多晶立方氮化硼之燒結研究:鋁矽合金之作用

Abstract

cBN是當今世上所知材料中第二硬的物質，也是在超硬材料中高溫穩定性最好的物質，在過去前人的研究中Si和cBN在高溫下(>1500℃)是無法發生化學反應，這也造成以Si和BN產生鍵結形成另一樣同是超硬物質的C-Si3N4的化合物的困難性，本研究以Al做為和BN的主要反應劑，Si做為主要的抑制劑，用以調節Al和BN反應時未來得及反應的N及反應後留下的N，本研究主要分成兩大部份，第一部份為高溫常壓下燒結(1350℃)，第二部份為高溫高壓下燒結(1350℃、5.5GPa)，在常壓下燒結出來的燒結體雖未能達到緻密良好的燒結體，但仍能提供是否能如預期讓Si在高溫下與N產生反應，並讓原本不與BN反應的Si成功的生成Si3N4化合物之資訊，以推論在高壓實驗下燒結成功的機會，據此本研究在高溫高壓實驗中，成功的合成出超高硬度的PcBN (Hv 5400)，在本實驗中另外使用了奈米等級的cBN重覆實驗，並且另行加入了B4C、AlN兩樣材料，結果顯示使用奈米等級的cBN並不一定能夠得到更好機械性質的PcBN燒結體，且若在cBN和Al-Si合金外另外加入B4C、AlN等化合物，並不會讓機械性質有所提升，反而會使燒結體的整體非晶質體數量增多，並破壞PcBN的機械性質，造成燒結體的整體機械強度下降。

Cubic boron nitride (cBN) is a high pressure synthetic material, having extreme hardness just next to diamond. Among superhard materials, cBN is the most stable when it applies at high temperature (>1000℃). Prior studies show that Silicon (Si) reacts hardly with cBN at temperatures as high as 1500℃, which imposes difficulty for the formation of superhard material, C-Si3N4. In this research, we use Aluminum (Al) as a major reagent to react with cBN and Silicon (Si) as an inhibitor for AlN formation to modulate free Nitrogen (N). The newly generated free N, which can not react timely with Al to form AlN, may react with Si to form Si3N4. There are two major parts in this research. Part I includes the sintering experiments at high temperature (1350℃) in vacuum, and part II is sintering under ultra-high pressure and high temperature (HTHP) (1350℃, 5.5 GPa). The results from low pressure experiments show that Si reacts successfully with BN to form Si3N4, although the density and mechanical property of the sintering body have not reach the requirement. This result infers that we can also successfully sinter Si3N4 –bearing body with better mechanical property at HTHP. In part II with HTHP experiment, we successfully synthesized superhard PcBN (Hv 5400) from Si-Al and cBN. We also repeated this experiment with nano cBN particle. The result indicates that mechanical property of PcBN sintering body using nano particles is no better than micro particles. Experiments by adding two materials B4C and AlN in Si-Al and cBN show the increase of amorphous materials in the sintering body which resulted in the observed deterioration of entire mechanical property of PcBN .