Elsevier

Ceramics International

Volume 43, Issue 17, 1 December 2017, Pages 15316-15331
Ceramics International

High speed and precision grinding of plasma sprayed oxide ceramic coatings

https://doi.org/10.1016/j.ceramint.2017.08.071Get rights and content

Abstract

In this paper, the effects of high speed grinding on six thermally sprayed ceramic coatings were investigated. Grinding was undertaken using four wheel speeds and two work speeds. The responses studied were the following; grinding forces, specific grinding energy, force ratio, surface roughness and surface residual stress. In addition, grinding in traverse mode was also considered in this investigation. An increase in wheel speed resulted in an appreciable reduction in grinding forces. The micro-brittle fracture was the predominant mode of material removal along with micro-cutting in some cases. Further, the surface roughness of traverse ground samples was superior to those obtained using plunge grinding. Finally, it was found that surface residual stresses of samples produced during high-speed grinding are lower than those of samples using precision grinding.

Introduction

Thermally sprayed coatings are applied to combat wear, corrosion and provide thermal insulation [1], [2], [3]. However, these coatings contain pores, unmelted or partially melted particles [4], [5], [6]. The roughness of the as-coated surface is of the order of 6–8 µm [7]. Certain applications like printing rolls, automobile parts, turbine components require high finish and accuracy [8], [9], [10], [11], [12]. The as–sprayed coatings are not suitable for such applications. Precision grinding of ceramics and ceramic coated samples resulted in an improvement of surface finish and in some cases, a reduction in the residual stress of the ground coated surface [13], [14], [15]. Ceramic coatings are hard and brittle. During precision grinding of such coatings, subsurface defect density increases. The wheel wear also increases during grinding of such hard coatings. However, ceramics finished using high-speed grinding, are free from these limitations [16], [17]. Klocke et al., reported that at higher grinding speed, both better surface finish and higher material removal rate were achievable. In addition, a considerable reduction in thermal damage to workpiece was also observed during high-speed grinding of metallic alloys [18]. Huang and Liu observed an improvement in surface finish followed by an increase in the grinding ratio and a decrease in subsurface damage, during high-speed grinding of sintered ceramics [19]. Chen et al., also observed that with an increase in the wheel speed, the grinding forces were drastically reduced for sintered ceramic workpieces. This was attributed to a significant reduction of uncut chip thickness at a higher wheel speed [20]. Some studies have also been conducted on finishing of thermally sprayed coatings. The researchers primarily studied thermally sprayed alumina, alumina–titania, tungsten carbide and chromia coatings [21], [22], [23], [24]. They have studied different grindability characteristics like forces, surface integrity, wheel wear, etc. In a detailed study, Kar et al. investigated grindability of different plasma sprayed oxide ceramics in the precision plunge surface grinding mode using single layer electroplated diamond wheels [15]. At a low grinding speed, it was observed that the mode of material removal is primarily micro-brittle fracture. Further, the grinding chips were observed to be irregular and blocky and their shape did not change with a variation in the grinding parameters. Finally, grinding-induced residual stresses of thermal origin were not detected on the ground surfaces, and this was attributed to retention of physical properties by ceramic coatings at grinding temperature.

From the above literature survey, it was observed that most of the work on high-speed grinding is related to the sintered ceramics only. Additionally, these investigations were limited to precision grinding employing either resin bonded or galvanic wheel with cBN and diamond grits [21], [22], [23], [24], [25], [26], [27]. Very few reports have been published on the finishability studies of thermally sprayed ceramics and cermets [14], [15], [23], [28], [29], [30], [31]. The advantages of high-speed grinding of ceramic coatings are yet to be explored. Thus, the aim of the present paper is to experimentally investigate the effect of work speed and wheel speed on various aspects of grindability characteristics of six different plasma sprayed oxide ceramic coatings using single layer electroplated diamond wheel. The study is restricted to a grinding speed of 150 m/s. The selected coatings represent a wide gamut of oxide ceramic coatings in the industry requiring finishing.

Section snippets

Experimental procedure

The coatings were deposited on 10 mm × 10 mm × 100 mm 1060 steel using a Sulzer Metco 9 MB plasma gun mounted on a CNC X–Y table. Argon, nitrogen and hydrogen gases, respectively, were used as the plasma initiator, primary and secondary plasma gases for all the coatings. The process parameters used for coating each powder are listed in Table 1. The details of the deposition process are described elsewhere [7].

The ultrasonically cleaned substrates were preheated to 150 °C and a layer of the bond

Results and discussion

Fig. 2(a) shows typical variation in the tangential and normal grinding forces, in a given grinding pass. These are raw signals as captured by the force measurement system. The processed signal is shown in Fig. 2(b). The normal grinding force is clearly much higher than the tangential grinding force as it is expected [32]. The ratio of normal grinding force to tangential grinding force is around 5. Similar observations have also been reported for precision grinding of thermally sprayed ceramic

Conclusion

Grinding forces generated during high–speed grinding of ceramic oxide coatings tend to decrease with an increase in wheel speed. In general, both grinding forces and specific grinding energy were very low during grinding of these coatings. The ratio of normal to tangential force, i.e., force ratio, was found to lie between 2 and 10. This ratio did not increase with an increase in wheel velocity, as expected. This is attributed to the predominance of brittle fracture as the mechanism of material

References (39)

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