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1、MULTI-SCALE AND MULTI-PHASE NANOCOMPOSITE CERAMIC TOOLS AND CUTTING PERFORMANCE*Abstract: An advanced ceramic cutting tool material AOs/TiC/TiN (LTN) is developed by incorporation and dispersion of micro-scale TiC particle and nano-scale TiN particle in alumina matrix. With the optimal dispersing an
2、d fabricating technology, this multi-scale and multi-phase nanocomposite ceramic tool material can get both higher flexural strength and fracture toughness than that of Al203ATiC (LT) ceramic tool material without nano-scale TiN particle, especially the fracture toughness can reach to 7.8 MPa m05. T
3、he nano-scale TiN can lead to the grain fining effect and promote the sintering process to get a higher density. The coexisting transgranular and intergranular fracture mode induced by micro-scale TiC and nano-scale TiN, and the homogeneous and densified microstructure can result in a remarkable str
4、engthening and toughening effect. The cutting performance and wear mechanisms of the advanced multi-scale and multi-phase nanocomposite ceramic cutting tool are researched.Key words: Multi-scale and multi-phase Ceramic tool material Mechanical properties Cutting performance0 INTRODUCTIONAs ceramic t
5、ool materials have a let o: advantages, such as high melting point, high hardnfss, and (;ood chcmicjl stability at high temperature, they can significji:tiy increase the tool-life and hence reduce tlie production time and costs. But the extremely low fracture toughi;e:s associated with these materia
6、ls has limited their applications, particularly in the intermittent or discontinuous cutting processes such as milling. Therefore, considerable research work has been carried out to improve the fracture toughness of ceramic tool materials by incorporating one or more compositions into the matrix mat
7、erial to form ceramic matrix composites. The reinforcing composition is often in the form of particles or whiskers, and it was proved that the whisker might better improve the fracture toughness than the particle dispersion1. And many kinds of micro-scale additives dispersed alumina matrix ceramic t
8、ool materials have been successfully developed and used in machining operation21. However, the whisker is not very practicable for the high price and its toxicity, and the comprehensive properties for the micro-scale particle reinforced ceramic composite should be further improved.In recent years, w
9、ith the development of the nano-scale powders, a great breakthrough in ceramic nanocomposites has been achieved in the world. NIIHARA3 first reported the significant improvement in the flexural strength and fracture toughness of alumina by the incorporation of nano-scale SiC particles. The research
10、results by LI, et al4, showed that adding nano-scale AI2O3 to micro-scale AI2O3 matrix might cause a toughening and strengthening effect. LIU, et al 51, and BARON, et al6, studied the multi-phase and single-nano-scale ceramics by sintering nano-scale AI2O3 with nano-scale SiC. It is shown that the f
11、lexural strength and the fracture toughness of multi-phase and single-nano-scale ceramics was greatly improved as compared to the monolithic alumina ceramics, and the major strengthening and toughening mechanisms are the transition from intergranular to transgranular fracture mode as a result of the
12、 contribution of nano-scale SiC located on the boundary or in the matrix. LIU, et al7, studied the multi-scale nanocomposites ceramic tool material by adding micro-scale and nano-scale SiC together into the micro-scale A1203 matrix. It is proved that the multi-scale i;anocomposit? can get both highe
13、r flexural strength and fracture toughness than that of the single-scale composite ceramics, and the improved mechanical properties may be mainly attributed to the intergranular-intragranular microstructure with a lot of micro-scale SiC particles located on the grain boundary and a few nano-scale Si
14、C particles located in the matrix grain. But so far, little research work on multi-phase and multi-scale nanocomposite ceramic tool materials has been carried out and reported.The present work attempts to study the possibility of improving the mechanical properties of micro-scale AI2O3 matrix cerami
15、cs by adding micro-scale TiC and nano-scale TiN together into the micro-scale AI2O3 matrix.1 FABRICATION OF MULTI-SCALE ANDMULTI-PHASE NANOCOMPOSITE CERAMIC TOOL MATERIAL AhO/TiC/TiNLTN)1.1 ExperimentThe compositions of the three compared materials are shown in Table 1. The raw nano-scale TiN was ul
16、trasonically dispersed in the medium of deionized water with polymethacrylic acid ammonia (PMAA-NH4) dispersant so as to avoid agglomeration, and then the mixed powders were ball-milled for 48 h in polyethylene jar. The mixed slurry was ultrasonically dispersed and electro-dynamically stirred simult
17、aneously for 30 min so as to get further homogeneous slurry, and dried in a vacuum chamber. The dried powders were screened through a sieve in glove-box for use. The compact powders were hot pressed for a certain time with a pressure of 30 MPa in N2 atmosphere and a sintering temperature of 1 700 C
18、to produce ceramic disks.The flexural strength, Vickers hardness and fracture toughness of the new ceramic tool materials were tested and compared with the other tool materials. Artificially fractured surfaces of the new ceramic tool materials were observed under a scanning electron microscope (SEM)
19、 Hitachi S-2500.1.2 Microstructure and mechanical propertiesThe effect of the fabrication duration time on the mechanical properties of the composite ceramic tool materials at the same sintering temperature of 1 700 C is shown in Table 2. It can be seen that the densities of the material LT and LTN1
20、 are increased with an increase in the duration time and the mechanical properties of the material LT and LTN1 fabricated at the duration time of 40 min are better than those at the duration time of 20 min. But the mechanical properties of the material LTN fabricated at the duration time of 30 min a
21、re the best one. By comparison of the fracture toughness and the flexural strength of the materials LTb, LTNb and LTN lb shown in Table 2, it can be seen that the fracture toughness and the flexural strength are initially increased and then reduced with an increase in the amount of nano-scale TiN po
22、wder. This is attributed to the microstructure agglomeration caused by the excessive amount of nano-scale TiN powder.It can be observed from Fig. 1 that the prolonged duration time can result in a higher relative density and more homogenous microstructure. Because the melting point of TiC is much hi
23、gher, it can inhibit the AOj graii from growing and the material LTb can get higher mechanical properties than material LTa. On the other hand, Table 2 and Fig. 2 indicate that although the fracture toughness of the material LTN 1 is improved with an increase in the duration time, the flexural stren
24、gth of LTN1 is reduced due to the abnormal grain growth shown in Fig. 2b. It is known that the toughening effect of dispersing particles was attributed to the large-sized particles, so the larger TiC particles may lead to the grain bridging and crack deflection which will improve the fracture toughn
25、ess by consuming more fracture energy during the crack propagation. While the larger grain size can lead to the reduction of flexural strength. It can be observed from Fig. 3 that the homogeneous microstructure of the material LTNb with a fine grain size and a high relative density was obtained at t
26、he sintering duration time of 30 min. Accordingly, the mechanical properties of LTNb are the best one of all, which is resulted from the synergism effect of micro-scale TiC particles and nano-scale TiN particles. The fractured surface of LTNb is evidently characterized by transgranular fracture show
27、n in Fig. 4a which can absorb more fracture energy and improve the mechanical properties rather than the material LTNc with intergranular fracture shown in Fig. 4b.2 WEAR RESISTANCE IN MACHINING TlOA STEELThe wear resistance was assessed by cutting tests turning one kind of steel on a 7.5 kW, 400 mm
28、 lathe (CA6140). The workpiece material was a kind of carbon tool steel (TlOA steel) bar with a hardness of 62 HRC. Four types of ceramic inserts, LTN, ASs, LT55 and SG4, the mechanical properties of which are shown in Table 3, were used in order to assess and compare the wear resistance of the tool
29、 materials. All the lathe tools were prepared to have an identical geometry, i.e., normal rake angle fir-5, normal clearance angle a=50, major cutting edge inclination angle =-5, major cutting edge angle ,=45, and the nose radius re=0.2 mm. In addition, all tools have a chamfered edge of 0.2 mm long
30、 with a -15 normal rake. All the tools used had no chip breakers and no coolant was used for all the cutting tests.When cutting the tool steel (TlOA steel), a cutting speed v of 97, 137 and 217 m/min, a feed rate/of 0.08 mm/rev, and a depth of cut ap of 0.1 mm were used. During the course of the cut
31、ting tests, the tool wear (wearland) was checked frequently with a tool makers microscope. Three inserts for each tool material are used for each workpiece material are their wearland measured at the same cutting time intervals. The average readings were then taken and plotted in Fig. 5. It is shown
32、 by the figures that the wearland size growth for the four tool materials follow the common patterns whereby a rapid increase is noticed at the initial cutting stage due to the asperity of the tool surface, followed by a nearly steady increase with the actual cutting time.As shown in Figs. Sa and Sb
33、 at the low and medium cutting speed, the initial wear rate for all tools is greater but is then reduced after about 2.0 min of cutting. It is believed that the surface of the tool was not well prepared, which contributed to this rapid tool wear. . Nevertheless, in the stable wear region, the wear r
34、ates for LTN, ASs and SG4 ceramics are reduced and comparable. The overall wear for the LTN and ASs ceramic tool is lower man that of the SG4 and LTSS ceramic tool, indicating an improved wear resistance by the nano-reinforcing effect. But the wear rates for three ceramic tools LTN, ASs and SG4 afte
35、r about 4 min of cutting show an identical pattern, as is the case in Fig. Sb.As shown in Fig. Sc, the wear and wear rates of the four tools are almost similar at the high cutting speed but are sharply increased after about 2.0 min of cutting. It can also be seen that the wear of the new ceramic too
36、l LTN is the smallest one.3 CONCLUSIONSThe multi-scale and multi-phase nanocomposite tool material AlO/TiC/TiN (LTN) is successfully fabricated by adding both micro-scale TiC particle and nano-scale TiN particle dispersion. The comprehensive mechanical properties of LTN are higher than that of A1203
37、/T:C (LT). Because of ihe addition of micro-scale TiC with high elastic modulus and high hirdacss, the framework microstructure is easily formed and the particles are enchased wch other with AijG3 rtnd then improves the flexural strength of composite. Proper content of nano-scale TiN can lead to the
38、 further refinement of matrix grains, the remarkable strengthening aid toughening effects are resulted from the crack pinning, deflection and bridging effect as well as dislocations. The homogeneous and densified microstructure as well as the coexisting transgranular and intergranular fracture mode
39、induced by micro-scale TiC and nano-scale TiN can result in the remarkable strengthening and toughening effect. While excessive content of nano-scale TiN may lead to the deterioration of the mechanical properties. The machining tests have shown that the new materials have a superior wear resistance over the other ceramics in the normal wear stage.
