Theoretical and experimental investigation of material removal rate in shape adaptive grinding of HVOF sprayed WC-Co coating
Introduction
Thermal spray coating is an economical method to alter the surface characteristics of various components without modifying the substrate properties [1,2]. It is mostly used for wear and corrosion resistance applications to enhance the lifespan of the machine components [3,4]. Since past few decades, WC based coating has been broadly used in aerospace, pump, textile, mining, automobile and power generation industries [5,6]. Some applications of WC based coatings such as cylinder and piston rod of the landing gear of aircraft, automotive engine cylinder and calendar roll of the printing devices require nanolevel surface finish [7,8].
High velocity oxy-fuel (HVOF) spraying process is vastly used for the fabrication of cermet coating as it can produce good quality of coating (i.e., high bonding strength and low porosity, etc.) [9]. As-deposited cermet coating exhibits high surface roughness and a suitable finishing technique requires to be engaged to achieve the desired surface roughness. Grinding is mostly employed to perform finishing operation of WC based coating. Murthy et al. [10] conducted surface grinding of WC based coatings using a diamond grinding tool. It was observed that the residual stress of the ground coating was substantially larger than that of the as-sprayed coating. Liu et al. [11] conducted grinding of the nanostructured carbide coating and investigated the material removal mechanism associated with the surface grinding operation. It was found that the material was removed through both the plastic flow and brittle fracture. Maiti et al. [12] conducted surface grinding of WC based coating using a diamond abrasive tool. It was observed that the hardness of coating could be enhanced by performing grinding of the coating. This is attributed to the evolution of high compressive residual stress during the grinding operation. Masoumi et al. [13] studied the grinding force and material removal mechanism in grinding of WC based coating. It was observed that the percentage of brittle fracture was substantially increased with an increase in depth of cut and table speed. Zoei et al. [14] investigated the functional performance of the WC based coating after surface grinding operation. It was observed that the wear resistance of the ground coating was considerably higher than that of as-sprayed coating. Pishva et al. [15] studied the corrosion behaviours of the as-sprayed and ground WC-Co-Cr coatings. A higher corrosion resistance was observed in the case of ground coating. This is attributed to comparatively lower surface roughness of the ground coating.
A few literatures are available where efforts were made to achieve nanofinishing of sintered carbide. Kim and Lee [16] developed an in-process electrolytic dressing system coupled with metal bonded grinding wheel to attain nanoscale surface roughness of the sintered carbide. However, a complex system is needed for this process. Jahan et al. [17] conducted die-sinking micro-electro-discharge (EDM) machining of sintered WC-Co composite using silver tungsten (AgW) electrode having a diameter of 500 μm. The events of arcing and short-circuiting were frequently occurred even at a lower gap voltage owing to the existence of a small working gap. As a result, few surface or sub-surface damages were found on the finished surface. Yin et al. [18] carried out the ultraprecision grinding of sintered carbide (i.e., WC-Co) and high surface finish was achieved. This process largely relied on the rigidity and stiffness of the machine tool, and the accuracy of the feedback control system that involves a very high capital investment. Besides, it is difficult to achieve nanolevel surface roughness on thermal spray coating using a rigid abrasive tool. The higher cutting force associated with the conventional machining may promote the pull out of particles and may damage the coated surface due to the truly mechanical bonding of the splats and the lower heat dissipation rate from the contact region [19].
Shape adaptive grinding (SAG) has the ability to perform nanofinishing over various materials. Beaucamp et al. [20] performed SAG of titanium based alloy (Ti6AL4V) produced by selective laser sintering (SLS). A highly irregular layer was found on the outer surface of the as-deposited titanium alloy, and the surface roughness was around 5 μm (). It was observed that the layer could be successfully removed by SAG and a highly finished surface was obtained. Beaucamp et al. [21] conducted SAG of SiC optics using diamond pellet abrasive pads. A higher material removal rate (MRR) was observed owing to the existence of rigid diamond pellets on the SAG tool. A model was proposed to correlate the average indentation depth of the abrasive particles with the process parameters. Zhu et al. [22] developed a spring-grain model for better understanding the interactions between the abrasive particles and workpiece in SAG. Material removal mechanism for the individual abrasive particles situated at the different locations of the SAG tool was revealed using the proposed model. Yang et al. [23] performed the finishing of thin-walled (deformable) aluminium sheets using a SAG tool of 12 mm diameter. A control strategy was proposed to perform uniform material removal over the entire aluminium sheet.
Material removal rate (MRR) is extensively investigated for various finishing processes using Preston's equation. Preston's equation presents a classic theory of material removal that indicates MRR varies proportionally with both the contact pressure and velocity of the polishing tool [24]. Matsuo et al. [25] conducted chemical mechanical polishing (CMP) of copper and presented a revised MRR model. In the proposed model, the frictional force was employed instead of the contact pressure. This study proposed a convenient method for better assessing the removal mechanism in polishing process. Wang et al. [26] proposed a modified model that presents a non-linear dependency of the contact pressure with MRR. Apart from the polishing experiments, finite element method was also employed to verify the proposed model. Zeng and Blunt [27] developed a MRR model for the bonnet polishing based on the Preston's equation. All the important process parameters of the bonnet polishing were introduced in the proposed model to assess the effect of individual parameter on the MRR. Ren et al. [28] observed that the MRR model could be developed based on the modified Preston's equations even for the continuous polishing process. They also incorporated the influence of process vibration during polishing with various rotational speeds on the MRR model. However, a huge numbers of experiments were needed to determine the Preston coefficient. Xia et al. [29] reported that apart from the polishing pressure and relative velocity of tool, several other factors such as properties of the finishing tool and workpiece material, abrasive particle size, and concentration or density of abrasive particles have a significant influence on the MRR. Kum et al. [30] employed computational fluid dynamics (CFD) simulation to develop a MRR model for abrasive flow finishing process. It was found that the extrusion pressure, velocity of media, properties and geometry of the workpiece material were found as the dominant parameters. Lin et al. [31] incorporated the effect of abrasive pad topography on MRR in computer controlled polishing process. It was found that the abrasive wear had a significant effect on the MRR and the corresponding results were verified with the simulation. Su et al. [32] investigated the shape preserving capability of bonnet tool using simulation as well as experimental observations. The contact condition was identified as the main influential parameter. It was found that the change in form of the bonnet tool after finishing could be restricted to 20 nm under suitable condition. Cheung et al. [33] conducted simulation and theoretical analysis for the generation of different surface patterns in bonnet polishing. It was observed that the proposed model could successfully predict the form error in the generated surface texture.
Although nanofinished cermet coating has significant industrial applications, ultra-high finishing of the coating is rarely addressed. In the present study, shape adaptive grinding (SAG) of the thermally sprayed carbide coating (i.e., WC-Co) is conducted with diamond abrasive pad. The effects of tool speed, tool compression, backing pad hardness and abrasive particle size of the polishing pads on the normal force, material removal rate (MRR) and surface roughness are studied. By considering indentation of active abrasive particle, cross-sectional area and length of the finishing grooves, a MRR model is proposed. The model is verified with the experimental findings. By performing SAG with the abrasive pads with decreasing particle sizes in sequence, the surface roughness () was reduced to around 7 nm. The cross-section of the finished coating is analysed to study the sub-surface condition and residual stress of the finished coating is also measured.
Section snippets
Experimental procedures
In the present study, HVOF technique (HIPOJET 2700) was used for the fabrication of WC-Co coating on low carbon steel substrate (50 mm × 50 mm × 5 mm). A commercially available WC-12Co powder (Metco 72F-NS) was used as the coating materials. The process parameters of HVOF spraying were reported in the earlier study [34]. The thickness of the as-deposited coating was in the range of 300–400 μm. The coating properties were measured in previous study and those are listed in Table 1. The
Modeling of material removal rate (MRR)
In the present study, SAG tool contains a flexible rubber backing covered with the diamond polishing pad. During finishing action, it acts as an elastic deformable body and interacts with the workpiece material (i.e., a rigid flat surface). The shape of the SAG tool is roughly cylindrical with a diameter (d = 2r) and height (H) of around 10 mm. The compression tests of all the three rubber pads (i.e., S = 30 A, 40 A and 50 A) are conducted using a DMA. The rubber pads are compressed from 0 to
Results and discussion
The pseudo-colour view of the as-sprayed coating is displayed in Fig. 7 (a). The coated surface consists of many randomly oriented and distributed big asperities and valleys. The mean protrusion height of the peaks and the depth of valleys are significantly higher and in micron scale. The peak-to-valley height () of the coated surface is found to be around 53 μm. The coated surface exhibits a surface roughness () as high as 5.1 μm. At first, grinding of the as-deposited coating is conducted
Conclusions
The purposes of the current study are to conduct ultra-high finishing of WC-Co coating, to develop a theoretical model of material removal rate (MRR) in SAG, and to assess the effects of process parameters on normal force, MRR and surface roughness. The important outcomes of the current investigation are listed below.
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The average penetration depth of the abrasive particles can be evaluated by considering the contact load on SAG tool and the number of active abrasive particle present over the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors acknowledge the funding support from the Indian Institute of Technology Kharagpur under ISIRD grant, Board of Research in Nuclear Sciences (BRNS) under young scientist research award (34/20/10/2015/BRNS) and Science and Engineering Research Board (SERB) under young scientist scheme (YSS-2015-001163).
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