真空燒結及熱均壓處理對奈米碳化鎢超硬合金燒結性質與顯微結構之研究

Study on the Sintered Properties and Microstructures of Nano-WC Hard Materials Produced by Vacuum Sintering and HIP Processes

張伯瑜(Po-Yu Chang)

指導教授 ： 張世賢

國立臺北科技大學/工程學院/材料科學與工程研究所/碩士(2013年)

https://doi.org/10.6841/NTUT.2013.00107

摘要

鈷基碳化鎢硬質合金(WC-Co)被廣泛使用於不同的切削、鑽孔以及其他方面的應用；碳化鎢提供了必需的強度及耐磨耗性，而鈷則提供合金韌性與延展性。奈米材料能夠提供高的強度、硬度以及優越的延展性與韌性，而奈米材料的這些優越性質，則可歸因於奈米尺寸的晶粒與高體積分率的晶界。此外，真空燒結是製造碳化鎢材料的一項有效的方法，而熱均壓技術為一種同時結合了高溫與高壓的熱處理方式，並已被廣泛應用於粉末冶金工業上，以去除工件內部的封閉孔隙與缺陷，來改善材料的機械及物理性質。
本研究主要運用不同的真空燒結溫度(1250°C、1300°C、1350°C及1400°C)，以尋求微米(Micro-)與奈米(Nano-)碳化鎢合金最佳的燒結溫度。此外，將更進一步比較兩種不同粉末尺寸對碳化鎢硬質合金性質的差異；試片的製作是利用粉末冶金真空燒結方式，結合熱均壓(1250°C 100 min 125 MPa)製程，析出相的顯微結構分析是利用SEM、XRD與EDS等技術。另一方面，並以硬度測試和橫向破裂強度(TRS)量測機械性質，腐蝕電位分析作為耐腐蝕性測試，以及材料破裂韌性(K1C)等，來評估真空燒結及熱均壓製程對於商用製造奈米碳化鎢硬質合金之可行性。
實驗結果顯示微米與奈米碳化鎢皆有良好的液相燒結且有較低的孔隙率，並擁有良好的機械性質。對於微米與奈米碳化鎢兩者的最理想真空燒結溫度皆為1350°C持溫一小時。其孔隙率可以降低到0.36%與0.8%，硬度分別可以提高到 HRA 90與91以上，橫向破裂強度TRS則可以增加到1441.62 MPa 與1540.56 MPa；同時，燒結後的奈米碳化鎢破裂韌性可以達到12.71 MPa m1/2，這些結果顯示燒結的奈米碳化鎢擁有較好的機械性質。此外，經過熱均壓處理後，微米與奈米碳化鎢的橫向破裂強度皆可以分別增加到1627.32與1842.69 MPa，同時只減少些微的硬度(微米碳化鎢由90.11降至88.76 HRA；奈米則由91.25降至90.24 HRA)，這顯示了熱均壓處理可以有效的改善抗彎強度。另一方面，在3.5wt% 氯化鈉溶液中的腐蝕測試結果顯示，經熱均壓處理之奈米碳化鎢擁有最低的腐蝕電流(1.1756 × 10-5 Amps) 以及最高的極化阻抗(2718.0 Ω•cm2)；同時，其破裂韌性也可維持在12.61 MPa m1/2。

關鍵字

真空燒結 ； 熱均壓 ； 奈米碳化鎢 ； 腐蝕電位 ； 破裂韌性

並列摘要

Tungsten carbide-cobalt hard alloys (WC-Co) are widely used for a variety of cutting, drilling and other applications. Tungsten carbide imparts the alloys with necessary strength and wear resistance, whereas cobalt contributes to the toughness and ductility of the alloys. Nano-materials possess high strength, high hardness and excellent ductility and toughness. The superior mechanical properties of nano-scale materials also have been attributed to the nano-sized grains and high volume fraction of grain boundaries. In addition, vacuum sintering is a useful method for manufacturing WC-Co material from powders. The hot isostatic pressing (HIP) technique is a heat treatment method which combines high temperature and high pressure. Recently, the HIP technique has been widely used in the power metallurgy industry to eliminate isolated pores and defects on the inside of work-pieces, thus, improving the mechanical and physical properties of materials.
In the present research, different sintering temperatures (1250°C, 1300°C, 1350°C and 1400°C) were explored in order to find the optimal parameters of Micro- and Nano-WC sintered materials, as well as to compare the different properties of two sizes for WC materials. The specimens were fabricated by using vacuum sintering of the powder metallurgy technique combined with the HIP process (1250°C, 100 min, 125 MPa). The precipitate phases presented in the microstructures were analyzed using SEM, XRD and EDS techniques. In addition, the hardness test and transverse rupture strength (TRS) were used for the mechanical property test, the corrosion potential analysis for the corrosion test and the K1C test for the fracture toughness. Finally, the feasibility of commercial manufacturing of nano-WC cement carbides via vacuum sintering and HIP processes was evaluated.
The experimental results showed that micro-WC and nano-WC specimens possessed good liquid-phase sintering and lower porosities, and thus, exhibited excellent mechanical properties. The optimal vacuum sintering temperature of micro- and nano-WC was 1350°C for 1 h. The porosities were decreased to 0.36% and 0.8%, the hardness was enhanced to HRA 90 and 91 and the TRS was increased to 1441.62 and 1540.56 MPa, respectively. Meanwhile, the value of K1C for sintering nano-WC increased to 12.71 MPam1/2. According to the above, these results indicate that sintering nano-WC has superior properties. In addition, the TRS of micro- and nano-WC increased to 1627.32 and 1842.69 MPa after HIP treatments, respectively. Meanwhile, hardness only decreased slightly (Micro-WC HRA 90.11 → 88.76; Nano-WC HRA 91.25 → 90.24). This shows that the HIP treatment has a more effective improvement in TRS. Moreover, the corrosion test results show that HIP treated Nano-WC had the lowest corrosion current (Icorr) of 1.1756 × 10-5 Amps and the highest polarization resistance (Rp) of 2718.0 Ω•cm2 in 3.5 wt% NaCl solution. Simultaneously, the fracture toughness of K1C remained at about 12.61 MPa m1/2.

並列關鍵字

Vacuum Sintering ； HIP ； Nano-WC ； Corrosion Potential ； K1C

參考文獻

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