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Precision superabrasive grinding of plasma sprayed ceramic coatings
Author’s Accepted Manuscript Precision superabrasive grinding of plasma sprayed ceramic coatings Simanchal Kar, P P Bandyopadhyay, S. Paul
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S0272-8842(16)31633-9 http://dx.doi.org/10.1016/j.ceramint.2016.09.100 CERI13760
To appear in: Ceramics International Received date: 17 August 2016 Revised date: 13 September 2016 Accepted date: 14 September 2016 Cite this article as: Simanchal Kar, P P Bandyopadhyay and S. Paul, Precision superabrasive grinding of plasma sprayed ceramic coatings, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.09.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Precision superabrasive grinding of plasma sprayed ceramic coatings Simanchal Kar, P P Bandyopadhyay*, S. Paul Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, West Bengal 721 302, India. *
Corresponding author: phone: +91 3222 282950. e-mail:[email protected] Abstract: In this investigation, a set of plasma sprayed ceramic oxide coatings were surface ground using super abrasive wheel. The surface integrity and grindability aspects of the coated surface were studied. During the grinding process, low grinding forces and specific grinding energy were observed and these observations pointed toward micro brittle fracture to be the predominant material removal mechanism in all cases. SEM and FIB milling of the ground samples also revealed that material removal is primarily owing to micro-fracture rather than micro-cutting. This is also corroborated by short broken chip formation during grinding. Further, the nature of the chips did not vary with change in grinding parameters. The surface topography showed signatures of micro-fracture, ploughing and rubbing only. The ground surface harbours residual stress. However, this stress, unlike metals, does not have a thermal origin. This is attributed to retention of material properties by the ceramic coatings at the grinding temperature. Keywords: plasma spray, ceramic oxide, super abrasive, grinding chips, residual stress, critical depth.
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1. Introduction: Plasma sprayed coatings are constituted by layers of deformed particles or splats. Defects like porosity and voids are commonplace in such coatings. Other major problems in these coatings are high surface roughness and poor dimensional tolerance of the coated components, as identified by Kanouff et al [1]. Plasma sprayed coating finds applications in different industries. Some examples of application of these coatings are combating wear [2,3] or thermal insulation [4,5] etc. Gerard has discussed application of thermal spray coating in automobile industries [6]. Such applications require acceptable level of surface finish and dimensional tolerance. Ceramic coatings deposited by plasma spraying can only be finished by grinding. In grinding of metallic alloys, material removal mechanism is based on shear deformation [7–9]. In ceramic grinding, while both brittle fracture and plastic deformation occur, the former action is predominant [10–13]. In ceramic grinding, force ratio i.e., FN / FT is high [14]. Chips are blocky in shape, bearing signature of fracture. The ground surface is susceptible to alteration in residual stresses that are present in an as-sprayed coating and formation of micro-cracks induced during the grinding process itself. Further, conventional wheels are ineffective as the hardness of the wheel and the coating are comparable, and wheel wear is high [15]. In order to overcome the problem of wheel wear, super abrasive wheels like cBN and diamond wheels are used during grinding of plasma sprayed ceramic samples [16–18]. cBN and diamond grits have better strength, wear resistance and high hardness. Ramajani et al., [19] studied grindability of nanostructured and conventional air
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plasma sprayed coating of Al2O3 13wt% TiO2. Grinding was undertaken with cBN wheel and they observed both transverse and longitudinal cracks on the ground surface.
Jia and Fischer [20] evaluated the abrasion resistance of nanostructured WC-Co coating, by abrading it against a diamond grinding wheel. They did not study any of the grindability parameters. Liu and Zhang [17] experimentally investigated the effects of material removal rate and grit size of the abrasive on the surface damage upon grinding of plasma sprayed nanostructured Al2O3 13wt% TiO2 and WC-Co coatings using diamond grinding wheel. They observed transverse as well as longitudinal cracks which increased with an increase in downfeed and workspeed. The same authors [21] in a sequel work studied the state of stress on these coatings. They concluded, grinding significantly changes the state of residual stress of an as-sprayed surface. Liu et al. [22], experimentally studied grinding of the same set of coatings also using a diamond grinding wheel. They studied the effect of downfeed and workspeed on normal grinding force, surface roughness and surface damage. They observed ductile flow and brittle fracture on the surface of the coating during grinding.
In a follow up investigation, Zhang et al., [23] studied the effect of grinding parameters on the surface roughness of thermally sprayed nanostructured Al2O3 13wt% TiO2 and WCCo coatings. They did not observe a significant change in the surface roughness with downfeed. Grinding induced sub-surface damage and depth of the same increased with an increase in the downfeed.
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Most of the above works are restricted to Al2O3 13wt% TiO2 and WC-Co coatings. The grinding wheels are either resin bonded or vitrified bonded. There has been no attempt to study other plasma sprayed coatings like Al2O3, TiO2, Cr2O3, YSZ etc. Usage of a single layered diamond grinding wheel that has more chip accommodation space has not been reported. Though the effects of downfeed and workspeed have been studied on different grindability parameters, but there is a lack of comprehensive discussion on grindability. Thus, the aim of the present paper is to systematically study the effect of downfeed, and workspeed on the different aspects of grindability of six different types of plasma sprayed coatings using single layer electroplated diamond wheel. These six coatings constitute a near exhaustive set of thermally sprayed ceramic coatings.
2. Experimental procedure: Low carbon steel coupons of dimensions 100 ×10 ×10 mm, served as substrate. Six ceramic powders were used to deposit top coat (Table 12). An air plasma spray system (SULZER METCO 9MC series spray unit with 9MB Gun (80 kW)) has been used for spraying purpose. A bond coat of thickness of around 100 µm has been applied first, followed by a thicker top coat (300-700 µm) [24]. The coating parameters are shown in Table 1. These parameters are recommended by the powder manufacturer (Sulzer Metco). The particle size and morphology are listed in Table 2. In this investigation alumina powders with two cut sizes were used, namely, Al2O3 NS (-45+15) and Al2O3 SFP (-31+3.9). These two powders were deposited using two arc currents; 500 A and 600 A. Therefore, the total numbers of coatings investigated in the present work is eight.
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Grinding was performed on a semi-automatic surface grinding machine (COSMOS EARTH, India) using mono layer nickel bonded diamond wheel. The grinding parameters are shown in Table 3. The forces was measured using a high-resolution piezoelectric type (make- KISTLER Instrumente AG Winterthur Switzerland, model: 9254) dynamometer equipped with two charge amplifiers (make- KISTLER Instrumente AG Winterthur Switzerland, model: 5011B10). The readings were recorded from an oscilloscope (make-Agilent Technologies, model: DSO X 2014A) connected to the amplifiers. For scanning electron metallography, slices of 10 mm × 10 mm × 10 mm has been cut off from the sprayed samples using a low speed diamond saw. The samples were placed in a resin bond and polished in the cross section using silicon carbide abrasive papers and diamond pastes of decreasing grit size. The polished cross sections were observed under scanning electron microscope (SEM) (Zeiss EVO 60, Germany and JEOL JSM 5800, USA). Residual stress of the as sprayed and ground sample was measured by sin2 method using a high resolution Xray diffractometer (Philips PW1710, Netherlands) equipped with a Co-Kα (λCo = 1.79 nm) target.
Hardness of the coating was measured using a hardness testing machine (LECO
LM700, USA) equipped with a Vicker’s indenter under 200 gf load ( 1.961N). Depth sensing indentation were performed using an instrumented hardness tester (Micro combi tester (MCT), CSM instruments, Switzerland) equipped with a Vickers indenter. The indentation measurements were undertaken under a maximum load of 2000 mN, loading and unloading rate of 4000 mN/min and with a dwell time of 15 s [25]. The elastic moduli of the coatings were calculated from the slope of the unloading curve. Indentation fracture
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toughness (KIC) values were calculated using the following expression proposed by Evans and Charles [26]
c K IC 0.15k a
3/2
H a
(1)
This method can be used very effectively for comparison of toughness of brittle ceramic coatings having similar microstructure and order of toughness [24,27]. Surface roughness of the coating has been measured using a contact type TAYLOR HOBSON Surtronic 3+ ‘2D’ surface profilometer and a TAYLOR HOBSON FORM TALYSURF 50 (INTRA 2) ‘3D’ profilometer (TAYLOR HOBSON, UK). In both cases a traverse length of 4 mm was used. 3. Results and Discussion: 3.1 Grinding forces versus downfeed: Fig.1 shows the variation in the tangential and normal forces, while grinding the plasma sprayed Al2O3 NS sample at a grinding velocity of 26 m/s, work speed of 6 m/min and downfeed of 38 µm with single layer electroplated diamond wheel. The raw force signals, as shown in Fig.1, have been further processed using a user developed MATLAB code and the processed grinding forces are depicted in Fig. 2. It clearly shows that the normal grinding force is higher than the tangential grinding force component. From Fig. 2, the average value of grinding forces has been determined using the said MATLAB code. The average and maximum force values are shown in Fig. 2.
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Fig. 3 and Fig. 4 show the variation in specific tangential and normal grinding forces respectively, with downfeed for two different work speeds. Each of the eight plasma sprayed coatings was taken into consideration.
3.2 Grinding forces versus maximum uncut chip thickness An increase in downfeed and work speed leads to an increase in the maximum uncut chip thickness according to the following equation [28]:
[
√ ]
(2)
As the maximum chip thickness increases, the chip load per grit also increases. Thus, in Fig. 3 and Fig. 4, increase in downfeed and work speed has led to an increase in tangential and normal grinding forces.
Fig. 5 shows the micrograph of the grits on the grinding wheel. The grit size used in the current wheel is 150 µm and the static grit density has been calculated to be 33 grits/mm2.
Fig. 6 and Fig. 7 respectively, show the variation in specific tangential and normal grinding forces with respect to the maximum chip thickness for the four selected coatings. Both the figures show an increase in the grinding forces with an increase in the maximum uncut chip thickness. Similar observations have been reported by previous researchers while grinding 7
metallic alloys [29]. Though maximum uncut chip thickness could unify grindability results across work speed, particularly for metallic alloys [30], but Fig. 6 and Fig. 7 seem to depict some individual effect of work speed on grinding forces, irrespective of the choice of plasma sprayed coating. This is attributed to micro-fracturing being the predominant mode of material removal in the present case. 3.3 Grinding force ratio and specific grinding energy:
Fig. 8 reveals the variation in grinding force ratio, FN / FT with downfeed for different work speeds. The indentation fracture toughness of most of the coatings investigated varied between 1.5 2 MPa-m1/2 (Table 4). Only chromia had a fracture toughness of 3 MPa-m1/2. However, individual or combined effect of fracture toughness or hardness of the coatings has not been observed on the force ratio. For all the material taken together, force ratio varied between 3 6. Chen et al. [12,31] employed brazed monolayer diamond wheel for grinding of alumina and zirconia sintered ceramics and observed force ratio FN / FT to be in the range of 5 10 for these two materials. Lower force ratio FN / FT in the present work can be attributed to the inherent defect in the plasma sprayed ceramic coating. It is well known that porosity and defects are commonplace in plasma sprayed coating [24]. In a plasma sprayed coating, porosity is generated owing to the following causes: 1) shadow effect that occurs during splashing of a second particle over a previously arrived one, and this in turn, produces porosity within a lamellar layer, 2) narrow holes or trapped gas inclusions between two consecutive lamellae, 3) unmelted particle inclusions, and 4) 8
impact of high velocity particles resulting in disruptive shock waves [32]. In addition, a plasma sprayed coating is formed by splats that are held together by weak cohesive force [33–35]. The weak cohesion also contributes to the low grinding force ratio. On the other hand, sintered ceramic components have high cohesive strength and larger grinding force ratio.
Fig. 9 reveals the variation in the force ratio against maximum uncut chip thickness for four selected coatings. Typically the force ratio reduced slightly with an increase in the maximum uncut chip thickness in the range of 0.9 to 1.7 µm. As the uncut chip thickness increases, the effect of edge radius of the grit on material removal reduces. For a low uncut chip thickness, the rake angle posed by the grit is more negative. This explains the reduction in force ratio with an increase in the uncut chip thickness. This effect is more pronounced in the case of a metallic material [36,37]. In a plasma sprayed coating, material removal is primarily micro-fracturing and effects of micro-cutting, ploughing and rubbing are not very significant. This is why hm has a limited influence on FN / FT in this case.
Specific grinding energy ( u g ) consists of contribution from rubbing, ploughing as well as chip formation owing to micro-cutting, and material removal owing to micro-fracture. Typically, u g reduces with an increase in the downfeed and approaches specific energy required for chip formation. The specific chip formation energy has been empirically found
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to be E/14 where E is the elastic modulus of the material expressed in GPa and u g is expressed in J/mm3 [38]. However, in case of brittle materials, contribution of microcutting for chip formation is very insignificant towards material removal [8]. For sintered brittle materials like alumina, silicon nitride, silicon carbide and yttria stabilized zirconia, the u g is much less than that of metallic alloys [11,28]. This is primarily attributed to insignificant contribution from ploughing and chip formation owing to micro-cutting. The major contribution towards u g in the present case has been from micro-fracture that is more energy efficient as compared to micro-cutting. Thus for all the materials taken together, u g has been observed to be as small as 0.5 to 5 J/mm3 as shown in Fig. 10. It may be noted that for sintered ceramics like alumina, the typical range of specific grinding energy in precision grinding is around 10 to 100 J/mm3 [11]. The specific grinding energy for sintered ceramics is higher than the present coatings owing to the presence of defects commonplace in plasma sprayed coating and its low cohesive strength. There is also no systematic effect of downfeed or workspeed on u g . This is also attributed to the porous and inhomogeneous nature of plasma sprayed coatings [24].
Fig. 11 depicts the effect of hm on u g for four of the selected coatings. There is no significant effect of hm on u g since the material removal mechanism is predominantly micro-fracturing.
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3.4 Grinding chips morphology and ground surface topography: Morphology of grinding chips can indicate mechanism of chip formation. In metallic alloys, primarily three types of chips are obtained and they are (1) spherical chips, (2) irregular shaped micro-chips, and (3) relatively long (around a millimeter long) chips which are produced by micro-cutting [9].
Fig. 12 shows the morphology of grinding chips for all the eight coatings at a downfeed of 14 µm and workspeed of 12 m/min. Close examination reveals complete absence of spherical chips and chips produced by micro-cutting. This indicates material removal to be primarily by micro-fracture. Most of the chips are irregular in shape and are as large as tens of microns which are much larger than hm. This clearly provides evidence of microfracturing which is the predominant mechanism of material removal.
Fig. 13 shows the effect of material removal rate on chip formation. The material removal rate gradually increased from 1.4 mm3/mm-s to 3.8 mm3/mm-s, but that did not provide any change in the chip morphology for one of the typical coatings (Al2O3 SFP sprayed at 600 A). This, once again indicates the predominance of micro-fracturing as the mode of material removal. Fig. 14 shows the morphology of chips while grinding sintered Al2O3. These chips are of more uniform size distribution as compared to the chips obtained from the coating. This
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may be attributed to presence of porosity in the coating leading to non-uniform size of chips during material removal.
Fig. 15 reveals the 3-D ground surface topography over an area of 4 mm 4 mm for three typical coating which are Al2O3 NS, Al2O3 – 13 wt% TiO2 and TiO2. All the topographies clearly reveal the micro-cutting marks. Fig. 16 shows the secondary electron image of the ground surface topography for the same coatings, captured using the same magnification. In this figure, the grinding marks are not visible as the magnification is much higher compared to lateral magnification of Fig. 15. Grinding marks appear at the inter-grit pitch in the transverse direction. In the present case, grit density is 33 /mm2, thus the average inter-grit distance is around 0.175 mm (√
). Thus the grinding marks owing to micro-cutting action
are not readily visible at a higher magnification. However, Fig. 16 clearly reveals the dissimilarities between ground surface topographies of Al2O3 – 13 wt% TiO2 and TiO2 coating in comparison to Al2O3.
Table 4 shows the different material properties for the present set of coatings and a parameter called ‘critical depth’. The critical depth has been determined using the approach proposed by Malkin and Hwang [11]. If the critical depth is high for a given coating, it has more metallic characteristics. The table clearly shows that TiO2 and Al2O3 – 13 wt% TiO2 have higher critical depth as compared to all other coatings. This observation corroborates
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to the findings of Fig. 16. The plastic deformation and ploughing marks are clearly visible for TiO2 coatings. These marks are also visible to a lesser extent in Al2O3 – 13 wt% TiO2 coating for which critical depth is slightly less. For Al2O3 coating, it seems material removal has primarily occurred owing to micro-fracture. In Fig. 16, for the same coatings the three figures are arranged in the increasing order of material removal rate. The material removal rate does not seem to have any significant effect on ground surface topography and possibly on the mechanism of material removal. Similar observations have been also reported while studying the effect of material removal rate on chip morphology in a previous section.
Fig. 17 shows the ground surface topography of all eight coatings at downfeed of 14 µm and workspeed of 12 m/min. The SEM photographs clearly show presence of microfracturing to be one of the mechanisms of material removal. Presence of pores and defects promotes micro-fracturing as mechanism of material removal. Plasma sprayed coating provide significant porosity owing to partial melting of powders and splat by splat built up of coating [39]. This also promotes formation of internal cracks and defects.
Fig. 18 shows the subsurface characteristics of Al2O3 SFP and Al2O3 NS coatings sprayed at 600 A as revealed from the secondary electron images of FIB milled pockets, before and after grinding. On the coated sample, some of the defects are visible. Upon grinding, the
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density of defects and cracks increases significantly, owing to the grinding action of the grits and micro-fracturing being the predominant mode of material removal. 3.5 Ground sub-surface characteristics and surface roughness:
Fig.19 depicts the sub-surface characteristics of ground alumina SFP 600 A as compared to coated samples under SEM. The sub-surface damages upon grinding have been highlighted by a rectangular box. The sub-surface damages are micro-crack networks very similar to the ones observed in Fig.18 (b) and (d).
Fig. 20 shows the effect of downfeed and workspeed on the surface roughness, both across and along the grinding direction. In the present case, the process parameters and the material combination seem to have almost no effect on the surface roughness. The domain of variation of surface roughness across the grinding direction is between 3 and 5 µm. Only for TiO2, it varies between 1.5 to 2.5 µm, possibly owing to less micro-fracturing and more micro-cutting by the abrasive grits. This is attributed to its favorable fracture toughness.
Another interesting fact is the ratio between surface roughness across and along the grinding direction. In the present case, this ratio lies in 12.8 range. For sintered ceramic grinding, this can be as high as 10 as can be seen in Fig. 21. This indicates micro-fracturing
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to be the dominant material removal mechanism for these porous and inhomogeneous coatings.
Table 5 reveals the effect of grinding on the surface residual stress, which was measured only at the downfeed of 14 µm and at a workspeed of 6 m/min. In metallic alloys, residual stress come into play owing to metallurgical changes, micro-cutting action and thermal cycle [36]. In grinding of the present coatings, the specific grinding energy is much lower leading to low grinding temperature as compared to metallic alloys and retention of material property by the ceramic coatings at the grinding temperature. Hence, there is limited chance of metallurgical changes and induction of residual stress of thermal origin. Micro-cutting action of the grits may change the surface residual stress to a limited extent as micro-fracturing is the dominant mechanism of material removal. Thus, Table 5 expectedly reveals insignificant effect of grinding on residual stresses. 4. Conclusion: In ceramic grinding, the forces were observed to increase with an increase in the undeformed chip thickness. Force results indicated, a micro-brittle fracture to be the predominant mode of material removal. The ratio of normal and tangential grinding force components for the coatings varied in 36 range. The same for the sintered ceramics is around 510. This lower value of force ratio for the coatings is ascribed to the presence of coating defects like porosity. Specific grinding energy ( u g ) of grinding for the coatings was found to be very small (0.5 to 5 J/mm3) as compared to sintered ceramics (10 to 100
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J/mm3). This is attributed to porous, and inhomogeneous structure of the plasma sprayed coatings. The grinding chips are blocky and irregular, and this is credited to micro-fracture being the predominant mode of material removal. Again with an increase in MRR, no change in chip morphology was observed. Surface topography revealed traces of microbrittle fracture on the surfaces of all coatings. However, in TiO2 and Al2O3 – 13 wt% TiO2 coatings, signatures of ploughing and plastic deformation along with micro-fracture were also observed. FIB pocketing milling revealed an increase in the subsurface defect density upon grinding. This phenomenon is attributed to the grinding action of the grits and microfracturing. In the TiO2 coating, low surface roughness indicates the dominance of microcutting as the mechanism of material removal. However, signatures of micro-fracturing were also detected. The latter effect was offset by the former. Grinding has limited effect on the pre-existing surface residual stress of the coatings. This is also ascribed to microbrittle fracture and not plastic deformation being the predominant mechanism of material removal. Residual stresses in grinding, especially in metals, are thermal in origin. Ceramics, on the other hand, could retain their properties at high temperature and hence, did not acquire thermally induced residual stress.
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References [1]
M.P. Kanouff, R.A. Neiser, T.J. Roemer, Surface Roughness of Thermal Spray Coatings Made with Off-Normal Spray Angles, 7 (1998) 219–228.
[2]
K. Sabiruddin, P.P. Bandyopadhyay, G. Bolelli, L. Lusvarghi, Variation of splat shape with processing conditions in plasma sprayed alumina coatings, Journal of Materials Processing Technology. 211 (2011) 450–462. doi:10.1016/j.jmatprotec.2010.10.020.
[3]
P.P. Bandyopadhyay, M. Hadad, C. Jaeggi, S. Siegmann, Microstructural, tribological and corrosion aspects of thermally sprayed Ti–Cr–Si coatings, Surface and Coatings Technology. 203 (2008) 35–45. doi:10.1016/j.surfcoat.2008.07.026.
[4]
K. Sabiruddin, J. Joardar, P.P. Bandyopadhyay, Analysis of phase transformation in plasma sprayed alumina coatings using Rietveld refinement, Surface & Coatings Technology. 204 (2010) 3248–3253. doi:10.1016/j.surfcoat.2010.03.026.
[5]
S. Das, T.K. Bandyopadhyay, S. Ghosh, A.B. Chattopadhyay, P.P. Bandyopadhyay, Processing and characterization of plasma-sprayed ceramic coatings on steel substrate: Part II. On coating performance, Metallurgical and Materials Transactions A. 34 (2003) 1919–1930. doi:10.1007/s11661-003-0157-2.
[6]
B. Gérard, Application of thermal spraying in the automobile industry, Surface and Coatings Technology. 201 (2006) 2028–2031. doi:10.1016/j.surfcoat.2006.04.050.
[7]
S. Malkin, J.E. Ritter, Grinding Mechanism and Strength Degradation for Ceramics, ASME J. of Eng. for Lnd. 111 (1989) 167–173.
[8]
I. Inasaki, Grinding of Hard and Brittle Materials, CIRP Annals - Manufacturing Technology. 36 (1987) 463–471. doi:10.1016/S0007-8506(07)60748-3.
[9]
S. Paul, P.P. Bandyopadhyay, A.B. Chattopadhyay, Effects of cryo-cooling in grinding steels, Journal of Materials Processing Technology. 37 (1993) 791–800. doi:10.1016/0924-0136(93)90137-U. 17
[10] A. Macwan, M. Marr, O. Kesler, D.L. Chen, Microstructure, hardness, and fracture toughness of suspension plasma sprayed yttria-stabilized zirconia electrolytes on stainless steel substrates, Thin Solid Films. 584 (2014) 6–11. doi:10.1016/j.tsf.2014.11.052. [11] S. Malkin, T.W. Hwang, Grinding Mechanisms for Ceramics, CIRP Annals Manufacturing Technology. 45 (1996) 569–580. doi:10.1016/S0007-8506(07)605113. [12] J. Chen, H. Huang, X. Xu, An experimental study on the grinding of alumina with a monolayer brazed diamond wheel, The International Journal of Advanced Manufacturing Technology. 41 (2009) 16–23. doi:10.1007/s00170-008-1459-8. [13] D. Zhang, C. Li, D. Jia, Y. Zhang, Investigation into Engineering Ceramics Grinding Mechanism and the Influential Factors of the Grinding Force, 7 (2014) 19–34. [14] K. Kitajima, N. Kurnagai, Y. Tanaka, G.Q. Cai, N. Kurnagai, Y. Tanaka, et al., Study on Mechanism of Ceramics Grinding, CIRP Annals - Manufacturing Technology. 41 (1992) 367–371. doi:10.1016/S0007-8506(07)61224-4. [15] B. Zhang, J. Wang, F. Yang, Z. Zhu, The effect of machine stiffness on grinding of silicon nitride, International Journal of Machine Tools and Manufacture. 39 (1999) 1263–1283. doi:10.1016/S0890-6955(98)00083-2. [16] X. Liu, B. Zhang, Z. Deng, Grinding of nanostructured ceramic coatings: Surface observations and material removal mechanisms, International Journal of Machine Tools and Manufacture. 42 (2002) 1665–1676. doi:10.1016/S0890-6955(02)001153. [17] X. Liu, B. Zhang, Grinding of nanostructural ceramic coatings: damage evaluation, International Journal of Machine Tools and Manufacture. 43 (2003) 161–167. doi:10.1016/S0890-6955(02)00157-8. [18] T. Kubohori, Y. Inui, T. Ikuta, Evaluation of Grinding Characteristics of Thermal
18
Spraying Ceramics Film, MATERIALS TRANSACTIONS. 48 (2007) 1050–1054. doi:10.2320/matertrans.48.1050. [19] M. Ramazani, J. Khalil-Allafi, R. Mozaffarinia, Grindability Evaluation and Fatigue and Wear Behavior of Conventional and Nanostructured Al2O3-13 Wt.% TiO2 Air Plasma Sprayed Coatings, Journal of Thermal Spray Technology. 19 (2010) 611– 619. doi:10.1007/s11666-009-9459-2. [20] K. Jia, T.E. Fischer, Abrasion resistance of nanostructured and conventional cemented carbides, 200 (1996) 206–214. [21] X. Liu, B. Zhang, Effects of grinding process on residual stresses in nanostructured ceramic coatings, Journal of Materials Science. 37 (2002) 3229–3239. doi:10.1023/A:1016174731658. [22] X. Liu, B. Zhang, Z. Deng, Grinding of nanostructured ceramic coatings: surface observations and material removal mechanisms, International Journal of Machine Tools and Manufacture. 42 (2002) 1665–1676. doi:10.1016/S0890-6955(02)001153. [23] B. Zhang, X. Liu, C.A. Brown, T.S. Bergstrom, Microgrinding of Nanostructured Material Coatings, CIRP Annals - Manufacturing Technology. 51 (2002) 251–254. doi:10.1016/S0007-8506(07)61510-8. [24] S. Kar, S. Paul, P.P. Bandyopadhyay, Processing and characterisation of plasma sprayed oxides: Microstructure, phases and residual stress, Surface and Coatings Technology. 304 (2016) 364–374. doi:10.1016/j.surfcoat.2016.07.043. [25] P.P. Bandyopadhyay, D. Chicot, B. Venkateshwarlu, V. Racherla, Mechanics of Materials Mechanical properties of conventional and nanostructured plasma sprayed alumina coatings, International Journal of Mechanics of Materials. 53 (2012) 61–71. doi:10.1016/j.mechmat.2012.05.006. [26] A.G. EVANS, E.A. CHARLES, Fracture Toughness Determinations by Indentation,
19
Journal of the American Ceramic Society. 59 (1976) 371–372. doi:10.1111/j.11512916.1976.tb10991.x. [27] G.. Anstis, P. Chantikul, B.. Lawn, D.. Marshall, A critical evaluation of indentation techniques for measuring fracture toughness: I Direct crack measurements, Journal of the American Ceramic Society. 46 (1981) 533–538. doi:10.1111/j.11512916.1981.tb10320.x. [28] T.W. Hwang, S. Malkin, Grinding Mechanisms and Energy Balance for Ceramics, Journal of Manufacturing Science and Engineering. 121 (1999) 623. doi:10.1115/1.2833081. [29] M. Vashista, S. Ghosh, S. Paul, Application of Micromagnetic Technique in Surface Grinding for Assessment of Surface Integrity, Materials and Manufacturing Processes. 24 (2009) 488–496. doi:10.1080/10426910802714456. [30] S. Ghosh, a. B. Chattopadhyay, S. Paul, Modelling of specific energy requirement during high-efficiency deep grinding, International Journal of Machine Tools and Manufacture. 48 (2008) 1242–1253. doi:10.1016/j.ijmachtools.2008.03.008. [31] J. Chen, J. Shen, H. Huang, X. Xu, Grinding characteristics in high speed grinding of engineering ceramics with brazed diamond wheels, Journal of Materials Processing Technology. 210 (2010) 899–906. doi:10.1016/j.jmatprotec.2010.02.002. [32] R.B. Heimann, Plasma spray coating: principles and applications, Advanced Materials. (1996) 1–327. doi:10.1002/9783527614851. [33] L.C. Erickson, R. Westergård, U. Wiklund, N. Axén, H.M. Hawthorne, S. Hogmark, Cohesion in plasma-sprayed coatings — a comparison between evaluation methods, Wear. 214 (1998) 30–37. doi:10.1016/S0043-1648(97)00216-0. [34] K.-S. SHI, Z.-Y. QIAN, M.-S. ZHUANG, Microstructure and Properties of Sprayed Ceramic Coating, Journal of the American Ceramic Society. 71 (1988) 924–929. doi:10.1111/j.1151-2916.1988.tb07559.x.
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[35] R. McPherson, On the formation of thermally sprayed alumina coatings, Journal of Materials Science. 15 (1980) 3141–3149. doi:10.1007/BF00550387. [36] S. Malkin, Grinding of metals: Theory and application, Journal of Applied Metalworking. 3 (1984) 95–109. doi:10.1007/BF02833688. [37] M. Vashista, S. Paul, Study of the effect of process parameters in high-speed grinding on surface integrity by Barkhausen noise analysis, Proceedings of the Institution of Mechanical Engineers Part B-Journal of Engineering Manufacture. 222 (2008) 1625–1637. doi:Doi 10.1243/09544054jem1214. [38] S. Malkin, R.B. Anderson, Thermal Aspects of Grinding: Part 1—Energy Partition, Journal of Engineering for Industry. 96 (1974) 1177. doi:10.1115/1.3438492. [39] R. McPherson, A review of microstructure and properties of plasma sprayed ceramic coatings, Surface and Coatings Technology. 39–40 (1989) 173–181. doi:10.1016/0257-8972(89)90052-2.
21
List of Figures: 1. Fig 1. Raw signal plot of forces obtained during grinding of Al2O3 NS sprayed at 500 A. 2. Fig 2. Analysed forces plots of grinding forces data set of Al2O3 NS sprayed at 500 A. 3. Fig 3. Tangential forces obtained during grinding of plasma sprayed coating using electroplated monolayer diamond wheel.
4. Fig 4. Normal forces obtained during grinding of plasma sprayed coating using electroplated monolayer diamond wheel.
5. Fig 5. Optical image of diamond grits taken at 100 . 6. Fig 6. Variation of normal forces with respect to uncut chip thickness. 7. Fig 7. Variation of normal forces with respect to uncut chip thickness. 8. Fig 8. Variation of force ratio with downfeed. 9. Fig 9. Variation of force ratio with uncut chip thickness, 10. Fig 10. Variation of specific energy ( u g ) with downfeed. 11. Fig 11. Variation of specific energy ( u g ) with uncut chip thickness (hm). 12. Figure 12. SEM images of grinding chips of (a) fine alumina (SFP) and (b) alumina (NS) coated 500 A, (c) fine alumina and (d) alumina coatings deposited at 600 A (e) Alumina13 wt% titania (f) titania (g) yttria stabilized zirconia coating coated at 500 A and (h) chromia coating deposited at 600 A obtained at downfeed 14 and at workspeed 12 m/min.
13. Figure 13. SEM images of grinding chips of fine alumina deposited at 600 A. 14. Fig 14. SEM image of grinding chips of sintered alumina. 15. Fig 15. Ground surface of Al2O313 wt% TiO2 at 14µm-6 m/min and at 38µm-6 m/min. 16. Figure 16. SEM images of ground surface of (a) alumina (NS) (b) Alumina13 wt% titania (c) titania coated at 500 A obtained at downfeed 14 and 38 µm and at workspeed 6 and 12 m/min.
17. Figure 17. SEM images of ground surface of (a) fine alumina (SFP) and (b) alumina (NS) coated 500 A, (c) fine alumina and (d) alumina coatings deposited at 600 A (e) Alumina13 wt% titania (f) titania (g) yttria stabilized zirconia coating coated at 500 A and (h) chromia coating deposited at 600 A obtained at downfeed 14 and at workspeed 12 m/min.
22
18. Fig 18. SEM microphotographs of FIB pocket milled coated and ground samples ((a) and (b) alumina SFP at 600 A and (c) and (d) alumina NS at 600 A; (a) and (c) – coated samples and (b) and (d) – ground samples).
19. Fig 19. SEM images of cross section of ground sample of Alumina SFP at 600 A ((a) 100x and (b) 1000x)
20. Fig 20. Roughness profile of ground surfaces at workspeed 6 m/min and 12 m/min. 21. Fig 21. Roughness ratio of ground surfaces of plasma sprayed coating.
23
Tables: Table 1 Spray parameters for coating Sl. No.
Powder
Nozzle diameter (mm)
SOD (mm)
1
Al2O3 SFP
5
100
2
Al2O3 NS
5
125
3 4 5 6 7 8
YSZ TiO2 Al2O3-TiO2 Cr2O3 Ni-Al NiCrAlY
5 5 5 5 5 5
100 130 130 125 125 125
Current (A)
Voltage (V)
N2 Flowrate (scfh)
H2 Flowrate (scfh)
Spray rate (g/min)
74
50
5
~ 26
75
50
5
~ 64
82.4 68 68 71 67 77
50 50 50 50 50 50
5 5 5 5 Nil Nil
~ 53 ~ 50 ~ 25 ~ 55 ~ 42 ~ 56
500 600 500 600 500 500 500 600 450 450
Table 2 Particle morphology of ceramic powders Sl No. 1. 2. 3. 4. 5. 6.
Formula
Size (microns)
Morphology
Al2O3 SFP Al2O3 NS Al2O3 13 wt% TiO2 TiO2 Cr2O3 YSZ
-31+3.9 -45+15
Crushed Crushed
-45+5
Clad
-88+7.8 -125+11 -75+45
Crushed Crushed Spheroidal
24
Table 3 Grinding parameters Sl No 1. 2. 3. 4. 5. 6. 7.
Parameters Machine specification: Wheel specification: Downfeed (a) in µm Work speed (Vw) in m/min Wheel speed (Vs) in m/sec Enviroment Coolant Fluid
Description Cosmos Earth E 6030 D200U10D126GN333 (Make: Wendt India) 14, 28, 32, 38 6, 12 26 Wet Servocut S 1:20 dilution (IOCL)
Table 4 Measure of critical depth of cut for crack initiation in ceramic coatings during grinding process Coating
HV200
E(GPa)
K1C (MPa m1/2)
critical depth dc (microns)
1.
SFP @ 500A
1066 86
85.94
1.955
0.0431
2.
NS @500
983 ± 42
137.28
1.719
0.0678
3.
SFP @600
1113 ± 112
90.73
1.726
0.0311
4.
NS @600
1098.9 ± 206
132.17
1.835
0.0534
5.
YSZ
913 ± 130
74.78
1.43
0.0795
6.
Al2O3 13wt% TiO2
744±63
185.76
1.902
0.1401
7.
CrO
1253 ± 87
134.36
2.983
0.0966
8.
TiO
673.7 ± 41
127.61
1.806
0.1606
Sl. No.
25
Table 5 Residual stress of As-sprayed [24] and ground samples Sl. No. 1. 2. 3. 4. 5. 6. 7. 8.
Coating
Al2O3 (NS) @ 500 A Al2O3 (NS) @ 600 A Al2O3 (SFP) @ 500 A Al2O3 (SFP) @ 600 A Al2O3 13 wt% TiO2 TiO2 Cr2O3 YSZ
Residual stress (MPa)
Experimental Elastic modulus (GPa)
As-sprayed coating
137.28 132.17 85.94 90.73 185.76 127.61 134.36 74.78
480 5 479 11.5 325 6.3 371 4 416 19.1 544 15.9 357 13.1 264 4.3
ground Vw = 6m/min, a = 14 µm 535 54 505 84 396 16 332 125 572 111 594 10 683 131 288.2 30
26
Figure 1
6 Al2O3 NS sprayed at 500 A
tangential - Ft normal - Fn
Fn
5 4
grinding force (N)
3 Ft
2 1 0
-1 -2 -3 -4 -5
0
1
2
3
time (s)
4
5
6
7
Fig 1. Raw signal plot of forces obtained during grinding of Al2O3 NS sprayed at 500 A.
Figure 2
20
Al2O3 NS sprayed at 500A Avg Ft = 1.79 N, Max Ft = 5.18 N Avg Fn = 2.1 N, Max Fn = 5.59 N
tangential force - Ft normal force - Fn
18 16
grinding force (N)
14 12 10 8
Fn
6 4
Ft
2 0 -2
2.6
2.8
3
3.2
3.4
3.6
3.8
4
4.2
time (s)
Fig 2. Analysed forces plots of grinding forces data set of Al2O3 NS sprayed at 500 A
Figure 3
a
25
sample: Al2O3 SFP
6 m/min 12 m/min
tangential grinding force, FT (N)
wheelspeed: 26 m/s enviroment: wet 20
b
tangential grinding force,FT (N)
25
15
10
5
15
10
5
downfeed (m)
30
0
40
10
Sample: Fine Al2O3 @ 600 A
6m/min 12m/min
wheel speed: 26 m/s enviroment: wet
15
15
10
10
5
downfeed (m) 30
40
10
25
sample: Al2O3-13 wt% TiO2
6 m/min 12 m/min
f
tangential grinding force,FT (N)
20
wheel speed: 26 m/s 20 enviroment: wet
tangential grinding force,FT (N)
5
0 10
15
10
5
20
20
30
40
downfeed (m)
sample: TiO2
6 m/min 12 m/min
wheelspeed: 26 m/s enviroment: wet
15
10
5
0
0 10
25
20
downfeed (m) 30
6m/min 12m/min
25
15
10
5
20
downfeed (m)
30
sample: Cr2O3
h tangential grinding force,FT (N)
20
10
40
sample: YSZ wheelspeed: 26 m/s enviroment: wet
g tangential grinding force,FT (N)
40
6 m/min 12 m/min
wheel speed: 26 m/s enviroment: wet
20
0
e
30
downfeed (m)
sample: Al2O3 NS @ 600 A
d tangential grinding force,FT (N)
20
25
20
25
25
c tangential grinding force,FT (N)
20
6m/min 12m/min
wheel speed: 26 m/s enviroment: wet
20
0 10
sample:Al2O3 NS
6m/min 12m/min
wheelspeed: 26 m/s enviroment: wet
20
40
15
10
5
0
0 10
20
downfeed (m)
30
40
10
20
downfeed (m)
30
40
Fig 3. Tangential forces obtained during grinding of plasma sprayed coating using electroplated monolayer diamond wheel.
Figure 4
a
25
25
sample: Al2O3 SFP
15
15
10
10
5
0 20
downfeed (m)
30
40
25
10
10
5
15
10
5
0
10
20
downfeed (m)
30
40
10
25
f
15
20
30
downfeed (m)
sample: TiO2
40
6 m/min 12 m/min
wheelspeed: 26 m/s 20 enviroment: wet
normal grinding force, FN (N)
wheelspeed: 26 m/s 20 enviroment: wet
normal grinding force, FN (N)
25
6 m/min 12 m/min
sample: Al2O3- 13 wt% TiO2
15
10
10
5
5
0 10
20
30
0
40
10
downfeed (m)
20
downfeed (m)
30
40
25
25
6 m/min 12 m/min
sample: YSZ wheelspeed: 26 m/s enviroment: wet
g
sample:Cr2O3
h
wheelspeed: 26 m/s enviroment: wet
20
6m/min 12m/min
normal grinding force, FN (N)
20
normal grinding force, FN (N)
40
6 m/min 12 m/min
wheel speed: 26 m/s enviroment: wet
20
0
e
30
downfeed (m)
sample: Al2O3 NS sprayed at 600 A
normal grinding force, Fn(N)
15
20
25
d
6 m/min 12 m/min
Alumina SFP sprayed at 600 A wheel speed: 26 m/s 20 enviroment: wet
normal grinding force, FN (N)
5
0 10
c
6 m/min 12m/min
wheel speed: 26 m/s enviroment: wet
20
normal grinding force, FN (N)
normal grinding force, FN (N)
wheel speed: 26 m/s 20 enviroment: wet
sample: Al2O3 NS
b
6m/min 12m/min
15
15
10
10
5
5
0
0 10
20
30
downfeed (m)
40
10
20
30
40
downfeed (m)
Fig 4. Normal forces obtained during grinding of plasma sprayed coating using electroplated monolayer diamond wheel.
Figure 5
Fig 5. Optical image of diamond grits taken at 100 and 40
2.0
a
1.6
2.0
sample: Al2O3 - 13wt% TiO2 wheel speed: 26 m/s enviroment: wet
b
1.6
F'T (N/mm)
F'T (N/mm)
6 m/min 12 m/min 1.2
0.8
0.4
sample: TiO2 sprayed at 500 A wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
1.2
0.8
0.4
0.0 1.2
1.5
1.8
2.1
2.4
2.7
0.0
3.0
1.2
Uncut chip thickness (m) 2.0
c
1.8
2.1
2.4
2.7
3.0
2.7
3.0
Uncut chip thickness (m)
sample: Cr2O3 sprayed at 600 A
2.0
d
wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
1.6
sample: YSZ sprayed at 500 A wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
1.2
F'T (N/mm)
F'T (N/mm)
1.6
1.5
0.8
1.2
0.8
0.4
0.4
0.0
0.0 1.2
1.5
1.8
2.1
2.4
2.7
3.0
1.2
1.5
1.8
2.1
2.4
Uncut chip thickness (m)
Uncut chip thickness (m)
Fig 6. Variation of specific tangential forces with respect to uncut chip thickness
sample: Al2O3 -13 wt% TiO2 3.6
F'n(N/mm)
3.0
sample: TiO2 sprayed at 500 A
b
wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
3.6
3.0
F'n(N/mm)
a
2.4
1.8
2.4
1.8
1.2
1.2
0.6
0.6
0.0
wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
0.0 1.2
1.5
1.8
2.1
2.4
2.7
3.0
1.2
1.5
uncut chipthickness (m)
sample: Cr2O3 sprayed at 600 A
c F'n(N/mm)
3.0
wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
d
3.6
3.0
F'n(N/mm)
3.6
1.8
2.1
2.4
2.7
3.0
uncut chipthickness (m)
2.4
1.8
sample: YSZ sprayed at 500 A wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
2.4
1.8
1.2
1.2
0.6
0.6
0.0
0.0 1.2
1.5
1.8
2.1
2.4
uncut chipthickness (m)
2.7
3.0
1.2
1.5
1.8
2.1
2.4
2.7
uncut chipthickness (m)
Fig 7. Variation of specific normal forces with respect to uncut chip thickness
3.0
Figure 8
10
wheelspeed: 26 m/s enviroment: wet Fn: normal grinding force
8
10
6 m/min 12 m/min
sample: Al2O3 SFP
a
wheel speed:26 m/s enviroment: wet Fn: normal grinding force
8
Ft: tangential grinding force
Ft: tangential grinding force 6
FN/FT
FN/FT
6
4
4
2
2
0
0 10
c
6 m/min 12 m/min
sample: Al2O3 NS
b
20
downfeed (m)
30
40
10
d
6m/min 12m/min
sample: Al2O3 SFP sprayed 600 A wheelspeed: 26 m/s enviroment: wet Fn: normal grinding force
25
30
35
40
10
sample: Al2O3 NS sprayed at 600 A wheel speed: 26 m/s enviroment: wet
8
Ft: tangential grinding force
6 m/min 12 m/min
6
FN/FT
FN/FT
6
20
downfeed (m)
10
8
15
4
4
2
2
0
0 10
20
30
10
40
20
10
e
sample: Al2O3 - 13 wt% TiO2
10
6m/min 12m/min
wheelspeed: 26 m/s enviroment: wet 8 Fn: normal grinding force
wheelspeed: 26 m/s enviroment: wet Fn: normal grinding force
8
Ft: tangential grinding force
6
FN/FT
FN/FT
6
4
4
2
2
0 10
20
downfeed (m)
30
0
40
10
10
20
downfeed (m)
30
40
10
sample: YSZ wheel speed: 26 m/s enviroment: wet 8 Fn: normal grinding force
6 m/min 12 m/min
6m/min 12m/min
sample: Cr2O3
h
wheelspeed: 26 m/s enviroment: wet Fn: normal grinding force
8
Ft: tangential grinding force
Ft: tangential grinding force
6
6
FN/FT
FN/FT
40
6m/min 12m/min
sample: TiO2
f
Ft: tangential grinding force
g
30
downfeed (m)
downfeed (m)
4
4
2
2
0
0 10
20
30
downfeed (m)
40
10
Fig 8. Variation of force ratio with downfeed.
20
downfeed (m)
30
40
Figure 9
10 9
a
8
10
sample: Al2O3 -13 wt% TiO2
b
wheel speed: 26 m/s enviroment: wet
9 8
6m/min 12m/min
7
FN/FT
FN/FT
4
5 4
3
3
2
2
1
1
0 1.2
1.5
1.8
2.1
2.4
2.7
0
3.0
1.2
uncut chip thickness (m)
9 8
1.5
1.8
2.1
2.4
2.7
3.0
2.7
3.0
uncut chip thickness (m) 10
sample: Cr2O3
d
wheel speed: 26 m/s enviroment: wet
9 8
6m/min 12m/min
7
7
sample: YSZ wheel speed: 26 m/s enviroment: wet 6m/min 12m/min
6
FN/FT
6
FN/FT
6m/min 12m/min
6
5
10
wheel speed: 26 m/s enviroment: wet
7
6
c
sample: TiO2
5
5
4
4
3
3
2
2
1
1 0
0 1.2
1.5
1.8
2.1
2.4
uncut chip thickness (m)
2.7
3.0
1.2
1.5
1.8
2.1
2.4
uncut chip thickness (m)
Fig 9. Variation of force ratio with uncut chip thickness
Figure 10
7
sample: Al2O3 SFP
b
6 m/min 12 m/min
wheel speed: 26 m/s 6 enviroment: wet
specific grinding energy (J/mm3)
specific grinding energy (J/mm3)
a
5
4
3
2
1
0
7
6
sample:Al2O3 NS
5
4
3
2
1
0
0
10
20
30
40
0
10
20
downfeed (m)
wheel speed: 26 m/s enviroment: wet
d specific grinding energy (J/mm3)
specific grinding energy (J/mm3)
7
6 m/min 12 m/min
sample: Al2O3 SFP sprayed at 600 A 6
5
4
3
2
1
0
10
20
30
wheel speed: 26 m/s enviroment: wet
5
4
3
2
1
0
40
10
20
7
sample: Al2O3- 13wt% TiO2
6 m/min 12 m/min
wheel speed: 26 m/s enviroment: wet
sample: TiO2
f
5
4
3
2
1
0
6m/min 12m/min
wheelspeed: 26 m/s enviroment: wet
6
5
4
3
2
1
10
20
30
10
40
20
downfeed (m)
30
40
downfeed (m)
sample: YSZ wheel speed: 26 m/s 6 enviroment: wet
6m/min 12m/min
5
4
3
2
1
7
h specific grinding energy (J/mm3)
7
specific grinding energy (J/mm3)
40
0
0
g
30
downfeed (m)
specific grinding energy (J/mm3)
specific grinding energy (J/mm3)
6
6 m/min 12 m/min
sample:Al2O3 NS sprayed 600A
6
downfeed (m) 7
40
0
0
e
30
downfeed (m)
7
c
6 m/min 12 m/min
wheel speed: 26 m/s enviroment: wet
6
sample: Cr2O3
6m/min 12m/min
wheel speed: 26 m/s enviroment: wet
5
4
3
2
1
0
0 0
10
20
downfeed (µm)
30
40
10
20
30
downfeed (m)
Fig 10. Variation of specific energy (Ug) with downfeed.
40
Figure 11
7
sample: Al2O3 -13 wt% TiO2
b
wheel speed: 26 m/s enviroment: wet
specific grinding energy Ug (J/mm3)
specific grinding energy Ug (J/mm3)
8
6 m/min 12 m/min
6 5 4 3 2 1
8 7
sample: TiO2 wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
6 5 4 3 2 1
0 1.2
1.5
1.8
2.1
2.4
2.7
3.0
0 1.2
uncut chip thickness (m)
1.5
1.8
2.1
2.4
2.7
3.0
2.7
3.0
uncut chip thickness (m)
8 7
d
sample: Cr2O3
specific grinding energy Ug (J/mm3)
c specific grinding energy Ug (J/mm3)
a
wheel speed: 26 m/s enviroment: wet
6
6 m/min 12 m/min
5 4 3 2 1
8 7 6
sample: YSZ wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
5 4 3 2 1 0
0 1.2
1.5
1.8
2.1
2.4
uncut chip thickness (m)
2.7
3.0
1.2
1.5
1.8
2.1
2.4
uncut chip thickness (m)
Fig 11. Variation of specific energy ( u g ) with uncut chip thickness (hm)
Figure 12
a = 14 µm, Vw = 12 m/min
a = 14 µm, Vw = 12 m/min
a
b
c
d
e
f
g h Figure 12. SEM images of grinding chips of (a) fine alumina (SFP) and (b) alumina (NS) coated 500 A, (c) fine alumina and (d) alumina coatings deposited at 600 A (e) Alumina13 wt% titania (f) titania (g) yttria stabilized zirconia coating coated at 500 A and (h) chromia coating deposited at 600 A obtained at downfeed 14 and at tablespeed 12 m/min.
Figure 13
a = 14 µm, Vw = 6m/min
a = 14 µm, Vw = 12 m/min
a = 38 µm, Vw = 6m/min
a b c Figure 13. SEM images of grinding chips of fine alumina deposited at 600 A.
Figure 14
Fig 14 SEM image of grinding chips of sintered alumina
Figure 15
a
b
Fig 15. Ground surface of Al2O313 wt% TiO2 at 14µm-6 m/min and at 38µm-6 m/min
Figure 16
a = 14 µm, Vw = 6m/min
a
e
a = 14 µm, Vw = 12 m/min
b
a = 38 µm, Vw = 6m/min
c
f
g
h j i Figure 16. SEM images of ground surface of (a) alumina (NS) (b) Alumina13 wt% titania (c) titania coated at 500 A obtained at downfeed 14 and 38 µm and at tablespeed 6 and 12 m/min.
Figure 17
a = 14 µm, Vw = 12 m/min
a = 14 µm, Vw = 12 m/min a
b
c
d
e
f
g
h
Figure 17. SEM images of ground surface of (a) fine alumina (SFP) and (b) alumina (NS) coated 500 A, (c) fine alumina and (d) alumina coatings deposited coating coated at 500 A and (h) chromia coating deposited at 600 A obtained at downfeed 14 and at workspeed 12 m/min.
Figure 18
Coated sample
Ground sample (Vw= 6 m/min, a= 38 µm)
Fig 18 SEM microphotographs of FIB pocket milled coated and ground samples ((a) and (b) alumina SFP at 600 A and (c) and (d) alumina NS at 600 A; (a) and (c) – coated samples and (b) and (d) – ground samples)
Figure 19
Mould Subsurface cracks Top Coat
Fig 19 SEM images of cross section of ground sample of Alumina SFP at 600 A ((a) 100x and (b) 1000x)
Figure 20
8
a
sample: Al2O3 SFP
6 m/min - perpendicular 12 m/min - perpendicular 6 m/min - parallel 12 m/min - paralel
wheel speed: 26m/s enviroment: wet
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roughness (Ra) (m)
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sample: Al2O3- 13 wt% TiO2
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sample: Cr2O3 wheel speed: 26 m/s enviroment: wet
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sample: YSZ wheel speed: 26 m/s enviroment: wet
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e
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6 5 4 3 2 1
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Fig 20. Roughness profile of ground surfaces at workspeed 6 m/min and 12 m/min
40
Figure 21
3.6
Roughness ratio (R/R//)
3.2
AlTiO, sfp 500,
CrO, NS 500, sfp 600, TiO,
NS 600 YSZ
2.8 2.4 2.0 1.6 1.2 0.8 0.4 0.0 14-6 20-6 26-6 32-6 38-6 14-12 20-12 26-12 32-12 38-12
--
Downfeed(m)workspeed (m/min)
Fig 21. Roughness ratio of ground surfaces of plasma sprayed coating
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PII: DOI: Reference:
S0272-8842(16)31633-9 http://dx.doi.org/10.1016/j.ceramint.2016.09.100 CERI13760
To appear in: Ceramics International Received date: 17 August 2016 Revised date: 13 September 2016 Accepted date: 14 September 2016 Cite this article as: Simanchal Kar, P P Bandyopadhyay and S. Paul, Precision superabrasive grinding of plasma sprayed ceramic coatings, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.09.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Precision superabrasive grinding of plasma sprayed ceramic coatings Simanchal Kar, P P Bandyopadhyay*, S. Paul Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, West Bengal 721 302, India. *
Corresponding author: phone: +91 3222 282950. e-mail:[email protected] Abstract: In this investigation, a set of plasma sprayed ceramic oxide coatings were surface ground using super abrasive wheel. The surface integrity and grindability aspects of the coated surface were studied. During the grinding process, low grinding forces and specific grinding energy were observed and these observations pointed toward micro brittle fracture to be the predominant material removal mechanism in all cases. SEM and FIB milling of the ground samples also revealed that material removal is primarily owing to micro-fracture rather than micro-cutting. This is also corroborated by short broken chip formation during grinding. Further, the nature of the chips did not vary with change in grinding parameters. The surface topography showed signatures of micro-fracture, ploughing and rubbing only. The ground surface harbours residual stress. However, this stress, unlike metals, does not have a thermal origin. This is attributed to retention of material properties by the ceramic coatings at the grinding temperature. Keywords: plasma spray, ceramic oxide, super abrasive, grinding chips, residual stress, critical depth.
1
1. Introduction: Plasma sprayed coatings are constituted by layers of deformed particles or splats. Defects like porosity and voids are commonplace in such coatings. Other major problems in these coatings are high surface roughness and poor dimensional tolerance of the coated components, as identified by Kanouff et al [1]. Plasma sprayed coating finds applications in different industries. Some examples of application of these coatings are combating wear [2,3] or thermal insulation [4,5] etc. Gerard has discussed application of thermal spray coating in automobile industries [6]. Such applications require acceptable level of surface finish and dimensional tolerance. Ceramic coatings deposited by plasma spraying can only be finished by grinding. In grinding of metallic alloys, material removal mechanism is based on shear deformation [7–9]. In ceramic grinding, while both brittle fracture and plastic deformation occur, the former action is predominant [10–13]. In ceramic grinding, force ratio i.e., FN / FT is high [14]. Chips are blocky in shape, bearing signature of fracture. The ground surface is susceptible to alteration in residual stresses that are present in an as-sprayed coating and formation of micro-cracks induced during the grinding process itself. Further, conventional wheels are ineffective as the hardness of the wheel and the coating are comparable, and wheel wear is high [15]. In order to overcome the problem of wheel wear, super abrasive wheels like cBN and diamond wheels are used during grinding of plasma sprayed ceramic samples [16–18]. cBN and diamond grits have better strength, wear resistance and high hardness. Ramajani et al., [19] studied grindability of nanostructured and conventional air
2
plasma sprayed coating of Al2O3 13wt% TiO2. Grinding was undertaken with cBN wheel and they observed both transverse and longitudinal cracks on the ground surface.
Jia and Fischer [20] evaluated the abrasion resistance of nanostructured WC-Co coating, by abrading it against a diamond grinding wheel. They did not study any of the grindability parameters. Liu and Zhang [17] experimentally investigated the effects of material removal rate and grit size of the abrasive on the surface damage upon grinding of plasma sprayed nanostructured Al2O3 13wt% TiO2 and WC-Co coatings using diamond grinding wheel. They observed transverse as well as longitudinal cracks which increased with an increase in downfeed and workspeed. The same authors [21] in a sequel work studied the state of stress on these coatings. They concluded, grinding significantly changes the state of residual stress of an as-sprayed surface. Liu et al. [22], experimentally studied grinding of the same set of coatings also using a diamond grinding wheel. They studied the effect of downfeed and workspeed on normal grinding force, surface roughness and surface damage. They observed ductile flow and brittle fracture on the surface of the coating during grinding.
In a follow up investigation, Zhang et al., [23] studied the effect of grinding parameters on the surface roughness of thermally sprayed nanostructured Al2O3 13wt% TiO2 and WCCo coatings. They did not observe a significant change in the surface roughness with downfeed. Grinding induced sub-surface damage and depth of the same increased with an increase in the downfeed.
3
Most of the above works are restricted to Al2O3 13wt% TiO2 and WC-Co coatings. The grinding wheels are either resin bonded or vitrified bonded. There has been no attempt to study other plasma sprayed coatings like Al2O3, TiO2, Cr2O3, YSZ etc. Usage of a single layered diamond grinding wheel that has more chip accommodation space has not been reported. Though the effects of downfeed and workspeed have been studied on different grindability parameters, but there is a lack of comprehensive discussion on grindability. Thus, the aim of the present paper is to systematically study the effect of downfeed, and workspeed on the different aspects of grindability of six different types of plasma sprayed coatings using single layer electroplated diamond wheel. These six coatings constitute a near exhaustive set of thermally sprayed ceramic coatings.
2. Experimental procedure: Low carbon steel coupons of dimensions 100 ×10 ×10 mm, served as substrate. Six ceramic powders were used to deposit top coat (Table 12). An air plasma spray system (SULZER METCO 9MC series spray unit with 9MB Gun (80 kW)) has been used for spraying purpose. A bond coat of thickness of around 100 µm has been applied first, followed by a thicker top coat (300-700 µm) [24]. The coating parameters are shown in Table 1. These parameters are recommended by the powder manufacturer (Sulzer Metco). The particle size and morphology are listed in Table 2. In this investigation alumina powders with two cut sizes were used, namely, Al2O3 NS (-45+15) and Al2O3 SFP (-31+3.9). These two powders were deposited using two arc currents; 500 A and 600 A. Therefore, the total numbers of coatings investigated in the present work is eight.
4
Grinding was performed on a semi-automatic surface grinding machine (COSMOS EARTH, India) using mono layer nickel bonded diamond wheel. The grinding parameters are shown in Table 3. The forces was measured using a high-resolution piezoelectric type (make- KISTLER Instrumente AG Winterthur Switzerland, model: 9254) dynamometer equipped with two charge amplifiers (make- KISTLER Instrumente AG Winterthur Switzerland, model: 5011B10). The readings were recorded from an oscilloscope (make-Agilent Technologies, model: DSO X 2014A) connected to the amplifiers. For scanning electron metallography, slices of 10 mm × 10 mm × 10 mm has been cut off from the sprayed samples using a low speed diamond saw. The samples were placed in a resin bond and polished in the cross section using silicon carbide abrasive papers and diamond pastes of decreasing grit size. The polished cross sections were observed under scanning electron microscope (SEM) (Zeiss EVO 60, Germany and JEOL JSM 5800, USA). Residual stress of the as sprayed and ground sample was measured by sin2 method using a high resolution Xray diffractometer (Philips PW1710, Netherlands) equipped with a Co-Kα (λCo = 1.79 nm) target.
Hardness of the coating was measured using a hardness testing machine (LECO
LM700, USA) equipped with a Vicker’s indenter under 200 gf load ( 1.961N). Depth sensing indentation were performed using an instrumented hardness tester (Micro combi tester (MCT), CSM instruments, Switzerland) equipped with a Vickers indenter. The indentation measurements were undertaken under a maximum load of 2000 mN, loading and unloading rate of 4000 mN/min and with a dwell time of 15 s [25]. The elastic moduli of the coatings were calculated from the slope of the unloading curve. Indentation fracture
5
toughness (KIC) values were calculated using the following expression proposed by Evans and Charles [26]
c K IC 0.15k a
3/2
H a
(1)
This method can be used very effectively for comparison of toughness of brittle ceramic coatings having similar microstructure and order of toughness [24,27]. Surface roughness of the coating has been measured using a contact type TAYLOR HOBSON Surtronic 3+ ‘2D’ surface profilometer and a TAYLOR HOBSON FORM TALYSURF 50 (INTRA 2) ‘3D’ profilometer (TAYLOR HOBSON, UK). In both cases a traverse length of 4 mm was used. 3. Results and Discussion: 3.1 Grinding forces versus downfeed: Fig.1 shows the variation in the tangential and normal forces, while grinding the plasma sprayed Al2O3 NS sample at a grinding velocity of 26 m/s, work speed of 6 m/min and downfeed of 38 µm with single layer electroplated diamond wheel. The raw force signals, as shown in Fig.1, have been further processed using a user developed MATLAB code and the processed grinding forces are depicted in Fig. 2. It clearly shows that the normal grinding force is higher than the tangential grinding force component. From Fig. 2, the average value of grinding forces has been determined using the said MATLAB code. The average and maximum force values are shown in Fig. 2.
6
Fig. 3 and Fig. 4 show the variation in specific tangential and normal grinding forces respectively, with downfeed for two different work speeds. Each of the eight plasma sprayed coatings was taken into consideration.
3.2 Grinding forces versus maximum uncut chip thickness An increase in downfeed and work speed leads to an increase in the maximum uncut chip thickness according to the following equation [28]:
[
√ ]
(2)
As the maximum chip thickness increases, the chip load per grit also increases. Thus, in Fig. 3 and Fig. 4, increase in downfeed and work speed has led to an increase in tangential and normal grinding forces.
Fig. 5 shows the micrograph of the grits on the grinding wheel. The grit size used in the current wheel is 150 µm and the static grit density has been calculated to be 33 grits/mm2.
Fig. 6 and Fig. 7 respectively, show the variation in specific tangential and normal grinding forces with respect to the maximum chip thickness for the four selected coatings. Both the figures show an increase in the grinding forces with an increase in the maximum uncut chip thickness. Similar observations have been reported by previous researchers while grinding 7
metallic alloys [29]. Though maximum uncut chip thickness could unify grindability results across work speed, particularly for metallic alloys [30], but Fig. 6 and Fig. 7 seem to depict some individual effect of work speed on grinding forces, irrespective of the choice of plasma sprayed coating. This is attributed to micro-fracturing being the predominant mode of material removal in the present case. 3.3 Grinding force ratio and specific grinding energy:
Fig. 8 reveals the variation in grinding force ratio, FN / FT with downfeed for different work speeds. The indentation fracture toughness of most of the coatings investigated varied between 1.5 2 MPa-m1/2 (Table 4). Only chromia had a fracture toughness of 3 MPa-m1/2. However, individual or combined effect of fracture toughness or hardness of the coatings has not been observed on the force ratio. For all the material taken together, force ratio varied between 3 6. Chen et al. [12,31] employed brazed monolayer diamond wheel for grinding of alumina and zirconia sintered ceramics and observed force ratio FN / FT to be in the range of 5 10 for these two materials. Lower force ratio FN / FT in the present work can be attributed to the inherent defect in the plasma sprayed ceramic coating. It is well known that porosity and defects are commonplace in plasma sprayed coating [24]. In a plasma sprayed coating, porosity is generated owing to the following causes: 1) shadow effect that occurs during splashing of a second particle over a previously arrived one, and this in turn, produces porosity within a lamellar layer, 2) narrow holes or trapped gas inclusions between two consecutive lamellae, 3) unmelted particle inclusions, and 4) 8
impact of high velocity particles resulting in disruptive shock waves [32]. In addition, a plasma sprayed coating is formed by splats that are held together by weak cohesive force [33–35]. The weak cohesion also contributes to the low grinding force ratio. On the other hand, sintered ceramic components have high cohesive strength and larger grinding force ratio.
Fig. 9 reveals the variation in the force ratio against maximum uncut chip thickness for four selected coatings. Typically the force ratio reduced slightly with an increase in the maximum uncut chip thickness in the range of 0.9 to 1.7 µm. As the uncut chip thickness increases, the effect of edge radius of the grit on material removal reduces. For a low uncut chip thickness, the rake angle posed by the grit is more negative. This explains the reduction in force ratio with an increase in the uncut chip thickness. This effect is more pronounced in the case of a metallic material [36,37]. In a plasma sprayed coating, material removal is primarily micro-fracturing and effects of micro-cutting, ploughing and rubbing are not very significant. This is why hm has a limited influence on FN / FT in this case.
Specific grinding energy ( u g ) consists of contribution from rubbing, ploughing as well as chip formation owing to micro-cutting, and material removal owing to micro-fracture. Typically, u g reduces with an increase in the downfeed and approaches specific energy required for chip formation. The specific chip formation energy has been empirically found
9
to be E/14 where E is the elastic modulus of the material expressed in GPa and u g is expressed in J/mm3 [38]. However, in case of brittle materials, contribution of microcutting for chip formation is very insignificant towards material removal [8]. For sintered brittle materials like alumina, silicon nitride, silicon carbide and yttria stabilized zirconia, the u g is much less than that of metallic alloys [11,28]. This is primarily attributed to insignificant contribution from ploughing and chip formation owing to micro-cutting. The major contribution towards u g in the present case has been from micro-fracture that is more energy efficient as compared to micro-cutting. Thus for all the materials taken together, u g has been observed to be as small as 0.5 to 5 J/mm3 as shown in Fig. 10. It may be noted that for sintered ceramics like alumina, the typical range of specific grinding energy in precision grinding is around 10 to 100 J/mm3 [11]. The specific grinding energy for sintered ceramics is higher than the present coatings owing to the presence of defects commonplace in plasma sprayed coating and its low cohesive strength. There is also no systematic effect of downfeed or workspeed on u g . This is also attributed to the porous and inhomogeneous nature of plasma sprayed coatings [24].
Fig. 11 depicts the effect of hm on u g for four of the selected coatings. There is no significant effect of hm on u g since the material removal mechanism is predominantly micro-fracturing.
10
3.4 Grinding chips morphology and ground surface topography: Morphology of grinding chips can indicate mechanism of chip formation. In metallic alloys, primarily three types of chips are obtained and they are (1) spherical chips, (2) irregular shaped micro-chips, and (3) relatively long (around a millimeter long) chips which are produced by micro-cutting [9].
Fig. 12 shows the morphology of grinding chips for all the eight coatings at a downfeed of 14 µm and workspeed of 12 m/min. Close examination reveals complete absence of spherical chips and chips produced by micro-cutting. This indicates material removal to be primarily by micro-fracture. Most of the chips are irregular in shape and are as large as tens of microns which are much larger than hm. This clearly provides evidence of microfracturing which is the predominant mechanism of material removal.
Fig. 13 shows the effect of material removal rate on chip formation. The material removal rate gradually increased from 1.4 mm3/mm-s to 3.8 mm3/mm-s, but that did not provide any change in the chip morphology for one of the typical coatings (Al2O3 SFP sprayed at 600 A). This, once again indicates the predominance of micro-fracturing as the mode of material removal. Fig. 14 shows the morphology of chips while grinding sintered Al2O3. These chips are of more uniform size distribution as compared to the chips obtained from the coating. This
11
may be attributed to presence of porosity in the coating leading to non-uniform size of chips during material removal.
Fig. 15 reveals the 3-D ground surface topography over an area of 4 mm 4 mm for three typical coating which are Al2O3 NS, Al2O3 – 13 wt% TiO2 and TiO2. All the topographies clearly reveal the micro-cutting marks. Fig. 16 shows the secondary electron image of the ground surface topography for the same coatings, captured using the same magnification. In this figure, the grinding marks are not visible as the magnification is much higher compared to lateral magnification of Fig. 15. Grinding marks appear at the inter-grit pitch in the transverse direction. In the present case, grit density is 33 /mm2, thus the average inter-grit distance is around 0.175 mm (√
). Thus the grinding marks owing to micro-cutting action
are not readily visible at a higher magnification. However, Fig. 16 clearly reveals the dissimilarities between ground surface topographies of Al2O3 – 13 wt% TiO2 and TiO2 coating in comparison to Al2O3.
Table 4 shows the different material properties for the present set of coatings and a parameter called ‘critical depth’. The critical depth has been determined using the approach proposed by Malkin and Hwang [11]. If the critical depth is high for a given coating, it has more metallic characteristics. The table clearly shows that TiO2 and Al2O3 – 13 wt% TiO2 have higher critical depth as compared to all other coatings. This observation corroborates
12
to the findings of Fig. 16. The plastic deformation and ploughing marks are clearly visible for TiO2 coatings. These marks are also visible to a lesser extent in Al2O3 – 13 wt% TiO2 coating for which critical depth is slightly less. For Al2O3 coating, it seems material removal has primarily occurred owing to micro-fracture. In Fig. 16, for the same coatings the three figures are arranged in the increasing order of material removal rate. The material removal rate does not seem to have any significant effect on ground surface topography and possibly on the mechanism of material removal. Similar observations have been also reported while studying the effect of material removal rate on chip morphology in a previous section.
Fig. 17 shows the ground surface topography of all eight coatings at downfeed of 14 µm and workspeed of 12 m/min. The SEM photographs clearly show presence of microfracturing to be one of the mechanisms of material removal. Presence of pores and defects promotes micro-fracturing as mechanism of material removal. Plasma sprayed coating provide significant porosity owing to partial melting of powders and splat by splat built up of coating [39]. This also promotes formation of internal cracks and defects.
Fig. 18 shows the subsurface characteristics of Al2O3 SFP and Al2O3 NS coatings sprayed at 600 A as revealed from the secondary electron images of FIB milled pockets, before and after grinding. On the coated sample, some of the defects are visible. Upon grinding, the
13
density of defects and cracks increases significantly, owing to the grinding action of the grits and micro-fracturing being the predominant mode of material removal. 3.5 Ground sub-surface characteristics and surface roughness:
Fig.19 depicts the sub-surface characteristics of ground alumina SFP 600 A as compared to coated samples under SEM. The sub-surface damages upon grinding have been highlighted by a rectangular box. The sub-surface damages are micro-crack networks very similar to the ones observed in Fig.18 (b) and (d).
Fig. 20 shows the effect of downfeed and workspeed on the surface roughness, both across and along the grinding direction. In the present case, the process parameters and the material combination seem to have almost no effect on the surface roughness. The domain of variation of surface roughness across the grinding direction is between 3 and 5 µm. Only for TiO2, it varies between 1.5 to 2.5 µm, possibly owing to less micro-fracturing and more micro-cutting by the abrasive grits. This is attributed to its favorable fracture toughness.
Another interesting fact is the ratio between surface roughness across and along the grinding direction. In the present case, this ratio lies in 12.8 range. For sintered ceramic grinding, this can be as high as 10 as can be seen in Fig. 21. This indicates micro-fracturing
14
to be the dominant material removal mechanism for these porous and inhomogeneous coatings.
Table 5 reveals the effect of grinding on the surface residual stress, which was measured only at the downfeed of 14 µm and at a workspeed of 6 m/min. In metallic alloys, residual stress come into play owing to metallurgical changes, micro-cutting action and thermal cycle [36]. In grinding of the present coatings, the specific grinding energy is much lower leading to low grinding temperature as compared to metallic alloys and retention of material property by the ceramic coatings at the grinding temperature. Hence, there is limited chance of metallurgical changes and induction of residual stress of thermal origin. Micro-cutting action of the grits may change the surface residual stress to a limited extent as micro-fracturing is the dominant mechanism of material removal. Thus, Table 5 expectedly reveals insignificant effect of grinding on residual stresses. 4. Conclusion: In ceramic grinding, the forces were observed to increase with an increase in the undeformed chip thickness. Force results indicated, a micro-brittle fracture to be the predominant mode of material removal. The ratio of normal and tangential grinding force components for the coatings varied in 36 range. The same for the sintered ceramics is around 510. This lower value of force ratio for the coatings is ascribed to the presence of coating defects like porosity. Specific grinding energy ( u g ) of grinding for the coatings was found to be very small (0.5 to 5 J/mm3) as compared to sintered ceramics (10 to 100
15
J/mm3). This is attributed to porous, and inhomogeneous structure of the plasma sprayed coatings. The grinding chips are blocky and irregular, and this is credited to micro-fracture being the predominant mode of material removal. Again with an increase in MRR, no change in chip morphology was observed. Surface topography revealed traces of microbrittle fracture on the surfaces of all coatings. However, in TiO2 and Al2O3 – 13 wt% TiO2 coatings, signatures of ploughing and plastic deformation along with micro-fracture were also observed. FIB pocketing milling revealed an increase in the subsurface defect density upon grinding. This phenomenon is attributed to the grinding action of the grits and microfracturing. In the TiO2 coating, low surface roughness indicates the dominance of microcutting as the mechanism of material removal. However, signatures of micro-fracturing were also detected. The latter effect was offset by the former. Grinding has limited effect on the pre-existing surface residual stress of the coatings. This is also ascribed to microbrittle fracture and not plastic deformation being the predominant mechanism of material removal. Residual stresses in grinding, especially in metals, are thermal in origin. Ceramics, on the other hand, could retain their properties at high temperature and hence, did not acquire thermally induced residual stress.
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References [1]
M.P. Kanouff, R.A. Neiser, T.J. Roemer, Surface Roughness of Thermal Spray Coatings Made with Off-Normal Spray Angles, 7 (1998) 219–228.
[2]
K. Sabiruddin, P.P. Bandyopadhyay, G. Bolelli, L. Lusvarghi, Variation of splat shape with processing conditions in plasma sprayed alumina coatings, Journal of Materials Processing Technology. 211 (2011) 450–462. doi:10.1016/j.jmatprotec.2010.10.020.
[3]
P.P. Bandyopadhyay, M. Hadad, C. Jaeggi, S. Siegmann, Microstructural, tribological and corrosion aspects of thermally sprayed Ti–Cr–Si coatings, Surface and Coatings Technology. 203 (2008) 35–45. doi:10.1016/j.surfcoat.2008.07.026.
[4]
K. Sabiruddin, J. Joardar, P.P. Bandyopadhyay, Analysis of phase transformation in plasma sprayed alumina coatings using Rietveld refinement, Surface & Coatings Technology. 204 (2010) 3248–3253. doi:10.1016/j.surfcoat.2010.03.026.
[5]
S. Das, T.K. Bandyopadhyay, S. Ghosh, A.B. Chattopadhyay, P.P. Bandyopadhyay, Processing and characterization of plasma-sprayed ceramic coatings on steel substrate: Part II. On coating performance, Metallurgical and Materials Transactions A. 34 (2003) 1919–1930. doi:10.1007/s11661-003-0157-2.
[6]
B. Gérard, Application of thermal spraying in the automobile industry, Surface and Coatings Technology. 201 (2006) 2028–2031. doi:10.1016/j.surfcoat.2006.04.050.
[7]
S. Malkin, J.E. Ritter, Grinding Mechanism and Strength Degradation for Ceramics, ASME J. of Eng. for Lnd. 111 (1989) 167–173.
[8]
I. Inasaki, Grinding of Hard and Brittle Materials, CIRP Annals - Manufacturing Technology. 36 (1987) 463–471. doi:10.1016/S0007-8506(07)60748-3.
[9]
S. Paul, P.P. Bandyopadhyay, A.B. Chattopadhyay, Effects of cryo-cooling in grinding steels, Journal of Materials Processing Technology. 37 (1993) 791–800. doi:10.1016/0924-0136(93)90137-U. 17
[10] A. Macwan, M. Marr, O. Kesler, D.L. Chen, Microstructure, hardness, and fracture toughness of suspension plasma sprayed yttria-stabilized zirconia electrolytes on stainless steel substrates, Thin Solid Films. 584 (2014) 6–11. doi:10.1016/j.tsf.2014.11.052. [11] S. Malkin, T.W. Hwang, Grinding Mechanisms for Ceramics, CIRP Annals Manufacturing Technology. 45 (1996) 569–580. doi:10.1016/S0007-8506(07)605113. [12] J. Chen, H. Huang, X. Xu, An experimental study on the grinding of alumina with a monolayer brazed diamond wheel, The International Journal of Advanced Manufacturing Technology. 41 (2009) 16–23. doi:10.1007/s00170-008-1459-8. [13] D. Zhang, C. Li, D. Jia, Y. Zhang, Investigation into Engineering Ceramics Grinding Mechanism and the Influential Factors of the Grinding Force, 7 (2014) 19–34. [14] K. Kitajima, N. Kurnagai, Y. Tanaka, G.Q. Cai, N. Kurnagai, Y. Tanaka, et al., Study on Mechanism of Ceramics Grinding, CIRP Annals - Manufacturing Technology. 41 (1992) 367–371. doi:10.1016/S0007-8506(07)61224-4. [15] B. Zhang, J. Wang, F. Yang, Z. Zhu, The effect of machine stiffness on grinding of silicon nitride, International Journal of Machine Tools and Manufacture. 39 (1999) 1263–1283. doi:10.1016/S0890-6955(98)00083-2. [16] X. Liu, B. Zhang, Z. Deng, Grinding of nanostructured ceramic coatings: Surface observations and material removal mechanisms, International Journal of Machine Tools and Manufacture. 42 (2002) 1665–1676. doi:10.1016/S0890-6955(02)001153. [17] X. Liu, B. Zhang, Grinding of nanostructural ceramic coatings: damage evaluation, International Journal of Machine Tools and Manufacture. 43 (2003) 161–167. doi:10.1016/S0890-6955(02)00157-8. [18] T. Kubohori, Y. Inui, T. Ikuta, Evaluation of Grinding Characteristics of Thermal
18
Spraying Ceramics Film, MATERIALS TRANSACTIONS. 48 (2007) 1050–1054. doi:10.2320/matertrans.48.1050. [19] M. Ramazani, J. Khalil-Allafi, R. Mozaffarinia, Grindability Evaluation and Fatigue and Wear Behavior of Conventional and Nanostructured Al2O3-13 Wt.% TiO2 Air Plasma Sprayed Coatings, Journal of Thermal Spray Technology. 19 (2010) 611– 619. doi:10.1007/s11666-009-9459-2. [20] K. Jia, T.E. Fischer, Abrasion resistance of nanostructured and conventional cemented carbides, 200 (1996) 206–214. [21] X. Liu, B. Zhang, Effects of grinding process on residual stresses in nanostructured ceramic coatings, Journal of Materials Science. 37 (2002) 3229–3239. doi:10.1023/A:1016174731658. [22] X. Liu, B. Zhang, Z. Deng, Grinding of nanostructured ceramic coatings: surface observations and material removal mechanisms, International Journal of Machine Tools and Manufacture. 42 (2002) 1665–1676. doi:10.1016/S0890-6955(02)001153. [23] B. Zhang, X. Liu, C.A. Brown, T.S. Bergstrom, Microgrinding of Nanostructured Material Coatings, CIRP Annals - Manufacturing Technology. 51 (2002) 251–254. doi:10.1016/S0007-8506(07)61510-8. [24] S. Kar, S. Paul, P.P. Bandyopadhyay, Processing and characterisation of plasma sprayed oxides: Microstructure, phases and residual stress, Surface and Coatings Technology. 304 (2016) 364–374. doi:10.1016/j.surfcoat.2016.07.043. [25] P.P. Bandyopadhyay, D. Chicot, B. Venkateshwarlu, V. Racherla, Mechanics of Materials Mechanical properties of conventional and nanostructured plasma sprayed alumina coatings, International Journal of Mechanics of Materials. 53 (2012) 61–71. doi:10.1016/j.mechmat.2012.05.006. [26] A.G. EVANS, E.A. CHARLES, Fracture Toughness Determinations by Indentation,
19
Journal of the American Ceramic Society. 59 (1976) 371–372. doi:10.1111/j.11512916.1976.tb10991.x. [27] G.. Anstis, P. Chantikul, B.. Lawn, D.. Marshall, A critical evaluation of indentation techniques for measuring fracture toughness: I Direct crack measurements, Journal of the American Ceramic Society. 46 (1981) 533–538. doi:10.1111/j.11512916.1981.tb10320.x. [28] T.W. Hwang, S. Malkin, Grinding Mechanisms and Energy Balance for Ceramics, Journal of Manufacturing Science and Engineering. 121 (1999) 623. doi:10.1115/1.2833081. [29] M. Vashista, S. Ghosh, S. Paul, Application of Micromagnetic Technique in Surface Grinding for Assessment of Surface Integrity, Materials and Manufacturing Processes. 24 (2009) 488–496. doi:10.1080/10426910802714456. [30] S. Ghosh, a. B. Chattopadhyay, S. Paul, Modelling of specific energy requirement during high-efficiency deep grinding, International Journal of Machine Tools and Manufacture. 48 (2008) 1242–1253. doi:10.1016/j.ijmachtools.2008.03.008. [31] J. Chen, J. Shen, H. Huang, X. Xu, Grinding characteristics in high speed grinding of engineering ceramics with brazed diamond wheels, Journal of Materials Processing Technology. 210 (2010) 899–906. doi:10.1016/j.jmatprotec.2010.02.002. [32] R.B. Heimann, Plasma spray coating: principles and applications, Advanced Materials. (1996) 1–327. doi:10.1002/9783527614851. [33] L.C. Erickson, R. Westergård, U. Wiklund, N. Axén, H.M. Hawthorne, S. Hogmark, Cohesion in plasma-sprayed coatings — a comparison between evaluation methods, Wear. 214 (1998) 30–37. doi:10.1016/S0043-1648(97)00216-0. [34] K.-S. SHI, Z.-Y. QIAN, M.-S. ZHUANG, Microstructure and Properties of Sprayed Ceramic Coating, Journal of the American Ceramic Society. 71 (1988) 924–929. doi:10.1111/j.1151-2916.1988.tb07559.x.
20
[35] R. McPherson, On the formation of thermally sprayed alumina coatings, Journal of Materials Science. 15 (1980) 3141–3149. doi:10.1007/BF00550387. [36] S. Malkin, Grinding of metals: Theory and application, Journal of Applied Metalworking. 3 (1984) 95–109. doi:10.1007/BF02833688. [37] M. Vashista, S. Paul, Study of the effect of process parameters in high-speed grinding on surface integrity by Barkhausen noise analysis, Proceedings of the Institution of Mechanical Engineers Part B-Journal of Engineering Manufacture. 222 (2008) 1625–1637. doi:Doi 10.1243/09544054jem1214. [38] S. Malkin, R.B. Anderson, Thermal Aspects of Grinding: Part 1—Energy Partition, Journal of Engineering for Industry. 96 (1974) 1177. doi:10.1115/1.3438492. [39] R. McPherson, A review of microstructure and properties of plasma sprayed ceramic coatings, Surface and Coatings Technology. 39–40 (1989) 173–181. doi:10.1016/0257-8972(89)90052-2.
21
List of Figures: 1. Fig 1. Raw signal plot of forces obtained during grinding of Al2O3 NS sprayed at 500 A. 2. Fig 2. Analysed forces plots of grinding forces data set of Al2O3 NS sprayed at 500 A. 3. Fig 3. Tangential forces obtained during grinding of plasma sprayed coating using electroplated monolayer diamond wheel.
4. Fig 4. Normal forces obtained during grinding of plasma sprayed coating using electroplated monolayer diamond wheel.
5. Fig 5. Optical image of diamond grits taken at 100 . 6. Fig 6. Variation of normal forces with respect to uncut chip thickness. 7. Fig 7. Variation of normal forces with respect to uncut chip thickness. 8. Fig 8. Variation of force ratio with downfeed. 9. Fig 9. Variation of force ratio with uncut chip thickness, 10. Fig 10. Variation of specific energy ( u g ) with downfeed. 11. Fig 11. Variation of specific energy ( u g ) with uncut chip thickness (hm). 12. Figure 12. SEM images of grinding chips of (a) fine alumina (SFP) and (b) alumina (NS) coated 500 A, (c) fine alumina and (d) alumina coatings deposited at 600 A (e) Alumina13 wt% titania (f) titania (g) yttria stabilized zirconia coating coated at 500 A and (h) chromia coating deposited at 600 A obtained at downfeed 14 and at workspeed 12 m/min.
13. Figure 13. SEM images of grinding chips of fine alumina deposited at 600 A. 14. Fig 14. SEM image of grinding chips of sintered alumina. 15. Fig 15. Ground surface of Al2O313 wt% TiO2 at 14µm-6 m/min and at 38µm-6 m/min. 16. Figure 16. SEM images of ground surface of (a) alumina (NS) (b) Alumina13 wt% titania (c) titania coated at 500 A obtained at downfeed 14 and 38 µm and at workspeed 6 and 12 m/min.
17. Figure 17. SEM images of ground surface of (a) fine alumina (SFP) and (b) alumina (NS) coated 500 A, (c) fine alumina and (d) alumina coatings deposited at 600 A (e) Alumina13 wt% titania (f) titania (g) yttria stabilized zirconia coating coated at 500 A and (h) chromia coating deposited at 600 A obtained at downfeed 14 and at workspeed 12 m/min.
22
18. Fig 18. SEM microphotographs of FIB pocket milled coated and ground samples ((a) and (b) alumina SFP at 600 A and (c) and (d) alumina NS at 600 A; (a) and (c) – coated samples and (b) and (d) – ground samples).
19. Fig 19. SEM images of cross section of ground sample of Alumina SFP at 600 A ((a) 100x and (b) 1000x)
20. Fig 20. Roughness profile of ground surfaces at workspeed 6 m/min and 12 m/min. 21. Fig 21. Roughness ratio of ground surfaces of plasma sprayed coating.
23
Tables: Table 1 Spray parameters for coating Sl. No.
Powder
Nozzle diameter (mm)
SOD (mm)
1
Al2O3 SFP
5
100
2
Al2O3 NS
5
125
3 4 5 6 7 8
YSZ TiO2 Al2O3-TiO2 Cr2O3 Ni-Al NiCrAlY
5 5 5 5 5 5
100 130 130 125 125 125
Current (A)
Voltage (V)
N2 Flowrate (scfh)
H2 Flowrate (scfh)
Spray rate (g/min)
74
50
5
~ 26
75
50
5
~ 64
82.4 68 68 71 67 77
50 50 50 50 50 50
5 5 5 5 Nil Nil
~ 53 ~ 50 ~ 25 ~ 55 ~ 42 ~ 56
500 600 500 600 500 500 500 600 450 450
Table 2 Particle morphology of ceramic powders Sl No. 1. 2. 3. 4. 5. 6.
Formula
Size (microns)
Morphology
Al2O3 SFP Al2O3 NS Al2O3 13 wt% TiO2 TiO2 Cr2O3 YSZ
-31+3.9 -45+15
Crushed Crushed
-45+5
Clad
-88+7.8 -125+11 -75+45
Crushed Crushed Spheroidal
24
Table 3 Grinding parameters Sl No 1. 2. 3. 4. 5. 6. 7.
Parameters Machine specification: Wheel specification: Downfeed (a) in µm Work speed (Vw) in m/min Wheel speed (Vs) in m/sec Enviroment Coolant Fluid
Description Cosmos Earth E 6030 D200U10D126GN333 (Make: Wendt India) 14, 28, 32, 38 6, 12 26 Wet Servocut S 1:20 dilution (IOCL)
Table 4 Measure of critical depth of cut for crack initiation in ceramic coatings during grinding process Coating
HV200
E(GPa)
K1C (MPa m1/2)
critical depth dc (microns)
1.
SFP @ 500A
1066 86
85.94
1.955
0.0431
2.
NS @500
983 ± 42
137.28
1.719
0.0678
3.
SFP @600
1113 ± 112
90.73
1.726
0.0311
4.
NS @600
1098.9 ± 206
132.17
1.835
0.0534
5.
YSZ
913 ± 130
74.78
1.43
0.0795
6.
Al2O3 13wt% TiO2
744±63
185.76
1.902
0.1401
7.
CrO
1253 ± 87
134.36
2.983
0.0966
8.
TiO
673.7 ± 41
127.61
1.806
0.1606
Sl. No.
25
Table 5 Residual stress of As-sprayed [24] and ground samples Sl. No. 1. 2. 3. 4. 5. 6. 7. 8.
Coating
Al2O3 (NS) @ 500 A Al2O3 (NS) @ 600 A Al2O3 (SFP) @ 500 A Al2O3 (SFP) @ 600 A Al2O3 13 wt% TiO2 TiO2 Cr2O3 YSZ
Residual stress (MPa)
Experimental Elastic modulus (GPa)
As-sprayed coating
137.28 132.17 85.94 90.73 185.76 127.61 134.36 74.78
480 5 479 11.5 325 6.3 371 4 416 19.1 544 15.9 357 13.1 264 4.3
ground Vw = 6m/min, a = 14 µm 535 54 505 84 396 16 332 125 572 111 594 10 683 131 288.2 30
26
Figure 1
6 Al2O3 NS sprayed at 500 A
tangential - Ft normal - Fn
Fn
5 4
grinding force (N)
3 Ft
2 1 0
-1 -2 -3 -4 -5
0
1
2
3
time (s)
4
5
6
7
Fig 1. Raw signal plot of forces obtained during grinding of Al2O3 NS sprayed at 500 A.
Figure 2
20
Al2O3 NS sprayed at 500A Avg Ft = 1.79 N, Max Ft = 5.18 N Avg Fn = 2.1 N, Max Fn = 5.59 N
tangential force - Ft normal force - Fn
18 16
grinding force (N)
14 12 10 8
Fn
6 4
Ft
2 0 -2
2.6
2.8
3
3.2
3.4
3.6
3.8
4
4.2
time (s)
Fig 2. Analysed forces plots of grinding forces data set of Al2O3 NS sprayed at 500 A
Figure 3
a
25
sample: Al2O3 SFP
6 m/min 12 m/min
tangential grinding force, FT (N)
wheelspeed: 26 m/s enviroment: wet 20
b
tangential grinding force,FT (N)
25
15
10
5
15
10
5
downfeed (m)
30
0
40
10
Sample: Fine Al2O3 @ 600 A
6m/min 12m/min
wheel speed: 26 m/s enviroment: wet
15
15
10
10
5
downfeed (m) 30
40
10
25
sample: Al2O3-13 wt% TiO2
6 m/min 12 m/min
f
tangential grinding force,FT (N)
20
wheel speed: 26 m/s 20 enviroment: wet
tangential grinding force,FT (N)
5
0 10
15
10
5
20
20
30
40
downfeed (m)
sample: TiO2
6 m/min 12 m/min
wheelspeed: 26 m/s enviroment: wet
15
10
5
0
0 10
25
20
downfeed (m) 30
6m/min 12m/min
25
15
10
5
20
downfeed (m)
30
sample: Cr2O3
h tangential grinding force,FT (N)
20
10
40
sample: YSZ wheelspeed: 26 m/s enviroment: wet
g tangential grinding force,FT (N)
40
6 m/min 12 m/min
wheel speed: 26 m/s enviroment: wet
20
0
e
30
downfeed (m)
sample: Al2O3 NS @ 600 A
d tangential grinding force,FT (N)
20
25
20
25
25
c tangential grinding force,FT (N)
20
6m/min 12m/min
wheel speed: 26 m/s enviroment: wet
20
0 10
sample:Al2O3 NS
6m/min 12m/min
wheelspeed: 26 m/s enviroment: wet
20
40
15
10
5
0
0 10
20
downfeed (m)
30
40
10
20
downfeed (m)
30
40
Fig 3. Tangential forces obtained during grinding of plasma sprayed coating using electroplated monolayer diamond wheel.
Figure 4
a
25
25
sample: Al2O3 SFP
15
15
10
10
5
0 20
downfeed (m)
30
40
25
10
10
5
15
10
5
0
10
20
downfeed (m)
30
40
10
25
f
15
20
30
downfeed (m)
sample: TiO2
40
6 m/min 12 m/min
wheelspeed: 26 m/s 20 enviroment: wet
normal grinding force, FN (N)
wheelspeed: 26 m/s 20 enviroment: wet
normal grinding force, FN (N)
25
6 m/min 12 m/min
sample: Al2O3- 13 wt% TiO2
15
10
10
5
5
0 10
20
30
0
40
10
downfeed (m)
20
downfeed (m)
30
40
25
25
6 m/min 12 m/min
sample: YSZ wheelspeed: 26 m/s enviroment: wet
g
sample:Cr2O3
h
wheelspeed: 26 m/s enviroment: wet
20
6m/min 12m/min
normal grinding force, FN (N)
20
normal grinding force, FN (N)
40
6 m/min 12 m/min
wheel speed: 26 m/s enviroment: wet
20
0
e
30
downfeed (m)
sample: Al2O3 NS sprayed at 600 A
normal grinding force, Fn(N)
15
20
25
d
6 m/min 12 m/min
Alumina SFP sprayed at 600 A wheel speed: 26 m/s 20 enviroment: wet
normal grinding force, FN (N)
5
0 10
c
6 m/min 12m/min
wheel speed: 26 m/s enviroment: wet
20
normal grinding force, FN (N)
normal grinding force, FN (N)
wheel speed: 26 m/s 20 enviroment: wet
sample: Al2O3 NS
b
6m/min 12m/min
15
15
10
10
5
5
0
0 10
20
30
downfeed (m)
40
10
20
30
40
downfeed (m)
Fig 4. Normal forces obtained during grinding of plasma sprayed coating using electroplated monolayer diamond wheel.
Figure 5
Fig 5. Optical image of diamond grits taken at 100 and 40
2.0
a
1.6
2.0
sample: Al2O3 - 13wt% TiO2 wheel speed: 26 m/s enviroment: wet
b
1.6
F'T (N/mm)
F'T (N/mm)
6 m/min 12 m/min 1.2
0.8
0.4
sample: TiO2 sprayed at 500 A wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
1.2
0.8
0.4
0.0 1.2
1.5
1.8
2.1
2.4
2.7
0.0
3.0
1.2
Uncut chip thickness (m) 2.0
c
1.8
2.1
2.4
2.7
3.0
2.7
3.0
Uncut chip thickness (m)
sample: Cr2O3 sprayed at 600 A
2.0
d
wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
1.6
sample: YSZ sprayed at 500 A wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
1.2
F'T (N/mm)
F'T (N/mm)
1.6
1.5
0.8
1.2
0.8
0.4
0.4
0.0
0.0 1.2
1.5
1.8
2.1
2.4
2.7
3.0
1.2
1.5
1.8
2.1
2.4
Uncut chip thickness (m)
Uncut chip thickness (m)
Fig 6. Variation of specific tangential forces with respect to uncut chip thickness
sample: Al2O3 -13 wt% TiO2 3.6
F'n(N/mm)
3.0
sample: TiO2 sprayed at 500 A
b
wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
3.6
3.0
F'n(N/mm)
a
2.4
1.8
2.4
1.8
1.2
1.2
0.6
0.6
0.0
wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
0.0 1.2
1.5
1.8
2.1
2.4
2.7
3.0
1.2
1.5
uncut chipthickness (m)
sample: Cr2O3 sprayed at 600 A
c F'n(N/mm)
3.0
wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
d
3.6
3.0
F'n(N/mm)
3.6
1.8
2.1
2.4
2.7
3.0
uncut chipthickness (m)
2.4
1.8
sample: YSZ sprayed at 500 A wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
2.4
1.8
1.2
1.2
0.6
0.6
0.0
0.0 1.2
1.5
1.8
2.1
2.4
uncut chipthickness (m)
2.7
3.0
1.2
1.5
1.8
2.1
2.4
2.7
uncut chipthickness (m)
Fig 7. Variation of specific normal forces with respect to uncut chip thickness
3.0
Figure 8
10
wheelspeed: 26 m/s enviroment: wet Fn: normal grinding force
8
10
6 m/min 12 m/min
sample: Al2O3 SFP
a
wheel speed:26 m/s enviroment: wet Fn: normal grinding force
8
Ft: tangential grinding force
Ft: tangential grinding force 6
FN/FT
FN/FT
6
4
4
2
2
0
0 10
c
6 m/min 12 m/min
sample: Al2O3 NS
b
20
downfeed (m)
30
40
10
d
6m/min 12m/min
sample: Al2O3 SFP sprayed 600 A wheelspeed: 26 m/s enviroment: wet Fn: normal grinding force
25
30
35
40
10
sample: Al2O3 NS sprayed at 600 A wheel speed: 26 m/s enviroment: wet
8
Ft: tangential grinding force
6 m/min 12 m/min
6
FN/FT
FN/FT
6
20
downfeed (m)
10
8
15
4
4
2
2
0
0 10
20
30
10
40
20
10
e
sample: Al2O3 - 13 wt% TiO2
10
6m/min 12m/min
wheelspeed: 26 m/s enviroment: wet 8 Fn: normal grinding force
wheelspeed: 26 m/s enviroment: wet Fn: normal grinding force
8
Ft: tangential grinding force
6
FN/FT
FN/FT
6
4
4
2
2
0 10
20
downfeed (m)
30
0
40
10
10
20
downfeed (m)
30
40
10
sample: YSZ wheel speed: 26 m/s enviroment: wet 8 Fn: normal grinding force
6 m/min 12 m/min
6m/min 12m/min
sample: Cr2O3
h
wheelspeed: 26 m/s enviroment: wet Fn: normal grinding force
8
Ft: tangential grinding force
Ft: tangential grinding force
6
6
FN/FT
FN/FT
40
6m/min 12m/min
sample: TiO2
f
Ft: tangential grinding force
g
30
downfeed (m)
downfeed (m)
4
4
2
2
0
0 10
20
30
downfeed (m)
40
10
Fig 8. Variation of force ratio with downfeed.
20
downfeed (m)
30
40
Figure 9
10 9
a
8
10
sample: Al2O3 -13 wt% TiO2
b
wheel speed: 26 m/s enviroment: wet
9 8
6m/min 12m/min
7
FN/FT
FN/FT
4
5 4
3
3
2
2
1
1
0 1.2
1.5
1.8
2.1
2.4
2.7
0
3.0
1.2
uncut chip thickness (m)
9 8
1.5
1.8
2.1
2.4
2.7
3.0
2.7
3.0
uncut chip thickness (m) 10
sample: Cr2O3
d
wheel speed: 26 m/s enviroment: wet
9 8
6m/min 12m/min
7
7
sample: YSZ wheel speed: 26 m/s enviroment: wet 6m/min 12m/min
6
FN/FT
6
FN/FT
6m/min 12m/min
6
5
10
wheel speed: 26 m/s enviroment: wet
7
6
c
sample: TiO2
5
5
4
4
3
3
2
2
1
1 0
0 1.2
1.5
1.8
2.1
2.4
uncut chip thickness (m)
2.7
3.0
1.2
1.5
1.8
2.1
2.4
uncut chip thickness (m)
Fig 9. Variation of force ratio with uncut chip thickness
Figure 10
7
sample: Al2O3 SFP
b
6 m/min 12 m/min
wheel speed: 26 m/s 6 enviroment: wet
specific grinding energy (J/mm3)
specific grinding energy (J/mm3)
a
5
4
3
2
1
0
7
6
sample:Al2O3 NS
5
4
3
2
1
0
0
10
20
30
40
0
10
20
downfeed (m)
wheel speed: 26 m/s enviroment: wet
d specific grinding energy (J/mm3)
specific grinding energy (J/mm3)
7
6 m/min 12 m/min
sample: Al2O3 SFP sprayed at 600 A 6
5
4
3
2
1
0
10
20
30
wheel speed: 26 m/s enviroment: wet
5
4
3
2
1
0
40
10
20
7
sample: Al2O3- 13wt% TiO2
6 m/min 12 m/min
wheel speed: 26 m/s enviroment: wet
sample: TiO2
f
5
4
3
2
1
0
6m/min 12m/min
wheelspeed: 26 m/s enviroment: wet
6
5
4
3
2
1
10
20
30
10
40
20
downfeed (m)
30
40
downfeed (m)
sample: YSZ wheel speed: 26 m/s 6 enviroment: wet
6m/min 12m/min
5
4
3
2
1
7
h specific grinding energy (J/mm3)
7
specific grinding energy (J/mm3)
40
0
0
g
30
downfeed (m)
specific grinding energy (J/mm3)
specific grinding energy (J/mm3)
6
6 m/min 12 m/min
sample:Al2O3 NS sprayed 600A
6
downfeed (m) 7
40
0
0
e
30
downfeed (m)
7
c
6 m/min 12 m/min
wheel speed: 26 m/s enviroment: wet
6
sample: Cr2O3
6m/min 12m/min
wheel speed: 26 m/s enviroment: wet
5
4
3
2
1
0
0 0
10
20
downfeed (µm)
30
40
10
20
30
downfeed (m)
Fig 10. Variation of specific energy (Ug) with downfeed.
40
Figure 11
7
sample: Al2O3 -13 wt% TiO2
b
wheel speed: 26 m/s enviroment: wet
specific grinding energy Ug (J/mm3)
specific grinding energy Ug (J/mm3)
8
6 m/min 12 m/min
6 5 4 3 2 1
8 7
sample: TiO2 wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
6 5 4 3 2 1
0 1.2
1.5
1.8
2.1
2.4
2.7
3.0
0 1.2
uncut chip thickness (m)
1.5
1.8
2.1
2.4
2.7
3.0
2.7
3.0
uncut chip thickness (m)
8 7
d
sample: Cr2O3
specific grinding energy Ug (J/mm3)
c specific grinding energy Ug (J/mm3)
a
wheel speed: 26 m/s enviroment: wet
6
6 m/min 12 m/min
5 4 3 2 1
8 7 6
sample: YSZ wheel speed: 26 m/s enviroment: wet 6 m/min 12 m/min
5 4 3 2 1 0
0 1.2
1.5
1.8
2.1
2.4
uncut chip thickness (m)
2.7
3.0
1.2
1.5
1.8
2.1
2.4
uncut chip thickness (m)
Fig 11. Variation of specific energy ( u g ) with uncut chip thickness (hm)
Figure 12
a = 14 µm, Vw = 12 m/min
a = 14 µm, Vw = 12 m/min
a
b
c
d
e
f
g h Figure 12. SEM images of grinding chips of (a) fine alumina (SFP) and (b) alumina (NS) coated 500 A, (c) fine alumina and (d) alumina coatings deposited at 600 A (e) Alumina13 wt% titania (f) titania (g) yttria stabilized zirconia coating coated at 500 A and (h) chromia coating deposited at 600 A obtained at downfeed 14 and at tablespeed 12 m/min.
Figure 13
a = 14 µm, Vw = 6m/min
a = 14 µm, Vw = 12 m/min
a = 38 µm, Vw = 6m/min
a b c Figure 13. SEM images of grinding chips of fine alumina deposited at 600 A.
Figure 14
Fig 14 SEM image of grinding chips of sintered alumina
Figure 15
a
b
Fig 15. Ground surface of Al2O313 wt% TiO2 at 14µm-6 m/min and at 38µm-6 m/min
Figure 16
a = 14 µm, Vw = 6m/min
a
e
a = 14 µm, Vw = 12 m/min
b
a = 38 µm, Vw = 6m/min
c
f
g
h j i Figure 16. SEM images of ground surface of (a) alumina (NS) (b) Alumina13 wt% titania (c) titania coated at 500 A obtained at downfeed 14 and 38 µm and at tablespeed 6 and 12 m/min.
Figure 17
a = 14 µm, Vw = 12 m/min
a = 14 µm, Vw = 12 m/min a
b
c
d
e
f
g
h
Figure 17. SEM images of ground surface of (a) fine alumina (SFP) and (b) alumina (NS) coated 500 A, (c) fine alumina and (d) alumina coatings deposited coating coated at 500 A and (h) chromia coating deposited at 600 A obtained at downfeed 14 and at workspeed 12 m/min.
Figure 18
Coated sample
Ground sample (Vw= 6 m/min, a= 38 µm)
Fig 18 SEM microphotographs of FIB pocket milled coated and ground samples ((a) and (b) alumina SFP at 600 A and (c) and (d) alumina NS at 600 A; (a) and (c) – coated samples and (b) and (d) – ground samples)
Figure 19
Mould Subsurface cracks Top Coat
Fig 19 SEM images of cross section of ground sample of Alumina SFP at 600 A ((a) 100x and (b) 1000x)
Figure 20
8
a
sample: Al2O3 SFP
6 m/min - perpendicular 12 m/min - perpendicular 6 m/min - parallel 12 m/min - paralel
wheel speed: 26m/s enviroment: wet
7
roughness (Ra) (m)
roughness (Ra) (m)
6
b
5 4 3 2 1
8
sample: Al2O3 NS 500
7
wheel speed: 26 m/s enviroment: wet
6 5 4 3 2 1
0
0
0
10
20
30
40
0
10
downfeed (m) 8
c
sample: Al2O3 SFP @ 600A
8
d
5 4 3 2 1
6 5 4 3 2
10
20
30
40
0
10
downfeed (m) 8
sample: Al2O3- 13 wt% TiO2
7
wheel speed: 26 m/s enviroment: wet
20
30
40
downfeed (m)
6 m/min - perpendicular 12 m/min - perpendicular 6 m/min - parallel 12 m/min - paralel
8
f
5 4 3 2 1
6 m/min - perpendicular 12 m/min - perpendicular 6 m/min - parallel 12 m/min - paralel
sample: Cr2O3 wheel speed: 26 m/s enviroment: wet
7
6
roughness (Ra) (m)
roughness (Ra) (m)
6 m/min - perpendicular 12 m/min - perpendicular 6 m/min - parallel 12 m/min - paralel
wheel speed:26 m/s enviroment: wet
0 0
6 5 4 3 2 1
0
0
0
10
20
30
40
0
10
downfeed (m) 8
wheel speed: 26 m/s enviroment: wet
8
h
30
40
5 4 3 2 1
6 m/min - perpendicular 12 m/min - perpendicular 6 m/min - parallel 12 m/min - paralel
sample: YSZ wheel speed: 26 m/s enviroment: wet
7
roughness (Ra) (m)
6
20
downfeed (m)
6 m/min - perpendicular 12 m/min - perpendicular 6 m/min - parallel 12 m/min - paralel
sample: TiO2
7
roughness (Ra) (m)
40
1
0
g
30
sample: Al2O3 NS @ 600 A
7
roughness (Ra) (m)
roughness (Ra) (m)
6
20
downfeed (m) 6 m/min - perpendicular 12 m/min - perpendicular 6 m/min - parallel 12 m/min - paralel
wheel speed: 26 m/s enviroment: wet
7
e
6 m/min - perpendicular 12 m/min - perpendicular 6 m/min - parallel 12 m/min - paralel
6 5 4 3 2 1
0 0
10
20
downfeed (m)
30
40
0 0
10
20
30
downfeed (m)
Fig 20. Roughness profile of ground surfaces at workspeed 6 m/min and 12 m/min
40
Figure 21
3.6
Roughness ratio (R/R//)
3.2
AlTiO, sfp 500,
CrO, NS 500, sfp 600, TiO,
NS 600 YSZ
2.8 2.4 2.0 1.6 1.2 0.8 0.4 0.0 14-6 20-6 26-6 32-6 38-6 14-12 20-12 26-12 32-12 38-12
--
Downfeed(m)workspeed (m/min)
Fig 21. Roughness ratio of ground surfaces of plasma sprayed coating