TÍTULO

In terms of tool life, KO60 gave the best performance under all conditions used for these experiments (Figures 25, 56 and 78). At low speeds of 50 to

200 the tests were stopped after 15 minutes of machining because of very low flank face wear observed and to avoid wasting the work materials available. It is expected that very long tool lives would have been achieved when cutting with the pure oxide ceramic tools under the low speed conditions. At intermediate speeds of 250-450 m/min KO60 tools gave the best tool life, a tool life of 36 minutes was recorded when machining EN24 steel at speeds of 250 m/min, tool life of 35 minutes was recorded when machining D2 tool steel at a speed of 350 m/min and 35 minutes tool life was recorded when machining EN8 steel at a speed of 450 m/min (Figure 100). As cutting speeds was increased, the tool life decreased due to the high temperatures generated at the cutting edges. This coupled with relatively high stresses (both tensile and compressive) as well as low thermal conductivity of the pure oxide ceramic tools resulted in poor tool lives. The low thermal conductivity of the pure oxide ceramic tools will minimise heat conduction from the cutting edge thus increasing the temperature gradient between the hot and cold area of an insert.

In terms of tool life, KO90 were the second best tools when machining different grades of steels: EN8, D2 Tool steel and EN24 (Figures 25, 5 6 ,7 8 and 101). KO90 gave better tool life when it was used to machine D2 tool steel (Figure 101) at low speeds of 50 - 250 m/min; due to the presence of TiC which increases their resistance to attrition wear and diffusion wear. At moderate and high speed conditions, tool life was found to be shorter than that given by KO60 tools. This was due to the metallic attraction of titanium to carbon contents of steels. A summary of the wear mechanisms in the mixed ceramic tools show attrition wear as being dominant at lower speeds and diffusion wear became dominant at higher speeds. Other types of wear mechanisms like plucking, cracks and plastic deformation were also

observed when machining at high speeds. The optimum cutting conditions tor mixed ceramics were obtained at low to moderate speeds.

The SiC whisker reinforced ceramic tools (KYON 2500) gave the third best performance when cutting steels (Figures 25, 5 6 ,7 8 and 102). It was found that KYON 2500 could only be used satisfactorily to machine different grades of steel at lower speeds eg. 50 to 300 m/min and as the speed was increased, the tool life was shortened drastically. Better tool life as given by SiC whisker reinforced ceramic tools (KYON 2500) compared with the sialon ceramic tools at lower speeds due to superior combination of wear resistance and fracture resistance. An addition of SiC whiskers to ceramic tools which improved their hot hardness and toughness retarded flank face wear especially at lower speeds in comparison with the sialon ceramic tools (ie. KYON 2000 and KYON 3000). It is true to say that the addition of SiC whiskers to ceramic tools resulted in an increased performance by about two to three fold when machining steel work materials.

Sialon ceramic tools (KYON 2000 and KYON 3000) gave somewhat disappointing results when machining different grades of steels, at speeds above 150 m/min (Figures 103 and 104). There was a slight improvement in performance when this tool was used at lower speeds ie.50, 100, 150 m/min. This can be due to the condition of low temperature and stress taking place when machining with KYON 2000 and KYON 3000 tools. The pure oxide and mixed oxide ceramic tools have higher resistance to diffusion wear than the nitride based ceramic tools because of their higher energy of formation. They therefore form more stable bonds at high temperatures. The low energy of formation of nitride based ceramics explain why shorter tool lives were recorded when cutting with the KYON 2000 tools unlike the pure oxide and mixed oxide ceramic tools. KYON 3000 inserts gave even worse

performance than the KYON 2000 at all the conditions used to machine steels. This was because of the lower aluminium oxide percentage in KYON 3000 tools which may have resulted in enhanced diffusion process. The five different grades of ceramic tools investigated all follow the same wear pattern. Diffusion and attrition wear mechanisms were found to be operating during the machining processes. In the case of pure oxide ceramic (AlgOg) tools, attrition wear was the dominant wear mechanism. In particular, extensive chipping and plucking was apparent in the KO60 cutting tools. In the case of mixed oxide (AlgO^ TiC) tools having hard TiC particles attrition and diffusion wear mechanisms occurred at slow and higher speeds. Wear of the S L N . based ceramic (KYON 2000 and KYON 3000) tools was

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mainly dominated by diffusion wear mechanism. The AlgOg-SiC whisker reinforced ceramic (KYON 2500) tools showed an improvement in performance

compared with the sialon tools. They exhibited similar wear mechanisms as the pure oxide and mixed oxide ceramic tools.

CHAPTER 6