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1 Grindiability comparison between conventional and nanostructured material coatings. Bi Zhang, Xianbing Liu, Zhaohui Deng and Jian Meng Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USA Email: zhang@engr.uconn.edu ABSTRACT This paper compares the grindability of conventionaland nanostructured material coatings in terms ofnormal grinding force, specific grinding energy, surfacefinish and surface topography. Material removalmechanism is correlated with the microstructures of thematerials such as material grain size. The effect of thedecreasing material grain size in nanostructuredmaterials on the grindability is studied. 1 INTRODUCTION In grinding of ceramics, the effects of grindingprocess parameters have been extensively studied (e.g.,Kirchner and Conway, 1985;T?nshoff and Brinksmeier,1988; Blake et al., 1988). It is of equal importance tostudy the influence of material microstructure. One ofthe material microstructures is the grain size, whichinfluences the mechanical properties, such as hardnessand toughness, and therefore the grindability ofceramics. Few works have been done on this aspect. Intheir work, Roth and T?nshoff (1993) studied thegrindability of alumina with different grain sizes increep feed grinding and conventional surface grinding.Both hardness and toughness for n-Al2O3/13TiO2 andn-WC/12Co coatings are found higher than theirconventional counterparts due to the reduced grain sizeand richer binder phases. Hardness of a material is itsability to resist plastic deformation. Plastic deformationis induced by the dislocation movement. The richer binder phases in nanostructured materials constrainmaterial flow and therefore plastic deformation. Unlike in the conventional materials, the increase of hardness in nanostructured materials does not lead to thedecrease of toughness due to more bridging ligaments,higher in-situ flow stress and higher rupture strength (Jia, et al., 1998). The difference in hardness and toughness between nanostructured and conventional materials can be expected to influence the grindability of their coatings. The large quantities of voids, cracks and microcracks induced by the thermal spray process greatly influence the properties of coatings made of these materials. The difference in hardness and toughness between conventional and nanostructured material coatings are not as much as in their bulk counterparts. Table 1 shows the typical properties of conventional and nanostructured WC/12Co and Al2O3/13TiO2 coatings. Normally, grindability is evaluated based on material removal rate, grinding force, surface finish and integrity of ground samples. In this paper, normal grinding force, specific grinding energy and surface finish are compared for nanostructured and conventional coatings. In addition, the scanning electronic microscopy (SEM) is used to assess the effects of grain sizes on the material removal mechanisms. Table 1 Typical Properties of the Conventional and Nanostructured WC/12Co and Al2O3/13TiO2 Coatings. c-WC /12Co n-WC /12Co c-Al2O3 /13TiO2 n-Al2O3 /13TiO2 Bonding strength, MPa 82.7 89.6 15.5 20.7 Powder grain size, mm 1.3 0.04 2.5 0.05 Mass density, g/cm3 14.2 14.5 3.5-4.0 3.7-4.1 Vickers hardness, GPa 12.00 12.50 10.44 10.57 Toughness, MPa m1/2 16.0 16.5 3.3 3.5 2 EXPERIMENTAL CONFIGURATION 2.1 Sample preparation and characterization Conventional and nanostructured material coatings were made on low carbon steel substrates of dimensions of 25 ′ 75 ′ 4 mm3 that were cleaned and blasted before thermal spray. The conventional and nanostructured WC/12Co coatings were produced using the high velocity oxygen fuel method, and the conventional and nanostructured Al2O3/13TiO2 coatings were prepared by the plasma thermal spray method. All the coatings had a thickness of around 0.5mm. The coated samples were cut into 25 ′ 4 ′ 4 mm3 for grinding. Fig.1 shows the SEM observations of c/n-WC/12Co coatings. Fig.1 (b) indicates that the smaller grains of WC are bonded together by the binder material cobalt and a large quantity of porosities can be observed in both conventional and nanostructured WC/12Co coatings. There are no obvious cracks found in conventional and nanostructured WC/12Co coatings. Fig.2 shows the typical surface features of the thermally sprayed conventional and nanostructured Al2O3/13TiO2 coatings: pores, cracks, microcracks and segmented structures formed by the connected microcracks perpendicular to the coating surface. Prior to the formal grinding test, the coatings were preground 2 with a diamond wheel of a mean grit size of 15 mm under minimum loading to avoid damaging the coatings. This preparation process was effective in getting rid of the random influence from the thermal spray process and making the samples more uniform, although it was time-consuming and effortsdemanding. 2.2 Grinding experiments Grinding experiments were conducted on a precision grinding machine (Dover Model 956-S) with the computer numerical control (CNC). The machine had aerostatic bearings for its spindle and x, y, z slideways. The spindle had an axial run-out of 0.05 mm and the three slideways had a straightness error of 0.1 mm/25mm. A laser interferometer was equipped to the machine that formed feedback loops for the x, y, z slideways with a resolution of 0.07 mm. The loop stiffness of the machine was measured to be 50 N/mm. In this study, a diamond grinding wheel SD600N100V (600V) was used to grind the coatings under different conditions, and the ground samples were compared. The wheel speed was set to 33 m/s or 3500 rpm. In order to investigate the effect of material removal rate (MRR) on residual stresses, depths of cut were set at 2, 5, 15 and 30 mm and feedrates at 1, 4, and 8 mm/s for the grinding experiments. Water-based synthetic solution (ITW fluid products Group, Rustlick G-10066D) was used as grinding coolant. 2.3 Post-grinding evaluation A surface profilometer (Federal Products, Surfanalyzer 5000) was used to measure surface finish (Ra) of ground coatings along the directions perpendicular to the grinding direction. An SEM (JOEL, Model JSM 840) was used to observe the surfaces of the ground coatings. One issue in SEM observations was to differentiate grinding damage from the coating defects. A large quantity of defects such as voids, unmolten particles, cracks and microcracks were identified in the as-sprayed coatings (Fig.1 and Fig.2). Because some of these defects can be easily mistaken as grinding damage, SEM examinations of the assprayed coatings were conducted to identify the defects from the spray process. It can be found that the voids from the thermal spray process normally appeared with smooth edges. From Fig.1 and Fig.2 the cracks or microcracks on the as-sprayed coatings were connected to each other without obvious directionality. With the as-sprayed coatings as a reference, the grinding damage was identified. 3 RESULTS AND DISCUSSIONS 3.1 Comparison of normal grinding force The normal grinding forces are important in characterizing a grinding process. Fig.3 compares the normal grinding forces in grinding c/n-Al2O3/13TiO2 and c/n-WC/12Co coatings at the same grinding conditions. The normal grinding force is higher for n-Al2O3/13TiO2 than for its conventional counterpart. One can also observe that the break-in force for n-Al2O3/13TiO2 is larger. This shows that the resistance to wearing for n-Al2O3/13TiO2 is higher due to its enhanced mechanical properties such as hardness and toughness when compared to its conventional counterpart. A similar trend is observed in grinding c/n-WC/12Co coatings: higher grinding force and break-in force for nanostructured coatings. Fig.3 also shows that the difference between the grinding forces for the nanostructured and conventional coatings becomes smaller at a large wheel depth of cut. This means that the material grain size exerts stronger influence on the grinding force at a low material removal rate. When the wheel depth of cut or material removal rate increases, the influence of grain size becomes second to that of grinding process parameters. 3.2 Comparison of tangential grinding force and specific grinding energy Tangential grinding force is much smaller than normal grinding force due to large negative rake angles of abrasive grits in grinding. The grinding force ratio (a) c-WC/12Co (b) n-WC/12Co (a) c-Al2O3/13TiO2 (b) n-Al2O3/13TiO2 Fig. 2 SEM observations of as-sprayed c/n-Al2O3/13TiO2 coatings. Fig. 1 SEM observations of as-sprayed c/n-WC/12Co coatings. 222222000000 __mm 222222000000 _mm 1200 _ _mm 1200 __ mm 3 indicates the relative magnitude of the normal grinding force Fn to the tangential grinding force Ft and is defined as t n F F l = (1) Fig.4 shows the grinding force ratio vs. wheel depth of cut for the four coatings. The grinding force ratio is higher for c/n-Al2O3/13TiO2 coatings than for c/n-WC/12Co coatings. c/n-Al2O3/13TiO2 coatings are more brittle than c/n-WC/12Co coatings. Under the same grinding conditions, brittle fracture is more obvious for c/n-Al2O3/13TiO2 coatings. The dominant ductile flow in grinding c/n-WC/12Co coatings results in a relatively high tangential grinding force and therefore a lower grinding force ratio. It is observed that the grinding force ratio of the n-Al2O3/13TiO2 coatings is distinctly different from that of the c-Al2O3/13TiO2 coatings while the difference for the grinding force ratios of n-WC/12Co and c-WC/12Co coatings is insignificant. The grinding force ratios for the four coatings decrease with the increase of material removal rate or wheel depth of cut. On the other hand, the grinding force ratios for the four coatings change over a relatively narrow range, which suggests that the material removal mechanism does not change much for the given range of the wheel depth of cut. The specific grinding energy U is defined as the energy required to remove a unit volume of material, which is derived from the tangential grinding force, f t c W d v F v U × × × = (2) where vc the grinding speed; W the width of a workpiece; d the wheel depth of cut; vf is feedrate. Fig.5 presents the effect of grain size on the specific grinding energy and the change of specific grinding energy with wheel depth of cut. The specific grinding (a) Conventional and nanostructured Al2O3/13TiO2. (b) Conventional and nanostructured WC/12Co. Fig. 3 Comparison of normal grinding force in grinding conventional and nanostructured coatings. 0 10 20 30 0 2 4 6 8 Depth of cut, mm Normal grinding force, N/mm2 Nano. Conv. Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V 0 10 20 30 0 2 4 6 8 Depth of cut, mm Normal grinding force, N/mm2 Nano. Conv. Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V Fig. 4 Comparison of grinding force ratio. 00 10 20 30 3 6 9 12 Depth of cut, mm Grinding force ratio, l n-WC/12Co c-WC/12Co n-Al2O3/13TiO2 c-Al2O3/13TiO2 Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V Fig. 5 Comparison of specific grinding energy. 0 10 20 30 0.0 0.5 1.0 1.5 2.0 2.5 Depth of cut, mm Specific grinding energy, ′103 J/mm3 n-WC/12Co c-WC/12Co n-Al2O3/13TiO2 c-Al2O3/13TiO2 Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V 4 Energy for four coatings decreases with the wheel depth of cut and asymptotically reaches a limit. The high value of specific grinding energy at small depth of cut suggests that only a part of the energy is associated with the chip formation (Malkin, 1989). Generally, the specific grinding energy consists of chip-forming energy Uch, sliding energy Upl and plowing energy Usl, ch pl sl U =U +U +U (3) Except Uch, the rest of the specific grinding energy is attributed to sliding and plowing between the workpiece and abrasive grits at a small depth of cut. At a larger depth of cut, sliding becomes insignificant and chip formation commen. However, plowing still exists, which reflects by the grinding marks and material pile-up on the ground surface. Only Uch is actually used in removing material and forming new surface. Theoretically, the asymptotical limit in Fig.5 is Uch. Relative flat curves for c/n-Al2O3/13TiO2 coatings in Fig.5 suggest that the energy expended in plowing is not dominant due to their high brittleness. The reduced grain size in nanostructured coatings apparently increases the specific grinding energy. More energy is needed for plowing due to enhanced hardness in nanostructured material coatings. The higher toughness also means more energy required for new surface formation in grinding. 3.3 Comparison of surface roughness Surface roughness was measured to characterize the ground coatings. Fig.6 shows that the influence of grain size on surface roughness of both ground coatings is significant. Opposite to the grinding force and specific grinding energy, the reduced grain size results in the decrease of the surface roughness for both nanostructured coatings, which can be explained by material removal mechanism. As observed in SEM photos (Fig.7 and Fig.8), brittle fracture dominates in grinding c-Al2O3/13TiO2 coatings while ductile flow plays a main role in grinding n-Al2O3/13TiO2 coatings. Although ductile flow is the major material removal mechanism in grinding both c/n-WC/12Co coatings, the observed transgranular fracture may partially contribute to surface roughness in c-WC/12Co coatings.Similar to the grinding force, surface roughness for ground conventional and nanostructured coatings are closer to each other at a larger wheel depth of cut, which means that the effect of reduced grain size in nanostructured coatings disappears at a higher material removal rate. 3.4 SEM surface observations and comparison Fig.7 shows the SEM observations of ground c/n-WC/12Co coatings under the same grinding conditions. The ground c-WC/12Co coating surface is more segmented and larger WC grains can be observed when compared to the ground n-WC/12Co coating surface. The ground n-WC/12Co coating surface is completely covered with a layer of plastically deformed material and the WC grain boundary is hardly observable. The comparison of the SEM surface observations of ground c/n-Al2O3/13TiO2 coatings is shown in Fig.8. Although the defects from thermal spray process are observable, the sound and smooth surface of ground n- Fig. 6 Comparison of surface roughness. n-WC/12Co c-WC/12Co n-Al2O3/13TiO2 c-Al2O3/13TiO2 Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V 0 10 20 30 0 0.3 0.6 0.9 1.2 1.5 Depth of cut, mm Surface roughness Ra, ′102 nm (a) n-WC/12Co (b) c-WC/12Co Fig. 7 SEM observations of ground c/n-WC/12Co coatings. (a) n-Al2O3/13TiO2 (b) c-Al2O3/13TiO2 Fig. 8 SEM observations of ground c/n- Al2O3/13TiO2 coatings. 2121210 __mm 2121210 __mm 2121210 __mm 2121210 _mm 5 Al2O3/13TiO2 coating suggests that ductile flow is a predominant material removal mechanism. Brittle fracture results in rough and fractured surface of ground c-Al2O3/13TiO2 coatings. Chipping and transgranular fracture dominate the surface of ground c-Al2O3/13TiO2 coating. The surface observations explain the above difference in roughness. 4 CONCLUSIONS From the comparisons done on grinding force, specific grinding energy, surface finish and surface topography of nanostructured and conventional coatings, it is concluded that the grain size plays a significant role in material removal for grinding. Grinding force, break-in force and specific grinding energy vary inversely with the grain size while the surface roughness increases with the grain size. Both ductile flow and brittle fracture occur during grinding. The grain size influences the extent of ductile flow in grinding, which dominates the final appearance of ground surface. The reduced grain size and richer binder phases enhance both hardness and toughness in nanostructured materials, and therefore influence the grinding of these materials. However, at a higher material removal rate, the influence of grain size becomes insignificant. 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Nanostructured Materials, Vol.10, No.5, 1998, pp. 875-891 [6] S.Malkin. “Grinding technology, theory and application of machining with abrasives”. Ellis Horwood Limited, Chichester, England, 1989