Introducing Meso-scale channels on Ceramics by Machining

Transcription

1 Introducing Meso-scale channels on Ceramics by Machining H-W Shin, C.K. Kok, E. Case*, and P. Kwon Department of Mechanical Engineering Department of Chemical Engineering and Materials Science* Michigan State University East Lansing, Michigan EXTENDED ABSTRACT Conventional wisdom dictates that ductile metals are easier to machine than brittle materials such as engineering ceramics, glasses, semiconductors, diamonds, and metal composites. In fact, Sreejith et al. [1] attributed the ease of machining metals to their inherent properties such as their non-directional bonding, high symmetry of crystal structure, high thermal conductivity, low density, zero porosity, high fracture toughness, large strain to fracture and high impact energy. They also explained the nature of bonding impact with material removal mechanism. Ceramics, on the other hand, possess many important attributes that cannot be attained from metals. Most covalent-bonded ceramics are hard, strong, and possess high melting temperature, exhibiting low symmetry of crystal structure, limited or inadequate slip systems for plastic deformation, low thermal conductivity, low fracture toughness and low breaking energy. Due to the brittleness of ceramics, the requirements on high-temperature processing, and the difficulty in traditional forming and machining techniques, the use of ceramic materials for engineering applications has been severely limited. As the usages of advanced ceramics are growing increasingly for applications in aerospace, automotive and electronics industries, one of most important key issues hindering the utilization of ceramic materials is in its processing. The present study tackles this problem by exploring a novel technique to shape ceramic materials by combining traditional ceramic-powder processing techniques with those of machining and joining. The recent studies [1, 2, 3] show that it is possible to machine brittle materials like ceramics in a ductile manner under certain machining parameters. Bifano et al. [4] explain when the critical resolved shear stress within a brittle material exceeds the elastic yield stress during machining, it is possible to change the mechanism of deformation. Kumbera et al. [2] analyzed the process by running the numerical simulation. They showed that silicon carbide change its phase from brittle to ductile under high pressure at contact interface between the work piece and the cutting tool by investigating the effects of cutting speed, tool tip radius, rake angle and feed on the machining of silicon carbide. By increasing the cutting speed, temperature and pressure can be increased, which causes thermal softening of materials. As the silicon nitride material undergoes a high-pressure phase transformation at about its hardness value of 22GPa under hydrostatic stress state conditions Winter Topical Meeting - Volume 28 20

2 In conjunction with our current research in the fabrication of a meso-scale heat exchanger, we have explored the processing possibility of introducing meso-scale channels on the surface of ceramics. The motivation for the complex network of channels is to circulate cooling fluids in the meso-scale heat exchangers to be developed. In our previous work, internal meso-scale channels have been introduced in the bulk of ceramic materials by embedding a fugitive phase inside the ceramic powder during compaction. During sintering, the fugitive phase burns away leaving the channels in the bulk of ceramics. A similar process has been used to make surface channels as well. The materials with the surface channels can be joined to form internal channels. However, this technique possesses a major disadvantage as it is only capable to make simple channels. As the shape of the channels becomes complex, powder flow around the complex fugitive phase becomes increasingly limited. This leads to the forming of large cracks around the fugitive phase in the final ceramic parts. By testing with tungsten carbide drills and a diamond grinding wheel, David et al. [5] showed that two-phase mixtures of consisting of refractory oxides, such as Al 2 O 3, ZrO 2, Mullite, and rare-earth phosphates, such as LaPO 4, CePO 4, can be cut and drilled due to the weakness of their bonding and formation and linking of cracks at the weak interfaces between the two phases. Similar to our work, Halcomb et al. [6] and Klocke et al. [7] machined advanced ceramics before final sintering and was able to reduce the expensive cost of processing hard materials. To demonstrate this, they successfully carried out the machining of green compact, for which a binder phase was use to consolidate the compact for machining, and white compact, which is the product after pre-sintering. The advantage of machining before final sintering is attributed to the lower inherent strength and hardness of green and white compacts as compared to their finally sintered counterparts. In addition, the desirable surface quality was obtained by decreasing the feed rate in green and white machining and increasing the feed rate increases relative wear and decreases tool life [ 8 ]. During machining, the powder-debris from the specimen can cause wear damage on sideways and bearings. Therefore, the debris should be cleaned by extraction equipment such as vacuum removal for good machining precision. Increasing the revolution speed (in rpm) of the cutting tool not only decrease the machining time, but also reduce the amount of tool wear [9] because tool loads are higher at the slower cutting speeds thus it is desirable to machine in high cutting speed. Most engineering ceramic materials after they have been fully sintered are too brittle and hard to machine with conventional machine tools. Mixing a binder phase with the powder and Binder-burnout are undesirable. A new meso-scale processing method is needed to introduce a complex network of channels in a ceramic material. One of the flexible methods of achieving this objective is the CNC milling process. Machining is controlled to arrive at the specific component shape, size for application, desired finish and strength, and so on, at a low cost. Therefore, we have prepared partially sintered ceramics (PSC) that have barely formed necks among the powders. Fig. 1 shows the partially stabilized zirconia samples about the size of a dime with the complex channels machined and subsequently sintered. PSC are easily handled and mounted on a conventional table-top CNC machine. American Society for Precision Engineering 21

3 Figure 1: Complex Channels and Cavity Achieved by CNC Milling In our work, a range of machining parameters including various pre-sintering temperatures, RPM and feed rates have been tried to determine the optimal processing condition. In particular, pre-sintering temperatures of 600, 800, 1000 C, feed-rate increments between 2.5 to 25 mm/min, and rpm varying from 500 to 2500, have been used to mill a series of channels on pre-sintered ceramic specimens made of partially stabilized zirconia (PSZ). The milling tool used in this study was made of stainless steels and the diameter is 0.625mm. This allows us to observe the finished surfaces in and near the channels at various rpm and feed rate in order to narrow down the recommended processing range. In our experiments, the green compacts without binder phase are too unwieldy for machining. Not only they are hard to handle but also they have a tendency to fragment during machining. Consequently, green machining is discounted in making a meso-scale heat exchanger. A set of recommended parameters governing both pre-sintering and machining processes is arrived based on SEM observations on fully sintered samples. Fig. 3 and 4 shows two extreme cases. As shown in Fig. 4, the finished channels would lead to the absence of surface fractures near the edges of the channels. Based on the observations, higher rpm, low feed rate and pre-sintering at a temperature above 800ºC for 4 hrs are generally recommend for machining the pre-sintered PSZ. Even though more experimental data are needed to understand machining the pre-sintered alumina samples, the similar machining condition may be recommended except there was a major difference in the recommended pre-sintering temperature. The channels on the samples pre-sintered at 600ºC were much better. Fig. 5 shows the cross-section of the channel. Machining, more specifically milling of channels, causes some effect on the material near the channels. The SEM observations were made on the surfaces inside of channels and the surface away from the channels. Unlike typical machining of metals, machining the PSC does not cause plastic deformation but breaking of the necks formed during the presintering process. Because the PSC are subsequently fully sintered (typically up to 98% theoretical density), the samples undergo substantial shrinkage (about 50% in volume or about 80% in length) during sintering. However, the shrinkage on the size of channels was much larger (around 47% reduction in size) than the shrinkage of the overall samples (around 25% reduction in diameter) in all our samples Winter Topical Meeting - Volume 28 22

4 5mm 1mm Figure 2: Excessive Surface Damage Caused by Milling at 500rpm and feed rate of 25mm/min on the Pre-sintered sample at 600oC for 4 hr. The Size of A channel 5mm 1mm Figure 3: No Pronoun Surface Damage Caused by Milling at 1500rpm and feed rate of 25mm/min on the Pre-sintered sample at 1000 o C for 4 hr. The discrepancy may come from the subsurface damage around the channels during machining. Depending on the pre-sintering temperature, the extent of neck formation between the powders affects the integrity of the samples. Milling PSCs cause the small subsurface cracks around the cracks. Final sintering heals these cracks as the cracks after final sintering cannot be observed. After milling with the milling tool with the same diameter (0.625mm) used in the experiment, the sizes of the channels after pre-sintering for all samples (processes at different sintering temperature were practically the same between 0.713mm and 0.794mm. After final sintering the size between 0.516mm and 0.605mm were observed. The data were profoundly depended on the pre-sintering temperature. This can be explained by the difference in the region affected by small subsurface cracks. American Society for Precision Engineering 23

5 Figure 4: The Cross-section of a Channel In the meso scale machining of metals, the major problem lies in the plastic deformation of the metals. Extensive burr formation [10] or chip formation [11] has been observed even with the special machine tool designed to rotate up to 670,00 rpm for meso-scale machining. Machining PSCs does not suffer from these problems. To form complex networks of internal channels, the specimens are joined which can be used to circulate coolants. Joining requires a precursor of silicate film, which is deposited on to the surfaces to be joined. Silicate film spin-coated from a liquid containing silicate is then cured. Joining takes place with MaCor on the conventional oven at around 1400 o C. The joined sample is then sectioned to observe the integrity of the join via SEM as shown in Fig. 5. Figure 5: The Cross-section of the Joined Sample 1 Kumbera, T.G., Cherukuri, H.P., Patten, J.A., Brand, C.J. and Marusich, T.D., 2001, Numerical simulations of ductile machining of silicon nitride with a cutting tool of defined geometry, Machining Science and Technology, 5[3], pp Winter Topical Meeting - Volume 28 24

6 2 Morris, J.C., Callahan, D.L., Kulik, J., Patten, J.A., and Scattergood, R.O., 1995 Origins of the Ductile Regime in Single-Point Diamond Turning of Semiconductors, J. Am. Ceram. Soc., 78[8], pp Chandra, A., Wang, K., Huang, Y., Subhash, G., Miller, M.H., and Qu, W., 2000, Role of Unloading in Machining of Brittle Materials, Journal of Manufacturing Science and Engineering, 122, pp Bifano, T.G., Dow, T.A. and Scattergood, R.O., 1991, Ductile-regime Grinding: A New Technology for Machining Brittle materials, Journal of Engineering for Industry, 113[2], pp Davis, J. B., Marshall, D. B., Housley, R. M., and Morgan, P. E. D., 1998, Machinable Ceramics Containing Rare-Earth Phosphates, Journal American Ceramic Society, 81[8], pp Halcomb L. D. and Rey, M. C., 1982, Ceramic Cutting Tools for Machining Unsintered Compacts of Oxide Ceramics, Ceramic Bulletin, 61[12], pp Klocke, F., Gerent, O. and Schippers, C., 1997, Machining Advanced Ceramics in the Green State, Ceramic Forum International(CFI) /Berichte der DKG, 74[6], pp ,. 8 King, A. G. and Wheildon, W. M., Ceramics in Machining Processes, Academic Press, Klocke, F., Gerent, O. and Schippers, C., 1999, Machining of Ceramics and Composites, ed. Jahanmir, S., Ramulu, M., Koshy, P., Manufacturing Engineering and Materials Processing, Marcel Dekker, pp Lee, K. and Dornfeld, D. A., A Experimental Study on Burr Formation in Micro Milling Aluminum and Copper, Transactions of NAMRI, XXX, pp Kim, C.-J., Bono., M. and Ni, J., 2002, Experimental Analysis of Chip Formation in Micro-Milling, Transactions of NAMRI, XXX, pp American Society for Precision Engineering 25