真空燒結及熱均壓製程對具不同黏結相的奈米碳化鎢硬質合金其燒結性質與顯微組織之研究

Study on the sintered Properties and Microstructure of Nanostructured WC-based Hardmetals with Variant Binders Produced by Vacuum Sintering and HIP Processes

張明弘(Ming-Hung 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(2014年)

https://doi.org/10.6841/NTUT.2014.00215

摘要

奈米化結構的碳化鎢硬質合金(WC-Co hardmetals)是使用在高精密和微製造加工的一種相當重要材料，主要是由於奈米化碳化鎢合金具有優越的高硬度、高強度和高韌性的機械性質。另一方面，鈷是目前已發現之多數碳化鎢最佳黏結相，它具有獨特性質，是硬質合金的主要黏結相；然而，使用鈷做為黏結劑的缺點，包括鈷的產量稀少且昂貴、市場價格的變動和鈷本身具有毒性。由於鐵、鎳和鈷屬於同一週期，均可以利用熱處理強化，被認為亦是理想的替代黏結相。本研究中，奈米化鈷基與鎳鐵鈷碳化鎢硬質合金的製備，將利用真空燒結及熱均壓等粉末冶金方式來探討該合金之性質。
實驗中將先利用不同的真空燒結溫度(1250°C、1300°C、1350°C及1400°C)，經由後續性質分析中找出兩種燒結硬質合金(N-WC-(Fe, Co, Ni)與N-WC-Co)之最佳燒結溫度後，並搭配兩種不同壓力之熱均壓處理，分別為125 MPa之低壓熱均壓(1250°C, 100分鐘, 125 MPa)和175 MPa之高壓熱均壓(1250°C, 100分鐘, 175 MPa)。試片的製作是經由真空燒結和熱均壓處理之製程；析出相的顯微結構微分析是利用SEM、XRD與EDS等技術。另一方面，以硬度測試和三點抗彎強度(TRS)量測其機械性質，並比較材料之破裂韌性(KIC)及動態電位腐蝕與磁性分析等。最後，評估以真空燒結及熱均壓製程，對於商用奈米化鎳鐵鈷碳化鎢硬質合金製造之可行性。
實驗結果顯示N-WC-(Fe, Co, Ni)與N-WC-Co硬質合金的最佳燒結溫度分別為1300°C及1350°C一小時；在此同時，N-WC-(Fe, Co, Ni)與N-WC-Co硬質合金產生良好的液相燒結，並展現優秀的機械性質。兩種不同燒結硬質合金所呈現出來的燒結性質分別為鄰接度(Contiguity)是0.44和0.42；硬度為HRA 90.83和90.92；三點抗彎強度(TRS)為2567.97與2860.08 MPa；破裂韌性(KIC)達到16.23與12.33 MPa•m1/2；腐蝕電流為3.288×10-5與2.467×10-5 A×cm-2。根據上述之實驗結果顯示，雖然N-WC-(Fe, Co, Ni)的三點抗彎強度相較之下略低了一些，但其破裂韌性明顯較高，亦有不錯的硬度表現，再加上其可以在較低溫度下燒結的特性，顯然，利用鐵、鎳來取代鈷作為黏結相是指日可待的。另一方面，本實驗中經熱均壓處理後之試片，由於產生大量黏結相團聚以及晶粒異質成長，因此在性質上並無明顯的改善。

關鍵字

奈米化硬質合金 ； 黏結相 ； 真空燒結 ； 熱均壓 ； 破裂韌性 ； 鄰接度

並列摘要

Nanostructured WC-Co hardmetals are one of the most important materials used in high-precision and micro-fabrication machining. Nanostructured WC-Co alloys possess excellent mechanical properties, such as high hardness, strength and toughness. Cobalt is considered to be the optimal binder metal for most applications, although using cobalt as the binder has several disadvantages, including its market-price fluctuations and environmental toxicity. However, since Fe and Ni belong to the same group as Co and can be strengthened by heat treatment, they are considered to be an ideal alternative binder. In this study, nanostructured N-WC-Co and N-WC-(Fe, Co, Ni) hardmetals were prepared in an innovative way by means of the vacuum sintering and hot isostatic pressing processes.
In the present research, four different sintering temperatures (1250°C, 1300°C, 1350°C and 1400°C) were studied to determine the optimal process parameters of N-WC-(Fe, Co, Ni) and N-WC-Co sintered hardmetals. Specimens were fabricated by vacuum sintering, combined with HIP processes of the powder metallurgy technique. The different procedures of HIP treatments included low (1250°C, 100 min and 125 MPa) and high pressure (1250°C, 100 min and 175 MPa) treatments, respectively. The specimens were produced by vacuum sintering and the hot isostatic pressing process. The precipitate phases present in the microstructures were analyzed using SEM, XRD and EDS techniques. In addition, hardness and transverse rupture strength (TRS) tests were used for the mechanical properties. Moreover, the fracture toughness was by means of KIC tests, and an electrochemical analysis and magnetic properties tests were also performed. Finally, an evaluation of the feasibility of nanostructured N-WC-(Ni, Fe, Co) hardmetals commercial manufacture was conducted via vacuum sintering and HIP processes.
Experimental results showed that the optimal sintering temperatures for N-WC-(Fe, Co, Ni) and N-WC-Co hardmetals were 1300°C and 1350°C for 1 h, respectively. The N-WC-(Fe, Co, Ni) and N-WC-Co hardmetals underwent a successful liquid-phase sintering and were shown to exhibit excellent mechanical properties. In addition, the sintered N-WC-(Fe, Co, Ni) and N-WC-Co hardmetals showed a contiguity of 0.44 and 0.42; hardness was enhanced to HRA 90.83 and 90.92; the TRS increased to 2567.97 and 2860.08 MPa; KIC was 16.23 and 12.33 MPa•m1/2 and the corrosion current (Icorr) was 3.288×10-5 and 2.467×10-5 A×cm-2. According to the experimental results, although the N-WC-(Fe, Co, Ni) hardmetals possessed a slightly lower TRS value, they exhibited the superior fracture toughness (KIC), with almost the same hardness as that of the N-WC-Co material. Significantly, N-WC-(Fe, Co, Ni) hardmetals could be sintered at a relatively lower temperature, but still had excellent mechanical properties. On the other hand, due to the present experiment’s obvious binder agglomeration and rapid grain growth of specimens by HIP treatment, the HIP process did not seem to have a positive effect on the properties of the two samples in this study.

並列關鍵字

Nanostructured Hardmetals ； Binder ； Vacuum Sintering ； HIP ； Kic ； Contiguity

參考文獻

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