stabilize the wear at high heat generation.

• Maximum adhesive strength exhibited by WC-12Co coating is 10.5% higher than Cr₃C₂-25NiCr

Abstract

An experimental investigation followed by a comparative study of HVOF sprayed WC-12Co, WC- 10Co4Cr and Cr₃C₂-25NiCr coatings were conducted at three different loads of 20, 40 and 60 N. Sliding wear of coated specimens against hardened EN-32 disc was performed as per the G 99–5 standard at room temperature. The feedstock powders and corresponding coatings were characterized for microstructural studies along with porosity, microhardness, and adhesive bond strength. The experimental results suggest that the stability of the transfer layer plays a significant role in stable wear. The WC-12Co coating shows the best sliding wear resistance, maximum microhardness which was found to be 56.6% and 9.6% higher than WC-10Co-4Cr and Cr₃C₂-25NiCr coatings. The maximum adhesive bond strength was found to be 2.03% and 10.5% higher in WC-12Co than WC-10Co-4Cr and Cr₃C₂-25NiCr. Tribo oxide layer counterbalances to stabilize the wear during high heat generation at higher loads. Various aspects and mechanisms of these improvements are discussed in this paper.

Introduction

Surface modification and its optimization is a challenging aspect for modern industrial application to increase the wear resistance. Wear reduces component life, decreases the cycle time and increases the power consumption resulting in enhancement of the operating cost of the machine. Coating deposition is the best method to enhance wear resistance and is presently applied in various industrial applications such as turbine, cylinder and valve, aerospace, oil refinery, and roller in paper mill [1,2]. Coating by thermally sprayed techniques includes flame spray, high-velocity oxy-fuel coating (HVOF), plasma spray, HVAF and detonation gun [3]. High-Velocity Oxy-Fuel (HVOF) is so far the best competent route for coating deposition to fulfill the modern industrial requirements. HVOF coatings offer good mechanical and microstructural properties [4]. HVOF coatings withstand in harsh conditions such as moisture, penetration of abrasive and erosive particles [5]. HVOF is extensively used for the deposition of hard metal powder and metal powder composite [[6], [7], [8]]. HVOF coating is preferably an appropriate method especially for tungsten carbide, chromium carbide and their matrix [9]. HVOF coatings minimize the porosity and decarburization owing to low flash temperature and higher spray velocity, resulting in excellent wear resistance, toughness and bond strength [4,10]. WC–Co and WC-Co-Cr coatings retain excellent sliding and abrasive wear resistance, better hardness and toughness [11,12]. The Cr₃C₂-25NiCr coating exhibits superior wear behaviour at higher temperatures [13]. Mechanical, wear and microstructural properties of coatings are governed by parameters like feedstock powder, residual stress, resultant porosity, binder fraction, and grain size of powder [14]. The small grain size of powder not only decreases the porosity but also increases the wear-resistance and hardness [[15], [16], [17]].

Tungsten, nickel and chromium-based coatings effectively enhance the wear resistance and other mechanical properties. Cobalt and Nickel binders are widely used in coatings which increase toughness, but cobalt is extensively used with WC due to higher bonding strength and superior microstructural properties [18]. Nickel binder provides better corrosion resistance than Co binder [19,20].

WC-12Co, WC-10Co-4Cr and Cr3C2-25NiCr coatings show different wear behaviour due to the formation of new phases of W2C, Co3W3C and Cr7C3 during spraying [21,22]. Hardness and bond strength are also affected by these new phases. Consequently, in this specific circumstance, extensive examinations that add to a better comprehension of a sliding wear phenomenon of WC-12Co, WC–10Co–4Cr and Cr₃C₂-25NiCr coatings are important. Researchers investigated the abrasive and erosive wear behaviour of WC-12Co, WC–10Co–4Cr and Cr₃C₂-25NiCr coatings, and reported that WC-12Co coating exhibited highest abrasive wear and erosion rate [23]. The sliding wear behaviour of WC-12Co and WC-10Co-4Cr was examined using a ball and disc tester to find volume losses of specimen [24,25]. The diameter of the pin specimen was very less as compared to the disc specimen. Sliding wear analysis using a pin on disc wear tester is more complex than the ball on disc wear tester because of very small volume losses [26]. The formation of thick tribo oxide layer on pin specimen varies during wear which makes it difficult to describe wear mechanism.

However, in earlier studies, the sliding wear examination and substantial wear mechanisms of WC- 12Co, WC-10Co-4Cr and Cr₃C₂-25NiCr coatings under three different loads of 20, 40 and 60 N using pin specimen and hardened EN 32 counter disc has not been reported. To fulfill this gap, the wear performance of WC-12Co, WC-10Co-4Cr and Cr₃C₂-25NiCr coatings against hardened EN-32 counter disc under the sliding wear condition was examined. Three different loads were taken and compared to understand different wear mechanisms among these coatings.